Synthesis of alkynylgold(III) complexes with bis-cyclometalating ligand derived from ethyl 2,6-diphenylisonicotinate and their structural, electrochemical, photo- and electroluminescence studies

Article abstract of DOI:10.1016/j.jorganchem.2015.02.037

Abstract A series of alkynylgold(III) complexes with bis-cyclometalating ligand derived from ethyl 2,6-diphenylisonicotinate has been synthesized. The single crystal structure of one of the complexes revealed an unprecedentedly short Au(III)...Au(III) distance of 3.44 ?. These complexes were found to exhibit rich electrochemistry. Upon photoexcitation, the alkynylgold(III) complexes were found to show rich emission properties in various states at both ambient and low temperatures. One of the complexes have also been utilized as the light-emitting layer in the fabrication of organic light-emitting diodes, demonstrating the potentials and versatility of alkynylgold(III) complexes.

Full text of DOI:10.1016/j.jorganchem.2015.02.037

Accepted Manuscript
Synthesis of Alkynylgold(III) complexes with Bis-cyclometalating Ligand Derived from
Ethyl 2,6-Diphenylisonicotinate and their Structural, Electrochemical, Photo- and
Electroluminescence Studies
Vonika Ka-Man Au, Daniel Ping-Kuen Tsang, Yi-Chun Wong, Mei-Yee Chan, Vivian
Wing-Wah Yam
PII:
S0022-328X(15)00115-1
10.1016/j.jorganchem.2015.02.037
JOM 18938
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 21 January 2015
Revised Date: 18 February 2015
Accepted Date: 24 February 2015
Please cite this article as: V.K.-M. Au, D.P.-K. Tsang, Y.-C. Wong, M.-Y. Chan, V.W.-W. Yam,
Synthesis of Alkynylgold(III) complexes with Bis-cyclometalating Ligand Derived from Ethyl 2,6-
Diphenylisonicotinate and their Structural, Electrochemical, Photo- and Electroluminescence Studies,
Journal of Organometallic Chemistry (2015), doi: 10.1016/j.jorganchem.2015.02.037.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
Synthesis of Alkynylgold(III) complexes with Bis-cyclometalating Ligand
Derived from Ethyl 2,6-Diphenylisonicotinate and their Structural,
Electrochemical, Photo- and Electroluminescence Studies
Vonika Ka-Man Au, Daniel Ping-Kuen Tsang, Yi-Chun Wong, Mei-Yee Chan,* and
Vivian Wing-Wah Yam*
Institute of Molecular Functional Materials (Areas of Excellence Scheme, University
Grants Committee (Hong Kong)) and Department of Chemistry, The University of
Hong Kong, Pokfulam Road, Hong Kong
E-mail: chanmym@hku.hk, wwyam@hku.hk
Dedicated to Prof. D. M. P. Mingos on the occasion of his 70th Birthday
Graphical Abstract
Synopsis
A series of alkynylgold(III) complexes has been synthesized and their
electrochemical and photophysical properties have been studied. The crystal structure
…
of one of the complexes revealed a short Au(III) Au(III) distance of 3.44 Å. One of
the complexes has also been utilized as the light-emitting layer in the fabrication of
OLEDs.

ACCEPTED MANUSCRIPT
Synthesis of Alkynylgold(III) complexes with Bis-cyclometalating Ligand
Derived from Ethyl 2,6-Diphenylisonicotinate and their Structural,
Electrochemical, Photo- and Electroluminescence Studies
Vonika Ka-Man Au, Daniel Ping-Kuen Tsang, Yi-Chun Wong, Mei-Yee Chan,* and
Vivian Wing-Wah Yam*
Institute of Molecular Functional Materials (Areas of Excellence Scheme, University
Grants Committee (Hong Kong)) and Department of Chemistry, The University of
Hong Kong, Pokfulam Road, Hong Kong
E-mail: chanmym@hku.hk, wwyam@hku.hk
Dedicated to Prof. D. M. P. Mingos on the occasion of his 70th Birthday
Abstract
A series of alkynylgold(III) complexes with bis-cyclometalating ligand derived
from ethyl 2,6-diphenylisonicotinate has been synthesized. The single crystal
…
structure of one of the complexes revealed an unprecedentedly short Au(III) Au(III)
distance of 3.44 Å. These complexes were found to exhibit rich electrochemistry.
Upon photoexcitation, the alkynylgold(III) complexes were found to show rich
emission properties in various states at both ambient and low temperatures. One of the
complexes have also been utilized as the light-emitting layer in the fabrication of
organic light-emitting diodes, demonstrating the potentials and versatility of
alkynylgold(III) complexes.
Keywords: Gold, photoluminescence, electroluminescence, electrochemistry
1. Introduction
Despite a long history of the chemistry of luminescent gold(I) complexes,1
increasing attention has only been directed towards gold(III) luminescence in the
recent years.2 This is mainly attributed to the difficulty to observe luminescence in
gold(III) complexes due to the electrophilicity of the gold(III) metal center and the
presence of low-lying d-d ligand field states which would quench the luminescent
1

