Cyclometallated platinum(ii) complexes of 1,3-di(2-pyridyl)benzenes: Tuning excimer emission from red to near-infrared for NIR-OLEDs

Article abstract of DOI:10.1039/c1jm12716k

The Pt(ii) complex N^C²^N-1,3-di(2-pyridyl)benzene platinum chloride (PtL¹Cl) is known to display efficient triplet luminescence in the green region of the spectrum, and to form an unusually emissive excimer that emits around 690 nm.

Full text of DOI:10.1039/c1jm12716k

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PAPER
Cyclometallated platinum(II) complexes of 1,3-di(2-pyridyl)benzenes: tuning
excimer emission from red to near-infrared for NIR-OLEDs†
Ester Rossi,a Lisa Murphy,b Phillipa L. Brothwood,b Alessia Colombo,a Claudia Dragonetti,a
ac
a
d
b
*
*
Renato Ugo, Massimo Cocchi and J. A. Gareth Williams
Dominique Roberto,
Received 13th June 2011, Accepted 28th July 2011
DOI: 10.1039/c1jm12716k
The Pt(II) complex N^C2^N-1,3-di(2-pyridyl)benzene platinum chloride (PtL1Cl) is known to display
efficient triplet luminescence in the green region of the spectrum, and to form an unusually emissive
excimer that emits around 690 nm. In this contribution, the introduction of trifluoromethyl groups into
either the 4- or 5-position of the pyridyl rings of the ligand is shown to lead to a red-shift in the excimer
band, moving it into the near infra-red (NIR) region. The new ligands, synthesised by either Suzuki or
Stille cross-coupling methods, are 1,3-bis(4-(trifluoromethyl)pyridin-2-yl)benzene HL27, 1,3-bis(4-
(trifluoromethyl)pyridin-2-yl)-4,6-difluorobenzene HL28, and 1,3-bis(5-(trifluoromethyl)pyridin-2-yl)-
4,6-difluorobenzene HL29, from which the corresponding Pt(II) complexes PtLnCl have been prepared.
The monomer and excimer emission energies in solution are compared with those of PtL1Cl and
PtL22Cl {HL22 ¼ 1,3-di(2-pyridyl)-4,6-difluorobenzene}. The order for the monomer can be
rationalised in terms of the stabilising effects of the F atoms and the CF3 groups on the HOMO and
LUMO respectively. The order of excimer emission proves to be subtly different, but the most red-
shifted complex in both cases is PtL27Cl. The electroluminescence of neat films of the complexes as
emitting layers in OLEDs displays uniquely excimer-like emission, extending well into the
technologically important NIR region.
energy conversion.4 Long lifetimes under ambient conditions are
Introduction
also being exploited in sensor technology, where they allow
discrimination from short-lived background fluorescence.5,6
In this field, complexes of platinum(II), a d8 ion, differ from
those of d6 metals such as Re(I), Ru(II), Os(II), Rh(III) and Ir(III),
in that they are normally 4-coordinate complexes with square-
planar geometry. This favours face-to-face intermolecular
interactions, including the formation of excimers and/or aggre-
gates, which emit at lower energy than the isolated monomeric
molecules.7 The combination of monomer and excimer emission
can lead to broad coverage of the emission spectrum. In terms of
applications to OLEDs, this behaviour may offer an attractive
way forward in the development of white light-emitting devices
(WOLEDs) that require only a single metal complex as dopant,8
as opposed to the use of separate red, green and blue dopants
that must be segregated within a device.9 The diversity of plat-
inum(II) complexes known to emit efficiently in solution at room
temperature has increased greatly over recent years.10 Cyclo-
metallating ligands have proved to be particularly successful
for generating luminescent complexes, apparently because
their strong ligand fields ensure that the severely distorted
metal-centred (d–d) states—population of which is responsible
for non-radiative decay of simple Pt(II) complexes such as
[Pt(bipyridine)2Cl2]—are displaced to high energies.10a,f Various
Pt(II) complexes with bidentate cyclometallating ligands,
Coordination complexes that emit efficiently from triplet excited
states are the subject of a great deal of current research interest
due to the numerous applications of such compounds. For
example, the new generation of display screen technology—
organic light-emitting devices (OLEDs)—can benefit from the
incorporation of such materials to induce emission from other-
wise wasted triplet states.1–3 Meanwhile, triplet excited states of
coordination complexes may be sufficiently long-lived for other
processes to occur following absorption of light, such as energy
and electron transfer, central to the field of light-to-chemical
aDipartimento di Chimica Inorganica, Metallorganica
e Analitica
ꢀ
‘‘Lamberto Malatesta’’ dell’Universita degli Studi di Milano and UdR
INSTM di Milano, via Venezian 21, 20133 Milano, Italy. E-mail:
dominique.roberto@unimi.it
bDepartment of Chemistry, University of Durham, South Road, Durham,
DH1 3LE, United Kingdom. E-mail: j.a.g.williams@durham.ac.uk
cISTM-CNR, via Golgi 19, 20133 Milano, Italy
dISOF-CNR, via P. Gobetti 101, 40129 Bologna, Italy. E-mail: cocchi@
isof.cnr.it
† Supporting Information available: 1H and 19F NMR spectra, cyclic
voltammograms, emission spectra as a function of concentration, plots
of the observed decay constant as a function of concentration, emission
spectra at 77 K, emission spectra in the solid state. See DOI:
10.1039/c1jm12716k
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 15501–15510 | 15501

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typically based on 2-arylpyridines, have been found to be quite
intensely luminescent,11 whilst related N^N deprotonated azolate
ligands are proving to be very attractive alternatives for opti-
mising emission.12
primarily to the HOMO) or the pyridyl ring (where the LUMO is
predominantly localised).10,21 Similar considerations apply to Pt
(N^C^N)Cl complexes, supported by DFT calculations.16 The
factors governing the excimer energy are less clear-cut, having
been studied to a lesser extent and being governed by a combi-
nation of steric as well as electronic factors, but it has been noted
to date that electron-donating groups in the pyridyl rings do tend
to blue-shift the excimer, as for the monomer.19b
