Blue-shifting the monomer and excimer phosphorescence of tridentate cyclometallated platinum(ii) complexes for optimal white-light OLEDs

Article abstract of DOI:10.1039/c2cc31330h

By using a tridentate N^C^N-coordinating ligand, the luminescence of a cyclometallated Pt(ii) complex can be shifted into the blue region, without the problematic drop-off in quantum yield observed for bidentate analogues. The combination of blue-shifted monomer and excimer allows white-emitting OLEDs with high colour rendering index to be produced.

Full text of DOI:10.1039/c2cc31330h

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Chemical Communications
www.rsc.org/chemcomm
Volume 48 | Number 47 | 14 June 2012 | Pages 5801–5940
ISSN 1359-7345
COMMUNICATION
Massimo Cocchi, J. A. Gareth Williams et al.
Blue-shifting the monomer and excimer phosphorescence of tridentate
cyclometallated platinum(II) complexes for optimal white-light OLEDs
1359-7345(2012)48:47;1-1

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COMMUNICATION
Blue-shifting the monomer and excimer phosphorescence of tridentate
cyclometallated platinum(II) complexes for optimal white-light OLEDsw
Lisa Murphy,a Pierpaolo Brulatti,bc Valeria Fattori,b Massimo Cocchi*bc and
J. A. Gareth Williams*a
Received 22nd February 2012, Accepted 26th March 2012
DOI: 10.1039/c2cc31330h
By using a tridentate N^C^N-coordinating ligand, the lumines-
cence of a cyclometallated Pt(II) complex can be shifted into the
blue region, without the problematic drop-off in quantum yield
observed for bidentate analogues. The combination of blue-
shifted monomer and excimer allows white-emitting OLEDs
with high colour rendering index to be produced.
of obtaining white emission from a single triplet-emitting
species.5 Here again, however, the need to blue-shift the
emission bands sufficiently to obtain optimal white light
emission has proved a challenge.
Cyclometallated complexes of N^C-coordinating aryl
pyridines have been widely investigated as emitters, both with
Pt(II) and Ir(III). In their landmark paper, Thompson and
Forrest showed how a range of colours could be obtained
The demonstration that luminescent metal complexes can be
used as phosphors for organic light-emitting devices (OLEDs)
has led to much interest in strategies for controlling the
excited-state energy and optimising the emission efficiency of
such complexes.1 Green-emitting phosphors tend to be the
most efficient. Good blue emitters are scarce, often owing in
part to the energetic proximity of strongly distorted metal-
centred states that can provide a deactivation pathway for the
emissive states.2 While the best available emitters are typically
those based on iridium(III), there is increasing interest in
platinum(II) complexes, particularly with respect to white-
light-emitting OLEDs (WOLEDs) for lighting applications.3
In contrast to the pseudo-octahedral geometry associated
with d6 metal ions like Ir(III), complexes of the d8 Pt(II) ion are
normally square-planar. This geometry favours face-to-face
interactions between molecules, involving overlap of orbitals
orthogonal to the molecular plane (e.g. 5dz2 and 6pz on the
metal, p and p* orbitals of conjugated ligands). Such interactions
in the ground-state lead to aggregate species that may display
low-energy absorption and emission bands that are not observed
in the isolated molecules, whilst the interaction between
excited and ground-state monomers can lead to the concen-
tration-dependent formation of excimers whose emission is
also necessarily red-shifted.4 At an appropriate concentration
in the emitting layer of an OLED, the combination of mono-
mer emission in the blue/green region and excimer/aggregate
emission in the red region of the spectrum can provide a means
from complexes based on Pt(ppy)(dpm) (for which lmax
=
486 nm in CH2Cl2), simply by introducing appropriate sub-
stituents into the ppy ligand (dpm = dipivaloylmethane).6 By
inserting a strongly-donating amine into the pyridine ring and
electron-withdrawing fluorine atoms into the aryl ring, a
complex (Fig. 1a) emitting deep into the blue could be
obtained, at least at low temperature (lmax = 440 nm at 77 K).
