Dependence of electron transfer dynamics in wire-like bridge molecules on donor-bridge energetics and electronic interactions

Electron transfer (ET) reactions in donor-bridge-acceptor (DBA) molecules that occur by means of superexchange interactions between the donor (D) and the bridge (B) molecules depend on the vertical energy gap separating D and B. This dependence modulates the electronic coupling matrix element for ET, and hence the ET rate. However, when the energy gap between D and B is small, the assumptions intrinsic to the simplest superexchange model break down and charge injection from D to B may occur. To investigate this range of possibilities, we synthesized a family of DBA molecules based on a 2,5-bis(2'-ethylhexyloxy)-1,4-distyrylbenzene (OPV3) bridge. Three electron donors, zinc 5,10,15,20-tetraphenylporphyrin (ZnTPP), perylene (PER) and tetracene (TET) as well as two electron acceptors naphthalene-1,8:4,5-bis(dicarboximide) (NI) and pyromellitimide (PI) were attached to the OPV3 bridge. The observed ET dynamics of these molecules are sensitive not only to the donor-bridge energy gap, but also to the excited state torsional dynamics between the donor and bridge.