Highly efficient blue phosphorescence from triarylboron-functionalized platinum(II) complexes of N-heterocyclic carbenes

Article abstract of DOI:10.1021/ja3048656

The first examples of BMes₂-functionalized NHC chelate ligands have been achieved. Their Pt(II) acetylacetonate complexes have been synthesized and fully characterized. These NHC-chelate Pt(II) compounds display highly efficient blue or blue-gr

Full text of DOI:10.1021/ja3048656

Communication
pubs.acs.org/JACS
Highly Efficient Blue Phosphorescence from Triarylboron-
Functionalized Platinum(II) Complexes of N‑Heterocyclic Carbenes
Zachary M. Hudson,† Christina Sun,† Michael G. Helander,‡ Yi-Lu Chang,‡ Zheng-Hong Lu,‡
and Suning Wang*,†
†Department of Chemistry, Queen’s University, Kingston, Ontario, K7L 3N6 Canada
‡Department of Materials Science and Engineering, University of Toronto, Ontario, M5S 3E4 Canada
S
* Supporting Information
materials may be prepared with impressive quantum yields and
ABSTRACT: The first examples of BMes2-functionalized
NHC chelate ligands have been achieved. Their Pt(II)
acetylacetonate complexes have been synthesized and fully
characterized. These NHC-chelate Pt(II) compounds
display highly efficient blue or blue-green phosphorescence
in solution (Φ = 0.41−0.87) and the solid state (Φ =
0.86−0.90). Highly efficient electroluminescent devices
based on these new Pt(II) compounds have also been
fabricated.
charge-transporting properties. As a result, phosphorescent
triarylboranes have found use as anion sensors,10 vapochromic
materials,11 and phosphorescent emitters in OLEDs,12
including the most efficient Pt(II)-based OLED reported to
date.
By combining the emissive and electron-transporting proper-
ties of the Lewis acidic triarylboron group with the strong
ligand field of N-heterocyclic carbenes, triarylboron-function-
alized NHCs would be versatile ligands that could find use in
both catalysis and advanced materials. Furthermore, in contrast
to blue phosphorescent Ir(III) compounds which have been
extensively explored, examples of efficient blue-emissive Pt(II)
compounds in OLEDs remain rare.13 Herein we report the
synthesis of the first examples of triarylboron-functionalized
NHC chelate ligands and their blue and blue-green
phosphorescent Pt(II) complexes. These compounds are highly
stable to air, moisture, and UV irradiation and have been used
as triplet emitters in efficient electrophosphorescent devices.
The synthesis of the boron-functionalized imidazolium salts 1d
and 2d are readily achieved by Debus/Radziszewski cyclization of
the phenylimidazole π-skeleton from the respective triarylboron-
functionalized anilines, which were prepared by a modified method
of Glogowski and co-workers14 (Scheme 1). Benzyl protection of
the appropriate bromoanilines followed by metal−halogen exchange
with n-BuLi and substitution with BMes2F affords the desired
amines in good overall yield after deprotection of the benzyl groups.
Cyclization with glyoxal, ammonia, and formaldehyde provides the
boron-functionalized phenylimidazoles, and upon stirring with
CH3I at room temperature the asymmetric imidazolium salts 1 and
2 may be easily isolated as white precipitates.
