Photo Redox Research Articles

Application of a novel, efficient and recyclable photo redox catalyst (Zn–Al layered double hydroxide/eosin) for the synthesis of substituted pyridine derivatives under visible light irradiation

Layered double hydroxides (LDHs) are a class of anionic clays with brucite‐like layers and interlayer anions and varying in composition and morphology. LDHs show potential as supports for the immobilization of catalytically active species, to synthesize recyclable catalysts, in which catalytic sites can be preferentially orientated, highly dispersed and stabilized to afford high catalytic efficiency and recyclability in reaction media. Redox active organic dyes such as eosin Y with strong absorption in the visible region of the spectrum have been successfully used. In this work, an intercalated photo redox catalyst (Zn–Al LDH/eosin) was synthesized by co‐precipitation of an aqueous solution of zinc nitrate and aluminium nitrate simultaneously with eosin Y as anionic dye. Zn–Al LDH/eosin as a recyclable catalyst was used in the reaction of various aryl ketones and benzylamines under molecular oxygen and visible light, giving good yields of substituted pyridines at room temperature. Also, the Zn–Al LDH/eosin catalyst could be reused three times without any significant changes in the reaction yields.

ChemInform Abstract: Photo Redox Mediated Inexpensive One‐Pot Synthesis of 1,4‐Diphenyl Substituted Butane‐1,4‐dione from Styrene Using Polyoxometalate as a Catalyst.

1,4-diphenyl substituted butane 1,4-dione is one of the important compounds in heterocyclic chemistry as well as natural product chemistry. It is a challenge to develop a simple and cheap route for the synthesis of 1,4-diphenyl substituted butane-1,4-dione. Here we demonstrate tail to tail dimerization of styrene followed by oxidation of dimerized product in one pot under UV-light to synthesize 1,4-diphenyl substituted butane 1,4-dione. In our present work as catalysts we have used different Mo and W based polyoxometalates such as Mo132, Mo154, Mo368, PW12O40. The polyoxometalates used here possess large number of active metal centres which lead to the generation of radicals in reaction leading to photoredox mediated organic coupling reaction. Catalyzed organic compound is characterized by single crystal X-ray diffraction analysis, as well as 1H-NMR, 13C-NMR, and MALDI-MS spectroscopy. Our reaction methodology to synthesize substituted 1,4-diphenylbutane-1,4-dione is also quite economic and scalable.

Photo Redox Mediated Inexpensive One-Pot Synthesis of 1,4-Diphenyl Substituted Butane-1,4-Dione from Styrene using Polyoxometalate as a Catalyst

1,4-diphenyl substituted butane 1,4-dione is one of the important compounds in heterocyclic chemistry as well as natural product chemistry. It is a challenge to develop a simple and cheap route for the synthesis of 1,4-diphenyl substituted butane-1,4-dione. Here we demonstrate tail to tail dimerization of styrene followed by oxidation of dimerized product in one pot under UV-light to synthesize 1,4-diphenyl substituted butane 1,4-dione. In our present work as catalysts we have used different Mo and W based polyoxometalates such as Mo132, Mo154, Mo368, PW12O40. The polyoxometalates used here possess large number of active metal centres which lead to the generation of radicals in reaction leading to photoredox mediated organic coupling reaction. Catalyzed organic compound is characterized by single crystal X-ray diffraction analysis, as well as 1H-NMR, 13C-NMR, and MALDI-MS spectroscopy. Our reaction methodology to synthesize substituted 1,4-diphenylbutane-1,4-dione is also quite economic and scalable.

Photoredox catalyst-mediated direct regioselective phosphonylation of indoles

ABSTRACTAn efficient hetero-cross-dehydrogenative-coupling (hetero-CDC) reaction between N-protected indoles and phosphites to 2-indolyphosphites is described. The regioselective methodology took place in the presence of photo redox catalyst Ru(bpy)3(PF6)2 combined with oxygen as an clean oxidant when exposed to visible light, furnishing the 2-indolyphosphites as the exclusive products in moderate to good yields with good functional group tolerance. Moreover, the phosphonylation protocol was proved by the control reaction to proceed through the radical pathway.

