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.

Scheme 1. Chemical Structures and Molecular Design of Nitrogen-Substituted DBP Compounds A1–A4

Figure 1

Chemical structures from theoretical calculations on compounds B1–B4. (a) Chemical structures of B1–B4. (b) B3LYP/6-31+G(d)-optimized ground-state (S₀) structures. (c) Calculated HOMO (H) and LUMO (L) charge density distribution plots for the excited states S₁ and T₁ obtained using the TDDFT/B3LYP/6-31+G(d) method in cyclohexane solution state.

Figure 2

Chemical structures from theoretical calculations on compounds C1–C4. (a) Chemical structures of C1–C4. (b) B3LYP/6-31+G(d)-optimized ground-state (S₀) structures. (c) Calculated HOMO (H) and LUMO (L) charge density distribution plots for the excited states S₁ and T₁ obtained using the TDDFT/B3LYP/6-31+G(d) method in cyclohexane solution state.

Figure 3

Chemical structures from theoretical calculations on compounds D1–D4. (a) Chemical structures of D1–D4. (b) B3LYP/6-31+G(d)-optimized ground-state (S₀) structures. (c) Calculated HOMO (H) and LUMO (L) charge density distribution plots for the excited states S₁ and T₁ obtained using the TDDFT/B3LYP/6-31+G(d) method in cyclohexane solution state.

Figure 4

Chemical structures from theoretical calculations on compounds E1–E4. (a) Chemical structures of E1–E4. (b) B3LYP/6-31+G(d)-optimized ground-state (S₀) structures. (c) Calculated HOMO (H) and LUMO (L) charge density distribution plots for the excited states S₁ and T₁ obtained using the TDDFT/B3LYP/6-31+G(d) method in cyclohexane solution state.

Figure 5

Plots of the calculated HOMO/LUMO energy levels and HOMO–LUMO gaps of the studied compounds.

Figure 6

Computed absorption spectra (a) and (c) and emission spectra (b) and (d) showing intensity for peak wavelengths for para-linked compounds B1–B4 and C1–C4 in cyclohexane solution state. Here the full width at half-maximum (fwhm) was fixed at 1200 and 3000 cm^–1 for the absorption and emission spectra, respectively.

Figure 7

Computed absorption spectra (a) and (c) and emission spectra (b) and (d) showing intensity for peak wavelengths for ortho-linked compounds D1–D4 and E1–E4 in cyclohexane solution state. Here the full width at half-maximum (fwhm) was fixed at 1200 and 3000 cm^–1 for the absorption and emission spectra, respectively.

Figure 8

Variation of calculated adiabatic singlet–triplet energy splitting (ΔE_ST^adia) with and without nitrogen substitution of the D–A compounds.

Figure 9

Optimized structures and calculated HOMO/LUMO energies, HOMO–LUMO gaps, and adiabatic singlet–triplet energy splittings ΔE_ST^adia of the studied tert-butylated donor- and four-nitrogen-substituted acceptor-based D–A compounds.

Figure 10

Schematic representation of excited state characteristics, including singlet/triplet energy levels, adiabatic singlet–triplet energy splittings (ΔE_ST^adia), spin–orbit coupling (SOC), ISC/RISC rate constants, radiative decay rates (k_f), and delayed fluorescence emission wavelengths for the higher-nitrogen-substituted (4N) D–A compounds.