Highly efficient deep-blue emitters based on cis and trans N-heterocyclic carbene PtII acetylide complexes: Synthesis, photophysical properties, and mechanistic studies

Article abstract of DOI:10.1002/chem.201302196

We have synthesized cis and trans N-heterocyclic carbene (NHC) platinum(II) complexes bearing σ-alkynyl ancillary ligands, namely [Pt(dbim) ₂(Ci￡CR)₂] [DBIM=N,N′- didodecylbenzimidazoline-2-ylidene; R=C₆H₄F (4), C ₆H₅ (5), C₆H₂(OMe)₃ (6), C₄H₃S (7), and C₆H₄Ci￡ CC₆H₅ (8)] and [Pt(ibim)₂(Ci￡CC ₆H₅)₂] (9) (ibim=N,N′- diisopropylbenzimidazoline-2-ylidene), starting from [Pt(cod)(Ci￡CR) ₂] (COD=cyclooctadiene) and 2equivalents of [dbimH]Br ([ibimH]Br for complexes 9) in the presence of tBuOK and THF. Mechanistic investigations aimed at uncovering the cis to trans isomerization reaction have been performed on the representative cis complex 5 a [Pt(dbim)₂(Ci￡CC ₆H₅)₂] and revealed the isomerization to progress smoothly in good yield when 5 a was treated with catalytic amounts of [Pt(cod)(Ci￡CR)₂] at 75 °C in THF or when 5 a was heated at 200 °C in the solid state under an inert atmosphere. Detailed examination of the reactions points to the possible involvement, in a catalytic fashion, of a solvent-stabilized Pt^II dialkyne complex in the former case and a Pt⁰ NHC complex in the latter case, for the transformation of the cis isomer to the corresponding trans complex. Thermal stability and the isomerization process in the solid state have been further investigated on the basis of TGA and DSC measurements. X-ray diffraction studies have been carried out to confirm the solid-state structures of 4 b, 5 a, 5 b, and 9 b. All of the synthesized dialkyne complexes 4-9 exhibit phosphorescence in solution, in the solid state at room temperature (RT), and also in frozen solvent glasses at 77K. The emission wavelengths and quantum yields have been found to be highly tunable as a function of the alkynyl ligand. In particular, the trans isomer of complex 9 in a spin-coated film (10wt % in poly(methyl methacrylate), PMMA) exhibits a high phosphorescence quantum yield of 80 %, which is the highest reported for Pt^II-based deep-blue emitters. Experimental observations and time-dependent density functional theory (TD-DFT) calculations are strongly indicative of the emission being mainly governed by metal-perturbed interligand (³IL) charge transfer. Deep-blue emitters: Triplet emitters based on cis and trans N-heterocyclic carbene platinum(II) complexes have been synthesized and characterized (see scheme). The structural features of the complexes offer highly tunable emission properties. A deep-blue emitter with a quantum yield of 80 % has been achieved. Copyright

Full text of DOI:10.1002/chem.201302196

FULL PAPER
DOI: 10.1002/chem.201302196
Highly Efficient Deep-Blue Emitters Based on cis and trans N-Heterocyclic
Carbene PtII Acetylide Complexes: Synthesis, Photophysical Properties, and
Mechanistic Studies
Yuzhen Zhang, Olivier Blacque, and Koushik Venkatesan*[a]
ꢁ
Abstract: We have synthesized cis and
trans N-heterocyclic carbene (NHC)
platinum(II) complexes bearing s-al-
kynyl ancillary ligands, namely [Pt-
ꢁ
N
N
state structures of 4b, 5a, 5b, and 9b.
All of the synthesized dialkyne com-
plexes 4–9 exhibit phosphorescence in
solution, in the solid state at room tem-
perature (RT), and also in frozen sol-
vent glasses at 77 K. The emission
wavelengths and quantum yields have
been found to be highly tunable as
a function of the alkynyl ligand. In par-
ticular, the trans isomer of complex 9
in a spin-coated film (10 wt% in poly(-
methyl methacrylate), PMMA) exhibits
a high phosphorescence quantum yield
of 80%, which is the highest reported
for PtII-based deep-blue emitters. Ex-
perimental observations and time-de-
pendent density functional theory (TD-
DFT) calculations are strongly indica-
tive of the emission being mainly gov-
when 5a was heated at 2008C in the
solid state under an inert atmosphere.
