Picosecond dynamics of excitation-energy relaxation of dye molecules in Langmuir–Blodgett monolayers

Fluorescence-decay curves and time-resolved fluorescence spectra of N, N′-dioctadecylrhodamine B in Langmuir–Blodgett monolayers have been measured by means of a picosecond time-resolved fluorescence spectrophotometer. The excitation energy relaxation of the dye molecule in the monolayers was compared at 295 and 80 K in the concentration range from 0.02 to 31 mol%. The relaxation of the dynamic Stokes shift was found to decay logarithmically with time in longer time regions when the concentration of N, N′-dioctadecylrhodamine B < 5 mol%. The fluorescence decays can be analyzed by an equation including a stretched exponential function. The mechanisms of singlet excitation-energy relaxation were interpreted in terms of (i) the energy migration among energetically disordered monomer sites represented by ultrametric space (hierarchical energy distribution) or the Gaussian distribution of the density of excited states and (ii) the energy trapping by two-dimensional aggregates. It was found that the relaxation of the dynamic Stokes shift and the fluorescence decay can be estimated by the diffusion length of the excitation-energy transport.