Rational design of CN substituted dibenzo[a,c]phenazine acceptor for color tuning of thermally activated delayed fluorescent emitters

Abstract

• Rational color tuning of thermally activated delayed fluorescence emitters by molecular engineering. • CN substituent management of dibenzophenazine acceptor to manage the light-emitting properties. • High external quantum efficiency from yellow to deep red organic light-emitting diodes. Thermally activated delayed fluorescent (TADF) emitters with emission wavelength beyond 600 nm and high external quantum efficiency (EQE) above 20 % are rarely reported because of the inherent limitation of the energy gap law. Rigid and planar dibenzo[ a,c ]phenazine (BP) core was proven to be a useful acceptor to develop highly efficient red TADF emitters by minimizing non-radiative loss mechanism. In this work, by varying the number and substitution position of donor/acceptor unit attached to the BP acceptor, four long wavelength TADF emitters (3-AcBPmCN, 3-AcBPdCN, 3-AcBPtCN and 11-ACBPdCN) were developed to control the emission wavelength from yellow (552 nm) to deep-red (637 nm) region. A directly attached 9,9-dimethyl-9,10-dihydroacridine (Ac) donor and newly developed 3-CNBP, 11,12-CNBP, 3,11,12-CNBP and 3,6-CNBP acceptor units were utilized in the synthesis of TADF emitters of 3-AcBPmCN, 3-AcBPdCN, 3-AcBPtCN and 11-AcBPdCN respectively. With the increasing number of cyano units on BP acceptor, decreased HOMO-LUMO energy gap and singlet–triplet energy gap (Δ E ST ) were reported for three TADF emitters of 3-AcBPmCN, 3-AcBPdCN and 3-AcBPtCN. Further, it was demonstrated that in between the two donor-substitution positions of BP acceptor (3 and 11), 11-position substituted 11-AcBPdCN TADF emitter exhibited higher EQE (23.1 %) than the 3-position substituted 3-AcBPdCN TADF emitter (17.3 %) with the similar color coordinate (0.53, 0.46).