A chiral cyclotrisazobiphenyl: Synthesis and photochemical properties

Article abstract of DOI:10.1021/ol202556z

A chiral cyclotrisazobiphenyl macrocycle was synthesized conveniently in three steps from the literature known 3,3′-diaminobimesityl in 37-38% overall yield. Irradiation with 302 nm, 365 nm or visible light allows access to different photostationary state

Full text of DOI:10.1021/ol202556z

ORGANIC
LETTERS
2011
Vol. 13, No. 21
5908–5911
A Chiral Cyclotrisazobiphenyl: Synthesis
and Photochemical Properties
Raphael Reuter and Hermann A. Wegner*
Department of Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel,
Switzerland
Hermann.wegner@unibas.ch
Received September 21, 2011
ABSTRACT
A chiral cyclotrisazobiphenyl macrocycle was synthesized conveniently in three steps from the literature known 3,30-diaminobimesityl
in 37ꢀ38% overall yield. Irradiation with 302 nm, 365 nm or visible light allows access to different photostationary states (PSSs). These
PSSs can be conveniently read out by CD-spectroscopy as each of them exhibits a positive, a negative, or no signal, respectively, at
275 nm.
Azobenzenes have been known for over 170 years.1
Especially, their characteristic colorant properties led to their
fame as dyes for various applications to the present day.2
Later, it was recognized that azobenzenes can be switched
reversibly from the usually thermodynamically more stable
E-isomer to the Z-isomer upon irradiation or heating.3,4
This dramatic structural change has led to numerous
applications5 such as photoresponsive hostꢀguest systems,6
controlling the activity of catalysts,7 influencing the
orientation and folding of peptides in biochemistry8 or
neuronal activity9 as well as in materials chemistry and on
surfaces.10
However, in most of these cases only one azobenzene
moiety is incorporated into the system giving access to two
distinct stages. The construction of compounds consisting
of two or more azobenzene units offers the possibility to
access multiple different states.11,12 In order to do so, it
must be possible to access these states selectively and, even
more challenging, to conveniently read out the actual
status. We recently showed that the construction of an
azobenzene macrocycle,13 a cyclotrisazobiphenyl, in which
(1) Mitscherlich, E. Ann. Phys. Chem. 1834, 32, 224.
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r
10.1021/ol202556z
Published on Web 10/11/2011
2011 American Chemical Society

