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Direct observation of light-driven, concerted electron–proton transfer

Significance Concerted proton-coupled electron transfer (EPT) reactions in which both electrons and protons transfer in tandem are at the heart of many chemical and biological conversions including photosystem II. We report here the direct observation of absorption bands arising from photoEPT transitions, in this case, in H-bonded complexes between N-methyl-4,4′-bipyridinium cation and biologically relevant donors including tyrosine. The importance of these observations follows from the earlier experimental observations by Taube and coworkers on intervalence transfer in mixed-valence complexes. The observation of these photoEPT transitions and the appearance of reactive radical products also points to a possible, if inefficient, role in DNA photodamage and, possibly, in the formation of reactive oxygen intermediates. The phenols 4-methylphenol, 4-methoxyphenol, and N-acetyl-tyrosine form hydrogen-bonded adducts with N-methyl-4, 4′-bipyridinium cation (MQ+) in aqueous solution as evidenced by the appearance of low-energy, low-absorptivity features in UV-visible spectra. They are assigned to the known examples of optically induced, concerted electron–proton transfer, photoEPT. The results of ultrafast transient absorption measurements on the assembly MeOPhO-H---MQ+ are consistent with concerted EPT by the instantaneous appearance of spectral features for MeOPhO·---H-MQ+ in the transient spectra at the first observation time of 0.1 ps. The transient decays to MeOPhO-H---MQ+ in 2.5 ps, accompanied by the appearance of oscillations in the decay traces with a period of ∼1 ps, consistent with a vibrational coherence and relaxation from a higher υ(N-H) vibrational level or levels on the timescale for back EPT.