Investigation of Aggregation Induced Emission Mechanism of Tetrabenzoheptafulvalene Derivative by Spin‐Flip Time‐Dependent Density Functional Theory (SF‐TDDFT)

This study explores the mechanism of aggregation-induced emission (AIE) in the tetrabenzoheptafulvalene derivative, 10,10',11,11'-tetrahydro-5,5'-bidibenzo[a,d][7]annulenylidene (abbreviated as THBDBA) in tetrahydrofuran (THF) solution. THBDBA is AIE-active because in THF solution, it emits significantly less emission (or almost non-emissive) and the fluorescence quantum yield increases by 230 times in aggregate state. We adopted spin-flip time-dependent density functional theory (SF-TDDFT), widely acknowledged for its ability to locate the conical intersection (CI) in medium to large-sized molecules (due to its balanced and reliable description of both ground and excited states and ability to capture double excitation and multireference characters at low computational cost). The functional used is long-range corrected ωPBEh (i. e., LRC-ωPBEh). The strategies used are the excited state deactivation processes by taking into account the S₁/S₀ surface crossing, referred to as the 'minimum energy conical intersection' (MECI). Reduction of oscillator strength near the minimum energy gap (MEG) structure or CI is also another parameter used to study fluorescence quenching. For the monomer (i. e., in solution), our findings reveal a significant reduction in oscillator strength (f) for de-excitation near the MEG structure and CI, which led us to conclude that in solution, the flapping motion of the phenyl rings plays a vital role to reach the CI. In a smaller scale, a dimer system was chosen to represent the aggregate state. The higher energy gap as well as higher f-value at MEG structure with just the model dimer system indicates that in the actual aggregate (or the crystal) the MECI might be absent. This is because in the aggregate the flapping motion of the phenyl rings will be highly restricted (because of the steric and electrostatic confinements by a large number of monomers from all sides), thereby favoring radiative transitions for energy dissipation. This study consequently elucidates the AIE mechanism of the chosen tetrabenzoheptafulvalene derivative, shedding light on its photophysical properties.