A bridged azobenzene derivative as a reversible, light-induced chirality switch

Article abstract of DOI:10.1002/anie.200906731

(Figure Presented) A new trick for an old dog: The azoben-zene derivative 1 was used to demon-strate the light-induced spatially directed (unidirectional) tran cis isomerization for the first time. This important effect expands the range of applications f

Full text of DOI:10.1002/anie.200906731

Communications
DOI: 10.1002/anie.200906731
Chirality Switch
A Bridged Azobenzene Derivative as a Reversible, Light-Induced
Chirality Switch**
Gebhard Haberhauer* and Christine Kallweit
Dedicated to Professor Rolf Gleiter
Photochromic molecules, which can be switched reversibly
between two isomeric forms having different structures and
properties, are of great interest for the development of optical
one of the chiral cis isomers would extend the range of
applications of azobenzenes tremendously. Such a process
would provide a switch with which, in addition to the
amplitude change, a useful helical chirality element can be
switched on and off.
[
1]
[2]
[3]
memory devices, and molecular motors and machines.
A
prominent example of a light-induced switching process is the
[
4–6]
trans!cis isomerization of azobenzene and its derivatives.
Azobenzene derivatives have been used for several
switching processes in which chirality is critical, for example
The high-amplitude change between the stretched trans form
and the compact cis isomer, the relatively high reversibility,
and the photostability which allows a multitude of switching
cycles, make azobenzene derivatives among the most fre-
[
7]
[8]
in molecular scissors, switchable peptides, and chiral
[
9]
nematic phases, and for the stabilization of helical struc-
tures. However, the unidirectional switching of the
azobenzene unit and thus a targeted use of the helical
chirality element has not been possible so far. The use of
circularly polarized light leads to a partially unidirectional
switching of the azobenzene unit; however, in solution this
is followed by fast racemization of the cis isomers. Only in the
case of tetrasubstituted alkenes with sterically demanding
substituents could unidirectional light-induced switching be
[
10,11]
[2–6]
quently used switching devices.
A closer look at the
[
11]
switching process of azobenzene shows that the transition of
the trans to the cis isomer implies not only a structural change
but also the generation of helical chirality; that is, two
enantiomeric cis isomers are formed (Scheme 1). The devel-
opment of a unidirectional switching process that takes place
exclusively between the achiral, planar trans isomer and only
[
12]
[
13,14]
achieved without subsequent racemization.
By implementation of a chiral clamp this could be, in
principle, also achieved with azobenzenes (2 in Scheme 1).
The clamp should be flexible enough to allow a strong
amplitude change upon trans!cis isomerization, but should
simultaneously destabilize one of the cis conformations (here
the cis-(M) isomer) to such an extent that only one isomer
(
here the cis-(P)-isomer) is present in solution under standard
conditions.
As we had already succeeded in accomplishing a unidirec-
tional switching of bipyridine derivatives by means of chiral
[
15]
cyclic imidazole peptides,
clamp 3
we decided to use the chiral
[16]
also for the synthesis of the unidirectionally
switchable azobenzene 2 (Scheme 2). Simple alkylation of 3
with dibromide 4 using Cs CO as the base in acetonitrile
2
3
provided the desired azo compound 2 in 22% yield. To clarify
whether the azo compound 2 combines both desired proper-
ties—high amplitude change along with the energetic dis-
crimination of one of the cis isomers—the structures of trans-
Scheme 1. Light-induced switching of azobenzene (1; bidirectional)
and of the chiral azobenzene derivative 2 (unidirectional).
1, trans-2, cis-(P)-1, cis-(P)-2, and cis-(M)-2 were determined
by geometry optimization using B3LYP and the 6-31G* basis
[
17]
[
*] Prof. Dr. G. Haberhauer, C. Kallweit
Institut fꢀr Organische Chemie, Fakultꢁt fꢀr Chemie
Universitꢁt Duisburg-Essen
Universitꢁtsstrasse 7, 45117 Essen (Germany)
E-mail: gebhard.haberhauer@uni-due.de
set. We found that the difference in energy between trans-1
ꢀ1
and cis-(P)-1, which amounts to 63.4 kJmol , is similar to
that between trans-2 and cis-(P)-2 (57.7 kJmol ) (Table 1).
ꢀ1
Hence, also for azobenzene 2, a trans!cis isomerization
should be possible under standard conditions. The high
amplitude change found in azobenzene 1, which is reflected,
for example, in the change of the C2–C2’ and C5–C5’
distances, is also present in the switching process from trans-
[
**] This work was supported financially by the Deutsche Forschungs-
gemeinschaft (DFG). We thank Dr. A. Schuster for helpful
discussions and Dr. T. Balgar and Prof. E. Hasselbrink for the use of
their laser.
2
to cis-(P)-2. For both azobenzenes (1 and 2), the trans!cis
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906731.
isomerzation results in a reduction of the C5–C5’ distance by
2
418
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Scheme 3. Unidirectional switching of the cyclic azo compound 2.
compound 2 is present in a trans/cis ratio of 95:5. At the
photostationary state at 355 nm a trans/cis ratio of 15:85 is
achieved. The reverse isomerization leads back to the starting
mixture without the formation of any by-products. Even after
the switching cycle was repeated ten times we could not
observe any sign of photochemical degradation of 2 in the
NMR spectra. The lifetime of the cis isomer is about 5 days at
2
98 K in the dark. In the 2D NOESY spectra of both isomers
only the cis isomer gives a coupling between the protons H2
and H3. This effect is due to the structural change that occurs
