Photo-induced molecular alignment of trisazobenzene derivatives

Abstract

Optically active small-molecular trisazobenzene derivatives are explored that allow facile photo-induced fabrication of holographic volume gratings which are unusually stable over time compared to structures based on other small-molecular organic compounds. The origin of this favorable characteristic of such architectures is investigated with three compounds that structurally differ only in the length of an alkyl spacer positioned between the molecular core and the active azobenzene chromophores. Species comprising spacers of sufficient length, and that exhibit a latent liquid-crystalline phase, feature efficient formation of stable, ordered domains in which the three side groups orient perpendicular to the polarization of the inscribing light beam. It is demonstrated that molecular order in these domains can be significantly improved by annealing at temperatures between the glass transition temperature and the clearing point of the specific compounds. This phenomenon allows combining the highly desirable processing characteristic of relatively short holographic writing times with small molecules in the fabrication of stable volume gratings, the latter feature so far having been reserved predominantly for polymeric species.