transfer and donor-anion interaction

IR absorption spectra from various K-phases of BEDT·TfF radical cation salts as well as from a-(BEDT·TIFhI, arc presented. From tbese spectra one special mode l'n representing the asymmetric ring C-C stretching vibration is assigned and the dependence of its frequency from the average charge on the donor molecule is given. The investigation of the metal-insulator phase transition of a-(BEDT·TrF)2I, at 135 K with respect to this vibrational model lin gives strong indication of a charge localization in at least one of the two crystallographic different stacks. Furthennore, we correlate the frequencies of the Cfh stretching modes of a variety of BEDT·TIF radical salts witb tbe superconducting phase transition temperatures. These frequencies are evidently not charge dependent, but significanUy reOect the strength of the interaction of the donor molecule with the respecti'Ye anion. It could be shown that phases with a higher Tc of their superconducting transition exhibit a smaller red shift of these frequencies, i.e., a less attractive donor-anion interaction