Two-dimensional infrared spectroscopy study on phase transition and structural variations of a hydrogen-bonded liquid crystal

Abstract

Infrared (IR) spectra have been measured for a liquid crystal (LC) consisting of one trans-butene diacid (BD) molecule as a proton donor and two 4-(2,3,4-tridecyloxybenzoyloxy)-4′-stilbazoles (DBS) molecules as a proton acceptor (DBS:BD:DBS) linked together with each other by inter-molecular hydrogen bonds over a temperature range from 20 to 120 °C to explore its phase transition and heat-induced structural variations. The temperature-dependent IR spectra have shown that the inter-molecular hydrogen bonds are stable in the liquid crystalline phase but become slightly decoupled with temperature increasing. Two kinds of two-dimensional (2D) correlation spectroscopy, variable–variable (VV) and sample–sample (SS) 2D spectroscopy, have been employed to analyze the observed temperature-dependent spectral variations more efficiently. The SS 2D correlation analysis in the spectral range of 2700–1800 cm −1 has demonstrated that a change in hydrogen bonds in the LC starts from 40 °C, which is not clarified by differential scanning calorimetry (DSC) and conventional IR and Raman spectroscopic analyses. On the other hand, the phase transition of LC revealed by SS 2D spectroscopy in the specific spectral regions of 1750–1650 and 3000–2700 cm −1 is in a good agreement with that revealed by DSC for the heating process. The VV 2D correlation spectroscopy analysis has provided information about the structural variations of inter-molecular hydrogen bonds. The different species of hydrogen-bonded and free COOH and COO groups in the LC have been clarified by the VV 2D correlation analysis. It has also elucidated the specific order of the temperature-induced structural changes in the intra- and inter-molecular hydrogen bonds concerning with the COOH and/or COO groups in the LC.