Controlling aggregation in highly emissive Pt(II) complexes bearing tridentate dianionic N^N^N ligands. synthesis, photophysics, and electroluminescence

Article abstract of DOI:10.1021/cm2010902

Neutral Pt(II) complexes bearing tridentate dianionic 2,6-bis(1H-1,2,4- triazol-5-yl)pyridine and ancillary alkyl-substituted pyridine ligands have been synthesized and characterized. They show bright green emission, reaching 73% photoluminescence quantum yield in deareated chloroform solution, which can be assigned to a predominantly metal-perturbed ligand-centered phosphorescence. We have followed two strategies to preserve the spectral purity of the monomeric species by varying the substituents on the chromophoric or on the ancillary ligands. However, variations in the substitution patterns only modestly affected the radiative and radiationless deactivation rate constants of the monomers. Photophysical and electrochemical properties have been measured for all the complexes and correlated with calculations using time-dependent density functional theory. The electroluminescence spectra of the brightest, nonaggregating derivative showed a better color purity than that of iridium(III) tris(phenylpyridine), thus proving that aggregation was hindered in a running electroluminescent device.

Full text of DOI:10.1021/cm2010902

ARTICLE
pubs.acs.org/cm
Controlling Aggregation in Highly Emissive Pt(II) Complexes Bearing
Tridentate Dianionic N∧N∧N Ligands. Synthesis, Photophysics, and
Electroluminescence
Mathias Mydlak,† Matteo Mauro,† Federico Polo,† Michael Felicetti,‡ Jens Leonhardt,‡ Gerhard Diener,‡
Luisa De Cola,*,† and Cristian A. Strassert*,†
†Physikalisches Institut and Center for Nanotechnology (CeNTech), Westf€alische Wilhelms-Universit€at M€unster, Mendelstrasse 7,
D-48149 M€unster, Germany
‡Sensient Imaging Technologies GmbH, Chemie Park Bitterfeld-Wolfen, Areal A, Sensientstrasse 3, 06766 Bitterfeld-Wolfen, Germany
S Supporting Information
b
ABSTRACT: Neutral Pt(II) complexes bearing tridentate dianionic 2,6-bis(1H-1,2,4-
triazol-5-yl)pyridine and ancillary alkyl-substituted pyridine ligands have been synthesized
and characterized. They show bright green emission, reaching 73% photoluminescence
quantum yield in deareated chloroform solution, which can be assigned to a predomi-
nantly metal-perturbed ligand-centered phosphorescence. We have followed two strate-
gies to preserve the spectral purity of the monomeric species by varying the substituents
on the chromophoric or on the ancillary ligands. However, variations in the substitution
patterns only modestly affected the radiative and radiationless deactivation rate constants
of the monomers. Photophysical and electrochemical properties have been measured for
all the complexes and correlated with calculations using time-dependent density func-
tional theory. The electroluminescence spectra of the brightest, nonaggregating derivative
showed a better color purity than that of iridium(III) tris(phenylpyridine), thus proving
that aggregation was hindered in a running electroluminescent device.
KEYWORDS: platinum(II) complexes, tridentate ligand, green emission, 1,2,4-triazoles
light when assembled into the emitting aggregates. Furthermore,
the nitrogen-rich tetrazole rings make it thermally unstable and,
therefore, unsuitable for vapor-processed devices.63ꢀ66
1. INTRODUCTION
The search for electroluminescent metal complexes without
the relatively scarce iridium has led to several classes of emitters
based on transition metals such as rhenium,1ꢀ4 osmium,5ꢀ8
platinum9ꢀ33 or gold34,35 or on cheaper and less toxic ones, e.g.,
copper36ꢀ38 and zinc.39ꢀ51
To control aggregation, which leads to undesired broadening of
spectra, and to replace the decomposing tetrazole rings, we have
designed and synthesized a new class of neutral Pt(II) complexes
bearing tridentate dianionic 2,6-bis(1H-1,2,4-triazol-5-yl)pyridine
chelates and ancillary alkyl-substituted pyridine ligands. 1,2,
4-triazoles can be conveniently functionalized at the C3 position
to tune the emission and aggregation properties and possess a
strong σ donating character that favors luminescence.
Tuning of color, aggregation, and processability are key pa-
rameters for applications in electroluminescent devices. There-
fore, the substitution pattern of the triazole rings was varied by
the alternative introduction of bulky, aliphatic adamantyl groups
or conjugated, aromatic tolyl moieties, and their influence is
discussed in detail. The photophysical properties have been
investigated and supported by computational calculations, and
vapor-processed devices were prepared with the most promising
derivative, displaying an electroluminescence spectrum with no
Pt(II) complexes have also been recently considered as triplet
emitters. They have a d8 electronic configuration and, as a result,
exhibit a square planar coordination geometry, which makes
them prone to formation of aggregates or excimers, originating
shifts in the emission and influencing the photoluminescence
quantum yields (PLQYs).13,16,52 Although this can be advanta-
geous for the realization of white organic light-emitting diodes
(WOLEDs), it also represents a drawback for applications where
color purity is desired.11,12,15,33 Terpyridine ligands53ꢀ56 and
their N∧C∧N and N∧N∧C analogues have been coordinated to
Pt(II),18,19,24,57ꢀ61 yielding neutral or singly or doubly charged
complexes, some of them showing bright luminescence. We have
recently reported a straightforward one-pot synthesis of neutral,
soluble Pt(II) coordination compounds bearing a dianionic tri-
dentate ligand. The coordination of an alkylpyridine ancillary
moiety to the 2,6-bistetrazolylpyridine-based complex allowed us
to enhance the solution processability, reaching 87% PLQY in thin
films.62 Unfortunately, this derivative is only able to emit yellow
Received: April 15, 2011
Revised:
June 24, 2011
Published: July 22, 2011
r
2011 American Chemical Society
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Chemistry of Materials
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evidence of aggregation and, consequently, even better color
purity than iridium(III) tris(phenylpyridine) (hereafter Ir(ppy)3).
the reference electrodes, the feedback correction was employed. The
electrochemical experiments were performed by using a homemade
3 mm diameter glassy carbon disk electrode (from a glassy carbon rod,
Tokai Inc.). The working electrodes were stored in ethanol and before
the experiments were polished with a 0.05 μm diamond suspension
(Metadi Supreme Diamond Suspension, Buehler) and ultrasonically
rinsed with ethanol for 5 min. The electrode was electrochemically
activated in the background solution by means of several voltammetric
cycles at 0.5 V sꢀ1 between the anodic and cathodic solvent/electrolyte
discharges until the same quality features were obtained. The reference
electrode was a silver quasi-reference electrode (Ag-QRE), which was
separated from the catholyte by a glass frit (Vycor). The reference
electrode was calibrated at the end of each experiment against the
ferrocene/ferricenium couple, whose formal potential is 0.460 V against
the KCl saturated calomel electrode (SCE); in the following, all
potential values are reported against the SCE. A platinum ring or coil
served as the counter electrode.
