New electroluminescent bipolar compounds for balanced charge-transport and tuneable colour in organic light emitting diodes: Triphenylamine-oxadiazole- fluorene triad molecules

Article abstract of DOI:10.1039/b604543j

This work describes bipolar 2,5-diaryl-1,3,4-oxadiazole-fluorene hybrids which incorporate triphenylamine or carbazole units within the π-electron system, viz. compounds 7, 8, 14 and 16. A related bipolar bis(oxadiazolyl) pyridine system 20 is reported. T

Full text of DOI:10.1039/b604543j

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
New electroluminescent bipolar compounds for balanced charge-transport
and tuneable colour in organic light emitting diodes: triphenylamine–
oxadiazole–fluorene triad molecules{
Kiran T. Kamtekar,a Changsheng Wang,a Sylvia Bettington,a Andrei S. Batsanov,a Igor F. Perepichka,{a
Martin R. Bryce,*a Jin H. Ahn,b Mohammad Rabinalb and Michael C. Petty*b
Received 29th March 2006, Accepted 25th July 2006
First published as an Advance Article on the web 24th August 2006
DOI: 10.1039/b604543j
This work describes bipolar 2,5-diaryl-1,3,4-oxadiazole–fluorene hybrids which incorporate
triphenylamine or carbazole units within the p-electron system, viz. compounds 7, 8, 14 and 16. A
related bipolar bis(oxadiazolyl)pyridine system 20 is reported. The syntheses of these five new
materials are discussed, along with their optoelectronic absorption and emission properties, and
their solution electrochemical redox properties. Anodic electropolymerisation of 20 was observed.
Calculations using DFT (density functional theory) establish that they all possess a significantly
higher HOMO energy level (by 0.60–1.02 eV) than 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-
5-yl]benzene (OXD-7) due to the presence of electron-rich amine moieties and increased
conjugation lengths, thereby leading to more balanced charge-transport characteristics. Devices
were fabricated by spin-coating techniques using the bipolar compounds as the emitters in the
simple device architecture ITO:PEDOT-PSS:X:Ca/Al (X = 7, 8, 14, 16 or 20). The turn-on
voltages were 2.9, 5.5, 3.6, 4.5 and 3.4 V for the devices incorporating 7, 8, 14, 16 and 20,
respectively. The highest external quantum efficiency (EQE) was observed for compound 7: viz.
EQE 0.36%; current efficiency 1.00 cd A21; power efficiency 0.56 lm W21 at 5.7 V. The EQE of
the device fabricated from 8 was considerably lower than for devices using other materials due to
low light emission. The EL emission peaked at lmax 430, 487, 487 and 521 nm for 8, 14 and 16,
and 7, respectively. For the 20 device lmax = 521 nm and 564 nm. Thus the HOMO–LUMO gap
has been modified, allowing the colour of the emitted light to vary from light blue through to
green by the systematic chemical modification of the molecular subunits. The high chemical and
thermal durability of these materials combined with the simplicity of the device structure and low
turn-on voltages offers considerable potential for OLED applications.
current LCD materials.3 The design and synthesis of materials
Introduction
which possess balanced hole-transport (HT) and electron-
The discovery of electroluminescence (EL) in low molecular
weight organic molecules1 and in conjugated polymers,2 has
transport (ET) properties remain a major challenge. Most
emissive polymers are predominantly hole-transporting (i.e.
led to unabated intense interest in new materials for
p-dopable), viz. derivatives of poly(p-phenylene vinylene) and
incorporation into organic light emitting diodes (OLEDs) for
poly(fluorene). In a device structure this creates an imbalance
display applications, ranging from small portable devices
of electron injection (from the low work-function cathode) and
to large area screens which consume much less power than
hole injection (from the high work-function anode) with the
consequence that charge recombination occurs near the
polymer/cathode interface which lowers the EL efficiency
aDepartment of Chemistry and Centre for Molecular and Nanoscale
due to quenching of excitons by the metal electrode. This
Electronics, University of Durham, Durham, UK DH1 3LE.
problem can be overcome by using a lower work function
metal (e.g. Ca) as the cathode. However, such metals are
highly reactive and are unstable in the atmosphere.
Alternatively, electron-deficient (n-type) polymeric or low
molecular weight materials have been incorporated into
devices in the following ways: (i) as an additional electron-
transporting hole-blocking (ETHB) layer between the cathode
and the emissive polymer; (ii) by covalently bonding the
electron-deficient segments to the emissive polymer4 or (iii) by
blending the ET material into the emissive polymer prior to
deposition.5 Much work has concerned multilayer structures
with electron-deficient 2,5-diaryl-1,3,4-oxadiazole (OXD)
E-mail: m.r.bryce@durham.ac.uk
bSchool of Engineering and Centre for Molecular and Nanoscale
Electronics, University of Durham, Durham, UK DH1 3LE.
E-mail: m.c.petty@durham.ac.uk
{ Electronic supplementary information (ESI) available: Synthesis of
11; cyclic voltammetry data for compounds 7, 8, 14, 16 and 20, X-ray
crystallographic data for compounds 11, 12, 13 and 20 including
diagrams and discussion of the structures; B3LYP/6-31G(d) optimised
geometries (Figures and Tables of coordinates); orbital energy level
diagrams and frontier orbital localisation for compounds 7a, 8a, 14a,
16a and 20; EL spectra of blended layer devices of MEH-PPV and
compound 7. See DOI: 10.1039/b604543j
{ On leave from the L. M. Litvinenko Institute of Physical Organic
and Coal Chemistry, National Academy of Sciences of Ukraine, R.
Luxemburg Street 70, Donetsk 83114, Ukraine.
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 3823–3835 | 3823

View Article Online
derivatives6 or Alq3 [tris(8-hydroxyquinoline)aluminium]7
serving as the ET material. While this strategy can lead to
very bright devices (sometimes requiring additional transport
layers to provide tri- and tetra-layer structures)8 there are
limitations in their practical applicability. Notably, multilayer
devices require more complex fabrication procedures than
single-layer devices and exciplex formation at the interface of
the organic layers can reduce operating lifetimes and lead to
broad red-shifted emission.9
used by Antoniadis et al. as a component of vapour-deposited
OLEDs with a very complicated device structure involving
several additional hole-transport and electron-transport
layers.17 Rapid degradation of these devices was ascribed
to a combination of exciplex formation at the HTL/ETL
interfaces and instability of the excited state of the fluorene–
oxadiazole hybrid.
In this article we present the synthesis of the new
compounds 7, 8, 14 and 16 which comprise varying juxtaposi-
tions of OXD, fluorene and triphenylamine units along with a
related bipolar bis(oxadiazolyl)pyridine system 20. We discuss
their optoelectronic properties and their applications as
emitters in OLEDs fabricated using simple spin-coating
techniques. In particular, we have established that within this
series of molecules the colour of the emitted light is tuned from
light blue through to green. These materials exhibit good
durability, unlike the analogue of 7 studied by Antoniadis et al.
which was reported to show instability in the excited state.17
Our strategy has been to develop single-layer devices
(sometimes with an additional PEDOT-PSS layer) fabricated
using solution-based processes which offer advantages to
commercialisation by such techniques as ink-jet printing. We
have chemically tailored functional molecules for balanced
charge-injection and charge-transport. For example, new
OXD derivatives, e.g. 1, enhance electron injection in single-
layer devices as blends with poly[2-(2-ethylhexyloxy)-5-meth-
oxy-1,4-phenylenevinylene] (MEH-PPV) as the emissive
material.10 The modest increase in efficiency for the same
device when Al was replaced by a Ca/Al cathode suggests that,
for some combinations of materials, the two methods of
enhancing electron injection into the MEH-PPV emitter are
mutually exclusive, with blended layers offering an attractive
alternative to using Ca electrodes, which are highly reactive
and are unstable in the atmosphere.11
Results and discussion
Synthesis
The route to compounds 7 and 8 is shown in Scheme 1. The
known dihydrazide 4 (obtained in three high yielding steps
from readily available 2,7-dibromo-9,9-dihexylfluorene)10 was
condensed with 4-iodobenzoyl chloride to give derivative 5
(90% yield) which undergoes dehydrative cyclisation18 in
phosphorus oxychloride to give compound 6 (86% yield).
Reaction of 6 with diphenylamine or carbazole under copper-
catalysed C–N bond-forming conditions19 gave the desired
products 7 and 8 in 83 and 85% yields, respectively.
We have now extended these studies by combining OXD,
triarylamine and fluorene units in the same molecule to serve
as ET, HT and emitter segments, respectively. Bipolar triad
molecules of this type are largely unexplored, although diads
comprising ET and HT moieties have been studied. For
example, Adachi and co-workers combined 1,3,4-oxadiazole
with triphenylamine moieties in small molecules.12 More
recently, Zhang et al. reported a luminescent PPV-type
polymer containing 1,3,4-oxadiazole and triphenylamine
units in the main chain.13 Thomas et al. synthesised various
combinations of oxadiazole and quinoxaline as ET segments,
with carbazole and triarylamine as HT segments,14 including
the fluorene-containing system 2. Vacuum deposition techni-
ques were used to fabricate multilayer OLEDs incorporating 2
and additional HT or ET layers. An OXD–spirobifluorene–
triphenylamine hybrid has been reported but no device
studies were mentioned.15 The EL properties of the blue-green
emitting dye 3 as a blend with poly(9-vinylcarbazole) in the
single-layer device ITO/PVK:3/Ca/Al have been reported.16
An analogue of 7 (with octyl chains instead of hexyl) has been
The route to compounds 14 and 16 is shown in Scheme 2.
We have previously shown that the fluorene derivative 9,
containing a trimethylsilyl substituent, is a versatile reagent
which is readily synthesised from 2,7-dibromofluorene.20 The
boronic ester 10 was readily obtained by lithiation followed by
reaction with 2-isopropyl-4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lane (69% yield from 2,7-dibromofluorene). Cross-coupling of
10 with the 2,5-diaryl-1,3,4-oxadiazole derivative 1121 under
Pd-catalysed Suzuki–Miyaura conditions afforded product 12
(78% yield). Bromo-desilylation of 12 with bromine in the
presence of sodium acetate22 gave 13 in 93% yield. Coupling
of 13 with diphenylamine using a palladium catalyst gave
product 14 (90% yield). The attempted two-fold reaction of 13
with aniline gave an impure product. However, when two
3824 | J. Mater. Chem., 2006, 16, 3823–3835
This journal is ß The Royal Society of Chemistry 2006

