Understanding electron-molecular vibrational coupling in organic molecular solids: Experimental evidence for strong coupling of the890−cm−1mode in ET-based materials

We report a detailed analysis of the vibrational response of several model organic molecular conductors and superconductors. We demonstrate that the $890\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ mode, originally assigned and discussed as ${\ensuremath{\nu}}_{60}\phantom{\rule{0.3em}{0ex}}({B}_{3g})$ in the ${D}_{2h}$ point group symmetry, displays an unusual sensitivity to both chemical and physical tuning for a wide variety of materials. Based upon the analysis of these combined spectral results as well as an assessment of the local molecular and crystallographic structure, we find ample experimental evidence for vibronic coupling of the $890\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ mode. This mode is not vibronically active within the planar dynamics framework, and its presence in the optical conductivity demonstrates that local symmetry breaking effects are important in layered organic molecular materials.