Approaching 800 nm near-infrared thermally activated delay fluorescence materials via simple malononitrile condensation strategy

Abstract

The development of organic materials with near-infrared (NIR) thermally activated delayed fluorescence (TADF) has drawn growing attention in recent years, but most of the reported NIR TADF emitters have emission peaks less than 780 nm with considerable visible emission remnants. To achieve pure NIR emission, we developed a strong electron acceptor fluoren-9-ylidene malononitrile ( FM ) by malononitrile condensation with fluorenone. Exploiting FM as acceptor, two novel molecules 3TPAFM and 36DTPAFM with pure NIR TADF were facilely synthesized and displayed high thermal stability. Compared with the orange emissive non-TADF fluorenone precursors, 3TPAFM and 36DTPAFM demonstrated significantly red-shifted emission of up to 217 nm and largely reduced energy gaps between the lowest singlet and triplet states, leading to pure NIR TADF with emission peak at 802 nm in neat films. The photophysical study revealed that their nonradiative decay rates are comparable with fluorenone counterparts even with such significantly reduced optical energy gap, manifesting largely attenuated limitation from the energy gap law. The non-doped OLEDs based on 3TPAFM and 36DTPAFM achieved pure NIR emission with λ EL at 792 and 788 nm, and external quantum efficiencies of 0.21% and 0.32% at 1 mA cm −2 , respectively. This work provides a simple and powerful strategy to achieve NIR TADF by malononitrile condensation with ketone group and presents FM group as a desirable acceptor candidate to construct pure and efficient NIR emitting materials. • Two novel NIR TADF emitters were constructed through simple malononitrile condensation strategy. • They achieved significant redshifts of 217 nm while maintaining similar nonradiative decay rates with fluorenone precursors. • Their non-doped OLED devices realized pure and efficient NIR emission with peaks around 790 nm.