One-Way Cyclic Transformation of Three Forms of Biphenyl-Hinged Schiff Base Crystals: Proposal of a New Concept “Kyklotropic” Polymorphism

A biphenyl-hinged Schiff base compound exhibited three polymorphs with contrasting photophysical properties, i.e., yellow-emissive Form I, red-emissive Form II, and nonemissive yet photochromic Form III. Every two of the polymorphs interconverted only irreversibly under a single stimulus: heating drove the monotropic transition of I → III, supercooling-cold crystallization afforded conversion of III → II, and mechanical grinding induced the II → I change. Linking these one-way steps under distinct external stimuli achieved a closed polymorphic loop, which we tentatively name it as “kyklotropic” transformation, enabling pseudoreversible multistate switching among all three forms.