Photosynthetic Antenna−Reaction Center Mimicry: Sequential Energy- and Electron Transfer in a Self-assembled Supramolecular Triad Composed of Boron Dipyrrin, Zinc Porphyrin and Fullerene

CHEMPHYSCHEM MINIREVIEWS DOI: 10.1002/cphc.201300715

Photosynthetic Antenna–Reaction Center Mimicry by Using Boron Dipyrromethene Sensitizers Mohamed E. El-Khouly,*[a] Shunichi Fukuzumi,*[b, c] and Francis D’Souza*[d] Various molecular and supramolecular systems have been synthesized and characterized recently to mimic the functions of photosynthesis, in which solar energy conversion is achieved. Artificial photosynthesis consists of light-harvesting and charge-separation processes together with catalytic units of water oxidation and reduction. Among the organic molecules, derivatives of BF2-chelated dipyrromethene (BODIPY), “porphyrin’s little sister”, have been widely used in constructing these artificial photosynthetic models due to their unique properties. In these photosynthetic models, BODIPYs act as not only excellent antenna molecules, but also as electron-donor and

-acceptor molecules in both the covalently linked molecular and supramolecular systems formed by axial coordination, hydrogen bonding, or crown ether complexation. The relationships between the structures and photochemical reactivities of these novel molecular and supramolecular systems are discussed in relation to the efficiency of charge separation and charge recombination. Femto- and nanosecond transient absorption and photoelectrochemical techniques have been employed in these studies to give clear evidence for the occurrence of energy- and electron-transfer reactions and to determine their rates and efficiencies.

1. Introduction There has been considerable interest in mimicking photosynthesis by means of artificial photosynthetic models.[1–20] In this marvelous process, sunlight is harvested by the antenna complexes and the excitation energy is funneled to the reaction center (RC), where multistep electron-transfer reactions occur to generate a potential that can drive chemical reactions (Figure 1). Constructing chemical systems capable of fast energy- and electron-transfer reactions and slow charge recombination has been a very challenging goal for chemists over the past two decades. The efficiency of such systems depends on their fast energy transfer, fast charge separation, and relatively slow charge recombination. These studies are impor-

[a] Prof. Dr. M. E. El-Khouly Department of Chemistry, Faculty of Science Kafrelsheikh University Kafr ElSheikh 33516 (Egypt) E-mail: [email protected] [b] Prof. Dr. S. Fukuzumi Department of Material and Life Science Graduate School of Engineering Osaka University, ALCA, JST Suita, Osaka 565-0871 (Japan) Fax: (+ 81) 6-6879-7370 E-mail: [email protected] [c] Prof. Dr. S. Fukuzumi Department of Bioinspired Science Ewha Womans University Seoul 120-750 (Korea) [d] Prof. Dr. F. D’Souza Department of Chemistry, University of North Texas 1155 Union Circle, #305070, Denton TX 76203-5017 (USA) Fax: (001) 940-565-4318 E-mail: [email protected]

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Figure 1. Representation of an artificial photosynthetic system for water splitting (solar fuel production). An antenna absorbs sunlight and transfers excitation energy to a reaction center (RC), which generates a charge-separated state by electron transfer. Electrons transferred from the RC enable the reduction of hydrogen ions to hydrogen gas at the proton reduction catalyst (PRC), whereas generated holes oxidize water into oxygen in the water oxidation catalyst (WOC).

tant not only for photochemical conversion of solar energy into fuels,[21–33] but also for the construction of various optoelectronic devices.[34–36] Through a survey of the literature, one finds that tetrapyrrole macrocycles, such as porphyrins[37–53] and phthalocyanines,[54–65] have extensively been used as lightharvesting and electron-transfer units in artificial photosynthetic systems to perform photoinduced energy and electron transfer due to their close resemblance to natural pigments, namely, chlorophyll and bacteriochlorophylls, which act as light-harvesting antenna and electron donors in the electrontransfer cascade. Toward the goal of constructing new artificial photosynthetic systems that have the ability to convert light into chemical energy, BF2-chelated dipyrromethene compounds (abbreviated to BODIPYs), which are derived from 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a diaaza-s-indacene, and its structural anaChemPhysChem 2014, 15, 30 – 47

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CHEMPHYSCHEM MINIREVIEWS logue BF2-chelated tetraarylazadipyrromethanes (azaBODIPYs), have attracted much attention in recent years as antenna molecules and as electron donors and electron acceptors. Herein, we focus on the design principles, syntheses, and photophysical behavior of promising BODIPYs that make them useful in many practical applications of chemistry, biology, and medicine.[66–70] We provide an overview of photosynthetic antenna–RC models based on BODIPYs developed recently to mimic light-harvesting and charge-separation functions in the photosynthetic RC. The key findings in the areas of photoinduced excitation transfer, charge separation, and charge recombination, mainly from our laboratories, are discussed. For confirmation of the energy- and electron-transfer processes, femto- and nanosecond transient absorption methods have been widely used. The kinetic data obtained by these techniques are useful for the prediction of the efficiencies of the BODIPY-based molecular and supramolecular systems as lightharvesting materials.

2. Structure and Photophysical Properties of BODIPYs The basic structure of BODIPY depicted in Figure 2 consists of two pyrrole rings linked by methene and BF2 groups. The first report related to these dyes dates from 1968 by Treibs and

Figure 2. Molecular structures of BODIPY and azaBODIPY.

Kreuzer,[71] but they did not gain recognition from the scientific community until the end of nineties when initial reports by the groups of Boyer and Pavlopoulos showed their potential application in tunable lasers.[72–75] The structures of azaBODIPYs are analogues of BODIPYs, in which the 8-position carbon atom has been replaced with a nitrogen atom. Knowledge of the photophysical behavior of BODIPYs and azaBODIPYs is essential to understand their behavior in newly prepared artificial photosynthetic models that have the ability to convert solar energy into chemical energy. BODIPYs are stable compounds; this allows a wide range of reactions to be performed on them. This high stability is partially caused by the boron, nitrogen, and fluorine atoms all being first row elements, which allow efficient orbital overlap and promote delocalization of the p system. Typical UV/Vis absorption and fluorescence spectra of BODIPYs are shown in Figure 3. The major absorption of BODIPYs in the l = 500–520 nm range is complementary to those of porphyrins (Soret band at l = 430 nm and Q bands at l = 530–  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 3. Steady-state absorption and emission spectra of BODIPYs.

