[Structure: see text]. Molecular devices incorporating azobenzene units represent active components of smart systems, as they are capable of exhibiting photoregulated cooperative molecular motion. Herein, we describe the synthesis, X-ray crystal analysis, and photochemical and thermal studies of a xanthene based cyclic azobenzene dimer and its precursor. The trans-trans isomer of the azobenzene dimer upon photoirradiation transforms to the cis-cis isomer through an intermediate trans-cis isomer. The X-ray crystal structures of the trans-trans isomer (open) and the cis-cis isomer (closed) provide unambiguous proof for the hinge-like molecular motion in this class of molecules. The inferences drawn from photochemical and thermal studies shed light on the effect of varied substitution and cyclic structures on the different transitions. The lifetime of the cis-cis isomer is estimated to be 6.43 years, whereas the trans-cis isomer is short-lived (2.73 min) at 303 K. A rational explanation for the relative stability of the different isomers is derived from the isokinetic plot and theoretical calculations.

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We report the occurrence of a reentrant twist grain boundary phase, which we designate as Re- TGB(A). Microscopic observations on a nonsymmetric dimer showed the phase sequence Iso- N*- TGB(A)-Sm- A-Re- TGB(A)- TGB(C*). Here N* and Sm- A stand for the chiral nematic and smectic- A phases, TGB(A) is the twist grain boundary phase with smectic- A blocks, and TGB(C*) that with smectic- C* blocks and exhibiting features of both the smectic- C* and TGB phases. The reentrance of the TGB(A) phase is unambiguously demonstrated using x-ray diffraction, selective reflection, and optical rotation data.

The first achiral bent core banana-shaped mesogen consisting of a thermally and hydrolytically stable salicylaldimine (2-hydroxybenzylideneamine) mesogenic segment has been synthesized and evaluated for its liquid crystalline behavior. This novel compound exhibits a single switchable mesophase over 60 uC temperature range. The textural and electro-optic switching characteristics suggest that the phase could be of the antiferroelectric type.

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