The synthesis of the Pt(II) complexes BC1 and BC2 is
challenging, as literature methods proved incompatible with
ligands 1 and 2. To date only two reports of carbene compounds
with the general formula [Pt(C∧C)(O∧O)] have appeared in
literature, prepared by first reacting the imidazolium salts of the
NHC ligands with Ag2O.9c,f In these cases, subsequent
transmetalation at 80 °C with [PtCl2(COD)] followed by sub-
stitution with acetylacetone in the presence of KOtBu at 100 °C
afforded the desired complexes. Unfortunately, ligands 1 and 2
were found to be unstable to Ag(I) as well as to extended heating
at high temperature under basic conditions. However, we
ince the X-ray crystal structure of a stable N-heterocyclic
S
carbene (NHC) was first determined by Arduengo in
1991,1 these compounds have gone from highly specialized
ligands to among the most widely used in organometallic
chemistry.2,3 Characterized by exceptionally strong σ-bonding
and readily tunable steric and electronic properties, NHCs have
found extensive use in both organic and transition-metal
catalysis.3 More recently, the rapid expansion of NHC research
has led to their incorporation into a wide variety of functional
materials, including antimicrobial agents,4 liquid crystals,5
supramolecular structures,6 and luminescent compounds.7
Phosphorescent transition-metal compounds are currently of
great interest as biological imaging agents, chemical sensors,
and emitters for organic light-emitting diodes (OLEDs).8 In
particular, blue phosphorescent materials represent a consid-
erable challenge, as they often suffer from poor stability and low
emission quantum yields. Transition-metal phosphors contain-
ing NHC ligands, however, show promise as a solution to this
problem. The strong ligand field of the carbene can raise the
energy of nonradiative d−d excited states on the metal center,
increasing the energy spacing with emissive excited states and
improving quantum yields.7b Furthermore, the stability of
metal−carbene bonds may also increase the operational lifetime of
these materials in organic electronic devices. On the basis of these
principles, a handful of reports have described the use of NHC
complexes of Pt(II) and Ir(III) as phosphorescent emitters for
OLEDs.9
We and others have recently shown that the phosphor-
escence of many metal complexes may be dramatically
enhanced by functionalization with a triarylboron group.10−12
This moiety is a powerful π-electron acceptor due to the empty
p orbital on boron, facilitating both charge-transfer lumines-
cence and electron transport. When protected from nucleo-
philic attack by appropriate bulky substituents, highly stable
Received: May 23, 2012
© XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
Scheme 1. Synthesis of Boron-Functionalized C∧C-chelate
a
Carbene Complexes
Figure 1. The crystal structures of BC1 (left) and BC2 (right) with
50% thermal ellipsoids and labeling schemes for key atoms. Important
bond lengths (Å) and angles ( deg), BC1: Pt(1)−C(1) 1.947(4),
Pt(1)−C(10) 1.989(4), Pt(1)−O(1) 2.095(3), Pt(1)−O(2) 2.040(3);
C(1)−Pt(1)−O(2) 170.68(13), C(10)−Pt(1)−O(1) 176.53(12).
BC2: Pt(1)−C(1) 1.912(8), Pt(1)−C(5) 1.983(13), Pt(1)−O(1)
2.068(7), Pt(1)−O(2) 2.030(8); C(1)−Pt(1)−O(2) 172.5(4), C(5)−
Pt(1)−O(1) 175.6(4).
a
Reagents and conditions: i) n-BuLi, THF, −78 °C; ii) FBMes2, THF,
−78 °C to RT; iii) Pd/C, TsOH, EtOH, 25 °C; iv) glyoxal THF/
MeOH, 25 °C; v) H2CO, NH4Cl, 25 °C; vi) H3PO4, reflux; vii)
NaOH(aq), 0 °C; viii) MeI, THF, 25 °C; ix) [PtMe2(SMe2)]2, −78 to
55 °C; x) TsOH, 25 °C; xi) Na(acac), THF/MeOH, −78 °C.
recently reported a simple one-pot procedure for the synthesis
of N∧C- and C∧C-chelate Pt(II) β-diketonate compounds,15
which was readily adapted to the synthesis of BC1 and BC2.
Deprotonation of the imidazolium salts with n-BuLi gives the
corresponding free carbenes, which are then coordinated to
Pt(II) on the addition of [PtMe2(SMe2)]2. Mild heating at
55 °C affords the corresponding Pt(C∧C)Me(SMe2) complexes
with irreversible loss of CH4. Addition of p-toluenesulfonic acid
(TsOH) induces loss of the remaining methyl group as CH4,
and addition of Na(acac) at low temperature gives the desired
complexes which can then be isolated by column chromatog-
raphy as yellow solids.
The molecular design of these complexes is intended to
achieve high-energy blue phosphorescence with maximum
quantum yield (ΦP). The C∧C chelate backbone presents a
strong ligand field to the Pt(II) center, raising the energy of
nonradiative d−d excited states and reducing thermal
quenching. The acetylacetonate (acac) ancillary ligand provides
good solubility as well as solution and solid-state stability, while
its rigid structure and high triplet energy level help to increase
ΦP. The incorporation of a BMes2 group serves to greatly
enhance metal-to-ligand charge-transfer phosphorescence.
