A family of borondifluorohydrazone (BODIHY) dyes have been characterized as fluorescent molecular rotors with potential applications such as OLEDs or biosensors. Understanding the aggregation induced emission (AIE) of the dyes was greatly enhanced through time-dependent density functional theory (TDDFT) studies. This computational investigation led to the proposal of a novel mechanism responsible for the dyes’ fluorescent emission: suppression of Kasha’s rule (SOKR). The SOKR mechanism describes the AIE of the BODIHY dyes using three electronic states: ground, emissive, and dark. Circumventing radiation from the dark S1 state in the solid state or highly viscous solution results in enhanced emission from a higher energy emissive excited state. A suite of spectroscopic experiments was employed to validate the SOKR mechanism. We propose that the SOKR mechanism is not only responsible for the emission of other hydrazone-based rotors, but also for the enhanced emission of fluorophores outside the aforementioned BODIHY family. In order to identify other fluorophores within the BODIHY family that follow the SOKR mechanism, the photophysical properties of several hydrazones were predicted in silico and attractive synthetic targets were identified. Secondly, a diverse set of fluorophores have been documented in the literature as having “anomalous emission”: the emissive excited state is higher in energy compared to S1. This attribute satisfies one of the requirements of SOKR. Thus, selected fluorophores have been screened to determine if their “anomalous emission” results in AIE due to the SOKR mechanism.

Novel mechanism of molecular rotor fluorescence: Suppression of Kasha’s rule

253rd National Meeting (2017)

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