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, (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.

Picosecond Energy Transfer in Quantum Dot Langmuir−Blodgett Nanoassemblies

“…Light-harvesting antenna functions have been reproduced in artificial energy-gradient assemblies based on organic molecules such as porphyrins (see, e.g., Ref. 2 and references therein). An alternative approach to engineered light-harvesting systems is the use of inorganic semiconductor nanocrystals [nanocrystal quantum dots (NQDs)].…”

Meso-Linked Multiporphyrins as Model for Light Harvesting Systems: Review

“…Lindsey et al have reported the synthesis of numerous phenylene-and diphenyl ethynyl-bridged multiporphyrins for mimicking the biological light-harvesting antenna systems and their photo physical properties have been studied [46][47][48]. These multiporphyrins show efficient energy transfer that involves a throughbond process mediated by the bridge.…”

Principles of Directed Electronic Energy Transfer

“…Following photoexcitation of the peripheral zinc porphyrin (antenna), energy migrates to the central zinc porphyrin (donor) from which it transfers to free base porphyrin (acceptor), initiating electron transfer to the fullerene (reaction centre). In this system, the charge-separated excited state is generated with an impressively high quantum yield of 0.90, based on the light absorbed by the zinc porphyrin antenna [45,46]. Energy harvesting dendrimers are increasingly being developed for use as organic light-emitting diode materials [47][48][49][50].…”