Direct observation of 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 by using ultrafast spectroscopy. In transient absorption spectra both singlet and triplet states were observed to exhibit PCET behavior upon laser excitation of HO-Bp-NO₂. When we focused on the PCET in the triplet state, a new sharp band attracted us. This band was recorded upon excitation of HO-Bp-NO₂ in aprotic polar solvents, and has not been observed for p-nitrobiphenyl which is without hydroxyl substitution. In order to find out what the new band represents, acidic solutions were used as an additional proton donor considering the acidity of HO-Bp-NO₂. With the help of results in strong (~10⁻¹ mol/L) and weak (~10⁻⁴ mol/L) acidic solutions, the new band is identified as 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 that 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 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 compounds.