Single-pulse measurements of fluorescence lifetimes: The influence of solvent on the isomerization of trans-stilbene included in zeolites

The fluorescence lifetime (tau (f)) of trans-stilbene has been evaluated in zeolites Y and ZSM-5 under various conditions, e.g., at low temperature an d in the presence of coadsorbed solvents. To make these measurements, an in direct method was developed to measure short-lived (<100 ps) fluorescence l ifetimes with a single laser pulse. In dehydrated NaY, <tau>(f,TS) was 52 p s compared to 66 ps in hexane and 32 ps in methanol. When the NaY sample wa s subsequently bathed in either water or cyclohexane, tau (f,TS) increased by a factor of 4 and 5, respectively, reflecting decreased mobility of tran s-stilbene because of blocking effects of the solvent. However, with small amounts of coadsorbed water equivalent to 13 H2O/supercage, tau (f,TS) decr eased by a factor of 2, reflecting increased mobility of trans-stilbene. Sm all amounts of coadsorbed cyclohexane only increased Z(f,TS) Values of tau (f,TS) in the solvent-free zeolites were significantly smaller (higher degr ee of probe mobility) than those published a decade ago and contradicted th e earlier conclusions regarding the mobility of stilbene in zeolites. The p ossible reasons for this are discussed.