Conformation‐Dependent Luminescence and Thermal Phase Transition of Crystals of a Tetrahydropentalene Derivative Bearing Aromatic Rings with Frustrated Free Rotation

Abstract Results of a study exploring the solid‐state photoluminescence properties of two crystalline polymorphs ( O ‐form (orange prisms) and R ‐form (red prisms)) of 1,4‐bis(dicyanomethylidene)‐2,2,5,5‐tetramethyl‐3,6‐bis(4‐methoxyphenyl)‐1,2,4,5‐tetrahydropentalene ( BTP‐An ) are reported. The O ‐form was found to display more quantum‐efficient photoluminescence in a shorter‐wavelength region compared to that of the R ‐form . X‐ray crystallographic analysis suggests that a unique conformation with largely rotated 4‐methoxyphenyl (anisyl) groups and suppression of free rotation of the dicyanomethylene groups are likely responsible for the shorter wavelength and higher quantum yield for photoluminescence from the O ‐form . Also, the thermally metastable O ‐form undergoes a phase transition to generate the R ‐form upon either heating or exposure to organic solvent vapors.