View Article Online
Scheme 1 (i) 4-Iodobenzoyl chloride, pyridine, 20 uC; (ii) POCl3, 105 uC; (iii) diphenylamine or carbazole, K2CO3, Cu powder, 18-crown-6, 1,2-
dichlorobenzene, reflux.
equivalents of aniline were used in this reaction, the mono-
arylated secondary amine 15 was obtained cleanly (81% yield).
The subsequent reaction of 15 with a second equivalent of 13
gave the tertiary amine 16 (71% yield). The X-ray molecular
structures of 11, 12 and 13 are discussed in the ESI{.
calorimetry (DSC). Both materials are stable up to 500 uC with
a weight loss of ,5%. Measured by DSC the glass transition
temperatures (Tg) were 76 and 146 uC, respectively. Melting
points for both compounds were .400 uC.
To explore the effect of replacing the central fluorene unit
of compound 7 with a pyridyl unit (which is more electron-
deficient)23 compound 20 was synthesised in three steps
The thermal properties of 14 and 16 were studied by
thermal gravimetric analysis (TGA) and differential scanning
Scheme 2 (i) n-BuLi, 278 uC, then 2-isopropyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane; (ii) 11, Pd(PPh3)4, K2CO3, 80 uC; (iii) Br2, NaOAc, THF,
0 uC; (iv) diphenylamine, Pd2(dba)3, tBu3P, NaOtBu, PhMe, 80 uC; (v) aniline, Pd2(dba)3, tBu3P, NaOtBu, PhMe, 80 uC; (vi) 13, Pd2(dba)3, PtBu3,
NaOtBu, PhMe, 80 uC.
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 3823–3835 | 3825

View Article Online
Scheme 3 (i) 4-Iodobenzoyl chloride, pyridine, 100 uC; (ii) polyphosphoric acid, 150 uC; (iii) diphenylamine, K2CO3, Cu powder, 18-crown-6, 1,2-
dichlorobenzene, reflux.
Fig. 1 X-Ray molecular structure of 20 (primed atoms are generated
by the inversion centre, thermal ellipsoids are drawn at the 50%
probability level).
(overall yield ca. 60%) from the known pyridyldihydrazide 1723
(Scheme 3). The structure of 20 was confirmed by a single-
crystal X-ray analysis (Fig. 1).§
Fig. 2 B3LYP/6-311G(2d,p)//B3LYP/6-31G(d) orbital energy level
diagrams for compounds used in OLEDs in the present work and a
comparison with OXD-7.
Theoretical calculations
DFT calculations were performed to elucidate the geometry
and the electronic state of the new compounds 7, 8, 14, 16 and
20. To decrease the computational time for the molecules with
a 9,9-dihexylfluorene fragment we performed calculations on
molecules 7a, 8a, 14a and 16a in which C6H13 was replaced by
C2H5 (denoted by the suffix ‘‘a’’ after the compound name; see
Figure S12 in the ESI{). For comparison, the HOMO–LUMO
energy levels for 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-
5-yl]benzene (OXD-7), which we recently calculated at the
same level of theory, are also shown (Fig. 2).10b OXD-7 is a
benchmark ET material in OLEDs.5
amine moieties and increased conjugation lengths, thereby
explaining their better hole-transport properties. The lower
LUMO energy of 20 compared to its fluorene analogue 7 is a
predictable consequence of the electron-deficient pyridyl ring
in the backbone: this low-lying LUMO should facilitate
acceptance of electrons from the cathode.
The external quantum efficiencies (EQEs) of the single-layer
devices based on compounds 7, 14, 16, and 20 are all similar,
however, the efficiency of the device with carbazole derivative
8 is ca. 1 order of magnitude lower than that for other
compounds (Fig. 9). To understand this we can compare the
HOMO–LUMO energy levels for carbazole derivative 8 with
its diphenylamino analogue 7. As seen from Fig. 2, compound
8 possesses much lower LUMO and HOMO levels compared
to 7, by 0.27 and 0.38 eV, respectively. Indeed, both LUMO
and HOMO are the lowest values in this series of new
compounds. Thus one can speculate that hole injection/
transport is a critical issue for these single layer devices and
lowering the HOMO energy level in the molecules results in a
decrease in the device performance. On the other hand,
electron injection/transport seems to be not so critical:
compound 8, being the strongest electron acceptor in the
fluorene series (i.e. having the lowest LUMO energy) shows
the lowest EL efficiency.
DFT calculations indicate that 7, 8 and 20 have lower
LUMO energy levels than OXD-7, whereas 14 and 16 have
higher LUMO energies. All the new compounds in Table 1
(see also Fig. 2) possess a higher HOMO energy level (by 0.60–
1.02 eV) than OXD-7, due to the presence of the electron-rich
§ CCDC reference numbers 602976–602979. For crystallographic data
in CIF format see DOI: 10.1039/b604543j
3826 | J. Mater. Chem., 2006, 16, 3823–3835
This journal is ß The Royal Society of Chemistry 2006

