The polarized infrared and optical reflectance spectra of several members of the bis-͑ethylenedithio͒tetrathiafulvalene ͑ET͒ complexed with perrhenate class ͓(ET) 2 (ReO 4 ), ␣-(ET) 3 (ReO 4 ) 2 , ␤-(ET) 3 (ReO 4 ) 2 ] of organic molecular solids have been measured as a function of temperature. For the compounds studied, the spectra are highly anisotropic, with data in the good conductivity polarization dominated by totally symmetric electron-phonon activated A g modes of ET. The low-temperature phase of each material is characterized by the opening of a semiconducting gap and vibronic symmetry breaking, as evidenced by vibrational fine structure in varying degrees. The nature and splitting of the 900-cm Ϫ1 perrhenate mode through the phase-transition temperatures implies that anion ordering drives the phase transition in the two 3:2 phases whereas the 82-K metal → insulator transition in (ET) 2 (ReO 4 ) is a weak structural modification in which anion reordering plays a minimal role. Correlation effects in this set of compounds are important, suggesting that a Hubbard gap picture may be appropriate for interpreting the low-lying electronic structure in these organic molecular solids.

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