The alignment of nematic liquid crystals (LC) can be regulated by controlling the photochromic reaction of azobenzene (Az) chromophores being attached to the LC substrate surface. Responding to the trans-cis photoisomerization of Az, the LC alignment switches between the perpendicular state and the parallel state with respect to the substrate surface. As demonstrated, the system exhibits another significant feature, novel in-plane rearrangement of the parallel state induced by exposure to the linearly polarized UV light, to attain formation of a homogeneous domain with a specific orientation axis. The induced orientation axis was found to be perpendicular to the electric polarization of the incident UV light. Thus, the orientation axis was rotated intentionally by changing the incident polarization plane. The induced anisotropy of the homogeneous domain was quite high, providing a dichroic ratio of more than 6. Present in-plane regulation of the nematic orientation was explained in terms of the polarization-selective photochromism of surface Az.
The absorption and fluorescence spectra, fluorescence quantum yields (φflu), and lifetimes (τs) for the
microcrystals and vacuum deposited thin films of the p,p
‘-disubstituted trans,trans,trans-1,6-diphenyl-1,3,5-hexatrienes (DPHs) 1−10 have been measured and compared with those in dilute solutions at room temperature
and in organic glasses at 77 K. The positions of the peaks at the shortest wavelengths in the fluorescence
main bands, which are mainly determined by effective π-conjugation lengths (ECLs) in the excited state, are
tuned in the visible spectral range 463−587 nm by introduction of various substituents. The fluorescence
main bands shift to longer wavelengths as the strength of electron-donating/withdrawing substituents on the
aromatic ring increases. The fluorescence spectra of the microcrystalline samples are more strongly dependent
on substituents than those of samples in solution, because of strong charge transfer (CT) type intermolecular
interactions induced by substituents in the solid state. The solid-state fluorescence spectra of the derivatives
with electron-donating N,N-dimethylamino and methoxy groups (1 and 2, respectively) are red-shifted and
structured, probably due to the formation of ground-state aggregates. The spectrum for the thin film of 2
shows a close resemblance to that for the thin film of high molecular weight poly(1,4-phenylenevinylene)
(PPV), suggesting similar ECLs for 2 and PPV in the excited states. The spectra of the derivatives with
electron-withdrawing substituents, on the other hand, are red-shifted and structureless. The broad fluorescence
observed for the diformyl derivative (6) is assigned to excimer emission.
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