Computer Simulation of Energy Relaxation and -Transport in Organized Porphyrin Systems

• Main Author: Yatskou, Mikalai M
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2001
• Subjects: Computer science; Information Technology
• ISBN: 9798516017537

The general goal of this Thesis was the development of an advanced computer simulation approach to improve the analysis of the complex fluorescence- and fluorescence anisotropy decay of porphyrin oligomers in solution as well as in thin solid films in the presence of energy transfer processes. With the application to light-harvesting antenna's in solar cells in mind, the energy transfer- and relaxation properties of several artificial antenna's, consisting of self-organized porphyrins in ultra-thin, spin coated films and as oligomers in different solutions, have been studied by the designed computer simulation approach.Chapter 1 contains a general description of the photophysical processes in artificial photosynthesis, the steady state- and time-resolved fluorescence spectroscopy, the methods used to analyze the time correlated single photon counting data and the mathematical modeling methods. The theoretical framework presented in this Chapter forms the background for understanding the basics of the studied porphyrin systems and the experimental and calculational methods to investigate these systems.Chapter 2 describes the analyzed porphyrin systems, the experimental steady state- and time-resolved absorption and fluorescence methods, technical details of the equipment, and typical results of the photophysics of some porphyrin systems.Chapter 3 compares the various optimization methods for fitting the fluorescence decay with the aim to review their properties. These properties are important when applying Monte Carlo simulations to the investigated energy transfer processes. A number of methods have been found to be promising for practical application in data analysis using computer simulation approach: the Nealder and Mead method and its simulated annealing modification, the Marquardt method, the Powell, Box , and Levenberg-Marquardt methods and modified Gauss and Newton method. Also, the % criterion turned out to be one of the best criteria for judging the goodness of the simulation-based fits as well as for parameter fitting. By the method of asymptotic standard errors an optimal equilibrium could be achieved between CPU and the accuracy of the calculated parameters requiring a confidence interval evaluation procedure.In Chapter 4 a computer simulation approach, based on Monte Carlo and optimization methods, is described for the investigation of energy transport and -relaxation, as these processes affect the fluorescence- and fluorescence anisotropy decay. This novel type of analysis of time-resolved fluorescence data is demonstrated for two differently organized porphyrin systems. Equations for 2 the % criterion and its weighting factors, used in Monte Carlo simulation fitting procedure, are presented and modified for different types of time-resolved data analysis. This Chapter presents some illustrative examples demonstrating several fits of the fluorescence- and the fluorescence anisotropy decay using Monte Carlo simulation fitting. Also, the conceptual properties behind the fitting procedures are discussed.Chapter 5 reports the results of a study of self-organized [Zn(4-Py)TrPP]4 tetramers in toluene and polystyrene/toluene solutions and in solid films, as well as of ordered ZnTOPP domains in solid films. The effects of lowering the temperature or increasing the concentration of a ZnM(4- Py)TrPP/toluene solution on the absorption- and fluorescence spectra are the result of porphyrin aggregation. A simple exciton model can describe the optical spectra of porphyrin aggregates in solution as the result of porphyrin ligation within the symmetrical tetramer structure. The shifts, found in both the Q-band region of the absorption spectrum and in the fluorescence spectrum can be ascribed to coordination of the metal center of one ZnM(4-Py)TrPP molecule to the pyridyl substituent of a second, neighboring molecule. Excitonic interactions can be neglected for the Si state. The splitting of the Soret band in the absorption spectrum can be readily explained by an excitonic interaction in the S2-state between the four mutually perpendicular ZnM(4-Py)TrPP monomers in the tetramer. This symmetrical tetramer is the smallest complex that can account for all of the observed shifts in the optical spectra. The experimental thermodynamic parameters are in good agreement with those expected for a tetramer. The global analysis of the time-resolved fluorescence decays of ZnM(4-Py)TrPP in toluene and PS/Tol at 10 °C yields two fluorescence components with 1.95 - 2.22 ns and 1.53 - 1.56 ns lifetime, where the shorter component is assigned to ligated monomers in the fraction of cyclic tetramers in the solution. The ~1.5 ns fluorescence component is associated with a rotational correlation time of- 1 ns, as compared to -100 ps for the monomer at 10°C, in agreement the size of the tetramer.The fluorescence- and anisotropy decay analysis by Monte Carlo simulations yields a singlet-singlet energy transfer rate constant of 40x109 s-1 for the average energy transfer within the tetramer.Applying the Monte Carlo simulations to ZnTOPP films, the results of fluorescence- and anisotropy decay can be explained by assuming a multi-domain model of parallel porphyrin stacks. In each stack the porphyrin planes are perpendicular to the substrate and form an angle of 45° with the long stack axis. Monte Carlo simulation analysis yields the rate constants for intra stack and inter-stack energy transfer as ~ 1 x 1012 s-1 and-80x109 s-1; whereas the fluorescence lifetime is ~ 1.8 x10-9 s.The results of fluorescence and fluorescence anisotropy decay measurements of Zn(4-Py)TrPP films are qualitatively described. A preliminary analysis of these results indicates that the optical spectra and the fluorescence anisotropy decay of the Zn(4-Py)TrPP films are basically in agreement with the tetramer porphyrin structure being the assembling unit in the solid film.