Single crystals of both BC1 and BC2 were successfully obtained
and have been examined by X-ray diffraction analyses.16 (Figure 1)
Both molecules display highly planar geometries about the Pt(II)
center with minimal strain apparent in either structure, important
for the maximization of phosphorescent quantum yields. The
strength of the carbene donor is evident in both cases, exhibiting
C−Pt bond lengths shorter than those observed between the
Pt(II) center and the phenyl ring. The considerable trans influence
of the carbene can also be observed, with the Pt−O bond trans to
the carbene lengthened by as much as 0.05 Å relative to more
common nitrogen donors in similar N∧C chelate cyclometalated
systems.12b The crystal structures of both BC1 and BC2 show
discrete dimeric Pt−Pt stacking, with Pt−Pt distances of 3.389(2)
and 3.505(2) Å, respectively (see Supporting Information [SI]).
Both BC1 and BC2 display weak absorption bands around
375 nm (Figure 2), which are absent in the nonborylated
phenylimidazole Pt(II) complex.9f TD-DFT calculations at the
B3LYP level of theory demonstrate that the lowest triplet state
involves mainly HOMO to LUMO character (see SI). The
Figure 2. (Left) Absorption (dashed) and emission (solid) spectra of
BC1 and BC2 at 10−5 M in CH2Cl2. (Right) Phosphorescence color of
10−5 M solutions of BC1 (green-blue) and BC2 (blue) in CH2Cl2
(top) and thin films of BC1 and BC2 on quartz substrates doped at
10 wt % in PMMA (bottom), λex = 365 nm.
HOMO levels include significant electron density on the Pt(II)
centers, whereas the LUMO levels show large contributions from
the empty p orbital on boron, with significant participation of the
π-backbone of the ligands in both orbitals. The lowest-energy
electronic transitions are thus of mixed MLCT/LC character, and
it can be seen that the BMes2 group significantly enhances MLCT
in these C∧C chelate systems. The enhancement in MLCT is
accompanied by a large increase in phosphorescent quantum yield
for both complexes, relative to the parent nonborylated
Pt(C∧C)(acac) complex. Doped PMMA films (10 wt %) of
BC1 and BC2 exhibit impressive quantum yields of 90 and 86%,
respectively, compared to only 13% for an analogous control
compound lacking the BMes2 group. Despite the difficulty in
developing blue-emissive Pt complexes, the research groups of
Strassner9h and Chi13g respectively have recently reported blue-
emissive complexes with quantum yields of 50% and 56% in the
solid state, although higher values are known for blue-green
emitters.9f,13f The solid-state quantum yield of BC2 thus represents
the highest observed to date for a blue phosphorescent Pt(II)
complex.13 BC1 exhibits blue-green phosphorescence in the solid
state and solution, with an emission maximum of 478 nm in CH2Cl2.
This emission is blue-shifted by 20 nm in BC2, resulting in sky-blue
emission from the complex at λmax = 462 nm (Figure 2, Table 1).
DFT results indicate that the 5-position of the phenyl ring
makes a large atomic orbital contribution to the HOMO of the
B
dx.doi.org/10.1021/ja3048656 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Table 1. Photophysical Properties of BC1 and BC2
a
a
b
a
c
a
b
d
e
f
λmax, abs (nm), (ε [104 M−1cm−1])
λmax, EM (nm) solution /solid
τP (μs) ΦP solution /solid
E1/2 (V)
HOMO (eV) LUMO (eV)
red
BC1
BC2
381 (0.38), 344 (1.16), 316 (2.15)
371 (0.76), 356 (0.80), 324 (1.48)
478/482
6.9
3.4
0.87/0.90
0.41/0.86
−2.50
−2.49
−5.73
−5.86
−2.64
−2.65
462/464
a
b
c
Measured in degassed CH2Cl2 at 1 × 10−5 M. Doped into PMMA at 10 wt %. Solution quantum efficiencies were measured in CH2Cl2 relative to
Ir(ppy)3 = 0.97.17 Solid state quantum yields were measured using an integration sphere. All QYs are 10%. In DMF relative to FeCp2
.
