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

Abstract

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[a,c]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 (ΔE_ST) in comparison to non-nitrogen-substituted compounds. The investigated ortho-linked D-A compounds show relatively large donor-acceptor twisting separation and small ΔE_ST compared to their para-linked counterparts. For higher number nitrogen (4N)-substituted emitters, we predict small adiabatic ΔE_ST (ΔE_ST^adia) in the range 0.01-0.13 eV, and with the tert-butylated donors, we even obtained ΔE_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 ΔE_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 para-linked compound B4 at 670 nm and the ortho-linked compound D4 at 713 nm and delayed NIR emission at 987 and 1217 nm for the ortho-linked compounds D3 and E3, respectively.