Photochemical light-energy conversion

These observations are relevant to the general problem of photochemical light-energy conversion. Thus, retardation of the back electron-transfer reaction is a major prerequisite in any artificial photosynthetic system (9). In the specific Ir(II) /DMB (sol-gel) system the absence of an effective back reaction allows (at acidic pH) the strong reductant Ir(II) to react with water yielding molecular H2 according to (7) ... 

Much of the work on the photoreduction of carbon dioxide centres on the use of transition metal catalysts to produce formic acid and carbon monoxide. A large number of these catalysts are metalloporphyrins and phthalocyanines. These include cobalt porphyrins and iron porphyrins, in which the metal in the porphyrin is first of all photochemically reduced from M(ii) to M(o), the latter reacting rapidly with CO to produce formic acid and CO. ° Because the M(o) is oxidised in the process to M(ii) the process is catalytic with high percentage conversion rates. However, there is a problem with light energy conversion and the major issue of porphyrin stability. 

During the 1970s and 1980s much work was carried out on photochemical solar energy conversion by semiconductors or sensitizers to produce fuels by solar energy.2-4 The most typical system for such fuel production is water cleavage by light. 

The emission spectrum of the sun consists approximately of 9 % UV light, 40 % visible, and 51 % IR [1]. Only UV, visible, and a small fraction of the near-IR can cause electronic excitation. Furthermore, we have to take into account that the excitation energy collected should be enough to drive the chemical reactions we would like to use to store the solar energy. Therefore, any practical photochemical solar energy conversion system has to be based on visible light absorption. 

The photochemical properties of various nanoassemblies discussed in this chapter highlight the ways in which the metal and semiconductor nanopartides interact with light. Furthermore, one can fine tune these responses by subjecting nano-stmctures to an externally applied electrochemical bias. The ability to functionalize these nanopartides with photoactive molecules has opened new avenues to utilize these nanoassemblies in light energy conversion and catalytic applications. By suitably modulating the fluorescence of the surface bound fluorophore these... 

Fig. 1 Types of Photochemical Reactions Employed for Light-Energy Conversion.

The photosynthetic process thus provides us with an example of a complex, light-powered photochemical molecular device which uses light as an energy supply in order to facilitate energy conversion. In green plants, this molecular device is located within a specially-adapted photosynthetic membrane. 

Within the kinetic and thermodynamic limitations on the conversion of light energy to chemical energy, I have shown that a reasonable efficiency goal would be 25 - 28% for conversion to electricity and 10 - 13% for storage as chemical energy. Five types of photochemical converters have been defined and described with examples where possible. 

This limiting value depends on the ambient temperature. The results suggest that the decomposition is mainly thermal due to the conversion of the light energy into heat. There is a small (5 to 10%) photochemical effect particularly when the nitrogen iodide is irradiated with blue or red light... 

Although the conversion of light energy to chemical energy via the electron transfer reactivity of [Ir(u pz)(COD)]2 is rather facile, the photochemical products rapidly return to starting materials because the back electron transfer reactions are highly exothermic. For example, back electron transfer between the oxidized iridium... 

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