Luminescent platinum(II) complexes of 1,3-bis(N-alkylbenzimidazol- 2′-yl)benzene-type ligands with potential applications in efficient organic light-emitting diodes

Article abstract of DOI:10.1002/chem.201203290

A series of luminescent platinum(II) complexes of tridentate 1,3-bis(N-alkylbenzimidazol-2′-yl)benzene (bzimb) ligands has been synthesized and characterized. One of these platinum(II) complexes has been structurally characterized by X-ray crystallography. Their electrochemical, electronic absorption, and luminescence properties have been investigated. Computational studies have been performed on this class of complexes to elucidate the origin of their photophysical properties. Some of these complexes have been utilized in the fabrication of organic light-emitting diodes (OLEDs) by using either vapor deposition or spin-coating techniques. Chloroplatinum(II)-bzimb complexes that are functionalized at the 5-position of the aryl ring, [Pt(R-bzimb)Cl], not only show tunable emission color but also exhibit high current and external quantum efficiencies in OLEDs. Concentration-dependent dual-emissive behavior was observed in multilayer OLEDs upon the incorporation of pyrenyl ligand into the Pt(bzimb) system. Devices doped with low concentrations of the complexes gave rise to white-light emission, thereby representing a unique class of small-molecule, platinum(II)-based white OLEDs. Copyright

Full text of DOI:10.1002/chem.201203290

FULL PAPER
DOI: 10.1002/chem.201203290
Luminescent Platinum(II) Complexes of 1,3-Bis(N-alkylbenzimidazol-
2’-yl)benzene-Type Ligands with Potential Applications in Efficient Organic
Light-Emitting Diodes
Elizabeth Suk-Hang Lam, Daniel Ping-Kuen Tsang, Wai Han Lam,
Anthony Yiu-Yan Tam, Mei-Yee Chan,* Wing-Tak Wong, and Vivian Wing-Wah Yam*[a]
Abstract: A series of luminescent plati-
num(II) complexes of tridentate 1,3-
bis(N-alkylbenzimidazol-2’-yl)benzene
(bzimb) ligands has been synthesized
and characterized. One of these plati-
num(II) complexes has been structural-
ly characterized by X-ray crystallogra-
phy. Their electrochemical, electronic
absorption, and luminescence proper-
ties have been investigated. Computa-
tional studies have been performed on
this class of complexes to elucidate the
origin of their photophysical properties.
Some of these complexes have been
utilized in the fabrication of organic
light-emitting diodes (OLEDs) by
using either vapor deposition or spin-
aryl ring, [PtACTHNGUTERN(UNG R-bzimb)Cl], not only
show tunable emission color but also
exhibit high current and external quan-
tum efficiencies in OLEDs. Concentra-
tion-dependent dual-emissive behavior
was observed in multilayer OLEDs
upon the incorporation of pyrenyl
coating
techniques.
Chloroplati-
ꢀ
AHCTUNGTRENGNnUN um(II) bzimb complexes that are
functionalized at the 5-position of the
ligand into the Pt
vices doped with low concentrations of
the complexes gave rise to white-light
emission, thereby representing
unique class of small-molecule, plati-
AHCTUNGTERGnNNUN um(II)-based white OLEDs.
ACHTUNGTREN(NUGN bzimb) system. De-
AHCTUNGTRENNUNG
Keywords: density functional calcu-
lations · cyclometalation · lumines-
cence · organic light-emitting di-
odes · platinum
a
Introduction
Among various metal complexes, cyclometalated plati-
AHCTUNGTRENNUNG
Organic light-emitting diodes (OLEDs) have recently
become popular candidates for research into lighting tech-
nology, owing to their potential applications in solid-state
lighting and flat-panel displays, because they offer the dis-
tinct advantages of high luminous efficiency, a full-color
gamut, and low manufacturing costs.[1] In particular, phos-
phorescent organic light-emitting diodes (PHOLEDs) are of
growing interest because of their ability to harvest both sin-
glet and triplet excitons for light emission through electron–
hole recombination upon electrical excitation,[2–4] thereby in-
creasing their internal quantum efficiency, which can theo-
retically reach unity at room temperature.[3] Platinum, as a
heavy atom and with strong spin-orbit coupling that can fa-
cilitate intersystem crossing for harvesting all singlet and
triplet excitons, has attracted much attention over the past
few decades.
cient OLEDs with
a
maximum luminance of
63000 cdmꢀ2.[4b] Another class of platinum(II) N^C^N com-
plexes of 1,3-dipyridylbenzene has also been reported to ex-
hibit strong phosphorescence with excellent PHOLED per-
formance.[4d–f] It has been shown that the emission color and
energy of these devices can be readily tuned by incorporat-
ing different functional groups onto the phenyl moiety.[4d,e]
Despite the use of 6-phenyl-2,2’-bipyridine and 1,3-dipyri-
dylbenzene as cyclometalating ligands, another new class of
cyclometalating ligands, namely 1,3-bis(N-alkylbenzimid-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
reports on the synthesis and spectroscopic properties of plat-
inum(II) complexes of this ligand until very recently, when
we reported its coordination to the platinum(II) system.[4a]
This new class of luminescent platinum(II) complexes exhib-
its bright-green EL and good PHOLED performance, with
a high current efficiency of 38.9 cdAꢀ1 and an external
quantum efficiency (EQE) of 11.5%.[4a]
[a] E. S.-H. Lam, D. P.-K. Tsang, Dr. W. H. Lam, Dr. A. Y.-Y. Tam,
Dr. M.-Y. Chan, Prof. Dr. W.-T. Wong, Prof. Dr. V. W.-W. Yam
Department of Chemistry, The University of Hong Kong
Pokfulam Road (Hong Kong)
Fax : (+852)2857-1586
E-mail: chanmym@hku.hk
wwyam@hku.hk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203290.
Chem. Eur. J. 2013, 19, 6385 – 6397
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6385

Although these platinum(II) complexes of the bzimb
ligand have interesting photophysical properties, there is
still room for improvement and optimization to develop
high-efficiency OLEDs. Challenges relating to color stability
and tunability, as well as the selection of suitable metal com-
plexes for high-efficiency PHOLEDs with low manufactur-
ing costs, are yet to be overcome.
MeOH under refluxing conditions, similar to that reported
for complex 6,[4g] to give the desired complexes (Scheme 1).
ꢀ
The identities of all of the newly synthesized platinum(II)
bzimb complexes were confirmed by H NMR spectroscopy,
1
MS (FAB), and elemental analysis. Complex 7 was also
structurally characterized by X-ray crystallography.
Herein, we report the synthesis, characterization, and
electrochemical and photophysical properties of a class of
luminescent Pt
plexes were prepared by either thermal evaporation or spin-
coating techniques. With the aim of tuning emission energies
without sacrificing the performance of devices based on the
[Pt(bzimb)Cl] system, we functionalized the bzimb ligand at
ACHTUNGTRENNUNG(bzimb) complexes. PHOLEDs of these com-
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
the 5-position of the phenyl ring and successfully fine-tuned
the emission energy and fabricated PHOLEDs with high
current efficiencies and EQEs. Interestingly, upon incorpo-
ration of a pyrenylalkynyl ligand, one of the mononuclear
organometallic complexes was found to exhibit dual emis-
ACHTUNGTRENNUNGsion in the red and green regions, the color of which could
be varied by controlling the concentration of the dopant.
Apart from the utilization of blue, green, and red molecular
emitters in multiple layers,[1a,6] or the combination of simul-
taneous monomer and excimer EL[7] to generate white light,
Scheme 1. Synthetic route to and molecular structures of complexes 4–8.
there are only a limited number of complexes of iridiumACHTUNTRGNEUNG(III)
that can emit white light from a single molecule.[8] This work
should provide an interesting insight into the future design
and development of white OLEDs (WOLEDs). In addition,
two devices that exhibit green emission with high efficien-
cies have been fabricated by using alkynylcarbazole-contain-
X-ray crystallography: Similar to complex 1,[4a] complex 7
adopts a distorted square-planar geometry, owing to the
ꢀ
steric demands of the bzimb ligand, as shown by the C15
ꢀ
ꢀ
ꢀ
ꢀ
Pt1 N1 and C15 Pt1 N2 angles (78.6–79.98) and the N1
ꢀ
ꢀ
Pt1 N2 angle (158.48). The Pt CACTHUNTGRNEUNG(alkynyl) bond length
(2.031 ꢁ) in complex 7 is longer than that in the related
platinum(II) bzimpy system. This difference has been at-
tributed to a stronger trans influence exerted by the anionic
ing Pt
N
[9]
ꢀ
and time-dependent density functional theory (TDDFT) cal-
culations have also been performed on this class of com-
plexes to gain a deeper understanding of their photophysical
properties.
phenyl ring on the bzimb ligand compared to the pyridyl
ꢀ
ꢀ
unit on the bzimpy ligand. The Pt N and Pt C
lengths are 2.006–2.027 ꢁ and 1.955 ꢁ, respectively. The Pt
(bzimb) bond length in complex 7 is longer than that in
ACHTUNGTRENUN(NG bzimb) bond
ꢀ
CACHTUNGTRENNUNG
Results and Discussion
complex 1 (1.931 ꢁ),[4a] which is due to a stronger trans in-
fluence of the alkynyl ligand than that of the chloro ligand.
The crystal data are summarized in the Supporting Informa-
tion, Table S1. The structure and crystal packing of complex
7 are shown in Figure 2 and the Supporting Information,
Figure S1, respectively. The interplanar angle between the
two alkynyl aryl rings is 15.18 and that between the carba-
zole unit and the adjacent aryl ring is 79.88. The intermolec-
ular Pt···Pt distance between two adjacent molecules is
5.18 ꢁ, which indicates the absence of Pt···Pt interactions.
The interplanar distance between the bzimb planes of two
adjacent molecules is 3.46 ꢁ, which may indicate the pres-
Synthesis and characterization: Complexes 2 and 3 were
synthesized according to a modified procedure for the syn-
thesis of complex 1, which was reported recently.[4a] The re-
action mixture was heated under reflux in glacial acetic acid
for 3 days (Figure 1). Complexes 4, 5, 7, and 8 were synthe-
sized by the reaction of complex 1 with the corresponding
alkynes in the presence of sodium hydroxide in CH2Cl2 and
ꢀ
ence of weak p p interactions between the two bzimb
planes.
Electrochemistry: Cyclic voltammetry was performed for
these complexes in CH2Cl2 with 0.1m nBu4NPF6 as a sup-
porting electrolyte. The electrochemical data are summar-
ized in Table 1 and a cyclic voltammogram of complex 5 in
CH2Cl2 (with 0.1m nBu4NPF6) is shown in the Supporting
ꢀ
Figure 1. Molecular structures of chloroplatinum(II) bzimb complexes 1–
3.
6386
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 6385 – 6397