Detailed examination of the reactions
points to the possible involvement, in
a catalytic fashion, of a solvent-stabi-
lized PtII dialkyne complex in the
former case and a Pt0 NHC complex in
the latter case, for the transformation
of the cis isomer to the corresponding
trans complex. Thermal stability and
the isomerization process in the solid
state have been further investigated on
the basis of TGA and DSC measure-
ments. X-ray diffraction studies have
been carried out to confirm the solid-
ACHTUNGTRENNUNG(dbim)2ACHTUNGTRENNUNG(C CR)2] [DBIM=N,N’-dido-
decylbenzimidazoline-2-ylidene;
R=
C6H4F (4), C6H5 (5), C6H2A(OMe)3 (6),
CTHUNGTRENNUNG
ꢁ
C4H3S (7), and C6H4C CC6H5 (8)] and
ꢁ
[Pt
N
G
N,N’-diisopropylbenzimidazoline-2-yli-
ꢁ
dene), starting from [Pt
(cod)
N
(COD=cyclooctadiene) and 2 equiva-
lents of [dbimH]Br ([ibimH]Br for
complexes 9) in the presence of tBuOK
and THF. Mechanistic investigations
aimed at uncovering the cis to trans
isomerization reaction have been per-
formed on the representative cis com-
ꢁ
Keywords: acetylides
plex 5a [PtACHTUNGTRENNUNG(dbim)2ACHTUNGTRENNUNG(C CC6H5)2] and re-
tion N-heterocyclic carbenes
·
vealed the isomerization to progress
smoothly in good yield when 5a was
treated with catalytic amounts of [Pt-
erned by metal-perturbed inter
(3IL) charge transfer.
ACHTUNGTRENNUNGliACHTUNGTRENNUNGgand
OLEDs · phosphorescence · plati-
Introduction
metal-centered d–d energy states, which in turn lowers the
HOMO–LUMO energy gap resulting in emissive com-
Small-molecule triplet emitters and polymers based on plati-
num(II) complexes have received considerable attention
due to their potential applications in phosphorescent organic
light-emitting devices (PhOLEDs),[1] chemosensors,[2] non-
linear optical (NLO) materials,[3] photocatalysts,[4] optical
limiting materials,[5] and photovoltaic devices.[6] This wide
range of applications can be attributed to platinumꢀs spin-
orbit coupling constant of 4481 cmꢀ1,[7] which promotes effi-
cient intersystem crossing (ISC) from singlet to triplet excit-
ed states. When the fast ISC rates of Pt complexes are com-
bined with a strong field ligand like an alkyne, it creates
a strong interaction through pp–dp overlap and raises the
plexes.[1i,8]
Since the time of the first report of a phosphine-ligated
PtII acetylide complex, trans-[Pt
(PEt3)
G
ꢁ
and Shaw in 1959,[9] intensive research investigations have
been carried out on the photophysical properties of these
complexes (Figure 1, top). Studies of the phosphorescent
emission properties of small molecules,[10] oligomers,[11] den-
drimers,[3c,12] and polymers[1b,13] based on phosphine PtII ace-
tylide fragments have indicated moderate to low quantum
yields with emission only in the low-energy part of the visi-
ble spectrum. The bipyridine PtII acetylide complexes first
reported by Che and co-workers showed a metal–ligand
charge transfer (3MLCT) [5d(Pt)!p*phen]-centered emission
in fluid solution at RT.[14] Subsequently, the groups of Eisen-
berg,[4a,15] Schanze,[16] Castellano,[4b,8a,17] and Yam[3b,c,8b,12,18]
carried out extensive investigations on similar molecular en-
tities and in due course expanded their diversity with the
[a] Y. Zhang, Dr. O. Blacque, Dr. K. Venkatesan
Inorganic Chemistry, University of Zꢁrich,
Winterthurerstrasse 190, 8057 Zꢁrich (Switzerland)
Fax : (+41)44 6356803
+
ꢁ
use of polypyridine-type ligands [Pt
(N^N^N)
N
E-mail: venkatesan.koushik@aci.uzh.ch
[4a,4c,5b,15f,19]
ꢁ
and [Pt
C
(C CR)] (Figure 1, botto-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302196.
