Cyclometallated platinum(ii) complexes of 1,3-di(2-pyridyl)benzenes for solution-processable WOLEDs exploiting monomer and excimer phosphorescence

Article abstract of DOI:10.1039/c1jm10193e

Two cyclometallated platinum(ii) complexes, N^C^N-5-fluoro-1,3-di(2- pyridyl)benzene platinum(ii) chloride, FPtCl, and N^C^N-5-methyl-1,3-di(2- pyridyl)benzene platinum(ii) isothiocyanate, MePtNCS, have been synthesized and characterized. Both complexes are highly efficient phosphorescent green emitters which can also display excimer emission in the red region. They have been studied as triplet emitters in solution-processed, multilayer organic light-emitting diodes (OLEDs), together with the known complex of 5-methyl-1,3-di(2-pyridyl)benzene, MePtCl, for comparison. The trend in efficiencies of the OLEDs prepared correlates with the charge-trapping properties of the complexes. The most efficiently emitting complex, FPtCl, was used as the dopant in a solution-processed white OLED, employing monomer and excimer emission.

Full text of DOI:10.1039/c1jm10193e

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PAPER
Cyclometallated platinum(II) complexes of 1,3-di(2-pyridyl)benzenes for
solution-processable WOLEDs exploiting monomer and excimer
phosphorescence†‡
a
a
a
b
b
b
Wojciech Mr ꢀo z,* Chiara Botta, Umberto Giovanella, Ester Rossi, Alessia Colombo, Claudia Dragonetti,
bc
b
c
d
Dominique Roberto,* Renato Ugo, Adriana Valore and J. A. Gareth Williams*
Received 13th January 2011, Accepted 31st March 2011
DOI: 10.1039/c1jm10193e
Two cyclometallated platinum(II) complexes, N^C^N-5-fluoro-1,3-di(2-pyridyl)benzene platinum(II)
chloride, FPtCl, and N^C^N-5-methyl-1,3-di(2-pyridyl)benzene platinum(II) isothiocyanate,
MePtNCS, have been synthesized and characterized. Both complexes are highly efficient
phosphorescent green emitters which can also display excimer emission in the red region. They have
been studied as triplet emitters in solution-processed, multilayer organic light-emitting diodes
(
OLEDs), together with the known complex of 5-methyl-1,3-di(2-pyridyl)benzene, MePtCl, for
comparison. The trend in efficiencies of the OLEDs prepared correlates with the charge-trapping
properties of the complexes. The most efficiently emitting complex, FPtCl, was used as the dopant in
a solution-processed white OLED, employing monomer and excimer emission.
Introduction
excimers or aggregates, they are of particular interest for the
production of white OLEDs (WOLEDs). The combination of
efficient triplet monomer and excimer emission to generate
broad-band light across the visible spectrum has been put
forward as an attractive route to single-dopant white-emitting
To construct efficient organic light-emitting diodes (OLEDs),
emitters possessing high quantum efficiencies under electrical
operation are required. Since the introduction of phosphorescent
metal complexes into this field, which utilize both singlet and
10
devices. WOLEDs are expected to be more efficient and less
1
triplet excitons to generate electroluminescence (EL), many new
expensive than fluorescent and incandescent illumination light
11,12
sources.
phosphorescent materials have been synthesized for potential
2,3
OLED applications. Among them, a class of cyclometallated
platinum(II) complexes with N^C^N-coordinated 1,3-di(2-pyr-
idyl)benzene ligands is gaining increasing interest, owing to their
They also offer useful properties like tunable correlated colour
temperature (CCT) and can be obtained as completely flexible
13,14
large area devices.
Excimers possess a broad EL spectrum,
4–7
high emission phosphorescence quantum yields.
These
a desirable feature for illumination purposes as it increases the
colour rendering index (CRI). Moreover, among many strategies
for obtaining white light from organic or organometallic
molecular materials, excimer emission is of particular interest as
it allows simplified device architectures to be used. This is
because excimers lack a bound ground state and, as a result,
energy transfer between multiple dopants is significantly
compounds have been tested as phosphors in multilayer OLEDs,
8,9
leading to high external quantum efficiencies. Additionally,
owing to their propensity to form highly emissive bimolecular
a
Istituto per lo Studio delle Macromolecole, Consiglio Nazionale delle
Ricerche, via Bassini 15, 20133 Milano, Italy. E-mail: w.mroz@ismac.
cnr.it
15
reduced.
b
Dip. di Chimica Inorganica, Metallorganica e Analitica ‘‘Lamberto
Most examples of devices with phosphorescent excimer emis-
sion use cyclometallated platinum(II) complexes with bidentate,
Malatesta’’ dell’Universit aꢁ degli Studi di Milano and UdR INSTM di
Milano, via Venezian 21, 20133 Milano, Italy. E-mail: dominique.
roberto@unimi.it
N^C-coordinating aryl-pyridines and b-diketonate co-
10,15–17
c
Istituto di Scienze e Tecnologie Molecolari, Consiglio Nazionale delle
Ricerche, Via Venezian 21, 20133 Milano, Italy
ligands.
