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

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.