Crystals of 4,7-Bis(2,5-dimethyl-[1,1'-biphenyl]-4-yl)benzothiadiazole and Its Derivative with Terminal n-Hexyl Substitutes: Growth, Structure, Thermal and Absorption/Fluorescent Properties

The crystallization and absorption/fluorescent properties of linear conjugated molecules derived from 2,1,3-benzothiadiazole, specifically, 4,7-bis(2,5-dimethyl-[1,1'-biphenyl]-4-yl)benzothiadiazole (Ph-Xy-BTD) and 4,7-bis(4'-hexyl-2,5-dimethyl-[1,1'-biphenyl]-4-yl)benzothiadiazole (Hex-Ph-Xy-BTD), have been studied. The synthesis of a new derivative of Hex-Ph-Xy-BTD is described. It was found that the presence of terminal n-hexyl substituents in Hex-Ph-Xy-BTD reduces the melting temperature, increases solubility, and has a positive effect on crystallization as compared to Ph-Xy-BTD. Hex-Ph-Xy-BTD single crystals were grown from a hexane solution, and their structure was elucidated using single-crystal X-ray diffraction, confirming a monoclinic system (sp. gr. P2₁/c, Z = 4). Absorption and fluorescence spectra were obtained and analyzed for solutions in tetrahydrofuran, as well as for the Ph-Xy-BTD and Hex-Ph-Xy-BTD crystals, along with the study of the quantum yield and fluorescence lifetime.