Photochemical Conversion of Solar Energy

DOI: 10.1002/cssc.200700087

Photochemical Conversion of Solar Energy Vincenzo Balzani,* Alberto Credi, and Margherita Venturi[a] In memory of Giacomo Ciamician (1857–1922) in the 150th anniversary of his birth

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Photochemical Conversion of Solar Energy

Energy is the most important issue of the 21st century. About 85 % of our energy comes from fossil fuels, a finite resource unevenly distributed beneath the Earth’s surface. Reserves of fossil fuels are progressively decreasing, and their continued use produces harmful effects such as pollution that threatens human health and greenhouse gases associated with global warming. Prompt global action to solve the energy crisis is therefore needed. To pursue such an action, we are urged to save energy and to use energy in more efficient ways, but we are also forced to find alternative energy sources, the most convenient of which is solar energy for several reasons. The sun continuously provides the Earth with a huge amount of energy, fairly distributed all

over the world. Its enormous potential as a clean, abundant, and economical energy source, however, cannot be exploited unless it is converted into useful forms of energy. This Review starts with a brief description of the mechanism at the basis of the natural photosynthesis and, then, reports the results obtained so far in the field of photochemical conversion of solar energy. The “grand challenge” for chemists is to find a convenient means for artificial conversion of solar energy into fuels. If chemists succeed to create an artificial photosynthetic process, “… life and civilization will continue as long as the sun shines!”, as the Italian scientist Giacomo Ciamician forecast almost one hundred years ago.

1. Introduction Energy is the most important issue of the 21st century.[1] Fossil fuels have offered astounding opportunities during the 20th century in the rich countries of the western world, but now reserves of fossil fuels are progressively decreasing[2, 3] and their continued use produces harmful effects such as pollution that threatens human health and greenhouse gases associated with global warming.[4] Currently, the world’s growing thirst for oil amounts to almost 1000 barrels a second, which corresponds to about 2 liters per day per person living on Earth.[5] The global energy consumption is equivalent to 15 trillion watts (15 TW) of power demand, which is expected to increase by 50 % by 2030.[6, 7] The goal of ecological sustainability is even more imperative if we consider the problem of disparity. As an example, a US citizen consumes as much energy as two Europeans, 10 Chinese, 20 Indians, or 30 African people.[8] Disparity is, indeed, the most prominent characteristic among Earth’s inhabitants and also the most difficult problem to solve. We are well aware that the stability of human society decreases with increasing disparities. How long can we keep running this road? Here is the fundamental challenge we face; here are many vital and entangled questions that we are called to answer.[1] As informed citizens, we have the duty to speak up with decision-makers and politicians on the key issues of the irresponsible depletion of resources, the reckless increase of pollution, and the intolerable and ever-increasing disparity between the rich and the poor. As chemists, we can help by improving energy technologies and, hopefully, finding a scientific breakthrough capable of solving the energy problem at its root. We are lucky that spaceship Earth, which is otherwise a closed system, receives an inexhaustible power flow from the sun: 120 000 TW of electromagnetic radiation. It is a quantity of energy far exceeding human needs. Covering 0.16 % of the land of the Earth with 10 % efficient solar-conversion systems would provide 20 TW of power,[6] nearly twice the world’s consumption rate of fossil energy and the equivalent of 20 000 nuclear fission plants of 1 GWe each. Sunlight is our ultimate energy source, and we need to learn not only how sunlight is used by nature to power life[9] but also how we can convert ChemSusChem 2008, 1, 26 – 58

sunlight into forms of energy useful for the development of our civilization.[4, 10–14] Sunlight in the geological eras has also provided us with fossil fuels, the nonrenewable energy source that we are so eagerly consuming. In contrast, we are not yet able to take full advantage of the extraordinary amount of energy that the sun supplies us with every day. This paradox was first pointed out by the Italian scientist Giacomo Ciamician in a famous lecture entitled “The Photochemistry of the Future” delivered in New York at the VIII International Congress of Pure and Applied Chemistry (1912):[15] “So far human civilization has made use almost exclusively of fossil solar energy. Would it not be advantageous to make a better use of radiant energy?” Ciamician also realized that a civilization based on solar energy could re-equilibrate the economic gap, already existing at that time, between northern and southern regions of the world: “Solar energy is not evenly distributed over the surface of the earth. There are privileged regions, and others that are less favored by the climate. The former ones would be the prosperous ones if we should become able to utilize the energy of the sun. The tropical countries would be conquered by civilization which would in this manner return to its birth-place”. The final sentence of that paper presents a concept quite meaningful even today (we should only add oil, gas, and nuclear energy to coal): “If our black and nervous civilization, based on coal, shall be followed by a quieter civilization based on the utilization of solar energy, that will not be harmful to the progress and to human happiness.” Solar energy has an enormous potential as a clean, abundant, and economical energy source, but cannot be employed as such; it must be captured and converted into useful forms of energy. Because solar power is diffuse (ca. 170 W m
2) and intermittent, conversion should involve concentration and storage. The ultimate challenge remains the production of a fuel [a] Prof. V. Balzani, Prof. A. Credi, Prof. M. Venturi Dipartimento di Chimica “G. Ciamician” Universit> di Bologna Via Selmi 2, 40126 Bologna (Italy) Fax: (+ 39) 051-209-9456 E-mail: [email protected]

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V. Balzani et al. capable of being stored and transported, such as hydrogen, a process that would solve both the energy crisis and the environmental emergency. Light excitation can induce a variety of chemical reactions. For energy-conversion purposes, photoinduced electron transfer is by far the preferred reaction in nature. This process generates a charge-separated state, which is then used to prepare the various high-energy molecules that fuel an organism. After the energy crisis of the 1970s, several types of endoergonic photochemical reactions (e.g. photodissociation, valence pho-

Vincenzo Balzani was born in Forlimpopoli (Italy) in 1936. He received his “Laurea” in chemistry at the University of Bologna in 1960. Following an assistant professorship at the University of Ferrara, he joined the faculty at the University of Bologna in 1969 and has remained there to this day. His scientific activity is documented by three monographs and about 500 papers in the fields of photochemistry, supramolecular chemistry, molecular-level devices and machines, and solar energy conversion. He is one of the 50 most-cited scientists in the field of chemistry (ISI). Alberto Credi was born in Bologna (Italy) in 1970. He received his “Laurea” (1994) from the University of Bologna, where, after a research period in the U.S., he later earned his PhD (1999). He is currently Associate Professor of Chemistry there. He has received several scientific awards, including the IUPAC Prize for Young Chemists (2000) and the Grammaticakis–Neumann Prize for Photochemistry (2006), and co-authored about 140 scientific papers and several books in the fields of molecular and supramolecular photochemistry and electrochemistry. Margherita Venturi is Professor of Chemistry at the University of Bologna. From 1972 to 1991, she worked at the National Research Council of Bologna, where she mainly studied, by means of pulsed and continuous radiolytic techniques, electron-transfer processes involved in model systems for the conversion of solar energy. She joined the group of Prof. Balzani in 1992. Her present research interests are in the field of supramolecular photochemistry and electrochemistry. She is the co-author of about 150 publications in international journals as well as the monograph “Molecular Devices and Machines” (with V. Balzani and A. Credi).

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toisomerization)[16] were proposed for the artificial conversion and storage of solar energy, but the results have been disappointing. Once the mechanism of natural photosynthesis was at least in part elucidated, mimicry of this natural process for artificial solar energy conversion began to be pursued by several research groups, as forecast by Ciamician:[15] “The photochemical processes, that hitherto have been the guarded secret of the plants, will have been mastered by human industry, which will know how to make them bear even more abundant fruit than nature, for nature is not in a hurry but mankind is.” It should be pointed out, however, that only hopes, not fruit, are abundant so far. Hopefully, a more detailed knowledge of natural photosynthesis coupled with a much stronger research effort in the field of chemistry will succeed to create an artificial photosynthetic process.

2. Natural Photosynthesis 2.1. Introduction Photosynthetic organisms are ubiquitous in nature; they are responsible for the development and sustenance of all life on Earth. They may be quite different, but all of them use the same basic strategy, in which light is initially absorbed by antenna proteins containing many chromophores, followed by energy transfer to a specialized reaction center protein, in which the captured energy is converted into chemical energy by means of electron-transfer reactions.[17]

2.2. Natural Antenna Systems The better-known natural antennae are the light-harvesting complexes of photosynthetic purple bacteria.[18] A major breakthrough in the field was the high-resolution X-ray crystal structure of the light-harvesting antenna complex LH2 of the photosynthetic unit of Rhodopseudomonas acidophila (Figure 1).[19] The complex is composed of two rings of bacteriochlorophyll (BChl) molecules, namely 1) a set of 18 molecules close to the membrane surface in almost a face-to-face arrangement like a turbine wheel, and 2) another set of nine molecules all lying in a plane that is perpendicular to the earlier ring of BChl molecules, in the middle of the bilayer. These structures are con-

Figure 1. Structure of the LH2 light-harvesting antenna system of R. acidophila which contains rings of 18 (a) and nine (b) bacteriochlorophyll molecules. See text for details. Reprinted with permission from Ref. [19].

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Photochemical Conversion of Solar Energy tained within the walls of two protein cylinders with radii of 1.8 and 3.4 nm. Because of the different chemical environments, the two sets of BChl molecules have different absorption and photophysical properties. The 18 BChl molecules belonging to the larger wheel have the lowest-energy absorption maximum at 850 nm (and are therefore named B850), and the nine BChl molecules in the middle of the bilayer have the lowest-energy absorption maximum at 800 nm (B800). There are other significant differences between the two sets of pigments:[20] the B800 species are largely monomeric, whereas the B850 molecules are strongly exciton-coupled, with the exciton state delocalized over several (presumably four) BChl molecules. All the BChl molecules are maintained in a fixed spatial relationship by the surrounding polypeptides. Carotenoids are also associated within the LH2 structure with the dual function of contributing to light harvesting and protecting the system against photooxidation by quenching the singlet oxygen molecules produced by photosensitization. The light absorbed by the B800 array is transferred to the B850 wheel within 1 ps. Energy migration among the various exciton states of B850 then occurs on the 300-fs timescale. The energy collected by the LH2 antennae is then transferred to another antenna complex, LH1, which surrounds the reaction center (RC). The reaction center is the final destination of the collected energy, and it is the site where charge separation takes place. A schematic view of the overall light-harvesting process is shown in Figure 2. The structure of LH1 is not

from approximately eight sites of LH1, each one comprising four delocalized BChl subunits, to RC (assuming one RC per 32 BChl LH1 molecules).[20] In conclusion, in natural light-harvesting antennae ultrafast energy migration within almost isoenergetic subunits of a single complex is followed by fast energy transfer to a lowerenergy complex with minimal losses. All processes are believed to occur by a Fçrster mechanism. Recent studies have provided evidence for wavelike energy transfer through quantum coherence.[22] Superposition states allow excitation to reversibly sample relaxation rates from all component exciton states, thereby efficiently directing the energy transfer to find the most effective sink for excitation energy. When viewed in this way, the system is essentially performing a single quantum computation, sensing many states simultaneously and selecting the correct answer, as indicated by the efficiency of energy transfer. The light-harvesting complexes of green plants are not well known and, likely, they are more complicated than those of bacterial photosynthesis.[23, 24] There are good reasons to believe, however, that the governing principles of operation are similar to those discussed above. 2.3. Natural Reaction Centers The simplest and best understood reaction center is that found in purple bacteria, which can be taken as a model of all the photosynthetic reaction centers.[25–27] The most important solar-energy-conversion process, however, is that occurring in green plants,[28–30] where the reaction center of Photosystem II has an electron-acceptor site quite similar to that of the bacterial reaction center and a very peculiar donor site, which can use water as an electron source and produce dioxygen as a “waste” product. Because this peculiar feature is particularly relevant to the design of artificial systems capable of performing photoinduced water splitting (Section 3.7), the donor site of the reaction center of Photosystem II will also be illustrated. 2.3.1. Bacterial Photosynthesis

Figure 2. Schematic representation of the overall light-harvesting process by LH2 and LH1 antenna complexes in bacterial photosynthesis. RC denotes the reaction center.

known at the same level of definition as that of LH2, but an analysis by electron crystallography of two-dimensional crystals of the LH1 complex of Rhodospirillum rubrum[21] has revealed that, although LH1 is much larger, there is a clear similitude between LH1 and LH2: its 32 BChl molecules are indeed arranged as the B850 molecules of LH2. LH1 absorbs at 880 nm (B880) and, because LH1 and LH2 are in close contact (estimated to be shorter than 30 N), LH2!LH1 energy transfer is quite fast (3 ps). The rate of the successive energy-transfer step from LH1 to the embedded RC is more than 10 times slower (35 ps). As the molecules of the LH1 wheel are exciton-coupled like those of B850, such an energy-transfer process should occur ChemSusChem 2008, 1, 26 – 58

The structures of several bacterial reaction centers are known precisely as a result of X-ray crystallographic investigations.[31, 32] The photosynthetic reaction centers of bacteria and other organisms consist mainly of a protein, which is embedded in and spans a lipid bilayer membrane. The basic photochemistry is performed by some cofactors buried within it.[33] A simplified view of the structure of the reaction center of Rhodopseudomonas viridis is sketched in Figure 3. Detailed photophysical studies of this reaction center have led to a precise picture of the sequence of events participating in photoinduced charge separation.[33, 34] The key molecular components are a bacteriochlorophyll “special pair” (P), a bacteriochlorophyll monomer (BC), a bacteriopheophytin (BP), a quinone (QA), and a four-heme ctype cytochrome (Cyt). These molecules are held in a fixed geometry by surrounding proteins, so that the twofold axis of P[35] is perpendicular to the membrane, the periplasmic face lies approximately between P and Cyt, and the cytoplasmic

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Figure 3. A simplified view of the structure of the reaction center of R. viridis. See text for details.

face lies at the level of QA. In the reaction center, excitation of P by absorption of light or, more commonly, by singlet–singlet energy transfer from various antenna systems, is followed by very fast (~ 3 ps) electron transfer to the BP “primary” acceptor (whether the interposed BC plays the role of mediator in a superexchange mechanism or directly intervenes as an intermediate electron acceptor has been the object of debate).[36] The next step involves fast (~ 200 ps) electron transfer from BP to QA, followed by slower (~ 270 ns) reduction of the oxidized P by the nearest heme group of Cyt.[37] At that stage, transmembrane charge separation has been achieved with an efficiency approaching unity and an extremely long lifetime with respect to charge recombination. The rate constants of the various electron-transfer steps involved in the charge-separation process are summarized in the approximate energy-level diagram of Figure 4, together with those of the non-occurring BP
!P + and QA
!P + charge-recombination steps (as deter-

Figure 4. Energy-level diagram and rate constants of the electron-transfer steps involved in the charge-separation process in the reaction center of R. viridis.

