Visible irradiation

Bare CuOx-supported nanostructures showed some activity in H 2 production from methanol-water mixture under UV-visible irradiation [180]. Ni is also used as a dopant, and small amounts (1 wt.%) of this element in mesoporous titania guarantee good activity in water-methanol mixtures under UV-visible light [181]. Indium-tantalum oxide Ni-doped materials also provided photocatalysts with promising efficiencies for direct water splitting [182]. TiOz nanotubes doped with Ir and Co nanopartides were effective for visible light water splitting even in the absence of... 

The carboxylic acid derivatives li-lm can only be matrix-isolated if the corresponding quinone diazides 2i-2m are irradiated with monochromatic blue light (k = 436 nm).81 91 92 UV or broad-band visible irradiation rapidly results in the decarboxylation of the carbenes. As expected, the IR and UV/vis spectra of the carbenes are very similar to that of la. Oxygen trapping results in the formation of the photolabile carbonyl oxides 7. Thus, the carbenes li-lm were identified both spectroscopically and by their characteristic reaction with molecular oxygen. 

To probe the origin of acetaldehyde in ethene oxidation, ethene oxide was admitted to the (TS-I/H2O2) system containing TiOOH groups. The formation of acetaldehyde was negligible even under the influence of UV-visible irradiation. Hence, the significant amount (10%) of acetaldehyde formed in the reaction of ethene with TS-1/H202 could not have been the product of the further reaction of ethene oxide. It is rather a primary product of oxidation at the vinylic carbon atom. 

Upon absorption of light an electron from the HOMO of the adsorbed dye, D, is raised to the LUMO from where it is injected into the conduction band of the n-type semiconductor and transferred to a counter electrode where an oxidant, O, is reduced. From the reduced species, R, the electron is transferred to the HOMO of the adsorbed dye to fill the electron vacancy, so that after the overall photoelectro-chemical process the dye is in its original oxidation state. Vlachopoulos et al. (1987) have reported on Ti02 photoelectrodes that were sensitized to visible light with various dyes and that showed high quantum yields of interfacial electron transfer under visible irradiation. 

Sulfur-based catalysts shows smaller band gaps with respect to the corresponding oxides. A ZnS-CuInS2-AgInS2 solid solution was recently proposed for H2 production under visible irradiation in the presence of aqueous solution containing sulfide and sulfite [129]. The main drawback of sulfide is photocorrosion. 

Y. Yang, Q. Wu, Y. Guo, C. Hu and E. Wang, Efficient degradation of dye pollutants on nanoporous polyoxotungstate anatase composite under visible irradiation. J. Mol. Catal.A Chem., 225 (2005) 203-212. 

The spiropyran derivative incorporated RMs showed good photo-controUed extraction of zwitter-ionic amino acid under UV irradiation and release under visible irradiation Unlike native enzyme, the modified counterpart tended to form reversible oligomeric structures in RMs, which allowed in realizing the role of carbohydrate moieties in the functioning of glycoproteins... 

Solar spectrum on the earth ranges from 250 to 2400 nm, having its maximum at 500 nm. The energy abundant ultraviolet region below 400 nm contributes only 5 % the visible region between 400 and 800 nm occupies about half the spectrum. The conversion of this visible irradiation is therefore important. 

Photosynthesis is the reduction of C02 by electrons from water with the help of visible irradiation producing carbohydrate and oxygen. The outline of the electrone flow is expressed by Fig. 1. The electron from water is pumped up twice by photosystems II and I (PS II and I), where chlorophyll (Chi) molecules play the main role for the excitation, energy concentration, and charge separation. 

Since photoinduced charge separation occurs easily at the semiconductor-water interface, its stabilization with polymer coating is one of the most promising approaches to water photolysis by visible irradiation. 

