Non-aggregating solvatochromic bipolar benzo[f]quinolines and benzo[a]acridines for organic electronics

Article abstract of DOI:10.1039/c2jm31052j

A novel series of light emitting and thermally stable non-aggregating benzannulated quinolines and acridines were designed and prepared from ketene-S,S-acetal under mild conditions through C-C bond formation. Photophysical and electrochemical analyses of

Full text of DOI:10.1039/c2jm31052j

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PAPER
Non-aggregating solvatochromic bipolar benzo[f]quinolines and benzo[a]
acridines for organic electronics†
a
Atul Goel, Vijay Kumar,a Salil P. Singh,a Ashutosh Sharma,a Sattey Prakash,b Charan Singhb
*
and R. S. Anandb
Received 20th February 2012, Accepted 21st May 2012
DOI: 10.1039/c2jm31052j
A novel series of light emitting and thermally stable non-aggregating benzannulated quinolines and
acridines were designed and prepared from ketene-S,S-acetal under mild conditions through C–C bond
formation. Photophysical and electrochemical analyses of these bipolar N-heterocyclic compounds
revealed intense solvatochromism due to the intramolecular charge-transfer character, exemplary for
9b, which covered PL ranging from blue (480 nm) to green (501 nm) to yellow (562 nm) to orange (589
nm) using aprotic solvents of varying polarity. Organic light emitting devices with a device
configuration of ITO/PEDOT:PSS (40 nm)/NPB (20 nm)/(N-heterocyclic compound) (50 nm)/BCP
(7 nm)/LiF (0.7 nm)/Al (200 nm) were successfully prepared. The optoelectronic properties of these
compounds were altered by controlled tuning of donor–acceptor and aromatic p-conjugation in benzo
[f]quinolines and benzo[a]acridines, which exhibited a low turn-on voltage with electroluminescence
ranging from blue (6c: lEL 455 nm) to green (8a: lEL 496 nm) to yellow (11: lEL 545 nm) to red (9b: lEL
630 nm).
face or edge-to-face p–p stacking not only leads to bathochromic
shifts in photoluminescence but also reduces their quantum
Introduction
Over the past two decades, organic fluorophores have received
efficiencies.9,10 To inhibit such p–p stacking in the quinoline class
overwhelming scientific and technological attention due to their
of organic compounds, we designed non-planar partially
tremendous potential as bio-imaging agents,1 chemosensors,2
fluorescent probes3 and as electroluminescent materials for
hydrogenated benzo[f]quinolines, which have not received much
attention for OLED applications (Fig. 1). In order to enhance the
organic electronics, particularly for energy saving and harvesting
hole–electron recombination efficiency, a nice strategy is to equip
devices like thin-film transistors,4 photovoltaic cells,5 and organic
the molecule with bipolar transporting character such as elec-
light-emitting devices6 (OLEDs). Most of the organic fluo-
tron-donating and electron withdrawing moieties to permit the
rophores used for OLEDs are either based on small molecule
formation of both stable cation and anion radicals.11 We have
N-/S-/O-heterocyclic compounds, their metal complexes or
recently demonstrated how swapping of the electron donor,
polymers containing heterocyclic scaffolds.7
acceptor and chromophoric moieties onto an aromatic scaffold
After the discovery of 8-hydroxyquinoline aluminium
plays a crucial role in modulating the optical properties of
complexes (Alq3) as efficient emitting/electron transporting
fluorenes,12 fluoranthenes13 and pyrenylarenes,14 leading to
materials in OLEDs reported by Tang and VanSlyke,8 the
efficient light emitting materials.
development of electroluminescent quinoline derivatives and
their oligomers has accelerated enormously. Quinolines and
poly(quinoline)s have high electron mobilities, good thermal
stability, high photoluminescence efficiencies and good film
forming properties, which are crucial for their use in OLEDs.9
However the self-assembly of quinolines in a well-defined
architecture through non-covalent interactions such as face-to-
aDivision of Medicinal and Process Chemistry, Central Drug Research
Institute, CSIR, Lucknow 226001, India. E-mail: atul_goel@cdri.res.in
bDepartment of Electrical Engineering, Indian Institute of Technology,
Kanpur 208016, India
1
† Electronic supplementary information (ESI) available: H NMR, 13C
NMR and UV, FL, CV graphs of 6a–f, 8a,b, 9a,b, 10, 11. See DOI:
10.1039/c2jm31052j
Fig. 1 Design of nonplanar donor–acceptor benzannulated quinolines.
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Herein we report the efficient synthesis of non-aggregating
bipolar partially hydrogenated benzannulated quinoline deriva-
tives such as benzo[f]quinolines and benzo[a]acridines, their
interesting photophysical and electrochemical properties and
their potential use in organic light emitting diodes.
Therefore, we devised a new synthetic route to prepare novel
benzo[f]quinolines and benzo[a]acridines, as shown in Scheme 1.
To prepare newly designed molecules with donor–acceptor
characteristics, we found a key intermediate a-oxo-ketene-S,S-
acetal with electron acceptor groups suitable for preparing the
corresponding lactones 3 following the Tominaga protocol.18 In
order to incorporate a donor functionality such as the N,N-
dialkylamino functionality, a good leaving methylsulfanyl group
of lactones 3a–e was replaced by an amine moiety (piperidine/
pyrrolidine) to furnish 6-aryl-2-oxo-4-piperidin/pyrrolidin-1-yl-
2H-pyran-3-carbonitriles (4a–f) in good yields. The reactivity of
lactones 4a–f was explored by the Michael addition of a conju-
gate base of 7,8-dihydroquinolin-5(6H)-one (5)19 or 3,4-dihy-
droacridin-1(2H)-one20 (7a,b) to afford dihydro-benzo[f]
quinolines or 5,6-dihydro-benzo[a]acridines, respectively. Thus,
after optimizing the reaction conditions, stirring of an equimolar
mixture of 4a–f, N-heterocyclic ketone 5 or 7a,b in the presence
of NaH in DMF at room temperature furnished bipolar dihydro-
benzo[f]quinolines 6a–f or 5,6-dihydro-benzo[a]acridines 8a,b,
respectively, in good yields (Scheme 1). For the further tuning of
the electronic properties by increasing the conjugation, 8a,b were
treated with DDQ in dry benzene at 80 ꢀC, which afforded fully
aromatized substituted benzo[a]acridines 9a,b in good yields
(Scheme 1).
Results and discussion
Synthesis and characterization
Numerous synthetic methodologies are available in the literature
for the synthesis of quinoline, acridines and their p-annulated
compounds.15 Recently, a few multicomponent coupling reac-
tions have been employed to furnish benzo[f]quinolines in
moderate to good yields.16 We reported a convenient and general
methodology for preparing various N-heterocyclic compounds
through the base-catalyzed reaction of diverse cyclic or acyclic
a,b-unsaturated carbonyl/nitrile compounds with 9-methyl-
acridone.17 Unfortunately this methodology was unsuccessful for
the synthesis of benzo[a]acridines due to the unexpectedly high
reactivity of the methyl group at position 9 of the acridone.