d
0/+
e
f
Measured by UV photoelectron spectroscopy. Calculated from the HOMO level and the optical energy gap.
phenylimidazole π-skeleton, whereas the 4-position makes a large
contribution to the LUMO (see SI). Functionalization at either
position with the electron-withdrawing BMes2 thus decreases the
energy of the corresponding orbital, resulting in a wider optical
band gap and blue-shifted phosphorescence for BC2 compared to
those for BC1. In both cases the emission shows some vibronic
character, indicative of phosphorescence from an excited state of
both ligand-centered and MLCT character. Both BC1 and BC2
also undergo reversible reduction, indicative of electron-transporting
functionality imparted by the BMes2 group.
Because of the impressive quantum yields, both BC1 and BC2
were evaluated as phosphorescent emitters for OLEDs fabricated
by vacuum vapor deposition on ITO-coated glass substrates. Due
to the wide band gaps of these materials, care was taken to ensure
that the HOMO and LUMO energy levels of both emitters
were confined within the band gap of the host material, to facilitate
efficient trapping of both holes and electrons. Furthermore, it was
necessary to employ a host material with a sufficiently high triplet
level to ensure that excitons within the device were confined to the
dopant. On the basis of these considerations, preliminary devices
were fabricated using 4,4′-N,N′-dicarbazolylbiphenyl (CBP) as the
hole-transport layer, 1,3,5-tris(N-phenylbenzimidazole-2-yl)-
benzene (TPBI) as the electron-transport layer, and N,N′-
dicarbazolyl-3,5-benzene (mCP) as host. These devices have a
structure of ITO/MoO3 (1 nm)/CBP (35 nm)/mCP (5 nm)/
mCP:Pt-emitter (12%, 15 nm)/TPBI (65 nm)/LiF (1 nm)/Al,
(see SI). The performance of both devices is shown in Figure 3.
EL spectra of both devices, however, are broadened relative to the
PL spectra, an indication that excitons are likely not fully confined
to the dopant. As such, future studies will focus on optimization of
the device structures and the design of new host materials to
further improve device performance.
In summary, we have developed a synthetic route to the first
triarylboron-functionalized metal−carbene complexes, and have
shown that the presence of the boron moiety greatly increases
the phosphorescent quantum yield of these systems.
Furthermore, the strong ligand field exerted by the carbene
enables the preparation of Pt(II) complexes with efficient high-
energy blue phosphorescence. The utility of these complexes
has been further demonstrated in a series of preliminary
electroluminescent devices with high efficiencies (see Table 2.)
Table 2. Electroluminescent Device Data
device
BC1
BC2
Von/V
max/cd m−2
4.0
3.6
L
4165
2098
current efficiency, max./cd A−1
power efficiency, max./lm W1−
EQEmax/%
53.0
25.8
41.6
22.5
17.9
9.8
CIE/(x,y)
(0.34, 0.53)
(0.27, 0.50)
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthetic details and characterization data for all precursor
compounds and the Pt(II) compounds, crystal structural data,
CV, and EL data. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
wangs@chem.queensu.ca
■
Notes
Figure 3. Current and power efficiencies for OLEDs based on BC1
and BC2.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the Natural Sciences and Engineering Research
Council of Canada for financial support.
■
The device incorporating BC1 as an emitter shows peak current
and power efficiencies of 53.0 cd/A and 41.6 lm/W, remaining as
high as 49.6 cd/A and 33.6 lm/W at the display-relevant
brightness of 100 cd/m2. Devices based on the blue-emitting BC2
also show impressive performance, with peak efficiencies of 25.8
cd/A and 22.5 lm/W, remaining at 19.2 cd/A and 13.6 lm/W at
100 cd/m2. Although research on platinum−NHCs in OLEDs has
been limited, Strassner et al. have recently reported blue-green
devices with external quantum efficiencies (EQEs) reaching
6.2%.9f More recently, Li and co-workers have achieved up to
14.8% EQE in an OLED based on a Pt−NHC complex.13b To our
knowledge, the value of 17.9% EQE for the device based on BC1
is the highest reported to date for a Pt(II)−carbene complex. The
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