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.
Triskelion-shaped π-luminophores bearing three coumarin units 1 a and 1 b were successfully synthesized by the intramolecular Ullmann coupling reaction of triphenyl benzene-1,3,5-tricarboxylate derivatives. X-ray crystal structure analysis revealed that compound 1 adopts a unique curved and three-blade propeller-shaped structure. These crystals are a racemic mixture consisting of PPP-and MMM-1 due to the helicity resulting from the steric hindrance. A computational study suggests that this helical inversion process between PPP-and MMM-1 a occurs with comparative ease. The emission spectra of 1 a and 1 b in THF solution showed sharp emission bands at 454 nm and 492 nm, respectively. The Stokes shifts were relatively large at 7385 cm À 1 and 6556 cm À 1 due to their degenerate frontier molecular orbitals. A solution of 1 b in different organic solvents demonstrated solvatofluorochromism due to intramolecular charge transfer of the coumarin unit in the excited states. Interestingly, both triskelion-shaped molecules formed aggregates in a mixture of THF-H 2 O and exhibited aggregationinduced emission enhancement characteristics.
Janus-type triskelion-shaped fluorophores comprising coumarins bearing various electron-donating substituents (1aad, 1add, 1ccd, and 1cdd) were successfully synthesized via an intramolecular Ullmann coupling. Density functional theory (DFT) calculations indicated that all the compounds presented two different molecular surfaces, similar to Janus-type molecules. The absorption and fluorescence spectra of asymmetrical derivatives 1aad, 1add, 1ccd, and 1cdd exhibited a bathochromic shift due to their narrow highest occupied molecular orbital (HOMO) –lowest unoccupied molecular orbital (LUMO) gap. Natural transition orbital (NTO) analysis indicated that the excited state orbital overlaps differ among the C3 symmetrical and asymmetrical dyes. These triskelion-shaped fluorophores were found to form molecular nanoaggregates in THF/H2O mixtures and demonstrated aggregation-induced emission (AIE) enhancement characteristics as a result of restricting their molecular inversion. These results indicate that Janus-type AIE fluorophores are potentially applicable as solid-state fluorescent chiral materials, which can be optimized by controlling their molecular rearrangement in the solid state.
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