  Origin of aggregation-induced enhanced emission (AIEE) is investigated
considering cyano-substituted 1,2-bis(pyridylphenyl)ethene (CNPPE) 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 AIEE. The electron
density difference delocalized over molecules gives rise to small diagonal
vibronic coupling constants (VCCs), which suggests that the internal conversion
is more suppressed in solid phase than in solution phase. The reduction of the
off-diagonal VCCs owing to the packing effect is elucidated by vibronic
coupling density (VCD) analysis. The pseudo-degeneracy enables fluorescence
from the high singlet excited states against Kasha's rule because the electron
density difference and the overlap density between the excited states vanish. A
Hubbard model of a pseudo-degenerate electronic system is constructed to
explain the vanishing mechanism. We propose the following design principle for
AIEE: a candidate molecule for AIEE should have pseudo-degenerate adiabatic
electronic states in the aggregation phases originating from the excimer
formation.