ACCEPTED MANUSCRIPT
excited states of gold(III) complexes.1d,2
After the successful attempt by the group of Yam to prepare room temperature
luminescent gold(III) complexes through the incorporation of strong
σ-donating
3
ligands in 1993
and the subsequent preparation of highly luminescent
alkynylgold(III) complexes by the same group in the last decade,4 there has been a
continuing increase in interest in the exploration of light-emitting gold(III)
complexes.5−7 In particular, the rich photo- and electroluminescence properties of the
alkynylgold(III) system has rendered them good candidates as light-emitting materials
in organic light-emitting devices (OLEDs).4a,5 The versatility of gold(III) complexes
in supramolecular assemblies could also be exemplified by the formation of
metallogels,8ab supramolecular polymers8b and nanostructures,8c as well as the rare
7
…
observation of short Au(III) Au(III) distances in some cationic gold(III) complexes.
Herein, series of bis-cyclometalated alkynylgold(III)
a
complexes,
[Au{C^N(COOEt)^C}(C≡C-C6H4-R-p)] (R = N(C6H5)2 (1), C6H5 (2), F (3)), has been
synthesized and characterized, and the structures of two of the complexes were
crystallographically studied. The electrochemical and photophysical properties of the
complexes are also reported. The electroluminescence properites of one of the
complexes have also been explored.
2. Experimental
2.1 Materials and reagents
The tridentate ligand, ethyl 2,6-diphenylisonicotinate, was prepared by standard
acid-catalyzed esterification reaction between 2,6-diphenylisonicotinic acid and
ethanol. The chlorogold(III) precursor, [Au{C^N(COOEt)^C}Cl] was synthesized
according to a literature procedure of related bis-cyclometalated complexes.4
Potassium tetrachloroaurate(III) was purchased from ChemPur, copper(I) iodide was
from Lancaster, while 2,6-diphenylisonicotinic acid and triethylamine were from
Aldrich. 4-Ethynyltriphenylamine was synthesized according to a literature
procedure,9 whereas 1-ethynyl-4-phenylbenzene and 1-ethynyl-4-fluorobenzene were
from Maybridge. All other reagents were of analytical grade and used as received.
2

ACCEPTED MANUSCRIPT
Other solvents were purified and distilled using standard procedures before use. All
reactions were performed under anaerobic and anhydrous conditions using standard
Schlenk techniques under an inert atmosphere of nitrogen unless otherwise specified.
2.2 Synthesis
2.2.1
[Au{C^N(COOEt)^C}(C≡C−C6H4−N(C6H5)2-p)] (1)
(HC^N(COOEt)^CH = ethyl 2,6-diphenylisonicotinate)
The alkynylgold(III) complex was prepared according to a literature procedure.4,5
To
a
mixture of [Au{C^N(COOEt)^C}Cl] (267 mg, 0.50 mmol),
4-ethynyltriphenylamine (202 mg, 0.75 mmol) and a catalytic amount of copper(I)
iodide (10 mg, 0.05 mmol) was added dichloromethane (35 mL). Triethylamine (2 mL)
was added to the resulting suspension. After stirring under an inert atmosphere at
room temperature for 3 h, the solvent was removed in vacuo. The resulting residue
was purified by column chromatography on silica gel with dichloromethane. The
complex was then recrystallized by slowly diffusing diethyl ether vapor into the
concentrated dichloromethane solution to give orange-yellow crystals. Yield: 172 mg
1
(45 %). H NMR (300 MHz, CDCl3, 298 K)/ppm:
δ 1.46 (t, 3H, J = 7.1 Hz, −CH3),
4.51 (q, 2H, −ΟCH2−), 7.01 (d, 4H, J = 6.2 Hz, −C≡CC6H4−), 7.12 (m, 4H,
−N(C6H5)2), 7.29 (m, 6H, −N(C6H5)2), 7.39 (t, 2H, J = 7.4 Hz, phenyl of C^N^C),
7.49 (d, 2H, J = 7.4 Hz, phenyl of C^N^C), 7.63 (d, 2H, J = 7.4 Hz, phenyl of
C^N^C), 7.98 (s, 2H, pyridine of C^N^C), 8.07 (d, 2H, J = 7.4 Hz, phenyl of C^N^C).
Positive FAB-MS: m/z 767 [M]+. IR (KBr): 2147 cm−1 (
ν(C≡C)). Elemental analyses
calcd for C40H29AuN2O2•½CH2Cl2 (found): C, 60.12 (59.75 ); H, 3.74 (3.64); N, 3.46
(3.45).
2.2.2
[Au{C^N(COOEt)^C}(C≡C−C6H4−C6H5-p)] (2)
This was synthesized according to the synthetic procedure of 1 except that
1-ethynyl-4-phenylbenzene (134 mg, 0.75 mmol) was used in place of
4-ethynyltriphenylamine. The target complex 2 was obtained as yellow crystals. Yield:
1
122 mg (36 %). H NMR (300 MHz, CDCl3, 298 K)/ppm:
δ
1.49 (t, 3H, J = 7.2 Hz,
3

ACCEPTED MANUSCRIPT
−CH3), 4.53 (q, 2H, −ΟCH2−), 7.31 (2H, m, biphenyl), 7.37 (1H, d, J = 8.7 Hz,
biphenyl), 7.42−7.49 (m, 4H, biphenyl), 7.58-7.71 (m, 8H, biphenyl and phenyl of
C^N^C), 8.01 (s, 2H, pyridine of C^N^C), 8.11 (d, 2H, J = 7.3 Hz, phenyl of C^N^C).
Positive FAB-MS: m/z 675 [M]+. Elemental analyses calcd for C34H24AuNO2 (found):
C, 60.45 (60.49); H, 3.58 (3.55); N 2.07 (2.02).
2.2.3
[Au{C^N(COOEt)^C}(C≡C−C6H4−F-p)] (3)
This was synthesized according to the synthetic procedure of 1 except that
1-ethynyl-4-fluorobenzene (90 mg, 0.75 mmol) was used in place of
4-ethynyltriphenylamine. The target complex 3 was obtained as yellow crystals. Yield:
1
71 mg (23 %). H NMR (300 MHz, CDCl3, 298 K)/ppm:
δ 1.48 (t, 3H, J = 7.2 Hz,
−CH3), 4.50 (q, 2H, −ΟCH2−), 7.03 (t, 2H, J = 8.7 Hz, −C6H4−), 7.29 (t, 2H, J = 8.7
Hz, −C6H4−), 7.43 (t, 2H, J = 7.2 Hz, phenyl of C^N^C), 7.58 (m, 2H, phenyl of
C^N^C), 7.65 (d, 2H, J = 7.2 Hz, phenyl of C^N^C), 7.99 (s, 2H, pyridine of C^N^C),
8.06 (d, 2H, J = 7.2 Hz, phenyl of C^N^C). Positive FAB-MS: m/z 617 [M]+.
Elemental analyses calcd for C28H19AuFNO2•½H2O (found): C, 53.69 (53.84); H,
3.22 (3.10); N 2.24 (2.36).
2.3 Physical measurements and instrumentation
1H NMR spectra were recorded on a Bruker DPX-300 (300 MHz) or Bruker
DPX-400 (400 MHz) Fourier transform NMR spectrometer with chemical shifts
recorded relative to tetramethylsilane (Me4Si). Positive FAB mass spectra were
recorded on a Finnigan MAT95 mass spectrometer or a Thermo Scientific DFS
high-resolution magnetic sector mass spectrometer. IR spectra were obtained on KBr
disks on a Shimadzu IR Affinity-1 FTIR spectrophometer (4000-400 cm−1). Elemental
analyses for the metal complexes were performed on a Carlo Erba 1106 elemental
analyzer at the Institute of Chemistry, Chinese Academy of Sciences, Beijing.
Electronic absorption spectra of 1 were obtained on a Hewlett-Packard 8452A diode
array spectrophotometer while those of 2 and 3 were obtained on a Varian Cary 50
UV-vis spectrophotometer. Steady state excitation and emission spectra of 1 were
recorded on a Spex Fluorolog-2 Model F111 fluorescence spectrofluorometer
4

