Artificial photosynthetic antenna-reaction center complexes based on a hexaphenylbenzene core

Journal of Porphyrins and Phthalocyanines

ECS 2005 article

J. Porphyrins Phthalocyanines 2005; 9: 706-723

Published at http://www.u-bourgogne.fr/jpp/ N

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Artificial photosynthetic antenna-reaction center complexes based on a hexaphenylbenzene core

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Yuichi Terazono, Paul A. Liddell, Vikas Garg, Gerdenis Kodis, Alicia Brune, Michael Hambourger, Ana L. Moore*, Thomas A. Moore* and Devens Gust*∏ Department of Chemistry and Biochemistry, Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, Arizona 85287-1604, USA Received 18 October 2005 Accepted 23 November 2005 ABSTRACT: A hexaphenylbenzene scaffold has been used to organize the components of artificial photosynthetic antennas and antenna-reaction center mimics that feature bis(phenylethynyl)anthracene antenna moieties and porphyrin-fullerene charge-separation units. The five bis(phenylethynyl)anthracene chromophores absorb in the spectral region around 430-480 nm, where porphyrins have low extinction coefficients but solar irradiance is maximal. The hexaphenylbenzene core was built up by the well-known Diels-Alder reaction of diarylacetylenes with substituted tetraphenylcyclopentadienones. The latter were in turn prepared by condensation of substituted benzils and dibenzyl ketones, allowing flexibility in the design of the substitution pattern on the core. The spacing between the various chromophores is suitable for rapid singlet-singlet energy transfer among antenna moieties and the porphyrin, and the relatively rigid structure of the hexaphenylbenzene limits conformational heterogeneity that could reduce the efficiency of energy and electron transfer. NMR studies reveal a high barrier to rotation of the porphyirn plane relative to the hexaphenylbenzene. Copyright © 2005 Society of Porphyrins & Phthalocyanines. KEYWORDS: porphyrin, singlet energy transfer, photoinduced electron transfer, synthesis, hexaphenylbenzene.

INTRODUCTION Light-harvesting complexes of photosynthetic bacteria contain multiple chromophores that are arranged by the accompanying protein into symmetric circular arrays [1, 2]. The special structures of these antennas make possible efficient light-harvesting and transport of the captured photon energy to reaction centers, where the excitation energy is converted into useful chemical potential [3, 4]. The occurrence of these circular arrays has prompted the investigation of artificial antenna systems that incorporate porphyrins, as mimics for chlorophyll, in a ring-like motif [5-27]. ∏SPP full

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*Correspondence to: Devens Gust, email: [email protected], fax: +1 480-965-2747, Ana L. Moore, email: amoore@asu. edu and Thomas A. Moore, email: [email protected] Copyright © 2005 Society of Porphyrins & Phthalocyanines

In addition to bacteriochlorophyll, the bacterial lightharvesting antennas include carotenoid polyenes, which contribute substantially to the absorption cross section of the antenna. This contribution is especially important in the 430-520 nm region, close to the maximum of the solar spectrum, where the extinction coefficients of bacteriochlorophylls are generally low. Although carotenoids can be useful antennas in synthetic as well as natural artificial systems [28-34], the photophysical requirements for efficient singlet-singlet energy transfer from carotenoids to cyclic tetrapyrroles are demanding, due to the spectroscopically forbidden nature of the carotenoid S1 excited state and the short lifetimes of higher excited singlet states. Carotenoids are also subject to thermal and chemical decomposition. For these reasons, we have now investigated the use of an alternative chromophore, a bis(phenylethynyl)anthraPublished on web 02/22/2006

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ARTIFICIAL PHOTOSYNTHETIC ANTENNA-REACTION CENTER COMPLEXES

cene (BPEA) derivative. This moiety absorbs in a spectral region similar to that of carotenoids (although the absorption does not extend as far into the longwavelength region), is stable, and is chemically and photophysically more easily incorporated into antenna systems than are carotenes. It is also highly fluorescent, with a quantum yield of 1, and its emission spectrum overlaps the porphyrin Q-bands, providing the potential for rapid and efficient energy transfer. Here, we report the synthesis of artificial “antenna-reaction center complexes”, 13 and 13(Zn), which consist of a hexaphenylbenzene structural core that organizes five BPEA antenna moieties, a porphyrin (P) that acts as the energy sink and photoinduced electron transfer donor, and a fullerene electron acceptor. These molecules were designed so that light absorbed by the BPEA units would be efficiently transferred to the porphyrin, yielding the porphyrin first excited singlet state. Photoinduced electron transfer to the fullerene would then generate a charge-separated state, completing the conversion of the excitation into chemical potential energy. The spectroscopic investigation of these molecules and the photochemistry deduced therefrom will be discussed in a separate publication, which reports that the molecules do in fact function as designed [35]. Key to designing a functional antenna-reaction center complex is choosing a molecular architecture that achieves rapid and efficient singlet energy transfer from all of the BPEA antenna moieties to the porphyrin while precluding electronic coupling, energy transfer or electron transfer phenomena that interfere with charge separation and recombination within the porphyrinfullerene unit. The hexaphenylbenzene core was chosen as a framework because it imparts structural rigidity without too much interchromophore electronic coupling, and provides interchromophore separations that can promote rapid singlet-singlet energy transfer. The hexaphenylbenzene was built up using the wellknown Diels-Alder reaction of tetraphenylcyclopentadienones with diphenylacetylenes. The tetraphenylcyclopentadienones were in turn synthesized by the base-catalyzed reaction of benzils with dibenzyl ketones. This approach allows very flexible design of antenna-reaction center complexes bearing a variety of antenna and/or charge-separation moieties. We have used this strategy in the past to prepare unsymmetrically-substituted hexaphenylbenzenes of welldefined structure [36-39].

RESULTS AND DISCUSSION Synthesis Hexad and heptad heptad. Hexad 8, which lacks the fullerene electron acceptor moiety, and its precursors Copyright © 2005 Society of Porphyrins & Phthalocyanines

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were prepared as shown in Scheme 1. First, porphyrin 2 was prepared in 22% yield by acid catalyzed condensation of aldehyde 1 (prepared as indicated in step i), methyl-4-formylbenzoate, and 5-(2,4,6trimethylphenyl)dipyrromethane. Formation of the hexaphenylbenzene moiety by the Diels-Alder route required cyclopentadienone 3. This in turn was prepared by the base-catalyzed condensation of the dibrominated dibenzyl ketone and benzil derivatives shown in the scheme. Benzils and dibenzyl ketones may be readily synthesized, making this route to hexaphenylbenzenes with different substitution patterns particularly flexible. The Diels-Alder reaction of 2 and 3 at 259 °C was followed by spontaneous loss of carbon monoxide to give porphyrin 4 in 94% yield. Turning now to the bis(phenylethynyl)anthracene antenna moiety, palladium catalyzed coupling of 9bromoanthraldehyde with commercially available 3,5-dimethoxyethynylbenzene was performed to yield anthracene 5. The formyl group of this compound was converted to the dibromoethenyl group of compound 6 by a Wittig reaction [40], and the alkene was converted to the ethynyl group by treating 6 with butyllithium followed by water, yielding BPEA 7. Coupling porphyrin 4 and excess antenna 7 under mild coupling conditions (using allyl palladium chloride dimer, P(tBu) t 3, and quinuclidine) [41] gave tBu) an 82% yield of hexad 8. As shown in Scheme 2, the ester group of hexad 8 was hydrolyzed to the acid (9). The Prato reaction [42] of aldehyde 10 with C60 and sarcosine gave 11, which was readily deprotected to give 12. Coupling of 9 and 12 via an amide linkage produced heptad 13 (60% yield). Treatment of 13 with zinc acetate gave the corresponding zinc compound 13(Zn). Porphyrin-antenna models. In order to elucidate the various photochemical events following excitation of one of the chromophores of the hexads or heptads, model compounds were necessary. Scheme 3 shows the synthesis of three bromine-containing hexaphenylbenzene derivatives that are key intermediates for preparing model compounds that contain only the BPEA antenna. All of these were prepared by Diels-Alder cycloaddition using the corresponding cyclopentadienones and diarylacetylenes as described in the Experimental section. Cycloaddtion of 3-(4bromophenyl)-2,4,5-triphenylcyclopentadienone and 1-phenyl-2-(4-methoxycarbonylphenyl)ethyne (18) gave a mixture of the para- and the meta-substituted products (19) that could not be separated nor distinguished spectroscopically at this point in the synthesis. Scheme 4 shows the synthesis of the porphyrinantenna model compounds. The ortho model compound (21) was prepared by the same strategy employed for J. Porphyrins Phthalocyanines 2005; 9: 706-723

