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

Abstract

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 BF-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 S state, as predicted from Kasha's rule, but from a higher energy (>S) state. Furthermore, we found that suppression of internal conversion to the dark S 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.

Crystal Multi‐Conformational Control Through Deformable Carbon‐Sulfur Bond for Singlet‐Triplet Emissive Tuning

“…Nevertheless, regulation of emissive pathways (e.g., regarding singlet‐triplet excited state features or the transfer in between) on single materials by crystal engineering has received little attention, probably because such a excited state regulation often lacks sensitivity in a crystalline system . In contrast, it is known that emissive pathways can basically respond to diverse structural and self‐assembly factors in amorphous phases . This factor makes it desirable to develop an intrinsic strategy of enrolling molecular conformational deformability to control the emissive pathways at the crystal level.…”

Thermally Activated Delayed Fluorescence in CuI Complexes Originating from Restricted Molecular Vibrations

“…However, the mechanism of TADF in aggregated molecules is still unclear and has not been studied in detail, although it is essential for many of the desired applications of these emitters. Fundamental factors that lead to enhanced emission of dyes in viscous and solid‐state environments are of ongoing interest …”

Optoelectronic, Aggregation, and Redox Properties of Double‐Rotor Boron Difluoride Hydrazone Dyes

“…For example, derivatives of 7 undergo trans/cis isomerization triggered by visible and near‐IR light and by controlled aggregation . Derivatives of 7′ were also found to exhibit non‐classical fluorescence upon relaxation from high‐energy excited states, in violation of Kasha's rule, and undergo aggregation‐induced emission leading to PL in the solid state . The latter properties originate from restricted intramolecular motion of the N‐aryl substituent which hinders π‐stacking interactions upon aggregation and often leads to aggregation‐caused quenching…”