Based on the same principle, WOLEDs with
N^C^N coordinated platinum(II) complexes have since been
d
Department of Chemistry, University of Durham, Durham DH1 3LE, UK.
E-mail: j.a.g.williams@durham.ac.uk
18
created. Further improvements in device performance were
recently achieved by combining the monomer and excimer
emission of such complexes with exciplex emission involving the
†
Dedicated to the memory of Professor Jan Kalinowski, a pioneer of
electroluminescence, deceased 18 December 2010.
Electronic supplementary information (ESI) available: IR and NMR
‡
19
host material. However, until now, all such excimer WOLEDs
characterization of the complexes, contrasting influence of F atoms,
influence of PVK electromer and AFM data. See DOI:
have been vacuum-sublimed multilayer structures. Such fabri-
cation methods, based on evaporation, are generally expensive
10.1039/c1jm10193e
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processes, requiring precise control of evaporation rates when
two or more compounds are in the same layer (e.g. host and
dopant), and are less suitable for the production of large area
devices due to the higher cost.
evaporated to dryness under reduced pressure. The residue was
taken up into CH Cl (3 ꢀ 50 mL) and washed with aqueous
2
2
NaHCO (1 M, 2 ꢀ 50 mL). The organic phase was dried over
3
2 4
anhydrous Na SO and evaporated to dryness. The brown
In this contribution, we present electroluminescent devices
that use Pt(N^C^N)X complexes as phosphors, built by a simple
spin-coating technique. Two complexes have been investigated,
as shown in Chart 1: N^C^N-5-fluoro-1,3-di(2-pyridyl)-benzene
residue was purified by chromatography on a silica gel, gradient
elution using hexane/diethyl ether 90/10 / 50/50, giving the
1
product as a white solid (0.146 g, 53%). H NMR (400 MHz,
6
2
0
4 3 19 1 4
DMSO-d6): d
3
H
¼ 8.83 (2H, d, H ), 8.68 (1H, s, H ), 8.39 (2H, dd,
7
0
platinum(II) chloride, FPtCl, and N^C^N-5-methyl-1,3-di-
H ), 8.27 (2H, ddd, H ), 8.11 (2H, d, J( F– H) ¼ 10, H ), 7.72
5
(2H, ddd, H ). Anal. Calcd for C16
(
2-pyridyl)-benzene platinum(II) isothiocyanate, MePtNCS. The
H
1
FN
2
: C, 76.79; H, 4.43; N,
5
known chloro analogue of the latter complex, MePtCl, has also
been studied for comparison. The synthesis and photophysical
properties of the complexes are reported, together with studies of
their use as phosphors in spin-coated diodes. Finally, the
complex showing the best performance has been applied in
a WOLED employing monomer and excimer emission mecha-
nisms, in a simple solution-processed multilayer device.
11.19%. Found: C, 76.55; H, 4.46; N, 11.16%.
1
H NMR data of 5-methyl-1,3-di(2-pyridyl)benzene for
6
comparison (400 MHz, CDCl ): dH ¼ 8.73 (2H, d, H ), 8.39 (1H,
3
2
0
4
s, H ), 7.92 (2H, s, H ), 7.85 (2H, d, H ), 7.77 (2H, td, H ), 7.26
0
3
4
5
3
(2H, td, H ), 2.54 (3H, s, CH ) (Chart 2).
Synthesis of platinum(II) complexes
Experimental
MePtCl. A solution of K PtCl (337 mg, 0.815 mmol) in water
2
4
(
(
12.5 mL) was treated with a solution of 3,5-di(2-pyridyl)toluene
210 mg, 0.856 mmol) in acetonitrile (12.5 mL), and the mixture
General comments
Reagents were purchased from Sigma-Aldrich or Fluorochem.
Products were characterized by elemental analysis, mass spec-
13
trometry (FAB and MALDI-TOF) and by H and C NMR
2
heated at reflux under N for 3 days. After cooling to room
temperature, the orange residue formed was collected by filtra-
tion and washed with water (2 ꢀ 20 mL) and ethanol (2 ꢀ 20
mL). After being dried under reduced pressure, the product was
+
1
1
spectroscopy (Bruker Avance DRX-400 instrument). H NMR
1
spectra are referenced relative to residual proton solvent reso-
obtained as an orange-yellow solid (259 mg, 67% yield). H NMR
6
1
95
nances; d are in ppm and coupling constants in Hz. 5-Methyl-
5
(400 MHz, CDCl
3
): d
H
195 3 3
¼ 9.34 (2H, d, J( Pt) 38, H ), 7.93 (2H,
4
td, H ), 7.66 (2H, d, J( Pt) 6.3, H ), 7.29 (2H, s, H ), 7.28 (2H,
0
1
,3-di(2-pyridyl)benzene was prepared as reported.
