Host-guest blue light-emitting electrochemical cells

Article abstract of DOI:10.1039/c3tc31983k

Carbazole, a commonly used hole-transporter for organic electronics, has been modified with an imidazolium cation and a hexafluorophosphate counter-anion to give an ionic hole-transporter. It has been applied as one of the hosts in a host-guest blue light

Full text of DOI:10.1039/c3tc31983k

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Materials Chemistry C
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Host–guest blue light-emitting electrochemical
cells†
Cite this: DOI: 10.1039/c3tc31983k
a
b
Antonio Pertegas, Nail M. Shavaleev, Daniel Tordera,a Enrique Ortı,a
*
´
´
b
a
*
Mohammad K. Nazeeruddin and Henk J. Bolink
Carbazole, a commonly used hole-transporter for organic electronics, has been modified with an
imidazolium cation and a hexafluorophosphate counter-anion to give an ionic hole-transporter. It has
been applied as one of the hosts in a host–guest blue light-emitting electrochemical cell (LEC) with the
neutral blue emitter FIrPic. We have obtained efficient and bright blue LECs with an electroluminescence
maximum at 474 nm and efficacy of 5 cd Aꢀ1 at a luminance of 420 cd mꢀ2, thereby demonstrating the
potential of the ionic organic charge-transporters and of the host–guest architecture for LECs.
Received 8th October 2013
Accepted 25th November 2013
DOI: 10.1039/c3tc31983k
www.rsc.org/MaterialsC
emission, the performances are worse: 2.6 cd Aꢀ1 at 5.3 cd mꢀ2
for a sky-blue LEC23 and 0.65 cd Aꢀ1 at 39 cd mꢀ2 for the bluest
LEC reported so far.19 With polymers as the active material in a
Introduction
Electroluminescent devices using organic semiconductors are
becoming a serious alternative to conventional inorganic tech-
nology as their efficiencies and stabilities have improved
signicantly over the last few years.1 The most efficient and
stable organic light-emitting devices (OLEDs) are based on a
multi-stack of low molecular-weight components that use air-
sensitive charge-injection layers.2 The multi-layer architecture is
obtained by sequentially evaporating the active species under
vacuum. OLEDs require rigorous encapsulation to prevent
degradation of the charge-injection layers.3,4
Another type of electroluminescent device, referred to as the
light-emitting electrochemical cell (LEC), has a simpler archi-
tecture and does not rely on air-sensitive charge-injection
layers,5–10 which simplies its preparation and makes it more
cost-efficient. In its simplest form, the LEC consists of a single
emitting layer of either an ionic transition-metal complex
(iTMC)9 or a neutral light-emitting material (usually a polymer)
mixed with an ionic transporter and a salt.8,10 The presence of
mobile ions facilitates the formation of ionic junctions that
lower the barrier for charge injection and make the LEC inde-
pendent of the work function of the electrode material.11–17
Despite these advances, the lack of efficient blue LECs
remains a problem. Although several blue LECs have been
reported, they exhibit low efficiency, luminance and life-
time.18–23 Efficacies of up to 18.3 cd Aꢀ1 at a luminance of 14.5 cd
mꢀ2 have been reported for blue-green LECs.22 For deeper-blue
tri-layer structure, an efficient LEC was made (5.3 cd Aꢀ1).24
However, in general, the stability of blue LECs is low, from
minutes to a few hours; moreover, their colour stability is rarely
discussed.
In iTMC–LECs, the iTMC is the only component involved
both in charge transport and in emission. With blue LECs, these
processes are challenging, because they involve high-energy
excitons (blue emitters have a large energy gap between the
lowest unoccupied and the highest occupied molecular
orbitals). The same problem occurs in OLEDs, which is a reason
for the continued search for stable and efficient blue emitters.
