Reversible Specific Vapoluminescence Behavior in Pure Organic Crystals through Hydrogen‐Bonding Docking Strategy

Abstract The design of vapoluminescence materials with high contrast, selectivity, and reversibility toward certain vapors is extremely attractive but remains a long‐standing challenge. In this work, crystallization of squaraine dye (SQ) molecules in different solvents affords two organic crystals (R‐SQ and SQ‐2TFA) with the fluorescence quantum yield up to 0.81, which shows reversible phase transition of the crystals with different emission color via annealing treatment and solvent‐vapor adsorption. From spectral and powder X‐ray diffraction analysis, exposure of green‐emitting R‐SQ crystals to trifluoroacetic acid (TFA) vapor gives cyan‐emitting SQ‐2TFA crystals, while desorption of SQ‐2TFA crystals by heating recovers R‐SQ crystals. With a comparison of their crystal structures, this reversible luminescence alternation is ascribed to the crystal rearrangements with formation and collapse of hydrogen‐bonded organic frameworks between SQ and TFA molecules, thus leads to a docking strategy for specific detection of TFA vapor. Meanwhile, various emission behaviors such as aggregation‐induced emission, crystallization‐induced emission enhancement, and mechanochromism of the crystals are verified. The research not only inaugurates a novel strategy for detection of specific certain vapor in pure organic crystals but also demonstrates a pair of excellent crystalline‐to‐crystalline model for deeply understanding various organic emission behaviors.