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. A family of fluorescent molecular rotors has been developed and their mechanism for emission understood. It has been observed that, although most fluorescent molecules emit from their lowest energy excited state, S1 (in accordance with Kasha's rule) BODIHY dyes do not. Furthermore, their fluorescence is enhanced through restricted rotor rotation, which suppresses internal conversion to the dark S1 state.
Nature Chemistry