Robert Gerdes

Photo-oxidation of phenol and monochlorophenols in oxygen-saturated aqueous solutions by different photosensitizers

The photo-oxidation of phenol and 2-, 3- and 4-monochlorophenols by irradiation with visible light in aqueous alkaline solution in the presence of photosensitizers and oxygen is described. The reactions were carried out in the absence or presence of detergents. The photosensitizers used were Al(III), Zn(II) and Ga(III) complexes of 2,9,16,23-tetrasulphophthalocyanine (1a–1c), 5,10,15,20-tetrakis(4-carboxy-phenyl)porphyrin (2), a cationically charged perylene tetracarboxylic acid diimide (3), rose bengal (4) and methylene blue (5). The photosensitizers exhibit different activities and photo-oxidative stabilities which are strongly dependent on the pH, hydrolysis sensitivity and aggregation tendency. Singlet oxygen, obtained by photoinduced energy transfer from the excited photosensitizers, dominates the initial steps of photo-oxidation. A reaction sequence is proposed on the basis of the identification of carbon dioxide and maleic or fumaric acid as the products of photo-oxidation in the absence of detergent and the evaluated mass balances.

Efficient oxidations and photooxidations with molecular oxygen using metal phthalocyanines as catalysts and photocatalysts

Metal phthalocyanines can be very efficient as catalysts and photocatalysts in oxidation reactions using molecular oxygen as oxidant. Different types of soluble low molecular weight or oligomeric and insoluble heterogeneous catalysts and photocatalysts were developed. The heterogeneous metal phthalocyanines exist either impregnated on SiO2, Al2O3, charcoal and TiO2 or covalently and coordinatively bound on SiO2 and organic polymers or ionically bound on an organic ion exchanger. The catalytic oxidations of toxic sulfide and thiol derivative are studied. In addition, toxic phenols were employed as substrates for the photooxidation. Heterogeneous catalysts can exhibit higher activities then low molecular weight phthalocyanines. These systems exhibit a good stability for re-use. Photooxidations are more efficient than oxidations. A Si(IV) phthalocyanine derivative on a polymer ion exchanger is most active and stable. Also some examples for photooxidations in the direction of photochemical synthesis are given.

Photooxidation of sulfide, thiol, phenols, and cyclopentadiene by artificial light and solar light irradiation

The photooxidations of the toxic products sulfide, 2-mercaptoethanol and phenols under partial mineralisation in aqueous alkaline solution and the photooxidation of cyclopentadiene in ethanol in order to produce fine chemicals is described. Because the employed photosensitizers such as metal phthalocyanines, a tetraphenylporphyrine, rose bengal or methylene blue absorb in the visible region of light, an inexpensive slide projector was used under laboratory conditions.

Synthesis of charged triazatetrabenzcorroles, phthalocyanines and tetrapyridylporphyrin, and their activities in the co-sensitized photooxidation of 2-mercaptoethanol

Water soluble zwitterionic, as well as cationic and anionic triazatetrabenzcorroles (TBC), phthalocyanines (Pc) and a porphyrin (P) were synthesized and their sensitizer properties investigated. Monomerization of the sensitizers, including their mixtures, in the presence of a detergent was used as a basis for studying the fundamental effect of co-sensitization upon a known model reaction, viz. the photooxidation of 2-mercaptoethanol in aqueous alkaline solutions. Compared to single-component sensitizers, a remarkable improvement of the photocatalytic activity is observed in the case of co-sensitization employing molecules that absorb in different Q and Soret regions.