Platinum(II) complexes with terdentate cyclometallating
ligands based on 1,3-di(2-pyridyl)benzene (HL1), which offer the
metal ion an N^C^N coordination environment,13–15 are amongst
the brightest Pt-based emitters in solution at room temperature;
e.g. for the parent complex PtL1Cl, (Chart 1), flum ¼ 0.60 and s ¼
7.2 ms in deoxygenated dichloromethane.14 The luminescence in
these complexes is attributed to a primarily ligand-centred 3p–p*
state, in which the contribution of the metal is nevertheless
sufficient to promote the triplet emission.16 The complexes
readily form excimers at elevated concentrations in solution, and
either excimers or aggregates in the solid state, which emit with
unusually high efficiency in the red region of the spectrum (e.g.,
lmax ꢀ 690 nm for the excimer of PtL1Cl). They have been
successfully introduced as phosphors into OLEDs using vacuum
sublimation techniques.17,18 High device efficiencies have been
obtained, including WOLEDs offering CIE coordinates close to
pure white and with impressive colour rendition indices.19
Meanwhile, neat films of such complexes display exclusively
excimer/aggregate emission. The use of the pure complex as the
emitting layer in an OLED has been used to produce exclusively
red-to-NIR-emitting devices (lmax ꢀ 700 nm), displaying high
efficiencies (up to 10.7% photons/electron and forward light
output > 15 mW cmꢁ2).20
In the present contribution, we describe three new complexes
of this general type, incorporating trifluoromethyl-substituted
pyridines, PtL27Cl, PtL28Cl and PtL29Cl (Chart 1). We show that
the CF3 groups lead to a stabilisation of the excimer and hence to
a significant red-shift of its emission, the effect being greater for
the pyridyl-4 compared to 5-position. Our study reveals that the
emission can be shifted more squarely into the NIR using this
strategy, and NIR-OLEDs incorporating neat films of the new
compounds as emissive layers are described.
Experimental
General comments
1H NMR spectroscopy was carried out on a Bruker Avance
DRX-400 or 300 MHz instrument (Milan, for HL29 and PtL29Cl),
or on Varian 200, 400 or 700 MHz instruments (Durham, HL27-28
and PtL27-28Cl). Chemical shifts are in ppm, referenced to residual
protio solvent resonances, and coupling constants are in Hertz.
1
The numbering systems used in the assignment of the H NMR
Extensive studies on Pt(II) complexes of phenylpyridine have
revealed how the emission energy of isolated (i.e., monomeric)
molecules can be controlled in a reasonably predictable manner
through substitution of the phenyl ring (which contributes
Chart 1 Structures of the new complexes investigated, together with
those of PtL1Cl and PtL22Cl.
Scheme 1 Synthesis of the new ligands HL27, HL28 and HL29 and their
corresponding Pt(II) complexes.
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spectra of the ligands and complexes are shown in Scheme 1 and
Chart 1 respectively. Electrospray mass spectra were recorded
using a Thermo Finnigan LTQ FT spectrometer with methanol
as the carrier solvent. Elemental analyses were performed using
an Exeter Analytical E-440 analyser. 1,3-Dibromo-4,6-difluoro-
benzene (2) was prepared as previously reported.22
a nitrogen atmosphere for 3 h. After cooling to room tempera-
ture, the resulting black suspension was filtered through celite.
Ether (10 mL) was added and the solution was washed with water
(3 ꢃ 5 mL) and then dried over anhydrous Na2SO4. The solvent
was removed under reduced pressure, giving the desired product
as a pale yellow oil, which was used in the preparation of HL29
without further purification (225 mg, 82%). 1H NMR (400 MHz,
CDCl3): d 9.01 (s, 1H), 7.76 (t, 1H), 7.63 (d, 1H).
1,3-Bis(4-(trifluoromethyl)pyridin-2-yl)benzene HL27
A mixture of benzene-1,3-diboronic acid (152 mg, 0.915 mmol),
2-chloro-4-(trifluoromethyl)pyridine (0.24 mL, 1.83 mmol),
sodium carbonate (642 mg, 6.06 mmol), methanol (6 mL) and
toluene (6 mL) was degassed by four freeze-pump-thaw cycles.
Pd(PPh3)4 (70 mg, 0.061 mmol) was added under a stream of
nitrogen gas and the resulting mixture heated at 100 ꢂC (oil bath
temperature) under a nitrogen atmosphere for 24 h. The solvent
was then removed under reduced pressure, and the residue taken
up into a mixture of dichloromethane (30 mL) and water (20
mL). The organic phase was separated, dried over anhydrous
magnesium sulfate, and filtered. Al2O3 was added (500 mg,
chromatographic grade), the reaction shaken several times, and
the alumina filtered off. The solvent was removed under reduced
pressure, to give the analytically pure product (57 mg, 17%). 1H
NMR (400 MHz, CDCl3): dH ¼ 8.92 (2H, d, 3J ¼ 5.0, H6), 8.73
1,3-Bis(5-(trifluoromethyl)pyridin-2-yl)-4,6-difluoro-benzene HL29
A solution of 2-(trimethyltin)-5-(trifluoromethyl)pyridine (540
mg, 1.74 mmol) in toluene (10 mL) was added dropwise to
a mixture of 1,3-dibromo-4,6-difluorobenzene (158 mg, 0.58
mmol), lithium chloride (240 mg, 5.8 mmol) and bis(triphenyl-
phosphine)palladium(II) dichloride (83 mg, 0.12 mmol) in
toluene (25 mL). The mixture was degassed via five freeze–pump–
thaw cycles and then heated at reflux temperature under
a nitrogen atmosphere for 4 h. After cooling to room tempera-
ture, a saturated aqueous solution of potassium fluoride (10 mL)
was added to the dark solution and stirred for 30 min. The
insoluble residue formed was removed by filtration and washed
with toluene. The filtrate and washings were combined and the
solvent removed under reduced pressure. The residue was taken
up into dichloromethane and washed with a saturated solution of
aqueous NaHCO3 (2 ꢃ 20 mL). The organic phase was dried
over anhydrous Na2SO4 and evaporated to dryness under
reduced pressure. The brown residue was purified by chroma-
tography on silica gel (eluent: hexane) giving the new compound
HL29 as a white solid (94 mg, 40%). 1H NMR (400 MHz, CDCl3):
0
0
3
(1H, t, 4J ¼ 2.0, H2 ), 8.14 (2H, dd, J ¼ 8.0, 4J ¼ 2.0, H4 ), 8.04
0
(2H, t, 4J ¼ 1.0, H3), 7.66 (1H, t, 3J ¼ 8.0, H5 ), 7.50 (2H, dd, 3J ¼
5.0, 4J ¼ 1.0, H5). 19F NMR (470 MHz, CDCl3): dF ¼ ꢁ65.0 (s).
