The Role of the Exciplex State in Photoinduced Electron Transfer of Phytochlorin−[60]Fullerene Dyads

Abstract

The photoinduced electron transfer (ET) in five structurally different phytochlorin-fullerene dyads was studied in polar and nonpolar solvents using femtosecond fluorescence up-conversion and pump−probe transient-absorption techniques. Small changes in the structures of the dyads result in considerable changes in the ET properties and allow the determination of reorganization energies of the photoinduced reactions and electronic couplings between the initial and final states. After the excitation of the phytochlorin moiety to the second excited singlet state, the dyads relax rapidly to the first excited singlet state of phytochlorin. The first excited singlet state of phytochlorin is in equilibrium with an intramolecular exciplex state. In polar benzonitrile, the exciplex undergoes an electron transfer, and a complete-charge-separated (CCS) state is formed with a quantum yield close to unity. In contrast to the previously studied phytochlorin−fullerene dyads, the dyads in the present study form the CCS state also in nonpolar toluene with a yield influenced by minor changes in the molecular structure. The new dyads have a weaker phytochlorin−fullerene interaction due to longer separation distances between the two moieties. Therefore, the energies of the exciplex states are increased, and thus, their formation rates are reduced. In addition, the rates and yields of the complete charge separations are increased both in polar and nonpolar solvents. In benzonitrile, the reorganization energies for the transitions from the exciplex to the CCS and from the CCS to the ground state are 0.38 and 1.05 eV, respectively. The electronic couplings between the corresponding initial and final states of the two transitions mentioned above are 22 and 15 cm-1