Time-Resolved Observation of Isothermal Phase Transition of Liquid Crystals Induced by Photoisomerization of Azobenzene Dopant

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Abstract
The isothermal phase transition behavior of azobenzene guest/liquid crystal host mixtures induced by nanosecond laser palse was investigated. Two kinds of azobenzene derivatives were employed as dopant, 4-butyl-4′-methoxyazobenzene (BMAB) and 4-hydroxyazobenzene (HOAB), in order to examine the heat effect on the phase transition. BMAB showed a trans to cis photoisomerization while HOAB showed little isomerization even by prolonged photoirradiation. In the BMAB/5CB mixture, the photochemical nematic (N) → isotropic (I) phase transition was found to be completed within ∼ 100 ms at the reduced temperature of 0.998 on irradiation of a single laser pulse of 10 ns FWHM which gave the cis isomer of BMAB. Contrary to the BMAB/5CB mixtare, no phase transition of the HOAB/5CB mixture was observed by the laser flash, indicating that the N-I phase transition observed for the BMAB/5CB mixture is ascribed to the photochemical reaction of the dopant, not to the heat-mode process. The photochemical phase transition was found to depend on such factors as the concentration of the dopant, the power of the laser pulse and irradiation temperature. The large amount of the cis isomer simultaneously produced with a single pulse induced the phase transition rapidly.