Minor alkyl modifications for manipulating the fluorescence and photomechanical properties in molecular crystals

Abstract

Responsive fluorescent solid-state materials, especially light-driven mechanical motions, have received interest owing to the potential applications in advanced smart reponsive devices. However, control of the dynamic photoresponsive behavior and static fluorescence of molecular crystals based on the same molecule remains a great challenge. Herein, we present a concise molecular design by minor modification of the alkyl groups of cyanostilbene derivatives (CS), allowing control of the solid-state fluorescence and photomechanical behaviours in the crystal state. The CSEt crystal with shorter alkyl groups exhibits negligible fluorescence (Φ_F = 0.2%) due to the strong intermolecular coupling. The high [2+2] photocycloaddition reactivity of the CSEt microcrystal leads to the light-driven crystal bending and cracking. Meanwhile, the fluorescence of the CSEt crystal increased with UV irradiation and force grinding, exhibiting dual stimuli-responsive fluorescence “turn-on”. For propyl-modified CSPr, the intermolecular coupling is weakened by the longer alkyl groups, achieving high fluorescence quantum yields (Φ_F = 84%) and excellent photostability to UV light. Furthermore, the CSEt crystal exhibits dual stimuli-responsive fluorescence “turn-on” to UV light and force grinding. This work provides new insight for controlling the optical properties and investigating the effects of the molecular stacking pattern.