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mined from experiments with modified reaction centers lacking the possibility of competing forward processes).[38] Figure 3 and Figure 4 point out the importance of the supramolecular structure of the reaction center. The achievement of efficient photoinduced charge separation over a large distance is made possible by optimization of several aspects of this photochemical device: 1) the organization of the molecular components in space, 2) the thermodynamic driving force of the various electron-transfer steps, and 3) the kinetic competition between forward (useful) over back (dissipative) electrontransfer processes. How this occurs can be reasonably well understood in terms of electron-transfer theory.[25, 39] In particular, it can be noted that the high efficiency of the charge-separation process is due to the fact that the charge-recombination steps are slow because they lie in the Marcus inverted region.[40] Furthermore, in order to have a high efficiency of charge separation, the photoinduced electron-transfer process must proceed only along one of the two branches of the apparently symmetric reaction center (Figure 3). It is likely that mutations have broken the symmetry, imposing unfavorable Franck–Condon factors on the disfavored side. It has also been observed[25] that if the distance between BP and QA was just a few angstroms longer, or the driving force of this reaction was several tenths of an eV larger or smaller, then the quantum efficiency of the reaction center would suffer as charge recombination became more common. On the other hand, if the driving force for the BP
-to-P + ground-state reaction was decreased, then this inverted region reaction would accelerate and also lower the efficiency of the productive charge separation. Experiments with reaction centers oriented in an external electric field have provided some evidence that this is true.[41] In the process described above, the ultimate electron acceptor is a quinone QA. Then, the process continues with many other steps. The electron migrates to a second quinone QB, and, after reduction of the oxidized special pair P + by a c-type cytochrome (see below), the energy of a second photon is used to transfer a second electron to QB. Reduction of QB to its hydroquinone form involves the uptake of two protons from water on the internal cytoplasmic side of the membrane. The hydroquinone then diffuses to the next component of the apparatus, a proton pump, denoted the cytochrome bc1 complex (Figure 5). This complex oxidizes the hydroquinone back to a

Figure 5. Schematic representation of the bacterial photosynthetic membrane and of the different protein components.

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Photochemical Conversion of Solar Energy quinone and uses the energy released to translocate protons across the membrane and establish a proton concentration and charge imbalance (protonmotive force). The oxidation process is ultimately driven, through various cytochrome redox relays, by the oxidized special pair P + , which becomes reduced to its initial state. Finally, a rotary motor, the enzyme adenosine triphosphate (ATP) synthase,[42, 43] allows protons to Figure 6. a) Schematic representation of the charge-separation process in PSII. b) The five redox states (S0–S4) of flow back across the membrane, the Mn4Ca cluster. down the thermodynamic gradient, driving the release of ATP formed from adenosine diphoscies found in nature and reaches a potential of + 1.2 V relative phate (ADP) and inorganic phosphate (Pi ). The ATP fills the mato NHE. P680 + is rapidly reduced by TyrZ (t = 20–200 ns, dejority of the energy needs of the bacterium. pending on the state of the Mn4Ca cluster; see below) which, in its reduced state, is hydrogen-bonded to a nearby histidine residue.[46] Such a hydrogen bond facilitates oxidation of TyrZ 2.3.2. Photosystem II which occurs with concomitant deprotonation (proton-coupled Photosystem II (PSII) carries out all the processes needed for electron transfer, PCET). PCET plays indeed a fundamental role photosynthesis in green plants: light absorption in antenna in biological processes, as electron transfer in many proteins components, energy transfer to a reaction center, charge sepaand enzymes is supported along pathways exhibiting hydroration, and charge stabilization.[30] Furthermore, it is capable of gen-bond contact between amino acid residues and polypepusing water as the reductant of the quinone which is at the tide chains.[47, 48] Electron and proton transfer influence each end of the acceptor side. In order to do that, PSII must other thermodynamically and kinetically.[49] Extensive investigations have been perfomed in the last few years in an attempt 1) reach potentials high enough to oxidize water (> + 0.9 V relative to the normal hydrogen electrode, NHE), 2) handle such a to throw light on these complex processes.[50] high oxidation potential in fragile biological structures, and As illustrated in Figure 7,[51] PCET offers an energy advantage 3) couple the one-photon/one-electron charge-separation proof 9.5 kcal mol
1 compared to simple electron transfer because cess to the four-electron water oxidation process. The water it avoids oxidation of TyrZ to the high-energy radical cation inoxidation moiety of PSII (Figure 6 a)[28, 44, 45] consists of a triad termediate TyrzOH + . The oxidized TyrZ radical so obtained is recomposed of a multimer of chlorophylls (named P680), a redoxduced by electrons, which ultimately are derived from water. How this happens is still largely unknown. When PSII works at active tyrosine aminoacid (TyrZ, Y161 of the D1 polypeptide), and the so-called oxygen-evolving complex (OEC), a cluster full speed, approximately 200 water molecules can be oxidized containing four Mn atoms and a Ca atom (Mn4Ca) connected per second. This suggests that the kinetic barriers must be by mono-m-oxo, di-m-oxo, and/or hydroxo bridges. The specific very low. protein environment and one chloride ion are also essential for the water-splitting activity. PSII spans the thylakoid membrane in the chloroplasts, and the water-oxidizing triad is located closely to one side of the membrane. On direct absorption of a photon or energy transfer from the antenna units, P680 is excited and becomes a strong reductant. An electron is then transferred from excited P680 to the acceptor system (pheophytin and two quinones, QA and QB) on the other side of the membrane (Figure 6 a). The oxidized primary donor, P680 + , is one of the most oxidizing spe- Figure 7. Comparative energetics of oxidation of TyrZ by electron transfer and by histidine-assisted PCET.[51] ChemSusChem 2008, 1, 26 – 58

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V. Balzani et al. Oxidation of water to dioxygen is a four-electron process, so the results of four charge-separation events must be accumulated. This role is played by the Mn4Ca cluster, which is close to TyrZ and is oxidized stepwise by the TyrZ radical to a series of states Si (i = 0–4), as shown in Figure 6 b. Dioxygen evolution occurs when the most oxidized cluster state, S4, returns, in a four-electron reduction process, to the most reduced state, S0. This process involves the oxidation of two water molecules, which have probably been coordinatively bound to the manganese cluster. The structure of the Mn4Ca cluster has been the object of extensive investigation with a variety of techniques, including X-ray diffraction (XRD) studies of single crystals of PSII at 3.5[52] and 3.0 N resolution,[53] and Mn X-ray absorption near-edge structure (XANES)[54, 55] and Mn X-ray absorption fine structure (EXAFS) studies.[55] In the S1 state, the Mn–Mn distances are about 2.7 N, except for two Mn atoms which are separated by 3.3 N, and the Ca atom is 3.4 N from two Mn atoms. The available data constrain the Mn4Ca cluster geometry to a set of three similar high-resolution structures.[55] The nature of the interaction of the manganese cluster with the TyrZ radical is not fully understood. As mentioned above, a concerted electronproton transfer of the manganese-bound water molecules by the tyrosine radical is most likely on thermodynamic grounds as it would avoid the formation of high-energy intermediates.[56] Some theoretical clues to the reaction mechanism of dioxygen formation at the oxygen-evolving complex, OEC, have also been discussed.[57] A reliable mechanism for the very complex water oxidation process, however, will only be obtained when the structures of the various Si states are available.

3. Artificial Photosynthesis 3.1. Introduction The conversion of solar energy into fuel by artificial photosynthetic systems is certainly one of the most challenging goals in chemistry.[1, 58–68] For the production of solar fuel to be economically and environmentally attractive, the fuels must be formed from abundant, inexpensive raw materials such as water and carbon dioxide. Water should be split into molecular hydrogen and molecular oxygen, and carbon dioxide in aqueous solution should be reduced to ethanol with the concomitant generation of dioxygen.[69] From many points of view, the most attractive fuel-generating reaction is the cleavage of water into hydrogen and oxygen [Eq. (1)]: 2 H2 O þ 4 hn ! 2 H2 þ O2

ð1Þ

Such a process, of course, has to be sensitized as water cannot be electronically excited by sunlight.[58] Combustion of molecular hydrogen, H2, with oxygen produces heat and water, and combination of molecular hydrogen and oxygen in a fuel cell generates electricity, heat, and water. Once obtained, hydrogen could also be used to obtain methanol, a liquid fuel.[3]

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Clearly, if hydrogen could promptly replace oil, both the energy and the environmental problems of our planet would be solved. 3.2. Hydrogen Economy The media, press, and even policy-makers often talk about the “hydrogen economy” and sometimes describe hydrogen as a fuel available or obtainable for free from water. This (wrong) message suggests that the energy problem will soon be solved. Most scientists, however, believe that the shift to a hydrogen economy will not occur soon and might even not occur at all unless a large research effort is set up to overcome several scientific and technological obstacles.[1, 70, 71] Since there is no molecular hydrogen on the Earth, molecular hydrogen cannot be mined but instead has to be “manufactured”, starting from hydrogen-rich compounds, by using energy. Therefore, hydrogen is not an alternative fuel, but a secondary form of energy. This is the central (but not unique) problem of a hydrogen economy. Like electricity, hydrogen must be produced by using fossil, nuclear, or renewable energy, and then it can be used as an energy vector with the advantage, with respect to electricity, that it can be stored. Although a proper use of hydrogen is not expected to cause big environmental problems, one cannot say that hydrogen is a “clean” form of energy. In fact, hydrogen is “clean” or “dirty” depending of the primary energy form used to produce it. Hydrogen obtained by spending fossil fuels or nuclear energy incorporates all the problems of using those primary energy sources. Burning fossil fuels in remote regions to produce hydrogen as a clean fuel for metropolitan areas would be an ineffective solution owing to the trans-boundary nature of atmospheric pollution.[72] Clearly, clean hydrogen can only be obtained by exploiting renewable energies, and this can be done, in principle, by photochemical water splitting or through the intermediate production of electricity (e.g. by wind or photovoltaic cells) followed by water electrolysis. 3.3. Components of an Artificial Photosynthetic System The best way to construct artificial photosynthetic systems for practical solar fuels production is that of mimicking the molecular and supramolecular organization of the natural photosynthetic process: light harvesting should lead to charge separation, that must be followed by charge transport to deliver the oxidizing and reducing equivalents to catalytic sites, where evolution of oxygen and hydrogen (or CO2 reduction) should separately occur. Therefore, a plausible artificial photosynthetic system should include the following basic features (Figure 8):[65] 1) an antenna for light harvesting, 2) a reaction center for charge separation, 3) catalysts as one-to-multielectron interfaces between the charge-separated state and the substrate, and 4) a membrane to provide physical separation of the products. The complexity of the natural photosynthetic systems is clearly out of the reach of the synthetic chemist. This complex-

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Photochemical Conversion of Solar Energy

Figure 8. Schematic representation of photochemical water splitting (artificial photosynthesis).[65] Five fundamental components can be recognized—an antenna for light harvesting, a charge-separation triad D-P-A, a catalyst for hydrogen evolution, a catalyst for oxygen evolution, and a membrane separating the reductive and the oxidative processes.

ity, however, is largely related to their living nature. Today, we know that single photosynthetic functions, such as photoinduced energy and electron transfer, can be duplicated by simple artificial systems. The important lesson from nature is that the achievement of efficient conversion of light into chemical energy requires the involvement of supramolecular structures with very precise organization in the dimensions of space (relative location of the components), energy (excitedstates energies and redox potentials), and time (rates of competing processes). Such an organization, which in natural systems comes as a result of evolution and is dictated by intricate intermolecular interactions, can be imposed in artificial systems by molecular engineering exploiting covalent or noncovalent bonding.[73] Today, while some progress has been made on each aspect of artificial photosynthesis, integration of the various components in a working system has not yet been achieved.