2.1.5. Device Preparation. All layers were prepared by thermal
evaporation in a high-vacuum system with a pressure of less than 10ꢀ4
Pa without breaking the vacuum. ITO-coated glass plates with a surface
resistivity of e15 Ω/0 were used as substrates. They were ultrasonically
cleaned and treated with oxygen plasma for work function tuning and
chemical cleaning. During the evaporation, the deposition rates were
monitored by controllers, which were calibrated using a Dektak 6 M
profiler. The deposition rate was 0.1 nm/s, and the layer thickness was
checked by quartz balances. The JꢀVꢀL characteristics were measured
by using a Keithley 238 (high-current source measure unit) and Keithley
2000 (multimeter) with a calibrated silicon photodiode under nitrogen.
Spectra were recorded on a tec5 spectrometer unit LOE-USB. The
luminance values were measured at normal incidence, and the total flux
was estimated by assuming a Lambertian distribution.
2.1.6. Synthesis and Characterization. All reagents were analytical
grade and used as received. Column chromatography (CC) was per-
formed with silica gel 60 (particle size 63ꢀ200 µm, 230ꢀ400 mesh,
Merck). NMR spectra were recorded on an ARX 300 or an AMX 400 from
Bruker Analytische Messtechnik (Karlsruhe, Germany). The 1H NMR
chemical shifts (δ) of the signals are given in parts per million and
referenced to residual protons in the deuterated solvents: chloroform-d1
(7.26 ppm), dimethyl sulfoxide-d6 (2.50 ppm), or methylene chloride-d2
(5.32 ppm). The signal splittings are abbreviated as follows: s, singlet; d,
doublet; t, triplet; q, quartet; m, multiplet. All coupling constants (J) are
given in hertz. Mass spectrometry was performed in the Department of
Chemistry, University of M€unster. Electrospray ionization (ESI) mass
spectra were recorded on a Bruker Daltonics (Bremen, Germany)
MicroTof with loop injection. Elemental analysis was recorded at the
University of Milan, Italy.
2. EXPERIMENTAL SECTION
2.1. Experimental Techniques. 2.1.1. Photophysics. Absorption
spectra were measured on a Varian Cary 5000 double-beam UVꢀ
visꢀNIR spectrometer and baseline corrected. Steady-state emission
spectra were recorded on an Edinburgh FS920 spectrometer equipped
with a 450 W xenon-arc lamp, excitation and emission monochromators
(1.8 nm/mm dispersion, 1800 grooves/mm blazed at 500 nm), and a
Hamamatsu R928 photomultiplier tube. Emission and excitation spectra
were corrected for source intensity (lamp and grating) by standard
correction curves. Time-resolved measurements were performed using
the multichannel scaling (MCS) single-photon-counting option on the
HORIBA Jobin-Yvon IBH FL-322 Fluorolog 3. A pulsed xenon lamp was
used to excite the sample. The excitation sources were mounted directly
on the sample chamber at 90° to a double-grating emission monochro-
mator (2.1 nm/mm dispersion, 1200 grooves/mm) and collected by a
TBX-4-X single-photon-counting detector. The photons collected at the
detector are correlated by a time-to-amplitude converter (TAC) to the
excitation pulse. Signals were collected using an IBH Data Station Hub
photon-counting module, and data analysis was performed using the
commercially available DAS6 software (HORIBA Jobin Yvon IBH). The
quality of the fit was assessed by minimizing the reduced χ2 function and
by visual inspection of the weighted residuals.
All solvents were spectrometric grade. Deaerated samples were
prepared by the freezeꢀpumpꢀthaw technique.
2.1.2. UV Photoelectron Spectroscopy (UPS). UPS was measured on a
Riken Keiki AC-2 system using the low-energy electron counter method
with a deuterium lamp as the light source and a grating type mono-
chromator as the spectrometer.
2.1.3. Computational Details. Geometries were optimized by means
of density functional theory (DFT). The parameter-free hybrid func-
tional PerdewꢀBurkeꢀErzenrhof67ꢀ69 was employed along with the
standard valence double-ζ polarized basis set 6-31G(d,p)70 for C, H, F,
and N. For Pt, the StuttgartꢀDresden (SDD) effective core potentials
were employed along with the corresponding valence triple-ζ basis set.
All the calculations were done assuming Cs symmetry. The nature of all
the stationary points was checked by computing vibrational frequencies,
and all the species were found to be true potential energy minima, as no
imaginary frequency was obtained (NImag = 0). To simulate the
absorption electronic spectrum down to 250 nm, for each complex
the lowest 20 singlet (S0 f Sn) as well as the 3 lowest triplet (S0 f Tn)
excitation energies were computed on the optimized geometry at S0 by
means of time-dependent density functional theory (TD-DFT)
calculations.71,72 Oscillator strengths were deduced from the dipole
transition-matrix elements (for single states only). All the calculations
were performed in vacuum and with the Gaussian 09 program package.73
2.1.4. Electrochemistry. The electrochemical characterization (cyclic
voltammetry) for the metal complexes herein reported was performed in
methylene chloride (dichloromethane, DCM)/0.1 M tetrabutylammo-
nium hexafluorophosphate (TBAH). The concentration of the samples
was 0.5 mM. Glassy carbon was employed as the working electrode,
platinum wire as the counter electrode, and silver wire as the quasi-
reference electrode (QRE). DCM (Acros Organics, 99.8%, extra dry
over molecular sieves) was used as received without any further
purification. TBAH (electrochemical grade, g99%, Fluka) was used as
the supporting electrolyte, which was recrystallized from a 1:1 ethanolꢀ
water solution and dried at 60 °C under vacuum.
2.2. Synthesis. 2.2.1. Pyridine-2,6-bis(carboximidhydrazide) (1).
To a solution of 2,6-pyridinedicarbonitrile (20 g, 0.155 mol) in ethanol
(1 L) was added hydrazine monohydrate (151 mL, 3.1 mol). The
reaction mixture was stirred at room temperature (rt) overnight, yielding
a pale yellow precipitate in a yellow solution. The precipitate was filtered
1
off, washed with cold ethanol, and dried (12 g, 40%). H NMR (300
MHz, dimethyl sulfoxide (DMSO)): δ 7.82 (d, J = 0.8 Hz, 1H),
7.80ꢀ7.78 (m, 2H), 7.65 (dd, J = 8.5, 7.1 Hz, 2H), 6.07 (s, 4H), 5.28
(s, 4H). MS (ESI+, MeOH, m/z): calcd for [M + H]+, 194.11487;
found, 194.11472 ([M + H]+).
2.2.2. N0,N000-(Pyridine-2,6-diylbis(iminomethylene))bis(adamantane-
1-carbohydrazide) (2). A flame-dried, nitrogen-purged Schlenk tube
was loaded with 1 (5 g, 26 mmol) and sodium carbonate (6.1, 58 mmol),
evacuated, gently heated, and refilled with nitrogen after being cooled to
rt. Next, dry N,N-dimethylformamide (N,N-DMF; 160 mL) was added,
and the suspension was cooled to 0 °C. In a separately prepared Schlenk
tube, a 1-adamantanecarbonyl chloride (10.3 g, 52 mmol) solution in dry
N,N-DMF (60 mL) was prepared the same way as above. This solution
For the electrochemical experiments, a CHI750C electrochemical
workstation (CH Instruments, Inc., Austin, TX) was used. The electro-
chemical experiments were performed in a glass cell under an Ar
atmosphere. To minimize the ohmic drop between the working and
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Scheme 1. General Synthetic Procedure for Compounds 1ꢀ5
was then slowly added to the cooled suspension under strong stirring. The
pale yellow suspension cleared for some time to a darker yellow solution,
except for the sodium carbonate, followed by the precipitation of a yellow
solidovernight while the reactionmixture was allowed towarm to rt. Then
the reaction volume was doubled by the addition of water, yielding more
precipitate. This suspension wasstirred strongly for another 1.5h and then
filtered and washed with water thoroughly. The yellow solid was dried
under vacuum and used as is for the next step. (11 g, 84%).
in a small amount of DCM and precipitated with hexane. The fine
precipitate was collected via centrifugation and dried under vacuum.
