Ketone-enol tautomerism, polymorphism, mechanofluorochromism and solid-state acidochromism of isoquinolinone-arylidenehydrazine derivatives

Four isoquinolinone-arylidenehydrazine derivatives with different aryl groups, such as benzene (BHIQ), naphthalene (NHIQ), anthracene (AHIQ), and triphenylamine (TPHIQ), have been designed and synthesized. These compounds exhibit solid-state fluorescence with emission wavelengths covering the whole visible-light region. Most of them have two crystalline structures with different fluorescence colours. Interestingly, although all the compounds adopt a keto-form structure in solvents, keto- and enol-form crystalline structures are obtained separately forBHIQ, while the two polymorphs ofNHIQandAHIQexist in the keto-form and the enol-form, respectively. Moreover, the difference in the emissions of theNHIQpolymorphs is attributed to different stacking arrangements, while that of theAHIQpolymorphs is ascribed to different molecular conformations. These results indicate that crystalline structures with different emissions can be obtainedviapolymorphism and keto-enol tautomerism, respectively. Furthermore, some crystalline structures of these compounds exhibit obvious mechanofluorochromic activities due to the crystalline-to-amorphous transition and solid-state acidochromic properties owing to the change in intramolecular charge transfer caused by protonation. This work provides a strategy for the development of solid-state fluorescent stimulus-responsive materials with keto-enol tautomerism.