Synthesis, Structures, and Properties of Triskelion-Shaped Fluorophores Bearing Coumarins with Nitrogen-Containing Donor Groups

Triskelion-shaped π-fluorophores bearing coumarins with nitrogen-containing donor groups (1a, 1b, and 1c) were successfully synthesized via intramolecular Ullmann coupling. X-ray crystal structure analysis revealed that 1a and 1b adopt curved propeller-shaped structures similar to that of parent compound 1-H. A theoretical study suggested that compounds 1a–1c possess two different molecular surfaces, as do Janus-type molecules. The emission spectra of 1a–1c in toluene showed broad emission bands at 570, 599, and 600 nm with a large Stokes shift at 4430–5930 cm−1. Although compounds 1a and 1b exhibited more intense fluorescence emission compared with that of parent compound, the emission intensity decreases in high-polarity solvents due to broken symmetry resulting from the twisting of the donor groups. Conversely, compound 1c showed weak emission in all tested solvents. These curved and triskelion-shaped fluorophores were found to form nanoaggregates in THF/H2O mixtures and demonstrated outstanding aggregation-induced emission enhancement (AIEE) properties.