Design, synthesis, and optoelectronic properties of dendrimeric Pt(II) complexes and their ability to inhibit intermolecular interaction

Article abstract of DOI:10.1021/ic402099x

Dendrimeric Pt(II) complexes [(C^N)Pt(dpm)] and [Pt(C^N)₂] (Hdpm = dipivaloylmethane, HC^N = 1,2-diphenylbenzoimidazole and its derivatives containing the carbazole dendrons) have been synthesized and characterized systematically. All of the complexes display green emission in the range of 495-535 nm that originated from the 360-440 nm absorption bands, which are assigned to dπ(Pt)→π*(L) metal-to-ligand charge transfer (MLCT) mixed with intraligand π(L)→π*(L) transition. Solution photoluminescence quantum yield (φ_p 0.26-0.31) of the heteroleptic complexes [(C^N)Pt(dpm)] obviously increases when compared with that of complex [(C^N)Pt(acac)]. Organic light-emitting diode devices based on these Pt(II) complexes with a multilayer configuration were fabricated and gave desirable electroluminescent (EL) performances, such as non- or less red-shifted EL spectra, in comparison with the photoluminescence spectra and slow efficiency roll-off with increasing brightness or current density. Complex [(t-BuCzCzPBI)Pt(dpm)] (where t-BuCzCzPBI = 1-(4-(3,6-di-(3,6-di-t-butyl- carbazol-9-yl))carbazol-9-yl)phenyl-2-phenylbenzoimidazole) showed the best performance, with a maximum current efficiency of 29.31 cd/A and a maximum external quantum efficiency (EQE) of 9.04% among the fabricated devices. Likewise, for homoleptic [Pt(t-BuCzCzPBI)₂] dendrimer, the powder φ_p (0.14) and maximum EQE (0.74%) improve by 7 and 7.4 times, respectively, as high as they do for nondendrimeric [Pt(1,2- diphenylbenzoimidazole)₂] (0.02, 0.10%), although its efficiency is still lower than that of the heteroleptic counterpart due to the severely distorted square-planar geometry of the emitting core. These results reveal that large steric hindrance from ancillary ligand (dpm) or the homoleptic conformation can effectively inhibit intermolecular interaction for these dendrimeric Pt(II) complexes.

Full text of DOI:10.1021/ic402099x

Article
pubs.acs.org/IC
Design, Synthesis, and Optoelectronic Properties of Dendrimeric
Pt(II) Complexes and Their Ability to Inhibit Intermolecular
Interaction
Hui Li,† Jing Li,†,‡ Junqiao Ding,† Wei Yuan,†,‡ Zilong Zhang,† Luyi Zou,†,§ Xingdong Wang,†
Hongmei Zhan,*,† Zhiyuan Xie,† Yanxiang Cheng,† and Lixiang Wang†
†State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, China
‡University of Chinese Academy of Sciences, Beijing 100039, China
§State Key Laboratory of Theoretical and Computational Chemistry, Institution of Theoretical Chemistry, Jilin University,
Changchun 130023, China
S
* Supporting Information
ABSTRACT: Dendrimeric Pt(II) complexes [(C∧N)Pt(dpm)] and [Pt(C∧N)2] (Hdpm = dipivaloylmethane, HC∧N = 1,2-
diphenylbenzoimidazole and its derivatives containing the carbazole dendrons) have been synthesized and characterized
systematically. All of the complexes display green emission in the range of 495−535 nm that originated from the 360−440 nm
absorption bands, which are assigned to dπ(Pt)→π*(L) metal-to-ligand charge transfer (MLCT) mixed with intraligand π(L)→
π*(L) transition. Solution photoluminescence quantum yield (φp 0.26−0.31) of the heteroleptic complexes [(C∧N)Pt(dpm)]
obviously increases when compared with that of complex [(C∧N)Pt(acac)]. Organic light-emitting diode devices based on these
Pt(II) complexes with a multilayer configuration were fabricated and gave desirable electroluminescent (EL) performances, such
as non- or less red-shifted EL spectra, in comparison with the photoluminescence spectra and slow efficiency roll-off with
increasing brightness or current density. Complex [(t-BuCzCzPBI)Pt(dpm)] (where t-BuCzCzPBI = 1-(4-(3,6-di-(3,6-di-t-butyl-
carbazol-9-yl))carbazol-9-yl)phenyl-2-phenylbenzoimidazole) showed the best performance, with a maximum current efficiency
of 29.31 cd/A and a maximum external quantum efficiency (EQE) of 9.04% among the fabricated devices. Likewise, for
homoleptic [Pt(t-BuCzCzPBI)2] dendrimer, the powder φp (0.14) and maximum EQE (0.74%) improve by 7 and 7.4 times,
respectively, as high as they do for nondendrimeric [Pt(1,2-diphenylbenzoimidazole)2] (0.02, 0.10%), although its efficiency is
still lower than that of the heteroleptic counterpart due to the severely distorted square-planar geometry of the emitting core.
These results reveal that large steric hindrance from ancillary ligand (dpm) or the homoleptic conformation can effectively inhibit
intermolecular interaction for these dendrimeric Pt(II) complexes.
strated the ability to fabricate highly efficient electrolumines-
cent devices. However, Pt(II) complexes have strong axial
intermolecular interaction due to their square-planar coordina-
tion geometry33 and always exhibit a tendency to form
aggregates or excimers among emissive molecules. The
emissions from aggregates/excimers, attributed to metal−
INTRODUCTION
■
The study of luminescent Pt(II) complexes has obtained
additional momentum since the complexes were found to be
efficient as phosphorescent luminophores for harvesting both
electrogenerated singlet and triplet excitons to achieve 100%
internal quantum efficiency in organic light emitting diodes
(OLEDs).1−4 A recent research effort using blue,5−12
green,13−18 red,19−28 and white9,13,29−32 phosphorescent Pt(II)
complexes based on judiciously chosen ligands has demon-
Received: August 18, 2013
© XXXX American Chemical Society
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Figure 1. Schematic diagram illustrating molecular structure design.