each of the possible isomers [(E,E,E), (E,E,Z), (E,E,Z),
(Z,Z,Z)] can be accessed as the major component in a
photostationary state (PSS) selectively by choosing the
right conditions.14 Although all isomers can be distin-
guished and analyzed by NMR, the similarity in the
absorption property rules out an easy analysis by UV-
spectroscopy. Therefore, an additional source of informa-
tion enabling an easy readout has to be introduced in the
macrocycle. If a chiral entity would be placed in the
cyclotrisazobenzene, this information should be highly
dependent on the conformation of the different azoben-
zenes allowing a facile analysis via circular dichroism (CD)
spectroscopy. There have been reports on the combination
of a monomeric azobenzene unit with chiral molecules
and the photochromic behavior of such a system.15 In
these cases, though, only two states could be addressed.
To validate the concept a chiral biphenyl was chosen as
a chiral source leading to the target compound 1
(Figure 1).
according to a procedure described by Rossnagel and
co-workers.17 Instead of using explosive acetyl nitrate, the
nitrating reagent was prepared in situ from fuming nitric
acid and acetic anhydride. Chiral resolution was also done
as described in the procedure from Moyer and Adams
by a recrystallization of the diastereomeric D-camphorsul-
fonic acid salt. The absolute configuration of 3,30-diami-
nobimesityl (3) was already assigned by Bloch et al.18
Both enantiomeric forms and the racemic mixture were
transformed to the corresponding macrocycle 1 in three
convenient steps. The Mills reaction with 1-bromo-3-
[(methoxymethoxy)methyl]-5-nitrosobenzene (4) pro-
duced bisazocompund 5 in a high yield of 90% (for rac-5;
90% for (S)-5 and 89% for (R)-5). After Suzuki reaction
with boronic acid pinacolate 6, the obtained diamine was
oxidatively cyclized using Pb(OAc)4 to furnish macrocycle
1 (41ꢀ42%).
Scheme 1. Synthesis of Both Enantiomers and the Racemic
Mixture of Macrocycle 1
Figure 1. (R) and (S) enantiomer of bimesitylcyclotrisazobiphe-
nyl macrocycle 1.
Chiral macrocycle 1 was prepared in three steps start-
ing with the literature known 3,30-diaminobimesityl (3)
(Scheme 1), which was first synthesized by Moyer and
Adams from iodomesitylene.16 Their procedure included
an Ullmann coupling of iodomesitylene followed by nitra-
tion with acetylnitrate and reduction of the nitro groups
with zinc powder. We replaced the Ullmann coupling by a
Scholl reaction of mesitylene (2) with ferric chloride,
The absorption spectrum of 1 showed the characteristic
features observed for azobenzenes with three maxima: one
at 444 nm, corresponding to the nꢀπ* transition; one at
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Barrett, C. J.; Mamiya, J.; Yager, K. G.; Ikeda, T. Soft Matter 2007, 3,
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C.; Hecht, S. Angew. Chem. 2006, 118, 1912. Angew. Chem., Int. Ed.
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De Cola, L.; Mayor, M. Eur. J. Org. Chem. 2009, 2562. (i) Reuter, R.;
Hostettler, N.; Neuenburg, M.; WegnerH. A. Eur. J. Org. Chem. 2009,
5647. (j) Reuter, R.; Hostettler, N.; Neuenburg, M.; Wegner, H. A. Chimia
2010, 64, 180. (i) For a recent review, see: Reuter R.; Wegner, H. A. Chem.
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Takaishi, K.; Yamashita, T. Chem. Commun. 2010, 46, 8344. (c)
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Chem. Ber. 1991, 124, 783.
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Org. Lett., Vol. 13, No. 21, 2011
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326 nm representing the πꢀπ* transition; and the third
absorption of the σꢀπ* transition at 256 nm. Upon
irradiation with UV light, a slight increase and a hypso-
chromic shift of the nꢀπ* absorption to 437 nm was
observed. This was combined with the typical sharp de-
crease of the πꢀπ* transition maximum. An interesting
feature was the splitting of the absorption maximum at
256 nm into two distinct bands at 248 and 270 nm. This is
probably the result of a bathochromic shift of the σꢀπ*
transition in the Z-azobenzene moieties, whereas the ab-
sorption at 248 nm can be attributed to the biphenyl units.
Comparing the optical rotation of 3,30-diaminobimesityls
(3) and chiral macrocycle 1, a large increase in the angle is
observed (for (S)-1: [R]20 = 2128°, for (R)-1: [R]20
=
D
D
ꢀ2077°). This behavior corresponds to a helical shape of
the macrocyclic species. For the measurement of the CD
spectra of chiral macrocycles (S)-1 and (R)-1, the samples
were heated at 45 °C overnight to effect thermal ZfE
isomerization; in this case only the all-E isomer waspresent
in the solutions. The CD spectra of the two enantiomers
are mirror images with four different absorption maxima
(Figure 2).
We irradiated a CD sample of (S)-1 with three light
sources of different wavelength (302 nm, 365 nm, and
visible light from a halogen floodlight).19 In every irradia-
tion experiment a photostationary state was obtained after
approximately 15 min. 1H NMR experiments revealedthat
(S)-1 could adopt seven different isomeric species. Six of
these isomers could be assigned to the different E/Z
isomers [one (E,E,E), two (E,E,Z), two (E,Z,Z), and one
(Z,Z,Z)], whereas one of them is thought to be a stable
conformer of the (E,E,Z)-isomer where a diazobond next
to the bimesityl unit has isomerized to its Z-form.20
In every investigated PSS a different ratio of these
isomers is obtained. This fact was also expressed in the
CD spectra of the irradiated mixtures. Upon compar-
ison of the three different states, it was observed that
they mostly vary in intensity with a similar overall shape
(Figure 3). However, a significant difference can be seen
for the values at about 275 nm. The sample which was
irradiated at 302 nm shows a positive value, while the one
irradiated with visible light shows almost no circular di-
chroism, and the one irradiated at 365 nm shows a negative
value (Figure 3, inset). This observation opens the possi-
bility of a ternary switch with three very distinct outputs
with a convenient readout þ, ꢀ, and 0.21 Although such
behavior is in principle not unique, it is essential to be able
to access three metastable states, which display different
chirality and hence CD spectra at a given wavelength.
A sample of (S)-1 can be irradiated with 302 nm, 365 nm,
or visible light to set the molecule to the desired state. Since
Figure 2. Absorption spectrum of (rac)-1 with change upon
irradiation at 365 nm from 0 to 3 min (2.0 ꢁ 10ꢀ5 M) and CD
spectra of (S)-1 (solid line) and (R)-1 (dashed line) after both enan-
tiomers were heated in the dark for 20 h at 45 °C (5.9 ꢁ 10ꢀ5 M).
the switching of azobenzenes is known to be fully rever-
sible, several cycles were done, switching between the
different PSSs without any sign of fatigue (Figure 4). It
does not matter from which starting state the switching is
done; in every case reliable CD signals were obtained.
(19) The time required to reach the PSS is dependent on the irradia-
tion source as well as the concentration. Irradiations with 302 and 365
nm were done with a 8 W hand-held UV lamp. Irradiation with visible
light was done with a tungsten-halogen lamp with 500 W. For the UV-
isomerization experiments the PSS was obtained after 3 min as the
concentration was three times lower than that for the CD measurements.
(20) For a detailed NMR analysis, see Supporting Information.
(21) For a recent example of a tristable fluorescent switch, see:
Figure 3. CD spectra at different PSSs at 302 nm (solid line),
visible light (dashed line), and 365 nm (pointed line) (5.9 ꢁ 10ꢀ5m),
with an enlarged graph for the region 270ꢀ280 nm (inset).
Irradiated samples of macrocycle (S)-1 were also tested
for thermal ZfE isomerization. The samples were stored
ꢀ
Ferreira, R.; Remon, P.; Pischel, U. J. Phys. Chem. C 2009, 113, 5805.
5910
Org. Lett., Vol. 13, No. 21, 2011