m).[5a,20] The origin of the triplet emission in diimine PtII ace-
Chem. Eur. J. 2013, 19, 15689 – 15701
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
15689

properties of the cis NHC PtII
dialkyne complexes in solution
and the solid state, the trans
NHC PtII dialkyne complexes
were expected to be suitable
candidates as novel building
blocks for the preparation of
linear organometallic oligomers,
dendrimers, macrocycles, and
polymers. NHC ligands bearing
dodecyl chains have been used
to enhance the solubility of the
final complexes.
Figure 1. Platinum(II) dialkynyl complexes bearing phosphine ligands (top) and nitrogen ligands (bottom).
In this context, we report
herein detailed investigations
on the synthesis, solid-state
structures, and luminescent
tylide complexes has been ascribed to a mixture of intra
G
properties of new trans bis-NHC PtII bisacetylide complexes.
Different reaction conditions have been employed to pre-
pare the trans complexes in high yields, and studies have
been carried out aimed at elucidating the mechanism under-
pinning their formation. Since the electronic properties of
the alkyne ligands were expected to strongly influence the
luminescent properties, complexes bearing different alkyne
ligands were targeted, and experimental and theoretical in-
vestigations were carried out to analyze the nature of the
excited states. The remarkable quantum yields of the trans
complexes of 4 and 5 in the solid state are the highest re-
ported for deep-blue emitters. The readily tunable emission
properties hold great promise for the potential use of this
fragment in the construction of light-emitting macromole-
cules for varied applications.
3
gand charge transfer
3[p!p*
ACHTUNGTRENNUNG
ꢁ
ligand charge transfer 3[5d(Pt)!p*
(NN)] (3MLCT) giving
ACHTUNGTRENNUNG
rise to moderate to good emission properties, albeit only at
longer wavelengths.[21]
Since phosphine- and diimine-bound PtII acetylide small
molecules show mostly moderate to good emission proper-
ties in the low-energy part of the visible spectrum, the de-
velopment of highly emissive and stable small-molecule
emitters at the high-energy end of the spectrum is highly de-
sirable. Such systems could serve as key building blocks for
the construction of new polymers and dendrimers that meet
the demands of a number of applications.[16,17b,c,22] N-Hetero-
cyclic carbene (NHC) ligands are considered as an interest-
ing alternative to phosphine and pyridine ligands. Given
that NHCs are strong s-donors, their strong Lewis basicity
II
ꢀ
would be expected to result in stronger Pt ligand bonds,
which would significantly reduce the likelihood of nonradia-
tive deactivation of the excited states through dissociation
of the ligand upon excitation.[1f,7c,23] In addition, the strong
s-donation of the NHC ligands is expected to be effective in
preventing thermal access to metal-centered d–d states by
pushing these to higher energies.[1h,24] The versatile coordina-
tion modes of NHCs would further allow the preparation of
molecules with large conformational diversity.[13c] Exquisite
control of the steric environment and electronic properties
can be achieved by rational modification of the substituents
on the nitrogen atoms and the NHC framework.[25] Follow-
ing the report of Strassner and co-workers on dicationic
blue-light-emitting tetra-NHC PtII complexes,[1d,26] we re-
cently described the preparation and photophysical investi-
gation of a series of neutral bis-NHC PtII acetylide and bis-
NHC PdII acetylide complexes.[27] Some of the bis-NHC PtII-
acetylides displayed blue emission at room temperature and
the luminescent properties were found to be tunable by
changing the functional group on the alkyne.[28] TD-DFT