not fully resolved due to overlap with the former signal and
5
solvent, m, H ), 2.36 (3H, s, CH ). H NMR (400 MHz, CD Cl ):
1
3
6
2
2
Synthesis of 5-fluoro-1,3-di(2-pyridyl)benzene
195
4
dH ¼ 9.29 (2H, d, J( Pt) 38, H ), 8.00 (2H, td, H ), 7.74 (2H, d, J
1
95
3
( Pt) 6, H ), 7.39 (2H, s, H ), 7.35 (2H, ddd, H ), 2.42 (3H, s,
3
0
5
A mixture of commercial 1,3-dibromo-5-fluorobenzene (0.279 g,
+
+
1
.10 mmol), 2-(tri-n-butylstannyl)pyridine (1.00 g, 2.72 mmol),
CH ). MS FAB : 440 (M ꢁ Cl) . Anal. Calcd for C H ClN Pt:
3
17 13
2
lithium chloride (0.074 g, 1.66 mmol) and bis(triphenylphos-
phine)-palladium(II) chloride (0.203 g, 0.287 mmol) in toluene
C, 42.91; H, 2.75; N, 5.89%. Found: C, 43.10; H, 2.86; N, 5.72%
(Chart 3).
(
7 mL) was degassed via five freeze–pump–thaw cycles and then
1
FPtCl. FPtCl was prepared similarly, in 60% yield. H NMR
heated at reflux under an atmosphere of N2 for 72 h. After
cooling, a saturated aqueous solution of potassium fluoride
195
6
(400 MHz, DMSO-d6): dH ¼ 9.10 (2H, d, J( Pt) 32, H ), 8.24
4
3
19
(2H, td, H ), 8.17 (2H, d, H ), 7.78 (2H, s, J( F) 10, H ), 7.60
3
0
(
10 mL) was added and the mixture stirred for 30 min. The
5
13
insoluble residue formed was removed by filtration and washed
with toluene. The combined filtrate and washings were
(2H, ddd, H ). C NMR (400 MHz, CD Cl ): d ¼ 152.4, 139.2,
2
2
+
123.8, 119.8, 111.5 (quaternary not detected). MS FAB : 444 (M
+
ꢁ
Cl) . Anal. Calcd for C H ClFN Pt: C, 40.05; H, 2.10; N,
16 10 2
5
.83%. Found: C, 40.12; H, 2.10; N, 5.76%.
MePtNCS. Sodium thiocyanate (18 mg, 0.210 mmol) was
added to a suspension of MePtCl (50 mg, 0.105 mmol) in
methanol/acetone (5/20 v/v). The reaction mixture was stirred at
room temperature for 1 day. The crude compound was dissolved
in CH
2 2
Cl , filtered, and precipitated twice from dry pentane,
1
giving the pure product as a red solid (26 mg, 50% yield). H
1
95
6
NMR (400 MHz, CD
2
Cl ): d
2
H
¼ 8.77 (2H, d, J( Pt) 40, H ),
Chart 1 Structures of the complexes investigated (top) and of the
1
Chart 2 Numbering system for H NMR assignment of ligands.
unsubstituted and difluorinated analogues.
8
654 | J. Mater. Chem., 2011, 21, 8653–8661
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PMT after passage through a monochromator. The estimated
uncertainty in the quoted lifetimes is ꢃ10% or better. Bimolec-
O
2
ular rate constants for quenching by molecular oxygen, kQ
,
were determined from the lifetimes in degassed and air-equili-
brated solution, taking the concentration of oxygen in CH Cl at
2
2
ꢁ3 20
0
2
.21 atm O to be 2.2 mmol dm .
1
Chart 3 Numbering system for H NMR assignment of complexes.
Device fabrication and characterization
Glass substrates coated with indium tin oxide (ITO) were cleaned
ultrasonically in distilled water, acetone and 2-propanol.
Subsequently, a 50 nm layer of poly(3,4-ethylene-dioxythio-
phene)/poly(styrenesulfonate) (PEDOT:PSS) (H.C. Starck Cle-
vios P VP.AI 4083) was spin-coated from a water solution filtered
4
.04 (2H, td, H ), 7.73 (2H, d, J( Pt) 6, H ), 7.36 (2H, ddd, H ),
195
3
5
8
7
6
0
13
.33 (2H, s, H ), 2.42 (3H, s, CH
Cl
): d ¼ 152.3, 140.2, 124.9, 123.1, 119.1, 22.1 (CH
quaternary not detected). IR (n/cm ): for SCNPt, 2096 and
3
). C NMR (400 MHz,
CD
2
2
3
)
ꢁ1
(
+
2
089 in CHCl and Nujol, respectively. MS FAB : 440 (M ꢁ
3
+ +
ꢂ
through nylon (pore size 0.45 mm), and annealed at 100 C for 10
min under a N
NCS) ; MS MALDI-TOF (with LiCl): 505 (M + Li) . Anal.
Calcd for C H N PtS: C, 43.37; H, 2.63; N, 8.43%. Found: C,
2
atmosphere. In the case of the green-emitting
1
8
13
3
OLEDs (5% dopant in the emissive layer), a film of poly-
6
4
3.57; H, 2.70; N, 8.52%.