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Scheme 1. Reaction conditions: i) (PPh3)2PdCl2, CuI, Et3N, benzene, rt, 17 h; ii) mesityldipyrromethane, methyl-4-formylbenzoate, BF3OEt2, CHCl3, rt, 2 h; then DDQ, rt, 16 h; iii) KOH, EtOH, reflux, 20 min; iv) Ph2O, reflux, 4 h; v) (PPh3)4Pd, CuI, DMF/Et3N, 100 °C, 3 h; vi) PPh3, CBr4, CH2Cl2, 0 °C, 10 min; vii) n-BuLi, THF, -78 °C, 1 h; then rt, 1.5 h, H2O; viii) (allyl Pd Cl)2, (t-Bu)3P, quinuclidine, toluene, 55 °C; 120 h

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ARTIFICIAL PHOTOSYNTHETIC ANTENNA-REACTION CENTER COMPLEXES

Scheme 2. Reaction conditions: i) KOH, MeOH/THF, 40 °C, 24 h; ii) Xant phos, Cs2CO3, Pd(OAc)2, THF, 45 °C, 20 h; iii) C60, sarcosine, toluene, reflux, 24 h; iv) TFA, rt, 10 min; v) DMAP, EDCI, CH2Cl2, rt, 24 h

the preparation of hexad 8; cycloaddition of porphyrin 2 and tetraphenylcyclopentadienone was followed by a palladium cross-coupling reaction with antenna 7. In order to prepare the para (24) and the meta (27) model compounds, aldehydes 22 and 25 were first prepared from precursors 17 and 19, respectively. Porphyrins 23 and 26 were prepared in yields of 12 and 26%, respectively, by acid catalyzed condensation with aldehyde 17 or 19, 5-(2,4,6-trimethylphenyl)dipyrroCopyright © 2005 Society of Porphyrins & Phthalocyanines

methane, and methyl-4-formylbenzoate. Palladium cross-coupling reactions yielded the para model compound (24), and model 27 as a mixture with 24. Although the meta and the para porphyrin-antenna compounds could not be completely separated chromatographically, they are readily distinguishable by 1H NMR spectroscopy. Repeated chromatography yielded the meta compound 27 at 75% enrichment, which was sufficient for spectroscopic studies. J. Porphyrins Phthalocyanines 2005; 9: 706-723

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Scheme 3.

Scheme 5 shows the synthesis of BPEA model compounds 28, 29, and 30. These compounds were prepared using similar palladium cross-coupling reaction conditions, as described in the Experimental section. Zn complexes of porphyrin-hexaphenylbenzeneantenna compounds. Zn complexes 8(Zn), 13(Zn), 21(Zn), 24(Zn), and 27(Zn) were prepared by the conventional metal insertion method using zinc acetate dihydrate. NMR investigation and restricted rotation The proton NMR spectra of the hexads, the heptads, and the ortho, meta, and para porphyrinantenna dyads are complicated in the aromatic region because of large number of similar aromatic residues. However, in the mid to high field region (4-1.5 ppm) the spectra are simple enough to clearly distinguish the various molecules via the resonances for the methyl and methoxyl substituents. A particularly interesting feature was observed in the proton NMR resonances of the molecules bearing only a single BPEA antenna moiety ortho to a porphyrin (compounds 21 and 21(Zn)). Figure 1 shows the 500 MHz 1H NMR spectrum of 21(Zn) in toluene-d8 at 20 °C. The expected single peaks are observed for the methyl ester at 3.70 ppm and the methoxyl at 3.35 ppm. However, two sets of singlets were observed for both the para- and the orthomethyl groups of the mesityl groups associated with the porphyrin. Similar splitting patterns were also observed in CDCl3. These large splittings suggest that rotation about the two single bonds in the phenyl linkage between the porphyrin macrocycle and the benzene ring at the core of the hexaphenylbenzene Copyright © 2005 Society of Porphyrins & Phthalocyanines

scaffold (labeled a and b in Scheme 4) is slow on the NMR time scale at ambient temperatures. This is not unexpected, as the dihedral angles between the phenyl rings in this linkage are large (typically 6590º [43-45]), indicating significant steric hindrance that precludes a planar conformation. To verify this possibility, 1H NMR spectra were measured at various temperatures in toluene-d8. As shown in Fig. 2, the two peaks from the para methyl groups of the two mesityl rings appear at 20 °C as sharp singlets at 2.53 and 2.30 ppm. As the temperature is raised, the resonances broaden due to exchange resulting from rotation about one or both of the single bonds in the linkage. At 105 °C they have essentially coalesced to a single broad peak. Line shape analysis based on a two-site exchange (Fig. 2) yielded ΔG298≠ = 16.8 kcal/mol for the rotational barrier. It is interesting to compare this rotational barrier with those obtained for related compounds. In previous work, we have found that the rotational barrier for the peripheral rings of a substituted hexaphenylbenzene in which the peripheral rings lack substituents at ortho positions is ΔG294≠ = 16.4 kcal/mol [36]. Barriers to rotation of a meso aryl ring bearing only hydrogen atoms in the ortho positions in metalated tetraarylporphyrins have also been measured, and found to be on the order of 14.3-18.6 kcal/mol, depending upon the metal and any porphyrin substituents [46, 47]. We therefore conclude that the rotational barriers for bonds a and b in Scheme 3 are very similar to one another, ~16 kcal/mol, and that the observed coalescence of resonances in 21(Zn) is the result of rapid rotation about either of these bonds on the NMR time scale at high temperature. Similar restricted rotation is doubtless also present in many of the other molecules investigated, including J. Porphyrins Phthalocyanines 2005; 9: 706-723

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Scheme 4. Reaction conditions: i) Ph2O, reflux, 4 h; ii) (allyl Pd Cl)2, (t-Bu)3P, quinuclidine, toluene, 55 °C; iii) n-BuLi, THF, -78 °C to rt; then DMF, H2O; iv) methyl-4-formylbenzoate, mesityldipyrromethane, BF3OEt2, NaCl, CH2Cl2, rt, 3 h, then DDQ; v) DIBAL-H, THF, 0 °C, 15 min; then H2O

heptads 13 and 13(Zn), but would not be observed in the NMR spectra because the symmetry of these Copyright © 2005 Society of Porphyrins & Phthalocyanines

molecules renders the methyl groups in question isochronous. J. Porphyrins Phthalocyanines 2005; 9: 706-723

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Fig. 1. The 500 MHz 1H NMR spectrum of model BPEA-porphyrin hexad 21(Zn) in toluene-d8 at 20 °C. The expansion shows the resonances of the diastereotopic pairs of ortho and para mesityl methyl groups (A, B, C, D in the structural drawing) that result from restricted rotation about the single bonds in the linkage between the porphyrin macrocycle and the central phenyl group of the hexaphenylbenzene moiety

UV-visible absorption spectra Figure 3 shows UV-visible absorption spectra of the porphyrin-free model compounds (28, 29, and 30) in dichloromethane. Compound 30, which lacks the hexaphenylbenzene core, has an almost identical absorption profile to that of 9,10-bis(phenylethynyl)anthracene [48]. The spectrum of compound 28, Copyright © 2005 Society of Porphyrins & Phthalocyanines

which has the hexaphenylbenzene core appended to the BPEA chromophore, has similar features to that of 30, but the peak maxima in the visible region are shifted by about 10 nm to longer wavelengths, appearing at 445 and 469 nm. In addition, two bands appear in the UV region at 309 and 319 nm. Compound 30 has a single, rather sharp peak in this region. Thus, there is some electronic interaction J. Porphyrins Phthalocyanines 2005; 9: 706-723

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Scheme 5. i) Reaction conditions: (allyl Pd Cl)2, (t-Bu)3P, quinuclidine, toluene, 55 °C; ii) (PPh3)4Pd, CuI, DMF/Et3N, 95 °C, 2h

Fig. 2. Experimental (left) and simulated (right) 1H NMR spectra of the para methyl resonances of the mesityl groups of 21(Zn) at various temperatures in toluene-d8. The coalescence behavior signifies increasingly rapid rotation about the single bonds in the linkage between the porphyrin macrocycle and the central phenyl group of the hexaphenylbenzene

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between the hexaphenylbenzene and the antenna moieties. This occurs even though π-π interactions between the BPEA and the central benzene ring are limited due to the large dihedral angles between the two directly bonded rings. This spectral shift moves the BPEA absorption further into the region where the porphyrin moiety does not absorb strongly, and enhances the properties of the BPEA unit as an antenna and carotenoid “mimic”. Addition of a second BPEA unit to the hexaphenylbenzene core does not significantly perturb the absorption spectrum, as illustrated by the spectrum for dyad 29 in Fig. 3. The peak positions and relative intensities are similar to those for 28. There are no excitonic or other interactions that alter the energies of the excited states. Absorption spectra of porphyrin-antenna compounds featuring free base (8, 21, 24, and 27) and zinc porphyrins (8(Zn), 21(Zn), 24(Zn), and 27(Zn)) are shown in Figs 4 and 5, respectively. For the free base compounds, the absorption spectra display the typical porphyrin Soret (~419 nm) and Q-bands (~515, 549, 590 and 647 nm), with the superposition of the BPEA absorption with maxima at ~446 and 472 nm. Although the BPEA features in the visible are fairly similar for 24 and 27, these bands have slightly lower extinction coefficients in 21 (see inset in Fig. 4). In 21, the BPEA

