THE EFFECTS OF COUPLINGS TO SYMMETRICAL AND ANTISYMMETRIC MODES AND MINOR ASYMMETRY ON THE SPECTRAL PROPERTIES OF MIXED-VALENCE AND RELATEDCHARGE-TRANSFER SYSTEMS

The most common methods used to describe the energy levels of charge-t ransfer systems (including mixed-valence systems) are the linear respo nse approach of Rice and co-workers and the essentially equivalent PKS model described initially by Piepho, Krausz, and Schatz, While these methods were quite successful, in their original form they omitted the effects of overall symmetric vibrations. As a consequence. in particu lar they were not capable of adequately describing the electronic band width in the strong-coupling limit: Hush and later Ondrechen et al. d emonstrated that symmetric modes are essential in this case, and moder n versions of these models now include them. Here, we explore the rela tionship between symmetric and antisymmetric modes, concentrating on h ow this is modified by the presence of weak (e,g., environmentally or substitutionally induced) asymmetry, For the symmetric case, we show t hat when the electronic Hamiltonian operators are transformed from the ir usual localized diabatic representation into a delocalized diabatic representation, the effects of the symmetric and antisymmetric modes are interchanged. The primary effect of weak asymmetry is to mix the p roperties of the various modes, and possible consequences of this for the spectroscopy of bacterial photosynthetic reaction centre and subst ituted Creutz-Taube cations are discussed. We also consider the proble m from an adiabatic Born-Oppenheimer perspective and examine the regio ns in which this approach is appropriate.