Design of a New Mechanism beyond Thermally Activated Delayed Fluorescence toward Fourth Generation Organic Light Emitting Diodes

Usually, development of organic molecules with efficient thermally activated delayed fluorescence (TADF) focuses on minimizing the energy gap between the lowest singlet and triplet state. However, although this is crucial, it is not sufficient for optimizing the emitter’s molecular and electronic structure for OLED use. Here, we present a design strategy that leads us not only to a new type of emitter but also to a new exciton harvesting mechanism. This concept is realized (i) by drastically reducing the energy gap between the lowest singlet and triplet energy states, (ii) by rigidifying the molecular structure to reduce inhomogeneity effects that usually induce long emission decay tails lying even in the millisecond time range, (iii) by maximizing the Franck–Condon factors that govern intersystem crossing (ISC), (iv) by shifting the charge transfer states, ¹CT and ³CT, to become the lowest energy states applying polarity tuning, and (v) by providing energetically nearby lying states for spin–orbit coupling (SOC) and configuration interaction (CI) paths to speed up ISC. Using this concept, we design an “almost zero-gap” compound showing ΔE(¹CT–³CT) ≈ 16 cm^–1 (2 meV). Thus, thermal activation is no longer a time delaying key problem at T = 300 K. Moreover, if the emitter is applied in an OLED, fast ISC will allow us to harvest all singlet and triplet excitons through emission from the lowest excited CT singlet state. This benchmark mechanism, the direct singlet harvesting (DSH) mechanism, offers the great advantage of a significant reduction of the overall emission decay time to the submicrosecond range. This is a shorter decay than found for TADF emission so far. Accordingly, this mechanism leads us to beyond TADF toward a new era in the design of OLED emitters and opens the way for reducing stability problems and roll-off effects.