3.4. Artificial Antenna Systems 3.4.1. Introduction The antenna effect can only be obtained in supramolecular arrays suitably organized in the dimensions of time, energy, and space. Each molecular component has to absorb the incident light, and the excited state so obtained (donor) has to transfer electronic energy to a nearby component (acceptor) before undergoing radiative or nonradiative deactivation (organization in the time dimension). For energy transfer to occur, the energy of the acceptor excited state has to be lower or, at most, equal to the energy of the excited state of the donor (organization in the energy dimension). Finally, the successive donor-to-acceptor energy-transfer steps must result in an overall energy-transfer process that leads the excitation energy to a selected component of the array (organization in the space dimension). In recent years, the development of supramolecular chemistry (particularly, of dendrimer chemistry) and the high level of experimental and theoretical efficacy ChemSusChem 2008, 1, 26 – 58

reached by photochemistry have enabled scientists to design and construct many artificial antenna systems. Dendrimers[74] (Figure 9) constitute a class of well-defined macromolecules exhibiting a treelike, nanometer-size architecture, reminiscent of the architecture of natural light-harvesting complexes. Therefore, dendrimer structures are very attractive for the construction of artificial antennae,[75–78] also because their convergent and/ or divergent synthesis[74] allows the assembly, in a few synthetic steps, of a large number of chromophores in a restricted

Figure 9. Schematic representation of a dendrimer.

space and with high topological control. Photoactive units can be directly incorporated or appended with covalent or coordination bonds in different regions of a dendritic structure and can also be noncovalently hosted in the cavities of a dendrimer. Because of their proximity, the various functional groups of a dendrimer may easily interact with one another. Collecting light by an antenna systems may also be useful for purposes different from artificial photosynthesis, for example, for signal amplification in luminescence sensors,[79] photodynamic cancer therapy,[80] and up-conversion processes.[81] A large system, where an array of chromophoric units absorb light and transfer energy to a luminescent center, can also be considered a spatial and spectral energy concentrator (“molecular lens”).[82] We will illustrate a few selected examples of artificial antenna systems. 3.4.2. Dendrimers Based on Metal Complexes Oligopyridine ligands have extensively been used to build up polynuclear complexes with dendritic structures.[75, 83–86] In such

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V. Balzani et al. dendrimers (see, for example, Figure 10), the metal units are linked together by bridging ligands. The choice of suitable bridging ligands is crucial in determining the properties of dendrimers, because 1) their coordinating sites (together with

Figure 10. Schematic representation of a dendrimer containing Ru and/or Os complexes in each branching site.[75] The formulae of the 2,3- and 2,5bis(2-pyridyl)pyrazine (2,3- and 2,5-dpp) bridging ligands and of the 2,2’-bipyridine (bpy) and 2,2’-biquinoline (biq) terminal ligands are also shown.

those of the “terminal” ligands) influence the spectroscopic and redox properties of the active metal-based units, 2) their structure and the orientation of their coordinating sites determine the architecture of the dendrimer, and 3) their chemical nature controls the electronic communication between the metal-based units. The more carefully investigated dendrimers of this kind are those containing RuII and OsII as metal ions, 2,3- and 2,5-bis(2pyridyl)pyrazine (2,3- and 2,5-dpp, respectively) as bridging ligands, and 2,2’-bipyridine (bpy) and 2,2’-biquinoline (biq) as terminal ligands (Figure 10).[75] The typical strategy used to prepare these dendrimers is the so-called “complexes as metals and complexes as ligands” approach,[87] which has enabled the construction of species containing four, six, 10, 13, and 22 metal-based units. A docosanuclear dendrimer of that family, such as that schematically shown in Figure 10, is a cationic species carrying a charge of 44 + and comprising 1090 atoms, with an estimated size of 5 nm. Besides 22 metal atoms, it contains 24 terminal ligands and 21 bridging ligands.

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From a photophysical viewpoint, such dendrimers, which can be viewed as ordered ensembles of [M(L)n(BL)3-n]2 + complexes (M = RuII or OsII ; L = bpy or biq; BL = 2,3- or 2,5-dpp), are known to have 1) intense ligand-centered (LC) absorption bands in the UV region and moderately intense metal-toligand charge-transfer (MLCT) bands in the visible region, and 2) a relatively long-lived luminescence in the red spectral region, originating from the lowest 3MLCT level. In the dendrimers of this family, there is only a small electronic interaction between nearby mononuclear units and, therefore, the absorption spectrum is practically the “sum” of the spectra of the constituent units. In the dendrimers of higher nuclearity, as a consequence, the molar absorption coefficient is huge throughout the entire UV/Vis spectral region (e = 202 000 L mol
1 cm
1 at 542 nm for a docosanuclear dendrimer in which all the metal ions are RuII), so that most of the photochemically active part of sunlight can be absorbed. The small but not negligible electronic interaction between nearby units is sufficient to cause in these dendrimers a very fast energy transfer that leads to the quenching of the potentially luminescent units having higher-energy 3MLCT levels and the sensitization of the luminescence of the units having lower-energy 3 MLCT levels. Energy transfer between nearby units, however, occurs mainly in the femtosecond timescale from non-thermalized singlet excited states, in competition with intersystem crossing.[88] The energy of the excited states of each unit depends on metal and ligands in a predictable way, and the modular synthetic strategy[75] enables high synthetic control in terms of the nature and position of metal centers, bridging ligands, and terminal ligands. Such synthetic control translates into a high degree of control on the direction of energy flow within the dendritic array. On increasing nuclearity, however, a unidirectional gradient (center-to-periphery or vice versa) for energy transfer cannot be obtained with only two types of metals (RuII and OsII) and ligands (bpy and 2,3-dpp). An extension of this kind of antennae is represented by heterometallic dendrimers with appended organic chromophores.[89] For the heptanuclear complex made up of a [Cl2Ru(m-2,3dpp)2] core, two [Ru(m-2,3-dpp)3]2 + branch units, and four [(bpy)2Ru(m-2,3-dpp)]2 + in the periphery, it has been shown that the peripheral units transfer energy to the core through the intermediate higher-energy units taking advantage of a sequential two-step electron-transfer process.[90] 3.4.3. Dendrimers Based on Porphyrins Porphyrins, the main chromophores of natural photosynthesis, are obvious candidates for the design of artificial antenna systems. In this section, we illustrate a few typical examples; more extensive coverage can be found in several reviews.[91] By means of a modular approach and the use of an ethyne linkage between aryl groups on adjacent tetraarylporphyrin macrocycles, a variety of di-, tri-, tetra-, and pentameric porphyrin arrays have been obtained.[91c] Photophysical investigations have shown[92] that 1) singlet excited-state energy transfer from Zn porphyrin to free-base porphyrin is extremely effi-

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Photochemical Conversion of Solar Energy cient (95–99 %), 2) competitive electron-transfer reactions are not observed, 3) the mechanism of energy transfer predominantly involves through-bond communication via the ethyne linker, and 4) energy transfer between two isoenergetic Zn porphyrins is very fast. These studies demonstrate that extended multiporphyrin arrays can be designed in a rational manner with predictable photophysical features and efficient light-harvesting properties. Efficient excitation energy transfer has been shown to occur in giant wheels (about 7nm diameter) composed of 24 porphyrin units with a rate of 35 ps
1 for energy hopping between neighboring tetraporphyrins moieties.[93] Attempts to build up artificial antennae by self-assembling of porphyrin components have also been reported.[94, 95] Morphology-dependent antenna properties have been revealed[96] for a series of dendrimers that have the general formula (L)nP, where P is a free-base porphyrin core bearing different numbers (n = 1–4) of poly(benzyl ether) dendrons (L) at its meso positions (Figure 11 a). In dichloromethane solutions, excitation of the chromophoric groups of the dendrons causes singlet–singlet energy-transfer processes that lead to the excitation of the porphyrin core. The (L)4P dendrimer, which has a Figure 11. Light-harvesting dendrimers with the general formula (L)nP, where P is a free-base porphyrin core bearof dendrons, L: a) L = poly(benzyl ether) dendrons;[96] b) L = compound 1 containing seven spherical morphology, exhibits a ing different numbers Zn-porphyrin units.[97] much higher energy-transfer quantum yield (0.8) than the partially substituted (L)1P, (L)2P, and (L)3P species (quantum cooperativity between dendrons, which decreases with increasyields less than 0.32). Fluorescence polarization studies on (L)4P ing conformational mobility, is necessary for efficient energy showed that the excitation energy migrates very efficiently transfer.[96] Such a behavior would mimic that of natural photoover the dendrons within the excited-state lifetime, so that the synthetic systems, where energy migration within “wheels” of four dendrons can be viewed as a single, large chromophore chromophoric groups results in an efficient energy transfer to surrounding the energy trap. Temperature-dependent effects the reaction center. The morphology effect has also been insuggested that increased flexibility and conformational freevestigated by appending to the free-base porphyrin core P of dom are responsible for the decreased energy-transfer efficiendendrimers (L)nP up to four, much larger dendrons each concy on decreasing the number of dendrons. Only the highly taining seven Zn-porphyrin units (e.g. L is compound 1 shown crowded (L)4P dendrimer retains a constant level of energy in Figure 11 b).[97] The presence of poly(benzyl ether) dendritic transfer, even at high temperatures. It was also postulated that wedges at the periphery makes such dendrimers soluble in ChemSusChem 2008, 1, 26 – 58

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V. Balzani et al. common organic solvents. In the star-shaped (1)4P dendrimer, energy transfer from the excited singlet states of dendrons 1 to the focal P core takes place with a rate constant of 1.0 T 109 s
1 and 71 % efficiency, whereas in the conically shaped (1)1P dendrimer the energy-transfer rate constant is 10 times smaller and the efficiency is 19 %. This result shows that morphology has indeed a noticeable effect on the energy-transfer rate. Excitation of (1)4P at 544 nm with polarized light results in a highly depolarized fluorescence from the Zn-porphyrin units (fluorescence anisotropy factor 0.03, to be compared with 0.19 of a monomeric reference compound), indicating an efficient energy migration among the Zn-porphyrin units before the energy is transferred to the free-base core. In the case of the conically shaped (1)1P compound, the fluorescence anisotropy factor is much higher (0.10). These results suggest a cooperation of the four dendrons of (1)4P in facilitating the energy migration among the Zn-porphyrin units. Clearly, the (1)4P system, which incorporates 28 light-absorbing Zn-porphyrin units into a dendritic scaffold that has an energy-accepting core, mimics several aspects of the natural light-harvesting LH1 complex. A study on a nonameric porphyrin assembly made up of a central free-base and eight peripheral Zn porphyrins connected by flexible nucleoside linkers has provided evidence for the presence of several nonequilibrated conformations.[98] Both singlet–singlet and triplet–triplet energy transfer from the peripheral Zn porphyrins to the free-base porphyrin and triplet– triplet annihilation have been detected. Association of bidentate bases increases the rigidity of the structure and improves the energy collection ability.

three peryleneimide and one terryleneimide chromophores are attached to the dendrimer rim, energy transfer from the former to the latter units takes place with an efficiency of over 95 %. All the observed energy-transfer processes can be interpreted on the basis of the Fçrster mechanism. Polyphenylene dendrimers with a perylene diimide as a luminescent core have also been investigated.[102] In films, the dendrons suppress the interaction of the emissive cores that causes loss and red shifting of the emission. Several studies on single dendrimer molecules have been reported.[103–106] In polyphenylene dendrimer 2 (Figure 12), which consists of a terrylenediimide (TDI) core, four perylenemonoACHTUNGREimides (PMIs) attached to the scaffold, and eight naphthalenemonoimides (NMIs) at the rim, the antenna effect has been studied at the ensemble and single-molecule level.[106] Efficient energy transfer from the PMIs to the core and from the NMIs directly or via PMIs to the core has been observed. In singlemolecule experiments, the NMI chromophores are the first to bleach.

3.4.4. Dendrimers Based on Organic Molecules Many antenna systems based on organic molecules have been constructed.[99] Energy transfer in a series of shape-persistent polyphenylene dendrimers substituted with peryleneimide and terryleneimide chromophoric units has been investigated in toluene solution.[100] Energy hopping among the peryleneimide chromophores, revealed by anisotropy decay times,[101] occurs with a rate constant of 4.6 T 109 s
1. When

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Figure 12. Dendrimer 2 consisting of a terrylenediimide (TDI) core, four perylenemonoimides (PMIs) attached to the scaffold, and eight naphthalenemonoimides (NMIs) at the rim.[106]

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Photochemical Conversion of Solar Energy Oligo(p-phenylene vinylene) (OPV) units are extensively studied as ideal model compounds for the corresponding poly(pphenylene vinylene) (PPV) polymers that can be used for lightemitting diodes (LEDs),[107] field-effect transistors (FETs),[108] and solar cells.[109] OPV units are also increasingly used to obtain photoactive dendrimers.[110] Energy transfer in single OPV vesicles,[111] chiral co-assemblies of hydrogen-bonded OPV and porphyrin,[112] and self-assembled OPV functionalized with perylene-bisimide units[113] have been recently investigated.

3.4.5. Dendrimers Based on Host–Guest Systems An important property of dendrimers is the presence of internal cavities where ions or neutral molecules can be hosted.[114] Such a property can potentially be exploited for a variety of purposes, which include catalysis[115] and drug delivery.[116] Energy transfer from the numerous chromophoric units of a suitable dendrimer to an appropriate guest may result in a light-harvesting antenna system.[117, 118] An advantage shown by such host–guest light-harvesting systems is that the collected energy can be delivered by the same dendrimer to suitably tuned guests. An interesting example is given by dendrimer 3 (Figure 13), which consists of a hexaamine core surrounded by eight dansyl-, 24 dimethoxybenzene-, and 32 naphthalene-type units.[118] In dichloromethane solution, compound 3 exhibits

the characteristic absorption bands of the component units and a strong dansyl-type fluorescence. Energy transfer from the peripheral dimethoxybenzene and naphthalene units to the fluorescent dansyl units occurs with over 90 % efficiency. When the dendrimer hosts a molecule of eosin (3eosin), the dansyl fluorescence, in its turn, is quenched and sensitization of the fluorescence of the eosin guest can be observed. Quantitative measurements showed that the encapsulated eosin molecule collects electronic energy from all 64 chomophoric units of the dendrimer with an efficiency of over 80 % (partial overlapping between dansyl and eosin emissions precludes a better precision). Both intramolecular (i.e. within dendrimer) and intermolecular (i.e. dendrimer host!eosin guest) energytransfer processes occur very efficiently by a Fçrster-type mechanism because of the strong overlap between the emission and absorption spectra of the relevant donor and acceptor units. Dye molecules can also be hosted into poly(propylene amine) dendrimers surface-modified with OPV units.[119] In these systems, energy transfer from the OPV fluorescent units (lmax = 492 nm) to the enclosed dye molecules is not efficient in solution (40 % efficiency at maximal loading), but is very efficient in spin-coated films of dendrimer/dye assemblies. Energy transfer has been found to occur from 1,3,4-oxodiazole dendrons to hydrogen-bonded OPV derivatives[120] and has been investigated at the single-molecule level in host–guest systems consisting of a second-generation polyphenylene dendrimer and a cyanine dye.[121] Investigations on dendrimers capable of hosting metal ions have been recently reviewed.[122] 3.4.6. Other Systems

Figure 13. Schematic representation of the energy-transfer processes occurring in dendrimer 3, which contains three different types of light-harvesting chromophoric units.[118] All the excitation energy can be channelled to a hosted eosin molecule.

