Hyperfine Coupling Quantifies Hole Delocalization in Triarylamine Radical Cations of D-χ-A Molecules

Spin-charge delocalization can reshape the hyperfine landscape that governs magnetic-field response in spin-correlated radical pairs (SCRPs), but delocalization is often hard to quantify experimentally. Here we show the isotropic ¹⁴N hyperfine coupling (A_N) of triarylamine radical cations provides an experimental metric of hole delocalization in donor-chiral bridge-acceptor molecules, where triarylamine acts as an electron donor. We modulate delocalization through a donor series that varies the N-aryl π-manifold (extension/fusion) and its torsion control. Room-temperature cw-EPR resolves the ¹⁴N three-line pattern for radical cations generated chemically and electrochemically at room temperature. A_N decreases as spin density moves off the N-centered aryl and into distal rings, correlating with both density functional theory (DFT) Fermi-contact terms and global delocalization metrics. In contrast, the visible-NIR radical-cation absorption contains two overlapping transitions that depend primarily on local N-bound aryl identity and therefore do not uniquely track delocalization. These findings provide a practical route to systematically tuning the effective hyperfine scale relevant to SCRP magnetic response.