In OLEDs, charge transport is usually not performed by the
emitter but by the specially designed electron- and hole-trans-
porters.25 In contrast, in LECs, charge transport occurs via the
emitter and involves its reduction and oxidation. Because
charge transport in blue LECs forms high-energy species, effi-
cient blue LECs are hard-to-make. Hence, in LECs, it is of
interest to decouple the charge transport and the emission by
using different molecules for these processes. LECs rely on ionic
movement to reduce the charge injection barriers; therefore, in
the LEC, the charge-transporter must be mixed with the emitter
and, more importantly, the ionic movement must be main-
tained; for example, host–guest orange and red LECs have been
made with moderate band-gap iTMCs as the host.26–29
Here, we report a wide band-gap ionic hole-transporter
NMS25 suitable for blue light-emitting guests. NMS25 is an aryl-
carbazole modied with an imidazolium cation and a hexa-
uorophosphate counter-anion (Scheme 1). NMS25 was mixed
with a neutral polar electron-transporter SPPO13 and a neutral
blue-phosphorescent iridium(III) emitter FIrPic (Scheme 1). We
show that the combination of these low molecular weight
compounds, when they are sandwiched between two air-stable
a
´
´
Instituto de Ciencia Molecular, Universidad de Valencia, C/Catedratico J. Beltran 2,
ES-46980 Paterna, Valencia, Spain. E-mail: henk.bolink@uv.es
bLaboratory of Photonics and Interfaces, Institute of Chemical Sciences and
´
´ ´
Engineering, Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne,
Switzerland. E-mail: shava@mail.ru; Fax: +41 21 693 4111; Tel: +41 21 693 6124
† Electronic supplementary information (ESI) available: Spectroscopy and
electrochemistry of NMS25. See DOI: 10.1039/c3tc31983k
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To nd the optimum ratio of hole- and electron-trans-
porters, we evaluated devices with various mass ratios of
NMS25:SPPO13 (matrix; Fig. 1). The device conguration was:
ITO/PEDOT:PSS (80 nm)/matrix:FIrPic (80 nm; with 10 wt%
FIrPic) with the ionic liquid (IL) 1-butyl-3-methylimidazolium
tetrauoroborate added in a 4 : 1 molar ratio of matrix:IL/Al.
The LECs were run with a block-wave pulsed current at a
frequency of 1000 Hz and a duty cycle of 50% at an average
density of 100 A mꢀ2
.
All of the devices exhibit blue electroluminescence from
FIrPic with a maximum at 474 nm and with the typical char-
acteristics of LECs (Fig. 1–4). In pulsed-current mode, the LECs
exhibit a rapid decrease of the initially high driving voltage to a
lower steady-state value (Fig. 1–4). This is caused by the
decrease in injection barriers for electrons and holes as ions
migrate to the electrodes. The maximum luminance (Lmax) and
the time when it is reached (tmax) vary signicantly for the
different LECs, thereby complicating the lifetime comparison
(as lifetime, for example, dened by the time it takes for the
luminance to decrease to half of its maximum value, t1/2
,
counted from t ¼ 0, depends on the luminance). Kalyuzhny
et al. suggested30 to characterize the LEC by the total emitted
energy (Etot) up to the time when the luminance decreases to the
1/5 of its maximum.
Scheme 1 Synthesis of NMS25: (a) AlCl3, tert-butyl chloride, CH2Cl2,
under Ar, 0 ꢁC to RT; (b) 3-iodoanisole, Cs2CO3, Cu2O, DMF, under Ar,
120 ꢁC; (c) pyridine hydrochloride, under Ar, 200 ꢁC; (d) 1, K2CO3, DMF,
under Ar, 60 ꢁC. Structures of SPPO13 and FIrPic.
electrodes, gives efficient blue-electroluminescent devices that
function as LECs, with a slow turn-on determined by the
movement of ions, and that reach an efficacy of 5 cd Aꢀ1 at a
luminance of 420 cd mꢀ2
.
Results and discussion
Synthesis and characterization
NMS25 was prepared in a multi-step procedure, which was up-
scaled to give up to 4 g of the product (Scheme 1). In NMS25, the
tert-butyl groups were added to improve the solubility and to
block electrochemically-reactive C3 and C6 positions of the
carbazole; a long hexyloxy-chain was introduced to prevent
interaction of the imidazolium cation with the carbazole. Imi-
dazolium was chosen because it is optically-transparent and
electrochemically-inert. NMS25 is a white solid that is soluble in
polar organic solvents; it exhibits electronic absorption with a
cut-off at 365 nm in dichloromethane solution (Fig. S1, ESI†).
The redox potentials for NMS25 were measured by cyclic vol-
tammetry. In acetonitrile, NMS25 undergoes reversible oxida-
tion of the carbazole at 0.77 V (against ferrocene couple),
conrming its hole-transport properties, but no reduction down
to ꢀ2.7 V (Fig. S2, ESI†).