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Energy transfer in derivatized polymers with attached chromophores has been extensively investigated.[123, 124] RuII and OsII polypyridine complexes have been attached by amide linkages to a 1:1 styrene-p-aminomethylstyrene copolymer with a polydispersivity of 1.5 and an average of 16 repeating units.[123] A mixed polymer was prepared by sequential coupling, first with a limited amount of the less reactive OsII complex, and then with the more reactive RuII complex to fill all the remaining free sites. In a mixed polymer containing the lower-energy OsII complex and the higher-energy RuII complex in a 3:13 ratio[123a] (subsequently corrected to 5:11[123d]),

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V. Balzani et al. triplet–triplet energy transfer from the excited RuII complex to the OsII complex was observed with an efficiency higher than 90 % in acetonitrile solution. Poly(amino acids) have also been used as backbones to construct antenna systems.[123e] Zeolite L, a crystalline aluminosilicate in which the SiO4 and AlO4 tetrahedra give rise to one-dimensional channels arranged in a hexagonal structure, has been used as a host for the organization of dyes to furnish antenna properties.[125] A zone of the zeolite nanocrystal (e.g. the central zone in each channel) can be filled with molecules of a specific dye (dye 1; e.g. 1,2-bis-(5-methyl-benzoxazol-2-yl)ethene); then, under appropriate experimental conditions, a second (dye 2; e.g. pyronine) and a third (dye 3; e.g. oxonine) dye are successively inserted into the channels. If the three dyes are suitably chosen, light excitation of dye1 located in the middle part leads to energy-migration processes on both sides of the channels. Closure (stopcock) molecules can be used both to prevent the dyes from leaving the channels and to interface the dye molecules contained in the channels with the external environment. The linear [RuACHTUNGRE(bpy)2ACHTUNGRE{bpy-(Ph)4-SiACHTUNGRE(CH3)3}]2 + complex can be used as a functional stopcock that transfers excitation energy to the acceptor dye oxazine contained inside the zeolite.[126] Energy transfer along a specified direction has been obtained[127] and dye–zeolite crystals have been organized as oriented monolayers.[128] Light harvesting has also been investigated in a variety of other systems such as multichromophoric cyclodextrins,[129] phthalocyanines,[130] metallosupramolecular squares,[131] rotaxanes,[132] and polyelectrolytes.[133] A combination of self-organizing biological structures and synthetic building blocks has been used as a flexible method for the construction of antenna systems. Recently,[134] building blocks were prepared by attaching fluorescent chromophores to cysteine residues introduced on tobacco mosaic virus coat protein monomers. When placed under the appropriate buffer conditions, these conjugates could be assembled into stacks of disks or into rods that reached hundreds of nanometers in length. Efficient energy transfer from a large number of donors to a single acceptor was observed in such systems.

3.5. Artificial Reaction Centers 3.5.1. Introduction As we have seen in Section 2.3, photoinduced charge separation taking place in a reaction center (Figure 4) is the key process that converts light energy into chemical energy in nature. In recent years, many attempts have been performed to construct artificial systems capable of mimicking the function of the natural reaction center. The minimum model for a charge-separation system is a dyad, which consists of an electron-donor (or -acceptor) chromophore, an additional electron-acceptor (or -donor) moiety, and an organizational principle that controls their distance and electronic interactions (and therefore the rates and yields of electron transfer). Dyads of this type have been constructed and investigated.[34, 91a,c,d, 135–140] The energy-level diagram for a

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dyad is shown in Figure 14. All the dyad-type systems suffer to a greater or lesser extent from rapid charge recombination (process 4).

Figure 14. Schematic energy-level diagram for a dyad.

As we have also seen in Section 2.3, the problem of rapid charge recombination has been overcome in nature with a series of short-range, fast, and efficient electron-transfer processes that lead to a charge separation over a long distance. This model has inspired the construction of systems consisting of three or more components. Charge-separation in three-component systems (triads) is illustrated in Figure 15 a. The functioning principles are shown in the orbital-type energy diagrams in the right part of the figure. Excitation of a chromophoric component (step 1) is followed by a primary photoinduced electron transfer to a primary acceptor (step 2). This process is followed by a secondary thermal electron-transfer process (step 3), namely, electron transfer from a donor component to the oxidized chromophoric component. The primary process competes with excitedstate deactivation (step 4), whereas the secondary process competes with primary charge recombination (step 5). Finally, charge recombination between remote molecular components (step 6) leads the triad back to its initial state. The sequence of processes indicated above (1-2-3) is not unique. Actually, the alternate sequence 1-3-2, would also lead to the same chargeseparated state. The performance of a triad for energy conversion purposes is related to the quantum yield (F) of formation of the charge-separated state (depending on the competition between forward and back processes, F = [k2/ACHTUNGRE(k2+k4)]ACHTUNGRE[k3/ACHTUNGRE(k3+k5)]), the lifetime (t) of charge separation (depending on the rate of the final charge-recombination process, t = 1/ k6 ), and the efficiency of energy conversion (hen.conv. = F T F, where F is the fraction of the excited-state energy conserved in the final charge-separated state). To put things in a real perspective, it should be recalled that the “triad portion” of the reaction center of bacterial photosynthesis discussed in Section 2.3.1 converts light energy with t ~ 10 ms, F = 1, and hen.conv. ~ 0.6. Several mechanistic investigations on photoinduced charge separation in artificial systems have been performed on covalently linked organic compounds. Recent reviews can be consulted for a detailed discussion of the various aspects of this

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Figure 15. a) Schematic representation of charge separation in a triad. Other arrangements of the components are possible. b) Triads 43 + ,[142] 5,[143] and 6.[135]

topic (through-bond compared with through-space transfer, interference effects in through-bond coupling, orientation effects, modulation of electron-transfer dynamics by electric fields, etc.).[141] 3.5.2. Triads Three examples of triads are shown in Figure 15 b. The 4-nmlong triad 43 + consists of an IrIII bis-terpyridine complex connected to a triphenylamine electron donor (D) and a naphthalene-bisimide electron acceptor (A).[142] Upon excitation of the electron donor D (or even the Ir-based moiety), a charge-separated state D + -Ir
-A is formed with 100 % yield in less than 20 ps that successively leads to D + -Ir-A
with 10 % efficiency in 400 ps. Remarkably, the fully charge-separated state D + -Ir-A
has a lifetime of 120 ms at room temperature in deaerated acetonitrile solution. Triad 5 (Figure 15 b) consists of a porphyrin as light-absorbing chromophore, naphthoquinone as acceptor, dimethylaniChemSusChem 2008, 1, 26 – 58

line as donor, and triptycene bridges as connectors.[143] Charge separation is very efficient (F = 0.71) and long-lived (t = 2.5 ms) at room temperature in butyronitrile, and the chargeseparated state has an energy of 1.39 eV. The charge-separation function is completely suppressed by cooling the system to low-temperature rigid solutions, behavior that is contrary to what happens for natural photosynthetic systems. The reason is that blocking the reorientation of solvent dipoles strongly destabilizes the chargeseparated states relative to fluid solution.[144] In particular, the forward secondary electrontransfer step 3 (which in fluid solution is exergonic by 0.14 eV) becomes endergonic in rigid matrix. As proof, in a triad containing a higher-energy chromophore and better electronacceptor and -donor components (the driving force of secondary electron-transfer step in fluid solution is 0.45 eV), photoinduced charge separation proceeds even at 5 K in 2-methyltetrahydrofuran.[145] Interestingly, under these conditions the charge-separated state can be detected as a spin-polarized transient EPR signal, which gives information about donor–

acceptor electronic coupling. In triad 6 (Figure 15 b), that will later be compared to a pentad of the same family, the quantum yield of charge separation is 4 %, the efficiency of energy conversion is 2 %, and the lifetime of the charge-separated excited state is 300 ns in dichloromethane solution.[135] Triad 7 (Figure 16) consists of a porphyrin (P) bearing a fullerene C60 and a carotenoid (C) secondary electron donor. Excitation of the porphyrin unit of the triad causes in 2-methyltetrahydrofuran the events shown schematically in the energylevel diagram shown in Figure 16.[146, 147] The C-1P-C60 excited state decays almost exclusively by electron transfer (step 2) with formation of C-P + -C60
with k2 = 3.3 T 1011 s
1 and a quantum yield of unity. A small fraction of C-P-1C60 excited states is also obtained (step 3), but these also decay to C-P + -C60
by electron transfer (step 4). Experiments performed on dyad PC60 show that C-P + -C60
can return to the ground state by charge recombination with k7 = 2.1 T 109 s
1. Electron transfer from the carotenoid to the porphyrin is, however, much faster

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V. Balzani et al. transfer steps, which, in suitably designed systems, produce charge separation over larger and larger distances. As the number of molecular components increases, also the mechanistic complexity increases and charge separation may involve energy-transfer steps. A tetrad comprising a ferrocene group, a Zn porphyrin, a free-base porphyrin, and a C60 unit, Fc-ZnP-P-C60, has been reported.[151] In this system, excitation of the Zn or free-base porphyrin causes electron transfer, leading to Fc-ZnP-P + -C60
; successive charge-shift processes produce the final charge-separated state, Fc + -ZnP-P-C60
, with F = 0.24 and hen.conv. = 0.13 (benzonitrile solution). Lifetime measurements reveal that, in solution, charge recombination Figure 16. Structure of triad 7, and the corresponding energy-level diagram for the charge-separation processoccurs mainly through bimoleces.[146] ular processes, that is, the intramolecular charge recombination (k8 = 1.5 T 1010 s
1). The C + -P-C60
state is therefore produced is too slow to compete with diffusion-limited intermolecular with an overall quantum yield of 0.88. It decays slowly by electron transfer. The lifetime of charge separation has been charge recombination to yield the carotenoid triplet (k9 = 2.9 T measured in frozen matrix by time-resolved EPR experiments; it was found to be remarkably long (380 ms in benzonitrile at 106 s
1). In conclusion, the two-step electron-transfer sequence 193 K). In the related tetrad Fc-(ZnP)2-C60,[152] the final chargein triad 7 has increased the lifetime of charge separation by a 3 factor of nearly 10 relative to that of dyad P-C60. Triad 7 has separated state, Fc + -(ZnP)2-C60
, is obtained with F = 0.80 also been found to have other properties that are present in (hen.conv. ~ 0.4) by excitation of the zinc-porphyrin dimer in benthe reaction centers but not common in artificial biomimetic zonitrile solution at 295 K. In this case, charge recombination systems.[148, 149] For example, the formation of C + -P-C60
occurs (t = 19 ms) apparently occurs through a reversed stepwise proeven in a glass at 8 K. Charge recombination of C + -P-C60
cess, that is, a rate-limiting electron transfer from (ZnP)2 to Fc + 3 yields C-P-C60 with a unique EPR-detectable spin-polarization followed by a fast electron transfer from C60
to (ZnP)2 + , which [148] as obpattern and occurs by a radical-pair mechanism, regenerates the ground state. served in natural reaction centers. Transfer of triplet excitation Pentad 8 (Figure 17) is formally obtained[135, 153] by introducenergy by a relay mechanism related to that found in some reing in triad 6 (Figure 15b) a secondary acceptor and a seconaction centers is, furthermore, also observed for this triad. In dary donor/chromophore. The various charge-separation pathcontrast with what happens in 2-methyltetrahydrofuran, in tolways of 8 are indicated in the state-energy diagram shown,[154] 1 uene solution the C-P- C60 excited state of 7 does not undergo which features also an energy-transfer step. The improvement electron transfer, decaying instead by intersystem crossing to in performance with increasing complexity can be seen by yield C-P-3C60. This triplet excited state decays with a rate concomparing data for the triad and the pentad: for 6, t = 300 ns, F = 0.04, and hen.conv. = 0.02 (dichloromethane); for 8, t = 55 ms, stant of 9 T 106 s
1 to give the carotenoid triplet, 3C-P-C60, via the slightly endergonic formation of the C-3P-C60 excited F = 0.83, and hen.conv. = 0.5 (chloroform). state.[150] It can be recalled that in nature the quenching of the Several multicomponent systems do exhibit both energyand electron-transfer photoinduced processes. However, they chlorophyll triplets by carotenoids is an important protective do not properly couple antenna and reaction-center compomechanism against photodegradation caused by formation of nents as needed to mimic the natural photosynthetic apparasinglet oxygen. tus. Examples of systems in which light harvesting and charge separation can be clearly identified are illustrated in the next 3.5.3. More Complex Systems section. The introduction of further molecular components (tetrads, pentads, hexads) leads to the occurrence of further electron-

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Figure 17. Structure of pentad 8, and the corresponding energy-level diagram for the charge-separation processes.[154]