View Article Online
Table 1 Cyclic voltammetry data,a electrochemical and optical band gaps, and DFT calculated HOMO–LUMO gaps for compounds 7, 8, 14, 16,
and 20.
b
b
)
Compound
E
ox1/V, (DEpa2pc
)
Eox2 [V]
E
red/V, (DEpa2pc
ECg V/Vc
Eogpt/eVd
EDg FT/eVe
7
8
14
16
20
a
Epa = 0.88 Epc = 0.77
Epa = 1.13
E1/2 = 0.60 (60 mV)
E1/2 = 0.53 (81 mV)
Epa = 0.89 Epc = 0.78
Epc # 22.06
2.71
2.93
2.55
2.37
2.48
2.92
3.13
2.89
2.79
2.72
3.14
3.24
3.21
3.09
2.99
E1/2 = 21.99 (50 mV)
22.15
22.02, 22.16
Epa = 1.24
Epa = 1.18
E1/2 = 21.76 (62 mV)
CV in benzonitrile, 0.1 M Bu4NPF6, scan rate 100 mV s21; potentials are vs. Ag/Ag+ reference electrode. Difference between Epa and Epc
b
c
for reversible processes. Electrochemical band gap estimated from the onsets of oxidation and reduction processes. Optical band gap
d
e
estimated from the red edge of absorption spectra in chloroform (Fig. 7). HOMO–LUMO gap from DFT calculations (Fig. 2).
Electrochemistry
observed at a lower potential (for 7 at Epc = +0.62 V, Fig. 4;
see also Fig. S4, S5a, S6 in the ESI{). Cycling between 0 and
+1.0 V (for 7) or +1.2 V (for 8) showed that starting from the
second cycle an additional peak grew in at potentials lower
than the oxidation of the starting compounds (Fig. S5, ESI{).
This was due to electropolymerisation of these bifunctional
triarylamine derivatives and deposition of the polymer films on
the electrode surface. However, in the case of 7 and 8, the
stability of the electropolymerised films was not very good
(particularly, because of solubilising groups on the fluorene
moiety, which can facilitate dissolution of polymeric films on
redox cycling). Moreover, the reduction process for 7 is
irreversible (or only partly reversible, Fig. 4).
To test the electrochemical properties of compounds 7, 8, 14,
16, and 20, cyclic voltammetry (CV) studies were performed in
benzonitrile solution, using tetrabutylammonium hexafluoro-
phosphate (Bu4NPF6) as supporting electrolyte. Compounds
14 and 16 with one triarylamine moiety (on which HOMO
orbital is localized, see Fig. S20 in the ESI{) undergo a
reversible single-electron oxidation to form radical cations
followed by a second, electrochemically irreversible oxidation
process (Fig. 3 and Fig. S1–S3 in the ESI{). The extended
conjugation length in 16 compared to 14 lowers the first
oxidation potential by 0.07 V (from +0.60 to 0.53 V), which is
in good agreement with theoretical calculations (an increase of
the HOMO energy level by 0.05 eV, Fig. 2). Reduction of both
14 and 16 occurs at high negative potentials of ,22 V and is
an irreversible process (Fig. 3 and Fig. S2 in the ESI{).
The most interesting results were obtained for compound 20.
ox
pa
Similar to 7 and 8, 20 shows partly reversible oxidation (E
ox
=
+0.89 V, E = +0.78 V) with the appearance of an additional
polym
pc
pc
peak at lower potentials on re-reduction (E
= 0.63 V).
The electrochemical behaviour of bifunctional compounds
7, 8 and 20, having two terminal triarylamine moieties is
similar to compounds 14 and 16. A feature is that the oxida-
tion process is irreversible or partly reversible. The electro-
chemical irreversibility was established by studies at different
scan rates: on increasing the scan rate to 2000 mV s21, the
Being a good electron acceptor due to the central bis(oxadia-
zolyl)pyridine moiety, compound 20 undergoes a reversible
red
pc{pa
single-electron reduction (DE
= 62 mV, see also inset
red
in Fig. 5) at less negative potentials compared to 7 (E
=
1=2
21.76 V, Fig. 5). Scanning the potentials between 22.05 and
+1.0 V, i.e. from the reduction to the oxidation process in 20,
shows good reversibility and reproducibility of the redox
processes on cycling. The growth of a small additional peak at
ox
pa
cathodic peak i on the re-reduction process increased (Fig. 4
and Fig. S4 in the ESI{ for compound 7). At lower scan rates,
the intensity of i (compared to the intensity of iopcx) was much
ox
pc
smaller and an additional peak on the re-reduction process was
Fig. 4 Cyclic voltammograms of compound 7 in benzonitrile, 0.1 M
Bu4NPF6 at different scan rates. CV at scan rate of 100 mV s21 is
normalised to that for scan rate of 2000 mV s21 at E (by multiplying
ox
pa
polym
pc
Fig. 3 Cyclic voltammogram of compound 14 in benzonitrile, 0.1 M
Bu4NPF6 at 100 mV s21 (E
ox1
1=2
ox1
pa{pc
ox2
ox
ox
pc
ox
= 0.60 V, DE
= 60 mV, E
=
pa
ipa 63.53). E = +0.88 V, E = +0.77 V, E
= +0.62 V
pa
ox
pa
(100 mV s21); E = +0.89 V, E = +0.78 V (2000 mV s21).
1.24 V). Inset shows the deconvoluted CV.
pc
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 3823–3835 | 3827

View Article Online
Fig. 5 Cyclic voltammogram of compound 20 in benzonitrile, 0.1 M
Bu4NPF6 at scan rate 100 mV s21, 4 consecutive scans (starting at
red
pc
red
pa
ox
21.2 V, scan to negative): E = 21.79 V, E = 21.73 V, E =
pa
ox
pc
Fig. 6 Cyclic voltammogram of a film of poly(20) on Pt disk,
+0.89 V, E = +0.78 V. Inset: deconvoluted CV for the oxidation
acetonitrile, 0.1 M Bu4NPF6, scan rate 100 mV s21, 4 consecutive
scans. Inset: potentiodynamic electropolymerisation of 20 in benzoni-
trile, 0.1 M Bu4NPF6, 100 mV s21 (10 ox/red cycles); arrows show the
growth of the polymer film on Pt electrode.
process. Arrows show electrodeposition of the polymer on the Pt
electrode after the first oxidation scan.
ca. +(0.65–0.7) V corresponds to a partial electropolymerisa-
tion process for 20.
electrochemical process occurs in the film deposited on the
electrode (Fig. S9, ESI{). The formation of poly(20) is
represented in Scheme 4; it is a non-conjugated polymer.
Couplings at triphenylamine units will increase the extent of
conjugation at these sites (at which the HOMO is located,
see Fig. S20, ESI{) resulting in a lowering of the HOMO
energy and a cathodic shift of its oxidation wave by y0.15–
0.2 V (cf. Fig. 5 and 6). Reduction of both 20 and poly(20)
occurred at almost the same potentials, which is under-
standable considering the similar localisation of their LUMOs
(Fig. S20, ESI{) and the interrupted conjugation in poly(20)
(cf. Fig. 5 and 6).
More detailed studies of the anodic electropolymerisation of
20 were performed in potentiodynamic conditions by scanning
the potentials for a solution of 20 in benzonitrile between 0
and +0.9 V. Cycling resulted in the growth of a new peak (at
ca. 0.65–0.70 V, inset in Fig. 6) of a polymer film on the Pt
electrode (yellow film). After rinsing the electrode with
acetonitrile, the CV recorded in monomer-free acetonitrile
(with 0.1 M Bu4NPF6 as electrolyte) showed the electroactivity
towards both p- and n-doping, with high stability on cycling
(Fig. 6). The electroactivity of poly(20) towards p-doping (at
ca. 0.6–0.8 V) demonstrated a good linear dependence on the
scan rate in the range of 10 to 200 mV s21 proving that an
Scheme 4 Electrochemical polymerisation of 20 in benzonitrile, 0.1 M Bu4NPF6 on a Pt disk electrode; left hand (smaller) and right hand (larger)
dashed ovals show the localisation of LUMO and HOMO, respectively.
3828 | J. Mater. Chem., 2006, 16, 3823–3835
This journal is ß The Royal Society of Chemistry 2006

View Article Online
CV data for compounds 7, 8, 14, 16, and 20 are collated in
Table 1. The band gaps estimated from the onsets of oxidation
and reduction processes in the electrochemical experiments are
different from those estimated from the spectroscopic data
(from the red edge of the longest wavelength absorption,
Fig. 7), which is not surprising taking into account that they
correspond to different processes. Moreover, there are two
additional factors: (i) being bipolar compounds, they show
some solvent polarity dependence of their absorption spectra
(and hence Eogpt), and (b) the electrochemistry is complicated
by irreversibility of one of the redox process (either oxidation
or reduction) limiting the accuracy of the ECg V value. Neverthe-
less, the general trend is clear and correlation coefficients
between the ECg V 2 Egopt and Eogpt 2 EDg FT are y0.95–0.96.
Table 2 Data for solution state photophysical studies
Compound
lmax abs (nm)
lmax PL (nm)
Wfa
7
8
14
16
20
a
385
361
380
399
402
452
416
478
483
533
0.86
0.87
0.92
0.93
0.55
in DCM solution, measured against quinine sulfate and fluorescein
standards.
14 and 16 fall within 30 nm of each other and their relative
colours are predicted by the CV and optical data.
The fluorescence quantum yields are very high for the
fluorene containing compounds, ranging from 0.86–0.93,
whilst the pyridine containing compound 20 gave a Wf value
of 0.55. The photophysical data are summarised in Table 2.
The fluorescence quantum yields were measured in DCM
solution against quinine sulfate and fluorescein standards.
Devices were fabricated using the new compounds 7, 8, 14,
16 and 20 in the architecture ITO:PEDOT-PSS:compound
X:Ca/Al. The current versus voltage (I–V) and light output
versus voltage (L–V) characteristics of these devices are
shown in Fig. 8 (positive bias applied to the ITO electrode).
Optical properties and device performance
The UV-Vis absorption and photoluminescence (PL) spectra
in chloroform solution (Fig. 7) show an incremental red-shift
in the sequence: lmax (abs): 8, 361; 14, 380; 7, 385; 16, 399 and
20, 402 nm; lmax (PL): 8, 416; 7, 452; 14, 478; 16, 483; 20,
533 nm. As predicted by the HOMO–LUMO gap calculations
(Fig. 2) compound 8 provides the bluest emission and 20 shows
the most red shifted emission which is also notably broadened
compared to the other compounds. The lmax (PL) values for 7,
Fig. 7 Normalised UV-Vis absorption (top) and photoluminescence
(bottom) spectra of compounds 7, 8, 14, 16 and 20 in chloroform. For
the PL measurements the samples were excited at 370 nm, except for 8
which was excited at 350 nm.
Fig. 8 Current–voltage (a) and light output–voltage (b) charac-
teristics of the single-layer devices using 7, 8, 14, 16 and 20.
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 3823–3835 | 3829