Overall yields after purification were close to 10%.
Data for 6a. 1H NMR (400 MHz, CD2Cl2): δ 10.01 (dd, J = 5.4, 1.3
Hz, 2H), 7.87 (dd, J = 8.1, 7.7 Hz, 1H), 7.58 (d, J = 7.9 Hz, 2H), 7.43 (d,
J = 6.7 Hz, 2H), 2.81ꢀ2.70 (m, 2H), 2.10 (s, 14H), 1.82 (s, 12H), 1.74
(s, 3H), 1.57 (s, 6H), 1.42ꢀ1.33 (m, 4H). 13C NMR (75 MHz,
CD2Cl2): δ 171.92 (s), 163.16 (s), 156.54 (s), 154.13 (s), 150.91 (s),
142.76 (s), 126.59 (s), 116.23 (s), 42.50 (s), 37.47 (s), 35.93 (s), 35.84
(s), 31.86 (s), 30.03 (s), 29.37 (s), 22.98 (s), 14.26 (s). MS (ESI+,
MeOH, m/z): calcd for [M + H]+, 824.37; found, 824.3721 ([M + H]+);
calcd for [M + Na]+, 846.35; found, 846.3505 ([M + Na]+). Anal. Calcd
for C39H48N8Pt: C, 56.85; H, 5.87; N, 13.60. Found: C, 56.51; H, 5.73;
N, 13.53.
2.2.3. N0,N000-(Pyridine-2,6-diylbis(iminomethylene))bis(4-methylbenzo-
hydrazide) (3). The synthesis was similar to the preparation of 2 using
p-tolyl chloride. The product was pale yellow (10 g, 88%).
2.2.4. 2,6-Bis(3-((3R,5R,7R)-adamantan-1-yl)-1H-1,2,4-triazol-5-yl)-
pyridine (4). Product 2 was suspended in ethylene glycol in a 1 mL/100
mg ratio in an open round-bottom flask. This suspension was heated to
180 °C, eliminating water. Once the solution was clear, the reaction
mixture was heated under reflux for another hour. After the reaction
mixture was cooled to rt, the product was precipitated with water and
stirred strongly for 1 h more. The white solid was filtered, washed with
Data for 6b. 1H NMR (300 MHz, CD2Cl2): δ 10.15 (d, J = 6.9 Hz,
2H), 7.85 (s, 1H), 7.60 (d, J = 7.9 Hz, 2H), 7.54 (d, J = 7.0 Hz, 2H), 2.12
(s, 12H), 2.11ꢀ2.06 (m, 6H), 1.84 (s, 12H), 1.79ꢀ1.69 (m, 6H),
1.32ꢀ1.25 (m, 6H), 1.13ꢀ0.98 (m, 6H), 0.88 (t, J = 7.3 Hz, 9H). 13C
NMR (75 MHz, CD2Cl2): δ 171.23 (s), 162.82 (s), 162.17 (s), 153.52
(s), 150.37 (s), 142.24 (s), 124.62 (s), 116.02 (s), 44.53 (s), 42.37 (s),
37.34 (s), 36.97 (s), 35.65 (s), 29.33 (d, J = 15.1 Hz), 26.04 (s), 23.77
(s), 14.21 (s). MS (ESI+, MeOH, m/z): calcd for [M + H]+, 936.50;
1
water, and dried under vacuum (8.5 g, 85%). H NMR (300 MHz,
DMSO): δ 8.08 (s, 2H), 8.07ꢀ7.99 (m, 1H), 4.50 (s, 2H), 2.05 (s, 6H),
2.02 (s, 12H), 1.76 (s, 12H). MS (ESI+, MeOH, m/z): calcd for [M +
H]+, 482.30; found, 482.3026 ([M + H]+).
2.2.5. 2,6-Bis(3-(p-tolyl)-1H-1,2,4-triazol-5-yl)pyridine (5). The pre-
paration was similar to that of 4, starting from 3. (6 g, 65%). 1H NMR
(300 MHz, DMSO): δ 14.50 (s, 2H), 8.26ꢀ8.09 (m, 4H), 8.03 (dd, J =
8.1, 3.1 Hz, 3H), 7.34 (d, J = 7.0 Hz, 4H), 2.38 (s, 6H). MS (ESI+,
MeOH, m/z): calcd for [M + H]+, 394.18; found, 394.1775 ([M + H]+);
calcd, for [M + Na]+, 416.43; found, 416.1593 ([M + Na]+).
2.2.6. General Procedure for the Synthesis of the Pt(II) Complexes
6a, 6b, 7a, and 7b). The tridentate ligand (1.1 equiv), PtCl2-
(DMSO)2, and the solvent mixture (2-methoxyethanol/water, 3:1)
were placed in a round-bottom flask. The according alkyl-substituted
pyridine derivative was added, and the mixture was then heated to
83 °C under nitrogen overnight. The precipitation of the product
started within 30 min. After the solution was cooled to rt, its volume
was doubled by adding water to precipitate all the product. It was
collected by filtration, washed with water, and dried. The crude
product was then dissolved in DCM and adsorbed onto neutral
alumina. This was loaded onto a column packed with neutral alumina,
and a column chromatographic separation was performed using
DCM/methanol (99:1) as the eluent. The first fraction was the
product, which was collected and dried. After that, it was dissolved
found, 936.4994 ([M + H]+). Anal. Calcd for C47H64N8Pt 0.5CH2Cl2: C,
3
58.30; H, 6.69; N, 11.45. Found: C, 58.19; H, 6.43; N, 12.11.
Data for 7a. 1H NMR (400 MHz, CD2Cl2): δ 9.82 (d, J = 6.6 Hz,
2H), 8.01 (t, J = 11.0 Hz, 4H), 7.56ꢀ7.50 (m, 1H), 7.43 (d, J = 7.5 Hz,
2H), 7.28 (d, J = 7.9 Hz, 4H), 7.21 (d, J = 6.4 Hz, 2H), 2.43 (s, 6H), 1.49
(s, 2H), 1.31 (ddd, J = 22.7, 15.5, 8.5 Hz, 6H), 0.92 (t, J = 7.1 Hz, 3H).74
MS (ESI+, CHCl3, m/z): calcd for [M + H]+, 736.25; found, 736.24929
([M + H]+). Anal. Calcd for C33H32N8Pt 0.5CH2Cl2: C, 51.20; H,
3
4.27; N, 14.40. Found: C, 50.93; H, 3.9; N, 14.66.