600 nm). In other words, the main absorption band of BODIPY is located in the unabsorbed region of porphyrins between the Soret and Q bands. This major absorption band, corresponding to the electronic transition from the ground to the first excited state (S0 !S1 transition), is characterized by a high transition probability with a high molar absorption coefficient (e105 m1 cm1). Other absorption bands, corresponding to transitions to higher singlet excited states (S2, S3, …), appear in the UV region and are less probable.[76, 77] Theoretical results indicate that this visible absorption band can be assigned to the promotion of an electron from the HOMO to the LUMO.[78] The shape of this absorption band is nearly independent of the dye concentration (at least up to 2  103 m), which suggests the absence of significant intermolecular interactions, including dye aggregation.[76] This low tendency of BODIPYs to self-associate is a very important advantage of these dyes with respect to other dyes because the formation of non- or poorly fluorescent aggregates drastically reduces the fluorescent intensity in concentrated solutions, which are generally required to record the laser signal.[79] Compared with BODIPYs, azaBODIPY show a strong absorbance in the visible–near-infrared (NIR) region with a sharp band in the l = 600–800 nm range. The extinction coefficients for simple azaBODIPYs range from 104 to 105 m1 cm1. The absorption maxima of the azaBODIPYs are relatively insensitive to solvent polarity; only small blue shifts tend to be observed (6–9 nm) upon switching solvents from nonpolar toluene to polar ethanol. The strong absorbance of azaBODIPYs in the visible-NIR region and their ability to produce singlet-oxygen have made them potentially useful photosensitizers for photodynamic therapy applications. Additionally, the low-energy redlight absorption of azaBODIPYs allow the imaging of live cells/ tissues for deeper penetration without the need to use potentially damaging shorter wavelength light.[80] The BODIPYs are highly fluorescent relative to porphyrins and phthalocyanines with fluorescence quantum yields that sometimes approach unity.[81–88] The fluorescence spectrum of BODIPY with a maximum emission at l  518 nm, from which the energy of the singlet state of BODIPY (1BODIPY*) is ChemPhysChem 2014, 15, 30 – 47

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CHEMPHYSCHEM MINIREVIEWS  2.4 eV, is practically the mirror image of the S0 !S1 absorption band (Figure 3); this suggests similar vibrational levels in both electronic states. Indeed, the results of quantum mechanical calculations indicate that the optimized geometry in the S1 excited state is similar to that of the S0 ground state. Consequently, the fluorescent band is characterized by a small Stokes shift in the range of n˜ = 520 cm1. The shape of the fluorescence band is independent of the excitation wavelength; this indicates that the emission is from the lowest vibrational level of the S1 excited state, which is independent of the electron-vibrational level directly populated in the excitation process.[76] Therefore, a very fast internal conversion process occurs from the upper excited states to populate the fluorescent excited state. The fluorescence quantum yield (F) can be very high, reaching values close to unity for some BODIPYs in certain media.[76, 77] Generally, the fluorescence decay curves of BODIPYs can be analyzed as monoexponential decays, with fluorescence lifetimes (t) of around 4–6 ns. The lifetime is independent of the excitation and emission wavelengths, which confirms simple emission from the locally excited state.[76] The high fluorescence quantum yield of BODIPY is a consequence of the structure of the chromophoric unit. The BF2 group acts as a linking bridge to provide a rigid delocalized p system. The BF2 group does not participate in the aromaticity of the p system, but avoids any cyclic electron flow around the chromophoric ring.[89] Consequently, BODIPY dyes can be classified as quasi-aromatic dyes and are characterized by low intersystem crossing. This low population of the triplet state[90] is a significant advantage for the potential application of BODIPYs in photonics because the triplet–triplet absorption, which is one of the most important losses in the resonator cavity, is drastically reduced. It is taken as a general rule that by increasing the degree of substitution on the pyrrole carbon atoms of the BODIPY, the absorption and emission wavelengths undergo a bathochromic shift. In contrast to the absorption process of BODIPY, the fluorescent band and decay curves of BODIPY dyes are dependent on the dye concentration. Indeed, an augmentation in the optical density of the samples leads to a bathochromic shift in the fluorescent band, an important decrease of its intensity, and a change of the lifetime.[91] These effects are attributed to the inner filter effect (i.e. reabsorption/reemission phenomena) rather than to any intermolecular dye–dye interactions that affect the emission characteristics (i.e. excimer formation). All of these novel features of BODIPYs make them useful in a range of applications, including artificial photosynthesis, luminescent labeling agents,[92–94] fluorescent switches,[95, 96] chemosensors,[97–100] and laser dyes.[101]

www.chemphyschem.org In addition, in search for new BODIPY derivatives, some specific synthetic routes have recently been developed. For example, the synthesis of a BODIPY from glutaric anhydride (Scheme 1) follows the use of boron trifluoride diethyl etherate as a Lewis acid to initiate the reaction to form the dipyrrin, which is then converted into the BODIPY.[102] The benefit of this reaction is that a free carboxylic acid is produced, which allows for the BODIPY to be attached to other molecules.

Scheme 1. Synthesis of BODIPY from glutaric anhydride.

The azadipyrromethenes are synthesized by two different methods.[80] In the first method, the conversion of 2,4-diarylpyrroles into their 5-nitroso derivatives is followed by acid-catalyzed condensation with a second molecule of pyrrole. The second method involves the Michael addition products of chalcones reacting with an ammonia source.[103–105] Subsequent coordination to the boron center produces the azaBODIPY.[106] Due to the nature of the synthetic procedure (Scheme 2), and the instability of certain pyrrolic intermediates, azaBODIPYs almost always have aryl groups at the 1-, 3-, 5-, and 7-positions. Thus far, fully unsubstituted azaBODIPY has not been synthesized.

Scheme 2. Synthesis of azaBODIPY.

4. Energy-Transfer Processes of BODIPY-Based Artificial Photosynthetic Complexes 4.1. BODIPYs as Antenna Molecules

3. Synthesis of BODIPYs BODIPYs are generally synthesized from the initial preparation of dipyrromethene by using an appropriate pyrrole and aldehyde followed by treatment with boron trifluoride diethyl etherate in the presence of an organic base. Often, the relatively unstable dipyrrin salts are not isolated and are directly used in the boronation reaction.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

An energy-transfer cassette consists of two or more fluorescent units attached to the same (or similar) molecule. One unit acts as an energy donor and the other as an energy acceptor. The energy donor absorbs light and this energy is then transferred to the acceptor, which then emits the light at a longer wavelength. The efficiency of the energy-transfer process depends on the spectral overlap of the donor emission with the acceptChemPhysChem 2014, 15, 30 – 47

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CHEMPHYSCHEM MINIREVIEWS or absorbance, the distance between the donor and the acceptor, the relative orientation of donor and acceptor, and the effectiveness of other de-excitation modes (e.g. emission from donor and nonradiative processes).[107] The energy-transfer efficiency is calculated by comparing the amount of emission from the donor with the emission from the acceptor. If no, or very little, emission is observed from the donor, it is implied that the majority of the energy is transferred from the donor to the acceptor, that is, efficiencies of over 99 %. Due to their intense fluorescence, BODIPYs are useful in building new energy-transfer cassettes. Because of the similarity of their structures, energy-transfer cassettes containing BODIPY and porphyrin units have received much attention for their applications as solar energy harvesting systems. A simple 8-(4-hydroxyphenyl)-BODIPY has been coordinated to an antimony tetraphenylporphyrin (Sb(TPP)) through an alkyl chain to form a through-space energy-transfer cassette 1 (Figure 4).[108]

www.chemphyschem.org BODIPY, H2P, and ZnP, the lack of interaction between the components in the ground state was established. In addition, luminescence and transient absorption experiments showed that the excitation of the BODIPY unit of 2 to the first singlet excited state resulted in rapid and efficient BODIPY-to-porphyrin energy transfer with rate constants of 2.9  1010 and 2.2  1010 s1 for BODIPY–H2P and BODIPY–ZnP, respectively, generating the porphyrin-based singlet excited state. The resulting porphyrin singlet excited states gave fluorescence in addition to competitively undergoing intersystem crossing to the corresponding triplet excited states. Both energy- and electron-transfer reactions have been shown in a novel photosynthetic antenna–RC complex constructed through a self-assembled supramolecular methodology.[110] For this, a supramolecular triad was assembled by axially coordinating C60Im to the zinc center of a covalently linked zinc porphyrin–boron dipyrrin dyad (ZnP–BODIPY) to form supramolecular triad 3 (C60Im!ZnP–BODIPY; Figure 6). The

Figure 4. Structure of antimony porphyrin–BODIPY array 1.