Electroluminescence devices
Two-layer LECs were prepared by spin-coating from solution. An
80 nm layer of PEDOT:PSS was spin-coated on ITO-glass to
increase the reproducibility of the devices. Then, an 80 nm layer
of NMS25, SPPO13 and FIrPic was deposited from an anisole
solution. Aluminium was used as the top electrode.
Fig. 1 Time-dependence of voltage and luminance for LECs with
various mass ratios of NMS25:SPPO13 (matrix) with 10 wt% FIrPic and
with a 4 : 1 molar ratio matrix:IL 80 nm emitting layer (block-wave
pulsed current; 1000 Hz; 50% duty cycle; average density 100 A mꢀ2).
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Table 1 Performance of LECs at a pulsed current of 100 A mꢀ2 (Fig. 1)
average current density31 from 25 to 100 A mꢀ2, with the
optimum determined to be 75 A mꢀ2
.
NMS25:SPPO13 tmax
(mass)
Lmax
t1/2
(h)
Etot
(J)
Effmax
EQEmax
(%)
There are two salts in the optimized LEC: the ionic hole-
transporter and the IL. A device was also prepared without IL to
conrm that it can function as a LEC by using only the ions
from NMS25 (Fig. 3). The LEC without the IL exhibits a higher
maximum luminance and efficiency, but a higher driving
voltage and a lower stability of the luminance than does the LEC
with the IL (Fig. 3).
(min) (cd mꢀ2
)
(cd Aꢀ1
)
0 : 1
0.4
6.6
4.2
14
41
176
165
205
124
67
0.1 0.01 1.7
0.6 0.02 1.6
1.1 0.03 1.9
3.9 0.07 1.2
0.6
0.6
0.8
0.5
0.3
0.5 : 9.5
2.5 : 7.5
3.5 : 6.5
1 : 1
14
0.13 0.6
Here, we demonstrate the potential of the host–guest LECs
in terms of efficiency and luminance (in previous reports,19,23
the stability of blue LECs was low and the focus was on the
efficiency). The LEC (without the IL) was optimized by anneal-
ing the emitting layer for 1 h at 100 ꢁC prior to the evaporation
of the top contact (the annealing changes the morphology and
increases the photoluminescence quantum yield of the active
layer32); the thus manufactured blue LEC exhibits a current
efficacy of 5 cd Aꢀ1 at a luminance of 420 cd mꢀ2 (Fig. 4). These
values are signicantly higher than are those previously repor-
ted for blue LECs.19,21,23 The lifetime of the optimized LEC
(without the IL) is short (Fig. 4); however, it is possible to
increase it by adding the IL (Fig. 3 and Table 1).
Table 1 summarizes the performance of LECs. The LEC with
the longest lifetime and the highest total emitted energy had an
emitting layer with a 1 : 1 mass ratio of NMS25:SPPO13
(Table 1); its steady-state driving voltage was the lowest, indi-
cating optimized charge-injection/transport (Fig. 1). Hence, this
LEC was chosen for optimization.
The thickness of the emitting layer changes the LEC
performance;20 therefore, we varied it from 40 to 80 and to 100
nm (Fig. 2). Although the driving voltage increases for thicker
layers, the highest luminance was achieved with an 80 nm layer
(Fig. 2). The driving conditions were evaluated by changing the
Fig. 2 Time-dependence of voltage and luminance for LECs with Fig. 3 Time-dependence of voltage and luminance for LECs with and
various thicknesses of the emitting layer: NMS25:SPPO13 (1 : 1 by without an ionic liquid (IL; in molar ratio): 80 nm emitting layer;
mass) with 10 wt% FIrPic (without IL); pulsed current (1000 Hz; 50% NMS25:SPPO13 (matrix; 1 : 1 by mass) with 10 wt% FIrPic; pulsed
duty cycle; average density 100 A mꢀ2).
current (1000 Hz; 50% duty cycle; average density 100 A mꢀ2).
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transporters with a suitable ionic group to optimize their
performance in organic electronics.
Experimental
Materials and methods
Purication and handling of all compounds were carried out
under air. All products were stored in the dark. Chemicals from
commercial suppliers were used without purication. Chro-
matography was performed on a column with an i.d. of 30 mm
on silica gel 60 (Fluka, Nr 60752). The progress of reactions and
the elution of products were followed on TLC plates (silica gel 60
F254 on aluminum sheets, Merck).