Luminescent Platinum(II) Complexes with Applications in OLEDs
FULL PAPER
idative wave at about +1.77 V versus SCE was found to be
rather insensitive to the nature of the alkynyl ligands that
were employed, possibly resulting from an irreversible
PtII!PtIII oxidation. A quasi-reversible oxidation couple
was observed in complexes 5–7. The wave at +1.00 V versus
SCE in complex 5 is probably attributed to the oxidation of
the triphenylACTHNUTRGNEUNG
amine unit,[10] whereas the wave at about
+1.37 V in complexes 6 and 7 is tentatively assigned to the
oxidation of carbazole.[4g,11]
Electronic absorption spectroscopy: Complexes 1–3 show a
higher-energy band at about 306 nm and a lower-energy
band at around 352–432 nm, as shown in Figure 3. The pho-
tophysical data of complexes 1–8 are summarized in Table 2.
A significant red-shift is observed in the lower-energy band
on going from complex 1 to complex 3, probably because
the electron-donating substituent on the benzene unit of
Figure 2. Perspective drawing of complex 7 with atom numbering; hydro-
gen atoms are omitted for clarity. Thermal ellipsoids are set at 30%
probability.
Table 1. Electrochemical data for complexes 1–8.
Complex
Oxidation E1/2 (Epa) [V]
Reduction Epc [V]
versus SCE[a,b]
versus SCE[c]
1
2
3
4
U
ꢀ1.97
ꢀ1.97
ꢀ1.85
ꢀ2.05
ꢀ2.08
ꢀ1.99
ꢀ1.99
ꢀ2.00
5
6[d]
7
8
[a] E1/2 refers to the quasi-reversible anodic peak, calculated according to
the equation E1/2 =(Epa+Epc)/2, in which Epa and Epc are the peak anodic
and cathodic potentials, respectively. [b] Values in parentheses refer to
the anodic peak potential (Epa) for the irreversible oxidation waves.
[c] Epc refers to the cathodic peak potential for the irreversible reduction
waves. [d] Taken from ref. [4g].
Information, Figure S2. An irreversACTHNUTRGNEUNGible reduction wave for
complexes 1–8 was observed between ꢀ1.85 and ꢀ2.08 V
versus the standard calomel electrode (SCE). This irrevers-
Figure 3. Electronic absorption spectra of complexes 1–3 in CH2Cl2 at
ACHTUNGTRENNUNGible reduction wave was found to be insensitive towards the
room temperature.
nature of the alkynyl ligands. This result could possibly be
due to a bzimb-based reduction, as supported by the DFT
calculations (see below). Slight variation with the substitu-
ents on the bzimb ligand was observed, which may be due
to the inductive effect of the substituents at the 5-position
of the aryl ring (see below). In complexes 1–3, three
bzimb destabilizes the HOMO energy level, thereby leading
to a narrowing of HOMO–LUMO energy gap (see below).
The electronic absorption spectra of most of the alkynyl
complexes show a higher-energy absorption band at around
284–345 nm and a lower-energy absorption band at around
360–409 nm (see the Supporting Information, Figure S3).
irreversACHTUNGTRENNUNGible oxidation waves were observed, whereas, in
complexes 4–8, an irreversible wave was observed within the
range +0.60 to +0.79 V (versus SCE). The potential for the
oxidation wave was found to vary with the electronic effects
of the alkynyl ligands. For example, complex 5, which pos-
sesses the more electron-rich triphenylaminoalkynyl ligand,
has a less-positive potential (+0.60V versus SCE) than that
of complex 4, which possesses a phenylalkynyl ligand
(+0.79V versus SCE). This suggests that the oxidation was
alkynyl-ligand based with mixing of a metal-centered contri-
bution. This assignment was further supported by the obser-
vation that complex 8, with a more p-conjugated pyrenylal-
kynyl ligand, was more readily oxidized than complex 4, be-
ꢁ ꢀ
The higher-energy absorptions are assigned to intraACHTUNGTRENNUNGliACHTUNGTRENNUNGgand
(IL) p!p* transitions (bzimb and alkynyl units), whereas
the lower-energy absorptions are ascribed to IL p!p* tran-
sitions in the bzimb ligand, with substantial mixing of metal-
to-ligand charge transfer (MLCT) [dp(Pt)!p*
ACHTUNGTRENUN(NG bzimb)] and
ꢁ
ligand-to-ligand charge transfer (LLCT) [p
G
(bzimb)] character.[4g,9] For complexes 7 and 8, the absorp-
tion band in the lower-energy region has a larger molar ex-
tinction coefficient than that of complex 4. It is possible that
the absorption band in complexes 7 and 8 would also consist
of a metal-perturbed IL p!p* transition of the alkynyl
ligand. This result is further supported by the TDDFT calcu-
lations (see below).
cause its extended p-conjugation would render the pACTHNUTRGNEUNG(C C
R)/dp(Pt) orbital higher-lying in energy. The irreversible ox-
Chem. Eur. J. 2013, 19, 6385 – 6397
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6387

