Three-Dimensional Photoreorientation of Self-Organized Azobenzene Chromophores in Liquid Crystalline Polymer Films
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C-CH2---O-(CI),-OaNý,/= N 137 1 G76 S 95 (G; glassy, S; smectic, N; nematic, I; Isotropic) 159
Mat. Res. Soc. Symp. Proc. Vol. 559 01999 Materials Research Society
EXPERIMENT A toluene solution of a liquid crystalline polymer, poly[ {6-(4methoxyphenylazo)phenyloxy}hexyl methacrylate], was spin-coated on quartz plates (for UV-Vis absorption measurements), CaF2 plates (for I transmission measurements) or Au plates (for IR RA measurements). Thickness was ca. 50-60 nm. The thin films were irradiated with 436 nm light from a Hg-Xe lamp through a combination of color filters of Y-43 and V-44 (Toshiba). Evaluation of photoinduced dichroism were carried out by polarized UV-Vis absorption spectra taken on a Hewlett-Packard diode array spectrometer 8452A. A hot stage (Mettler FP800) was employed to control heating temperatures. Tilted alignment of azobenzenes was followed by monitoring UV-Vis absorption spectra with p- and s-polarized probe light. Infrared spectra were recorded on an FTS spectrometer (BIO-RAD) equipped with a DTGS detector. All the data were gained at a spectral resolution of 4 cm- 1. IR-RA spectra were obtained at the incident angle of 80'. RESULTS Molecular reorientation by 436 nm light irradiation and successive annealing When a spin-coated film was irradiated with non-polarized 436 nm light from the surface normal, the absorption band due to the 7r-7r* transition of trans isomer decreased gradually and A, was red-shifted. Moreover, annealing at 75 'C or above the glass transition temperature resulted in the reduction of 7r-7-* absorption band at around 360 nm. Azobenzene has transition moment approximately parallel to its molecular axis' 6"17 so that it is possible to determine its orientational behavior by following UV-Vis absorption spectra using p- and s-polarized probe light. Absorption spectra at various incident angles (Z,,•) of p-polarized probe light displayed a pronounced blue-shift of /. up to 320 nm and increase of 7r-7r* absorption band, when a film of the polymer was exposed to 436 nm light and successively annealed at 75 *C (see Figure 2). 0.403-'
0
0.3 Initial
0
0.2-
0.10200
S0.10Jcm -annealing 300 400 500 Wavelength (nm)
300 200
00. 0- . 200
600
FIG. 1. Absorption spectral changes in a thin film of the azobenzene liquid crystalline polymer.
.
. .i. . , , 300 400 500 600 Wavelength (nm)
FIG. 2. Absorption spectra of a film of' the azobenzene liquid crystalline polymer exposed to 436 nm light of 10 J cm-2 and successively annealed at 75 °C at various incident angle (0e) of p-polarized probe light.
160
The results mean that the azobenzene undergoes perpendicular photoreorientation with respect to the surface plane, which is enhanced by annealing. This spatial reorientation is accompanied by the formation of H-aggregation as a result of strong side-by-side interactions among the azobenzene side chains. On the other hand, UV-Vis absorption spectra with spolarized probe light did not depend on the incident angles of probe light, reflecting that the electric vector of the s-po
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