Sequential Energy and Electron Transfer in an Artificial Reaction Center:  Formation of a Long-Lived Charge-Separated State

Abstract

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.

Primary charge-recombination in an artificial photosynthetic reaction center

“…The fluorescence lifetime (1.6 ns) in the triad is much shorter than the S 1 state lifetime (Ϸ10 ns) of H 2 P. Fig. 3B shows time profiles of the transient absorptions obtained by the 532-nm laser (full width at half maximum Ϸ 18 ps) irradiation (8). The initial decays (lifetime, 60 ps) in Fig.…”

“…The synthesis of ZnP-H 2 P-C 60 is described in ref. 8. Benzonitrile (PhCN) was used as received (Aldrich, 99ϩ% anhydrous).…”

“…2) in a linked triad, ZnP-H 2 P-C 60 (8,9), in which the zinc tetraphenylporphyrin (ZnP), free-base tetraphenylporphyrin (H 2 P), and fullerene (C 60 ) moieties (N-methyl-2-phenyl-3,4-fulleropyrrolidine) are rigidly linked by amide spacers (Fig. 1A).…”

“…T o construct nanoscale devices or molecular wires that efficiently convert photon energies to chemical potentials, extensive studies have produced photoinduced, long-distance charge separation (CS) states by using donor (D)-acceptor (A)-linked multiarray systems that mimic natural photosynthesis (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11). In each intramolecular, sequential electron transfer (ET) step, an electron or a hole tunnels between D and A over substantial distances (Ͼ10 Å) as in the natural photosynthetic reaction centers (12).…”

C₆₀-Fullerene Bonded Silica Monolithic Capillary for Specific Separations of Aromatic Compounds

“…We chose C 60 -fullerene as a nano-carbon material because of its higher reactivity, uniform size, and better solubility [38][39][40][41][42][43]. Also, silica-based monoliths are well known as an efficient separation medium in LC [44,45] and easily modified with simple chemical reaction due to silanol groups on the surface of the pores.…”