Page 153 - Photoreactive Organic Thin Films

J 3 2 ZOUHEIR SEKKAT AND WOLFGANG KNOLL

the rate of cis-^trans thermal isomerization is quite high, a feature that shows
that the azo-chromophore is in the trans form most of the time during
trans«-»cis cycling. The near-pure photo-orientation observed in PUR-1 is in
clear contrast to photo-orientation observations in all of the azo-poiymers
studied to date, including PUR-2, PUR-3, and PUR-4, wherein both 'Abs/ f and
Abs ± change in the same direction upon photo-orientation (OHB), as shown
in Figure 4.19 for PUR-3, and Abs± can exceed Ab$ 0 only for weak pump
intensities to minimize the concentration, of the cis population. Indeed, weak
pump intensities minimize the cis concentration, thereby favoring orientational
redistribution over OHB.
Near-pure photo-orientation occurs in PUR-1 and not in PUR-2, PUR-3
and PUR-4 because of a fast cis->trans isomerization of PUR-1 versus PUR-3
and PUR-4 and a seemingly small difference into the polymer backbone of
PUR-1 versus PUR-2. Although PUR-1 and PUR-3 have the same polymer
backbone, the CN electron-withdrawing group of the azo chromophore in
PUR-3 and PUR-4 slows the cis-»trans thermal isomerization. The isotropic
absorbance of PUR-1 is recovered more quickly than that of PUR-3 upon
cis-»trans thermal isomerization after the end of irradiation, with 0.45 and
0.14 s-1 as the fastest isomerization rates for PUR-1 and PUR-3, respectively
(see Table 4.2). This table also shows that cis-~»trans thermal isomerization
proceeds with faster rates for PUR-1 and PUR-2 versus PUR-3 and PUR-4,
and that the rates of polymers with the same electron-withdrawing groups
are similar. The cis—Hrans thermal isomerization is slowed down in PUR-3
and PUR-4 because of a higher energy barrier that needs to be crossed by the
cis form to isomerize back to the trans form, a feature that decreases the
number of cycles per unit time for the azo-chromophore in PUR-3 and PUR-4
versus PUR-1 and PUR-2.
The effect of the structure of the polymer backbone on photo-orientation
can be seen from the dynamic behavior as well as from the steady-state values
of the photoinduced anisotropy in all azo-PURs. The photo-orientation
dynamics of PUR-2 resemble but also contrast with those of PUR-1. In
PUR-2, Abs ± exceeds Abs 0, but not quite, as is the case for PUR-1, and the
photostationary-state anisotropy is smaller than that of PUR-1, as can be
seen in Figure 4.20. PUR-1 and PUR-2 exhibit exactly the same extinction
coefficient at the analysis wavelength because they have the same azo
chromophore; furthermore, the rate of the cis—»trans thermal isomerization is
nearly the same in both polymers. The seemingly small difference into the

TABLE 4.2 Rate Constants, k,, and Weighting Coefficients, a,, i= 1,2,3, of the
Cis-VTrans Thermal Isomerization of the Chromophore in Azo-PURs

PUR-1 PUR-2 PUR-3 PUR-4
1
•i(s- ); a, 0.450; 0.75 0.410; 0.67 0.140; 0.18 0.27; 0.16
-1 0.031; 0.31 0.024; 0.35 0.007; 0.47 0.01; 0.46
•2(s ); a 2
__ _
1 0.0003; 0.3 0.0002; 0.40
^(s" ); a 3