Fluoranthene-based triarylamines as hole-transporting and emitting materials for efficient electroluminescent devices

Article abstract of DOI:10.1039/c0nj00415d

Electroluminescent materials based on the fluoranthene core and containing triarylamine segments were synthesized and characterized by IR, NMR, UV-Vis, and emission spectroscopic, electrochemical, and thermal studies. The electronic absorption and emissio

Full text of DOI:10.1039/c0nj00415d

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Fluoranthene-based triarylamines as hole-transporting and emitting
materials for efficient electroluminescent devicesw
Neha Kapoor and K. R. Justin Thomas*
Received (in Victoria, Australia) 1st June 2010, Accepted 5th July 2010
DOI: 10.1039/c0nj00415d
Electroluminescent materials based on the fluoranthene core and containing triarylamine
segments were synthesized and characterized by IR, NMR, UV-Vis, and emission spectroscopic,
electrochemical, and thermal studies. The electronic absorption and emission characteristics
of the new functional materials were affected by the nature of the chromophore present on the
fluoranthene nucleus. Incorporation of amine functionality red shifted the absorption and
emission profiles significantly, while a marginal bathochromic shift was noticed for the cyano
substituent. The redox propensity of the dyes was also altered by the nature of the substituents.
The electron donating amino group imparted facile oxidation while the electron-withdrawing
cyano unit rendered reduction capability. The decomposition temperatures (Td) observed for the
dyes are exceptionally high (406–527 1C). Electroluminescent devices incorporating two amines,
N-(4-tert-butylphenyl)-N-(naphthalen-1-yl)-7,8,9,10-tetraphenylfluoranthen-3-amine (7c) and
N-phenyl-N-(7,8,9,10-tetraphenylfluoranthen-3-yl)pyren-1-amine (7d) with the configurations
ITO/7c or 7d (40 nm)/TPBI or Alq3 (40 nm)/LiF (1 nm)/Al (150 nm) were fabricated and
displayed bright greenish yellow emission originating from the dyes.
problem with such systems is the molecular aggregation via p–p
stacking which can suppress their emission in the solid state.
Nonplanarity is the best solution to overcome the aggregation
and nonplanar configuration can be easily achieved with the use
of star shaped molecules or incorporation of bulky non-planar
trigonal moieties in the molecules.8 Shirota et al.9–11 and
Schmidt et al.12,13 have reported several families of high Tg
starburst aromatic amines which served as hole transporting
materials for OLEDs. Inclusion of rigid polyaromatic segments
has been additionally beneficial for increasing the thermal
properties of the compounds.14 Various molecules incorporated
Introduction
Molecular materials featuring multiple functional characteristics
have been developed in recent years to use in electronic devices
such as organic field effect transistors (OFETs), non-linear
optics, solar cells and organic light emitting diodes (OLEDs).1
OLEDs have gained significant interest due to their potential
application in flat panel and full color range displays ever since
the breakthrough demonstrations by Tang and Van Slyke2 for
vapor deposited multi-layered small molecule OLEDs
(SMOLED) and Friend et al.3 for spin coated polymer based
OLEDs (PLED). Most of the electro-optical devices require
materials featuring multiple functionalities. For instance,
OLED requires organic materials possessing luminescent and
charge transporting characteristics.4,5 Embedding multiple
functional entities in a single molecule is synthetically challenging
and often leads to molecules dominating one function due to
the imbalanced communication between them. In this direction
dendrimers are being used as a suitable platform for rendering
multiple functions at molecular level.6 Multifunctional
molecules are designed, synthesized and evaluated by many
groups to diminish the steps in the cumbersome deposition of
multi-layers of organic materials and reduce the production
cost.7 Evaluation of structure-property relationship in
polyfunctional materials is also beneficial to improve the
molecular design understanding for such applications.
with polyphenyl-based dendrons (Mullen dendrons) have been
¨
demonstrated to be detrimental for the molecular aggregation
or p–p stacking in the solid state.15 Thus, conjugated
dendrimers incorporating polyaromatics may be potential
candidates for applications in electronic devices.
Fluoranthene segments have attracted a surge of interest
recently as a promising scaffold for making high quality
conducting polymers16,17 and fluoranthene cored small
molecules18–21 were developed as effective materials for
OLEDs, dye-sensitized solar cells and field effect transistors.
In this paper, we report the synthesis of new fluoranthene
derivatives containing multiple phenyl groups and an amine
donor featuring an additional aromatic segment such as
pyrene or naphthalene. As the newly developed materials
contained amine functionality they were used as emitting hole
transporters in OLEDs. This is in contrast to the earlier
reports in which the fluoranthene derivatives were only applied
as an emitting layer.22 Also, the materials reported here show
red-shifted emission in the greenish yellow region owing to the
auxochromic effect of the amine unit. Additionally they
exhibit very high thermal stability and display exceptionally
high glass transition temperature owing to the presence of
rigid structural units.
