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

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We present the polarized reflection spectra of (BEDT-ATD)2X(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 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)2BF4(THF) and (BEDT-ATD)2PF6(DHF). The low-temperature structure has the 4kF-modulated lattice (dimerized structure). The magnetic susceptibility supports this 4kF modulation as well. We present the low-temperature crystal structures of (BEDT-ATD)2BF4(THF) and (BEDT-ATD)2PF6(DHF). BF4− and THF are fully ordered in (BEDT-ATD)2BF4(THF) and DHF is almost ordered in (BEDT-ATD)2PF6(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.