1
Data for 7b. H NMR (300 MHz, CD2Cl2): δ 10.17ꢀ10.12 (m,
2H), 8.12 (d, J = 8.1 Hz, 4H), 7.99ꢀ7.94 (m, 1H), 7.73 (d, J = 8.0 Hz,
2H), 7.65ꢀ7.59 (m, 2H), 7.29 (d, J = 7.9 Hz, 4H), 2.42 (s, 6H), 1.74
(s, 6H), 1.34ꢀ1.28 (m, 6H), 1.09 (s, 6H), 0.90 (t, J = 7.3 Hz, 9H). 13C
NMR (75 MHz, CD2Cl2): δ 164.24 (s), 162.69 (s), 161.80 (s), 153.47
(s), 150.18 (s), 142.77 (s), 138.96 (s), 130.42 (s), 129.72 (s), 126.55 (s),
124.89 (s), 116.75 (s), 44.72 (s), 37.12 (s), 26.20 (s), 23.93 (s), 21.72
(s), 14.40 (s). MS (ESI+, CHCl3, m/z): calcd for [M + H]+, 848.37;
found, 848.37243 ([M + H]+); calcd for [M + Na]+, 870.35; found
870.35567 ([M + Na]+). Anal. Calcd. for C41H48N8Pt CH2Cl2: C,
3
54.07; H, 5.40; N, 12.01. Found: C, 53.25; H, 5.56; N, 13.24.
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Scheme 2. Synthesis of Complexes 6a, 6b, 7a, and 7b
lead to insoluble, nonluminescent products, as previously reported
for our analogous bistetrazolylplatinum(II) complexes.62
Complexes 6a and 6b are more soluble in chlorinated solvents
than the tolyl-terminated ones due to the two bulky adamantyl
substituents on the triazoles. To further enhance the solubility
without affecting the emission color, we introduced either a linear
(C5) or a branched (C13) alkyl substituent in the 4-position on
the ancillary pyridine ligand. Complexes 6a and 6b are both
soluble in chloroform, and the branched tail was especially useful
to enhance the solubility of 7b, if compared with 7a. The
complexes have been characterized by high-resolution mass
1
spectrometry, H and 13C NMR spectroscopy, and elemental
analysis (see the Experimental Section and Figure S1, Supporting
Information).
3.2. Absorption Spectroscopy and Frontier Orbitals. The
absorption spectra of complexes 6a and 7a in chloroform are
depicted in parts a and b, respectively, of Figure 1 (for the sake of
clarity, the similar absorption spectra of 6b and 7b are shown in
the Supporting Information, Figure S2). The molar absorption
coefficients are collected in Table 1.
Complexes 6a and 7a were also investigated by means of DFT,
and the corresponding computational data are summarized in
Tables S1 and S2 of the Supporting Information. Their ground-
state geometries were optimized (see parts a and b, respectively,
of Figure S3, Supporting Information), along with other sub-
stitution patterns on the triazole moieties (namely methyl,
hydrogen, and phenyl, parts a, b, and c, respectively, of Figure S4,
Supporting Information), to verify a significant correlation with
the calculated frontier orbitals (see Figure S5, Supporting
Information) and geometries obtained by DFT.
Moreover, to gain deeper insight into the nature of the
electronic transitions involved in the absorption spectra, the
complexes were investigated by using TD-DFT and the com-
puted vertical transitions described in terms of molecular orbitals
of the corresponding ground-state geometry. The calculated
energies are listed in Table S3, Supporting Information, together
with the nature of the involved orbitals and their expansion
coefficients. It can be seen that they nicely agree with the
experimental values (see Figure 1; further computational details
can be found in the Experimental Section).
The experimental spectra clearly show a broad and featureless
weak band (ε = 0.49ꢀ0.91 ꢁ 103 Mꢀ1 cmꢀ1) between 400 and
500 nm, which can be attributed to the energetically lowest lying
metal-to-ligand charge-transfer (MLCT) transitions, as already
observed for comparable compounds.60 These bands are the
S0 f S1 transitions and mainly involve the platinum d orbitals
(partially mixed with the tridentate-ligand-centered π orbitals)
and π* orbitals of the chelate (HOMO f LUMO). The more
intense bands (ε = 1.1ꢀ4.3 ꢁ 104 Mꢀ1 cmꢀ1) observed at higher
Figure 1. Absorption spectra of 6a (a) and 7a (b) in chloroform
solutions. Calculated absorption peaks in vacuum are shown for
comparison.
3. RESULTS AND DISCUSSION
3.1. Synthesis. A general overview of the synthetic strategy is
depicted in Scheme 1.
Starting from 2,6-pyridinebiscarbonitrile, the bisamidrazone 1 is
obtained upon reaction with hydrazine monohydrate. This inter-
mediate 1 is further reacted with the corresponding acyl chloride,
either adamantanecarbonylchloride to yield 2 or p-tolylcarbonyl
chloride, leading to 3. These are the direct precursors to the
substituted 1,2,4-triazoles, which are formed in a condensation
reaction in ethylene glycol at 180 °C.75 The tridentate ligands 4
and 5 and the ancillary pyridine derivatives were reacted with
PtCl2(DMSO)2, yielding complexes 6a, 6b, 7a, and 7b, as can be
seen in Scheme 2. It is interesting to notice that only the one-pot
approach yields the desired products upon purification by column
chromatography and crystallization. Attempts to first coordinate
the tridentate ligand and to leave a labile solvent molecule only
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Table 1. Photophysical Properties of Compounds 6a, 6b, 7a, and 7b
λabs [nm]
(ε ꢁ 10ꢀ3
/
λemRTa,b,c
λem77 Kd
Φem
aera,c,e deaera,c,e
Φem
τ(rt) deaera τ(rt) aera
τ(77 K)d
[μs]
kr ꢁ 10ꢀ4 knr ꢁ 10ꢀ4
complex
[Mꢀ1 cmꢀ1]) a,b
[nm]
[nm]
[μs]
[μs]
[sꢀ1
]
[sꢀ1
]
6a
265 (22),
507, 540 579 (sh) 493, 529, 569
507, 539, 574 (sh) 493, 529, 568
530, 560, 608 (sh) 514, 555,604
0.02
0.02
0.02
0.73
0.64
0.73
14.14
0.39
14.59
5.13
4.69
4.54
1.95
2.65
1.68
316 (20),
420 (0.49)
266 (33),
6b
7a
13.62
16.07
0.42
0.44
14.19
316 (30),
421 (0.91)
273 (41), 285
(43), 320 (24),
342 (8.6, sh),
433 (0.64)
14.84
(39%)
3.48
(61%),f
13.59
(81%)
3.17
(81%)g
7b
272 (40), 283
(38), 321 (23),
338 (10.5, sh),
436 (0.71)
527, 560, 603 (sh) 513, 550, 593
0.01
0.67
15.64
0.32
17.72
4.28
2.11
a In chloroform. b The abbreviation “sh” denotes a shoulder. c λexc = 315 nm (6a, 6b) and 320 nm (7a, 7b) . d In a 2-MeTHF matrix. e Quantum yields
were measured in an integrating sphere system. f Monitored at λem = 515 nm. g Monitored at λem = 600 nm.
Table 2. Frontier Orbitals Obtained from Electrochemistry
and UPSa
For complexes 6a and 6b, thus in absence of π-conjugated
tolyl moieties, the lowest LC bands are observed at about 316 nm
(Figure 1). They can be attributed to the πꢀπ* transitions of the
tridentate ligand without any contribution from the aliphatic
substituents. Nevertheless, these transitions still display a slight
MLCT character, as reflected by TD-DFT calculations (see
Table S3 and Figure S5a, Supporting Information). Finally, the
energetically higher bands are due to πꢀπ* transitions involving
the occupied orbital located on the triazole rings and the virtual
orbital mainly located on the pyridine of the tridentate ligand
(HOMO ꢀ 1 f LUMO + 2, computed at 271 nm for 6a; see
Figure S5c). Similar explanations can be adduced for the higher
energy absorption bands displayed by 7a and 7b.
