LIGHT-HARVESTING HOST-GUEST ANTENNA MATERIALS FOR NEW PHOTONIC DEVICES

Gion Calzaferri

Department of Chemistry and Biochemistry, University of Bern, CH-3012 Bern
gion.calzaferri@iac.unibe.ch

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 neighbour molecule in such a way that the excitation energy reaches the reaction centre with high probability. Trapping occurs there. The anisotropic arrangement of the chlorophyll molecules is important for efficient energy migration. In natural antennae 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 and by choosing conditions such that the cavities are able to uptake only monomers but not aggregates. In most of our experiments we have been using zeolite L as a host because it was found to be very versatile. Its crystals consist of an extended one-dimensional tube system and can be prepared in wide size range. We have filled the individual tubes with successive chains of different dye molecules and we have shown that photonic antenna materials can be prepared, not only for light harvesting within the volume, but also for radiationless transport of electronic excitation energy to a target molecule fixed at the ends of the nanochannels as well as with an injector molecule fixed at their „entrances”. The molecule which has been excited by absorbing an incident photon transfers its electronic excitation to another one. After a series of such steps the electronic excitation reaches a luminescent trap. The energy migration is in competition with spontaneous emission, radiationless decay, quenching, and photochemically induced degradation. Fast energy migration is therefore crucial if a trap should be reached before other processes can take place. - 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 nano-crystalline zeolite and radiationless transport to both ends of the cylinder or from the ends to the centre. The second stage of organization is the coupling to an external 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. 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.[1-3]

[1] G. Calzaferri, S. Huber, H. Maas, C. Minkowski, Photon-Harvesting Host-Guest Antenna Materials, Angew. Chem. Int. Ed. Engl. 2003, 42, 3732.
[2] M. Yatskou, M. Pfenniger, M. Meyer, S. Huber, G. Calzaferri, Electronic Excitation Energy Migration in a Photonic Dye-Zeolite Antenna, ChemPhysChem, 2003, 4, 567.
[3] a) S. Huber, G. Calzaferri, Sequential Functionalization of the Channel Entrances of Zeolite L Crystals, Angew. Chem. Int. Ed. Engl. 2004, 43, 6738. b) O. Bossart, L. De Cola, S. Welter,  G. Calzaferri,  Injecting Electronic Excitation Energy Into an Artificial Antenna System by a Ru2+ Complex, Chem. Eur. J. 2004, 10, 5771.