Scheme 1. (a) Structures of Pt(II) Complexes and (b) Synthetic Route of [Pt(C∧N)2] Complexes
metal-to-ligand charge transfer (MMLCT) or excimeric ligand-
to-ligand charge transfer states,34 can be used to develop
simplified single-doped excimer-based white OLEDs,35−38 but
they also result in the occurrence of self-quenching at higher
doping concentrations and significantly affect luminescent
efficiency, color purity, stability, lifetime, etc.39,40 Effectively
inhibiting the intermolecular interaction by shielding the
emitting cores could solve the aforementioned problems and
obtain the highly monochromatic emissive Pt(II) complexes.41
For example, employing rigid and bulky architecture,42 such as
the incorporation of a camphor43/trimethylsilane44-based
substituent, is an effective method. This structure breaks the
columnar alignment of complex molecules or weakens the π−π
stacking in the solid state.45 Accordingly, the Pt(II) complexes
exhibit better device performance.15 In addition, dye
encapsulation and steric protection may also prevent the
intermolecular interaction by the isolation of the Pt(II)
complex molecule. Ikai et al. reported that the “double
decamethylene straps” facially encumbered porphyrin Pt(II)
complex is more effective in confining triplet excitons and
remarkably improved the external quantum efficiency (EQE)
values as high as 8.2%.23 Therefore, the modification of ligand
by introducing the functional groups with unique features can,
to some extent, successfully suppress the intermolecular
interaction of Pt(II) complexes and improve luminescent
performance.
Dendrimers with well-defined structure and three-dimen-
sional geometry display the ability to encapsulate emitting core
and create specific site-isolated nanoenvironments; they have
therefore been applied in phosphorescent organo-transition
metal complexes.46 Corresponding dendrimeric Ir(III), Ru(III),
and Re(I) complexes have been successfully explored as
OLEDs materials. For instance, the green-emitting OLEDs
based on a class of Ir(III) dendrimer, developed by Burn and
co-workers, have been prepared with a high current efficiency
(CE) of 55 cd/A.47 Our group has also synthesized carbazole-
based dendrimeric green48 and red49 Ir(III) emitters with the
CE of 45.7 cd/A (EQE 13.4%) and 13.0 cd/A (EQE 11.8%),
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respectively, in which carbazolyl groups act as host material
besides encapsulating emitting core and isolating emitting
molecules.46 However, there is still a little doubt that the
chemistry of Ir(III) dendrimers is directly applicable to Pt(II)
complexes with open coordination sites and fewer coordination
numbers,2 especially the ability to suppress the intermolecular
interaction. Recently, the dendrimeric Pt(II) complex [(t-
BuCzPBI)Pt(acac)] (where t-BuCzPBI indicates 1-(4-(3,6-di-t-
butylcarbazol-9-yl))phenyl-2-phenylbenzoimidazole and acac
indicates the acetylacetonato ligand), reported previously by
our group, exhibited an improved CE of 17.55 cd/A (EQE
5.62%), which was 1.6 times as high as that of nondendrimeric
[(PBI)Pt(acac)].50 The increased efficiency illustrated that the
dendrons can restrain the formation of aggregates and excimers,
but the naked ancillary β-diketonate ligand (acac) of complex
[(t-BuCzPBI)Pt(acac)] still keeps a great tendency to generate
intermolecular interaction at higher concentration (Figure 1),
which indicates that further modification work is necessary. In
light of this consideration, we designed and synthesized a new
series of the dendrimeric Pt(II) complexes, which involved
altering the ligand from acac to dipivaloylmethane (dpm) with
the bulky t-butyl group or adopting the homoleptic
conformation to achieve two-sided dendron function (Figure
1). Finally, some beneficial emitting behavior of these
dendrimeric Pt(II) complexes in doped matrixes was observed.
Using heteroleptic complex [(t-BuCzPBI)Pt(dpm)] as a
dopant, OLED devices showed a maximum CE of 24.76 cd/
A and a maximum EQE of 7.77%, significantly higher than
those doped with the [(t-BuCzPBI)Pt(acac)] complex. Further
extending the size (or generation number) of the dendrons
increased the CE and EQE of complex [(t-BuCzCzPBI)Pt-
(dpm)] (where t-BuCzCzPBI = 1-(4-(3,6-di-(3,6-di-t-butyl-
carbazol-9-yl))carbazol-9-yl)phenyl-2-phenylbenzoimidazole)
to 29.31 cd/A and 9.04%, respectively. In addition, for
homoleptic complex [Pt(t-BuCzCzPBI)2], the approach
dramatically enhanced its powder photoluminescence (PL)
quantum yield and electroluminescent (EL) performance,
although the distorted square-planar coordination geometry
of the central Pt(II) emitting core affected the efficiency of
spin−orbit coupling (SOC),51 which were thoroughly discussed
based on X-ray diffraction analysis and density functional
theory (DFT) calculations.
reported literatures.53,54 All Pt(II) complexes exhibit good
solubility in organic solvents, and their structures were verified
using 1H and 13C NMR spectroscopy, elemental analysis (EA),
and matrix-assisted laser desorption-ionization time-of-flight
(MALDI-TOF) mass spectrometry. Their thermal properties
were characterized by thermogravimetric analysis (TGA), and
the data are summarized in Table 1. These complexes have high
thermal stability, and the decomposition temperatures (278−
422 °C) are comparable to those of previously reported
[(C∧N)Pt(acac)] complexes.50
The crystal structure of complex [Pt(PBI)2], established by
X-ray diffraction (Figure 2), has a nearly cis configuration. The
bond distances and angles around the Pt atom are similar to
those of reported complexes [Pt(bpy)2], [Pt(bhq)2], and
[Pt(thq)2].51,53,54 The mutual steric congestion between H
atoms on C(2) and C(21), C(9) and C(28) [H(2)−H(21) =
3.322 Å, H(9)−H(28) = 3.313 Å] causes sensible distortion of
the two ligands in an X-shaped fashion.51 The Pt atom resides
in a deformed square-planar environment with the dihedral
angle of 30.9° between the planes of PtC(1)C(6)C(7)N(1)
and PtC(20)C(25)C(26)N(3). The large torsion from the
ideal square-planar configuration of center Pt(II) emitting core
might result in weaker emission for those [Pt(C∧N)2]
complexes at room temperature (RT).41
In the crystal packing, [Pt(PBI)2] molecules stacked as
dimers, one molecule of which oriented at a center of inversion
relative to the other, and are similar to [(C∧N)Pt(acac)] and
other Pt(II) species (Figure 2b).5,50 The shortest Pt−Pt
separation was determined to be 7.903 Å, ruling out the
presence of any significant Pt−Pt interaction (Figure 2). Part of
the overlap between the benzoimidazole fragments in the
dimeric units is noted with C atom-to-phenyl separation of
3.579 Å. Other interactions are from N-phenyl with the
benzoimidazole unit and Pt atom, with the distances of 3.341
and 3.601 Å, respectively. The weak π−π and d-π interactions,
in principle, are unfavorable for emission,33,39,40,55 which could
be suppressed by introducing dendron groups as discussed
below in the complexes [Pt(t-BuCzPBI)2] and [Pt(t-
BuCzCzPBI)2] presented in this work.
