Multi-branch effect on aggregation-induced emission enhancement and tunable emission of triphenylamine fluorophores

We designed and synthesized three triphenylamine-based compounds (MOTPAs), namely 4-(((4-methoxyphenylimino)methyl)phenyl)-diphenylamine (SMOTPA), di(4-((4-methoxy- phenylimino)methyl)) phenylamine (DMOTPA), and tris(4-((4-methoxyphenylimino)methyl)) phenylamine (TMOTPA). Initially, MOTPAs possessed aggregation- and crystallization-induced emission enhancement (AIEE and CIEE) properties via the restrained intramolecular charge transfer (ICT) state, and by preventing π-π stacking and restricted intramolecular rotation. It was also demonstrated that multi-branch effect of triphenylamine was benefit to enhance AIEE and CIEE. The phenomenon was explained by X-ray diffraction analysis and calculations. Interestingly, the single branch fluorophores (SMOTPA) presented a tunable emission. The green emission of SMOTPA originating from the twisted intramolecular charge transfer (TICT) state in the strong polar solvent (methanol) transformed into a strong blue fluorescence assigned to the local excited (LE) state in the aggregated and crystalline states due to the restraint of the TICT state. Fluorescence imaging experiments of TMOTPA in living A549 cells proved its potential for practical applications.