PHOTOCHEMICAL CHARACTERIZATION OF COVALENTLY‐LINKED PORPHYRIN‐QUINONE COMPLEXES

Abstract— The fluorescence properties of a covalently‐linked porphyrin‐quinone complex and its zinc derivative were studied in a variety of organic solvents. The kinetics of fluorescence decay for both the quinone and hydroquinone oxidation states were measured in acetonitrile, dichloromethane, dimethyl‐formamide, and pentane. The fluorescence yield and kinetics of decay at room temperature were little affected in the porphyrin or zinc porphyrin complexes when the attached quinone was reduced. However, for these complexes the fluorescence yield and lifetimes were both substantially decreased in acetonitrile and dichloromethane when the quinone was in its oxidized state. These latter decay kinetics were not explainable by a process having a single exponential decay. On the other hand, little fluorescence quenching or lifetime shortening was observed in dimethylformamide or pentane, indicating unique solvent dependencies for the quenching process. Evidence was obtained for photoproduced charge separation from EPR measurements on the covalently‐linked zinc porphyrin‐quinone complex. The EPR data showed equivalent concentrations of a Zn porphyrin cation radical and a benzoquinone anion radical in acetonitrile or dichloromethane at both room temperature and 77 K. The charge separated state rapidly decayed at room temperature (in sub‐millisecond times) but was quite stable at 77 K. It is concluded that light‐induced charge separation in acetonitrile and dichloromethane at room temperature may occur from the excited singlet state with a high quantum efficiency. A photoproduced charge separated state also occurred when the covalently‐linked complexes were incorporated into egg yolk phosphatidylcholine liposomes. The quantum yield for radical formation in this latter system was 0.1 and the lifetimes of the radical species formed were many minutes.