Crystals of Diphenyl-Benzothiadiazole and Its Derivative with Terminal Trimethylsilyl Substituents: Growth from Solutions, Structure, and Fluorescence Properties

Abstract

Linear conjugated molecules consisting of benzothiadiazole (BTD) and phenyl rings are highly efficient organic luminophores. Crystals based on these compounds have great potential for use as light-emitting elements, in particular, scintillation detectors. This paper compares the peculiarities of growth, structure, and fluorescent properties of crystals based on 4,7-diphenyl-2,1,3-benzothiadiazole (P 2 -BTD) and its organosilicon derivative 4,7-bis(4-(trimethylsilyl)phenyl) BTD ((TMS-P) 2 -BTD). The conditions for the formation of centimeter-scale single crystals were found for the former, while it was possible to prepare also bulky faceted individual crystals for the latter. The structures of P 2 -BTD and (TMS-P) 2 -BTD crystals at 85 and 293 K were investigated by single-crystal X-ray diffraction. The crystal structure of P 2 -BTD has been refined (sp. gr. P1̅ , Z = 4), and for (TMS-P) 2 -BTD crystals, the structure has been solved for the first time (sp. gr. P2 1 /c, Z = 32). Experimental and theoretical investigations of the absorption-fluorescent properties of solutions and crystals of the molecules have been carried out. The luminophores are characterized by a large Stokes shift for both solutions and crystals with a high fluorescence quantum yield of 75− 98% for solutions and 50−85% for the crystals. A solvatochromic effect was observed for solutions of both luminophores: an increase in the values of the fluorescence quantum yield and the excited state lifetime were established with increasing the solvent polarity. Fluorescence properties of solutions and crystals have been analyzed using the data on crystal structure and conformation structure of the molecules as well as density functional theory calculations of their electronic structure. The results have shown that the crystal packing of P 2 -BTD molecules exhibits uniformity in conformational states, while (TMS-P) 2 -BTD molecules display a variety of conformational structures in the crystals. This unique combination of features makes them a remarkable example among the other molecular systems for identifying the relationship between the structure and absorption-fluorescence properties through comparative analysis.