Page 41 - Photoreactive Organic Thin Films

2(J HERMANN RAU

13,2 Isomerization of Molecules of the Azobenzene Type
1.3.2.1 Thermal Isomerization
The E-form of azobenzenes is by 50 to 55 kj moH thermodynamically
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more stable than the Z-form. ' ' For azobenzene, the heats of formation
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are AH^ 9g(E) = 311 kJmoH and AH5 98(Z) = 367 kjmol- . The Z-fprm can
1
be isolated and crystallized, it is kinetically rather stable and is protected by
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an activation barrier that depends on the environment. In homogeneous and
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1
liquid crystalline solution, E a » 95 kj mo!" is reported, a bit more in the
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melt (E a = 105 kj moH) and a value more than doubly as high in the
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81
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crystal (E a = 233 kj mol" ). Andersson et al. have determined the activa-
tion parameters in all three phases and find a linearity in the AH* vs. AS* plot,
which leads them to conclude that the mechanism is the same in all three
phases (inversion; see Section VI.) The thermal Z —> E isomerization can be
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85 86
accelerated by iodine ' and electron donating and accepting catalysts. '
For a series of substituted azobenzene-type azobenzenes in solution, Talaty
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and Fargo give activation energies between 85 and 100 kj mol" and pre-
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exponential factors of 10 to 3-10 s" . The dependence of the thermal
isomerization rate on pressure is small. It increases less than 20% at 2100 bar
1
and is not very dependent on the solvent (AY* = -3.3 ml mol" in hexane,
1
-0.7 ml mol" in methanol). 88
The thermal Z —> E isomerization of azobenzene has been widely used to
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determine free volume in polymers at room and temperatures as low as
90 91
4 K. ' The thermal reaction is also important in the context of photo-
response, as an information written or a signal or state produced by switch-
ing E to Z is slowly fading. However, the Z-lifetime is strongly modified
by strain in the molecule: Z-azobenzene in solution at room temperature has
a half life of about 2 days; the Z,E —> E,E isomerization in the [3.3](4,4*}azo-
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benzenophane 9 has a half life of ca. 4 min.; the [2.2](4,4')azobenzenophane
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7 has a half life of ca. 15 seconds; and in dibenzo[2.2]14.4*)-azobenzeno-
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phane 8 the life of the E,Z-isomer drops to 1 s. On the other hand, the Z,Z
-> Z,E isomerization in these phanes is slowed down enormously: Z,Z-7 lives
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2.5 days, Z,Z-9 about 5 days, and Z,Z-10 about 1 year at room tempera-
ture. Activation energies are available in the publications. The Z,E -» E,E
isomerization in most azobenzenophanes is very fast. However, in 2,19-
Dioxo[3.3](3,3'}azobenzolophane 12, the Z,E-form is relatively stable. 94
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The remarkable differences in these and other structures ' are not due to
different activation enthalpies but to different activation entropies.
Of course, the surrounding matrix influences the lifetimes. Double and
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multiple decays of the Z-form concentration are found in polymer matrices
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and tnonolayers.
Electrochemical reduction of azobenzene leads to very fast isomerization
of the resulting radical anion. Thus, there is no difference in the polaro-
graphic behavior of the two isomers. 97
1.3.2.2 Photoisomerization
According to the absorption coefficients, irradiation in the UV drives the
E-Z ratio toward Z and irradiation in the visible toward the E-form (Figure