Conversion of radiation energy

The rearrangement of phenyl- er -butyl nitrone to the isomeric oxazirane (9) occurred in 95% yield on irradiation in acetonitrile for 2 hr. Because 2-ieri-butyl-3-phenyloxazirane (9) can be reconverted into the more stable nitrone, the photochemical reaction involves the conversion of radiation energy into chemical energy. 

Photovoltaic energy conversion also makes use of bandgap materials for the direct conversion of radiation energy into work, electricity. For C = 1, the best value for w = 0.31, or 31%, at an optimal bandgap Eg = 1.30 eV with w = 25% reached in the laboratory and 15% in industrial production. For C = 400, w increases to 35%. For multicolor conversion, where many different bandgap materials are applied simultaneously, w can be calculated to... 

The most common method for detection of radiation in the clinical laboratory is by scintillation. This is the conversion of radiation energy into photons in the visible or near-UV range. These are detected with photomultiplier tubes. 

In order to effect the conversion of radiation energy into chemical energy, multielectron processes will frequently be necessary. As a means to achieve such processes, photoredox at a multiplicity of metal centers may be necessary. Such complexes have been synthesized using a cyanide bridge from cobalt to other metal centers. One of the earliest examples is the complex (NH3)5Co° 0/-NC)Ru°(CN)5. This complex shows an intervalence transfer (IT) band due to a Ru° Co° transition at 375 nm, and irradiation into this band results in photoredox. Similar photoredox reactions are observed in complexes containing cyanide-bridged Co Os°, Cr Fe° and Fe°Os° combinations. ... 

The ultraviolet absorber could dispose of absorbed energy by radiation, by conversion into heat or by chemical changes leading to stable products. The most important commercial absorbers, such as the o-hydroxybenzophenones, o-hydroxyphenylbenzotriazoles and salicylates, appear to function by conversion of electronic energy into heat. The properties of the main types of ultraviolet absorbers are summarised in Table 7.7. 

Electrical motors are less than 100% efficient in the conversion of electrical energy into useful work, since a part of that energy is converted into frictional heat which radiates away. 

Electromagnetic radiation, besides being a probe of surface structure, can excite electrons in the species in solution (especially in organic compounds) or in the electrode itself (especially in semiconductor electrodes). This photon excitation can lead to electron transfer between electrode and solution. The study of these phenomena is photoelectrochemistry and can be very important in conversion of solar energy into electricity in order to convert substances (photoelectrolysis). 

The role of photochemistry in nature is difficult to overestimate especially when we consider conversion of solar energy into the chemical energy in natural photosynthesis or ozone production, which protects the Earth from hazardous UV radiation. Much less is known about the role of sunlight in environmental self-cleaning processes. 

The concerns for changes in atmospheric ozone can be divided into two major categories changes in total column of ozone, and changes in the concentrations at particular altitudes. The penetration of ultraviolet radiation to the surface of the earth is determined almost entirely by the total amount of ozone in the atmospheric column, with very litde dependence on the altitude distribution of this ozone. However, if the prime concern is with processes such as the conversion of ultraviolet energy into heat after absorption by ozone (i.e. with the temperature structure of the stratosphere), then a redistribution of ozone to different altitudes is extremely important. 

After excitation by visible or UV radiation, molecules relax back to the ground state by dissipating the excitation energy through molecular vibrations and collisions with solvent molecules. This process is called nonradiative decay and is typically a very fast set of processes for molecules in fluid solution at room temperature. Alas, this process, which is simply the conversion of photonic energy to heat, is rarely useful. 

---