3.6. Coupling Artificial Antennas and Reaction-Center Building Blocks 3.6.1. Introduction As we have seen in Section 2.3, in natural systems the solar energy collected by the antenna devices at the end of the energy-transfer chain is used to induce a charge-separation reaction, that is, to obtain redox energy. Coupling energy- and electron-transfer processes is a very demanding task. In this section, we examine attempts to couple these two functions in artificial systems. Several types of chromophores have been used as antennas, whereas a porphyrin–fullerene moiety is often used as a charge-separation device as it leads, with high efficiency, to long-lived charge-separated states.[147] 3.6.2. Systems Based on Metal Complexes Because there are significant limitations in the preparation of molecular assemblies by sequential covalent-bond formation, an alternative strategy involves the derivatization of preformed polymers, particularly with metal complexes.[123e] A derivatized polystyrene assembly has been constructed with appended [RuACHTUNGRE(bpy)3]2 + -type units (Figure 18).[155] In some of the metal complexes (three out of 20), a bpy ligand bears an electron donor (phenothiazine, PTZ) and an electron acceptChemSusChem 2008, 1, 26 – 58

or (a derivative of 1,1’-dimethyl-4,4’-bipyridinium, methylviologen, MV2 + ) to yield a structure that mimics the reaction center. Following excitation of the metal complexes by visible laser flash photolysis, the PTZ + -MV + state was observed, with a significant (ca. 30 %) contribution from excitation at antenna Ru sites not adjacent to the functionalized Ru units. In the overall reaction, the 2.13 eV excited energy of the antenna excited states is transferred to the reaction-center model, where it is converted into 1.15 eV of stored redox energy. The efficiency of formation of the redox-separated state varies from 12 to 18 % depending on the laser intensity. At high intensities, multiphoton excitation and excited-state annihilation compete with sensitized electron transfer. The lifetime of the chargeseparated state is 160 ns. It should be noted, however, that a small amount (0.5 %) of a long-lived transient (about 20 ms) was also observed and attributed to polymers in which PTZ + and MV + were formed on spatially separated sites. This observation is potentially important because it suggests that photochemically generated redox equivalents can be created and stored on the polymers for extended periods.[123e] 3.6.3. Systems Based on Organic Compounds and Porphyrins Light-harvesting and charge-separation coupling has been obtained in modified windmill porphyrin arrays.[156] In the com-

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V. Balzani et al. by hole transfer from the oxidized ZnPc + units to a peripheral Zn porphyrin, as indicated in the following reactions [Eqs. (2)–(5)] (for the sake of clarity, only one ZnPp, ZnPc, and A units are indicated): 1

ZnPp -ZnPc -A ! ZnPp -1 ZnPc -A

ð2Þ

ZnPp -1 ZnPc -A ! ZnPp -ZnPc þ -A


ð3Þ

ZnPp -ZnPc þ -A
! ZnPp þ -ZnPc -A


ð4Þ

ZnPp þ -ZnPc -A
! ZnPp -ZnPc -A

ð5Þ

The charge-separation efficiency is, however, low because the hole-transFigure 18. Sequence of energy- and electron-transfer processes in a polystyrene derivatized with [Rufer reaction [Eq. (4)] is slower than the ACHTUNGRE(bpy)3]2 + -type units.[155] charge-recombination reaction in the ZnPp-ZnPc + -A
species. pounds shown in Figure 19 which bear a naphthalenetetracarAn interesting attempt to couple light-harvesting antennas boxylic diimide or a meso-nitrated free-base porphyrin electron and a charge-separation module is represented by compound acceptor, A, attached to the two core ZnPc units, energy trans9 (Figure 20), which consists of four covalently linked zinc tetfer from the peripheral porphyrins, ZnPp, to the two ZnPc porraarylporphyrins, (ZnPp)3-ZnPc (p stands for peripheral, c stands phyrins is followed by electron transfer to the A unit, and then for central), covalently joined to a free-base-porphyrin–fullerene dyad, P-C60, to form the (ZnPp)3-ZnPc-P-C60 hexad.[157] Results obtained from time-resolved emission and absorption investigations in 2-methyltetrahydrofuran solution compared with those obtained for some model compounds have led to the following picture: 1) excitation of any peripheral zinc porphyrin is followed by singlet–singlet energy transfer to the central zinc porphyrin [Eq. (6)] with a rate constant k6 = 2.0 T 1010 s
1; 2) singlet–singlet energy transfer from the central zinc porphyrin to the free-base porphyrin [Eq. (7)] occurs with a rate constant k7 = 4.1 T 109 s
1; 3) electron transfer from the excited free-base unit to the fullerene unit [Eq. (8)] is very rapid, k8 = 3 T 1011 s
1; and 4) the lifetime of the charge-separated state is 1.3 ns [Eq. (9), k9 = 7.5 T 108 s
1]. 1

Figure 19. Antenna–reaction-center complexes based on a windmill porphyrin array.[156]

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ðZnPp Þ3 -ZnPc -P-C60 ! ðZnPp Þ3 -1 ZnPc -P-C60

ð6Þ

ðZnPp Þ3 -1 ZnPc -P-C60 ! ðZnPp Þ3 -ZnPc -1 P-C60

ð7Þ

ðZnPp Þ3 -ZnPc -1 P-C60 ! ðZnPp Þ3 -ZnPc -Pþ -C60


ð8Þ

ðZnPp Þ3 -ZnPc -Pþ -C60
! ðZnPp Þ3 -ZnPc -P-C60

ð9Þ

The quantum yield of the charge-separated state is unity on excitation of the free-base porphyrin because of the very large rate constant for photoinduced electron transfer [Eq. (8)]; on excitation of the Zn-porphyrin units, however, the quantum yield drops to 0.70 because of the competition between intrinsic decay and energy transfer to the central free-base porphyrin. The non-unity quantum yield of charge separation and the short lifetime of the charge-separated state left room for improvement of the performance of the hexad by clever molecular engineering of the free-base porphyrin unit. Replacement of the free-base diaryloctaalkylporphyrin in 9 with a meso-tetraarylporphyrin gives compound 10 (Figure 20).[158] In 2-meth-

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Photochemical Conversion of Solar Energy fullerene electron-donor–acceptor module. The BPEA antenna chromophore was chosen because it absorbs strongly in the 430–475 nm region, as do carotenoid polyenes in photosynthetic organisms. Energy transfer from the five antennas to the porphyrin occurs on the picosecond timescale with a quantum yield of 1.0, comparable to those seen in some photosynthetic antenna systems. The Fçrster mechanism plays the major role in energy transfer, but a through-bond, electron-exchange mechanism also contributes. After light harvesting, the first singlet excited state of the porphyrin donates an electron to the attached fullerene to yield a P + -C60
charge-separated state, which has a lifetime of several nanoseconds. The quantum yield of charge separation based on the light absorbed by the antenna chromophores is 80 % for the free-base compound 11 and 96 % for the zinc analogue 12. The rate constants of the energy- and electrontransfer processes are indicated in Figure 21. Figure 20. Schematic representation of the energy- and electron-transfer processes that occur in hexads 9[157] and Biomimetic reaction centers 10.[158] have also been constructed by self-assembly of a porphyrin dimer with functionalized fullerenes.[161] yltetrahydrofuran solution, this hexad leads to faster energy transfer from the central Zn porphyrin to the free-base porphyrin as compared with 9, thereby increasing the overall yield of 3.7. Coupling Single-Photon Charge Separation with charge separation. Because the tetraarylporphyrin employed in 10 has a higher oxidation potential than its octaalkylporphyrin Multielectron Redox Processes analogue in 9, migration of the positive charge from the free3.7.1. Introduction base porphyrin to the Zn-porphyrin system [Eq. (10)] occurs, The main problem of artificial photosynthesis is perhaps the moreover, with a rate constant k10 = 2.6 T 109 s
1. The lifetime coupling of photoinduced charge separation, which is a oneof the final charge-separated state is increased to 240 ns photon, one-electron process, with oxygen evolution, which is [Eq. (11), k11 = 4.2 T 106 s
1]. a four-electron process. As we have seen in Section 2.3.2, naðZnPp Þ3 -ZnPc -Pþ -C60
! ½ðZnPp Þ3 -ZnPc þ -P-C60
ð10Þ ture’s answer is the Mn4Ca cluster, a catalyst for multielectron transfer that is capable of 1) releasing electrons in a stepwise
þ ½ðZnPp Þ3 -ZnPc  -P-C60 ! ðZnPp Þ3 -ZnPc -P-C60 ð11Þ manner at constant potential, and 2) oxidizing water molecules in a concerted way, so as to avoid the formation of highenergy intermediates. The design of specific multielectron A further improvement has been obtained with heptads 11 redox catalysts is a fascinating and challenging problem of and 12 (Figure 21),[159, 160] in which the hexaphenylbenzene modern chemistry.[51, 162–164] scaffold provides a rigid and versatile core for organizing antenna chromophores and coupling them efficiently with a From the standard redox potentials of the two correspondcharge-separation moiety. Such compounds contain five bising half-reactions, the free energy demand for water splitting ACHTUNGRE(phenylethynyl)antracene (BPEA) antennas and a porphyrin– [Eq. (1), Section 3.1] is 1.23 eV. For many charge-separated ChemSusChem 2008, 1, 26 – 58

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43

V. Balzani et al. The answer to this general problem lies in the possibility of accelerating multielectron redox reactions by the use of artificial catalysts.[51, 164] A catalyst for multielectron redox processes is essentially a “charge pool”, that is, a species capable of 1) acquiring electrons (or holes) from a one-electron-reducing (or -oxidizing) species in a stepwise manner at constant potential, and 2) delivering these electrons (or holes) to the substrate in a “concerted” manner to avoid the formation of high-energy intermediates. From the field of heterogeneous catalysis, metals and metal oxides are known to be good candidates for this type of processes. Photochemical “water splitting” cycles based on bimolecular reactions between molecular photosensitizer, acceptor, and donor species were actively studied some time ago.[59–62] The main result of such studies (which did not lead to any practical success with regard to water splitting) was the optimization of several heterogeneous catalysts.[165] For example, colloidal platinum was found to be a superior catalyst for photochemical hydrogen evolution and colloidal RuO2 was identified as a moderately efficient catalyst for photochemical oxygen formation. Apart from solid-state materials, discrete supramolecular species can also be conceived as catalysts for multielectron redox reactions. This is what hapFigure 21. Energy- and electron-transfer processes occurring in heptads 11 and 12[160] pens, as we have seen in Section 2.3.2, in green(subscripts: e = energy transfer, o = ortho, m = meta, p = para, cs = charge separation, plant photosynthesis whereby an enzyme (still not cr = charge recombination). well characterized, but containing four manganese centers) catalyzes the oxidation of water. A supramolecular catalyst for multielectron redox processes must constates discussed in previous sections, the difference in redox tain several equivalent redox centers (at least as many as the potentials of the oxidized and reduced molecular components electrons to be exchanged), with the appropriate redox propis larger than this number. Thus, by the use of such systems, erties to mediate between the charge-separated state and the light-energy conversion by means of photoinduced charge substrate. The electronic coupling between such centers separation followed by the reactions shown in Equations (12) should be not too strong, otherwise the “charging” process and (13) is thermodynamically feasible. (stepwise one-electron transfer to the catalyst) could not occur at a reasonably constant potential. The centers should, on the ð12Þ Dþ -P-A
þ H2 O ! Dþ -P-A þ 1=2 H2 þ OH
other hand, be sufficiently close to be able to cooperate in binding and reducing (or oxidizing) the substrate. Apparently, ð13Þ Dþ -P-A þ 1=2 H2 O ! D-P-A þ 1=4 O2 þ Hþ electron transfer alone cannot satisfy these requirements. The lesson to be learnt from nature (Section 2.3.2) is that multiple electron-transfer processes can be profitably accomNone of these systems, however, would evolve hydrogen plished when accompanied by proton transfer. This can occur and oxygen upon simple irradiation in aqueous solution. There by a sequence of two distinct reactions or by a concerted prois, in fact, a fundamental kinetic problem. The photoinduced cess (proton-coupled electron transfer, PCET).[49] PCET processcharge separation is a one-electron process (i.e. D + and A
are one-electron oxidants and reductants). On the other hand, the es are advantageous from the thermodynamic viewpoint, but reactions depicted in Equations (12) and (13), although written they are inevitably more complex than either electron or in one-electron terms for stoichiometric purposes, are inherproton transfer because both electrons and protons must be ently multielectron processes: two electrons for [Eq. (12)] and transferred simultaneously. For example, electron–proton transfour electrons for [Eq. (13)]. Thus, although relatively long-lived fer between the relatively simple cis-[RuIVACHTUNGRE(bpy)2(py)(O)]2 + and charge separation can be achieved with supramolecular syscis-[RuIIACHTUNGRE(bpy)2(py)ACHTUNGRE(H2O)]2 + complexes involves electron transfer tems, the reactions depicted in Equations (12) and (13) are from a dpACHTUNGRE(RuII) orbital to a dpACHTUNGRE(RuIV) orbital and proton transfer hopelessly slow to compete with charge recombination. This from sO
H to a lone pair on the oxo group (Figure 22).[51, 166] problem is common to any conceivable fuel-generating proTherefore, stepwise and concerted electron-transfer processes cesses. may compete, depending on the particular conditions

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Photochemical Conversion of Solar Energy electron-rich substrates.[171] Reaction of two ferrous porphyrin subunits with O2 reforms the diironACHTUNGRE(III) m-oxo complex.[172] Figure 22. Concerted electron–proton transfer between cis-[RuIVACHTUNGRE(bpy)2(py)(O)]2 + and cis-[RuIIACHTUNGRE(bpy)2(py)ACHTUNGRE(H2O)]2 + (py = pyridine).[51]

(pH, driving force, reorganizational barrier, dielectric constant).[167] To obtain photoinduced evolution of hydrogen and oxygen, catalysts must be coupled with a charge-separation unit. Much has to be learnt in this field, but some interesting studies have started to appear.

