Ultraviolet-visible irradiation

Wang, B., Li, C, Hirabayashi, D., and Suzuki, K. (2010) Hydrogen evolution by photocatalytic decomposition of water under ultraviolet-visible irradiation over K2La2Ti3L M Oio+6 perovskite. Int. J. Hydrogen Energy, 35, 3306-3312. 

The ultraviolet inactivation spectrum for gramicidin has been published by Setlow and Doyle121. Sugimoto and coworkers have presented infrared, ultraviolet, visible and ESR spectra for gramicidin solutions irradiated with various amounts of ultraviolet light122. 

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. 

Research Focus Preparation of superoxide anions generated using water-soluble fullerenes by irradiating with ultraviolet-visible (UV-Vis) light. 

The photochemical and thermal stabilities of Ru complexes have been investigated in detail [8,153-156]. For example, it has been reported that the NCS ligand of the N3 dye, cri-Ru(II)(dcbpy)2(NCS)2 (dcbpy = 2,2 -bipyridyl-4,4 -dicarboxylic acid), is oxidized to produce a cyano group (—CN) under irradiation in methanol solution. It was measured by both ultraviolet-visible (UV-vis) absorption spectroscopy and nuclear magnetic resonance (NMR) [8,153]. In addition, the intensity of the infrared (IR) absorption peak attributed to the NCS ligand starts to decrease at 135°C, and decarboxylation of N3 dyes occurs at temperatures above 180°C [155]. Desorption of the dye from the 2 surface has been observed at temperatures above 200°C. 

Finally, it should be pointed out that methods used to study short-lived chemical intermediates in fast thermal reactions may be applicable also to photochemical studies. Radical intermediates, however generated, can be studied by CIDNP ichemically induced dynamic nuclear spin polarization), in which the n.m.r. spectrum of the reaction mixture is recorded during the reaction period. II a substrate is continuously irradiated with ultraviolet/visible light in the cavity of an n.m.r. spectrometer, the resulting n.m.r. spectrum of the substrate/product mixture exhibits intensity variations as compared with the normal spectrum—intensity enhancement, reduction or even reversal (i.e. emissionl. Note that the spectrum involved is not... 

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. 

COLOR CENTERS. Certain crystals, such as the alkali halides, can be colored by the introduction of excess alkali metal into the lattice, or by irradiation with x-rays, energetic electrons, etc. Thus sodium chloride acquires a yellow color and potassium chloride a blue-violet color. The absorption spectra of such crystals have definite absorption bands throughout the ultraviolet, visible and near-infrared regions. The term color center is applied to special electronic configurations in the solid. The simplest and best understood of these color centers is the F center. Color centers are basically lattice defects that absorb light. 

Figure 14 (Top) Irradiance profile of ambient light measured on the laboratory bench (Sylvania Octron F043/741 fluorescent bulb, with plastic cover) (bottom) irradiance profile of the light measured on the laboratory windowsill (daylight passing through glass window). Ultraviolet/visible absorption spectra for solutions of dmg A in water are included for comparison.

The research was initiated by Kato et al. [51] in 1976, who used an acetyl cellulose film containing photochromic spirobenzopyran and phosphatidyl diloride. Figure 23 shows a schematic diagram of the apparatus used for the measurement of the membrane potential. The concentration ratio, y = C1/C2, of the electrolytes in compartments I and II is a parameter to vary the potential in the dark. In the dark before photoirradiation, the membrane exhibited a steady state potential difference Aq> of —28mv. The membrane potential shifted to — lOmv when the membrane was irradiated with ultraviolet light, and it reverted to the initial value upon visible irradiation. The change in the membrane potential was thus reversible. 

The conformation change of polyamide with pendant azobenzene grou (6) in aqueous solution also induced a change in pH [14]. Ultraviolet irradiation caused the pH of an aqueous solution of the polyamide to decrease and visible irradiation returned the pH to the initial value. 

