ENHANCEMENT OF THE PHOSPHORESCENCE YIELDS OF XANTHONE INCLUDED IN ALKALI-METAL-CATION-EXCHANGED ZEOLITES - EXTERNAL HEAVY-ATOM EFFECT ON THE SINGLET-TRIPLET TRANSITIONS

The phosphorescence properties of xanthone included in alkali-cation-exchanged zeolites have been investigated to determine the effect of the microenvironment of the host adsorbent on the electronic excited states of the guest molecules included in the restricted void spaces. An enhancement of the phosphorescence yield is observed on changing the exchangeable alkali cation from Li+ to Cs+. Simultaneously, the phosphorescence lifetimes are continuously reduced by changing the cation from Li+ to Cs+. IR investigation of xanthone included in alkali-cation-exchanged Y-zeolite shows that xanthone molecules are stabilized in zeolite cavities by the direct interaction between the molecules and the alkali cations anchored in the cavity. These results are well explained in terms of the external heavy-atom effect, derived from the alkali cations Li+, Na+, K+, Rb+ and Cs+, on the singlet-triplet transitions of xanthone molecules included in the alkali-cation-exchanged zeolites.