Experimental and Theoretical Studies towards the Charge Recombination of Organic Radicals in Inorganic Mesostructures

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The photoinduced charge separation between two redox centers is one of the basic steps in electron transfer (ET) processes. For this purpose the study of model processes of charge separation and recombination, as exemplified with large polycyclic aromatic hydrocarbons (PAH) as probe molecules for which stable radical centers are well documented, might allow fundamental insights into the kinetics of light conversion into chemically usable work, DG, and for charge recombination. Well defined compartmented reaction spaces where PAH probe molecule redox centers like e.g. dicoronenylene or quaterylene are held in intermolecular proximity to each other allow to study fundamental charge recombination processes under geometrically constrained conditions. Recombination parameters are dependent on the size of the given host space as well as the individual orientation of the inorganic radicals under study. As hosts, one dimensional nano- and mesoporous inorganic oxides like alumina or mesoporous hosts of the MCM 41 family that offer a highly ordered parallel pore arrangement will be used, making available a constrained reaction space for radical formation and recombination of large PAHs. To study the fate of charge separated species transient spectroscopic measurements are necessary. Timeresolved laser spectroscopy, will be used to investigate the kinetics of charge recombination by optical and/or conductivity detection. Within these experiments employed pulse widths of the lasers control the time resolution of the experiments. Radical properties like spin densities and g values as well as dynamic charge exchange phenomena are subjects of detailed ESR- and ENDOR studies. Understanding the microscopic situation and modeling the time-resolved spectra will be facilitated by theoretical studies employing quantum chemistry and molecular simulations. That part of the project is aimed towards a determination of structural and energetic characteristics of PAH adsorption on model surfaces of nano- and mesoporous zeolite- and aluminatype materials under various conditions mimicking the experimentally chosen geometrically constrained reaction spaces of porous alumina and MCM 41. Equilibrium surface density distributions of PAH guest molecules along with information about their orientational ordering are expected research results. Ultimately, chemical, geometrical (in the sense of utilizing the variable confinement conditions represented by the porous host material), as well as thermodynamic control of charge transfer kinetics based on microscopic insight is aspired.