Polarization-Induced Inversion of Singlet and Triplet Excited States: Towards New Photocatalytic and Optoelectronic Materials

The phenomenon of inversion between singlet and triplet excited states (INVEST), which runs counter to Hund's rule, holds great potential for revolutionizing the mechanisms underlying photocatalysis and optoelectronics. Despite the recent theoretical advancements in understanding this phenomenon, there is a dearth of experimentally feasible approaches for implementing INVEST, which impedes its systematic integration in future applications. This work presents a compelling instance of INVEST that is prompted by environmental polarization effects. By utilizing a simple heptazine molecule as a model chromophore, we demonstrate the switching of the delayed fluorescence mechanism from the conventional thermal activation to one that involves INVEST upon phase transition of solvents. The freezing medium alters the local electrostatics surrounding the chromophore, as evidenced by the observed luminescence rigidochromic shift, which leads to the reversal of the relative spin energetics as measured by temperature-dependent time-resolved PL kinetics. Our preliminary computation suggests that there may exist a differential polarization effects on singlet and triplet excited states as they possess disparate polarity. Ultimately, this work lays a new mechanistic foundation for utilizing local polarization to achieve INVESTThe phenomenon of singlet-triplet inversion, which defies Hund's rule, holds tremendous potential for revolutionizing the mechanisms underlying photocatalysis and optoelectronics.