Flat aromatic molecules and linear p-conjugated systems are
highly fluorescent and exhibit potential applications in various
electronic devices. However, an enfeebling and long standing
Department of Chemistry, Indian Institute of Technology Roorkee,
Roorkee—247 667, India. E-mail: krjt8fcy@iitr.ernet.in;
Fax: +91 1332 286202; Tel: +91 1332 285376
w Electronic supplementary information (ESI) available: Spectral data
for the newly synthesized compounds. See DOI: 10.1039/c0nj00415d
c
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electron-withdrawing groups on the optical and electro-
chemical properties. The amine containing compounds are
yellow in color and are soluble in common organic solvents
and insoluble in methanol, hexane, and acetonitrile.
Results and discussion
Synthesis and characterization
The fluoranthene containing bromo derivative, 3-bromo-
7,8,9,10-tetraphenylfluoranthene required for the present
study was made by an established route (Scheme 1).23 In this
method 1,3-diphenyl-2-propanone was condensed with the
appropriate 5-bromoacenaphthylene-1,2-dione24 to obtain
the 3-bromo-7,9-diphenyl-8H-cyclopenta[a]acenaphthylene-8-
one. This underwent Diels–Alder reaction with diphenyl
acetylene in refluxing diphenyl ether to generate the desired
precursor, 3-bromo-7,8,9,10-tetraphenylfluoranthene (5b).
The syntheses of fluoranthene incorporated diarylamines
(7a–7d) were accomplished by Pd-catalyzed C–N coupling
strategy developed by Hartwig and co-workers.25 To the best
of our knowledge, animation reactions have not been demon-
strated for the substrate which we used. For comparison, we
have also synthesized 7,8,9,10-tetraphenylfluoranthene-3-
carbonitrile(6) from 3-bromo-7,8,9,10-tetraphenylfluoranthene
(5b) by means of a Rosenmund-Von Braun reaction26 in which
CuCN was used as reagent and DMF as solvent. This
compound has been used in this work to evaluate the role of
Optical properties
The absorption spectra of the compounds were recorded in
dichloromethane and toluene. Representative absorption
spectra recorded for the toluene solutions are presented in
Fig. 1 along with the parent compounds for comparison. The
data are listed in Table 1. The parent compound (5a) exhibited
two absorption peaks centered at 295 and 373 nm, respectively
originating from the p–p* transitions. It is interesting to note
that the data is not significantly different from those observed
for the basic unit fluoranthene (289 and 360 nm).27 It suggests
that outer phenyl rings do not extend the conjugation of the
core dramatically. This is possibly due to the twisting of the
phenyl groups out of the plane of the aromatic p-system which
inhibits an effective p-conjugation with the fluoranthene core
(see below for theoretical results). The absorption spectra were
almost similar for 4, 5a and 5b. Though the bromo substituent
did not influence the absorption pattern in 5b, the cyano
Scheme 1 Synthesis of the fluoranthene derivatives 4–7.
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ca. 300 nm in these compounds seems to be originating from
the diphenylamine chromophore. The absorptions in the higher
wavelength region (347 and 395 nm) are attributed to the charge
transfer transition from the amine donor to the aromatic acceptor
segment. On a similar note the absorption peak appearing at
the low energy side of the spectrum in the amine substituted
compounds (7a–7d) is assigned to the charge transfer transition.
On comparing the absorption features of the amine deriva-
tives few salient points emerge: (1) naphthyl phenyl amine
derivative 7b displays slighter hypsochromic shift (approx. 7 nm)
than that of 7a. This is because of the competitive delocaliza-
tion of the lone pair of nitrogen by the naphthyl ring, (2)
compound 7c absorbs red-shifted (442 nm) when compared to
that of 7b and approximately the same as 7a even though it
contains a naphthyl unit. The reason for this might be the
presence of a t-butyl group on the phenyl ring which probably
alters the donor strength of the phenyl moiety, (3) absorption
spectra of 7b and 7c show a hump at 335 nm which is due to
the naphthalene unit and similarly two humps at 381 and
398 nm in the absorption spectrum of compound 7d are
probably arising due to the pyrene unit. The latter assignments
are consistent with the fact that N,N-diphenylnaphthalen-1-
amine and N,N-diphenylpyren-1-amine cores show absorption
peaks at 347 and 395 nm, respectively.
Fig. 1 Absorption spectra of the compounds (4, 5a, 5b, 6, 7a–7d)
measured for toluene solutions.
bearing compound 6 showed bathochromic shift of 13 nm when
compared to the parent compound (5a) which probably
indicates a presence of charge transfer between the p-rich
fluoranthene segment and the electron-withdrawing cyano unit.
