Page 96 - Photoreactive Organic Thin Films

3, PHOTO-ORIENTATION BY PHOTOISOMERIZATION 75

40 60 80 tOO
Normalized Irradiation Intensity
FIGURE 3.5 Theoretical simulations of the effect of polarized irradiation on the variation of the
cis population (a) and die cis (oAf) and trans ({I - a)A*) orientational anisotropies.

Figure 3.5) of the cis and trans population and orientation under linearly
polarized light irradiation shows that for high-irradiation times or high-
irradiation intensities, the system should be saturated and the isotropy should
be restored, because even molecules that are perpendicular to the irradiating
light polarization present an appreciable probability for being photoisomerized.
Such a behavior has been observed in a 9A ultra-thin (optically thin) SAM
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containing azobenzene molecules, and in an azobenzene-polyglutamate
polymer, the dichroism decreased with increasing irradiation times. The
behavior illustrated in Figure 3.5 was also observed for DR1 as well as for
spiropyran and diarylethene-type molecules in PMMA films (vide infra).
Since both the cis and trans isomers of these chromophores present strong
spectral overlap at the irradiation wavelength, it is important to note that
double-exponential functions in the form of Equation 3.10 can be used to
approximate the dynamics of photo-orientation for systems in which all
three isomerizations (i.e., photo-induced and thermally activated) can occur.
However, care must be taken in comparing fitted values to actual physical
quantities outside the approximations framework discussed previously.
3.3.2.3.2. Relaxation of Photo-orientation
When the irradiating light is switched off, the thermal cis—>trans
isomerization rate, k, governs mainly the population change from cis to trans
isomers, and the cis and trans rotational diffusion rates, (i.e., k^ and &p,
respectively) influence only the relaxation of the isomers' orientation. This
orientation diffuses in both cis and trans forms during the cis lifetime, and
after that time it diffuses only in the trans form. If the irradiating light is
turned off at the time t' = 0, the cis and trans population and orientation
changes are rigorously described by: