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

In understanding the mechanism of aggregation-inducedemission(AIE), the multilevel ONIOM framework has been demonstrated as oneof the efficient tools that can capture the essential mechanisticinformation by choosing a single fluorophore as the quantum mechanics(QM) model and putting all surrounding molecules in the low-levelregion. Recently, the ionic styryl-pyridine salt (namely, SPH) hasbeen reported as a new class of AIEgen with a high fluorescence yield.In the SPH crystal, a pair of ionic SPH molecules are closely stackedwith each other in an antiparallel, head-to-tail pattern, thus thechoice of QM models (an individual or dimeric structure) becomes criticalin the ONIOM study. Herein we report the AIE mechanism of the ionicSPH at the QM ((TD)-CAM-B3LYP) and ONIOM(QM:MM) levels. As usual,the fluorescence quenching of SPH in tetrahydrofuran (THF) solutionis attributed to a nonradiative relaxation via the central C Cbond rotation, with a rather low barrier of 2.7 kcal/mol. In crystals,either with a monomer or dimer model, the fluorescence quenching channelis found to be restricted due to the obvious C C rotation barriers.Compared with the monomer model, the dimer model, by treating theorbital interaction of the two SPH molecules at the QM level, providessignificantly increased barriers and a red-shifted emission wavelengththat better matches the experimental value. In addition, the calculatedexciton coupling in the fluorescence emission state can be discoveredonly by a dimer model. The findings here emphasize not only the importanceof choosing a proper model in the ONIOM study of AIE but also expandingour understanding of novel AIE systems.

Automated summary

The multilevel ONIOM framework has been proven to be an efficient tool for understanding the mechanism of aggregation-induced emission (AIE). The ionic styryl-pyridine salt (SPH) has been identified as a new class of AIEgen with high fluorescence yield. Choosing the appropriate QM model (monomer or dimer) is crucial for studying the AIE mechanism of SPH. The dimer model provides increased barriers and a red-shifted emission wavelength, while also revealing the exciton coupling in the fluorescence emission state.