A new carbazole-based helically chiral architecture: Synthesis and physical properties

Article abstract of DOI:10.1016/j.tetlet.2016.05.030

A short and efficient synthesis leading to a new helically chiral heptacyclic system, containing a carbazole unit, was achieved via a Knoevenagel condensation-photocyclization strategy. The optical properties of the carbazole-based material were investigated by UV-vis absorption and photoluminescence spectroscopy and an emission in the visible region was observed. From the cyclic voltammetry analysis, the electrochemical band gap of the target organic material was estimated to be 2.38 eV indicating that it might be a good candidate for electron-injection hole-blocking layers in organic light-emitting diodes.

Full text of DOI:10.1016/j.tetlet.2016.05.030

Accepted Manuscript
A new carbazole-based helically chiral architecture: Synthesis and physical
properties
Mourad Ben Braiek, Faouzi Aloui, Béchir Ben Hassine
PII:
S0040-4039(16)30544-5
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.05.030
TETL 47649
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
21 March 2016
4 May 2016
9 May 2016
Please cite this article as: Braiek, M.B., Aloui, F., Hassine, B.B., A new carbazole-based helically chiral architecture:
Synthesis and physical properties, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.05.030
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A new carbazole-based helically chiral architecture:
Synthesis and physical properties
Mourad Ben Braiek, Faouzi Aloui, Béchir Ben Hassine*
University of Monastir, Laboratory of Asymmetric Organic Synthesis and Homogenous Catalysis
(UR11ES56), Faculty of Sciences, Avenue of Environment, 5019-Tunisia
*Corresponding author. Tel.: 0021673500279, Fax: 0021673500278
e-mail: bechirbenhassine@yahoo.fr
Abstract- A short and efficient synthesis leading to a new helically chiral heptacyclic system,
containing a carbazole unit, was achieved via a Knoevenagel condensation-photocyclization
strategy. The optical properties of the carbazole-based material were investigated by UV-visible
absorption and photoluminescence spectroscopy and an emission in the visible region was
observed. From the cyclic voltammetry analysis, the electrochemical band gap of the target
organic material was estimated to be 2.38 eV indicating that it might be a good candidate for
electron-injection hole-blocking layers in organic light-emitting diodes.
Keywords: Chirality, Azahelicenes, Carbazole, Fused-rings, Knoevenagel condensation,
Photocyclization.
Helicenes have been investigated since 1956 as aromatic molecules in which the ortho-
condensed ring system gives rise to a helical structure.1 Thirty years later, numerous studies on
the synthesis and physico-chemical properties of helicenes have been published.2 More recently,
helicenes have sparked interest due to the use of their intrinsic properties (e.g. chirality, strong
rotatory power) in various fields such as catalysis,3 non-linear optics,4 circulary polarized
luminescence5 and organic light-emitting materials.6 Besides typical carbohelicenes composed of
an all-carbon ring framework, heterohelicenes incorporating one or more heteroaromatic units in
the skeleton have also gained increasing attention due to their additional propreties.7
Pyridohelicenes, as a kind of typical azahelicenes, are prime examples.8 However, the pyrrole-
alternatives have been rarely focused on.9 That is partially because pyrrolohelicenes, with a low
number of fused aromatic rings (<6), are supposed to be stereochemically less stable resulting
from the small overlap at the terminal aromatic nuclei.10
1

The classical oxidative photocyclization of stilbene derivatives has been optimized and
widely applied. However, many helicene derivatives consisting of fused aromatic rings, including
benzene,11 thiophene,12 benzothiophene,13 pyridine,8 quinoline14 and phosphindole15 have
previously been reported. To the best of our knowledge, few examples of helicenes in which a
large π-conjugated polycyclic aromatic hydrocarbon (PAH) core such as carbazole, have been
used to obtain helical structures. In an independent study, Katz and co-workers16 reported the
synthesis of the azahelicenebisquinone 1 in nonracemic form (Fig. 1). Later, Nozaki17 reported
the synthesis of aza[7]helicene 2 via Pd-catalyzed coupling reactions. Recently, Liu and co-
workers18 described the synthesis of the carbazole-based diaza[7]helicene 3 and aza[6]helicene 4
through ‘‘3+3’’ and “3+2” approaches respectively, and described their application as a deep-
blue emitter in an organic light-emitting diode (OLED). More recently, our research group
successfully prepared novel pentacyclic helicene 5 containing a carbazole unit, using an oxidative
intramolecular photocyclization through a “3+1” approach.19 More recently, Bedekar and co-
workers20 have synthesized the carbazole derived aza[7]helicene 6. This class of helicene, with a
carbazole framework, could open new possibilities for the utilization of N-containing helicenes in
OLEDs and could also have potential applications in other organic electronic fields. For these
reasons, there is interest to extend our approach to higher helicene frameworks containing the
carbazole core. Moreover, we selected carbazole as a starting material because it has excellent
fluorescence features21 and carbazole derivatives are commonly used as fluorescence
probes/sensors.22
2