The efficiency of energy transfer varied from 13 to 40 %. By increasing the length of the alkyl chain spacer unit, the efficiency was decreased. It was found that the BODIPY acted as the energy donor and not Sb(TPP), that is, no quenching of the singlet excited state of the porphyrin (energy of 1MP* = 1.9– 2.1 eV) by donation of energy to the BODIPY (energy of 1 BODIPY*  2.4 eV) was found because of unfavorable energetic conditions.[108] Guldi et al. reported that BODIPY could easily be combined with either a free-base or a zinc porphyrin moiety (H2P and ZnP, respectively) through a cyanuric chloride bridging unit to yield the dyads BODIPY–H2P and BODIPY–ZnP 2 (Figure 5).[109] Through a comparison of the absorption spectra and cyclic voltammograms of 2 with those of their model compounds

Figure 5. Structure of BODIPY–H2P and BODIPY–ZnP dyads 2.

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Figure 6. Molecular structure of supramolecular triad 3 utilized to mimic the “combined antenna–RC” events of photosynthesis.

major absorptions of BODIPY (l  500 nm) were complementary to those of porphyrins (l  430 and 500–600 nm). Therefore, the resulting conjugate absorbed over a broad range in the visible region. For this conjugate, there was also good spectral overlap between the BODIPY (i.e. energy donor) emission and the porphyrin (i.e. energy acceptor) absorption. Both of these features are desirable for efficient intramolecular energy transfer. Hence, a combination of BODIPYs, porphyrins, and fullerenes is highly desirable to enhance light-harvesting efficiency throughout the solar spectrum, and to convert the harvested light into the high-energy state of charge separation by photoinduced electron transfer. Upon forming the supramolecular triad 3 (C60Im!ZnP– BODIPY) in non-coordinating o-dichlorobenzene, selective excitation of the BODIPY subunit resulted in the occurrence of efficient energy transfer from the singlet-excited state of BODIPY to ZnP (kENT = 9.2  109 s1; FENT = 0.83). The formed 1ZnP* moiety resulted in efficient electron transfer to the coordinated fullerene, resulting in a C60CIm!ZnP· + –BODIPY charge-separated state, as observed clearly from the nanosecond transient absorption measurements. The characteristic absorption band ChemPhysChem 2014, 15, 30 – 47

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CHEMPHYSCHEM MINIREVIEWS of the C60C was observed at l = 1000 nm, from which the rate constant of charge recombination and lifetime of the chargeseparated state were found to be 2.0  108 s1 and 5.0 ns, respectively. The observed electron transfer followed by energy transfer in the supramolecular triad mimicked the events of natural photosynthesis, that is, the BODIPY entity acts as an antenna that absorbs light energy and spatially transports it to the photosynthetic RC, whereas electron transfer from the excited zinc porphyrin to fullerene mimicked the primary events of the RC, where conversion of the electronic excitation energy to chemical energy in the form of charge separation occurs. The important feature of model system 3 is its relative “simplicity” because of the supramolecular approach utilized to mimic rather complex combined antenna–RC events of photosynthesis. To increase the amount of the light absorbed in the solar spectrum by the antenna, supramolecular tetrads 4 and 5 (Figure 7) were synthesized by assembling different molecular entities (antenna1–antenna2–electron donor–electron acceptor).[111] The covalently linked triad featured triphenylamine (TPA; antenna1, absorbing light in the UV region), BODIPY (antenna2, absorbing light in the visible region), and ZnP (electron donor) entities positioned in such a fashion that excitation of either TPA or BODIPY resulted in ultrafast transfer of energy to the terminal energy acceptor and electron donor, ZnP. In the presence of an electron acceptor (fullerene C60 or naphthalenediimide), which was combined with ZnP through a coordination

www.chemphyschem.org bond, resulted in photoinduced electron-transfer-generating charge-separated states. The photochemical events studied by the femtosecond transient absorption technique in nonpolar toluene showed that tetrad 4 exhibited a longer lifetime of the radical ion pair (5.0 ns) than that of 5 (1.2 ns), which was rationalized by 1) the fairly rigid two-dimensional electron-acceptor naphthalenediimide, compared with the three-dimensional electron acceptor C60 and 2) the longer center-to-center distance between C60 and ZnP through an imidazole linker compared with that of ZnP and naphthalendiimide through pyridine linker. Similarly, model 6 utilized a metal–ligand axial coordination approach to link imidazole-functionalized fullerene to ZnP of the covalently linked BODIPY–ZnP dyads having up to four BODIPY entities (Figure 8).[112] In the case of the triad of the

Figure 8. Molecular structure of supramolecular pentad 6 utilized to mimic the combined antenna–RC events of photosynthesis.

Figure 7. Molecular structure of supramolecular tetrads 4 and 5 utilized to mimic the combined antenna–RC events of photosynthesis.

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type 6 with varying number of BODIPY units (1, 2, and 4), the singlet–singlet energy transfer was quick and occurred on the timescale ranging from 28 to 48 ps. Additionally, a decrease in time constants with an increasing number of BODIPY units was observed and revealed a better antenna effect of the dyads with a higher number of BODIPY entities. The same type of coordination–covalent binding was employed to build C60py!ZnPc–BODIPY (C60py = N-(4-pyridyl)-3,4fulleropyrrolidine; ZnPc = zinc phthalocyanine) supramolecular triad 7 through covalent–coordination bonds, in which the BODIPY unit was tethered to the peripheral position of a ZnPc (Figure 9). The electron acceptor, (C60py) was utilized to coordinate the zinc atom of ZnPc.[113] Triad 7 exhibited intense absorptions of Pc, not only in the near-UV region (Soret bands at l = 350 nm), but also in the near-IR region (Q band at l = 700 nm), where the maximum of the solar photon flux occurs, in addition to the strong absorption of BODIPY in the visible spectral region of l = 500–550 nm and the weak absorption of C60py at l = 430 and 700 nm (Figure 9). Upon being subjected to photoexcitation at l = 480 nm, the wavelength at which BODIPY was selectively excited, an intramolecular transduction ChemPhysChem 2014, 15, 30 – 47

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Figure 9. Supramolecular triad 7 and its absorption spectral features.

of singlet-excited-state energy occurred; this funneled the excitation from the light-absorbing, energy-donating BODIPY (2.30 eV) to the energy-accepting ZnPc (1.80 eV) subunit. When such an acceptor was present, specifically C60 provided through pyridine coordination to the ZnPc metal center (resulting in the formation of triad 7), a sequence of energy- and electron-transfer reactions occurred following photoexcitation. Irradiation of the resulting supramolecular ensembles within the visible range led to a charge-separated BODIPY–ZnPcC + –C60C radical ion pair state through a sequence of the formation of excited and charge-separated states, which were characterized by a long lifetime of 39.9 ns in toluene.[113] In an alternative way to combine BODIPY, porphyrin, and C60 in a supramolecular fashion, D’Souza et al. reported that both energy- and electron-transfer reactions occurred sequentially in a BODIPY–zinc porphyrin–crown ether triad bound to a alkylammonium-derivatized fulleropyrrolidine.[114] For construction, first boron dipyrrin was covalently attached to a zinc porphyrin entity with a benzo-[18]-crown-6 host segment at the opposite end of the porphyrin ring. Next, an alkyl ammonium functionalized fullerene was used to self-assemble the crown ether entity through ion–dipole interactions to form the supramolecular triad 8 (Figure 10). The advantage of 8 was its relative simplicity and its higher binding constant (4.6  104 m1) in polar benzonitrile. It should be noted here that the supramolecular triads 3–7 exhibited moderate stability only in non-coordinating solvents, for example, o-dichlorobenzene and toluene, but not in polar solvents. The intramolecular processes of 8 were summarized as follows: the initial step was excitation of the BODIPY followed by energy transfer to the zinc porphyrin. The rate of energy transfer obtained from the decay measurements of time-correlated singlet photon counting (TCSPC) and pump–probe techniques revealed efficient photoinduced energy transfer in the dyad  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org (time constant in the order of 10–60 ps, depending upon the conformer) with an efficiency of about 97 %. This step can be bypassed by directly exciting the zinc porphyrin with l = 420 or 550 nm excitation laser light. This was followed by charge separation to give the ZnP radical cation (l  650 nm) and the C60 radical anion (l = 1000 nm), which then underwent charge recombination to give the ground-state compound again. A small amount of the singletexcited-state zinc porphyrin undergoes intersystem crossing to produce the triplet-excited zinc porphyrin, which can either phosphoresce or still undergo charge separation to give the charge separated species before