3.7.2. Two-Electron Mixed-Valence Systems An interesting approach is that pursued by expanding the reactivity of metal complexes in electronically excited states beyond the conventional one-electron transfer. The types of two-electron mixed valency that have been investigated are shown in Figure 23:[164] a) bimetallic complexes that rely on

Figure 23. Types of two-electron photocatalysts (see text for details).[164]

ligand sets favoring a ground state Mn-Mn + 2 species, which is stabilized relative to its Mn + 1-Mn + 1 congener; b) porphyrinogens that store two-electron equivalency in the framework of a macrocyclic ligand; and c) MIII-O-MIII macrocycles tethered to a rigid spacer that upon excitation produce a two-electron mixed-valence intermediate, which is a reactive oxidant. Rh complexes offer interesting examples of binuclear mixedvalence systems (Figure 23 a). Hydrogen has been produced from hydrohalic acid (HX) solutions upon irradiation of a dirhodium complex LRh0-Rh0L with generation of a two-electron mixed-valence LRh0-RhIIX2 species. In the presence of a halogen trap, LRh0-RhIIX2 can be converted back by light irradiation into the coordinatively unsaturated species LRh0-Rh0L, thereby generating a photocycle for H2 production without any heterogeneous electron mediator.[168] The detailed kinetic aspects and the intermediate compounds involved in the photocycle have been investigated.[169] Iron porphyrinogens (Figure 23 b) have been prepared that display one-electron metal-based and four-electron ligandbased chemistry.[170] A series of cofacial “Pacman” bisporphyrins (Figure 23 c) bridged by xanthene and diphenyl-furan have also been prepared. For the case of di-ironACHTUNGRE(III) m-oxo porphyrins, light excitation breaks the FeIII-O-FeIII bond to produce a (PFeII)ACHTUNGRE(PFeIV=O) cofacial intermediate, which oxidizes simple ChemSusChem 2008, 1, 26 – 58

3.7.3. Systems Based on Ru ACHTUNGREOligopyridine Complexes

RuII oligopyridine complexes, in particular [RuACHTUNGRE(bpy)3]2 + , were again chosen[173–179] because of their well-known photochemical, photophysical, and electrochemical properties.[180] A variety of mono- and multinuclear MnII complexes have been covalently linked to a Ru complex. Furthermore, inspired by the presence of tyrosine as a mediator in the photooxidation of the Mn cluster in the natural process (Section 2.3.2), tyrosine itself or another type of phenolate moiety (hereafter indicated by ArOH) were introduced in the model systems designed. Some of the investigated systems are shown in Figure 24 and Figure 25. It should be noted that the oxidizing species is never the excited state of [RuACHTUNGRE(bpy)3]2 + , a relatively weak oxidant (*[RuACHTUNGRE(bpy)3]2 + /[RuACHTUNGRE(bpy)3] + = + 0.84 V relative to NHE), but is the [RuACHTUNGRE(bpy)3]3 + unit ([RuACHTUNGRE(bpy)3]3 + /[RuACHTUNGRE(bpy)3]2 + = + 1.26 V relative to NHE) generated by bimolecular reaction of the excited [RuACHTUNGRE(bpy)3]2 + moiety with a sacrificial electron acceptor, methylviologen (MV2 + ) or [CoACHTUNGRE(NH3)5Cl]2 + . In the example illustrated in Figure 24 a,[173, 175] the intermolecular photoinduced electrontransfer process (step 2) is followed by electron transfer from the tyrosine moiety to the [RuACHTUNGRE(bpy)3]3 + unit with generation of a tyrosyl radical (as revealed by EPR experiments), which is able to oxidize the dinuclear Mn complex. In fact, competing energy- and electron-transfer processes also occur. In the system shown in Figure 24 b,[175] a dinuclear Mn complex is linked to the RuII complex. Flash photolysis experiments revealed that the excited state of the RuII complex is quenched by intermolecular electron transfer to MV2 + or [CoACHTUNGRE(NH3)5Cl]2 + (step 2) and the RuIII complex obtained is reduced by rapid (k > 1 T 107 s
1) intramolecular electron transfer from the Mn complex that is oxidized to the MnII-MnIII state. Figure 25 a shows a complex consisting of three [RuACHTUNGRE(bpy)3]2 + -type units attached to a MnIV complex in which Mn is coordinated to electron-rich phenols.[178] Photoexcitation in the presence of MV2 + or [CoACHTUNGRE(NH3)5Cl]2 + leads to the formation of a [RuACHTUNGRE(bpy)3]3 + -type unit (step 2). Intramolecular electron transfer (k 5 T 107 s
1) from the phenolate ligands to the oxidized Ru complex then occurs, with formation of a (complexed) phenoxyl radical. In the system shown in Figure 25 b, however, in which [RuACHTUNGRE(bpy)3]2 + -type units are linked to a trinuclear MnII complex, the source of the electron that reduces the photochemically generated [RuACHTUNGRE(bpy)3]3 + -type species is not a phenolate but a MnII ion.[178] 3.7.4. Other Systems Other studies have been performed in an attempt to achieve photoinduced water splitting. Some binuclear Ru-Ir,[181] RuRh,[182] and Ru-Pt[183] species undergo a photochemical twoelectron reduction process, and a binuclear complex based on

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Figure 24. Model systems for the PSII center: a) intermolecular and b) intramolecular photochemical oxidation of binuclear MnII complexes.[173, 175, 176]

the [RuACHTUNGRE(phen)2]2 + (phen = 1,10-phenanthroline) moiety[184] can be reversibly photoreduced on the bridging ligand by four electrons. Several dendrimers are capable of storing several redox equivalents[185] and, in some cases, the reduction process can be carried out by photoexcitation.[186] Artificial photosynthetic systems could be realized by connecting catalysts for water reduction and oxidation to a photovoltaic cell (Section 5.2).[13, 187] In such constructs, the spatially separated electron–hole pairs provided by the photovoltaic junction are captured by the catalysts, and the energy is stored in the bond rearrangement of water to H2 and O2. Systems in which light is converted into chemical energy through electricity as a discrete intermediate are, however, affected by constraints associated with electrical contacts. To overcome these problems, a more intimate integration of the charge-separation and chemical bond-forming functions is required. Systems based on the semiconductor–liquid interface have been studied[68, 188–190] starting from pioneering work on singlecrystal TiO2 electrodes.[191] The most difficult problems are stability of the semiconductors and their band gap, which should be small enough to absorb visible light. Alternatively, the process can be sensitized by dyes.[192] Recently, aqueous suspensions of oxides or (oxy)nitrides semiconductors capable of absorbing visible light,[193] sometimes impregnated with metal oxide nanoparticles,[194] have been used.

3.8. Assembly Strategies 3.8.1. Introduction Any efficient artificial photosynthetic system must satisfy another requirement—the oxidized and reduced products should

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be produced in physically separated compartments of the system to avoid uncontrolled energy-wasting back reactions and to facilitate collection and storage of the fuel. As occurs in natural systems, therefore, some kind of membrane is needed to separate the oxidative and reductive parts of the process (Figure 8). This requirement, in turn, needs that every chargeseparating molecular device is specifically organized and oriented with respect to such a membrane. This problem should be addressed by research on self-assembling processes and organized media. As we will see in Section 4, artificial triads have already been successfully inserted into bilipid membranes. 3.8.2. Self-Assembly

Once organization at the supramolecular level has been achieved by covalent synthesis of appropriate building blocks, the supramolecular entities should self-assemble (or be assembled) into structures that can bridge length scales from nanometers to macroscopic dimensions. When the building blocks used to obtain light harvesting and/or charge separation have particular shapes, sizes, and capacity to give hydrogen bonds or p-p interactions, they can self-assemble.[141b, 195, 196] For example, compound 13 (Figure 26), which is made up of four perylenediimide peripheral units (PDIp) attached to a perylenediimide core (PDIc), self assembles into stacked dimers (13)2 in soluACHTUNGREtion.[195a] Femtosecond transient absorption spectroscopy showed that energy transfer from (PDIp)2 to (PDIc)2 occurs with t = 21 ps, followed by excited-state symmetry breaking of 1*ACHTUNGRE(PDIc)2 to produce PDIc + -PDIc
quantitatively with charge-separation time of 7 ps. The ion-pair recombines with t = 420 ps. Electron transfer occurs only in the dimeric system and does not occur in the disassembled monomer, thus mimicking both antenna and special pair functions in natural photosynthesis. A photoactive layer consisting of electron-donating zinc-porphyrin and electron-accepting fullerene arrays was constructed by using dendrimers appended with multiple porphyrin units (DPm, m = 6, 12, 24) capable of hosting bis-pyridine compounds carrying multiple fullerene units (BFn, n = 1–3).[197] Compounds of the type DPmBFn were obtained in which photoexcitation of the zinc-porphyrin units results in electron transfer to the fullerene. The charge-separation rate constant (109-1010 s
1) increases with increasing m and n values, whereas the charge-recombination process is much slower (about 5 T 106 s
1) in all cases. Langmuir–Blodgett films containing zinc-porphyrin–fullerene dyads have also been constructed.[198]

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Photochemical Conversion of Solar Energy from boron-dipyrrin to porphyrin is followed by electron transfer from porphyrin to fullerene and then by electron transfer from ferrocene to the oxidized porphyrin, with formation of the Fc + -P-C60
charge-separated state of the triad. In the presence of an electron carrier such as methylviologen, MV2 + , the reduced fullerene is reoxidized in a bimolecular process and the ferrocenium ion receives an electron from the gold electrode, resulting in current generation with an incident photon-to-current efficiency of 1–2 %.[201] Rigid p-octiphenyl rods were used to create tetrameric p stacks of blue–red-fluorescent naphthaline-diimides that can span lipid bilayer membranes.[202] In lipid vescicles containing quinone as electron acceptor that is surrounded by ethylenediaminetetracetic acid as hole acceptor, the p-stacked arrays accept an electron from the outer sacrificial electron donor and transport the electron to the quinone contained inside the vesicle which is thus reduced to hydroquinone. By adding an electron-rich dialkoxybenzene derivative to the system, the naphthalene-diimide array transforms in a Figure 25. Model systems for the PSII center. Intramolecular photochemical oxidation of a) a phenolate ligand and hollow photoinactive coassemII [178] b) a trinuclear Mn complex. bly. Under such conditions, the electron-transport function becomes disabled and the scaffold is transformed into an ion channel. Mixed self-assembled siloxane monolayers containing coumarin-2 and coumarin-343 have been constructed on a silicon wafer.[199] Single coumarin-2 molecules and dendron-type structures containing two and four coumarin-2 donor units were used. The energy-transfer efficiency from excited coumarin-2 3.8.3. Bilayer Membranes to coumarin-343 was found to depend, as expected, on the A bilayer membrane made of two amphiphiles has been concomposition of the mixture and on the branched nature of the structed.[203] One amphiphile contains an N-ethylcarbazolyl energy donor. Mixed self-assembled monolayers (SAMs) have been prelight-absorbing group, A, and the other has an energy-acceptpared on gold surfaces to study light harvesting and photoor anthryl group, B, appended to an electron-accepting violocurrrent generation.[200] Pyrene or boron-dipyrrin were used as gen group, C. Light excitation of the absorbing species A is followed by energy migration among the A groups until energy light-harvesting units and porphyrins as acceptors. Energyis irreversibly transferred to the B group with 70 % efficiency. transfer efficiency was 100 % for a donor/acceptor ratio of 7:3. The excited B group then transfers an electron to the appendIn SAMs containing the boron-dipyrrin energy donor and a fered viologen unit with 95 % efficiency. rocene-porphyrin-fullerene (Fc-P-C60) triad, energy transfer ChemSusChem 2008, 1, 26 – 58

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V. Balzani et al. faster (102-103-fold) recombination rate. The longer lifetime of the 146 + derivative is attributed to the involvement of protons in the electron-transfer process.

4.2. Conversion of Light to Proton-Motive Force

Figure 26. Structural formula of compound 13. See text for details.[195a]

Although the conversion of light into chemical energy by means of artificial systems seems to be a somewhat distant goal, a hybrid natural–artificial system capable of using light to obtain protonmotive force and then ATP synthesis has been constructed.[91a, 206, 207] These results have been achieved by coupling the photoinduced electron-transfer capability of artificial triads with the movement of protons across a membrane. As illustrated in Figure 28,[206] the molecular triad 6 (Figure 15 b), consisting of a light-absorbing tetraarylporphyrin (P) covalently linked to a quinone ac-

4. Hybrid Systems 4.1. Semibiological Photosynthetic Reaction ACHTUNGRECenters The artificial photosynthetic reaction center 146 + (Figure 27), in which a crown ether containing a [RuACHTUNGRE(bpy)3]2 + -type unit is mechanically linked in a catenane fashion to a cyclobis(paraquat-p-phenylene) (CBPQT4 + ) moiety, has been constructed on a protein surface by cofactor reconstitution.[204, 205] Recostitution of apo-myoglobin (Mb) with 146 + afforded the Mb-based artificial triad MbACHTUNGRE(FeIIIOH2)-[RuACHTUNGRE(bpy)3]2 + -CBPQT4 + , in which excitation of the [RuACHTUNGRE(bpy)3]2 + moiety in aqueous solution causes photoinduced electron transfer from the Ru complex to the CBPQT4 + electron-acceptor unit. This process is followed by a proton-coupled electron transfer that leads, with a quantum yield of 0.005, to the final charge-separated state containing a porphyrin cation radical and a reduced viologen radical, MbACHTUNGRE(FeIV=O)-[RuACHTUNGRE(bpy)3]2 + -CBPQT3 + . This species lies 1 eV above the ground state and has a lifetime (> 2 ms) comparable to that of the charge-separated state in the natural photosynthetic process. In the case of the analogous Mb(Zn)-[RuACHTUNGRE(bpy)3]2 + CBPQT4 + species, the charge-separated state MbACHTUNGRE(Zn+)-[RuACHTUNGRE(bpy)3]2 + -CBPQT3 + is obtained with the same excess of energy, a much higher quantum yield (0.08), and moreover a much

Figure 27. Structural formula of compound 146 + .[204]

