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

New electroluminescent bipolar compounds for balanced charge-transportand 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.

Introduction

The discovery of electroluminescence (EL) in low molecular

weight organic molecules1 and in conjugated polymers,2 has

led to unabated intense interest in new materials for

incorporation into organic light emitting diodes (OLEDs) for

display applications, ranging from small portable devices

to large area screens which consume much less power than

current LCD materials.3 The design and synthesis of materials

which possess balanced hole-transport (HT) and electron-

transport (ET) properties remain a major challenge. Most

emissive polymers are predominantly hole-transporting (i.e.

p-dopable), viz. derivatives of poly(p-phenylene vinylene) and

poly(fluorene). In a device structure this creates an imbalance

of electron injection (from the low work-function cathode) and

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

due to quenching of excitons by the metal electrode. This

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)

aDepartment of Chemistry and Centre for Molecular and NanoscaleElectronics, University of Durham, Durham, UK DH1 3LE.E-mail: [email protected] of Engineering and Centre for Molecular and NanoscaleElectronics, University of Durham, Durham, UK DH1 3LE.E-mail: [email protected]{ Electronic supplementary information (ESI) available: Synthesis of11; cyclic voltammetry data for compounds 7, 8, 14, 16 and 20, X-raycrystallographic data for compounds 11, 12, 13 and 20 includingdiagrams and discussion of the structures; B3LYP/6-31G(d) optimisedgeometries (Figures and Tables of coordinates); orbital energy leveldiagrams and frontier orbital localisation for compounds 7a, 8a, 14a,16a and 20; EL spectra of blended layer devices of MEH-PPV andcompound 7. See DOI: 10.1039/b604543j{ On leave from the L. M. Litvinenko Institute of Physical Organicand Coal Chemistry, National Academy of Sciences of Ukraine, R.Luxemburg Street 70, Donetsk 83114, Ukraine.

PAPER www.rsc.org/materials | Journal of Materials Chemistry

This journal is � The Royal Society of Chemistry 2006 J. Mater. Chem., 2006, 16, 3823–3835 | 3823

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

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

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

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

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.

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 � The Royal Society of Chemistry 2006

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{.

The thermal properties of 14 and 16 were studied by

thermal gravimetric analysis (TGA) and differential scanning

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

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.

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.


(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).§

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

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

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.

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).

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.

§ CCDC reference numbers 602976–602979. For crystallographic datain CIF format see DOI: 10.1039/b604543j


Electrochemistry

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 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

cathodic peak ioxpa on the re-reduction process increased (Fig. 4

and Fig. S4 in the ESI{ for compound 7). At lower scan rates,

the intensity of ioxpc (compared to the intensity of iox

pc) was much

smaller and an additional peak on the re-reduction process was

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).

The most interesting results were obtained for compound 20.

Similar to 7 and 8, 20 shows partly reversible oxidation (Eoxpa =

+0.89 V, Eoxpc = +0.78 V) with the appearance of an additional

peak at lower potentials on re-reduction (Epolympc = 0.63 V).

Being a good electron acceptor due to the central bis(oxadia-

zolyl)pyridine moiety, compound 20 undergoes a reversible

single-electron reduction (DEredpc{pa = 62 mV, see also inset

in Fig. 5) at less negative potentials compared to 7 (Ered1=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

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.

Compound Eox1/V, (DEpa2pc)b Eox2 [V] Ered/V, (DEpa2pc)

b ECVg /Vc Eopt

g /eVd EDFTg /eVe

7 Epa = 0.88 Epc = 0.77 Epc # 22.06 2.71 2.92 3.148 Epa = 1.13 E1/2 = 21.99 (50 mV) 2.93 3.13 3.2414 E1/2 = 0.60 (60 mV) Epa = 1.24 22.15 2.55 2.89 3.2116 E1/2 = 0.53 (81 mV) Epa = 1.18 22.02, 22.16 2.37 2.79 3.0920 Epa = 0.89 Epc = 0.78 E1/2 = 21.76 (62 mV) 2.48 2.72 2.99a CV in benzonitrile, 0.1 M Bu4NPF6, scan rate 100 mV s21; potentials are vs. Ag/Ag+ reference electrode. b Difference between Epa and Epc

for reversible processes. c Electrochemical band gap estimated from the onsets of oxidation and reduction processes. d Optical band gapestimated from the red edge of absorption spectra in chloroform (Fig. 7). e HOMO–LUMO gap from DFT calculations (Fig. 2).

Fig. 3 Cyclic voltammogram of compound 14 in benzonitrile, 0.1 M

Bu4NPF6 at 100 mV s21 (Eox11=2 = 0.60 V, DEox1

pa{pc = 60 mV, Eox2pa =

1.24 V). Inset shows the deconvoluted CV.

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 Eoxpa (by multiplying

ipa 63.53). Eoxpa = +0.88 V, Eox

pc = +0.77 V, Epolympc = +0.62 V

(100 mV s21); Eoxpa = +0.89 V, Eox

pc = +0.78 V (2000 mV s21).


ca. +(0.65–0.7) V corresponds to a partial electropolymerisa-

tion process for 20.