Figure 10. Sructure of BODIPY–porphyrin–fullerene supramolecular array 8.

charge recombination. Nanosecond transient absorption studies yielded a lifetime of the charge-separated state of 23 ms, indicating charge stabilization in the supramolecular triad. These results of the supramolecular triad 8 system represented another successful model to mimic the rather complex combined antenna–RC events of photosynthesis. As seen from Figure 10, the donor ZnP and acceptor C60 entities of 8 are adjacent to each other to facilitate sequential energy- and electron-transfer processes. Although charge stabilization was observed to some extent, a long-lived chargestabilized state was difficult to attain. Thus, this approach was extended by designing a new supramolecular triad 9 composed of the same subunits BODIPY, ZnP, and C60, but with a different spatial arrangement. The authors intentionally placed the BODIPY antenna unit between the electron-donor ZnP and C60 to increase the distance between them in triad 9 (Figure 11).[115] The excitation spectrum recorded for the crownBODIPY–ZnP dyad revealed energy transfer from 1BODIPY* to ZnP to populate the singlet ZnP, which in turn donated its electron to the attached BODIPY to form the crown-BODIPYC– ChemPhysChem 2014, 15, 30 – 47

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Figure 11. Zinc porphyrin–BODIPY–fullerene supramolecular array 9 and its optimized structure.

ZnPC + . Although subsequent electron migration to form C60C :crown-BODIPY–ZnPC + seemed possible, charge recombination occurred to populate 1C60*, which was converted by intersystem crossing into the triplet-excited state (3C60*). A relatively slow photoinduced electron transfer subsequently occurred from ZnP to 3C60* within the triad, resulting in the formation of C60C :crown-BODIPY–ZnPC + as the final charge-separated state with a lifetime in the order of 100 ms, which was nearly four times longer than that of 8. To increase the antenna effect of the BODIPY units in the photosynthetic antenna–RC complex, multi-BODIPY–porphyrin–fullerene C60 pentad 10 (Figure 12) was synthesized by Ng and co-workers by connecting three BODIPY units with zinc porphyrin through click chemistry, which in turn was connected to C60 moiety by using the Prato procedure.[116] The BODIPY units served as antennae that absorbed light in the visible

Figure 12. Molecular structure showing the intramolecular events of the pentad 10.

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www.chemphyschem.org region and transferred it to the ZnP unit, which acted as an energy acceptor and also an electron donor, and finally C60 acted as an excellent electron acceptor. In this covalent pentad 10, the photogenerated charge-separated state revealed significant stability. The absorption of BODIPYs (l = 500 nm), porphyrins (l  430 and 500–600 nm), and fullerene (l = 300–450 and 700 nm) showed that pentad 10 absorbed over a broad range in the visible region. Both the steady-state emission and picosecond transient absorption measurements showed clearly the occurrence of energy transfer from the singlet excited BODIPY (2.42 eV) to populate the singlet ZnP (2.08 eV), with a rate constant of 2.70  1010 s1, which in turn donated its electron to the covalently linked C60 to yield (BODIPY)3–ZnPC + –C60C with a rate constant of 1.70  1011 s1. From the decay of the ZnPC + and C60C profiles, the rate constant of charge recombination and the lifetime of the charge-separated state were found to be 1.00  109 s1 and 1 ns, respectively. Nanosecond transient absorption studies showed that a small amount of the singletexcited-state zinc porphyrin undergoes intersystem crossing to populate the triplet-excited zinc porphyrin (3ZnP*; 1.56 eV), which underwent charge separation to give the triplet chargeseparated species with a lifetime of 385 ns and a quantum yield of 0.27. Such a relatively long-lived charge-separated state can be attributed to its triplet character, which makes charge recombination back to the singlet ground state a spinforbidden process. The fast rates of energy- and electron-transfer reactions, as well as the slow rate of charge recombination, suggest the usefulness of pentad 10 as a solar energy harvesting system.

5. BODIPY–AzaBODIPY/Styryl BODIPY Complexes Energy-transfer arrays have also been constructed by covalently linking BODIPY and azaBODIPYs (11; Figure 13).[117] Although structurally similar, the BODIPY and azaBODIPY macrocycles differ substantially in their optical absorption and emission properties in such a fashion that energy transfer from the singlet-excited BODIPY to azaBODIPY, which is a near-IR emitter, is achievable. In these two arrays, both BODIPY units acted as energy donors for the central azaBODIPY acceptor. The central azaBODIPY unit has the effect of red shifting the absorption and emission wavelengths of this unit; thus lowering the energy of the singlet excited states of BODIPY. These properties result in azaBODIPY acting as an energy acceptor. Photochemical studies showed that triad 11 absorbed light over the wide range of wavelengths in the UV/Vis/NIR region with intense absorption bands for BODIPY and azaBODIPY at l = 503 and 670 nm, respectively. By exciting the BODIPY unit with l = 505 nm excitation light, the emission band of singlet BODIPY at l = 517 nm was strongly quenched with the appearance of the emission band of singlet azaBODIPY at l = 674 nm; this provides direct proof of singlet-singlet energy transfer from singlet excited BODIPY to the covalently linked azaBODIPY. By changing the solvent from toluene to more polar benzonitrile, the formed 1azaBODIPY* was further quenched by ChemPhysChem 2014, 15, 30 – 47

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Figure 13. Molecular structure of the azaBODIPY–(BODIPY)2 triad 11 and a molecular electrostatic potential map. Arrows show photochemical processes.

As an extension to this work, an electron donor, ZnP, replaced one of the two BODIPY units. The resulting BODIPY–azaBODIPY–zinc porphyrin triad 12 (Figure 15) exhibited a broadband capturing and emitting light (l = 300–800 nm) that would be useful for solar energy harvesting systems.[118] The photochemical studies showed that upon excitation of BODIPY at l = 490 nm, energy transfer occurs from the singlet BODIPY to populate the singlet

electron transfer from BODIPY to the singlet excited state of azaBODIPY to yield the charge-separated state BODIPYC + – azaBODIPYC . The recorded intramolecular events of 11 are summarized as follows (Figure 14): selective excitation of the BODIPY entity populates the singlet excited state of BODIPY, which transfers

Figure 15. Structure of the BODIPY–azaBODIPY–ZnP triad 12 and illustration of the intramolecular photoinduced processes. ET = electron transfer, EnT = energy transfer.

