Photoinduced charge separation (CS) and charge recombination (CR) processes have been examined in various porphyrin-fullerene linked systems (i.e., dyads and triads) by means of time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. The investigated compounds comprise a homologous series of rigidly linked, linear donor-acceptor arrays with different donor-acceptor separations and diversified donor strength: freebase porphyrin-C60 dyad (H2P-C60), zincporphyrin-C60 dyad (ZnP-C60), ferrocene-zincporphyrin-C60 triad (Fc-ZnP-C60), ferrocene-freebase porphyrin-C60 triad (Fc-H2P-C60), and zincporphyrin-freebase porphyrin-C60 triad (ZnP-H2P-C60). Most importantly, the lowest lying charge-separated state of all the investigated systems, namely, that of ferrocenium ion (Fc+) and the C60 radical anion (C60.-) pair in the Fc-ZnP-C60 triad, has been generated with the highest quantum yields (close to unity) and reveals a lifetime as long as 16 micros. Determination of CS and CR rate constants, together with the one-electron redox potentials of the donor and acceptor moieties in different solvents, has allowed us to examine the driving force dependence (-DeltaG0ET) of the electron-transfer rate constants (kET). Hereby, the semilogarithmic plots (i.e., log kET versus -DeltaG0ET) lead to the evaluation of the reorganization energy (lambda) and the electronic coupling matrix element (V) in light of the Marcus theory of electron-transfer reactions: lambda = 0.66 eV and V = 3.9 cm(-1) for ZnP-C60 dyad and lambda = 1.09 eV and V = 0.019 cm(-1) for Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H2P-C60 triads. Interestingly, the Marcus plot in Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H2P-C60 has provided clear evidence for intramolecular CR located in both the normal and inverted regions of the Marcus parabola. The coefficient for the distance dependence of V (damping factor: betaCR = 0.58 A(-1) is deduced which depends primarily on the nature of the bridging molecule.

An extremely long-lived charge-separated state has been achieved successfully using a ferrocene-zincporphyrin-freebaseporphyrin-fullerene tetrad which reveals a cascade of photoinduced energy transfer and multistep electron transfer within a molecule in frozen media as well as in solutions. The lifetime of the resulting charge-separated state (i.e., ferricenium ion-C(60) radical anion pair) in a frozen benzonitrile is determined as 0.38 s, which is more than one order of magnitude longer than any other intramolecular charge recombination processes of synthetic systems, and is comparable to that observed for the bacterial photosynthetic reaction center. Such an extremely long lifetime of the tetrad system has been well correlated with the charge-separated lifetimes of two homologous series of porphyrin-fullerene dyad and triad systems.

A novel molecular triad, representing an artificial reaction center, was synthesized via linking a fullerene moiety to an array of two porphyrins (i.e., a zinc tetraphenyl porphyrin (ZnP) and a free base tetraphenyl porphyrin (H 2 P)). In this ZnP-H 2 P-C 60 triad, the ZnP performs as an antenna molecule, transferring its singlet excited state energy to the energetically lower lying H 2 P. In benzonitrile, this energy transfer (k ) 1.5 × 10 10 s -1 ) is followed by a sequential electron-transfer relay evolving from the generated singlet excited state of H 2 P to yield ZnP-H 2 P •+ -C 60 •and subsequently ZnP •+ -H 2 P-C 60 •with rate constants of 7.0 × 10 9 s -1 and 2.2 × 10 9 s -1 , respectively. The final charge-separated state, formed in high yield (0.4), gives rise to a remarkable lifetime of 21 µs in deoxygenated benzonitrile and decays directly to the singlet ground state. In contrast, in nonpolar toluene solutions the deactivation of the porphyrin chromophores (ZnP and H 2 P) takes place via singlet-singlet energy transfer leading to the fullerene singlet excited state. This stems from the unfavorable free energy changes for an intramolecular electron-transfer event in toluene from the singlet excited state of H 2 P to the adjacent fullerene acceptor.

Four different kinds of C60-linked zincporphyrins have been prepared by changing systematically the linking position at meso-phenyl ring from ortho to para and their photophysical properties have been investigated. Regardless of the linkage between the two chromophores, photoinduced charge separation (CS) and subsequent charge recombination (CR) were observed in a series of zincporphyrin-C60 dyads by picosecond fluorescence lifetime measurements and time-resolved transient absorption spectroscopy. In THF the CS occurs from both the excited singlet state of the porphyrin and the C60 moieties, implying that the increase of the absorption cross section by both the chromophores results in the efficient formation of the ion pair (IP) state. On the other hand, in benzene the IP state generated by the photoinduced CS from the excited singlet state of the porphyrin to the C60 produces or energetically equilibrates with the locally excited singlet state of the C60. Both the CS and CR rates for the meta isomer are much slower than those for the other porphyrin-linked C60. Linkage dependence of the electron transfer (ET) rates can be explained by superexchange mechanism via spacer. These results demonstrate that C60 is a new promising building block as an acceptor in artificial photosynthetic models.

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Three different kinds of mixed self-assembled monolayers have been prepared to mimic photosynthetic energy and electron transfer on a gold surface. Pyrene and boron-dipyrrin were chosen as a light-harvesting model. The mixed self-assembled monolayers of pyrene (or boron-dipyrrin) and porphyrin (energy acceptor model) reveal photoinduced singlet-singlet energy transfer from the pyrene (or boron-dipyrrin) to the porphyrin on the gold surface. The boron-dipyrrin has also been combined with a reaction center model, ferrocene-porphyrin-fullerene triad, to construct integrated artificial photosynthetic assemblies on a gold electrode using mixed monolayers of the respective self-assembled unit. The mixed self-assembled monolayers on the gold electrode have established a cascade of photoinduced energy transfer and multistep electron transfer, leading to the production of photocurrent output with the highest quantum yield (50 +/- 8%, based on the adsorbed photons) ever reported for photocurrent generation at monolayer-modified metal electrodes and across artificial membranes using donor-acceptor linked molecules. The incident photon-to-current efficiency (IPCE) of the photoelectrochemical cell at 510 and 430 nm was determined as 0.6% and 1.6%, respectively. Thus, the present system provides the first example of an artificial photosynthetic system, which not only mimics light-harvesting and charge separation processes in photosynthesis but also acts as an efficient light-to-current converter in molecular devices.

Redox-active fullerenes can be covalently bound to a variety of donors; their photophysical properties have been investigated. Their photochemical processes, including electron transfer and energy transfer, are varied, depending on the donor, linkage between the donor and C~O , and solvent. Regardless of the solvent and linkage, the charge-separated state is produced efficiently in zinc porphyrin-C60 systems, showing that C6o is a good electron acceptor. The most intriguing characteristic of C60 in electron transfer is that C~O accelerates photoinduced charge separation and retards charge recombination in the dark. The long-lived charge-transfer state of the (260-porphyrin dyad was successfully converted to photocurrent using a self-assembled monolayer technique. These findings will provide a new strategy for the design and synthesis of artificial photosynthetic systems and photoactive materials using CbO as a building block.

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