However, the shift was accompanied by a dramatic drop in the
room-temperature photoluminescence quantum yield,
reported to be o10À3, compared to much higher values of
0.15 for Pt(ppy)(dpm) (lmax = 486 nm)6 and 0.36 for
Pt(thpy)(acac) (lmax = 554 nm).7 This is probably due to a
combination of molecular distortion and the aforementioned
d–d states promoting non-radiative decay.
Pt(II) complexes based on the tridentate, N^C^N-coordinating
ligand 1,3-di(2-pyridyl)benzene (HL1) (see caption to Fig. 1b),
are amongst the brightest Pt-based emitters in solution at
room temperature.8 They also form highly emissive excimers
at elevated concentrations. Related N^C^N-coordinated com-
plexes of bis(benzimidazol-2-yl)benzene ligands are likewise
very good emitters.9 The high efficiency seems to be due to a
combination of factors, primarily the rigidity imposed by
tridentate coordination (e.g. the distortion from D4h towards
a Department of Chemistry, University of Durham, Durham,
DH1 3LE, UK. E-mail: j.a.g.williams@durham.ac.uk;
Fax: 44 191 384 4737
b ISOF, Consiglio Nazionale delle Ricerche, 40129 Bologna, Italy.
E-mail: cocchi@isof.cnr.it; Fax: +39 051 6399844
Fig. 1 Structures of (a) the previously described bidentate complex
that emits in the blue with very low quantum yield at ambient
temperature, and (b) the intensely luminescent new blue-emitting
complex PtL30Cl. The complex PtL1Cl is the analogue with no
substituents in the rings.
c Consorzio MIST E-R via P. Gobetti 101, 40129 Bologna, Italy
w Electronic supplementary information available: Synthesis and char-
acterisation of PtL30Cl; details of the OLED architecture and experi-
mental methods employed. See DOI: 10.1039/c2cc31330h
c
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Chem. Commun., 2012, 48, 5817–5819 5817

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D2d local symmetry open to bis-bidentate complexes is prohibited)
and to the unusually short Pt–C bond enforced by the flanking
heterocyclic rings.10 We reasoned that these factors should allow
more efficient blue-emitting complexes to be obtained and, in
combination with the excimer emission, high-quality white light.
The complex PtL30Cl was targeted (Fig. 1b). The substitution
pattern is rationalised as follows. Inspection of the frontier
orbital plots of PtL1Cl obtained from TD-DFT calculations in
previous work (see Fig S1w)11 reveals that the 4-position of the
pyridyl rings plays a role in the LUMO but lies on a node in
the HOMO, so that electron-donating amino groups at these
positions should destabilise the former without affecting the
latter. In contrast, the positions in the central aryl ring meta to
the metal make a major contribution to the HOMO but not to
the LUMO, and so electron-withdrawing fluorine atoms at
these positions should independently allow the HOMO to be
lowered without affecting the LUMO. The net effect should be
a large increase in the HOMO–LUMO gap and hence blue-
shifted emission.
The absorption and emission spectra of the new complex in
dichloromethane solution at room temperature are shown
in Fig. 2, together with those of PtL1Cl for comparison
(tabulated photophysical data are given in Table S1w). It can
be seen immediately that the substitution pattern leads to the
desired blue-shifting effect. The lowest spin-allowed absorp-
tion band shifts from 401 to 370 nm, indicating an increase in
energy of the lowest singlet state of around 2100 cmÀ1. The
spin-forbidden S0 - T1 band, sharp and well-resolved in each
case despite the low extinction coefficient, is shifted from 483
to 441 nm (DE = 1950 cmÀ1).
The new complex is intensely luminescent in degassed
solution at ambient temperature. Dilute solutions display
sky-blue luminescence (lmax = 453 nm compared to 491 nm
for PtL1Cl) with a high quantum yield (Flum = 0.60; t = 4.7 ms).
