Conformation-dependent molecular association and spectral properties of 4-pentyl-4′-cyanobiphenyl liquid crystal in different phases

The 4-pentyl-4′-cyanobiphenyl (5CB) molecule conformation and structure alignment have been investigated by means of temperature variable Fourier transform infrared (FTIR) and photoluminescence (PL) spectroscopy in a wide temperature range (from −90 to 50 °C) covering the 5CB glass-like, metastable and stable crystalline, nematic and isotropic phases defined by differential scanning calorimetry measurements. The corresponding FTIR absorption and PL emission bands reflect a strong dipolar association and a formation of at least two types of 5CB dimeric structures with antiparallel or parallel orientation in the planar configuration. No formation of hydrogen bonds between the cyano groups and hydrogen atoms of the adjacent benzene ring has been found. Besides, a small amount of the 5CB monomer structure has been shown to be present in all phases. The observed temperature-induced changes in the IR and PL spectra can be used as an independent source of experimental information on conformational equilibria and structure alignment of 5CB in different states under the phase transitions.