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

Iain McKenzie,Upali A. Jayasooriya,Andrew N. Cammidge,Hemant Gopee,Herbert Dilger,Alexey Stoykov,Robert Scheuermann

doi:10.1103/physreve.87.012504

Abstract

Avoided level crossing muon spin resonance (ALC-μSR) spectroscopy was used to study radicals produced by the addition of the light hydrogen isotope muonium (Mu) to the discotic liquid crystal (DLC) 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT). Mu adds to the secondary carbon atoms of HHTT to produce a substituted cyclohexadienyl radical, whose identity was confirmed by comparing the measured hyperfine coupling constants with values obtained from DFT calculations. ALC-μSR spectra were obtained in the isotropic (I), hexagonal columnar (Col(h)), helical (H), and crystalline (Cr) phases. In the I and Col(h) phases the radicals, which are incorporated within the stacks of HHTT molecules as isolated paramagnetic defects, undergo extremely rapid electron spin relaxation, on the order of a hundredfold faster than in the H or Cr phases. The electron spin relaxation rate increases significantly with increasing temperature and appears to be caused by the liquidlike motion within the columns, which modulates the overlap between the π system of the radical and the π systems of the neighboring HHTT molecules, and hence, the hyperfine coupling constants. Rapid electron spin relaxation should occur for any π radical incorporated within the columns of a DLC, which may limit the utility of DLCs for future spin-based technologies.

Highlights

Liquid crystals are materials with phases that have anisotropic properties normally found in solids and a degree of fluidity characteristic of liquids [1,2,3]
Mu adds to the secondary carbon atoms of HHTT to produce a substituted cyclohexadienyl radical, whose identity was confirmed by comparing the measured hyperfine coupling constants with values obtained from DFT calculations
Rapid electron spin relaxation should occur for any π radical incorporated within the columns of a discotic liquid crystal (DLC), which may limit the utility of DLCs for future spin-based technologies

Summary

INTRODUCTION

Liquid crystals are materials with phases that have anisotropic properties normally found in solids and a degree of fluidity characteristic of liquids [1,2,3]. Additional information about the behavior of HHTT at the molecular level can be obtained by spin labeling the HHTT molecules with positive muons and studying the resulting radicals with muon spectroscopic techniques collectively known as μSR for muon spin rotation, resonance, and relaxation [11]. Several studies of the Mu adducts of polyaromatic hydrocarbons have shown that Mu does not add to tertiary carbons because this causes significant distortion of the ring system and is energetically unfavorable [25,26,27] In these experiments the HHTT molecules were partially aligned by cooling from the isotropic phase in a large (2.5 T) magnetic field. The motivation for this study is to probe the dynamics of the HHTT at the molecular level and to determine how an isolated paramagnetic defect interacts with neighboring molecules within the HHTT columns

EXPERIMENT

DFT calculations

Analysis of the ALC-μSR spectra

Resonance linewidth and amplitude in the Colh and I phases

CONCLUSION