Muon spin spectroscopy of the rod-like liquid crystal 4-n-octyl-4'-cyanobiphenyl (8CB)

Avoided level crossing muon spin resonance (ALC-μSR) spectroscopy has been used to study the four cyclohexadienyl-type radicals produced by the addition of muonium (Mu) to the rod-like liquid crystal 4-n-octyl-4'-cyanobiphenyl (8CB). ALC-μSR spectra have been obtained over a wide temperature range in the isotropic (I), nematic (N), smectic-A (Sm-A) and crystalline (Cr) phases. The ALC-μSR spectra of the Mu adducts of 8CB are very similar to those of the Mu adducts of 4-n-pentyl-4'-cyanobiphenyl (5CB). Four Δ0 resonances were observed in the ALC-μSR spectra in the I, N and super-cooled Sm-A phases, from which the methylene proton hyperfine coupling constants (hfcs) of the Mu adducts of 8CB were determined as a function of temperature. The methylene proton hfcs of two of the radicals have unusual temperature dependence in the N and Sm-A phases and have smaller values than would be predicted from extrapolating the data in the isotropic phase. The shift of the hfcs is due to the dipolar hyperfine coupling, the ordering of the molecules along the applied magnetic field and the fluctuations of the molecules about the director. Δ1 resonances appear just below the Cr-Sm-A phase transition, which indicates that this transition is associated with cessation of rotation around the long molecular axes.