Static and Dynamic Fluorescence from α,ω-Di(1-pyrenyl)alkanes in Polyethylene Films. Control of Probe Conformations and Information about Microstructure of the Media

Static and dynamic fluorescence from four α,ω-bis(1-pyrenyl)alkanes (P n P, where n, the number of carbons atoms in the alkane chain, is 3, 5, 7, and 12) has been examined in two polyethylene films of 31% (LDPE) and 71% (HDPE) crystallinity. Fluorescence decays of four P n P at ∼10 -5 M bulk concentrations, including that of P7P which is monoexponential in hexane, are biexponential in the films (τ 1 ∼ 200 ns; τ 2 ∼ 40−60 ns). Since 1-ethylpyrene, a monolumophoric model for the P n P, provides monoexponential decays (τ ∼ 200 ns) in both hexane and the films, the P n P interchromophoric interactions in the PE films are intra molecular. Dynamic excimer formation is evident only for P3P in LDPE; static excimeric emissions are observed for P3P in LDPE and HDPE and for P5P in LDPE. Especially in interfacial sites of the PE films, the lumophoric groups of the P n P appear to adopt specific conformations that are not detectable in “normal” isotropic liquid or glassy media. A model to account for these results that involves these conformations and the lateral surfaces of PE microcrystallites is presented.