The in-plane alignment photocontrol of a nematic liquid crystal (LC) was performed by irradiation of thin films of polymethacrylates with benzylidenephthalimidine (BPI) side chains with linearly polarized UV light. The orientational direction of LC molecules was perpendicular to the electric vector of polarized actinic light and in line with that of BPI moieties in polymer films which were exposed to linearly polarized UV light. Prolonged polarized-light-irradiation leading to sufficient photodimerization did not result in the generation of a homogeneous alignment indicating unequivocally that the photoalignment is caused specifically by polarized-light-induced E/Z photoisomerization. The level of the photoinduced LC alignment was improved by p-methoxyl substitution at the benzylidene residue and by an appropriate choice of an irradiation wavelength giving Z-isomer as a predominant component at a photostationary state. Both effects are interpreted in terms of the molecular shape of BPI favorable for the generation of LC photoalignment. Photodimerization under controlled polarized-light-irradiation conditions enhanced considerably the thermal stability of LC photoalignment owing to the formation of crosslinks.
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Suh et al. (1998) studied this question.