All the amine derivatives 7a–7d showed a new prominent band
located at 442–444 nm when compared to that of the parent
compound (5a). The diarylamine could either act as a strong
auxochrome or be involved in a facile charge transfer
transition.28,29 The appearance of absorption features in the
higher wavelength has been noticed earlier for diarylamine
substituted pyrene28 or anthracene derivatives.29 The triphenyl-
amine, N,N-diphenylnaphthalen-1-amine and N,N-diphenylpyren-
1-amine cores show absorption peaks at 303, 294 & 347 and
300 & 395 nm, respectively in dichloromethane solutions
(see supporting informationw).30,31 The common peak at
It is evident from the data listed in Table 2, that the energy
gap of these materials can be fine tuned by appropriate
substitution in the 3rd position of the fluoranthene core.
Substitution of an electron-withdrawing unit such as cyano
widens the band gap while the electron-donating amine
segment shrinks it. Among the amines the pyrene derivative
7d displays the smaller band gap. All these points suggest
that the fluoranthene core acts as a p–acceptor and the
incorporation of amine functionality brings about dipolar
character to the structure. Enhanced donor–acceptor inter-
action is expected to decrease the band gap.
Table 1 Photophysical properties of compounds
Toluene
Dichloromethane
l
em/nm
Stokes
shift/cmÀ1
l
em/nm
Stokes
shift/cmÀ1
l
max/nm (emax/MÀ1 cmÀ1) Â 103)
(FF/%)
l
max/nm (emax/MÀ1 cmÀ1) Â 103)
297.0 (28.6), 329.0 (7.9), 375.0 (9.9)
(FF/%)
Compound
4
298.0 (31.4), 329.0 (9.3), 374.0 (11.0)
297.0 (38.3), 329.0 (10.6), 374.5 (12.8)
300.5 (23.1), 334.0 (6.2), 382.5 (10.5)
295.5 (34.3), 323.5 (8.3), 339.0 (14.2),
387.5 (14.9)
460.5
457 (37)
469.5
4856
4987
4845
460.5 (36) 4880
295.0 (42.2), 328.0 (10.4), 373.0 (13.9) 459 (23) 5094
299.0 (28.4), 333.0, (5.9), 380.5 (12.3) 474 (12) 5184
294.0 (36.5), 323.0 (8.7), 339.0 (14.6), 503 (7)
386.0 (16.2)
5a
5b
6
494.5 (39) 5584
6026
7a
7b
7c
7d
302.5 (40.3), 443.0 (10.6)
521.5 (41) 3398
300.0 (41.5) 442.0 (10.3)
544.5 (27) 4259
304.0 (33.7), 335.5 (12.3), 437.0 (12.5)
305.5 (26.4), 341.5 (10.2), 442.5 (10.5)
306.5 (40.5), 382.5 (18.4), 401.0 (17.6),
446.5 (15.7)
518 (36)
524 (37)
530 (28)
3578
3515
3528
302.0 (35.3), 335.0 (12.5), 435.0 (13.0) 538.5 (26) 4418
303.0 (29.9), 339.5 (11.6), 443.0 (12.0) 549.5 (24) 4375
304.0 (37.8), 381.0 (17.2), 398.5 (16.2), 558 (11) 4601
444.0 (14.9)
Table 2 Electrochemical and thermal properties of the functional materials (6, 7a–7d)
Compounds
Eox, mV (DEp, mV)
HOMO, eV
LUMO, eV
E0À0/eV
Tm, 1C
Tg, 1C
Td, 1C
6
À1864.0
6.66
5.29
5.32
5.26
5.23
3.80
2.84
2.86
2.82
2.81
2.86
2.45
2.46
2.44
2.42
—
—
198
—
192
205
449
469
519
502
527
7a
7b
7c
7d
492.0 (62)
516.0 (67)
460.0 (63)
432.0 (67)
345
320
—
401
c
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in the ground state. Similarly bromo derivative (5b) also
displays a small Stokes shift when compared to the cyano
derivative (6) because of the presence of a heavy bromine atom
which makes the compound more rigid (Table 1). The Stokes
shift is significantly larger in dichloromethane when compared
to that in toluene indicating a possible role of dipole–dipole
relaxation to the excited state.
Theoretical investigations
In order to understand the absorption characteristics of the
dyes we have performed a theoretical calculation for the
molecules 5a, 6 and 7a using density functional theory with
the B3LYP functional and 6-31G(d,p) basis set. The prominent
higher wavelength vertical transitions predicted by the theory
are collected in Table 3 and the observed frontier molecular
orbital diagrams are displayed in Fig. 3. The phenyl groups are
significantly twisted from the fluoranthene core and display
a lesser p-interaction. The higher wavelength absorption peaks
observed for these compounds in toluene agree well with the
theoretically forecast values except for a minor solvent role.
