Chemistry (Jan 2021)

Synthesis and Photostability of Cyclooctatetraene-Substituted Free Base Porphyrins

  • Joanna Buczyńska,
  • Agnieszka Gajewska,
  • Aleksander Gorski,
  • Barbara Golec,
  • Krzysztof Nawara,
  • Renata Rybakiewicz,
  • Jacek Waluk

DOI
https://doi.org/10.3390/chemistry3010008
Journal volume & issue
Vol. 3, no. 1
pp. 104 – 115

Abstract

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A series of free base meso-tetraarylporphyrins functionalized with substituents containing one, two, and four cyclooctatetraene (COT) moieties have been obtained and characterized by spectral and photophysical studies. Three COT-free porphyrins served as reference compounds. COT is a triplet quencher, well-known to enhance the photostability of several, but not all, fluorophores. In the case of porphyrins, substitution with COT improves photostability in zinc derivatives, but for free bases, the effect is the opposite. We show that placing the COT moiety further from the free base porphyrin core enhances the photostability when the COT group lies in the direct vicinity of the macrocycle. The quantum yields of photobleaching inversely correlate with porphyrin oxidation potentials. An improvement in photostability in both COT-containing and COT-free porphyrins can be achieved by screening the porphyrin core from oxygen by switching from tolyl to mesityl substituents. This leads to a decrease in the photobleaching quantum yield, even though triplet lifetimes are longer. The results confirm the involvement of oxygen in the photodegradation of porphyrins.

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