Page 89 - Photoreactive Organic Thin Films

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3J PHOTO-ORIENTATION BY PHOTOISOMERIZATION

3.3.1 Base Ground Work
3
2
Experiments by Neoport and Stolbova, and Todorov et al. demonstrated
that methyl-red and congo-red and methyl-orange chromophores, which are
azobenzene and diazo derivatives, are oriented perpendicular to the
irradiation light polarization. The understanding of photo-orientation by
photoisomerization has been advanced by means of intensity-dependent,
real-time anisotropy experiments in DR1-containing poly-methyl-metha-
19 20
crylates. ' Figure 3.3 shows the observed induced anisotropy, i.e.,
dichroism, by polarized light irradiation into a guest-host film of PMMA
containing DR1 for low- and high-irradiation intensities. For low-irradiation
intensity, the cis concentration is small, and Abs// (absorbance of an analysis
light polarized parallel to the irradiation light polarization) and Abs ±
(absorbance of an analysis light polarized perpendicular to the irradiation
light polarization) evolve in opposite directions, indicative of a near-pure
orientation of the trans isomer. For high irradiation intensities, the cis
concentration is appreciable, and both Abs// and Abs ± evolve in the same
direction. When the irradiation light is turned off, cis—Hrans thermal
isomerization converts back all cis to trans isomers, as can be seen from the
total recovery of the isotropic absorbance in Figure 3.3. A net remnant
anisotropy, i.e., orientation of the trans isomer, is also observed. These
observations imply that (1) the molecular linear polarizability of the cis form
being appreciably smaller than that of the trans form, photoselection
resulting from polarized light irradiation burns a hole into the molecular

100 200 300
1 10 100
(A) (B)
FIGURE 3,3 Variation of the absorbance, O.D., of a film of DR I -doped PMMA (2.5% w/w) with (A)
+
low, 0.28 mW/cm , and (B) high, 28 mW/cm , irradiation (488 nm Ar laser) intensity. The analysis light
+
was the 5 1 4 nm from the same Ar laser, and both A// and A^ are measured. A m represents the isotropic
absorbance (A//+2AJJ/3. (C) Variation of the absorbance — parallel, perpendicular, and isotropic — with
the irradiation light intensity. The data were taken 3 minutes after the irradiation was turned on. At low
irradiation intensity, the cis population is small and A t increases as a consequence of the orientational
distribution. At high irradiation intensity, saturation of the orientational hole-burning process tends to
equalize A// and A ±. (After reference 20, by permission.)