Rational designs of structurally similar TADF and HLCT emitters with benzo- or naphtho-carbazole units as electron donors.

Thermally activated delayed fluorescence (TADF) and hybridized local and charge transfer (HLCT) emitters are two types of highly efficient electroluminescent materials which could improve their internal quantum efficiency (IQE) by converting triplet excitons to singlet ones. However, the molecular designs of TADF and HLCT materials are usually carried out separately because of their distinct emission mechanisms. In this work, we report a rational design strategy for the realization of switching between HLCT and TADF emissions in structurally similar donor-acceptor (D-A) type molecules, which are constructed with the same electron donors (benzo- or naphtho-carbazole) and acceptors with tunable electron-withdrawing abilities (benzonitrile (BN) and benzene-1,2,3,4,5-pentacarbonitrile (BPN)). Such switching of two types of emissions could be attributable to the modulation of the intramolecular charge transfer (ICT) and twist between donor and acceptor units. In the theoretical framework of the state hybridization, the excited-state properties are analyzed to reveal the intrinsic structure-property relationships for the donor-based HLCT and TADF molecules. This work not only offers an in-depth understanding of the excited-state properties of HLCT/TADF molecules, but also provides theoretical guidelines for the designing and screening of highly efficient electroluminescent materials.