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

Abstract

The design and development of long-wavelength deep-red emitters have gained significant attention due to their potential prospective applications in optical communication, nightvision devices, and sensors. However, due to the intrinsic limitations of the energy gap law, creating high-performing deepred 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.