Synthesis, structure, properties, and application of a carbazole-based diaza[7]helicene in a deep-blue-emitting OLED

Article abstract of DOI:10.1002/chem.201200068

A carbazole-based diaza[7]helicene, 2,12-dihexyl-2,12-diaza[7]helicene (1), was synthesized by a photochemical synthesis and its use as a deep-blue dopant emitter in an organic light-emitting diode (OLED) was examined. Compound 1 exhibited good solubility and excellent thermal stability with a high decomposition temperature (T_d=372.1 °C) and a high glass-transition temperature (T_g, up to 203.0 °C). Single-crystal structural analysis of the crystalline clathrate (1)₂· cyclohexane along with a theoretical investigation revealed a non-planar-fused structure of compound 1, which prevented the close-packing of molecules in the solid state and kept the molecule in a good amorphous state, which allowed the optimization of the properties of the OLED. A device with a structure of ITO/NPB (50 nm)/CBP:5 % 1 (30 nm)/BCP (20 nm)/Mg:Ag (100 nm)/Ag (50 nm) showed saturated blue light with Commission Internationale de L'Eclairage (CIE) coordinates of (0.15, 0.10); the maximum luminance efficiency and brightness were 0.22 cd A^-1 (0.09 Lm W^-1) and 2365 cd m^-2, respectively. This new class of helicenes, based on carbazole frameworks, not only opens new possibilities for utilizing helicene derivatives in deep-blue-emitting OLEDs but may also have potential applications in many other fields, such as molecular recognition and organic nonlinear optical materials. Copyright

Full text of DOI:10.1002/chem.201200068

DOI: 10.1002/chem.201200068
Synthesis, Structure, Properties, and Application of a Carbazole-Based
Diaza[7]helicene in a Deep-Blue-Emitting OLED
Longqiang Shi,[a] Zhi Liu,*[a] Guifang Dong,[b] Lian Duan,[b] Yong Qiu,[b] Jiong Jia,[c]
Wei Guo,[a] Dan Zhao,[a] Deliang Cui,[a] and Xutang Tao[a]
Abstract: A carbazole-based diaza[7]-
helicene, 2,12-dihexyl-2,12-diaza[7]heli-
cene (1), was synthesized by a photo-
chemical synthesis and its use as
a deep-blue dopant emitter in an or-
ganic light-emitting diode (OLED) was
examined. Compound 1 exhibited good
solubility and excellent thermal stabili-
ty with a high decomposition tempera-
ture (Td =372.18C) and a high glass-
transition temperature (Tg, up to
203.08C). Single-crystal structural anal-
ysis of the crystalline clathrate
(1)2·cyclohexane along with a theoreti-
cal investigation revealed a non-planar-
fused structure of compound 1, which
prevented the close-packing of mole-
cules in the solid state and kept the
molecule in a good amorphous state,
which allowed the optimization of the
properties of the OLED. A device with
Internationale de LꢀEclairage (CIE)
coordinates of (0.15, 0.10); the maxi-
mum luminance efficiency and bright-
ness were 0.22 cdAꢀ1 (0.09 LmWꢀ1)
and 2365 cdmꢀ2, respectively. This new
class of helicenes, based on carbazole
frameworks, not only opens new possi-
bilities for utilizing helicene derivatives
in deep-blue-emitting OLEDs but may
also have potential applications in
many other fields, such as molecular
recognition and organic nonlinear opti-
cal materials.
