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.