Structures and Redox Characteristics of Electron-Deficient Vanadyl Phthalocyanines

The first single-crystal X-ray structures of substituted vanadyl phthalocyanine materials reveal the high-valence vanadium ions (denoted as V(IV)), whose coordination by a highly electron-deficient ligand is facilitated by an axial oxo group. The metal center of the hydrophilic V═O core, encapsulated in F-rich hydrophobic pockets, reaches a coordination number of 6 by binding an additional H(2)O that, in turn, hydrogen-bonds with ketones, resulting in solvent-induced variable solid-state architectures. Fluoroalkyl (R(f)) ligand substituents hinder π-π stacking interactions and favor ordered long-range packing, as well as the facile formation of film materials that exhibit high thermal stability and oxidation resistance. Reversible redox chemistry and spectroscopic studies in both solution and the solid-state indicate single-site isolation in both phases and an R(f)-induced propensity for electron uptake and inhibition of electron loss. Repeated redox cycles reorganize the thin films to accommodate Li(+) ions and facilitate their migration. The facile reduction, combined with high stability and ease of sublimation imparted by the R(f) scaffold that suppresses oxidations, recommends the new materials for sensors, color displays, electronic materials, and redox catalysts, as well as other applications.

Photo-oxidation of 1,3-cyclopentadiene using partially quaternized poly(1-vinylimidazole)-bound ruthenium(II) complexes

Photo-oxidation of cyclopenta-1,3-diene (CP) with singlet oxygen using tris(2,2′-bipyridine)ruthenium(II) [Ru(bpy)32+] and polymer-bound ruthenium(II) complexes as photosensitizers was investigated in oxygen-saturated ethanol. The polymer-bound ruthenium(II) complexes consist of a bis(2,2′-bipyridine)ruthenium(II) complex coordinated with two imidazolyl residues on partially quaternized poly(1-vinylimidazole) with hexyl groups (C6RuQPIm) and hexadecyl groups (C16RuQPIm) as the alkyl side chains. The photo-oxidation of CP using the polymer photosensitizers effectively occurred in a comparable manner to the Ru(bpy)32+ system: the degree of reaction, being given by the ratio of reacted CP and initial CP concentrations, was high. In particular, all of the CP added was oxidized in the C16RuQPIm system even when the CP concentration was low. This was attributed to the concentration of CP species into the heterogeneous reaction field formed by the polymer photosensitizers. The Ru(bpy)32+ photosensitizer showed excellent stability and was repeatedly able to be used for the photo-oxidation reaction. During the repeated experiments, the reaction activity for the polymer photosensitizer systems gradually decreased because the polymer photosensitizers changed to monochloro complexes [CnRu(Cl)QPIm (n=6 or 16)] in which one imidazolyl residue was substituted by a chloride ion. However, these polymer photosensitizers also had excellent stability. Compared with other photosensitizers, such as Rose Bengal and zinc(II) phthalocyaninetetrasulfonic acid, the stability of the ruthenium(II) complex photosensitizers was excellent.

Environmental Cleaning by Molecular Photocatalysts

Visible light absorbing organic photosensitizers and photocatalysts active in photodegradation of pollutants were described in this chapter. An important reactive is singlet oxygen obtained by energy transfer from the photoexcited sensitizer to triplet oxygen (photosensitized oxidative degradation). The other possibility when the photosensitizer is on the surface of inorganic semiconductors like TiO2 is formation of superoxide radicals (photocatalytic oxidative degradations). The degree of degradation depending on the reactivity of the pollutants was discussed against these reactive species. It was shown that visible light photons have sufficient energy which can be transformed into highly reactive species of oxygen for degradation of pollutants. Technical applications were shown. In general the use of solar visible light is relatively open field for oxidative degradation of pollutants. In the Sect. 11.2 oxidative methods for photodegradation of pollutants were described. At first a short overview on UV processes for water cleaning was given in the Sect. 11.2.1. More details of photodegradation of pollutants with oxygen by the visible light were described in the Sect. 11.2.2.

Molecular and macromolecular photosensitizers in photooxidation reactions and photovoltaic cells

Based on low molecular and macromolecular phthalocyanines as light absorbing compounds different light driven reactions are shortly discussed. These processes include under irradiation with visible light energy transfer in photooxidation reactions and electron transfer in photosensibilisation cells or photovoltaic cells. Catalytic oxidation reactions are also mentioned to compare the different mechanisms of dark and light oxidations.