Beyond the three-state picture: when higher-lying excited states become quantitatively indispensable

Higher-lying excited states beyond S₁ and T₁ are widely recognized in many photophysical systems, including thermally activated delayed fluorescence (TADF). However, their explicit and quantitative impact on photophysical observables such as photoluminescence quantum yields (PLQY) and lifetimes is difficult to be attained experimentally and it has not been systematically assessed within a fully ab initio kinetic modeling framework. To address this gap, we developed KinLuv, a multistate excited state kinetic model that includes higher-lying excited states (S₂, T₂) and all possible monomolecular interconversion processes between all the electronic states, whose rate constants were computed using Fermi's golden rule (FGR) explicitly including the Herzberg-Teller (HT) vibronic coupling effect. We applied KinLuv to prototypical multi-resonance TADF (MR-TADF) emitters and their derivatives, as well as other representative organic chromophores, demonstrating its broad applicability across diverse photophysical playgrounds beyond TADF. The resulting simulations quantitatively reproduce key experimental observables, including PLQY and prompt/delayed fluorescence lifetimes. Beyond its predictive power, the present results establish clear criteria for identifying when higher-lying excited states influence the excited-state decay and when simplified models remain adequate. This framework enables rational selection of minimal kinetic models that balance physical insight with numerical robustness, with direct implications for the in silico design of high-performance organic emitters.