Time-resolved EPR investigation of charge recombination to a triplet state in a carotene-diporphyrin triad

Abstract

Photoinduced charge separation and recombination were investigated in a triad consisting of a carotenoid (C), a tetraarylporphyrin (P) and a tris(heptafluoropropyl)porphyrin (P_F), C–P–P_F, by means of time-resolved electron paramagnetic resonance. The electron transfer process was studied in a glass of 2-methyltetrahydrofuran at 10 K, in the crystalline phase at 150 K and in the liquid nematic phase of the uniaxial LC E-7 at 295 K, and in the nematic phase of the LC ZLI-1167 at 300 K. In all the different media and in the different phases, the molecular triad undergoes two-step photoinduced electron transfer, with the generation of a long-lived charge-separated state (C^•+–P–), and charge recombination to the triplet state, localized in the carotene moiety ³C–P–P_F. Low-temperature charge separation and triplet recombination are common features of both fullerene-based and diporphyrin molecular triads, proving that the large delocalized π-electron system of the porphyrin electron acceptor leads to low total reorganization energy and low sensitivity to solvent stabilization of the radical ions in a similar way as for fullerene systems.

Acknowledgements

This work was supported by the PRIN project no. 2002031443, by the CNR project no. CNRG004079 and by the US Department of Energy (DE-FG02-03ER15393). The authors thank Dr Michael Fuhs for providing the simulation program for the spin-correlated radical pair spectra.