Involvement of weak C[BOND]H···X hydrogen bonds in metal-to-semiconductor regime change in one-dimensional organic conductors (o-DMTTF)2X (X = Cl, Br, and I): combined IR and Raman studies

Authors

  • Damian Jankowski,

    1. Institute of Molecular Physics, Polish Academy of Sciences, ul. Mariana Smoluchowskiego 17, 60-179 Poznań, Poland
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  • Roman Świetlik,

    Corresponding author
    1. Institute of Molecular Physics, Polish Academy of Sciences, ul. Mariana Smoluchowskiego 17, 60-179 Poznań, Poland
    • Institute of Molecular Physics, Polish Academy of Sciences, ul. Mariana Smoluchowskiego 17, 60-179 Poznań, Poland.
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  • Eric W. Reinheimer,

    1. Sciences Chimiques de Rennes, Université de Rennes 1, UMR CNRS 6226, Campus de Beaulieu, 35042 Rennes, France
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  • Marc Fourmigué

    Corresponding author
    1. Sciences Chimiques de Rennes, Université de Rennes 1, UMR CNRS 6226, Campus de Beaulieu, 35042 Rennes, France
    • Sciences Chimiques de Rennes, Université de Rennes 1, UMR CNRS 6226, Campus de Beaulieu, 35042 Rennes, France
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Abstract

We report on the infrared (IR) and Raman studies of the three isostructural quasi-one-dimensional cation radical salts of 3,4-dimethyl-tetrathiafulvalene (o-DMTTF)2X (X = Cl, Br, and I), which all exhibit metallic properties at room temperature and undergo transitions to a semiconducting state in two steps: a soft metal-to-semiconductor regime change in the temperature region Tρ = 5–200 K and then a sharp phase transition at about TMI = 50 K. Polarized IR reflectance spectra (700–16 000 cm−1) and Raman spectra (50–3500 cm−1, excitation λ = 632.8 nm) of single crystals were measured as a function of temperature (T = 5–300 K) to assess the eventual formation of a charge-ordered state below 50 K. Additionally, the temperature dependence of the IR absorption spectra of powdered crystals in KBr discs was also studied. The Raman spectra and especially the bands related to the C[DOUBLE BOND]C stretching vibration of o-DMTTF provide unambiguous evidence of uniform charge distribution on o-DMTTF down to the lowest temperatures, without any modification below 50 K. However, the temperature dependence of Raman spectra indicates a regime change below about 200 K. Temperature dependence of both electronic dispersion and vibrational features observed in the IR spectra also clearly confirms the regime change below about 200 K and shows the involvement of C[BOND]H···X hydrogen bonds in the electronic localization; some spectral changes can be also related with the phase transition at 50 K. Additionally, using density functional theory methods, the normal vibrational modes of the neutral o-DMTTF0 and cationic o-DMTTF+ species, as well as their theoretical IR and Raman spectra, were calculated. The theoretical data were compared with the experimental IR and Raman spectra of neutral o-DMTTF molecule. Copyright © 2011 John Wiley & Sons, Ltd.

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