Theory of charge–transfer excitations at the neutral–ionic interface

Abstract

Diagrammatic valence bond (DVB) theory is applied to charge–transfer (CT) excitations in ion–radical organic solids that, like TTF–chloranil, are close to the neutral–ionic (n–i) interface. Small but finite Mulliken integrals ‖t‖ in mixed regular donor–acceptor stacks lead to charge resonance among segments containing 2r adjacent ions and to a linear lowering of the CT absorption. Finite t and long range Coulomb interactions also lead, up to a critical value, to an ionicity jump Δq=qi−qn <1 between the neutral and ionic phases. The TTF–chloranil ionicities and CT absorption are fit with ‖t‖=0.23 eV. Neither attractive nor repulsive interstrand Coulomb interactions affect the CT spectra at fixed ionicity in the mean field approximation. Direct DVB solutions to finite (2N≤18) donor–acceptor strands yield the n–i interface while 2N≤16 strands suffice for the CT absorption and intensity.