[(C^N^N)Pt(C≡C)nR] (HC^N^N = 6-aryl-2,2′-bipyridine, n = 1-4, R = aryl, SiMe3) as a new class of light-emitting materials and their applications in electrophosphorescent devices

Article abstract of DOI:10.1039/b108793b

Tridentate cyclometalated platinum(II) complexes bearing σ-alkynyl ligands exhibit tunable photoluminescence and enhanced stability during vacuum deposition; OLEDs based on these materials display orange to red electrophosphorescence with low turn-on volt

Full text of DOI:10.1039/b108793b

[(C^N^N)Pt(C·C)nR] (HC^N^N = 6-aryl-2,2A-bipyridine, n = 1–4,
R = aryl, SiMe3) as a new class of light-emitting materials and their
applications in electrophosphorescent devices†
Wei Lu,a Bao-Xiu Mi,b Michael C. W. Chan,a Zheng Hui,a Nianyong Zhu,a Shuit-Tong Leeb and
Chi-Ming Che*a
a Department of Chemistry and the HKU-CAS Joint Laboratory on New Materials, The University of Hong
Kong, Pokfulam Road, Hong Kong SAR, China. E-mail: cmche@hku.hk
b Centre of Super-Diamond and Advanced Films and Department of Physics and Materials Science, City
University of Hong Kong, Kowloon, Hong Kong SAR, China
Received (in Cambridge, UK) 28th September 2001, Accepted 23rd November 2001
First published as an Advance Article on the web 7th January 2002
Tridentate cyclometalated platinum(II) complexes bearing
s-alkynyl ligands exhibit tunable photoluminescence and
enhanced stability during vacuum deposition; OLEDs based
on these materials display orange to red electrophosphores-
cence with low turn-on voltages ( ~ 4 V), maximum lumi-
nance approaching 10000 cd m22 and efficiency up to 4.2 cd
A21
.
Light-emitting metal–organic compounds have numerous po-
tential applications in chemosensing and optoelectronic de-
vices.1 In this regard, we have focused our attention upon
organometallic compounds with medium-sensitive phosphores-
cent properties. Recent studies have highlighted the employ-
ment of luminescent cyclometalated platinum(II) complexes in
OLEDs2 and as luminescent probes for biomolecules.3 In
particular, the diverse photoluminescent properties of
Fig. 1 ORTEP plots of 2 (right) and 7 (left) (30% probability ellipsoids).
orbitals across the Pt–tolylacetylide fragment can engage in
favorable overlap in the crystal lattice. The alternating length of
the butadiynyl moiety in 7 is consistent with poor Pt ? alkynyl
back-bonding. Dimeric p–p stacking interactions of around 3.5
Å are observed in the crystal lattices of 2 and 7. We contend that
[Pt(C^N^N)L]+ (L
Fischer carbene ligands) have been reported.4 We envisaged
that incorporation of acetylide into cyclometalated Pt(II
= phosphine, pyridine, isocyanide or
)
complexes would destabilize non-radiative d–d transitions and
maintain strict stereochemical integrity, while the Pt–C·CR
interaction may facilitate tuning of the 3MLCT energies by
variation of the R substituent.5 We now report the new acetylide
derivatives 1–13 (Chart 1). These neutral organometallic
[(C^N^N)Pt(C·C)nR] complexes are sufficiently stable with
respect to sublimation and thus are suitable for vacuum
deposition in OLED fabrication.
the conjugation based on a s–p interaction between the Pt(II
)
and arylacetylide moieties confers stability and rigidity to this
system.
The electronic absorption spectra of 1–13 in CH2Cl2 solution
at 298 K show an intense and broad band at lmax 430–470 nm
(e ~ 5 3 103 dm3 mol21 cm21), which tails beyond 500 nm.
Excitation of complexes 1–13 in fluid solution at room
temperature results in orange to red luminescence (Fig. 2). The
emission energies are red-shifted from related [Pt(C^N^N)L]+
derivatives.4 This is in accordance with the strong s-donating
strength of the alkynyl ligand, which destabilizes the dp(Pt)
HOMO to yield relatively low-energy MLCT 5d(Pt) ?
p*(C^N^N) transitions. The emission maxima in dichloro-
methane at 298 K can be modified from 630 nm for 3 to 560 nm
for 6 (range 1980 cm21). This illustrates the tunability of the
emission energies, depending on the nature of (a) the 4-aryl-
acetylide substituent (i.e. electron-withdrawing groups stabilize
the Pt-based HOMO to yield blue-shifted emissions), and (b)
Complexes 1–13 were prepared by treatment of the corre-
sponding Cl-ligated precursor with R(C·C)nH or R(C·C)nCu in
a dichloromethane/amine (10/1, v/v) solution in the presence of
CuI. As established by thermogravimetric analysis, 1, 2, 12 and
13 are thermally stable up to 400 °C and decompose to give
metallic platinum only at temperatures above 420 °C. The
molecular structures of 2 and 7 were confirmed by X-ray
crystallography (Fig. 1).‡The tolylacetylide phenyl ring and the
Pt(C^N^N) unit in 2 are virtually coplanar; this implies that p
Chart 1
† Electronic supplementary information (ESI) available: General experi-
mental procedure, analytical and spectral characterizations, OLED fabrica-
tion and performance. See http://www.rsc.org/suppdata/cc/b1/b108793b/
Fig. 2 Normalized photoluminescence spectra for selected complexes in
CH2Cl2 solution at 298 K, illustrating tunability of emission energies.
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CHEM. COMMUN., 2002, 206–207
This journal is © The Royal Society of Chemistry 2002

