Photochemistry Without Light

Reactions that come under this heading have been dealt with so extensively in the literature that no attempt to cover them within the confines of this book could posibly do them justice. In the following few pages, we will restrict our attention to the basic features of the two best known examples, concentrating on the question of how much light considerations of orbital and geminal symmetry can cast on their mechanism. 

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Energy is transferred from molecules electronically excited in a chemical reaction to other molecules which emit the accepted excitation energy in the form of light alternatively the accepting molecules can undergo photochemical transformations. First examples of this photochemistry without light were described by E. H. White and coworkers 182>. Thus the trans-stilbene hydrazide 127, on oxidation, yielded small amounts of the cis- 128 beside the trans-stilbene dicarboxylate in a luminol-type reaction. 

Another type of photochemistry without light involves the actual formation of the triplet reactant excited state by thermolysis of appropriately selected dioxetanes as depicted in equation 210 ". 

The thermal generation of reactive excited states ("photochemistry without light") has been reported before (36). 

Dark photochemistry (photochemistry without light) Chemical reactions involving electronically excited molecular entities which are generated thermally rather than by absorption of electromagnetic radiation. The use of this term is discouraged. 

The rate constants for reactions of all-trans retinal and retinol with have been measured in various solvents. Rate constants increase with dielectric constant, thereby suggesting that charge transfer is involved. Electronically excited triplet species generated during peroxide-catalysed aerobic oxidation of substrates gives rise to fluorescence from acceptors in micelles. Micelle-solubilized chlorophyll is an excellent detector of enzyme-generated triplet carbonyl species, and micelles make photochemistry without light possible for... 

Electrochemical excitation, photochemistry without light, exhibits many phenomena that are unique to ECL as compared to photochemistry. The efficient production of emission from excimers or exciplexes as compared to excited monomers, efficient generation of excited triplet states, and intense delayed fluorescence caused by triplet-triplet annihilation are the most typical examples. On the other hand, the method offers a chance to populate the excited states that are inaccessible by the processes following photoexcitation. 

Nardello, V. et al.. Photochemistry without light Oxidation of ruhrene in a microemulsion with a chemical source of singlet molecular oxygen ( O2, Dg), J. Chemical Education, 76, 1285, 1999. 

Photophysics involves the absorption, transfer, movement, and emission of electromagnetic, light, energy without chemical reactions. By comparison, photochemistry involves the interaction of electromagnetic energy that results in chemical reactions. Let us briefly review the two major types of spectroscopy with respect to light. In absorption studies, the detector is placed... 

A major challenge for research in future, the control of chemical reactions as stated by the Pimentel Report, can be approached by various methods light-driven processes are among the most important ones. Without interaction of the diverse scientific disciplines, recent progress in photochemistry, as well as future developments would scarcely be possible. This is particularily true for the study of electron transfer processes. Herein lies a challenge for science and economy and the special fascination of this topic — at least for the guest editor. 

The global field of photochemistry has been defined in Coxon Halton as the study of chemical reactions initiated by light. 13 Although succinct, it may be too restrictive for the purposes of vision. The field might be better described by replacing the expression chemical reactions by chemical processes or chemical interactions. The revised definition would allow mechanisms, such as sensitization of one species by another without any change in species to occur. This... 

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