A TIME-RESOLVED FLUORESCENCE STUDY OF ELECTRONIC EXCITATION-ENERGY TRANSPORT IN CONCENTRATED DYE SOLUTIONS

Singlet-state radiative and nonradiative energy transport among randomly distributed donor and acceptor molecules in solutions of ethylene glycol has been investigated by using time-resolved fluorescence spectroscopy. Radiative energy transfer in moderately concentrated solutions of rhodamine 6G contained in a 1 cm pathlength cuvette results in nonexponential fluorescence decay curves which can be described in terms of the relative fluorescence emission yield of the ith transfer process, phi-i, the average number of transfer steps, <n>, and the apparent fluorescence lifetime, <tau>. The fluorescence decay profiles measured from thin films (less-than-or-equal-to 20-mu-m) of solutions of rhodamine 6G donors in the presence of malachite green acceptors are also nonexponential and at low donor concentrations (1.0 x 10(-4) M) the decay curves can be described by using Forster's dipole-dipole model for nonradiative energy transfer, where the value for the critical transfer distance, R0, is calculated to be (5.9 +/- 0.1) nm. At higher donor concentrations (3.0 x 10(-3) M) the Forster model is inappropriate. However, the model proposed by Loring, Anderson and Fayer (LAF), which allows for the effects of nonradiative energy transfer among donors, provides excellent fits to the experimentally determined fluorescence decay curves for this donor concentration and results in a value of R0 for nonradiative energy transfer between rhodamine 6G chromophores of (5.8 +/- 0.1) nm. The LAF model also provides a satisfactory description of the kinetics of the quenching by rhodamine 6G dimers of the fluorescence from thin films of highly concentrated solutions of rhodamine 6G. The value of R0 for nonradiative energy transfer from monomer to dimer and the equilibrium constant for rhodamine 6G dimerization are calculated to be (3.2 +/- 0.1) nm and 19 M-1, respectively.