Highly Efficient Red-Orange Delayed Fluorescence Emitters Based on Strong pi-Accepting Dibenzophenazine and Dibenzoquinoxaline Cores: toward a Rational Pure-Red OLED Design

Organic luminescent materials that exhibit thermally activated delayed fluorescence (TADF) can harvest both singlet and triplet excitons for light emission, leading to high electroluminescence (EL) quantum efficiencies in organic light-emitting diodes (OLEDs). However, efficient red TADF materials are still very rare because of their restricted molecular design based on the energy gap law. To address this issue, elaborate pi-conjugated donor-acceptor (D-A) systems that can simultaneously achieve a large fluorescence radiative rate and small singlet-triplet energy splitting should be strategically designed. In this study, to produce high-efficiency pure-red TADF materials, a remarkably strong pi-accepting dicyanodibenzo[a,c]phenazine (CNBPz) unit has been introduced in a D-pi-A molecular framework, and combined with a phenylene-linked p-ditolylamine or 9,9-dimethylacridan moiety. The steady-state and time-resolved photophysical measurements revealed intense genuine red TADF emissions of these CNBPz-based molecules in both solution and doped thin films. The OLEDs incorporating the CNBPz-based TADF emitters achieve the desired high-efficiency pure-red EL, centered at 670 nm with color coordinates of (0.66, 0.34), accompanied by a high maximum external EL quantum efficiency of 15.0%. Therefore, it is concluded that CNBPz, with its expanded pi-conjugation and strong electron-accepting characteristics, is a particularly useful building unit to design long-wavelength TADF materials that can overcome the intrinsic energy gap law obstacle.