Property and application of novel ferrocenyl–azobenzene labeled peptide nucleic acid monomers with the dual stimulus–response characteristics

In this work, three novel types of peptide nucleic acid (PNA) monomers labeled by ferrocenyl–azobenzene units (Fc–Azo–PNAs, 1a, 1b, and 1c) have been synthesized and they all exhibited good reversible photoswitch and redox properties. In the different solvents of ethanol (EtOH) and acetonitrile (ACN), photoisomerization rate constants ( K) of three PNA monomers under UV light irradiation have also been analyzed and compared, discovering that the values of K are closely correlative to their molecular structures. The electrochemical properties of the synthesized PNA monomers are also investigated and the typical redox properties are displayed, demonstrating that they can be regarded as the anchor to be grafted at the end of PNA oligomers for detecting themselves. Finally, according to the analyses of UV–Vis absorption spectrum and differential pulse voltammetry (DPV), the synthesized PNA monomers afford the best detection limit down to 10 −8 M in EtOH. The results also illuminate that PNA monomers with new photo–redox dual stimulus–response characteristics can be realized to detect themselves by the metal complex unit together with azo moiety; moreover, the hybridization with DNA also can be tuned by the isomerization of azo moiety.

Studies of solar effect of safranine, methylene blue and Azur-B with reductants and their photogalvanic effect

Photochemical studies of dyes has contributed to the understanding of the mechanism of electron transfer reactions in photoelectro-chemical devices 1-6 . A umber of photoinduced redox reactions have been studied. These reactions prove to be helpful in electrochemical cell for the conversion of solar energy to electrical energy by chemical means 7-9 . It has been found that phenazyne dyes in conjunction with different reducing agents is efficient photosensitizers for probing charge transfer in photochemical cells 1-8 . Studies carried out by Isabel et al. 9 on the photo redox reactions of thionine with iron(11), cobalt(n) and manganese(11) by flash photolysis techniques provided better understanding of the mechanism of thionine based photogalvanic cell. A detailed literature survey 1-23 reveals that different photosensitizers mainly cationic and neutral dyes and reductants have been used in photogalvanic cells but the use of combination of anionic, cations and neutral dyes as photosensitizers in the photogalvanic cell for solar energy conversion and storage is rare, and hence the present work is undertaken. The system consisting of anionic dye safranine in aqueous solution generates photovoltage when studied in photogalvanic cell. The open circuit photovoltage, the short circuit current and solar energy efficiency of these systems have been determined. The effect of different parameters on electrical output of the cell were observed and a mechanism has also been proposed for the generation of photocurrent in photogalvanic cell.

Redox abilities of rutile TiO2 ultrafine powder in aqueous solutions

Redox abilities of rutile TiO2 powder with an acicular primary particle, having about a 200 m2/g BET surface area obtained by a homogeneous precipitation process in aqueous TiOCl2 solution at 50°C for 4 hrs, were investigated using a photocatalytic reaction in aqueous 4-chlorophenol, Cu- and Pb-EDTA solutions. Its abilities were then compared with those of commercial P-25 powder, together with investigating those of anatase TiO2 powder obtained from the aqueous TiOCl2 solution, having a similar surface area and primary particle shape as those of the rutile powder. The powder was more effective than the anatase or P-25 powder, while the anatase powder unexpectedly showed the slowest decomposition rate and the smallest amount in the same experiments in spite of similar particle shapes and surface area. From the results, it is found that the excellent photo redox abilities of the powder is likely to be caused by the specific powder preparation method regardless of crystalline structures even when having similar particle shapes and values in the surface area. Also, many OH− attached to the surface of the TiO2 particle appeared to interfere with the adsorption of decomposing target materials to the TiO2 surface in the solution during the photocatalytic reaction, resulting in a delay in the reaction.

