Fluorescence behavior of dyes in thin films of various polymers

We have studied the fluorescence behavior of a dye, 6-(N-(7-nitrobenz-2- oxa-1,3-diazol-4-yl)amino)hexanoic acid (NBD), in thin films of polymers with various polarities, such as poly(methyl methacrylate) (PMMA), Arton, poly(styrene) (PS), hydrogenated polystyrene (H-PS), and Zeonex. In the case of well-dispersed systems, the fluorescence behavior of NBD could be explained in terms of the mobility of the polymer matrix. This was the case for PMMA, Arton, and PS. On the other hand, when H-PS or Zeonex was used, NBD aggregated in the films, leading to unusual fluorescence behavior: the fluorescence lifetime and maximum wavelength increased with a decrease in the film thickness. Time- and space-resolved fluorescence spectroscopy using an evanescent wave excitation revealed that the aggregation states of NBD near the interface varied with the film thickness. While NBD molecules near the interface aggregated in a thick film, such was not the case in a thin film. Angular-dependent X-ray photoelectron spectroscopy (ADXPS) structurally confirmed the above observations; that is, the film thickness greatly influenced the depth profile of the NBD composition in the polymer films.