INTERMOLECULAR 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 fluorescenc e quenching and electron transfer. Due to the very fast thermal back r eaction, no stable charge separation can be achieved. In contrast, J-a ggregates of cyanine dyes (1,1'-diethyl-2,2'-cyanine iodide = 1) conta ining some molecules with covalently linked viologen groups inolinium- 1-yl]propyl]-1'-methyl-4,4'-bipyridinium triiodide = 2 1,1'-(4,4'-bipy ridinium-1, [(1-ethyl-2(1H)-chinolinyliden)methyl]chinolinium] tetrach loride = 3) show an increase in charge separation. Hydrogen production experiments in these organized assemblies, both via inter- (with meth yl viologen) and intramolecular (with 2 or 3) charge separation were s uccessful. Hydrogen production experiments based on intramolecular cha rge separation gave even higher quantum yields than via intermolecular reaction steps. In contrast, hydrogen production experiments with dil ute aqueous solutions (no aggregates) with 2 or 3 failed in the absenc e of methyl viologen (MV2+) due to the fast intramolecular back reacti on. In the presence of MV2+ compounds 1-3 were able to produce H-2 onl y when irradiated with near UV-light (lambda > 280 nm). Irradiation of these solutions with visible light (lambda > 400 nm) resulted in no H -2 production. This is attributed to competing nonradiative desactivat ion processes. Dilute aqueous solutions of 1-3 gave also no visible em ission (lambda > 400 nm) in the absence of (additional) quenching mole cules. So a further progress was achieved by using the J-aggregates: c ompeting nonradiative desactivation processes (of the excited dye mole cules) were diminished in J-aggregates.