Liquid crystalline transition or liquid-solid interface vibrational dynamics of diphenylacetylene in solutions by Raman spectroscopy?

Raman spectra of the stretching mode nu (S)(C=C) (A(g)) at 2224 cm(-1), the C-Ph bending in plane mode (B-3g) at 535 cm-l, the low frequency Raman reg ion from 15-200 cm(-1) of diphenylacetylene (DPA) in methylcyclohexane and acetonitrile as well. as the rocking mode of methylcyclohexane (A') at 547 cm(-1) in isotropic liquids, liquid ordered phases, and solid matrices as a function of temperature have been recorded in the range 293-77 K. We have found that DPA-methylcyclohexane system exhibits isotropic liquid-anisotrop ically aligned liquid phase transition at 258 K and liquid-solid transition at 150 K. Mechanisms of vibrational relaxation and band broadening around 258 and 150 K transitions have been discussed. We have found that DPA molec ules modify significantly reorientation order of the host solvent molecules in the phase below 258 K that strongly deviates from isotropic alignment o bserved in the liquid phase above 258 K. This anisotropic alignment may com e from liquid crystalline properties of DPA or liquid-solid interface inter actions between methylcyclohexane molecules adsorbed on surfaces of DPA cry stals. (C) 2000 Published by Elsevier Science B.V.