For the parent compound 5a, the intense band at 360 nm is
thought to arise from the HOMO to LUMO electronic
transition. The HOMO and LUMO orbitals of 5a are p-type
and mainly localized on the fluoranthene core. From this it can
be safely concluded that the low energy transition observed for
the parent compound 5a originates from the fluoranthene
based p–p* transition. For 6 a slight red-shift in absorption
is proposed by theory which is faithfully observed in the
experiment. The 380 nm vertical transition calculated for 6
shows approximately equal contributions from HOMO to
LUMO and HOMO-1 to LUMO electronic transitions. The
LUMO orbital in 6 is being contributed to from the cyano-
fluoranthene segment and the HOMO and HOMO-1 orbitals
are spread over the fluoranthene and phenyl segments. A red
shift in the absorption is consequently attributed to the
extension of conjugation in the molecule as there is a spread
of orbitals over the larger section of the molecule. The
diphenylamine containing derivative 7a shows an intense
vertical transition at 469 nm which is primarily composed of
a HOMO to LUMO electron transfer. As the HOMO and
LUMO for 7a are contributed by the triarylamine and fluor-
anthene units, respectively, the transition at 469 nm is assigned
to an amine to fluoranthene charge transfer.
Fig. 2 Emission spectra of the compounds (4, 5a, 5b, 6, 7a–7d)
measured in toluene.
The emission spectra recorded for the compounds in toluene
are shown in Fig. 2. The emission peak appears in the range
460–558 nm. The emission characteristics were affected by the
auxochromes. Incorporation of amine functionality red shifts
the emission profiles. The emission colors of the amine
derivatives are greenish yellow or yellow and are dependent
on the nature of the diarylamine unit. The pyrene derivative
displayed strong yellow emission. We have also recorded the
emission of the vapor deposited films of 7c and 7d which
showed emission peaks at 535 and 555 nm, respectively, and a
more clear vibronic fine structure in the case of 7d illustrating
the rigidity and retention of the intermolecular segregation.
The suppression of aggregation in the solid film is attributed to
the presence of the polyphenylated chromophore which decre-
ments tendency for molecular aggregation or p–p stacking.
No significant solvent dependent absorption behavior was
observed for the compounds. However, the excited state
solvatochromism observed for the compounds is noteworthy.
The parent compound (5a) and 5b, and 6 showed unaltered
emission profiles on increasing the solvent polarity. On the
contrary, the amine derivatives (7a–7d) displayed positive
solvatochromism for the emission spectra. This indicates that
the excited state of amine derivatives is more stable in polar
solvent probably due to the separation of charges in the higher
energy state.
Thermal properties
Quantum yield was also measured for the compounds in
toluene and dichloromethane (see Table 1). The fluorescence
efficiency of the samples was high in non-polar solvents such as
toluene and it decreased in dichloromethane. It has been
noticed that the compounds that display comparatively lower
oxidation potential exhibit less quantum yield when recorded
in polar solvents. Thus, among amine derivatives (7a–7d), the
pyrene derivative displays relatively lesser quantum efficiency
in dichloromethane (FF = 11%). This clearly indicates that
the amino groups is effectively involved in an electron-transfer
quenching of the excited states.
The thermal properties of the compounds were studied by
both thermogravimetric analysis and differential scanning
Table 3 Predicted (TDDFT B3LYP/6-31G(d,p)) vertical transitions
and their assignments
Compound
labs, nm
f
Assignment
5a
375
360
411
0.02
0.25
0.02
HOMO-1 - LUMO (93%)
HOMO - LUMO (92%)
HOMO - LUMO (53%)
HOMO-1 - LUMO (40%)
HOMO-1 - LUMO (54%)
HOMO - LUMO (40%)
HOMO - LUMO (98%)
HOMO-1 - LUMO (85%)
6
7
380
0.31
The Stokes shifts of the amine derivatives are small when
compared to the parent compound 5a, which probably
indicates a more planar arrangement in the excited state than
469
376
0.28
0.09
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Fig. 3 Frontier molecular orbitals of the compounds 5a, 6 and 7a.
calorimetry measurements (see Table 2 for relevant parameters).
All the compounds exhibited excellent thermal stability and
the decomposition temperatures are in the range of 406–527 1C.
The marked thermal robustness of the compounds is attrib-
uted to the fused polyaromatic architecture. The thermal
stability trend observed for compounds 4, 5a, 5b and 6 may
be assigned to the differences in the number of non-hydrogen
atoms and molecular heaviness. Diarylamine derivatives
(7a–7d) display thermal decomposition temperatures above
450 1C and the order of stability is 7a o 7c o 7b o 7d. Among
the naphthylamine derivatives (7b and 7c) improved thermal
stability is observed for 7b. This difference probably arises due
to the presence of the tert-butyl group in 7c which makes the
molecule floppy. The stability order realized based on the
diarylamine units, diphenylamine (7a) o 1-naphthylphenyl-
amine (7b & 7c) o 1-pyrenylphenylamine (7d), is in accordance
with the earlier observations which revealed that molecular
materials containing flat polyaromatic extended skeletons
were prone to resisting thermal degradation. Glass transition
temperatures of 7c and 7d were recorded as 192 1C and 205 1C,
respectively. The present amine derivatives (7a–7d) were found
to show exceptionally higher thermal stability and a prolonged
glassy state than the reported blue emitting fluoranthene
derivatives, 7,10-diphenyl-8-(1-naphthyl) fluoranthene [DPNF]
and 7,10-diphenyl-8-(9-phenanthrenyl)fluoranthene[DPPF].22
This clearly highlights the importance of amine functionality
in improving the emission and thermal properties.
