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 optoelectronics and photocatalysis. However, 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 role of vibrational effects in four azaphenalene-based molecules. Our results reveal that maximizing the thermodynamic driving force for reverse intersystem crossing is not necessarily a desired strategy for efficient triplet harvesting. Nevertheless, due to the vibronic coupling, molecular cores with small negative gaps are a promising starting point as appropriate functionalization can decrease delayed fluorescence lifetimes despite the gap moving toward positive values. 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.