Electrochemical and UPS measurements were performed to
experimentally estimate the HOMO and LUMO levels
(Table 2). The results show that the introduction of conjugated
moieties reduces the HOMOꢀLUMO gap as expected by
destabilizing the HOMO and lowering the LUMO. Theoretical
calculations support these observations, confirming that the
destabilization of the HOMO is more pronounced than the
stabilization of the LUMO (0.29 eV vs 0.10 eV, respectively; see
Table S2, Supporting Information).
ΔEHOMOꢀLUMO
HOMOd
(eV)
complex Eox(HOMO) Ered(LUMO)
(eV)
6a
6b
7a
7b
+1.51 Vb
ꢀ1.64 Vc
3.15
3.14
2.99
2.99
ꢀ5.45
ꢀ5.71
ꢀ5.54
ꢀ5.59
(ꢀ5.84 eV)
+1.52 Vb
(ꢀ2.69 eV)
ꢀ1.61 Vc
(ꢀ5.86 eV)
+1.44 Vb
(ꢀ2.72 eV)
ꢀ1.56 Vc
(ꢀ5.77 eV)
+1.44 Vb
(ꢀ2.78 eV)
ꢀ1.55 Vc
(ꢀ5.78 eV)
(ꢀ2.79 eV)
a Samples of 0.5 mM concentration in DCM/0.1 M TBAH solutions
were investigated. Glassy carbon was employed as the working electrode,
a platinum ring as the counter electrode, and a silver wire as the reference
electrode. The scan rate was varied in the range of 0.1ꢀ5 Vsꢀ1
.
b Irreversible peak, E = Ep. Reversible peak, E = E°, calculated as the
c
mean value between the cathodic and the anodic peaks averaged in the
scan rate range of 0.1ꢀ0.5 V sꢀ1
.
d The HOMO levels of drop-casted
neat films were measured by UPS.
energy (ca. 265ꢀ321 nm) can be described as spin-allowed,
ligand-centered (LC) in nature, involving mainly the tridentate
ligand (π f π*).
The redox processes were studied by varying the scan rate in
the range of 0.1ꢀ5 V sꢀ1, and the peak currents were found to
depend linearly on the square root of the scan rate as expected for
a diffusion-controlled redox process.76 The values are collected in
Table 2; cyclic voltammograms are shown in the Supporting
Information (see Figures S6 and S7). The electrochemical
behavior in dichloromethane of the two pairs 6a/6b and 7a/7b
showed similar patterns in the negative bias, i.e., a reversible peak
in the potential range from ꢀ1.55 to ꢀ1.64 V. The first reduction
process for 6a and 6b (ꢀ1.64 V) occurs at a more negative potential
than the same process for complexes 7a and 7b (ꢀ1.55 V).
As far as complexes 7a and 7b are concerned, the absorption
bands at about 340 nm can be ascribed to a transition with mixed
MLCT/LC character (HOMO f LUMO + 1, computed at
341 nm for 7a). Indeed, the HOMO of the tolyl-substituted
triazole complexes extends beyond the triazole rings, also invol-
ving the tolyl substituents, while the LUMO + 1 is mostly
localized on the pyridine moiety of the chelate (see Figure S5a,
Supporting Information).
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Figure 3. Emission spectra of 6a (solid line), 7a (dashed line), and 7b
(dotted line) in 2-MeTHF glass matrixes at 77 K (λexc = 305 nm for 6a
and 310 nm for 7a and 7b).
Figure 2. Emission spectra of 6a (solid line) and 7a (dashed line) in
degassed chloroform solutions (λexc = 315 nm for 6a and 320 nm for 7a).
which, to the best of our knowledge, represents the highest value
reported so far for N∧N∧N Pt(II) coordination compounds and
among the highest for tridentate chelates.18,19,24,53ꢀ61 A summary
of the photophysical data is shown in Table 1.
However, this small difference represents only a weak modula-
tion, which indicates that the LUMO is mainly centered on the
pyridine moiety of the tridentate ligand. This evidence is also
supported by computational calculations (see Table S2 and
Figure S5a).
At room temperature, the complexes show identical emission
energies within the same family of compounds, i.e., 6a and 6b or 7a
and 7b. The lack of noticeable shifts within the emission spectra in
frozen 2-MeTHF matrixes at 77 K (see Table 1, Figure 3, and
Figure S11ꢀS13 in the Supporting Information) is indicative of the
In the positive bias, an irreversible oxidation wave is observed
in the range between +1.44 and +1.52 V and is mainly related to
the oxidation of the metal core.10,11,16,19,24,27,29,32,53,59,61,77ꢀ82
The results show that a shift is observed upon a change of the
functional group on the triazole moieties of the tridentate ligand.
This can be rationalized taking into consideration that the
triazole moieties contribute to the HOMO of the complexes. If
they exclusively involved the d orbitals of the Pt center, they
would be less sensitive to the change of the triazole substituents.
3.3. Photophysics. Complexes 6a and 6b show emission
spectra peaking at 509 nm in chloroform at room temperature,
whereas 7a and 7b emit at 530 nm (see Figure 2 and Figures
S8ꢀS10 in the Supporting Information). To simulate the phos-
phorescent spectra, the vertical (FranckꢀCondon) excitation
energies from the optimized ground state (S0) to the lowest lying
triplet manifold (T1) were computed at the TD-DFT level.
Indeed, this method has been already successfully used for other
phosphorescent platinum complexes.83,84 The S0 f T1 transi-
tions display mainly HOMO f LUMO character and were
calculated at 481 and 526 nm for 6a and 7a, respectively, in good
agreement with theexperimental values (see Table 1 andTable S3
in the Supporting Information).
3
weak MLCT character of the emissive excited state, which is
mainly 3LC, as can be deduced from the clear vibrational progres-
sion and long excited-state lifetimes. The only exception is
represented by 7a, which in frozen matrixes displays an additional
broad, unstructured, and red-shifted emission band centered
around 605 nm. This dual emission can be explained by the
formation of emissive aggregates. Therefore, the structured emis-
sion of the monomeric form displaying a clear vibrational progres-
sion is observed along the featureless band of the aggregates at
lower energy, leading also to biexponential excited-state lifetimes.
Thelonger component(13.6μs) corresponds to the monomer and
the shorter one (3.2 μs) to the aggregated species (see Table 1).
Ground-state aggregation is evidenced by the broad additional
metalꢀmetal-to-ligand charge-tranfer (MMLCT) band in the
excitation spectrum of the red-shifted band and cannot be
observed when monitoring the monomeric emission (see Figure
S12, Supporting Information). Indeed, the corresponding frozen
matrix appears visibly yellow, while the dilute fluid solution at
room temperature is colorless. Contrastingly, compound 7b
displays only one structured emission at 77 K, which has been
assigned to the monomeric species, indicating the lack of
aggregate formation. It is clear from this set of spectroscopic
data that adamantyl groups are bulky enough to effectively avoid
aggregation. Complexes bearing tolyl substituents, which are
planar and therefore prone to stacking, require the introduction
of a bulky, branched chain on the ancillary ligand to retain the
monomeric emission.