Clearly, the strategy of employing tridentate ligands is successful
in allowing a large blue-shift to be induced without compromising
the quantum yield. More concentrated solutions appear vividly
white under UV irradiation. Under these conditions, the
monomer emission is accompanied by strong excimer emission,
centred at 596 nm. This represents a substantial shift to higher-
energy compared to the excimer of PtL1Cl (which is centred
close to 700 nm). Apparently, the presence of the amino
substituents destabilises the interaction between pairs of mole-
cules that is responsible for the formation of the excimer. The
clear consequence of this effect is that the excimer more
effectively covers the mid-range part of the visible spectrum
than PtL1Cl, and much less energy is wasted in the NIR region.
The efficient coverage of the visible spectrum offered by the
combination of monomer and excimer emission renders
PtL30Cl of particular interest as a phosphor for WOLEDs.
Multi-layer devices have been prepared using PtL30Cl as a
phosphorescent guest within the emitting layer (EML). The
device architecture—shown in Fig. S2w together with full
names of the components—comprises an ITO anode, hole-
transporting layers of TPD:PC and TCTA, an electron trans-
porting layer of TAZ and an Al/LiF cathode. The 10 nm thick
TCTA layer, characterized by a triplet-exciton energy ET of
2.85 eV, has been used to block triplet-exciton transfer from
the Pt complex (ET B 2.7 eV) to the non-radiative triplet of
TPD (ET = 2.45 eV). Meanwhile, the 25 nm-thick layer of
TAZ (ET = 2.75 eV) plays a similar role of inhibiting
quenching of the triplet excitons at the cathode while at the
same time blocking holes, thus enabling the recombination
To date, dipyridylbenzene derivatives have been prepared
by Pd-catalysed cross-coupling of dihalogenated benzenes
with either pyridyl-zinc reagents (Negishi coupling)12 or
pyridylstannanes (Stille coupling).8 In the present case,
4-dimethylamino-2-tri-n-butylstannane was prepared from
4-di-methylamino-2-bromopyridine by lithiation followed by
reaction with tri-n-butyl tin chloride. However, repeated attempts
to prepare HL30 by Pd-catalysed coupling of the stannane with
1,3-dibromo-4,6-difluorobenzene led to almost quantitative
conversion of the stannane to 4-dimethylaminopyridine
(DMAP). The introduction of the electron-donating amino
substituent clearly renders the compound thermally unstable
with respect to decomposition to DMAP.
We therefore applied a new approach to such ligands,
namely through the intermediacy of a 1,3-benzene-diboronic
ester to undergo Suzuki coupling with a halogenated pyridine
(Scheme 1). Thus, Pd-catalysed reaction of 1,3-dibromo-4,6-
difluorobenzene with bispinacolatodiboron under the standard
conditions established by Miyaura for aryl boronate ester
synthesis {Pd(dppf)Cl2 as the catalyst, in the presence of
KOAc, DMSO solvent} led to the difluorinated diboronate
ester in 86% yield (ESIw). This compound was then cross-
coupled with 2 equiv. of 4-dimethylamino-2-bromopyridine to
give HL30 in 51% yield after purification. Finally, reaction of
this proligand with K2PtCl4 in MeCN/H2O led to the desired
complex PtL30Cl in 67% yield.
Fig. 2 Absorption and emission spectra of PtL30Cl (blue) and PtL1Cl
(red) in CH2Cl2 at 298 K. Concentration = 4 Â 10À4 M for the
emission spectra. The dotted lines show the weak S0 - T1 bands on an
expanded scale.
Scheme 1 Synthesis of HL30 via the diboronate ester.