In order to inhibit the possible p–p stacking9a in planar benzo
[a]acridines 9, we incorporated diaryl substituents by exploiting
the two C–Br bonds in 8b using ‘Nobel’ Suzuki coupling.21 Thus
a mixture of 8b and phenylboronic acid under basic conditions
furnished product 10 in 75% yield (Scheme 2), which was
aromatized into 4,9,11-triphenyl-2-(piperidin-1-yl)benzo[a]acri-
dine-1-carbonitrile (11) in 70% yield using DDQ as an oxidant in
dry benzene (Scheme 2).
Optical properties
The photophysical and electrochemical data such as lmax of their
UV-vis absorption and photoluminescence (PL) spectra, fluo-
rescence quantum yield, HOMO/LUMO energy levels, and the
electrochemical band gap (Eg) of compounds 6a–f, 8a,b, 9a,b, 10
and 11 are shown in Table 1 and Table 2. Fig. 2 shows the
normalized UV-vis and PL spectra of selected substances 6a,
8a,b, and 9a,b. The PL maxima of all benzo[f]quinolines 6a–f
showed strong blue fluorescence at 455–458 nm with a good
fluorescence quantum yield up to 69%. In the case of increasing
p-conjugation from benzo[f]quinoline 6a to benzo[a]acridines 8a,
a red shift of 12 nm in the PL maxima was observed, while
a remarkable red shift of 40 nm was noticed after aromatization
of 8a (PL lmax 468 nm) to 9a (PL lmax 508 nm). Interestingly
Scheme 2 Synthesis of p-conjugated benzo[a]acridines 10 and 11.
Reaction conditions: (i) PdCl2(PPh3)2, 2M Na2CO3, 80 ꢀC, DMF, 45 min.;
(ii) DDQ, dry benzene, 80 ꢀC, 1 h.
Scheme 1 Synthesis of benzo[f]quinolines 6a–f and benzo[a]acridines
8a,b and 9a,b.
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Table 1 Spectroscopically determined photophysical data of 6a and 9b in various solvents
6a
9b
Solvents (Df)
lmax;abs (nm)
lmax;em (nm)
Stoke’s shift (cmꢁ1
)
lmax; abs (nm)
lmax;em (nm)
Stoke’s shift (cmꢁ1
)
Cyclohexane (ꢁ0.0014)
Toluene (0.0134)
THF (0.2096)
DMSO (0.2640)
DMF (0.2745)
360
364
360
367
364
436
448
456
471
468
4800
5200
5800
6000
6100
343
350
348
352
361
480
501
540
562
589
8300
8600
10000
10600
10700
dibromo-substituted benzo[a]acridines 8b and 9b showed PL
maxima in the regions of green (8b: lmax 508 nm) and yellow (9b:
lmax 540 nm), respectively, in THF. The replacement of 9,11-
dibromo groups in 8b with 9,11-diphenyl substituents (10)
showed a hypsochromic shift of about 20 nm, while the aroma-
tization of partially hydrogenated benzo[a]acridines 10 to fully
aromatized benzo[a]acridines 11 exhibited a bathochromic shift
of 40 nm due to an increase in p-conjugation.
The solid state fluorescence of the benzo[f]quinolines 6a–f and
the benzo[a]acridines 8a,b, 9a,b, 10, and 11 were examined in
powder form and the data is presented in Table 2. The powder of
all benzo[f]quinolines 6a–f showed blue emission characteristics
with emission maxima in the range of 456–471 nm. In compar-
ison to the emission spectra of the benzo[f]quinolines 6a–f in
dilute solution, the solid state emission data indicates that there is
no aggregation in solid state. In order to confirm the non-
aggregating behaviour of the synthesized benzoquinolines, we
performed classical concentration-dependent photoluminescence
analysis of 6c in THF at different concentrations, as shown in
Fig. 2 Normalized absorbance and fluorescence spectra of 6a, 8a,b and
9a,b in THF (ꢃ10ꢁ6 M).
with a maximum at 460 nm, which was only 4 nm red shifted
from the emission maxima in dilute solution (lPL,max 456 nm).
The solution and solid state PL spectra of 6c showed that indeed
there is no aggregation in solid phase. It is remarkable that all
benzo[f]quinolines and benzo[a]acridines exhibited strong solid-
state fluorescence with a wide range of colour emissions due to
increasing p-conjugation.
Fig. 3. By increasing the concentration from 10ꢁ5 M to 10ꢁ2
M
(Fig. 3), the intensity of the PL band increased gradually up to
a concentration of 2.5 ꢂ 10ꢁ4 M and then reduced at 10ꢁ3 M and
10ꢁ2 M due to concentration quenching. Importantly, at all
concentrations, the emission corresponded only to the monomer
at 456 nm (Fig. 3). No red shift or additional peak for excimer
formation was observed in benzoquinoline 6c.
Due to the difference in dipole moments of the donor–acceptor
molecules in the ground and excited states and their stabilization
by polar or non-polar solvent molecules through various non-
covalent interactions like hydrogen bonding, solvation or
dipole–dipole interactions, bipolar compounds generally exhibit
solvatochromic behaviour. To investigate the stabilization of the
The non-aggregating properties of 6c were further investigated
using a thin film prepared by vacuum evaporation on a glass
substrate (see ESI, Fig. S3†). The thin film of 6c on the glass
substrate was continuous and exhibited strong blue emission
Table 2 Optical and electrochemical properties of 6a–f, 8a,b, 9a,b, 10 and 11
a
lmax;abs (nm)
b
lmax;em (nm)
Entry
lmax;emc (nm)
Ffd (%)
HOMO (eV)
LUMO (eV)
Ege (eV)
6a
6b
6c
6d
6e
6f
8a
8b
9a
9b
10
11
257, 360
274, 390
281, 360
257, 362
265, 363
277, 345
260, 367
269, 376
267, 347
279, 348
268, 369
283, 348
456
455
456
458
457
456
468
490
508
540
470
510
467
456
468
470
471
471
480
481
525
557
465
518
63
62
69
60
59
58
46
40
33
31
51
37
ꢁ5.45
ꢁ5.28
ꢁ5.37
ꢁ5.39
ꢁ5.38
ꢁ5.41
ꢁ5.36
ꢁ5.40
ꢁ5.37
ꢁ5.39
ꢁ5.39
ꢁ5.39
ꢁ2.67
ꢁ2.57
ꢁ2.65
ꢁ2.66
ꢁ2.64
ꢁ2.67
ꢁ2.73
ꢁ2.96
ꢁ2.96
ꢁ3.41
ꢁ2.79
ꢁ2.99
2.78
2.71
2.72
2.73
2.74
2.74
2.63
2.44
2.41
1.98
2.60
2.40
a
Longest wavelength absorption maximum. b PL emission maximum in THF. c Fluorescence emission maximum in powder. d Fluorescence quantum
yield relative to harmine in 0.1 M H2SO4 as a standard (F ¼ 45%). e Band gap from CV.
