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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 106; no. 10; pp. 2036 - 2048
• Main Authors: Kodis, Gerdenis, Liddell, Paul A, de la Garza, Linda, Clausen, P. Christian, Lindsey, Jonathan S, Moore, Ana L, Moore, Thomas A, Gust, Devens
• Format: Journal Article
• Language: English
• Published: American Chemical Society 14-03-2002
• DOI: 10.1021/jp012133s
• EISSN: 1520-5215
• ISSN: 1089-5639

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, (PZP)3−PZC, has been coupled to a free-base porphyrin−fullerene artificial photosynthetic reaction center, P−C60, to form a (PZP)3−PZC−P−C60 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 (PZP)3−PZC−P−C60 with a time constant of 50 ps. The excitation is passed on to the free-base porphyrin in 32 ps to produce (PZP)3−PZC−1P−C60, which decays by electron transfer to the fullerene with a time constant of 25 ps. The resulting (PZP)3−PZC−P•+−C60 •- 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 ((PZP)3−PZC)•+−P−C60 •- 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 (a2u versus a1u 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.