Suppression of Kasha's rule as a mechanism for fluorescent molecular rotors and aggregation-induced emission

Although there are some proposed explanations for aggregation-induced emission, a phenomenon with applications that range from biosensors to organic light-emitting diodes, current understanding of the quantum-mechanical origin of this photophysical behaviour is limited. To address this issue, we assessed the emission properties of a series of BF₂-hydrazone-based dyes as a function of solvent viscosity. These molecules turned out to be highly efficient fluorescent molecular rotors. This property, in addition to them being aggregation-induced emission luminogens, enabled us to probe deeper into their emission mechanism. Time-dependent density functional theory calculations and experimental results showed that the emission is not from the S₁ state, as predicted from Kasha's rule, but from a higher energy (>S₁) state. Furthermore, we found that suppression of internal conversion to the dark S₁ state by restricting the rotor rotation enhances fluorescence, which leads to the proposal that suppression of Kasha's rule is the photophysical mechanism responsible for emission in both viscous solution and the solid state.