Theoretical exploration of NIR-II circularly polarized luminescence in open-shell boron embedded pyrene-naphthalene helicenes

Abstract

The incorporation of heteroatoms into helically twisted conjugated system has attracted considerable attention due to their intrinsic chirality and tunable circularly polarized luminescence (CPL) in the second near-infrared (NIR-II) window. In this work, using a pyrene-naphthalene-based [3]helicene as parent structure, we design open-shell radicals by embedding boron (B) at nine distinct positions of the helical skeleton and systematically evaluate their NIR-II-CPL performances through density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, along with vibronic analysis. The position-dependent influence of B incorporation is examined across four chiral configurations (PP, PM, MP, MM) of each derivative by assessing energetic stability, geometrical change, charge distribution, aromaticity, spin density, reorganization energy, and dissymmetry factor during the electron excitation process. Among the studied systems, the central boron-embedded structure (B-6-PP) was identified as an optimal NIR-II-CPL candidate for balancing structural stability and chiroptical performance, which exhibits NIR-II emission at 1075 nm with high luminescence dissymmetry factor (|glum|) of 1.27 × 10-3. This enhanced performance arises from the synergistic effect of a reduced transition electric dipole moment (2.57 × 10-18 esu cm) and a non-orthogonally oriented angle (θ = 80.89°) with magnetic transition dipole moments. Additionally, B-6-PP exhibits the lowest reorganization energy (200.95 meV) and a relatively small Huang-Rhys factor for D1 → D0 process among the studied systems, which is favorable for narrow-band emission. This work provides insights into the structure-property relationships governing open-shell stability, excited-state behavior, and chiroptical performance, offering valuable guidance for designing helical organic radicals with tailored chiroptical properties.