Temperature-dependent fluorescence emission of 4-cyano-4′-pentylbiphenyl and 4-cyano-4′-hexylbiphenyl liquid crystals and their bulk phase transitions

Abstract

This study examined the fluorescence emission properties of bulk 4-Cyano-4′-pentylbiphenyl (5CB) and 4-Cyano-4′-hexylbiphenyl (6CB) and their variation with temperature. The temperature-dependent fluorescence emission spectra of these two thermotropic liquid crystals (LCs) were measured between 4 and 80 °C and the changes in fluorescence intensities and wavelengths were monitored. The fluorescence spectra of both liquid crystals are characterized by emissions of two structurally different excimers centred around 380–386 nm and 412 nm. The results show that the specific changes in intensity and wavelength differ between the two LCs and suggest presence of two different molecular conformers as the temperature changes. During nematic-isotropic phase transition, the fluorescence spectra show significant intensity increase, which can be exploited for sensitive analytical applications. The crystalline-nematic phase transition of 5CB was found to be 22 °C and the nematic-isotropic liquid transition occurred at 35.5 °C. On the other hand, the crystalline-nematic phase transition of 6CB occurred at 16 °C, while its nematic-isotropic phase transition was observed at 29 °C. The dramatic temperature-dependent phase changes observed in the bulk LCs were completely absent in a solution of 6CB. The observed temperature-dependent spectral evolutions are discussed in the context of possible excimer conformations that exist in different mesophases of the two liquid crystals as well as change in the angle between the two benzene rings, which can contribute to excimer behaviour during phase transitions.