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Introduction: Host-guest materials based on zeolites and incorporated dye molecules are usually idealized as highly organized systems in which the arrangement of molecules adapts to the crystal structure of the host material, for example the dye molecules aligning to the channel structure. This chapter presents an SMS study that, for the first time and in a direct way, revealed the orientational distribution of various guest molecules of

different size encapsulated into the unidimensional channels of AlPO4-5 during synthesis.

Figure 58: Chemical structures of dye molecules incorporated into the channels of AlPO4-5. The sizes have been calculated from the bond lengths [Dewar77] and van-der- Waals radii [Bondi64].

AlPO4-5 has unidimensional, nearly cylindrical pores with a diameter of 0.73 nm which are oriented parallel to the main crystal axis. Molecules incorporated in this matrix may align more or less well to the pores, in dependence of their respective sizes. Four different Oxazine dyes, that is, molecules with a comparable chemical structure but differing in size (cf. fig.58 for the structures and sizes) were incorporated into AlPO4-5 crystals during microwave assisted synthesis. The central question addressed by this study is how the interplay between guest and host-size determines the structural arrangement in the

From the structures of AlPO4-5 and of the Oxazine molecules the expected orientational distribution would be as follows: The slimmest molecules, Oxazine-1 and -4, (estimated width 0.85 and 0.75 nm respectively) tightly fit the pores of AlPO4-5 (0.73 nm),

considering that the molecules and the host structure also have a certain degree of flexibility. Oxazine-1 and -4 should therefore become incorporated aligned more or less perfectly with the pore orientation. The larger dyes, Oxazine-170 and -750, (estimated width 1.0 and 1.1 nm respectively) are not expected to align with the pores in the

resulting host-guest material, as their size significantly exceeds the pore size of AlPO4-5. Instead, these molecules are likely to induce defects upon encapsulation and thus become incorporated in random orientations. Schematically:

Figure 59: Encapsulation process of a guest species (dye) into a porous host material such as AlPO4-5. a) If the guest is small enough its encapsulation will not significantly affect the host structure. b) If the size mismatch is significant the molecular guests are likely to induce defects in the host structure and may become incorporated in random orientations.

The orientational distribution of the dyes with respect to the crystal main axis is obtained directly via polarization dependent SMS measurements. Before turning to the individual results it is important to note here that polarization dependent measurements conducted on the level of molecule ensembles, while possibly pointing to the presence of a

preferential orientation of the guests, will fail to describe the distribution of orientations. [Hellriegel03]

The orientational distribution of a dye in the pores of AlPO4-5 is in the following described in detail for the oxazine-4 / AlPO4-5 sample. The orientational distribution of the other dyes (described for example in [Hellriegel03] and [Seebacher02]) are acquired in the same way.

Orientational distributrion of Oxazine-4 in AlPO4-5: The orientational angle of an

individual Oxazine-4 molecule is determined by analyzing the polarization dependent emission data (see figure 60b). The polarization angle of the excitation light is constantly modulated as the fluorescence intensity of an individual fluorophore is recorded (cf. section 3.2..3.). The orientation angle of the pores is obtained from transmission images of the AlPO4-5 crystals with an accuracy of ±3°.

Figure 60: Orienations of individual Oxazine-4 molecules in AlPO4-5. a) Typical trasnsmission and fluorescence images (cross-sections as indicated in the schematical drawing) showing the diffraction limited patterns produced by single molecules. b) Three examples for polarization dependent data used to determine the angle of a molecule with respect to the crystal axis. (Note the abrupt photobleaching / blinking steps in the curves that point to the presence of individual molecules)

The angles of 62 individual molecules in 5 different crystals are plotted in figure 61a. The histogram in figure 61b shows that the dyes have a preferential orientation along the main crystal axis with a narrow distribution width. The half-width at half-maximum of the curve is determined to be 14°± 2 using a Gaussian to model the curve. It is also possible to notice that a small population of moleucles is found oriented transversally to the main pore direction.

Figure 61: Orientational distribution of Oxazine-4 in AlPO4-5. a) Raw data b) Histogram showing a narrow (ξ = ±14°) distribution around the main crystal axis.

Orientational distribution of oxazine dyes in AlPO4-5: The histograms for all studied

molecules are shown in figure 62. The histogrammes confirm that the distribution of the slimmer dyes, oxazine-1 and oxazine-4 is narrow, whereas the distribution of the largest molecule oxazine-750 is random. Interestingly, oxazine-170, with a 'thickness' of

~1.0 nm - clearly exceeding the pore size (0.7 nm) shows a broad distribution, but still with a preferential orientation along the pores.

In all cases the incorporation of dyes into the channels of AlPO4-5 is not perfect. Even the slimmest molecules oxazine-1 and oxazine-4 become incorporated transversally to the pores, possibly in pronounced defects.

Figure 62: Orientational distribution histogrammes for the four studied dye molecules. (An angle of 0° is defined as parallel to the pore orientation). ξ denotes the half width at half maximum of the model curves.

Summary: The orientational distribution of four differently sized oxazine dyes incorporated in AlPO4-5 crystals was reconstructed from the orientations of many individual molecules. It is shown that a host system like AlPO4-5 can still have a

directing influence on the incorporation of guest molecules, even when the sizes of these molecules exceed the diameter of the channels, as is the case for oxazine-170. There is, however, an upper size limit for this effect, the largest dye, oxazine-750 with

approximately 1.1 nm width does not become incorporated with a noticeable preferential orientation. A comparison between this method and conventional ensemble studies clearly shows that the SMS approach allows a description of the dye orientational distribution at a more fundamental level, leading to the actual orientational distribution function.