# Enhancing Photostability by Thermodynamic and Kinetic Factors: Free-Base and Palladium meso-Aryl-octaethylporphyrins

## TL;DR

This paper shows how altering the structure of porphyrin compounds can dramatically improve their resistance to light-induced degradation.

## Contribution

The study reveals that nonplanar palladium porphyrins exhibit exceptionally low photobleaching due to shortened triplet lifetimes and reduced oxidation.

## Key findings

- Nonplanar palladium porphyrins show photobleaching quantum yields below 10–9 in degassed toluene.
- Triplet lifetimes in nonplanar palladium porphyrins are reduced by 3 orders of magnitude compared to planar ones.
- Free-base nonplanar porphyrins are less stable than planar ones in nondegassed toluene.

## Abstract

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.