Page 145 - Photoreactive Organic Thin Films

§24 ZOUHEIR SEKKAT AND WOLFGANG KNOLL

PI-3a

FIG. 4.11 Chemical structures of the azo-containing polyimide polymers.

stable population of the cis isomer. The stability of the cis isomer in Pl-1 and
PI-2 is unusual, because donor-acceptor substituted azo chromophores usually
undergo rapid cis—»trans isomerization on a time scale of seconds to minutes.
This effect is shown in Figure 4.13. In this case, the mean absorbance at 'k max
(A y+2A x)/3 first drops substantially upon irradiation then finally recovers to
near its initial value after ~ 25 h. The order parameter, S=(A y-A x)/(A y4-2A x),
however, does not return to its original value even after 350 h, suggesting the
orientational birefringence is retained even after the cis—»trans thermal
isomerization is completed. A similar effect is observed for the irradiated sample
of PI-2. In this case, AE ~ 0 after 26 h at room temperature although the film
still remained quite birefringent, An x = 0.014, An y = - 0.028, An z = 0.013. The
kinetics of the return of the trans isomer from the cis isomer in both PI-1 and
PI-2 exhibit a fast component of few seconds (not shown) and a longer one
on the order of minutes to hours, which can be fitted with a triexponential
decay with a long time constant of 6 to 8 h (see Figure 4.14 for PI-1).
By means of comparison, in related experiments on PI-3a and PI-3b, the
mean absorbance returned to its initial value within two minutes after
removal of the irradiation light. Clearly, the polymer rigidity intrinsic to the
donor-embedded samples must affect the rate of isomerization of the cis
azobenzene derivative, suggesting that motion of the chromophore and the
polymer backbone are somehow coupled.