Proton coupled electron transfer in tri-molecular system involving 7-Methylbenzo [a] pyrene and different phenol–base pairs: A case study for determining hydrogen bonding equilibrium constant

Photo induced proton-coupled electron transfer (PCET) reactions comprehend the concerted transfer of an electron from donor to acceptor and a proton along the hydrogen bonded axis in same time scale. Photo-oxidations of hydrogen-bonded phenols by excited-state polyarene are utilized to extract concerted proton−electron transfer reactions rate and hydrogen bonding equilibrium constant. Experiments have examined fluorescence quenching of 7-Methyl benzo[a]pyrene (7-MeB[a]P) in all possible ways in solution phase by phenol-bases pairs (hydrogen bonded adducts) for two phenols, 4-Methoxy phenol (4-MeO PhOH) and 2,6- Dimethoxy phenol (2,6-DiMeo PhOH), and different pyridine derivatives, such as simple Pyridine (Py), 2,4-Dimethylpyridine (DMPy), 2,4,6-Trimethylpyridine (TMPy), 4-(N,N-Dimethylaminopyridine) (DMAPy), along with 1-Methylimidazole (IM). Detection of 7-MeB[a]P anion in femtosecond transient absorption studies and positive kinetic isotope effect confirm the fluorescence quenching of 7-MeB[a]P is due to PCET reaction where photo-oxidations of phenols proceed by intermolecular proton movement of phenolic proton to the base along the hydrogen bond axis concerted with electron transfer to the photo excited 7-MeB[a]P. The systematic fluorescence quenching experiments of 7-MeB[a]P by hydrogen bonded adducts are performed in two different ways: Case-1, variation of base concentration for fixed and dilute solution of 7-MeB[a]P and phenol mixture and Case-2, changing phenol concentration for fixed and dilute solution of 7-MeB[a]P and base. These two data sets are thus well described by non-linear Stern-Volmer type relation, providing a strong validation for the use of these approaches for determining hydrogen bonding equilibrium constants (KHB) along with rate constant (kQB) for PCET reaction. The estimated KHB values are fairly consistence to that observed in typical spectrophotometric method following Mataga-Tsuno relation. Thus, this is a very clear demonstration that the PCET reactions can be used to determine KHB in favourable cases where ground state hydrogen bonding is the essential and necessary condition for PCET reaction to occur.