a
structure of ITO/NPB (50 nm)/
CBP:5% (30 nm)/BCP (20 nm)/
1
Mg:Ag (100 nm)/Ag (50 nm) showed
saturated blue light with Commission
Keywords: deep-blue emission
·
helical structures · luminescence ·
OLEDs · photochemistry
Introduction
been a few reported deep-blue emitters and the realization
of highly efficient OLEDs remains a big challenge because
blue light is generated from wide-gap excited states.[3] It is
well-known that the doping approach, which uses considera-
ble amounts of blue-emitting dopants, is an effective strat-
egy for improving device performance in terms of electrolu-
minescence (EL) efficiency, pure emissive color, and opera-
tional lifetime.[4] Moreover, to avoid current leakage from
the device, a homogeneous film without grain boundaries or
pin holes is required.[5] To achieve this result, the introduc-
tion of suitable lengths of alkyl substituents onto organic
molecules is a key approach that can also ensure the solubil-
ity of these molecules in solvents for the fabrication of devi-
ces in solution.[6]
Over the past couple of decades, organic light-emitting
diodes (OLEDs) have been of great interest as promising
candidates for applications in displays and solid-state light-
ing.[1] To meet the requirement of full-color displays, suita-
ble blue-light-emitting materials with saturated color puri-
ties, high solid-state photoluminescence (PL) quantum
yields, and good stabilities still require further development
to match the advancements in red- and green emitters.[2] An
ideal blue-light material would have CIE coordinates of
(0.14, 0.08), as specified in the National Television System
Committee (NTSC) standard. Until now, there have only
Helicene derivatives are a class of chiral helical molecules
that are structurally characterized by their ortho-fusion of
aromatic rings. Their unique structures and properties have
led to them being extensively explored for use in various ap-
plications, such as asymmetric catalysis,[7] enantioselective
fluorescent sensors,[8] chiral discotic liquid-crystalline materi-
als,[9] and organic materials that have nonlinear optical prop-
erties;[10] however, their use as emitters in OLEDs has been
rarely reported.[11] Their twisted and non-coplanar helical
configuration can prevent close-packing interactions, there-
by effectively relieving excited-state quenching. Further-
more, their highly fused conjugated geometry could extend
the p-system, which could balance the efficiency and
charge-transport in OLEDs. Therefore, helicenes are prom-
ising candidates for emitting materials.
[a] L. Shi, Prof. Dr. Z. Liu, W. Guo, D. Zhao, Prof. Dr. D. Cui,
Prof. Dr. X. Tao
State Key Laboratory of Crystal Materials
Shandong University, Jinan, 250100 (P. R. China)
Fax : (+86)531-8836-2135
E-mail: lz@sdu.edu.cn
[b] Prof. Dr. G. Dong, Dr. L. Duan, Prof. Dr. Y. Qiu
Key Lab of Organic Optoelectronics &
Molecular Engineering of Ministry of Education
Department of Chemistry, Tsinghua University
Beijing 100084 (P.R. China)
[c] Dr. J. Jia
School of Chemistry and Chemical Engineering
Shandong University, Jinan, 250100 (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200068.
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FULL PAPER
The carbazole group is a popular functional unit in conju-
Results and Discussion
ꢀ
gated systems owing to its good planarity and N H bond,
which can be easily substituted for other functional
groups.[12] Herein, by using carbazole as the starting materi-
al, we synthesized a nitrogenous helicene that was com-
prised of seven rings, namely 2,12-dihexyl-2,12-diaza[7]heli-
cene (1), by using a traditional oxidative-photocyclization
method (Scheme 1). Photocyclized compound 1 was ob-
Synthesis and characterization: The synthesis of compound
1 is shown in Scheme 1. First, compound 2 was formylated
to give compound 3 by a Vilsmeier–Haack reaction with
DMF and POCl3. Then, compound 3 was subjected to
a McMurry coupling reaction with TiCl4, Zn powder, and
pyridine in THF under an argon atmosphere to obtain com-
pound 4. Finally, the target compound (1) was successfully
achieved by oxidative photocyclization using compound 4,
I2, and propylene oxide in benzene under irradiation from
a Hanovia high-pressure mercury lamp. The compounds
1
were characterized by H NMR, 13C NMR, and FTIR spec-
troscopy, MS, elemental analysis, and single-crystal X-ray
diffraction.
Simple syntheses and satisfactory yields in each step
would allow compound 1 to be synthesized on a large scale.
Indeed, photocyclized compound 1 was obtained in an ex-
ceedingly short reaction time (about 10 min) and high yield
(80.5%), which afforded us with an efficient and convenient
route for synthesizing other diaza[7]helicene derivatives. Im-
portantly, compound 1 was readily soluble in common or-
ganic solvents, owing to its hexyl substituents, as well as its
non-coplanar helical structure. Such good solubility not only
overcame the formidable synthetic problem of long heli-
cenes, which typically form highly rigid and annelated struc-
tures,[13] but also made it possible for us to afford homogene-
ous films[5] and the fabrication of devices in solution.[6]
Scheme 1. Synthesis of compound 1.
