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ABSTRACT
Activity of tetrapyrrolic macrocycles such as chlorins and phthalocyanines with magnesium, aluminum and zinc in hydrogen peroxide decomposition in dark and under visible light irradiation was studied. Using adsorption isotherms the conditions for obtaining photocatalytic systems with varying degrees of adsorption of phthalocyanines were revealed. In adsorption systems, the activity of metal complexes depends on their molecular association. The principles allowing to obtain the most effective photocatalysts were found.
Keywords: Tetrapyrrolic macrocycles, metal phthalocyanines, chlorophyll, adsorption layers, molecular associates, hydrogen peroxide
AIM AND BACKGROUND
In this study catalytic and photocatalytic properties of metal tetrapyrrolic complexes were investigated. Special attention was paid to the molecular association of complexes.
INTRODUCTION
Many metal complexes of tetrapyrrole macrocycles exhibit catalytic and photocatalytic activity in redox processes. It is known that high-spin FeIII-porphyrins and FeIII-phthalocyanine with a distorted rhombic coordination structure are active in hydrogen peroxide decomposition that is also characteristic for catalase or peroxidase. It was found that FeIII-octacarboxyphthalocyanine supported on an amorphous concentrated fiber of artificial silk core element is a remarkably effective catalyst for the decomposition of hydrogen peroxide [1]. Both phthalocyanines in a solution of concentrated sulfuric acid and polymeric structure of phthalocyanines have the catalase activity. Phthalocyanines with metals of VIIIB group have the largest activity. Kinetic data for the H2O2 decay catalyzed by phthalocyanines, for a large number of metal complexes are summarized in [2-3].
However, high rate constants of excited states radiationless deactivation are observed for complexes of tetrapyrrole macrocycles with open-d-shell metals because of strong exchange interaction between metal unpaired electron and tetrapyrrole ligand molecular orbitals [4]. In contrast, complexes of porphyrins and phthalocyanines with non-transition metals (Mg, Al, Zn) are capable to generate long-lived (up to 1 ms) triplet excited states with high quantum yield (60-90%) [5]. So it is possible to realize light-controlled processes in the presence of these metal complexes. Chlorophyll (Chl) also belongs to this group of tetrapyrrolic compounds, because its molecule contains a magnesium ion.
The study of the catalytic activity of adsorbed chlorophyll and phthalocyanines-metal complexes in the hydrogen peroxide decay could be used for photodynamic therapy, artificial photosynthesis and molecular photonics [3, 6]. In this regard, in this paper the catalytic and photocatalytic activity of Chl and metal phthalocyanines adsorbed on silica in...