MS (ES+): m/z ¼ 369 [M + H]+. Anal. calcd. for C18H10N2F6: C,
58.7; H, 2.7; N, 7.6%. Found: C, 58.0; H, 3.0; N, 6.9%.
0
dH ¼ 9.02 (2H, s, H6), 8.87 (1H, t, J ¼ 8.5, H2 ), 8.05 (2H, d, J ¼
1,3-Bis(4-(trifluoromethyl)pyridin-2-yl)-4,6-difluoro-benzene HL28
0
8.5, H4), 7.97 (2H, d, J ¼ 8.5, H3), 7.13 (1H, t, J ¼ 11, H5 ). MS
A mixture of 2-chloro-4-trifluoromethylpyridine (49 mg, 0.27
mmol), the pinacol ester of 1,3-difluoro-4,6-diboronic acid (45
mg, 0.12 mmol), aqueous sodium carbonate solution (1M, 2 mL)
and dimethoxyethane (2 mL), was degassed via four freeze-
pump-thaw cycles, and Pd(PPh3)4 (17 mg, 0.02 mmol) was added
under a stream of nitrogen. The mixture was heated at 100 ꢂC (oil
bath temperature) for 24 h and the reaction worked up as
described for HL27. The crude product was purified by column
chromatography on silica gel (eluant: hexane/EtOAc 100 : 0 /
80 : 20), and was obtained as a white oily solid (40 mg, 0.11
(FAB): m/z 405 [M + H]+.
PtL27Cl
A mixture of HL27 (40 mg, 0.11 mmol) and K2PtCl4 (50 mg, 0.12
mmol) in glacial acetic acid (3 mL) was degassed by three freeze-
pump-thaw cycles, and then heated under a nitrogen atmosphere
for 3 days. After cooling to room temperature, the precipitated
solid was washed successively with methanol, water, ethanol and
diethyl ether (3 ꢃ 5 mL of each) and then extracted into
dichloromethane. Removal of the solvent led to the product as
1
mmol, 89%), Rf 0.42 (silica, hexane/EtOAc 80 : 20). H NMR
(200 MHz, CDCl3): dH ¼ 8.93 (2H, d, J ¼ 5.0, H6), 8.83 (1H, t, J
1
a bright yellow precipitate (30 mg, 46%). H NMR (700 MHz,
CDCl3): dH ¼ 9.55 (2H, d, 3J ¼ 6.0, 3J(195Pt) ¼ 42, H6), 7.88 (2H,
0
¼ 9.0, H2 ), 8.04 (2H, s, H3), 7.51 (2H, dd, J ¼ 5.0 and 0.5, H5),
0
0
7.12 (1H, t, J ¼ 11, H5 ). 19F NMR (188 MHz, CDCl3): dF
¼
s, H3), 7.60 (2H, d, 3J ¼ 7.0, H3 ), 7.52 (2H, d, 3J ¼ 6.0, H5), 7.34
0
ꢁ111.1 (2F, t, J ¼ 11), -65.2 (6F, s). HRMS (ES+): m/z 405.06356
(1H, t, 3J ¼ 8.0, H4 ). 19F NMR (658 MHz, CDCl3): dF ¼ ꢁ65.0.
[15%, M + H+]; calcd for [C18H9F8N2], 405.06325.
MS (ES+): m/z ¼ 599 [M + H]+. Anal. Calcd for C18H9N2F6ClPt:
C, 36.16; H, 1.52; N, 4.69%. Found: C, 35.78; H, 1.45; N, 3.98%.
2-(Trimethyltin)-5-(trifluoromethyl)pyridine
PtL28Cl
The preparation of this compound was based upon a previously
reported synthesis,23 but modified using a higher temperature
and shorter reaction time leading to an improvement in selec-
tivity and yield.
A mixture of ligand HL28 (40 mg, 0.10 mmol) and K2PtCl4 (41
mg, 0.10 mmol) in acetic acid (2 mL) was degassed by three
freeze-pump-thaw cycles and heated at reflux under a nitrogen
atmosphere for 3 days. Upon cooling to room temperature, the
precipitate that formed was separated and treated as described
for PtL28Cl, giving the product as a bright yellow solid (32 mg,
51%). 1H NMR (700 MHz, CDCl3): dH ¼ 9.54 (2H, d, J ¼ 6.0, 3J
Bis(triphenylphospine)palladium dichloride (25 mg, 0.035
mmol) and hexamethylditin (580 mg, 1.77 mmol) were added to
a solution of 2-bromo-5-(trifluoromethyl)-pyridine (200 mg, 0.89
mmol) in anhydrous dioxane (10 mL). The mixture was degassed
via four freeze-pump-thaw cycles, and then heated at reflux under
(
195Pt) ¼ 41, H6), 8.09 (2H, s, H3), 7.55 (2H, dd, J ¼ 5.5 and 1.5,
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0
H5), 6.84 (1H, t, 3J(19F) ¼ 11, H4 ). 19F NMR (658 MHz, CDCl3):
at 374 nm with an EPL-375 pulsed-diode laser. The emitted light
was detected at 90ꢂ using a Peltier-cooled R928 PMT after
passage through a monochromator. The estimated uncertainty in
the quoted lifetimes is ꢄ 10% or better. Bimolecular rate
constants for quenching by molecular oxygen, kQ, were deter-
mined from the lifetimes in degassed and air-equilibrated solu-
tion, taking the concentration of oxygen in CH2Cl2 at 0.21 atm
dF ¼ ꢁ105.2 (2F, d, J ¼ 11, J(195Pt) ¼ 49), ꢁ65.6 (6F, s, CF3).
