Nonradiative excitation energy transport in one-component disordered systems

High-accuracy Monte Carlo simulations of the time-dependent excitation probabilityG ^s (t) and steady-state emission anisotropyr _M /r _0M for one-component three-dimensional systems were performed. It was found that the values ofr _M /r _0M obtained for the averaged orientation factor $\overline {\kappa ^2 } $ only slightly overrate those obtained for the real values of the orientation factor κ _ik ² . This result is essentially different from that previously reported. Simulation results were compared with the probability coursesG ^s (t) andR(t) obtained within the frameworks of diagrammatic and two-particle Huber models, respectively. The results turned out to be in good agreement withR(t) but deviated visibly fromG ^s (t) at long times and/or high concentrations. Emission anisotropy measurements on glycerolic solutions of Na-fluorescein and rhodamine 6G were carried out at different excitation wavelengths. Very good agreement between the experimental data and the theory was found, with λ_ex≈λ_0-0 for concentrations not exceeding 3.5·10⁻² and 7.5·10⁻³ M in the case of Na-fluorescein and rhodamine 6G, respectively. Up to these concentrations, the solutions investigated can be treated as one-component systems. The discrepancies observed at higher concentrations are caused by the presence of dimers. It was found that forλ _ex <λ_0-0 (Stokes excitation) the experimental emission anisotropies are lower than predicted by the theory. However, upon anti-Stokes excitation (λ_ex>λ_0-0), they lie higher than the respective theoretical values. Such a dispersive character of the energy migration can be explained qualitatively by the presence of fluorescent centers with 0-0 transitions differing from the “mean” at λ_0-0.