M.-Y. Chan, V. W.-W. Yam et al.
Table 2. Photophysical data for complexes 1–8.
Complex
Medium (T [K])
Electronic absorption
Emission
lem [nm] (t0 [ms])
FPL
lmax [nm] (e [dm3 molꢀ1 cmꢀ1])
in solution
0.19[e]
1
CH2Cl2[a] (298)
solid[b] (298)
solid[b] (298)
glass[c] (77)
306 (30960), 352 (7730), 384 (15820), 402 (17865)
506, 544, 587 (5.3)
520, 546, 593 (0.5)
520, 543, 564 (5.5)
491, 530, 574 (7.9)
513, 551, 603
507, 547, 595 (6.4)
509, 544, 592, 641 (1.5)
525, 568, 621 (5.5)
495, 534, 579, 637 (9.6)
514, 552, 602
thin film[d] (298)
CH2Cl2[a] (298)
solid[b] (298)
solid[b] (77)
0.78
0.25
2
3
306 (27880), 352 (6770), 388 (12340), 407 (14665)
glass[c] (77)
thin film[d] (298)
CH2Cl2[a] (298)
0.70
0.15
306 (27360), 362 (5760), 388 (7865), 408 (9710),
432 (12780)
530, 572, 624 (5.8)
solid[b] (298)
solid[b] (77)
526, 565, 617 (1.2)
538, 580, 634 (8.8)
518, 560, 610 (12.8)
535, 580, 628
glass[c] (77)
thin film[d] (298)
CH2Cl2[a] (298)
0.55
0.22
4
284 (35540), 310 (32115), 364 (14030), 390 (13070),
409 (12050)
509, 548, 593
solid[b] (298)
solid[b] (77)
505, 541, 590 (2.2)
519, 563, 613 (3.8)
492, 532, 576, 630 (8.2)
514, 551, 602
glass[c] (77)
thin film[d] (298)
CH2Cl2[a] (298)
solid[b] (298)
solid[b] (77)
0.77
0.67
5
6
300 (34745), 312 (38285), 380 (17660), 403 (13290)
510, 547, 595
510, 550, 615 (3.2)
434, 521, 564, 614, 730 (2.5)
493, 533, 577, 625 (6.9)
514, 552, 603
glass[c] (77)
thin film[d] (298)
0.71
0.51
[a,f]
CH2Cl2 (298)
292 (90770), 306 (78330), 345 (25120), 366 (26690),
385 (23610), 408 (18005)
509, 548, 595
solid[b,f] (298)
solid[b,f] (77)
505, 540, 584 (1.7)
500, 540, 625 (7.6)
490, 531, 575 (8.1)
514, 551, 600
glass[c,f] (77)
thin film[d] (298)
CH2Cl2[a] (298)
0.78
0.46
7
8
294 (46755), 316 (52340), 338 (49840), 370 (42605),
404 (17260)
507, 535, 595
solid[b] (298)
solid[b] (77)
468, 512, 538, 664 (1.2)
531, 567, 594, 651 (5.3)
493, 525, 557, 573, 593, 629 (8.4)
514, 543, 600
507, 547, 593, 659, 719, 737
467, 497, 645 (4.7)
467, 497, 645 (5.3)
490, 526, 564 (8.8) 650, 669, 707,
725, 752, 776 (0.8)
glass[c] (77)
thin film[d] (298)
CH2Cl2[a] (298)
solid[b] (298)
solid[b] (77)
0.52
0.09
293 (63045), 360 (35330), 384 (26370), 402 (26775)
glass[c] (77)
thin film[d] (298)
661, 736
0.02
[a] Measured at a concentration of 1ꢂ10ꢀ5 m. [b] Solid-state emission was recorded after grinding. [c] In butyronitrile glass. [d] 5 wt.% in MCP. [e] Taken
from ref. [4a]. [f] Taken from ref. [4g].
Luminescence spectroscopy: All of these complexes, except
complex 8, exhibit strong green luminescence in solution,
77 K glass, and in the solid state. The large Stokes shifts sug-
gest that the emissions are of triplet parentage. The photolu-
minescence quantum yields (FPL) of chloro complexes 1–3
range from 0.15 to 0.25, whilst alkynyl complexes 4–7 have
higher FPL (0.22–0.67). All of the complexes give higher FPL
than their bzimpy counterparts (bzimpy=2,6-bis(N-alkyl-
benzimidazol-2’-yl)pyridine).[9] This result may be attributed
to the presence of an anionic phenyl ring on the bzimb
tivation pathways. In CH2Cl2, complexes 1–6 exhibit a vi-
bronic-structured emission at around 507–530 nm, with vi-
brational progressional spacings of about 1380 cmꢀ1. The vi-
brational progressional spacings correspond to typical aro-
matic vibrational modes of the bzimb ligand. The emission
probably originated from the triplet IL excited state of the
cyclometalated ligand,[4a] which is further supported by its
sensitivity towards the substituent at the 5-position of the
bzimb ligand, as demonstrated by complexes 1–3. Although
the emission is only slightly perturbed by the electron-do-
nating tert-butyl group on the benzene unit of the bzimb
ligand (complex 1 versus complex 2), a clear red-shift in
ꢀ
ligand, which substantially raises the energy of the d d ex-
cited state, thereby decreasing the rate of nonradiative deac-
6388
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 6385 – 6397

Luminescent Platinum(II) Complexes with Applications in OLEDs
FULL PAPER
emission is observed for complex 3 (Figure 4) with respect
to complex 1. The electron-donating butoxy group on the
benzene unit raises the energy of the HOMO level to a
larger extent than that of the LUMO, thus leading to the
red-shift in energy (see below).
ligand, a vibronic band is also observed at approximately
659–737 nm (Figure 5). This band is possibly due to the
3
ꢁ ꢀ
emission from the IL p!p*(C C pyrenyl) excited state, as
suggested by previous studies on pyrene-containing plati-
AHCTUNGTRENNUNG
num(II) complexes.[12] This dual emission might result from
inefficient coupling between these two excited states.
In a butyronitrile glass at 77 K, complexes 4–6 display a
well-resolved vibronic-structured emission band at around
3
490–630 nm, which is assigned to emission from the IL [p!
p*
G
ACHTUNGERTN(NUNG bzimb)] excited state. In
complex 8, apart from the 3IL [p!p*
(bzimb)]/3MLCT
ACHTUNGTRENNUNG
[dp(Pt)!p*
ACHTUNGTREN(NUNG bzimb)] vibronic-structured emission band at
around 490–564 nm, an additional vibronic-structured band
3
at around 650–776 nm is observed, which is assigned as a IL
ꢁ
[p!p*(C C-pyrenyl)] emission.
Computational studies: DFT and TDDFT calculations were
performed for chloro complexes 1–3 and alkynyl complexes
4–8 to gain further insight into the photophysical properties
of this series of cyclometalated platinum(II) complexes. In
the computational study, model complexes were used (1’–
8’), in which the long alkyl chains on the nitrogen atoms of
the benzimidazolyl groups were replaced by methyl groups.
For chloro complexes 1’–3’, the S1 excited state arises
from a one-electron excitation from the HOMO to the
LUMO. The HOMO is a mixture of the p orbital that is lo-
calized on the bzimb ligand and the metal dp orbital, whilst
the LUMO is the p* orbital that is localized on the bzimb
ligand (see the Supporting Information, Figure S4). Selected
singlet–singlet transitions for complexes 1’–8’ are listed in
the Supporting Information, Tables S2–S9. The HOMO
energy level increases from complex 1’ to complex 2’ and
complex 3’, in line with the electron-donating ability of the
substituents on the benzene unit, that is, butoxy>tert-
butyl>H. However, the energy of the LUMO level is less
influenced by the substituents, because the LUMO has a
node on the substituent-bearing carbon atom, which is in
agreement with the slight variation in the potential of the
reduction waves in our electrochemical studies. The S0!S1
transition is computed to show a red-shift in energy, in the
order 1’>2’>3’, which is in good agreement with the trend
for the observed lowest-energy absorption band in com-
plexes 1–3 in their UV/Vis absorption spectra and can be as-
Figure 4. Normalized emission spectra of complexes 1–3 in degassed
CH2Cl2 at room temperature.
Complexes 4–6 show a vibronic-structured emission band
at about 510 nm, which is similar to that of complex 1.
3
Therefore, this emission most likely originates from the IL
[p!p*
(bzimb)]/MLCT [dp(Pt)!p*
ACHTUNGERTN(NUNG bzimb)] excited state.
Interestingly, complex 8, which contains an alkynylpyrenyl
moiety, shows two distinct emission bands. In addition to the
vibronic-structured emission band at around 507–593 nm,
which is derived from the 3IL excited state of the bzimb
ꢀ
signed as an IL [p p*
transition.
(bzimb)]/MLCT [dp(Pt)!p*
ACHTUNGTRENNUNG(bzimb)]
For alkynyl complexes 4’–6’, several electronic transitions
are computed in the region of the low-energy absorption
band. The Supporting Information, Figure S5, shows the mo-
lecular orbitals that are involved in these transitions for
ꢁ ꢀ
complex 4’. The HOMO is mainly the p orbital of the C C
R ligand, whereas the HOMOꢀ1 and HOMOꢀ2 are the re-
spective p orbitals of the bzimb ligand mixed with platinum
ꢀ
ꢀ
dp orbitals having Pt CACTHUNGTRENNG(U bzimb) and Pt N antibonding char-
ꢁ
acter. For the HOMOꢀ1, some mixing of the C C p-bond-
ing orbital can be found. The LUMO and LUMO+1 are the
p* orbitals of the bzimb ligand. On the basis of the TDDFT
calculations, the lower-energy absorption band in complexes
Figure 5. Normalized emission spectra of complexes 6–8 in degassed
CH2Cl2 at room temperature.
Chem. Eur. J. 2013, 19, 6385 – 6397
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6389

