Optical properties of beta ''-(ET)(2)SF5RSO3 (R = CH2CF2, CHFCF2): Changing physical properties by chemical tuning of the counterion

We report the polarized infrared reflectance of beta "-(ET)(2)SF5CHFCF2SO3 as a function of temperature. This salt is a derivative of the first fully organic superconductor, beta "-(ET)(2)SF5CH2CF2SO3, where the nature of the alkyl group has been modified to contain a chiral center. This small chemi cal modification has a dramatic effect on the spectral and charge transport properties. The beta "-(ET)(2)SF5CHFCF2SO3 complex displays highly anisotr opic behavior compared with the superconducting material; the essential dif ference is observed in the perpendicular to b direction where the interdime r interaction is weak, in agreement with recent theoretical calculations. T he temperature dependence of key anion vibrational modes is discussed in te rms of intermolecular hydrogen bonding within the anion pocket. That small chemical modifications of the anion template result in such striking change s in the physical properties suggests that the beta "-(ET)(2)SF5RSO3 system is quite close to a Mott phase boundary.