Developing Red and Near-Infrared Delayed Fluorescence Emission in Nitrogen-Substituted Donor–Acceptor Polycyclic Hydrocarbon OLED Emitters: A Theoretical Study

Nitrogen substitutions have shown a great impact for the development of thermally activated delayed fluorescence (TADF)-based organic light-emitting diode (OLED) materials. In particular, much focus has been devoted to nitrogen-substituted polycyclic aromatic hydrocarbons (PAHs) for TADF emitters. In this context, we provide here a molecular design approach for symmetric nitrogen substitutions in fused benzene ring PAHs based on the dibenzo­[<i>a</i>,<i>c</i>]­picene (DBP) molecule. We designed possible donor–acceptor (D–A) compounds with dimethylcarbazole (DMCz) and dimethyldiphenylamine (DMDPA) donors and studied the structure and photophysics of the designed D–A compounds. The twisted and extended D–A-type PAH emitters demonstrate red and near-infrared (NIR) TADF emission. Nitrogen substitutions lead to significant LUMO stabilization and reduced HOMO–LUMO energy gaps as well. Additionally, we computed significantly smaller singlet–triplet energy splittings (Δ<i>E</i>_ST) in comparison to non-nitrogen-substituted compounds. The investigated <i>ortho</i>-linked D–A compounds show relatively large donor–acceptor twisting separation and small Δ<i>E</i>_ST compared to their <i>para</i>-linked counterparts. For higher number nitrogen (4N)-substituted emitters, we predict small adiabatic Δ<i>E</i>_ST (Δ<i>E</i>_ST^adia) in the range 0.01–0.13 eV, and with the <i>tert</i>-butylated donors, we even obtained Δ<i>E</i>_ST^adia values as small as 0.007 eV. Computed spin–orbit coupling (SOC) for the T₁ triplet state on the order of 0.12–2.28 cm^–1 suggests significant repopulation of singlet charge transfer (¹CT) excitons from the triplet CT and locally excited (³CT+LE) states. Importantly, the small Δ<i>E</i>_ST^adia and large SOC values induce a reverse intersystem crossing (RISC) rate as high as 1 × 10⁶ s^–1, which will cause red and NIR delayed fluorescence in the 4N-substituted D–A emitters. Notably, we predict red TADF emission for the <i>para</i>-linked compound <b>B4</b> at 670 nm and the <i>ortho</i>-linked compound <b>D4</b> at 713 nm and delayed NIR emission at 987 and 1217 nm for the <i>ortho</i>-linked compounds <b>D3</b> and <b>E3</b>, respectively.