Origin of Aggregation-Induced Enhanced Emission: A Role of Pseudo-Degenerate Electronic States of Excimers Formed in Aggregation Phases

Origin of aggregation-induced enhanced emission (AIEE) is investigated taking cyano-substituted 1,2-bis(pyridylphenyl)ethene (CNBE) as an example. On the basis of ONIOM calculations using the time-dependent density functional theory (TD-DFT), we found that pseudo-degeneracy of excimers formed in solid phase plays an important role in the appearance of the AIEE. The electron density difference delocalized over molecules gives the small diagonal vibronic coupling constants (VCCs), which suggests that the internal conversion is more suppressed in solid phase than in solution phase. The decrease of the off-diagonal VCCs by the packing effect is visualized by the vibronic coupling density (VCD) analysis. The pseudo-degeneracy enables the fluorescence from the high singlet excited states against the Kasha's rule because the electron density difference and the overlap density between excited states vanish. A Hubbard model of a pseudo-degenerate electronic system is constructed to elucidate the vanishing mechanism. We propose the following design principle for AIEE: a candidate molecule for AIEE should have pseudo-degenerate adiabatic electronic states because of the excimer formation in aggregation states.