In situ planar photoalignment of liquid crystals: two-step interfacial modifications through light–matter interactions actuated by linearly polarized UV-light†
Photopolymerizable, photoisomerizable, photodimerizable and photoluminescent dichroic chromophores are deliberately designed and used for comprehending the sequential realization of the polyimide-free in situ photoalignment of liquid crystals (LCs). Upon a linearly polarized UV-light (LPUV) treatment, the multifunctional photoaligning additive, homogeneously dissolved in a LC mixture, is interfacially polymerized at the inner surfaces as a thin photo-responsive polymer layer. Subsequently, the molecular orientational anisotropy is induced by the dichroic photochromic responses, resulting in a uniaxial planar LC alignment. The reversible trans/cis-isomerization plays a crucial role for the LC alignment, rather than irreversible [2+2] dimerization. Therefore, the LC aligning effect is strongly influenced by the wavelengths of LPUV. Under the longer wavelength UV-irradiation (λ > 350 nm), the reversible isomerization is more preferable to the irreversible dimerization, resulting in a rewritable LC alignment with much better alignment quality. Such sequential processes are evidenced by monitoring spatial and orientational distributions of the self-labeled dichroic fluorescent monomers during the process. The conclusions are further corroborated by the wavelength dependencies of the photochromic responses, LC aligning effect, and rewritable LC alignment.