Ni(COD)2 coupling of 3,6-dibromocarbazoles as a route to all-carbazole shape persistent macrocycles

Article abstract of DOI:10.1016/j.tetlet.2015.08.048

The number of applications for carbon-based materials has experienced tremendous growth over the last decade arising from their low toxicity, straight-forward chemical modification, and interesting electronic properties. Among these materials, self-assembled structures based on shape persistent macrocycles are perhaps the most exciting as they offer a means to prepare a wide range of morphologies through reversible assembly of these molecular precursors. In this letter, we report on the preparation of a novel family of all-carbazole shape persistent macrocycles through the simple single-step reaction of the corresponding 3,6-dibromocarbazoles over Ni(COD)₂. The resultant macrocycles display optical properties characteristic of the parent N-alkyl polycarbazoles, with quantum yields ranging from 11% up to 21% suggesting that certain substituents induce the formation of highly emissive aggregates, which could potentially provide a mechanism for the preparation of functional self-assembled nanomaterials.

Full text of DOI:10.1016/j.tetlet.2015.08.048

Tetrahedron Letters 56 (2015) 5595–5598
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Ni(COD) coupling of 3,6-dibromocarbazoles as a route
2
to all-carbazole shape persistent macrocycles
⇑
⇑
Leah S. Coumont , Jonathan G. C. Veinot
University of Alberta, 11227 Saskatchewan Dr., Edmonton T6G 2G2, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
The number of applications for carbon-based materials has experienced tremendous growth over the last
decade arising from their low toxicity, straight-forward chemical modification, and interesting electronic
properties. Among these materials, self-assembled structures based on shape persistent macrocycles are
perhaps the most exciting as they offer a means to prepare a wide range of morphologies through rever-
sible assembly of these molecular precursors. In this letter, we report on the preparation of a novel family
of all-carbazole shape persistent macrocycles through the simple single-step reaction of the correspond-
Received 30 June 2015
Revised 18 August 2015
Accepted 20 August 2015
Available online 21 August 2015
Keywords:
Carbazole
Tetramer
Macrocycle
2
ing 3,6-dibromocarbazoles over Ni(COD) . The resultant macrocycles display optical properties character-
istic of the parent N-alkyl polycarbazoles, with quantum yields ranging from 11% up to 21% suggesting
that certain substituents induce the formation of highly emissive aggregates, which could potentially
provide a mechanism for the preparation of functional self-assembled nanomaterials.
Ó 2015 Elsevier Ltd. All rights reserved.
In recent years shape persistent macrocycles (SPMs) have been
the subject of intense research because of their useful properties,
and their propensity to self-assemble into interesting micro and
nano-scale structures.1 While molecules based upon a carbazole
Alkylated carbazole monomers (2a–g, Scheme 1) were prepared
by straightforward room temperature deprotonation of 3,6-dibro-
mocarbazole followed by reaction with appropriate alkyl halides
to produce N-alkylated 3,6-dibromocarbazoles with alkyl groups
–5
6
20
substructure have found widespread application in electronics,
ranging from 3 to 12 carbons in length (Table 1). The alkylated
7
–9
10,11
sensing,
and biological applications,
relatively few examples
1
2–16
of carbazole-based SPMs have been reported.
Notable success
has been achieved with arylene-ethynylene and carbazyl-ethyny-
lene monomers that readily react via precipitation driven alkyne
metathesis to produce macrocycles.2
,5,17,18
This methodology
allows control over macrocycle size and shape, and has facilitated
the development of a new family of compounds with a rigid core
conformation that self-assemble into a variety of supramolecular
3
,13,19
structures.
A brief report published in 2003 suggested the polymerization
of N-alkylated 3,6-dibromocarbazole yielded substantial quantities
of cyclic oligomers when the reactions were performed under
1
5
dilute conditions.
Given recent interest in shape-persistent
macrocycles and the straightforward N-alkyl functionalization
and C–C coupling of 3,6-dibromocarbazole, we have prepared a
family of all-carbazole cyclic tetramers. The tetramers were pre-
pared bearing N-alkyl solubilizing side-groups of varied chain
length and steric bulk with the intent of investigating their opto-
electronic properties.
⇑
Tel.: +1 780 232 3729 (L.S.), +1 780 492 7206 (J.G.C.).
E-mail addresses: lcoumont@ualberta.ca (L.S. Coumont), jveinot@ualberta.ca
(
J.G.C. Veinot).
Scheme 1. Synthetic scheme used to prepare compounds 2b–g and 3a–g.
http://dx.doi.org/10.1016/j.tetlet.2015.08.048
040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
0

