Photophysics and photochemistry of kinetically labile, water-soluble porphyrin complexes

Abstract

Most metalloporphyrins contain a metal ion in the center of the planar tetrapyrrolic ring system, resulting in a kinetically inert complex. If, however, the ionic radius of the metal ions is too large (over ca. 75–80 pm) to fit into the hole in the center of the macrocycle, they are located out of the ligand plane, distorting it. These kinetically labile sitting-atop (SAT) complexes display characteristic structural and photoinduced properties that strongly deviates from those of the regular metalloporphyrins. In this paper we review the results of studies on water-soluble TPPS 6− (H 2TPPS 4− = 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin anion) complexes of some metal ions, each with two different oxidation states (Fe 3+, Fe 2+, Tl 3+, Tl +, Hg 2+, and Hg 2 2+) performed in order to reveal how the charge, and especially the size of the metal center influence their composition, structure and photoinduced behavior. While the heme-like (1:1) iron(II) complex displays typical SAT properties (red-shifted Soret absorption band and blue-shifted emission and Q absorption bands, irreversible photoinduced porphyrin ligand-to-metal charge transfer reaction), the corresponding iron(III) porphyrin is a regular one with no emission and photoredox behavior. Moreover, in the synthesis of the latter complex Fe 2+ ions act as catalyst via formation of kinetically labile iron(II) porphyrin as intermediate. In contrast to iron, thallium (Tl 3+, Tl +) and mercury (Hg 2+, Hg 2 2+) ions form exclusively SAT complexes, regardless of their oxidation state. These metalloporphyrins, however, are of various composition with considerable variation in the efficiencies of their emission and photoredox activities. While thallium(III) forms 1:1 TPPS 6− complex, the composition of the corresponding thallium(I) species is 2:1. Despite their similar absorption and emission spectra, the quantum yield for the photoredox degradation of the thallium(I) complex is significantly larger, due to its much lower stability. Mercury ions (both Hg 2+ and Hg 2 2+) form also bis-porphyrin complexes, which are, however, not fluorescent at room temperature. Interestingly, the energy of the Q-band excitation is enough to promote their photoinduced dissociation, which can only be observed upon Soret-band irradiation in the case of the monoporphyrin complexes. All these results clearly indicate that the photoinduced behavior (both the irreversible LMCT reaction and the reversible dissociation) of these metalloporphyrins basically depends on the size (instead of the charge) of the metal center, determining the structure of these coordination compounds. Deviating from the photochemistry of the regular (coplanar) metalloporphyrins, the out-of-plane structure and the kinetic lability of the SAT complexes facilitate the photodissociation as well as the separation of the primary products of the photoinduced LMCT reaction, promoting an irreversible ring-opening of the oxidized porphyrin.