As described in Section 3 of Chapter 4, the stabilization of n-Si electrode by coating with poly(pyrrole) has attracted much attention. The stabilization of a small bandgap n-semiconductor electrode against oxidation is of great value not only to convert visible light into chemical energy, but also to construct liquid-junction solar cells operated under visible irradiation. The poly(pyrrole) film is usually electropolymerized on the semiconductor electrode dipped in the aqueous solution of pyrrole. The remarkable stabilizing effect of poly(pyrrole) film on polycrystalline n-Si is shown in Fig. 22 67). The photocurrent obtained under irradiation in the aqueous solution of... 

To realize the above system, it is required to design a polymer which reversibly changes the molecular properties, such as hydrophilicity, by the external stimulation. Many molecules are known to be reversibly transformed to other isomers by external stimulation, such as photons, electrons or chemicals. Table 1 lists a few examples. Azobenzene shows the property change by photoirradiation. It isomerizes from the trans to the cis form by ultraviolet irradiation, and the dipole moment increases from 0.5 to 3.1 deb ye. The polar cis form returns to the less polar tram form by visible irradiation. Electrochemical oxidation of ferrocene changes the hydrophilicity. When it is oxidized from Fe(II) to Fe(III), the hydrophilicity increases. The Feflll) state returns to the Fe(II) state by either electrochemical or chemical reduction. Host molecules also change the properties in the presence of suitable guest ions. Benzo[18]crown-6, for example, captures potassium ions in the cavity, and increases the hydrophilicity. 

The phase separation temperature shift by photo-isomerization implies that in the temperature range between 19.4 and 26.0 °C, ultraviolet irradiation solubilizes the polymer or expands the polymer chain, while visible irradiation decreases the solubility, or shrinks the chain. Figure 3 shows the photo-stimulated phase separation at 19.5 °C accompanied by the conformation change of the polymer chain. Upon exposure to UV light (350 <%< 410 nm), the opaque solution became transparent, while visible irradiation (>. > 470 nm) again decreased the transmittance of the solution. The polymer chain conformation changed concomitantly with the phase separation. 

Fig. 2. Subdiploid DNA peak assay. Burkitt lymphoma cells were analyzed for cell cycle 24 h after irradiation with 4 Gy. Control cells (A) show a cycling population with the Gl. S, and G2M phases clearly visible. Irradiated cells (B) have an additional peak with decreased fluorescence (the subdiploid peak [SD]) representing apoptotic cells.

The polymerization is initiated (1) by generating free radicals R using an initiator, a molecule that decomposes under heating or under UV or visible irradiation. A radical attacks the double bond of a monomer, resulting in the formation of an intermediate radical Mi . This is the rate-limiting step of the process. 

The diphenylphenanthroline complex 23 is emissive in fluid solutions, with quantum yields of ca. 10-4 and lifetimes of 0.4-0.7 ps. The estimated excited state reduction potential of 2.2 V (vs. normal hydrogen electrode (NHE)) suggested that the complex is a strong photooxidant, which was demonstrated with the formation of the 1,4-dimethoxybenzene radical cation (DMB+) upon UV-visible irradiation of an MeCN solution of the complex with DMB.22... 

Fig. 12 Reflectivity recorded as a function of after UV irradiation. The crosses represent the external incidence angle (O ofthe measur- experimental data and the dotted lines are the ing device above) forTM polarized light at 785 theoretical curves obtained from the fitting nm wavelength, after visible irradiation and procedure.

Brenowitz, S. and Castenholz, R. W., Long-term effects of UV and visible irradiance on natural populations of a scytonemin-containing cyanobacterium (Calothrix sp.), FEMS Microbiol. Ecol., 24, 343, 1997. 

Photoelectron spectra have been reported for [RhH(PF3)4] and some related tri-fluorophosphine species.120 Comparison with related carbonyl complexes suggests similar 7t-acceptor properties for CO and PF3 ligands. Studies have shown that the activity of the hydrogenation catalyst [RhH(CO)2(PPh3)3], which decreases with time, can be regenerated using weak u.v.-visible irradiation.121... 

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