Synthesis and optical properties of dithieno-1,4-thiaborins bearing electron-donating amino groups

Dibenzo-1,4-heteraborins and their pi-extended derivatives have been paid increased attention to because of their potential as optical and electronic functional materials such as organic electroluminescence emitters. However, 1,4-heteraborin compounds with heteroaromatic ring systems remain scarce, even though Liu et al. recently reported the synthesis, properties, and functionalization of dithieno-1,4-thiaborins (DTTBs). In this work, DTTBs with 10H-phenothiazine-10-yl (Pz), 9H-carbazol-9-yl (Cz), and p-(Ph2N)C6H4 groups as electron-donating amino groups at the alpha-positions of their thiophene moieties were synthesized using palladium-catalyzed coupling reactions. X-ray crystallographic analysis revealed the molecular structure of a DTTB substituted with Pz groups. This analysis showed that the 2 Pz moieties adopted extra (quasi-axial) conformations. The Cz- and p-(Ph2N)C6H4-substituted DTTBs showed red-shifted absorption and emission when compared to the original DTTBs. This shift is due to donor-acceptor interactions between the amino groups and the DTTB cores. The Pz-substituted DTTB exhibited dual fluorescence emissions, originating from the locally excited (LE) and twisted intramolecular charge transfer (TICT) states. The intensity and nature of these emissions varied based on solvent polarity, temperature, and viscosity, suggesting the potential of the Pz-substituted DTTB to act as a fluorescent environment sensor. Dithieno-1,4-thiaborins (DTTBs) bearing electron-donating amino groups at the alpha-positions of their thiophene moieties were synthesized, and their optical properties were investigated. Phenothiazine (Pz)-substituted DTTB exhibited dual fluorescence emission behavior, which may stem from the twisted intramolecular charge transfer excited states. The dual emissions responded to the polarity, temperature, and viscosity of the solvents.