5818 Chem. Commun., 2012, 48, 5817–5819
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associated Pt complexes. The EL spectrum of a neat film of
the Pt complex (100%) is practically devoid of blue monomer
emission (Fig. S3bw). The CIE coordinates show an apparent
monotonic trend towards the red with increasing concentration
of the phosphor dopant (see inset to Fig. 3). A remarkable
increase in the colour rendering index (CRI) is observed
between 12 and 25 wt% phosphor content in the EML, reaching
values of 87 (at 20 wt%) and 88 (at 25 wt%) with CIE
coordinates very close to white: (0.36, 0.37) and (0.37, 0.39),
respectively. For the 20% doped device, the quantum efficiency is
3.7% ph/e and the luminous efficiency is 7.4 cd AÀ1. Further
device and performance characteristics are listed in Table S2.w
In summary, whilst green-emitting Pt(II) complexes based
on N^C^N-coordinating ligands are known to be modestly
superior, in terms of luminescence quantum yield, to analo-
gues with bidentate ligands,8 we have shown here that the
superiority of tridentate systems is much more dramatic in
the blue region. PtL30Cl is over 600 times more emissive than
the analogue based on a similarly substituted phenylpyridine,
probably due to the greater rigidity ensuring that deactivating
d–d states are kept at higher energy. The complex also performs
well as an OLED phosphor, and the accompanying blue-shift in
the excimer of this complex ensures good cover-age of the
visible region, such that the CRI values are the highest hitherto
reported for triplet excimer-based WOLEDs.
Fig. 3 Electroluminescence spectra and CIE diagram (inset) with
different concentrations of PtL30Cl in the EML. The host material
for the devices shown is a 1 : 1 mixture of TCTA and TCP by weight.
process to be confined to the EML layer. The choice of host is
particularly important for blue-emitting phosphors: materials
with high-energy triplet states are required to avoid guest-to-
host energy transfer. A material with a low energy barrier for
hole transport (TCTA), one with low energy barrier for
electrons (TCP), and a 1 : 1 mixture (by weight) of the two
were tested as hosts. Both materials have triplet energies which
are sufficiently high not to quench the PtL30Cl exciton by
energy transfer (ET = 2.85 and 2.95 eV respectively).
The electroluminescence efficiencies and spectra of OLEDs
having 6 wt% PtL30Cl in different hosts are reported in
Fig. S3.w When the EML is a mixed matrix of TCTA and
TCP, better charge-balancing properties are anticipated as the
mixture combines low barriers for both holes and electrons
and charge recombination is not restricted to a narrow zone
close to the junctions with TAZ or TCTA buffer, but spreads
over the entire EML (energy levels of the constituent materials
of the device are shown in Fig. S2w). Hence, concentration-
dependent quenching processes, such as exciton-exciton and
exciton-charge interactions, and excitonic migrations toward
the quenching TPD layer, are expected to be less effective. In
fact, the overall electroluminescence efficiency when the mixed
host is employed is higher through the entire current range
examined than using either of the pure hosts, as shown in
Fig. S3a.w The use of TCP alone is seen to give poor results:
due to the same barrier for holes and electrons at the TCTA/
TCP junction (0.4 eV), charge recombination is not confined
to the EML but can also occur in the TCTA and TPD layers.
This is evident from the TPD emission band present in the EL
spectrum (Fig. S3bw).
This work was supported by Durham University, Consorzio
MIST E-R (project FESR-Tecnopolo AMBIMAT) and
Italian MISE (project Industria 2015 ‘‘ALADIN’’).
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Having established its superiority, subsequent studies on the
tuning of the blue-to-red ratio with guest concentration were
performed using this mixed TCTA:TCP host system. Fig. 3
shows EL spectra and CIE coordinates for different PtL30Cl
concentrations in the EML. It is clear that a reduction in
dopant concentration leads to a relative increase of the blue-
light emission band originating from the radiative decay of
molecular triplet excitons of the isolated Pt complex molecules,
while increasing dopant concentration increases the contribu-
tion of the broad, red-light band emanating from the triplet
excimers or aggregates that comprise two or more closely
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