Zeolite nano crystals can act as hosts for supramolecular organization of molecules, complexes and clusters, thus encouraging the design of precise functionalities. The main role of the zeolite framework is to provide the desired geometrical properties for arranging and stabilizing the incorporated species [1]. Focusing on supramolecularly organized dye molecules in the channels of hexagonal zeolite L crystals we have shown that they provide fascinating possibilities for building an artificial antenna device which consists of highly concentrated monomeric dye molecules with a large Förster energy transfer radius and a high luminescence quantum yield in a specific geometrical arrangement [2]. Organic dye molecules have the tendency to form aggregates already at low concentration. Such aggregates are known to cause fast thermal relaxation of electronic excitation energy. The role of the zeolite is to prevent this aggregation and to superimpose a specific organization, which is possible as we have shown recently. In such an antenna, light is absorbed by one of the strongly luminescent chromophores. Due to short distances and the alignment of the electronic transition dipole moments of the dyes along the channels, the excitation energy is transported by Förster type energy migration preferentially along the axis of the cylindrical antenna to a specific trap. The ideal dimension of a microcrystal containing several million chromophores is less than a micrometer. We have recently demonstrated that the intercalation of pyronine and oxonine molecules into the linear channels of zeolite L can be visualized with the help of a fluorescence microscope [3]. One can observe the alignment of the dyes in the channels by means of a polarizer, because maximum luminescence appears parallel to the longitudinal axis of the crystals, and extinction appears perpendicular to it. Furthermore the cylindrical nano crystals appear always dark at their base. A simple experiment for the visual proof of the energy transfer from pyronine to oxonine in zeolite L is based on the observation that both dyes are absorbed from an aqueous solution within about the same time leading to short donor-acceptor distances, which allow efficient energy transfer between them. The formation of aggregates which can act as unwanted traps is prohibited for spatial reasons. Hence, there are no such traps inside the material. The dyes are arranged with their long molecular axis along the linear channels and they cannot glide past each other because the linear channels are too narrow. This allows the filling of specific parts of the microcrystals with a desired type of dye.

SCHEME 1

Theoretical considerations of energy migration as a series of Förster energy transfer steps have shown that in material of this kind energy migration rate constants of up to 30 steps/ps can be expected [4]. This is faster than in the antenna system of natural photosynthetic organisms, where approximately 0.2 steps/ps have been reported. In our first report on experimentally observed energy migration along the axis of pyronine loaded cylindrical zeolite L microcrystals we have shown that it is possible to realize these theoretical predictions [2]. The principle of the system investigated is illustrated in Scheme 1, where the empty bars represent pyronine molecules located in the channels of zeolite L and the dashed bars are oxonine molecules which act as luminescent traps at both ends of the cylinder. We define the occupation probability p as the ratio between the number of sites occupied by a dye and the total number of sites available. Hence, p adopts values between 0 for an unloaded zeolite and 1 for a zeolite loaded to its maximum. The enlargement shows a zeolite L channel with a pyronine molecule (X = C-H), the S0,S1 electronic transition moment which is aligned along the channel axis. The dashed bars at the front and end symbolize an oxonine molecule located at the front and the back of each channel. A schematic view illustrating the parallel lying channels filled with pyronine monomers is given in Scheme 2. The primitive vector c corresponds to the channel axis, while the primitive vectors a and b are perpendicular to it, enclosing an angle of 60^o. A feeling for the material is obtained by realizing that a zeolite L nano crystal of i.e. 700 nm length and a diameter of 600 nm gives rise to about 95'000 parallel lying channels each of which bears a maximum of 470 sites for the pyronine molecules.
In the experiments reported in ref [2], light is absorbed by a pyronine molecule located somewhere in one of the channels. The excitation energy then migrates along the axis of the nano crystal, as indicated by the arrows in Scheme 1, and is eventually trapped by an oxonine located at the front or at the back of the cylinder. The electronically excited oxonine then emits the excitation with a quantum yield of approximately one. This process which we call front-back trapping has been investigated theoretically for energy migration occurring by Förster energy transfer. We found that a total front trapping efficiency of up 99.8 % can be obtained for nano crystals of 50 nm length, if all sites are occupied by a pyronine chromophore [4]. Donor-donor self-absorption and re-emission were not considered in this theoretical study which focused on very small crystals. However, for crystals of larger dimension, particularly larger diameter, self-absorption and re-emission are expected to contribute significantly to the energy migration properties. The experiments reported in ref [2] are based on the fact that pyronine loaded zeolite L nano crystals, modified withone oxonine molecule at the front and at the back of each channel on average, can be prepared, as illustrated in Scheme 1, and that it is possible to synthesizes zeolite L nano crystal-cylinders of different average length with narrow size distribution. Nano crystals with an average length of 200 to 1500 nm have been investigated so far.

SCHEME 2

For some applications it is desirable or even necessary to arrange the nano crystals as monolayers on a substrate such as a semiconductor, a conducting glass or a metal. This is explained in Fig. 1 where as an example a speculation on a new type of a dye-sensitized solar cell is illustrated. In such a device all incoming light is absorbed within the volume of the nano crystals of less than 1 mm length containing appropriate dye molecules for light harvesting. The excitation energy is then transported via very fast energy migration to the contact surface with the semiconductor, where, by efficient (radiationless) energy transfer from an excited dye to the semiconductor, it creates an electron hole pair in the semiconductor. This means that the absorption of light and the creation af an electron-hole pair are spatially separated similar as in the natural antenna system of green plants. The semiconductor could for instance consist of a very thin Si layer which by itself would be much to thin to absorb a significant amount of light. The electron hole pair can then be separated as in an ordinary Si based solar cell. This results in a very thin solar cell only about 2 mm thickness; see ref [1]. We have shown that monocrystal-layers, of the type needed for realizing such a device, can be formed with zeolite A and I am confident that the same will be possible with zeolite L and other zeolite nano crystals bearing the appropriate morphology.

Figure 1

[1]	Calzaferri, G. Chimia 1998, 52, 525.
[2]	Gfeller, N.; Megelski, S.; Calzaferri, G. J. Phys. Chem B 1999, 103, 1250.
[3]	Gfeller, N.; Megelski, S.; Calzaferri, G. J. Phys. Chem B 1998, 102, 2433.
[4]	Gfeller, N.; Calzaferri, G. J. Phys. Chem. B 1997, 101, 1396.

[BACK] [MAIN] [FURTHER]