Page 100 - Photoreactive Organic Thin Films

3. PHOTO-ORIENTATION BY PHOTOISOMERIZATION JO,

and Equation 3.24 resumes to Fisher's stationary state relation for thermally
irreversible systems:

,,, ,, c-
(J.2.J I
a
So, when photo-orientation analysis is not performed at the irradiation
wavelength, the order parameter at infinite irradiation intensity of spectrally
overlapping isomers reads:

2 PI (cos M B) + aP 2 (cos a> A) . , e A
^ = — __ - _ - wicn a = r. / « / s j.z,oj
S
13 1 + a B e Acf) AB
? 2 (cos a) A) and P 2 ( cos W B) are tne second-order Legendre polynomials of <t) A
and W B, which are the angles between the irradiation and analysis transitions
of isomers A and B, respectively. a) A and <M B can be experimentally determined
by a two-step photo-orientation experiment: irradiate at a wavelength Aj and
analyze at A 2; irradiate at A 2 and analyze at Aj. Although S changes for each
step, F 2 (cos a> B] and P 2 (cos a) A) remain unchanged, and their determination is
straightforward, a can be determined beforehand by determining the cis
spectrum by Fisher's method and the quantum yields by photo-orientation
analysis at the irradiation wavelength.

3.4 PHOTO-ORIENTATION OF AZOBENZENES: INDIVIDUALIZABLE ISOMERS

In spectrally overlapping isomers, such as cis- and trans-DRl, both forms
may be simultaneously isomerized and the molecule may rotate in each
isomerization reaction, i.e., the trans—»cis photoisomerization, and the photo-
induced and thermally activated back cis->trans reactions. All three reactions
may happen simultaneously given that both the trans and cis isomers exhibit
their absorption maximum in the same visible region (see Figure 3.2 for
trans- and cis-DRl), and the lifetime of the cis isomer is relatively short (4 to
16
5 seconds in polymethyl-methacrylates). In contrast, azobenzene-containing
polymeric films are more appropriate for closely probing this reorientation
process in each of the three isomerization reactions. The trans and cis isomers
of azobenzene present different absorption bands in the UV (around 360 nm)
and the visible (around 450 nm) regions (see Figure 3.6). The lifetime of the
azobenzene cis isomer is generally on the order of hours, depending on the
polarity of the host material, which may be a polymer or a solvent; therefore,
the azobenzene cis isomer can be considered stable on a time scale of minutes,
i.e., the time scale of the experiment. It will be shown that azobenzene
molecules are oriented within both the direct trans—>cis photoisomerization
and the thermal cis—Mrans back isomerization.

3.4.1 Reorientation within the transacts Photoisomerization
Figure 3.7 shows the dichroism observed in spin-cast films of an azo-
polyglutamate (see the formula in the caption for Figure 3.6). These UV-vis
spectra were obtained after irradiation with linearly polarized UV light