Elemental analyses were performed by Dr E. Solari, Service
for Elemental Analysis, Institute of Chemical Sciences and
1
Engineering (ISIC EPFL). H and 13C NMR spectra were recor-
ded with Bruker AV400 (400 MHz) and AVIII-400 (400 MHz)
spectrometers. Mass spectra were recorded with Q-TOF Ultima
(Waters) and TSQ7000 (Thermo Fisher) spectrometers (Mass-
Spectroscopy Service, ISIC EPFL).
Synthesis of NMS25
Imidazolium salt, 1. The reaction was performed under air
while protected from the moisture with a CaCl2-lled tube. 1,6-
Dibromohexane (5 mL, 7.9 g, 32 mmol, excess, Fluka) was added
to a solution of 1-methylimidazole (1 mL, 1.03 g, 12.5 mmol,
Fluka) in acetone (10 mL) at RT. The mixture was stirred at 40 ꢁC
overnight to give a white suspension. It was ltered to remove
white solid, which is the bis-substituted by-product. The solid
was washed with acetone and discarded. The combined ltrates
were rotor-evaporated to remove the acetone. Chromatography
was performed on silica (20 g) with CH2Cl2 to remove the
starting material and with 5–10% of CH3OH in CH2Cl2 to
recover the product (TLC were developed with I2; the bis-
substituted imidazolium salt follows the product). Viscous pale
yellow oil: 2.56 g (7.85 mmol, 63%; C10H18Br2N2; MW 326.07). 1H
NMR (400 MHz, DMSO-d6): d ¼ 9.12 (s, 1H), 7.79–7.75 (m, 1H),
7.72–7.68 (m, 1H), 4.15 (t, J ¼ 7.2 Hz, 2H), 3.84 (s, 3H), 3.52 (t,
J ¼ 6.8 Hz, 2H), 1.83–1.73 (m, 4H), 1.45–1.35 (m, 2H), 1.30–1.20
(m, 2H) ppm. ESI+ MS: m/z 245.3 ({M ꢀ Br}+, 100%).
Fig. 4 Time-dependence of electroluminescence spectrum, lumi-
nance, voltage and efficacy of optimized LEC (1 : 1 by mass of
NMS25:SPPO13 with 10 wt% FIrPic; 80 nm emitting layer; annealed at
100 ꢁC for 1 h; without IL; block-wave pulsed current; 1000 Hz; 50%
duty cycle; average density 75 A mꢀ2).
Table 2 CIE coordinates for optimized LEC (Fig. 4)
t (min)
CIE (x)
CIE (y)
lmax (nm)
3
5
10
30
60
0.214
0.217
0.219
0.230
0.242
0.377
0.379
0.380
0.389
0.400
473
474
474
473
474
Scaled-up synthesis. 1-Methylimidazole (2 mL, 2.06 g, 25
mmol) and 1,6-dibromohexane (15 mL, 24 g, 97 mmol, excess)
in acetone (25 mL) gave 6 g (18.4 mmol, 74%) of the product.
Chromatography was performed on silica (30 g) with 0–1%
CH3OH in CH2Cl2 to remove the starting material and with
7.5% CH3OH in CH2Cl2 to recover 1.