Photoinduced electron transfer and interfacial photocatalysis behaviour of ZnS/CdS binary co-colloid system

The interfacial photoinduced electron transfer and related secondary photochemical behaviour in the system of ZnS/CdS co-colloid superfine particles were studied by means of ESR and fluorescence spectroscopy techniques. The phoroinduced charge-separation and the radical intermediates produced in the secondary redox reactions initiated via charge separation, as well as the mechanism of reaction processes, were investigated in detail through simultaneous excitation of two colloid components or only one of them. Research results indicated that, as Eg(ZnS) > Egl > Eg(CdS), only CdS in co-colloid system might be excited. The transfer process of electron from the conduction band of CdS to the conduction band of ZnS is forbidden, and under the excitation wavelength range used, the electron transfer of cocolloid system was impossible, thus the photo redox reactions of the substrate in co-colloid system had no obvious difference from those reactions happening in single colloid system. While the excitation wavelength met the conditionEλ>Eg(ZnS)>Eg(CdS), both components of ZnS/CdS co-colloid system might be simultaneously excited, so the electron of ZnS conduction band could transfer to the conduction band of CdS, leading to an effective separation of charge, thereby the extent iind approach for the photoredox rcactions of various substrates in co-colloid system showed an extinct difference from that of single colloid system. Some reactions might be weakened or restrained entirely, and some reactions might be strengthened clearly; therefore, thew characteristics plays an important role in controlling the direction and mechanism of interfacial reactions.

Luminescent Ruthenium Polypyridyl Complexes Containing Pendant Pyridinium Acceptors

Direct attachment of putative electron acceptors ((bi)pyridinium ions) to either Ru(bpy)32+ or Ru(1,10-phenanthroline)32+ (excellent photo-excited-state electron donors) leads to enhancement of excited-state lifetimes, rather than the diminution anticipated from electron transfer quenching. An evaluation of photo redox and emission energetics suggests that charged “quencher” attachment lengthens luminescence lifetimes by (a) diminishing the reductant strength of the photo excited state, (b) inhibiting intersystem crossing to a short-lived d−d state, and/or (c) diminishing Franck−Condon factors for nonradiative decay.

Photokatalytische Systeme. XLVIII. Ungewöhnliche lichtinduzierte Redoxreaktionen von Cobalt(III)‐, Eisen(III)‐ und Kupfer(II)‐Komplexverbindungen in Gegenwart von Chromsäureestern

Photocatalytic Systems. XLVIII. Unusual Light‐Induced Redox Reactions of Cobalt(III), Iron(III) and Copper(II) Complexes in Presence of Chromic Acid EstersThe coordination compounds[Co(NH3)5H2O]3+,[Co(NH3)4(H2O)2]3+,[Co(en)2(H2O)Cl]2+ (en = ethylendiamine), [L2(H2O)Fe‐O‐Fe(H2O)L2]4+ (L = 1,10‐phenanthroline; 2,2′‐bipyridine) and [CuL2]2+ (L = 2,9‐dimethyl‐1,10‐phenanthroline) show no or neglegible photo redox reactions upon irradiation wave‐length above 350 nm. However, in the presence of chromate(VI), water, and different alcohols like methanol, ethanol, i‐propanol and n‐propanol a very efficient light‐induced formation of cobalt(II), iron(II) and copper(I) (ϕ = 0,7; Λirr = 375 nm) can be observed together with reduced generation of chromium(III). The participation of chromium(V) in the primary step of the photolysis of chromic acid esters was shown to be the reason for these unusual light‐induced redox reactions. The coordination compounds under discussion behave as very efficient scavengers for chromium(V) connected with simultaneous reoxidation to chromate(VI) and reduction to the appropriate cobalt(II), iron(II) and copper(I) compounds. The general importance of the participation of chromium(V) in the primary step of the photolysis of chromate(VI) in non‐aqueous solvents is also shown by investigations of the photochemical behaviour of the system metal complex/CrO4—, Cr2O7—/dimethylsulfoxide. Possible mechanisms of the light‐induced reductions of the metal complexes are discussed.