trans-and ris-azobenzene form with -, ß-, and -cyclodextrin inclusion complexes which can be evidenced by UV and induced circular dichroism spectra. The inclusion in the cyclodextrin cavity has different consequences on the photoreactivity of the two isomers. The reactivity of the cis isomer is practically unaffected while the photoisomerization quantum yields of the trans isomer are strongly reduced with respect to the aqueous medium; moreover, the wavelength dependence of the yields tends to vanish. A partial or total block of the torsional motion about the -N=Ndouble bond and the inversion at one of the nitrogens as sole isomerization coordinate are the most likely rationalization of the experimental findings.
The photochemistry of benzophenone aqueous solutions in the presence of cyclodextrins (CDx) has been studied by stationary and pulsed techniques. Quenching of the phosphorescence is the consequence of hydrogen abstraction following the inclusion in the cyclodextrin cavity of the triplet benzophenone. The H-abstraction process has close analogies with the same reaction in micelles. Radical pair decay is controlled by the dimensions of the CDx cavity: intersystem crossing prevails in the presence of ß-CDx, radical exit in the presence of a-and -CDx. The fate of the escaped radical also depends on the cavity dimensions, as revealed by the photoreduction quantum yields.
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