Organic molecules with inverted gaps between first excited singlet and triplet states and appreciable fluorescence rates

Summary

One recent proposal for designing state-of-the-art emissive materials in organic light-emitting diodes (OLEDs) is the principle of thermally activated delayed fluorescence (TADF). The idea is to enable facile thermal upconversion of excited-state triplets, which are generated upon electron-hole recombination, to excited-state singlets. Minimizing the corresponding singlet-triplet energy difference enables devices with up to 100% internal quantum efficiency. Ideal emissive materials potentially surpassing TADF emitters in other performance parameters should have both negative singlet-triplet gaps and substantial fluorescence rates to maximize reverse intersystem crossing (ISC) rates from excited triplets to singlets while minimizing ISC rates and triplet state occupation, leading to long-term operational stability. However, molecules with negative singlet-triplet gaps are extremely rare, and none of them emissive. In this work, based on computational studies, we describe the first molecules with negative singlet-triplet gaps and considerable fluorescence rates and show that they are more common than hypothesized previously.