4
MS (MALDI): m/z 633.0 [M], 1231.0 [2M-Cl]. HRMS (ASAP):
m/z ¼ 631.9783 [M]; calcd for [C18H7ClF8N2194Pt] m/z ¼
631.9797. Anal. Calcd for C18H7F8N2ClPt: C, 34.11; H, 1.11; N,
4.42%. Found: C, 33.78; H, 1.25; N, 4.13%.
O2 to be 2.2 mmol dmꢁ3 25
.
PtL29Cl
A mixture of HL29 (30 mg, 0.074 mmol) and K2PtCl4 (32 mg,
0.074 mmol) in acetic acid (23 mL) was degassed by four freeze-
pump-thaw cycles and then heated at reflux temperature under
nitrogen for 3 days. Upon cooling to room temperature, the
yellow precipitate that had formed was separated, washed with
MeOH and diethyl ether (4 ꢃ 5 mL of each), and then dried
under reduced pressure, giving the new complex [PtL29Cl] (29 mg,
50%). 1H NMR (300 MHz, d8-THF): dH ¼ 9.67 (2H, s, 3J(195Pt) ¼
Details of OLED fabrication and assessment
OLEDs were fabricated by growing a sequence of thin layers on
clean glass substrates pre-coated with a 120 nm-thick layer of
indium tin oxide (ITO) with a sheet resistance of 20 U per square.
A 60 nm thick hole-transporting layer of a blend of N,N0-
diphenyl-N,N0-bis(3-methyl)-1,10-biphenyl-4,40-diamine (TPD)
and bisphenolol-A-polycarbonate (MW 32000–36000) (PC)
(Aldrich and Polysciences Inc., respectively) was spun on top of
the ITO from a 10 mg mLꢁ1 dichloromethane solution at room
temperature. All remaining organic layers were deposited in
succession by thermal evaporation under vacuum of ꢀ 10ꢁ6 hPa,
followed by high vacuum (ꢀ 10ꢁ6 hPa) thermal-evaporation of
the cathode layer consisting of 0.5 nm-thick LiF and by a 100
nm-thick Al cap. The emitting layer (EML) was evaporated by
co-deposition of the Pt complex and 4,40,400-tris(N-carbazolyl)
triphenylamine (TCTA) to form a 30 nm-thick blend film (5 wt%
Pt complex: 95 wt% TCTA) or by single deposition of Pt complex
to form a 30 nm neat film. We have used the series of emitting
phosphors PtLnCl where n ¼ 27, 28, 29 (Chart 1), and fabricated
four-layer OLEDs in this way.
48, H6), 8.53 (2H, d, J ¼ 9.0, H4), 8.15 (2H, d, J ¼ 9.0, 3J
0
(
195Pt) ¼6.0, H3), 7.06 (1H, t, 3J(19F) ¼ 12, H5 ). 19F NMR
(282 MHz, d8-THF): dF ¼ ꢁ106.3 (2F, d, J ¼ 12, 4J(195Pt) ¼ 48),
ꢁ63.8 (6F, s, CF3). MS (FAB+): m/z ¼ 598 [M–Cl]+. Anal. Calcd
for C18H7F8N2ClPt: C, 34.11; H, 1.11; N, 4.42%. Found: C,
34.23; H, 1.00; N, 4.44%.
Electrochemical measurements
Cyclic voltammetry was carried out using a mAutolab Type III
potentiostat with computer control and data storage via GPES
110 Manager software. Solutions of concentration 1 mM in
CH2Cl2 were used, containing [Bu4N][PF6] as the supporting
inert electrolyte. A three-electrode assembly was employed,
consisting of a platinum working electrode, platinum wire
counter electrode and platinum flag reference electrode. Solu-
tions were purged for 5 min with solvent-saturated nitrogen gas
with stirring, prior to measurements being taken without stirring.
The voltammograms were referenced to a ferrocene-ferrocenium
couple as the standard (E1/2 ¼ 0.42 vs. SCE).
The current–voltage characteristics were measured with
a Keithley Source-Measure unit, model 236, under continuous
operation mode, while the light output power was measured with
an EG&G power meter, and electroluminescence (EL) spectra
recorded with a StellarNet spectroradiometer. All measurements
were carried out at room temperature under argon atmosphere
and were reproduced for many runs, excluding any irreversible
chemical and morphological changes in the devices. The
performance of the emissive layer has been optimized by locating
the EML between exciton blocking layers of TCTA and
3-phenyl-4-(10-naphthyl)-5-phenyl-1,2,4-triazole (TAZ), the
latter acting also as an electron-transporting and hole-blocking
layer.
Photophysical measurements
Absorption spectra were measured on a Biotek Instruments XS
spectrometer, using quartz cuvettes of 1 cm pathlength. Steady-
state luminescence spectra were measured using a Jobin Yvon
FluoroMax-2 spectrofluorimeter, fitted with a red-sensitive
Hamamatsu R928 photomultiplier tube; 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 contained within
quartz cuvettes of 1 cm pathlength 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; final vapour pressure
at 77 K was < 5 ꢃ 10ꢁ2 mbar, as monitored using a PRE10K
Pirani gauge. Luminescence quantum yields were determined
relative to two independent standards: [PtL3Cl] (HL3 is 4-methyl-
1,3-di(2-pyridyl)benzene) in CH2Cl2 as a secondary standard
emitting in the same region (Flum ¼ 0.68 at 298 K),14a and Ru
(bpy)3Cl2 in degassed aqueous solution (Flum ¼ 0.042 at 298
K).24 The luminescence lifetimes of the complexes were measured
by time-correlated single-photon counting, following excitation
Results and discussion
(i) Synthesis of ligands and complexes
Three new proligands based on the 1,3-di(2-pyridyl)benzene
skeleton have been prepared (Scheme 1), incorporating tri-
fluoromethyl groups in the pyridyl rings, either at the 4-position
(HL27 and HL28) or at the 5-position (HL29). Ligands HL28 and
HL29 also contain fluorine atoms at the 4 and 6 positions of the
central aryl ring.
