In situ Second-Harmonic Generation Probing of Magnetic-Field-Induced Chiral Stacking in a Radical-Doped Liquid Crystal Monolayer

Abstract

Functional open-shell π-radicals offer attractive optical and magnetic tunability but suffer from poor ambient stability, limiting their deployment in condensed-phase architectures. Here we circumvent this limitation by embedding the photolabile radical precursor HTTM-PDMAC into a fluid nematic liquid-crystal host (5CT), where the matrix suppresses radical quenching while retaining orientational responsiveness. Using a combination of surface pressure–area isotherms, UV–vis and fluorescence spectroscopy, in situ polarization-resolved second-harmonic generation (SHG), and atomic force microscopy, we systematically investigate the molecular stacking and dynamic response of this doped system under coupled photo- and magnetic-field stimuli. Upon 420 nm photoexcitation, HTTM-PDMAC efficiently generates TTM-PDMAC radicals, whose fluorescence half-life (∼10,000 s) is 3 orders of magnitude longer than that of bare TTM radicals, confirming dramatically enhanced photostability. Application of an external magnetic field (10–60 mT) modulates the orientation of the radicals’ unpaired electrons; through the anisotropic spin–lattice interaction and the collective elastic response of the nematic matrix, this minute perturbation is amplified to reorient neighboring 5CT molecules from an average tilt angle of 48° ± 2° to 82° ± 2°, as quantified by SHG polarization analysis. Chiral-resolved SHG further reveals a monotonic increase in the degree of chiral excess (DCE) with magnetic-field strength, from 0.005 (0 mT) to 0.252 (60 mT). AFM topography corroborates the formation of stable, ∼ 90 nm-high spherical core–shell supramolecular assemblies, with radical-rich cores enveloped by an ordered 5CT shell. This work elucidates the cooperative interaction between open-shell radicals and liquid-crystal hosts under external fields and provides a feasible approach for this system to program interfacial chirality and optical response without relying on intrinsic chiral building blocks.