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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.

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 (T m = 305.5°C, ΔH m = 52.2 kJ mol−1) and the presence of a liquid-crystalline mesophase (T LC = 336.3°C, ΔH LC = 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.

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Studies of the growth of large-area free-standing single-crystalline films of p-quinquephenyl are presented. High-quality crystals were grown by slow cooling of a hot chlorobenzene solution. Worse quality large-area free-standing single crystals of p-quinquephenyl were also grown by using physical vapor transport and used for comparison. The crystal structure of p-quinquephenyl at 293 K and 85 K was refined by single-crystal X-ray diffraction. The optical absorption and photoluminescence spectra of solutions and crystalline films were obtained and analyzed; a positive solvatochromic effect was detected.

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