M.-Y. Chan, V. W.-W. Yam et al.
ꢀ
ꢁ
4–6 is mainly due to the IL [p p*
p* (bzimb)] and LLCT [p
(bzimb)]/MLCT [dp!p*
R)!p* (bzimb)] transitions.
ACHTUNGTRENNUNG
G
E
ACHTUNGERTN(NUNG C C
(bzimb)]/MLCT [dp(Pt)!p*
ACHTUNGTRENNUNG
Unlike in complexes 4’–6’, a very intense S0!S2 transi-
tion, which arises from the HOMO!LUMO+1 excitation,
is found in complexes 7’ and 8’. Because the HOMO and
ꢁ ꢀ
LUMO+1 are the respective p and p* orbital of the C C
R ligand (see the Supporting Information, FigACTHNUTRGENUGuN res S6 and
ꢀ
S7), this transition can be assigned as an IL p p* transition
ꢁ ꢀ
of the C C R ligand. This result is in line with the more in-
tense absorption bands observed for complexes 7 and 8 than
for complex 4 in the lower-energy region, thus supporting
ꢀ
ꢁ ꢀ
the involvement of the IL [p p*
absorption band.
N
Most of these complexes exhibit strong phosphorescence
at room temperature in CH2Cl2. To gain more insight into
the nature of this emission for this class of complexes, the
singlet–triplet transitions were computed for complexes 1’–
4’, 7’, and 8’ by using TDDFT/CPCM with CH2Cl2 as the sol-
vent. Because complexes 5 and 6 show similar emission
properties to complex 4, the triplet excited state for complex
4 will be investigated. In the Supporting Information,
Tables S10 and S11 list the two lowest-energy singlet–triplet
transitions for these complexes. For complexes 1’–4’, the T1
3
ꢀ
excited state has IL [p p*
(bzimb)]/3MLCT [dp(Pt)!p*-
Figure 6. Plot of spin density (isovalue=0.001) of the lowest-energy trip-
let-excited states of a) complex 1’, b) complex 4’, c) complex 7’, and
d) complex 8’.
ACHTUNGTRENNUNG(bzimb)] character. However, the T1 excited state for com-
plexes 7’ and 8’ has IL [p p*ACHTGNUTRENNUNG
3
ꢀ
ꢁ ꢀ
(C C R)] character and the
3IL [p p*
C
ACHTUNGTRENNUNG
ꢀ
state becomes the T2 state. Geometry optimizations of the
lowest-energy triplet excited states for these complexes
were performed by using the unrestricted Kohn–Sham ap-
proach (UPBE0/CPCM). The optimized structures of the
triplet excited states for complexes 1’–4’ show that the dis-
tortion occurs mainly on one of the two benzimidazolyl
units and on the central phenyl ring in the bzimb ligand,
whereas, for complexes 7’ and 8’, the distortion is localized
Table 3. Relative energy of the optimized triplet excited states of the
model complexes.
Complex
DE
G
Complex
DE
486[b]
(T1ꢀS0)[a]
1’
2’
3’
487[b]
491[b]
530[b]
4’
7’
8’
485,[b] 547[c]
483,[b] 679[c]
[a] Energy difference between the triplet excited states and the ground
states at their corresponding optimized geometries in CH2Cl2. [b] 3IL-
ꢁ ꢀ
ꢁ ꢀ
on the C C R ligand. Figure 6 shows a plot of the spin den-
ACHUTNGREUNNG ACHTUNGTNER(NUGN C C R) excited state.
(bzimb)/3MLCT excited state. [c] 3IL
sities of the triplet excited state for complexes 1’, 4’, 7’, and
8’.
The calculated emission wavelengths of the triplet excited
states of these complexes, which are approximated from the
differences in their solvent-corrected energies at the opti-
mized geometries of the ground and triplet excited states,
are listed in Table 3. There is a noticeable red-shift on going
from complex 1’ to complex 3’, which is in line with the ob-
served red-shift in the experiments. This result further sup-
ports the tuning of the emission color by modification of the
substituents on the benzene unit. A similar emission wave-
length of the triplet excited state for complex 4’ to that of
complex 1’ is observed, which is in agreement with the ex-
perimental findings and further supports the origin of the
plexes 7’ (485 nm) and 8’ (483 nm) are similar to that of
3
ꢀ
ꢁ ꢀ
complex 4’, whilst the IL [p p*ACHTNUGTRENNUNG
(C C R)] triplet excited
states of complexes 7’ and 8’ are computed to be 547 and
679 nm, respectively. As mentioned earlier, two emissions
were observed in complex 8. On the basis of these calcula-
tions, as well as the observed trend in the emission bands,
the two emissions for complex 8 at around 507–593 and
659–737 nm are assigned to the ILACHTUNGTRNEUNG
(bzimb)/3MLCT and IL-
3
3
ꢁ ꢀ
AHCTUNGTREG(NNUN C C R) excited states, respectively. Notably, the vibronic-
structured emission band for complex 7 at around 507–
595 nm is broadened compared to those of complexes 4–6.
3
It is possible that this emission originates from both the IL-
3
emission as being derived from the 3IL [p p*-
(bzimb)/3MLCT and IL [p p*
N
ꢀ
ꢀ
ꢁ ꢀ
G
G
For complexes 7’ and 8’, geometry optimizations of the
(bzimb)] triplet ex-
cited states were also performed. The emission wavelengths
3
Multilayer green OLEDs fabricated by vacuum deposition:
As mentioned above, we have recently reported the fabrica-
tion of multilayer PHOLEDs by using complex 1 as the
for the 3IL
ACHTUNGTRENNUNG
(bzimb)/3MLCT triplet excited states of com-
6390
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 6385 – 6397

Luminescent Platinum(II) Complexes with Applications in OLEDs
FULL PAPER
emissive layer (EML) and we have demonstrated that they
exhibited high current efficiency and EQE (38.9 cdAꢀ1 and
11.5%, respectively).[4a] As an extension of our previous
work, complexes 2 and 3 were used as phosphorescent
dopant materials to prepare multilayer PHOLEDs by vapor
deposition with the configuration of indium tin oxide (ITO)/
a-naphthylphenylbiphenyldiamine (NPB; 70 nm)/4,4’,4’’-
tris(carbazole-9-yl)-triphenylamine (TCTA; 5 nm)/x% PtII:
N,N’-dicarbazolyl-3,5-benzene (MCP; 30 nm)/3-(4-biphenyl-
doped into a MCP layer at a current density of 20 mAcmꢀ2.
Both types of devices exhibit a vibronic-structured emission
band that is independent of the dopant concentration (see
the Supporting Information, Figure S8).[13] Moreover, their
EL spectra are similar to their photoluminescence (PL)
spectra, thus confirming that they originate from the same
3
ꢀ
IL [p p*] excited state of the cyclometalated bzimb ligand.
However, an undesirable blue emission from the adjacent
electron-transporting BAlq layer is observed at 480 nm
when the dopant concentration of the PtII complexes is low,
which may be due to hole leakage from the emissive layers
into the BAlq layer for light emission.
yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole
5 nm)/bis(2-methyl-8-quinolinato)-4-phenylphenolate)
(TAZ;
alumi-
N
ACHTUNGTRENNUNG
ACHTUNGTRENNUNGnumACHTUNGTRENNUNG(III) (BAlq; 30 nm)/LiF/Al with different concentra-
tions of platinum(II) complexes, that is, x=2, 4, 6, and 8%.
Similar to the device with complex 1,[4a] NPB and BAlq
were used as hole- and electron-transporting layers, respec-
tively, whilst TCTA and TAZ were used as the carrier-con-
finement layers. For the devices with 2 and 3, MCP was
used as a host material instead of 4,4’-bis(9-carbazolyl)-2,2’-
biphenyl (CBP), because of its higher triplet energy. The
Supporting Information, Figure S8, shows the normalized
EL spectra of the devices in which complexes 2 and 3 were
To improve the color purity, devices with dual EMLs
were fabricated, in which the platinum(II) complex was
doped into both wide-band-gap host materials, that is, MCP
and TAZ,[14] to form dual EMLs. In particular, the dopant
concentration of the PtII complexes that were doped into
the MCP layer was kept constant (i.e. 8% for complex 2
and 10% for complex 3), whilst different concentrations (i.e.
6, 8, 10, and 12%) of the PtII complexes were doped into
the TAZ layer. Optimized devices with complexes 2 and 3
Figure 7. Plots of current efficiency (top), power efficiency (middle), and EQE (bottom) of the dual-EML devices as a function of current density by
using a) complex 2 and b) complex 3 as a phosphorescent dopant.
Chem. Eur. J. 2013, 19, 6385 – 6397
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6391