tained within a very short reaction time (about 10 min) in
high yield (80.5%). Such an efficient and convenient reac-
tion rendered it feasible to synthesize other carbazole-based
diaza[7]helicene derivatives. We confirmed that compound
1 exhibited good solubility in common organic solvents,
which overcame the formidable synthetic problem of long
helicenes, which typically form highly rigid and annelated
structures.[13] Meanwhile, compound 1 showed excellent
thermostability, with a decomposition temperature (Td) of
372.18C and a high glass-transition temperature (Tg) of up
to 203.08C. Single-crystal structure analysis and theoretical
investigation revealed a non-planar-fused structure of com-
pound 1, which decreased the close-packing arrangement
and kept the molecule in a good amorphous state. These fa-
vorable properties enabled the optimization of the electrolu-
minescent behavior of the OLED. An OLED device with
the structure ITO/NPB (50 nm)/CBP:5% 1 (30 nm)/BCP
(20 nm)/Mg:Ag (100 nm)/Ag (50 nm) was fabricated and
emitted saturated blue light with CIE coordinates of (0.15,
0.10), which were quite close to the NTSC standard blue
CIE coordinates, and maximum luminance efficiency and
brightness of 0.22 cdAꢀ1 (0.09 LmWꢀ1) and 2365 cdmꢀ2, re-
spectively. Moreover, by slow evaporation from a mixture of
cyclohexane and CH2Cl2, we fortunately obtained the crys-
talline clathrate, that is, (1)2·cyclohexane, which has great
potential in various supramolecular-chemistry applications,
such as crystal engineering[14] and molecular recognition.[15]
Carbazole-based helicenes may allow us to utilize helicene
derivatives in deep-blue OLEDs and in many other fields,
such as molecular recognition and organic nonlinear optical
materials.
Crystal-structure analysis: A yellow bulk crystal (see the
Supporting Information, Figure S1) was obtained by crystal-
lization of compound 1 from CH2Cl2/cyclohexane. The mo-
lecular structure and its packing arrangement are given in
Figure 1. The crystal data and intensity collection parame-
ters are summarized in Table 1. Unexpectedly, XRD showed
that this sample was a host-guest complex with a 2:1 clath-
rate stoichiometry ((1)2·cyclohexane), rather than a single
crystal of compound
1 (Figure 1a). More specifically,
a right-handed helicene interacted with a left-handed heli-
cene through various short contacts to form a porous struc-
ture and molecules of cyclohexane were inserted into the
holes (Figure 1b). This finding has important implication be-
cause clathrates have great potential in supramolecular-
chemistry applications, such as crystal engineering[14] and
molecular recognition.[15] The dihedral angles between two
planar carbazole segments in right-handed (containing N1,
N2) and left-handed helicenes (containing N3, N4) were
27.48 and 30.18, respectively. The non-coplanar-fused crystal
structure of compound 1 impeded the close-packing ar-
rangement and kept the molecule in a good amorphous
state. Moreover, there were no classical hydrogen-bonding
ꢀ
or classical face-to-face p p interactions between the neigh-
boring molecules owing to non-coplanar fused rings and
a steric effect of the long substituted alkyl chains.
Thermal properties: Next, we investigated the thermal prop-
erties of the recrystallized clathrate of compound 1,
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Z. Liu et al.
Figure 2. TG curve of (1)2·cyclohexane, recorded under a nitrogen atmos-
phere, in the temperature range 25–6008C at
a heating rate of
108Cminꢀ1
.
porting Information, Figure S10). A broad endothermic
peak was observed at 96.08C, which corresponded to the de-
solvation of cyclohexane. Indeed, the observed mass-loss
value of 7.42% (Figure 2) was in agreement with the calcu-
lated weight percentage of cyclohexane in (1)2·cyclohexane
(7.43%). The desolvation temperature was much higher
than the normal boiling point of cyclohexane (80.78C),
which indicated that guest molecules were rather strongly
held within the compact host lattice. This clathrate showed
no melting point but instead showed a high glass-transition
temperature at about 203.08C (Tg) from the first to the third
heating processes. The TG curve showed that host molecule
1 possessed a high thermal-decomposition temperature (Td,
corresponding to 5% weight loss) of 372.18C. It should be
emphasized that a high Td value is a great asset to OLED
applications, because Joule heating occurs under typical op-
erating conditions.[16] These thermal analysis data confirmed
that compound 1 was thermally stable and suitable for
vacuum thermal sublimation for OLED fabrication.
Figure 1. a) Molecular structure of (1)2·cyclohexane; ellipsoids set at 30%
probability. b) Crystal-packing diagram along the crystallographic a axis;
hydrogen atoms are omitted for clarity.
