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Origin of aggregation-induced enhanced emission (AIEE) is investigatedconsidering cyano-substituted 1,2-bis(pyridylphenyl)ethene (CNPPE) as anexample. On the basis of ONIOM calculations using the time-dependent densityfunctional theory (TD-DFT), we found that pseudo-degeneracy of excimers formedin solid phase plays an important role in the appearance of AIEE. The electrondensity difference delocalized over molecules gives rise to small diagonalvibronic coupling constants (VCCs), which suggests that the internal conversionis more suppressed in solid phase than in solution phase. The reduction of theoff-diagonal VCCs owing to the packing effect is elucidated by vibroniccoupling density (VCD) analysis. The pseudo-degeneracy enables fluorescencefrom the high singlet excited states against Kasha's rule because the electrondensity difference and the overlap density between the excited states vanish. AHubbard model of a pseudo-degenerate electronic system is constructed toexplain the vanishing mechanism. We propose the following design principle forAIEE: a candidate molecule for AIEE should have pseudo-degenerate adiabaticelectronic states in the aggregation phases originating from the excimerformation.

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