Figure 1. Chemical structures of azahelicenes 1-7.
Herein, we report an efficient synthetic strategy towards a new symmetric carbazole-based
heptahelicene 7 bearing two bromine atoms and two cyano groups at selected positions on the
aromatic rings. The cyano groups could increase the solubility of the helicene derivative and
improve its optical properties. Our synthetic strategy to construct the target functionalized
heptahelicene 7 was based on a Knoevenagel condensation, followed by an oxidative
photocyclization.
The synthesis of the helically chiral heptacyclic framework 7 was performed as shown in
Scheme 1. To ensure good solubility of the final organic π-conjugated material in common
organic solvents, we chose to incorporate a flexible aliphatic chain onto the carbazole unit. Thus,
the commercially available tricyclic ring system, 9-H-carbazole (8), was reacted with
hexylbromide in DMF to produce N-hexylcarbazole (9) in 95% yield. The latter was converted
into N-hexylcarbazole-3,6-dicarbaldehyde (10) in 79% yield via a Vilsmeier-Haack reaction.23
Compound (10) underwent double Knoevenagel condensation with 4-bromophenylacetonitrile
(11), using potassium tert-butoxide as a base, in THF by heating at 45 °C for 12 hours to afford
3,6-bis(p-bromo-α-cyanostyryl)-9-hexylcarbazole (12) in 86% yield.24
In order to obtain the helically chiral heptahelicene 7, bis-cyanovinylene 12 was subjected to
a double photocyclization using a 500 W high-pressure mercury immersion lamp. Photolysis was
performed on a dilute solution (2.10-1 g/L) for approximately 6 hours, in the presence of
3

stoichiometric iodine as an oxidizing agent, in one litre mixture of toluene and THF (95:05) at
room temperature. THF was used as a hydrogen iodide scavenger and also to increase the
solubility of alkene 12.25 The heptacyclic helicene 7 was obtained as a pure yellow solid in 72%
yield, after purification by column chromatography.26
Scheme 1. Synthetic route to the helically chiral heptacyclic system 7.
The helicene 7 obtained was characterized by NMR spectroscopy and HRMS, and was found
to show good solubility in common organic solvents including dichloromethane, chloroform,
toluene, ethyl acetate and tetrahydrofuran. The product was highly stable in air and to light. In the
1H NMR spectrum, it displayed typical patterns for the H-1/17 and H-3/15 protons of the helicene
moiety: high field signals at δ = 7.14 (J = 1.8 Hz) and 7.42 ppm (J1 = 8.7 Hz, J2 = 1.8 Hz) for H-
1/17 and H-3/15, respectively (Scheme 1).
No other regioisomer was isolated from the reaction mixture, indicating that the ring closure
of alkene 12 had occurred from the opposite side of the tricyclic moiety. Importantly, the
anthracene-like derivatives 13 and 14 (Fig. 3) were not formed during the cyclization step, as
these regioisomers would be expected to display characteristic signals for both H-1 and H-8 at
low field in the 1H NMR spectrum.27
4

Figure 3. Chemical structures of the anthracene-like derivatives 13 and 14.
The optical properties of the heptahelicene derivative 7 were investigated using UV-Vis
absorption and photoluminescence (PL) spectroscopy at room temperature (Table 1). The UV-
Vis absorption spectra were recorded using a dilute solution of 7 in dichloromethane (1 × 10-5 M).
As shown in Figure 4, the UV-Vis spectra were characterized by several structured bands
between 250 nm and 425 nm. Specifically, the absorption spectrum of compound 7 contained one
maxima (266 nm), four medium peaks (308, 332, 347 and 362 nm), and two minima (398 and
415 nm). The absorption bands in the range of 250-350 nm are associated with π-π* and n-π*
electronic transitions, while the absorption peaks at the longer wavelength region (375-425 nm)
can be ascribed to the intramolecular charge transfer (ICT) transition (π-π* transition) from the
N-hexylcarbazole unit as an electron donor to the electron withdrawing cyano group and are
similar to the characteristic vibration patterns of other helicene derivatives.20,28 Moreover, the
optical energy band gap Eg-op, calculated from the absorption band edge of the UV-Vis absorption
spectra, is approximately 2.82 eV.
Upon excitation, a dilute solution (ca. 1 × 10-6 M) of compound 7 in dichloromethane at
room temperature, the material showed two main emissions in the green region at 507 nm and
540 nm with a shoulder peak at 467 nm. This emission behaviour is similar to that observed for
other aza[7]helicenes.28
5