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Fig. 5 Lippert–Mataga plot of 6a and 9b in solvents of increasing
polarity.
as dimethylsulfoxide and dimethylformamide, which exhibited
a gradual increase in the Stoke’s shift upon increasing the
polarity. The slope of the Lippert plot reflects the solvent sensi-
tivity of the fluorophore. For both the dyes, a nice linear corre-
lation (6a: r2 ¼ 0.94, 9b: r2 ¼ 0.99) with the orientation
polarizability (Df) was observed (Fig. 5), but the benzo[a]acridine
dye 9b exhibited a much steeper slope (8257 cmꢁ1) than benzo[f]
quinoline 6a (4017 cmꢁ1), indicating a larger fluorescence sol-
vatochromism for the benzo[a]acridine 9b. The positive sol-
vatochromism indicated that the donor–acceptor benzo[f]
quinolines and benzo[a]acridines exhibited a strongly stabilized
excited state with high intramolecular charge transfer (ICT)
character and larger dipole moments compared to the ground
state by the surrounding solvent molecules.
Fig.
3 Fluorescence spectra of 6c at different concentrations
(10ꢁ5 to 10ꢁ2 M).
ground and excited states of the synthesized bipolar molecules in
various solvents, we examined the absorption and PL spectra of
selected N-heterocyclic derivatives 6a and 9b using solvents of
varying polarity (for 6a see the ESI†). The benzo[f]quinoline 6a
and benzo[a]acridine 9b showed closely resembling absorption
profiles upon increasing the polarity of the solvent.
Photoluminescence spectra of the benzo[f]quinoline 6a and
benzo[a]acridine 9b were also investigated in a series of solvents
with varying polarity index to examine the effect of the polarity
of the solvent on the excited state. Interestingly large sol-
vatochromic shifts were observed in the PL emission spectra for
the benzo[f]quinoline and benzo[a]acridine derivatives. For
example in the case of 9b (Fig. 4), the PL maximum increased
dramatically with increasing polarity from cyclohexane
(lPL,max ¼ 480 nm) to toluene (lPL,max ¼ 501 nm) to
THF (lPL,max ¼ 540 nm) to DMSO (lPL,max ¼ 562 nm) to DMF
(lPL,max ¼ 589 nm). Further information regarding the solvent
sensitivity of the emission spectra of 6a and 9b was obtained by
evaluation of its Stoke’s shift in terms of a Lippert–Mataga
plot.22 Table 1 revealed the changes in emission wavelength and
Stoke’s shift with increasing solvent polarity.
Electrochemical and thermal properties
Electrochemical studies were carried out to ascertain the redox
behavior of the benzo[f]quinolines 6a–f and the benzo[a]acridines
8a,b, 9a,b, 10 and 11. All measurements were performed in
a three-electrode cell setup using Ag/AgCl as reference electrode
and a Pt disc as the working electrode, using a 2 mM solution of
compound, and 0.2 M of the electrolyte tetrabutylammonium
hexafluorophosphate (Bu4NPF6) dissolved in DMF. The cyclic
voltammograms obtained were employed to evaluate the highest
occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) energy levels (Fig. 6 and also see
ESI†). The HOMO and LUMO levels were readily estimated
from the onset oxidation (Eox) and onset reduction potentials
(Ered), using the equations EHOMO ¼ ꢁ(Eox + 4.8) eV and
ELUMO ¼ ꢁ(Ered + 4.8) eV, respectively, where Eox and Ered are
the onset oxidation and reduction potentials relative to the
ferrocene/ferrocenium couple (EFOC ¼ 0.53 V versus Ag/AgCl
electrode).
The correlation of the Stoke’s shift (Dn in cmꢁ1) with the
orientation polarizability (Df) for 6a and 9b is represented in
Fig. 5. The emission profile of 6a and 9b revealed bath-
ochromically shifted emission maxima in the polar solvents such
Fig. 4 Solvatochromism shown by 9b in solvents of varying polarity.
This journal is ª The Royal Society of Chemistry 2012
Fig. 6 Cyclic voltammograms of 6c, 8a, 9b and 11 (in DMF).
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During the anodic oxidization voltage sweep measured in
DMF, irreversible oxidation peaks at 1.47, 1.44, 1.29 and 1.33 V
with onset potentials at 1.10, 1.09, 1.12, and 1.12 V for 6c, 8a, 11,
and 9b, respectively, were observed (Fig. 6).
efficiency, multilayer devices were fabricated to investigate the
performance of multi-colour light-emitting 6c, 8a, 9b and 11
compounds with the device configuration of ITO/PEDOT:PSS
(40 nm)/NPB(20 nm)/(N-heterocyclic compound) (50 nm)/BCP
(7 nm)/LiF (0.7 nm)/Al (200 nm). The patterned ITO glass plate
was cleaned in 6 : 1 : 1 RCA-I solution, rinsed in DI water
a number of times and then dried. The ITO surface was treated in
ozone for 15 minutes. Immediately, the first layer of poly(3,4-
ethylenedioxythiophene) doped with poly(styrenesulfonic acid)
(PEDOT:PSS) was spin-coated onto the patterned ITO to form
a hole injection layer. The PEDOT:PSS was vacuum dried at
120 ꢀC for 1 hour. All other organic layers and metal layers were
sublimed under high vacuum (ꢃ1 to 5 ꢂ 10ꢁ6 mbar). A bilayer
metal cathode of LiF (0.7 nm)/Al (200 nm) was deposited in
a separate vacuum chamber. Then, the devices were sealed with
a covering glass plate using UV epoxy. The ILV characteristics of
the sealed OLED devices were obtained using a Keithley source-
measuring unit. EL spectra were recorded with an Ocean Optics
USB 2000 fibre optic spectrometer.
The HOMO energy levels of 6c, 8a, 11, and 9b were estimated
from the oxidation onset potentials, which were found to
be ꢁ5.37, ꢁ5.36, ꢁ5.39, and ꢁ5.39 eV, respectively (Table 2).