ꢁ1
(
N-vinylcarbazole) (PVK-h, average M
w
¼ 10 g mol ) was then
ꢁ1
spin-coated from CH Cl solution (12 mg mL ) onto the layers
2
2
Electrochemical measurements
ꢂ
prepared as above. The samples were annealed at 150 C for 30
min. The emitting layer, consisting of PVK-l (Mw ¼ 42 000 g
Cyclic voltammetry was carried out using an Autolab PG-Stat 30
potentiostat with computer control and data storage via GPES
Manager software. Solutions of concentration 1 mM in CH Cl
ꢁ
1 21
mol ),
2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadia-
zole (PBD), and either FPtCl, MePtNCS or MePtCl, was then
2
2
ꢁ1
2 2
spin-coated from a deaerated CH Cl solution (8 mg mL )
were used, containing [Bu N][PF ] 0.1 M as the supporting inert
4
6
comprised of 65% PVK-l:30% PBD:5% Pt complex by mass. The
average total thickness of PVK-h and the emitting layer was
equal to 165 nm for all three devices. Finally barium (5 nm layer)
and aluminium (60 nm layer) were deposited by thermal vacuum
electrolyte. A three-electrode assembly was employed, consisting
2
of a platinum wire working electrode (0.012 cm ), a platinum
wire counter electrode and a saturated calomel electrode (SCE)
as the reference electrode. The polishing procedure for the
working Pt electrode consisted of surface cleaning with diamond
powder (Aldrich, diameter 1 mm) on a wet cloth (DP-Nap,
Struers). Solutions were purged for 5 min with solvent-saturated
ꢁ
6
evaporation at 10 mbar.
For the WOLED, a film of PVK-h mixed with poly(9,9-dio-
ctylfluorenyl-2,7-diyl) end-capped with N,N-bis(4-methyl-
phenyl)-4-aniline (PFO, American Dye Source), with ratio 52%
PVK-h : 48% PFO by mass, was spin-coated from toluene
2
N gas with stirring, prior to measurements being taken without
stirring. The ohmic drop (310 U) was compensated by the posi-
tive feedback technique. The voltammograms were referenced to
ꢁ1
solution (15 mg mL ) onto the PEDOT:PSS-covered ITO
ꢂ
1/2
substrates. Samples were successively annealed at 100 C for 30
a ferrocene–ferrocenium couple as the standard (E ¼ 0.42 vs.
minutes. Then the second layer, comprised of 40% PVK-l, 20%
SCE).
PBD, 40% FPtCl by mass, was spin-coated from CH
2
Cl
2
solu-
ꢁ1
tion (2 mg mL ). Finally calcium (10 nm layer) and aluminium
Photophysical measurements
ꢁ6
(
60 nm) were deposited by thermal vacuum evaporation at 10
Absorption spectra were measured on a Biotek Instruments XS
spectrometer, using quartz cuvettes of 1 cm path length. Steady-
state luminescence spectra were measured using a Jobin Yvon
FluoroMax-2 spectrofluorimeter, fitted with a red-sensitive
Hamamatsu R928 photomultiplier tube (PMT); the spectra
shown are corrected for the wavelength dependence of the
detector, and the quoted emission maxima refer to the values
after correction. Samples for emission measurements were con-
tained within quartz cuvettes of 1 cm path length, modified with
appropriate glassware to allow connection to a high-vacuum
line. Degassing was achieved via a minimum of three freeze–
pump–thaw cycles whilst connected to the vacuum manifold;
mbar.
Electroluminescence (EL) and photoluminescence (PL)
spectra of the devices were recorded using a liquid-nitrogen-
cooled CCD camera in conjunction with a monochromator
(Spex 270M). The light source for PL was a monochromated
xenon lamp. The spectra were corrected for the instrument
response functions. Current–voltage (I–V) device characteriza-
tion was performed with a Keithley 2602 source meter combined
with a calibrated photodiode. External quantum efficiency
(EQE) was calculated by measuring the light emitted in the
22
forward direction with Lambertian source assumption. Device
preparation and characterization was performed under a N
2
ꢁ
2
final vapour pressure at 77 K was <5 ꢀ 10 mbar, as monitored
using a Pirani gauge. Luminescence quantum yields were deter-
mined relative to MePtCl as a secondary standard emitting in the
atmosphere inside a M-Braun glovebox system. CCT was eval-
uated according to ref. 23. Luminance (L), luminance efficiency
(LE) and power efficiency (PE) were calculated using formulae
from ref. 24.
5
same region. The luminescence lifetimes of the complexes were
measured by time-correlated single-photon counting, following
excitation at 374 nm with an EPL-375 pulsed-diode laser. The
AFM investigations were performed with a NT-MDT NTE-
GRA instrument in non-contact mode under ambient
conditions.
ꢂ
emitted light was detected at 90 using a Peltier-cooled R928
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the Pt ion in m-F PtCl (structures in Chart 1), as opposed to the
2
Results and discussion
para-disposed F atom in FPtCl. In m-F PtCl, the emission is
2
26
blue-shifted by 20 nm compared to HPtCl.
