Page 28 - Photoreactive Organic Thin Films

I. PHOTOISOMERiZATIONOFAZOBENZENES J

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zene type where (njt*) and (n,n*) are at comparable energies (see also
Figure 1.11), and a pseudo-stilbene type with a (n,n*) state as the lowest-
excited state (see also Figure 1.13). The assignment of an azo molecule or an
azobenzene-containing macromolecule or system to one of these classes can
be made by a simple inspection of the absorption spectrum. The spectroscop-
ic properties of these types of azo compounds will be covered in Sections 1.3
to 1.5 of this chapter.

1.2.2 isomerization
A major feature of the azo group is its capability to isomerize; this is the
property used widely in photoresponsive organic thin films. The E-Z. photo-
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isomerization of azobenzene was detected by Hartley, who created the
Z-form by irradiation of E-azobenzene. Generally, the E-forms of azo com-
pounds are more stable than the Z-forms. The parent E-azobenzene is by ca.
1 19 20
50 kj mol~ the more thermodynamically stable isomer. ' Isomerization is
the main photoreaction of most aromatic azo compounds. Other thermal- and
photoreactions lose the competition with isomerization.
If the processes occurring during thermal- and photoisomerization are to
be analyzed and understood a kinetic analysis is very useful. This can help
give answers to the following questions: How fast and how effective is the
isomerization reaction? What is the molecular mechanism? and Do all units
isomerize according to the same mechanism and kinetics? Some tools for this
analysis are presented here.
i .2.2,1 Concentration/Time Relations: How Fast?
The general scheme of geometric photoisomerization is given by three
elementary reactions of Scheme I (see also Figure 1.1)
hv
E ^Z=^ Z
hv A
' (Scheme I)
Isomerization can be induced by light in both directions or by heat in the
Z —> E direction. The reverse thermal reaction is not observed at normal tem-
peratures. Any one of the elementary reactions can be missing. Z-azobenzene
in solution 1>21 has a thermal Z —> E activation enthalpy AH* ~96 kj moi" 1
and a half life time of 2 to 3 days at room temperature. Thus, the thermal
reaction is irrelevant for the photoisomerization at usual irradiation intensi-
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ties (for comparison: Z-stilbene has E a ~ 180 kj mol" , is liquid, and is
kinetically stable). On the other hand, one of the photoreactions may not be
active (e.g., when an irradiation wavelength is selected where one form does
not absorb or when the quantum yield is too small). Inspection of Figure
LIB shows that E- and Z-azobenzene have virtually no spectral region with-
out overlapping absorption.
Photoisomerization can be induced by direct irradiation and by triplet
sensitization. Monitoring by UV7VTS spectroscopy is a convenient means of
following the kinetics. According to Scheme I, a photostationary state of
different E/Z compositions is reached in which the composition is determined