Electronic structure and conduction mechanism of donor-bridge-acceptor systems where PPV acts as a molecular wire

摘要

p-Phenylenevinylene (PPV) oligomers behave as molecular wires, although in different ways in different experiments. The donor-bridge-acceptor systems of Davis et al., where a tetracene (TET) donor and a pyromellitimide (PI) acceptor are linked by PPV oligomers of varying lengths, have revealed an unusual donor-acceptor distance dependence in the rate constant for photo-induced electron transfer (ET). Sikes et al. in an electrochemical experiment have found fast tunneling over 28 A through PPV oligomers. In the present paper, we have applied density functional theory and other quantum chemical methods to study the electronic structure of PPV-systems and used these data to make conclusions regarding the conduction mechanisms. Consistent with the earlier interpretations we find that the PPV bridges act as a tunneling medium for ET in the thermal case and for the shorter bridges in the photo-induced case. In the three longer molecules the photo-induced ET is coherent to the bridge molecule, and is followed by fast draining to the acceptor. The electron transfer does not appear to be connected to torsional activation of bridge plane or side groups.