M.-Y. Chan, V. W.-W. Yam et al.
Table 4. Key parameters of devices with a single EML or with dual
EMLs.
with a single EML show maximum current efficiencies of
15.8 and 23.5 cdAꢀ1, respectively, as well as luminous power
efficiencies of 11.1 and 18.4 Lm Wꢀ1, respectively. These
values correspond to peak EQEs of 5.8% and 6.8% for de-
vices with complexes 2 and 3, respectively. Upon the intro-
duction of dual EMLs, both the current efficiency and EQE
were improved. Devices with complexes 2 and 3 showed
maximum current efficiencies of 40.3 and 34.7 cdAꢀ1 and
EQEs of 11.8% and 10.0%, respectively (Figure 7). More
importantly, the EL spectra of all of these devices showed a
pure vibronic-structured emission that originated from the
PtII complexes in the absence of any residual emission from
BAlq, as shown in Figure 8. Table 4 summarizes the key pa-
Complex
Device
Current
efficiency
Power
EQE
[%]
efficiency
A
]
[lmWꢀ1
ACHTUNGTERNNUNG ]
1[a,b]
2
single EML
dual EMLs
single EML
dual EMLs
single EML
dual EMLs
17.8
38.9
15.8
40.3
23.5
34.7
12.4
27.2
11.1
28.1
18.4
26.4
5.7
11.5
5.8
11.8
6.8
3
10.0
[a] Taken from ref. [4a]. [b] CBP was used as a host material.
Solution-processable multilayer OLEDs: As observed in its
PL spectra, complex 8 showed an interesting dual emission
(simultaneous green and red emission) in very dilute solu-
tions ([Pt]=10ꢀ6 m), and we envisaged that complex 8 would
be a good candidate for a white-light-emitting material in a
single EML that could be used for the fabrication of
WOLEDs. To test this hypothesis, multilayer PHOLEDs
with the configuration of ITO/poly(ethylenedioxythio-
AHCTUNGTREGpNNNU hene):poly(styrene sulfonic acid) (PEDOT:PSS; 70 nm)/
y% 8:MCP (60 nm)/TAZ (5 nm)/BAlq (30 nm)/LiF/Al (y=
5, 10, and 20%) were fabricated by using a spin-coating
technique. Interestingly, the EL is strongly dependent on
the concentration of complex 8. As shown in Figure 9a, at a
current density of 60 mAcmꢀ2, white light can be generated
by the superposition of two vibronic-structured emission
bands in the green and red regions, which are consistent
with the PL observation in dilute CH2Cl2 at 298 K. Upon in-
creasing the concentration of complex 8, the EL spectrum is
dominated by the vibronic-structured emission band in the
red region and the color of light changes from white to
yellow and eventually to red at a dopant concentration of
20% (Figure 9b). Table 5 summarizes the key characteristics
of this device. Notably, the device with 5% complex 8 gives
white-light emission with Commission Internationale de
Lꢃꢄclairage (CIE) coordinates of (0.35, 0.39), which are
very close to those of pure white light (0.33, 0.33). In sharp
contrast to other platinum(II) systems that show white-light
EL upon dopant aggregation at high concentrations, this
class of platinum(II) complexes can give white-light EL
emission at low dopant concentrations. To the best of our
knowledge, this device represents the first example of white-
light EL emission at low concentrations, based on small mo-
lecular platinum(II) systems.
Figure 8. Normalized EL spectra of dual EML devices at a current densi-
ty of 20 mAcmꢀ2 by using a) complex 2 and b) complex 3 as a phospho-
A
E
Complexes 6 and 7 have also been utilized as phosphores-
cent dopants in solution-processable OLEDs, in which the
device structures were the same as that of complex 8. Their
EL spectra were also found to be independent of the dopant
concentration, even up to 20% (Figure 10). Bright-green
emission was observed, with vibronic-structured bands at
512, 552, and 600 nm, which corresponded to CIE coordi-
nates of (0.31, 0.55). The lack of distinctive dual-emissive
behavior in complexes 6 and 7 led to discrepancies between
the emission colors of devices with complex 8 and those
rameters of devices with single EML and dual EMLs. It is
believed that the dual EMLs could effectively confine the
triplet excitons within the EML, thereby leading to a sup-
pression of the unwanted BAlq emission. In addition, no sig-
nificant spectral shift was observed upon increasing the
dopant concentration. This result is probably due to a lower
tendency of this class of platinum(II) complexes to form ag-
gregates in the solid thin films.[4a]
6392
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 6385 – 6397

Luminescent Platinum(II) Complexes with Applications in OLEDs
FULL PAPER
Figure 10. Normalized EL spectra of devices at a current density of
20 mAcmꢀ2 by using a) complex 6 and b) complex 7 as a phosphorescent
dopant.
Figure 9. a) EL spectra of devices with complex 8 as a phosphorescent
dopant at a current density of 60 mAcmꢀ2. b) Chromaticity diagram,
which shows the CIE coordinates of devices at different concentrations.
Conclusion
In conclusion, a class of [Pt
ACHTUNGTNER(NUNG bzimb)Cl] complexes with tun-
Table 5. CIE coordinates of devices with complex 8 as an emissive layer.
AHCTUNGTREGaNNNU ble emission color has been synthesized and their photo-
physical properties have been characterized. By varying the
functional groups on the benzene unit in chloro complexes
[PtACTHNUGRTENUNG(R-bzimb)Cl], the emission color can be readily tuned.
Dopant concentration [%]
CIE coordinates (x,y)
5
10
20
(0.35, 0.39)
(0.37, 0.43)
(0.43, 0.38)
Moreover, by incorporating an alkynyl ligand into com-
ꢁ ꢀ
plexes [PtACTHNUTRGENNU(G bzimb)ACHUTTGNREN(NUNG C C R)], an enhancement in PL quan-
tum efficiency can be achieved. Multilayer PHOLEDs have
been fabricated based on these platinum(II) complexes. De-
vices with complex 2 demonstrated high current efficiencies
and EQEs of up to 40.3 cdAꢀ1 and 11.8%, respectively,
whilst solution-processable devices with complexes 6 and 7
exhibited EQEs of 3.4% and 1.8%, respectively. Interest-
ingly, white light can be obtained by using complex 8 as a
phosphorescent dopant in solution-processable PHOLEDs.
Dual emission can be observed in devices with complex 8, in
which green and red emissions originate from 3IL-
with complexes 6 and 7. On the other hand, the perform-
ance of devices that were doped with complexes 6 and 7 was
much better than that of devices doped with complex 8. As
shown in Figure 11, the device with complex 6 exhibited a
maximum current efficiency of 11.0 cdAꢀ1, which corre-
sponded to an EQE of 3.4%, whereas the device with com-
plex 7 show a maximum current efficiency of 5.5 cdAꢀ1,
which corresponded to an EQE of 1.84%. The reason for
this better performance by doping with complex 6 than with
complex 7 may be attributed to reduced hole leakage from
the emissive layers into the BAlq layer in devices with com-
plex 6.
3
3
ACHUTNGRENNUG CAHTUNGTREN(NNGU C C R)/ MLCT excited states, re-
(bzimb)/3MLCT and IL
ꢁ ꢀ
spectively. This interesting observation may be helpful for
providing new insight into the development of PHOLEDs
Chem. Eur. J. 2013, 19, 6385 – 6397
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6393

M.-Y. Chan, V. W.-W. Yam et al.
Figure 11. Plots of current efficiency (top), power efficiency (middle), and EQE (bottom) of the devices as a function of current density by using a) com-
plex 6 and b) complex 7 as a phosphorescent dopant.
and WOLEDs. We envisage that, with a suitable choice of
p-conjugated alkynyl ligands, multiple emissions may result
that could generate white light from a single-molecule emis-
sive material.
were added to distilled triethylamine (60 mL) under a nitrogen atmo-
AHCTUNGERTGsNNUN phere and the mixture was heated at reflux for 1 day. After the removal
of the solvent, the solid was purified by column chromatography on silica
gel by using n-hexane/CH2Cl2 (4:1 v/v) to obtain the product as a pure
white solid. Yield: 0.47 g (45% yield). 1H NMR (400 MHz, CDCl3): d=
0.33 (s, 9H; CH3), 7.30 (t, J=7.8 Hz, 2H; carbazole H), 7.46 (m, 8H;
benzene H), 7.58 (d, J=8.5 Hz, 2H; carbazole H), 7.75 (d, J=8.5 Hz,
2H; carbazole H), 8.15 (d, J=7.8 Hz, 2H; carbazole H); MS (FAB, posi-
tive-ion mode): m/z 440 [M+H]+; HRMS (EI, positive-ion mode): m/z
calcd for C31H25N28Si: 439.1751; found: 439.1751.
Experimental Section
Materials: Sodium hydroxide, 1-bromobutane, 1-ethynylpyrene, and phe-
Synthesis of complex 2: The product was synthesized according to a
nylacetylene were obtained from Aldrich. 9-(4-Ethynylphenyl)carba-
modified procedure for the synthesis of [PtACTHNUTRGNEUNG
(nBuBzimb)Cl],[4a] except that
ACHTUNGTRENNUNG
zole,[15a] 4-ethynyl-N,N-diphenylaniline,[15b] ((4-bromophenyl)ethynyl)tri-
1,3-bis(N-butylbenzimidazol-2’-yl)-4-tert-butylbenzene (0.5 g, 1.0 mmol)
was used in the place of 1,3-bis(N-butylbenzimidazol-2’-yl)benzene.
Yield: 0.2 g (40%); 1H NMR (400 MHz, CDCl3): d=0.98 (t, J=7.6 Hz,
6H; CH3), 1.49 (m, 4H; CH2), 1.58 (s, 9H; CH3), 1.87 (m, 4H; CH2),
methylsilane,[15c] 1,3-bis(benzimidazol-2’-yl)benzene (bzimb),[4a] complex
1,[4a] and complex 6[4g] were synthesized according to literature proce-
dures. The chloroplatACHTUNGTRENNUNGinum(II) precursor complexes were synthesized ac-
cording to
chloroplast
a
modified literature procedures for the synthesis of
4.48 (t, J=7.6 Hz, 4H; CH2), 7.02 (d, J=8.0 Hz, 2H; benzimidazACHTUNGTRENNUNGolyl H),
ꢀ
inum(II) 2,6-bis(benzimidazol-2’-yl)pyridine (bzimpy) com-
plexes.[9] All solvents were purified and distilled by using standard proce-
dures before use. All other reagents were of analytical grade and were
used as received.
7.25 (m, 4H; benzimidazolyl H), 7.46 (s, 2H; C6H2), 8.85 (d, J=8.0 Hz,
2H; benzimidazolyl H); MS (FAB, positive-ion mode): m/z 709 [M+H]+;
elemental analysis calcd (%) for C32H37ClN4Pt: C 54.27, H 5.27, N 7.91;
found: C 54.19, H 5.35, N 7.84.
Synthesis of 9-(4-((4-((trimethylsilyl)ethynyl)phenyl)ethynyl)phenyl)-9H-
carbazole: 9-(4-Ethynylphenyl)carbazole (0.65 g, 2.4 mmol), CuI (0.02 g,
0.1 mmol), ((4-bromophenyl)ethynyl)trimethylsilane (1.23 g, 4.9 mmol),
and trans-dichlorobis(triphenylphosphine)palladium(II) (0.08, 0.1 mmol)
Synthesis of complex 3: The product was synthesized according to a
modified procedure for the synthesis of [Pt
bis(N-butylbenzimidazol-2’-yl)-5-butoxybenzene (0.15 g, 0.3 mmol) was
(nBuBzimb)Cl],[4a] except 1,3-
ACHTUNGTRENNUNG
6394
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 6385 – 6397

