Artificial antenna systems

Abstract

The work describes experimental and theoretical results on the water oxidation in absence of an externally added potential, on the charge transport in organized microporous media and on the transport of excitation energy in an antenna system. Water oxidation to O2 takes place at the solid/water phase boundary of a thin AgCl layer in the presence of a small excess of Ag+. This water oxidation step shows self-sensitization as the reaction proceeds, the sensitivity is extended from the near-UV-visible towards the red range. The quantum yield per redox equivalent for O2 evolution upon illumination with near UV light (340–390 nm) is ∼ 0·8 and it is the same upon illumination with blue light (420–480 nm). In the green range it is ∼ 0·5. We discuss parameters controlling these reactions. Zeolite microcrystals are investigated as hosts for supramolecular organization of clusters, complexes and molecules. The possibility to arrange zeolite microcrystals of good quality and narrow size distribution as dense monograin layers on different types of substrates allows the discovery of specific properties. In the present context, three functionalities are of special importance intrazeolite ion transport, intrazeolite charge transport and intrazeolite excitation energy transport. All of them have been clearly demonstrated experimentally although there are still some controversies going on. Highly concentrated dyes have the tendency to form aggregates which generally show very fast radiationless decay. In natural antenna systems the formation of aggregates is prevented by fencing the chlorophyll molecules in polypeptide cages. A similar approach is possible by enclosing dyes inside a microporous material such that the volume of the cages and channels is able to uptake monomers only, but not aggregates. We know a number of materials bearing linear channels running through the whole microcrystal which allow the formation of highly anisotropic, monomeric dye assemblies. A few cases based on zeolite L as a host and the cationic dye molecules pyronine and oxonine have been investigated experimentally to some extent for this purpose. While the molecules can penetrate the channels, the geometrical constraints of this system excludes aggregation and therefore self-quenching up to very high concentrations, namely 0·2 M. Microcrystals with cylinder morphology and a size in the range of 100 nm have been found to be optimal for realizing collection efficiencies in order of 99%.