Cyclotetraazocarbazole-a multichromic molecule

Article abstract of DOI:10.1039/c2cc37956b

A macrocyclic cyclotetraazocarbazole was prepared. Surprisingly, during the investigation of its photochromic behavior a drastic color change from yellow to green upon irradiation in chlorinated solvents was observed, which was temperature dependent and could also be induced by acid/base addition. With these three inputs an integrated molecular logic gate including an OR, a NOT and an AND function was build.

Full text of DOI:10.1039/c2cc37956b

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Cyclotetraazocarbazole – a multichromic molecule†‡
Luca Schweighauser and Hermann A. Wegner*
Cite this: DOI: 10.1039/c2cc37956b
Received 1st October 2012,
Accepted 26th November 2012
DOI: 10.1039/c2cc37956b
www.rsc.org/chemcomm
A macrocyclic cyclotetraazocarbazole was prepared. Surprisingly, during
the investigation of its photochromic behavior a drastic color change
from yellow to green upon irradiation in chlorinated solvents was
observed, which was temperature dependent and could also be induced
by acid/base addition. With these three inputs an integrated molecular
logic gate including an OR, a NOT and an AND function was build.
Controlling function at the molecular level opens immense
possibilities in all areas of science, but also in our day-to-day life.1,2
Therefore, we initiated a research program for the preparation of
multiphotochromic macrocyclic switches based on the azobenzene
scaffold.3 These structures offer the possibility to access multiple
different isomers,4 which allow creation of higher order switches with
more than two states, as we could recently show.5 Additionally, the
cyclic arrangement6 forces the geometry to deviate from the preferred
planarity. Ideally, a bowl-shaped structure will be adopted allowing
the creation of a molecular gripper.7,8 Unfortunately, the macrocyclic
cyclotrisazobiphenyl switches prepared so far by our group were too
flexible due to the rotability around the biphenyl unit to allow the
efficient build-up of a cavity by E - Z transformation.4 To overcome
this limitation it was envisioned to bridge the biphenyl unit to block
rotation. As a bridging unit the N-Alkyl group was chosen, as it allows
attachment of solubilizing groups without creating stereochemical
issues leading to a cyclooligoazocarbazole target structure.
Scheme 1 Synthesis of macrocycle 3 (R = C12H25).
This transformation led to the literature known 3,6-dinitro-9-
dodecyl-9H-carbazole (2).12,13 Finally, macrocycle 3 could be iso-
lated from a one-pot reaction using LiAlH4 as a reducing agent in
o1% yield (Scheme 1). The major products of the final cyclization
were linear polymers. Isolation of the cyclic product turned out to
be difficult and could finally be achieved by column as well as
recycling gel permeation chromatography.
In order to test our initial hypothesis to create a molecular
switchable container a sample of macrocycle 3 was irradiated in
chloroform with UV-light at 302 nm (hand held UV-lamp: UVP,
3UV-38 3UV Lamp, 8 Watt). Surprisingly, the color of the solution
changed from yellow to a deep green and finally to almost
colorless (Fig. 1).
In a first attempt to prepare such an azocarbazole macrocycle a
stepwise strategy via Mills reactions (condensation of an aromatic
amine with a nitroso compound)9,10 was planned. Unfortunately,
the Mills reaction was not successful and none of the desired
azocompounds were observed. Therefore, the strategy was changed
and a reductive coupling of 3,6-dinitrocarbazole (2) envisioned
(Scheme 1).11 Firstly, carbazole (1) was substituted with 1-bromo-
dodecane. Secondly, it was nitrated at the favored 3,6-positions.
To quantify this unexpected color change the process was
followed by UV/VIS-spectroscopy (Fig. 2). Initially, an increase in
the maximum at 420 nm could be observed after irradiation for
2 min. Further irradiation at 302 nm led to a decrease in the
maximum at 420 nm but an appearance of a new absorption band
at 570 nm. When irradiation was continued, the absorption
University of Basel, Department of Chemistry, St. Johanns-Ring 19, 4056 Basel,
Switzerland. E-mail: hermann.wegner@unibas.ch; Fax: +41-61-267-0976
† This article is part of the ChemComm ‘Emerging Investigators 2013’ themed
issue.
‡ Electronic supplementary information (ESI) available: Experimental
procedures and analytical data; UV spectra of irradiation of 4 and 5. See DOI:
10.1039/c2cc37956b
Fig. 1 Color change when macrocycle 3 was irradiated with UV-light (3 Â 10À5
M
solution in CHCl3).
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the irradiation process. The color of the solution did not undergo
any visible change. Nevertheless, the absorption of the solution of
azocarbazole 5 also decreased in the whole visible area of the
spectrum while longer irradiation with UV-light illustrated
decomposition.
It is known that CHCl3 upon irradiation with X-rays loses a Cl
radical which ultimately forms HCl.15 It has been reported that
carbazole itself can upon irradiation abstract Cl atoms from chlori-
nated compounds.16 In the case of 3–5 protonation could result in
the observed color change from yellow to green. The photoformation
of HCl could be observed not only in chloroform, but also in
1,2-dichloroethylene and dichloromethane, which indicates a differ-
ent mechanism from the simple decomposition of CHCl3 to phos-
gene and HCl as described above. No color change was observed in
