Conformational Polymorphism of 4,7-Bis(4-(trimethylsilyl)phenyl)-2,1,3-benzothiadiazole Crystals

Derivatives of 4,7-diphenyl-2,1,3-benzothiadiazole are highly stable compounds that fluoresce efficiently both in solutions and in the crystalline state. Thanks to their wide range of remarkable optoelectronic characteristics, they can rightly be called smart materials. This paper presents the results of an investigation into the polymorphism of 4,7-bis(4-(trimethylsilyl)phenyl)-2,1,3-benzothiadiazole (TMS-P-BTD) crystals under weakly and strongly non-equilibrium crystallization conditions from the vapor phase (PVD), solutions, and melt. Using single-crystal X-ray diffraction analysis at room temperature, two new polymorphic crystal modifications have been identified: orthorhombic II (sp. gr. Pnaa, Z/Z' = 12/1.5) and triclinic III (sp. gr. P-1, Z/Z' = 8/4). It was determined that the densest polymorph III melts at 154 °C. The least dense orthorhombic polymorph II dominates under kinetic growth conditions, melts independently at 151 °C, but transforms into polymorph III upon prolonged annealing. It has been established that the previously identified monoclinic polymorph I (P2₁/c, Z/Z' = 32/8) transforms into polymorph III upon heating in the range of 75-110 °C. In the series of polymorphs I→II→III, a blue shift in the fluorescence spectrum maximum is observed: approximately 375 cm^-1 for polymorph II and ~635 cm^-1 for polymorph III relative to the position of the maximum λ_max,I = 497 nm for polymorph I. The observed spectral-fluorescence features of the TMS-P-BTD crystal polymorphic phases are consistent with the structure of the flattest molecular conformers within the crystal unit cells.