The cathodic voltage sweeps for 6c and 8a were recorded in
DMF, which exhibited reversible reduction waves with half wave
potentials (E1/2) of ꢁ1.79 (Epc ¼ ꢁ1.84 V, peak–peak separation
DEp ¼ 87 mV) and ꢁ1.67 V (Epc ¼ ꢁ1.70 V, peak–peak sepa-
ration DEp ¼ 68 mV), respectively (Fig. 6). Benzo[a]acridines 9b
and 11 with longer aromatic p-conjugation showed positively
shifted reduction steps (9b: three reduction peaks at ꢁ0.97, ꢁ1.16
and ꢁ1.30 V; 11: two reduction peaks at ꢁ1.39 and ꢁ1.68 V).
Accordingly, the LUMO energy levels of 6c, 8a, 11, and 9b were
calculated from the reduction wave onset potentials to
be ꢁ2.65, ꢁ2.73, ꢁ2.99 and ꢁ3.41 eV, respectively. Careful
examination of the CV graphs revealed that the LUMO levels
can be tuned by altering the aromatic p-conjugation without
affecting the HOMO levels, as shown in Table 2. The corre-
sponding band gaps of 6c, 8a, 11 and 9b calculated from the
HOMO/LUMO values were 2.72, 2.63, 2.40, and 1.98 eV,
respectively (Fig. 6).
The EL characteristics of all four compounds are shown in
Fig. 7 and the performance data and Commission Internationale
de L’Eclairage (CIE) colour coordinates of the OLEDs 6c, 8a, 9b
and 11 are summarized in Table 3. The EL of benzo[f]quinoline
6c showed a sharp peak at 455 nm (blue) with a fwhm of 72 nm,
while benzo[a]acridines 8a, 11 and 9b showed ELs at 496 nm,
545 nm and 630 nm, respectively and an electroluminescence
color ranging from blue (CIE 0.18, 0.20) to green (CIE 0.26, 0.43)
to yellow (CIE 0.41, 0.53) to red (CIE 0.55, 0.42).
The thermal properties of 6c, 8a, 9b and 11 were gauged by
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) (Table 3). 6c, 8a, 9b and 11 are all thermally
ꢀ
stable up to 260 C, as shown by their decomposition tempera-
ture (5% weight loss temperature) under nitrogen atmosphere in
the TGA measurements. DSC experiments were performed by
The EL of 6c exhibited a blue-shift of 4 nm compare to the PL
of the thin film of 6c (lPL,film 460 nm), while similarity with the
scanning at a rate of 10 C minꢁ1 under nitrogen atmosphere in
ꢀ
ꢀ
the temperature range from 50 to 350 C. Benzo[f]quinoline 6c
and benzo[a]acridine 8a melted at 174 ꢀC and 256 ꢀC respec-
tively, while 11 with a higher molecular weight showed a higher
melting temperature of 284 ꢀC.
Electroluminescence properties
Benzo[f]quinoline 6c and benzo[a]acridines 8a, 9b, and 11 were
selected for further electroluminescence studies. Two single-layer
OLEDs (Diode 1: ITO/benzo[f]quinoline 6c (100 nm)/LiF
(0.7 nm)/Al (200 nm); Diode 2: ITO/PEDOT:PSS (40 nm)/6c
(100 nm)/LiF (0.7 nm)/Al (200 nm)) were initially fabricated to
check the bipolar characteristics of these compounds. The
performance of single-layer Diode 1 was not satisfactory while
Diode 2 showed a faint blue emission at higher voltage, indi-
cating that the holes and electrons are passing through the device
with less recombination. In order to facilitate the recombination
Fig. 7 The EL characteristics and chromaticity diagram showing the
CIE coordinates of the OLEDs of 6c, 8a, 9b, and 11.
Table 3 Thermal stability and performance data of OLEDs of 6c, 8a, 9b, and 11
Turn-on
voltage (V)
Max. brightness
(cd mꢁ2
Max. current
eff. (Cd Aꢁ1
b
Entry
Tda/Tm (ꢀC)
EL (nm) (lmax
)
)
)
CIE 1931
6c
8a
11
9b
260/174
300/256
320/284
315/260
3.5
4
6
455
496
545
630
461
39
4
0.34
0.78
0.40
0.57
0.0018
0.18, 0.20
0.26, 0.43
0.41, 0.53
0.55, 0.42
6
a
b
Decomposition temperature (5% weight loss) under nitrogen atmosphere in the TGA measurements. Melting temperature gauged by DSC.
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photoluminescence of 6c in THF (lPL 456 nm) was observed.
These results suggested that the EL was attributed to emission
from the charge-transfer excited state of benzo[f]quinoline 6c and
the partially hydrogenated benzo[f]quinoline 6c did not exhibit
aggregation behaviour in device operation. Furthermore, we
observed that due to an increase in the planarity by aromatiza-
tion of the partially hydrogenated benzo[f]quinolines to rigid
benzo[a]acridines, the EL spectra of 8a, 9b and 11 showed red-
shifts compare to their PL in both solution and solid state. These
results supported our design of non-planar partially hydroge-
nated benzo[f]quinolines for OLEDs.
Among the four fabricated multicolour OLEDs, the blue
OLED of benzo[f]quinoline 6c showed good stability and
performance. The relative energy-level alignments of the fabri-
cated multilayered OLED and the actual device picture of 6c are
shown in Fig. 8.
Fig. 9 EL vs. voltage graph of 6c and blue OLED display of donor–
acceptor benzo[f]quinoline 6c.
alumina column using 3% ethyl acetate in n-hexane as the eluent
to afford 6a (328 mg, 90%) as a light greenish solid: Rf ¼ 0.56
(n-hexane–ethyl acetate, 9 : 1, v/v); mp (n-hexane–ethyl acetate)
80–82 ꢀC; MS (ESI) 366 [M + H+]; IR (KBr) n ¼ 3063 (w), 2931
An unoptimized multilayer OLED of 6c showed bright blue
emission with a low turn-on voltage of 3.5 V and a maximum
brightness of 461 cd mꢁ2 at 10 V with a current efficiency of
0.78 cd Aꢁ1, without using any dopant. Low information content
displays like 7-segment and displays for signage and mobile
phones have a huge market due to the low operating voltage
required to derive these OLED displays with commercially
available drivers with 5 V outputs. In order to demonstrate the
commercial applications of blue emitter 6c, a flat panel OLED
display using 6c as emissive material with the device structure
ITO/PEDOT:PSS (40 nm)/NPB (20 nm)/6c (50 nm)/BCP (7 nm)/
LiF (0.7 nm)/Al (200 nm) was fabricated as shown in Fig. 9. The
dependence of the device EL on the applied voltage was inves-
tigated by increasing the voltage in intervals of 1 V (Fig. 9). The
display 6c showed no change in EL spectrum when the bias was
increased up to 12 V, which revealed that the device is electro-
optically stable. Further optimization of the device configura-
tion-performance is currently underway.
