Single-Pulse Measurements of Fluorescence Lifetimes: The Influence of Solvent on the Isomerization of trans -Stilbene Included in Zeolites

The fluorescence lifetime (τ f ) of trans -stilbene has been evaluated in zeolites Y and ZSM-5 under various conditions, e.g., at low temperature and in the presence of coadsorbed solvents. To make these measurements, an indirect method was developed to measure short-lived (<100 ps) fluorescence lifetimes with a single laser pulse. In dehydrated NaY, τ f,TS was 52 ps compared to 66 ps in hexane and 32 ps in methanol. When the NaY sample was subsequently bathed in either water or cyclohexane, τ f,TS increased by a factor of 4 and 5, respectively, reflecting decreased mobility of trans -stilbene because of blocking effects of the solvent. However, with small amounts of coadsorbed water equivalent to 13 H 2 O/supercage, τ f,TS decreased by a factor of 2, reflecting increased mobility of trans -stilbene. Small amounts of coadsorbed cyclohexane only increased τ f,TS . Values of τ f,TS in the solvent-free zeolites were significantly smaller (higher degree of probe mobility) than those published a decade ago and contradicted the earlier conclusions regarding the mobility of stilbene in zeolites. The possible reasons for this are discussed.