5
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L. S. Coumont, J. G. C. Veinot / Tetrahedron Letters 56 (2015) 5595–5598
Table 1
Preparation of 3,6-dibromo-N-alkyl carbazoles and their corresponding tetramers
The alkyl-free tetramer 3a readily oxidizes to produce the
green, insoluble quinoid form under acidic conditions making the
2
1
R
Monomer
Monomer
yield %
Tetramer
Tetramer
yield %
purification procedure described above ineffective. To avoid pro-
duct decomposition during purification, we turned to soxhlet
extraction; first washing the crude product with methanol, then
acetone. The product 3a was then obtained from the crude mixture
by extracting the cyclic products from insoluble impurities using
cold THF. Following evaporation of the THF, residual Ni impurities
were removed from the product by suspending the brown solid in
a 3% solution of sodium dimethylglyoximate (NaDMG) in ethanol,
H
1
—
3a
3b
3c
3d
3e
3f
67a
54
63
72
60
51
47
C
C
C
C
6
H
H
H
13
2b
2c
2d
2e
2f
2g
43
41
19
30
52
33
8
17
10
21
25
12
H
CH(CH
CH CH(C
3 2
)
2
2
H
5
)C
4
H
9
3g
a
2
followed by filtration to remove the precipitated Ni(DMG) . The
desired product, 3a, was recovered as a glassy tan solid in 67%
yield.
3
a reacts under the acidic conditions used to isolate the other compounds, as
such a different purification method was employed.
Purified 3a was evaluated using MALDI-TOF and showed a sin-
gle high-intensity peak consistent with 4 linked monomers
(Fig. 1a). Further analysis using high-resolution MALDI-FTICR
(Fig. 1b) showed the fragment of highest intensity occurred at
m/z = 660.231 with an isotopic distribution pattern corresponding
to that predicted for the expected cyclic structure. Of important
note, no evidence of an isotopic distribution pattern centered at
m/z = 662.2, as expected for the linear tetramer, was detected
(Fig. 1b). Consistent with the present MS results, and the proposal
products were isolated from unreacted carbazole using column
chromatography with the exceptions of compounds 2d and 2e,
which were readily recrystallized from ethanol.
Compounds 1 and 2b–2g were subsequently coupled using a
variation of a literature procedure previously employed to prepare
tetramer 3g (see Table 1).15 Briefly, the desired 3,6-dibromocar-
bazole monomer was coupled over Ni(Bpy)(COD) and quenched
in methanol to yield crude products as green-gray solids. The tar-
get compounds, 3a–g, were extracted and purified using one of
two methods depending on the substitution at N. The alkyl substi-
tuted 3b–g were purified by adding 1 M HCl and acetone to the
crude product in methanol in a 1:1:1 ratio followed by extraction
into toluene. This process removed the majority of the unreacted
catalyst, and Ni byproducts formed during the coupling reaction.
The cyclic compounds were then isolated from the crude product
mixture by extraction into hot acetone.
1
of a cyclic structure, the H NMR spectrum showed resonances
readily assigned to protons in the 4 and 5 positions of the carbazole
structural units that are shifted downfield to 8.51 ppm (Fig. 1c).15
Further discounting the presence of the linear tetramer, no reso-
nances at 8.04 ppm, corresponding to terminal protons on linear
1
22
oligomers, were detected at the sensitivity of H NMR technique.
MALDI-TOF spectrometry confirmed the solids isolated from
the extraction of the crude alkyl substituted products were also
primarily cyclic tetramers (3b–3g, Table 1). The spectra revealed
Figure 1. (a) Low resolution MALDI-TOF spectrum and (b) high resolution MALDI-FTICR trace obtained for 3a showing the mass-to-charge ratio and isotopic distribution of
1
the parent ion. (c) The aromatic region of the H NMR spectrum of 3a recorded in toluene-d8 including the proton NMR assignment.

