Dicyanodistyrylthiophene-Based Emissive Chiral Photoswitches: Effect of the Position of the Cyano Group on Reversible Photoisomerization and Fatigue Resistance

Emissive chiral molecular switches with reversible and tunable attributes are promising materials for both fundamental studies and device applications. In this study, we have developed two novel chiral fluorescent photoswitches bearing one dicyanodistyrylthiophene as the emissive scaffold and bridged binaphthyl moieties as chiral entities. The position of the cyano group in these photoswitches 1 and 2 is varied and its effect on the photochemical properties of the light-driven chiral molecular switches is investigated. Experimental and molecular simulation studies indicate that both of these two regio-isomeric dicyanodistyrylthiophene photoswitches initially exist in (Z,Z) configuration and exhibit partial reversible Z/E photoisomerization and variation of fluorescence intensity upon blue and UV light irradiations in solution as well as in a room-temperature liquid-crystal (LC) matrix. The helical twisting power (HTP) values of the switches in inducing cholesteric LCs (Ch-LCs) depend on the position of the cyano group, being higher for switch 1 than switch 2. However, the switch 2 doped Ch-LC system shows a better photoresponsive behavior and pronounced fatigue resistance, while the Ch-LC system fabricated using switch 1 undergoes a considerable degradation in performance after several switching cycles, which might be due to photodissociation of switch 1 with the cyano group close to the thiophene ring upon light irradiation. Finally, simultaneous phototuning of both the reflection band and fluorescence intensity in a Ch-LC sample doped with 14.0 wt % switch 2 is demonstrated. These results provide a route for designing chiral fluorescent photoswitches with reversible photoisomerization characteristics.