Enhancing Photostability by Thermodynamic and Kinetic Factors: Free-Base and Palladium meso-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 meso 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.