Photophysics and photochemistry of water-soluble, sitting-atop bis-thallium(i) 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin

Abstract

In aqueous solutions, thallium(I) ions and 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin form a kinetically labile metalloporphyrin of 2 ∶ 1 composition (Tl₂P⁴⁻). The formation constant of this sitting-atop (SAT) complex is relatively low (β₂/[H⁺]² = 3.55 × 10³ M⁻² at pH = 7), due to the large size and rather small charge of Tl⁺. As a consequence of the considerably weak metal–ligand interaction in this system, the 1 ∶ 1 species does not appear in detectable concentration. Both the absorption and the emission properties of the Tl₂P⁴⁻ complex are characteristic for the typical SAT metalloporphyrins. Compared to the corresponding values of the free-base porphyrin, the diminished fluorescence quantum efficiency (Q_fl = 0.0131 vs. 0.056) of Tl₂P⁴⁻ can be accounted for by the heavy-atom effect, while the larger Stokes shift (442 vs. 282 cm⁻¹) indicates a stronger distortion of the ligand plane. Both Soret- and Q-band irradiations of the Tl₂P⁴⁻ complex lead to the degradation of the porphyrin with quantum yields of magnitude 3 × 10⁻⁴. The primary photochemical step in this process is ligand-to-metal charge transfer reaction, which is unusual for normal (coplanar) metalloporphyrins. In the case of SAT complexes, the kinetic lability facilitates the separation of the primary redox products, followed by an irreversible ring-opening of the oxidized porphyrin. Photoinduced electron ejection as a considerable step in the degradation mechanism could be ruled out.