1,3-Bis(5-(trifluoromethyl)pyridin-2-yl)-4,6-difluoro-benzene
HL29 was prepared by a procedure similar to that used previously
in the synthesis of HL1, involving a palladium-catalysed Stille
cross-coupling between 1,3-dibromo-4,6-difluorobenzene and 2-
(trimethyltin)-5-(trifluoromethyl)pyridine (Scheme 1). Stannyl
pyridines are typically prepared by reaction of the corresponding
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lithiate with tributyltin chloride. However, we noted that the
reaction of 2-bromo-5-(trifluoromethyl)pyridine with hexame-
thylditin catalysed by palladium(0) provides more convenient
direct access to the requisite stannane without recourse to
lithiation.
For the preparation of HL27, we explored an alternative
procedure, not hitherto reported for the preparation of dipyr-
idylbenzene proligands,26 involving a Suzuki cross-coupling of
1,3-benzenediboronic acid with 2-chloro-4-(trifluoromethyl)
pyridine. The high reactivity of ortho-halogenated pyridines
allows the chloro derivative to be employed, even with the simple
catalyst Pd(PPh3)4. The cross-coupling proceeds readily without
the need for the more sophisticated catalysts that are typically
required when chlorinated benzenes are used in cross-coupling
reactions. The 4,6-difluorinated analogue, HL28, was prepared in
the same way from the pinacol ester of 1,3-difluoro-4,6-diboronic
acid.
Subsequent reaction of the proligands HL27, HL28 and HL29
with K2PtCl4 in acetic acid at reflux gave the corresponding
complexes PtL27Cl, PtL28Cl and Pt29Cl (structures shown in
Chart 1), which were readily isolated and purified by a simple
washing and extraction procedure.
(ii) Ground-state absorption spectra
The UV-visible absorption spectra of the three new complexes in
dichloromethane solution at room temperature are shown in
Fig. 1, together with those of the parent complex PtL1Cl and the
difluorinated complex PtL22Cl (Chart 1) for comparison. Data
are summarised in Table 1. We first consider the low-energy part
of the spectrum, the region between 450 and 550 nm (Fig. 1b).
Each complex displays a weak but sharp band in this region,
3 ꢀ 200 Mꢁ1cmꢁ1, which is attributed to the formally forbidden
S0 / T1 transition, facilitated by the spin–orbit coupling (SOC)
associated with the Pt ion. The position of the band varies
significantly according to the identity of the complex.
Fig. 1 UV-visible absorption spectra of the Pt(II) complexes investigated
in CH2Cl2 at 298 ꢄ 3 K. (a) Top: full range. (b) Bottom: expansion of the
low-energy region, showing the S0 / T1 bands.
The fluorine atoms in the 3 and 5 positions of the central aryl
ring lead to an increase in the energy of the triplet state: the S0 /
T1 band of PtL22Cl is blue-shifted by 19 nm (840 cmꢁ1) compared
to PtL1Cl. This effect is reminiscent of that observed in phenyl-
pyridine complexes of iridium(III), where fluorine atoms in the 3,5
position of the phenyl group are well-known to increase the
triplet energy, both for charge-neutral complexes such as Ir
(F2ppy)3 and related compounds,27 and cationic complexes such
as [Ir(F2ppy)2(bpy)]+.28
Comparing PtL27Cl with the parent PtL1Cl reveals that the
introduction of CF3 groups into the 4-position of the pyridyl
rings is accompanied by a red shift of 18 nm, corresponding to
a stabilisation of the triplet state of around 700 cmꢁ1. Similarly, if
we compare PtL28Cl with PtL22Cl, the introduction of CF3
groups into the 4-position of the pyridyl rings of the difluorinated
system again leads to a red-shift of 18 nm. On the other hand, the
effect of CF3 groups in the pyridyl 5-position is seen to be a little
smaller: PtL29Cl is red-shifted by 12 nm compared to PtL22Cl.
These effects can be interpreted with the aid of results from
time-dependent density functional theory calculations previously
carried out on the parent compound PtL1Cl.16,17 They reveal that
the T1 state has primarily HOMO / LUMO character. The
HOMO is based largely on the metallated aryl ring, the metal and
the chloride, with relatively minor contribution from the pyridyl
rings. Of particular relevance is the fact that the pyridyl 4-posi-
tion is a node in the HOMO, whereas the 5-position does
contribute to the HOMO. On the other hand, the LUMO is
heavily based on the pyridyl rings, with little contribution from
the central rings, whose 3,5 positions are nodes.
The blue shift upon introduction of the F atoms into the
central ring is thus due primarily to stabilisation of the HOMO,
increasing the HOMO–LUMO gap. Meanwhile, the red-shifting
effect of the CF3 groups in the pyridyl 4-position is clearly due to
stabilisation of the LUMO with little or no effect on the HOMO,
such that the HOMO–LUMO gap is decreased (e.g., for
PtL27Cl). Apparently, these two stabilising effects—of F and CF3
on HOMO and LUMO respectively—must be of a similar
magnitude, leading to the triplet absorption lmax of PtL28Cl
being essentially identical to that of PtL1Cl.
