Azo-silane

This chapter is organized as follows. Section 4.2 addresses the study of photoisomerization and photoinduced orientation of azobenzene molecules at the molecular level in SAMs of azo-silane molecules. Section 4.3 discusses photoinduced effects in supramolecular assemblies, i.e., LBK multilayer structures containing azobenzene molecules, and compares the photoinduced movement of azobenzenes in these structures to that observed in spin-cast films. Section 4.4 focuses on the isomerization and sub-Tg photoinduced orientation in a series of very high Tg (up to 350°C) nonlinear optical polyimide and thermoplastic donor-embedded polyurethane polymers containing azo dye, especially focusing on polymer structure-Tg-photoinduced molecular movement relationships. Section 4.5 describes pressure effects on photoisomerization and photo-orientation in films of a PMMA polymer containing azo dye. Finally, we make some concluding remarks in Section 4.6. 

In this section, we discuss the photo-orientation of azobenzenes in molecularly thin SAMs by means of UV-vis spectroscopy and surface plasmons (SPs). The structural formula of 4-(6-carboxy-(3-amidopropyl)triethoxysilane)-4 -pentylazobenzene, referred to as azo-silane, which leads to a self-assembled monolayer, is shown in Figure 4.1 (top). Azo-silane SAMs (see schematic in Figure 4.1, bottom) for the UV-vis spectroscopy and surface plasmons experiments were prepared as reported elswhere. ... 

UV (360 nm) and blue (450 nm) light irradiations of the ultrathin azo-silane SAMs clearly induce the forth, i.e., trans—>cis, and back, i.e., cis—>trans, photoisomerization of azobenzene molecules (see Figure 4.2A). The real-time dependence of the absorbance of the sample during the thermal cis—>trans back reaction is not a monoexponential decay (see Figure 4.2B). This decay shows a complex multiexponential relaxation behavior that could be fit neither by a monoexponential decay nor by a biexponential relaxation. Nevertheless, a monoexponential decay could be fit to the data acquired over... 

Figure 4.3 shows the photo-orientation, i.e., the dichroism, observed in the azo-silane SAMs. These spectra were obtained after 3 minutes of irradiation with linearly polarized UV light. It is clear that the absorption, Absx, recorded with the probe light linearly polarized perpendicular to the initial UV-light polarization, is higher than the absorption, Abs//, recorded with the probe light linearly polarized parallel to the initial UV-light polarization. Identical spectra were recorded for both Abs// and Absx prior to UV irradiation (only... 

FIG. 4.3 UV-vis absorption spectra of the azo-silane l r before (Initial) and after linearly polarized UV (360 nm. - 2rnW/cm ) irradiation. The probe li t was also linearly polaraed and spectra were obtained for both parallel and perpendicular orientations. The polarization of the irradiation UV light was rotated by 90 degrees from (A) for (8), After reference 39, redrawn by permission of ACS. 

For the azo-silane layers, assuming a value equal to 1.45, at 632.8 nm, for the refractive index, normal to the plane of the layer, our SAM could best be described by a layer thickness of 9 A (i.e., an optical thickness of 13.1 A). This is considerably thinner than would be expected for a fully extended azosilane molecule (ca. 30 A). This may be better understood by comparing the... 

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... 

Figure 8. Top Structural formula of 4-[6-carboxy-(3-amidopropyl)triethoxysilane]-4 -pentylazobenzene, referred to in the text as azo-silane, that leads to a self-assembled monolayer. Bottom idealized schematic drawing of a SAM on a Si02 substrate.

Figure 9. Top PSP resonance of the bare Ag substrate (dosed circles) after coating with a self-assembled monolayer the azo-silane (open circles) the inset shows the thickness of Ag, SiO,, and the SAM. Bottom Optical thickness change as obtained by recording the reflected intensity of the same sample at a fixed angle of incidence (6 = 45°, cf. top) while irradiating the moments of turning the irradiation light on and off are indicated by arrows.

Fignre 21. Top UV-VIS absorption spectra of the azo-silane layer before ( ) and after 3 min (O, ) of linearly polarized UV (360nm) irradiation. The probe light was also linearly polarized and spectra were obtained for both parallel, AbS, ( ) and perpendicular, Abs, (0,B) orientations. Identical spectra were obtained for both AbSj and AbSj prior to UV irradiation. For reasons of clarity, therefore, only Abs, ( ), is shown. Bottom As in the top, except that the polarization of the irradiating UV light has been rotated through 90°. Abs. and AbSj correspond to the same orientations as in the top. 

F absorption spectra of the azo-silane layer before and after various timed doses of UV (360 nm, 2mW/cm irradiation. Cis->trans photolsoinerizatlon by blue (4S0 nm) irradlaoon produced a reverse effect and restored the initial spectra. (B) Aeal-time kinetics of the thermally activated cis- trans Isomerization, with an inset showing an expanded view of the first points of the figure. After reference 39, redrawn by permission of ACS... 

The concentration of trimethylsilyldiazo-methane in hexane can be determined by adding 91mg (0.5mmol) of dibenzyl (see [103-29-7], Aromatic compounds) in 1ml of the hexane solution containing it, and measuring the H NMR. The concentration of the azo-silane (x mmol/ml) is calculated from the formula x = 2a b, where a is the integral value (mm) of the methine proton (5 2.58) of trimethylsilyldiazomethane, and b is the integral value (mm) of the methylene protons (5 2.99) of dibenzyl. [Shioiri Aoyama Org Synth Coll Vol Vin 612 1993, Mori et al. Chem Pharm Bull Jpn 30 3380 1982],... 

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