Results and Discussion
Synthesis and characterization: Transition metal alkynyl
complexes are mostly synthesized by well-known transme-
tallation procedures involving copper acetylides, alkynyl
stannanes, or lithium acetylides starting from the readily ac-
cessible metal halide complexes. However, previous studies
have shown that trans [PtACHTNUTGRNEUNG
(dbim)2Br2][29] (DBIM=N,N’-diiso-
propylbenzimidazoline-2-ylidene), particularly bearing non-
sterically demanding substituents at the nitrogen atoms, can
only be generated in a dismal yield (3%), and hence this ap-
ꢁ
proach was discarded. [PtACTHNUTRGENN(UG cod)ACHTUTGNREN(NUNG C CR)2] (cod=cycloocta-
diene) was considered as a suitable precursor, since the
labile COD ligand was expected to be readily displaced in
the presence of the NHC ligand due to the strong trans
effect of the alkynyl ligand. Based on an earlier literature
ꢁ
protocol, the precursor complexes of the type [Pt
G
ACHTUNGTRENNUNG(C
CR)2] (R=C6H4F (1), C6H5 (reported),[30a] C6H
CHTUNGTRENNUNG
(2), 2-thienyl (3), and C6H4C CC6H5 (reported)[30b]) were
ꢁ
calculations revealed that the emission was dominated by
obtained in good to moderate yields by treating [Pt
(cod)Cl2]
3
intra
E
C
G
(C CR)] ( IL), with lim-
with the corresponding sodium acetylides in ethanol.[31] Sub-
ꢁ
ꢁ
ited participation of the metal center in the excited state. In
view of the observed intriguing phosphorescence emission
sequent treatment of [Pt
lents of DBIM in THF at 758C resulted in a mixture of two
G
G
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Chem. Eur. J. 2013, 19, 15689 – 15701

Blue-Emitting N-Heterocyclic Carbene PtII Acetylide Complexes
FULL PAPER
Scheme 1.
1
complexes. The H NMR spectrum of the mixture featured
C6H5 (9a,b)) were synthesized (Scheme 1). The isolated
yields were between 9 and 42% for the cis isomers and be-
tween 4 and 27% for the trans isomers, depending on the
alkyne substituents. All of the cis and trans complexes were
three multiplets at d=4.89, 4.86, and 4.38 ppm, characteris-
tic resonances of the -CH2 protons bound to the a-C con-
nected to the benzimidazole nitrogen (NCH2C11H23). ESI-
MS analysis of the mixture gave a peak at m/z 1330.0
[M+Na]+. Based on detailed NMR studies, the multiplets at
d=4.89 and 4.38 ppm were assigned to the cis isomer 5a
and that at d=4.86 ppm to the trans isomer 5b (Figure 2).
1
characterized by H NMR, 13C NMR, and IR spectroscopy,
as well as ESI-MS. The structure of 4b was also confirmed
by a single-crystal X-ray diffraction study. In order to im-
prove the yield of the highly desired trans product and to ac-
count for the observation of
both cis and trans product
during the reaction, further in-
vestigations were carried out.
When the reaction was per-
formed at room temperature by
using 3.0 equivalents of the
NHC ligand instead of the orig-
inal 2.0 equivalents,
a
near-
quantitative yield of 5a was ob-
tained with only a trace of the
trans isomer. Significant im-
provements in the yields (47–
90%) of the respective cis iso-
mers were accomplished when
3.0 equivalents of the NHC
were used in the reaction.
Single-crystal X-ray diffraction
analysis: Single crystals suitable
for X-ray diffraction were
grown by slow evaporation of
the solvent from a concentrated
Figure 2. 1H NMR spectra of cis complex 5a and trans complex 5b.
solution of the compound in
The two products were obtained in yields of 27 and 24%,
respectively, after separation by column chromatography on
silica gel. Single-crystal X-ray diffraction studies further con-
firmed the structures of 5a and 5b. Analogously, various cis
ꢁ
a mixture of MeOH and CH2Cl2. The molecular structures
of complexes 4b, 5a, 5b, and 9b are shown in Figure 3.