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Fig. 3. UV-visible absorption spectra (~2 × 10-6 M) of BPEA model compounds 28 (– – –), 29 (—) and 30 (– . – . –) in dichloromethane

Fig. 4. UV-visible absorption spectra in dichloromethane (~3.7 × 10-6 M) of BPEA-porphyrin hexad 8 (—), and model BPEA-porphyrin dyads 21 (- - - - -), 24 (– . – . –), and 27 (– – –). The expansion in the inset shows that the extinction coefficient for the BPEA in the ortho-linked isomer 21 is slightly reduced in the 450 nm region, relative to those for the meta and para isomers 27 and 24, respectively

and porphyrin units are ortho to one another on the hexaphenylbenzene skeleton, and the close proximity of the moieties evidently leads to this small spectral perturbation. In hexad 8, the additive effect of the five BPEA antenna moieties greatly increases the Copyright © 2005 Society of Porphyrins & Phthalocyanines

Fig. 5. UV-visible absorption spectra in dichloromethane (~1.9 × 10-6 M) of BPEA-zinc porphyrin hexad 8(Zn) (—), and model BPEA-zinc porphyrin dyads 21(Zn) (- - - - -), 24(Zn) (– . – . –), and 27(Zn) (– – –). The expansion in the inset shows that the extinction coefficient for the BPEA in the ortho-linked isomer 21(Zn) is slightly reduced in the 450 nm region, relative to those for the meta and para isomers 27(Zn) and 24(Zn), respectively, as is also the case for the free base analogs

absorption cross section of the porphyrin in the regions where BPEA absorbs. The magnitude of this extra absorbance may be appreciated by comparison of the BPEA spectral region with the absorbance of the porphyrin visible absorption bands (Q-bands). Thus, the BPEA units have the potential to greatly increase the light harvesting ability of the porphyrin, provided that singlet energy transfer to the porphyrin is rapid. Results for the zinc series of compounds parallel those for the free base series (see Fig. 5). The spectra of the heptads are very similar to those of the hexads, with the addition of the absorbance of the fullerene. This absorbance is significant in the UV region, but very small in the visible, compared to the absorbance of the other chromophores. The fullerene absorbance is relatively featureless throughout the visible, with a small peak at about 705 nm (see Fig. 6). Thus, the fullerene adds little to the lightharvesting capabilities of the heptads although it is vital for their electron transfer function. Cyclic voltammetry Cyclic voltammetric measurements were made in order to obtain redox potentials that could be used to estimate the energies of charge-separated states that might be produced by electron transfer reactions. Potentials were determined in benzonitrile J. Porphyrins Phthalocyanines 2005; 9: 706-723

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EXPERIMENTAL General procedures H NMR spectra were recorded on Varian Unity spectrometers at 300 or 500 MHz. Samples were dissolved in deuteriochloroform with tetramethylsilane as an internal reference, or in toluene-d8. Mass spectra were obtained on an Applied Biosystems Voyager-DE STR matrix-assisted laser desorption/ionization timeof-flight spectrometer (MALDI-TOF). Ultravioletvisible ground state absorption spectra were measured on a Shimadzu UV2100U spectrometer. The solvents for all optical measurements were freshly distilled dichloromethane or 2-methyltetrahydrofuran. The HPLC grade benzonitrile for cyclic voltammetry was purchased from Aldrich and deoxygenated by bubbling with argon for 20 min.

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Fig. 6. UV-visible absorption spectra in dichloromethane (~3.6 × 10-6 M) of BPEA-porphyrin-fullerene heptad 13. The expansion shows the porphyrin Q-band region and the fullerene maximum at 705 nm

solution containing 0.1 M tetra-n-butylammonium hexafluorophosphate using a glassy carbon working electrode, platinum counter electrode, and Ag/AgNO3 pseudo-reference electrode. Ferrocene was used as an internal standard. Antenna model 30 (without the hexaphenylbenzene core) exhibited a reversible reduction wave at -1.37 V vs SCE and an irreversible two electron oxidation process at 1.06 V. Antenna model 28 (with a hexaphenylbenzene core) exhibited very similar behavior, with a reversible reduction at -1.37 V and an irreversible oxidation at 1.17 V. The first oxidation potential for a model for the porphyrin component of 13 is 1.03 V vs SCE [49], and the first reduction potential for a fullerene similar to the fullerene moiety of 13 and 13(Zn) is -0.59 V [39]. These data suggest that from a thermodynamic point of view, photoinduced electron transfer from the porphyrin first excited singlet state of 13 at 1.91 eV to the fullerene to generate a P•+-C60•− charge-separated state at ~1.62 eV should be facile, but that migration of the radical cation into the BPEA antenna array would be thermodynamically unfavorable. A similar process is expected in the zinc analog. The zinc porphyrin first excited singlet state lies 2.07 eV above the ground state, and the first oxidation potential of a model zinc porphyrin is 0.76 V vs SCE [49], giving an energy of 1.35 eV for the charge-separated state. Such photoinduced electron transfer has indeed been observed [35].

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Materials Dichloromethane and toluene used for synthesis were distilled from calcium hydride. Tetrahydrofuran for synthesis was distilled from sodium/benzophenone. Dimethylformamide and triethylamine for palladium coupling reactions were dried over activated 4 Å molecular sieves. The 4-iodobenzaldehyde, 4-bromobenzaldehyde, methyl 4-formylbenzoate, methyl 4-iodobenzoate, 4,4’-dibromobenzil, tetraphenylcylopentadienone, 1-ethynyl-3,5-dimethoxybenzene, phenylacetylene, allylpalladium(II) chloride dimer, bis(triphenylphosphine)palladium(II) chloride, tetrakis(triphenylphosphine)palladium(0), quinuclidine, piperidine, tri-tert-butylphosphine, diisobutylaluminum hydride (1 M in hexane), n-butyllithium (2.86 M in hexane), tert-butylcarbamate, and C60 were commercially available and were used without purification. The 5-(2,4,6-trimethylphenyl)dipyrromethane [50], 3-(4-bromophenyl)-2,4,5-triphenylcyclopentadienone [51], 1,3-bis(4-bromophenyl)-2propanone [52], and 9-bromoanthraldehyde [53, 54] were synthesized by published methods. Synthesis 4-(4-bromophenylethynyl)benzaldehyde (1). To a heavy-walled glass tube were added 1.0 g (4.3 mmol) of 4-iodobenzaldehyde, 0.82 g (4.5 mmol) of 4bromo-1-ethynylbenzene, 22 mL of benzene, 3.6 mL (0.026 mol) of triethylamine and 82 mg (0.43 mmol) of copper(I) iodide. The mixture was flushed with argon for 10 min, 182 mg (0.259 mmol) of dichlorobis(triphenylphosphine)palladium(II) was then added, and the argon flushing was continued for 5 min. The tube was sealed with a teflon screw plug and the reaction mixture was stirred at room temperature for 17 h. After this period, the contents of the tube were