Figure 14. Energy-level diagram of triad 11. kENT = rate of energy transfer, kCS = rate of charge separation, kCR = rate of charge recombination, kS-S = decay rate of the azaBODIPY singlet.

its energy to the attached azaBODIPY with a rate constant of 1.1  1011 s1. In toluene, the formed 1azaBODIPY* decayed to its ground state without any evidence of additional subsequent energy- and/or electron-transfer reactions (kdecay = 5.7  108 s1). In benzonitrile, however, electron transfer from BODIPY to 1azaBODIPY* was clearly observed, yielding BODIPYC + –azaBODIPYC with a rate constant of 1.40  109 s1. The electron-transfer product, BODIPYC + –azaBODIPYC , decayed to populate the ground state without the intermediate formation of either the triplet state of BODIPY or azaBODIPY, as supported by the transient absorption measurements. These examples demonstrate the utilization of a near-IR-emitting sensitizer, azaBODIPY, as a suitable candidate to build new types of donor– acceptor dyads for excitation- and electron-transfer studies.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

azaBODIPY, which in turn accepts one electron from the ground state of ZnP, generating BODIPY–azaBODIPYC–ZnPC + , with a lifetime of 1.3 ns. Upon excitation of the ZnP at l = 420 nm, excited ZnP transfer its energy to lower energy 1 azaBODIPY*, followed by electron transfer from the ground ZnP to 1azaBODIPY* to yield the radical ion pair. Moreover, triad 12 has the advantage of absorbing light near the IR region by the azaBODIPY unit to populate its singlet state, which then accepts an electron from ZnP to form the chargeseparated state. Recently, a novel photosynthetic antenna–RC model was designed and characterized by Ng and co-workers.[119] The model contains the near-IR-absorbing azaBODIPY, which is connected to a monostyryl-BODIPY through a click reaction and to a fullerene (C60) by using the Prato reaction (Figure 16). The BODIPY– azaBODIPY–C60 triad 13 absorbed light over a wide range of the solar spectrum. Upon excitation, the BODIPY moiety was significantly quenched due to energy transfer to the azaBODIPY core, which subsequently transferred an electron to the fullerene unit. By using femtosecond laser flash photolysis, concrete evidence was obtained for the occurrence of energy transfer from 1BODIPY* to azaBODIPY in the BODIPY–azaBODIPY dyad, followed by electron transfer from BODIPY to the ChemPhysChem 2014, 15, 30 – 47

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Figure 16. Structure of the triad BODIPY–azaBODIPY–C60 13 and illustration of the intramolecular photoinduced processes.

formed 1azaBODIPY* in polar media. Sequential energy and electron transfer have also been clearly observed in triad 13. By monitoring the rise of azaBODIPY emission, the rate constant of energy transfer was determined to be about 1011 s1. The dynamics of electron transfer from the singlet excited state of azaBODIPY to covalently attached C60 was also studied by monitoring the formation of the C60 radical anion at l = 1000 nm. A fast charge-separation process from 1azaBODIPY* to C60 was detected; this afforded relatively long-lived BODIPY–azaBODIPYC + –C60C with a lifetime of 1.47 ns. The nanosecond transient absorption spectra showed that the charge-separated state decayed slowly to populate mainly the triplet state of azaBODIPY before returning to the ground state. These findings show that BODIPY–azaBODIPY and BODIPY–azaBODIPY–C60 triad 12 are interesting artificial analogues that can mimic the antenna and RC of natural photosynthetic systems.[119]

6. BODIPYs as Energy Acceptors Energy transfer from or to BODIPY in a dyad can be achieved through careful selection of substituents on the periphery of BODIPY. For example, subphthalocyanine–BODIPY dyads 14 and 15 were synthesized, in which BODIPY was substituted with methyl and distyryl groups, respectively (Figure 17).[120] In 14, energy transfer from the subphthalocyanine to the BODIPY is energetically unfavorable, but the reverse is favorable. In the case of 15, with a distyryl-BODIPY, energy transfer to the BODIPY from the excited subphthalocyanine was feasible. For dyad 14, in which methyl groups were attached to the BODIPY, when excited at l = 470 nm (partial BODIPY excitation, avoiding excitation of the boron subphthalocyanine), energy transfer occurred onto the boron subphthalocyanine and emission was observed at l = 570 nm. However, when distyrylBODIPY 15 was used, excitation of the subphthalocyanine at  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

l = 515 nm caused energy transfer to the BODIPY and subsequent emission at l = 653 nm. The energy-transfer quantum yield for both processes was calculated to be 98 %. Unlike the above-mentioned examples of BODIPY–porphyrin molecular and supramolecular systems, in which energy transfer occurs from the higher energy of the singlet BODIPY to the lower energy of the singlet porphyrin, the reported blue BODIPY–porphyrin tweezers linked by triazole rings showed different features. The BODIPY is functionalized to connect two zinc porphyrin entities through triazole linkers. In BODIPY–(por-

Figure 17. Structures of BODIPY–subphthalocyanine dyads 14 and 15.

phyrin)2 tweezers 16 (Figure 18), two styryl groups were introduced at the 3- and 5-positions of the BODIPY core.[121] The resulting distyryl-BODIPY, which had a more extended p-conjugated system, could absorb and emit at longer wavelengths (l  650 nm) than those of the porphyrin moieties. Therefore, distyryl-BODIPY worked as an energy acceptor rather than as an energy donor in this system, whereas the ZnP unit acted an energy donor. The p-conjugated distyryl-BODIPY exhibited an absorption band at l  660 nm, which was much longer than those typically observed for BODIPY (l  510 nm), resulting in a green or blue color of distyryl-BODIPY. The fluorescence measurements of distyryl-BODIPY–(porphyrin)2 showed that, upon excitation of porphyrin unit at l = 420 nm, the emission band of the distyryl-BODIPY was clearly observed at l = 680 nm, from which the energy of the singlet state of distyryl-BODIPY was evaluated to be 1.84 eV. These observations highlight that energy transfer from the porphyrin to distyryl-BODIPY moiety occurs. Such inverse energy transfer shown in the model resulted from p elongation of BODIPY at the 3,5-positions. The fluorescence emission of the porphyrin moieties overlapped with the absorption of p-conjugated BODIPY. ChemPhysChem 2014, 15, 30 – 47

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Figure 18. BODIPY–(porphyrin)2 tweezers 16.

The photochemical events of 16 in toluene were summarized as follows: the initial step was the excitation of ZnP followed by efficient energy transfer to distyryl-BODIPY to generate the singlet excited state of distyryl-BODIPY with a rate constant of 1.30  1011 s1. The efficiency of energy transfer was altered by the conformation change due to coordination of the nitrogen atoms of the triazole rings to the zinc atom of porphyrins at low temperature. Upon changing the solvent from toluene to polar benzonitrile, energy transfer from 1ZnP* to BODIPY occurs with almost the same rate as that in toluene, generating the singlet BODIPY, which then undergoes charge separation by accepting an electron from the ground-state ZnP to generate distyryl-BODIPYC–(ZnP)2C + . These models provide a new strategy toward the design of photoswitching devices, which can be controlled by a conformation change, depending on the temperature and type of solvents. As mentioned earlier, the structure of azaBODIPY is analogue of BODIPYs in which the 8-position carbon atom has been replaced with a nitrogen atom. This small modification of the ring of azaBODIPY results in 1) a bathochromic shift of l  100 nm in both the absorption and emission maxima, 2) a lowering of the energy of the singlet state of azaBODIPY (1.85 eV) relative to that of BODIPY (2.42 eV), and 3) a shift in the reduction potential to more negative values than that of BODIPY. These properties result in azaBODIPY acting as a promising energy acceptor, as well as an electron acceptor

in the porphyrin–azaBODIPY complexes. By taking these properties of azaBODIPY into account, D’Souza and co-workers reported the synthesis and photodynamics of supramolecular polyads of a covalently linked azaBODIPY–bis-porphyrin molecular clip hosting fullerene, as shown in Figure 19.[122] The azaBODIPY was functionalized with one and two porphyrin entities to form ZnP–azaBODIPY 17 and (ZnP)2–azaBODIPY 18, respectively. The two ZnP entities of the triad were spatially arranged in a molecular clip structure to accommodate the bis-pyridine functionalized fullerene through two-point metal–ligand axial coordination to generate supramolecular tetrad C60(py)2 ! (ZnP)2–azaBODIPY 19. The examined triad and tetrad exhibited broad absorption over a wide range of the Vis–NIR region, where the major absorption bands of azaBODIPY and ZnP were located at l = 685

Figure 19. Molecular structures of dyad 17, triad 18, and tetrad 19.

