Quenching photoisomerization

Evidence that eliminates the triplet mechanism as the mode for the cis-trans isomerization of stilbene upon direct photolysis has been provided by azulene quenching studies.(48) Using the experimentally determined decay ratio a/(l — a) and the triplet mechanism, it is possible to calculate what the effect of azulene is upon the pss. The predicted and observed azulene effects on the direct photoisomerization are shown in Figure 9.6. The failure of the triplet mechanism in predicting the very small changes observed in the pss provides a crucial test that is the basis for rejecting the triplet mechanism. 

The synthesis reported by Bonjoch and co-workers utilizes a Ni-mediated reductive cyclization of vinyl iodide 82 with concomitant reductive indolenine formation quenching of the presumed intermediate (dehydrotubifoline, 83) with the Vilsmeier reagent affords A-formyl derivative 84 [36b]. Photoisomerization gives norfluorocurarine (22) in low yield. Similarly, Rawal and He produce... 

In a study of the photoisomerization of 4,4-dimethyl-2-cyclohexenone (8) to 6,6-dimethylbicyclo[3.1.0]-hexane-2-one (9) and 3-isopropyl-2-cyclo-pentenone (10), Chapman and Wampfler48 accounted for the pronounced effect of sensitizer concentration by such a self-quenching mechanism. The phenomenon was exhibited by four ketones with () lowest triplet states but not by benzophenone or acetophenone which have (w, n) triplet states.49... 

Photoisomerization of this type in acetonitrile is not quenched by dienes and the process, therefore, proceeds through the singlet but the yield is dependent on the type of substitution. For example, the 3,5-di-p-tolyl homologue of (365) gives a better yield (77%)... 

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... 

The formation of 1,2-dimethylcyclobutene (Formula 385) in the vapor phase irradiation of 2,3-dimethyl-l,3-butadiene (Formula 384) is not quenched by oxygen or nitric oxide (169). Addition of inert vapor (diethyl ether) increased the quantum efficiency in this reaction (169). The inert vapor presumably removes excess vibrational energy from the product cyclobutene thus stabilizing the product (169). Rate studies on the cis- and Jrans-isomers of 1,3-pentadiene in solution indicate that the iraras-isomer is the only source of 3-methylcyclobutene (169). The photoisomerization to 3-methylcyclobutene is faster than photoisomerization of trans- to m-l,3-pentadiene (169). 

However, not all the six prepared compounds (Scheme 3b) underwent photochemical isomerization the photoisomerization reactivity of these dithienylethene-bridged diporphyrins strongly depended on the distance between the porphyrin and dithienylethene. The three compounds, in which porphyrin and dithienylethene were directly linked (1), linked through a 1,4-phenylene (2), and linked through meso-ethynylene (5), did not exhibit any photochromism. The authors attributed it to the efficient quenching of the excited dithienylethene by the attached porphyrin moiety through intramolecular energy transfer. 

The Battelle group found that photoisomerization of various indigo derivatives could be used to record holograms which had low (<0.2%) scattering efficiencies (36). Their refractive-index data yield nQ = 2.9 x 10 cm for an N,N -dibenzoylindigo derivative. These systems were reversible. The quantum yields of isomerization were much lower in poly(methyl methacrylate) or polystyrene than in fluid solution, in accord with arguments presented earlier. Concentration quenching was also observed in solution. 

Rearrangements.—Various examples of Z,E-photoisomeri7ation about the carbon-nitrogen double bond have been reported. The E-esters (1) have been prepared by irradiation of the Z-isomers (2) in pentane-benzene, and quantum yields for the photochemically induced interconversions of the four stereoisomers of l-(methoxyimino)-3-phenylprop-2-ene have been determined. Particular interest has been shown in the isomerization of azines the E — Z photoisomerization of benzophenone-9-anthraldehyde azine is quenched by dyes with the appropriate singlet energy," with the quantum yield for this transformation being influenced by solvent polarity." The composition of the photostationary state has been determined for a number of arylazines "... 

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