Fluorescent lifetime measurement for dye molecules tagged to polystyrene in a thin film; analysis for thermal molecular motion of a polymer thin film

Fluorescence lifetime for 6-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino) hexanoic acid (NBD) tagged to polystyrene (PS) in a film on a sapphire substrate was examined as a function of temperature. Mode-locked Ti:Sapphire picosecond laser at 430 nm was irradiated from the substrate side. The substrate interface was selectively excited by evanescent wave excitation, lifetime decreased with increasing temperature because the fractional amount of non-radiative pathways to the ground state for excited NBD molecules increased with temperature. In general, as measuring temperature goes beyond a temperature at which molecular motion releases, the slope for the temperature-lifetime changes. In this experiment, the inflection point was higher in the interfacial region than in the internal bulk phase of the film. Thus, it was concluded that chain mobility in the interfacial region is restricted in comparison with that in the bulk phase.