Synthesis of ligands and complexes
These contrasting effects of the F atoms can be understood in
terms of the frontier orbitals in such complexes. Thus, time-
dependent density functional theory calculations on HPtCl have
The ligand 5-fluoro-1,3-di(2-pyridyl)benzene was easily obtained
by a Stille reaction of 1,3-dibromo-5-fluorobenzene with 2-(tri-n-
butylstannyl)pyridine (molar ratio 1 : 2.5) in the presence of
lithium chloride and bis(triphenylphosphine)-palladium(II)
1
revealed that the T state has predominantly HOMO–LUMO
27
character. The HOMO is based primarily on the aryl ring, the
metal ion and the chloride co-ligand, whilst the LUMO is
predominantly localised on the pyridyl rings. The inductive
electron-withdrawing effect of F atoms, irrespective of whether
they be meta or para to the metal, should lower the energy of
both HOMO and LUMO, as found, for example, with iridium
(III) complexes containing related fluorine-substituted terdentate
chloride as the catalyst, in toluene at reflux under N
2
. Further
reaction with K PtCl in refluxing CH CN/H O (v/v) affords
2
4
3
2
7
5
FPtCl. Reaction of the known complex MePtCl with NaSCN
gives the new isothiocyanate Pt(II) complex, MePtNCS. The
ꢁ
bonding of the SCN ligand to the Pt ion through the N atom is
ꢁ1
confirmed by the strong, sharp peak at 2096 cm in CHCl3
ꢁ
1
(
2089 cm in Nujol) a value typical of the n(SCN) stretching
28
25
ligands. However, when the F atom is para to the metal as in
FPtCl, the mesomeric electron-donating effect of F through the
p orbitals will slightly attenuate the inductive effect. In contrast,
mode of metal N-bonded isothiocyanate ligands (see Experi-
mental section and Scheme 1).
2
there is no such possibility in m-F PtCl, in which the F atoms are
Photophysical and electrochemical properties in solution
meta to the metal, so the HOMO is expected to drop further in
energy in this case. On the other hand, the extent to which the
LUMO is stabilised in the latter complex may be expected to be
slightly less than in the former, owing to the pyridyl rings (on
which the LUMO is localised) being ortho to the F atoms, and
therefore subject to the opposing mesomeric effect. These
combined effects—which are represented schematically in Fig. S6
of the ESI‡—lead to the net trend in energies: FPtCl < HPtCl <
Photophysical and electrochemical data for FPtCl, MePtNCS
and MePtCl in solution are reported in Table 1. Their electronic
absorption and photoluminescence spectra in diluted dichloro-
methane are complex, we attribute the intense absorption bands
1
at l < 300 nm to p–p* transitions localized on the ligands, while
those between 320 nm and 450 nm correspond to transitions of
5,6
mixed charge-transfer/ligand-centred character. A very weak
band appears at around 490 nm, due to the formally forbidden S
transition facilitated by the high spin–orbit coupling
2
m-F PtCl.
0
Meanwhile, the change in the monomeric co-ligand from
chloride in MePtCl to isothiocyanate in MePtNCS is seen to
have essentially no significant effect, probably reflecting the
/
T
1
associated with the platinum ion. The PL spectrum of complex
FPtCl is essentially identical to that of MePtCl. It displays
27
rather small participation of the co-ligand in such complexes,
ꢁ1
a vibrationally structured spectrum, n z 1300 cm , with the 0,0
and the similar positions of the NCS and Cl ligands in the
spectrochemical series.
vibrational component at 504 ꢃ 1 nm, representing a red-shift of
5
3 nm compared to the unsubstituted complex HPtCl. This
1
The relatively long lifetimes of several microseconds (Table 1)
are typical of the phosphorescence of complexes of this class. The
quantum yields of FPtCl and MePtNCS in degassed dichloro-
methane at room temperature were 0.47 and 0.60, comparable
with those of MePtCl (see Table 1) and other complexes of this
contrasts with the effect of fluorine atoms at the positions meta to
6
type. That such high quantum yields arise despite the relatively
long lifetimes (which are indicative of relatively low radiative rate
constants) indicates that non-radiative decay processes in these
complexes are particularly ineffective, probably reflecting the
high rigidity of the system and high ligand-field strength asso-
ciated with the N^C^N-coordinated ligand. The radiative k and
r
non-radiative Sknr decay constants can be estimated from the
lifetimes and quantum yields, assuming that the emitting state is
formed with unitary efficiency (see footnote to Table 1). The
rates of both processes are smaller than those typically found for
29
more commonly studied iridium(III)-based OLED emitters, for
example, but the fact that the values are comparable to one
another leads to the PL quantum yields of around 0.5.
The influence of increasing concentration on the absorption
and PL spectra of FPtCl in dichloromethane is shown in Fig. 1
(
b). While the shape of the absorption spectrum does not change,
the PL spectra of all three complexes at higher concentrations
reveal the appearance of a broad, structureless band at longer
wavelengths, due to emission from an excimer, as observed for
5,6
other members of this family of complexes. The associated self-
SQ
Scheme 1 Synthesis of the complexes FPtCl and MePtNCS.
