Efficient blue-green light emitting poly(1,4-phenylene vinylene) copolymers

Article abstract of DOI:10.1039/a909382f

2,3-Dialkoxy-substituted poly(1,4-phenylene vinylene) (PPV) homo- and co-polymers have been prepared by the Gilch dehydrohalogenation polycondensation of the corresponding bishalomethyl-substituted benzene monomers, and double layer light emitting devices

Full text of DOI:10.1039/a909382f

Efficient blue–green light emitting poly(1,4-phenylene vinylene) copolymers†
Rainer E. Martin,a Florence Geneste,a Robert Riehn,b Beng Sim Chuah,a Franco Cacialli,b Richard H. Friendb
and Andrew B. Holmes*a
a Melville Laboratory, Department of Chemistry, University of Cambridge, Pembroke Street, Cambridge,
UK CB2 3RA. E-mail: abh1@cus.cam.ac.uk
b Cavendish Laboratory, Department of Physics, University of Cambridge, Madingley Road, Cambridge,
UK CB2 0HE
Received (in Cambridge, UK) 29th November 1999, Accepted 18th January 2000
2,3-Dialkoxy-substituted poly(1,4-phenylene vinylene)
(PPV) homo- and co-polymers have been prepared by the
Gilch dehydrohalogenation polycondensation of the corre-
sponding bishalomethyl-substituted benzene monomers,
and double layer light emitting devices fabricated with these
materials exhibited high electroluminescence efficiencies
with low turn-on voltages.
Mannich reaction to introduce both carbon substituents and the
conversion of the diol formed from saponification of the
diacetate 7 with CBr4 and PPh3 into the crystalline 1,4-bis-
14
bromomethyl compound 8.‡ Statistical copolymersation under
Gilch conditions of monomer 8 with 99,10 and 10,11 using in
both cases a feed ratio of 1+1, in the presence of excess KOBut
in degassed THF at room temperature (Scheme 2) gave the
copolymers 11 and 12, respectively. The polymers were
purified by precipitation from methanol and acetone or mixtures
of methanol–water, respectively, and formed bright yellow
fibres displaying high luminescence. 1H NMR side-chain
analysis of DMOS-co-DB-PPV 11 gave the ratio n+m of 3+4
and for BDMOS-co-DB-PPV 12 of 4+3.
Semiconducting organic polymers exhibiting electrolumines-
cence (EL) have recently attracted considerable interest owing
to their high potential for application as the active layer in
polymer light-emitting devices (PLEDs).1–5 The 2,3-disub-
stituted derivatives of PPV are of interest owing to the potential
distortion of the intra- and inter-molecular chain interactions as
a result of steric effects.6 Poly(2,3-dibutoxy-1,4-phenylene
vinylene)7 1 exhibits a blue–green solid state photolumines-
cence (PL) with 40% efficiency which is considerably higher
than the prototypical 2,5-dialkoxy-substituted analogue, MEH-
PPV.8 We have also reported a new class of 2-silylated and
2,5-disilylated PPV derivatives which essentially behave as
organic solvent processible analogues of PPV itself, but which
are much more luminescent (PL efficiency 60%). However,
single layer (100 nm) devices fabricated from poly(2-dimethyl-
octylsilyl-1,4-phenylene vinylene) (DMOS-PPV)9,10 2 and
Scheme 1 Reagents and conditions: i, CH2O, morpholine, PriOH, reflux,
1 h. ii, C4H9Br, K2CO3, EtOH, reflux, 20 h. iii, Ac2O, NaOAc, HOAc,
reflux, 72 h. iv, K2CO3, MeOH–THF–H2O (50+9+3), 48 h. v, CBr4, PPh3,
THF, 4 h.