ACCEPTED MANUSCRIPT
equipped with a Hamamatsu R-928 photomultiplier tube, while those of 2 and 3 were
recorded on a Spex Fluorolog-3 Model FL3-211 spectrofluorometer, equipped with an
R2658P photomultiplier tube (PMT) detector. Unless specified otherwise, the
emission spectra were corrected for PMT response. Photophysical measurements in
low-temperature glass were carried out with the sample solution loaded in a quartz
tube inside a quartz-walled Dewar flask. Liquid nitrogen was placed into the Dewar
flask for low-temperature photophysical measurements at 77 K. Excited-state
lifetimes of solution samples were measured using a conventional laser system. The
excitation source used was the 355-nm output (third harmonic, 8 ns) of a
Spectra-Physics Quanta-Ray Q-switched GCR-150 pulsed Nd:YAG laser (10 Hz). All
solution samples for photophysical studies were freshly prepared under a high
vacuum in a 10-cm3 round-bottomed flask equipped with a side arm 1-cm
fluorescence cuvette and sealed from the atmosphere by a Rotaflo HP6/6
quick-release Teflon stopper. Solutions were rigorously degassed on a high-vacuum
line in a two-compartment cell with no less than four successive freeze-pump-thaw
cycles. Cyclic voltammetric measurements were performed by using a CH
Instruments, Inc. model CHI 600 A electrochemical analyzer. The electrolytic cell
used was a conventional two-compartment cell. Electrochemical measurements were
n
performed in dichloromethane solutions with 0.1 M Bu4NPF6 as supporting
electrolyte at room temperature. The reference electrode was a Ag/AgNO3 (0.1 M in
acetonitrile) electrode (CH Instruments, Inc.), and the working electrode was a glassy
carbon electrode (CH Instruments, Inc.) with a platinum wire as the counter electrode.
The working electrode surface was first polished with a 1-
µm alumina slurry (Linde),
followed by a 0.3- m alumina slurry, on a microcloth (Buehler Co.). The
µ
ferrocenium/ferrocene couple (FeCp2+/0) was used as the internal reference. 10
Luminescence quantum yields were measured by the optical dilute method reported
by Demas and Crosby.11a A degassed aqueous solution of quinine sulfate in 1.0 N
sulfuric acid (Φ = 0.546, excitation wavelength at 365 nm) was used as the reference
and corrected for the refractive index of the solution.11b All solutions for
electrochemical studies were deaerated with prepurified argon gas just before
measurements. Luminescence quantum yields of thin films were measured on a
Hamamatsu C9920-03 Absolute Photoluminescence Quantum Yield Measurement
System.
5

ACCEPTED MANUSCRIPT
2.4 Crystal structure determination
Single crystals of 1 and 2 suitable for X-ray diffraction studies were grown by
vapor diffusion of diethyl ether into a concentrated dichloromethane solution of the
complex. For complex 1, the X-ray diffraction data were collected on a Bruker Smart
CCD 1000 using graphite monochromatized Mo-K
Department of Chemistry, The University of Hong Kong. The X-ray diffraction data
for complex 2 were collected using synchrotron radiation ( = 0.85 Å) on beamline
α radiation (λ = 0.71073 Å) in the
λ
3W1A at the Beijing Synchrotron Radiation Facilities (BSRF) in the Institute of High
Energy Physics (IHEP), Chinese Academy of Sciences. The diffraction data reduction
and integration were performed by the HKL2000 software. The structures of both
complexes were solved by direct methods and refined using the SHELXS program.12
Full-matrix least-squares refinement on F2 was used in the structure refinement. The
positions of hydrogen atoms were calculated on the basis of the riding mode with
thermal parameters equal to 1.2 times those of the associated carbon atoms and
participated in the calculation of final R-indices. In the final stage of least-squares
refinement, all non-hydrogen atoms were refined anisotropically.
2.5 OLED fabrication and characterization
The OLEDs were fabricated on patterned indium-tin oxide (ITO)-coated glass
substrates with a sheet resisstance of 30 ꢀ/sq. The substrates were cleaned with
Decon 90, rinsed with deionized water, dried in an oven, and treated in an ultraviolet
ozone chamber. A 70-nm thick α-naphthylphenylbiphenyldiamine (NPB) was used as
the
holetransporting
layer,
and
a
5-nm
thick
4,4’4,”-tris(carbazole-9-yl)-triphenylamine (TCTA) was used as the carrier
confinement layer. A 30-nm thick emissive layer consisting of 2 %, 4 %, 6 %, and 8%
selected complex was doped into the N,N’-dicarbazolyl-3,5-benzene (MCP) layer via
thermal codeposition. A 30-nm thick bis(2-methyl-8-quinolinato)-4-phenylphenolate)
aluminum(III) (BAlq) was used as the electron-transporting layer; while LiF/Al was
used as metal cathode. All films were sequentially deposited at a rate of 0.1−0.2 nm
s−1 without vacuum break. A shadow mask was used to define the cathode and to
6