656 | J. Mater. Chem., 2011, 21, 8653–8661
quenching rate constants of the monomer emission, k
Q
, are of
8
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Table 1 Photophysical and electrochemical parameters at 298 K in dichloromethane
b
Absorption
max/nm
(
s
0
/ms
degassed
SQ
Q
ꢁ1 ꢁ1
9
O
2
8
red
p
e
ox
p
e
/
f
f
HOMO / LUMO /
l
Emission
max/nm
k
M
/10
s
k
M
Q
/10
s
k
10 s
r
(Sknr)/
E
V
/
E
V
ꢁ
1
3/L mol cm
ꢁ1
a
lum
b
ꢁ1 ꢁ1
c
4
ꢁ1
d
)
l
F
(aerated)
eV
eV
g
h
h
h
h
MePtCl
335 (5710)
505, 539, 578 0.68
7.8 (0.3)
3.3
16
8.7 (4.1)
ꢁ2.27
0.36
ꢁ5.16
ꢁ2.53
3
4
4
4
81 (6900)
12 (6780)
60 (190)
95 (130)
FPtCl
322sh (6170)
504, 542, 580 0.47
501, 533, 580 0.60
6.3 (0.6)
8.6 (0.7)
5.6
8.9
7.4
7.4 (8.5)
7.0 (4.6)
ꢁ2.19
ꢁ2.21
0.56
0.24
ꢁ5.36
ꢁ5.04
ꢁ2.61
ꢁ2.59
3
4
4
77 (7740)
22 (8270)
86 (312)
MePtNCS
333 (4760)
6.2
3
3
4
4
57 (2800)
82 (3890)
06 (3820)
99 (188)
a
b
SQ
Q
Luminescence quantum yield in degassed solution.
intercept and slope, respectively, of a plot of the measured emission decay rate constant against concentration. Bimolecular rate constant for quenching
s
0
is the lifetime at infinite dilution and k
the self-quenching rate constant, determined from the
c
ꢁ
3
by molecular oxygen, estimated using the lifetimes in degassed and air-equilibrated solutions, and taking [O
2
] ¼ 2.2 ꢀ 10 M in CH
and non-radiative (Sknr, in parenthesis) decay constants are made as follows: k . Sknr ¼ (1/s
¼ Flum/s
refer to peak potentials of oxidation and reduction, respectively; values
2 2
Cl at 1 atm
d
pressure of air. Estimates of radiative k
r
r
0
0
)
e
ox
p
red
p
ꢁ
k
r
.
All processes were electrochemically irreversible, hence E
and E
+
1/2
are reported relative to a ferrocenium/ferrocene (Fc /Fc) redox couple used as an internal reference (E ¼ +0.42 V vs. saturated calomel electrode
ꢁ
1
f
SCE); at 298 K, in the presence of 0.1 M [Bu
4
N][PF
6
] as the supporting electrolyte, scan rate 200 mV s
.
Energies of the highest occupied
g h
molecular orbital HOMO and lowest unoccupied molecular orbital LUMO as calculated from the redox potentials. Data from ref. 5 and 6. Data
from the present work.
a comparable magnitude in each case, as are the bimolecular
oxygen quenching rate constants, which show only small differ-
ences amongst the three complexes.
the radiative excitons from direct quenching by PEDOT:PSS,
removing a non-radiative decay channel introduced by this latter
22
6
ꢁ1
material. High molecular weight PVK-h (M
w
¼ 10 g mol ),
A comparison of the cyclic voltammetry data for complexes
FPtCl, MePtNCS and MePtCl is given in Table 1. As expected
for the higher electron withdrawing effect of F with respect to
a methyl group, FPtCl is more easily reduced and more difficult
to oxidize with respect to MePtCl. The first oxidation and
reduction potentials can be used to estimate the HOMO and
LUMO energy levels by means of equations EHOMO (eV) ¼
with diminished solubility upon thermal treatment, allows us to
fabricate multilayer architecture by successive solution deposi-
tion, as verified by AFM investigation. The top (emissive) layer is
composed of the platinum complex dispersed in a host matrix
comprised of PVK-l:PBD. PBD is an electron-transporting
material, introduced in order to improve the balance between
charges.
ꢁ
(EOX + 4.8) and ELUMO (eV) ¼ ꢁ(ERED + 4.8), which involve
the use of the internal ferrocene standard value of ꢁ4.8 eV with
respect to the vacuum level. The HOMO–LUMO gap is 2.45,
.63, and 2.75 eV for MePtNCS, MePtCl, and FPtCl, respec-
The EL spectra of OLEDs prepared in this way are shown in
Fig. 2 together with HOMO and LUMO levels of used materials,
and device performance data are compiled in Table 2. The EL
spectra are similar to the corresponding PL spectra of dilute
solutions. However, an additional long-wavelength component is
evident, around 600 nm, superimposed on the emission of the
complex and increasing in intensity in the order FPtCl < MePtCl
30
2
tively (Table 1).
Spin-coated OLEDs using new Pt(II) complexes
<
MePtNCS. This low-energy emission is attributed to a PVK
32
Using complexes FPtCl and MePtNCS as phosphorescent
dopants, we fabricated solution-processed multilayer electrolu-
minescent devices with a typical composition of 100% PVK-h in
the first layer and 65% PVK-l:30% PBD:5% Pt complex in the
emissive layer. The PEDOT:PSS layer, which is standardly used
in polymer based OLEDs, is considered as a part of the substrate.