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diluted with dichloromethane (100 mL) and washed first with 5% hydrochloric acid and then with brine solution (× 3). After evaporation of the solvent the crude material was chromatographed on silica gel (flash column, dichloromethane/hexanes 3/2 to 2/1) and then recrystallized from dichloromethane/hexanes to give 1.09 g (89% yield) of the desired aldehyde (1). 1 H NMR (300 MHz, CDCl3): δ, ppm 7.41 (2H, d, J = 9 Hz), 7.52 (2H, d, J = 8 Hz), 7.67 (2H, d, J = 8 Hz), 7.87 (2H, d, J = 8 Hz), 10.02 (1H, s). 15-{4-[2-(4-bromophenyl)ethynyl]-phenyl}5-(4-carbomethoxyphenyl)-10,20-bis(2,4,6-trimethylbenzene)porphyrin (2). To a flask were added 1.85 g (7.01 mmol) of 5-(2,4,6-trimethylbenzene)dipyrromethane, 0.58 g (3.51 mmol) of 4-carbomethoxybenzaldehyde, 1.0 g (3.51 mmol) of (1) and 670 mL of chloroform. The solution was flushed with argon for 20 min and 0.98 mL of 2.5 M boron trifluoride etherate in chloroform was added. After stirring the reaction mixture for 2 h, 1.5 g (6.6 mmol) of 2,3-dichloro-5,6-dicyanoquinone was added and stirring was continued for 16 h. The solution was washed with aqueous sodium bicarbonate (× 3) and then concentrated by distillation of the solvent at reduced pressure. The residue was chromatographed on silica gel (flash column, dichloromethane/hexanes: 3/2 to pure dichloromethane) to give 720 mg (22% yield) of porphyrin (2).1H NMR (300 MHz, CDCl3): δ, ppm -2.63 (2H, s), 1.84 (12H, s), 2.62 (6H, s), 4.10 (3H, s), 7.28 (4H, s), 7.53 (2H, d, J = 9 Hz), 7.57 (2H, d, J = 9 Hz), 7.91 (2H, d, J = 8 Hz), 8.22 (2H, d, J = 8 Hz), 8.31 (2H, d, J = 8 Hz), 8.43 (2H, d, J = 8 Hz), 8.70-8.81 (8H, m). MALDI-TOF-MS: m/z calcd. for C60H47N4O2Br1 936.3, obsd. 936.3. UV-vis (CH2Cl2): λmax, nm 647, 590, 550, 516, 421. Tetrakis(4-bromophenyl)cyclopentadienone (3). A flask containing 3.87 g (10.5 mmol) of 1,3-bis(4bromophenyl)-2-propanone, 4.33 g (11.76 mmol) of 4,4’-dibromobenzil and 25 mL of ethanol was warmed to reflux and a solution comprising 0.375 g of potassium hydroxide and 3 mL of ethanol was added dropwise. Refluxing was continued for an additional 20 min. Upon cooling, the dark slurry was dissolved in dichloromethane and washed with water, the solvent was evaporated and the residue was recrystallized from dichloromethane/ethanol to give 6.20 g (84% yield) of tetracyclone 3. 1H NMR (300 MHz, CDCl3): δ, ppm 6.77 (4H, d, J = 8 Hz), 7.06 (4H, d, J = 9 Hz), 7.35-7.41 (8H, m). MALDI-TOF-MS: m/z calcd. for C29H16Br4O1 699.79, obsd. 699.79. Porphyrin 4. A flask containing 94 mg (0.10 mmol) of porphyrin 2, 0.7 g (1.00 mmol) of tetrakis(4-bromophenyl)cyclopentadienone (3) and 12 mL of diphenyl ether was warmed at reflux for 4 h. The solvent was then evaporated under vacuum and the residue was chromatographed on silica gel (flash column, dichloromethane/hexanes: 3/2). Recrystallization of the Copyright © 2005 Society of Porphyrins & Phthalocyanines

product from dichloromethane/methanol gave 152 mg (94% yield) of porphyrin 4. 1H NMR (300 MHz, CDCl3): δ, ppm -2.12 (2H, s), 1.82 (12H, s), 2.63 (6H, s), 4.10 (3H, s), 6.74 (2H, d, J = 8 Hz), 6.79 (2H, d, J = 8 Hz), 6.94 (2H, d, J = 8 Hz), 7.08-7.17 (8H, m), 7.28 (4H, s), 7.33 (4H, d, J = 8 Hz), 7.77 (2H, d, J = 8 Hz), 8.29 (2H, d, J = 8 Hz), 8.36 (2H, d, J = 4 Hz), 8.41 (2H, d, J = 8 Hz), 8.67-8.72 (6H, m). MALDITOF-MS: m/z calcd. for C88H63N4Br5O2 1608, obsd. 1608. UV-vis (CH2Cl2): λmax, nm 647, 592, 550, 516, 418, 252. 10-(3,5-dimethoxyphenyl)ethynyl-9-formylanthracene (5). Portions of 9-bromoanthraldehyde (191 mg, 0.670 mmol) and 1-ethynyl-3,5-dimethoxybenzene (109 mg, 0.670 mmol) were dissolved in a mixture of deoxygenated, dry dimethylformamide (3 mL) and triethylamine (7 mL) in a flask. Tetrakis(triphenylphosphine)palladium(0) (16 mg, 0.014 mmol) and cuprous iodide (3 mg, 0.02 mmol) were added to the solution and the flask was capped with a septum which was secured with a copper wire. The mixture was stirred at 100 °C for 3 h. The reaction mixture was cooled, diluted with dichloromethane, and washed with water. The organic layer was concentrated by distillation at reduced pressure, and the product was chromatographed on a silica gel column (dichloromethane/hexanes: 1/1) to yield an orange powder of 5 (227 mg, 92%). 1H NMR (CDCl3): δ, ppm 11.52 (s, 1H), 8.96 (d, 2H, J = 8 Hz), 8.76 (d, 2H, J = 8 Hz), 7.69 (m, 4H), 6.93 (d, 2H, J = 2 Hz), 6.58 (t, 1H, J = 2 Hz), 3.89 (s, 6H). MALDITOF-MS: m/z calcd. for C25H18O3 366.126, obsd. 366.119. 9-(2,2’-dibromoethenyl)-10-(3,5-dimethoxyphenyl)ethynylanthracene (6). Triphenylphosphine (464 mg, 1.77 mmol) in dry dichloromethane (4 mL) was added to a solution of 5 (216 mg, 0.414 mmol) and carbon tetrabromide (294 mg, 0.885 mmol) in dry dichloromethane (5 mL) at 0 °C over 10 min. The mixture was stirred for 10 min. The solvent was evaporated at reduced pressure, and the product was chromatographed on a silica gel column (dichloromethane/hexanes: 1/1) to yield 6 (207 mg, 68%).1H NMR (CDCl3): δ, ppm 8.68 (m, 2H), 8.09 (s, 1H), 8.08 (2H, m), 7.60 (m, 4H), 6.90 (d, 2H, J = 2 Hz), 5.44 (t, 1H, J = 2 Hz), 3.87 (s, 6H). MALDITOF-MS: m/z calcd. for C26H18Br2O2 519.967, obsd. 519.967. 9-(3,5-dimethoxyphenyl)ethynyl-10-ethynylanthracene (7). n-butyllithium (2.5 M in hexane, 2 mL) was added to 6 (1.04 g, 1.98 mmol) in dry tetrahydrofuran (40 mL) at -78 °C over a period of 15 min. The mixture was stirred at -78 °C for 1 h and then at room temperature for 1.5 h. The reaction was quenched with water, and tetrahydrofuran was removed by distillation at reduced pressure. The remaining mixture was dissolved in dichloromethane and washed with J. Porphyrins Phthalocyanines 2005; 9: 706-723

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water. The product was chromatographed on a silica gel column (dichloromethane/hexanes: 1/1) to yield 7 (528 mg, 74%). 1H NMR (CDCl3): δ, ppm 8.64 (m, 4H), 7.64 (m, 4H), 6.92 (d, 2H, J = 2 Hz), 6.55 (t, 1H, J = 2 Hz), 4.08 (s, 1H), 3.88 (s, 6H). MALDI-TOFMS: m/z calcd. for C26H18O2 362.131, obsd. 362.133. Hexad 8. 5,15-bis(2,4,6-trimethylphenyl)-10-(4(2,3,4,5,6-pentakis(4-bromophenyl)phenyl)-20-(4methoxycarbonylphenyl)-porphyrin (4) (50 mg, 0.031 mmol), acetylene 7 (89 mg, 0.24 mmol), allylpalladium(II) chloride dimer (1.4 mg, 3.9 × 10-3 mmol), P(tBu) t 3 (3.1 mg, 0.016 mmol), and quinutBu) clidine (35 mg, 0.31 mmol) were stirred in argonsaturated toluene (5 mL) at 55 °C. After 30 h, 67 mg (0.60 mmol) of 7 were added, and after 96 h 60 mg (0.54 mmol) of 7, additional palladium catalyst (2.0 mg, 5.5 × 10-3 mmol ), P(tBu) t 3 (4.4 mg, tBu) 0.022 mmol), and quinuclidine (48 mg, 0.43 mmol) were added to the reaction mixture. The total reaction time was 120 h. After the reaction, the mixture was cooled to room temperature, diluted with chloroform (5 mL), stirred for 1 h, and filtered through a short silica gel column to remove compounds having low solubility. The solvent was evaporated and the residue chromatographed on a silica gel column (dichloromethane/methanol: 100/0 to 99/1) to yield 77 mg (82%) of the hexad. 1H NMR (CDCl3): δ, ppm 8.74-8.59 (m, 26H), 8.51 (d, 2H, J = 4 Hz), 8.36 (d, 2H, J = 8 Hz), 8.22 (d, 2H, J = 8 Hz), 7.97 (d, 2H, J = 8 Hz), 7.73 (d, 4H, J = 8 Hz), 7.65-7.48 (m, 26H), 7.41 (d, 4H, J = 8 Hz), 7.38 (d, 4H, J = 8 Hz), 7.22 (m, 4H), 6.97 (d, 4H, J = 2 Hz), 6.88 (m, 6H), 6.74 (s, 4H), 6.58 (t, 2H, J = 2 Hz), 6.53 (m, 3H), 4.07 (s, 3H), 3.91 (s, 12H), 3.86 (bs, 18H), 2.23 (s, 6H), 1.54 (s, 12H), -2.75 (bs, 2H). MALDI-TOF-MS: m/z calcd. for C218H148N4O12 3013.11, obsd. 3013.41. UVvis (CH2Cl2): λmax, nm 275, 310, 320, 420, 446, 472, 515, 552, 591, 647. Zn porphyrin-antenna 8(Zn). Zn porphyrin 8(Zn) was synthesized as described later for 21(Zn). The product was purified by two silica gel columns (dichloromethane/hexanes: 7/3 followed by dichloromethane) to yield the desired material in essentially quantitative yield. 1H NMR (CDCl3): δ, ppm 8.748.59 (m, 28H), 8.36 (d, 2H, J = 8 Hz), 8.24 (d, 2H, J = 8 Hz), 7.97 (d, 2H, J = 8 Hz), 7.73 (d, 4H, J = 8 Hz), 7.65-7.48 (m, 26H), 7.39 (m, 8H), 7.22 (m, 4H), 6.96 (d, 4H, J = 2 Hz), 6.88 (m, 6H), 6.77 (s, 4H), 6.58 (t, 2H, J = 2 Hz), 6.53 (m, 3H), 4.07 (s, 3H), 3.90 (s, 12H), 3.86 (bs, 18H), 2.27 (s, 6H), 1.54 (s, 12H). MALDI-TOF-MS: m/z calcd. for C218H146N4O12Zn: 3075.022, obsd. 3074.912. UV-vis (CH2Cl2): λmax, nm 274, 310, 320(sh), 422, 446, 472, 550, 588. Porphyrin 9. To a flask containing 40 mg (0.013 mmol) of porphyrin 8 was added 20 mL of tetrahydrofuran, 10 mL of methanol and 5 mL of 10% aqueous potassium hydroxide. The solution Copyright © 2005 Society of Porphyrins & Phthalocyanines

