Hopping versus Tunneling Mechanism for Long-Range Electron Transfer in Porphyrin Oligomer Bridged Donor–Acceptor Systems

Abstract

Achieving long-range charge transport in molecular systems is interesting to foresee applications of molecules in practical devices. However, designing molecular systems with pre-defined wire-like properties remains difficult due to the lack of understanding of the mechanism for charge transfer. Here we investigate a series of porphyrin oligomer-bridged donor-acceptor systems Fc-P-n-C-60 (n = 1-4, 6). In these triads, excitation of the porphyrin-based bridge generates the fully charge-separated state, Fc(center dot+)-P-n-C-60(center dot-) through a sequence of electron transfer steps. Temperature dependence of both charge separation (Fc-P-n*-C-60 -> Fc-P-n(center dot+)-C-60(center dot-)) and recombination (Fc(center dot+)-P-n-C-60(center dot-) -> Fc-P-n-C-60) processes was probed by time-resolved fluorescence and femtosecond transient absorption. In the long triads, two mechanisms contribute to recombination of Fc(center dot+)-P-n-C-60(center dot-) to the ground state. At high temperatures (>= 280 K), recombination via tunneling dominates for the entire series. At low temperatures (<280 K), unusual crossover from tunneling to hopping occurs in long triads. This crossover is rationalized by the increased lifetimes of Fc(center dot+)-P-n-C-60(center dot-); hence the higher probability of reforming Fc-P-n(center dot+)-C-60(center dot-) during recombination. We demonstrate that at 300 K, the weak distance dependence for charge transfer (beta = 0.028 angstrom(-1)) relies on tunneling rather than hopping.