the tridentate ligand (i.e. lower p* levels afford red-shifted
emissions). Extension of the alkynyl conjugation length in
[(C^N^N)Pt(C·C)nR] results in slight deviations of the emis-
sion energy; a tendency to blue-shift is generally recognized
[i.e. for R = Ph, 1 (n = 1, 17180 cm21) < 7 (n = 2, 17510
cm21); for R = SiMe3, 8 (n = 1, 17540 cm21) < 9 (n = 2,
17730 cm21) < 10 (n = 3, 17890 cm21)]. For complex 11 (n
= 4), a highly structured emission at lmax 589 nm with a sharp
vibronic progression of ~ 2100 cm21 is observed. This clearly
demonstrates that the nature of the excited state is different and
involves the acetylenic unit. All derivatives in the present study
undergo self-quenching at room temperature in CH2Cl2. Except
for 3, the luminescent quantum yields of these complexes in
CH2Cl2 compare well with those of classical [Ru(bpy)3]2+ (bpy
= 2,2A-bipyridine) salts.
The intense tunable orange–red phosphorescence of these
Pt(II) s-alkynyl materials, plus their thermal stability and
neutrality, render them good candidates as emitters in high-
efficiency OLEDs. The devices in the present study (top of Fig.
3: inset) were fabricated on indium–tin oxide (ITO) glass using
the vacuum deposition method. NPB (N,NA-di-1-naphthyl-N,NA-
diphenylbenzidine) and Alq3 [tris(8-quinolinonato)aluminium]
were used as the hole- and electron-transporting layers,
respectively. BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenan-
throline, bathocuproine) was used to confine excitons within the
luminescent zone. Magnesium silver alloy was applied as the
cathode. The [(C^N^N)Pt(C·C)nR] materials were doped into
the conductive host material CBP (4,4A-(N,NA-dicarbazole)bi-
phenyl) with mass ratios of 2, 4 or 6%. The performances and
optimal conditions of the devices using 1, 2, 12 and 13 as
emitters are listed in the ESI.† Upon stimulation of positive bias
voltage for devices with emitter ratios of 4 and 6%, intense
orange to red electrophosphorescence is observed while blue
fluorescence from the host and hole-transporting layers is
negligible,6 thus implying that energy transfer from singlet to
triplet excitons is complete.7 Notably low turn-on voltages in
the 3.6–4.5 V range are observed. The emission maxima are
independent of the doping level and applied voltage (for current
density up to 600 mA cm22). As shown in Fig. 3 (top), the EL
red shift from 1 and 2 to 13 is in agreement with that observed
for the solution photoluminescence. A maximum luminance of
7800 cd m22 at 11 V and a maximum efficiency of 2.4 cd A21
at 30 mA cm22 is obtained for an orange OLED (lmax 564 nm)
using 1 at 4% doping level (bottom of Fig. 3). For the red OLED
(lmax 612 (max), 656 nm; CIE coordinates x = 0.594, y =
0.341), a maximum luminance of 3100 cd m22 at 12 V and a
maximum efficiency of 1.0 cd A21 at 30 mA cm22 is observed
using 13 (4%) in CBP. These values are comparable with the
best red-light OLEDs in the literature1,2,7 and demonstrate the
great potential of platinum(II)–alkynyl complexes as electro-
phosphorescent emitters. It is important to note that luminescent
metal–alkynyl complexes have been attracting substantial
interest in recent years, yet practical applications have still to be
realized. We have previously reported that OLEDs based on
related platinum– and copper–arylacetylide complexes have
relatively low luminances and/or are unstable during vacuum
deposition.8
We are grateful for financial support from The University of
Hong Kong, the Research Grants Council of Hong Kong SAR,
China [HKU 7298/99P and CityU 8730009], and the Innovation
and Technology Commission of the Hong Kong SAR Govern-
ment (ITS/053/01). We thank Mr. Wai-Lim Chan for assistance
in device fabrication.
Notes and references
‡ Crystal data for 2: C25H18N2Pt, M = 541.50, hexagonal, R3, a = b =
¯
34.132(3), c = 8.6940(10) Å, a = b = 90, g = 120°, V = 8771.6(15) Å3,
Z = 18, Dc = 1.845 g cm23, m(Mo-Ka) = 7.221 mm21, F(000) = 4680,
T = 300(2) K, 2qmax = 51°, 3260 independent reflections, 253 variable
parameters, R1 = 0.0356 (I > 2s(I)), wR2 = 0.0950, GOF(F2) = 1.08.
Crystal data for 7: C26H16N2Pt, M = 551.50, monoclinic, P21/c, a =
15.268(3), b = 12.626(3), c = 10.929(2) Å, b = 107.33(3)°, V = 2011.2(7)
Å3, Z = 4, Dc = 1.821 g cm23, m(Mo-Ka) = 6.991 mm21, F(000) = 1056,
T = 300(2) K, 2qmax = 51°, 3584 independent reflections, 262 variable
parameters, R1 = 0.0340, wR2 = 0.0825, GOF(F2) = 0.956. CCDC
reference numbers 172097 and 161089. See http://www.rsc.org/suppdata/
cc/b1/b108793b/ for crystallographic data in CIF or other electronic
format.
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Fig. 3 Top: normalized electroluminescence spectra for 1, 2 and 13 at 4%
doping level (inset: multi-layer configuration of OLED). Bottom: current
density, voltage and luminance characteristics (inset: luminescent effi-
ciency vs. current density) for OLED using 1 as emitter at 4% doping
level.
CHEM. COMMUN., 2002, 206–207
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