Photochemistry of azobenzene in microporous aluminophosphate AlPO4-5

Photochemical studies of molecules in the organized voids of microporous frameworks is an area of considerable research interest. In this study, we have examined the spectroscopy and photochemistry of trans-azobenzene introduced into the 8 Angstrom channels of an aluminophosphate framework, AlPO4-5. The acidity of aluminophosphates is considered to be very weak, based on their poor catalytic performance in transformations of organic compounds. However, we report that the Bronsted acidity of the AlPO4-5 is enough to protonate the azo groups in azobenzene. This leads to appearance of a band at 318 nm. Also, room temperature emission observed at 518 nm, an unusual spectroscopic feature for azobenzene, is due to the protonated form. Photolysis of the azobenzene/AlPO4-5 sample leads to the formation of benzo[c]cinnoline and benzidine. The Lewis acid sites of AlPO4-5 are proposed to play an important role in the photocyclization chemistry. Because the number of such acid sites in AlPO4-5 is limited, only a fraction of the azobenzene molecules undergo photocyclization. The rest exhibit reversible cis-trans photoisomerization in the presence of visible and ultraviolet irradiation.