Analysis of Hydrogen-Bonding Effects on Excited-State Proton-Coupled Electron Transfer from a Series of Phenols to a Re(I) Polypyridyl Complex

In the present study of proton-coupled electron transfer (PCET) reactions, the excited-state of a fac-[(CO)₃Re^I(bpy)(4,4′-bpy)]⁺ (bpy = 2,2′-bipyridine and 4,4′-bpy = 4,4′-bipyridine) complex was reductively quenched by a series of phenols. A variation of substituents on the phenols substantially alters their pK_a and E° values and provides an opportunity to study photoinduced PCET as a function of their redox properties. Analyses of absorption spectral changes indicate that the phenols form a weak hydrogen bond with the pyridinic nitrogen of the 4,4′-bpy ligand in the ground-state, and ground-state association constant (K_A) values were determined. This H-bonded adduct quenches the excited Re complex by PCET from the phenol, to form the reduced and protonated Re complex. The K_A values obtained aid quantitative evaluation of the rate constant for the PCET reaction in the H-bonded adduct. Thus, photophysical studies and mechanistic analysis indicate that the reaction occurs via a concerted mechanistic pathway, for the unsubstituted phenol and phenols with electron-withdrawing substituents. Furthermore, the magnitude of the quenching varies systematically with the proton-coupled potentials of the phenols and not their hydrogen-bonding strength (as reflected in K_A). This study is one of the first detailed analyses of intermolecular H-bonding between a self-assembling metal complex and a series of substituted phenols in an effort to study their relationship with the kinetic parameters in a photoinduced CPET reaction.