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.