Optical properties and metal-insulator transitions in organic metals (BEDT-ATD)2X(solvent) (X = PF6, AsF6, BF4; solvent = THF, DHF, DO) [BEDT-ATD=4,11-bis(4′,5′-ethylenedithio-1′,3′-dithiol-2′-ylidene)-4,11-dihydroanthra[2,3-c][1,2,5]thiadiazole]

We present the polarized reflection spectra of (BEDT-ATD)(2)X(solvent) (X = AsF6, PF6, BF4; solvent = THF, DHF, DO), where THF is tetrahydrofuran, DHF 2,5-dihydrofuran, and DO 1,3-dioxolane. BEDT-ATD(+) has an intramolecular excitation in a very low-energy region (4000-5000 epsilon cm(-1)). These isostructural compounds are characterized as strongly correlated quasi-1D metals. The optical spectra in all these compounds suggest that the screw-axis symmetry is broken below the metal-insulator transition temperature. This broken symmetry is confirmed by the X-ray diffraction experiment in (BEDT-ATD)(2)BF4(THF) and (BEDT-ATD)(2)PF6(DHF). The low-temperature structure has the 4k(F)-modulated lattice (dimerized structure). The magnetic susceptibility supports this 4k(F) modulation as well. We present the low-temperature crystal structures of (BEDT-ATD)(2)BF4(THF) and (BEDT-ATD)(2)PF6(DHF). BF4- and THF are fully ordered in (BEDT-ATD)(2)BF4(THF) and DHF is almost ordered in (BEDT-ATD)(2)PF6(DHF). The vibronic modes are observed in the reflection spectra of all these compounds even above the metal-insulator transition temperature, which suggests that the screw-axis symmetry is locally broken.