Tuning the Frontier Molecular Orbitals of Aromatic Radicals via Chalcogen Substitution and Heterocyclic Stabilization

Abstract

Due to the intrinsic electron-donating effect of chalcogen lone pairs to the aromatic rings, well-characterized radicals, including phenoxy and thiophenoxy, demonstrate that sulfur substitution for oxygen generally raises the electron affinity (EA). Here we report a counterintuitive reversal using high-resolution photoelectron imaging of 4-pyridoxide (4-PyO–) and 4-pyridylthiolate (4-PyS–). Well-resolved vibronic features are obtained for their corresponding neutral radicals. The EAs are determined to be 23941 ± 7 cm–1 (2.9683 ± 0.0009 eV) for 4-PyO and 22975 ± 6 cm–1 (2.8485 ± 0.0007 eV) for 4-PyS, with EA(4-PyO) > EA(4-PyS). Term energies of the A2B2 excited state also follow this order (0.6599 vs. 0.2219 eV). Combined quantum chemical calculations and molecular orbital analyses show that the electron-withdrawing ring nitrogen stabilizes the entire π-manifold, and the compact oxygen 2p orbitals undergo stronger conjugation-induced stabilization than diffuse sulfur 3p orbitals. This heterocycle-mediated p-π conjugation modulation overrides the classical donating effect and provides a critical design principle for tuning frontier orbitals in heteroaromatic radicals.