Electrochemical properties
The redox propensity of the compounds was examined in
2 Â 10À4 M dichloromethane solutions by cyclic and
c
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Fig. 4 Cyclic voltammograms recorded for the amine functionalized
derivatives (7a–7d) in dichloromethane solutions (concentration: 2.0 Â
10À4 M; scan rate: 100 mV sÀ1; supporting electrolyte: TBAHP).
differential pulse voltammetric methods. The cyclic voltammo-
grams recorded for the amine derivatives (7a–7d) with the
internal standard ferrocene are displayed in the Fig. 4. The
pertinent parameters are collected in Table 2. All the amines
exhibited one quasi-reversible oxidation couple followed by
one irreversible oxidation peak. It is reasonable to assume that
the quasi-reversible oxidation wave arises from the amine
functionality while the irreversible oxidation may originate
from the fluoranthene core. It is interesting to note that this
irreversible wave attributable to the fluoranthene core is
not observed for the bromo (5b) and cyano derivatives (6).
Probably, the electron attracting nature of these groups either
positively shifts this potential and the wave probably appears
outside the electrochemical window or the core less prone to
oxidation. This argument is further supported by the fact that
the cyano derivative (6) shows a quasi-reversible reduction
couple.
Fig. 5 I–V–L characteristics of the devices fabricated using (a) TPBI
or (b) Alq3 as electron transporting layer.
1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) and
aluminium tris-8-hydroxyquinoline (Alq3) as electron trans-
porting materials to effect the optimization of charge balance
in the devices. The devices had the configuration as ITO/7c
(40 nm)/TPBI or Alq3 (40 nm)/LiF (1 nm)/Al (150 nm) [Device I]
and ITO/7d (40 nm)/TPBI or Alq3 (40 nm)/LiF (1 nm)/Al
(150 nm) [Device II]. Fig. 5 shows the current–voltage–
luminance (I–V–L) characteristics of the fabricated devices,
and their device characteristics at 100 and 20 mA cmÀ2 are
listed in Table 4. In general, the TPBI serves as a better
electron transporting material for these devices than Alq3,
which is clearly evident from the better efficiency and bright-
ness observed for the TPBI based devices. Particularly, device
II with TPBI as an electron transporting layer showed the
highest external quantum efficiency 1.86%, current efficiency
6.73 cd AÀ1 and power efficiency 4.37 lm/W at a voltage of
4.85 V and a current density of 20 mA cmÀ2. The improved
performance for pyrene containing the dye-TPBI device is
attributed to the larger barrier for the injection of holes from
the dyes (7d) to TPBI (0.97 eV) when compared to the
corresponding barrier (0.86 eV) at the interface of 7d-Alq3
(Fig. 6). This probably enhances the possibility of recombina-
tion and confinement of the exciton in the emitting layer.
The orbital energies, deduced from the oxidation potentials
and absorption edge, are also listed in Table 2. The HOMO
energy levels were observed to be 6.66, 5.29, 5.32, 5.26 and
5.23 eV for 6, 7a, 7b, 7c, and 7d, respectively. It was revealed
that the electron withdrawing substituent lowers the HOMO
energy level whereas the electron donating group increases
the HOMO energy level and decreases the LUMO energy
resulting in a smaller band gap. The conclusions about the
band gap derived from the optical spectra can be corroborated
with these findings (vide supra). All the amine derivatives show
comparable band gaps, out of which the pyrene derivative (7d)
shows a comparatively low band gap, which is evident from
the red-shifted absorption profile and cathodically shifted
oxidation potential.
Electroluminescence and OLED performance
As the amine derivatives have displayed favorable electro-
chemical, thermal and emission properties, we further examined
two selected derivatives (7c and 7d) as hole transporting
emitting materials in double layer OLEDs. We have used both
c
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Table 4 Electroluminescence characteristics of the devices constructed from 7c and 7d
7c/TPBI
7c/Alq3
7d/TPBI
7d/Alq3
lem (fwhm), nm
534 (70)
24120
534 (74)
15390
558 (56)
36750
558 (62)
26140
Max. brightness (Cd/m2)
CIE (x, y)
0.34, 0.60
1.21/1.48
2.132/3.300
4.315/5.287
6.37/5.04
4290/1050
0.34, 0.59
1.09/1.23
1.981/2.718
3.904/4.397
6.21/5.09
3860/870
0.42, 0.55
1.60/1.86
3.026/4.373
5.801/6.733
6.03/4.85
5790/1330
0.41, 0.55
1.53/1.66
2.933/4.010
5.413/5.889
5.81/4.62
5380/1170
External quantum efficiency (%) at (100/20 mA cmÀ2
)
Power efficiency (lm/W) at (100/20 mA cmÀ2
)
Current efficiency (Cd/A) at (100/20 mA cmÀ2
Voltage/V at (100/20 mA cmÀ2
Brightness (Cd/m2) at (100/20 mA cmÀ2
)
)
)
devices.32 In this work, by using amine functionality, we have
shown that the emission color can be tuned to green and
yellow depending on the nature of the diarylamine tethered
with the fluoranthene core.