during the isomerization process. The NOESY experiments
do not afford exact distances, but the spectra show that the
present cis isomer must be the cis-(P) conformer: There are
cross-coupling signals between H5 and H8, but none between
H5 and H6b, and this can be explained only by the presence of
exclusively cis-(P)-2.
Scheme 2. Synthesis of the chiral azo compound trans-2. Reaction
conditions: a) Cs CO , CH CN, D, 22%.
2
3
3
Table 1: Selected experimental data and values obtained by DFT
calculations (energy differences and absorption maxima in the CD
spectra) of the azobenzene compounds trans-1, trans-2, cis-(P)-1, cis-(P)-
2, and cis-(M)-2.
The trans!cis isomerization can also be observed in the
trans-1 cis-(P)-1 trans-2 cis-(P)-2 cis-(M)-2
UV spectrum by the decrease of the absorption band at
3
ꢀ
1 [a]
DE [kJmol
]
0.0
63.4
0.0
57.7
148.2
25 nm (Figure 1). This change in the p!p* band of the trans
n!p* transitions:
[
b]
c]
[18]
[18]
form is typical for isomerization of azobenzene derivatives.
The n!p* band is shifted from 446 nm to 413 nm as a result
of the trans-to-cis isomerization. A corresponding hypsochro-
mic shift of the negative Cotton effect of the n!p* band is
also observed in the CD spectrum (Table 1). Simultaneously,
a positive Cotton effect at 242 nm is observed for the cis
isomer. The reverse isomerization leads back to the original
spectrum.
l
[nm]
449
478
440
468
442
476
406
437
max,exp.
[
lmax,calcd [nm]
algebraic sign of De
algebraic sign of De
557
[
[
b]
c]
minus minus
minus minus
minus
plus
[
a] Energy differences of the respective stereoisomers calculated using
ꢀ4
DFT/6-31G*. [b] [2]=2.0ꢂ10 m in acetonitrile. [c] Calculated using TD-
DFT-PBE1PBE/6-31G* in CH CN as solvent.
3
For a better assignment and interpretation of the absorp-
tion spectra, the UV and CD spectra of trans-1, trans-2, cis-
(P)-1, cis-(P)-2, and cis-(M)-2 in acetonitrile as solvent were
simulated on the basis of time-dependent density functional
theory (TD-DFT) with the PBE1PBE functional and by
more than 2.5 ꢀ. Comparing the energy values of the cis
isomers (P)-2 and (M)-2 shows that the peptide clamp also
fulfills the second requirement, namely the energetic discrim-
ination of one cis isomer: the M isomer is destabilized by
ꢀ
1
[17]
9
0 kJmol relative to the P isomer. This high value implies
employing the 6-31G* basis set. As the intensities of the
that only the P isomer is present in solution under standard
conditions. The high destabilization of the M conformation
results from the strong repulsive interaction between the
aromatic ring of the azobenzene unit and the methyl group of
the imidazole rings. Accordingly, the unidirectional switching
of 2 should be possible (Scheme 3).
calculated curves are always higher, they were normalized to
the experimentally determined intensities. Both the UV and
the CD spectra for trans-2 are in good agreement with the
experimental spectra (Figures 1 and 2). In both CD spectra
there is a strong positive Cotton effect at roughly 200 nm, a
negative Cotton effect at about 250 nm, and in particular a
negative Cotton effect of the n!p* transition at values above
400 nm (Table 1).
The experimentally determined spectrum of the cis isomer
is in agreement only with the spectrum of cis-(P)-2, but not
with that of the cis-(M)-2. Accordingly, only the cis-(P)
isomer is present in solution, as already proved by the
calculated energy differences and the NMR data. Especially
noticeable are the absorption bands around 200 nm and the
The photoisomerization between trans-2 and cis-2 was
studied in dilute solution in chloroform for the NMR
experiments and in acetonitrile for the UV/CD analyses.
The trans!cis isomerization was induced by UV irradiation
using a laser (Spectra Physics Quanta Ray) with a wavelength
of 355 nm. The reverse isomerization process was achieved by
irradiation with visible light or by slight warming. The
1
recorded H NMR spectra show that after synthesis the azo
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Communications
band is negative and hypsochromically shifted relative to the
analogous band of the trans isomer. Since the n!p* bands of
the simple azobenzene cis-(P)-1 also show negative values
(
Table 1), this is definite evidence for the existence of the azo
unit in the P conformation. In the calculated spectrum of cis-
M)-2 the opposite effect is evident: The Cotton effect at
(
2
14 nm is strongly negative and the n!p* band is positive
and bathochromically shifted relative to the analogous band
of the trans isomer. The calculated spectra thus prove
unambiguously that the azo compound trans-2 is switched
unidirectionally to the cis-(P) isomer.
In summary we could show that by the use of a chiral
clamp it is possible to switch the azobenzene unit unidirec-
tionally by irradiation with light. Thus the range of applica-
tions of azobenzene derivatives has been extended by an
important effect: Beside the amplitude change, the switchable
chirality element can also be used in a deliberate way.
Because of the simple synthesis of the azobenzene derivative
2
2
and possibilities for further substitution (at C3, C4, and C5),
may be used for prospective novel switching processes, in
which chirality of the switched states plays an important role.
Figure 1. CD spectra (top) and UV spectra (bottom) of the chiral azo
switch 2 before (blue) and after (red) UV irradiation (l=355 nm)
Received: November 30, 2009
Published online: February 23, 2010
ꢀ4
(c=2.0ꢂ10 m in acetonitrile). The dashed lines at wavelengths
exceeding 380 nm represent the 20fold enlargement of the spectra.
Keywords: azo compounds · CD spectroscopy · isomerization ·
molecular switches · photochemistry
.
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