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.

A theoretical insight into excited-state decay and proton transfer ofp-nitrophenylphenol in the gas phase and methanol solution

“…34 Experimentally, transient absorption and kinetic analysis results showed that three methanol molecules were required to quench the free triplet followed by the formation of the anion triplet (see the ESI † for experimental and theoretical considerations for choosing the number of Paper PCCP methanol molecules). 18 Therefore, in our QM/MM calculations, the QM subsystem was composed of the NO 2 -Bp-OH chromophore and three hydrogen-bonded methanol molecules nearby the hydroxyl group, which were treated with the CASSCF method; whereas the remaining methanol molecules were all regarded as the MM subsystem and carried out with the modified CHARMM22 force field, referred to as QM(CASSCF)/ MM. 29 In addition, any molecules within 15 Å from the NO 2 -Bp-OH chromophore were allowed to move, while the outermost solvent layer including about 470 methanol molecules were fixed.…”

“…This anion form is consistent with the anion triplet observed in the transient absorption experiment. 18 In addition, as depicted in Fig. 8, we also computed the EPT reactions in the singlet 1 np* and 1 pp* states, which have localized and delocalized (i.e.…”

“…16 However, the long-lived and high-quantum-yield T 1 state possesses photoacid properties with a pK Ã a in acetonitrile of 16.8. 18,19 This means that EPT can be used on weak bases such as methanol. Kinetic analysis demonstrated that the formation of the hydrogen-bonded complex with methanol as proton acceptors involves their trimers (see the ESI † for details).…”

“…Kinetic analysis demonstrated that the formation of the hydrogen-bonded complex with methanol as proton acceptors involves their trimers (see the ESI † for details). 18,19 As a result, we put three methanol molecules near the hydroxyl group to form a hydrogen-bond network, which was regarded as the QM region and treated with high-level electronic structure calculations.…”

“…More recently, Xue and coworkers revisited the EPT reaction in the excited triplet state of NO 2 -Bp-OH and its alcohol adducts with transient absorption methods. 18,19 They concluded that the nitro group could enhance spin-orbit coupling and thus prompt the formation of the reactive T 1 state with a high quantum yield, as seen in many other nitro-substituted chromophores. 20,21 In addition, the long-lived T 1 state is capable of the EPT reaction to weak bases such as methanol and ethanol via the hydrogenbonded adducts between triplet NO 2 -Bp-OH and three methanol or ethanol molecules.…”