The chemical structures of the organic materials used are shown
in Chart 4. The multilayer OLED approach has many advan-
tages in comparison to single layer devices, but usually requires
the use of thermal evaporation which is generally an expensive
process. Here, we combined the simplicity of the low-cost spin-
coating technique with the advantages of multilayer architecture.
A PVK-h layer, directly deposited on PEDOT:PSS, plays
a multiple role in our OLEDs. This polymer is well known as
electromer, rather than to complex luminescence. The order of
increasing intensity of the electromer can be explained by the
charge-trapping properties of the complexes.
If a complex does not trap the charge carriers efficiently, they
can travel into the host more freely and the probability that they
will recombine on PVK-l:PBD matrix increases.
Poorer trapping ability of a complex will lead to a more
intense electromer band being observed. In contrast, efficient
trapping on a complex emitting centre will favour high EQE
16
of devices and also should decrease leakage current. In the
present case, FPtCl doped in PVK-l:PBD host matrix has the
best trapping abilities while MePtNCS has the poorest
(see inset in Fig. 2), according to the trend in electromer
intensities, and indeed this order is reflected in the EQE
values in Table 2.
31
a hole transporting material, and additionally is able to block
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Fig. 2 EL spectra of PVK-h/65% PVK-l:30% PBD:5% Pt complex
diodes:FPtCl (solid), MePtCl (dash), MePtNCS (circle). In the inset
HOMO and LUMO levels of used materials are shown.
Table 2 Electroluminescence performance of devices based on FPtCl,
MePtNCS and MePtCl with the device structure PVK-h/65% PVK-l:30%
PBD:5% Pt complex
FPtCl
MePtNCS
MePtCl
V
EQE (%)
on/V
12
0.28
91
0.94
0.18
755
15
0.21
17
0.62
0.11
161
18
0.23
21
0.68
0.097
195
(0.37, 0.58)
a
b
L /cd m
ꢁ2
b
LE /cd A
ꢁ1
b
PE /lm W
ꢁ1
Fig. 1 (a) Absorption and PL spectra of FPtCl, MePtNCS and MePtCl
in dichloromethane; (b) Absorption and PL spectra of FPtCl in
ꢁ
2
Lmax/cd m
CIE (x, y)
ꢁ
5
(0.32, 0.61)
(0.39, 0.55)
dichloromethane at concentrations 4.15 ꢀ 10 M (solid line), 2.075 ꢀ
ꢁ
4
ꢁ4
ꢁ3
10
M (dashed line), 4.15 ꢀ 10 M (circles), and 2.075 ꢀ 10
M
a
b
Maximum external quantum efficiency at the forward direction. At
a maximum external quantum efficiency.
(squares).
It is remarkable that the EQE values obtained for the various
devices (Table 2), although unsurprisingly lower than those
obtained for OLEDs based on complexes of the same family built
9
by evaporation techniques, are one order of magnitude higher
than for a recently reported OLED based on a substituted
0
4
,4 -stilbenoid N^C^N platinum(II), also built by the spin-coating
33
technique.
Fig. 3 EL spectra of single layer devices with different concentrations of
FPtCl dispersed in PVK-l:PBD matrix.
Chart 4 Molecular structures of PBD, PVK and PFO.
658 | J. Mater. Chem., 2011, 21, 8653–8661
8
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By increasing the concentration of the FPtCl complex in the
PVK-l:PBD matrix, we were able to tune the emission of the
single layer devices (in this case without PVK-h bottom layer)
from green to red (see Fig. 3), owing to the appearance of an
excimer band centred at 700 nm. For the diode with the
composition 40% PVK-l:20% PBD:40% FPtCl, balanced green
and red emission was observed, and the same composition was
therefore employed for the construction of a multilayer
WOLED, as described in the next section.
WOLED
In this section, we present a multilayer WOLED prepared using
a simple solution-processing technique. As diodes with a bottom
layer consisting of PVK-h or PVK-h:PBD did not emit in the
blue region, we introduced PFO (Chart 4) as an efficient blue
34
emitter. The selected structure for the device was: ITO/PEDOT:
PSS/52% PVK-h:48% PFO/40% PVK-l:20% PBD:40% FPtCl/
Ca/Al. A schematic diagram illustrating the HOMO and LUMO
levels of each of the materials used is shown in Fig. 4. The bottom
layer consists of a mixture of PVK-h and PFO, and serves as
a blue-emitting and hole-transporting layer. PFO is known to
35
provide good hole mobility. Additionally, the HOMO level of
PVK-h is lower compared to the PEDOT:PSS level, which
facilitates injection of holes from the anode to PFO. The top
layer is composed of the platinum complex FPtCl dispersed in
PVK-l:PBD matrix. FPtCl is an effective hole-trapping centre
(
Fig. 4), collecting holes that arrive from the bottom layer and
preventing their migration towards the cathode, automatically
decreasing leakage current.
