Crystals of isomers of thiophene-phenyl-benzothiadiazole oligomers with terminal trimethylsilyl substituents: Growth from solutions, structure and fluorescent properties

• Published in: Optical materials 2025-03, Vol.160, p.116764, Article 116764
• Main Authors: Postnikov, Valery A., Sorokina, Natalia I., Yurasik, Georgy A., Kulishov, Artem A., Sorokin, Timofei A., Lyasnikova, Maria S., Borshchev, Oleg V., Skorotetsky, Maxim S., Svidchenko, Evgeniya A., Surin, Nikolay M., Ponomarenko, Sergey A.
• Format: Article
• Language: English
• Subjects: Benzothiadiazole; Crystal growth from solution; Fluorescence; Large Stokes shift; Single-crystal X-ray diffraction; Solvatochromism; Solvatocrystals; Thiophene-phenylene oligomers
• ISSN: 0925-3467
• DOI: 10.1016/j.optmat.2025.116764

The work presents the results of investigation of the phase behavior, crystal growth from solutions, structure, and absorption-fluorescent properties of the crystals of 4,7-bis(5-(4-(trimethylsilyl)phenyl)thien-2-yl)benzothiadiazole (TMS-P-T-BTD) and its isomer 4,7-bis(4-(5-(trimethylsilyl)thien-2-yl)phenyl)benzothiadiazole (TMS-T-P-BTD). The structure of the toluene-grown single crystals of both TMS-P-T-BTD and TMS-T-P-BTD was investigated using single-crystal X-ray diffraction at temperatures of 295 and 90 K. In the crystal unit cell of TMS-P-T-BTD, which is triclinic (space group P1, Z = 1), statistical disorder in the arrangement of molecules was established, resulting in the emergence of a so-called “average” molecule with dual (50 %) orientation of the central 2,1,3-benzothiadiazole fragment., The molecules of TMS-P-T-BTD in the crystal exhibit an almost planar conformation. It was found that crystals of the isomer TMS-T-P-BTD of triclinic modification (space group P-1, Z = 2) are solvated: in the unit cell there is one solvent molecule - toluene - per two molecules of the basic compound. In the asymmetric block of the unit cell of the solvated crystals of TMS-T-P-BTD, there are two molecules with different conformations. Studies of the absorption-fluorescent properties of TMS-P-T-BTD and TMS-T-P-BTD molecules in the crystals showed that swapping the thiophene and phenyl fragments significantly alters the energy and probability of electronic transitions, as well as the relative position of nπ∗ and ππ∗ levels. Both the probability of singlet - singlet transitions and the probability of singlet - triplet conversion change. The splitting of nπ∗ levels, caused by the arrangement of thiophene fragments in the TMS-P-T-BTD molecule, leads to a strong dependence of the fluorescent (kr) and phosphorescent (knr) characteristics of the isomer on the immediate environment. In the TMS-P-T-BTD crystal, this results in the complete suppression of fluorescence. [Display omitted] •Single crystals of 4,7-bis(5-(4-(trimethylsilyl)phenyl)thien-2-yl)benzothiadiazole (TMS-P-T-BTD) and its isomer 4,7-bis(4-(5-(trimethylsilyl)thien-2-yl)phenyl)benzothiadiazole (TMS-T-P-BTD) were obtained from the solution.•In the TMS-P-T-BTD crystal, statistical disorder in the arrangement of molecules is established. The TMS-T-P-BTD crystals are solvated.•The quantum yield of both compounds in solution reaches 90 %, while in the crystals it decreases to 47% for TMS-T-P-BTD and to 2 % for T