Crystallographic parameters are summarized in Table S1 in
the Supporting Information and selected bond lengths and
angles are listed in Table 1. The four complexes exhibit dis-
torted square-planar coordination geometries about the PtII
and trans complexes of the type [PtACHTNUGTRENNUG(dbim)2ACHTUNGTRENN(UGN C CR)2] (R=
C6H4F (4a,b); C6H5 (5a,b); C6H2ACHTNUTRGNEUNG(OCH3)3 (6a,b); 2-thienyl
ꢁ
ꢁ
ꢀ
(7a,b); C6H4C CC6H5 (8a,b)) and [Pt
(ibim)
G
core. The Pt Ccarb distances in the complexes range from
Chem. Eur. J. 2013, 19, 15689 – 15701
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15691

K. Venkatesan et al.
Figure 3. Molecular structures of 4b, 5a, 5b, and 9b showing selected atomic numbering. Ellipsoids are set at the 50% probability level. Hydrogen
atoms and solvent molecules are omitted for clarity
ꢀ
ꢁ
2.0168(18) to 2.029(3) ꢃ and the Pt Calk distances from
thus comparable to those of reported [PtACTHUGNRTEN(NUG diimine)CAHTUNGTERN(NUNG C CR)2]
ꢀ
1.996(3) to 2.0092(18) ꢃ. The Pt Ccarb distances in the cur-
complexes. In comparison with the spectra of our previously
reported bidentate NHC PtII acetylide complexes, the low-
energy absorption bands are hypsochromically shifted by ap-
proximately 40 nm and overlap with the high-energy bands.
Varying the solvent in further UV/Vis studies on these com-
plexes showed no significant shifts in the absorption wave-
lengths.
rent complexes are longer than those in previously reported
analogous structures, which may be due to the trans influ-
ence of the strongly electron-donating acetylene ligands.[29,32]
ꢀ
The Pt Calk distances are shorter than those in reported
[33]
ꢁ
[Pt(phosphine)
G
but 0.04 ꢃ longer
[15a,21]
than those in [Pt(bipyridine)
(C CR)2] complexes.
Pt Ccarb distance in 5a is 0.01 ꢃ longer than that in 5b, but
the reverse is true for the Pt Calk distances. The shortest
Pt···Pt distance in these structures was found to be
8.6169(2) ꢃ, and hence any intermolecular Pt···Pt interaction
in the solid state can be ruled out.
The
ꢀ
Based on the spectroscopic studies and the DFT and TD-
DFT calculations, the low-energy absorption bands in these
ten complexes are assigned to a mixture of metal-perturbed
ligand-to-ligand 1LLCT (palk!p*carb) and metal-to-ligand
1MLCT (5d(Pt)!p*carb) transitions.[15a,21] The high-energy
absorption bands that display the characteristic p!p* tran-
UV/Vis absorption and emission: The UV/Vis absorption
spectra of complexes 4–9 in solution at 298 K are shown in
Figure 4 and Figure S1 in the Supporting Information. All
ten complexes exhibit similar spectra, with an intense ab-
sorption band at 284–300 nm and an additional shoulder
band (low-energy absorption bands) at 305–358 nm. The
molar absorption coefficients (e) for the intense absorption
bands are in the range 2.0ꢄ104 to 6.0ꢄ104 mꢀ1 cmꢀ1, and are
gand 1LLCT
sition are assigned to metal-perturbed intraACTHUNGTRENNUlG iHCATUNGTRENNNUG
(palk!p*alk) transitions.[16,18b,20a] There are no significant dif-
ferences in the patterns of the absorption bands and absorp-
tion maxima for complexes 4–7 and 9. However, in the case
ꢁ
of complexes 8a and 8b bearing the conjugated ligand C
CC6H4C CC6H5, the absorption bands are further redshifted
to 343 and 353 nm, respectively. The observed redshifts in
the absorptions of 8a and 8b in comparison to those of the
ꢁ
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Blue-Emitting N-Heterocyclic Carbene PtII Acetylide Complexes
FULL PAPER
Table 1. Selected bond lengths [ꢃ] and angles [8] of complexes 4b, 5a,
5b, and 9b.
