Electronic and Intramolecular Structural Localizations in Conducting Organic Salts

We examined the energy factors governing intramolecular structural localization, which leads to a mixed-valence structure for molecular stacks and hence to a 4k_f CDW in small-Ulimit (i.e., U/4|β| < 1). Intramolecular structural localization is found to occur when E_R/4|β| > 1, where the intramolecular relaxation energy E_R is a measure of the tendency for a stacking molecule to relax its geometry upon losing or gaining an electron. For a donor stack E_Ris found to be approximated by 2(IP^v – IP^a), given the vertical and adiabatic ionization potentials of the donor as IP^v and IP^a, respectively. On the basis of these ionization potentials and the estimates of bandwidth 4|β|, the intramolecular structural localization condition was tested for several well-studied organic salts. For a donor molecule, the tendency for intramolecular relaxation is found to increase as antibonding character of its HOMO increases. Thus, sulfur-based donor molecules have a stronger tendency for intramolecular relaxation than do their selenium analogues, which appears to be why a 4k_f CDW is more likely to be found from sulfur-based donors. To examine how electronic localization (i.e., U/4|β| > 1) and intramolecular structural localization (i.e., E_R/4|β| > 1) affect the electronic structures of molecular stack, we performed band electronic structure calculations on several mono-valence and mixed-valence structures of TTF^1/2+ and TTF^2/3+ stacks. Analysis of these results led us to formulate simple models of 2k_fand 4k_f CDW's in terms of intramolecular and intermolecular distortions.