Efficient Energy Transfer and Electron Transfer in an Artificial Photosynthetic Antenna−Reaction Center Complex

Abstract

A highly efficient functional mimic of a photosynthetic antenna−reaction center complex has been designed, synthesized, and studied spectroscopically. The antenna, consisting of four covalently linked zinc tetraarylporphyrins, (P ZP ) 3 −P ZC, has been coupled to a free-base porphyrin−fullerene artificial photosynthetic reaction center, P−C 60, to form a (P ZP ) 3 −P ZC −P−C 60 hexad. As revealed by time-resolved absorption and emission studies in 2-methyltetrahydrofuran solution at ambient temperature, excitation of a peripheral zinc porphyrin moiety is followed by singlet−singlet energy transfer to the central zinc porphyrin to give (P ZP ) 3 − 1 P ZC −P−C 60 with a time constant of 50 ps. The excitation is passed on to the free-base porphyrin in 32 ps to produce (P ZP ) 3 −P ZC − 1 P−C 60, which decays by electron transfer to the fullerene with a time constant of 25 ps. The resulting (P ZP ) 3 −P ZC −P •+ −C 60 •- charge-separated state is generated with a quantum yield of 0.98 based on light absorbed by the porphyrin antenna. Direct excitation of the free-base porphyrin moiety or the fullerene also generates this state with a yield near unity. Thermodynamically favorable migration of positive charge into the zinc porphyrin array transforms the initial state into a long-lived ((P ZP ) 3 −P ZC ) •+ −P−C 60 •- charge-separated state with a time constant of 380 ps. The final charge-separated state, formed in high yield (∼0.90), decays to the ground state with a lifetime of 240 ns. In benzonitrile, the lifetime is 25 μs. A previous hexad, which differs from the current hexad solely in the nature of the free-base porphyrin, gave a charge-separated state with a lower yield (0.69) and a shorter lifetime (1.3 ns). The difference in performance is attributed to differences in electronic composition (a 2u versus a 1u HOMO), conformation, and oxidation potential (1.05 versus 0.84 V) between the meso -tetraarylporphyrin and the β-octaalkylporphyrin of the current and former hexads, respectively. These results can be explained on the basis of an understanding of factors that affect through-bond energy-transfer and electron-transfer processes. The results demonstrate that it is possible to design and prepare synthetic, porphyrin-based antenna−reaction center complexes that mimic the basic photochemical functions of natural photosynthetic light-harvesting antennas and reaction centers in simple, structurally well-defined constructs.