Distance [ꢃ]
Angle [8]
Complex 4b
ꢀ
C(1) Pt(1)
2.020(2)
2.0092(18)
C(1)-Pt(1)-C(32)
C(1)-Pt(1)-C(32)i
88.85(8)
91.15(8)
ꢀ
C(32) Pt(1)
Complex 5a
ꢀ
C(1) Pt(1)
2.026(3)
2.029(3)
1.996(3)
2.006(3)
C(63)-Pt(1)-C(71)
C(71)-Pt(1)-C(1)
C(63)-Pt(1)-C(32)
C(1)-Pt(1)-C(32)
84.14(13)
91.42(12)
92.16(12)
92.37(11)
ꢀ
C(32) Pt(1)
ꢀ
C(63) Pt(1)
ꢀ
C(71) Pt(1)
Complex 5b
ꢀ
C(1) Pt(1)
2.0043(19)
2.0168(18)
C(1)-Pt(1)-C(9)
C(1)-Pt(1)-C(9)i
88.60(7)
91.40(7)
ꢀ
C(9) Pt(1)
Complex 9b
ꢀ
C(1) Pt(1)
2.008(2)
2.027(2)
C(1)-Pt(1)-C(9)
C(8)-Pt(1)-C(1)
93.03(8)
86.97(8)
ꢀ
C(9) Pt(1)
[i]ꢀx,ꢀy,ꢀz.
other eight complexes reflect the smaller HOMO–LUMO
gaps arising from the increased conjugation of the alkyne
ligand. Comparison of the UV/Vis absorption bands be-
tween the isomers revealed those of the trans isomers to be
slightly bathochromically shifted with lower molar absorp-
tion coefficients than those of the corresponding cis isomers.
All of the synthesized dialkyne complexes were found to
be emissive both in solution and the solid state. The emis-
sion spectra of the complexes recorded in CH2Cl2 featured
bands in the range 431–530 nm (Table 2). The large Stokes
shifts (ꢂ150 nm), along with emission lifetimes of the order
of microseconds and rapid quenching of the emission in air,
are highly indicative of emission from a triplet manifold.
The emission wavelengths of the cis complexes 4a–6a are
bathochromically shifted with increasing electron-donating
character (F<H<(OMe)3) of the substituents on the al-
kynes. A similar trend was seen for the trans isomers 4b–6b.
All of the trans isomers show a 3–6 nm redshift in their
emission maxima compared to the corresponding cis iso-
mers. This can be attributed to the efficient charge delocal-
Figure 4. Top) Electronic absorption and normalized emission spectra of
4a (light-gray dotted line), 4b (light-gray solid line), 5a (black dotted
line), 5b (black solid line), 6a (dark-gray dotted line), 6b (dark-gray
solid line) in CH2Cl2 at RT. Bottom) Electronic absorption and normal-
ized emission spectra of 7a (light-gray dotted line), 7b (light-gray solid
line), 8a (black dotted line), and 8b (black solid line) in CH2Cl2 at RT.
sion due to the thienyl group. Their emission maxima are
also bathochromically shifted by about 70 nm compared to
those of complexes 6a and 6b. In accordance with the differ-
ent electron donicities and electron delocalization of the al-
kynyl substituents, the emission maxima of complexes 8a
ACHTUNGTRENNUNGizaACHTUNGTRENNUNGtion in the highly symmetrical trans isomer, resulting in
a decrease in the HOMO–LUMO gap. Complexes 7a and
7b exhibit broad bands along with a strong vibronic progres-
ꢁ
ꢁ
and 8b bearing C CC6H4C CC6H5 are significantly redshift-
Table 2. Photophysical properties of complexes 4a–9a and 4b–9b.