Table 1. Crystallographic
data
and
structure
refinement
of
(1)2·cyclohexane.
(1)2·cyclohexane
empirical formula
formula weight
crystal system
space group
a [ꢀ]
ACHTUNGTRENNUNG
monoclinic
P21/c
Optical and electrochemical properties: The UV/Vis absorp-
tion and photoluminescence (PL) spectra of compound 1 in
CH2Cl2 (1ꢂ10ꢀ5 m) and in the thin film are shown in Fig-
ure 3a. The UV/Vis spectra were characterized by several
structured bands between 280 and 450 nm. Specifically, the
absorption spectrum of compound 1 in CH2Cl2 contained
two maxima (300 and 328 nm), one medium peak (313 nm),
two shoulders (348 and 363 nm), and two minima (410 and
436 nm). Similar absorption behavior was found in the thin
film except that the corresponding peaks and shoulders
were red-shifted by about 5–8 nm. The main absorption
peaks of compound 1 were blue-shifted compared with
those of linear carbazole dimers 4 (see the Supporting Infor-
mation, Figure S11) and with literature data.[17] This shift
suggested that the twisted and non-coplanar helical configu-
ration prevented electron-delocalization to some extent.
Compound 1 in CH2Cl2 emitted pure blue fluorescence with
emission peaks at 444 and 471 nm and a shoulder peak at
498 nm. By comparison, the PL spectrum of the neat film
showed only a small red-shift (about 8 nm), thereby indicat-
ing the formation of weak J-aggregates in the solid state. We
concluded that neither specific nor additional intermolecular
interactions that involved the p-systems occurred when the
8.3476(4)
27.5396(14)
28.7473(15)
93.4150(10)
6597.0(6)
4
1.138
0.065
2436
b [ꢀ]
c [ꢀ]
b [8]
V [ꢃ3]
Z
1calcd [gcmꢀ3
]
m (MoKa) [mmꢀ1
F (000)
]
crystal size [mm]
2qmax [8] (completeness)
total reflns
0.50ꢂ0.45ꢂ0.10
54.8 (99.4%)
40277
unique reflns
14897 [RACTHNGUTER(NNUG int)=0.0252]
data/restraints/parameters
14897/0/779
1.216
GOF, F2
final R indices [I>2s(I)]
R indices (all data)
largest diff. peak/hole [eꢃꢀ3
R1=0.0551, wR2=0.1669
R1=0.0803, wR2=0.1840
0.747/ꢀ0.320
]
(1)2·cyclohexane. Thermal transitions and stability were in-
vestigated by simultaneous TG-DSC. DSC scans were per-
formed in the temperature range 25–2408C for the first
heating and cooling cycle, from 25–2508C for the second
cycle, and from 25–2608C for the third cycle (see the Sup-
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Carbazole-Based Diaza[7]helicene in a Deep-Blue-Emitting OLED
FULL PAPER
Figure 4. CV of compound 1 in CHCl3 (0.1m Bu4NClO4) at a scanning
rate of 200 mVsꢀ1
.
2,FOC)ꢀ4.8 eV.[20] Correspondingly, the energy of the LUMO
level, as calculated by subtraction of the optical band gap
(2.79 eV) from the HOMO level, was ꢀ2.27 eV.
Quantum-chemical calculations: To gain an insight into the
electronic properties of compound 1, quantum-chemical cal-
culations were performed by using the Gaussian 09 pro-
gram.[21] The geometry was optimized by using density func-
tional theory (DFT) with the B3LYP function and the 6-
31G(d) basis sets. The optimized geometry, molecular orbi-
tals, and energy diagrams are shown in Figure 5. As de-
Figure 3. a) Normalized absorption and PL spectra of compound 1 in
CH2Cl2 and in a thin film. b) Normalized absorption and PL spectra of
compound 1 in different solutions (1ꢂ10ꢀ5 m). All of the PL emission
spectra were excited at 325 nm.
film was formed because both the UV/Vis absorption and
the PL spectra in the film state were identical to that in so-
lution. Moreover, small Stokes’ shifts (about 10 nm) and
good mirror images between the absorption and fluores-
cence spectra revealed almost no conformational changes
between the ground state and the excited state. Moreover,
the absorption and PL emission spectra of compound 1 in
different solvents exhibited solvent-polarity independence
and PL quantum yields of 9–10% (Figure 3b; also see the
Supporting Information, Table S12). Absorption at long
wavelengths when the PL spectra were excited at 325 nm—
thereby generating intermolecular energy transfer—was de-
termined to be the main reason for the low quantum yields.