ACCEPTED MANUSCRIPT
make four 0.1 cm2 devices on each substrate. Current density−voltage−luminance
(J−V−L) characteristics and EL spectra were measured simultaneously with a
programmable Keithley 2420 power source and a Photoresearch PR-655
spectroradiometer.
3 Results and discussion
3.1 Synthesis and characterization
The alkynylgold(III) complexes were synthesized according to a literature
procedure, in which the chlorogold(III) precursor, [Au{C^N(COOEt)^C}Cl], was
allowed to react with the respective alkyne in dichloromethane in the presence of a
catalytic amount of copper(I) iodide and triethylamine under an inert atmosphere.4,5
The target complexes were purified by column chromatography on silica gel using
dichloromethane as eluent, followed by recrystallization from slow diffusion of
diethyl ether into the concentrated dichloromethane solution of the complex. Complex
1 was isolated as orange crystals while complexes 2 and 3 were collected as yellow
crystals. All of the complexes were soluble in dichloromethane and exhibited good
1
air- and thermal-stabilities. H NMR spectroscopy, FAB-mass spectrometry and
satisfactory elemental analyses were performed to confirm the identities of the
gold(III) complexes.
3.2 X-ray crystal structure
Single crystals of 1 and 2 were obtained by diffusion of diethyl ether vapor into a
concentrated dichloromethane solution of the respective complex. The crystal
structures were determined by X-ray crystallography. Crystal structure determination
data are summarized in Table 1 while the selected intermolecular parameters are
shown in Table 2. Figure 1 depicted the perspective views of the crystal structures of
1 and 2, and the crystal packing diagrams of the complexes are shown in Figure 2.
In the crystal structure, the complex molecules were found to align in a
head-to-tail fashion with some degree of
π
-
π
stacking interactions between adjacent
7

ACCEPTED MANUSCRIPT
[Au{C^N(COOEt)^C}] moieties, as revealed by the short interplanar distances of
about 3.63 and 3.45 Å in complexes 1 and 2, respectively. Both gold(III) complexes
exhibited square planar geometries with slight distortions from the ideal arrangement
in the crystal structures. The C−Au−N angles were in the range of 80.53(19)−81.6(2)°,
resulting from the steric requirements of the tridentate bis-cyclometalating
{C^N(COOEt)^C} ligands. Similar observations were also reported in the related
cyclometalated gold(III) complexes.4,5 The [Au{C^N(COOEt)^C}] motif was
essentially coplanar, and the Au−C (2.064(5)−2.071(6) Å) and Au−N
(1.988(4)−1.995(6) Å) bond distances were similar to those found in other related
gold(III) complexes.4,5 In these alkynylgold(III) complexes, the dihedral angles
between the [Au{C^N(COOEt)^C}] plane and the plane of the phenyl ring of the aryl
alkynyl ligand were found to be about 31.4° in 1 and 2.9° in 2. The Au−C≡C and
C≡C−C angles of 172.3(5)−174.3(6)° and 174.6(6)−177.7(6)° showed only a minor
deviation from the ideal 180°, establishing a slightly distorted linear arrangement,
with the Au−C (1.946(9)−1.952(5) Å) and C≡C bond distances (1.196(7)−1.222(12) Å)
similar to those found in the related cyclometalated alkynylgold(III) systems.4,5
As in other related organogold(III) complexes,4,5 the shortest Au Au distances
between adjacent molecules in complexes 2 (4.6284(9) Å) were found to be longer
than the sum of van der Waals radii for two gold(III) centers, revealing that no
significant aurophilic interactions occurred in the crystal lattices of the complex.
Interestingly, complex 1 was found to display an unprecedentedly short
intermolecular Au(III) Au(III) distance of 3.4435(3) Å, which was smaller than the
theoretical Au(III) Au(III) distance of 3.7 Å for the possible occurrence of aurophilic
interactions between the gold(III) metal centers as predicted by Pyykkö and
co-workers.13 In spite of the presence of extended π−π stacking interactions that
would assist the alkynylgold(III) complexes to be arranged in close proximity, related
bis-cyclometalated complexes in the literature have not been observed to show a
7
…
Au(III) Au(III) distance shorter than 3.7 Å. Recent DFT calculations have
…
supported the occurrence of weak intermolecular Au(III) Au(III) interactions in the
solid-state structure of a cationic alkynylgold(III) complex in the literature.7a To the
best of our knowledge, the Au(III) Au(III) distance of complex 1 in the present work
8

ACCEPTED MANUSCRIPT
is the first observation of such a short Au(III) Au(III) distance in neutral
alkynylgold(III) complexes.
3.3 Electrochemical studies
The electrochemical properties of the alkynylgold(III) complexes 1−3 have been
studied by cyclic voltammetry in dichloromethane (0.1 mol dm−3 nBu4NPF6). The
complexes were found to show a quasi-reversible first reduction couple at −1.11 to
−1.14 V versus saturated calomel electrode (SCE). An irreversible second reduction
couple was also observed at −1.71 to −1.75 V versus SCE. In addition, the complexes
were found to exhibit an irreversible to quasi-reversible couple at +0.88 to +1.52 V
versus SCE. The electrochemical data of complexes 1−3 are collected in Table 3.
With reference to the electrochemical studies of related gold(III) complexes in
the literature,4,5 the first reduction couple could be assigned to the ligand-centered
reduction of the C^N(COOEt)^C ligand. The reduction potential of complexes 1−3
were found to be essentially the same, with values lying between −1.11 and −1.14 V
versus SCE. The reduction potentials were found to be less negative than those of
analogous alkynylgold(III) complexes with the unsubstituted 2,6-diphenylpyridine
(C^N^C) ligand.4 This could be attributed to the presence of the electron-withdrawing
ethyl ester group directly substituted on the pyridine moiety of the cyclometalating
C^N(COOEt)^C ligand. The electron-deficient substituent would make the π* orbital
much more lower-lying in energy and would thus increase the ease of reduction for
the complex.
The anodic waves of complexes 1−3, which ranged from +0.88 to +1.52 V
versus SCE, were found to be rather sensitive to the nature of the alkynyl ligand. In
view of the electron-deficient nature and the unlikely redox activitiy of the gold(III)
metal center, the oxidation wave was assigned to the alkynyl ligand-centered
oxidation of the complexes. The anodic waves were found to show potentials in the
order 1 < 2 < 3, in line with the electron-donating ability of the alkynyl group:
C≡C−C6H4−N(C6H5)2
>
C≡C−C6H4−C6H5-p
>
C≡C−C6H4−F-p.
A
better
electron-donating ability of the alkynyl group would be associated with a less positive
9

