Excited state proton transfer of triplet state p -nitrophenylphenol to amine and alcohol: a spectroscopic and kinetic study

Hydroxyaromatic compounds (ArOHs) have a wide range of applications in catalytic synthesis and biological processes due to their increased acidity upon photo-excitation. The proton transfer of ArOHs via the excited singlet state has been extensively studied. However, there has still been a debate on the unique type of ArOH that can undergo an ultrafast intersystem crossing. The nitro group in p -nitrophenylphenol (NO 2 -Bp-OH) enhances the spin–orbit coupling between excited singlet states and the triplet manifold, enabling ultrafast intersystem crossing and the formation of the long-lived lowest excited triplet state (T 1 ) with a high yield. In this work, we used time-resolved transient absorption to investigate the excited state proton transfer of NO 2 -Bp-OH in its T 1 state to t -butylamine, methanol, and ethanol. The T 1 state of the deprotonated form NO 2 -Bp-O − was first observed and identified in the case of t -butylamine. Kinetic analysis demonstrates that the formation of the hydrogen-bonded complex with methanol and ethanol as proton acceptors involves their trimers. The alcohol oligomer size required in the excited state proton transfer process is dependent on the excited acidity of photoacid.