Analysis of thermal molecular motion on substrate interface by time- And space-resolved fluorescence spectroscopy

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. Picosecond pulse with the wavelength of 430 nm generated by a mode-locked Ti:Sapphire laser was irradiated from the substrate side. Information near the substrate interface was successfully extracted on the basis of evanescent wave excitation. The lifetime for NBD monotonically decreased with increasing temperature because the fractional amount of non-radiative pathways to the ground state for excited species 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 related to the slope 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 was restricted in comparison with that in the bulk phase.