ACCEPTED MANUSCRIPT
potential of the alkynyl ligand-centered oxidation. Such an assignment was also in
agreement with those of related alkynylgold(III) complexes in the literature.4,5
3.4 Electronic absorption properties
In dichloromethane solutions at room temperature, the complexes were generally
found to exhibit a high energy absorption band at 325−340 nm region, and a
moderately intense vibronically structured absorption band at 405−430 nm region.
The extinction coefficients (ε
) of the absorption bands were on the order of 103−104
dm3mol−1cm−1, which were in agreement with those of related alkynylgold(III)
complexes reported in the literature.4,5 The absorption spectra of complexes 1−3 are
depicted in Figure 3, while the photophysical data of the complexes are summarized
in Table 4. The higher energy band at 325−340 nm region was assigned to the
intraligand π-π* transition of the alkynyl ligands. On the other hand, the lower-energy
absorption band was found to be at essentially the same wavelengths at about
405−430 nm for complexes 1−3. With reference to the observations as well as
literature results, the lower-energy band was assigned to a metal-perturbed intraligand
π
-
π
* transition of the tridentate bis-cyclometalating C^N(COOEt)^C ligand, with
some charge-transfer character from the phenyl moiety to the central ester-substituted
pyridine For complex containing the electron-rich
moiety.4,5
4-ethynyltriphenylamine ligand, the absorption shoulder beyond 450 nm could be
assigned to ligand-to-ligand charge transfer (LLCT)
(C≡C-R)→ *{C^N(COOEt)^C}] transition from the alkynyl moiety to the
1
a
[
π
π
cyclometalating ligand.4,5
The presence of an ethyl ester group (−COOEt) on the central pyridine ring of
the tridentate ligand was observed to result in a substantial red shift in the absorption
energy (ca. 405−430 nm) when compared to the literature complexes that contained
the unsubstituted 2,6-diphenylpyridine (C^N^C) ligand (ca. 360−410 nm).4 This
could be attributed to the fact that the presence of the electron-withdrawing ester
group would lower the energy level of the pyridine-based LUMO
π* orbital to a much
larger extent than the phenyl-based HOMO orbital. This greatly reduced the energy
π
difference between the HOMO and LUMO, and thus the energy of the resulting IL
10

ACCEPTED MANUSCRIPT
π-π*[C^N(COOEt)^C] absorption.
3.5 Photoluminescence properties
The complexes were observed to be emissive at both ambient and low
temperatures in various media. Table 4 summarizes the photophysical properties of
complexes 1−3 and the representative emission spectra in dichloromethane solution
and in low-temperature glass are respectively shown in Figures 4 and 5.
Upon excitation at
λ ≥ 350 nm at 298 K, all complexes were found to exhibit
photoluminescence in dichloromethane solution at 298 K. Complexes 2 and 3 were
observed to exhibit a vibronic-structured emission band at 511−538 nm, and the
emission energies were rather insensitive to the variation of the nature of the alkynyl
ligand. This emission band could be assigned to a metal-perturbed triplet intraligand
π-π*excited state of the C^N(COOEt)^C ligand, probably with some phenyl to
ester-substituted pyridine character.
On the contrary, complex 1 was observed to show an unusual blue-shifted
emission band at ca. 463 nm. With reference to the emission properties of related
gold(III) complexes with the same 4-ethynyltriphenylamine ligand,4,5 an assignment
of the emission of 1 to an origin from an alkynyl-to-tridentate ligand LLCT excited
state would be unlikely. It was because the presence of the electron-withdrawing ethyl
ester group on the pyridine moiety should lower the energy of the pyridine-based π*
orbital to result in a red-shifted emission band of lower energy. In view of the close
resemblance of the emission energy of 1 to that of the free 4-ethynyltriphenylamine
ligand, the emission of 1 was tentatively assigned to be originated from an
alkynyl-centered intraligand excited state.
In alcoholic glass at 77 K, complexes 1−3 were all found to exhibit
vibronic-structured emission bands at 494−575 nm, and the emission energies were
observed to be insensitive to the nature of the alkynyl ligands. The vibrational
progressional spacings of 1200−1400 cm−1 were characteristic of the stretching
frequencies of the C=C and C=N bonds in the tridentate ligands, suggestive of the
11

