Theoretical Insights into Aggregation-Induced Emission with the Ionic π Fluorophore: The Importance of Choosing the Dimer QM Model in the ONIOM Study

Abstract

In understanding the mechanism of aggregation-induced emission (AIE), the multilevel ONIOM framework has been demonstrated as one of the efficient tools that can capture the essential mechanistic information by choosing a single fluorophore as the quantum mechanics (QM) model and putting all surrounding molecules in the low-level region. Recently, the ionic styryl-pyridine salt (namely, SPH) has been reported as a new class of AIEgen with a high fluorescence yield. In the SPH crystal, a pair of ionic SPH molecules are closely stacked with each other in an antiparallel, head-to-tail pattern, thus the choice of QM models (an individual or dimeric structure) becomes critical in the ONIOM study. Herein we report the AIE mechanism of the ionic SPH at the QM ((TD)-CAM-B3LYP) and ONIOM(QM:MM) levels. As usual, the fluorescence quenching of SPH in tetrahydrofuran (THF) solution is attributed to a nonradiative relaxation via the central CC bond rotation, with a rather low barrier of 2.7 kcal/mol. In crystals, either with a monomer or dimer model, the fluorescence quenching channel is found to be restricted due to the obvious CC rotation barriers. Compared with the monomer model, the dimer model, by treating the orbital interaction of the two SPH molecules at the QM level, provides significantly increased barriers and a red-shifted emission wavelength that better matches the experimental value. In addition, the calculated exciton coupling in the fluorescence emission state can be discovered only by a dimer model. The findings here emphasize not only the importance of choosing a proper model in the ONIOM study of AIE but also expanding our understanding of novel AIE systems.

Photoluminescent organic crystals and co-crystals

“…58 Computational studies further evaluated this phenomenon employing models such as the Duschinsky rotation effect, reorganisation energy, conical insertion to include excited-state effects, based on which in 2013 Blancafort introduced the restricted access to conical intersection (RACI) model. [59][60][61][62][63][64][65][66][67][68] Conical intersections are regions in the potential energy surface (PES) where ground and excited state energies result in non-radiative internal conversions. The RACI model can explain the emission of boranes, ESIPT and conjugated molecules.…”