An Artificial Photosynthetic Antenna-Reaction Center Complex

Abstract

A model photosynthetic antenna consisting of four covalently linked zinc tetraarylporphyrins, (PZP)3−PZC, has been joined to a free base porphyrin-fullerene artificial photosynthetic reaction center, P−C60, to form a (PZP)3−PZC−PC60 hexad. As revealed by time-resolved absorption and emission studies, excitation of any peripheral zinc porphyrin moiety (PZP) in 2-methyltetrahydrofuran solution is followed by singlet−singlet energy transfer to the central zinc porphyrin to give (PZP)3−1PZC−P−C60 with a time constant of ∼50 ps. The excitation is passed on to the free base porphyrin in 240 ps to produce (PZP)3−PZC−1P−C60, which decays by electron transfer to the fullerene with a time constant of 3 ps. The (PZP)3−PZC−P•+−C60 •- charge-separated state thus formed has a lifetime of 1330 ps, and is generated with a quantum yield of 0.70 based on light absorbed by the zinc porphyrin antenna. The complex thus mimics the basic functions of natural photosynthetic antenna systems and reaction center complexes.

Dependence of the energy transfer to graphene on the excitation energy

“…Whenever a donor is placed sufficiently close to an acceptor, assuming their spectral properties and relative orientations do not inhibit it, they can couple via electrostatic interactions and the energy is funneled down to the acceptor [1]. Such a scheme evolved in natural photosynthesis [3] for efficient capturing and transport of the sunlight energy, and has been recently implemented in artificial light-harvesting assemblies [4,5]. The energy transfer between molecules with precisely designed optical spectra has also been useful in studying and understanding molecular mechanisms responsible for protein folding [6], intracellular transport [7], etc., as the efficiency of this process is extremely sensitive to the distance between a donor and an acceptor [1].…”

Approaches for biological and biomimetic energy conversion

“…A significant challenge for artificial photosynthesis is the ability to achieve long-lived charge separation and prevent charge recombination (41,52,57,58). One way to address this challenge has been to integrate successive energy gradients into covalently linked molecular dyads that will quickly shuttle electrons away from the excitation site to more stable sites.…”

“…A molecular linear pentad consisting of carotenoid-polyene-metallated (Zn) porphyrin-freebase porphyrindiquinone [C-P Zn -P-Q A -Q B ] has been shown to perform efficient photoinitiated electron transfer and produce a charge-stabilized state with an overall quantum yield of 0.83 and a lifetime of 55 s. The molecular order of the pentad was critical for charge stabilization because it was constructed such that all of the possible electron transfer pathways would converge to the same final charge-separated structure (60). Much of this work has recently been extended using fullerenes (C 60 ) as the electron acceptors; examples include triads such as a porphyrin-bearing fullerene covalently linked to a carotenoid polyene (41,57,(61)(62)(63)(64)(65).…”

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].…”