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was warmed to 40 °C under an argon atmosphere. After 24 h, the reaction mixture was diluted with dichloromethane (100 mL) and washed with dilute citric acid, and then with water. The solvent was evaporated at reduced pressure and the residue was chromatographed on silica gel (deactivated with 2% w/w water) using dichloromethane/3-5% methanol as the eluent. This gave 36 mg (90% yield) of the hexad acid (9). MALDI-TOF-MS: m/z calcd. for C217H146O12N4 3001.1, obsd. 3001.3. UV-vis (CH2Cl2): λmax, nm 647, 591, 551, 516, 472, 446, 420, 321, 310, 275. tert-butyl-4-formylphenyl carbamate (10). A tube containing 3.70 g (20.0 mmol) of 4-bromobenzaldehyde, 174 mg (0.30 mmol) of Xant phos™, 2.81 g (24.0 mmol) of tert-butyl carbamate, 9.26 g (28.0 mmol) of cesium carbonate and 20 mL of tetrahydrofuran was flushed with argon. After 15 min, 45 mg (0.20 mmol) of palladium(II) acetate was added and the argon gas flow continued for an additional 10 min. The tube was then sealed with a teflon screw plug and the reaction mixture warmed to 45 °C for 20 h. The suspension was then diluted with dichloromethane (120 mL), washed with water and dried over sodium sulfate. The solvent was evaporated at reduced pressure and the residue was chromatographed on silica gel (flash column, dichloromethane). Recrystallization of the product from dichloromethane/hexanes gave 3.86 g (87% yield) of the desired compound. 1H NMR (300 MHz, CDCl3): δ, ppm 1.54 (9H, s), 6.77 (1H, s), 7.53 (2H, d, J = 9 Hz), 8.72 (2H, d, J = 9 Hz), 9.89 (1H, s). MALDI-TOF-MS: m/z calcd. for C12H15N1O3 221.10, obsd. 222.11 (M+H)•+ and 244.09 (M+Na)•+. Fullerene 11. A flask containing 44 mg (0.20 mmol) of tert-butyl-4-formylphenyl carbamate (10), 288 mg (0.40 mmol) of C60, 178 mg (2.00 mmol) of sarcosine and 80 mL of toluene was warmed to reflux under an argon atmosphere. After 24 h, the solvent was evaporated and the residue was chromatographed on silica gel (flash column, toluene/carbon disulfide/ ethyl acetate: 75/20/5) to give 112 mg (58% yield) of the desired fullerene (11). 1H NMR (300 MHz, CDCl3): δ, ppm 1.50 (9H, s), 2.78 (3H, s), 4.24 (1H, d, J = 9 Hz), 4.87 (1H, s), 4.95 (1H, d, J = 9 Hz), 6.41 (1H, s), 7.90 (2H, d, J = 9 Hz), 7.68 (2H, d, J = 9 Hz). MALDI-TOF-MS: m/z calcd. for C74H20N2O2 968.1, obsd. 968.1. UV-vis (CH2Cl2): λmax, nm 431, 704. Fullerene 12. To a flask containing 110 mg (0.114 mmol) of fullerene 11 was added 20 mL of trifluoroacetic acid. The suspension was sonicated and then stirred at room temperature for 10 min. The solvent was evaporated and the residue was suspended between carbon disulfide (60 mL) and aqueous sodium bicarbonate (60 mL). After sonicating the mixture until all the solid had dissolved the organic phase was recovered and the solvent was evaporated. The J. Porphyrins Phthalocyanines 2005; 9: 706-723

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residue was then chromatographed on silica gel (flash column, toluene/4% ethyl acetate) to give 88 mg (89% yield) of fullerene 12. 1H NMR (300 MHz, CDCl3): δ, ppm 2.77 (3H, s), 3.64 (2H, s), 4.20 (1H, d, J = 9 Hz), 4.79 (1H, s), 4.92 (1H, d, J = 9 Hz), 6.65 (2H, d, J = 9 Hz), 7.50 (2H, brs). MALDI-TOF-MS: m/z calcd. for C69H12N2 868.1, obsd. 868.1. UV-vis (CH2Cl2): λmax, nm 431, 702. Heptad 13. To a flask containing 30 mg (0.010 mmol) of hexad acid (9) was added 2 mL of dichloromethane, 10.4 mg (0.012 mmol) of fullerene 12, 2.0 mg (0.02 mmol) of 4-dimethylaminopyridine and 3 mg (0.015 mmol) of EDCI. The solution was stirred at room temperature under an argon atmosphere. After 24 h the reaction mixture was diluted with dichloromethane (60 mL) and first washed with dilute citric acid and then with aqueous sodium bicarbonate. The solution was then dried over sodium sulfate and the solvent was evaporated. The residue was chromatographed on silica gel (flash column, dichloromethane/0.5-0.8% acetone) to give 23 mg (60% yield) of compound 13. 1H NMR (500 MHz, CDCl3): δ, ppm -2.74 (2H, s), 2.23 (12H, s), 2.35 (3H, s), 2.78 (6H, s), 3.85 (18H, s), 3.89 (12H, s), 4.17 (1H, d, J = 9 Hz), 4.86 (1H, s), 4.91 (1H, d, J = 9 Hz), 6.52 (3H, m), 6.57 (2H, m), 6.74 (4H, s), 6.87 (6H, m), 6.96 (4H, m), 7.14-7.22 (4H, m), 7.24 (4H, s), 7.37 (4H, d, J = 8 Hz), 7.41 (2H, d, J = 8 Hz), 7.48-7.63 (26H, m), 7.73 (4H, d, J = 8 Hz), 7.84 (4H, brs), 7.97 (2H, d, J = 8 Hz), 8.15 (2H, d, J = 9 Hz), 8.17 (1H, s), 8.22 (2H, d, J = 9 Hz), 8.50 (2H, d, J = 5 Hz), 8.568.64 (16H, m), 8.67-8.72 (8H, m). MALDI-TOF-MS: m/z calcd. for C286H156O11N6 3852.19, obsd. 3852.12. UV-vis (CH2Cl2): λmax, nm 702, 647, 591, 550, 517, 471, 447, 421, 320, 310, 275. 3,4-bis(4-bromophenyl)-2,5-diphenylcyclopentadiene (14). To a 50 mL round bottomed flask fitted with a reflux condenser were added 1,3-diphenylacetone (1.00 g, 4.75 mmol), 4,4’-dibromobenzil (1.75 g, 4.75 mmol) and 15 mL of absolute ethanol. The suspension was heated to reflux and a solution of KOH (0.15 g, 2.6 mmol) in 3 mL of absolute ethanol was added in portions over 15 min. The resulting deep purple solution was refluxed under N2 for 30 min. The reaction mixture was cooled to 5 °C in an ice bath and the dark crystalline product was removed by filtration, washed with cold ethanol (2.5 mL × 2) and dried under suction to yield 2.06 g (80%) of 14. 1 H NMR (300 MHz, CDCl3): δ, ppm 7.41 (d, 4H, J = 8 Hz), 7.25 (m, 6 H), 7.19 (m, 4H), 6.79 (d, 4H, J= J 8 Hz). MALDI-TOF-MS: m/z calcd. for C29H18OBr2 539.972, obsd. 540.000. 2,5-bis(4-bromophenyl)-3,4-diphenylcyclopentadienone (15). Portions of 1,3-bis(4-bromophenyl)-2propanone (1.10 g, 3.00 mmol) and benzil (631 mg, 3.00 mmol) were dissolved in absolute ethanol (110 mL) and the mixture was refluxed. Potassium Copyright © 2005 Society of Porphyrins & Phthalocyanines

