Photocatalytic application of watersoluble metalloporphyrins

Metalloporphyrins are key compounds in nature, but their advantageous coordination, redox and photoinduced properties can also be exploited in artificial systems, e.g., in photocatalytic reactions. According to our preliminary results water-soluble sitting-atop (SAT- metalloporphyrins containing large size metal ions located out of the plane of the porphyrin ligand exhibit unique photochemistry. They undergo efficient ligand-to-metal charge-transfer reactions followed by strong absorption of visible light. This special photoactivity allows their utilization as photocatalysts in cyclic processes for chemical synthesis and in solar energy conversion reaction mechanisms. The main goal of our proposed collaboration is the elucidation of these photoredox reaction mechanisms of such water- soluble SAT metalloporphyrins. On the basis of these new results, the catalytic systems and also in-plane complexes will be checked and utilized for preparative or photochemical water-splitting procedures for renewable energy technology and green chemistry in the future. For tuning the photoactivity of SAT metalloporphyrins, we shall synthesize water-soluble SAT complexes of a series of lanthanide ions with a wide range of ionic radii, and of other larger-sized metal ions to control the out-of-plane position of the metal center. The distortion of the ligand plane will also be governed by appropriate substituents (e.g. via bromination). The photophysical and photochemical properties of the newly prepared complexes will be determined using steady-state and time-resolved absorption, emission, resonance Raman and CW- and time-resolved ESR spectroscopy as well as quantum chemical calculations. The mechanism of their photoredox reactions will be elucidated by systematically testing wide ranges of both reductive and oxidative quenchers interacting with either the excited-state metalloporphyrins or their primary photochemical intermediates (but not with each other) . The optimal metalloporphyrin/oxidant/ reductant systems will be selected for the realization of the photocatalysis in a scalable reactor designed for solar energy utilization. We plan to use the complementary equipment of the participating Austrian and Hungarian laboratories.

Metal-porphyrins due to their special coordination and redox features play important roles in nature. Their advantageous photoinduced behavior can also be exploited in medical treatments by photodynamic therapy and in various photocatalytic procedures. Therefore, they are considered to be very promising molecules also for the utilization of solar radiation.Metal ions can form normal in-plane metal-porphyrins fitting the metal ion into the central hole of the porphyrin ring or locating the metal ion out of the ligand plane resulting in sitting-at-top (SAT) complexes. For catalytic purposes both, kinetically labile SAT-complexes as well as a normal in-plane metalloporphyrins are promising candidates for reducing the metal ions via photoinduced outer-sphere charge transfer reactions in the presence of suitable electron donors. The size and the electron configuration of the metal-ion center strongly influence the photophysical and photochemical features of these metal-porphyrins.Several new water-soluble metal-porphyrins have been synthesized and tested for their application for photo-catalytical reactions, especially for water-splitting reactions.Comprehensive structural, time-resolved laser spectroscopic, photophysical and photochemical as well as Electron-Spin Resonance (ESR)-spectroscopic studies of both labile out-of-plane and kinetically inert in-plane metal-porphyrins were accomplished in detail. Quantum-chemical (DFT)-calculations confirm that, in accordance to expectations due to the lanthanide contraction, the stability constants of the lanthanide(III)-complexes with 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin ligands decrease upon increasing ionic radius of the metal center. This is because of the increasing out-of-plane distance of the lanthanide ions. For the same reason, the efficiency of their photolysis causing predominantly irreversible redox (MLCT) reaction increases, due to the favored charge separation. Such a behavior is found for both, the 1:1 and 3:2-metal to ligands compositions. The structure of the 3:2-bisporphyrin complexes was concluded from photophysical results.Photocatalytic cycles were realized by application of kinetically inert Mn(III), Co(III) and Ni(II) complexes with cationic 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin, which generated methylviologen radical cations in the presence of triethanolamine acting as an electron donor. These radicals can be used for hydrogen evolution from water. The cycle of the Mn(III) and Co(III) complexes are significantly different from that of the Ni(II) porphyrin. The previous ones transfer the electron to the acceptor in a two-step reaction, whereas the latter species proceed in a single-step reaction. Both anionic and amphiphilic cationic Mn(III) porphyrins were also checked and used for the oxygenation and the oxidative degradation of organic compounds.