Evidence for Encaging Luminescent Guest Molecules in the Inner Cages of Zeolite Host

Abstract

Clear evidence for encaging guest molecules in a host material was obtained for FAU-type zeolite containing fluorescein (FL) as a guest by employing the luminescence-quenching phenomenon of FL with C 60 . Whereas fluorescence of FL molecules adsorbed on the outer surface of the zeolite was effectively quenched as well as that in a homogeneous solution, that of the most FL molecules in the cages of the zeolite did not suffer from quenching by C 60 , giving clear evidence that FL molecules are located in the inner cages, which are inaccessible to C 60 . Only the fluorescence of the FL molecules located in the cages in the outermost unit cells of the zeolite was partially quenched. This partial quenching can be understood by considering the structure of the zeolite and the critical distance of electron transfer inducing the fluorescence quenching.Nanohybrid materials using host-guest systems are currently attractive for scientists working in chemistry and biology, because of their special properties and functions that cannot be obtained from independent constituents. To make use of these functions, optical functional materials based on nanohybrid systems have widely been studied, and a large number of papers on nanohybrid systems have been published. [1][2][3][4][5][6][7][8] Recently, our research group has reported that rare earth metal ions and a photosensitizer encaged in zeolite cages show rainbow-color photoluminescence. 9 The change in the color of the photoluminescence induced by changing the temperatures and the excitation wavelength is a good example of the special functions that cannot be obtained from independent constituents. Distinction between guests encaged in hosts and those adsorbed on the surface of hosts is essential for systematizing nanohybrid chemistry. In order to systematize nanohybrid chemistry using host-guest systems, it is required to elucidate the origin of the specific properties of nanohybrid systems, which usually appear only when the guests are encaged into the hosts. NMR, [10][11][12][13]12,14,15 SEM,16,17 optical microscopy,18 and TEM 1,16,17,19 are used as the conventional methods for the distinction. NMR and IR are important tools that give information of local structures or environments of guests and hosts in the range from atomic size to molecular size. In studies on nanohybrid systems using organic host materials (e.g., cyclodextrin 20 or crown ether 21 ), NMR and IR measurements provide us clear information on the position of guest molecules, because these organic hosts have small and simple structures compared to inorganic host materials (e.g., zeolite 9-15 or mesoporous silica 2,16

Visible Light-Induced Catalytic Oxidation Using Zeolite Encapsulating Two Distinct Metal Complexes

“…Considering the cage structure of the Y-type zeolite, there are both Ru(bpy) 3 and Mn(saloph) units within 2-3 supercages. The importance of the distance between a luminescent molecule and a quencher in the zeolite has been pointed out by Tanaka et al 36) The effective quenching behavior of 1+2@Y is due to the positions of 1 and 2 in zeolite.…”