The LUMO/HOMO energy gap was estimated from the ab-
sorption edge of the optical absorption spectra.[18] From the
spectrum of the thin film, the optical band gap of compound
1 was 2.79 eV.
Cyclic voltammetry (CV) was used to identify the electro-
chemical behavior of compound 1 (Figure 4). A convention-
al three-electrode cell with a Pt working electrode, a Ag
wire counter electrode, and a Ag/AgCl reference electrode
was used. The potential of the Ag/AgCl reference electrode
in CHCl3 (0.1m Bu4NClO4) was calibrated by using the fer-
rocene/ferrocenium (Fc/Fc+) redox system (ꢀ4.8 eV with re-
spect to the vacuum level).[19] In this arrangement, the half-
wave potential of Fc/Fc+ (E1/2,FOC) was 0.269 V versus Ag/
AgCl. The oxidation peaks of compound 1 were at 0.658
and 1.086 V and its onset oxidation potential of 0.53 V
(Eonset(ox)) was adopted to determine the HOMO level. Thus,
the HOMO level of compound 1 was estimated to be
Figure 5. a) Optimized geometry of compound 1. b) Molecular orbitals
and energy diagrams [eV] of compound 1, calculated by using the
B3LYP/6-31G(d) method.
scribed above, the molecular geometry was divided into two
planar segments, containing five aromatic rings at one end
and two at the other end, by the dihedral angle, and the dis-
tribution of the HOMO revealed that the delocalization of
the electrons was on the main part of the five-membered
plane owing to its extended p-electron system and good pla-
narity, whilst the LUMO was spread over the whole con-
densed nucleus and exhibited limited delocalization.
Vertical electronic transitions for compound 1 were calcu-
lated by using TD-DFT methods at the B3LYP/6-31G(d)
level. The transition energies, oscillator strength, and assign-
ments for the most-relevant singlet excited states are listed
in the Supporting Information, Table S13. The UV/Vis spec-
tra were calculated by using the SWizard program, revision
ꢀ5.06 eV by using the equation HOMO=ꢀe
N
4.6,[22] with the Gaussian model. The half-bandwidths (D1/2,I
)
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Z. Liu et al.
Figure 6. TD-DFT-calculated electronic spectrum of compound 1 com-
pared with the experimental electronic spectrum (in CH2Cl2, 10ꢀ5 m).
were taken to be equal to 3000 cmꢀ1 and the simulated spec-
trum (Figure 6) accurately reproduced the experimental
data. These results indicated that the most-intense absorp-
tion band at 332 nm (30100 cmꢀ1) was mainly due to the
HOMO!LUMO+1 transition, whilst the HOMO!LUMO
transition at 409 nm (24500 cmꢀ1) was characteristic of a for-
bidden transition and possessed a small oscillator strength
of 0.0320, which we believed to be unfavorable for efficient
fluorescence. The absorptions with calculated absorption
wavelengths of 290 nm (34200 cmꢀ1) and 359 nm
(27900 cmꢀ1) corresponded to the HOMOꢀ2!LUMO+1
and HOMOꢀ1!LUMO electronic excitations, respectively.
Figure 8. a) EL spectrum of the device at 8 V compared with the PL
spectrum in CH2Cl2. b) EL spectra of the device at different driving vol-
tages.
the PL spectrum of compound 1 in CH2Cl2 (Figure 8). This
result suggested that the host–guest CBP/1 system was just
like a solid solution, thereby maintaining compound 1 in
a good amorphous state. As shown in Figure 9, the corre-
Electroluminescence properties: The device properties of
the blue material (1) were examined by doping compound
1 in a CBP host at a concentration of 5% with the following
structure: ITO/NPB (50 nm)/CBP:5%
1
(30 nm)/BCP
(20 nm)/Mg:Ag (100 nm)/Ag (50 nm); the device configura-
tion and the energy diagram are shown in Figure 7. The EL
properties are summarized in Table 2. The device emitted
pure blue light (lmax =446 nm), which was consistent with
Figure 9. CIE coordinates of the OLED along with the NTSC blue.
sponding CIE coordinates were (0.15, 0.10) at a voltage of
8 V, which was quite close to the NTSC blue standard. With
a turn-on voltage of 5.30 eV, the device attained a maximum
brightness of 2365 cdmꢀ2 (Figure 10a), maximum current ef-
ficiency of 0.22 cdAꢀ1, and
a
power efficiency of
Figure 7. a) Device configuration: ITO/NPB (50 nm)/CBP:5% 1 (30 nm)/
BCP (20 nm)/Mg:Ag (100 nm)/Ag (50 nm). b) Energy-level diagram of
the OLED components (relative to the vacuum energy level).
