Muon spin spectroscopy of the discotic liquid crystal HAT6

Abstract

Avoided level crossing muon spin resonance (ALC-muSR) 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 pi-systems and hence on the relative orientation of the triphenylene rings. The two Delta(1) resonances in the ALC-muSR 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-muSR 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.

Muoniated spin probes in the discotic liquid crystal HHTT: Rapid electron spin relaxation in the hexagonal columnar and isotropic phases

“…Nor is this related to the different timescales probed by μSR and NMR. A similar broad 1 resonance was observed for the Mu adduct of HAT6 in the Col h and I phases [24] and we propose that the same mechanism is at work in both systems.…”

“…The majority of the unpaired electron spin density is located on the six-membered ring to which Mu has added. The calculated muon and methylene proton hfccs are slightly lower than the corresponding hfccs in the structurally similar Mu adducts of triphenylene (A μ = 422.10 MHz) or HAT6 (A μ = 420.29 MHz) calculated at the UB3LYP/6-311G(d,p) level [24].…”

“…Muon spin spectroscopy has been used to study a wide range of soft matter systems, such as the calamitic liquid crystals 4-n -pentyl-4-cyanobiphenyl (5CB) [14][15][16], 4-n -octyl-4-cyanobiphenyl (8CB) [17], cholesterol nonanoate [18], 4-(trans-4-pentylcyclohexyl)benzonitrile (PCH5) [19], and the partitioning of cosurfactants in lamellar phase dispersions [20][21][22][23]. ALC-μSR has been used to study the DLC 2,3,6,7,10,11-hexahexyloxytriphenylene (HAT6) [24]. Muoniated radicals were formed by Mu addition to the unsaturated secondary carbon of the triphenylene ring of HAT6.…”

Probing Columnar Discotic Liquid Crystals by EPR Spectroscopy with a Rigid‐Core Nitroxide Spin Probe

“…HAT6 and other members of the HAT n series have been studied by many methods including broad-band dielectric spectroscopy, DSC, X-ray diffraction, Muon spectroscopy, deuteron NMR spectroscopy, and quasi-electric neutron scattering. [ 8 ]–[ 11 ]…”

Probing Columnar Discotic Liquid Crystals by EPR Spectroscopy with a Rigid‐Core Nitroxide Spin Probe

“…HAT6 and other members of the HATn series have been studied by many methods including broad-band dielectric spectroscopy, DSC, X-ray diffraction, Muon spectroscopy, deuteron NMR spectroscopy, and quasi-electric neutron scattering. [8][9][10][11] EPR spectroscopy of nitroxide spin probes (SPs) is a valuable method for the study of both structure and dynamics of complex partially disordered systems such as proteins and their complexes, DNA/RNA, biological membranes, nanoparticles, and soft matter. [12][13][14][15][16][17][18][19][20][21][22] EPR spectroscopy has the ability to resolve directly molecular re-orientational dynamics over time scales of 10 À11 -10 À7 s through the variation in spectral line shapes.…”