View Article Online
Fig. 9 The EQE data of the devices using 7, 8, 14, 16 and 20 in the
structure ITO:PEDOT-PSS:compound X:Ca/Al.
Fig. 10 Luminance versus bias voltage diagram which compares the
operation voltages of the devices using 7, 8, 14, 16 and 20.
Compounds 7, 14 and 20 showed similar current with each
other. The current of 8 and 16 was lower at the same bias. The
voltage required for a current of 30 mA cm22 was about 7 V
for 7, 14 and 20; about 9 V for 8 and about 10 V for 16. The
light emission from 16 was notably lower than other devices.
The relative light emission at 7 V normalised to that of 16 was
157, 89, 7 and 64 for 7, 14, 8 and 20, respectively.
showed peaks at lmax 521 nm (2.38 eV) and 564 nm (2.20 eV).
Several lower intensity peaks are present at lmax 411 nm
(3.02 eV), 457 nm (2.72 eV), 610 nm (2.04 eV) and 665 nm
(1.87 eV) in the EL of all compounds beside the main peaks
of the materials. CIE coordinates were (0.235, 0.432), (0.214,
0.16), (0.191, 0.352), (0.393, 0.549) and (0.213, 0.358) for 7, 8,
14, 20 and 16, respectively.
Fig. 9 shows the EQE vs. current data for the devices in
Fig. 8. The EQE of the 8 device was more than one order of
magnitude lower than the values for devices using other
materials. This low efficiency results from the very low light
emission of the 8 device as shown in Fig. 8b. The light emission
of the 16 device was lower than the 7, 14 and 20 devices.
However, the EQE of the 16 device was similar because the
lower current compensated for the lower light emission. Table 3
summarises and compares the efficiency of the devices using
each material.
We have previously reported that the efficiency of devices
could be greatly increased by blending poly[2-(2-ethylhexyl-
oxy)-5-methoxy-1,4-phenylenevinylene] MEH-PPV with elec-
tron transporting, hole blocking (ETHB) materials.10,11 When
compound 7 was blended with MEH-PPV, the efficiencies of
the 50 and 95% (by the weight of 7) devices using an Al
electrode were 0.04 and 0.26%, respectively. These efficiencies
are similar to those reported using other types of ETHB
materials10,11 and are two orders of magnitude higher than the
pure MEH-PPV reference device without 7. As well as the
efficiency, the current and light output behaviour and EL
spectrum of the blend devices were similar to our previous
reports. Light output was solely from the MEH-PPV for the
50% device, whereas an additional minor peak in the emission
Fig. 10 compares the operation voltages of each device. The
turn on voltage was defined as the bias required to achieve a
brightness of 0.1 cd m22 which was observed visually. The
turn-on voltages were low, viz. 2.9, 5.5, 3.6, 4.5 and 3.4 V for
the devices incorporating 7, 8, 14, 16 and 20, respectively.
Fig. 11 shows the EL spectra of the devices fabricated using
7, 8, 14, 16 and 20. The emission peaked at lmax 430 nm
(2.89 eV), 487 nm (2.55 eV), 487 nm (2.55 eV) and 521 nm
(2.38 eV) for 8, 14 and 16, and 7, respectively. The 20 device
Table 3 Data for OLEDs using the device structure ITO:PEDOT-
PSS:compound X:Ca/Al.a
External
quantum
Current
efficiency/ Power efficiency/lm W21
Compound efficiency (%) cd A21
(operating voltage/V)
7
8
14
16
20
a
0.36
0.015
0.26
0.22
0.18
1.00
0.019
0.60
0.49
0.54
0.56 (5.7)
0.005 (10.87)
0.29 (6.5)
0.17 (9.3)
0.25 (6.9)
EQE was measured at 100 cd m22 except the 8 device which was
measured at 15 cd m22
.
Fig. 11 The EL spectra of the devices using 7, 8, 14, 16 and 20.
This journal is ß The Royal Society of Chemistry 2006
3830 | J. Mater. Chem., 2006, 16, 3823–3835

View Article Online
spectrum of the 95% blend device can be assigned to emission
from compound 7 (Figure S23 in the ESI{). The implication
here is that the components of the blended layers (i.e. MEH-
PPV and 7) are intimately mixed and there is very efficient
energy transfer to the emissive polymer.
in thickness) was dried for 12 h in nitrogen at room
temperature to remove residual solvent. The compounds (7,
8, 14, 16 and 20) were dissolved in chloroform (15 mg cm23
)
and which were spin-coated onto the ITO to make 110 nm
thick layers. Following the spin-coating, Ca (40 nm) and Al
(100 nm) electrodes with 1 mm radius were thermally
evaporated at a pressure of ca. 1026 mbar, respectively.
Electrical measurements were conducted in a vacuum
chamber (1021 mbar). A D.C. bias was applied and the
current was measured by a Keithley 2400 source measure unit
and the light emitted from the device was collected by a large
area photodiode (1.5 cm diameter) connected to a Keithley 485
digital picoammeter. For external quantum efficiency mea-
surements, the light power was calculated using the photo-
current and the conversion factor (wavelength dependent)
of the photodiode (ampere/watt). Electroluminescence (EL)
spectra were measured using an Ocean Optics USB2000
Miniature Fibre Optic Spectrometer.
Conclusions
We have synthesised new bipolar molecules 7, 8, 14 and 16
comprising linearly p-extended fluorene derivatives with 1,3,4-
oxadiazole fragments for electron injection and arylamine
substituents to enhance hole transport and established that
they are suitable emitters for OLEDs. A related bipolar
bis(oxadiazolyl)pyridine system 20 is reported. The DFT
calculations establish that they all possess a higher HOMO
energy level (by 0.60–1.02 eV) than OXD-7 due to the presence
of the electron-rich amine moieties and increased conjugation
lengths, giving rise to more balanced charge-transport
characteristics. Cyclic voltammetry studies showed reversible
(14 and 16) or partly reversible (7, 8 and 20) oxidation process
and reversible (8 and 20) or irreversible (7, 14 and 16)
reductions, with electrochemical band gaps in the range of
2.37 to 2.93 eV. There are generally good correlations
between the optical data, electrochemical data and DFT
calculations. Single-layer devices were fabricated by using
solution-based, spin-coating techniques using these new
compounds as emitters in the simple architecture
ITO:PEDOT-PSS:X:Ca/Al (X = compound 7, 8, 14, 16 and
20). The lowest turn on voltage (2.9 V) and the highest external
quantum efficiency (EQE) was observed for compound 7: viz.
EQE 0.36%; current efficiency 1.00 cd A21; power efficiency
0.56 lm W21 at 5.7 V. The colour of the emitted light has been
tuned from light blue through to green by chemical modifica-
tion within this series of compounds. The lower device
efficiencies in comparison with the best reported systems is
offset by the high chemical and thermal stability of the
materials, the simplicity of the device architecture and their
low turn-on voltages. These are promising materials for further
OLED applications.
Cyclic voltammetry
Cyclic voltammetry experiments were carried out using a BAS-
CV50W electrochemical workstation in a three-electrode cell
equipped with a platinum disk (Ø 1.6 mm) working electrode,
platinum wire counter electrode and a non-aqueous Ag/Ag+
reference electrode (0.01 M AgNO3 in dry MeCN), with iR
compensation. CV data for compounds 7, 8, 14, 16 and 20
were obtained in dry benzonitrile with 0.1 M tetrabutylam-
monium hexafluorophosphate (Bu4NPF6) as a supporting
electrolyte, under an argon atmosphere. The potential of the
reference electrode in benzonitrile (0.1 M Bu4NPF6) was
checked against the ferrocene/ferrocenium couple (Fc/Fc+),
which showed the average potential against the reference
electrode of +0.187 V. CV of polymers electrodeposited on Pt
working electrode from solutions of either 14, 16 and 20 in
benzonitrile were studied (after rinsing in dry acetonitrile
to remove traces of monomers) in dry acetonitrile with 0.1 M
Bu4NPF6.
Computational procedures
The ab initio computations of the geometries of compounds
7a, 8a, 14a, 16a, 20 and OXD-7 were carried out with the
Gaussian 0324 package of programs at density functional
theory (DFT) level using Pople’s 6-31G split valence basis set
supplemented by d-polarisation functions on heavy atoms.
DFT calculations were carried out using Becke’s three-
parameter hybrid exchange functional25 with Lee–Yang–Parr
gradient-corrected correlation functional (B3LYP).26 Thus,
the geometries were optimised with a B3LYP/6-31G(d) level
of theory and the electronic structures were then calculated for
a single point at B3LYP/6-311G(2d,p)// B3LYP/6-31G(d)
level. Whereas HOMO and LUMO orbital energies calculated
at B3LYP/6-311G(2d,p)//B3LYP/6-31G(d) level are ca.
0.17–0.23 eV lower than that for B3LYP/6-31G(d) optimised
geometries, the HOMO–LUMO gaps are quite similar,
showing a deviation of only 0.01–0.03 eV. Space-filled
structures for optimised geometries and contours of HOMO
and LUMO orbitals were visualised using the Molekel v.4.3
program.27
Experimental
General
The procedures and equipment used for the synthesis and
characterisation of materials are the same as those reported
recently.10
Device fabrication and measurement
MEH-PPV was purchased from Aldrich whereas the ET
materials were synthesised as described below. Indium–tin
oxide (ITO) coated glass from Merck with a sheet resistance
21
of 9 V % was used as the anode. The ITO coated glass was
cleaned by ultrasonification in acetone and isopropyl alcohol
for 30 min each and dried with a nitrogen gun. For the hole
injection layer, poly(3,4-ethylenedioxythiophene) was doped
with polystyrene sulfonated acid (PEDOT:PSS, purchased
from Bayer AG) and spin-coated onto the ITO prior to the
deposition of the organic materials. The PEDOT layer (40 nm
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 3823–3835 | 3831