In the optimized device, FIrPic exhibits only small red-shi
of electroluminescence, that is, the host–guest LEC gives stable
blue electroluminescence during the 60 min of work (Fig. 4 and
Table 2). In contrast, in previous reports, the electrolumines-
cence of blue iTMC–LECs underwent signicant red-shi.33,34
Di-tert-butyl carbazole, 2. The reaction was performed under
argon in dry solvents. Carbazole (5 g, 29.9 mmol) was sus-
pended in CH2Cl2 (100 mL) at RT. AlCl3 (4 g, 29.9 mmol, Sigma-
Alꢁdrich) was added to give a brown suspension. It was cooled to
0 C and a solution of tert-butyl chloride (6.51 mL, 5.53 g, 59.8
Conclusions
Ionic derivatives of charge-transport hosts are successfully mmol, Aldrich) in CH2Cl2 (20 mL) was added drop-wise over 10
applied in host–guest LECs. The performance of these LECs min to give a dark yellow suspension. It was allowed to warm to
depends on the amount and type of the ions. For a blue LEC, we RT, and it was stirred overnight to give brown solution. It was
achieve high colour stability and a maximum efficacy of 5 cd Aꢀ1 cooled in an ice bath and very cautiously quenched with ice
at a luminance of 420 cd mꢀ2. One can modify neutral charge- [Caution: the quenching is highly exothermic and has an
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induction period (!); external cooling (ice bath) is recom- column; a yellow impurity follows the product; intensely col-
mended]. Extraction with water and CH2Cl2 provided at rst an oured by-products remain at the top of the column). Colourless
orange cloudy organic layer that aer multiple washings with or pale yellow oil which easily foams and slowly solidies to
water became pale yellow and clear. Hexane (100 mL) was added white or pale green solid: 256 mg (0.69 mmol, 59%; C26H29NO;
and CH2Cl2 was rotor-evaporated to provide a suspension of the MW 371.51). The yield can be improved by extending the reac-
product in hexane. It was cooled to RT. The solid was ltered tion time. 1H NMR (400 MHz, DMSO-d6): d ¼ 9.9 (s, br, 1H, OH),
(the ltrate was yellow) and washed with a small volume of 8.27 (d, J ¼ 1.6 Hz, 2H), 7.47 (dd, J ¼ 8.8, 2.0 Hz, 2H), 7.43 (t, J ¼
hexane. White crystalline solid: 2.90 g (10.4 mmol, 35%). Anal. 8.0 Hz, 1H), 7.32 (dd, J ¼ 8.8, 0.4 Hz, 2H), 6.99 (ddd, J ¼ 8.0, 2.0,
calcd for C20H25N (MW 279.42): C, 85.97; H, 9.02; N, 5.01. Found: 0.8 Hz, 1H), 6.93 (t, J ¼ 2.0 Hz, 1H), 6.88 (ddd, J ¼ 8.0, 2.4, 0.8
1
C, 86.06; H, 8.94; N, 4.98. H NMR (400 MHz, DMSO-d6): d ¼ Hz, 1H), 1.41 (s, 18H) ppm. Solid 4 turns green upon long
10.90 (s, 1H), 8.12 (d, J ¼ 1.2 Hz, 2H), 7.41 (dd, J ¼ 8.4, 1.6 Hz, exposure to air at RT, but it can be safely stored for indenite
2H), 7.34 (d, J ¼ 8.4 Hz, 2H), 1.39 (s, 18H) ppm. 13C NMR (100 time in a freezer.
MHz, CDCl3): d ¼ 142.5, 138.3, 123.8, 123.5, 116.4, 110.3, 35.0,
32.3 ppm. ESI+ MS: m/z 280.3 ({M + H}+, 100%).
Scaled-up synthesis. Aryl carbazole 3 (2.12 g, 5.49 mmol) and
Py$HCl (40 g, 0.35 mol) were stirred at 200 C for 7.5 h. Chro-
ꢁ
Aryl carbazole, 3. The reaction was performed under argon in matography on silica (30 g) gave 1.25 g (3.36 mmol, 61%) of 4.
dry solvents.35 3-Iodoanisole (0.45 mL, 0.88 g, 3.77 mmol, small
Scaled-up synthesis. Aryl carbazole 3 (4.6 g, 11.9 mmol) and
excess, Aldrich), di-tert-butyl carbazole 2 (1 g, 3.58 mmol), Py$HCl (80 g, 0.69 mol) were stirred at 200 ꢁC for 8 h. Chro-
Cs2CO3 (3.7 g, 11 mmol, excess, Aldrich), Cu2O powder (<5 mm, matography on silica (30 g) gave 3.18 g (8.56 mmol, 72%) of 4.
