Solid-state photochromic properties of indole thiosemicarbazones with aggregation-induced emission

Abstract

Four novel indole thiosemicarbazone derivatives, 1-(indole-2-methylene)-4-substituted-thiosemicarbazides, were synthesized via a one-step reaction by introducing substituents with different conjugacy (methyl, ethyl, cyclohexyl, phenyl). Different from the traditional Schiff-base photochromic compounds, these compounds in this work present dual response signals to UV light, and they exhibit not only reversible solid-state photochromic behavior but also aggregation-induced emission (AIE) characteristics. Furthermore, the structure–activity relationships were revealed by experimental studies, and their photochromism and fluorescence emission were enhanced efficiently with the increase of the conjugation. Furthermore, based on the single crystal structure, FT-IR spectra and theoretical calculations, the proposed photochromic mechanism originates from the intermolecular proton transfer between the thio–keto and thio–enol configurations, and the AIE property is closely related to the restriction of intramolecular rotation. Furthermore, the picture of anti-counterfeiting suggests that the dual responses of these compounds are extremely suitable for multiple encryptions. This work enriches the AIE system and provides a reference for constructing excellent multi-photoresponsive materials.