isomerization is negligibly low, compared to the irradia-
tion times needed to reach a certain photostationary state.
In summary, we presented the synthesis of two enantio-
mers as well as the racemic form of a chiral cyclotrisazo-
benzene macrocyle incorporating a chiral bismesityl unit
into the molecule. In this manner the compound can be set
to three different PSSs by irradiation with 302 nm, 365 nm,
or visible light. The actual status of the system can be
conveniently read out by CD spectroscopy as the CD
signal of each of the PSSs shows a different sign at 275 nm.
The concept will be applied in the future to control
molecular function by incorporating this macrocycle into
materials.
Acknowledgment. The authors thank the Swiss Na-
tional Science Foundation for project funding. H.A.W. is
indebted to the Fonds der Chemischen Industrie for a
Liebig Fellowship.
Figure 4. Switching between different PSSs with the value taken
at 275 nm starting with a heated sample. 302 nm (2), 365 nm (b),
and vis (9) (5.9 ꢁ 10ꢀ5 m; 15 min irradiation time).
Supporting Information Available. Experimental pro-
cedures, characterization data, assignment of different
photoisomers, isomer ratio at different PSSs and thermal
stability charts. This material is available free of charge
via the Internet at http://pubs.acs.org.
at 4 °C in the dark, and all of them showed a small decrease
in θ of about 1 mdeg per day. The characteristic shape
of the different PSSs was still clearly visible after four days
of irradiation. Therefore, it can be said that the thermal
Org. Lett., Vol. 13, No. 21, 2011
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