Multiplying the efficiency of red thermally activated delayed fluorescence emitter by introducing intramolecular hydrogen bond

As one of the three primary colors, red organic light-emitting diodes (OLEDs) are indispensable in practical applications. However, red emitters are generally subject to severe non-radiative exciton loss due to their narrow energy gap. In this work, three new thermally activated delayed fluorescence (TADF) emitters were developed, namely 4-(acenaphtho[1,2-b]quinoxalin-9-yl)-N,N-diphenylaniline (TPA-AP), 4-(acenaphtho[1,2-b]pyrido[2,3e]pyrazin-10-yl)-N,N-diphenylaniline (TPA-APy), and 4-(acenaphtho[1,2-b]pyrazino[2,3-e]pyrazin-9-yl)-N,Ndiphenylaniline (TPA-APm), employing a series of finely modified acenaphtho[1,2-b]quinoxaline (AP) derivatives as acceptor units. Among three TADF emitters, an intramolecular hydrogen bond is formed between the donor (D) and acceptor (A) units in TPA-APm. Consequently, the overlap of the frontier molecular orbitals (FMOs) of TPA-APm can increase appropriately, and the fluorescence radiative rate (kF) of TPA-APm is nearly twofold than that of TPA-AP and TPA-APy. Furthermore, the non-radiative decay rate (knr) of TPA-APm is less than that of TPA-AP and TPA-APy by an order of magnitude, which is attributed to the improved molecular rigidity caused by intramolecular hydrogen bond. As a result, TPA-APm-based OLEDs achieved a multiplied external quantum efficiency (EQE) of 21.1% with the electroluminescence peak at 590 nm, comparing to only 7.0% and 11.5% for the TPA-AP-based and TPA-APy-based devices, respectively. These results demonstrate appropriate intramolecular hydrogen bond can suppress the influence of non-radiative decay by simultaneously enhancing molecular rigidity and facilitating the fluorescence process, and have great potential in the design of efficient red TADF emitters.

Automated summary

In this study, three new thermally activated delayed fluorescence (TADF) emitters were developed with improved molecular rigidity and enhanced fluorescence process by forming appropriate intramolecular hydrogen bond. These emitters showed significantly improved radiative rate and reduced non-radiative decay rate, leading to multiplied external quantum efficiency (EQE) in red organic light-emitting diodes (OLEDs).