non-chlorinated solvents such as THF. A similar photoreaction was
reported for other heterocycles such as b-carbolines,17 or indoles.18
It is proposed that the main process is an electron transfer from
singlet-excited carbazole to CHCl3 via an exciplex formation.
As expected from this proposition the addition of HCl to a
solution of macrocycle 3, dimer 4 or azocarbazole 5 yielded a colored
HCl salt showing strong pH dependence (Fig. 4). The addition of
sodium hydroxide to the acidic solutions led to the free base and a
color change back to yellow. Upon adding diluted HCl to macrocycle
3 or dimer 4, the initial absorbance band increased first, and a
new red shifted absorbance band occurred similar to irradiation
with UV-light. Based on these observations it is proposed that first a
carbazole unit and secondly the azo unit are protonated. The latter
protonation led to the new red shifted absorbance band. The fact
that no increase of the initial absorption band for azocarbazole 5
could be observed when irradiating with UV-light or when adding
diluted HCl to a solution was a strong indication that only the azo
unit was protonated. Additional 1H NMR titration experiments
revealed the appearance of one additional signal (B7.9 ppm), which
vanished upon treatment of the sample with D2O. A second peak
(B8.0) emerged on further addition of HCl before the signals
broadened considerably (see ESI‡ for details). The protonation of
azo units and the accompanied red shift were described in the
literature for different aminoazobenzenes.19 Methyl yellow is also
reported to undergo a color change when irradiated in the presence
of chlorinated compounds.20
Fig. 2 UV/VIS-spectra of macrocycle 3.
Fig. 3 Linear azocarbazoles 4 and 5.
decreased in the whole visible area of the spectrum. The latter
behavior was most probably due to the decomposition of macrocycle
3 because the process was no longer reversible. If irradiation was
stopped after 10 min, the green color was stable for days in an open
flask. Analysis by 1H NMR spectroscopy only showed a broadening
of the signal excluding the possibility of isomerization. However,
attempts to isolate the green species on silica gel restored immedi-
1
ately the initial yellow color and produced a H NMR spectrum
identical to that of the starting compound. In order to elucidate
if this unanticipated property results due to the macrocyclic
arrangement or the special features of the carbazoles two additional
azocarbazole substrates were prepared, the bisazocarbazole 4
(ref. 14) and the azophenylcarbazole 5 (Fig. 3; see ESI‡ for synthesis
details).
When a solution of dimer 4 in chloroform was irradiated with
UV-light a similar effect was observed in the absorbance spectrum.
The color changed from yellow to light green and a new absorption
band at 610 nm was observed. Also, the absorption of 4 decreased in
the whole visible area, when irradiation was continued. Irradiation
of azocarbazole 5 with UV-light, however, led only to a very small
new absorption band at 540 nm. No increase of the primary
absorption band at 390 nm could be observed at the beginning of
As such an acid–base equilibrium should show temperature
dependence, protonated samples of 3, 4 and 5 were heated.
Remarkably, an irradiated solution of macrocycle 3 in chloroform
Fig. 4 UV/VIS-spectra of macrocycles 3 (a), 4 (b) and 5 (c) in CHCl3 when treated with 0.054 M HCl in methanol.
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absorbance band at 570 nm for macrocycle 3 or 610 nm for dimer 4
is the visible logic output of such a system (Fig. 6).
Several applications, for example electronic devices such as
transistors, require such complex systems with more than two
terminals.23 Input and output were simply described ‘‘low’’ or
‘‘high’’, respectively, as digital ‘‘0’’ and ‘‘1’’. These simplifications
allowed setting up a truth table. The logic array consists of several
simpler logic gates including an OR, a NOT and an AND function.
The combination of these logic gates described the functions
performed by each of the two azocarbazoles in chloroform solution.
Monitoring the absorbance band at 570 nm when a solution of
macrocycle 3 in CHCl3 was subjected to all possible input operations
proved the principle. The absorbance perfectly corresponded to the
output expected.
Fig. 5 Temperature dependence of the absorption maximum at 570 nm of
macrocycle 3 and at 610 nm of dimer 4 after irradiation with 302 nm (left);
absorption of a solution of 3 at 570 nm in 10 heating and cooling cycles between
20 1C and 55 1C after irradiation with 302 nm (right).
In summary macrocyclic as well as linear azocarbazoles were
prepared. If the azo unit is flanked by two carbazoles a color change
can be induced by different stimuli (UV irradiation in chlorinated
solvents, pH and temperature). Based on this property an integrated
molecular logic gate was realized. Further investigations to explore
and apply azocarbazoles are ongoing.
We thank Prof. Dr Bernd Giese, University of Fribourg,
Switzerland, for the fruitful discussion, and the group of Prof.
Dr Stefan Weber, University of Freiburg, Germany, for EPR
measurements. Jens Hermes, Ulrike Fluch (GPC) and Pia
¨
Feinaugle (UV) from the University of Basel are gratefully
acknowledged for their help. Furthermore, we thank the Swiss
National Science foundation (SNSF) for financial support.
Notes and references
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¨
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.