Artificial photonic antenna systems have been realised by incorporating organic dyes in a nanoporous material.
We have been using zeolite L in most of our experiments as it has proven to be a very versatile host. Its crystals
are cylindrically shaped porous aluminosilicates featuring hexagonal symmetry. The size and aspect ratio of the
crystallites can be tuned over a wide range. A nanometre sized crystal consists of many thousand one-dimensional
channels oriented parallel to the cylinder axis. These can be filled with suitable organic guests.
Geometrical constrains of the host structure lead to supramolecular organisation of the guests in the channels.
Thus very high concentrations of non- or only very weakly interacting dye molecules can be realised. A special
twist is added to these systems by plugging the channel openings with a second type of fluorescent dye, which
we call stopcock molecule. The two types of molecules are precisely tuned to each other; the stopcocks are able
to accept excitation energy from the dyes inside the channel, but cannot pass it back. The supramolecular
organisation of dyes inside the zeolite channels is what we call the first stage of organization. It allows light
harvesting within the volume of a dye-loaded zeolite L crystal and also radiationless energy transport to either
the cylinder ends or centre. The second stage of organisation represents the coupling to an external acceptor or
donor stopcock fluorophore at the ends of the zeolite L channels, which can then trap or inject electronic
excitation energy. The third stage of organization is realised by interfacing the material to an external device via
a stopcock intermediate. We observed that electronic excitation energy transfer in dye-zeolite L materials occurs
mainly along the channel axis. This important finding means that macroscopically organised uni-directional
materials can be prepared. In order to achieve this, we prepared oriented zeolite L monolayers, filled them with
luminescent dyes, and finally added a stopcock. The new materials offer unique possibilities as building blocks
for optical, electro-optical and sensing 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 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 are of cylindrical shape and consist of an extended one-dimensional tube system. They 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. Moreover, fluorescent dye molecules can be bound covalently to the channel entrances. Dependent on the spectral properties of these stopcock molecules, the electronic excitation energy is transported radiationless to the stopcock fixed at the ends of the nanochannels or injected from the stopcock to the dyes inside the zeolite. The radiationless energy migration is in competition with spontaneous emission, thermal deactivation, quenching, and photochemically induced degradation. Fast energy migration is therefore crucial for an efficient antenna material. - The supramolecular organization of the dyes inside the channels is a first stage of organization. It allows light harvesting within the volume of a dye-loaded zeolite L crystal 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.
The luminescence quenching of the oxygen sensitive Ru2+ complex (Ru-ph4-TMS) used as a stopcock and attached to a zeolite L monolayer has been investigated. The luminescence lifetime of the attached Ru-ph4-TMS was the same under N2 and under O2 atmosphere. This remarkable result is attributed to the shielding provided by the channels of the zeolite L crystals arranged as a monolayer. The emitting 3MLCT state of the Ru-ph4-TMS stopcock is localized on the ligand bearing the phenyl groups forming the tail of this complex, which deeply penetrates into the zeolite L channel.
In device engineering, a high degree of supramolecular organisation is required to achieve certain desired macroscopic properties. Dye-loaded zeolite L host-guest materials have been successfully used in the realisation of efficient light-harvesting antenna systems. A new hierarchy of structural order is introduced by arranging the zeolite L crystals into densely packed, oriented monolayers on a substrate. We developed methods to synthesise such monolayers, to fill them with dyes and to terminate them with a luminescent stopcock. By the subsequent insertion of different types of dye molecules in a zeolite L monolayer, the first unidirectional antenna system was realised. Such antenna materials open possibilities for the design of a novel thin layer, silicon based solar cell, where the excitation energy can only migrate in one direction towards the zeolite-semiconductor interface. The electronic excitation energy is then transmitted to the semiconductor by Forster resonance energy transfer (FRET) via stopcock molecules attached to the channel ends. Direct transfer of electrons is prevented by an insulating layer. We report here on the UV-VIS absorption as well as NIR luminescence spectroscopy results obtained from such materials.
Dye-loaded zeolite L host-guest materials were already successfully used in the realisation of efficient light-harvesting
antenna systems. A new hierarchy of structural order is introduced by arranging the zeolite L crystals into densely
packed, oriented monolayers on a substrate. In device engineering, a high degree of supramolecular organisation is a
prerequisite for achieving desired macroscopic properties. The methods we developed to synthesise such monolayers, to
fill them with dyes and to terminate them with a luminescent stopcock will be discussed as well as their influence on the
design of novel materials. By subsequent insertion of two different types of dye molecules in a zeolite L monolayer, the
first unidirectional antenna system was realised. UV-VIS absorption as well as NIR luminescence spectroscopy was
carried out on dye-loaded zeolite L monolayers. We also report a novel concept for the preparation of thin layer, silicon
based solar cells.
We report the observed and calculated fundamental IR and Raman frequencies of H8Si8O12 and D8Si8O12. Assignments are based on normal coordinate analysis and given in terms of internal vibrations. The NIR FT-Raman spectrum of the D5h-H10Si10O15 molecule in the range 65 - 3000 cm-1 is shown.