poly[2,5-bis(dimethyloctylsilyl)-1,4-phenylene
vinylene]
(BDMOS-PPV)11 3 suffered from rather high turn-on voltages
of ca. 15 V. It has recently been reported that high efficiencies
may be realised in ternary statistical copolymers of PPV
derivatives,12 and that the degree of defects is determined in part
by non-regioregular coupling of the putative quinomethide
precursor in the Gilch polycondensation.13
Here, we report an improved synthesis of 1,4-bis(bromo-
methyl)-2,3-dibutoxybenzene 8 and the exploitation of two
important monomers to yield the statistical copolymers poly[(2-
dimethyloctylsilyl-1,4-phenylene vinylene)-co-(2,3-dibutoxy-
1,4-phenylene vinylene)] (DMOS-co-DB-PPV) 11 and poly-
{bis[(2,5-dimethyloctylsilyl)-1,4-phenylene
vinylene]-co-
(2,3-dibutoxy-1,4-phenylene vinylene)} (BDMOS-co-DB-
PPV) 12, respectively, that combine the intrinsically high
electroluminescence present in the three corresponding homo-
polymers 1–3 with the significantly reduced turn-on voltages of
2,3-dialkoxy-substituted PPVs.
The synthesis of the 2,3-dibutoxy monomer 8 is shown in
Scheme 1. The key improvements were the use of the double
Scheme 2 Reagents and conditions: i,KOBut, THF, 18 h.
Molecular weight determination of the purified polymers
using size-exclusion chromatography (SEC) calibrated with
polystyrene standards and multiple angle light scattering
revealed high molecular weights and polydispersities, typical of
dehydrohalogenation polycondensations (DMOS-co-DB-PPV:
Mn = 290 000, Mw/Mn = 7.1; BDMOS-co-DB-PPV: Mn
180 000, Mw/Mn = 5.5). DMOS-co-DB-PPV 11 and BDMOS-
† Electronic supplementary information (ESI) available: normal optical
absorption, PL and EL spectra (Fig. S1) and cyclic voltammograms (Fig.
S2) for BDMOS-DB-PPV. See http://www.rsc.org/suppdata/cc/a9/
a909382f/
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Chem. Commun., 2000, 291–292
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co-DB-PPV 12 readily dissolved in aprotic solvents such as
THF, chloroform or xylene and formed good uniform trans-
parent films on ITO-coated glass substrates. The thermal
properties of the polymers were investigated by thermal
gravimetry (TG) and differential scanning calorimetry (DSC)
under nitrogen. Analysis of the TG trace (heating rate 10 °C
min21) for DMOS-co-DB-PPV and BDMOS-co-DB-PPV re-
vealed a 5% weight loss at ca. 370 and 320 °C, respectively.
BDMOS-co-DB-PPV showed an exothermic phase transition at
ca. 170 °C.
Optical absorption measurements in CHCl3 solutions for
DMOS-co-DB-PPV and BDMOS-co-DB-PPV showed lmax of
440 and 442 nm, respectively. Interestingly, DMOS-co-DB-
PPV showed for the lmax in the solid-state a bathochromic shift
to 448 nm, whereas BDMOS-co-DB-PPV revealed a slightly
hypsochromically shifted value of 440 nm (Fig. S1 in ESI). The
longest wavelength absorption maxima reflected the statistical
composition of the respective monomeric building blocks; DB-
PPV 1: lmax = 454 nm; DMOS-PPV 2: lmax = 414 nm;
BDMOS-PPV 3: lmax = 436 nm. Both polymers are bright
yellow materials exhibiting high solid-state PL efficiencies of
35 and 28%, with the longest wavelength emission peaks at lem
= 548 nm (2.26 eV) and lem = 544 nm (2.28 eV, (Fig. S1 in
ESI)), respectively.
Cyclic voltammetry (CV) was perfomed on thin polymer
films of BDMOS-co-DB-PPV 12 spin-coated onto Pt disk
electrodes in MeCN using a Pt wire as counter electrode and Ag/
AgCl as a reference. In the anodic scan the onset of oxidation
occurred at ca. 1.2 V followed by three subsequent non-
reversible oxidations at 1.38, 1.63 and 1.80 V, respectively (Fig.
S2 in ESI). The cathodic sweep showed onset of reduction at ca.