0.09 LmWꢀ1 (Figure 10b), which were better in some as-
pects than previously reported carbazole-based EL materi-
als.[23] However, the external
quantum efficiency (EQE) of
Table 2. EL properties of compound 1 based device.
[a]
[b]
[c]
[f]
the device (0.23%) was inferior
to those of a handful of deep-
blue-emitting OLEDs. For ex-
ample, Bryce, Petty, and co-
workers reported that, by using
a carbazole moiety as the donor
Vonset [V]
Bmax [cdmꢀ2
]
hc,max [cdAꢀ1
]
hp,max[d] [lmWꢀ1
]
CIE[e] [x,y]
lmax[e] [nm]
hext,max [%]
5.30
2365
0.22
0.09
(0.15, 0.10)
446
0.23
[a] Vonset: Turn-on voltage at a brightness of 1 cdmꢀ2; [b] Bmax: maximum brightness at 14.6 V; [c] hc,max: maxi-
mum current efficiency at 11.8 V; [d] hp,max: maximum power efficiency at 7.1 V; [e] CIE coordinates and lmax
values were obtained at the voltage of 8 V; [f] hext,max: maximum external quantum efficiency (EQE), calculat-
ed from the corresponding current efficiency.
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Carbazole-Based Diaza[7]helicene in a Deep-Blue-Emitting OLED
FULL PAPER
Ag (50 nm) emitted saturated blue light with CIE coordi-
nates of (0.15, 0.10). The device attained a brightness of
2365 cdmꢀ2 and
a maximum luminance efficiency of
0.22 cdAꢀ1 (0.09 LmWꢀ1). This class of carbazole-based hel-
icenes is promising for the application of helicene deriva-
tives in deep-blue-emitting OLEDs and in many other
fields, such as molecular recognition and organic nonlinear
optical materials.
Experimental Section
Chemicals and instruments: THF was distilled over sodium/benzophe-
none before use. Other reagents and chemicals were commercially avail-
able and used as received. Hexylcarbazole (2) was synthesized according
to a literature procedure.[24] Photocyclization was performed by using
a Hanovia high-pressure mercury lamp (500 W). 1H NMR and 13C NMR
spectroscopy was performed on a Bruker Advance 300 spectrometer.
FTIR spectra were recorded on a Thermo-Nicolet NEXUS 670 spectrom-
eter (Thermo-Nicolet Company, USA). Matrix-assisted laser-desorption/
ionization time-of-flight (MALDI-TOF) MS were recorded on a Bruker
Biflex III mass spectrometer that was equipped with a 337 nm nitrogen
laser. HRMS spectra were recorded on a Q-TOF6510 spectrograph (Agi-
lent). UV/Vis spectra were recorded on a U-4100 (Hitachi). Elemental
analysis was performed on a German Vario EL III elemental analyzer.
X-ray diffraction intensity data were collected at 130 K on a Bruker
Smart Apex2 CCD area-detector diffractometer with graphite-monochro-
mated Mo Ka radiation (l=0.71073 ꢃ). Processing of the intensity data
was carried out by using the Bruker SMART routine and the structure
was solved by direct methods by using the SIR-97 program and refined
by a full-matrix least-squares technique based on F2 with the SHELXL-
97 program.[25]
Figure 10. a) Current-density–voltage–brightness (J–V–B) characteristics
of the device. b) Current-efficiency–current-density–power-efficiency
characteristics of the device.
group, an ambipolar molecule exhibited highly saturated
deep-blue emission with a maximum EQE of 4.7% because
of its balanced hole- and electron transport.[4e] We suggested
that the unbalanced carrier-transporting properties led to
this non-excellent device performance. The low hole-injec-
tion barrier (0.37 eV) between NPB and the light-emitting
layer facilitated hole transport, whilst a relatively high barri-
er (0.93 eV) hindered electrons transport from BCP to the
light-emitting layer. The EL and thermal stability, and the
interesting blue purity make carbazole-based helicene deriv-
atives promising candidates for applications in modern dis-
plays. Further study on the structure of the device and full
exploration of more carbazole-based helicene derivatives
would improve the performances of these OLEDs.
