Page 51 - Photoreactive Organic Thin Films

HERMANN RAU

Z ~» E isomerization of 4-dimethylamino-azobenzene and its o'~ and p*-
carboxylated derivatives (o- and p-methyl red) in the alkaline realm, with a
warning about "the interpretation of results when the rate of isomerization is
measured in organized media if the pH is not appropriately controlled,"

1.5.2.1.3 Influence of Pressure
Pressure dependence was thoroughly investigated by Asano and his
88 132 133
group. ' ' It turns out that the partial volumes of the Z-forms of 4-
3 1
dimethylamino-4'nitorazobenzene and related molecules are ca. 250 cm mol"
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in all solvents. Those of the E-forms are smaller and solvent-dependent.
Thermal isomerization rates are weakly dependent on pressure in nonpolar
solvents, but contrary to azobenzene- and aminoazobenzene-type compounds,
88
they are strongly dependent in polar solvents: in hexane 10%, in acetone
3
1
#
475% for 2100 bar (AV = -0.7 and -25.3 cm mol' , respectively). This has
implications for the discussion of the mechanism of isomerization (Section 1.6).
One concludes from these facts that pseudo-stilbenes are not suitable for
persistent switching of the molecular form. Any information based on E-Z
isomerization is quickly lost. If, however, fast interconversion of E- and Z-
forms is the aim, as it is in the alignment of the higher-order polarizability
tensor of donor/acceptor azobenzenes, then thermal isomerization supports
the photoisomerization process.

1.5.2.2 Photoisomerization
1.5.2.2.1 Protonated Azo Compounds
In a reaction analogous to that of stilbene, protonated azobenzene iso-
merizes at room temperature under irradiation with <J>E_»Z = 0-27 and <J>Z~»E =
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0.25. Subsequently, the dehydro-photocyclization starts from the photo-
137 138
stationary E/Z mixture to give protonated benzojcjcinnoline * with a
158
yield of § = 0.02. This is parallel to the stilbene-to-phenanthrene reaction,
and it proceeds not in concentrated sulfuric acid where the thermal Z —> E re-
isomerization is fast, but best in ca. 66% acid. Protonated benzo[c]cinnoline
and hydrazobenzene are formed, and the latter undergoes the benzidine
137 138 158
rearrangement. ' ' The complexation with metal ions in azobenzene
crown ethers under participation of the n-electrons of the azo group leads to
an increase of the E —» Z quantum yield from 0.25 of the metal free com-
2
2+
pound to 0.4 to 0.6 in the Ba and Ca * complexes. The Z -» E yield de-
creases from 0.18 to 0.13 and 0.05. If the crown is larger, the values increase
to ^ z « 0.35 and (j> z^ E « 0.45. 107
1.5.2.2.2 Donor!Acceptor-Substituted Azo Compounds
At room temperature, the isomerization of e.g. 4-dimethylarmno-4'-
nitroazobenzene cannot be observed reasonably, because the thermal back iso-
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merization of the Z-form is too fast. To determine the spectrum of the
160
Z-form, Gabor and Fischer applied low-temperature and extrapolation
techniques. The photoisomerization quantum yields are §E-^>Z " 0.20 and <{>Z-»E
« 0.75 at 163 K. They decrease to (|> E^ Z * 0.06 to 0.09 at 143 K, whereas <|> Z^ E
does not change much. Newer data regarding quantum yields are not available.