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

TL;DR

This study investigates the origin of aggregation-induced enhanced emission (AIEE) using computational methods, revealing that pseudo-degenerate excimer states and suppressed internal conversion in solid phases are key factors.

Contribution

It uncovers the role of pseudo-degenerate excimer states in AIEE and proposes a design principle based on pseudo-degenerate electronic states for developing AIEE-active molecules.

Findings

Pseudo-degeneracy of excimers is crucial for AIEE.

Solid phase suppresses internal conversion via small vibronic coupling.

Pseudo-degeneracy allows fluorescence from high singlet states, defying Kasha's rule.

Abstract

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.