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Figure 28. Schematic representation of the liposome-based proton pump powered by a photoinduced charge-separation process (PS = pyraninetrisulfonate).[206]

ceptor (Q) and a carotenoid donor (C), was incorporated into the bilayer of a liposome. When excited in various solvents, this triad undergoes photoinduced electron transfer from the singlet excited state of the porphyrin moiety to yield the intermediate charge-separated state C-P + -Q
with quantum yield of approximately one. Subsequent electron transfer from the carotenoid to the porphyrin cation competes with charge recombination to give the C + -P-Q
charge-separated state with a quantum yield of 0.15. Liposomes were prepared from a liquid mixture that contained the lipid-soluble 2,5-diphenylbenzo-

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Photochemical Conversion of Solar Energy quinone, Qs, and pyraninetrisulfonate (PS), a water-soluble dye whose fluorescence indicates the pH of the solution. Vectorial electron and proton transport requires asymmetric insertion of the triad into the liposomal bilayer membrane. This result was achieved because the negatively charged carboxylate group close to the quinone resides near the outside of the liposome surface, whereas the lipophilic carotenoid extends into the oily portion of the bilayer. It was found that excitation of the porphyrin moiety of 6 in liposomes with 5-ns light pulses results in the formation of C + -P-Q
, as detected by the characteristic transient absorbance of the carotenoid cation at 930 nm. The yield of this species is around 0.1; its lifetime, which is about 110 ns in the absence of Qs, is reduced to approximately 60 ns when Qs is present in the liposomes. Under the latter conditions, the PS dye indicates that light excitation drives protons into the interior aqueous phase of the liposomes, as expected on the basis of the shuttling mechanism involving Qs shown schematically in Figure 28. Step 1 includes excitation and twostep charge separation. In step 2, Qs, near the external aqueous phase, accepts an electron from the C + -P-Q
species, as expected on thermodynamic grounds. In step 3, Qs
accepts a proton from the aqueous phase, as required by the pKa value of Qs. The semiquinone so formed diffuses across the bilayer (step 4) and it is oxidized by the carotenoid cation (step 5). The protonated quinone then releases a proton (step 6) with the driving force related to its pKa, and Qs diffuses back to the exterior region of the bilayer (step 7). A pH gradient is thus created between the inside and outside of the liposome. The quantum yield of the proton transport in the first minute of irradiation was found to be around 0.004. The efficiency of the system can be, however, increased if an ionophore, such as valinomycin, is added to relax the membrane potential. In this system, photon energy is transduced into vectorial intramembrane redox potential and then into proton-motive force, that is, the biological analogue of electromotive force, by a chemically cyclic mechanism. It does not require sacrificial electron acceptors or donors and, as happens in natural systems, the redox potential remains confined to the membrane.

Figure 29. Schematic representation of the process leading to light-driven production of ATP.[207]

ferin–luciferase fluorescence assay. The results show that the synthesis of ATP occurs against an ATP chemical potential of approximately 12 kcal mol
1 and with a quantum yield of more than 7 %. One molecule of ATP is synthesized per 14 absorbed photons of 633 nm light, an observation which means that up to 4 % of the initial energy incident on the sample is stored by the system. The photocyclic system operates efficiently over a timescale of hours with a turnover number of seven ATP molecules per F0F1 per second. This number is similar to that observed in bacteriorhodopsin/ATP synthase constructs.[208] This is the first complete biomimetic system which effectively couples electrical potential, derived from photoinduced electron transfer, to the chemical potential associated with the ADP-ATP conversion, thereby mimicking the entire process of bacterial photosynthesis. It constitutes a synthetic biological motor that, in principle, can be used to power anything which requires a proton gradient or ATP to work, for example, to pump calcium ions across a lipid bilayer membrane[209] or even nanomachines.

5. Conversion of Light into Electricity 4.3. Light-Driven Production of ATP

5.1. Introduction

In principle, proton-motive force generated by the light-driven process described above can be used to perform work. This result has been achieved[207] by the system illustrated in Figure 29. F0F1-ATP synthase has been incorporated, with the ATP-synthesizing portion extending out into the external aqueous solution, into liposomes containing the components of the proton-pumping photocycle. F0F1-ATP synthase is a molecularscale rotary motor moved by a proton gradient and capable of synthesizing ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi ).[42, 43] Irradiation of the membrane (Figure 29) with visible light leads to the charge-separation process that causes the above-described proton translocation, with generation of a proton-motive force. On accumulation of sufficient proton-motive force, protons flow through the F0F1ATP synthase, with the formation of ATP from ADP and Pi. The functioning of the system was monitored by means of the luci-

Solar power can be converted directly into electrical power by photovoltaic (PV) cells and photoelectrochemical cells.[13, 14, 68, 187, 189, 190] Solar electricity can be profitably exploited in developing as well as developed countries.[1] By 2005, more than 2 million households in developing countries received electricity from solar home systems. One gets an idea of how necessary development in this field is and how huge this market is from the estimation that 350 million households worldwide do not have access to central power networks. The spreading of decentralized electricity-generation systems could eliminate the need to build up an extensive and costly transmission grid, in the same way as mobile telecommunications has allowed the leapfrogging of cabled telephone lines in some developing regions of the world. In wise developed countries, grid-connected PV systems grow so much that production does not satisfy demand. The potential of solar energy

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V. Balzani et al. generation in the European Union member states has been recently evaluated in detail.[210] Here, we will not discuss the solid-state PV cells based on inorganic semiconductors, present on the market,[13, 14, 187, 211] and those based on organic semiconductors[212, 213] or organic–inorganic hybrids,[68, 214] which are the object of extensive investigations. We will only mention a few issues related to the improvement of PV cells and describe briefly the photoelectrochemical cells which exploit scientific principles quite similar to those used in the above-described photosynthetic processes.

5.1. Photovoltaic Cells Photovoltaic cells capture photons by exciting electrons across the band gap of a semiconductor. This process creates electron–hole pairs that are subsequently separated, typically by p–n junctions introduced by doping.[211, 215] In the n-type regions of the device, conduction-band electrons can flow easily to and from cell contacts, whereas valence holes cannot; the p-type regions have the opposite properties. Such an asymmetry causes a flow of photogenerated electrons and holes in opposite directions, which generates a potential difference at the external electrodes. First-generation PV cells, which make up 85 % of the current commercial market,[14] are based on expensive (poly)crystalline silicon wafers. Shipped PV modules have efficiencies of 15 to 20 % and a lifetime on the order of 30 years.[187] However, for solar electricity to be cost-competitive with fossil-based electricity at utility scale, manufacturing costs must be substantially reduced. This goal has been partly reached with secondgeneration cells, which are based on thin films of less expensive materials such as amorphous or nanocrystalline Si, CdTe, or CuInSe2. However, research is needed to improve the efficiency of these cells in order to render them economically competitive. A key issue in the manufacturing of PV cells is the trade-off between material purity and device performance.[187] A minimum thickness of the cell is set by the thickness of the material required to absorb most of the incident sunlight. However, the thickness of the material imposes a constraint on the required purity, because the photoexcited charge carriers must live sufficiently long within the absorbing material to arrive at the electrical junction, where they can be separated to produce an electrical current. Impure absorber materials are characterized by short charge-carrier lifetimes and are therefore unable to convert effectively the absorbed sunlight energy into electricity. On the other hand, materials with the necessary purity are expensive to produce and manufacture. This cost–efficiency compromise could be circumvented in systems in which the collection of the charge carriers takes place along a direction orthogonal to the direction of light absorption. Highaspect-ratio nanorods, for example, can provide a long dimension for light absorption, while charge carriers move radially along the short axle of the nanorod to be separated by the junction and collected as electricity (Figure 30).[187, 216]

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Figure 30. Scheme of a PV cell based on nanorod arrays, illustrating an approach to orthogonalization of the directions of light absorption (along the length of the rods) and charge-carrier collection (radially outward to the surface of the rods).[187, 216]

5.3. Photoelectrochemical Cells Photoelectrochemical cells, often called GrWtzel cells after the scientist who has developed them,[217] are based on the sensitization of wide-gap semiconductors by dyes capable of exploiting sunlight (i.e. visible light). Although the basic principles of dye sensitization of semiconductors have long been established,[215] the application of such techniques to light-energy conversion became appealing only when new nanocrystalline semiconductor electrodes of very high surface area were developed.[218–221] The working principle of a dye-sensitized solar cell is shown in Figure 31 a.[222] The system comprises a photosensitizer (P) linked in some way (usually, by -COOH, -PO3H2, or -B(OH)2 functional groups) to the semiconductor surface, a solution containing a redox mediator (R), and a metallic counter electrode. The sensitizer is first excited by light absorption. The excited sensitizer then injects, on the femto- to picosecond timescale,[223] an electron into the conduction band of the semiconductor (step 1 in Figure 31 a). The oxidized sensitizer is reduced by a relay molecule (step 2), which then diffuses to discharge at the counter electrode (step 3) which is a conductive glass. As a result, a photopotential is generated between the two electrodes under open-circuit conditions, and a corresponding photocurrent can be obtained on closing the external circuit by use of an appropriate load. A great number of photosensitizers of the Ru-oligopyridine family, which display metal-toligand charge-transfer excited states, have been employed. The most efficient ones are those bearing two NCS
and two substituted bpy ligands (see, for example, compound 15 in Figure 31[224]) which show intense absorption bands in the visible region. A variety of solvents of different viscosity and of redox mediators have been used, the most common being the I3
/I
couple in acetonitrile solution. A global efficiency up to 11 % has been reported.[190] Zn porphyrins have also proved to be promising sensitizers.[225] Recently, corroles[226] and cyclometalated IrIII dyes displaying ligand-to-ligand charge-transfer excited states have been used.[227]

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Photochemical Conversion of Solar Energy Photoelectrochemical cells might look at first sight quite different from the photosynthetic systems discussed in Section 2. On closer inspection, however, analogies are apparent. The system is clearly based on photoinduced charge separation. From this viewpoint, it can be regarded as a heterogeneous “pseudo-triad”, in which the semiconductor surface acts as the primary acceptor and the relay as the secondary donor. As in any triad, the efficiency of charge separation and energy conversion depends critically on the kinetic competition between the various forward processes and charge-recombination steps. The main difference from photosynthetic systems is simply that the redox potential energy of the charge-separated state is not stored in products of subsequent reactions, but rather it is directly used to produce a photocurrent.[228] Hybrid photoelectrochemical biofuel cells have also been constructed.[229] Taking this comparative analysis a step further, one might consider applying some of the strategies of photosynthesis to increase the efficiency of photoelectrochemical cells.[230–232] In this regard, note that several possible processes can provide “short-circuit” paths within the photoelectrochemical cell. The most important of such dissipative processes is the charge recombination between the hole in the oxidized sensitizer and the injected electron (step 4, Figure 31 a). This process is always thermodynamically allowed and can be avoided only if it is disfavored by kinetic reasons compared with the other “useful” processes. To prevent the detrimental charge-recombination step, a simple heterotriad system, as shown schematically in Figure 31 b, could be used to produce the hole at spatially remote sites. For this idea to be implemented, of course, several nontrivial problems must be solved.[233] Experiments have been performed on TiO2 electrodes with dyads consisting of a [RuACHTUNGRE(bpy)3]2 + -type complex linked to phenothiazine,[234] RuII

and OsII oligopyridine compounds,[235] a RuII-RhIII dyad,[236] and Ru complexes with one (compound 16 in Figure 31)[237] or two[238] triphenylamine moieties appended. The antenna effect could also find useful application in these systems. With a conventional semiconductor electrode and a simple molecular sensitizer, light absorption is often quite inefficient at monolayer coverage. Multilayer adsorption, on the other hand, does not help because the inner layers tend to act as insulators relative to the outer layers.[239] Although this type of limitation is now much less severe because of the introduction of nanostructured electrodes of exceptionally high surface area,[219] the search for sensitizers with high intrinsic light-harvesting efficiency is still of considerable interest in the field. One possibility in this direction is to replace the sensitizer molecule at the semiconductor–solution interface with an antenna sensitizer molecular device (Figure 31 c).[240] A further advantage offered by antenna devices is that appropriate selection of the spectral properties of the various chromophoric groups can lead to better matching between the absorption spectrum and the solar emission spectrum. The trinuclear complex 172 + (Figure 31) has been developed[240] in a first attempt to demonstrate the applicability of the antenna effect in semiconductor sensitization. The presence of the carboxylate groups, besides being relevant to the energetics of the system, is essential for grafting the complex through its central component to a TiO2 surface. Experiments performed using TiO2-coated electrodes (aqueous solution, pH 3.5, NaI as electron donor) showed that the photocurrent spectrum reproduces closely the absorption spectrum of the complex. This indicates that, as a consequence of efficient energy transfer to the central unit bound to the semiconductor, all the light energy absorbed by the trinuclear complex, including that absorbed by the peripheral units, is used for elec-

Figure 31. a) Working principle of a photosensitized (n-type) semiconductor cell; P denotes a sensitizer linked to the semiconductor electrode, and R represents an electron relay molecule.[222] b) Photosensitization of a semiconductor by a dyad.[233] c) Photosensitization of a semiconductor by an antenna system.[240] Compounds 15,[224] 16,[237] and 172 + [240] are examples of a photosensitizer, a dyad, and an antenna system, respectively.