CCDC-860240 (1)2·cyclohexane contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data request/cif.
TG/DSC were measured on a SDT Q600 V8.0 Differential Scanning Cal-
orimeter under a nitrogen atmosphere with heating and cooling rates of
108Cminꢀ1. UV-absorption spectroscopy was performed on a TU-1800
spectrophotometer and photoluminescence (PL) measurements were col-
lected on a Hitachi F-4500 fluorescence spectrophotometer with a 150 W
Xe lamp. The concentration of the solutions was 10ꢀ5 m and the neat film
was prepared by using the spin-coating method. PL efficiencies were cal-
culated by using quinine sulfate in 0.1m H2SO4 as a reference.[26] Cyclic
voltammetry (CV) of compound 1 in 0.1m [Bu4N]ACHTNUTRGNE[UNG ClO4]CHCl3 solution
was carried out on a CHI660D electrochemical workstation. A conven-
tional three-electrode cell with a Pt working electrode, a Ag wire counter
electrode, and a Ag/AgCl reference electrode was employed. The experi-
ment was corrected by using the ferrocene/ferrocenium (Fc/Fc+) redox
system. DFT and TD-DFT calculations were performed by using the
Gaussian 09 program. The target molecule was assumed to be an isolated
molecule in the gas phase.
Conclusion
A new carbazole-based diaza[7]helicene, namely, 2,12-dihex-
yl-2,12-diaza[7]helicene (1), was prepared by a photochemi-
1
cal synthesis and characterized by H NMR, 13C NMR, and
FTIR spectroscopy, HRMS, and X-ray diffraction. Its ther-
mal, photophysical, electrochemical, and electroluminescent
behavior, along with its single-crystal structure and electron-
ic structure were also reported. Compound 1 exhibited good
solubility and excellent thermal stability. Absorption and PL
spectroscopic analysis, as well as a crystal structure of the
crystalline clathrate (1)2·cyclohexane, showed that the non-
planar conformation was beneficial for impeding the close-
packing interactions and keeping the molecule in a good
amorphous state. An OLED that incorporated compound
1 as the dopant emitter with the configuration ITO/NPB-
9-hexyl-9H-carbazole-3-carbaldehyde (3): POCl3 (1.17 mL, 12.55 mmol)
was added dropwise into DMF (1.13 mL, 14.60 mmol) at 08C under an
argon atmosphere. Then,
a solution of hexylcarbazole (2; 3.145 g,
12.50 mmol) in dry CHCl3 (7.5 mL) was added and the reaction mixture
was heated at reflux for 16 h and then cooled to RT. The mixture was
then poured into ice water and neutralized to pH 8 with sodium bicar-
bonate. The aqueous solution was extracted several times with CHCl3.
The CHCl3 solution was washed with water and dried with anhydrous
sodium sulfate. After filtration, the solvent was removed under reduced
pressure. The residue was purified by column chromatography on silica
gel (petroleum ether/EtOAc, 24:1 v/v) to give the product (2.551 g,
73.0%). 1H NMR (300 MHz, CDCl3, 258C, TMS): d=10.10 (s, 1H;
CHO), 8.62 (d, J=1.5 Hz, 1H; Ar-H), 8.16 (d, J=7.8 Hz, 1H; Ar-H),
8.01 (dd, J=8.6 Hz, 1H; Ar-H), 7.57–7.44 (m, 3H; Ar-H), 7.33 (m, 1H;
ACHTUNGTRENNUNG(50 nm)/CBP:5% 1 (30 nm)/BCP (20 nm)/Mg:Ag (100 nm)/
Chem. Eur. J. 2012, 18, 8092 – 8099
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8097

Z. Liu et al.
Ar-H), 4.34 (t, J=7.2 Hz, 2H; hexyl-H), 1.90 (m, 2H), 1.38–1.25 (m, 6H;
hexyl-H), 0.87 ppm (t, J=7.1 Hz, 3H; hexyl-H); 13C NMR (75 MHz,
CDCl3, 258C, TMS): d=191.22, 143.58, 140.68, 128.02, 126.63, 126.18,
123.46, 122.57, 122.51, 120.22, 119.77, 108.87, 108.42, 42.93, 30.99, 28.38,
26.40, 21.99, 13.44 ppm; FTIR (Smart iTR diamond ATR accessory): n˜ =
1688 cmꢀ1; elemental analysis calcd (%) for C19H21NO: C 81.68, H 7.58,
N 5.01; found: C 81.66, H 7.63, N 4.94.
deposited successively by thermal evaporation at a rate of 1 ꢃsꢀ1 under
5ꢂ10ꢀ4 Pa pressure. The EL spectrum and current-density–voltage–
brightness (J–V–B) characteristics were measured on a Photo Research
PR705 spectrophotometer and a Keithley 4200 semiconductor characteri-
zation system, respectively. All of the measurements were performed in
air at RT without encapsulation.