The above finding indicates that the gel-sol transition can be induced isothermally between —56 and —47 °C by changing the irradiation wavelength. Ultraviolet irradiation (400 nm > > 310 nm) converts the sol to the gel state, whereas visible irradiation X > 450 nm) induces the transition from the gel to the sol state. In fact, a reversible gel-sol transition was observed at —52 °C. 

Since we had proposed a similar experiment with irradiation in the ultraviolet-visible absorption band of the uranyl ion (15), we tried to reproduce these results, but without success (16). Collisions in the liquid phase occur so rapidly (about 10l2 s x) that vibrational excitation of the uranyl ions would be dissipated long before any significant fraction of excited uranyl ions could reach the interface and therefore change the distribution between the two phases. Rapid loss of vibrational excitation in relation to other processes is a generic problem for infrared laser effects in any system of condensed phases. However, differences between experimental setups may account for the differences in results,... 

Melanins have photoprotective properties. Paramagnetic changes occur upon ultraviolet or visible irradiation and have been studied by ESR. Two types of reactions have been identified [161] production of melanin free radicals and reduction of oxygen to O2. Oxygen consumption in the polymer solution is strongly wavelength dependent, as shown by spin-probe measurements of oxygen concentration [167],... 

Biacetyl diffused onto the pre-wetted fiber initially. Then, as monomer vapors were introduced and the surface was irradiated with ultraviolet-visible light, the excited biacetyl dissociated to yield acetyl radicals or decayed to its more stable triple state through intersystem crossing. These radical species abstract accessible hydrogens from the substrate or near the surface of the fiber substrate, which in turn react with monomer in the proximity or possibly within the solvent wetting the substrate. Growing pol)rmer chains were terminated in a manner whereby only limited initiation of homopol3nnerization occurred. 

Het = general designation for an aromatic heterocyclic nucleus HMDS = 1,1,1,3.3.3-hexamethyldisilazane [McsSiNHSiMes] hplc = high pressure liquid chromatography HOMO = highest occupied molecular orbital hv = ultraviolet or visible irradiation hy = high yield... 

A variety of pyrimidine dimer-cleaving photochemical model systems have been developed to aid in the study and elucidation of the DNA-PL reaction mechanism (752, 158, 159). Direct excitation of pyrimidine dimers does not occur on ultraviolet or visible irradiation, suggesting that enzyme cofactors might be involved in a photosensitization process 146). Model systems utilizing free flavin derivatives have recently been described 137, 160). Rokita and Walsh have demonstrated that lumiflavin, 5-deazariboflavin, and 8-methoxy-7,8-didemethyl-7V °-ethyl-5-deazaflavin (Scheme 31) are effective photosensitizers for the thymine dimer cleavage reaction 160). These reactions utilized cis-syn-thymine dimer as substrate with irradiation at the Xmax of the flavin derivative, under strict anaerobic conditions and high pH. Flavin derivatives that contain electron-rich substituents at the 8-position, such as 8-hydroxy-5-deazariboflavin,... 

Further, Wu et al. (2004) exploited radiation chemical technique to synthesize CdS/polystyrene nanocomposite hollow spheres with diameters between 240 and 500 nm under ambient conditions in which the polymerization of styrene and the formation of CdS nanoparticles were initiated by y-irradiation. It was demonstrated that the walls of the hollow spheres were porous and composed of polystyrene containing homogeneously dispersed CdS nanoparticles (Figure 23.14). The quantum-confined effect of the CdS/polystyrene nanocomposite hollow spheres was confirmed by the ultraviolet-visible (UV-vis) and PL spectra. They proposed that the walls of these nanocomposite hollow spheres originated from the simultaneous synthesis of polystyrene and CdS nanoparticles at the interface of microemulsion droplets. 

HOMO = highest occupied molecular orbital hv = ultraviolet or visible irradiation LDA = lithium diisopropylamide [LiN/-Pr2]... 

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