Extracting Design Principles for Efficient Thermally Activated Delayed Fluorescence (TADF) from a Simple Four-State Model

We introduce a simple quantum-mechanical model for thermally activated delayed fluorescence (TADF). The Hamiltonian is represented in the basis of four spin-mixed diabatic states representing pure charge transfer (CT) and local excitations (LE). The model predicts that it is possible to realize lowest-lying adiabatic singlet (S 1 ) and triplet (T 1 ) states with a small singlet–triplet gap, differing CT/LE contributions, and appreciable LE component in the S 1 state. These characteristics can explain the coexistence of fast T 1 → S 1 reverse intersystem crossing and S 1 → S 0 radiative decay in some chromophores. Through the sampling of the parameter space and statistical analysis of the data, we show which parameter combinations contribute the most to the TADF efficiency. We also show that conformational fluctuations of a single model donor–acceptor system sample a significant region of the parameter space and can enhance the TADF rate by almost 3 orders of magnitude. This study provides new guidelines for optimization of TADF emitters by means of electronic structure and conformation engineering.