Unveiling the Role of Substituent Positioning in Enhancing Circularly Polarized Luminescence of Azaborahelicenes: A Theoretical Insight

Abstract

Small organic molecules are highly promising candidates for circularly polarized luminescent (CPL) materials owing to their well-defined structures and excellent tunability, yet achieving high luminescence dissymmetry factors (g_lum) remains a critical challenge. In this work, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were employed to systematically investigate the effect of substituent position on the g_lum of helically chiral four-coordinate N,C-boron-chelated azaborahelicenes. The results reveal that shifting the strong electron-donating -NMe₂ group along the naphthalene ring significantly modulates the local torsional angle, which is closely associated with the microscopic angle θ between the transition electric dipole moment (μ) and transition magnetic dipole moment (m). Remarkably, when the -NMe₂ group is located at the C3 position, the strongest π-conjugation with the naphthalene framework is achieved, resulting in the most pronounced global helical asymmetry. This geometric imbalance promotes a more favorable alignment between μ and m, thereby reducing the angle θ and substantially increasing the g_lum value to 0.66 × 10^–3 for ABH-N3, approximately 1 order of magnitude higher than that of the parent ABH-N (0.07 × 10^–3). The present study elucidates how substituent position regulates local π-conjugation and global helical asymmetry, thereby modulating the relative orientation of the transition electric and magnetic dipole moments and ultimately the CPL response. These insights provide valuable theoretical guidance for the rational design of helical organic chiroptical materials.