Photophysical Properties. Figure 3 shows the absorption
spectra and the PL spectra of the Pt(II) complexes in DCM,
and the corresponding data are listed in Table 1. The other RT
and low-temperature (77 K) solution and solid-state emission
spectra were recorded for the Pt(II) complexes (Supporting
Information, Figures S1−S3). All of the complexes exhibit
intense absorption bands below 350 nm that originate from the
spin-allowed π→π* transitions localized on the cyclometalating
ligands (1LC), and relatively weak absorption peaks observed in
the 360−400 nm region are attributed to spin-allowed singlet
dπ(Pt)→π*(L) metal-to-ligand charge transfer (MLCT) mixed
with intraligand π(L)→π*(L) transition. Weaker absorption
shoulders at the long-wavelength tail of each complex are
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The structures and
synthetic routes of the new dendrimeric Pt(II) complexes are
shown in Scheme 1. The complexes [(C∧N)Pt(dpm)] were
synthesized by a two-step procedure, referring to previous
reports for [(C∧N)Pt(acac)] complexes.5,50 The homoleptic
complexes [Pt(C∧N)2] were obtained in moderate yield via
cyclometalation, as illustrated in Scheme 1b, which involved an
in situ generation of the lithiated ligands at low temperature,
followed by treatment with a solution of trans-Pt(SEt2)2Cl2 (Et
= ethyl), a desired Pt(II) precursor, in tetrahydrofuran (THF).
The necessary precautions should be adopted in the
purification of the objective products because they are
photoactive in several solvents such as dichloromethane
(DCM) and CHCl3.52 The formation of [Pt(C∧N)2]
complexes is completely geometric, and the cis isomers were
easily purified. This could be due to the strong steric hindrance
of ligands or to the trans effect exerted by the carbon donor
atom of the ligated benzene.2,43 The detailed mechanism for
the formation of [cis-Pt(C∧N)2] complexes was described in the
3
assigned to spin-forbidden MLCT transition, which becomes
partially allowed due to the strong SOC induced by the Pt
heavy metal atom.25
Like the absorption properties, the PL spectra of complex
[(C∧N)Pt(dpm)] are similar to those of the analogous complex
[(C∧N)Pt(acac)] (Figure 3).51 Three homologous complexes
with dpm ligand show a less vibronically structured RT
emission spectra between 497 and 532 nm in DCM solution at
10−5 M. Unlike previously reported nondendrimeric Pt(II)
complexes,17,33,41 the shoulder peak, which was produced by
intermolecular aggregation, nearly disappeared in the PL
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spectra (Figure 3). Moreover, a small red shift of 121−282
cm−1 in film (between 500 and 539 nm) and some
enhancement of solution PL quantum yield (φp 0.26−0.31)
are observed, compared with [(C∧N)Pt(acac)] (Supporting
Information, Figure S1 and Table 1). These results indicate that
the ancillary dpm ligand with bulky t-butyl groups does not
notably affect the photophysical properties of these [(C∧N)-
Pt(dpm)] complexes but to some extent does inhibit the
unfavorable intermolecular interaction and thus improves their
emissive efficiencies.
The homoleptic [Pt(C∧N)2] complexes show sharp vibronic
progressions of 505, 540, and 584 (shoulder) nm at 77 K,
respectively, which is the typical emission from 400 and 433 nm
absorptions of complexes, a predominant LC and MLCT triplet
state based on the absorption and excitation spectra
(Supporting Information, Figure S2). At room temperature,
only the weak emissive peaks in the region of 495−535 nm are
observed above 10−4 M in solution (Figure 3), and nearly
nonemissive peaks are observed in the film state because of self-
quenching effect (Supporting Information, Figure S4). The
SOC effect may be disturbed in severely distorted square-planar
[Pt(C∧N)2] complexes involving configuration varieties in the
excited state.42 It has been speculated that favorable MLCT
states were affected, which will be further discussed in the
section of DFT Calculation. However, they exhibit moderately
intense emission located at 514−565 nm in powder state
(Table 1), and the powder φp of complex [Pt(t-BuCzCzPBI)2]
is 0.14, which is almost 4.6 times that of complex [Pt(t-
BuCzPBI)2] and 7.0 times that of complex [Pt(PBI)2] (Table
1). The enhanced emissive quantum yield is obviously
attributed to the function of two dendrons on both sides. In
other words, although these [Pt(C∧N)2] complexes exhibit
poor phosphorescent emission arising from the distorted
emitting core, we can conclude that the homoleptic
coordination configuration of Pt(II) dendrimers is able to
address the problems produced by open coordination sites of
the square-planar Pt(II) complexes, which allow for inter-
molecular interaction or deactivating pathways with the
environment.