hydroxide (100 mg, 1.78 mmol) dissolved in ethanol (5 mL) was added to the refluxing mixture over 15 min. The dark red mixture was refluxed under N2 for an additional 30 min. The solution was reduced to about half its original volume (~50 mL) by distillation and the mixture was cooled to room temperature. Dark purple crystals formed, and were removed by filtration, washed with methanol, and dried under suction to yield 1.32 g (81%) of 15. 1H NMR (CDCl3): δ, ppm 7.37 (d, 4H, J = 8 Hz), 7.26 (m, 2H), 7.19 (m, 4H), 7.09 (d, 4H, J = 8 Hz), 6.49 (m, 4H). MALDITOF-MS: m/z calcd. for C29H18OBr2 539.972, obsd. 539.864. 1,2-bis(4-bromophenyl)-3,4,5,6-tetra phenylbenzene (16). Portions of 3,4-bis(4-bromophenyl)-2,5diphenylcyclopentadieneone (245 mg, 0.45 mmol) and diphenylacetylene (320 mg, 1.8 mmol) were dissolved in diphenyl ether (5 mL) and the mixture was refluxed under N2 for 18 h. After cooling to room temperature, the product was precipitated with ethanol (100 mL), removed by filtration, and recrystallized from toluene to yield 260 mg (82%) of 16. 1H NMR (300 MHz, CDCl3): δ, ppm 7.01 (d, 4H, J = 8 Hz), 6.89-6.76 (m, 20H), 6.68 (d, 4H, J = 8 Hz). MALDI-TOF-MS: m/z calcd. for C42H28Br2: 690.055, obsd. 690.064. 1,4-bis(4-bromophenyl)-2,3,5,6-tetra phenylbenzene (17). Cyclopentadienone 15 (1.30 g, 2.39 mmol) and diphenylacetylene (855 mg, 4.80 mmol) were dissolved in diphenyl ether (10 mL), and the mixture was refluxed under N2 for 16 h. After the mixture was cooled to 100 °C, diphenylacetylene (427 mg, 2.40 mmol) was added, and the mixture was refluxed under N2 for another 7 h. Diphenyl ether was removed by vacuum distillation. The product was recrystallized from dichloromethane/hexanes to yield 1.44 g (87%) of 17. 1H NMR (CDCl3): δ, ppm 6.98 (d, 4H, J = 9 Hz), 6.90-6.85 (m, 12H), 6.80-6.77 (m, 8H), 6.68 (d, 4H, J = 9 Hz). MALDI-TOF-MS: m/z calcd. for C42H28Br2: 690.055, obsd. 690.040. 1-(4-carbomethoxyphenyl)-2-phenyl-ethyne (18). Methyl 4-iodobenzoate (3.00 g, 11.4 mmol) and phenylacetylene (1.25 mL, 11.4 mmol) were dissolved in piperidine (5 mL). Bis(triphenylphosphine)palladium(II) dichloride (60 mg, 0.228 mmol) was added and the mixture was stirred overnight at room temperature. The solvent was removed by distillation at reduced pressure, and the residue was chromatographed on a silica gel column (hexanes/dichloromethane: 1/1) to yield 18 (1.80 g, 67%). 1H NMR (CDCl3): δ, ppm 8.02 (d, 2H, J = 8 Hz), 7.59 (d, 2H, J = 8 Hz), 7.55 (m, 2H), 7.37 (m, 3H), 3.93 (s, 3H). Hexaphenylbenzene 19. Portions of 3-(4-bromophenyl)-2,4,5-triphenylcyclopentadienone (500 mg, 1.08 mmol) and 18 (510 mg, 2.16 mmol) were refluxed in diphenyl ether (5 mL) under N2 for 28 h. The J. Porphyrins Phthalocyanines 2005; 9: 706-723

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ARTIFICIAL PHOTOSYNTHETIC ANTENNA-REACTION CENTER COMPLEXES

mixture was cooled and the solvent was distilled under reduced pressure. The residue was chromatographed on a silica gel column (dichloromethane/hexanes: 1/1) to yield 400 mg of 19 (55%) as a mixture of two regioisomers. MALDI-TOF-MS: m/z calcd. for C44H31BrO2 670.150, obsd. 670.165. Porphyrin 20. To a flask was added 94 mg (0.10 mmol) of porphyrin 2, 0.39 g (1.00 mmol) of tetraphenylcyclopentadienone and 10 mL of diphenyl ether. The mixture was warmed at reflux under an argon atmosphere. After 4 h the solvent was removed under vacuum and the residue was chromatographed on silica gel (flash column, dichloromethane/hexanes: 1/1) to give 117 mg (91% yield) of the desired product. 1 H NMR (300 MHz, CDCl3): δ, ppm -2.72 (2H, s), 1.82 (12H, s), 2.63 (3H, s), 2.64 (3H, s), 4.10 (3H, s), 6.91-6.96 (14H, m), 7.01 (2H, d, J = 8 Hz), 7.09-7.18 (8H, m), 7.27-7.30 (6H, m), 7.73 (2H, d, J = 8 Hz), 8.28 (2H, d, J = 8 Hz), 8.35 (1H, d, J = 5 Hz), 8.41 (2H,d, J = 8 Hz), 8.43 (1H, d, J = 5 Hz), 8.62 (1H, d, J = 5 Hz), 8.66-8.72 (5H, m). MALDI-TOF-MS: m/z calcd. for C88H67N4O2Br1 1292.45, obsd. 1292.43. UV-vis (CH2Cl2): λmax, nm 647, 591, 550, 515, 417, 246. BPEA-porphyrin 21. Dry toluene was deoxygenated by three freeze-pump-thaw cycles and then backfilled and purged with argon for 10 min. Porphyrin 20 (20 mg, 0.015 mmol), acetylene 7 (56 mg, 0.15 mmol), allylpalladium(II) chloride dimer (0.6 mg, 2 × 10-3 mmol), P(tBu) t 3 (1.3 mg, tBu) 6.4 × 10-3 mmol), and quinuclidine (11 mg, 9.9 × 10-2 mmol) were dissolved in the argon-saturated toluene (1 mL) in a round bottomed flask bearing a septum. The mixture was stirred at 55 °C for 6 h. The reaction mixture was then chromatographed on a silica gel column (dichloromethane/hexanes: 7/3) to yield 23 mg of 21 (97%). 1H NMR (CDCl3): δ, ppm 8.75 (d, 2H, J = 8.7 Hz), 8.70-8.62 (m, 6H), 8.55 (s, 2H), 8.52 (d, 2H, J = 5 Hz), 8.38 (m, 4H), 8.25 (d, 2H, J = 8 Hz), 7.80 (d, 2H, J = 8 Hz), 7.67 (m, 2H), 7.59 (d, 2H, J = 8 Hz), 7.53 (m, 2H), 7.29 (s, 2H), 7.27 (s, 2H), 7.17 (s, 6H), 7.03-6.91 (18H, m), 6.78 (s, 2H), 6.59 (t, 1H, J = 2 Hz), 4.08 (s, 3H), 3.92 (s, 6H), 2.64 (s, 3H), 2.67 (s, 3H), 1.81 (s, 6H), 1.55 (s, 6H), -2.75 (s, 2H). MALDI-TOF-MS: m/z calcd. for C114H84N4O4 1572.649, obsd. 1572.435. UV-vis (CH2Cl2): λmax, nm 274, 308, 317, 419, 447, 470, 515, 549, 591, 646. BPEA-porphyrin 21(Zn). Porphyrin 21 (2.0 mg, 1.3 × 10-3 mmol) was dissolved in CHCl3 (2 mL), and zinc acetate dihydrate (2.8 mg, 1.3 × 10-2 mmol) dissolved in a minimum amount of methanol was added. The mixture was stirred at room temperature for 1 h and then chromatographed on a silica gel column (dichloromethane/hexanes: 70/30 to 100/0) to yield 2.0 mg of 21(Zn) (94%). 1H NMR (CDCl3): δ, ppm 8.78-8.65 (m, 10H, pyrrβ + anthracene), 8.61 (d, 1H, J = 5 Hz, pyrrβ), 8.47 (d, 1H, J = 5 Hz, pyrrβ), 8.38 (d, Copyright © 2005 Society of Porphyrins & Phthalocyanines