View Article Online
Compound 5
2,7-Bis{2-[4-(N-carbazolyl)phenyl]-1,3,4-oxadiazol-5-yl}-9,9-
dihexylfluorene (8)
To a stirred solution of compound 410b (1.35 g, 3.0 mmol) in dry
pyridine (30 cm3) which was cooled with a cold water bath,
4-iodobenzoyl chloride (2.00 g, 7.5 mmol) was added. The
mixture was stirred for 15 min then heated to 50 uC and stirred
for an additional 15 min. The pyridine was removed by vacuum
evaporation and methanol (100 cm3) was added and the mixture
was gently boiled then left to cool. Compound 5 was isolated by
suction filtration and washing with methanol, as white shiny
crystals (2.45 g, 90%), mp: 272.0–275.0 uC. Anal. calcd for
C41H44I2N4O: C, 54.08; H, 4.87; N, 6.15. Found: C, 55.04; H,
5.07; N, 6.45; dH (DMSO-d6, 300 MHz) 0.49 (m, 2H), 0.71 (t, J =
6.8 Hz, 3H), 1.02 (m, 6H), 2.07 (m, 2H), 7.71 (d, J = 8.4 Hz, 2H),
7.94 (d, J = 8.4 Hz, 2H), 7.96 (d, J = 5.4 Hz, 1H), 8.03 (s, 1H),
8.04 (d, J = 7.5 Hz, 1H), 10.60 (s, 2H) ppm.
By analogy to the synthesis of 7, compound 6 (0.79 g,
0.90 mmol), carbazole (0.5 g, 3.0 mmol), potassium carbonate
powder (0.50 g), copper powder (0.23 g), 18-crown-6 (45 mg)
and 1,2-dichlorobenzene (20 cm3) were refluxed for 60 h.
Column chromatography on silica, eluting with DCM–diethyl
ether (95 : 5 v/v) and recrystallisation twice each from DCM–
ethanol mixture and toluene–cyclohexane mixtures yielded 8
as an off-white solid (0.78 g, 85%), mp: 194.4–195.5 uC. Anal.
calcd for C65H56N6O2: C, 81.90; H, 5.92; N, 8.82. Found: C,
82.00; H, 6.10; N, 8.56%; 1H NMR (CDCl3, 300 MHz): d 0.69
(m, 2H), 0.77 (t, J = 7.0 Hz, 3 H), 1.2–1.0 (m, 6 H), 2.21 (m,
2 H), 7.35 (td, J12 = 7.4 Hz, J13 = 1.2 Hz, 2H), 7.47 (td, J12
=
7.5 Hz, J13 = 1.2 Hz, 2H), 7.54 (d, J = 7.8 Hz, 2 H), 7.82 (d, J =
7.8 Hz, 2H), 7.98 (d, J = 8.4 Hz, 1H), 8.18 (d, J = 7.5 Hz, 2H),
8.24 (d, J = 7.2 Hz, 1H), 8.25 (s, 1H), 8.45 (d, J = 8.4 Hz, 2H)
ppm; 13C NMR (CDCl3, 75 MHz): d 165.2, 164.0, 152.4, 143.5,
140.9, 140.2, 128.6, 127.2, 126.3, 126.2, 123.8, 123.3, 122.4,
121.4, 121.1, 120.6, 120.5, 109.7, 56.0, 40.3, 31.5, 29.6, 23.8,
22.5, 14.0 ppm.
2,7-Bis[2-(4-iodophenyl)-1,3,4-oxadiazol-5-yl]-9,9-
dihexylfluorene (6)
A mixture of compound 5 (2.45 g, 2.69 mmol) and POCl3
(30 cm3) was stirred at reflux for 4 h. The POCl3 was removed
by vacuum evaporation, ethanol (100 cm3) was added and
mixture was boiled to dissolve the residue. Pale-yellow crystals
of compound 6 were obtained after cooling and a subsequent
suction filtration (2.02 g, 86%), mp: 202.6–203.1 uC. Anal. calcd
for C41H40I2N4O2: C, 56.31; H, 4.61; N, 6.41. Found: C, 56.01;
H, 4.58; N, 6.38%; 1H NMR (CDCl3, 400 MHz): d 0.61 (m, 2H),
0.73 (t, J = 7.2 Hz, 3H), 1.1–0.9 (m, 6H), 2.15 (m, 2H), 7.92 (m,
5H), 8.16 (m, 2H) ppm; 13C NMR (CDCl3, 75 MHz): d 165.1,
164.0, 152.3, 143.5, 138.4, 128.3, 126.3, 123.3, 123.2, 121.4,
121.1, 98.6, 56.0, 40.2, 31.5, 29.5, 23.8, 22.5, 13.9 ppm.
4,4,5,5-Tetramethyl-2-[9,9-dihexyl-2-(trimethylsilyl)fluoren-7-
yl]-1,3,2-dioxaborolane (10)
An argon purged flask was charged with 2,7-dibromo-9,9-
dihexylfluorene (9.00 g, 18.3 mmol) and dry THF (70 cm3) and
cooled to 278 uC. n-Butyllithium (2.5 M solution, 7.3 cm3,
18.3 mmol) was added dropwise and the resulting solution was
stirred at 278 uC for 0.5 h. Trimethylsilyl chloride (3.4 cm3,
26.8 mmol) was added in one portion and the solution was
stirred at 278 uC for 0.5 h before being allowed to warm to
room temperature for 1 h. The reaction was quenched with
brine and the crude product was extracted into diethyl ether,
which was dried over MgSO4, filtered and concentrated to
yield a yellow oil. The crude product was purified by column
chromatography on silica eluting with petroleum ether (bp 40–
60 uC) to yield 9 a colourless oil which was dried and used
without further purification.
The oil 9 was dissolved in dry THF (50 cm3) under argon
and cooled to 278 uC. n-Butyllithium (2.5 M solution, 7.4 cm3,
18.5 mmol) was added dropwise and the solution stirred at
278 uC for 0.5 h. 2-Isopropyl-4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane (4.2 cm3, 20.6 mmol) was added in one portion and
the reaction was allowed to warm slowly to room temperature
overnight. Brine was added and the mixture was stirred for a
further 3 h. The product was extracted into diethyl ether, the
extracts were washed with water, dried over MgSO4, filtered
and concentrated to yield a white solid. This was purified by
column chromatography on silica (eluent DCM–ethyl acetate
9 : 1 v/v) to yield 10 as a white solid (6.76 g, 69%), mp: 113.2–
115.1 uC. Anal. calcd for C34H53BO2Si: C, 76.66; H, 10.03.
Found: C, 76.73; H, 10.05%; 1H NMR (CDCl3): d 0.30 (s, 9H),
0.60 (m, 4H), 0.76 (t, J = 6.7 Hz, 6H), 1.03 (m, 12H), 1.39 (s,
12H), 1.98 (t, J = 8.4 Hz, 4H), 7.47 (s, 1H), 7.48 (d, J = 8.5 Hz,
1H), 7.77 (d, J = 8.5 Hz, 2H), 7.75 (s, 1H), 7.80 (d, J = 8.5 Hz, 1H)
ppm; 13C NMR (CDCl3): d 0.00, 14.86, 23.36, 24.48, 25.72, 25.84,
30.43, 32.22, 40.87, 55.91, 84.58, 119.96, 120.23, 128.58, 129.84,
132.58, 134.56, 140.50, 142.46, 145.00, 151.01, 151.35 ppm.
2,7-Bis[2-(4-diphenylaminophenyl)-1,3,4-oxadiazol-5-yl]-9,9-
dihexylfluorene (7)
A mixture of compound 6 (0.875 g, 1 mmol), diphenylamine
(0.508 g, 3 mmol), potassium carbonate powder (0.55 g),
copper powder (0.25 g, Aldrich, 99% for Org. Synth.) and
18-crown-6 (50 mg) in 1,2-dichlorobenzene (20 cm3) was
heated under argon to gentle reflux then stirred with heating
for 50 h. The dichlorobenzene was removed by distillation
under vacuum and chloroform (50 cm3) was added to the
residue. The mixture was sonicated for 5 min and the solids
were removed by suction filtration. The filtrate was vacuum
evaporated to dryness and the residue was purified by column
chromatography on silica, eluting with DCM–diethyl ether
(95 : 5 v/v). The yellow fraction was crystallised from DCM–
ethanol and then recrystallised from cyclohexane to afford
compound 7 as bright yellow needles (0.79 g, 83%), mp: 190.0–
190.5 uC. Anal. calcd for C65H60N6O2: C, 81.56; H, 6.32; N,
1
8.78. Found: C, 81.65; H, 6.28; N, 8.84%; H NMR (CDCl3,
300 MHz): d 0.61 (m, 2H), 0.73 (t, J = 6.9 Hz, 3 H), 1.10–0.99
(m, 6 H), 2.12 (m, 2 H), 7.13 (d, J = 7.2 Hz, 2H), 7.15 (t, J =
7.8 Hz, 2H), 7.19 (d, J = 8.4 Hz, 4 H), 7.32 (d, J = 7.8 Hz, 2H),
7.35 (d, J = 7.8 Hz, 2H), 7.89 (d, J = 7.8 Hz, 1H), 7.99 (d, J =
8.1 Hz, 2H), 8.13 (d, J = 7.2 Hz, 1H), 8.14 (s, 1H) ppm; 13C
NMR (CDCl3, 75 MHz): d 164.6, 164.4, 152.2, 150.9, 146.6,
143.2, 129.6, 128.0, 126.0, 125.7, 124.4, 123.5, 121.2, 121.1,
121.0, 116.0, 74.7, 55.9, 40.3, 31.5, 29.6, 23.8, 22.5, 14.0 ppm.
3832 | J. Mater. Chem., 2006, 16, 3823–3835
This journal is ß The Royal Society of Chemistry 2006

