Kinetic evidence for methanol trimer assisted proton transfer: Transient absorption study on excited triplet state of 4-hydroxy-4′-nitrobiphenyl

Abstract

Hydroxyaromatic compounds have a wide range of applications in catalytic synthesis and biological processes due to their enhanced acidity upon photo-excitation. Most hydroxyaromatic compounds with a medium excited state acidity are unable to deprotonate in non-aqueous solvents such as alcohol due to their short-lived excited singlet states. The nitro group in 4-hydroxy-4′-nitrobiphenyl (NO2-Bp-OH) increases the spin-orbit coupling between excited singlet states and the triplet state, resulting in ultrafast intersystem crossing and the formation of the long-lived lowest excited triplet state (T1) with a high yield. Using transient absorption spectroscopy and kinetic analysis, we discover that, despite its moderate acidity, the T1 state of NO2-Bp-OH (3NO2-Bp-OH) is able to transfer proton to methanol. Following the formation of the hydrogen-boned complex between 3NO2-Bp-OH and three methanol molecules in a consecutive process, proton transfer occurs very fast. This finding suggests that the long lifetime of the photoacid excited state allows for the formation of alcohol oligomer with sufficient basicity to induce photoacid deprotonation.