Comparison with Symmetric Dyes: SW-TDI and DIP-TDI

In order to obtain a better understanding of the interactions between the dye and the surrounding host matrix, two symmetric dye molecules, DIP-TDI and SW-TDI (see Figure 7.2), were incorporated in the same way into the mesopores of thin Brij-templated films. These dyes were selected, as their terrylene diimide body is identical to the asymmetricAS-TDI. Both dyes are symmetrical,DIP-TDIhas the identical head group asAS-TDIon both ends, andSW-TDIa swallow like double alkyl-tail compa-rable to the octyl tail ofAS-TDI. The investigations of these dyes should elucidate if there is a correlation between the excess of forward jumps observed withAS-TDIand the asymmetry of the dye. Furthermore, when incorporated into the different phases of the host, single-molecule experiments on these dyes provide additional information about the interactions of the dye with its environment, e.g. which specifications a dye

needs to align perpendicular with respect to the substrate. However, these investiga-tions are in a very initial stage, thus here only some tentative examples and preliminary results will be discussed. They will be continued in a future project.

In the hexagonal phase, the diffusion behaviour of the three TDI dyes was similar. All dyes diffuse in a highly structured manner, with similar diffusion coefficients. A ten-tative ranking of the diffusion coefficients is D_SW-TDI > D_AS-TDI > D_DIP-TDI. Moreover, polarization dependent measurements with the confocal setup showed that none of the dyes keeps a constant orientation while diffusing through the channels.

The diffusion behaviour in the lamellar phase differs strongly for the three different dyes. Whereas AS-TDI shows nearly exclusively doughnut patterns and some un-structured tracks on the surface,SW-TDIexhibits only some doughnut patterns and a high number of unstructured trajectories, of relatively fast molecules is observed. First experiments showed that these fast molecules can not be washed away from the sur-face. Therefore they must be in the bulk of the material. Contrary toAS-TDIand SW-TDI, the dye with two di-isopropyl-phenyl headgroups, DIP-TDI, did not show any doughnut patterns, but fast and unstructured diffusion. Like for the fast, unstructured molecules of SW-TDI, these molecules could not be washed away from the surface.

In addition, some highly structured trajectories ofDIP-TDI were observed in freshly prepared lamellar samples. The XRD of this sample was measured after the wide-field measurements, i.e. a few hours after the synthesis, and did show only one peak at the typical position of the pure lamellar phase. After about one day of ageing of the sample, only very few of these structured trajectories remained. One possible explana-tion is, that immediately after spin-coating a mixture of phases with a small amount of hexagonally arranged pores is present, which transforms within the first few minutes or hours into a purely lamellar structure. It is known from GISAXS measurements at a synchrotron source that the lamellar phase is buildviaan hexagonal phase⁰.

Interestingly, these structured trajectories show a large excess of forward steps and a higher diffusion coefficient than the molecules in the hexagonal phase of the pure samples or the phase mixture. This prooves that there is no relation between the asym-metry of the molecule and the bias towards forward steps. Similar trajectories could be observed in one tentative example ofAS-TDIin a sample synthesized with the in-termediate recipeB4. In this sample doughnuts and highly structured trajectories with large diffusion coefficients coexisted. The XRD of this sample showed a large peak around2θ = 1.45° and a small peak at2θ= 1.60°, indicating the coexistence of the two phases. The structured trajectories show a significant excess of forward steps, as the samples ofB5withDIP-TDI. Repeated experiments using recipeB4, however, showed

0personal communication by A. Zürner, Bein group, LMU Munich

mostly doughnuts and only sporadically some structured trajectories. Up to now, no satisfactory explanation could be found for the excess of forward steps in these struc-tured trajectories, as for the unstrucstruc-tured diffusion in the lamellar phase. As a trend it was observed, that mostly samples synthesized with a high surfactant/silica ratio, showed this phenomenon, either in highly structured trajectories of gaussian shaped patterns or in the slow diffusion of the doughnuts.

These first preliminary results show already that even small structural changes at the end groups of the dye molecules can result in major changes of the behaviour of the dyes within the porous environment. Inducing favourable interactions of the hydrophobic tails of the template with the dye, which align the dye perpendicular to the silica planes, needs apparently the presence of hydrophobic alkyl tails in the dye molecule. Nonetheless, the asymmetric dye with only one alkyl chain showed much more orientation, than the analogous dye with four alkyl chains. This shows, that the host-guest or, more precisely, host-template-guest interactions cannot be explained by simple hydrophobic/hydrophilic interactions.

Though tentative, these results indicate that further investigations of the different dye molecules will provide a deeper understanding of the host-guest interactions within such mesoporous structures.