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CHEMPHYSCHEM MINIREVIEWS and 422 nm, respectively. By exciting the Q band of ZnP of the triad with l = 530 nm excitation light, fluorescence emission studies in toluene showed that the emission of the ZnP emission was heavily quenched with the appearance of new emission bands corresponding to the azaBODIPY entity; this is suggestive of the occurrence of excitation transfer from the singlet excited porphyrin to azaBODIPY. This was expected to occur due to spectral matching and energetics. On the contrary, changing the solvent to polar benzonitrile revealed quenching of the singlet porphyrin emission (  97 %) with no emission corresponding to azaBODIPY; this indicates additional photochemical events in the polar benzonitrile solvent. The femtosecond transient absorption studies of 18 in toluene provided direct evidence of energy transfer from singlet ZnP to azaBODIPY with a rate constant of 6.6  1011 s1. The formed singlet azaBODIPY decayed to the ground state with a rate constant of 4.6  108 s1. By changing the solvent from toluene to benzonitrile, energy transfer occurred from singlet porphyrin to azaBODIPY with a rate constant of 6.4  1011 s1. Interestingly, the formed 1azaBODIPY* decayed much faster in benzonitrile than in toluene, indicating subsequent photoinduced electron transfer from the porphyrin to 1azaBODIPY* to produce the charge-separated state [(ZnP)2C + –azaBODIPYC]. The rate constant of charge separation (kCS) was found to be 4.5  1010 s1. The molecular clip like structure of 18 was utilized to build a novel supramolecular architecture with fullerene functionalized with two pyridine entities suitable for accommodating both ZnP entities of 18 through a two-point axial coordination approach. The binding constant, K, of the supramolecular tetrad, C60(py)2 :(ZnP)2–azaBODIPY 19 was found to be 1.85  105 m1. Upon exciting ZnP at l = 430 nm laser light, the femtosecond transient absorption studies showed the occurrence of electron transfer from excited ZnP to noncovalently bonded C60(py)2 to form the charge-separated state, but no energy transfer as in the triad. Charge stabilization, to some extent, has been observed (tCS = 5.5 ns), leading to the application of such supramolecular assemblies for direct light energy conversion, as we present in Section 8.

7. Electron Transfer of BODIPY-Based Artificial Photosynthetic Systems Due to the high stability and versatility of ferrocene (Fc) derivatives, they have also been incorporated into electron-transfer systems involving BODIPYs. In these molecules, Fc acts as an excellent electron donor for its low oxidation potential, whereas BODIPY acts as an antennae molecule and as an electron acceptor (the reduction potential = 1.61 V vs Fc/Fc + ). In the case of a closely spaced Fc–BODIPY dyad 20 (Figure 20), electron transfer occurred rapidly from the Fc unit to the singlet excited state of BODIPY to generate Fc + –BODIPYC .[123] Such electron transfer was confirmed by 1) fluorescence quenching of the singlet excited state of BODIPY by the attached Fc and 2) the fast decay of singlet BODIPY, as confirmed from the femtosecond transient absorption spectra. The close distance between Fc and BODIPY helped the charge-sep 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 20. BODIPY–ferrocene dyad 20 and BODIPY–ferrocene–C60 triad 21.

arated state to recombine extremely quickly to populate the ground state. The replacement of the fluorine substituents on the boron atom of dyad 20 provides another route towards the preparation of electron-transfer cassettes. As seen from Figure 20, electron acceptor C60 has been attached to the boron atom of dyad 20 resulting in triad 21.[123] The excitation of BODIPY promoted electron transfer from Fc to the singlet-excited state of BODIPY to generate Fc + –BODIPYC–C60, and this fast process was followed by an electron shift from BODIPYC (Ered = 1.61 V vs Fc/Fc + ) to the stronger electron acceptor C60 (Ered = 1.03 V vs Fc/Fc + ) to afford the final charge-separated state Fc + –BODIPY–C60C . As a result of the relatively distant positioning of Fc + and C60C , more charge stabilization (kCR = 5.8  1010 s1 and tCS = 17 ps) was achieved in the Fc–BODIPY–C60 triad. As described above, azaBODIPYs have been used as promising electron acceptors in electron-transfer reactions for their high absorption extinction coefficients (7–8  105 m1 cm1) and large fluorescence quantum yields beyond l = 700 nm. The reported electrochemical measurements showed that azaBODIPY was involved in a one-electron reduction process at 0.79 V versus Fc/Fc + , which was cathodically shifted by nearly 200 mV compared with the well-known electron-acceptor C60. In addition, the electron transfer dynamics in azaBODIPY-based molecular and supramolecular systems can be readily monitored because the strong, characteristic absorption bands of the singlet state at l  680 and 840 nm, and the one-electronreduced product at l  800 nm. These absorption bands are sufficiently far from the spectral region of the singlet–singlet and triplet–triplet absorption of most of the sensitizers. For their unique properties, the near-IR-emitting sensitizer, azaBODIPY, has been utilized in the construction of dyad 22 and triad 23 by attaching one and two Fc units, respectively (Figure 21).[124] The absorption spectra showed intense absorption bands in the range of l = 665–700 nm, depending upon substituents on the aromatic ring, in addition to a less intense band in the l = 465–480 region. The fluorescence spectra of the singlet azaBODIPY at l = 682 nm (Ff = 0.13) were heavily quenched by the attached Fc units, suggesting the occurrence of photochemical events of 22 (Ff = 0.005) and 23 (Ff = 0.002) ChemPhysChem 2014, 15, 30 – 47

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www.chemphyschem.org To examine the effect of the geometric positions of the donor and acceptor entities on the efficiencies of the electrontransfer processes, two novel tetrads 24 and 25 were reported recently our group by covalently linking a near-IR absorbing azaBODIPY as an electron acceptor sensitizer, Fc as an electron donor, and C60 as the second electron acceptor.[125] As seen from Figure 22, the geometric positions of these entities were controlled so that the Fc and C60 entities were in close proximity in the case of 24, whereas they were far apart in the case of 25. The difference in geometric structures resulted in slowing down the charge recombination of 25 (1.1  1010 s1) relative to that of 24 (8.4  1010 s1); this suggested the occurrence of fast charge recombination between Fc + and C60C owing to the shorter distance between them in 24. From the rates of charge

Figure 21. Structures of ferrocene–azaBODIPY arrays 22 and 23. The optimized structure and photochemical events of 23 are shown in the lower panel.

via the singlet azaBODIPY. DFT calculations revealed a molecular clip type structure for triad 23, wherein the donor and acceptor entities were separated by about 14 . Femtosecond transient absorption studies confirmed the electron-transfer character that yielded the charge-separated states of 22 and 23 with rate constants of 6.8  1010 and 1.1  1011 s1, respectively. Nanosecond transient absorption studies showed that the chargeseparate state decayed to populate low-lying triplet azaBODIPY (  1 eV), prior to returning to the ground state.