(s), 2820 (m), 2213 (s), 1589 (s) cmꢁ1 1H NMR (300 MHz,
;
CDCl3): d ¼ 1.54–1.64 (m, 2H), 1.72–1.86 (m, 4H), 2.68–2.78 (m,
2H), 2.82–2.94 (m, 2H), 3.14–3.26 (m, 4H), 6.94 (s, 1H), 7.27–
7.48 (m, 6H), 8.44–8.52 (m, 1H), 8.54–8.62 (m, 1H) ppm; 13C
NMR (75 MHz, CDCl3): d ¼ 23.9, 25.9, 26.1, 32.0, 53.8, 102.6,
118.9, 120.0, 121.7, 127.8, 127.9, 128.4, 128.8, 129.9, 134.5, 138.6,
140.1, 146.1, 148.7, 157.4, 159.2 ppm; HRMS calculated for
C25H24N3 [M + H+] 366.1970, found: 366.1968.
Synthesis of 7-phenyl-9-(pyrrolidin-1-yl)-5,6-dihydrobenzo[f]
quinoline-10-carbonitrile (6b)
A mixture of 2-oxo-6-phenyl-4-(pyrrolidin-1-yl)-2H-pyran-3-
carbonitrile (4b, 266 mg, 1 mmol), 7,8-dihydroquinolin-5(6H)-
one (5, 176 mL, 1.2 mmol) and NaH (60% dispersion in oil,
60 mg, 1.5 mmol) in dry DMF (5 mL) was stirred at room
temperature for 11 min. The progress of reaction was monitored
by TLC and on completion, the reaction mixture was poured
onto crushed ice with vigorous stirring and finally neutralized
with 10% HCl. The precipitate obtained was filtered and purified
on a neutral alumina column using 3% ethyl acetate in n-hexane
as the eluent to afford 6b (309 mg, 88%) as a light greenish solid:
Rf ¼ 0.54 (n-hexane–ethyl acetate, 9 : 1, v/v); mp (n-hexane–ethyl
acetate) 146–148 ꢀC; MS (ESI) 352 [M + H+]; IR (KBr) n ¼ 3037
Experimental
Synthesis of 7-phenyl-9-(piperidin-1-yl)-5,6-dihydrobenzo[f]
quinoline-10-carbonitrile (6a)
A mixture of 2-oxo-6-phenyl-4-(piperidin-1-yl)-2H-pyran-3-car-
bonitrile (4a, 280 mg, 1 mmol), 7,8-dihydroquinolin-5(6H)-one
(5, 176 mL, 1.2 mmol) and NaH (60% dispersion in oil, 60 mg,
1.5 mmol) in dry DMF (5 mL) was stirred at room temperature
for 10 min. The progress of reaction was monitored by TLC. On
completion, the reaction mixture was poured onto crushed ice
with vigorous stirring and finally neutralized with 10% HCl. The
precipitate obtained was filtered and purified on a neutral
(m), 2931 (s), 2856 (w), 2806 (s), 2214 (s), 1597 (s), 1488 (s) cmꢁ1
;
1H NMR (300 MHz, CDCl3): d ¼ 1.96–2.08 (m, 4H), 2.62–2.74
(m, 2H), 2.82–2.96 (m, 2H), 3.52–3.70 (m, 4H), 6.68 (s, 1H), 7.27–
7.50 (m, 6H), 8.42–8.54 (m, 2H) ppm; 13C NMR (50 MHz,
CDCl3): d ¼ 25.6, 25.9, 32.3, 50.9, 91.7, 115.4, 120.8, 121.5, 125.5,
127.8, 128.1, 128.3, 128.7, 135.0, 139.0, 140.6, 146.1, 148.5, 152.1,
159.5 ppm; HRMS calculated for C24H22N3 [M + H+] 352.1813,
found: 352.1801.
Synthesis of 7-(naphthalen-1-yl)-9-(piperidin-1-yl)-5,6-
dihydrobenzo[f]quinoline-10-carbonitrile (6c)
A mixture of 6-(naphthalen-1-yl)-2-oxo-4-(piperidin-1-yl)-2H-
pyran-3-carbonitrile (4c, 330 mg, 1 mmol), 7,8-dihydroquinolin-
5(6H)-one (5, 176 mL, 1.2 mmol) and NaH (60% dispersion in oil,
60 mg, 1.5 mmol) in dry DMF (5 mL) was stirred at room
temperature for 12 min. The progress of reaction was monitored
Fig. 8 Relative energy-level alignment and layer thickness, actual device
picture, and OLED characteristics of 6c.
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J. Mater. Chem., 2012, 22, 14880–14888 | 14885

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by TLC and on completion, the reaction mixture was poured
onto crushed ice with vigorous stirring and finally neutralized
with 10% HCl. The precipitate obtained was filtered and purified
on a neutral alumina column using 3% ethyl acetate in n-hexane
as the eluent to afford 6c (374 mg, 90%) as a light greenish solid:
Rf ¼ 0.52 (n-hexane–ethyl acetate, 9 : 1, v/v); mp (n-hexane–ethyl
acetate) 172–174 ꢀC; MS (ESI) 416 [M + H+]; IR (KBr) n ¼ 3050
(m, 4H), 2.76–2.98 (m, 4H), 3.12–3.28 (m, 4H), 6.99 (s, 1H), 7.30–
7.54 (m, 6H), 7.58–7.76 (m, 4H), 8.44–8.54 (m, 1H), 8.59 (d, J ¼
6.9 Hz, 1H) ppm; 13C NMR (75 MHz, CDCl3): d ¼ 24.0, 26.0,
26.1, 32.0, 53.9, 102.6, 118.9, 120.2, 121.8, 127.0, 127.1, 127.6,
127.8, 128.8, 129.3, 130.0, 134.5, 138.7, 139.0, 140.3, 140.9, 145.7,
148.8, 157.5, 159.3 ppm; HRMS calculated for C31H28N3
[M+ + H] 442.2283, found: 442.2257.
(w), 2930 (s), 2849 (m), 2208 (s), 1586 (s) cmꢁ1 1H NMR
;
(300 MHz, CDCl3): d ¼ 1.54–1.66 (m, 2H), 1.74–1.88 (m, 4H),
2.34–2.48 (m, 2H), 2.76–2.82 (m, 2H), 3.12–3.28 (m, 4H), 6.98 (s,
1H), 7.28–7.58 (m, 6H), 7.84–7.98 (m, 2H), 8.42–8.52 (m, 1H),
8.58–8.72 (m, 1H) ppm; 13C NMR (75 MHz, CDCl3): d ¼ 24.0,
25.7, 26.2, 32.0, 54.0, 102.8, 119.1, 120.8, 121.9, 125.3, 125.4,
126.2, 126.5, 126.7, 127.9, 128.4, 128.5, 131.4, 133.5, 134.6, 138.0,
138.3, 144.6, 148.9, 157.6, 159.4 ppm; HRMS calculated for
C29H26N3 [M+ + H] 416.2126, found: 416.2118.
