Azo-polyglutamates

Irradiation with unpolarized blue light does not erase the induced dichroism, because only the cis isomer has significant absorption in the blue region around 450 nm consequently, the trans molecules cannot be excited and reoriented. The in-plane isotropy in both the trans and the cis molecular distributions can be restored only after successive unpolarized UV and blue light irradiations. The initial spectra of a freshly prepared sample prior to irradiation is not restored by this procedure, however, because a net out-of-plane orientation of the azomolecules remains. It is noteworthy that spectra recorded with different analysis light polarizations do not intersect at isosbestic points because of the anisotropy that exists in the sample. Heating the azo-polyglutamate sample at 80°C for 30 minutes and 14.5 hours failed... 

FIGURE 3.7 UV-vis absorption spectra of the azo-polyglutamate film before (I) and after 35 min (2. 3) of linearly polarized UV (360 nm) irradiation. The probe light was also linearly polarized and spectra were obtained for both parallel (3) and perpendicular (2) orientations.The inset is an expanded view of the cis absorption. After reference 22, redrawn by permission of ACS. 

FIQtIRE 3.10 Relaxation in the dark of the dichroism induced by linearly polarized 3( 0 nm light irradiation in the Pjjo azo-polyglutamate. The spectra were obtained after 2 minutes unpolarized 450 nm irradiation (I and I and after 6 days relaxation in the dark (2// and 2jj. None that the absorbances have shifted to higher values due to the cis->trans isomerization, while tlte initial dichroism was increased.The inset is an expanded view of of the region around 450 nm. After reference 22, redrawn by permission of ACS. 

It is my pleasure to thank many friends and colleagues for their support and many helpful discussions. In particular , the work on DRl was done with J. A. Delaire and M. Dumont at the University of Orsay, and the work on azobenzene was done with W. Knoll and J. Wood at the Max-Planck Institute for Polymer Research in Mainz. H. Menzel from Hannover University provided the azo-polyglutamate polymer. The work on azo-polyurethanes and spiropyrans and diaryiethenes was done at Osaka University, Handai, with D. Yasumatsu, H. Ishitobi, and S. Kawata, and M. Irie from Kyushu University. I would like to thank the Japan Society for the Promotion of Science for research support under the Research for the Future Program and to thank the Max-Planck Society for financial support during my stay at the Max-Planck Institute for Polvmer Research in Mainz. 

PHOTOISOHERIZATION AND PHOTO-ORIENTATION OF AZOBENZENES IN SUPRAMOLECULAR ASSEMBUES PHOTO-CONTROL OFTHE STRUCTURALAND OPTICAL PROPERTIES OF LANGMUIR-BLODGETT-KUHN MULTILAYERS OF HAIRY ROD AZO-POLYGLUTAMATES... 

This work is based on collaborations with many friends and colleagues, and it is our pleasure to thank them for their support and for many helpful discussions. In particular, J. Wood participated in most of the work, Y. Geerts and K. Mullen provided the azo-silane compound, M. Biichel and S. Paul contributed to the azo-polyglutamate LBK experiments, and B. Weichart and H. Menzel from Hannover Universitat provided the azo-poiyglutamate polymers. The work on azo-polyimides was done at the University of California, Davis and IBM Almaden (San Jose) with... 

The thermal cis => trans back reaction was studied in detail by irradiating the azo-polyglutamate sample with UV light until the photostationary state was attained, and, subsequently, recording the absorbance of the sample at... 

V. LANGMUIR-BLODGETT-KUHN SUPRAMOLECULAR ASSEMBLIES OF HAIRY ROD AZO-POLYGLUTAMATES... 

For all the azo-polyglutamate LBK-polymer films, all the WGS experiments mentioned above were repeated by setting the UV and blue lights linearly polarized, in the plane of the sample, successively parallel and perpendicular to the dipping direction the same results were obtained for both UV and blue-light directions of polarization. In other words, no photoselection effect has been detected in these LBK structures. This is in contrast to the results obtained in a spin-cast film from the Pi.io polymer used in this study, and can be due to the subtle manner of the reorientation of the azobenzenes imposed by the LBK deposition technique, because efficient photoselection, following photoisomerization induced reorientation, can easily be achieved in spin-casted azo-polymer films [7-13]. This effect is discussed in the next section. 

Figure 24. (a) UV-VIS absorption spectra of the azo-polyglutamate layer before (1) and after 35 min (2,3) of linearly polarized UV (360 nm) irradiation. The probe light was also linearly polarized and spectra were obtained for both parallel, Abs, (3) and perpendicular, AbSj (2) orientations. Identical spectra were obtained for both Abs and AbSj prior to UV irradiation. For reasons of clarity, therefore, only Absj (1) is shown, (b) Spectra (1) obtained after unpolarized UV (360 nm) irradiation for 20 min further blue (450 nm) irradiation for 2 min leads to spectra (2,3). Note that the dichroism has been significantly erased in comparison to Figure 24a. The insets are an expanded view of the cis absorption.

Heating the azo-polyglutamate sample at 80°C for 30 min and 14.5 h failed to erase the UV irradiation induced dichroism in the sample. At this temperature, the sample is still at least 120°C below the sidechain isotro-pization temperature (T> 200°C) of the polymer [56]. Molecular movement in polymeric material is generally governed by the difference between the operating temperature T and the glass transition temperature (T ) of the polymer [67] in other words, the smaller the difference T -T, the higher the molecular movement. 

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