Luminescent Platinum(II) Complexes with Applications in OLEDs
FULL PAPER
used in place of 1,3-bis(N-butylbenzimidazol-2’-yl)benzene. Yield: 0.04 g
(33%); 1H NMR (400 MHz, CDCl3): d=0.96 (m, 9H; CH3), 1.46 (m,
6H; CH2), 1.60 (m, 2H; CH2), 1.88 (m, 4H; CH2), 3.90 (t, J=6.4 Hz, 2H;
performed on KBr disks on a Bio-Rad FTS-7 FTIR spectrometer (4000–
400 cmꢀ1). Elemental analysis of the new complexes was performed on a
Flash EA 1112 elemental analyzer at the Institute of Chemistry, the Chi-
nese Academy of Sciences.
ꢀ
O CH2), 4.51 (t, J=7.3 Hz, 4H; CH2), 6.97 (s, 2H; benzimidazolyl H),
7.14 (d, J=8.0 Hz, 2H; benzimidazolyl H), 7.30 (m, 4H; benzimidazol-
Electronic absorption spectroscopy was performed on a Hewlett–Packard
8452A diode-array spectrophotometer. The concentration of the samples
for the electronic absorption measurements were typically within the
range 2ꢂ10ꢀ4 to 1ꢂ10ꢀ6 moldmꢀ3. Steady-state excitation and emission
spectroscopy were performed on a Spex Fluorolog-2 Model F111 fluores-
cence spectrofluorimeter at RT and 77 K. Solid-state photophysical stud-
ies were performed on solid samples inside a quartz tube that was placed
inside a quartz-walled Dewar flask. Similarly, measurements of a butyro-
nitrile glass or solid-state sample at 77 K were performed by adding
liquid nitrogen into the optical Dewar flask. The concentration of the
complexes in butyronitrile glass for the emission measurements was typi-
cally of the order ꢂ10ꢀ6 moldmꢀ3. For the photophysical studies, all of
the solutions were degassed on a high-vacuum line in a two-compartment
cell that consisted of a Pyrex bulb (10 mL) and a quartz cuvette (path
length: 1 cm) and was sealed from the atmosphere by a Bibby Rotaflo
HP6 Teflon stopper. The solutions were rigorously degassed by at least
four successive freeze/pump/thaw cycles. Emission-lifetime measurements
were performed by using a conventional laser system. The concentrations
of the complexes in solution for the lifetime measurements were typically
about 2ꢂ10ꢀ5 moldmꢀ3. The excitation source was a 355 nm output (third
harmonic) of a Spectra-Physics Quanta-Ray Q-switched GCR-150–10
pulsed Nd-YAG laser. Luminescence-decay signals were detected on a
Hamamatsu R928 PMT, recorded on a Tektronix Model TDS-620A
(500 MHz, 2GS/s) digital oscilloscope, and analyzed by using a program
for the exponential fits.
ACHTUNGTRENNUNGyl H), 8.95 (d, J=8.0 Hz, 2H; benzimidazolyl H); MS (FAB, positive-ion
mode): m/z 725 [M+H]+; elemental analysis calcd (%) for
C32H38N4Pt·H2O: C 52.85, H 5.54, N 7.70; found: C 52.91, H 5.15, N 7.71.
Synthesis of complex 4: The product was synthesized according to a simi-
lar procedure for the synthesis of complex 6.[4g] A mixture of phenyl
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
a
ACHTUNGTRENNUNG
was stirred under heating at 508C for 2 days. After filtration, the yellow
precipitate was washed with water, MeOH, and Et2O. Slow diffusion of
Et2O vapor into a solution of the product in CH2Cl2 gave complex 4 as a
yellow powder. Yield: 0.07 g (64%); 1H NMR (400 MHz, CDCl3): d=
0.98 (t, J=7.4 Hz, 6H; CH3), 1.47 (m, 4H; CH2), 1.94 (m, 4H; CH2), 4.56
(t, J=7.4 Hz, 4H; CH2), 7.31 (m, 9H; benzimidazolyl H and C6H4), 7.53
(d, J=8.0 Hz, 2H; benzimidazolyl H), 7.70 (d, J=7.1 Hz, 2H; C6H4),
9.05 (d, J=8.0 Hz, 2H; benzimidazolyl H); IR (KBr): n˜ =2089 cmꢀ1 (w,
+
ꢁ
C C); MS (FAB, positive-ion mode): m/z 719 [M+H] ; elemental analy-
sis calcd (%) for C36H34N4Pt: C 60.24, H 4.77, N 7.81; found: C 60.33,
H 4.88, N 7.81.
Synthesis of complex 5: The product was synthesized according to a simi-
lar procedure for the synthesis of complex 4, except that 4-ethynyl-N,N-
diphenylaniline (0.25 g, 0.6 mmol) was used in place of phenylacetylene.
Yield: 0.09 g (66%); 1H NMR (400 MHz, CDCl3): d=0.99 (t, J=7.4 Hz,
6H; CH3), 1.52 (m, 4H; CH2), 1.98 (m, 4H; CH2), 4.60 (t, J=7.4 Hz, 4H;
CH2), 7.00 (t, J=7.5 Hz, 2H; triphenylamine), 7.09 (d, J=8.6 Hz, 2H; tri-
phenylamine), 7.17 (d, J=7.5 Hz, 4H; triphenylamine), 7.27 (m, 5H;
benzimidazolyl H), 7.33 (m, 4H; benzimidazolyl H), 7.40 (m, 2H; triphe-
nylamine), 7.58 (t, J=7.5 Hz, 4H; triphenylamine), 9.13 (d, J=7.9 Hz,
Cyclic voltammetry was performed on a CHI 620A (CH Instruments,
Inc.) electrochemical analyzer. Electrochemical measurements were per-
formed at RT in CH2Cl2 with 0.1 moldmꢀ3 nBu4NPF6 (TBAH) as the sup-
porting electrolyte. The reference electrode was an Ag/AgNO3
(0.1 moldmꢀ3 in MeCN) electrode, the working electrode was a glassy
carbon electrode (CH Instruments), and Pt wire was used as the counter
electrode. The surface of the working electrode was polished with a 1 mm
alumina slurry (Linde) on a microcloth (Buehler) and then with a 0.3 mm
alumina slurry. Then, the surface was rinsed with ultra-pure deionized
water and sonicated in a beaker that contained ultra-pure water for
5 min. The polishing and sonication steps were repeated twice and, final-
ly, the working electrode was rinsed under a stream of ultra-pure deion-
2H; benzimidazolyl H); IR (KBr): n˜ =2083 cmꢀ1 (w, C C); MS (FAB,
ꢁ
positive-ion mode): m/z 886 [M+H]+; elemental analysis calcd (%) for
C48H43N5Pt·0.5H2O: C 64.49, H 4.96, N 7.83; found: C 64.62, H 4.91,
N 7.85.
Synthesis of complex 7: The product was synthesized according to a simi-
lar procedure for the synthesis of complex 4, except that 9-(4-((4-((tri-
A
E
(0.13 g,
0.3 mmol) was used in place of phenylacetylene. Yield: 0.08 g (53%);
1H NMR (400 MHz, CDCl3): d=0.98 (t, J=7.4 Hz, 6H; CH3), 1.47 (m,
4H; CH2), 1.93 (m, 4H; CH2), 4.56 (t, J=7.4 Hz, 4H; CH2), 7.22 (m, 3H;
0
ized water. The ferrocenium/ferrocene couple (FeCp2+/FeCp2 ) was used
as an internal reference. All solutions for the electrochemical studies
were deaerated with pre-purified Ar gas prior to the measurements.
ꢀ ꢁ ꢀ
ꢀ ꢁ ꢀ
carbazole H and C6H4 C C C6H4), 7.30 (m, 4H; C6H4 C C C6H4 and
benzimidazolyl H), 7.38 (t, J=8.0 Hz, 2H; carbazole H), 7.47 (m, 6H;
benzimidazolyl and carbazole H), 7.60 (t, J=8.0 Hz, 4H; benzimidaz-
X-ray crystal-structure determination: Single crystals of complex 7 that
were suitable for X-ray diffraction studies were grown by layering Et2O
onto a concentrated solution of the complex in CH2Cl2. The X-ray dif-
fraction data were collected on a Bruker APEX II CCD detector with
graphite-monochromated MoKa radiation (l=0.71073 ꢁ). The raw frame
data were integrated by using the SAINT[16] program. Multi-scan
SADABS[17] was applied during the absorption correction. The structure
was solved by using heavy-atom Patterson methods[18] and expanded by
using Fourier techniques. Full-matrix least-squares refinement on F2 was
used in the structure refinement. The positions of the H atoms were cal-
culated on the basis of the riding mode with thermal parameters equal to
1.2 times that of the associated C atoms and these positions participated
in the calculation of the final R indices. In the final stage of least-squares
refinement, all non-hydrogen atoms were refined anisotropically. Crystal-
lographic and structural refinement data are given in the Supporting In-
formation, Table S1.
ACHTUNGTRENNUNGolyl H), 7.69 (d, J=8.0 Hz, 2H; carbazole H), 7.80 (d, J=8.0 Hz, 2H; car-
ꢀ ꢁ ꢀ
bazole H), 8.16 (d, J=8.0 Hz, 2H; C6H4 C C C6H4), 8.97 (d, J=8.0 Hz,
2H; benzimidazolyl H); IR (KBr): n˜ =2077 cmꢀ1 (w, C C); MS (FAB,
ꢁ
positive-ion mode): m/z 984 [M+H]+; elemental analysis calcd (%) for
C56H45N5Pt·0.5H2O: C 67.79, H 4.67, N 7.06; found: C 67.50, H 4.57,
N 6.92.
Synthesis of complex 8: The product was synthesized according to a simi-
lar procedure for the synthesis of complex 4, except that 1-ethynyl-
pyrene (0.1 g, 0.3 mmol) was used in place of phenylacetylene. Yield:
0.1 g (77%); 1H NMR (400 MHz, CDCl3): d=1.02 (t, J=7.4 Hz, 6H;
CH3), 1.55 (m, 4H; CH2), 2.02 (m, 4H; CH2), 4.64 (t, J=7.4 Hz, 4H;
CH2), 7.34 (m, 7H; benzimidazolyl H), 7.64 (d, J=7.8 Hz, 2H; benzimACTHNUTRGENUGiN d-
ACHTUNGTRENNUNGazolyl H), 8.1 (m, 7H; pyrene H), 8.39 (d, J=7.9 Hz, 1H; pyrene H), 9.27
(m, 3H; pyrene and benzimidazolyl H); IR (KBr): n˜ =2066 cmꢀ1 (w, C
C); MS (FAB, positive-ion mode): m/z 845 [M+H]+; elemental analysis
ꢁ
CCDC-880903 (7) contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif.
calcd (%) for C46H40N4Pt·0.5
ACTHUNGTREN(NUNG CH3CN)·0.5C6H14: C 66.17, H 5.08, N 6.95;
found: C 66.48, H 4.76, N 6.91.
Physical measurements and instrumentation: 1H NMR spectroscopy was
performed on a Bruker AVANCE 400 (400 MHz) Fourier-transform
NMR spectrometer; chemical shifts are reported relative to tetramethyl-
silane. Positive-ion FAB MS was performed on a Thermo Scientific DFS
high-resolution magnetic sector mass spectrometer. IR spectroscopy was
Computational details: Calculations were performed by using the Gauss-
AHCTUNGTRENNUNG
ian 09 software package.[19] The ground-state geometries of model com-
plexes 1’–8’ were fully optimized in CH2Cl2 by using DFT calculations
with the hybrid Perdew, Burke, and Ernzerhof functional (PBE0)[20] in
Chem. Eur. J. 2013, 19, 6385 – 6397
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6395

