Page 85 - Photoreactive Organic Thin Films

ZOUHE1R SEK.KAT

present contrasting series of photo-orientation experiments on azobenzene
derivatives, which undergo isomerization by molecular-shape change, and
chromophores, e.g., spiropyrans and diarylethenes, which photoisomerize by
reversible photocyclization. In all of these chromophores, both cis and trans
isomers are oriented by polarized photoisomerization, and in diarylethenes
and spiropyranes, the ultaviolet and the visible transitions of the cis isomer
are oriented perpendicularly to each other.

3.1 INTRODUCTION

Induction of photochemical anisotropy in certain materials upon irradiation
with polarized light (the Weigert effect) has been known since the beginning
1
of this century. In the 1960s, this effect was widely studied in viscous
solutions containing azo-dye molecules, which are known to undergo
2
cis<->trans photoisomerization upon light irradiation. During the 1980s, the
Weigert effect was used for polarization holography applications in azo-
3
dye-containing polymer films. Recently, there has arisen a great deal of
interest in azo-dye-containing materials, because of their possible application
4 6
in the areas of optoelectronics, photonics, and optical signal processing. " In
this regard, photoinduced anisotropy in azo-polymer films has been inten-
sively studied, and a tremendous amount of work focusing on applications in
rewritable data storage and photon-mode liquid crystal alignment, as well as
fundamental aspects of photo-orientation, have been done by a large number
6
of research groups. "" 15
Photo-orientation of azo-dye molecules within trans<-»cis photoisomeriz-
ation occurs when these photochromic molecules are photo-selected by
linearly polarized light of appropriate wavelength. The azo-dye molecules
then experience successive cycles of transocis isomerization and eventually
align perpendicular to the irradiating light polarization (vide infra). In
solutions, photo-orientation can be neglected when rotational diffusion has
enough time to randomize induced molecular orientation. Small molecules
diffuse rotationally in a few picoseconds in low-viscosity solutions. In solid
polymer films, however spontaneous molecular mobility can be strongly
hindered, depending on temperature and pressure, and photo-orientation
effects can be appreciable.
There are three equally important aspects of photo-orientation by
photoisomerization that need to be addressed. A first aspect is the bridging of
photo-orientation with the pure photochemical point of view of isomeriza-
tion. In pure photochemistry studies, photo-orientation, or photoinduced
birefringence, is generally disregarded, even when it is appreciable (and
isotropic absorbance can be properly measured when the polarizations of the
analysis and irradiation lights are at the magic angle, i.e., ~ 54.7 degrees, to
each other, thereby eliminating the contribution of anisotropy to measured
absorbances). However, the analytical expressions used for the extraction of
reaction parameters—which are based on absorbance values, such as the
photoisomerization quantum yields—were not corrected for anisotropy. In
this chapter, I will present the analytical theory that allows for the quantifica-