DFT Calculation. To understand deeply the fact that weak
emission of [Pt(C∧N)2] complexes caused by distorted square-
planar geometry of the Pt(II) emitting core, DFT calculations
on the complexes [Pt(PBI)2] and [(PBI)Pt(acac)], based on X-
ray single crystal structure, were carried out. The selected
diagrams and data are summarized in Table 2. Their lowest-
lying state (S0→T1) primarily involves the highest occupied
molecular orbital (HOMO)→the lowest unoccupied molecular
orbital (LUMO) transition {[Pt(PBI)2], 66%; [(PBI)Pt(acac)],
72%}, in which the HOMO is mainly localized at the central
Pt(II) atom and the C−H bond-actived phenyl moiety of the
C∧N ligand, while the LUMO is attributed to delocalized π*
orbital on the PBI ligand. Moreover, the contribution of d
orbital electron density (%) of Pt metal in the [Pt(PBI)2]
complex is larger than that of the complex [(PBI)Pt(acac)] for
HOMO, and smaller for LUMO. In addition, homoleptic
[Pt(C∧N)2] dendrimers possess stronger ability to suppress the
intermolecular interaction. Therefore, the homoleptic [Pt-
(C∧N)2] dendrimers would show stronger emission, compared
with heteroleptic [(C∧N)Pt(acac)] species in theory. However,
these results are inconsistent with the experimental results,
which show that the [(PBI)Pt(acac)] complex is a stronger
emitter than the [Pt(PBI)2] complex at RT. A contrast between
the two complexes is the coordination geometry: complex
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Figure 2. (a) Perspective view of complex [Pt(PBI)2] with thermal ellipsoids shown at the 30% probability level limit. Bond distances (Å): Pt−C(1)
= 1.966(4), Pt−C(20) = 2.006(4), Pt−N(1) = 2.121(3), Pt−N(3) = 2.111(3). Bond angles (deg): C(1)−Pt−C(20) = 100.14(1), N(1)−Pt−N(3) =
105.13(4), C(20)−Pt−N(3) = 79.55(2), N(1)−Pt−C(1) = 79.86(2), C(1)−Pt−N(3) = 166.93(1), C(20)−Pt−N(1) = 159.13(2); (b) Crystal
packing diagram of [Pt(PBI)2] (hydrogen atoms have been omitted for clarity).
minimum, which is an unfavorable state for luminescence
because nonradiative deactivation have more accessible
channels. As a result, the nonradiative rate constant (knr) of
complex [Pt(PBI)2] (17.34 × 105 s−1) is larger than that of
[(PBI)Pt(acac)] (3.00 × 105 s−1).56 Likewise, as shown in
Table 1, the complexes [Pt(C∧N)2] have much greater knr
(>106 s−1) with the increasing of dendrons. (2) The energy
differences between d1 and other occupied d orbitals d2, d3, or
d4 are distinctly larger, so the energy denominations which
govern the mixing between the MLCT states become greater in
the distorted square-planar (quasi-tetrahedron) configuration
than they do in the square-planar configuration; that is, the
efficiency of SOC decreased or will not be attained. This leads
to lower radiative rate constants (kr) for the transitions from T1
substates to the ground state S0 ([Pt(PBI)2], 0.52 × 105 s−1;
[(PBI)Pt(acac)], 5.50 × 105 s−1).41 The different distribution
Figure 3. Normalized absorption and PL spectra of Pt(II) complexes
diagrams of calculated energy levels for two complexes
in DCM solution at 10−5 M (PL spectra of the [Pt(C∧N)2] complexes
at 10−3 M).
supported this speculation (Table 2). Thus, according to the
equation φp = kr/(kr + knr), a smaller kr and a larger knr for
complex [Pt(PBI)2] would result in a smaller φp, and hence we
could understand why these homoleptic Pt(II) complexes
exhibit weaker emission at RT.
Electroluminescent Properties. Multilayer devices were
fabricated with a configuration of indium tin oxide/poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (ITO/PE-
DOT:PSS) (50 nm)/emissive layer (poly-(N-vinyl carbazole)
(PVK):30% [(C∧N)Pt(dpm)] complexes) (30 nm)/2,7-bis-
(diphenylphosphoryl)-9,9′-spirobi[fluorene] (SPPO13) (50
[(PBI)Pt(acac)] has ideal square-planar configuration, while
[Pt(PBI)2] displays antitwist square-planar fashion and nearly
becomes quasi-tetrahedron either in ground state or in excited
state (Table 2). This variation in geometry leads to two
consequences according to the ligand field theory: (1) the d−
d* energy splitting is reduced from the square-planar
configuration of complex [(PBI)Pt(acac)] to the quasi-
tetrahedron configuration of [Pt(PBI)2]. The d−d* excited
state of complex [Pt(PBI)2] may approach the energy
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Table 2. Summary of MO Frontier Orbitals and Energy Levels Distribution Diagram of [(PBI)Pt(acac)] and [Pt(PBI)2] by DFT
Calculation
nm)/LiF (1 nm)/Al (100 nm) (device A) or ITO/
PEDOT:PSS (50 nm)/emissive layer (4,4′-(N,N′-dicarbazole)-
biphenyl (CBP):16% [Pt(C∧N)2] complexes) (30 nm)/2,9-
dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (15 nm)/
tris(8-quinolinato)aluminium (Alq3) (40 nm)/LiF (1 nm)/Al
(100 nm) (device B) via a spin-coating process. For complexes
[(C∧N)Pt(dpm)], PVK was chosen as the host material and
SPPO13 as the electron-transporting material due to the
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obvious variation of HOMO and LUMO energy levels
compared to the [(C∧N)Pt(acac)] complexes (Table 1).50
Molecular structures of PVK and SPPO13 are shown in Figure
4. The devices’ performance characteristics are collected in
Figure 4. EL spectra of complexes [(C∧N)Pt(dpm)] and [(t-
BuCzPBI)Pt(acac)] at the driving voltage of 7 V, and the molecular
structures of SPPO13 and PVK used in fabricating the EL devices.
