Photogeneration of distant radical pairs in aqueous pyruvic acid glasses

The λ > 300 nm photolysis of h ₄- or d ₄-pyruvic acid aqueous glasses at 77 K yields identical electron magnetic resonance (EMR) spectra arising from distant (r ≳ 0.5 nm) triplet radical pairs. Spectra comprise: (1) well-resolved quartets, X, at g ∼ ∼ g _e, that closely match the powder spectra of spin pairs interacting across r ∼ 1.0 nm with D ∼ 3.0 mT, E ∼ 0 mT zero field splittings (ZFS), and (2) broad signals, Y, centered at g ∼ 2.07 that display marked g-anisotropy and g-strain, exclude D ≳ 20.0 mT values (i.e., r ≳ 0.5 spin nm separations), and track the temperature dependence of related g ∼ 4 features. These results imply that the n-π* excitation of pyruvic acid, PA, induces long-range electron transfer from the promoted carbonyl chromophore into neighboring carbonyl acceptors, rather than homolysis into contact radical pairs or concerted decarboxylation into a carbene. Since PA is associated into hydrogen-bonded dimers prior to vitrification, X signals arise from radical pairs ensuing intradimer electron transfer to a locked acceptor, while Y signals involve carbonyl groups attached to randomly arranged, disjoint monomers. The ultrafast decarboxylation of primary radical ion pairs, ³[PA ^+. PA ^-.], accounts for the release of CO ₂ under cryogenic conditions, the lack of thermal hysteresis displayed by magnetic signals between 10 and 160 K, and averted charge retrotransfer. All EMR signals disappear irreversibly above the onset of ice diffusivity at ∼190 K.