Optical investigation of a τ-phase organic molecular conductor:: τ-(EDO-(S,S)-DMEDT-TTF)2 (AuBr2)(AuBr2)y (y ≈ 0.75)

We report the infrared and optical properties of a quasi-two-dimensional organic conductor tau-(EDO-(S,S)-DMEDT-TTF)(2) (AuBr2) (AuBr2)(y). (y approximate to0.75), where EDO-(S,S)-DMEDT-TTF is ethylenedioxy-(S,S)-dimethylethylene-dithio-tetrathiafulvalene as a function of temperature. This cation radical salt, based on an unsymmetrical pi donor, displays an unusual temperature dependence of the low-lying electronic excitation, attributed to electronic localization effects. The two components of the low-lying electronic structure were studied by standard fitting procedures and show different behavior with temperature, suggesting that they may steal oscillator strength from a narrow Drude feature. The trends are interpreted within the Hubbard model. Vibrational CH2-related features show also signs of disorder, suggesting a connection with the electronic localization and a proposed magnetic phase transition at low temperature.