Accurate Prediction for Dynamic Hybrid Local and Charge Transfer Excited States from Optimally Tuned Range-Separated Density Functionals

Fine regulation of excited-state characteristics of organic molecules plays a vital role in the rational design of novel optoelectronic materials. Recently, the fluorescent emitters with a hybridized local and charge transfer (HLCT)-excited state have attracted significant interest in developing high-efficiency organic light-emitting diodes. The HLCT state generally consists of a mixture of local excitation and charge transfer (CT) characters that are known to be sensitive to molecular configuration and surrounding environment. Thus, both qualitative and quantitative characterizations of “dynamic” HLCT states remain challenging from a theoretical perspective. In this work, a series of donor–acceptor (D–A) molecules with HLCT excited-state characters were theoretically studied using density functional theory (DFT) and time-dependent DFT. Successful prediction of both vertical absorption and emission excitation energies (E VA and E VE) of the lowest singlet excited state (S1) is demonstrated when using the optimally tuned range-separated (RS) density functionals with the smallest average mean absolute deviations (MADs) of 0.07 eV for LC-ωPBE* and 0.09 eV for ωB97XD* compared to the available experimental data. The percentages of CT character (CT %), nature transition orbitals, hole–electron distribution, and transition density matrix maps are further analyzed qualitatively and quantitatively, highlighting the importance of right balance between localization and delocalization effects of electronic structures. The results indicate that a moderate amount of exact exchange (eX %) included in density functionals is key in reasonably predicting HLCT states. Thanks to the accuracy of the optimally tuned RS method, quantitative characterization of energy gaps and spin–orbit couplings between singlet and triplet excited states is performed to assess the possible “hot-exciton” paths. The present work provides a reliable and efficient theoretical tool for further developing novel HLCT-based optoelectronic materials.