Low-temperature photoluminescence of 5CB liquid crystal

Abstract

The photoluminescence spectra of the 4-n-pentyl-4′-cyanobiphenyl (5CB) liquid crystal have been investigated in detail at low temperatures 4.2–200 K, for the first time. The spectral data obtained are compared to the results of the luminescence study for the 5CB nematic phase at Т=300 K. The luminescence of the 5CB crystal state is characterized by emissions of both monomer and dimer structures. Besides, there are several energetically and conformationally non-equivalent types of monomers and dimers, and their amounts change with the temperature growth non-monotonously. The 5CB several crystal modifications, having different 5CB monomer and dimer conformers, have been found out below T=160 K. The 5CB crystal–crystal transition at Т=80 K is characterized with a total loss of the fine structure in the 5CB photoluminescence spectrum and a disappearance of the spectral band at 343 nm. At T=140 K, a new spectral band at 424 nm appears; it corresponds to the significantly overlapping 5CB dimers, its intensity grows under further heating. The present investigation gives a tool for the further characterization of the molecular alignments and changes in the 5CB molecular conformations, using the monomer and excimer fluorescence emissions as a probe. The conclusions made are confirmed by the IR-spectroscopy data, measured and analyzed for the 5CB in the same temperature region.