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 π-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 π-accepting dicyanodibenzo[a,c]phenazine (CNBPz) unit has been introduced in a D–π–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 π-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.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Atomic and Molecular Physics, and Optics