High-Temperature Reversible Martensitic Transition in an Excited-State Intramolecular Proton Transfer Fluorophore

Reversible martensitic transitions in molecular crystals are an unusual phenomenon, and most of the cases reported in the literature occur at relatively low temperatures. The title compound, 2-(5-(benzo[d]thiazol-2-yl)-1,3,4-oxadiazol-2-yl)-5-(diethylamino)phenol (DBOX), reveals an interesting and rich polymorphism, characterized by a reversible single-crystal-to-single-crystal transition occurring at 185 °C. The transition can be repeated several times on a single crystal without degradation and/or damage. The crystal structure of this high-temperature phase (II) and two low-temperature polymorphs (I and III) are solved by single-crystal X-ray analysis to fully elucidate the mechanism of the reversible I → II transition. Changes in the molecular packing are preceded by anomalous thermal expansion and evolve as slipping/rotation motions of molecular dimers, activated by steric disengagement along the anomalous expansion direction and resulting in the reversible contraction of the long crystal axis and lateral expansion. Uncommon panchromatic- and white-emitting fluorescence properties of the reported compound are also described.