Photoisomerization dynamics of a light-sensitive chiral dopant in a nematic medium

Azobenzene-based chiral dopants in cholesteric liquid crystals are of interest since the properties they induce in the liquid crystal could be tuned photochemically.Here, we use a substituted binaphthyl with a halogenated azobenzene as a chiral dopant to induce a photoswitchable cholesteric phase in the nematic 4-n-pentyl-4'-cyanobiphenyl.The azobenzene group chemically attached to the chiral dopant undergoes isomerisation from trans to cis upon irradiation with green light (wavelength 535 nm), and from cis to trans upon irradiation with blue light (wavelength 450 nm).The transition between the two isomers causes helicity inversion of the cholesteric, with a lefthanded trans isomer and a right-handed cis isomer.We report on the kinetics of photoisomerisation of both processes (trans-to-cis and cis-to-trans) in the nematic host by following the pitch evolution over time.We show that the kinetic mechanism corresponds to a two-step process: a first-order isomerisation followed by a second-order autocatalytic isomerisation.This mechanism differs from the typical first-order kinetics for cis-to-trans or trans-to-cis isomerisation in azobenzenes.The autocatalytic process is attributed to interactions between the chiral dopant and the nematic host.