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

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).

Fig. 5 Cyclic voltammogram of compound 20 in benzonitrile, 0.1 M

Bu4NPF6 at scan rate 100 mV s21, 4 consecutive scans (starting at

21.2 V, scan to negative): Eredpc = 21.79 V, Ered

pa = 21.73 V, Eoxpa =

+0.89 V, Eoxpc = +0.78 V. Inset: deconvoluted CV for the oxidation

process. Arrows show electrodeposition of the polymer on the Pt

electrode after the first oxidation scan.

Fig. 6 Cyclic voltammogram of a film of poly(20) on Pt disk,

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.

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.


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 Eoptg ), and (b) the electrochemistry is complicated

by irreversibility of one of the redox process (either oxidation

or reduction) limiting the accuracy of the ECVg value. Neverthe-

less, the general trend is clear and correlation coefficients

between the ECVg 2 Eopt

g and Eoptg 2 EDFT

g are y0.95–0.96.

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,

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).

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.

Table 2 Data for solution state photophysical studies

Compound lmax abs (nm) lmax PL (nm) Wfa

7 385 452 0.868 361 416 0.8714 380 478 0.9216 399 483 0.9320 402 533 0.55a in DCM solution, measured against quinine sulfate and fluoresceinstandards.

Fig. 8 Current–voltage (a) and light output–voltage (b) charac-

teristics of the single-layer 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.

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.

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

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.

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

Table 3 Data for OLEDs using the device structure ITO:PEDOT-PSS:compound X:Ca/Al.a

Compound

Externalquantumefficiency (%)

Currentefficiency/cd A21

Power efficiency/lm W21

(operating voltage/V)

7 0.36 1.00 0.56 (5.7)8 0.015 0.019 0.005 (10.87)14 0.26 0.60 0.29 (6.5)16 0.22 0.49 0.17 (9.3)20 0.18 0.54 0.25 (6.9)a EQE was measured at 100 cd m22 except the 8 device which wasmeasured at 15 cd m22.

Fig. 10 Luminance versus bias voltage diagram which compares the

operation voltages of the devices using 7, 8, 14, 16 and 20.

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. 11 The EL spectra of the devices using 7, 8, 14, 16 and 20.


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.

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.

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

of 9 V %21 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

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.

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


Compound 5

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.

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.

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,

8.78. Found: C, 81.65; H, 6.28; N, 8.84%; 1H 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.

2,7-Bis{2-[4-(N-carbazolyl)phenyl]-1,3,4-oxadiazol-5-yl}-9,9-

dihexylfluorene (8)

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.

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-(4-tert-Butylphenyl)-5-{4-[9,9-dihexyl-2-

(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+).

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 uCin 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,

7.23; N, 3.97%; 1H 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.

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

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+).

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,

7.90; N, 5.99. Found: C, 83.79; H, 7.90; N, 5.79%; 1H 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)

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),


7.60 (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).

Compound 18

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.

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.

2,5-Bis[2-(4-diphenylaminophenyl)-1,3,4-oxadiazol-5-yl]pyridine

(20)

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

solution. 1H 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,

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 =

1576.2(4) A3, T = 120 K, space group P21/n (no. 14, non-

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.

C46H58N2OSi 12, M = 683.03, monoclinic, a = 14.695(2), b =

13.330(1), c = 41.182(5) A, b = 94.32(1)u, V = 8044(2) A3,

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 =

93.84(1), c = 94.38(1)u, V = 1806(1) A3, 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 =

4787(2) A3, 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.

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

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. Muller, A. Falcou, N. Reckefuss,M. Rojahn, V. Wiederhirn, P. Rudati, H. Frohne, O. Nuyken,H. Becker and K. Meerholz, Nature, 2003, 421, 829; (c) OrganicElectroluminescence, ed. Z. H. Kakafi, CRC Press, Boca Raton,2005.

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. Bauerle,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.


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 andP. 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.

9 (a) H. Ogawa, R. Okuda and Y. Shirota, Appl. Phys. A, 1998, 67,599; (b) Y. Shirota, J. Mater. Chem., 2000, 10, 1.

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.

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. Taoand 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 andP. 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 andL. M. Alcazar-Roman, J. Org. Chem., 1999, 64, 5575.

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. Mullen, M. Gaal andE. J. W. List, Macromolecules, 2003, 36, 8240.

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.

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, GAUSSIAN03 (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.

26 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785.27 (a) P. Flukiger, H. P. Luthi, S. Portmann and J. Weber, Molekel,

Version 4.3, Swiss Center for Scientific Computing, Manno(Switzerland), 2002, http://www.cscs.ch/molekel/; (b) S. Portmannand H. P. Luthi, Chimia, 2000, 54, 766.


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

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