Synthesis and characterization of new carbazole-based materials for optoelectronic applications

Article abstract of DOI:10.1016/j.tetlet.2013.05.091

New triazolo-carbazole derivatives were synthesized by copper-catalyzed azide-alkyne cycloaddition (CuAAC) reactions. The chemical structures of these compounds were confirmed by NMR and FT-IR spectroscopic analysis. The optical properties of the triazolo-carbazoles were investigated by UV-visible absorption and photoluminescence spectroscopy; an emission in the ultraviolet region was observed. The energy levels of these organic materials were determined by cyclic voltammetry and showed a relatively high electronic affinity indicating that they might be good candidates for electron-injection hole-blocking layers in organic light-emitting diodes.

Full text of DOI:10.1016/j.tetlet.2013.05.091

Tetrahedron Letters 54 (2013) 4026–4029
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and characterization of new carbazole-based materials
for optoelectronic applications
Amira Bahy a, Mejed Chemli b, Béchir Ben Hassine a,
⇑
a Laboratoire de Synthèse Organique Asymétrique et Catalyse Homogène (O1UR1201), Faculté des Sciences, Avenue de l’environnement, 5019 Monastir, Tunisia
b Laboratoire des Interfaces et Matériaux Avancés, Faculté des Sciences, Avenue de l’environnement, 5019 Monastir, Tunisia
a r t i c l e i n f o
a b s t r a c t
Article history:
New triazolo-carbazole derivatives were synthesized by copper-catalyzed azide–alkyne cycloaddition
(CuAAC) reactions. The chemical structures of these compounds were confirmed by NMR and FT-IR spec-
troscopic analysis. The optical properties of the triazolo-carbazoles were investigated by UV–visible
absorption and photoluminescence spectroscopy; an emission in the ultraviolet region was observed.
The energy levels of these organic materials were determined by cyclic voltammetry and showed a
relatively high electronic affinity indicating that they might be good candidates for electron-injection
hole-blocking layers in organic light-emitting diodes.
Received 4 April 2013
Revised 28 April 2013
Accepted 17 May 2013
Available online 28 May 2013
Keywords:
Triazolo-carbazoles
CuAAC
Ó 2013 Elsevier Ltd. All rights reserved.
Optical properties
UV emission
High electronic affinity
The chemistry of carbazole and its functionalized derivatives
dates back nearly to 100 years.1–6 Historically, the synthetic appli-
cations of these compounds have been rooted primarily in the
preparation of alkaloids and other natural products.1–3 However,
renewed interest in photophysical properties of carbazole has led
to the incorporation of this moiety into the molecular structure
The triazolo-carbazoles TC1–TC4 were synthesized by a
standard CuAAC reaction procedure.14,16 Sodium ascorbate and
aqueous copper sulfate pentahydrate were added to 9-propargylc-
arbazole and the corresponding azides (A1–A4) in THF/H2O (1:1).
The mixture was stirred for 24 h at 60 °C. All the triazolo-carbaz-
oles were obtained in good yields after work-up (Scheme 2).23
The optical properties of compounds TC1–TC4 were investi-
gated by UV–vis and photoluminescence (PL) spectroscopy in
dilute chloroform solution. The absorption spectra showed similar
behavior with two maxima between 327 and 340 nm and a shoul-
der at about 350 nm (Fig. 2, Table 1). A systematic study of these
spectra allowed a comparison of materials in pair, that is, TC1/
TC3 and TC2/TC4. Indeed, we noted that compounds TC1 and
TC3 (linear type) presented quasi-identical spectra. On the other
hand, the comparison of TC2 and TC4 (dendrimer-type) revealed
broadened spectra and a higher absorption-onset for TC4. Thus,
TC4 exhibits a more important effective conjugation length which
of several p-conjugated organic materials for optoelectronic appli-
cations.4–13 The exploitation of carbazole-based compounds as
electron-injection hole-blocking layers in multi-layer organic
light-emitting diodes (OLEDs) has been widely reported.14,15 In
fact, such compounds are known to have high electron affinity,
thereby reducing the interface barriers caused by the band offset
between the organic material and the cathode and contributing
to the balance of holes and electron transport.16,17
Herein, we report a facile synthesis of four new carbazole-based
molecules containing the triazole moiety (TC1–TC4, Fig. 1), using
the Cu-catalyzed azide–alkyne cycloaddition (CuAAC) reaction,
commonly known as ‘click chemistry’.14,18
can be related to privileged intramolecular p–p interactions of the
Four carbazole-based azides (A1–A4) were prepared as shown in
conjugated systems. The same behavior of these materials was
observed in PL analysis with identical spectra for TC1 and TC3
and a slightly broadened TC4 spectrum compared to TC2. The
emissions of TC1–TC4 were in the ultraviolet region, with
relatively narrow spectra and consisted of two maxima at approx-
imately 358 and 378 nm (Fig. 3, Table 1).
Cyclic voltammetry (CV) was employed to estimate the HOMO
(highest occupied molecular orbital) and LUMO (lowest unoccu-
pied molecular orbital) energy levels of the triazolo-carbazoles.
Knowledge of these energy levels is important to determine the
energy barriers and to select cathode and anode materials for
Scheme 1. To ensure good solubility of the final organic
p-
conjugated materials, we chose to graft a flexible aliphatic chain
onto the carbazole unit. Two types of azide were prepared: the first
is linear (A1 and A3), whereas the second consists of dendrimer-
type structures (A2 and A4). They were obtained by formylation19
of alkylated carbazole using the Vilsmeier–Haack reaction followed
by reduction,20 chlorination,21 and azidation22 (A1–A4) (Scheme 1).
⇑
Corresponding author. Tel.: +216 73500279; fax: +216 73500278.
E-mail address: bechirbenhassine@yahoo.fr (B.B. Hassine).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.05.091

