Zeolites as single electron donors for photoinduced electron transfer reactions of guest aromatic species Diffuse reflectance laser photolysis study

Abstract

Photoinduced electron transfer reactions have been observed with transient absorption spectroscopy from zeolite framework to a few guest aromatic molecules with strong electron accepting nature represented by 1,2,4,5-tetracyanobenzene (TCNB): photoexcitation of the guest molecules adsorbed in cavities of faujasite zeolites, i.e. Y-type zeolite with Na⁺ as charge-balancing cations (NaY) and partially Cs⁺-exchanged Y-type zeolite (CsNaY), resulted in the formation of anion radicals of the guest species by a single photon process. These zeolites are considered to possess an electron donating property. No electron transfer appeared to take place for the same guest species in siliceous or ultrastable Y-type zeolite (USY) which is presumably a weaker electron donor than NaY or CsNaY. The electron transfer was considered to take place mainly in the excited singlet state of the guest species since the fluorescence quenching was observed for the guest species depending on the electron donor ability of the zeolites. In addition, the electron transfer to the triplet excited state of TCNB from the zeolite framework was also evidenced in the transient spectra in NaY and CsNaY. A possible role of zeolites as electron donors towards guest species was investigated extensively in the present study whereas this role of zeolites has been known only for a few cases in zeolite photochemistry.