Sign-alternating optical reorientation in nematic liquid crystals with low-molar-mass and polymeric absorbing bis-azobenzene dopants

Abstract

A comparative study of optical orientation in nematic liquid crystals (NLCs) doped with a low-molar-mass bis-azobenzene dye (monomer) and a comb-shaped polymer with side fragments similar in structure to the monomer has been carried out. Both types of bis-azobenzene dopants induced a sign-alternating nonlinearity in the NLC: the positive when the angle Ψ between the light field and the director is less than a certain critical value Ψc and the negative in the opposite case of Ψc < Ψ ≤ 90°. The transition from the monomer to polymer led to a decrease in the critical angle Ψc, i.e., to an expansion of the region of negative nonlinearity. At the same concentration of chromophores, an increase in both negative and positive nonlinearities occurs.The magnitudes of the optical-torque enhancement factors due to trans- and cis-isomers (ηT and ηC), and the ratios ηT/ηC for the low- and high-molar-mass dopants are compared.The results obtained show the possibilities of increasing the orientational optical nonlinearity when passing from low-molar bis-azobenzene dye dopants to the corresponding polymers and are important for elucidating the optimal architecture of absorbing additives.