Table 3. Analogously to the EL characterization results of
complex [(t-BuCzPBI)Pt(acac)], using devices A and B
indicates that the alteration of device structure has no obvious
effect on the EL performance of complex [(t-BuCzPBI)Pt-
(acac)]. So the data in Table 3 are comparable for complexes
[(C∧N)Pt(dpm)] and [(C∧N)Pt(acac)] in spite of adopting a
different device structure. The 30% doping concentration of
complexes [(C∧N)Pt(dpm)] was chosen to compare effectively
the EL performance of these species with the [(t-BuCzPBI)-
Pt(acac)] complex.
The turn-on voltage distinctly decreases going from
complexes [(t-BuCzPBI)Pt(acac)], [(PBI)Pt(dpm)], and [(t-
BuCzPBI)Pt(dpm)] to [(t-BuCzCzPBI)Pt(dpm)], owing to
better solubility and high-quality film for the latter (Table 3).
As shown in Figure 5a, the maximum brightness of device A is
between 3437 and 7492 cd/m2 and is lower than it is in
previously reported devices (exceed 10000 cd/m2). This should
be attributed to the PO group in the electron-transporting
material SPPO13, which can significantly quench the
luminescent brightness.57 Figure 4 shows the EL spectra of
[(C∧N)Pt(dpm)] complexes at the driving voltage of 7 V. The
devices exhibit a strong green emission at 496−538 nm,
resembling the corresponding PL spectra in solution in spite of
the relatively high doping concentration. Moreover, the
shoulder peak at 577 nm of complexes [(C∧N)Pt(dpm)] is
obviously lower than that of complex [(t-BuCzPBI)Pt(acac)].
Such findings rule out any significant intermolecular interaction
between the doping molecules in the device condition, which
means bulky dpm ligand can effectively inhibit the
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reduce the interaction between the emissive cores of Pt(II)
complexes.48−50 As shown in Figure 5, the luminescent
efficiency at the same brightness increases when going from
[(t-BuCzPBI)Pt(acac)], [(PBI)Pt(dpm)], and [(t-BuCzPBI)-
Pt(dpm)] to [(t-BuCzCzPBI)Pt(dpm)]. As we expected,
complex [(t-BuCzCzPBI)Pt(dpm)] gives the best performance,
with a maximum CE of 29.31 cd/A, a maximum power
efficiency of 21.72 lm/W, and a maximum EQE of 9.04%
among the Pt(II) complexes, which is comparable to the
reported efficient solution-processed OLEDs incorporating
Pt(II)-based emitter.58−60 In addition, these [(C∧N)Pt(dpm)]
complexes show a slow efficiency roll-off with increasing
brightness or current density (Table 3). For example, at
brightness of 100 cd/m2 and 1000 cd/m2, the CE of complex
[(t-BuCzCzPBI)Pt(dpm)] remains 27.05 cd/A (EQE 8.34%)
and 19.78 cd/A (EQE 6.12%) and only the decreases of 7.7%
and 32.5% are observed, respectively.
OLEDs based on three homoleptic dendrimeric [Pt(C∧N)2]
complexes doped into CBP at 16% by weight were fabricated
and characterized in device B structure. On the basis of their
solution behavior, at the outset we predicted that they would
give rise to worse EL properties. The best performances were
shown by complex [Pt(t-BuCzCzPBI)2], with the maximum
brightness of 5021 cd/m2, CE of 2.71 cd/A, power efficiency of
1.68 lm/W, and EQE of 0.74% (Table 3). Such poor efficiency
stems from the relatively low φP of the distorted emitting core;
however, the data obtained from these devices display two
notable features. As illustrated in Figure 6, a decreased red shift
from 1674 to 262 cm−1 in EL spectra compared to PL spectra
in solution (Tables 1 and 3) and an increased CE from 0.34
(EQE 0.10%) to 2.71 cd/A (EQE 0.74%) are observed in order
of nondendrimeric [Pt(PBI)2], dendrimeric [Pt(t-BuCzPBI)2],
and extended dendrimeric [Pt(t-BuCzCzPBI)2] complexes.
This is clearly attributed to the effect of dendrons on decreasing
aggregation and triplet−triplet (T−T) annihilation processes in
the solid. That is to say, dendrimeric homoleptic [Pt(C∧N)2]
complexes have such ability to inhibit intermolecular
interaction depending obviously on the size of the dendrons,
which is benefit to improve the optoelectronic properties of
Pt(II) complexes, as discussed before.
CONCLUSIONS
■
In this study, we reported the synthesis, structure, and
optoelectronic investigation of a series of dendrimeric Pt(II)
complexes, which were designed to avoid the formation of
aggregates or excimers through incorporating bulky t-butyl
group or adopting homoleptic configuration. Improved
emission properties have been realized, including increased
φp, hardly changed EL spectra, enhanced EL efficiency, and
slow efficiency roll-off. The complex [(t-BuCzCzPBI)Pt(dpm)]
showed the best performance, with a peak CE of 29.31 cd/A
(EQE 9.04%) in solution-processed OLED incorporating
Pt(II)-based dendrimers so far. Homoleptic dendrimeric
complexes [Pt(C∧N)2] should be more effective for suppress-
ing the intermolecular interaction relative to heteroleptic
[(C∧N)Pt(acac)] species. However, they exhibited poor
luminescent behavior due to severely distorted square-planar
geometry of the emitting core, which had been clarified by X-
ray single diffraction analysis and DFT calculations. Never-
theless, the powder φp and the EL performance of dendrimeric
emitter are still far superior to those of a nondendrimeric one,
because of the encapsulation and isolation effect of dendrons.