A. Bahy et al. / Tetrahedron Letters 54 (2013) 4026–4029
4027
N
N
N
N
N
N
N
N N
N
N
N
N
N
TC1
TC2
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
TC3
TC4
Figure 1. Molecular structures of TC1–TC4.
H
N
C6H13Br , NaH
NaBH4
SOCl2
THF , r.t.
NaN3
DMF , r.t.
POCl3 , DMF
N
N
N
N
N
DMF , r.t.
80%
100 °C
THF / EtOH , r.t.
O
1
OH
Cl
N3
H
R'
R'
R'
R'
2
A1 : R' = H , 98%
3a : R' = H , 55%
4a : R' = H , 92%
5a : R' = H
4b : R' = CH2OH , 90%
A2 : R' = CH2N3 , 95%
3b : R' = CHO , 50%
5b : R' = CH2Cl
R'
R'
R'
R'
O
N
N
N
N
N
Cl
H
OH
SOCl2
THF , r.t.
N3
C6H12Br2 , NaH
DMF , r.t.
85%
POCl3 , DMF
100 °C
NaBH4
THF / EtOH , r.t.
NaN3
DMF , r.t.
N
N
N
N
N
O
H
OH
Cl
N3
R'
R'
R'
R'
6
7a : R' = H , 46%
8a : R' = H , 96%
9a : R' = H
A3 : R' = H , 88%
8b : R' = CH2OH , 70%
7b : R' = CHO , 43%
9b : R' = CH2Cl
A4 : R' = CH2N3 , 80%
Scheme 1. Synthetic route to A1–A4.
OLEDs.24,25 CV analysis is reliable, as the electrochemical processes
are similar to those involved in charge injection and transport pro-
cesses in such electronic devices.26,27
that the energy level of ferrocene/ferrocenium is 4.8 eV below
the vacuum level, the HOMO and LUMO energy levels and the elec-
trochemical gap (Eg-el) can be calculated as follows:28,29
The organic films were drop-coated onto indium-tin oxide (ITO)
glass substrates and scanned both at positive and negative poten-
tials in 0.1 M (n-Bu)4NBF4/acetonitrile solution at a scan rate of
50 mV sꢀ1. According to an empirical method, and by assuming
EHOMO ðIP; ionization potentialÞ ¼ ꢀðVonset-ox ꢀ VFOC þ 4:8Þ eV
ELUMO ðEA; electron affinityÞ ¼ ꢀðVonset-red ꢀ VFOC þ 4:8Þ eV