21.6 V and a quasi-reversible reduction step at 21.83 V (Fig.
S2 in ESI). The electrochemically measured band gap was
2.8 V, which compares well with the HOMO–LUMO energy
gap of 2.82 eV as measured from UV–VIS spectroscopy. The
HOMO and LUMO energy levels of BDMOS-co-DB-PPV
were estimated from the oxidation and reduction onset poten-
for a structure with an 80 nm active layer (Fig. 1). This
compares well with a 6 V turn-on voltage for the BDMOS-PPV
3 based homopolymer device.11 The EL emission peak was
measured at lem = 533 nm (2.33 eV).
In summary, a new and versatile synthetic route for the
preparation of 2,3-dialkoxy-1,4-bis(bromomethyl) monomers
was developed. Improved EL performance of devices using
statistical copolymers such as BDMOS-co-DB-PPV 12 as the
active layer in comparison with the homopolymers was
demonstrated, making this material a promising candidate for
further optimisation.
We thank EPSRC for financial support and provision of the
Swansea Mass Spectrometry Service, the Swiss National
Science Foundation and Churchill College, Cambridge (Fellow-
ships to R. E. M.), the Royal Society (University Research
Fellowship to F. C.), the Commission of the European Union
(Marie Curie Fellowship to F. G., TMR network ‘SELOA’ and
Brite-Euram Contract BRPR-CT97-0469 ‘OSCA’), the Cam-
bridge Commonwealth Trust and the CVCP (ORS studentship
to B. S. C.) and Cambridge Display Technology (CDT) for
generous support.
Notes and references
‡ All new monomers were characterised fully by their melting points, IR, 1H
and 13C NMR spectroscopy, EI or CI MS spectrometry and elemental
analysis.
§ Absorption and PL spectroscopy were carried out on thin films deposited
onto spectrosil substrates by means of a Hewlett Packard B453 UV–VIS
spectrophotometer (absorption) and of a CCD UV-enhanced spectrograph
(PL). The PL efficiency was determined on films deposited on spectrosil
substrates, using a nitrogen purged integrating sphere. Excitation was by
means of the visible or multiline UV lines (ca. 351, 364 nm) of an Ar-ion
laser. LEDs were prepared on commercial ITO substrates (Asahi), treated
prior to use with a oxygen plasma for 10 min.16 The PEDOT+PSS (Bayer)
was coated from a water dispersion yielding a ca. 80 nm thick layer after
drying (ca. 90 °C; 1 h).17 Active layers were spin-coated from THF–p-
xylene solutions on either the bare ITO or on ITO/PEDOT substrates to give
film thickness of 80–100 nm. Cathodes (Al or Ca/Al) thermal evaporation
(at ca. 5 3 1026 mbar or less) completed the preparation of the diodes which
were then tested in a 1022 mbar vacuum.
tials to be HOMO
= 5.6 eV and LUMO = 2.8 eV,
respectively.15
Double layer devices with the configuration ITO/PED-
OT:PSS/polymer/cathode were fabricated, [PEDOT:PSS is
poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)].§
Both polymers exhibited blue–green light emission. Inter-
estingly, DMOS-co-DB-PPV LEDs only showed EL emission
for Ca cathodes evaporated at pressures of ca. 1023 mbar, but
little or no EL for evaporation at pressures @3 3 1026 mbar.
DMOS-co-DB-PPV 11 double layer LEDs with Al cathodes
showed turn-on voltages of 2.0–2.4 V (threshold = 0.01
cd m22) with a power efficiency of 0.05 cd A21 and a maximum
luminance of 36 cd m22 at 11 V. Substantially better
performance (both efficiency and luminance) was noted for
devices made with BDMOS-co-DB-PPV 12 and Ca cathodes
compared with devices made with DMOS-co-DB-PPV 11. The
power efficiency was up to 0.72 cd A21 with a maximum
luminance of 1384 cd m22 at 12 V and turn-on voltages of 4.0 V
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Fig. 1 Current–voltage–luminance (I–V–L) characteristics of an ITO/
PEDOT (80 nm)/BDMOS-DB-PPV (80 nm)/Ca LED. The active area of the
device is ca. 0.045 cm2. EL emission turns on at 4.0 V and the maximum
efficiency is 0.72 cd A21
.
Communication a909382f
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Chem. Commun., 2000, 291–292