Conclusions
In summary, we have synthesized electroluminescent materials
based on a fluoranthene core integrated with a polyphenylated
dendron-like structure. Incorporation of diarylamine red shifts
the absorption as well as the emission profiles. All four amine
derivatives are potential molecular materials displaying unique
absorption, emission, thermal and electrochemical properties.
Further, we have also demonstrated that these derivatives
can function as efficient hole transporting and emitting
material in OLEDs. Particularly, the device fabricated using
the pyrene containing derivative, 7d, displayed promising
external quantum efficiency 1.86% and maximum brightness
36750 cd mÀ2 with TPBI as an electron transporting layer,
which appear to be rather high among the previously reported
carbazole based derivatives capable of green or yellow
emission and hole transporting.33
Fig. 6 Energy level diagram of the four devices fabricated using the
dyes 7c and 7d with TPBI and Alq3.
Experimental
General methods
All commercially available materials were used as obtained
from their sources. Dichloromethane (DCM) and toluene were
distilled from phosphorus pentoxide. All chromatographic
separations were carried out with hexane : DCM on silica gel
(60–120 mesh, Rankem).
1H and 13C NMR spectra were recorded on a Bruker AV500
O FT-NMR spectrometer operating at 500 and 125 MHz,
respectively in CDCl3 and (CD3)2SO (chemical shifts (d) in
ppm and coupling constants (J) in Hz). Me4Si (0.00 ppm)
served as internal standard, or residual signals for CHCl3
(1HNMR, d = 7.26; 13CNMR, d = 77.36 ppm) or DMSO
(1HNMR, d = 2.5 ppm; 13CNMR d = 40.45 ppm). Mass
spectra were recorded on a JMS-700 double focusing mass
spectrometer (JEOL, Tokyo, Japan) or a Bruker ESI TOF
mass spectrometer. IR spectra were measured on NEXUS
FT-IR (THERMONICOLET). The electronic absorption
spectra were obtained with an Evolution 600, Thermo
Scientific UV-Vis spectrophotometer in dichloromethane and
toluene solutions. Emission spectra were recorded in dichloro-
methane and toluene with a Fluorologs3 spectrofluorimeter.
The fluorescence quantum yields (FF) were determined using
the classical formula: Fs = (Fr  Ar  Is  Zs2)/(As  Ir  Zr2)
Fig. 7 Comparison of EL spectra observed for the devices with the
PL spectra recorded for the thin films.
The leaking of holes into the TPBI layer is also confirmed by
the residual TPBI based emission observed in the EL (Fig. 7).
EL spectra show peaks at 534 and 558 nm for 7c and 7d
based devices, respectively. CIE coordinates for 7c and 7d
based devices indicated green and yellow emissions,
respectively. This suggests that the emission is originating
from the dye. Although fluorene bridged fluoranthene deriva-
tives have been applied in electroluminescent devices, they
were found to be blue emitting in the electroluminescent
c
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where A is the absorbance at the excitation wavelength, I is the
integrated area under the fluorescence curve and Z is the
refraction index. Subscripts r and s refer to the reference and
to the sample of unknown quantum yield, respectively.
Coumarin 6 in ethanol (FF = 0.78) or coumarin 1 ((FF = 0.99)
in ethyl acetate was used as the reference. The samples
were excited either at 340 nm (for coumarin 1 reference) or
420 nm (for coumarin 6 reference) during the quantum yield
measurements. Qualitative emission spectra of the samples
were obtained by exciting at the absorption maximum. Cyclic
Synthesis of 3-bromo-7,8,9,10-tetraphenylfluoranthene (5b)
mixture of 3-bromo-7,9-diphenyl-8H-cyclopenta[a]-
A
acenaphthylen-8-one (3b) (1 g, 2.3 mmol) and diphenyl-
acetylene (0.45 g, 2.53 mmol) in diphenylether (5 ml) were
heated to 220 1C and stirred for 7 h. After completion of the
reaction, it was quenched by adding hexane. A gray precipitate
was formed. It was filtered and adsorbed on silica gel for
subsequent purification by column chromatography. The
desired product was separated by eluting with a 5 : 1 hexane–
dichloromethane mixture as a pale yellow color solid. Yield:
0.96 g (71%). 1H NMR (DMSO, 500 MHz, ppm): d 7.85
(d, J = 8.5 Hz, 1 H)), 7.69 (d, J = 7.5 Hz, 1 H), 7.49 (t, J =
7.8 Hz, 1 H), 7.30–7.38 (m, 10 H), 6.96–6.98 (m, 4 H), 6.90
(t, J = 7.8 Hz, 4 H), 6.82–6.85 (m, 2 H), 6.42 (d, J = 7.5 Hz, 1 H),
6.25 (d, J = 7.5 Hz, 1 H). 13C (CDCl3, 125 MHz, ppm):
d 141.11, 141.04, 139.68, 139.64, 139.52, 137.38, 137.29,
136.85, 136.37, 136.18, 135.90, 134.48, 131.21, 131.00,
129.98, 129.93, 129.43, 128.85, 128.27, 127.07, 127.05,
126.69, 125.76, 125.54, 123.89, 123.72, 121.88. IR (KBr, cmÀ1):
3056, 1601, 1495, 1442, 1421, 837, 818, 769, 699.
