In search of mechanism of aggregation-induced emission in carbazole and triphenylamine substituted ethenes: An approach based on spin-flip time dependent density functional theory and optimally tuned range-separated hybrid functional

Aggregation-induced emission (AIE) has emerged as a groundbreaking advancement in the field of photoluminescence behavior. A thorough understanding of the AIE mechanism is essential for the rational design of innovative molecules exhibiting these exceptional properties. In this study, we report a quantum mechanical (QM) investigation through spin-flip time dependent density functional theory (SF-TDDFT) using optimally tuned range-separated hybrid (OT-RSH) functional to explore the cause of fluorescence quenching of the ethylene derivatives such as TPE-9CB, TPE-3CB, and tetraphenylethene-triphenylamine in tetrahydrofuran (THF) solution. The fluorescence enhancement in the crystalline state of an ethylene derivative TPE-9CB is studied by adopting a QM:MM based approach using the SF-TDDFT method and RSH functional. It is observed that the OT-RSH functional delivers impressive results regarding photophysical properties. The twisting of the central C=C bond has been identified as the phenomenon that quenches the photoexcited state of the ethylene derivatives in THF solution. Notably, the minimum-gap point (MGP) along the α-torsional coordinate of the S1 state in solution lies ∼0.1 eV below the Frank-Condon (FC) point for all the derivatives. Interestingly, all examined monomers exhibited nearly zero oscillator strength (f values) near the point of minimum-gap, indicating minimal radiative transitions in the solution state. Conversely, the excited state deactivation channels, which involve the twisting of the central C=C bond in the solution phase, are effectively restricted in the solid state by steric hindrance and electrostatic repulsion from the neighboring molecules. In the solid state, the MGP is 0.48 eV above the FC point, demanding high energy to prevent photoinduced rotary relaxation. This effectively blocks nonradiative deactivation pathways, resulting in a significant enhancement of the emission response. Our findings underscore the potential of AIE phenomena in advancing material design and applications.