Fig. 5(a) shows the EL spectrum of the device, with
Commission Internationale de l’Eclairage (CIE) coordinates x ¼
0
.28, y ¼ 0.35 and CCT equal to 8145 K, CRI equal 74 and EQE
ꢁ4
of about 4 ꢀ 10 %. The shoulder around 400–420 nm is most
likely due to the residual emission of PFO a-phase, almost
36–38
Fig. 5 (a) Comparison between EL at 7 V (solid) and PL (lexc ¼ 325 nm,
dashed) spectra of the diode of architecture ITO/PEDOT:PSS/52% PVK-
h:48% PFO/40% PVK-l:20% PBD:40% FPtCl/Ca/Al. The inset shows the
CIE coordinates of the EL spectrum. (b) Absorption spectrum of FPtCl
in dichloromethane (dashed line) and PL spectra of films of PFO (solid
line) and PBD:PVK-l (circles). Excitation wavelengths were 325 nm and
eliminated after thermal treatment of the PVK-h:PFO layer (see
Experimental part). The sharp peaks at 438 nm and 465 nm are
due to the vibrational 0 / 0 and 0 / 1 components of the
emission of the newly formed b phase PFO. In our diode, the
emitted blue light can exit the device immediately or after
reflection from the cathode. In the second case, partially absor-
bed light is used to excite the Pt(II) complex by an energy transfer
3
20 nm for PFO and PVK-l:PBD, respectively.
in the top layer. Bands at 509 nm and 544 nm, together with
a shoulder at 590 nm, originate from the phosphorescence of
single FPtCl molecules. The tail at low energy is due to complex
excimer emission. The lower intensity of the excimer band
observed in this WOLED in comparison to the single-emitting-
layer device with the same concentration of the complex,
described in the previous section, is probably caused by the
partial dissolution of the bottom layer during preparation on the
second film, as confirmed by AFM investigation. In this process,
the effective dispersion of the complex at the interface is
increased, changing the balance between accessible monomer
and dimer states. The combined EL from the two layers creates
white light, since green-red light emitted from the top layer is not
absorbed by polymers in the bottom layer and thus exits the
device freely, along with blue light from the bottom layer.
The corresponding PL spectrum of the device is also shown in
Fig. 5(a). A difference in the green-red region can be noticed,
with reversed intensities of monomer and excimer emission
39,40
process,
enabled by the overlap of the absorption of the
complex and blue emitter’s emission spectrum (Fig. 5(b)). Two-
colour emission was obtained with a high concentration of FPtCl
Fig. 4 Energy levels of materials used in the multilayer WOLED device.
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compared to EL. Such a discrepancy between EL and PL origi-
nates from the different mechanisms responsible for the exciton
and excimer generation in two types of luminescence. In PL,
energy transfer is the dominant phenomenon, while in EL, the
charge trapping properties of the system are the main contrib-
utor. PFO emission recorded in EL suggests that electrons may
enter into the bottom layer and bind there some holes on emitting
centres localized on PFO. In devices with a bottom layer con-
sisting of PVK-h or PVK-h:PBD, blue EL is not observed. This is
due to the good transporting properties of both materials, pre-
venting efficient trapping of charge carriers on emitting centres,
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4
41
and to their emission quantum yields being only modest. On the
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(a) S. J. Farley, D. L. Rochester, A. L. Thompson, J. A. K. Howard
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8
Z. Wang, E. Turner, V. Mahoney, S. Madakuni, T. Groy and J. Li,
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We have synthesized two phosphorescent green-emitting
N^C^N-coordinated platinum(II) complexes, FPtCl and
MePtNCS, and we have compared their properties with those of
MePtCl. Photophysical measurements revealed that, in addition
to green emission, the complexes also display an excimer band
centred around 700 nm. The emitters were tested in simple-to-
build, solution-processable OLEDs as dopants in a PVK:PBD
matrix. The obtained external quantum efficiencies were ten
times higher than for a recently reported OLED based on
2
005, 86, 153505.
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1
1
0
a substituted 4,4 -stilbenoid N^C^N platinum(II), also built by
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its double emission color, was used as a single dopant in solution
processed multilayer WOLED. Green and red light originated
from the monomer and excimer emission of the complex,
respectively, while blue came from the bottom layer. Application
of highly concentrated dopant displaying excimer emission
allows the device structure to be simplified, as energy transfer
between multiple dopants is reduced. Additionally we combined
the facility of the low-cost spin-coating technique with the
advantages of multilayer architecture. The study reveals that the
Pt(N^C^N) class of emitters are amenable to the production of
solution-processed OLEDs, in addition to the vacuum-sublimed
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Acknowledgements
We deeply thank Dr Luigi Falciola (Dip. di Chimica Fisica ed
Elettrochimica dell’Universit ꢁa degli Studi di Milano) for elec-
trochemical measurements and Dr Andrea Arcari for develop-
ment of a setup for OLED characterization. This work was
supported by the CARIPLO Foundation (2007, Diodi per illu-
minazione a luce bianca basati su nuovi complessi organo-
metallici), by MIUR (FIRB 2004: RBPR05JH2P and PRIN
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