Such molecular design strategy would be successful and
Figure 5. (a) Voltage−brightness, (b) brightness−CE, and (c)
brightness−EQE characteristics of complexes [(C∧N)Pt(dpm)] and
[(t-BuCzPBI)Pt(acac)].
intermolecular interaction in the presence or absence of
dendrons. It can be further verified by the brightness−efficiency
characteristics of complexes [(C∧N)Pt(dpm)]. As indicated in
Figure 5b,c, the EL performance of complex [(t-BuCzPBI)Pt-
(dpm)] with peak CE of 24.76 cd/A, power efficiency of 15.99
lm/W, and EQE of 7.77% was achieved and enhanced by about
7 cd/A, 9 lm/W and 2%, respectively, compared with complex
[(t-BuCzPBI)Pt(acac)], by simple alternation of ancillary
ligand structure. The efficiency of nondendrimeric [(PBI)Pt-
(dpm)] (19.45 cd/A, 9.52 lm/W, and 6.10%) is also much
higher than it is for dendrimeric [(t-BuCzPBI)Pt(acac)] (Table
3). In particular, dendrimeric molecular structure can further
H
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apparatus. The TGA was performed under a flow of nitrogen at a
heating rate of 10 °C/min with a Perkin-Elmer TGA 7 thermal
gravimetric analyzer. The experimental error is estimated to be 10%.
Ultraviolet−visible (UV−vis) absorption and PL spectra were
measured on a Perkin-Elmer Lambda 35 UV−vis spectrometer and
a Perkin-Elmer LS 50B spectrofluorometer in aerated solution,
respectively. Cyclic voltammetry experiments were performed with
an EG&G 283 (Princeton Applied Research) potentiostat/galvanostat
system at RT with a conventional three-electrode system consisting of
a Pt working electrode, a Pt counter electrode, and a Ag/AgCl
reference electrode. The supporting electrolyte was 0.1 M
tetrabutylammonium perchlorate (n-Bu4NClO4). The potentials are
quoted against the ferrocene standard. The lifetime of the samples was
obtained by an exponential fit of emission decay curves recorded with
the laser photolysis technique, in which the third harmonic of a
neodymium-doped yttrium aluminum garnet laser (355 nm output
and ca. ∼3 ns pulse width) was used as the excitation source, coupled
with a fast-response photomultiplier. The solution PL quantum
efficiencies were measured and calculated by a relative method using
[(ppy)Pt(acac)] (Φem = 0.15) as a reference.5 The powder and film
PL quantum efficiency was determined with an integrating sphere
under a 409 nm excitation wavelength of a He−Cd laser. The ligands
1,2-diphenylbenzoimidazole (H-PBI), 1-phenyl-2-(2-bromophenyl)-
benzoimidazole (Br-PBI), 1-(4-(3,6-di-t-butylcarbazol-9-yl))phenyl-2-
phenylbenzoimidazole (t-BuCz-H-PBI), 1-(4-(3,6-di-t-butylcarbazol-9-
yl))phenyl-2-(2-bromophenyl)benzoimidazole (t-BuCz-Br-PBI), 1-(4-
(3,6-di-(3,6 -di-t-butyl-carbazol-9-yl))carbazol-9-yl)phenyl-2-phenyl-
benzoimidazole (t-BuCzCz-H-PBI) and 1-(4-(3,6-di-(3,6-di-t-butyl-
carbazol-9-yl))carbazol-9-yl)phenyl-2-(2 -bromophenyl)-
benzoimidazole (t-BuCzCz-Br-PBI) were prepared according to the
literature procedures.48,50 trans-Pt(SEt2)2Cl2 was prepared according
to the procedure given by Kaufmann et al.61 The syntheses,
characterization, and optoelectronic properties of the [(PBI)Pt(acac)]
and [(t-BuCzPBI)Pt(acac)] complexes were previously reported by
our group.51
Preparation of Pt(II) Complexes. General procedures for the
construction of heteroleptic Pt(II) complexes: A mixture of K2PtCl4
and H−C∧N-chelate ligand (H-PBI, t-BuCz-H-PBI, t-BuCzCz-H-PBI)
(2.05 equiv.) in 2-ethoxyethanol and H2O (3:1 v/v) was stirred at 60
°C for 24, 48, and 72 h, respectively. After it was cooled to RT, the
solvent was removed by reduced pressure distillation, and the vacuum-
dried green-yellow chloro-bridged dimer was obtained. Then, the
dimer was suspended in 2-ethoxyethanol and reacted with dipivaloyl-
methane (Hdpm) (3 equiv) and anhydrous Na2CO3 (3 equiv) at 80
°C for 24, 48, and 72 h, respectively. The resulting mixture was poured
into water and extracted with DCM. The combined organic layers
were washed with water and dried over anhydrous sodium sulfate.
After the volatiles were completely removed under reduced pressure,
the residue was purified by column chromatography on silica gel using
DCM/petroleum (PE) (1:2 v/v) as the eluent to afford pure product.
[(PBI)Pt(dpm)]. Yield: 21.2%. 1H NMR (300 MHz, CDCl3, ppm): δ
8.67 (d, J = 8.1 Hz, 1H), 7.78 (d, J = 7.2 Hz, 1H), 7.69−7.67 (m, 3H),
7.53−7.49 (m, 2H), 7.38 (td, J = 8.0, 0.9 Hz, 1H), 7.25 (td, J = 8.1, 0.9
Hz, 1H), 7.10 (td, J = 7.5, 1.2 Hz, 1H), 7.03 (d, J = 7.8 Hz, 1H), 6.75
(t, J = 7.5 Hz, 1H), 6.45 (d, J = 7.5 Hz, 1H), 5.89 (s, 1H), 1.37 (s,
9H), 1.31 (s, 9H). 13C NMR (151 MHz, CDCl3, ppm): δ 193.7, 191.8,
162.5, 138.9, 138.5, 134.6, 133.8, 133.3, 129.9, 129.3, 129.2, 128.2,
127.0, 123.0, 122.9, 121.6, 115.8, 109.3, 91.9, 41.1, 40.0, 27.7, 27.6.
Anal. Calcd for C30H32N2O2Pt: C, 55.63; H, 4.98; N, 4.33. Found: C,
55.29; H, 5.13; N, 3.96%. MALDI-TOF (m/z): 648.2 [M+H]+.
Figure 6. (a) EL spectra at the driving voltage of 14 V. (b)
Brightness−CE and (c) brightness−EQE characteristics of [Pt-
(C∧N)2] complexes.
promising for developing the highly efficient Pt(II) complexes
emitting materials, once the Pt(II) complex core has perfect
square-planar geometry.
