STATIC AND DYNAMIC FLUORESCENCE FROM ALPHA,OMEGA-DI(1-PYRENYL)ALKANESIN POLYETHYLENE FILMS - CONTROL OF PROBE CONFORMATIONS AND INFORMATION ABOUT MICROSTRUCTURE OF THE MEDIA

Static and dynamic fluorescence from four alpha,omega-bis(1-pyrenyl)al kanes (PnP, 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 PnP at similar to 10(-5) M bulk concentrations, including that of P7P whic h is monoexponential in hexane, are biexponential in the films (tau(1) similar to 200 ns; tau(2) similar to 40-60 ns). Since 1-ethylpyrene, a monolumophoric model for the PnP, provides monoexponential decays (t au similar to 200 ns) in both hexane and the films, the PnP interchrom ophoric interactions in the PE films are intramolecular. Dynamic excim er formation is evident only for P3P in LDPE; static excimeric emissio ns are observed for P3P in LDPE and HDPE and for P5P in LDPE. Especial ly in interfacial sites of the PE films, the lumophoric groups of the PnP appear to adopt specific conformations that are not detectable in ''normal'' isotropic liquid or glassy media. A model to account for th ese results that involves these conformations and the lateral surfaces of PE microcrystallites is presented.