Formation, photophysics and photochemistry of thallium(III) 5,10,15,20-tetrakis(4-sulphonatophenyl)porphyrin: New supports of typical sitting-atop features

Zsolt Valicsek, Ottó Horváth

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27 Citations (Scopus)

Abstract

In aqueous solutions of thallium(III) and 5,10,15,20-tetrakis(4-sulphonatophenyl)porphyrin a kinetically labile metalloporphyrin of composition 1:1 (TlIIIP3-) is formed in a moderately fast bimolecular reaction (k = 270 M-1 s-1). The formation constant of this sitting-atop (SAT) complex is rather high (K′ = 9.5 × 106 M-1 at pH 6), due to the not too large size (compared to the core of the ligand) and the three-fold positive charge of Tl(III). The strong coordinative bond is manifested in the large shifts of the emission and absorption bands of the porphyrin. The directions of these shifts characteristically depend on the structure of the metalloporphyrin, making the identification of the sitting-atop type unambiguous. Compared to the corresponding values of the free-base porphyrin, the dramatically diminished fluorescence quantum efficiency of TlIIIP3- (Qfl = 0.00079 versus 0.075) can be accounted for by the heavy-atom effect and the distortion to saddle shape. Both Soret- and Q-band irradiations of the TlIIIP3- complex lead to the degradation of the porphyrin with quantum yields of (1-3) × 10-5 magnitude in both aerated and argon-saturated solutions. The primary photochemical step in this process is ligand-to-metal-charge-transfer reaction followed by the detachment of the reduced metal center. The latter step is favoured by the out-of-plane position of the metal in the case of SAT complexes. Subsequent to the demetallation, an irreversible ring-opening of the oxidized porphyrin takes place. The results on the photolysis of the TlIIIP3- complex confirm that the overall reaction is a four-electron process. Reversible dissociation of the TlIIIP3- complex also occurs upon irradiation.

Original languageEnglish
Pages (from-to)1-7
Number of pages7
JournalJournal of Photochemistry and Photobiology A: Chemistry
Volume186
Issue number1
DOIs
Publication statusPublished - Feb 5 2007

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Keywords

  • Charge transfer
  • Photochemistry
  • Photodissociation
  • Sitting-atop complex
  • Thallium(III) ion
  • Water-soluble porphyrin

ASJC Scopus subject areas

  • Chemistry(all)
  • Chemical Engineering(all)
  • Physics and Astronomy(all)

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