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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 ·OBp-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.