(E)-1,2-bis(9-hexyl-9H-carbazol-3-yl)ethene (4): To a suspension of Zn
powder (5.182 g, 79.2 mmol) in dry THF (200 mL) was added TiCl4
(4.42 mL, 40.21 mmol) by syringe at 08C under an argon atmosphere.
After addition of pyridine (3.2 mL), the suspension was heated at reflux
(808C) for 2 h. A solution of compound 3 (1.120 g, 4.01 mmol) in dry
THF (100 mL) was then added dropwise over 1.5 h to the gently reflux-
ing suspension. The resulting mixture was heated at reflux continuously
under stirring for 24 h, and, after cooling to RT, saturated aqueous
NaHCO3 (200 mL) was added. The reaction mixture was filtered and the
filtrate was extracted several times with CH2Cl2. Then, the CH2Cl2 solu-
tion was washed with water, dried with anhydrous sodium sulfate, fil-
tered, and concentrated under reduced pressure. The crude product was
purified by column chromatography on silica gel (petroleum ether/
CH2Cl2, 1:2 v/v) to give the product (0.655 g, 62.0%). M.p.: 1918C;
1H NMR (300 MHz, CDCl3, 258C, TMS): d=8.27 (d, J=1.2 Hz, 2H; Ar-
H), 8.15 (d, J=7.5 Hz, 2H; Ar-H), 7.72 (dd, J=8.6 Hz, 2H; Ar-H), 7.50–
7.44 (m, 2H; Ar-H), 7.39 (t, J=7.8 Hz, 6H; Ar-H), 7.28–7.22 (m, 2H;
Ar-H), 4.30 (t, J=7.4 Hz, 4H; hexyl-H), 1.88 (m, 4H; hexyl-H), 1.41–
1.29 (m, 12H; hexyl-H), 0.87 ppm (t, J=7.1 Hz, 6H; hexyl-H); 13C NMR
(75 MHz, CDCl3, 258C, TMS): d=140.86, 139.97, 129.22, 127.06, 125.66,
124.23, 123.00, 120.41, 118.85, 118.22, 116.93, 108.83, 108.12, 43.22, 31.59,
28.99, 26.98, 22.54, 13.99 ppm; FTIR (Smart iTR diamond ATR accesso-
ry): n˜ =3017, 1677 cmꢀ1; MS (MALDI-TOF): m/z: calcd for C38H42N2:
526.33; found: 526.1; elemental analysis calcd (%) for C38H42N2: C 86.65,
H 8.04, N 5.32; found: C 86.57, H 8.05, N 5.26.
Acknowledgements
This research was supported by the National Natural Science Foundation
of China (Grant Nos. 50990061, 51021062, and 21073107) and by the Nat-
ural Science Foundation of Shandong Province (No. ZR2010M017).
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2,12-dihexyl-2,12-diaza[7]helicene (1): Argon was bubbled through a solu-
tion of compound 4 (0.237 g, 0.45 mmol) and I2 (0.118 g, 0.46 mmol) in
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(1:10) to give the product (0.19 g, 80.5%).1H NMR (300 MHz, CDCl3,
258C, TMS): d=9.37 (s, 1H; Ar-H), 9.05 (d, J=8.1 Hz, 1H; Ar-H), 8.66
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1.94 (m, 4H; hexyl-H), 1.51–1.21 (m, 12H; hexyl-H), 0.90 (t, J=7.1 Hz,
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105.49, 43.02, 42.82, 31.12, 31.08, 28.69, 28.61, 26.54, 26.52, 22.14, 22.07,
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Carbazole-Based Diaza[7]helicene in a Deep-Blue-Emitting OLED
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Received: January 8, 2012
Published online: May 16, 2012
Chem. Eur. J. 2012, 18, 8092 – 8099
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8099