Photoinduced Processes in Fluorene-Bridged Rhenium–Phenothiazine Dyads – Comparison of Electron Transfer Across Fluorene, Phenylene, and Xylene Bridges

The photoinduced processes occurring after pulsed laser excitation of a series of donor–bridge–acceptor molecules comprising a phenothiazine electron donor, variable-length fluorene bridges, and a rhenium(I) electron acceptor were investigated. A dyad with a single fluorene bridge unit exhibits electron transfer from phenothiazine to the rhenium(I) complex upon photoexcitation, whereas in dyads with fluorene oligomers bridge-localized triplet excited states are formed rather than electron transfer products. In the monofluorene-bridged system with a donor–acceptor distance of ca. 15 Å, electron transfer occurs with a time constant of 1.9 ns. The equidistant electron transfer between the same donor and acceptor is considerably slower across a biphenyl bridge (3.9 ns) or a bi-p-xylene spacer (20 ns). This finding is interpreted in terms of different tunneling barrier heights associated with the charge transfer across the three different types of molecular bridges