Normalized Absorbance
Normalized PL intensity
1,0
0,8
0,6
0,4
0,2
0,0
1,0
0,8
0,6
0,4
0,2
0,0
250 300 350 400 450 500 550 600 650 700 750
Wavelength (nm)
Figure 4. UV/Vis absorption (black line) and PL spectra (red line) of heptahelicene 7
in dilute dichloromethane.
Table 1.
Photophysical properties of helicene 7.
Absorption
Photoluminescence
c (nm) FWHMd (nm)
a (nm)
abs
max
λ
onset (nm)
433
Eg-opb (eV)
2.86
λ
λ
ems
Helicene 7
266
491
132
a Absorption maxima measured in a dichloromethane solution (5 x 10-5 mol.L-1) at room temperature.
b The optical gap (Eg-op) was estimated from the onset point of the absorption spectra: Eg-op = 1240/ λ
.
onset
c Emission maxima measured in a dichloromethane solution (5 x 10-6 mol.L-1) at room temperature.
d Spectrum full width at half maximum.
Next, we investigated the electrochemical characteristics of the helicene 7 by cyclic
voltammetry (CV) using ferrocene (Fc/Fc+) as the internal standard with a scan rate of 100
mV.s−1 at room temperature. This material was scanned both positively and negatively,
separately, in 0.10M tetrabutylammonium tetrafluoroborate (Bu4NBF4) in anhydrous
dichloromethane at room temperature. As shown in the cyclic voltammogram in Figure 5,
compound 7 exhibits irreversible anodic and cathodic peaks, the onset of oxidation (Vonset-ox) was
found to occur at 1.45 V and the onset of reduction (Vonset-red) was at -0.93 V (vs SCE). According
to an empirical method,29 and by assuming that the energy level of the ferrocene/ferrocenium
couple was 4.8 eV below the vacuum level, the HOMO and LUMO energy levels were calculated
6

as follows:
EHOMO (ionization potential) = – (Vonset-ox – VFOC + 4.8)
ELUMO (electron affinity) = – (Vonset-red – VFOC + 4.8)
Eg-el = (ELUMO – EHOMO) eV
Where VFOC is the ferrocene half-wave potential (0.494 V), Vonset-ox is the oxidation onset
and Vonset-red is the material reduction onset, all measured versus (Ag/AgCl). The calculated
ELUMO, EHOMO and Eg-el values are summarized in Table 2. The lowest unoccupied molecular
orbital (LUMO) was 3.37 eV, while the highest occupied molecular orbital (HOMO) was 5.75
eV, giving electrochemical band gaps of 2.38 eV. The difference between the optical and
electrochemical band gap of 0.48 eV is attributed to the interface barrier between the electrode
and the helicene, and the exciton binding energy.30
Figure 5. Cyclic voltammogram of azahelicene 7 in 0.1 M (n-Bu)4NBF4/dichloromethane
at a scanning rate of 100 mVs-1.
Table 2.
Electrochemical properties of helicene 7.
Compound
Vonset-ox (V)
Vonset-red (V)
EHOMO a (eV) ELUMO b (eV)
-5.75 -3.37
Eg-el c (eV)
Helicene 7
1.45
-0.93
2.38
a Calculated from the oxidation potential.
b Calculated from the reduction potential.
c Band gap was calculated from the difference between HOMO and LUMO.
7

