Preorganization and Condensed State‐Dependent Photoisomerization and Varied Photochemical Reactions of Single Molecular Constrained Bis( α ‐Cyanostilbene) Liquid Crystalline Derivatives

Cyanostilbene derivatives constitute a variety of versatile molecular photoswitches and organic photofunctional materials, while prediction or control over their photochemical outcome represents yet a huge challenge. Here, in this work, two well‐preorganized single molecular constrained bis‐ α ‐cyanostilbene derivatives 1,8‐N‐2CNS‐C10 [1,8‐naphthalene bis(decyloxy‐ α ‐cyanostilbene)] and 1,5‐N‐2CNS‐C10 have been synthesized and fully characterized. Both 1,8‐N‐2CNS‐C10 and 1,5‐N‐2CNS‐C10 showed enantiotropic nematic liquid crystal (LC) mesophase and crystal polymorphism; more importantly, they exhibited distinct photophysical properties, especially the preorganization and condensed states‐dependent varied photochemical reactions. Specifically, 1,8‐N‐2CNS‐C10 in its U‐shaped configuration in solution or molecular aggregates, the reversible intra molecular [2 + 2] photocycloaddition predominates, accompanied by characteristic fluorescence color and intensity modulation, which is promising for fluorescent photoswitch application. In the bulk nematic LC mesophase, mainly photopolymerization occurs through the inter molecular [2 + 2] cycloaddition reaction (irreversible); while in the crystalline solid state, the reaction efficiency is extremely low, in sharp contrast to the reported common nonliquid crystalline α ‐cyanostilbene derivatives, whereas for 1,5‐N‐2CNS‐C10 in its extended but somewhat twisted stretching S‐shaped configuration, in solution or aggregates, after a prerequisite Z / E photoisomerization, mainly photocyclization reactions occur. These findings provide valuable insights into the design of α ‐cyanostilbene‐based multifunctional photoresponsive materials and offer an in‐depth understanding of the underlying mechanisms of their varied photochemical reactions.