Photochemistry of Anils in NaY Zeolite

Abstract

A series of anils (1−6) have been incorporated in NaY zeolites and the resulting samples characterised spectroscopically. Raman spectral analysis showed that anils exist in the zeolite microcavities predominantly in a zwitterionic form. In agreement, diffuse reflectance UV/Vis spectra showed the presence of a significant concentration of the zwitterionic form, characterised by an absorption band around 400 nm. A sharp contrast is drawn between their photochemistry in solution or in the crystalline form with that in NaY zeolites. Thus, steady state irradiation at 400 nm led to persistent changes in the diffuse reflectance UV/Vis spectra with decrease of the reflectance of the band at 400 nm and the appearance of a new absorption band at longer wavelengths. No photochromism is observed and this is attributed to the isomerization of the zwitterionic form to other stereoisomers, possibly by rotation about C−C or C−N bonds with partial double bond character. This photochemical isomerization has not been reported so far. Even though the hydrogen bonding ability and polarity of NaY zeolite has been reported to be similar to that of polyfluorinated alcohols, based on the enolisation equilibrium of anils, the photochemical behaviour of these compounds incorporated within NaY is different to that reported in polyfluorinated alcohols. Specifically, no excited-state proton transfer or proton catalysed re-enolization of the zwitterionic form is observed with anils in NaY. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)