Mechanistic photochemistry

The mechanistic photochemistry of cross-conjugated 2,5-cyclohexadienones has been comprehensively examined (Scheme 21). The photochemical isomerization of the... 

DR. WILLIAM WOODRUFF (University of Texas) I would like to make a comment not so much on your paper as on mechanistic photochemistry in general. I think most of us would agree that if we are going to draw mechanistic conclusions, we really need to know what the structures of the reactants and products are. One of the problems in photochemistry is that we generally do not know the structure of the reactant, which is the excited state. There aren t very many structure-specific probes in solution, in fact, none below about the millisecond time scale where esr and NMR cease to be applicable. In our laboratory, we have been able to obtain the resonant spectra of excited states. In two of the three kinds of systems that we have observed so far, the structures of the excited states are not predictable in a straightforward way, either from the ground state structures or from calculations. 

Polysilane high polymers possessing fully saturated all-silicon backbone have attracted remarkable attention recently because of their unique optoelectronic properties and their importance in possible applications as photoresists, photoconductors, polymerization initiators, nonlinear optical materials etc. A number of review articles have been published on this topic4-9. The studies in this field have stimulated both experimental and theoretical chemists to elaborate on understanding the excited state nature of polysilanes and oligosilanes and of their mechanistic photochemistry. 

The connection between synthesis and photochemistry is vital. As long as photochemistry is felt as a sanctuary of the small group of professional photochemists, many synthetic perspectives will be ignored, and this is a negative impact also on mechanistic photochemistry that loses part of its interest. As a matter of fact, this remark is not new. In a talk in Leipzig in 1908, Hans Stobbe, a pioneer of photochemistry (well known for his innovative studies on the photochromism of... 

The species absorbing and transferring the radiant energy is called the photosensitizer. In mechanistic photochemistry the term is limited to cases in which the photosensitizer is not consumed in the reaction [29]. 

Phosphorescence From a phenomenological point of view, the term has been used to describe long-lived luminescence. In mechanistic photochemistry, the term designates luminescence involving change in spin multiplicity, typically from triplet to singlet or vice versa. The luminescence from a quartet state to a doublet state is also phosphorescence. 

Analysis of a CIDEP pattern with time-resolved ESR (TR ESR) spectra provides a solid conclusion to be made on the spin multiplicity of molecular precursors of polarized free radicals (a singlet or a triplet excited molecule) and the tracking of fast reactions of polarized radicals leading to secondary radicals. Thus, TR ESR is a convenient method in mechanistic photochemistry and free radical chemistry. Continuous wave TR ESR (CW TR ESR) devices are widely used for detection of photogenerated radicals. They usually consist of a pulsed ns laser with detection of transients by their ESR spectra with a X band ESR spectrometer in the direct detection mode (no hied modulation).Time-resolved Eourier transform ESR (FT ESR) has some advantages and drawbacks with respect to CW TR ESR. Rather sophisticated FT ESR devices have become available, and FT ESR studies become more common. 

N. J. Turro, Modern Mechanistic Photochemistry, Benjamin Cummings Co., Menlo Park, California, 1978. 

A starting point for the discussion of experimental results in mechanistic photochemistry is the knowledge of the shapes of the ground-state (S ) and... 

Case Study 5.1 Mechanistic photochemistry - adiabatic proton transfer reactions of 2-naphthol and 4-hydroxyacetophenone... 

Case Study 5.2 Mechanistic photochemistry - cyclopentane-1,3-diyl biradicals... 

Case Study 5.3 Mechanistic photochemistry - the photo-Favorskii reaction of p-hydroxyphenacyl compounds... 

Case Study 6.3 Mechanistic photochemistry - previtamin D photochemistry... 

Case Study 6.9 Mechanistic photochemistry - photocyclization of NJV-dimethylaminoalkylstyrenes... 

Case Study 6.15 Mechanistic photochemistry - regioseiectivity of photosubstitution... 

Case Study 6.29 Mechanistic photochemistry - singlet-triplet interconversion of carbenes... 

Case Study 6.32 Mechanistic photochemistry - reactions from different excited states... 

Considerable data have been published on the deterioration of some specific properties of polyamides as a consequence of natural or artificial weathering but few papers have been concerned with mechanistic photochemistry of these polymers. The results of Sharkey and Mochel [101] on the photo-oxidation of amides have opened the way to a better understanding of the problem. These authors have suggested that the primary photochemical step in the photolysis of amides is the breaking of the CO—NH bond with the formation of two free radicals which preferentially abstract hydrogen atoms from the methylene groups adjacent to the nitrogen namely,... 

H. D. Roth, Chemically induced dynamic nuclear polarization as a tool in mechanistic photochemistry, Mol. Photochem. 5 (1973)91. 

Mechanistic photochemistry of metal )8-diketonate complexes has received increasing attention. Near-ultraviolet irradiation of diketonate complexes of Mn ", Fe , Co ", No" and Cu" eventuates in one-electron reduction by the ligand of the metal center whereas Cr " complexes photo-isomerize. Irradiation of franj-Rh(CF3COCHCOMe)3 appears to lead to two photoactive excited states which may have significant radical character. Photoisomerization to the cis form occurs in inert solvents whereas decomposition takes place in the presence of potential hydrogen atom donors such as alcohols. ... 

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