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2H, J = 8 Hz, methoxycarbonylphenyl), 8.26 (d, 2H, J = 8 Hz, methoxycarbonylphenyl), 7.80 (d, 2H, J = 8 Hz, hexaphenylbenzene), 7.66 (m, 2H, anthracene), 7.59 (d, 2H, J = 8 Hz, hexaphenylbenzene), 7.53 (m, 2H, anthracene), 7.29 (s, 2H, hexaphenylbenzene), 7.28 (s, 2H, mesityl-meta-H), meta-H), 7.17 (s, 6H,), 7.04-6.90 meta-H (m, 18Hhexaphenylbenzene + 3,5-methoxyphenylortho-H), H), 6.80 (s, 2H, mesityl-meta-H H meta-H), 6.57 (t, 1H, J = 2 Hz, 3,5-methoxyphenyl3,5-methoxyphenyl-para para-H), H), 4.07 (s, 3H, H -CO2CH H3), 3.91 (s, 6H, -OCH H3), 2.64 (s, 3H, mesitylpara-CH H3), 2.29 (s, 3H, mesitylmesityl-para-CH H3), 1.80 (s, 6H, mesityl-ortho-CH H3), 1.55 (s, 6H, mesityl-orthoCH H3). MALDI-TOF-MS: m/z calcd. for C114H82N4O4Zn 1634.562, obsd. 1634.506. UV-vis (CH2Cl2): λmax, nm 274, 310, 319, 421, 447, 471, 549, 588. 4-(4-bromophenyl)-1-(4-formylphenyl)-2,3,5,6tetraphenylbenzene (22). Compound 17 (500 mg, 0.720 mmol) was dissolved in dry tetrahydrofuran (500 mL). n-butyllithium (2.86 M in hexane, 0.389 mL, 1.11 mmol) was added to the solution at -78 °C over 15 min, the mixture was stirred at the same temperature for 30 min, and then DMF (730 μL, 9.4 mmol) was added. After the reaction mixture was brought to room temperature, the reaction was quenched with water (20 mL). Tetrahydrofuran was removed by distillation at reduced pressure. The product was extracted with dichloromethane and chromatographed on a silica gel column (dichloromethane) to yield 272 mg (59%) of 22. 1 H NMR (CDCl3): δ, ppm 9.751 (s, 1H), 7.38 (d, 2H, J = 8 Hz), 7.00 (d, 2H, J = 8 Hz), 6.99 (d, 2H, J = 8 Hz), 6.90-6.78 (m, 20H), 6.70 (d, 2H, J = 8 Hz). MALDI-TOF-MS: m/z calcd. for C43H29OBr 640.140, obsd. 640.144. Porphyrin 23. A solution of 5-(2,4,6-trimethylphenyl)dipyrromethane (562 mg, 2.13 mmol), methyl 4-formylbenzoate (280 mg, 1.70 mmol), and aldehyde 22 (274 mg, 0.427 mmol) in dry dichloromethane (200 mL) was purged with argon for 30 min. Dry powdered sodium chloride (0.79 g, 13.5 mmol) and boron trifluoride etherate (79 μL, 0.64 mmol) were added and the mixture was stirred for 3 h at room temperature. 2,3-dichloro-5,6-dicyanoquinone (753 mg, 3.32 mmol) was added and the mixture was stirred at room temperature overnight. The reaction mixture was filtered with a silica gel column (2 × 10 inch, dichloromethane) to remove polar impurities, and the product was purified by chromatography (silica gel, dichloromethane/hexanes: 1/1 and ethyl acetate/hexanes: 1/9) to yield 66 mg (12%) of 23. 1 H NMR (CDCl3): δ, ppm 8.71 (d, 2H, J = 5 Hz), 8.67 (d, 2H, J = 5 Hz), 8.62 (d, 2H, J = 5 Hz), 8.41 (d, 2H, J = 8 Hz), 8.36 (d, 2H, J = 5 Hz), 8.28 (d, 2H, J = 8 Hz), 7.69 (d, 2H, J = 8 Hz), 7.28 (s, 4H), 7.19-7.08 (m, 12H), 7.05 (d, 2H, J = 9 Hz), 6.96 (m, 10H), 6.80 (d, 2H, J = 9 Hz), 4.10 (s, 3H), 2.64 (s, 6H), 1.81 (s, 12H). MALDI-TOF-MS: m/z calcd. for C88H67N4O2Br J. Porphyrins Phthalocyanines 2005; 9: 706-723

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1290.444, obsd. 1290.419. UV-vis (CH2Cl2): λmax, nm 374, 420, 515, 550, 590, 648. BPEA-porphyrin 24. Porphyrin 23 (65 mg, 0.050 mmol), acetylene 7 (19 mg, 0.052 mmol), allylpalladium(II) chloride dimer (1.8 mg, 5.0 × 10-3 mmol), P(tBu) t 3 (4.3 mg, 0.020 mmol), and quinutBu) clidine (29 mg, 0.26 mmol) were stirred in argonsaturated toluene (3 mL) at 55 °C. After 4.5 and 24 h, 29 mg (0.080 mmol) and 20 mg (0.055 mmol), respectively, of 7 were added. After 7 h, and again after 24 h, additional palladium catalyst (1.8 mg) and P(tBu) t 3 (4.3 mg) were added to the reaction tBu) mixture. After 27 h the reaction temperature was raised to 70 °C. The total reaction time was 31 h. After the reaction, the mixture was diluted with dichloromethane, filtered with a short silica gel column, and chromatographed on a silica gel column (dichloromethane/hexanes: 1/1) to yield 50 mg (64%) of 24. 1H NMR (CDCl3): δ, ppm 8.72-8.62 (m, 10H), 8.42-8.37 (m, 4H), 8.28 (d, 2H, J = 8 Hz), 7.70 (d, 2H, J = 8 Hz), 7.62 (m, 4H), 7.38 (d, 2H, J = 8 Hz), 7.29 (s, 4H), 7.20-7.13 (m, 12H), 7.06-6.97 (m, 12H), 6.91 (d, 2H, J = 2 Hz), 6.55 (t, 1H, J = 2 Hz), 4.10 (s, 3H), 3.88 (s, 6H), 2.64 (s, 6H), 1.82 (s, 12H), -2.72 (s, 2H). MALDI-TOF-MS: m/z calcd. for C114H84N4O4 1572.649, obsd. 1572.439. UV-vis (CH2Cl2): λmax, nm 275, 309, 318, 419, 472, 514, 548, 590, 646. BPEA-porphyrin 24(Zn). Zn porphyrin 24(Zn) was synthesized as described for 21(Zn). The yield was 4.5 mg of 24(Zn) (89%). MALDI-TOF-MS: m/z calcd. for C114H84N4O4Zn 1634.562, obsd. 1634.556. UV-vis (CH2Cl2): λmax, nm 275, 308, 318, 421, 444, 471, 548, 588. Aldehyde 25. Compound 19 (399 mg, 0.595 mmol) was dissolved in dry tetrahydrofuran (5 mL). To this solution DIBAL-H (1 M in hexane, 1.8 mL, 1.8 mmol) was added slowly at 0 °C, and the mixture was stirred at the same temperature for 15 min. The reaction was quenched with a few drops of water, and the solvent was evaporated at reduced pressure. The residue was passed through a short silica gel column and the column was rinsed with dichloromethane. Evaporation of the dichloromethane at reduced pressure gave a white solid which was dissolved in a mixture of dichloromethane/tetrahydrofuran (5/10 mL) and stirred with manganese(IV) dioxide (1.4 g, 16 mmol) at room temperature for 2 h. The reaction mixture was filtered through celite 545, and evaporation of the solvents at reduced pressure yielded 340 mg of aldehyde 25 (89%). 1H NMR (CDCl3): δ, ppm 9.75 (s, 1H), 7.38 (d, 2H, J = 8 Hz), 7.00 (d, 2H, J = 8 Hz), 6.99 (d, 2H, J = 8 Hz), 6.90-6.78 (m, 20H), 6.69 (d, 2H, J = 9 Hz). MALDI-TOF-MS: m/z calcd. for C43H29BrO 640.140, obsd. 640.135. Porphyrin 26. A solution of 5-(2,4,6-trimethylphenyl)dipyrromethane (840 mg, 3.18 mmol), methyl 4-formylbenzoate (435 mg, 2.65 mmol), Copyright © 2005 Society of Porphyrins & Phthalocyanines