ACCEPTED MANUSCRIPT
involvement of the tridentate ligand in the excited state. Such emission bands were
similar to the solution emission bands of 2 and 3. The glass emission of complexes
3
1−3 were assigned to a metal-perturbed IL [
π
→
π
*{C^N(COOEt)^C}] excited state.
The observation of such IL emission band in complex 1 at low temperature suggested
that the emissive states of complex 1 were rather close-lying in energies. The
solid-state emission of the crystal and powder forms of complex 1 were also measured.
At 298 and 77 K, complex 1 was found to exhibit a structureless emission band at
578−615 nm in the solid state. With reference to literature reports of solid-state
emissions of related gold(III) complexes,4,5 as well as the close-packing of the
complex molecules in the crystal structure of 1, the solid-state emission of 1 could
tentatively be assigned to an excimeric nature. Complexes 2 and 3 were also observed
to show similar emission maxima in the solid state at 611−613 nm at room
temperature.
3.6 Electroluminescence properties
Multilayer OLEDs with the structures of ITO/NPB (70 nm)/ TCTA (5 nm)/x%
Au(III) complex:MCP (30 nm)/BAlq (30 nm)/LiF (0.8 nm)/Al had been prepared by
thermal evaporation at different concentrations (x) of 2, 4, 6, and 8 %. NPB and BAlq
were used as hole-transporting and electron-transporting layers, respectively; while
TCTA was employed as carrier confinement layer. Figure 6a depicts the
electroluminescence (EL) spectra of devices doped with 1 as phosphorescent dopant
at a current density of 100 mA cm–2. The devices exhibited a broad, structureless
emission band, and the EL peak maxima were found to shift from 520 to 548 nm with
increasing dopant concentration from 2 to 8 %. The concentration dependence might
be ascribed to the excimeric emission resulting from π-stacking of the cyclometalated
ligand and a better packing of molecules at higher concentrations, which is well
documented in square-planar metal complexes. Devices doped with complexes 1 were
found to exhibit current efficiencies of 7.9 cd A–1 and EQE of 2.8 %. The device
performance of the devices doped with complex 1 is depicted in Figure 6b-d.
4 Conclusion
12

ACCEPTED MANUSCRIPT
A
series of alkynylgold(III) complexes with an ester-substituted
bis-cyclometalating ligand has been synthesized. The single crystal structure of one of
…
the complexes revealed an unprecedentedly short Au(III) Au(III) distance of 3.44 Å.
The complexes were found to exhibit quasi-reversible reduction couples originated
from the tridentate ligand and oxidation couples originated from the alkynyl ligands.
In addition, the complexes were found to exhibit rich photo- and electroluminescence,
rendering them potential candidates for supramolecular assembly and functions.
Acknowledgements
V. W.-W. Y. acknowledges support from The University of Hong Kong under the
URC Strategic Research Theme on New Materials. This work has been supported by
the University Grants Committee Areas of Excellence Scheme (AoE/P-03/08) and a
General Research Fund (GRF) Grant from the Research Grants Council of the Hong
Kong Special Administrative Region, P. R. China (HKU 17305614). V. K.-M. A.
acknowledges the receipt of a University Postdoctoral Fellowship. Prof. W.-T. Wong
and Dr. L. Szeto are gratefully acknowledged for their technical assistance in the
determination of the crystal structure of complex 1, and Mr. L.-Y. Yao is gratefully
acknowledged for his technical assistance in the collection of crystal diffraction data
of complex 2.
Appendix A. Supplementary material
CCDC 1036010 (1) and 1036011 (2) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
References
1
(a) Schmidbaur, H. Chem. Soc. Rev. 1995, 24, 391; (b) Hutchings, G. J.; Brust,
M.; Schmidbaur, H. Chem. Soc. Rev. 2008, 37, 1759; (c) Ford, P. C.; Vogler, A.
Acc. Chem. Res. 1993, 26, 220; (d) Vogler, A.; Kunkely, H. Coord. Chem. Rev.
2001, 219−221, 489; (e) Fung, E. Y.; Olmstead, M. M.; Vickery, J. C.; Balch, A.
L. Coord. Chem. Rev. 1998, 171, 151; (f) Burini, A.; Mohamed, A. A.; Fackler, J.
P., Jr. Comments Inorg. Chem. 2003, 24, 253; (g) Puddephatt, R. J. Chem. Soc.
13

ACCEPTED MANUSCRIPT
Rev. 2008, 37, 2012; (h) Pyykko
̈
, P. Chem. Rev. 1988, 88, 563; (i) Yam, V.
W.-W.; Lo, K. K.-W. Chem. Soc. Rev. 1999, 28, 323-334.
2
(a) Yam, V. W.-W.; Cheng, E. C.-C. Top. Curr. Chem. 2007, 281, 269; (b) Yam,
V. W.-W.; Cheng, E. C.-C. Chem. Soc. Rev. 2008, 37, 1806; (c) Wong, K. M.-C.;
Chan, M. M.-Y.; Yam, V. W.-W. Adv. Mater. 2014, 26, 5558.
3
4
Yam, V. W.-W.; Choi, S. W.-K.; Lai, T.-F.; Lee, W.-K. J. Chem. Soc., Dalton
Trans. 1993, 1001.
(a) Wong, K. M.-C.; Zhu, X.; Hung, L.-L.; Zhu, N.; Yam, V. W.-W.; Kwok, H. S.
Chem. Commun. 2005, 2906; (b) Yam, V. W.-W.; Wong, K. M.-C.; Hung, L.-L.;
Zhu, N. Angew. Chem. Int. Ed. 2005, 44, 3107; (c) Wong, K. M.-C.; Hung, L.-L.;
Lam, W. H.; Zhu, N.; Yam, V. W.-W. J. Am. Chem. Soc. 2007, 129, 4350.
(a) Au, V. K.-M.; Wong, K. M.-C.; Tsang, D. P.-K.; Chan, M.-Y.; Zhu, N.; Yam,
V. W.-W. J. Am. Chem. Soc. 2010, 132, 14273; (b) Au, V. K.-M.; Tsang, D. P.-K.;
Wong, K. M.-C.; Chan, M.-Y.; Zhu, N.; Yam, V. W.-W. Inorg. Chem. 2013, 52,
12713; (c) Tang, M.-C.; Tsang, D. P.-K.; Chan, M. M.-Y.; Wong, K. M.-C.; Yam,
V. W.-W. Angew. Chem. Int. Ed. 2013, 52, 446; (d) Tang, M.-C.; Chan, C. K.-M.;
Tsang, D. P.-K.; Wong, Y.-C.; Chan, M. M.-Y.; Wong, K. M.-C.; Yam, V. W-W.
Chem. Eur. J. 2014, 20, 15233; (e) To, W.-P.; Chan, K. T.; Tong, G. S. M.; Ma,
C.; Kwok, W.-M.; Guan, X.; Low, K.-H.; Che, C.-M. Angew. Chem. Int. Ed.
2013, 52, 6648; (f) Cheng, G.; Chan, K. T.; To, W.-P.; Che, C.-M. Adv. Mater.
2014, 26, 2540.
5
6
(a) Au, V. K.-M.; Wong, K. M.-C.; Zhu, N.; Yam, V. W.-W. J. Am. Chem. Soc.
2009, 131, 9076; (b) Au, V. K.-M.; Wong, K. M.-C.; Zhu, N.; Yam, V. W.-W.
Chem. Eur. J. 2011, 17, 130; (c) Garg, J. A.; Blacque, O.; Venkatesan, K. Inorg.
Chem. 2011, 50, 5430; (d) Garg, J. A.; Blacque, O.; Fox, T.; Venkatesan, K.
Inorg. Chem. 2010, 49, 11463; (e) Herbst, A.; Bronner, C.; Dechambenoit, P.;
Wenger, O. S. Organometallics 2013, 32, 1807; (f) Smith, D. A.; Roşca, D.-A.;
Bochmann, M. Organometallics, 2012, 31, 5998; (g) Roşca, D.-A.; Smith, D. A.;
Bochmann, M. Chem. Commun. 2012, 48, 7247.
7
8
(a) Lu, W.; Chan, K. T.; Wu, S.-X.; Chen, Y.; Che, C.-M. Chem. Sci. 2012, 3,
752; (b) Au, V. K.-M.; Lam, W. H.; Wong, W.-T.; Yam, V. W.-W. Inorg. Chem.
2012, 51, 7537.
(a) Au, V. K.-M.; Zhu, N.; Yam, V. W.-W. Inorg. Chem. 2013, 52, 558; (b) Yim,
14

