Understanding the Control of Singlet-Triplet Splitting for Organic Exciton Manipulating: A Combined Theoretical and Experimental Approach

Developing organic optoelectronic materials with desired photophysical properties has always been at the forefront of organic electronics. The variation of singlet-triplet splitting (ΔE_ST) can provide useful means in modulating organic excitons for diversified photophysical phenomena, but controlling ΔE_ST in a desired manner within a large tuning scope remains a daunting challenge. Here, we demonstrate a convenient and quantitative approach to relate ΔE_ST to the frontier orbital overlap and separation distance via a set of newly developed parameters using natural transition orbital analysis to consider whole pictures of electron transitions for both the lowest singlet (S₁) and triplet (T₁) excited states. These critical parameters revealed that both separated S₁ and T₁ states leads to ultralow ΔE_ST; separated S₁ and overlapped T₁ states results in small ΔE_ST; and both overlapped S₁ and T₁ states induces large ΔE_ST. Importantly, we realized a widely-tuned ΔE_ST in a range from ultralow (0.0003 eV) to extra-large (1.47 eV) via a subtle symmetric control of triazine molecules, based on time-dependent density functional theory calculations combined with experimental explorations. These findings provide keen insights into ΔE_ST control for feasible excited state tuning, offering valuable guidelines for the construction of molecules with desired optoelectronic properties.