SYNTHESIS, LOCATION, AND PHOTOINDUCED TRANSFORMATION OF ZEOLITE-ENCAGED THIOINDIGO

NaX-encaged thioindigo was prepared either by incorporation of the dye during hydrothermal crystallization of the zeolite or by solid-state reaction between dehydrated NaX and dye. Rietveld refinement of X-ray diffraction patterns revealed the location of the dye molecules at the 12-membered rings connecting two neighbored supercages of the faujasite structure. Depending on the preparation conditions, the sodium cations in closest vicinity to the dye molecules populate (1) S(II) sites, (2) S(II*) sites, or (3) S(III) sites. The photoinduced trans-to-cis isomerization as well as the opposite thermal cis-to-trans conversion was followed by visible spectroscopy. No photoisomerization was observed for samples exhibiting Na+ in S(II*) or S(III) cation sites, thus blocking the torsional motion. The analysis of the kinetics of the cis-to-trans isomerization in a sample with Na+ in S(II) exhibited an increased energy barrier for the twisting mode of approximately 4 kJ mol-I as compared to thioindigo in solution. Decreased rates of photodegradation for the encaged dye in comparison with the dissolved one were observed.