Low-energy Metastable Electronic States of Donor-Acceptor Oligomers

摘要

A decade ago a class of molecules was proposed which might have a pair of lowest-energy electronic metastable states. The molecules of this sort are rigid oligomers built of N donor-acceptor pairs as monomers. One of these states (called NEU) is non-polarised; the other one (called ION) differs from NEU by a collective transfer of N electrons from the donors to the acceptors. Formally, ION is an N-tuply excited state with respect to NEU. Usually, such an electron transfer would require a sizable excitation energy; however, the electrostatic interactions of the charged monomers stabilise the ION state, making its energy close to that of the NEU state. The heuristic approach of the previous paper is presently replaced by a more quantitative quantum mechanical description based on a Pariser-Parr-Pople configuration interaction formalism. At this level of approximation it is proved that for realistic values of the lonisation potentials of the donors, electron affinities of the acceptors, and donor-acceptor distances, the two lowest-energy electronic states of the system are indeed linear combinations of the configurations representing the NEU and ION states (with some smaller admixtures of other configurations). It is also shown that the NEU and ION states are quasi-stationary, with the lifetimes sharply increasing with N.