half-life times of the Z-isomers of 4,4'-disubstituted azoben-
zenes 1p and 2p are shorter than those of 3,3'-disubstituted
azobenzenes although they are still much longer (three to four
order in magnitude) than a push±pull azobenzene such as
4-nitro-4'-dimethylaminoazobenzene (see also Table 2). It is
interesting to notice that the longer lifetimes of the ester
substituted 3,3'-disubstituted azobenzenes (2m and 4m) are not
due to higher activation energy (Ea) since their Ea are rather
low when compared to those of the others. Their lifetimes are
characterized by small pre-exponential factors A which are one
order smaller in magnitude. Their larger negative entropy (DS)
may be an indication of rather severe coulombic interactions
between the azo electronic systems and the ester groups in the
activated complexes or degree of steric restriction, which are
plausible explanations considering the results of Brown and
Granneman on pyridyl analogs of azobenzenes.2c
Isokinetic plots for DH and DS for azobenzenes of this work
were compared to those in the literature to evaluate the
mechanisms of the reactions (Fig. 5). The plots of azobenzenes
of this work are all in line with other non-push±pull
azobenzenes, indicating that the inversion mechanism occurs
for the thermal isomerization. Using a more polar solvent such
as acetone resulted in small decrease of reaction rates, which is
also characteristic of this mechanism.5b
In the glassy state of PMMA, the thermal isomerization
shows a deviation from eqn. (1) as is depicted in Fig. 6 for
reactions at 60 ³C. A portion of the initial reaction proceeds
faster than the rest. This fast reaction has been interpreted as
the relaxation of strained Z-isomers trapped in the matrix.11
This behavior can be expressed as a sum of two simultaneous
®rst order kinetics.10 However, this is not applied here due to
extremely slow isomerization of some of the samples for which
the relaxation times were spectroscopically traced only until a
little longer than the half-life times. Instead, by de®ning the
ratio (R) between the apparent activation energy at the half-
and quarter-life times, where (A`2At)/(A`2A0) are 0.75 and
0.50, respectively, a relative comparison among the behaviors
of these azobenzenes in glassy states can be done, noting that at
the quarter-life times, most fast reactions with low activation
energies were still in progress (the ordinate in Fig. 6 being less
than 0.30 and exactly 0.69 at the quarter- and half-life times,
respectively). Fig. 7 compares the ratio R for the azobenzenes.
It is found here that 1m±4m have Rw1.00, suggesting that the
portion of fast reactions are relatively smaller than those for 1p
or 2p. This may be attributed to the difference in the
conformations of the Z-isomers that affect the space for
isomerization. The Z-isomers of some 3,3'-disubstituted
azobenzenes have been indicated to prefer rod-like conforma-
tions, enhanced more by the presence of methyl substituents at
ortho-positions and need less space to isomerize with respect to
4,4'-disubstituted analogs.6 As a consequence, the formation of
strained Z-isomers should be suppressed, resulting in relatively
less portion of the fast isomerization.
Conclusion
The spectroscopic features and Z-to-E thermal isomerization
kinetics of some 3,3'-disubstituted azobenzenes have been
studied in comparison with other azobenzenes. From spectral
analysis and thermal isomerization behaviors, the 3,3'-disub-
stituted azobenzenes here seem to belong to ``azobenzene type''
azobenzenes,4 characterized by a large energy gap between
n±p* and p±p* bands, insensitivity to solvent polarity, and
relatively slow reaction rates. The inversion mechanism occurs
most likely for these azobenzenes as was shown by the
isokinetic plots. The differences in the conformations of the E-
and Z-isomers, as a consequence of different positional
substitutions of an azobenzene, affect the thermal Z-to-E
kinetics behaviors in the glassy states of polymeric systems.
References
1
See for example: (a) K. Ichimura, in Polymers as Electrooptical and
Photooptical Active Media, ed. V. Shibaev, Springer, New York,
1996, p. 138; (b) K. Horie and I. Mita, in Advances in Polymer
Science, Vol. 88, Springer, Berlin, 1989, p. 77; (c) H. Asanuma,
T. Ito, T. Yoshida, X. Liang and M. Komiyama, Angew. Chem.,
Int. Ed., 1999, 38, 2393.
2
(a) E. R. Talaty and J. C. Fargo, J. Chem. Soc., Chem. Commun.,
1967, 65; (b) J. P. Otruba III and R. G. Weiss, J. Org. Chem., 1983,
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3
4
H. Rau and S. Y. Quan, J. Photochem. Photobiol. A, 1988, 42, 321.
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5
6
7
8
C. Ruslim and K. Ichimura, J. Mater. Chem., 1999, 9, 673.
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9
Crystal structure determination of compound 3m. Crystal data.
C
c~15.344(9) A, b~102.29(4), U~1242(1) A , T~296 K, space
26H38N2O2, M~410.60, monoclinic, a~12.203(8), b~6.789(6),
Ê 3
Ê
group P21/c, Z~2,
measured, 2394 unique (Rint~0.021). The ®nal wR(F2) was
0.052. CCDC 1145/245.
m , 2509 re¯ections
(MoKa)~0.64 cm21
10 (a) C. D. Eisenbach, Makromol. Chem., 1978, 179, 2489;
(b) C. Barrett, A. Natansohn and P. Rochon, Chem. Mater.,
1995, 7, 899.
11 (a) C. S. Paik and H. Morawetz, Macromolecules, 1972, 5, 171;
(b) I. Mita, K. Horie and K. Hirao, Macromolecules, 1989, 22, 558.
J. Mater. Chem., 2000, 10, 2704±2707
2707
Ruslim