L. S. Coumont, J. G. C. Veinot / Tetrahedron Letters 56 (2015) 5595–5598
5597
Table 2
Relevant optical properties of the tetramers 3a–g determined in THF. kEM and kEX are
the maximum PL and PLE wavelengths, respectively
Compound
R
kEX (nm)
k
EM(nm)
U
fl
3
3
3
3
3
3
3
a
b
c
d
e
f
H
384
396
395
396
397
395
391
417
430
430
430
431
432
430
15
13
11
12
12
12
21
C
C
C
C
6
H
H
H
13
17
8
10
21
12
H
25
CH(CH
CH CH(C H )C H
3 2
)
g
2
2 5 4 9
alkyl substituent. In this case, the broad PL is accompanied by a
blue-shift of the PLE maximum which is consistent with the pres-
2
7
ence of H-aggregates (Table 2).
To further investigate the presence/influence of stable molecu-
lar aggregates and the origin of the broad PL spectrum of 3a and 3g,
PL quantum yields (i.e.,
fl
U ) were evaluated for all tetramers
(
Table 2). The measured for 3g is 21%, greater than those noted
U
fl
for the majority of the alkyl-substituted tetramers studied, and of
the same magnitude as those observed for macrocycles with short
alkyl chains (i.e.,
f
U = 15%, 13%, and 12% for 3a, 3b, and 3f, respec-
tively). It is likely that the influence of the relatively large 2-ethyl-
hexyl group is driving the aggregation process, which in turn
2
8,29
restricts twisting motions between carbazole units.
This effect
could be responsible for the large quantum yield of 3g when dis-
solved in THF when compared to both the other tetramers pre-
pared, and the literature values measured for linear carbazole
3
0–32
oligomers (Table 2).
The source of these observations, includ-
ing the influence of solvent polarity on the assembly of the mole-
cules into unique nanostructures, and its impact on optical
properties of the macrocycles, is the subject of ongoing
investigation.
In conclusion, we have prepared a novel family of carbazole-
based SPMs bearing pendant alkyl chains of varied lengths by cou-
Figure 2. PL and PLE spectra of the tetramers 3a–g recorded in THF solution for
tetramers with (a) linear R groups and (b) branched R groups.
2
pling the corresponding 3,6-dibromocarbazoles using Ni(COD) .
Subsequent evaluation of the optical properties of the tetramers
indicates the PL spectra of carbazole macrocycles depends upon
the nature of the N-alkyl substituent and is consistent with sub-
stituent-dependent aggregation. These findings suggest the macro-
cycles prepared herein could find use as building-blocks in highly
emissive assemblies.
peaks of high intensity at m/z that correspond to the expected
molecular ion [M+] of the target cyclic tetramer (spectra are pro-
vided in ESI). High resolution MALDI-FTICR further confirmed the
m/z ratio was consistent with the molecular formulas of 3b–g. In
addition, inspection of the 1H NMR spectra shows the resonances
of protons at the 2,7 and 4,5 positions are shifted downfield rela-
tive to the monomers; similar to what was observed in the proton
NMR spectrum of 3a discussed above. Based upon these data we
conclude 3b–g are primarily cyclic tetramers.
Acknowledgments
We acknowledge longstanding, generous financial assistance of
the National Sciences and Engineering Research Council of Canada,
and the University of Alberta. We thank the Analytical and Instru-
mentation, Mass Spectrometry, and NMR Labs at the University of
Alberta. Finally, we thank the members of the Veinot Research
Group for their helpful discussions.
Given the central role of optical properties in many applications
of carbazole-based materials, the excitation (PLE) and emission
(
PL) spectra of THF solutions of 3a–g were evaluated (Fig. 2). Qual-
itative inspection of the spectra reveals two trends; one trend
relating to the length of alkyl chains in tetramers with linear R
groups (Fig. 2a), and a second relating to the length of the pendant
group in samples prepared with branched R groups (Fig. 2b).
The PLE and PL spectra of macrocycles bearing linear R groups
Supplementary data
(
i.e., 3b–3e) are very similar, with nearly identical excitation and
Supplementary data (details of the synthesis, and quantum
emission maxima and similar definition of vibronic features. The
spectroscopic similarities indicate that 3b–3e are experiencing
similar chemical environments. This observation is not unexpected
given the optical properties of the tetramers originate from the all-
carbazole backbone.
1
13
yield determination, as well as H NMR, C NMR, MALDI TOF
and MALDI FTICR traces are provided online) associated with this
article can be found, in the online version, at http://dx.doi.org/10.
1
016/j.tetlet.2015.08.048.
In contrast, PL spectra of 3f and 3g are broad with more pro-
nounced vibronic features. It is reasonable these observations are
a consequence of fixed molecular conformations in solution, pos-
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