Page 86 - Photoreactive Organic Thin Films

3. PHOTO-ORIENTATiON BY PHOTOISOMERIZATION ££

tion of coupled photoisomerization and photo-orientation in A«-»B photo-
isomerizable systems where B is unknown. I will use this theory to study
quantitatively both the photoisomerization and the photo-orientation of a
series of photoisomerizable chromophores in films of polymer, including
azobenzene derivatives and photochromic spectrally distinguishable spiro-
pyrans and diarylethene-type chromophores. The way these chromophores
move upon isomerization and the symmetry of the isomers' transitions will
also be discussed.
A second aspect relates to the mechanisms of photo-orientation. Photo-
orientation by photoisomerization occurs through a polarization-sensitive
photoexcitation, i.e., photoselection, and the probability of exciting a
transition in an isomer is proportional to the cosine square of the angle
between that transition and the polarization of the excitation light.
Transitions that lie along the polarization of the irradiation light will be
excited with the highest probability, and molecules may be isomerized and
reoriented and may fade in the direction of the polarization of the irradiation
light. Now, and because photoisomerizable chromophores usually have two
isomers, cis and trans, that can be interconverted into each other by light or
heat, e.g., thermodynamically (vide infra), one can ask legitimate questions.
Why does the molecule change orientation upon isomerization? Which
isomer is oriented during which isomerization reaction? Do isomers orient
upon photoselection without isomerization? How may photoisomerization
quantum yields influence photo-orientation? Questions are also posed in
regard to the systematic choice of the type of the photochromic molecules for
photo-orientation studies.
A third aspect of photo-orientation in films of solid polymers is how
photo-orientation is influenced by the polymer structure, molecular
environment, and film configuration. Polymer structural effects on photo-
orientation as well as the effect of intermolecular interaction and free volume
are discussed in other chapters of this book.
Section 3.2 of this chapter recalls the pure photochemical point of view
of photoisomerization of azobenzene derivatives. Section 3.3 discusses the
theory of photo-orientation by photoisomerization and gives analytical
expressions for the measurement of coupled photoisomerization and photo-
orientation parameters. Sections 3.4 and 3.5 review observations of photo-
orientation in azobenzene and push-pull azobenzene derivatives, respectively.
Among other things, these sections address photo-orientation in both cis
and trans isomers and discuss the effect of trans<-»cis cycling, i.e., the photo-
chemical quantum yields, on photo-orientation. Section 3.6 discusses the
effect of the symmetry of photochemical transitions on photo-orientation in
spiropyran and diarylethene-type chromophores. Finally, I make some
concluding observations in Section 3.7.

3.2 PHOTOISOMERIZATION OF AZOBENZENES

The isomerization of azobenzenes is discussed in detail by Rau in Chapter 1.
In this section, I shall recall the basic features of photoisomerization of