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

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.

The crystal structure of N′-(2-chloro-6-hydroxybenzylidene)thiophene-2-carbohydrazide monohydrate, C12H11ClN2O3S

C12H11ClN2O3S, monoclinic, P21/n (no. 14), a = 4.7305(8) Å, b = 21.252(4) Å, c = 13.134(2) Å, β = 96.286(2)°, Z = 4, V = 1312.5(4) Å3, R gt (F) = 0.1002, wR ref (F 2) = 0.1118, T = 296 K.

UK, P. P. N, Chandrasekharan K. Effects of substituents on the enrichment of the optical limiting action of novel imidazo[2,1-b][1,3,4]thiadiazole fused thiophene-based small molecules

Schematic of the optical limiting action of a novel imidazo[2,1-b][1,3,4]thiadiazole based small molecule (ThITD3), which blocks high irradiance and transmits low-intensity (less harmful) light.

Aggregation induced emission by pyridinium–pyridinium interactions

Non-covalent intermolecular interactions between pyridinium subunits in a crystal-state are an efficient means to accomplish aggregation induced emission and avoid aggregation caused quenching.

Non-covalent intermolecular pyridinium–pyridinium and pyridinium–arene-π system interactions result in aggregation induced emission (AIE).