Red thermally activated delayed fluorescence emitters in organic light emitting diodes: a computational approach

This study is a computational analysis of thermally activated delayed fluorescence (TADF) properties for ten molecules emitting red color, together with three molecules having only fluorescent emission. Four different descriptors which are twisting angle (α), low lying singlet triplet energy difference (ΔEST), triplet states contribution to reverse intersystem crossing (RISC) and Φs index with Natural Transition Orbitals (NTOs) were assessed to be able to clearly distinguish TADF emitters from non-TADFs. The geometry optimizations of emitters were performed on ground, singlet and triplet states by Density Functional Theory (DFT) at the M06-2X/6-31G(d) level of theory. Conformation analysis was also carried out to determine the most stable and energetically favorable geometries. Excited state topologies were assessed through the most favorable conformations by Time Dependent Density Functional Theory (TD-DFT) and Tamm-Dancoff Approximation (TDA) at B3LYP/6-31+G(d) by considering solvent effects. Population analysis of NTOs have been performed following TD-DFT and TDA calculations, and Φs indices were tabulated by Lowdin and Mulliken charge distributions. As a result of quantum calculation assessments, it has been shown that TADF emitters are well separated from the emitters which perform only fluorescent emission by the four descriptors utilized in this study. This study has revealed the fact that these four descriptors can be further used for suggesting new potential TADF emitters. Thus, developing design strategy for red TADF organic light emitting diodes (OLEDs) can be accelerated.