Bent-core liquid crystalline cyanostilbenes: fluorescence switching and thermochromism

Abstract

Fluorescent bent-core molecules, bearing one or two cyanostilbene units in the lateral structure and different positions of the cyano group (α- or β-isomers), are described with the aim of modulating the molecular packing and fluorescence properties. These compounds give rise to a variety of crystal polymorphs and bent-core liquid crystalline phases (SmCP, Col_r and B₆), offering the unique chance to study the fluorescence properties of the cyanostilbene structure in different phases. Experimental and computational studies elucidate geometrical and electronic properties of these bent-core structures but especially the fluorescence properties (spectral positions, quantum yields and decay curves), in a detailed comparison between diluted solutions, in dichloromethane (DCM) or poly(methylmethacrylate) (PMMA), and condensed phases. Quantum yields as high as 70% have been obtained in some diluted solutions (PMMA) and condensed phases. Remarkably, the quantum yield values depend on the position of the cyano group, being higher for β- than for the α-isomers due to the higher radiative rates and lower non-radiative rates of the former. The photophysical characterization in the condensed phase focuses on RT studies with solid samples and different processing, and show that, upon aggregation, interactions between the cyanostilbene groups result in changes of the emission spectra and dynamics compared to the diluted systems in DCM and PMMA, giving rise to H-aggregations of varying strength. Furthermore, the compounds exhibit thermochromism, showing a green–yellow fluorescence in the pristine crystalline phase that changes to blue on heating to the liquid crystal phase.