Enhancing triplet harvesting in inverted singlet-triplet gap molecules through mechanistic understanding

Molecules with an inverted singlet-triplet gap violate Hund's rule and offer promising applications in photoactive materials. In particular, they are expected to improve the efficiency of triplet harvesting via delayed fluorescence. Understanding the mechanistic factors governing electronic transitions is crucial for optimizing their performance. In this work, we employ the nuclear ensemble method combined with high-level electronic structure calculations to investigate the key excited-state processes in four azaphenalene-based molecules. We demonstrate that minimizing the negative singlet-triplet gap is, contrary to the expectations based on the Arrhenius relation, not necessarily a desired strategy for decreasing the delayed fluorescence lifetimes. Nevertheless, due to the vibronic coupling, molecular cores with small negative gaps are a promising starting point for assuring fast reverse intersystem crossing. These findings provide new design principles for developing efficient triplet-harvesting materials, emphasizing the need to control vibrational and symmetry effects to balance radiative and non-radiative transitions effectively.