voltammetry experiments were performed with
a CH
Instruments electrochemical analyzer. All measurements were
carried out at room temperature with a conventional three-
electrode configuration consisting of a glassy carbon working
electrode, a platinum wire auxiliary, and a nonaqueous
acetonitrile Ag/AgNO3 reference electrode. The E1/2 values
a
c
a
c
were determined as (Ep + Ep )/2, where Ep and Ep are the
anodic and cathodic peak potentials, respectively. The
potentials are quoted against ferrocene internal standard.
The solvent in all experiments was dichloromethane and the
supporting electrolyte was 0.1
M tetrabutylammonium
hexafluorophosphate. Thermal studies such as TGA-DTA-
DTG measurements were performed on a Perkin-Elmer
(Pyris Diamond) at a heating rate of 10 1C minÀ1 under a
nitrogen atmosphere.
Synthesis of 7,8,9,10-tetraphenylfluoranthene-3-carbonitrile (6)
A mixture of 3-bromo-7,8,9,10-tetraphenylfluoranthene (5b)
(0.73 g, 1.25 mmol) and cuprous cyanide (0.16 g, 1.87 mmol)
and dimethylformamide (10 mL) were placed in a round
bottom flask and heated at 150 1C with efficient stirring for
48 h. The mixture was poured into 10 ml of ammonia solution
to yield a dark yellow precipitate. It was filtered and washed
thoroughly with water. This crude product was further
purified by column chromatography using a 2 : 3 mixture of
dichloromethane and hexane to yield a pale yellow solid.
Yield: 0.24 g (36%). 1H NMR (CDCl3, 500 MHz, ppm):
d 7.93 (d, J = 8.5 Hz, 1 H), 7.67 (d, J = 7.5 Hz, 1 H), 7.41
(t, J = 7.8 Hz, 1 H), 7.28–7.35 (m, 10 H), 6.85–6.91 (m, 10 H),
6.61 (d, J = 7 Hz, 1H), 6.55 (d, J = 7.0 Hz, 1 H); 13C NMR
(CDCl3, 125 MHz, ppm): d 141.63, 140.66, 138.44, 138.37,
138.23, 138.16, 137.58, 136.86, 136.36, 136.18, 134.40, 133.44,
131.92, 130.21, 130.14, 129.29, 128.93, 128.86, 128.11, 127.55,
127.51, 126.48, 126.38, 125.90, 124.85, 123.68, 123.12, 120.95,
116.90, 107.22. IR (KBr, cmÀ1): 3056, 2215, 1603, 1496, 1438,
1418, 765, 698.
OLED fabrication and performance evaluation
Prepatterned ITO substrates with an effective individual device
area of 3.14 mm2 were cleaned as described in a previous
report.34 The amine derivatives (7c or 7d) acting as hole
transporting emitting layers were first deposited (40 nm) on
the ITO substrate by vapor deposition. Then 40-nm-thick Alq3
or a TPBI layer as an electron transport layer was deposited.
Finally, a thin layer of LiF (10 A) followed by aluminium
(1500 A) was deposited as the cathode. The I–V curve was
measured on a Keithley 2400 Source meter in an ambient
environment. Light intensity was measured with a Newport
1835 optical meter.
Computational details
The ground state geometry of the compounds at the gas phase
were optimized using the density functional theory method
with the B3LYP functional in conjugation with the basis set
6-31G(d,p) as implemented in the Gaussian 09 package.35 The
default options for the self-consistent field (SCF) convergence
and threshold limits in the optimization were used. The
electronic transitions were calculated using the time-dependent
DFT (B3LYP) theory and the 6-31G(d,p) basis set. Even
though the time-dependent DFT method less accurately
describes the states with charge-transfer nature, the qualitative
trends in the TDDFT results can still offer correct physical
insights. At least 10 excited states were calculated for each
molecule.
General synthesis of the amine derivatives
The amine substituted fluoranthene derivatives (7a–7d) were
obtained by essentially following a similar procedure utilizing
the C–N coupling reaction involving the palladium catalyst.
