Optical studies of the β″-(ET)2SF5RSO3 (R = CH2CF2, CHFCF2 and CHF) system: chemical tuning of the counterion

We compare the polarized optical spectra of the organic metal β-(ET) 2 SF 5 CHFSO 3 and the β-(ET) 2 SF 5 CHCF 2 SO 3 metal/insulator material with those of the first fully organic superconductor β-(ET) 2 SF 5 CH 2 CF 2 SO 3 . The small chemical modification of the counterion has a dramatic effect on the spectral and charge transport properties of these materials, and we discuss their electronic structure in terms of band structure, many-body effects, and disorder. Based on structural differences in the anion pocket of the three salts, we conclude that the unusual electronic excitations observed in the β-(ET) 2 SF 5 CHFCF 2 SO 3 metal/insulator material are caused by disorder-related localization.