Through-Space C-Br···π Halogen Interaction: Efficient Modulation of Reaction-Based Photochromism and Photoluminescence at Crystalline States for Irradiation Time-Dependent Anti-Counterfeiting

Reaction-based photo-responsive molecular systems have been used as a reliable platform for building intelligent materials, but often suffer from high inactivity in terms of conversion efficiency and reversibility when it comes to solid-state applications. Herein, a class of Spiro-conjugated diphenylindene (DPI) aggregation-induced emission materials that exhibit solid-state photocyclization-based photochromism and photoluminescence, achieved by manipulating through-space C-Br···π halogen interactions is reported. In the crystalline state, the Spiro bridge enables a pair of perpendicularly configured C-Br···π interactions that suppress photoluminescence while activating a highly reversible (>10 000 cycles) and efficient photochromism based on photocyclization. After breaking this intermolecular C-Br···π interaction, for example, by grinding, the solid-state photochromic properties are sensitively de-activated. In addition, this spontaneously reversible photochromism behavior gives a controllable decolorization time (from a few seconds to a few minutes), depending on the light exposure conditions, while displaying a luminescence change. In a proof-of-principle study, these photo-function features demonstrate an attractive potential for novel anti-counterfeiting applications. This work reveals the efficient regulation of C-Br···π halogen interactions for spatial molecular packing, molecular electronic transitions, and chemical transformations, paving the way for the development of solid-state intelligent materials with good reversibility.