A new diabatization scheme is proposed to calculate the electronic couplings for the singlet fission process in multichromophoric systems. In this approach, a robust descriptor that treats single and multiple excitations on an equal footing is adopted to quantify the localization degree of the particle and hole densities of the electronic states. By maximally localizing the particles and holes in terms of predefined molecular fragments, quasi-diabatic states with well-defined characters (locally excited, charge transfer, correlated triplet pair, etc.) can be automatically constructed as the linear combinations of the adiabatic ones, and the electronic couplings can be directly obtained. This approach is very general in that it applies to electronic states with various spin multiplicities and can be combined with various kinds of preliminary electronic structure calculations. Due to the high numerical efficiency, it is able to manipulate more than 100 electronic states in diabatization. The applications to the tetracene dimer and trimer reveal that high-lying multiply excited charge transfer states have significant influences on both the formation and separation of the correlated triplet pair and can even enlarge the coupling for the latter process by 1 order of magnitude.
We comment on an excited‐state localization method recently proposed by Blanc et al. (J. Comput. Chem. 2023, 44, 105). Elaborate comparisons are made to demonstrate that their method is a less‐comprehensive version of the diabatization method proposed by us 2 years earlier (J. Phys. Chem. Lett. 2021, 12, 1032).
Pure organic materials with persistent and efficient room‐temperature phosphorescence have recently aroused great research interest due to their vast potential in applications. One crucial design principle for such materials is to suppress as much as possible the non‐radiative decay of the triplet exciton while maintaining a moderate phosphorescent radiative rate. However, molecular engineering often exhibits similar regulation trends for the two processes. Here, we propose that the quantum interference caused by aggregation can be utilized to control the phosphorescent and non‐radiative decay channels. We systematically analyze various constructive and destructive transition pathways in aggregates with different molecular packing types and establish clear relationships between the luminescence characters and the signs of the singlet and triplet excitonic couplings. It is shown that the decay channels can be flexibly switched on or off by regulating the packing type and excitonic couplings. Most importantly, an enhanced phosphorescent decay and a completely suppressed non‐radiative decay can be simultaneously realized in the aggregate packed with inversion symmetry. This work lays the theoretical foundation for future experimental realization of quantum interference effects in phosphorescence.
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