Radical Ions of 3‑Styryl-quinoxalin-2-one Derivatives Studied by Pulse Radiolysis in Organic Solvents

The absorption-spectral and kinetic behaviors of radical ions and neutral hydrogenated radicals of seven 3-styryl-quinoxalin-2­(1H)-one (3-SQ) derivatives, one without substituents in the styryl moiety, four others with electron-donating (R = −CH₃, −OCH₃, and −N­(CH₃)₂) or electron-withdrawing (R = −OCF₃) substituents in the para position in their benzene ring, and remaining two with double methoxy substituents (−OCH₃), however, at different positions (meta/para and ortho/meta) have been studied by UV–vis spectrophotometric pulse radiolysis in neat acetonitrile saturated with argon (Ar) and oxygen (O₂) and in 2-propanol saturated with Ar, at room temperature. In acetonitrile solutions, the radical anions (4R-SQ^•–) are characterized by two absorption maxima located at λ_max = 470–490 nm and λ_max = 510–540 nm, with the respective molar absorption coefficients ε_470–490 = 8500–13 100 M^–1 cm^–1 and ε_510–540 = 6100–10 300 M^–1 cm^–1, depending on the substituent (R). All 4R-SQ^•– decay in acetonitrile via first-order kinetics, with the rate constants in the range (1.2–1.5) × 10⁶ s^–1. In 2-propanol solutions, they decay predominantly through protonation by the solvent, forming neutral hydrogenated radicals (4R-SQH^•), which are characterized by weak absorption bands with λ_max = 480–490 nm. Being oxygen-insensitive, the radical cations (4R-SQ^•+) are characterized by a strong absorption with λ_max = 450–630 nm, depending on the substituent (R). They are formed in a charge-transfer reaction between a radical cation derived from acetonitrile (ACN^•+) and substituted 3-styryl-quinoxalin-2-one derivatives (4R-SQ) with a pseudo-first-order rate constant k = (2.7–4.7) × 10⁵ s^–1 measured in solutions containing 0.1 mM 4R-3-SQ. The Hammett equation plot gave a very small negative slope (ρ = −0.08), indicating a very weak influence of the substituents in the benzene ring on the rate of charge-transfer reaction. The decay of 4R-SQ^•+ in Ar-saturated acetonitrile solutions occurs with a pseudo-first-order rate constant k = (1.6–6.2) × 10⁴ s^–1 and, in principle, is not affected by the presence of O₂, suggesting charge–spin delocalization over the whole 3-SQ molecule. Most of the radiolytically generated transient spectra are reasonably well-reproduced by semiempirical PM3-ZINDO/S (for 4R-SQ^•–) and density functional theory quantum mechanics calculations employing M06-2x hybrid functional together with the def2-TZVP basis set (for 4R-SQ^•+).