Photoreactions, inorganic

Many solid surfaces efficiently convert light to long-lived electron-hole pairs that can induce the chemical changes leading to the reactions in Eqs. (66)-(68). In fact, inorganic photoreaction is one of the exciting new fields of surface science and heterogeneous catalysis. 

Inorganic salts participate in a number of photochemical reactions. The transfer of electrons from one ion to another ion, or to solvent molecules, plays an important role in the majority of inorganic photoreactions. A number of observations suggest that water of hydration participates in the electron transitions, e.g. ... 

Just as with organic combustion aerosols, the chemical and physical nature of inorganic solid substrates can have a dramatic impact on the photoreactivity of adsorbed PAH. In 1980, Korfmacher and co-workers reported that BaP, pyrene, and anthracene all pho-tolyzed efficiently in liquid solution but were resistant to photodegradation when adsorbed on coal fly ash. Subsequent studies confirmed this observation and revealed that the carbon content of the ash (and the associated darkening of color) is a key factor in establishing the photostability of these PAHs. Indeed, they were stabilized at relatively small percentages of carbon, e.g., 5% or less (Behymer and Hites, 1985, 1988 Yokley et al., 1986 Dunstan et al., 1989 Miller et al., 1990). 

In the photochemistry of coordination compounds and organometallics containing mono or bidentate ligands, photosubstitutions occupy virtually the first position in research activity and a number of published papers [1,98, 99]. Photosubstitutions are those processes for which the first rules (Adamson rules [100] enabling photochemists to predict the course and relative efficiency of photoreactions) of photoreactivity in the field of inorganic photochemistry were formulated. Yet there are still a lot of questions to be answered.. 

On the other hand, photosubstitutions of tetrapyrrole complexes are rare processes. The main reason for such distinctions between tetrapyrrole complexes and common inorganic compounds lies in the electronic structure of the two mentioned classes of compounds in their low-lying photoreactive excited states. 

Based on some interesting reactions in certain inorganic crystalline compounds, Kohlschutter [9,10] proposed that the nature and properties of the products obtained take place on the surface or within the solid state. Indeed, he coined the term topochemistry for such reactions in the solid state. However, systematic investigations of photoinduced reactions in crystals began from 1964 onward by Schmidt and Cohen [11], Their studies on the 2tt + 2tt photoreaction of cinnamic acid derivatives in the crystalline state and correlation with the molecular organization in these crystals led to what are now known as Topochemical Principles. The most important conclusions reached by them are as follows (1) The necessary conditions for the reactions to take place are that the reactive double bonds are parallel to one another and the center-to-center distance be within 4.1 A (2) there is one-to-one correspondence between the stereochemistry of the photoproduct and the symmetry relationship between the reactants. The centrosymmet-ric relationship (called the a-form) leads to centrosymmetric cyclobutane (anti-HT), whereas the mirror symmetric arrangements (called the (5-form) produce mirror symmetric dimer (yy -HH). 

The photophysical and photochemical properties of the binuclear [Au2 (dppf)(C=CR)2] (R = Ph and fBu) were studied [82]. The complexes were non-emissive in the solid state even at 77 K, but emission bands at 410 nm in dichloromethane could be observed. The photoreaction between [Au2(dppf)(C=CPh)2] and the dichloromethane solvent was probed, which led to the formation of the organic C-C coupled product, PhC=CC=CPh, and the inorganic [Au2(dppf)Cl2] complex. 

A lot of photoreactions occurring in the atmosphere, hydrosphere, and soils ensure the health, comfort, and welfare of human beings, creatures, and the environment. These processes are mostly driven by coordination compounds of transition metals, which play the role of (photo)catalysts or (photo)sensitizers. There is also increased understanding of the role of supramolecular inorganic systems in their interaction with light and the great variety of processes that may ensue. 

More recently, the various aspects of light-responsive and photoreactive inorganic compounds in biological and bioinspired systems have been addressed (3-6). In the present contribution, some fundamental principles and first advances of the rapidly evolving fields of bioinorganic photochemistry and biomimetic photocatalysis will be discussed. No attempt will be made to provide a comprehensive coverage of the relevant literature, which is widespread across a bunch of different disciplines... 

The most fundamental principle of photochemistry, which actually was discovered by exposing inorganic compounds to sunlight 21), states that light must be absorbed by a chemical substance for a photochemical reaction to occur. We will therefore start our discussion with a brief overview on some of the natural chromophores which are known to be involved in photosensory processes or could be useful for the construction of artificial photoreactive systems. 

Photoreactivity is a common feature of many components of biological systems and their synthetic functional counterparts. The important role of inorganic photochemistry and photocatalysis as a versatile tool for triggering, driving, and controlling molecular processes by light will therefore certainly be further increasing in the near future. 

---