Role of hydrogen bonding in the photophysical properties of isomeric tetrapyridylporphyrins in aprotic solvent

Extensive photophysical properties of isomeric tetra-2-pyridylporphyrin (TpyP(2)), tetra-3-pyridylporphyrin (TpyP(3)), and tetra-4-pyridylporphyrin (TpyP(4)) have been studied in the presence of a series of phenols of increasing hydrogen bonding power in dichloromethane solution by employing UV/vis spectroscopy; steady-state, time-resolved fluorescence spectroscopy; and transient absorption spectroscopic techniques. The change of absorption spectra of all three porphyrins as a function of different phenol concentrations established the preference of hydrogen bonded complex formation to the peripheral pyridyl nitrogen rather than the pyrrole nitrogen of the porphyrin macrocycle. The fluorescence behaviors of the porphyrins which were observed upon addition of different phenols point to a marked dependence on the nature of the added phenols. Phenols with an electron withdrawing group do not quench the fluorescence of porphyrins, whereas phenols with an electron donating group quench the singlet porphyrin both in static and dynamic pathways. A remarkable difference in quenching behaviors of singlet excited porphyrin by 4-methylphenol (4-MePhOH) and 4-MeOPhOH/4-EtOPhOH (4-EtOPhOH = 4-ethoxyphenol) are observed. The quenching of singlet excited porphyrins by 4-MePhOH is attributed to be purely static in nature, and the H-bond provides a strong nonradiative channel to singlet excited porphyrins. However, the quenching of singlet excited porphyrins by 4-MeOPhOH/4-EtOPhOH is mostly dynamic, and it is ascribed to be the reductive quenching of single excited porphyrins. Picosecond transient absorption study with TpyP(2) and 4-MeOPhOH provides the evidence of porphyrin radical anion and phenol radical cation of equal lifetime, which indicates the fact that electron transfer occurs from phenol to singlet excited porphyrin. The temperature effect on dynamic quenching by 4-MeOPhOH/4-EtOPhOH and kinetic deuterium isotope effect established the reaction to be a photoinduced concerted proton coupled electron transfer.