Dye molecules, zeolite L channels

SUPRAMOLECULARLY ORGANIZED LUMINESCENT DYE MOLECULES IN THE CHANNELS OF ZEOLITE L ... 

In this chapter, we describe the design and important properties of supra-molecularly organized dye molecules in the channels of hexagonal nanocrystals. We focus on zeolite L as a host. The principles, however, hold for other materials as well. As an example, we mention ZSM-12 for which some preliminary results have been reported [55], We have developed different methods for preparing well-defined dye-zeolite materials, working for cationic dyes, neutral dyes, and combinations of them [3, 22, 25, 52], The formula and trivial names of some dyes that so far have been inserted in zeolite L are reported in Section II.C. The properties of natural and commercially available zeolites can be influenced dramatically by impurities formed by transition metals, chloride, aluminiumoxide, and others. This fact is not always sufficiently taken care of. In this chapter, we only report results on chemically pure zeolites, the synthesis of which is described in [53]. 

Figure 1.20. (a) Angles 0 0y, and y, describing the relative orientation of the electronic transition dipole moments s between two dye molecules, (b) Relative orientations of the electronic transition dipole moments between two equal dye molecules in the channels of zeolite L. (c) Angular dependence of the orientation factor k2 under the anisotropic conditions (b) and averaged over y. 

Figure 1.30. Diffusion of dye molecules in the channels of zeolite L. Idealized initial state of a channel and state after diffusion has occurred for some time.

In this chapter, we describe equilibria between dyes inside the zeolite L channels and dyes outside either in gas phase or in solution. We consider dye molecules... 

Figure 5 shows the total concentration of dye molecules in the channels of zeolite L [DJtot expressed as occupation probability p versus the dye concentration in solution in units of the total number of available sites, uc. From the results illustrated, it follows that it is easy to prepare materials with low loading, but that sophisticated techniques are needed for high loading. 

In Fig. 12 in Ref 25, fluorescence microscopy images of different dye-loaded zeolite L single crystals are shown. Each line consists of three pictures of the same sample, but with different polarizations of the fluorescence observed. In the first one, the total fluorescence of the crystals is shown, and in the others, the fluorescence with the polarization direction indicated by the arrows is displayed. The zeolite was loaded with the following dyes (A) Py+, (B) PyGY", (C) PyB +, (D) POPOP (see Table 1). Most crystals show a typical sandwich structure with fluorescent dyes at the crystal ends and a dark zone in the middle. This situation can be observed when the diffusion of the dyes in the channels has not yet reached its equilibrium situation. It illustrates nicely how the molecules penetrate the crystals via the two openings on each side of the one-dimensional channels. 

First experimental results on dye-loaded zeolite L systems modified with commercial stopcock molecules on the external surface show that electronic excitation energy can be transferred from molecules inside the channels to the stopcocks and vice versa and that the stopcocks prefer to adsorb on the cylinder base instead of the coat [42]. 

Depending on the size of an incorporated dye, the angle of the transition dipole moment to the c axis lies between 0° for long molecules and 72° for smaller ones. Therefore, if a small molecule is inserted into the channels of zeolite L, part of the emission will be parallel to the c axis. Due to the flat and parallel ends of appropriately prepared zeolite crystals, one can envisage to arrange crystals between two mirrors or to add a reflecting layer on individual crystals. This might lead to a microlaser with a plane-parallel resonator. Apart from experimental difficulties, the realization of a dye-loaded zeolite L microlaser appears to be feasible. 

Figure 1.1. Representation of a cylindrical nanocrystal consisting of organized dye molecules acting as donors (empty rectangles) and an acceptor acting as a trap at the front and the back of each channel (shaded rectangles). The enlargement shows a detail of the zeolite L channel with a dye molecule and its electronic transition moment. The orientation of this electronic transition moment with respect to the long axis depends on the length and shape of the molecules [54].

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