Inter- und intramolekularer Energietransfer und Ladungstrennung in kovalent gebundenen Cyanin-Farbstoff-Viologen-Systemen / Inter- and Intramolecular Energy Transfer and Charge Separation in Covalently Linked Cyanine Dye-Viologen Systems

As described by many authors, covalently linked dye-electron accepting group systems may provide a very efficient intramolecular fluorescence quenching and electron transfer. Due to the very fast thermal back reaction, no stable charge separation can be achieved. In contrast, J-aggregates of cyanine dyes (1,1′-diethyl-2,2′-cyanine iodide = 1) containing some molecules with covalently linked viologen groups (1-[3-[2-[(1-ethyl-2(1 H)-chinolinyliden)methyl]-chinolinium-l-yl]propyl]-1′-methyl-4,4′-bipyridinium triiodide = 2, 1,1′-(4,4′-bipyridinium-1,1′-diyldi-5,1 -pentandiyl)bis[2-[(1-ethyl-2(1H)-chinolinyliden)methyl]chinolinium] tetrachloride = 3) show an increase in charge separation. Hydrogen production experiments in these organized assemblies, both via inter- (with methyl viologen) and intramolecular (with 2 or 3) charge separation were successful. Hydrogen production experiments based on intramolecular charge separation gave even higher quantum yields than via intermolecular reaction steps. In contrast, hydrogen production experiments with dilute aqueous solutions (no aggregates) with 2 or 3 failed in the absence of methyl viologen (MV2+) due to the fast intramolecular back reaction. In the presence of MV2+ compounds 1-3 were able to produce H2 only when irradiated with near UV-light (λ > 280 nm). Irradiation of these solutions with visible light (λ > 400 nm) resulted in no H2 production. This is attributed to competing nonradiative desactivation processes. Dilute aqueous solutions of 1-3 gave also no visible emission (λ > 400 nm) in the absence of (additional) quenching molecules. So a further progress was achieved by using the J-aggregates: competing nonradiative desactivation processes (of the excited dye molecules) were diminished in J-aggregates.