This red shift relative to the adamantyl-substituted com-
pounds can be rationalized considering the extended conjugation
caused by the aromatic substitution on the triazole ligand, which
leads to a smaller HOMOꢀLUMO separation (vide supra), i.e., a
lower emission energy. Extended conjugation would also account
for the slightly lower radiative rate constants of compounds 7a
and 7b, if compared with their adamantyl-substituted analogues
6a and 6b, respectively (see Table 1). On the other hand, the
radiationless deactivation is slightly slower for the comparatively
rigid aromatic moieties (see Table 1). All in all, the excited-state
lifetime is somewhat longer for tolyl-substituted complexes.
In chloroform solution, both classes of complexes display
Photoluminescence spectra of neat and 10 wt % doped PMMA
films (see Figure 4 and Figures S14ꢀS21 in the Supporting
Information) confirm the influence of the substituents. While the
adamantyl family of compounds (6a and 6b) display structured,
monomeric spectra resembling homogeneous solution, it is
evident that tolyl substitution (7a and 7b) leads to broadened
emission, proving that adamantyl moieties are useful to avoid
3
emissions from excited states with mainly LC character, which
correlates with the observed vibrational progression, long excited-
state lifetimes, and sensitivity to quenching by oxygen. The PLQYs
are remarkably high, above 65%, reaching 73% for 6a and 7a,
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Figure 4. Emission spectra of 6a (solid line) + 6b (dashed line) (a) and
7a (solid line) + 7b (dashed line) (b) in PMMA films (10 wt %, λexc
=
315 nm for 6a and 320 nm for 7a).
aggregation. The fact that the emission spectrum of 6b in PMMA
shows sharper vibrational features if compared with that of 6a,
and that the emission band of 7b is narrower than that of 7a,
indicates that the introduction of branched substituents on the
pyridine ancillary ligands favors the monomeric emission. Mea-
surements in neat films further confirm these observations.
3.4. Electroluminescence Spectra. We selected compound
6a due to the fact that it possesses the highest PLQY of the
adamantyl-substituted family, which is less prone to aggregation.
Vapor-processed electroluminescent devices were fabricated
with the following setup: ITO/40 nm of Li224/30 nm of mCP +
6% 6a/10 nm of ZrQ4/60 nm of VK953/1 nm of LiF/150 nm of
Al (ITO = indium tin oxide, mCP = 1,3-bis(9H-carbazol-9-yl)-
benzene). In this setup, Li224 (4,40-bis(dihydrodibenzazepin-
1-yl)biphenyl) is a hole transporter (HOMO, ꢀ5.09 eV; LUMO,
ꢀ1.65 eV) and VK953 (2,9-substituted anthracene) an electron
transporter (HOMO, ꢀ6.03 eV; LUMO, ꢀ2.87 eV). Both
materials are available for purchase by Sensient Imaging Tech-
nologies GmbH (see Figure S22, Supporting Information, for
the chemical formulas). The electroluminescence of our best
candidate was compared to that of a well-established green
emitter, namely, Ir(ppy)3, in a similar architecture.85 The device
characteristics are shown in Figure 5 and Table 3.
Figure 5. Device characteristics of 6a (solid line) and Ir(ppy)3 (dashed
line) in identical device architectures: (a) electroluminescence spectra
recorded at 8 V, (b) current densityꢀvoltageꢀluminance (JꢀVꢀL)
curve, (c) luminous efficiency (ηc) and power efficiency (ηp) versus
current density J.
Table 3. Device Characteristics
a
Von
Lmax
ηc
ηp
λmax
complex (V) (cd mꢀ2
)
(cd Aꢀ1
)
(lm Wꢀ1
)
(nm)
6a
4.75
5623
15.153
16.394
7.0108
7.5445
510 (1), 544 (0.74)
517 (1)
Ir(ppy)3 3.75
22545
a Von = turn-on voltage.
The aim of these tests was to observe the actual color and
aggregation tendency of 6a in a running device configuration to
assess the effectiveness of our design strategy. Even though this setup
is not optimized and therefore does not achieve the highest
performances reported for Ir(ppy)3, it provides a suitable way to
compare the two green emitters. As can be seen in Figure 5a,
compound 6a is able to match the electroluminescence spectrum of
Ir(ppy)3. Furthermore, color purity is improved by a double-peak
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emission with a maximum at 510 nm and a second one at 544 nm,
which is slightly blue-shifted and more structured than the reference
spectrum. It is clear that, in the running device configuration,
aggregation is hindered and the clean monomer emission is retained.
This is relevant for OLED applications showing that the color purity
can be improved by tailoring the substitution pattern of the ligand.
Comparing the device performances, a slightly higher turn-on voltage
(Von) relative to the reference can be observed, with 4.75 V for 6a
and 3.75 V for Ir(ppy)3 (Figure 5b). These values are in a
comparable range and show potential for optimization by using
matrix materials that are tailored for each emitter. Further differences
between the tested materials can be observed in Figure 5c depicting
the current efficiency (ηc) and the power efficiency (ηp). On one
hand, 6a shows a maximum current efficiency of 15.2 cd Aꢀ1, which
is similar to the value measured for Ir(ppy)3 (16.4 cd Aꢀ1), but the
efficiencies of 6a drop at higher current densities, whereas the
reference device only slightly decreases in the same range. A
comparable trend is apparent in the power efficiencies, with similar
maximum values at low current densities (6.8 lm Wꢀ1 for 6a and
7.5 lm Wꢀ1 for the reference device). The performance of 6a at a
current density of 67 mA cmꢀ2 is slightly lower than that for
Ir(ppy)3, demonstrating, however, that platinum complexes with
tridentate N∧N∧N ligands can indeed constitute interesting alter-
natives to iridium complexes with good color purity.
(5) Chien, C.-H.; Liao, S.-F.; Wu, C.-H.; Shu, C.-F.; Chang, S.-Y.;
Chi, Y.; Chou, P.-T.; Lai, C.-H. Adv. Funct. Mater 2008, 18, 1430.
(6) Angulo, G.; Kapturkiewicz, A.; Chang, S.-Y.; Chi, Y. Inorg. Chem.
Commun. 2009, 12, 378.
(7) Carlson, B.; Eichinger, B. E.; Kaminsky, W.; Bullock, J. P.;
Phelan, G. D. Inorg. Chim. Acta 2009, 362, 1611.
(8) Lee, T.-C.; Hung, J.-Y.; Chi, Y.; Cheng, Y.-M.; Lee, G.-H.; Chou,
P.-T.; Chen, C.-C.; Chang, C.-H.; Wu, C.-C. Adv. Funct. Mater. 2009,
19, 2639.
(9) Vezzu, D. A. K.; Deaton, J. C.; Jones, J. S.; Bartolotti, L.; Harris,
C. F.; Marchetti, A. P.; Kondakova, M.; Pike, R. D.; Huo, S. Inorg. Chem.
2010, 49, 5107.
(10) Wang, Z.; Turner, E.; Mahoney, V.; Madakuni, S.; Groy, T.; Li,
J. Inorg. Chem. 2010, 49, 11276.
(11) Yang, X.; Wang, Z.; Madakuni, S.; Li, J.; Jabbour, G. E. Adv.