An illustrative description is given below for the synthesis of
compound 7a.
Synthesis of N,N,7,8,9,10-hexaphenylfluoranthen-3-amine
(7a). A mixture of 3-bromo-7,8,9,10-tetraphenylfluoranthene
(1.17 g, 2 mmol), diphenylamine (0.41 g, 2.4 mmol), Pd(dba)2
(0.023 g, 0.04 mmol), dppf (0.022 g, 0.04 mmol), sodium
tert-butoxide (0.29 g, 3 mmol), and toluene (15 ml) was placed
in a pressure tube. It was heated at 80 1C and stirred for 48 h.
After completion of the reaction, as evidenced by the
disappearance of 3-bromo-7,8,9,10-tetraphenylfluoranthene,
it was quenched by the addition of water and extracted with
dichloromethane. The combined extracts were washed with
Synthesis
The parent compounds 7,8,10-triphenylfluoranthene36 (4) and
7,8,9,10-tetraphenylfluoranthene23 (5a) were synthesized by
the method reported in the literature.
c
2746 New J. Chem., 2010, 34, 2739–2748 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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brine solution and dried over anhydrous Na2SO4. Rotary
evaporation of the extracts gave the crude product which on
column chromatography purification using 1 : 4 hexane–
dichloromethane mixtures produced a yellow solid. Yield:
1.2 g (89%). 1H NMR (CDCl3, 500 MHz, ppm): d 7.53
(d, J = 8.5 Hz, 1 H), 7.27–7.32 (m, 8 H), 7.21–7.23 (m, 1 H),
7.16 (t, J = 8.0 Hz, 4 H), 7.04–7.07 (m, 1 H), 7.01 (d, J =
7.5 Hz, 4 H), 6.84–6.96 (m, 14 H), 6.49 (d, J = 7.0 Hz, 1 H),
6.46 (d, 7.5 Hz, 1 H). 13C (CDCl3, 125 MHz, ppm): d =
148.97, 144.46, 140.67, 140.25, 139.88, 139.81, 137.20, 136.86,
136.77, 136.55, 136.15, 133.74, 131.33, 131.28, 130.06, 130.04,
129.11, 128.16, 127.35, 127.29, 126.96, 126.90, 126.87, 126.62,
125.43, 124.17, 124.12, 123.24, 122.99, 122.16. ESI-TOF MS:
Calcd for C52H35N: 673.2770. Found: 674.2832 (M+H)+.
N-(Naphthalen-1-yl)-N,7,8,9,10-pentaphenylfluoranthen-3-
amine (7b) Yellow solid. Yield: 75%. 1H NMR (CDCl3,
500 MHz, ppm): d 7.98 (d, J = 9.0 Hz, 1 H), 7.84 (d, J =
8.0 Hz, 1 H), 7.67 (d, J = 8.5 Hz, 1 H), 7.61 (d, J = 8.0 Hz,
1 H), 7.42 (t, J = 7.3 Hz, 1 H), 7.27–7.36 (m, 7 H), 7.17–7.25
(m, 6 H), 7.09–7.12 (m, 2 H), 7.03–7.06 (m, 1 H), 6.79–6.92
(m, 14 H), 6.51 (d, J = 7.0 Hz, 1 H), 6.35 (d, J = 7.5 Hz, 1 H).
13C (CDCl3, 125 MHz, ppm): d = 150.73, 145.55, 145.09,
140.67, 140.12, 139.92, 139.86, 139.82, 137.18, 136.91, 136.65,
136.48, 136.21, 135.22, 135.12, 133.01, 131.35, 131.29, 130.30,
130.06, 129.22, 128.98, 128.39, 128.17, 128.12, 127.26, 126.91,
126.81, 126.61, 126.37, 126.33, 126.09, 126.06, 125.91, 125.82,
125.41, 124.67, 124.28, 124.20, 124.06, 123.34, 123.18, 121.52,
121.34. ESI-TOF MS: Calcd for C56H37N: 723.2926. Found:
724.2981 (M+H)+.
124.86, 124.26, 124.11, 123.29, 123.23, 121.58, 121.48. FAB
MS: Calcd for C62H39N: 797.3083. Found: 797.2 [M]+.
Acknowledgements
We thank the Council of Scientific and Industrial Research
(CSIR), New Delhi for financial support (grant no. 01(2111)/
07/EMR-II). NK acknowledges UGC for a research fellow-
ship. We are also thankful to Prof. J. T. Lin, Academia Sinica,
Taipei for the OLED characterization facility.
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1
phenylfluoranthen-3-amine (7c) Yellow solid. Yield: 84%. H
NMR (CDCl3, 500 MHz, ppm): d 7.99 (d, J = 8.0 Hz, 1 H),
7.83 (d, J = 8.5 Hz, 1 H), 7.65 (d, J = 8.0 Hz, 1 H), 7.59
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2748 New J. Chem., 2010, 34, 2739–2748 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010