Synthesis of 9-(piperidin-1-yl)-7-(pyren-1-yl)-5,6-dihydrobenzo
[f]quinoline-10-carbonitrile (6f)
A mixture of 2-oxo-4-(piperidin-1-yl)-6-(pyren-1-yl)-2H-pyran-
3-carbonitrile (4f, 404 mg, 1 mmol), 7,8-dihydroquinolin-5(6H)-
one (5, 176 mL, 1.2 mmol) and NaH (60% dispersion in oil,
60 mg, 1.5 mmol) in dry DMF (5 mL) was stirred at room
temperature for 12 min. The progress of reaction was monitored
by TLC and on completion, the reaction mixture was poured
onto crushed ice with vigorous stirring and finally neutralized
with 10% HCl. The precipitate obtained was filtered and purified
on a neutral alumina column using 3% ethyl acetate in n-hexane
as the eluent to afford 6f (416 mg, 85%) as a light greenish solid:
Rf ¼ 0.54 (n-hexane–ethyl acetate, 9 : 1, v/v); mp (n-hexane–ethyl
acetate) 214–216 ꢀC; MS (ESI) 490 [M + H+]; IR (KBr) n ¼ 2934
Synthesis of 7-(naphthalen-2-yl)-9-(piperidin-1-yl)-5,6-
dihydrobenzo[f]quinoline-10-carbonitrile (6d)
A mixture of 6-(naphthalen-2-yl)-2-oxo-4-(piperidin-1-yl)-2H-
pyran-3-carbonitrile (4d, 330 mg, 1 mmol), 7,8-dihydroquinolin-
5(6H)-one (5, 176 mL, 1.2 mmol) and NaH (60% dispersion in oil,
60 mg, 1.5 mmol) in dry DMF (6 mL) was stirred at room
temperature for 10 min. The progress of reaction was monitored
by TLC and on completion, the reaction mixture was poured
onto crushed ice with vigorous stirring and finally neutralized
with 10% HCl. The precipitate obtained was filtered and purified
on a neutral alumina column using 3% ethyl acetate in n-hexane
as the eluent to afford 6d (357 mg, 86%) as a light greenish solid:
Rf ¼ 0.53 (n-hexane–ethyl acetate, 9 : 1, v/v); mp (n-hexane–ethyl
acetate) 132–134 ꢀC; MS (ESI) 416 [M + H+]; IR (KBr) n ¼ 3032
(s), 2851 (w), 2212 (s), 1588 (s) cmꢁ1 1H NMR (300 MHz,
;
CDCl3): d ¼ 1.54–1.68 (m, 2H), 1.74–1.88 (m, 4H), 2.38–2.46 (m,
2H), 2.78–2.88 (m, 2H), 3.14–3.28 (m, 4H), 7.10 (s, 1H), 7.32–
7.38 (m, 1H), 7.73 (d, J ¼ 9.1 Hz, 1H), 7.88 (d, J ¼ 7.8 Hz, 1H),
7.97–8.32 (m, 7H), 8.46–8.54 (m, 1H), 8.69 (d, J ¼ 7.8 Hz, 1H)
ppm; 13C NMR (75 MHz, CDCl3): d ¼ 24.0, 25.8, 26.2, 31.9, 53.9,
102.8, 119.1, 121.1, 121.8, 124.5, 124.6, 124.6, 125.3, 125.6, 126.3,
126.7, 127.2, 127.9, 128.2, 128.7, 130.8, 131.1, 131.3, 131.5, 134.6,
135.1, 138.4, 145.0, 148.9, 157.5, 159.4 ppm; HRMS calculated
for C35H28N3 [M+ + H] 490.2283, found: 490.2213.
(w), 2930 (s), 2820 (m), 2213 (s), 1587 (s) cmꢁ1 1H NMR
;
(300 MHz, CDCl3): d ¼ 1.54–1.64 (m, 2H), 1.74–1.88 (m, 4H),
2.68–2.92 (m, 4H), 3.12–3.28 (m, 4H), 7.04 (s, 1H), 7.28–7.56 (m,
4H), 7.76–7.98 (m, 4H), 8.46–8.54 (m, 1H), 8.60 (d, J ¼ 7.9 Hz,
1H) ppm; 13C NMR (75 MHz, CDCl3): d ¼ 23.9, 26.0, 26.1, 31.9,
53.9, 102.6, 119.0, 120.2, 121.8, 126.5, 126.6, 126.7, 127.7, 127.8,
127.9, 128.0, 130.1, 132.6, 133.0, 134.6, 137.6, 138.6, 146.1, 148.7,
157.5, 159.2 ppm; HRMS calculated for C29H26N3 [M+ + H]
416.2126, found: 416.2135.
Synthesis of 4-phenyl-2-(piperidin-1-yl)-5,6-dihydrobenzo[a]
acridine-1-carbonitrile (8a)
A mixture of 2-oxo-6-phenyl-4-(piperidin-1-yl)-2H-pyran-3-car-
bonitrile (4a, 280 mg, 1 mmol), 3,4-dihydroacridin-1(2H)-one
(7a, 197 mg, 1 mmol) and NaH (60% dispersion in oil, 60 mg, 1.5
mmol, 1.5 equiv.) in dry DMF (10 mL) was stirred at room
temperature for 20 min. The progress of reaction was monitored
by TLC, and upon completion, the solvent was evaporated and
the reaction mixture was poured onto crushed ice with vigorous
stirring and finally neutralized with 10% HCl. The precipitate
obtained was filtered and purified on a silica gel column using
20% ethyl acetate in n-hexane as the eluent to afford 340 mg
(82%) of 8a as a light yellow solid: Rf ¼ 0.52 (ethyl acetate–n-
hexane, 1 : 9, v/v); mp (ethyl acetate–n-hexane) 254–256 ꢀC; MS
(ESI) 416 [M + H+]; IR (KBr) n ¼ 2937 (s), 2851 (m), 2207 (s),
Synthesis of 7-(biphenyl-4-yl)-9-(piperidin-1-yl)-5,6-
dihydrobenzo[f]quinoline-10-carbonitrile (6e)
A
mixture of 6-(biphenyl-4-yl)-2-oxo-4-(piperidin-1-yl)-2H-
pyran-3-carbonitrile (4e, 356 mg, 1 mmol), 7,8-dihydroquinolin-
5(6H)-one (5, 176 mL, 1.2 mmol) and NaH (60% dispersion in oil,
60 mg, 1.5 mmol) in dry DMF (6 mL) was stirred at room
temperature for 12 min. The progress of reaction was monitored
by TLC and on completion, the reaction mixture was poured
onto crushed ice with vigorous stirring and finally neutralized
with 10% HCl. The precipitate obtained was filtered and purified
on a neutral alumina column using 3% ethyl acetate in n-hexane
as the eluent to afford 6e (388 mg, 88%) as a light greenish solid:
Rf ¼ 0.50 (n-hexane–ethyl acetate, 9 : 1, v/v); mp (n-hexane–
ethyl acetate) 152–154 ꢀC; MS (ESI) 442 [M + H+]; IR (KBr)
1
1581 (s) cmꢁ1; H NMR (300 MHz, CDCl3): d ¼ 1.56–1.68 (m,
2H), 1.78–1.95 (m, 4H), 2.78–2.90 (m, 2H), 3.01–3.14 (m, 2H),
3.21–3.30 (m, 4H), 6.97 (s, 1H), 7.28–7.60 (m, 6H), 7.68–7.78 (m,
1H), 7.95 (d, J ¼ 7.9 Hz, 1H) 8.03 (d, J ¼ 8.5 Hz, 1H), 9.02 (s,
1H); 13C NMR (75 MHz, CDCl3): d ¼ 24.1, 26.1, 26.3, 33.3, 54.1,
102.9, 119.3, 120.3, 124.8, 125.9, 126.3, 127.3, 128.0, 128.2, 128.5,
128.6, 128.9, 130.4, 130.5, 134.6, 138.8, 140.2, 146.2, 147.2, 157.7,
160.4 ppm; HRMS calculated for C29H26N3 [M+ + H] 416.2127,
found: 416.2126.