In summary, we have developed a straightforward method for the preparation of a carbazole-
fused heptahelicene 7, from readily available 9-H-carbazole in four steps with an overall yield of
46% under mild conditions. The photophysical and electrochemical behaviors of the helicene
were also studied. This material showed relatively high electron affinity and may be a good
candidate for electron-injection hole-blocking layers. Work in this field and resolution of the
heptacyclic helicene are currently in progress.
Acknowledgements
The authors are grateful to the DGRS (Direction Générale de la Recherche Scientifique) of
the Tunisian Ministry of Higher Education and Scientific Research for the financial
support of this research. The authors thank Dr. Mourad Chmekh (Unité de recherche: Matériaux
et Dispositifs Electroniques Organiques, UR11ES55, Faculté des Sciences de Monastir-5000-
Tunisia) for the UV and PL analysis.
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24. Experimental procedure for diarylethene 12: A solution of 9-hexylcarbazole-3,6-
dicarbaldehyde 10 (1.5 g, 4.85 mmol) and 4-bromophenylacetonitrile 11 (2.10 g, 10.7 mmol) in
dry THF (30 mL) was placed in a two-necked flask, fitted with a septum, which was degassed and
purged with argon three times. To this solution was added a stoichiometric amount of
potassium tert-butoxide (2.64 g, 23.5 mmol) and the mixture was stirred vigorously for 12 hours
at 45 °C. After cooling, the mixture was poured into MeOH. The precipitate was collected using
a sintered glass filter, washed with MeOH, and dried under vacuum. The purification of 12 was
accomplished by chromatography on a silica gel column using CH2Cl2 as eluent to provide a
1
bright yellow solid (2.75 g, yield = 86 %); H NMR (CDCl3, 300 MHz): δ (ppm) = 0.87 (t, J =
7.2 Hz, 3H), 1.30-1.36 (m, 6H), 1.90-1.86 (m, 2H), 4.29 (t, J = 7.2 Hz, 2H), 7.45 (d, J = 8.4 Hz,
2H), 7.50-7.60 (m, 8H), 7.65 (s, 2H), 8.15 (d, J = 8.4 Hz, 2H), 8.60 (d, J = 1.2 Hz, 2H); 13C NMR
(CDCl3, 75 MHz): δ (ppm) = 13.97, 22.48, 26.86, 28.90, 31.42, 43.54, 106.90, 109.60, 118.53,
122.66, 122.91, 123.01, 125.43, 127.10, 127.57, 132.11, 133.84, 142.12, 142.98; HRMS
(MALDI-TOF) Calcd for C36H29Br2N3 [M]+: 663.0728. Found: 663.0733.
25. Talele, H. R.; Gohil, M. J.; Bedekar, A. V. Bull. Chem. Soc. Jpn. 2009, 82, 1182-1186.
26. Experimental procedure for the photocyclization reaction: To a solution of alkene 12 (200
mg, 0.30 mmol) in 1 L of toluene/THF (95:05) was added a stoichiometric amount of I2. The
solution was irradiated using a falling-film photoreactor and a high-pressure Hg-vapor lamp (500
W, Hanovia). The reaction was monitored by TLC. Following completion, the solvent was
removed under reduced pressure and the crude residue was purified by column chromatography
on silica gel with cyclohexane/CH2Cl2 (80:20) to give helicene 7 as a pale yellow solid (144 mg,
11

1
72%); H NMR (CDCl3, 300 MHz): δ (ppm) = 0.88 (t, J = 7.2 Hz, 3H, Me), 1.29-1.44 (m, 6H),
2.04-2.11 (m, 2H), 4.78 (t, J = 7.2 Hz, 2H), 7.14 (d, J = 1.8 Hz, 2H), 7.42 (dd, J1 = 8.7 Hz, J2 =
1.8 Hz, 2H), 8.05 (d, J = 8.7 Hz, 2H), 8.09 (d, J = 8.7 Hz, 2H), 8.17 (d, J = 8.7 Hz, 2H), 8.54 (s,
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2H); C NMR (CDCl3, 75 MHz): δ (ppm) = 14.04, 22.56, 27.05, 29.85, 31.54, 44.19, 105.96,
111.60, 115.68, 118.15, 118.72, 125.21, 126.04, 127.08, 127.83, 127.87, 129.56, 129.96, 130.65,
135.47, 141.12; HRMS (MALDI-TOF) Calcd for C36H25Br2N3 [M]+: 659.0415. Found: 659.0422.
27. (a) Martin, R. H.; Marchant, M.-J.; Baes, M. Helv. Chim. Acta 1971, 54, 358-360; (b)
Soudhakar, A.; Katz, T. J. Tetrahedron Lett. 1986, 27, 2231-2234; (c) Harrowven, D. C.; Nunn,
M. I. T.; Fenwick, D. R. Tetrahedron Lett. 2002, 43, 7345-7347; (d) Roose, J.; Achermann, S.;
Dumele, O.; Diederich, F. Eur. J. Org. Chem. 2013, 3223-3231.
28. Upadhyay, G. M.; Bedekar, A. V. Tetrahedron 2015, 71, 5644-5649.
29. (a) Bredas, J. L.; Silbey, R.; Boudreaux, D. S.; Chance, R. R. J. Am. Chem. Soc. 1983, 105,
6555-6559; (b) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.; Bassler, H.; Porsch, M.;
Daub, J. Adv. Mater. 1995, 7, 551-554.
30. (a) Zahn, D. R. T.; Gavrila, G. N.; Salvan, G. Chem. Rev. 2007, 107, 1161-1232; (b) Egbe, D.
A. M.; Carbonnier, B.; Paul, E. L.; Mühlbacher, D.; Kietzke, T.; Birckner, E.; Neher, D.;
Grummt, U.-W.; Pakula, T. Macromolecules 2005, 38, 6269-6275.
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Graphical abstract
A new carbazole-based helically chiral architecture: Synthesis and physical properties
Mourad Ben Braiek, Faouzi Aloui, Béchir Ben Hassine
N
Br
Br
N
N
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Highlights
- A photochemical approach leading to a new carbazole-based heptahelicene
- The optical properties of the carbazole-based material were investigated
- The electrochemical band gap of the target helicene was estimated to be 2.38 eV
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