Muon Spin Spectroscopy of the Nematic Liquid Crystal 4-n-Pentyl-4′-cyanobiphenyl (5CB)

Abstract

Avoided level crossing muon spin resonance (ALC-microSR) spectroscopy has been used to study the four cyclohexadienyl-type radicals produced by the addition of muonium (Mu) to the rodlike liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB). ALC-microSR spectra have been obtained over a wide temperature range in the isotropic, nematic, and crystalline phases. Four Delta0 resonances were observed in the ALC-microSR spectra, from which the methylene proton hyperfine coupling constants (hfcs) of the Mu adducts of 5CB were determined as a function of temperature. The methylene proton hfcs of two of the radicals have unusual temperature dependence in the nematic phase and have smaller values than would be predicted from extrapolating the data in the isotropic phase. We have used the Maier-Saupe theory for rodlike liquid crystals to explain the temperature dependence of the methylene proton hfcs, which results from the ordering of the 5CB molecules, the alignment of the molecules with the external magnetic field, and fluctuations that average the anisotropic hyperfine coupling constants. There are no Delta1 resonances in the ALC-microSR spectra of the nematic phase due to the radicals rotating rapidly around the long molecular axis and fluctuations about the local director. The Delta0 resonances broaden substantially as the temperature is lowered due to the slowing down of the fluctuations, which have an average activation energy of approximately 15.9 kJ mol(-1). Cooling the sample below 275 K stopped the rotation around the long molecular axis and led to the appearance of Delta1 resonances.