M.-Y. Chan, V. W.-W. Yam et al.
conjunction with the conductor-like polarizable continuum model
(CPCM).[21] On the basis of the ground-state-optimized geometries, the
TDDFT[22] method at the same level associated with the CPCM (CH2Cl2)
was employed to compute the singlet–singlet and singlet–triplet transi-
tions. To gain more insight into the emissive states, geometry optimiza-
tions of the triplet excited states were performed by using the unrestrict-
ed UPBE0/CPCM (CH2Cl2) method. The calculated emission wavelength
of the triplet excited state for the complexes was approximated from the
difference in the solvent-corrected energies at the optimized geometries
of the ground and triplet excited states. Vibrational frequency calcula-
tions were performed for all stationary points to verify that each was a
minimum (NIMAG=0) on the potential-energy surface. For all of the
calculations, the Stuttgart effective core potentials (ECPs) and the associ-
ated basis set were applied to describe Pt[23] with f-type polarization func-
[1] a) S. R. Forrest, Nature 2004, 428, 911; b) B. W. DꢃAndrade, S. R.
Forrest, Adv. Mater. 2004, 16, 1585; c) B. W. DꢃAndrade, Nat. Pho-
tonics 2007, 1, 33.
[2] a) M. A. Baldo, D. F. OꢃBrien, Y. You, A. Shoustikovm, S. Sibley,
M. E. Thompson, S. R. Forrest, Nature 1998, 395, 151; b) M. A.
Baldo, S. Lamansky, M. E. Thompson, S. R. Forrest, Appl. Phys.
Lett. 1999, 75, 4; c) M. A. Baldo, M. E. Thompson, S. R. Forrest,
Nature 2000, 403, 750; d) G. He, M. Pfeiffer, K. Leo, M. Hofmann, J.
Birnstock, R. Pudzich, J. Salbeck, Appl. Phys. Lett. 2004, 85, 3911.
[3] M. Cocchi, J. Kalinowski, L. Murphy, J. A. G. Williams, V. Fattori,
Org. Electron. 2010, 11, 388.
[4] a) A. Y.-Y. Tam, D. P.-K. Tsang, M.-Y. Chan, N. Y. Zhu, V. W.-W.
Yam, Chem. Commun. 2011, 47, 3383; b) S. C. F. Kui, I. H. T. Sham,
C. C. C. Cheung, C.-W. Ma, B. Yan, N. Zhu, C.-M. Che, W.-F. Fu,
Chem. Eur. J. 2007, 13, 417; c) W. Lu, B.-X. Mi, M. C. W. Chan, Z.
Hui, C.-M. Che, N. Y. Zhu, S.-T. Lee, J. Am. Chem. Soc. 2004, 126,
4958; d) Z. Wang, E, Turner, V. Mahoney, S. Madakuni, T. Groy, J.
Li, Inorg. Chem. 2010, 49, 11276; e) S. J. Farley, D. L. Rochester,
A. L. Thompson, J. A. K. Howard, J. A. G. Williams, Inorg. Chem.
2005, 44, 9690; f) D. L. Rochester, S. Develay, S. Zꢅlis, J. A. G. Wil-
liams, Dalton Trans. 2009, 1728; g) E. S. H. Lam, A. Y.-Y. Tam,
W. H. Lam, K. M.-C. Wong, N. Y. Zhu, V. W.-W. Yam, Dalton Trans.
2012, 41, 8773.
tions (z=0.993),[24] whereas, for all other atoms, the 6–31G
ACTHUNRTGNE(GNU d,p) basis
set[25] was used.
Fabrication and characterization of the PHOLEDs: The PHOLEDs were
fabricated on patterned ITO-coated glass substrates with a sheet resist-
ance of 30 W/sq. The substrates were cleaned with Decon 90, rinsed with
de-ionized water, dried in an oven, and treated in an ultraviolet ozone
chamber. For the PHOLEDs, NPB (thickness: 70 nm) was used as the
hole-transporting layer and TCTA (thickness: 5 nm) was used as the car-
rier-confinement layer. An emissive layer (thickness: 30 nm) that consist-
ed of Pt complexes doped into the MCP layer with different dopant con-
centrations was prepared by thermal co-deposition. For solution-process-
ed OLEDs, PEDOT:PSS (thickness: 70 nm) was spin-coated onto the
ITO-coated glass substrates as a hole-transporting layer. After that, the
emissive layer was formed by mixing the Pt complexes with MCP to pre-
pare 10 mgcmꢀ3 solutions in CHCl3 and spin-coating onto the PE-
DOT:PSS layer to give uniform thin films with a thickness of 60 nm. All
devices were completed by depositing TAZ (thickness: 5 nm) and BAlq
(thickness: 30 nm) as hole-blocking and electron-transporting layers, re-
spectively; LiF/Al was used as the metal cathode. All films were sequen-
tially deposited at a rate of 0.1–0.2 nmsꢀ1 without a vacuum break. A
shadow mask was used to define the cathode and to make four 0.1 cm2
devices on each substrate. Current-density–voltage–luminance character-
istics and EL spectra were measured simultaneously with a programma-
ble Keithley model 2400 power source and a Photoresearch PR650 spec-
trometer.
[5] a) R. F. Carina, A. F. Williams, G. Bernardinelli, J. Organomet.
Chem. 1997, 548, 45; b) T. Yutaka, S. Obara, S. Ogawa, K. Nozaki,
N. Ikeda, T. Ohno, Y. Ishii, K. Sakai, M. Haga, Inorg. Chem. 2005,
44, 4737.
[6] a) B. P. Yan, C. C. C. Cheung, S. C. F. Kui, H.-F. Xiang, V. A. L. Roy,
S.-J. Xu, C.-M. Che, Adv. Mater. 2007, 19, 3599; b) Z. Shen, P. E.
´
Burrows, V. Bulovic, S. R. Forrest, M. E. Thompson, Science 1997,
276, 2009.
[7] a) E. L. Williams, K. Haavisto, J. Li, G. E. Jabbour, Adv. Mater.
2007, 19, 197; b) X. Yang, Z. Wang, S. Madakuni, G. E. Jabbour,
Adv. Mater. 2008, 20, 2405; c) V. Adamovich, J. Brooks, A. Tamayo,
A. M. Alexander, P. I. Djurovich, B. W. D’Andrade, C. Adachi, S. R.
Forrest, M. E. Thompson, New J. Chem. 2002, 26, 1171.
[8] H. J. Bolink, F. De Angelis, E. Baranoff, C. Klein, S. Fantacci, E. Co-
ronado, M. Sessolo, K. Kalyanasundaram, M. Grꢆtzel, Md. K. Na-
zeeruddin, Chem. Commun. 2009, 4672.
[9] A. Y.-Y. Tam, W. H. Lam, K. M.-C. Wong, N. Zhu, V. W.-W. Yam,
Chem. Eur. J. 2008, 14, 4562.
[10] a) O. Yurchenko, D. Freytag, L. zur Borg, R. Zentel, J. Heinze, S.
Ludwigs, J. Phys. Chem. B 2012, 116, 30; b) S. C. Jones, V. Coropcea-
nu, S. Barlow, T. Kinnibrugh, T. Timofeeva, J. L. Brꢇdas, S. R.
Marder, J. Am. Chem. Soc. 2004, 126, 11782.
[11] a) B. R. Hsieh, M. H. Litt, Macromolecules 1986, 19, 521; b) Z. P.
Zhang, B. K. Jin, Y. P. Tian, S. J. Wang, Chin. Chem. Lett. 2005, 16,
1621; c) M. Ates, K. Yilmaz, A. Shahryari, S. Omanovic, A. S. Sarac,
IEEE Sens. J. 2008, 8, 1628.
[12] Q. Wang, F. Xiong, F. Morlet-Savary, S. Li, Y. Li, J.-P. Fouassier,
G. Q. Yang, J. Photochem. Photobiol. A 2008, 194, 230.
[13] Z. He, W.-Y. Wong, H.-S. Kwok, Z. Lin, Inorg. Chem. 2006, 45,
10922.
Supporting information: The Supporting Information, Table S1, lists the
X-ray crystallographic data for complex 7. Tables S2–S9 list selected sin-
glet–singlet transitions of model complexes 1’–8’, which were computed
by using TDDFT/CPCM (CH2Cl2). Tables S10 and S11 list the first two
lowest-lying triplet excited states, as computed by using TDDFT/CPCM
(CH2Cl2), for complexes 1’–4’, 7’, and 8’. The Supporting Information,
Figure S1, shows the crystal-packing diagram of complex 7. Figure S2
shows the cyclic voltammogram of complex 5. Figure S3 shows electronic
absorption spectra of complexes 4, 6, and 8. Figures S4–S7 show spatial
plots of selected molecular orbitals of complexes 1’, 4’, 7’, and 8’. Fig-
ure S8 shows normalized EL spectra of single EML devices with com-
plexes 2 and 3, as well as Cartesian coordinates of their optimized geo-
metries.
[14] H. Sasabe, E. Gonmori, T. Chiba, Y. J. Li, D. Tanaka, S. J. Su, T.
Takeda, Y. J. Pu, K. Nakayama, J. Kido, Chem. Mater. 2008, 20,
5951.
[15] a) F. Sanda, T. Kawaguchi, T. Masuda, Macromolecules 2003, 36,
2224; b) S. Ch. Suh, M. Ch. Suh, S. Ch. Shim, Macromol. Chem.
Phys. 1999, 200, 1991; c) R. J. Rahaim, J. T. Shaw, J. Org. Chem.
2008, 73, 2912.
Acknowledgements
V.W.-W.Y. acknowledges support from The University of Hong Kong and
the URC Strategic Research Theme on Molecular Materials. This work
has been supported by a grant from the Theme-Based Research Scheme
(TBRS) of the Research Grants Council of the Hong Kong Special Ad-
ministrative Region, China (Project No. T23–713/11). E.S.-H.L. acknowl-
edges the receipt of a Postgraduate Studentship and A.Y.-Y.T. acknowl-
edges the receipt of a University Postdoctoral Fellowship, both from The
University of Hong Kong. We are grateful to Dr. L. Szeto for her assis-
tance in X-ray crystal-structure data collection and determination and to
the Computer Center of The University of Hong Kong for providing
computational resources.
[16] SAINT+, SAX area detector integration program, version 7.34 A;
Bruker AXS, Inc., Madison.
[17] G. M. Sheldrick, SADABS, Program for empirical absorption correc-
tion of area-detector data, Universitꢆt Gçttingen, Germany, 2008.
[18] P. T. Beurskens, G. Admiraal, G. Beurskens, W. P. Bosman, R. de
Gelder, R, Israel, J. M. M. Smits, The DIRDIF-99 program system,
Technical Report of the Crystallography Laboratory, University of
Nijmegen, The Netherlands, 1999.
6396
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 6385 – 6397

