Excited-State Quenching of Porphyrins by Hydrogen-Bonded Phenol-Pyridine Pair: Evidence of Proton-Coupled Electron Transfer

A series of porphyrins containing methoxy-substituted phenols were treated with different pyridine bases. Besides hydrogen bonding (H-bonding), the pyridine bases have imparted oxidation to the phenol rings resulting in coupled electron and proton movement. It has been shown that reduction of an excited substrate/porphyrin macrocycle by phenols with adjacent methoxy groups is facilitated by the movement or transfer of the phenolic proton toward H-bonded bases. Rates of electron transfer are accomplished by associated proton displacements within the redox reaction complex. Demonstrated fluorescence quenching of meso-(4-hydroxyphenyl derivatives)substituted porphyrins in aprotic solvents is attributed to electron transfer from the phenol moiety by added bases (different pyridine derivatives), and rates of quenching are found to be correlated with Bronsted base strength rather than H-bonding equilibria. The rate of quenching is observed to be a function of the extent of hydroxy and methoxy substitutions to the phenyls and the solvent polarities. Replacement of 4-hydroxy by 4-methoxy completely eliminated the quenching indicating the disappearance of reduction in the porphyrin macrocycle. The dependence of the extent of fluorescence quenching of studied porphyrins on pyridine concentration led to phenol-pyridine H-bonding equilibrium constants, and these values closely resemble the values obtained directly from the corresponding absorption spectra. The quenching agent is thus revealed to be H-bonded phenol. Further, positive deuterium isotope effects on quenching upon deuteration of the hydroxyl confirm that the electron transfer is coupled to the proton movement.