4028
A. Bahy et al. / Tetrahedron Letters 54 (2013) 4026–4029
N
N
N
R
CuSO4.5H2O
Na ascorbate
N
N
R
+
H2O / THF 1 : 1
N3
60 °C
50 - 89% yield
TC1-TC4
A1-A4
Scheme 2. Copper-catalyzed click reactions of azides A1–A4 with 9-
propargylcarbazole.
Figure 3. Photoluminescence spectra of TC1–TC4 in dilute chloroform solution
(2 ꢁ 10ꢀ7 mol Lꢀ1).
Figure 2. Normalized UV–vis absorption spectra of TC1–TC4 in dilute chloroform
solution (5 ꢁ 10ꢀ5 mol Lꢀ1).
where, VFOC is 0.73 V, the ferrocene half-wave potential measured
versus Ag/AgCl, Vonset-ox is the material oxidation onset and
Vonset-red is the material reduction onset.
The obtained cyclic voltammogram of TC4 is shown in Figure 4
as an example, and the estimated EHOMO and ELUMO values of all the
products are summarized in Table 2.
A comparison of these compounds revealed that the HOMO en-
ergy levels were almost identical (Fig. 5). However, a higher elec-
tron affinity was observed for TC1. The LUMO levels of these
compounds were comparable with that of bathocuproine (BCP).
BCP is widely used as an electron-injection hole-blocking layer in
OLEDs.30,31 It prohibits excitons diffusing toward the Al electrode
where they would otherwise be quenched.32 However, it is known
that BCP crystallizes quickly, which results in degradation of the
device performance.33,34 For this reason, it is desirable to replace
BCP by other materials in such devices. In fact, the fused triazol-
o-carbazoles TC1–TC4 might be good replacements for BCP and
for the elaboration of multilayer devices.
Figure 4. Cyclic voltammogram of TC4 film coated on an ITO electrode in 0.1 M
(n-Bu)4NBF4/acetonitrile at a scanning rate of 50 mV sꢀ1
.
Table 2
Electrochemical data for TC1–TC4
Vonset-ox (V)
Vonset-red (V)
EHOMO (eV)
ELUMO (eV)
TC1
TC2
TC3
TC4
1.19
1.16
1.15
1.24
ꢀ1.10
ꢀ1.26
ꢀ1.30
ꢀ1.17
5.26
5.23
5.22
5.31
2.97
2.81
2.77
2.90
In summary, we have successfully employed the Cu-catalyzed
azide–alkyne cycloaddition (CuAAC) reaction to generate fused
triazolo-carbazole derivatives. The structures of the products were
confirmed through spectral analysis. Investigations of the optical
properties in dilute chloroform solution showed emissions in the
ultraviolet region for all products. These new materials show
relatively high electron affinity and would appear to be good can-
didates for electron-injection hole-blocking layers. Work is
Table 1
Optical data for TC1–TC4 in dilute chloroform solution
Absorption
Photoluminescence
kmax (nm)
emax (104 Mꢀ1 cmꢀ1
)
konset (nm)
Eg-op (eV)
kmax (nm)
FWHMb (nm)
Stokes shift (nm)
TC1
TC2
TC3
TC4
331; 340; 348a
327; 340; 352a
332; 340; 348a
328; 340; 352a
0.59; 0.65
0.63; 0.74
0.98; 1.06
2.34; 2.73
358
362
359
366
3.46
3.42
3.45
3.39
358; 374
361; 377
359; 375
362; 378
34.6
35.8
36.4
37.9
18
21
19
22
a
Shoulder.
Spectrum full width half maximum.
b

A. Bahy et al. / Tetrahedron Letters 54 (2013) 4026–4029
4029
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The authors are grateful to DGRSRT (Direction Générale de la
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Tunisian Ministry of Higher Education, Scientific Research, and
Technology for the financial support.
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a yellow solid; mp:
1
240 2 °C (EtOAc); H NMR (300 MHz, DMSO-d6): d 1.18 (m, H16,160), 1.55
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1457.6580 [M+H]+; IR (KBr) cmꢀ1: 745, 1321, 1460, 1481, 2845, 2929, 3051,
3134.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.05.
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