Mater. 2008, 20, 2405.
(12) Zhou, G.; Wang, Q.; Ho, C.-L.; Wong, W.-Y.; Ma, D.; Wang, L.
Chem. Commun. 2009, 3574.
(13) Williams, J. A. G. Top. Curr. Chem. 2007, 281, 205.
(14) Lu, W.; Mi, B.-X.; Chan, M. C. W.; Hui, Z.; Che, C.-M.; Zhu, N.;
Lee, S.-T. J. Am. Chem. Soc. 2004, 126, 4958.
(15) Ma, B.; Djurovich, P. I.; Garon, S.; Alleyne, B.; Thompson,
M. E. Adv. Funct. Mater. 2006, 16, 2438.
(16) Ma, B.; Djurovich, P. I.; Thompson, M. E. Coord. Chem. Rev.
2005, 249, 1501.
(17) Ma, B.; Li, J.; Djurovich, P. I.; Yousufuddin, M.; Bau, R.;
Thompson, M. E. J. Am. Chem. Soc. 2005, 127, 28.
(18) Rochester, D.; Develay, S.; Zꢀaliꢁs, S.; Williams, J. A. G. Dalton
Trans. 2009, 2009, 1728.
4. CONCLUSIONS
(19) Schneider, J.; Du, P.; Wang, X.; Brennessel, W. W.; Eisenberg,
R. Inorg. Chem. 2009, 48, 1498.
We have shown that neutral, sublimable Pt(II) complexes
bearing tridentate N∧N∧N ligands can be designed to avoid
aggregation in a running device setup, thus improving color
purity. The facile synthesis and high PLQYs encourage further
investigations with other substitution patterns to tune the color
of the emitters in the red and blue regions of the visible spectrum.
(20) Unger, Y.; Zeller, A.; Taige, M.; Strassner, T. Dalton Trans.
2009, 4786.
(21) Kim, D.; Brꢀedas, J.-L. J. Am. Chem. Soc. 2009, 131, 11371.
(22) Kwok, C.-C.; Ngai, H. M. Y.; Chan, S.-C.; Sham, I. H. T.; Che,
C.-M.; Zhu, N. Inorg. Chem. 2005, 44, 4442.
(23) Li, M.; Chen, W.-H.; Lin, M.-T.; Omary, M. A.; Shepherd, N. D.
Org. Electron. 2009, 10, 863.
(24) Develay, S.; Blackburn, O.; Thompson, A. L.; Williams, J. A. G.
Inorg. Chem. 2008, 47, 11129.
(25) D’Andrade, B.; Brooks, J.; Adamovich, V.; Thompson, M. E.;
Forrest, S. Adv. Mater. 2002, 14, 1032.
(26) Hudson, Z. M.; Sun, C.; Helander, M. G.; Amarne, H.; Lu,
Z.-H.; Wang, S. Adv. Funct. Mater. 2010, 20, 3426.
(27) Cho, J.-Y.; Domercq, B.; Barlow, S.; Suponitsky, K. Y.; Li, J.;
Timofeeva, T. V.; Jones, S. C.; Hayden, L. E.; Kimyonok, A.; South,
C. R.; Weck, M.; Kippelen, B.; Marder, S. R. Organometallics 2007,
26, 4816.
’ ASSOCIATED CONTENT
S
Supporting Information. Absorption and emission spec-
b
tra, NMR spectra, illustrations of the optimized geometries for
the calculated compounds, and tables containing all relevant
calculated data (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
(28) Cocchi, M.; Kalinowski, J.; Fattori, V.; Williams, J. A. G.;
Murphy, L. Appl. Phys. Lett. 2009, 94, 073309.
(29) Cummings, S.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118,
Corresponding Author
*E-mail: ca.s@uni-muenster.de (C.A.S.); decola@uni-muenster.de
(L.D.C.).
1949.
(30) Chang, S.-Y.; Cheng, Y.-M.; Chi, Y.; Lin, Y.-C.; Jiang, C.-M.;
Lee, G.-H.; Chou, P.-T. Dalton Trans. 2008, 6901.
(31) Chang, S.-Y.; Kavitha, J.; Li, S.-W.; Hsu, C.-S.; Chi, Y.; Yeh,
Y.-S.; Chou, P.-T.; Lee, G.-S.; Carty, A. J.; Tao, Y.-T.; Chien, C.-H. Inorg.
Chem. 2006, 45, 137–146.
(32) Chan, S.-C.; Chan, M. C. W.; Wang, Y.; Che, C.-M.; Cheung,
K.-K.; Zhu, N. Chem.—Eur. J. 2001, 7, 4180.
(33) Adamovich, V.; Brooks, J.; Tamayo, A.; Alexander, A. M.;
Djurovich, P. I.; D’Andrade, B. W.; Adachi, C.; Forrest, S. R.; Thompson,
M. E. New J. Chem. 2002, 26, 1171.
’ ACKNOWLEDGMENT
We thank the German Federal Ministry for Education and
Research (BMBF) for funding, Project 13N10529 (So-Light).
’ REFERENCES
(1) Duati, M.; Fanni, S.; Vos, J. G. Inorg. Chem. Commun. 2000, 3, 68.
(2) Duati, M.; Tasca, S.; Lynch, F. C.; Bohlen, H.; Vos, J. G.; Stagni,
S.; Ward, M. D. Inorg. Chem. 2003, 42, 8377.
(34) Elbjeirami, O.; Rashdan, M. D.; Nesterov, V.; Rawashdeh-Omary,
M. A. Dalton Trans. 2010, 39, 9465.
(3) Hage, R.; Prins, R.; Haasnoot, J. G.; Reedijk, J.; Vos, J. G. Dalton
Trans. 1987, 1987, 1389.
(35) Garg, J. A.; Blacque, O.; Fox, T.; Venkatesan, K. Inorg. Chem.
2010, 49, 11463.
(36) Armaroli, N.; Accorsi, G.; Cardinali, F.; Listorti, A. Top. Curr.
Chem. 2007, 280, 69.
(4) Mauro, M.; Quartapelle Procopio, E.; Sun, Y.; Chien, C.-H.;
Donghi, D.; Panigati, M.; Mercandelli, P.; Mussini, P.; D’Alfonso, G.;
De Cola, L. Adv. Funct. Mater. 2009, 19, 2607.
3666
dx.doi.org/10.1021/cm2010902 |Chem. Mater. 2011, 23, 3659–3667

Chemistry of Materials
ARTICLE
(37) Chowdhury, S.; Patra, G. K.; Drew, M. G. B.; Chattopadhyay,
N.; Datta, D. Dalton Trans. 2000, 235.
(71) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys.
1998, 109, 8218.
(38) Miller, A. J. M.; Dempsey, J. L.; Peters, J. C. Inorg. Chem. 2007,
46, 7244.
(72) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R.
J. Chem. Phys. 1998, 108, 4439.
(39) Benedix, R.; Hennig, H.; Kunkely, H.; Vogler, A. Chem. Phys.
Lett. 1990, 175, 483.
(40) Chang, K.-H.; Huang, C.-C.; Liu, Y.-H.; Hu, Y.-H.; Chou,
P.-T.; Lin, Y.-C. Dalton Trans. 2004, 1731.
