A New Design Strategy for Efficient Thermally Activated Delayed Fluorescence Organic Emitters: From Twisted to Planar Structures

Abstract

In the traditional molecular design of thermally activated delayed fluorescence (TADF) emitters composed of electron-donor and electron-acceptor moieties, achieving a small singlet–triplet energy gap (ΔE_ST) in strongly twisted structures usually translates into a small fluorescence oscillator strength, which can significantly decrease the emission quantum yield and limit efficiency in organic light-emitting diode devices. Here, based on the results of quantum-chemical calculations on TADF emitters composed of carbazole donor and 2,4,6-triphenyl-1,3,5-triazine acceptor moieties, a new strategy is proposed for the molecular design of efficient TADF emitters that combine a small ΔE_ST with a large fluorescence oscillator strength. Since this strategy goes beyond the traditional framework of structurally twisted, charge-transfer type emitters, importantly, it opens the way for coplanar molecules to be efficient TADF emitters. Here, a new emitter, composed of azatriangulene and diphenyltriazine moieties, is theoretically designed, which is coplanar due to intramolecular H-bonding interactions. The synthesis of this hexamethylazatriangulene-triazine (HMAT-TRZ) emitter and its preliminary photophysical characterizations point to HMAT-TRZ as a potential efficient TADF emitter.

A new strategy is proposed for the molecular design of efficient thermally activated delayed fluorescence (TADF) emitters that combine small singlet–triplet energy gaps and large fluorescence oscillator strengths. This strategy goes beyond the traditional framework of twisted TADF emitters and opens the way for coplanar molecules to be efficient TADF emitters.

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