“Molecular Designs Featuring Cyanobenzene-Decorated Phenazine Acceptor Units for the Highly Efficient Deep-Red Thermally Activated Delayed Fluorescent Emitters”

The design and development of long-wavelength deep-red emitters have gained significant attention due to their potential prospective applications in optical communication, night-vision devices, and sensors. However, due to the intrinsic limitations of the energy gap law, creating high-performing deep-red emitters is still found to be difficult. Herein, based on the auxiliary cyanobenzene core attached to the phenazine acceptor unit, we have reported two types of orange-red to deep-red emitting thermally activated delayed fluorescence (TADF) emitters, 4,4′-(3,6-bis(9,9-dimethylacridin-10(9H)-yl)dibenzo[a,c]phenazine-11,12-diyl)dibenzonitrile (Ac-PhCNDBPZ) and 4,4′-(3,6-di(10H-phenoxazin-10-yl)dibenzo[a,c]phenazine-11,12-diyl)dibenzonitrile (PXZ-PhCNDBPZ). A direct attachment of donor units to the phenazine acceptor unit was preferred for steric repulsion between the donor and acceptor units. Hence, more twisted molecular structures are necessary for a small singlet–triplet energy gap (ΔE_ST). Terminal cyanobenzene units helped to further shift the emission wavelength toward the long wavelength region and to minimize the intermolecular interaction to suppress the aggregation-caused emission quenching. Both these emitters exhibited a very small singlet–triplet energy gap (0.13 and 0.06 eV) and short DF lifetime (τ_d) values (2.62 and 1.63 μs). Vacuum-deposited organic light-emitting diodes using Ac-PhCNDBPZ and PXZ-PhCNDBPZ as emitters displayed orange-red and deep-red electroluminescence having maximum external quantum efficiencies of 10.5% and 9.9%, respectively. This work shows that high-efficiency deep-red TADF materials are efficiently produced by combining a cyano substituent with a strong and rigid acceptor.