On the other hand, CF3 groups in the 5-pyridyl position
(PtL29Cl) should stabilise the HOMO as well as the LUMO,
owing to the contribution of this position to both frontier
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Table 1 Photophysical and electrochemical data for the new complexes PtL27Cl, PtL28Cl and PtL29Cl and for PtL1Cl and PtL22Cl for comparisona
Emission 77 K
Absorbanceb lmax/nm
Complex (3/Mꢁ1cmꢁ1
Emissionc lmax/nm
monomer [excimer] Flum
kSQ
s0/msd e 109 Mꢁ1sꢁ1 108 Mꢁ1sꢁ1
/
kQ
/
e
f
O2
d
s/ms Epox/Vg Epred/Vg
,
)
lmax/nm
PtL1Cl
257 (27500), 290 (22400), 491, 524, 562 [691] 0.60 (0.04) 7.2 (0.5) 5.3
9.1
6.2
486, 516, 548
6.4 0.35
ꢁ2.14
332 (6510), 380 (8690),
401 (7010), 454 (270),
486 (240)
PtL22Cl 260 (27400), 286 (18800), 472, 503, 535 [677] 0.85 (0.09) 7.9 (0.7) 6.0
467, 478, 496, 526 5.8 0.31
ꢁ2.28
320 (6690), 334 (9180),
360 (7340), 374 (8500),
437 (143), 467 (139)
PtL27Cl 268 (34300), 284 (27900), 518, 548 [756]
348 (4690), 400 (11400),
0.37 (0.09) 5.0 (1.0) 3.2
0.59 (0.21) 4.8 (1.7) 2.3
3.6
1.7
511, 549, 591
487, 519, 562
6.0 0.38
4.9 0.55
ꢁ2.05
ꢁ2.15
503 (200)
PtL28Cl 267 (21700), 298sh
(14300), 334 (4470), 349
(6510), 383 (8190), 453
(221), 485 (171)
PtL29Cl 268 (30900), 293 (22600), 484, 515 [717]
330 (7070), 342 (10400),
388 (11100), 479 (521)
496, 528 [734]
0.53 (0.16) 3.8 (1.2) 3.6
2.6
478, 510, 542
4.4 0.67h
ꢁ1.90h
a
In CH2Cl2 except where stated otherwise. b Extinction coefficients measured from the plots of absorbance versus concentration over the range 1 ꢃ 10ꢁ5
c
d
to 1.5 ꢃ 10ꢁ4 M. Recorded in degassed solution at an optical density of 0.1 at 400 nm. In degassed solution; values in parenthesis are for air-
e
equilibrated solution. s0 is the lifetime at infinite dilution and kSQ is the self-quenching rate constant, determined from the intercept and slope,
respectively, of a plot of the measured decay rate constant against concentration [see supporting information]. The lifetime in a dilute (< 10ꢁ5 M)
f
aerated solution is given in parentheses. kQ is the bimolecular rate constant for quenching by molecular oxygen, determined from the lifetimes in
O2
g
degassed and aerated solution, taking [O2] ¼ 2.2 mM in CH2Cl2 at 298 K and 1 atm pressure air. Ep and Ep are the forward peak potentials
ox
red
h
for the electrochemically irreversible oxidation and partially reversible reduction respectively. In 5% DMSO/95% CH2Cl2 to achieve adequate
solubility.
orbitals, so that the red-shifting effect is attenuated compared to
its 4-CF3 isomer (PtL28Cl), accounting for the small displacement
of lmax towards the blue for the former complex compared to the
latter.
too optimistic to seek clear trends, especially given the rather
small differences expected. Nevertheless, there is at least some
evidence from the CV data in DMSO (Table 1) that the F atoms
in the aryl ring tend to make the oxidation potential more
positive (e.g., PtL28Cl and PtL29Cl) and that the CF3 groups in
the pyridyl rings can shift the reduction potential to rather less
negative values (e.g. PtL27Cl and PtL29Cl), corresponding to the
proposed stabilisation of HOMO and LUMO respectively by
these substituents.
These trends are partly mirrored in the more intense, spin-
allowed singlet bands in the 350–450 nm region (Fig. 1a). Thus
PtL27Cl displays the most red-shifted bands in this region, and
PtL22Cl the most blue-shifted, while PtL28Cl again has essentially
the same lmax as PtL1Cl. At first sight, PtL29Cl seems to follow
a different trend from that exhibited by the triplet bands, in that
it is now marginally red-shifted compared to PtL28Cl. However,
some caution has to be exercised in drawing such conclusions for
these bands, because they clearly comprise more than one tran-
sition (S0 / S1, S2 etc.), as supported by the TD-DFT calcula-
tions,16 and the S0 / S1 is not adequately resolved for the
CF3-containing complexes to establish the definitive trend.
The very intense bands at lmax < 300 nm are attributed to
p–p* transitions within the ligands, and are very similar for all
five complexes.
(iv) Photoluminescence
All three new complexes are intensely luminescent in dilute
solution at room temperature, displaying vibrationally struc-
tured emission spectra with maxima in the green region around
500 nm (Fig. 2 and Table 1). The profile of the spectrum is in each
case similar to that of the parent complex PtL1Cl. The trend in
emission energy PtL22Cl > PtL29Cl > PtL1Cl ꢀ PtL28Cl > PtL27Cl
is essentially the same as that observed in the energies of the S0
/ T1 absorption band. The Stokes shift between the latter and
the 0,0 emission band is small in each case, though increasing
slightly from PtL22Cl (230 cmꢁ1) to PtL27Cl (580 cmꢁ1). Clearly,
the emission emanates from the T1 state, promoted by the SOC
associated with the Pt(II) ion. The luminescence lifetimes are on
the microsecond timescale and the photoluminescence quantum
yields Flum under degassed conditions are around 0.5 (values are
given in Table 1).
(iii) Electrochemistry
Whilst electrochemical data is frequently useful in supporting
interpretations of the type described above, cyclic voltammetry
for this class of complexes proves to be quite troublesome, owing
to their inadequate solubility in the preferred solvents for elec-
trochemistry (e.g., CH2Cl2 and CH3CN); to the chemical and
electrochemical irreversibility of the oxidation process; and to
the lack of clear-cut reversibility in reduction.
An interesting feature of the trio of CF3-containing complexes
is that they are relatively insensitive to diffusional quenching by
dissolved molecular oxygen (see data in Table 1). Thus, the
lifetimes and quantum yields are reduced by a factor of only 3 to
The reproducibility of measurements is rather poor, and the
uncertainty on the potentials rather high, so that it is probably
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excimers,19b,31 we can tentatively infer a donor–acceptor-like
interaction, in which the pyridyl rings act as the acceptors. The
introduction of electron-withdrawing substituents in the 4-posi-
tion of the pyridyl rings then serves to stabilise the excimers,
leading to a red shift, whilst electron-donating groups have the
opposite effect.
The excimer: monomer intensity ratio is smaller for all three
CF3 complexes than for PtL1Cl at the same concentration. This
probably reflects a combination of the effects of shorter mono-
mer lifetimes, smaller self-quenching rate constants, and lower
efficiency of excimer emission once formed. The latter effect may
in turn be associated with the expected decreased radiative decay
rate and increased non-radiative decay as the energy drops.
