The dramatic changes of the lifetimes of the charge-separated (CS) states were confirmed in zinc porphyrin (ZnP)-oligothiophene (nT)-fullerene (C 60) linked triads (ZnP-nT-C60) with the solvent polarity. After the selective excitation of the ZnP moiety of ZnP-nT-C60, an energy transfer took place from the 1ZnP* moiety to the C 60 moiety, generating ZnP-nT-1C60*. In polar solvents, the CS process also took place directly via the 1ZnP* moiety, generating ZnṖ+-nT-C 60̇-, as well as the energy transfer to the C 60 moiety. After this energy transfer, an indirect CS process took place from the 1C60* moiety. In the less polar solvent anisole, the radical cation (hole) of ZnṖ+-nT-C 60̇- shifted to the nT moiety; thus, the nT moiety behaves as a cation trapper, and the rates of the hole shift were evaluated to be in the order of 108 s-1; then, the final CS states ZnP-nṪ+-C60̇- were lasting for 6-7 μs. In the medium polar solvent o-dichlorobenzene (o-DCB), ZnP-nT ̇+-C60̇- and ZnṖ+-nT- C60̇- were present as an equilibrium, because both states have almost the same thermodynamic stability. This equilibrium resulted in quite long lifetimes of the CS states (450-910 μs) in o-DCB. In the more polar benzonitrile, the generation of ZnP-nṪ+-C 60̇- was confirmed with apparent short lifetimes (0.6-0.8 μs), which can be explained by the fast hole shift to more stable ZnṖ+-nT-C60̇- followed by the faster charge recombination. It was revealed that the relation between the energy levels of two CS states, which strongly depend on the solvent polarity, causes dramatic changes of the lifetimes of the CS states in ZnP-nT-C60; that is, the most appropriate solvents for the long-lived CS state are intermediately polar solvents such as o-DCB. Compared with our previous data for H2P-nT-C60, in which H2P is free-base porphyrin, the lifetimes of the CS states of ZnP-nT-C60 are ∼30 times longer than those in o-DCB.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry