SQUARAINES INSIDE ZEOLITES - PREPARATION, STABILITY, AND PHOTOPHYSICAL PROPERTIES

A series of four symmetrical squaraines (ditoylyl, di-m-xylyl, dianisy l, and diresorcinyl) incorporated inside zeolites Y, mordenite, and ZS M-5 have been obtained by treating squaric acid and the corresponding arene in the presence of acid zeolites. Acid sites and high reaction t emperatures (150 degrees C) were found to be crucial for the success o f the preparation procedure. Surprisingly, this method failed for the preparation of the squaraine derived from N,N-dimethylaniline, which i s known to be readily formed from squaric acid in homogeneous phase wi thout a catalyst. The solid samples containing squaraines were charact erized by diffuse reflectance and Raman spectroscopies and by thermogr avimetry-differential scanning calorimetry. Among the hosts, mordenite was found to be the most general and convenient zeolite for the prepa ration of the squaraines, while in the other solids either the organic content adsorbed was comparatively smaller (ZSM-5) or some squaraines were not very stable (Y zeolite for ditolyl and dixylyl squaraines). The absorption spectra of the samples correspond to the J-aggregation state of the squaraines, except for some ZSM-5 samples, where simultan eous observation of the bands due to both monomers and aggregates occu rs. Aggregation also changes with the water content of the samples. Tr eatment of the zeolite-bound diresorcinyl squaraine with basic aqueous solutions leads to remarkable variations in the diffuse reflectance a nd Raman spectra. These changes in the Raman spectrum of the diresorci nyl squaraine were found to be reversible by basic or acid washings. L aser flash photolysis using the 355- or 532-nm output of a Nd-YAG lase r (<10 ns pulses; less than or equal to 20 mJ/pulse) allowed the detec tion of two different types of long-lived intermediates which depended on the zeolite host. On the basis of the similarity of the UV-vis abs orption spectra obtained in solution, these transients have been ident ified as the radical cation (HY) and the triplet excited state (HMor) of the incorporated squaraine.