n ¼ 3019 (w), 2939 (s), 2851 (w), 2851 (w), 2215 (s), 1589 (s) cmꢁ1
;
1H NMR (300 MHz, CDCl3): d ¼ 1.54–1.68 (m, 2H), 1.76–1.92
14886 | J. Mater. Chem., 2012, 22, 14880–14888
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Synthesis of 9,11-dibromo-4-phenyl-2-(piperidin-1-yl)-5,6-
1.98 (m, 4H), 3.40–3.52 (m, 4H), 7.32 (s, 1H), 7.43–7.60 (m, 5H),
7.88–7.96 (m, 2H), 8.25–8.38 (m, 2H), 10.54 (s, 1H); 13C NMR
(75 MHz, CDCl3): d ¼ 24.1, 26.2, 54.1, 99.3, 118.9, 120.9, 121.1,
123.6, 124.8, 125.1, 127.1, 127.5, 128.5, 128.7, 129.8, 130.3, 130.8,
132.8, 134.3, 137.0, 139.5, 143.8, 147.1, 150.3, 159.4 ppm; HRMS
calculated for C29H22Br2N3 [M+ + H] 570.0180, found: 570.0178.
dihydrobenzo[a]acridine-1-carbonitrile (8b)
A mixture of 2-oxo-6-phenyl-4-(piperidin-1-yl)-2H-pyran-3-car-
bonitrile (4a, 280 mg, 1 mmol), 6,8-dibromo-3,4-dihydroacridin-
1(2H)-one (7b, 355 mg, 1 mmol) and NaH (60% dispersion in oil,
60 mg, 1.5 mmol) in dry DMF (8 mL) was stirred at room
temperature for 24 min. The progress of reaction was monitored
by TLC and on completion, the solvent was evaporated and the
reaction mixture was poured onto crushed ice with vigorous
stirring and finally neutralized with 10% HCl. The precipitate
obtained was filtered and purified on a silica gel column using
20% ethyl acetate in n-hexane as the eluent to afford 580 mg
(84%) of 8b as a light yellow solid: Rf ¼ 0.72 (ethyl acetate–n-
hexane, 1 : 9, v/v); mp (ethyl acetate–n-hexane) 244–246 ꢀC; MS
(ESI) 572 [M + H+]; IR (KBr) n ¼ 2920 (s), 2805 (w), 2205 (s),
1584 (w) cmꢁ1; 1H NMR (300 MHz, CDCl3): d ¼ 1.58–1.68 (m,
2H), 1.79–1.90 (m, 4H), 2.78–2.90 (m, 2H), 3.10–3.30 (m, 6H),
7.00 (s, 1H), 7.30–7.38 (m, 2H), 7.40–7.51 (m, 3H), 8.02–8.16 (m,
2H), 8.89 (s, 1H); 13C NMR (75 MHz, CDCl3): d ¼ 24.0, 26.0,
26.2, 33.4, 53.9, 102.9, 119.0, 119.3, 120.8, 124.8, 127.6, 128.2,
128.6, 128.9, 129.2, 130.3, 130.7, 133.6, 136.2, 137.7, 139.9, 143.0,
146.5, 157.8, 161.7 ppm; HRMS calculated for C29H24Br2N3 [M+
+ H] 572.0337, found: 572.0337.
Synthesis of 2-(piperidin-1-yl)-4,9,11-triphenyl-5,6-dihydrobenzo
[a]acridine-1-carbonitrile (10)
The solution of 9,11-dibromo-4-phenyl-2-(piperidin-1-yl)-5,6-
dihydrobenzo[a]acridine-1-carbonitrile (8b, 569 mg, 1 mmol) and
phenylboronic acid (268 mg, 2.2 mmol) in DMF (8 mL) was
degassed with nitrogen for 20 min followed by addition of
Na2CO3 (3.0 mL, 2 M) under continuous flow of nitrogen.
PdCl2(PPh3)2 (280 mg, 0.40 mmol) was added to the reaction
mixture under a nitrogen atmosphere. The reaction mixture was
stirred at 80 ꢀC for 45 min. After completion, the reaction
mixture was diluted with H2O (5 mL), and then extracted four
times with ethyl acetate (10 mL). The combined organic layer
was dried over Na2SO4 and the solvent was removed in vacuo.
The crude product was purified on a silica gel column using
hexane : ethyl acetate (8 : 2, v/v) as eluent to afford 426 mg (75%)
of 10 as a yellow solid; Rf ¼ 0.74 (ethyl acetate–n-hexane, 1 : 9,
v/v); mp (ethyl acetate–n-hexane) > 250 ꢀC; MS (ESI) 568 [M +
H+]; IR (KBr) n ¼ 2929 (s), 2854 (w), 2215 (s), 1458 (w) cmꢁ1; 1H
NMR (300 MHz, CDCl3): d ¼ 1.60–1.70 (m, 2H), 1.78–1.90 (m,
4H), 2.74–2.84 (m, 2H), 2.98–3.05 (m, 2H), 3.20–3.32 (m, 4H),
6.97 (s, 1H), 7.32–7.60 (m, 11H), 7.75–7.88 (m, 4H), 8.02–8.06
(m, 1H), 8.13–8.16 (m, 1H), 9.09 (s, 1H); 13C NMR (75 MHz,
CDCl3): d ¼ 24.1, 26.2, 26.3, 33.4, 54.0, 102.9, 119.4, 120.1, 125.8,
126.0, 127.3, 127.4, 127.7, 127.9, 128.0, 128.1, 128.5, 129.0, 130.7,
130.8, 131.0, 134.9, 138.7, 138.9, 139.3, 140.1, 140.2, 140.3, 144.2,
146.2, 157.7, 159.6 ppm; HRMS calculated for C41H34N3
[M+ + H] 568.2753, found: 568.2744.
