Retention of arene photosystems in the polar-solvated cavities of the zeolites NaX and NaY

Abstract

This study describes the retention of photochemically interesting arenes in the internal cavities of polar-solvated zeolites (PSZ), or zeolites completely submerged in polar organic solvents. Initial observations at room temperature showed that aromatic species such as pyrene or perylene, when placed in the internal cavities of the zeolites NaX or NaY by adsorption from solution, were >90% retained following overnight agitation of samples in desorbing polar solvents such as methanol or acetonitrile. Percent retainment was further explored for a series of naphthalene and anthracene derivatives of variable size. Studies of extraction rates and the fluorescence of retained species at room temperature indicate two different retention mechanisms depending on the size of the arene. One mechanism applies to smaller arenes such as naphthalene, and involves diffusion of monomers to yield doubly occupied supercages in which the arene is immobilized and excimer fluorescence is observed. For naphthalene, values of percent retainment varied from 40% to 90% following overnight exposure to solvents. In general, smaller arenes were completely unretained in NaY and retention was only observed in NaX with certain solvents. The other retention mechanism applies to larger arenes having dimensions close to the 7.4-Å supercage aperture and involves inhibited diffusion in which the arene is essentially trapped in PSZ for months at room temperature. Additional studies showed that differences in rates of extraction can be utilized to achieve chemical separations by a process described as reverse molecular sieving. Of notable interest is that studies of photoinduced processes involving entrapped arenes in PSZ are possible. Such studies comment on solvation effects in the nanoconfines of zeolite cavities.