Photochemical isomerization rates

Burton WB, Pollard GE. 1974. Rate of photochemical isomerization of endrin in sunlight. Bull Environ Contain Toxicol 12 113-116. 

Thermal vs. Photochemical Behavior. The results of these investigations allow for comments at several levels of detail. First, it is quite evident that the photochemistry differs significantly from the thermal reaction chemistry. The latter displays isomerization as the predominant reaction for all forms, and perhaps for all temperatures, in the range normally available for aqueous systems. Thus, for the C-T system, the aquation mode remained relatively unimportant over a temperature range that provided a twenty-fold change in isomerization rate. 

Biacetyl sensitized photoisomerization of l,2-di-9-anthrylethane 7a does not lead to the 4n + 4n cyclomer 8a but yields exclusively the An + 2n cycloadduct 26 with a quantum yield of 0.1 [72]. Since the phosphorescence of biacetyl is quenched by dianthrylethane 7a at nearly diffusion controlled rate, the photochemical Diels-Alder reaction is explicable by triplet energy transfer from biacetyl to 7a. The photochemical isomerization of 10-benzoyl-l,2-di-9-anthrylethane 27 also proceeds exclusively by An + 2n cycloaddition and gives cycloadduct 28 with a quantum yield of 0.005 [73], The low fluorescence quantum yield of 27 (excited triplet state. Biacetyl sensitization of 27 leads to 28... 

In 2008, Fox and co-workers reported that frani-cyclooctene (TCO) could be produced in gram scale from c/i-cyclooctene by a circular photochemical isomerization procedure [45], which demonstrated an unusually fast second-order rate constant between TCO 5 and dipyridyl-tetrazine is unusually fast ( 2 > 10 s in 9 1 methanol/water) [20]. As the ring strain is essential to... 

Photochemical isomerization provides a practical testing ground for the above theory. Time resolved spectroscopy has been used to study the isomerization, i.e. a Icurge amplitude structural change, in different molecules Which are listed in Table I. In these molecules, the rate of isomerization was measured as a function of temperature, pressure amd solvent viscosity. 

The photochemical isomerization of various spiropyran molecules in different media (gas phase, solution, mono-layers, polymers) has been extensively experimentally [1-3, 20-30] and theoretically [31, 32] studied in order to understand the reaction mechanism. Less studied, however, are the effect of protonation, charge, and external fields on the isomerization pathway of this electrocyclic reaction. It was recently reported that the rate of isomerization of the SP form is considerably increased by protonation of the... 

SAMs containing an azobenzene moiety have also been used to modify the electrochemical properties of electrodes by several investigators. Thus, Uosaki and collaborators constructed a SAM by depositing an alkyl thiol molecule containing ferrocene and azobenzene moieties on gold electrode surface (Scheme 65). They found that the charge-transfer rate of the ferrocene group in the SAM could be reversibly controlled by electro- and photochemical isomerization between ,Z-forms of the azobenzene moiety in the SAM. 

Direct photochemical excitation of unconjugated alkenes requires light with A < 230 nm. There have been relatively few studies of direct photolysis of alkenes in solution because of the experimental difficulties imposed by this wavelength restriction. A study of Z- and -2-butene diluted with neopentane demonstrated that Z E isomerization was competitive with the photochemically allowed [2tc + 2n] cycloaddition that occurs in pure liquid alkene. The cycloaddition reaction is completely stereospecific for each isomer, which requires that the excited intermediates involved in cycloaddition must retain a geometry which is characteristic of the reactant isomer. As the ratio of neopentane to butene is increased, the amount of cycloaddition decreases relative to that of Z E isomerization. This effect presumably is the result of the veiy short lifetime of the intermediate responsible for cycloaddition. When the alkene is diluted by inert hydrocarbon, the rate of encounter with a second alkene molecule is reduced, and the unimolecular isomerization becomes the dominant reaction. 

Although the isomerization of allylic alcohols can be catalyzed by Fe(CO)s under thermal conditions, this reaction suffers from slow reaction rates, low yields, and high reaction temperature. To overcome these problems, photochemical activation of Fe(CO)s was investigated. By employing photochemical activation conditions, the isomerization of a wide variety of allylic alcohols proceeded in good to excellent yields using 1-10 mol% of Fe(CO)s in pentane (Scheme 9).32... 

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