UV and Visible Light

Modified ETEE films are used as wiadows ia greenhouses and conservatories due to their high transparency to both uv and visible light and excellent resistance to weatheriag (31). 

Fluorescent Dyestuffs. Very few dyes are of use in making daylight-fluorescent products. Of the dyes discovered up to 1920, only the brilliant ted and salmon dyes of the rhodamine and rosamine classes ate used in fluorescent materials in the 1990s. The first of these, Rhodamine B, was discovered in 1877. Fluorescence excited by both uv and visible light components in daylight was formally recognized as a notable property of certain dyed fabrics by the 1920s (1). 

Similar types of cross-linking reactions are observed for polymers to which photosensitive molecules ate chemically attached to the backbone of the polymer stmcture (Fig. 7). Radiation curing of polymers using uv and visible light energies is used widely in photoimaging and photoresist technology (Table 8) (58,59). 

Some application instruments possess light-absorbent covers to prevent or reduce the action of UV and visible light (Fig. 10). 

Radiation of a particular wavelength (monochromatic) is required to initiate a specific electronic transition, but most UV and visible light sources are polychromatic. For example, common mercury arc lamps emit around 50% of their energy in the 405 nm to 578 nm range. Hence, for most processes, well over half of the electrical energy... 

Solutions of tetrazolium salts, e.g., 53, have been reported to both become colored and bleached under the influence of both UV and visible light. Several workers have attributed this phenomenon to photoreduction to the corresponding formazan (51) and the formation of a fluorescent colorless compound (152) through photooxidation.240- 243 The reduction of 152 under UV or blue light to the intense green radical structure (153) has also been reported (Scheme 21).244 A one-electron reduction product (154) is proposed as a short-lived intermediate in the photoreduction.245... 

Lorret, O., Franco va, D., Waldner, G., and Stelzer, N. (2009) W-doped titania nanoparticles for UV and visible-light photocatalytic reactions. Applied Catalysis B Environmental, 91 (1-2), 39-46. 

An example of light-assisted transport of the first type involves (200) as the carrier in the liquid membrane. In this case, irradiation of the membrane alternatively with UV and visible light significantly increases the rate of K+ and Rb+ transport in the presence of picrate ion. This system also exhibits discrimination since the transport of K+ is favoured over Rb+ (Shinkai, Shigematsu, Sato Manabe, 1982). 

The peroxyl and sulfonyl radicals absorb light in different regions of UV and visible light Amax(R02 ) = 260 nm and Amax(RS 02) = 360 nm. Such concentrations of the reactants were chosen to create the ratio of initial concentrations of radicals [R02 ]o [RS 02]o- Therefore, the kinetics of sulfonyl radicals decay obeys the first-order equation ... 

A typical photochemical isomerization of the azobenzene amphiphile was found in an ethanol solution. A trans isomer converted to a cis isomer with ultraviolet irradiation. Back reaction from cis to trans was accelerated when a weak n-n absorption band of the cis isomer at ca.450nm was excited (Figure 21a). An alternative irradiation of uv and visible light to the ethanol solution gave reversible changes of the ji-ji transition between 355nm and 325nm attributed to the trans and cis isomers, respectively. 

Figure 22. Spectral change of CgAz.oCn>N+ Br cast film by alternative irradiation of uv and visible light on the cast film at 80°C in dry condition. An irradiation interval is 1 min for each light.

Most of the previous section concerned UV and visible light. In this section we will look in greater depth at the other common forms of light. From previous chapters, we are now familiar with the concept that different physical and chemical processes require differing amounts of energy. More specifically, it was shown in the previous section how the energies of photons can also vary. In this section, we see how the energies of different types of photon are manifested, and how their interactions may be followed. 

We have now looked at the way photons are absorbed. Photons of UV and visible light cause electrons to promote between orbitals. Infrared photons have less energy, and are incapable of exciting electrons between orbitals, but they do allow excitation between quantized vibrational levels. The absorption of microwaves, which are less energetic still, effects the excitation between quantized rotational levels. 

Dawson, B. and Torr, K. (1992). Spectroscopic and colour studies on acetylated radiata pine exposed to UV and visible light. In Pacific Rim Bio-Based Composites Symposium Chemical Modification of Lignocellulosics, Plackett, D.V. and Dunningham, E.A. (Eds.). ERI Bulletin, 176, pp. 41-51. 

Free radical initiation can occur through application of heat (thermal), UV and visible light (photochemical), ionizing light, redox reagents, electricity (electrochemical), etc., that is any process that creates the essential free radicals. 

Zou Z, Ye J, Arakawa H (2003) Photocatalytic water splitting into H2 and or O2 under UV and visible light irradiation with a semiconductor catalyst. Int J Hydrogen Energy 28 663-669... 

FIGURE 3.25 Calculated ratio of transmission of UV and visible light to the earth s surface in the presence of aerosol particles compared to that with no aerosol particles for typical continental aerosol particles at (a) average summer RH and (b) high summer RH and for urban aerosol particles with (c) average RH and (d) high RH. (Adapted from Erlick and Frederick, 1998.)... 

Photoinduced Changes in Phase Boundary Potentials. The photoinduced membrane potentials were measured by using PVC matrix liquid membranes in contact with a polypyrrole-coated Pt electrode [dibutyl phthalate (DBP) as the membrane solvent]. The polypyrrole layer allows to obtain a stable and sample-in-dependent potential drop between Pt and the PVC membrane. The phase boundary potential at the interface of a membrane containing ionophore and an aqueous RbCl or KCl solution could be reversibly altered by UV and visible light irradiation, as shown for ionophore 89 in Figure 23a,b. The values of the photoinduced potential... 

Figure 23. Photoresponse of the EMF induced by UV and visible light irradiation for membrane 89 in contact with (a) 0.1 M aqueous RbCI and (b) 0.1 M aqueous KCI. Membrane composition DBP/PVC = 2 1 wl/wt containing 1 mM of 89.

---