Light-harvesting host-guest antenna materials for quantum solar energy conversion devices

In natural photosynthesis, light is absorbed by photonic antenna systems consisting of a few hundred chlorophyll molecules. These devices allow fast energy transfer from an electronically excited molecule to an unexcited neighbor molecule in such a way that the excitation energy reaches the reaction center with high probability. Trapping occurs there. Systems with similar properties can be prepared by enclosing dyes inside a microporous material and by choosing conditions such that the cavities are able to uptake only monomers but not aggregates. The materials we discuss generally consist of cylindrical zeolite L crystals, in the size range of 30-7000 nm, the channels of which are filled with dye molecules. The synthesis is based on the fact that molecules can diffuse into individual channels. In some cases it is desirable to modify the channel openings afterwards by adding a closure molecule. Functionalization of the closure molecules allows fine-tuning of, for example, wettability, refractive index, and chemical reactivity. The supramolecular organization of the dyes inside the channels is a first stage of organization. It allows light harvesting within a certain volume of a dye-loaded nanocrystalline zeolite and radiationless transport to both ends of the cylinder or from the ends to the center. The second stage of organization is the coupling to an acceptor or donor stopcock fluorophore at the ends of the zeolite L channels, which can trap or inject electronic excitation energy. The third stage of organization is the coupling to an external device via a stopcock intermediate. Another host-guest system, which will be shortly presented, is zeolite A containing silver sulfide. Due to the small dimensions of the cages of zeolite A it is possible to stabilize Ag2S monomers and dimers. In contrast to bulk silver sulfide these small particles show photoluminescence in the visible. In both host-guest systems interesting optical phenomena such as laser action are observed. The wide-ranging tunability of these highly organized materials offers fascinating new possibilities for exploring excitation energy transfer phenomena, and challenges for developing new photonic devices for solar energy conversion and storage.