In the solid state, in thin films prepared by sublimation, the
emission spectra display uniquely excimer-like emission in the
red region. The spectra are essentially identical to the electrolu-
minescence of such pure films, as shown in Fig. 4 and described in
the next section. On the other hand, solid-state samples prepared
by evaporation from dichloromethane solution display differing
proportions of the monomer and excimer-like bands. Repre-
sentative spectra are shown in Section 6 of the Supporting
Information. PtL29Cl displays uniquely the red emission band
typical of the excimer. In contrast, for PtL27Cl and PtL28Cl, the
excimer-like band is accompanied by significant emission at
higher energy, resembling the monomeric emission in solution.
Moreover, the order of energies of the excimer-like band is now
different: PtL29Cl > PtL27Cl > PtL28Cl. Clearly, the observed
spectra reflect subtly different arrangements of molecules in the
sample, and possible heterogeneity in the molecular environ-
ment, leading to differing intermolecular interactions. In a recent
study, we found that a related brominated complex investigated
by X-ray diffraction could form three different polymorphs, two
crystallising from the same solution in dichloromethane, yet
differing substantially in the intermolecular packing in the
solid.32
Fig. 2 Photoluminescence spectra of the five Pt(II) complexes in CH2Cl2
(concentration ¼ 1.5 ꢃ 10ꢁ4 M) at 298 K. The weak excimer bands of
PtL27-29Cl have been expanded for clarity.
4 upon aeration, compared to factors of 10 to 30 times encoun-
tered for other members of the Pt(N^C^N)Cl class of
complexes.14,15 This seems to be due not only to the somewhat
shorter lifetimes of the new complexes (around half that of
PtL1Cl, for which s ¼ 7.2 ms), but also to their smaller bimo-
O2
lecular oxygen quenching rate constants kQ
(Table 1),
presumably associated in some way with the more electron-poor
nature of the CF3-substituted pyridyl rings.
As the concentration is increased, the lifetimes decrease - a self-
quenching effect commonly encountered for luminescent square
planar Pt(II) complexes.10 The observed emission decay constants
(¼ 1/sobs) increase linearly with the concentration of the complex,
as found previously for PtL1Cl and other derivatives. The
bimolecular self-quenching rate constant, as derived from the
gradient of the plot of 1/sobs versus concentration, is of the order
of 3 ꢃ 109 Mꢁ1sꢁ1 for the new complexes, viz. around half that of
PtL1Cl and PtL22Cl. The self-quenching of the monomeric
emission is accompanied by the appearance of a structureless
band at low-energy (lmax > 700 nm), which is attributable to an
excimer. The expansion of this zone in Fig. 2 reveals that the
excimers of the CF3-substituted complexes are all red-shifted
relative to that of PtL1Cl. This contrasts with the monomer
emission data, where PtL29Cl emitted at higher energy than
PtL1Cl, and suggests that the substituents in the pyridyl rings are
more important in determining the excimer energy. As for the
monomer, the most red-shifted system is PtL27Cl. Notably, the
excimer band of this complex falls almost entirely in the NIR
region. The molecular electronic properties that determine exci-
mer energies are less well understood than those which control
the excited state energies of the isolated molecules. For platinum
(II) complexes with aromatic ligands, excimers can in principle
form through ligand-ligand, metal-metal, or metal–ligand inter-
actions, and which of these is operative for a given system is
difficult to assess.29 Moreover, the state-of-the-art in TD-DFT
methods applied to excimer formation remains limited by issues
such as the correct treatment of dispersive interactions.30 From
the results of this study and our related work on blue-shifted
(v) Electroluminescence: NIR-OLEDs
The extension of efficient OLEDs into the technologically
important near-infrared (NIR) spectral range is a challenge for
the photonics community. The approach of using increasingly
extended conjugation in order to shift emission to the red is not
necessarily viable for metal-complex triplet emitters, since the
excited state will tend to become localised on the ligand, with
little metal character to promote ISC to the triplet state.33 The
low-energy emission from aggregates or excimers of square
planar platinum complexes is thus a potentially attractive alter-
native. Recently, the use of neat films of dipyridylbenzene Pt(II)
complexes, such as PtL1Cl, has enabled the fabrication of highly
efficient red/NIR OLEDs through exclusive excimeric emission
[efficiency ca. 10% ph/e at 10 mA cmꢁ2, peaking at ca 700 nm].20
The high electroluminescence (EL) quantum efficiency (QE) of
excimer-based NIR OLEDs was ascribed to the high photo-
luminescence (PL) efficiency of excimeric emission of neat PtLnCl
films and to the optimization of the OLED configuration.20,34
Given the longer-wavelength excimer emission of PtL27-29Cl in
solution, unequivocally in the NIR in the case of PtL27Cl, these
complexes have been examined as phosphors for NIR-OLEDs.
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to be confined to a 30 nm-thick EML layer. HOMO and LUMO
energy levels of the Pt complexes were estimated from redox
potentials (Table 1). HOMO values of 5.50, 5.65, 5.75 eV and
LUMO values of 3.05, 2.95, 3.20 eV are found for PtL27Cl,
PtL28Cl and PtL29Cl, respectively.
Fig. 4 shows the electroluminescence (EL) spectra of OLEDs
having (i) 5 wt% PtLnCl in TCTA as EML, and (ii) neat PtLnCl
films as EML. While the EL spectra of the former are mainly
characterized by monomolecular electronic transitions, the
spectra of the neat-film devices exhibit uniquely excimer-like
emission. Note that in both cases, however, we are dealing with
phosphorescent triplet states.
The emission colour of all the 5 wt% OLEDs is green-yellow-
biased while the neat-film emitter OLEDs fall in the deep red/
NIR region.
Table 2 Performance of OLEDs with 5 wt% and 100 wt% of Pt complex
in the emitting layer, at ca. 500 cd mꢁ2 and 500 mW cmꢁ2 respectively
Bias j mA QE
PE
LE
CIE
Pt complex
(V)
10.8
8.6
cmꢁ2
ph/e lm/W cd Aꢁ1 (x,y)
Fig. 3 OLED configuration and molecular structures of the materials
used. These are complemented by an energy level diagram shown for
understanding the exciton and electronic traffic within the devices. Full
compound names are given in the Experimental section.
