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

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) 2 X (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 T ρ = 5–200 K and then a sharp phase transition at about T MI = 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 CC 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 CH···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 ‐DMTTF 0 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.