1
EXPERIMENTAL SECTION
[(t-BuCzPBI)Pt(dpm)]. Yield: 27.0%. H NMR (300 MHz, CDCl3,
■
ppm): δ 8.72 (d, J = 8.1 Hz, 1H), 8.19 (s, 2H), 7.90 (d, J = 8.7 Hz,
2H), 7.82 (d, J = 7.5 Hz, 1H), 7.74 (d, J = 8.4 Hz, 2H), 7.55 (s, 4H),
7.42 (t, J = 7.4 Hz, 1H), 7.33 (t, J = 7.4 Hz, 1H), 7.20 (d, J = 8.4 Hz,
1H), 7.16 (t, J = 7.2 Hz, 1H), 6.85 (t, J = 7.5 Hz, 1H), 6.64 (d, J = 7.8
Hz, 1H), 5.91 (s, 1H), 1.50 (s, 18H), 1.38 (s, 9H), 1.32 (s, 9H). 13C
NMR (151 MHz, CDCl3, ppm): δ 193.8, 191.9, 162.7, 142.7, 139.1,
139.0, 138.7, 137.6, 134.6, 133.2, 131.6, 130.1, 128.5, 128.4, 126.8,
123.3, 122.9, 122.86, 122.8, 122.5, 121.8, 116.0, 115.5, 109.3, 108.0,
92.0, 41.2, 40.0, 33.7, 30.9, 27.7, 27.6. Anal. Calcd for C50H55N3O2Pt:
Methods and Materials. All reactions and manipulations were
carried out under argon atmosphere. All chemicals and reagents
(analytical reagent grade) for chemical synthesis were used as received
from commercial sources. Some solvents, such as DCM, THF, toluene,
and chlorobenzene were distilled carefully by standard procedures
1
prior to use. The H and 13C NMR spectra were recorded on Bruker
Avance 300/600 MHz NMR spectrometer. Elemental analyses were
carried out using a Bio-Rad Company elemental analytical instrument.
The MALDI-TOF mass spectra were obtained on an Autoflex III TOF
I
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Inorganic Chemistry
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C, 64.92; H, 5.99; N, 4.54. Found: C, 64.63; H, 6.08; N, 4.22%.
MALDI-TOF (m/z): 925.4 [M+H]+.
were assigned with anisotropic displacement parameters, whereas
hydrogen atoms were placed at calculated theoretical positions and
included in the final cycles of refinement in a riding model along with
attached carbons.
[(t-BuCzCzPBI)Pt(dpm)]. Yield: 39.0%. 1H NMR (300 MHz,
CDCl3, ppm): δ 8.75 (d, J = 8.1 Hz, 1H), 8.30 (d, J = 2.1 Hz, 2H),
8.18 (d, J = 1.8 Hz, 4H), 8.11 (d, J = 8.7 Hz, 2H), 7.90 (d, J = 8.7 Hz,
2H), 7.86−7.82 (m, 3H), 7.72 (dd, J = 8.7, 2.1 Hz, 2H), 7.50−7.36
(m, 10H), 7.25 (d, J = 7.5 Hz, 1H), 7.18 (td, J = 7.4, 1.2 Hz, 1H), 6.91
(td, J = 7.5, 0.9 Hz, 1H), 6.72 (dd, J = 7.8, 0.6 Hz, 1H), 5.92 (s, 1H),
1.48 (s, 36H), 1.39 (s, 9H), 1.33 (s, 9H). 13C NMR (151 MHz,
CDCl3, ppm): δ 193.8, 192.0, 162.7, 141.7, 139.1, 139.0, 138.7, 138.1,
134.5, 132.9, 130.6, 130.2, 129.0, 128.5, 127.6, 125.2, 123.5, 123.4,
122.7, 122.6, 122.5, 122.1, 121.8, 118.5, 116.1, 115.2, 109.9, 109.2,
108.0, 92.0, 41.2, 40.0, 33.7, 31.0, 27.7, 27.6. Anal. Calcd for
C82H85N5O2Pt: C, 72.01; H, 6.26; N, 5.12. Found: C, 71.87; H, 6.35;
N, 4.96%. MALDI-TOF (m/z): 1367.6 [M+H]+.
DFT Calculations. All calculations have been performed on an
Origin/3900 server, using the Gaussian 03 program package.64 The
ground-state structures were optimized by the DFT using the B3PW91
hybrid functional. The “double-ξ” basis set LANL2DZ,65−67 consisting
of 18 valence electrons associated with the pseudopotential, was
employed for Pt atom and 6-31G(d) basis sets for H, C, O and N
atoms. Following each ground state optimization, the vibrational
frequencies were calculated, and the results showed that all optimized
structures were stable geometric structures. The calculated electronic
density plots for HOMO and LUMO orbitals were prepared using
GaussView 4.1.2 software. In addition, compositions of molecular
orbitals and overlap populations between molecular fragments were
analyzed using the PYMOLYZE program.68
OLED Fabrication and Measurements. The OLEDs were
fabricated with a structure of ITO/PEDOT:PSS (50 nm)/emissive
layer (PVK:30% [(C∧N)Pt(dpm)] complexes) (30 nm)/SPPO13 (50
nm)/LiF (1 nm)/Al (100 nm) (device A) or ITO/PEDOT:PSS (50
nm)/emissive layer (CBP:16% [Pt(C∧N)2] complexes) (30 nm)/BCP
(15 nm)/Alq3 (40 nm)/LiF (1 nm)/Al (100 nm) (device B). The
PEDOT:PSS and emissive layers (chlorobenzene was used as the
solvent) were spin-coated in sequence onto the precleaned and UV−
ozone-treated ITO substrate, respectively, followed by thermal
evaporation of the SPPO13 (50 nm)/ or BCP (15 nm)/Alq3 (40
nm)/, LiF (1 nm)/Al (100 nm) structure in vacuum chamber at a base
pressure of less than 4 × 10−4 Pa. The current-density−voltage and
brightness−voltage curves of the devices were measured using a
Keithley 2400 source meter with a calibrated silicon photodiode. The
EL spectra and CIE coordinates were recorded using the PR650
spectra colorimeter. All the experiments and measurements were
carried out at RT under ambient conditions.