PtL27 Cl 5%
2.1
7.7
3.4
7.0
0.3
4.9
0.4
4.9
1.2
6.6
_
4.9
_
3.0
_
22.5
_
13.3
_
11.6
_
0.35,0.62
0.65,0.33
0.30,0.59
0.65,0.35
0.31,0.54
0.67,0.32
100% 16
PtL28 Cl 5%
100% 15.6 10.7
5% 11 4.0
100% 10.8 24.0
PtL29Cl
The architecture of the OLEDs manufactured is described in the
experimental section and shown schematically in Fig. 3.
The devices consist of an ITO-coated glass anode transparent
to the light generated in the emitting layer (EML). Holes are
injected from the ITO anode, and pass through TPD:PC and
TCTA hole-transporting layers. They recombine in the EML
with electrons injected from an Al/LiF cathode and transported
through the electron-transporting layer of TAZ. 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 complexes (ET < 2.6 eV) to the non-radiative triplet of TPD
(ET ¼ 2.45 eV). The 30 nm-thick layer of TAZ (ET ¼ 2.75 eV)
plays a similar role of inhibiting quenching of the Pt complex
triplet excitons at the cathode while at the same time blocking
holes, both of which properties enable the recombination process
Fig. 4 Electroluminescence spectra from the 5 wt% PtLnCl complex-
doped TCTA emitter layers of blend devices (left) and PtLnCl neat film
emitter devices (right).
Fig. 5 Luminance and current density (j) vs. applied voltage (a) and
external EL QE vs. driving current (b) for the OLEDs based on the EMLs
made of TCTA doped with 5 wt% of the three platinum complexes.
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No significant differences between the PL and EL spectra for
both types of EML (blend or neat film) are found. The colour of
emission from the blends does not change significantly in the
brightness range studied of 100–5000 cd mꢁ2. The CIE coordi-
nates of each of the OLEDs studied are given explicitly in the
seventh column of Table 2. The OLED performance parameters
are also summarized in Table 2, at a brightness of ca. 500 cd mꢁ2
for the 5 wt% devices, and at an EL intensity ca. 500 mW cmꢁ2 for
the NIR OLEDs. The luminance and EL efficiency as a function
of the applied voltage and current density, respectively, are
shown for the 5% Pt complex in TCTA in Fig. 5 and for the pure
films in Fig. 6. The current dependence of EL efficiency observed
is due to the interplay between the current-density-dependent
electron-hole recombination rate and EL quenching processes,
such as exciton interactions and electric field-assisted dissocia-
tion of excitons occurring simultaneously within the EMLs.35
The QE and NIR emission emanating from the OLEDs is seen
to depend on the substituents and their positions in the central
and lateral rings of the terdentate N^C^N ligand.
The excellent NIR emission of the PtL27Cl-based OLEDs is
associated with the CF3 substituents in the pyridyl rings leading
to a red shift in both molecular exciton and excimer emission, as
shown in Fig. 4 in comparison with the unsubstituted complex,
PtL1Cl, and also mirroring the behaviour in solution. Never-
theless, the PL and EL QEs of PtL27Cl in neat film are lower, ca.
2% and < 0.35% (EL QEmax), respectively. An enhancement of
EL QE (EL QEmax ¼ 0.55% at ca. 0.1 mA cmꢁ2) is obtained with
PtL28Cl, by introducing F substituents into the central ring,
whilst the OLED with neat PtL29Cl film as EML gives an EL
QEmax of 1.7% (at ca. 0.1 mA cmꢁ2). These improvements come
at the expense of the emission colour, however, falling partly in
the visible rather than the NIR. The trend may be associated in
part to the expected increase in non-radiative decay rates as the
excited state energy decreases, according to the so-called energy-
gap law that typically applies quite well in series of metal
complexes having excited states of similar orbital parentage.36
Nevertheless, there is also the possibility of subtle differences in
the packing arrangement of these molecules in the solid state
films, which may influence both charge-capture and emission
efficiencies. As noted above, crystal structures of closely related
complexes have revealed the existence of, in some cases, as many
as three polymorphs with differing intermolecular separations.32
Conclusions
In summary, we have demonstrated a simple way of spectral
tuning from green to yellow and from red to NIR using Pt
(N^C^N)Cl complexes as monomolecular and excimer-like
emitters respectively. The present study reveals how the mono-
mer and excimer excited-state energies of such complexes can be
displaced towards the red through the introduction of electron-
withdrawing trifluoromethyl groups into the pyridyl rings. The
effect is greatest for the 4-position, a result which can be
understood, for the monomeric excited state, in terms of the
significant contribution of this position to the LUMO but not to
the HOMO, such that the LUMO is stabilised and the HOMO–
LUMO gap decreased. The bimolecular excimeric state is also
stabilised by the CF3 groups in the 4-position, compared to the
parent complex, leading to emission in the NIR region. By using
neat films of the complexes as emitting layers, NIR-OLEDs have
been produced, whose emission falls almost completely in the
NIR region. Nevertheless, the further shift to the red is
compromised by decreasing device efficiencies, and further effort
will be required to optimize EL QE in the NIR.
Acknowledgements
This work was supported by MIUR (FIRB 2004: RBPR05JH2P,
FIRB ‘‘LUCI’’ - ‘‘NODIS’’ and PRIN 2008: 2008FZK5AC_002),
by the CNR (INSTM-PROMO 2006: Sistemi molecolari e
ꢀ
nanodimensionali con proprieta funzionali: Nanostrutture
organiche, organometalliche, polimeriche ed ibride; ingegner-
ꢀ
izzazione supramolecolare delle proprieta fotoniche e dis-
positivistica innovativa per optoelettronica), and by the
University of Durham (doctoral fellowship to LM). E.U. COST
Action D35 is also acknowledged supporting our trilateral
activities.
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15510 | J. Mater. Chem., 2011, 21, 15501–15510
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