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

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 (P2-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 P2-BTD and (TMS-P)2-BTD crystals at 85 and 293 K were investigated by single-crystal X-ray diffraction. The crystal structure of P2-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. P21/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 P2-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.

A new linear phenyloxazole-benzothiadiazole luminophore: crystal growth, structure and fluorescence properties.

A new linear luminophore consisting of five conjugated units of oxazole, phenylene and a central benzothiadiazole fragment, 4,7-bis[4-(1,3-oxazol-5-yl)phenyl]-2,1,3-benzothiadiazole, has been synthesized and characterized. Needle-like single-crystal samples up to 10 mm in length were obtained by physical vapor transport. The crystal structure was determined at 95 K and 293 K using single-crystal X-ray diffraction. With decreasing temperature, the space group P21/n does not change, but the unit-cell volume of the crystal decreases. The presence of intra- and intermolecular hydrogen bonds was established. Melting parameters (Tm = 305.5°C, ΔHm = 52.2 kJ mol-1) and the presence of a liquid-crystalline mesophase (TLC = 336.3°C, ΔHLC = 1.4 kJ mol-1) were determined by differential scanning calorimetry and in situ thermal polarization optical microscopy studies. The presence of linear chains of hydrogen bonds ensures high stability of the crystal structure in a wide temperature range. The luminophore is characterized by a large Stokes shift (5120-5670 cm-1) and a high quantum yield of fluorescence, reaching 96% in solutions (λmax = 517 nm) and 27% in thin crystalline films (λmax = 529 nm). The calculated absorption and emission spectra are in good agreement with the experimental data. Because of the excellent optical properties and high thermal stability, the new linear luminophore has great potential for application in organic photonics and optoelectronic devices.

Highly luminescent crystals of a novel linear π-conjugated thiophene-phenylene co-oligomer with a benzothiadiazole fragment.

The synthesis, growth from solutions and structure of crystals of a new linear thiophene-phenylene co-oligomer with a central benzothiadiazole fragment with a conjugated core, (TMS-2T-Ph)2-BTD, are presented. Single-crystal samples in the form of needles with a length of up to 7 mm were grown and their crystal structure was determined at 85 K and 293 K using single-crystal X-ray diffraction. The conformational differences between the crystal structures are insignificant. The parameters of melting and liquid crystalline phase transitions of (TMS-2T-Ph)2-BTD were established using differential scanning calorimetry and the thermal stability of the crystals was investigated using thermogravimetric analysis. The optical absorption and photoluminescence spectra of the solutions and crystals of (TMS-2T-Ph)2-BTD were obtained, and the kinetics of their photodegradation under the action of UV radiation were studied.