Triphenylamine-based N,O-bidentate BF₂ complexes: synthesis and photophysical properties

In this contribution, we report the design and synthesis of two N,O-bidentate BF2 dyes (TTBKI-1 and TTBKI-2), with excellent optical properties like large Stokes shifts, strong solvatofluorochromism, and bright emission in solution, aggregated and solid states. Formation of a push-pull electron system resulted from the introduction of a triphenylamine unit as an excellent donor and a NBF2O moiety as an acceptor. Meanwhile, by further extending pi-conjugation, the introduction of a thiophene unit resulted in a red-shift of absorption/emission wavelengths. Higher fluorescence quantum yields were obtained by TTBKI-1 and TTBKI-2 in nonpolar solvents. However, with an increase in the solvent polarity, a large bathochromic shift was observed in emission wavelengths but with a dramatic decrease in emission efficiency. Furthermore, as a typical aggregation-induced emission (AIE) unit, triphenylamine could reduce intermolecular pi-pi stacking in the aggregated state. TTBKI-1 and TTBKI-2 exhibited AIE properties in THF/H2O mixtures with high water fractions and showed bright solid-state emission.