Nonisotropic excitation energy transport in organized molecular systems: Monte Carlo simulation-based analysis of fluorescence and fluorescence anisotropy decay.

An improved application is presented of the Monte Carlo method including simultaneous parameter fitting to analyze the experimental time-resolved fluorescence and fluorescence anisotropy decay of two organized molecular systems exhibiting a number of different, nonisotropic energy transfer processes. Using physical models and parameter fitting for these systems, the Monte Carlo simulations yield a final set of parameters, which characterize the energy transfer processes in the investigated systems. The advantages of such a simulation-based analysis for global parametric fitting are discussed. Using this approach, energy transfer processes have been analyzed for two porphyrin model systems, i.e., spin-coated films of zinc tetra(octylphenyl)-porphyrins (ZnTOPP) and the tetramer of zinc mono(4-pyridyl)triphenylporphyrin (ZnM(4-Py)TrPP). For the ZnTOPP film energy transfer rate constants of similar to1 x 10(12) s(-1) and similar to 80 x 10(9) s(-1) have been found, and are assigned to intra- and interstack transfer, respectively. For the tetramers, the transfer rate constants of 38 x 10(9) and 5 x 10(9) s(-1) correspond to energy transfer to nearest and next nearest neighbor molecules, respectively. The results are in agreement with a Forster type energy transfer mechanism.