Direct Observation of the Transient Species Generated from Protonation and Deprotonation of the Lowest Triplet of p-Nitrophenylphenol

Photo-induced proton coupled electron transfer (PCET) is essential in the biological, photosynthesis, catalysis and solar energy conversion processes. Recently, p-nitrophenylphenol (HO-Bp-NO₂) has been used as a model compound to study the photo-induced PCET mechanism using ultrafast spectroscopy. In transient absorption spectra both singlet and triplet exhibited PCET behavior. When we focused on the PCET in the triplet state, a new sharp band attracted us. This band had not been observed for p-nitrobiphenyl which is without hydroxyl substitution. To assign the new band, acidic solutions were used as an additional proton donor. Based on results in strong (~10^-1 M) and weak (~10^-4 M) acidic solutions, the new band is identified as the open shell singlet O-Bp-NO₂H, which is generated through protonation of nitro O in ³HO-Bp-NO₂ followed by deprotonation of hydroxyl. Kinetics analysis indicates the formation of radical •O-Bp-NO₂ competes with O-Bp-NO₂H in the way of concerted electron-proton transfer and/or proton followed electron transfers and is responsible for the low yield of O-Bp-NO₂H. The results in the present work will make it clear that how the ³HO-Bp-NO₂ deactivates in aprotic polar solvents and provide a solid benchmark for the deeply studying the PCET mechanism in triplets of analogous aromatic nitro compound.