General procedures for the synthesis of homoleptic Pt(II)
complexes: To a stirred solution of Br−C∧N-chelate ligand (Br-PBI,
t-BuCz-Br-PBI, t-BuCzCz-Br-PBI) (2.1 equiv) in anhydrous THF was
slowly added n-butyllithium (2.2 equiv, 2.5 M in hexane) at −78 °C.
After it was stirred for 45 min, a solution of trans-Pt(SEt2)2Cl2 in
anhydrous THF was added dropwise to the reaction mixture. The
resulting mixture was stirred continuously for 30 min at −78 °C; then
the temperature was allowed to rise slowly to RT. The reaction
mixture was hydrolyzed with methanol. The volatiles were removed
under reduced pressure, and the residue was recrystallized from diethyl
ether and ethanol, respectively. If the precipitate was still impure,
further purification by flash chromatography (alkaline Al2O3 filler, PE/
diethyl ether = 3:2 v/v) was carried out.
[Pt(PBI)2]. Yield: 56.5%. 1H NMR (300 MHz, CDCl3, ppm): δ 8.25
(d, J = 8.1 Hz, 1H), 7.89 (dd, J = 7.1, 1.8 Hz, 1H), 7.72−7.68 (m, 3H),
7.59−7.56 (m, 3H), 7.23 (td, J = 6.8, 1.5 Hz, 1H), 7.18 (td, J = 7.2, 1.2
Hz, 1H), 7.09 (dd, J = 10.0, 1.8 Hz, 1H), 6.82 (t, J = 7.2 Hz, 1H), 6.66
(dd, J = 7.7, 1.2 Hz, 1H). 13C NMR (75 MHz, CDCl3): δ 164.5, 150.3,
141.2, 138.1, 137.9, 136.2, 135.8, 131.3, 130.8, 130.5, 128.7, 125.4,
123.7, 123.5, 122.5, 119.4, 110.9. Anal.Calcd for C38H26N4Pt: C, 62.20;
H, 3.57; N, 7.64. Found: C, 62.57; H, 3.34; N, 7.26%. MALDI-
TOF(m/z): 733.1 [M+H]+.
ASSOCIATED CONTENT
* Supporting Information
■
S
[Pt(t-BuCzPBI)2]. Yield: 48.6%. 1H NMR (300 MHz, CDCl3, ppm):
δ 8.32 (d, J = 8.4 Hz, 1H), 8.20 (s, 2H), 7.98 (d, J = 8.4 Hz, 1H), 7.93
(d, J = 8.7 Hz, 2H), 7.81 (d, J = 8.7 Hz, 2H), 7.57 (d, J = 1.2 Hz, 4H),
7.35−7.23 (m, 4H), 6.95 (t, J = 7.5 Hz, 1H), 6.86 (dd, J = 9.0, 1.2 Hz,
1H), 1.51 (s, 18H). 13C NMR (75 MHz, CDCl3): δ 164.7, 150.4,
144.2, 141.9, 141.2, 140.4, 139.1, 138.1, 135.7, 134.1, 131.0, 130.1,
128.4, 125.4, 124.4, 124.3, 124.0, 123.8, 122.8, 119.6, 116.9, 110.9,
109.6, 35.2, 32.4. Anal. Calcd for C78H72N6Pt: C, 72.71; H, 5.63; N,
6.52. Found: C, 72.42; H, 5.85; N, 6.59%. MALDI-TOF(m/z): 1288.7
[M+H]+.
X-ray crystallographic data for [(PBI)Pt(acac)] and [Pt(PBI)2]
in CIF format; crystal data and structure refinement details;
additional photophysical data and spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: hmzhan@ciac.jl.cn.
■
[Pt(t-BuCzCzPBI)2]. Yield: 32.0%. 1H NMR (300 MHz, CDCl3,
ppm): δ 8.35 (d, J = 8.4 Hz, 1H), 8.30 (d, J = 1.8 Hz, 2H), 8.18 (d, J =
1.5 Hz, 4H), 8.13 (d, J = 8.7 Hz, 2H), 8.02 (d, J = 8.4 Hz, 1H), 7.98
(d, J = 8.7 Hz, 2H), 7.86 (d, J = 8.7 Hz, 2H), 7.74 (dd, J = 8.7, 1.8 Hz,
2H), 7.49 (dd, J = 8.7, 1.8 Hz, 4H), 7.40−7.29 (m, 8H), 7.01 (t, J = 7.5
Hz, 1H), 6.94 (d, J = 6.9 Hz, 1H), 1.48 (s, 36H). 13C NMR (75 MHz,
CDCl3): δ 164.8, 150.4, 143.1, 141.4, 140.5, 140.4, 139.4, 138.0, 135.6,
135.4, 132.1, 131.2, 130.7, 129.2, 126.7, 124.9, 124.0, 123.6, 122.8,
120.0, 116.7, 111.4, 109.5, 35.2, 32.5. Anal. Calcd for C142H132N10Pt:
C, 78.46; H, 6.12; N, 6.44. Found: C, 78.53; H, 6.12; N, 6.71%.
MALDI-TOF(m/z): 2173.1 [M+H]+.
Crystal Structure Determination. Crystals of complexes [(PBI)-
Pt(acac)] and [Pt(PBI)2] were obtained by slow evaporation of
DCM/MeOH solution at RT. The crystal data and details of data
collection and refinement were summarized in Supporting Informa-
tion, Table S3. The single-crystal X-ray diffraction experiments were
carried out using a Bruker Smart APEX diffractometer with CCD
detector and graphite monochromator, Mo Kα radiation (λ = 0.71073
Å). The intensity data were recorded with ω scan mode (187 K).
Lorentz polarization factors were made for the intensity data, and
absorption corrections were performed using SADABS program.62
The crystal structure was determined using the SHELXTL program
and refined using full matrix least-squares.63 All non-hydrogen atoms
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The work was financially supported by the National Natural
Science Foundation of China (No. 51073152 and 21204083)
and 973 Project (No. 2009CB623600).
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