ACCEPTED MANUSCRIPT
K.-C.; Lam, E. S.-H.; Wong, K. M.-C.; Au, V. K.-M.; Ko, C.-C.; Lam, W. H.;
Yam, V. W.-W. Chem. Eur. J. 2014, 20, 9930; (c) Zhang, J.-J.; Lu, W.; Sun, R.
W.-Y.; Che, C.-M. Angew. Chem. Int. Ed. 2012, 51, 4882; (d) Xiao, X.-S.;
Kwong, W.-L.; Guan, X.; Yang, C.; Lu, W.; Che, C.-M. Chem. Eur. J. 2013, 19,
9457.
9
McIlroy, S. P.; Cló, E.; Nikolajsen, L.; Frederiksen, P. K.; Nielsen, C. B.;
Mikkelsen, K. V.; Gothelf, K. V.; Ogilby, P. R. J. Org. Chem. 2005, 70, 1134.
10 Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877.
11 (a) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991; (b) van Houten, J.;
Watts, R. J. Am. Chem. Soc. 1976, 98, 4853.
12 (a) Sheldrick, G. M. Acta Cryst. 2008, A64, 112; (b) Sheldrick, G. M. SHELXS
97: Programs for Crystal Structure Analysis (Release 97-2); University of
Göttingen: Göttingen, Germany, 1997.
13 Mendizabal, F.; Pyykkö, P. Phys. Chem. Chem. Phys. 2004, 6, 900.
15

ACCEPTED MANUSCRIPT
Table 1
Crystal and structure determination data of complexes 1 and 2
[Au{C^N(COOEt)^C}(C≡C- [Au{C^N(COOEt)^C}(C≡C-
complex
C6H4-N(C6H5)2-p)] (1)
C40H29AuN2O2
766.62
C6H4-C6H5-p] (2)
C34H24AuNO2
675.51
empirical formula
formula weight
temp, K
301(2)
293(2)
wavelength, Å
crystal system
space group
a, Å
0.71073
monoclinic
P 21/c
0.85
monoclinic
P 21/c
9.8039(4)
9.9712(4)
32.0807(14)
90
12.748(3)
8.5030(17)
24.151(5)
90
b, Å
c, Å
α
β
, deg
, deg
92.2930(10)
90
98.44(3)
90
γ, deg
volume, cm3
Z, Å3
3133.6(2)
4
2589.5(9)
4
density (calcd),
gcm−3
1.625
1.733
crystal size
0.40 mm × 0.26 mm
× 0.05 mm
0.05 mm × 0.10 mm
× 0.20 mm
index ranges
−12 ≤ h ≤ 13
−13 ≤ k ≤ 13
−42 ≤ l ≤ 38
−16 ≤ h ≤ 16
−11 ≤ k ≤ 11
−31 ≤ l ≤ 31
reflections
23234 / 7556
12086 / 4092
collected / unique
GOF on F2
1.134
1.058
final R indices
R1 = 0.0435
wR2 = 0.0860
R1 = 0.0511
wR2 = 0.1416
[I > 2σ(I)]
largest diff peak
and hole, e Å−3
1.313 and −1.180
1.458 and −2.626
16

ACCEPTED MANUSCRIPT
Table 2 Selected bond lengths (Å) and angles (deg) for complexes 1 and 2 with
estimated standard deviations (esd’s) given in parentheses
[Au{C^N(COOEt)^C}(C≡C-C6H4- [Au{C^N(COOEt)^C}(C≡C-C6
N(C6H5)2-p)] (1)
H4-C6H5-p] (2)
Bond Lengths (Å)
Au(1)−C(21)
Au(1)−C(37)
Au(1)−N(2)
Au(1)−C(1)
C(1)−C(2)
2.065(5)
2.064(5)
1.988(4)
1.952(5)
1.196(7)
Au(1)−C(7)
2.069(6)
2.071(6)
1.995(6)
1.946(9)
1.222(12)
Au(1)−C(13)
Au(1)−N(1)
Au(1)−C(18)
C(18)−C(19)
Bond Angles (deg)
C(21)−Au(1)−C(37)
N(2)−Au(1)−C(1)
Au(1)−C(1)−C(2)
N(2)−Au(1)−C(21)
N(2)−Au(1)−C(37)
C(1)−C(2)−C(3)
161.8(2)
177.65(19)
172.3(5)
C(7)−Au(1)−C(13)
162.4(2)
177.9(2)
174.3(6)
81.6(2)
N(1)−Au(1)−C(18)
Au(1)−C(18)−C(19)
N(1)−Au(1)−C(7)
N(1)−Au(1)−C(13)
C(18)−C(19)−C(20)
80.53(19)
81.33(19)
174.6(6)
80.8(2)
177.7(6)
17

