Page 52 - Photoreactive Organic Thin Films

PHOTOISOMERIZATION OF AZOBENZENES 3 J

1.5.2.2.3 Influence of Solvent
The dependence of the photoisomerization process on the polarity of the
environment varies greatly for differently substituted compounds. King et
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al. found that p-nitro- or cyanosubstituted azobenzenes had photostation-
ary states with similar E/Z ratios, but that an additional p'-amino function
stopped photoisomerization in acetonitrile. This was not true, however, in
methylcyclohexane, where lifetimes of Z-isomers were determined to be in
the order of seconds at room temperature. These findings are retained at
-35°C, which is taken as proof that it is not the fast thermal Z -» E isomeriza-
tion that fakes the lack of photoisomerization. This conclusion may be ques-
tionable, however, considering the weak temperature dependence for
azobenzene-type molecules (Figure 1.10).
Pseudo-stilbene type azobenzenes are not as stable under UV irradiation
159 162 163 164
as the azobenzenes. ' ' ' kick and Pacifici found that in de-aererated
alcoholic solvents, photoreduction gives the hydrazo compound with a yield
4
165
159
of ca. IO"" at 254 nm irradiation. Albini et al found yields of one order
higher, but at 313 nm and longer wavelength irradiation, the decomposition
is virtually absent. Some caveat is necessary when triplet-sensitizing additives
are present, because irradiation under air does not lead to photoreduction. 165
Benzophenone-sensitized reduction in benzene proceeds with a yield of
164
0.17.

1.6 THE ISOMERIZATION MECHANISM

! .6.! Azobenrene-Type Molecules
The close similarity between isosteric azobenzene and stilbene suggested that
their isomerization mechanisms might be the same and that this would be by-
rotation around the central double bond. As early as 1941, however, Magee
166
et al. speculated about a different mechanism for azobenzene. They
proposed a planar transition state with a single bond between the N-atoms of
the azo group. The "lateral shift mechanism," today called "inversion," was
167
proposed by Curtin et al. a rehybridization of the n- and 0-electrons of the
azo group should create a planar transition state that would only weakly
influence the ft-system. Such a mechanism is established in the ground state
168 169
isomerization of imines. ' The two alternative paths from Z- to E-azoben-
zene are visualized in Figure 1.14. For the two routes, the simple coordinates:
twist angle or N-N-C-bond angle, should be good approximations of the true
reaction coordinates. They have different potential energy profiles.
Photoreactions proceed on the potential energy surfaces of both the
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excited and the ground state. Interesting features of the potential energy
diagrams are the maxima and minima as well as the loci where the system
changes from one potential energy surface to another, usually from an
excited-state curve to the ground-state curve. Note that the gradient of the
potential energy curves gives the force exerted on the molecular system along
the respective coordinate.
Even after the advent of femtosecond spectroscopy, a potential energy