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 (S(1)) and triplet (T(1)) excited states. These critical parameters revealed that both separated S(1) and T(1) states leads to ultralow ΔE(ST); separated S(1) and overlapped T(1) states results in small ΔE(ST); and both overlapped S(1) and T(1) 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.