Page 45 - Photoreactive Organic Thin Films

24 HERMANN RAU

llj
Gegiou et al. found "only a very slight viscosity effect, both in the n-n*
and in the n-n* absorption bands" on the isomerization quantum yield. They
used glycerol as a viscous solvent, but the result may also be transferred to
polymer matrices. In solid matrices, several photoisomerization modes are
observed {see the preceding section on the influence of temperature). A com
parison between azobenzene isomerization in liquid methylmethacrylate and
the slow mode in poly (methylmethacrylate) showed that the difference in
quantum yields on Si (0.17) and S 2 excitation (0.03) is retained in the solid
28 28
matrix. The fast process is not observed in n —> n* excitation. These data
are important in relation to the use of the azobenzene isomerization method
for the determination of the free volume in a polymer.
Confinement of azobenzene in defined structures changes the quantum
yields. In the cavity of 6-cyclodextrine, the <J>E-»Z are reduced and become
are
nearly wavelength-independent, whereas the §Z-*E practically unaffected
114
by the inclusion. Photoisomerization is found for azobenzene-type mole-
115
cules in ZSM-5 and sicalite-1 zeolites. E-stilbene, in comparison, does
not isomerize on direct excitation in the channels of narrow pore zeolites
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(the fluorescence yields increase by a factor of 10}, ' and the triplet-
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sensitized isomerization of Z-stilbene in zeolites is inhibited. These findings
are relevant to the discussion of the isomerization mechanism in Section 1.6.
In monolayers of azobenzene containing amphiphiles, the characteristics
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of photoisomerization are dependent on the chain length.
There is one report of a concentration dependence of the 313 nm photo-
stationary state of azobenzene and 4-methoxyazobenzene in cyclohexane—
120
not in benzene or CC1 2F-CC1F 2—in the literature. A bimolecular excimer
intermediate was postulated. Further work is needed to elucidate whether the
absorption coefficients or the quantum yields are concentration-dependent,
for instance by ground or excited-state association (cf. Equation 1.3).

1.3.2.2.4 Triplet-State Isomerization
Isomerization of azobenzenes may also be sensitized by triplet sensitizers.
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Jones and Hammond and Fischer came up with different results: 2% of
Z-form versus 25% in the photostationary state. A thorough study by
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Lemaire and coworkers ' showed that two triplet states at 195 and 180 kj
1
4
mol"" in the E- and at 190 and ca. 140 kj mol" in the Z-isomer are involved
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in the reaction. According to Bortolus and Monti, sensitizers with high
(>190 kj moH) triplet energy transfer their energy efficiently {diffusion con-
trolled) to both E- and Z-azobenzene. Still, the isomerization yield is small—
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<|>E-»Z = 0.015—in agreement with Jones and Hammond. On the other
hand, the sensitized Z —> E isomerization has <|>Z-»E = 1.0. Azobenzenophane 9
also undergoes triplet-sensitized isomerization. 52
1.3.2.2.5 The Azobenzene Radical Anion
One-electron reduction of azobenzene yields the azobenzene radical
anion. Its Z-form thermally isomerizes fast. This explains the cleavage/recom-
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bination mechanism reported for azosulfides and the reduction/oxidation
mechanism of azobenzene derivatives in Langmuir-Blodgett monlayers