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

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-Pn-C60 (n = 1-4, 6). In these triads, excitation of the porphyrin-based bridge generates the fully charge-separated state, Fc(•+)-Pn-C60(•-), through a sequence of electron transfer steps. Temperature dependence of both charge separation (Fc-Pn*-C60 → Fc-Pn(•+)-C60(•-)) and recombination (Fc(•+)-Pn-C60(•-) → Fc-Pn-C60) processes was probed by time-resolved fluorescence and femtosecond transient absorption. In the long triads, two mechanisms contribute to recombination of Fc(•+)-Pn-C60(•-) 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(•+)-Pn-C60(•-), hence the higher probability of reforming Fc-Pn(•+)-C60(•-) during recombination. We demonstrate that at 300 K, the weak distance dependence for charge transfer (β = 0.028 Å(-1)) relies on tunneling rather than hopping.