Figure 23. Molecular structures of supramolecular conjugates 26 and 27, which operate in the near-IR region.

Figure 22. Structures and cartoons of short (close) and tall (distant) versions of tetrads 24 and 25.

recombination, the lifetime of the charge-separated state of 25 (88 ps) was much longer than that of 24 (12 ps). Supramolecular conjugates azaBODIPY–zinc phthalocyanine (ZnPc)2 26 and azaBODIPY– (ZnNc)2 (ZnNc = zinc naphthalocyanine) 27 have also been reported by using a metal–ligand axial coordination approach, in which both donor and acceptor entities are very good near-IR absorbing and emitting molecules (Figure 23).[126] To accomplish this task, azaBODIPY was functionalized to possess two pyridine or imidazole axial ligat-

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CHEMPHYSCHEM MINIREVIEWS ing entities. The X-ray crystal structure of one such derivative revealed the presence of axial-binding-capable pyridine entities on the azaBODIPY macrocycle. The boron atom assumed a tetrahedral geometry with the two ring nitrogen atoms and two fluorine atoms. The distance between the boron atom to the coordinating nitrogen of the pyridine entities were 11.4 and 11.5  for 26 and 27, respectively. The two coordinating nitrogen atoms of pyridine were spatially separated by 14.35 , which was needed to accommodate two donor metal macrocycles. No intermolecular interactions were observed in the crystal packing. By exciting the azaBODIPY unit, fluorescence studies revealed that the singlet state of azaBODIPY was mostly quenched without any evidence of the formation of singlet ZnNc, which suggested that electron transfer from ground ZnNc to singlet azaBODIPY might have been a dominant process, but not energy transfer. Femtosecond transient absorption spectra showed clear evidence for the formation of the charge-separated state by recording characteristic absorption bands of the ZnPc radical cation and the ZnNc radical anion at l = 840 and 963 nm. The higher kCS/kCR ratio for 26 (180) and 27 (130) suggest the usefulness of these supramolecular conjugates in solar energy harvesting systems. In the molecular and supramolecular models 20–27, in which BODIPYs acted as an electron acceptor, the BODIPYs could also work as an electron donor when combined with appropriate electron acceptors, as shown in the following examples. As seen from the reported series of 28–31, the substituted units (pyrene, anthracene, fluorine, and naphthalene) acted as antenna units, BODIPY acted as an electron donor, and C60 acted as an electron acceptor (Figure 24).[127] In each compound of this series, efficient energy transfer occurred from the aromatic antenna unit to the BODIPY, for which emission from the BODIPY core was only observed. This process was followed by electron transfer from the singlet BODIPY to the C60 subunit to yield the charge-separated states with lifetimes of 770, 900, 390, and 770 ps for 28, 29, 30, and 31, respectively.

Figure 24. Antenna–BODIPY–C60 arrays 28–31.

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www.chemphyschem.org The recently reported distyryl-BODIPY–fullerene 32[128, 129] and viologen-linked BODIPY 33[130] dyads reflected the ability of the BODIPYs to work as an electron donor in electron-transfer reactions (Figure 25). The reason for choosing distyryl-

Figure 25. Structures of distyryl-BODIPY–C60 32 and BODIPY–viologen 33.

BODIPY is to overcome the relatively short-wavelength absorption and emission of unsubstituted BODIPY (l < 600 nm). This can be addressed by introducing two styryl groups at the 3- and 5-positions of the BODIPY core. The resulting distyrylBODIPYs, with a more extended p-conjugated system, exhibited a strong S0–S1 transition at l = 648 nm and a higher-energy S0–S2 transition at l = 375 nm.[128] The photodynamic studies of 32 showed that the fluorescence of singlet BODIPY at l = 662 nm was significantly quenched (Ff = 0.04) relative to that of the control BODIPY (Ff = 0.56); this suggested the occurrence of electron transfer. The considerably low oxidation potential of distyryl-BODIPY at 0.80 and 1.05 V versus a saturated calomel electrode (SCE) rendered it as a reasonable electron donor. Upon excitation at the distyryl-BODIPY moiety, photoinduced electron transfer was witnessed from distyryl-BODIPY to C60, giving a relatively long-lived charge-separated state (BODIPYC+–C60C) with lifetimes of 476 and 730 ps in polar benzonitrile and nonpolar toluene, respectively. Charge recombination occurred to populate the triplet state of C60, as supported by the nanosecond transient absorption studies. The electron-donating ability of BODIPY was also observed when it was covalently linked with viologen to form BODIPY– viologen dyad 33 (Figure 25),[130] which exhibited an absorption in the visible region with a maximum at l = 517 nm and an extinction coefficient reaching 29 000 m1 cm1. No fluorescence was observed for the singlet BODIPY of 33 due to electron transfer from the BODIPY to the viologen subunit.[130] The ChemPhysChem 2014, 15, 30 – 47

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CHEMPHYSCHEM MINIREVIEWS first oxidation potential of BODIPY and reduction potential of viologen were recorded at + 1.41 and 0.33 V versus SCE, respectively. From this data, a free energy value of about 590 mV for photoinduced electron transfer from the BODIPY to the viologen fragment was estimated. The thermodynamic driving force was clearly in favor of an electron-transfer mechanism and explained quenching of the luminescence in dyad 33. The transient absorption technique demonstrated the occurrence of the electron-transfer reaction by recording the formation of the methyl viologen radical at l = 396 nm and the weak, broad absorbance at l = 603 nm. The growth of the methyl viologen radical absorbance reached its maximum approximately 2 ps after the laser pulse and decayed immediately. The time constant for electron transfer forming the chargeseparated species was approximately 0.8 ps with charge recombination occurring at 6.3 ps, as calculated from the transient absorption spectra at l = 615 nm.

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Figure 26. Structure of compound 34.

8. Applications of BODIPY as Light-Harvesting Materials in Electronic and Energy-Harvesting Devices Now that the versatility of BODIPYs as components of energytransfer cassettes is quite clear, investigations into their practical use in electronic devices, particularly in solar cells, is now receiving attention.[131–142] Photocurrent generation for practical use in solar cells can be achieved by employing the array as a sensitizer for titanium dioxide [dye-sensitized solar cells (DSSCs)] or by the formation of bulk heterojunction (BHJ) solar cells consisting of a donor–acceptor system bound between conducting surfaces [usually indium tin oxide (ITO), and other metallic electrodes]. As highlighted herein, BODIPY dyes exhibit high absorption strength and good chemical stability with a tunable absorption range by means of the substituents; this qualifies them as electron donors in organic solar cells. For these devices to be efficient, the dye must absorb over a wide range throughout the electromagnetic spectrum, which would appear to be a problem for BODIPY dyes given their relatively sharp absorption bands. However, as seen for some of the energy-transfer cassettes, various groups can be attached to BODIPY to maximize absorption of the dye, and thus, increase photocurrent generation. One of the earlier examples of BODIPY as a sensitizer involved a BODIPY with two triphenylamine groups attached through a styryl linker (34; Figure 26).[131] The styryl groups caused the BODIPY unit to absorb in the red region, while the triphenylamine units absorbed in the blue and green regions. Given that the electron density of a BODIPY moves from the BODIPY core (HOMO) to the 5-phenyl group (LUMO) upon excitation, this allows a relatively simple site for charge injection. Cyclic voltammetry of 34 revealed that it possessed a LUMO of 3.517 eV, which allowed thermodynamically favorable electron transfer into the titanium dioxide conduction band at 3.9 eV. Compound 34 was shown to have a photon-to-current efficiency of 22 %. Roncali and co-workers investigated compounds 35 and 36 (Figure 27) in combination with [6,6]-phenyl-C61-butyric acid  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 27. Structures of compounds 35–37.

