Low temperature Raman study of stable and metastable structures of phenylacetylene in benzene. Vibrational dynamics in undercooled liquid solutions, crystals, and glassy crystals

Abstract

Raman spectra of the ν s(≡C–H) and ν s(C≡C) stretching modes as well as Raman spectra in the lattice region 15–200 cm −1 of phenylacetylene (PA) dissolved in benzene in homogeneous liquid solutions, undercooled liquid state, crystal, and glassy matrices as a function of concentration, temperature, and quenching rate have been recorded in the temperature range of 77–293 K. The optical measurements were complemented by the differential scanning calorimetry (DSC) measurements. One has been demonstrated that the Raman spectroscopy combined with the DSC method is a powerful tool to obtain information about the nature of phase transitions at the molecular level. The results reveal some dramatic changes with concentration, temperature, and quenching rate and are of potential relevance both to fundamental condensed phase modelling and to liquid crystal technology. We have discussed the origin of a vibrational substructure observed for the stretching modes of phenylacetylene in benzene. We have found that the components of the vibrational structure correspond to stable, equilibrium crystal phase, and two non-equilibrium glassy crystal phases. Low temperature polymorphism of phenylacetylene–benzene mixtures in a broad temperature range of 77–293 K has been characterized. We have found that the low temperature polymorphism of phenylacetylene strongly depends on concentration, temperature, and quenching rate. We hope this paper is an important step in understanding mechanisms of nucleation and irreversible phase transitions.