Localiztion in Molecular Solids

Abstract

The local or extended nature of molecular ion or exciton states in molecular solids is determined by a competition between fluctuations in the local site energies of these states (which tend to localize them) and the hopping integrals for inter-site excitation transfer (which tend to delocalize them). This paper constitutes a brief survey of a body of work in which the parameters governing the localization of molecular ion states are evaluated spectroscopically with the aid of molecular orbital theory and subsequently utilized to assess the character of these states in a variety of molecular solids, including pendant-group polymers, Van der Waals crystals, and segregatedstack, quasi-onedimensional charge transfer salts. To illustrate the utility of this approach the example of electron localization in polystyrene (PS) and poly(2-vinyl pyridine)(PVP) is developed. Specifically, a model of electron interactions with longitudinal polarization fluctuations is described and shown to predict the (relaxationenergy) shifts and broadening of the photoemission lines from these polymers relative to those from the model molecules ethyl benzene and 2-vinyl pyridine, respectively. This example reveals the power of the methodology because the relaxation energy shifts in PS and PVP are identical although their low-frequency dielectric constants differ by a factor of two: a result which was unexplicable within the context of previous models of solvation in dielectric media. Applications of the methodology to predict successfully extended states in segregated-stack charge transfer salts, relaxation-energy-induced surface states in Van der Waals solids, and a localized-extended state transition in Van der Waals crystals are indicated.