Tris(2-hydroxyphenyl)triazasumanene: Bowl-shaped excited-state intramolecular proton transfer (ESIPT) fluorophore coupled with aggregation-induced enhanced emission (AIEE)

Tris(2-hydroxyphenyl)triazasumanene ((A)-(+)-1), a bowl-shaped molecule, which possesses 2-(2⁰-hydroxyphenyl)pyridine moieties, was successfully synthesised. (A)-(+)-1 showed a single peak emission in CH₂Cl₂, which overlapped with the emission of the triazasumanene skeleton and diminished at a high concentration, corresponding to the formation of the excited enol form. In contrast, single crystals of (A)-(+)-1 exhibited a dual emission with a large Stokes shift, indicating the presence of the excited keto form by the excited state intramolecular proton transfer (ESIPT) process. In the solution of (A)-(+)-1 containing a mixture of a large amount of a poor solvent (hexane or MeOH) and a small amount of CH₂Cl₂, colloidal aggregates emerged with the continuous increment of emission intensity by further addition of the poor solvent, demonstrating aggregation-induced enhanced emission (AIEE). The analysis of the morphology and the structure of the aggregates using a scanning electron microscope (SEM) and powder X-ray diffraction (PXRD) revealed the well-ordered structure of the aggregates, which possessed a molecular packing pattern similar to that of an (A)-(+)-1 single crystal.