Electrostatically driven electronic molecular bistability: Ab initio calculation for a model (LiF)(n) system

摘要

Semiempirical theories predict that some donor-acceptor molecules may exhibit long-lived electronic metastable states. The states result from a cooperative effect of successive electron transfers from several donor to acceptor moieties of the molecule. Calculations within the Moller-Plesset second-order perturbation theory following the unrestricted Hartree-Fock, projected unrestricted Hartree-Fock, and complete active space self-consistent field procedures confirm the effect for the first time at the ab initio level. An equidistant linear chain (DA)(n), where the donor (D) and acceptor (A) subunits are the lithium and fluorine atoms, respectively, has been chosen as a model for a molecule with fixed in space D and A substituents. The nearest-neighbor LiF distance is set to be sufficiently large to assure the isolated DA pair has lower energy in the neutral DA state than in the ionic D(+)A(-) one, i.e., a single electron transfer to occur requires energy. In the (DA)(n) system, a single electron transfer from D to the nearest A requires a comparable amount of energy (Delta E-n(1)). It is shown, however, that, due to the cooperative nature of the excitations, the excited state corresponding to m such electron transfers (m > n(crit)) may have an excitation energy Delta E-n(m) lower than Delta E-n(1). Due to this a multiply excited state may be close in energy scale to the nonexcited one, both states separated by energy barrier related to n(crit). The effect has been checked against perturbations that mimic dimerization of the chain and a lateral extension of the D+ and A(-) charge distribution. It turned out that the cooperative effect is likely to survive these perturbations. (C) 2000 American Institute of Physics. [S0021-9606(00)30506-2]. [References: 28]