74 mg, 0.52 mmol, catalyst, Aldrich) in DMF (4.5 mL; 99.8%,
extra dry over molecular sieve, AcroSeal, Acros) were stirred at
Carbazole host, NMS25
Batch 1. The reaction was performed under argon in dry
120 ꢁC for 24 h to give a brownish-green suspension. It was solvents. Phenol-carbazole 4 (241 mg, 0.65 mmol), imidazolium
cooled to RT, and it was extracted with water and ether (dark salt 1 (212 mg, 0.65 mmol), and K2CO3 (90 mg, 0.65 mmol) were
brown solid remains at the phase interface, and the aqueous stirred in DMF (4 mL; 99.8%, extra dry over molecular sieve,
phase remains a brown suspension). The organic layer was AcroSeal, Acros) at60 ꢁCfor 24hto givea white suspension. Itwas
washed with water and evaporated. Chromatography was per- diluted with water (100 mL) containing KPF6 (for anion
formed on silica (15 g) with hexane–CH2Cl2 (6/1) to provide the exchange; 0.55 g, 2.99 mmol, excess). The resulting suspension
crude product as colourless oil (the starting carbazole is one of was stirred for 30 min and ltered. The solid was washed with
the main impurities). It was dissolved in a minimum volume (5 water and hexane. It was extracted with water–CH2Cl2. The
mL) of CH2Cl2. Ethanol (15 mL) was added and CH2Cl2 was organic layer was washed with water and evaporated. Chroma-
rotor-evaporated leaving a suspension of the product in ethanol tography was performed on silica (15 g) with 0.5% CH3OH in
(the compound easily forms oversaturated ethanol solution; CH2Cl2 to remove impurities and with 2% CH3OH in CH2Cl2 to
sonication, scratching and seeding may be required to induce recover the pure product. The product may separate as thick and
ꢁ
the precipitation). The suspension was cooled to 0 C, ltered, easily foaming oil; in this case, mixing with ether and sonication
and washed with a small volume of cold (ꢀ15 ꢁC) ethanol. White followed by evaporation of the solvent converts the oil to a solid.
solid: 927 mg (2.40 mmol, 67%; C27H31NO; MW 385.54). 1H NMR White solid: 300 mg (0.44 mmol, 68%). Anal. calcd for
(400 MHz, CDCl3): d ¼ 8.16 (d, J ¼ 2.0 Hz, 2H), 7.56–7.46 (m,
C36H46F6N3OP (MW 681.73): C, 63.42; H, 6.80; N, 6.16. Found: C,
3H), 7.41 (d, J ¼ 8.8 Hz, 2H), 7.20–7.16 (m, 1H), 7.13 (t, J ¼ 2.0 63.66; H, 6.84; N, 6.24. 1H NMR (400 MHz, CD2Cl2): d ¼ 8.55 (s,
Hz, 1H), 7.00 (dd, J ¼ 8.4, 2.4 Hz, 1H), 3.88 (s, 3H), 1.50 (s, 18H) 1H), 8.18 (d, J ¼ 2.0 Hz, 2H), 7.55–7.47 (m, 3H), 7.40 (d, J ¼ 8.4 Hz,
ppm. 3 is soluble in hexane.
2H), 7.25 (s, br, 1H), 7.22 (s, br, 1H), 7.17 (dd, J ¼ 7.6, 1.2 Hz, 1H),
Scaled-up synthesis. 3-Iodoanisole (1.8 mL, 3.53 g, 15.1 mmol), 7.09 (t, J ¼ 2.4 Hz, 1H), 7.00 (dd, J ¼ 8.0, 2.0 Hz, 1H), 4.20 (t, J ¼ 7.2
di-tert-butyl carbazole 2 (4 g, 14.3 mmol), Cs2CO3 (17 g, 52 Hz, 2H), 4.04 (t, J ¼ 6.0 Hz, 2H), 3.94 (s, 3H), 2.01–1.91 (m, 2H),
mmol), Cu2O powder (296 mg, 1.55 mmol) in DMF (12 mL; 1.90–1.80 (m, 2H), 1.65–1.40 (m, 4H, obscured by signal of water
99.8%, extra dry over molecular sieve, AcroSeal, Acros) were protons), 1.48 (s, 18H) ppm. 1H NMR (400 MHz, DMSO-d6): d ¼
stirred at 120 ꢁC for 24 h. Chromatography on silica (20 g) and 9.08 (s, 1H), 8.28 (d, J ¼ 2.0 Hz, 2H), 7.76 (t, J ¼ 1.6 Hz, 1H), 7.67 (t,
re-crystallization from CH2Cl2 and ethanol (20 mL) gave 4.6 g J ¼ 1.6 Hz, 1H), 7.54 (t, J ¼ 8.0 Hz, 1H), 7.47 (dd, J ¼ 8.4, 1.6 Hz,
(11.9 mmol, 83%) of 3.