View Article Online
2-(4-tert-Butylphenyl)-5-{4-[9,9-dihexyl-2-
solution which was heated at 80 uC with stirring for 16 h. After
cooling to room temperature water was added and the layers
separated. The aqueous layer was extracted with diethyl ether
and the combined organic extracts were washed with brine,
dried over MgSO4, filtered and concentrated to yield yellow
oil. The product was purified by column chromatography on
silica eluting with DCM to yield 14 as a yellow solid which was
dried under vacuum mp: .400 uC; Tg 76 uC (306 mg, 90%).
Anal. calcd for C55H59N3O: C, 84.90; H, 7.64; N, 5.40. Found:
C, 84.68; H, 7.66; N, 5.12%; 1H NMR (CDCl3): d 0.71 (s, 4H),
0.80 (t, J = 7.2, 6H), 1.07 (m, 12H), 1.39 (s, 9H), 1.90 (m, 4H),
7.03 (m, 3H), 7.14 (m, 4H), 7.24 (m, 5H), 7.58 (m, 5H), 7.70 (d,
J = 8.0, 1H), 7.83 (d, J = 8.8, 2H), 8.10 (d, J = 8.4, 2H), 8.23 (d,
J = 8.8, 2H) ppm; 13C NMR (CDCl3): d 14.01, 22.54, 23.82,
29.62, 31.15, 31.51, 35.11, 40.26, 55.25, 119.27, 119.53, 120.62,
121.24, 122.47, 122.60, 123.52, 123.92, 126.07, 126.12, 126.82,
127.35, 127.60, 127.70, 129.19, 135.58, 137.79, 141.24, 144.91,
147.49, 147.98, 151.60, 152.51, 155.34, 164.40, 164.67 ppm; MS
(ES+): m/z 778.7 (M+).
(trimethylsilyl)fluoren-7-yl]phenyl}-1,3,4-oxadiazole (12)
A mixture of 10 (3.00 g, 5.6 mmol), 11 (2.01 g, 5.6 mmol)
(synthesised as described in the ESI{), potassium carbonate
(1 M solution, 30 cm3) and THF (50 cm3) was degassed for
1 h before tetrakis-(triphenylphosphine)palladium (330 mg,
5 mol %) was added and the mixture was refluxed under argon
at 80 uC for 65 h. After cooling to room temperature, the
layers were separated and the aqueous layer was extracted with
ethyl acetate. The combined organic extracts were washed with
brine, dried over MgSO4, filtered and concentrated to yield a
dark oil. This was purified by column chromatography on
silica eluting with DCM. The product was recrystallised from
ethanol to yield 12 as a white crystalline solid (3.00 g, 78%),
mp: 144.0–144.7 uC. Anal. calcd for C46H58N2OSi: C, 80.89;
H, 8.56; N, 4.10. Found: C, 81.11; H, 8.57; N, 4.10%; 1H NMR
(CDCl3): d 0.34 (s, 9H), 0.75 (m, 4H), 0.78 (t, J = 6.4 Hz, 6H),
1.09 (m, 12H), 1.40 (s, 9H), 2.03 (m, 4H), 7.52 (m, 2H), 7.58 (d,
J = 8.8 Hz, 2H), 7.63 (m, 2H), 7.73 (d, J = 7.6 Hz, 1H), 7.81 (d,
J = 8.0 Hz, 1H), 7.84 (d, J = 8.4 Hz, 2H), 8.11 (d, J = 8.4 Hz,
2H), 8.24 (d, J = 8.4 Hz, 2H) ppm; 13C NMR (CDCl3): d 0.00,
14.85, 23.35, 24.59, 30.44, 32.03, 32.23, 36.00, 41.02, 56.07,
120.07, 121.11, 122.09, 122.39, 123.46, 126.96, 127.71, 128.24,
128.53, 128.58, 132.79, 139.59, 140.40, 141.95, 142.25, 145.81,
151.11, 152.76, 156.24, 165.27, 165.57 ppm; MS (EI): m/z
682 (M+).
7-{4-[5-(4-tert-Butylphenyl)-1,3,4-oxadiazol-2-yl]phenyl}-9,9-
dihexyl-N-phenylfluoren-2-amine (15)
A mixture of 13 (500 mg, 0.72 mmol), aniline (0.13 cm3,
1.4 mmol), sodium tert-butoxide (80 mg, 0.83 mmol),
tris(dibenzylideneacetone)dipalladium (35 mg 5 mol %), tri-
tert-butylphosphine (y15 mg) and dry toluene (30 cm3) was
heated at 80 uC for 16 h. Workup as described for 14 yielded a
brown oil which was purified by column chromatography on
silica eluting with DCM. The product was recrystallised from
ethanol to yield 15 as a yellow crystalline solid (0.41 g, 81%),
mp: 169.2–170.2 uC. Anal. calcd for C49H55N3O: C, 83.84; H,
2-(4-tert-Butylphenyl)-5-[4-(9,9-dihexyl-2-bromofluoren-7-
yl)phenyl]-1,3,4-oxadiazole (13)
A mixture of 12 (3.00 g, 4.4 mmol), sodium acetate (0.75 g,
9.0 mmol) and dry THF (40 cm3) was cooled in the dark to 0 uC
in an ice bath. Bromine (0.90 g, 17.5 mmol) was added and the
solution was stirred for 2 h with the ice bath removed. The
reaction was then quenched with triethylamine (4.9 cm3,
8 equiv.) producing a white precipitate. A saturated sodium
thiosulfate solution was then added to quench the excess
bromine. The product was extracted with diethyl ether and
the combined organic extracts were washed with water, dried
over MgSO4, filtered and concentrated. The resulting solid
was recrystallised from ethanol to yield 13 as shiny white
crystals (2.82 g, 93%), mp: 133.9–134.8 uC. Anal. calcd for
C43H49BrN2O: C, 74.87; H, 7.16; N, 3.97. Found: C, 75.23; H,
1
7.90; N, 5.99. Found: C, 83.79; H, 7.90; N, 5.79%; H NMR
(CDCl3): d 0.70 (m, 10H), 1.05 (m, 12H), 1.36 (s, 9H), 2.00 (m,
4H), 6.86 (t, J = 7.1, 1H), 7.12 (m, 4H), 7.26 (t, J = 8.2, 2H),
7.68 (d, J = 8.2, 2H), 7.73 (m, 2H), 7.80 (2H), 8.01 (d, J = 8.2,
2H), 8.10 (d, J = 8.2, 2H), 8.22 (d, J = 8.2, 2H), 8.38 (s, 1H)
ppm; 13C NMR (CDCl3): d 13.74, 21.88, 23.43, 28.88, 30.81,
30.86, 34.83, 38.30, 54.64, 111.88, 116.61, 117.33, 119.85,
120.41, 121.38, 121.53, 121.77, 122.42, 126.51, 126.98, 127.29,
127.90, 128.03, 129.90, 132.87, 136.64, 142.11, 143.89, 144.15,
144.54, 151.28, 152.75, 155.71, 164.51, 164.72 ppm.
7-{4-[5-(4-tert-Butylphenyl)-1,3,4-oxadiazol-2-yl]phenyl}-N-{2-
[4-(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)phenyl]-9,9-
dihexyl-fluoren-7-yl}-9,9-dihexyl-N-phenyl-9H-fluoren-2-amine
(16)
1
7.23; N, 3.97%; H NMR (CDCl3): d 0.74 (m, 10H), 1.04 (m,
12H), 1.37, (s, 9H), 2.00 (m, 4H), 7.49 (m, 2H), 7.59 (m, 5H),
7.82 (m, 3H), 8.08 (d, J = 7.9, 2H), 8.22 (d, J = 7.9, 2H) ppm;
13C NMR (CDCl3): d 13.96, 22.54, 23.73, 29.61, 31.14, 31.44,
35.11, 40.28, 55.61, 120.24, 121.18, 121.26, 121.40, 121.45,
122.76, 126.08, 126.27, 126.31, 126.82, 127.38, 127.72, 130.11,
139.12, 139.52, 140.26, 144.65, 151.30, 153.30, 155.40, 164.32,
164.73 ppm.
A mixture of 15 (360 mg, 0.51 mmol), 13 (0.360 mg,
0.52 mmol), sodium tert-butoxide (80 mg, 0.83 mmol),
tris(dibenzylideneacetone)dipalladium (25 mg, 5 mol %), tri-
tert-butylphosphine (y15 mg) and dry toluene (30 cm3) was
heated at 80 uC for 16 h. Workup as described for 14 yielded a
brown oil which was purified by column chromatography on
silica (eluent DCM–ethyl acetate 9 : 1 v/v). The product was
recrystallised from ethanol to yield 16 as a yellow solid (0.48 g,
71%) mp: .400 uC; Tg 146 uC. Anal. calcd for C92H103N5O2:
C, 84.30; H, 7.92; N, 5.34. Found: C, 84.10; H, 7.94; N, 5.35%;
1H NMR (CDCl3): d 0.77 (m, 20H), 1.70, (m, 24H), 1.39 (s,
18H), 1.95 (m, 8H), 7.07 (m, 3H), 7.20 (m, 4H), 7.28 (m, 2H),
7-{4-[5-(4-tert-Butylphenyl)-1,3,4-oxadiazol-2-yl]phenyl}-9,9-
dihexyl-N,N-diphenylfluoren-2-amine (14)
A mixture of 13 (300 mg, 0.4 mmol), diphenylamine (74 mg,
0.4 mmol), sodium tert-butoxide (58 mg, 0.6 mmol), tris(di-
benzylideneacetone)dipalladium (5 mg), tri-tert-butylphos-
phine (y2 mg) and dry toluene (25 cm3) gave a dark red
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 3823–3835 | 3833

