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

Weak-field magnetic response in spin-correlated radical pairs (SCRPs) is set by the radical-pair hyperfine couplings, making synthetic handles that reshape these couplings important to SCRP design. Here we show the isotropic 14N hyperfine coupling (AN) of triarylamine radical cations provides an experimental metric of hole delocalization in donor–chiral bridge–acceptor molecules. 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 triplet for radical cations generated chemically and electrochemically. AN decreases as spin density moves off the N-centered aryl and into distal rings, correlating with both 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.