CHARGE-TRANSFER TRANSITIONS IN EXTENDED CORRELATED ELECTRONIC SYSTEMS

The positions are intensities of dipole-allowed charge transfer (CT) excitations are found exactly for model hamiltonians with one valence state per site, nearest-neighbor electron transfer, and equal number of electrons Ne and sites N. Diagrammatic valence bond (VB) methods yield the complete analysis of configuration interactions in free-radical stacks of N3 = N = 4 and 6 sites and in donor-acceptor stacks of Ne = N = 4, 6 and 8 sites, which suffice for N → ∞ extrapolarations. The previously used dimer model (Ne = N = 2) of CT transitions is significantly modified in extended systems when, as in π-molecular free-radical or CT solids or in linear polyenes, the correlations are comparable to the bandwidth and there is extensive configuration interaction mixing of VB states with different electron distributions. The roles of correlations, of nearest-neighbor interactions, and of temperature are discussed, and constrasted to dimer results, for CT transition in both segregated an mixed stacks. © 1979.