EPR Investigation of Photoinduced Radical Pair Formation and Decay to a Triplet State in a Carotene−Porphyrin−Fullerene Triad

Abstract

The photochemistry of a molecular triad consisting of a porphyrin (P) covalently linked to a carotenoid polyene (C) and a fullerene derivative (C 60 ) has been studied at 20 K by time-resolved EPR spectroscopy following laser excitation. Excitation of the porphyrin moiety yields C-1 P-C 60 , which decays by photoinduced electron transfer to yield C-P •+ -C 60 •-. This state rapidly evolves into a final charge-separated state C •+ -P-C 60 •-, whose spin-polarized EPR signal was observed and simulated. There is a weak exchange interaction between the electrons in the radical pair (J ) 1.2 G). The C •+ -P-C 60 •state decays to give the carotenoid triplet in high yield with a time constant of 1.2 µs. The spin polarization of 3 C-P-C 60 is characteristic of a triplet formed by charge recombination of a singlet-derived radical pair. The kinetics of the decay of 3 C-P-C 60 to the ground state were also determined. The photoinduced electron transfer from an excited singlet state at low temperature and the high yield of charge recombination to a spin-polarized triplet mimic similar processes observed in photosynthetic reaction centers.

Triplet–triplet energy transfer in artificial and natural photosynthetic antennas

“…. These ZFS parameters are in very good agreement with previous triplet EPR studies on similar carotenoids in organic solvents (23)(24)(25). In light of the good agreement between dyads as well as with the literature data, it seems unlikely that there is any significant (>5%) spin delocalization from the carotenoid triplet into the porphyrin of either of the two dyads.…”

The effect of structural changes on charge transfer states in a light-harvesting carotenoid-diaryl-porphyrin-C60 molecular triad

“…33 Since many dyad-based artificial reaction centers suffer from rapid charge recombination, the triad succeeded in retarding charge recombination by the addition of a secondary donor (carotenoid) molecule which allowed for an increased separation between the particle and hole states. 14,33 The large distance between the donor and acceptor components in the CPC 60 triad leads to weak electronic coupling and slows the charge recombination process. 14 The synthesis and photochemistry of the CPC 60 molecular triad established that fullerenes can act as effective primary electron acceptors in multi-component systems larger than porphyrin-fullerene dyads.…”

“…14,33 The large distance between the donor and acceptor components in the CPC 60 triad leads to weak electronic coupling and slows the charge recombination process. 14 The synthesis and photochemistry of the CPC 60 molecular triad established that fullerenes can act as effective primary electron acceptors in multi-component systems larger than porphyrin-fullerene dyads. 33 The molecular triad generated long-lived charge-separated states with high quantum yields even at low temperature.…”

“…The excited state triplet radical pair recombines to yield a 3 C-P-C 60 triplet carotenoid state. 14 The main charge-transfer transition pathway between the ground-state and the final + CPC 60 -charge-separated state involves a local excitation on the porphyrin moiety, followed by electron transfer to the adjacent C 60 component. Next, the carotenoid transfers an electron to the positively charged porphyrin to yield the final charge-separated state.…”

Mimicking bacterial photosynthesis

“…4 does not yield the ground state, but rather the carotenoid triplet state, 3c-P-c60. This triplet formation by the radical pair mechanism involves evolution of the initially formed singlet biradical into the triplet biradical, which then recombines to yield the carotenoid spectroscopic triplet, as confirmed by epr spectroscopy (14). Such recombination is unusual in model systems, but has been observed in natural photosynthetic reaction centers.…”