Substituent and Matrix Effects in Electron-Induced One- and Two-Electron Reduction of Quinones

The one- and two-electron reduction chemistry of p-benzoquinone 1 and tetrafluoro-p-benzoquinone (fluoranil) 4 was investigated in argon and amorphous water ice matrices at cryogenic temperatures (3-9 K). Sodium atoms served as the electron source, and reaction products were characterized by matrix-isolation FTIR and UV-vis spectroscopy. In argon matrices, co-deposition of quinones with sodium vapor followed by visible-light excitation of matrix-isolated sodium atoms generated the corresponding radical anions. Photoexcitation of the p-benzoquinone radical anion 2 at 430 nm resulted in photoionization and regeneration of the parent quinone 1, whereas the fluoranil radical anion 5 was photostable under identical conditions. In amorphous water ice, sodium atoms underwent spontaneous ionization to form hydrated electrons, which reacted with embedded quinones. p-Benzoquinone 1 yielded both the radical anion 2 and the dianion 3, with subsequent photochemistry revealing a reversible interconversion between 1, 2, and 3. In contrast, fluoranil 4 underwent preferential two-electron reduction to form the dianion 6, which also displayed further photochemical transformations analogous to those observed for p-benzoquinone. These results highlight the pronounced substituent and matrix effects on low-temperature electron-transfer and photochemical behavior of quinones.