Modulating the embedding position of nitrogen atom in [5]helicene:the design of hot-exciton materials

Abstract

The introduction of helical chirality is of great significance for improving the luminescence performance of circularly polarized thermally activated delayed fluorescence (CP-TADF) materials. The current work provides valuable design strategy to develop efficient “hot” exciton TADF molecules with CPL characteristics. A novel class of nitrogen-embedded [5]helicene compounds is designed by adjusting the positions of the nitrogen atom, named [5]H-PTZ, 1-aza[5]H-PTZ, 2-aza[5]H-PTZ, and 3-aza[5]H-PTZ. By employing density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, we focus on investigating the profound impact of specific nitrogen atom positions within the [5]helicene framework on the electronic properties. Intriguingly, when the nitrogen atom resides near the donor position in 3-aza[5]H-PTZ, the higher-lying singlet-triplet energy gap (ΔE_ST) approaches almost 0 eV. The small ΔE_ST values (the ΔE_S1T2 = 0.07 eV, ΔE_S1T3 = 0.0003 eV, ΔE_S1T4 = 0.03 eV, ΔE_S1T5 = 0.04 eV) facilitate a heightened participation of triplet excited states in the luminescence process. Notably, 3-aza[5]H-PTZ exhibits the radiation rate (k_r) value as large as 4.79 × 10⁷ s⁻¹. Particularly, the luminescence asymmetry factor (g_lum) of the 3-aza[5]H-PTZ reaches 3.90 × 10⁻³. Therefore, the advantage of [5]helicene as a chiral molecule exhibits CPL characteristic. By precisely controlling nitrogen atom positions, this work highlights design strategies for modulating CPL and “hot” exciton TADF properties.