Enhancing Photostability by Thermodynamic and Kinetic Factors: Free-Base and Palladium <i>meso</i>-Aryl-octaethylporphyrins

Photostability is a crucial parameter in applications based on light-matter interactions. In this work, we demonstrate that photodegradation efficiency can be strongly decreased by altering the thermodynamic and kinetic characteristics of a chromophore. Photobleaching quantum yields have been determined for a series of free-base octaethylporphyrins and their palladium metallocomplexes gradually substituted by phenyl groups at the <i>meso</i> positions. Due to increased oxidation potential of palladium porphyrins, photostability is improved in comparison with zinc or magnesium derivatives. A spectacular effect is observed for nonplanar palladium derivatives in which the triplet lifetime in deoxygenated solution is shortened by 3 orders of magnitude with respect to planar porphyrins. Comparison of photodestruction efficiencies in oxygen-containing and degassed toluene samples shows a hundred-fold decrease of photobleaching quantum yields for nonplanar palladium porphyrins, reaching an extremely low value of less than 10^-9. In contrast, free-base, nonplanar porphyrins are less stable than the planar analogues in nondegassed toluene. Finally, planar free-base and palladium porphyrins become significantly less photostable in the degassed solution because the triplet decay time increases by 3 orders of magnitude compared to oxygen-containing samples.