Chemistry Reference
In-Depth Information
10.6.2.2 Photo-Isomerization of the Azo Group
Azobenzene is a well-known photo-responsive chromophore, and its photo-induced and thermal
geometric isomerizations have been extensively explored [
200
]. Azobenzene and its derivatives
assume both
trans
and
cis
conformations with respect to the
azo
linkage.
Azo p ! p
* excitation
and
azo
n
! p
* excitation trigger
trans
-to-
cis
and
cis
-to-
trans
isomerizations [
203
-
205
]. The
azo
linkage normally exists in the more stable
trans
form. Also, the
trans
isomer of azobenzene exhibits
an intense absorption around 320 nm due to the
p ! p
* transition, while the
cis
isomer shows a weak
absorption of the n
! p*
transition, around 430 nm [
206
]. Reversible isomerizations between
cis
and
trans
structures are due to these transitions. Photo-isomerization can proceed almost quantitatively
[
207
]. By comparison, the thermal isomerizations from
cis
to
trans
configurations take place due to
low activation energy of the
chromophore in
compounds are often accompanied by drastic changes in a number of properties such as, for instance,
changes in the dipole moments [
208
]. The isomerization back to the
cis
-to-
trans
process. Isomerizations of the
azo
configuration can be readily
carried out either thermally, or by visible light irradiation. Changes in the molecular structure, such as
cis-trans
isomerization in polymers, can induce contraction and expansion of the polymeric chains on
both microscopic and macroscopic scale. This was demonstrated on a polymers with
trans
azo
linkages.
Exposure from dark to light can result in a contraction of as much as 0.5% [
209
]:
dark
NN
light
NN
One example is a copolymer of styrene, where azobenzene structures are attached in the comono-
mer to the benzene portion through amide linkages [
210
]:
n
n
heat or visible light
NH
λ = <380 nm
NH
O
O
N
N
N
N
After 10 min of irradiation with ultraviolet light, the photo-stationary state is reached, consisting of
79% of the
of the sample is slow in the dark(less than 10% in
60 min), but is much faster when exposed to visible light [
210
].
cis
isomer. Back isomerization to
trans