View Article Online
7.60 (m, 10H), 7.72 (d, J = 8.9, 2H), 7.84 (d, J = 8.9, 2H), 8.10
(d, J = 8.9, 2H), 8.23 (d, J = 8.9, 2H) ppm; 13C NMR (CDCl3):
d 14.34, 22.85, 24.12, 29.92, 31.40, 31.83, 35.37, 40.56, 55.51,
119.03, 119.77, 120.88, 121.41, 121.50, 122.68, 122.85, 123.53,
123.96, 126.34, 126.40, 127.06, 127.60, 127.85, 129.47, 135.75,
138.02, 141.49, 145.13, 147.64, 148.35, 151.76, 152.73, 155.61,
164.64, 164.93 ppm; MS (ES): m/z 1312 (M+ + 2H).
J = 8.7 Hz, 2H), 8.44 (d, J = 8.4 Hz, 1H), 8.58 (dd, J12 = 8.1 Hz,
J13 = 2.1 Hz, 1H), 9.48 (s, 1H) ppm; 13C NMR (CDCl3,
75 MHz): d 151.5, 151.4, 148.0, 146.5, 146.4, 145.5, 134.9,
129.7, 129.6, 128.5, 128.3, 125.9, 125.8, 124.7, 124.6, 123.0,
120.7, 115.2, 115.0 ppm; HRMS (EI): m/z found: 701.25400;
calcd for C45H31N7O2 701.25392.
Crystal data: C18H17BrN2O 11, M = 357.25, monoclinic, a =
˚
7.357(1), b = 6.139(1), c = 34.909(6) A, b = 91.43(2)u, V =
3
˚
1576.2(4) A , T = 120 K, space group P21/n (no. 14, non-
Compound 18
standard setting), Z = 4, m(Mo Ka) = 2.61 mm21, 14799
reflections, 3504 unique, Rint = 0.067, R(F) = 0.076 [3114 data
with F2 ¢ 2s(F2)], wR(F2) = 0.171 (all data), CCDC 602976.
2,5-Pyridinedicarboxylic dihydrazide23 (1.00 g, 5.12 mmol) was
suspended in pyridine (30 cm3). The suspension was heated to
100 uC with stirring followed by the addition of 4-iodobenzoyl
chloride (3.41 g, 12.8 mmol) in one portion and the mixture
was heated and stirred for 2.5 h to yield a white suspension.
Ethanol (50 cm3) was added followed by a suction filtration to
collect the white solid, which was purified by recrystallisation
from DMSO–ethanol mixture to afford compound 18 as a
white solid (2.56 g, 76%), mp: .320 uC. Anal. calcd for
C21H15I2N5O4: C, 38.50; H, 2.31; N, 10.69. Found: C, 38.47;
H, 2.29; N, 10.78; 1H NMR (DMSO-d6, 300 MHz): d 7.70 (dd,
J12 = 8.4 Hz, J13 = 3.0 Hz, 4H), 7.92 (m, 4H), 8.21 (d, J =
8.1 Hz, 1H), 8.48 (dd, J12 = 8.1 Hz, J13 = 2.1 Hz, 1H), 9.15 (d,
J = 2.0 Hz, 1H), 10.67 (s, 1H), 10.76 (s, 1H), 10.87 (s, 1H),
10.96 (s, 1H) ppm; 13C NMR (DMSO-d6, 75 MHz): d 165.2,
164.8, 163.7, 162.5, 151.5, 147.5, 137.5, 137.4, 137.2, 131.9,
131.7, 130.6, 129.4, 129.3, 122.5, 99.9, 99.7 ppm.
C46H58N2OSi 12, M = 683.03, monoclinic, a = 14.695(2), b =
3
˚
˚
13.330(1), c = 41.182(5) A, b = 94.32(1)u, V = 8044(2) A ,
T = 120 K, space group C2/c (no. 15), Z = 8, m(Mo Ka) =
0.09 mm21, 34477 reflections, 7093 unique, Rint = 0.078, R(F) =
0.045 [4312 data with F2 ¢ 2s(F2)], wR(F2) = 0.118 (all data),
CCDC 602977. C43H49BrN2O 13, M = 689.75, triclinic, a =
˚
10.945(5), b = 12.714(3), c = 13.128(4) A, a = 96.10(1), b =
˚
3
93.84(1), c = 94.38(1)u, V = 1806(1) A , T = 120 K, space
group P1 (No. 2), Z = 2, m(Mo Ka) = 1.17 mm21, 32533
¯
reflections, 10531 unique, Rint = 0.042, R(F) = 0.035 [7932 data
with F2 ¢ 2s(F2)], wR(F2) = 0.094 (all data), CCDC 602978.
C45H31N7O2?3CDCl3 20, M = 1062.89, monoclinic, a =
˚
21.462(6), b = 9.581(3), c = 23.428(6) A, b = 96.48(2)u, V =
3
˚
4787(2) A , T = 120 K, space group I2/a (No. 15, non-standard
setting), Z = 4, m(Mo Ka) = 0.58 mm21, 21479 reflections,
4224 unique, Rint = 0.031, R(F) = 0.055 [3572 data with F2 ¢
2s(F2)], wR(F2) = 0.166 (all data), CCDC 602979.
Compound 19
Compound 18 (1.69 g, 2.58 mmol) was mixed with poly-
phosphoric acid (ca. 30 cm3, Aldrich) and the flask was heated
with an oil bath at 150 uC and stirred for 12 h. The solution
was allowed to cool then hydrolysed with water (100 cm3).
Suction filtration and washing sequentially with water and
methanol yielded compound 19 as a pale-yellow solid (1.60 g),
mp: .320 uC. The solid was essentially insoluble in organic
solvents and no further purification or any solution spectro-
scopy was possible. This product was used in the subsequent
reaction.
Acknowledgements
This work was funded by EPSRC (K.T.K., C.S.W. and
improvements to the X-ray instrumentation), Durham County
Council under the Science and Technology for Business and
Enterprise Programme SP/082, and CENAMPS (S.L.B.) and
the Postdoctoral Fellowship Programme of Korea Science
and Engineering Programme (KOSEF) (for J.H.A.). We thank
Professor J. A. K. Howard for the use of X-ray facilities.
References
2,5-Bis[2-(4-diphenylaminophenyl)-1,3,4-oxadiazol-5-yl]pyridine
(20)
1 C. W. Tang and S. A. van Slyke, Appl. Phys. Lett., 1987, 51, 913.
2 J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks,
K. McKay, R. H. Friend, P. L. Burn and A. B. Holmes, Nature,
1990, 347, 539.
3 (a) S. M. Kelly, Flat Panel Displays, Royal Society of Chemistry,
Cambridge, 2000; (b) C. D. Mu¨ller, A. Falcou, N. Reckefuss,
M. Rojahn, V. Wiederhirn, P. Rudati, H. Frohne, O. Nuyken,
H. Becker and K. Meerholz, Nature, 2003, 421, 829; (c) Organic
Electroluminescence, ed. Z. H. Kakafi, CRC Press, Boca Raton,
2005.
By analogy to the synthesis of compounds 7 and 8, a mixture
of compound 19 (2.05 g, 3.31 mmol), diphenylamine (1.70 g,
10.0 mmol), potassium carbonate powder (1.83 g), copper
powder (0.84 g, Aldrich, 99%) and 18-crown-6 (0.25 g) and 1,2-
dichlorobenzene (60 cm3) were refluxed for 50 h. Chloroform
(100 cm3) was added to the cooled reaction mixture, followed
by suction filtration to remove the solids. The filtrate was
evaporated to dryness in vacuo and the residue was column
chromatographed on silica eluting with DCM–ethyl acetate
9 : 1 v/v then recrystallised sequentially from chloroform–
ethanol mixture, toluene and chlorobenzene to yield
compound 20 as orange–yellow crystals (1.83 g, 79%), mp:
269.5–270.3 uC. A single crystal suitable for X-ray structural
analysis was obtained by slow evaporation of its chloroform
4 Q. Peng, E. T. Kang, K. G. Neoh, D. Xiao and D. Zou, J. Mater.
Chem., 2006, 16, 376.
5 Reviews: (a) A. Kraft, A. C. Grimsdale and A. B. Holmes, Angew.
Chem., Int. Ed., 1998, 37, 402; (b) U. Mitschke and P. Ba¨uerle,
J. Mater. Chem., 2000, 10, 1471; (c) A. P. Kulkarni, C. J. Tonzola,
A. Babel and S. A. Jenecke, Chem. Mater., 2004, 16, 4556; (d)
G. Hughes and M. R. Bryce, J. Mater. Chem., 2005, 15, 94.
6 (a) C. Adachi, T. Tsutsui and S. Saito, Appl. Phys. Lett., 1990, 56,
799; (b) A. R. Brown, D. D. C. Bradley, J. H. Burroughes,
R. H. Friend, N. C. Greenham, P. L. Burn and A. Kraft, Appl.
Phys. Lett., 1992, 61, 2793.
1
solution. H NMR (CDCl3, 300 MHz): d 7.20–7.09 (m, 16H),
7.34 (t, J = 7.7 Hz, 8H), 7.96 (d, J = 8.7 Hz, 2H), 8.02 (d,
3834 | J. Mater. Chem., 2006, 16, 3823–3835
This journal is ß The Royal Society of Chemistry 2006

