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 (NO2-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 (T1) with a high yield. In this work, we used time-resolved transient absorption to investigate the excited state proton transfer of NO2-Bp-OH in its T1 state to t-butylamine, methanol, and ethanol. The T1 state of the deprotonated form NO2-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.

Excited state proton transfer in the triplet state of 8-hydroxy-5-nitroquinoline: a transient absorption and time-resolved resonance Raman spectroscopic study.

8-Hydroxy-5-nitroquinoline (NO2-QN-OH) is an antimicrobial, anti-inflammatory and anticancer agent, and has been approved for use in the treatment of diseases. Its photosensitivity, however, cannot be overlooked. The photochemistry of 8-hydroxy-5-nitroquinoline in acetonitrile is investigated using transient absorption and time-resolved resonance Raman spectroscopies. By identifying the short-lived intermediates during the photoreaction, it is clear that the Tn state NO2-QN-OH is generated in 0.8 ps via an ultrafast ISC, followed by the IC in 8.5 ps to produce the T1 state. In neat acetonitrile, the T1 state NO2-QN-OH undergoes intramolecular proton transfer and tautomerizes to form T1 state NO2-QNH-O. To our knowledge, this is the first time that the intramolecular excited state proton transfer of hydroxyl-quinolines in an aprotic polar solvent is observed.