2H), 7.32 (d, J ¼ 8.4 Hz, 2H), 7.15 (d, J ¼ 7.6 Hz, 1H), 7.10 (t, J ¼ 2.0
Phenol-carbazole, 4. The reaction was performed under Hz, 1H), 7.04 (dd, J ¼ 8.4, 2.4 Hz, 1H), 4.15 (t, J ¼ 6.8 Hz, 2H), 4.04
argon. Aryl carbazole 3 (450 mg, 1.17 mmol) and pyridine (t, J ¼ 6.4 Hz, 2H), 3.82 (s, 3H), 1.86–1.69 (m, 4H), 1.41 (s, 18H),
hydrochloride (Py$HCl, 15 g, 0.13 mol, excess, ꢁused as a reagent 1.52–1.20 (m, 2H), 1.37–1.26 (m, 2H) ppm. 13C NMR (100 MHz,
and as a solvent, Aldrich) were stirred at 200 C for 6 h to give DMSO-d6): d ¼ 160.6, 143.2, 139.2, 139.2, 137.2, 131.5, 124.3,
pale green solution. It was cooled to RT. The solid reaction 124.3, 123.6, 122.9, 118.9, 117.3, 114.3, 112.7, 109.8, 68.3, 49.4,
mixture was extracted with CH2Cl2–water. The organic layer was 36.4, 35.1, 32.5, 30.0, 29.1, 26.0, 25.6 ppm. ESI+ TOF MS: m/z
thoroughly washed with water to remove Py$HCl. Evaporation 536.36 ({M ꢀ PF6}+, 100%).
provided pale-violet foam. Chromatography was performed on
Batch 2, scaled-up synthesis. Phenol-carbazole 4 (1.25 g, 3.36
silica (12 g) with hexane–CH2Cl2 (3/1) to remove the starting mmol), imidazolium salt 1 (1.10 g, 3.37 mmol), and K2CO3 (0.47
material and with hexane–CH2Cl2 (1/2) to recover the product as g, 3.40 mmol) in DMF (8 mL; 99.8%, extra dry over molecular
a colourless fraction (the fraction may look pale yellow on the sieve, AcroSeal, Acros) at 60 ꢁC for 24 h gave a pale orange
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Journal of Materials Chemistry C
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suspension. Work-up with KPF6 (for anion exchange; 4 g, 22 voltage and the luminance simultaneously by a True Colour
mmol) and chromatography on silica (35 g) with 0.5–2.0% Sensor MAZeT (MTCSICT Sensor) using a Lifetime Test System
CH3OH in CH2Cl2 (pale green impurity precedes the product) designed by BoTEST (Botest OLT OLED Lifetime-Test System).
gave 1.45 g of the product that contained a small amount of Electroluminescence spectra were recorded with an Avantes
impurity. The product was dissolved in CH2Cl2 (5 mL) and ber-optics photo-spectrometer. The devices were not encap-
poured into ether (>200 mL). A solution formed at rst, but a sulated and were characterized inside the glovebox.
ne dense precipitate appeared aer 1 min of stirring. It was
ltered and washed with ether to provide the pure product: 1.38
g (2.03 mmol, 60%).
Acknowledgements
Batch 3, scaled-up synthesis. Phenol-carbazole 4 (3.18 g, 8.56 This work is supported by the European Union (CELLO, STRP
mmol), imidazolium salt 1 (2.79 g, 8.56 mmol), and K2CO3 (1.20 248043; http://www.cello-project.eu/) and the Spanish Ministry
g, 8.68 mmol) in DMF (9 mL; 99.8%, Extra Dry over Molecular of Economy and Competitiveness (MINECO) (MAT2011-24594,
Sieve, AcroSeal, Acros) at 60 ꢁC for 24 h gave a pale orange CSD2007-00010, and CTQ2009-08790). A.P. and D.T. acknowl-
suspension. Work-up was performed with KPF6 (for anion edge the support of a FPI and FPU grant of the MINECO and
exchange; 13 g, 71 mmol) and by chromatography on silica (30 MECD, respectively.
g) with 0.5–2.0% CH3OH in CH2Cl2. The product was dissolved
in CH2Cl2 (15 mL) and poured into ether (>350 mL). A solution
formed at rst, but a ne dense precipitate appeared aer 1 min
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