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

Abstract

In aqueous solutions of thallium(III) and 5,10,15,20-tetrakis(4-sulphonatophenyl)porphyrin a kinetically labile metalloporphyrin of composition 1:1 (Tl IIIP 3−) 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 × 10 6 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 Tl IIIP 3− ( Q fl = 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 Tl IIIP 3− 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 Tl IIIP 3− complex confirm that the overall reaction is a four-electron process. Reversible dissociation of the Tl IIIP 3− complex also occurs upon irradiation.