Luminescent Platinum(II) Complexes with Applications in OLEDs
FULL PAPER
[19] Gaussian 09 (Revision C.01), M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V.
Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X.
Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J.
Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Ko-
bayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant,
S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gom-
perts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Po-
melli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzew-
ski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D.
Daniels, ꢈ. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J.
Fox, Gaussian, Inc., Wallingford CT, 2010.
[21] a) V. Barone, M. Cossi, J. Phys. Chem. A 1998, 102, 1995; b) M.
Cossi, N. Rega, G. Scalmani, V. Barone, J. Comput. Chem. 2003, 24,
669.
[22] a) R. E. Stratmann, G. E. Scuseria, M. J. Frisch, J. Chem. Phys. 1998,
109, 8218; b) R. Bauernschmitt, R. Ahlrichs, Chem. Phys. Lett. 1996,
256, 454; c) M. E. Casida, C. Jamorski, K. C. Casida, D. R. Salahub,
J. Chem. Phys. 1998, 108, 4439.
[23] D. Andrae, U. Hꢆussermann, M. Dolg, H. Stoll, H. Preuss, Theor.
Chim. Acta. 1990, 77, 123.
[24] A. W. Ehlers, M. Bçhme, S. Dapprich, A. Gobbi, A. Hçllwarth, V.
Jonas, K. F. Kçhler, R. Stegmann, A. Veldkamp, G. Frenking, Chem.
Phys. Lett. 1993, 208, 111.
[25] a) W. J. Hehre, R. Ditchfield, J. A. Pople, J. Chem. Phys. 1972, 56,
2257; b) P. C. Hariharan, J. A. Pople, Theor. Chim. Acta. 1973, 28,
213; c) M. M. Francl, W. J. Pietro, W. J. Hehre, J. S. Binkley, M. S.
Gordon, D. J. Defrees, J. A. Pople, J. Chem. Phys. 1982, 77, 3654.
[20] a) J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77,
3865; b) J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1997,
78, 1396; c) C. Adamo, V. Barone, J. Chem. Phys. 1999, 110, 6158.
Received: September 14, 2012
Published online: March 19, 2013
Chem. Eur. J. 2013, 19, 6385 – 6397
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6397