Synthesis of 4-phenyl-2-(piperidin-1-yl)benzo[a]acridine-1-
carbonitrile (9a)
4-Phenyl-2-(piperidin-1-yl)-5,6-dihydrobenzo[a]acridine-1-car-
bonitrile (8a, 415 mg, 1 mmol) was treated with DDQ (554 mg,
2 mmol) in refluxing benzene for 60 min. After completion, the
reaction mixture was filtered and the filtrate was evaporated
under reduced pressure. The crude product obtained was purified
by column chromatography on neutral alumina using 10% ethyl
acetate in n-hexane as the eluent to afford 289 mg (70%) of 9a as
a yellow solid: Rf ¼ 0.58 (ethyl acetate–n-hexane, 1 : 9, v/v); mp
(ethyl acetate–n-hexane) 240–242 ꢀC; MS (ESI) 414 [M + H+]; IR
(KBr) n ¼ 2929 (s), 2839 (m), 2206 (s), 1563 (m) cmꢁ1; 1H NMR
(300 MHz, CDCl3): d ¼ 1.65–1.76 (m, 2H), 1.82–1.97 (m, 4H),
3.40–3.52 (m, 4H), 7.30 (s, 1H), 7.42–7.70 (m, 6H), 7.82–7.92 (m,
3H), 8.18–8.28 (m, 2H), 10.66 (s, 1H); 13C NMR (75 MHz,
CDCl3): d ¼ 24.1, 26.2, 54.1, 99.2, 120.4, 121.1, 122.6, 124.4,
125.9, 126.1, 127.4, 128.3, 128.5, 128.6, 129.2, 129.4, 129.8, 131.2,
133.5, 134.8, 139.8, 146.9, 148.4, 149.5, 159.3 ppm; HRMS
calculated for C29H24N3 [M+ + H] 414.1970, found: 414.1977.
Synthesis of 2-(piperidin-1-yl)-4,9,11-triphenyl-benzo[a]acridine-
1-carbonitrile (11)
2-(Piperidin-1-yl)-4,9,11-triphenyl-5,6-dihydrobenzo[a]acridine-
1-carbonitrile (10, 567 mg, 1 mmol) was treated with DDQ
(554 mg, 2 mmol) in refluxing benzene for 60 min. After
completion, the reaction mixture was filtered and dried over
Na2SO4 and the solvent was removed in vacuo. The crude
product obtained was purified by column chromatography on
neutral alumina using 10% ethyl acetate in n-hexane as the eluent
to afford 396 mg (70%) of 11 as a yellow solid: Rf ¼ 0.78 (ethyl
acetate–n-hexane, 1 : 9, v/v); mp (ethyl acetate–n-hexane) >
250 ꢀC; MS (ESI) 566 [M + H+]; IR (KBr) n ¼ 2926 (s), 2855 (m),
2207 (s), 1590 (m) cmꢁ1; 1H NMR (300 MHz, CDCl3): d ¼ 1.66–
1.78 (m, 2H), 1.80–1.96 (m, 4H), 3.40–3.52 (m, 4H), 6.98 (s, 1H),
7.35–7.64 (m, 11H), 7.70–8.01 (m, 6H), 8.19–8.22 (m, 1H), 8.36–
8.42 (m, 1H), 10.71 (s, 1H); 13C NMR (75 MHz, CDCl3): d ¼
24.1, 26.3, 54.2, 99.4, 120.4, 121.1, 125.8, 126.1, 127.4, 127.5,
127.9, 128.0, 128.3, 128.6, 128.9, 129.1, 129.8, 131.1, 131.5, 133.4,
134.9, 138.5, 139.3, 140.0, 140.2, 140.3, 145.7, 146.3, 147.0, 149.3,
157.7, 159.2 ppm; HRMS calculated for C41H32N3 [M+ + H]
566.2596, found 566.2561.
Synthesis of 9,11-dibromo-4-phenyl-2-(piperidin-1-yl)benzo[a]
acridine-1-carbonitrile (9b)
9,11-Dibromo-4-phenyl-2-(piperidin-1-yl)-5,6-dihydrobenzo[a]
acridine-1-carbonitrile (8b, 571 mg, 1 mmol) was treated with
DDQ (554 mg, 2 mmol) in refluxing benzene for 70 min. After
completion, the reaction mixture was filtered and the filtrate was
evaporated under reduced pressure. The crude product obtained
was purified by column chromatography on neutral alumina
using 10% ethyl acetate in n-hexane as the eluent to afford 387
mg (68%) of 9b as a red solid: Rf ¼ 0.76 (ethyl acetate–n-hexane,
1 : 9, v/v); mp (ethyl acetate–n-hexane) 256–258 ꢀC; MS (ESI)
570 [M + H+]; IR (KBr) n ¼ 2930 (s), 2813 (m), 2214 (s), 1572 (w)
cmꢁ1; 1H NMR (300 MHz, CDCl3): d ¼ 1.65–1.75 (m, 2H), 1.85–
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J. Mater. Chem., 2012, 22, 14880–14888 | 14887

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Conclusions
In summary, we have developed a new efficient methodology for
the synthesis of novel bipolar benzo[f]quinolines and benzo[a]
acridines from easily accessible ketene-S,S-acetal in good yields.
We demonstrated that the donor–acceptor partially hydroge-
nated benzo[f]quinolines exhibited non-aggregating behaviour
and intense solvatochromism due to intramolecular charge
transfer (ICT). We showed that the emission color can be
effectively controlled from blue to green to yellow to red by
modulating the aromatic p-characteristics not only in solution
(photoluminescence) but also in solid electroluminescent devices.
Among the four fabricated blue–green–yellow–red devices, the
blue organic light emitting device using partially hydrogenated
benzo[f]quinoline 6c as an emissive layer showed good I–L–V
characteristics and stability. An efficient blue flat-panel display
was successfully fabricated using dihydrobenzo[f]quinoline 6c,
which opens a new avenue for the usage of these nonplanar
compounds as emissive materials for further research and
development in organic electronics.
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Acknowledgements
We gratefully acknowledge financial support from the Depart-
ment of Science and Technology (DST Ramanna Fellowship to
AG & project sanctioned vide DST no. SR/S2/CMP-0050/2008),
New Delhi, India. VK and SPS are thankful to UGC and CSIR,
New Delhi for research fellowships. The CDRI communication
number for the manuscript is 8255. Spectroscopic data provided
by SAIF, CDRI is acknowledged.
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