View Article Online
7 Y. Shirota, M. Kinoshita, T. Noda, K. Okumoto and T. Ohara,
J. Am. Chem. Soc., 2000, 122, 11021 and references therein.
8 (a) Y.-T. Tao, E. Balasubramaniam, A. Danel, A. Wisla and
P. Tomasik, Chem. Mater., 2001, 11, 768; (b) M. Guan, Z. Q. Bian,
Y. F. Zhou, F. Y. Li, Z. J. Li and C. H. Huang, Chem. Commun.,
2003, 2708.
20 M. Tavasli, S. Bettington, M. R. Bryce, H. A. Al Attar, F. B. Dias,
S. King and A. P. Monkman, J. Mater. Chem., 2005, 15, 4963.
21 S. W. Cha, S.-H. Choi, K. Kim and J.-I. Jin, J. Mater. Chem.,
2003, 13, 1900.
22 J. Jacob, J. Zhang, A. C. Grimsdale, K. Mu¨llen, M. Gaal and
E. J. W. List, Macromolecules, 2003, 36, 8240.
9 (a) H. Ogawa, R. Okuda and Y. Shirota, Appl. Phys. A, 1998, 67,
599; (b) Y. Shirota, J. Mater. Chem., 2000, 10, 1.
23 C. Wang, G.-Y. Jung, Y. Hua, C. Pearson, M. R. Bryce,
M. C. Petty, A. S. Batsanov, A. E. Goeta and J. A. K. Howard,
Chem. Mater., 2001, 13, 1167.
10 (a) J. H. Ahn, C. Wang, C. Pearson, M. R. Bryce and M. C. Petty,
Appl. Phys. Lett., 2004, 85, 1283; (b) S. Oyston, C. Wang,
G. Hughes, A. S. Batsanov, I. F. Perepichka, M. R. Bryce,
J. H. Ahn, C. Pearson and M. C. Petty, J. Mater. Chem., 2005, 15,
194.
11 S. Oyston, C. Wang, I. F. Perepichka, A. S. Batsanov, M. R. Bryce,
J. H. Ahn and M. C. Petty, J. Mater. Chem., 2005, 15, 5164.
12 (a) Y. Hamada, C. Adachi, T. Tsutsui and S. Saito, Jpn. J. Appl.
Phys., 1992, 31, 1812; (b) N. Tamoto, C. Adachi and K. Nagai,
Chem. Mater., 1997, 9, 1077.
24 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven,
K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi,
V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,
J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth,
P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich,
A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz,
Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov,
G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin,
D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson,
W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN
03 (Revision B.04), Gaussian, Inc., Wallingford, CT, 2004.
25 A. D. Becke, Phys. Rev. A, 1988, 38, 3098; A. D. Becke, J. Chem.
Phys., 1993, 98, 5648.
13 Z. Zhang, Y. Hu, H. Li, L. Wang, X. Jing, F. Wang and D. Ma,
J. Mater. Chem., 2003, 13, 773.
14 (a) K. R. J. Thomas, J. T. Lin, Y.-T. Tao and C.-H. Chuen, Chem.
Mater., 2002, 14, 3852; (b) K. R. J. Thomas, J. T. Lin, Y.-T. Tao
and C. H. Chuen, J. Mater. Chem., 2002, 12, 3516; (c) K. R. J.
Thomas, J. T. Lin, Y.-T. Tao and C.-H. Chuen, Chem. Mater.,
2004, 16, 5437.
15 Y.-Y. Chien, K.-T. Wong, P.-T. Chou and Y.-M. Cheng, Chem.
Commun., 2002, 2874.
16 A. Patra, M. Pan, C. S. Friend, T.-C. Lin, A. N. Cartwright and
P. N. Prasad, Chem. Mater., 2002, 14, 4044.
17 H. Antoniadis, M. Inbasekaran and E. P. Woo, Appl. Phys. Lett.,
1998, 73, 3055.
18 (a) F. N. Hayes, B. S. Rogers and D. J. Ott, J. Am. Chem. Soc.,
1955, 77, 1850; (b) A. Kraft, Chem. Commun., 1996, 77.
19 J. F. Hartwig, M. Kawatsura, S. I. Hauck, K. H. Shaughnessy and
L. M. Alcazar-Roman, J. Org. Chem., 1999, 64, 5575.
26 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785.
27 (a) P. Flu¨kiger, H. P. Lu¨thi, S. Portmann and J. Weber, Molekel,
Version 4.3, Swiss Center for Scientific Computing, Manno
(Switzerland), 2002, http://www.cscs.ch/molekel/; (b) S. Portmann
and H. P. Lu¨thi, Chimia, 2000, 54, 766.
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 3823–3835 | 3835