and aldehyde 25 (340 mg, 0.530 mmol) in dry dichloromethane (318 mL) was purged with argon for 30 min. Dry powdered sodium chloride (1.00 g, 17.2 mmol) and boron trifluoride etherate (110 μL, 4.58 mmol) were added and the mixture was stirred for 3 h at room temperature. 2,3-dichloro-5,6-dicyanoquinone (1.08 g, 3.32 mmol) was added and the mixture was stirred at room temperature overnight. The reaction mixture was filtered through a silica gel column (2 × 10 inch, dichloromethane) to remove polar impurities, and the product was purified by chromatography (silica gel, dichloromethane/ hexanes: 1/1) to yield 150 mg of 26 (meta + para isomers, 22%). MALDI-TOF-MS: m/z calcd. for C88H67N4O2Br 1290.444, obsd. 1290.459. UV-vis (CH2Cl2): λmax, nm 374, 420, 516, 550, 591, 648. BPEA-porphyrin 27. Porphyrin 26 (150 mg, 0.116 mmol), acetylene 7 (67 mg, 0.185 mmol), allyl palladium(II) chloride dimer (2.5 mg, 7.0 × 10-3 mmol), P(tBu) t 3 (5.8 mg, 0.028 mmol), and quinuclidine tBu) (34 mg, 0.30 mmol) were stirred in argon-saturated toluene (5 mL) at 55 °C. After 20 h, the reaction temperature was raised to 70 °C, and the mixture was stirred for another 4 h. The reaction mixture was then cooled and filtered through a short silica gel column, and the column was washed with dichloromethane to collect the porphyrin starting material and products. A silica gel column chromatography (dichloromethane/ hexanes: 70/30 to 100/0) gave 55 mg (30% yield) of a para/meta-mixture of 24 and 27. TLC of the product using dichloromethane/hexanes: 45/55 as a solvent mixture revealed that 27 eluted slightly more slowly than 24. A mixture enriched in 27 (meta/para / /para = 75/25) was obtained after two preparative TLC runs to yield 14 mg (7.7%) of total product. 1H NMR (CDCl3): δ, ppm 8.71-8.62 (m, 10H), 8.44-8.38 (m, 4H), 8.26 (d, 2H, J = 8 Hz), 7.70-7.58 (m, 6H), 7.39 (m, 2H), 7.27 (s, 4H), 7.21-7.16 (m, 12H), 7.11-6.91 (m, 14H), 6.55 (m, 1H), 4.12 (s, 2.3H, CH H3CO2-(meta)), 4.10 (s, 0.7H, CH H3CO2-(para -( )), 3.88 (m, 6H), 2.62 (m, 6H), 1.83 (s, 12H), -2.72 (bs, 2H). MALDI-TOF-MS: m/z calcd. for C114H84N4O4 1572.694, obsd. 1572.654. UV-vis (CH2Cl2): λmax, nm 274, 308, 318, 419, 444, 471, 514, 548, 590, 646. BPEA-porphyrin 27(Zn). Zn porphyrin 27(Zn) was synthesized as described for 21(Zn). The product was purified by chromatography (silica gel, dichloromethane/hexanes = 50/50). MALDI-TOF-MS: m/z calcd. for C114H82N4O4Zn 1634.562, obsd. 1634.572. UV-vis (CH2Cl2): λmax, nm 274, 308, 318, 421, 471, 549, 587. Antenna model 28. Dry toluene was deoxygenated by three freeze-pump-thaw cycles, and backfilled and purged with argon for 10 min. Portions of 1,2-(4-bromophenyl)-3,4,5,6-tetraphenylbenzene (50 mg, 0.070 mmol), acetylene 7 (30 mg, 0.083 mmol), allyl palladium(II) chloride dimer (1.5 mg, J. Porphyrins Phthalocyanines 2005; 9: 706-723

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ARTIFICIAL PHOTOSYNTHETIC ANTENNA-REACTION CENTER COMPLEXES

4.1 × 10-3 mmol), P(tBu) t 3 (3.4 mg, 0.017 mmol), tBu) and quinuclidine (16 mg, 0.14 mmol) were dissolved in the argon-saturated toluene (3 mL) in a round bottomed flask bearing a septum. The mixture was heated at 55 °C for 24 h. The product was purified by a silica gel column (dichloromethane/hexanes: 1/1) to yield 15 mg (22%) of 28. 1H NMR (CDCl3): δ, ppm 8.63 (m, 4H), 7.61 (m, 5H), 7.34 (d, 2H, J = 8 Hz), 7.06 (d, 2H, J = 8 Hz), 6.94-6.80 (m, 23H), 6.76 (d, 2H, J = 8 Hz), 6.55 (t, 1H, J = 2 Hz), 3.88 (s, 6H). MALDI-TOF-MS: m/z calcd. for C68H45BrO2 972.2597, obsd. 972.1673. UV-vis (CH2Cl2): λmax, nm 275, 309, 319, 446, 471. Antenna model 29. Dry toluene was deoxygenated by three freeze-pump-thaw cycles and then backfilled and purged with argon for 10 min. Portions of 1,2-(4-bromophenyl)-3,4,5,6-tetraphenylbenzene (26 mg, 0.037 mmol), acetylene 7 (40 mg, 0.11 mmol), allyl palladium(II) chloride dimer (3.7 mg, 7.4 × 10-3 mmol), P(tBu) t 3 (4.3 mg, tBu) 0.015 mmol), and quinuclidine (23 mg, 0.21 mmol) were dissolved in the argon-saturated toluene (3 mL) in a round bottomed flask bearing a septum. The mixture was heated at 55 °C for 10 h in the sealed flask. The reaction mixture was introduced directly onto a silica gel column (dichloromethane/hexanes: 5/5 to 7/3) to obtain a crude product. The product was further purified by chromatography (silica gel, ethyl acetate/hexanes: 2/8) to yield 6.5 mg (36%) of 29. 1 H NMR (CDCl3): δ, ppm 8.60 (m, 8H), 7.55 (m, 8H), 7.38 (d, 4H, J = 8 Hz), 7.01 (d, 4H, J = 8 Hz), 6.956.85 (m, 24H), 6.53 (t, 2H, J = 2 Hz), 3.86 (s, 12H). MALDI-TOF-MS: m/z calcd. for C94H62O4 1254.464, obsd. 1254.513. UV-vis (CH2Cl2): λmax, nm 275, 310, 318, 446, 470. Antenna model 30. Acetylene 7 (32 mg, 0.088 mmol) and bromobenzene (10 μL, 0.095 mmol) were dissolved in a mixture of deoxygenated DMF/triethylamine (1/1 mL). Tetrakis(triphenylphosphine)palladium(0) (3 mg, 0.002 mmol) and cuprous iodide (1 mg, 0.005 mmol) were added to the solution and the flask was capped with a septum. The mixture was stirred at 95 °C for 2 h. The reaction mixture was cooled, diluted with dichloromethane, and washed with water. The organic layer was evaporated at reduced presssure, and the product was purified by chromatography (silica gel, dichloromethane/ hexanes: 1/1 and ethyl acetate/hexanes: 1/9) to yield 33 mg (85%) of 30. 1H NMR (CDCl3): δ, ppm 8.69 (m, 4H), 7.78 (m, 2H), 7.65 (m, 4H), 7.44 (m, 3H), 6.92 (d, 2H, J = 2 Hz), 6.56 (t, 1H, J = 2 Hz), 3.89 (s, 6H). MALDI-TOF-MS: m/z calcd. for C32H22O2 438.161, obsd. 438.169. UV-vis (CH2Cl2): λmax, nm 274, 312, 440, 464.

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CONCLUSION Hexaphenylbenzene is a useful framework for the assembly of artificial photosynthetic antennareaction center compounds due to its rigidity, coupled with a lack of strong π-conjugation effects enforced by the large dihedral angles between the central and peripheral rings. The spacing between chromophores linked to the periphery of the hexaphenylbenzene is ideal for rapid singlet-singlet energy transfer, as revealed by spectroscopic studies [35] of the heptads, hexads and model compounds whose synthesis has been described above. Substituted hexaphenylbenzenes may be readily synthesized by trimerization of substituted diphenylacetylenes [22, 5], but this method is best suited to the preparation of symmetric structures, since the use of mixtures of diphenylacetylenes or of unsymmetrical diphenylacetylenes will lead to mixtures of products. It also requires metalation of the porphyrin rings. The approach described here requires more synthetic steps, but allows the preparation of unsymmetrical structures in which different antenna molecules may be placed at different positions on the hexaphenylbenzene core. Thus, the door is opened for the preparation of a variety of antenna motifs, including those with multiple chromophores that harvest a larger range of wavelengths of light. Acknowledgements This work was supported by a grant from the U. S. Department of Energy (DE-FG02-03ER15393). This is publication No. 644 from the ASU Center for the Study of Early Events in Photosynthesis.

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