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V. Balzani et al. tron injection. High conversion efficiency (~ 7 %) of incident light to electricity has been obtained by use of this trinuclear complex on high-surface-area nanocrystalline TiO2 films.[217] Substantial efficiencies have also been achieved with related compounds.[241] One-dimensional systems that could stack perpendicularly to the surface (with the different antenna units working in series rather than in parallel) would be an even more interesting means of increasing the ratio of chromophoric components to the occupied surface area. Supramolecular arrays of porphyrin with fullerene have also been used.[242]

6. Conversion of Light into Mechanical Work by Molecular Machines 6.1. Introduction In the past decade, there has been an extraordinary development of studies aimed at using light to cause mechanical motions at the molecular level.[243–248] Although these processes will hardly be exploited to convert sunlight into mechanical work, it is worthwhile to illustrate a few of the most recent achievements.[249] 6.2. Molecular Rotary Motors Based on -C=C- Photoisomerization In suitably designed alkene-type compounds containing stereogenic centers, the relative direction of the movement leading to geometrical photoisomerization can be controlled.[250, 251] Each of the two helical subunits of compound 18 (Figure 32) can adopt a right-handed (P) or a left-handed (M) helicity. As a result, a total of four stereoisomers are possible for this compound. The cis–trans isomerizations are reversible and occur on irradiation at appropriate wavelengths. In contrast, the inversions of helicities, while maintaining a cis or a trans configuration, occur irreversibly under the influence of thermal energy because of the strain associated with the equatorial methyl susbstituents. Thus, a sequence of light- and temperature-induced isomerizations can be exploited to move this molecular rotor in one direction only. Like natural motor proteins,[252] a system of this type can operate autonomously; that is, in a constant environment and without the intervention of an operator, as long as the energy source (continuous light irradiation at a suitable temperature) is available. As the photoisomerization process in such systems is extremely fast (picosecond timescale), the rate-limiting step is the slowest of the thermally activated isomerization (relaxation) reactions. The effect of molecular structure on this rate has been investigated in a series of derivatives.[253] The rotary motor was then redesigned so that it had distinct upper and lower parts, with the lower half that could be connected to other molecules or surfaces. Rates of up to 44 rotations per second were achieved, and a derivative bearing donor and acceptor substituents was prepared capable of operating with visible light.[254, 255] Photoinduced unidirectional rotation has also been observed for molecular motors anchored on the surface of Au nanoparticles[256] and of a quartz plate.[257]

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Figure 32. Compound 18 undergoes unidirectional rotation in four steps; each light-driven, energetically uphill process is followed by a thermal, energetically downhill process.[250]

A molecular rotary motor of the above-described kind was employed to construct a prototype of a light-powered “nanocar” designed to move on an atomically flat surface (19, Figure 33).[258] The molecule comprises the motor unit, an oligo(phenylene ethynylene) chassis, and an axle system with four carborane wheels. It was shown that the light-powered motor of 19 does work in solution, but light-driven movement across a surface poses several problems and has not yet been demonstrated. It was observed[259] that on doping a liquid-crystal film with a chiral light-driven molecular motor related to 18, the helical organization induced by the dopant results in a fingerprint-like structure to the surface of the film. Irradiation of the film with light changes the distribution of the isomers, and, as they have different helical twisting power, the organization of the liquid crystal is changed. This process results in a rotational reorgani-

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Photochemical Conversion of Solar Energy

Figure 33. Prototype of the light-powered nanocar 19 based on a photochemical unidirectional motor, an oligo(phenylene ethynylene) chassis, and an axle system with four carborane wheels.[258]

zation of the surface structure, which can be followed by a glass rod sitting on the film.[259] When a photostationary state is reached, the rod’s rotary motion ceases. Removing the light excitation enables the population of the unstable isomer to decay, returning the system to its starting state, accompanied by rotation of the rod in the opposite direction. Very recently, a molecular rotary motor was attached to the terminus of a dynamically racemic helical polymer, containing equal amounts of left- and right-handed helices.[260] Effective chiral induction by the molecular motor allows fully reversible control of the preferred helical sense of the polymer backbone by photoand thermal isomerization of the alkene chromophore. All these effects, however, are related to the state of the system (i.e. the distribution of its various forms) modified by the photoinduced rotary motion, rather than to the unidirectional trajectory of the motor. In other words, the operation of the system is related to its switching properties.[247] Strategies to obtain unidirectional light-induced ring rotation in catenanes have also been explored.[261]

6.3. A Molecular Shuttle Powered by Sunlight Molecular shuttles, that is, rotaxanes in which the ring component can be controllably displaced between “stations” located along the axle, constitute the most common examples of artificial molecular machines.[243–248, 262] Photoinduced ring shuttling in a rotaxane containing two different recognition sites in the axle component has been achieved with the compound 206 + (Figure 34).[263] This compound consists of six molecular components suitably chosen and assembled to achieve the devised function. It comprises a bis-p-phenylene-34-crown-10 electrondonor macrocycle R (hereafter called the ring), and a dumb-

bell-shaped component which contains two electron-acceptor recognition sites for the ring, namely a 4,4’-bipyridinium (A12 + ) and a 3,3’-dimethyl-4,4’-bipyridinium (A22 + ) unit, that can play the role of “stations” for the ring R. Furthermore, the dumbbell-shaped component incorporates a [RuACHTUNGRE(bpy)3]2 + -type electron-transfer photosensitizer P2 + , which is able to operate with visible light and also plays the role of a stopper, a p-terphenyltype rigid spacer S, which has the task of keeping the photosensitizer far from the electron-acceptor units, and finally a tetraarylmethane group T as the second stopper. The stable translational isomer is that in which the R component encircles the A12 + unit because this station is a better electron acceptor than the other. In acetonitrile solution, the absorption of a visible photon by the Ru-based unit of 206 + causes the forward and backward shuttling of the ring R between the two stations. The operation of this system is based on a “four-stroke” synchronized sequence of electronic and nuclear processes.[263] The quantum yield of ring shuttling is 0.02, and it can be estimated that about 10 % of the photon’s energy is used for the mechanical motion. The somewhat disappointing ring-shuttling efficiency is compensated by the fact that the investigated system is a unique example of an artificial molecular machine because it gathers together the following features: 1) it is powered by visible light (in other words, sunlight); 2) it exhibits autonomous behavior; 3) it does not generate waste products; 4) its operation can rely only on intramolecular processes, allowing in principle operation at the single-molecule level; 5) it can be driven at a frequency of about 1 kHz; 6) it works under mild environmental conditions (i.e. fluid solution at ambient temperature); and 7) it is stable for at least 103 cycles. Although the system in its present form cannot develop a net mechanical work in a full cycle of operation (as for any reversible molecular shuttle, the work done in the forward stroke would be cancelled by that performed in the backward stroke[264]), it shows that the structural and functional integration of different molecular subunits in a multicomponent assembly is a powerful strategy to construct light-powered nanoscale machines.[265, 266]

7. Concluding Remarks About 85 % of our energy comes from fossil fuels (“fossil solar energy”),[15] a finite resource unevenly distributed beneath the Earth’s surface. Reserves of fossil fuels are progressively decreasing,[2, 3] and their continued use produces harmful effects such as pollution that threatens human health and greenhouse

Figure 34. Structural formula of the multicomponent rotaxane 206 + designed to work as an autonomous molecular shuttle powered by sunlight.[263]

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V. Balzani et al. gases associated with global warming.[4] Prompt global action to solve the energy crisis is needed. Such an action should be incorporated in a more general strategy based on the consciousness that the Earth’s resources are limited.[1] We are urged to save energy and to use energy in more efficient ways, but we are also forced to find alternative energy sources as soon as possible. The ultimate choice is between nuclear energy and renewable energies (essentially, solar energy).[267] Nuclear energy obtained with the currently available technologies is neither clean nor inexhaustible. It must be produced under severe technical, political, and military control because of its high capital cost, possible catastrophic accidents, huge difficulties to dispose waste, possible misuse of nuclear material, and proliferation of nuclear armaments.[268, 269] Poor countries will not be able to develop an independent energy policy based on nuclear energy. The sun continuously provides the Earth with a huge amount of energy, fairly distributed all over the world. The amount of energy mankind uses annually, about 4.6 T 1020 J, is delivered to Earth by the sun in one hour. The enormous potential of sunlight as a clean, abundant, and economical energy source, however, cannot be exploited unless it is converted into useful forms of energy. As solar energy is diffuse and intermittent, conversion should involve concentration and storage. These two requirements are hardly met by the currently available artificial conversion technologies, namely conversion into thermal and electrical energy. Chemists can play a key role in improving thermal and electrical conversion technologies by finding new materials and new processes. There is also plenty of room for improvement in increasing solar biomass production.[270–273] But the “grand challenge” of chemistry is to find a convenient means for artificial conversion of solar energy into fuels.[10, 13, 67]

Acknowledgements Financial support from the University of Bologna is gratefully acknowledged. We thank Dr. Fausto Puntoriero for his help with the preparation of the frontispiece image. Keywords: artificial photosynthesis · electron transfer energy transfer · photochemistry · photosynthesis

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[8] V. Smil, Energy, Oneworld, Oxford, 2006. [9] This fundamental issue of scientific research has been beautifully expressed in poetical sentences by Wilhelm Ostwald: “Life is a water mill: the effect produced by the falling water is achieved by the rays of the sun. Without the sun the wheel of life cannot be kept going. But we have to investigate more closely which circumstances and laws of Nature bring about this remarkable transformation of the sunrays into food and warmth” [translated from German “Die Rolle des fallenden Wasser aber wird bei der Maschine des Lebens von den Sonnenstralen Zbernommen; ohne die Sonnenstralen kann das rad des Lebens nicht im Gang erhalten werden und wir werden noch genauer erforschen mZssen, auf welchen VerhWltnissen und Naturgesezen diese merkwZrdige Umwandlung der Sonnenstralen in Nahrungsmittel und WWrme beruht”, W. Ostwald, Die Muehle des Lebens, Thomas, Leipzig, 1911) into English by Horst Hennig (Leipzig): H. Hennig, R. Billing, H. Knoll, in Photosensitization and Photocatalysis (Eds.: K. Kalyanasundaram, M. GrWtzel), Kluwer, Dordrecht, 1993]. [10] R. F. Service, Science 2005, 309, 548. [11] Basic Energy Sciences Report on Basic Research Needs for Solar Energy Utilization, Office of Science U.S. Department of Energy, Washington, DC, 2005; http://www.sc.doe.gov/bes/reports/files/SEU_rpt.pdf. [12] Special issue on “Forum on Solar and Renewable Energy”: Inorg. Chem. 2005, 44(20). [13] N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. USA 2006, 103, 15729. [14] G. W. Crabtree, N. S. Lewis, Phys. Today 2007, 60(3), 37. [15] G. Ciamician, Science 1912, 36, 385. This paper has been published in four languages (English, German, French, and Italian). Giacomo Ciamician, one of the most important pioneers of photochemistry, was Professor of Chemistry at the University of Bologna, where the chemistry department is now named in his honor. For more information about Ciamician, see, for example: N. D. Heindel, M. Pfau, J. Chem. Educ. 1965, 42, 383. [16] See, for example: Solar Energy: Chemical Conversion and Storage (Eds.: R. R. Hautala, R. B. King, C. Kutal), Humana Press, Clifton, 1979. [17] R. E. Blankenship, Molecular Mechanisms of Photosynthesis, Blackwell Science, Oxford, 2002. [18] X. Hu, A. Damjanovic, T. Ritz, K. Schulten, Proc. Natl. Acad. Sci. USA 1998, 95, 5935. [19] G. McDermott, S. M. Prince, A. A. Freer, A. M. Hawthornthwaite-Lawless, M. Z. Papiz, R. J. Cogdell, N. W. Isaacs, Nature 1995, 374, 517. [20] T. Pullerits, V. Sundstrçm, Acc. Chem. Res. 1996, 29, 381, and references therein. [21] S. Karrash, P. A. Bullough, R. Ghosh, EMBO J. 1995, 14, 631. [22] G. S. Engel, T. R. Calhoun, E. L. Read, T.-K. Ahn, T. Mancal, Y.-C. Cheng, R. E. Blankeship, G. R. Fleming, Nature 2007, 446, 782. [23] W. KZhlbrandt, D. N. Wang, Y. Fujiyoshi, Nature 1994, 367, 614. [24] a) E. Formaggio, G. Cinque, R. Bassi, J. Mol. Biol. 2001, 314, 1157; b) H. Rogl, R. Schodel, H. Lokstein, W. KZhlbrandt, A. Schubert, Biochemistry 2002, 41, 2281; c) A. N. Melkozernov, V. H. R. Schmid, S. Lin, H. Paulsen, R. E. Blankenship, J. Phys. Chem. B 2002, 106, 4313. [25] C. C. Moser, C. C. Page, P. L. Dutton, in Electron Transfer in Chemistry, Vol. 3 (Ed.: V. Balzani), Wiley-VCH, Weinheim, 2001, p. 25. [26] T. Ritz, A. Damjanovic, K. Schulten, ChemPhysChem 2002, 3, 243. [27] W. Zinth, J. Wachtveitl, ChemPhysChem 2005, 6, 871. [28] Oxygenic Photosynthesis: The Light Reactions (Eds.: D. R. Ort, C. F. Yocum), Kluwer, Dordrecht, 1996. [29] K.-H. Rhee, E. P. Morris, J. Barber, W. KZhlbrandt, Nature 1998, 396, 283. [30] N. Krauss, Curr. Opin. Chem. Biol. 2003, 7, 540. [31] a) J. Deisenhofer, O. Epp, K. Miki, R. Huber, H. Michel, Nature 1985, 318, 618; b) J. Deisenhofer, H. Michel, Angew. Chem. 1989, 101, 872; Angew. Chem. Int. Ed. Engl. 1989, 28, 829; c) R. Huber, Angew. Chem. 1989, 101, 849; Angew. Chem. Int. Ed. Engl. 1989, 28, 848. [32] J. P. Allen, G. Feher, T. O. Yeates, H. Komiya, D. C. Rees, Proc. Natl. Acad. Sci. USA 1987, 84, 5730. [33] J. R. Norris, M. Schiffer, Chem. Eng. News 1990, 68(31), 22. [34] V. Balzani, F. Scandola, Supramolecular Photochemistry, Horwood, Chichester, 1991, Ch. 5, and references therein. [35] Of the two structurally equivalent branches, usually called A and B, only one (branch A) is used for electron transfer; see, for example: E. Katilius, Z. Katiliene, S. Lin, A. K. W. Taguchi, N. W. Woodbury, J. Phys. Chem. B 2002, 106, 1471.

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