ACCEPTED MANUSCRIPT
Table 3 Electrochemical data for 1−3a
Oxidation Epa
Reduction
/ V vs SCEb
Complex
E / V vs SCEb (∆Ep / mV)
½
[Epc / V vs SCEb]
−1.14 (63), [−1.75]
−1.11 (55), [−1.72]
−1.12 (56), [−1.71]
+0.88
+1.41
+1.52
1
2
3
a In dichloromethane solution with 0.1 M nBu4NPF6 as supporting electrolyte at room
temperature; working electrode, glassy carbon; scan rate 100 mV s−1.
b Epa refers to the anodic peak potential for the irreversible oxidation waves, whereas
Epc referes to the cathodic peak potential for the irreversible reduction waves. E =
½
(Epa + Epc)/2; Epa and Epc are peak anodic and peak cathodic potentials, respectively.
∆Ep = |Epa − Epc|.
18

ACCEPTED MANUSCRIPT
Table 4 Photophysical data for complexes 1−3
absorptiona
emission
max/nm (
463 (0.6)
b
λ
max/nm (εmax
complex
1
Φlum
medium (T/K)
λ
τ0/µs)
/dm3mol−1cm−1)
328 (41610),
410 (7370),
< 0.1 × 10−3
CH2Cl2 (298)
Glass (77)c
494, 570 (95)
615 (0.1)
605 (4.4)
612 (0.1)
578 (5.3)
430 (7660)
Crystal (298)
Crystal (77)
Powder (298)
Powder (77)
Thin Film (298)
5% in MCP
557
0.30
0.45
0.44
339 (12330),
388sh (5620),
406 (5270),
431 (3690)
CH2Cl2 (298)
Glass (77)c
513, 538, 567 (1.2)
499, 537, 575 (80)
611, 655sh (4.2)
2
3
Powder (298)
Thin Film (298)
5% in MCP
514, 545sh
327 (11860),
388sh (3820),
408 (4390),
431 (3600)
CH2Cl2 (298)
Glass (77)c
511, 535, 570 (1.4)
499, 531, 566, 607sh (92)
613, 658 (6.0)
Powder (298)
Thin Film (298)
5% in MCP
513, 543sh
a In dichloromethane at 298 K.
b For CH2Cl2 solutions, the luminescence quantum yield, measured at 298 K using
quinine sulfate as a standard; for thin films, the absolute photoluminescence
quantum yield, measured under an excitation wavelength of 350 nm at 298 K.
c In EtOH−MeOH−CH2Cl2 (40:10:1 v/v).
19

ACCEPTED MANUSCRIPT
Scheme 1
Structures of alkynylgold(III) complexes 1−3
20

ACCEPTED MANUSCRIPT
(a)
(b)
Figure 1
Perspective view of (a) 1 and (b) 2 with atomic numbering scheme.
Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are
drawn at the 30 % probability level
21

ACCEPTED MANUSCRIPT
(a)
(b)
22

ACCEPTED MANUSCRIPT
(c)
Figure 2
Crystal packing diagrams showing (a) the head-to-tail configuration of
1, (b) the short gold-gold distances in 1 and (c) the head-to-tail
configuration of 2.
23

ACCEPTED MANUSCRIPT
8
6
4
2
0
1
2
3
400
450
500
550
Wavelength / nm
Figure 3
Electronic absorption spectra of complexes 1−3 in dichloromethane at
298 K
24

ACCEPTED MANUSCRIPT
1
2
3
*
400
500
600
700
Wavelength / nm
Figure 4
Normalized emission spectra of complexes 1−3 in dichloromethane at
298 K (Asterisk indicates the instrumental artifact)
25

ACCEPTED MANUSCRIPT
500
600
700
Wavelength / nm
Figure 5
Emission spectrum of complex 3 in EtOH-MeOH-CH2Cl2 (40:10:1 v/v)
glass at 77 K
26

ACCEPTED MANUSCRIPT
(a)
ITO/NPB (70 nm)/TCTA (5 nm)/x% 1:MCP (30 nm)/BAlq (30 nm)/LiF/Al
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2%
4%
6%
8%
380
480
580
680
780
Wavelength (nm)
(b)
ITO/NPB (70 nm)/TCTA (5 nm)/x% 1:MCP (30 nm)/BAlq (30 nm)/LiF/Al
100
10
2%
4%
6%
8%
7.9 cd/A
1
0.1
0.01
0
50
100
150
200
250
Current density
(
mA/cm2
)
27

ACCEPTED MANUSCRIPT
(c)
ITO/NPB (70 nm)/TCTA (5 nm)/x% 1:MCP (30 nm)/BAlq (30 nm)/LiF/Al
100
10
2%
4%
6%
8%
4.9 lm/W
1
0.1
0.01
0
50
100
150
200
250
Current density
(
mA/cm2
)
(d)
ITO/NPB (70 nm)/TCTA (5 nm)/x% 1:MCP (30 nm)/BAlq (30 nm)/LiF/Al
100
10
2%
4%
6%
8%
2.8 %
1
0.1
0.01
0
50
100
150
200
250
Current density
(
mA/cm2
)
Figure 6
(a) Normalized EL spectra of devices doped with 1 as phosphorescent
dopant at a current density of 100 mA cm−2. (b) Current and (c) power
efficiencies and (d) external quantum efficiencies (EQE) of the devices
with different concentrations of 1 doped into the MCP layer as a
function of current density.
28