Photo-oxidation of metalloporphyrins in aqueous solution

Abstract

Water-soluble, diamagnetic metalloporphyrins have been prepared which contain either zinc@), palladium(@ or tin(1v) ions as the central metals and their photophysical properties have been measured in dilute aqueous solution. All the compounds undergo efficient intersystem crossing to form long-lived excited triplet states that can participate in electron-transfer reactions. Thus excitation of the metalloporphyrin in dilute aqueous solution containing an appropriate electron acceptor, such as iron(m), may result in formation of the metalloporphyrin n-radical cation in quite high yield. These n-radical cations are powerful oxidants, in some cases Eo > 1 y us NHE, but they undergo secondary reactions that lead to formation of n-dications and isoporphyrins. Despite the high redox potentials, it has not been possible to couple the one-electron reduction of the n-radical cations to the four-electron oxidation of water to molecular oxygen, even in the presence of a redox catalyst such as RuO,. Therefore these compounds appear to possess little promise as water oxidants in homogeneous photosystems.

Platinum(ii)–porphyrin as a sensitizer for visible-light driven water oxidation in neutral phosphate buffer

“…56 The photophysical and electrochemical data were used to construct energy level diagrams including combinations of WOC, PS and sacrificial electron acceptor (Na 2 S 2 O 8 ) in phosphate buffer at pH 7.0 ( Figure 4). Highly exothermic triplet-state one-electron transfer (ΔG = −1.01 eV) from photogenerated 3 Pt(II)-TCPP (E 1/2 (Pt III/II ) = -0.40 V) to S 2 O 8 2in buffer solution 16 …”

A Bioinspired System for Light‐Driven Water Oxidation with a Porphyrin Sensitizer and a Tetrametallic Molecular Catalyst

“…200 mV over that of the corresponding neutral species13 and provides good solubility in water 14. Earlier studies focused on the use of ZnTMePyP 4+ as a photosensitizer for light‐driven water oxidation in the presence of heterogeneous RuO 2 /TiO 2 or homogeneous Co 2+ catalysts 15. These attempts failed, and oxygen was not detected as a product.…”

Syntheses, properties and cellular studies of metallo-isoporphyrins

“…The existence of this type of compound was first suggested by Woodward [1] in 1961, whose prediction was confirmed about a decade later when Dolphin et al [2] reported the first synthesis of a metalloisoporphyrin ( 4 ) from zinc 5,10,15,20-tetraphenylporphyrin by way of electrochemical oxidation. Other methods for directly generating metallo-isoporphyrins since then include peroxide oxidation [3, 4] and photooxidation [5–8]. …”