methyl ester (PCBM).[132] Compound 35 had an excitation of l = 572 nm, whereas 36 had an excitation of l = 646 nm. When cast as thin-films on indium tin oxide (ITO) glass and bound in poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT–PSS; polymer support), a slight redshift in the absorption maxima was observed. BHJs of these compounds were produced by spin-coating a mixture of BODIPY with PCBM in a 1:2 weight ratio onto ITO glass treated with a 40 nm thick layer of PEDOT–PSS. These cells showed distinct photocurrent generation around the major absorption peaks. A power conversion efficiency (PCE)[133–135] of 1.2–1.3 % was calculated for these cells. One limitation of BODIPY dyes are the narrow absorption bands, which prevent high short-circuit densities. Roncali and co-workers found that the PCE increased when using mixtures of both 35 and 36 with PCBM in PEDOT–PSS, and coating ITO glass with the mixture.[136] The combination of the two different BODIPY dyes allowed photons to absorb over a broader spectral range. The PCE for these mixtures was calculated to be 1.7 %. Due to interest in oligothiophenes as donors in BHJs, a bisthiophene analogue of compound 36 was recently prepared by attaching a bithiophene unit to the peripheries of BODIPY to enhance the light-harvesting properties of the distyrylBODIPY with the electron–hole transport properties of oligoChemPhysChem 2014, 15, 30 – 47

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CHEMPHYSCHEM MINIREVIEWS thiophene 37.[137] Due to the nonplanarity of the system, this modulation in compound 37 only marginally influenced the optical and electrochemical properties relative to 35. Compound 37 displayed a strong absorption between l = 300 and 700 nm with major peaks for the BODIPY chromophore and strong absorption for the aromatic groups in the UV region. As predicted, the photocurrent generation profile matched the absorption profile and the PCE was calculated to be 2.2 % when mixed with PCBM in a PEDOT–PSS matrix. This improvement could originate from the higher hole mobility of 37 relative to that of 35. Donor–p–acceptor-type BODIPY, with two pyridyl groups as electron-withdrawing/anchoring groups at the end of the 3- and 5-positions, and a carbazolediphenylamine moiety as an electron donor at the 8position on the BODIPY core 38 (Figure 28), was designed and developed for DSSCs.[138] This complex possesses a light-harvesting efficiency in the red/ near-IR region and a good adsorption ability on TiO2 film. DSSCs based on 38 exhibit an incident photonto-current conversion efficiency (IPCE) of about 10 % over a range of l = 500 to 700 nm, with an onset at l = 800 nm.

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Figure 29. a) Pictures of fluorine tin oxide (FTO)/SnO2-modified electrodes, b) IPCE [%] curves, and c) photocurrent on–off switching of i) 19, ii) 18, and iii) C60py2 :(ZnP)2 donor– acceptor systems electrophoretically deposited on the electrode surface in a solution of o-dichlorobenzene containing 0.50 m LiI and 0.10 m I2 as the I/I3 redox mediator.

Figure 28. Molecular structure of 38.

Due to their unique structures and their relatively long lifetime of charge-separated states, the photoelectrochemical cells were constructed by electrophoretic deposition of triad 18 and supramolecular tetrad 19, discussed previously, on an FTO/ SnO2-modified surface (Figure 29).[122] As predicted, higher IPCE values (nearly 17 % at the peak maxima) for the C60py2 :(ZnP)2– azaBODIPY-modified electrode than those of the (ZnP)2–azaBODIPY-modified electrode (  11 %) were observed. These models successfully demonstrate control over energy and electron transfer in molecular polyads and their direct usage in lightenergy conversion applications.

9. Summary and Outlook As demonstrated herein, the design of new molecular structures based on the BODIPY chromophore with specific photophysical and photochemical properties is essential for the application of these dyes in different, interesting fields, such as  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

solar-energy conversion, fluorescent probes, and photodynamic therapy. The high fluorescence yield of BODIPY ensures that these dyes have important applications in analytical chemistry, organic light-emitting diodes, solar concentrators, and as registers in molecular electronics.[139–142] The presence of functional groups in the molecular structure of BODIPY allows the modulation of the absorption and emission of BODIPYs. An extension of the p system through styryl groups at the 3- and/or 5-position shifts the absorption and emission signals to the red; a very interesting spectral region from a technological point of view. The recent finding that the fluorine atoms could be replaced by organic moieties (as seen in 24 and 25), thereby extending the range of derivatives and facilitating the attachment of multiple chromophores, was recognized as a good accomplishment in this field.[143, 144] The incorporation of adequate substitution patterns allows covalent and noncovalent linkages of the BODIPY chromophoric system to electron-donor units (such as porphyrin) and/or electron-acceptor units (such as fullerene C60); an adequate strategy to develop efficient photoactive systems. In most of the reported artificial photosynthetic models, BODIPYs act as excellent antenna units that absorb visible light and transfer it very efficiently to the RC (models 1–14). Additionally, BODIPYs have the advantage of acting as electron acceptors (as in models 20–27) and electron donors (as in models 28–33) to provide a variety of combinations in both covalently linked molecular and supramolecular systems to mimic photoinduced energy- and electron-transfer processes in the photosynthetic RC. Also, the donor–acceptor units were attached to BODIPYs at the meso, pyrrole, or boron site; each of which has advantages. The meso sites favor orthogonal geometries for attached aromatic residues, which can minimize electronic coupling. The pyrrole site allows coplanar geometries that maximize elecChemPhysChem 2014, 15, 30 – 47

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tronic communication between the subunits, whereas the boron center allows free rotation.[144] The antenna–RC complexes 3–10 were characterized by their ability to capture and emit light over a wide range of the UV/Vis/NIR region and their higher charge separation/charge recombination ratio. This behavior suggests the usefulness of BODIPY-based photosynthetic models in solar-energy conversion. Photoelectrochemical cells constructed with some of these compounds revealed direct light-to-current conversion with an efficiency of 22 %. Further modifications of BODIPY and azaBODIPY would lead to finer regulation of photoinduced energy- and electron-transfer processes for more efficient solar-energy conversion and molecular-scale devices. For example, logic gates[145] and photochromic switches[146] are known, and BODIPY-based units are often used as input/output signalers in optoelectronic devices.[147] Furthermore, substitution of BODIPY with a polymeric chain is an adequate strategy to develop a tunable dye laser in the solid state.

Acknowledgements We gratefully acknowledge the contributions of the collaborators and co-workers mentioned in the cited references. This work was supported by the National Science Foundation (grant 1110942 to F.D.), a Grant-in-Aid (no. 20108010) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and NRF/ MEST of Korea through the WCU (R31-2008-000-10010-0) and GRL (2010-00353) Programs. Keywords: charge transfer · dyes/pigments conversion · photosynthesis · porphyrinoids

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