Page 101 - Photoreactive Organic Thin Films

ZOUHEIR SEKKAT

(A)

(B)
1,4 -
1,2 -
1,0
c
.9. 0,8
o.
| 0.6
0,4
0,2
0,0
200 250 300 350 400 450 500 550 600 850

FIGURE 3.6 (A) Structure formula and (B) UV-vis spectra of a poly-(5-(2-(4-(4-decyIoxyphenylazo)
phenoxy) ethyl)-L-glutamate), denoted by P 2ito, solution in CHCI 3 in the dark (trans) and at the
photostationary state (cis) of a 360 nm UV light irradiation. After reference 21, redrawn by permission.

2
(360nm; 0.2 mW/cm ) for 35 minutes, and recording both Abs// and Abs ±. It
is clear that the absorption Abs ± is higher than the absorption Abs//. Identical
spectra were recorded for both Abs// and Ab$ L before UV irradiation (only
Abs L shown), demonstrating that the sample was in-plane optically isotropic
at that time. These findings are true for the trans absorption band in the UV
region around 360 nm as well as for the cis absorption band in the visible
region around 450 nm (see the inset in Figure 3.7). This shows clearly that
both the trans and cis azobenzene molecules are preferentially distributed
perpendicular to the initial UV polarization and that the cis isomer aligns
perpendicular to the initial UV polarization within the trans—»cis
isornerization.
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