Muon spin spectroscopy of the discotic liquid crystal HAT6

Abstract

Avoided level crossing muon spin resonance (ALC-μSR) has been used to study the cyclohexadienyl-type radicals produced by the addition of muonium (Mu) to the discotic liquid crystal HAT6 (2,3,6,7,10,11-hexahexyloxytriphenylene) in the crystalline (Cr) phase, the hexagonal columnar mesophase (Col_h) and isotropic (I) phase. In the Cr phase unpaired electron spin density can be transferred from the radical to neighboring HAT6 molecules depending on the overlap of their π-systems and hence on the relative orientation of the triphenylene rings. The two Δ₁ resonances in the ALC-μSR spectra of the Cr phase indicate that the neighboring HAT6 molecules have two preferred orientations with respect to the radical: one which results in negligible spin density transfer and a second where 17% of the unpaired spin density is transferred. The ALC-μSR spectra in Col_h and I phases are substantially different from those of the Cr phase in that there are two narrow resonances superimposed on an extremely broad and intense resonance. The narrow resonances are due to highly mobile radicals located in the aliphatic region between the columns and the broad resonance is due to radicals incorporated within the columns of HAT6 molecules. The large width and amplitude of this resonance indicates that the radicals within the columns are undergoing rapid electron spin relaxation but the mechanism that causes this relaxation is unknown.