Photo-orientation and Electron Paramagnetic Resonance Spectra of a Nitroxide and Azobenzene-Containing Hydrogen-Bonded Complex

Abstract

Photochromism and light-induced orientation of a hydrogen-bonded paramagnetic mesogenic complex were investigated. The azopyridine chromophore included in the chemical structure of the complex is capable of reversible trans-to-cis photoisomerization with the lifetime of the cis-isomer being on the order of seconds. The action of polarized light on the solid film of the complex results in its in-plane photo-orientation, wherein the azopyridine chromophores become preferentially aligned perpendicular to the electric vector of the incident light. The photoorientation kinetics strongly depends on the morphological properties and thermal history of the film and has an unusual Sshaped form due to the topochemical nature of the process (Avrami kinetics). The observed peculiarities of the photoinduced alignment of the complex corroborate the mechanism of photoorientation by means of microscopic structure rearrangement. The polarized optical absorbance and electron paramagnetic resonance spectra of the photoaligned film of the material prove that the sample is characterized by macroscopic biaxiality due to the presence of two competing orienting forces, namely, the polarized light and the orienting influence of the surface. Electron paramagnetic resonance spectra of the complex in aligned crystalline and nematic samples are interpreted in terms of the macroscopic order of the material and intermolecular spin−spin interactions of the nitroxyl fragments.

Photoinduced Split of the Cavity Mode in Photonic Crystals Based on Porous Silicon Filled with Photochromic Azobenzene-Containing Substances

“…The lowest values obtained for the composite with smectic polymer P2 are explained by an influence of ordered smectic phase strongly preventing rotational diffusion of the chromophores. A similar effect was observed repeatedly in previous studies showing crucial effect of smectic phase formation on photo-orientation.…”

“…Another efficient way of phototuning the structure and optical properties of photochromic glass-forming substances is the so-called Weigert effect representing photo-orientation of the azobenzene chromophores under polarized-light action. − Photoinduced repetitive cycles of E–Z–E isomerization accompanied by rotational diffusion of chromophores align the chromophores in the direction orthogonal to the polarization of incident excitation light (Figure ). In amorphous polymers or glass-forming low-molar-mass substances this effect enables to obtain high values of dichroism or birefringence. − In polydomain LC samples, uniaxial alignment of mesogenic groups could be achieved (although an ordered LC state in some cases prevents the photo-orientation process, especially for the smectic phase).…”