(41) Deda, M.; Ghedini, M.; Aiello, I.; Grisolia, A. Chem. Lett. 2004,
33, 1060.
(42) Ferguson, J.; Krausz, E. Inorg. Chem. 1987, 26, 1383.
(43) Gui-ying, D.; Wen-long, J.; Jin, W.; Li-Zhong, W.; Guang-de,
W.; Lin, C.; Xin-hua, O.; He-Ping, Z. Semicond. Sci. Technol. 2009,
24, 025016.
(44) Highland, R. G.; Brummer, J. G.; Crosby, G. A. J. Phys. Chem.
1986, 90, 1593.
(73) Gaussian 09, Revision B.1; Frisch, M. J.; Trucks, G. W.;
Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani,
G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato,
M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.;
Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.;
Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov,
V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant,
J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene,
M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski,
V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels,
A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.
Gaussian, Inc.: Wallingford, CT, 2009.
(45) Ho, K.-Y.; Yu, W.-Y.; Cheung, K.-K.; Che, C.-M. Dalton Trans.
1999, 1999, 1581.
(46) Liu, Q.-D.; Wang, R.; Wang, S. Dalton Trans. 2004, 2004, 2073.
(47) Roh, S.-G.; Kim, Y.-H.; Seo, K. D.; Lee, D. H.; Kim, H. K.; Park,
Y.-I.; Park, J.-W.; Lee, J.-H. Adv. Funct. Mater. 2009, 19, 1663.
(48) Sapochak, L. S.; Benincasa, F. E.; Schofield, R. S.; Baker, J. L.;
Riccio, K. K. C.; Fogarty, D.; Kohlmann, H.; Ferris, K. F.; Burrows, P. E.
J. Am. Chem. Soc. 2002, 124, 6119.
(74) The corresponding 13C NMR spectrum of the monomer could
not be measured due to aggregation of the compound at the concentra-
tion necessary for this measurement. These characteristics will be
discussed in a separate publication.
(49) Son, H.-J.; Han, W.-S.; Chun, J.-Y.; Kang, B.-K.; Kwon, S.-N.;
Ko, J.; Han, S. J.; Lee, C.; Kim, S. J.; Kang, S. O. Inorg. Chem. 2008,
47, 5666.
(75) Orselli, E.; Kottas, G. S.; Konradsson, A. E.; Coppo, P.;
Fr€ohlich, R.; De Cola, L.; van Dijken, A.; B€uchel, M.; B€orner, H. Inorg.
Chem. 2007, 46, 11082.
(50) Vellis, P. D.; Mikroyannidis, J. A.; Lo, C.-N.; Hsu, C.-S.
(76) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals
J. Polym. Sci., Part A 2008, 46, 7702.
and Applications, 2nd ed.; Wiley: New York, 2001.
(51) Zhang, H.-Y.; Ye, K.-Q.; Zhang, J.-Y.; Liu, Y.; Wang, Y. Inorg.
Chem. 2006, 45, 1745.
(52) Develay, S.; Williams, J. A. G. Dalton Trans. 2008, 4562.
(53) Tang, R. P.-L.; Wong, K. M.-C.; Zhu, N.; Yam, V. W.-W. Dalton
Trans. 2009, 2009, 3911.
(77) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.;
Bau, R.; Thompson, M. E. Inorg. Chem. 2002, 41, 3055.
(78) Danilov, E. O.; Rachford, A. A.; Goeb, S.; Castellano, F. N.
J. Phys. Chem. A 2009, 113, 5763.
(79) Garner, K. L.; Parkes, L. F.; Piper, J. D.; Williams, J. A. G. Inorg.
Chem. 2010, 49, 476.
(54) Jarosz, P.; Lotito, K.; Schneider, J.; Kumaresan, D.; Schmehl, R.;
Eisenberg, R. Inorg. Chem. 2009, 48, 2420.
(55) Zhang, H.; Zhang, B.; Li, Y.; Sun, W. Inorg. Chem. 2009, 48,
3617.
(80) Hua, F.; Kinayyigit, S.; Cable, J. R.; Castellano, F. N. Inorg.
Chem. 2006, 45, 4304.
(81) Rachford, A. A.; Hua, F.; Adams, C. J.; Castellano, F. N. Dalton
Trans. 2009, 2009, 3950.
(56) Tam, A. Y.-Y.; Wong, K. M.-C.; Yam, V. W.-W. J. Am. Chem. Soc.
2009, 131, 6253.
(82) Hirani, B.; Li, J.; Djurovich, P. I.; Yousufuddin, M. Inorg. Chem.
2007, 46, 3865.
(83) Hua, F.; Kinayyigit, S.; Rachford, A. A.; Shikhova, E. A.; Goeb,
S.; Cable, J. R.; Adams, C. J.; Kirschbaum, K.; Pinkerton, A. A.;
Castellano, F. N. Inorg. Chem. 2007, 46, 8771.
(84) Wang, K.-W.; Chen, J.-L.; Cheng, Y.-M.; Chung, M.-W.; Hsieh,
C.-C.; Lee, G.-H.; Chou, P.-T.; Chen, K.; Chi, Y. Inorg. Chem. 2010,
49, 1372.
(57) Williams, J. A. G. Chem. Soc. Rev. 2009, 38, 1783.
(58) Williams, J. A. G.; Beeby, A.; Davies, E. S.; Weinstein, J. A.;
Wilson, C. Inorg. Chem. 2003, 42, 8609.
(59) Farley, S. J.; Rochester, D. L.; Thompson, A. L.; Howard,
J. A. K.; Williams, J. A. G. Inorg. Chem. 2005, 44, 9690.
(60) Shao, P.; Li, Y.; Azenkeng, A.; Hoffmann, M. R.; Sun, W. Inorg.
Chem. 2009, 48, 2407.
(61) Schneider, J.; Du, P.; Jarosz, P.; Lazarides, T.; Wang, X.;
Brennessel, W. W.; Eisenberg, R. Inorg. Chem. 2009, 48, 4306.
(62) Strassert, C. A.; Chien, C.-H.; Galvez Lopez, M. D.; Kourkou-
los, D.; Hertel, D.; Meerholz, K.; De Cola, L. Angew. Chem., Int. Ed. 2011,
50, 946.
(85) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.;
Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4.
(63) Yuen, M.-Y.; Roy, V. A. L.; Lu, W.; Kui, S. C. F.; Tong, G. S. M.;
So, M.-H.; Chui, S. S.-Y.; Muccini, M.; Ning, J. Q.; Xu, S. J.; Che, C.
Angew. Chem., Int. Ed. 2008, 47, 9895.
(64) Lu, W.; Chui, S. S.-Y.; Ng, K.-M.; Che, C.-M. Angew. Chem., Int.
Ed. 2008, 47, 4568.
(65) Lu, W.; Chen, Y.; Roy, V. A. L.; Chui, S. S.-Y.; Che, C.-M.
Angew. Chem., Int. Ed. 2009, 48, 7621.
(66) Chen, Y.; Li, K.; Lu, W.; Chui, S. S.-Y.; Ma, C.-W.; Che, C.-M.
Angew. Chem., Int. Ed. 2009, 48, 9909.
(67) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77,
3865.
(68) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1997, 78,
1396.
(69) Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110, 6158.
(70) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon,
M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654.
3667
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