The effect of ion-exchanged alkali metal cations on the excited states of xanthone and the photolysis of 2-pentanone included within zeolite cavities

Abstract

In order to clarify the effect of the microenvironment of the host adsorbents on the photophysical and photochemical properties of the guest molecules in restricted void spaces, the characteristic properties of the phosphorescence of xanthone and photolysis of 2-pentanone included within alkali metal cation-exchanged zeolites were investigated. An enhancement of the phosphorescence yield of xanthone included within the zeolite cavities and a reduction in the phosphorescence lifetime were ovserved on changing the exchangeable alkali metal cation from Li + to Cs +. These results suggest an external heavy-atom effect derived from the alkali metal cation on the singlet-triplet transitions of xanthone molecules within the zeolite cavities. During the photolysis of 2-pentanone included within the zeolite cavities, the selectivity of the Norrish type II reaction was found to decrease remarkably due to the steric hindrance caused by the restricted void space of the zeolite cavities. IR and photoluminescence investigations indicated that 2-pentanone consists of two different adsorption types: one is adsorbed on the surface OH group by hydrogen bonding and the other is directly adsorbed onto the alkali metal cations. On changing the cation from Li + to Cs +, a decrease in the strength of the interaction between the alkali metal cation and 2-pentanone caused an increase in the yield of photolysis of 2-pentanone. Thus changes in the zeolite cavities, modified by the exchange of cations, have a significant effect on the excited states and chemical reactions of ketones.