Room temperature solution (CH2Cl2)
77 K Glass
ACTHNUTRGNEUNG(2-MeTHF) Emission
lmax [nm]
10 wt% PMMA Film
Absorption lmax [nm]
Emission lmax [nm]
Solution Solid state
t
f
em ꢄ10ꢀ2
[ms] CH2Cl2 Solid
kr
[ꢄ103 sꢀ1
knr
[ꢄ105sꢀ1
fem ꢄ10ꢀ2
G
E
]
G
]
lmax [nm]
4a 284 (51634), 293 (50335) 431, 462 sh 432, 450 sh, 472 sh 0.22 0.4
5a 285 (49082), 300 (39978) 432, 465 sh 430, 450 sh, 475 sh 0.24 0.9
6a 289 (55406), 310 (33877) 454, 478 sh 454, 471 sh, 505 sh 3.48 1.2
7.1
1.6
0.2
0.5
3.4
19
36
3.4
0.4
0.5
10
70
2.7
7.5
1.4
1.4
8.1
45
41
2.8
1.3
1.4
9.9
422, 440 sh
425, 447 sh
442, 475 sh
488, 503
516, 548 sh
420, 448 sh
426, 445 sh
430, 451 sh
440, 473 sh
488, 504
429, 449
431, 449
454, 484
506, 531
523, 558
431, 450
434, 457 sh 52.6
440, 460 sh 44.7
453, 484
511, 535
525, 563
436, 456
65.1
60.8
7.5
5.0
5.2
7a 293 (64198), 322 (41740) 529, 533 sh 496, 511, 531, 550
8a 327 (43462), 343 (40726) 522, 554 sh 526, 558, 573 sh
7.90 0.4
6.80 0.4
9a 285 (47827), 302 (34208) 432, 465 sh 432, 450 sh, 460 sh 0.10 0.1
4b 281 (48381), 311 (26955) 436, 471 sh 416, 431 sh, 457 sh 0.37 2.6
5b 288 (31751), 315 (15709) 439, 461 sh 443, 465 sh, 487 sh 3.58 1.0
6b 289 (45128), 318 (23190) 457, 498 sh 463, 481 sh, 523 sh 2.52 1.9
0.8
53.6
44.5
46.6
0.2
0.4
5.7
26
2.8
3.9
4.1
1.8
8.9
7.7
1.5
6.9
7b 289 (41746), 333 (26741) 505 sh, 534 502, 516, 538, 556
8b 291 (19531), 353 (39222) 527, 561 529, 561, 576 sh
9b 283 (34110), 310 (18219) 439, 455 sh 438, 451 sh
2.41 0.3
5.30 0.8
1.11 0.9
522, 555
423, 450 sh
4.1
80.0
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15693

K. Venkatesan et al.
ed to 522 and 527 nm, respectively. The triplet emissions are
assigned to the metal-perturbed ligand-to-ligand (palk
9b. The absorption and emission spectra of complexes 9a
and 9b were the same as those of complexes 5a and 5b.
However, the quantum yields of 5b and 9b at 10 wt% in
PMMA differed significantly.
!
p*alk) transition. An increase in ligand donicity raises the al-
kynyl-based HOMO energy level, thereby decreasing the
energy gap between the HOMO and LUMO, which in turn
results in a bathochromic shift of the emission. The solution
quantum yields of complexes 4–9 were found to lie in the
range 1.0ꢄ10ꢀ3 to 2.6ꢄ10ꢀ2 by using quinine sulfate as a ref-
erence (fem =0.55). These are comparable to those of
ꢁ
Based on the above results, we can conclude that in-
creased molecular rigidity and weaker intermolecular inter-
actions in the solid state lead to a significant enhancement
in quantum efficiency in molecules of this type. The devel-
opment of blue-emitting PtII complexes has been notoriously
difficult, and only recently have quantum yields of 90 and
86% with lem =464 nm been reported by the groups of
Strassner[1d] and Wang[1f] by using cyclometalated ligands. To
the best of our knowledge, the emission quantum yield of
80% for 9b with lem =436 nm is the highest reported for
a deep-blue emitter in this range. This work provides further
impetus to the design of highly emissive deep-blue emitters
devoid of a cyclometalating ligand.
known [PtACHTUNGTRENNUNG(diimine)ACHTUNGTRENNUNG(C CR)2] complexes and our previously
reported chelating NHC Pt acetylide complexes.
In a rigidified glass medium (2-methyltetrahydrofuran, 2-
MeTHF) at 77 K, the spectra of 4–9 are well-resolved, as
shown in Figures S2 and S3 in the Supporting Information.
The emission maxima of the complexes are in the wave-
length range 422–522 nm, shifted by 5–58 nm compared with
those at RT. Unlike the broad and unsymmetrical emission
profiles at room temperature, the spectra at 77 K exhibit
a rigidochromic shift and resolvable vibronic components of
the emission. The vibrational progressional spacings of 7a
are 650 and 1450 cmꢀ1, which can be assigned to the charac-
teristic skeletal vibration of the thienyl ring, and 2041 cmꢀ1,
Electrochemistry: Cyclic voltammograms of complexes 4–8
were measured in DMF by using [nBu4N]ACTHUNTRGNENG[U PF6] (0.1m) as
supporting electrolyte. The electrochemical data are listed in
Table 3. There is only one irreversible oxidation wave for
each complex and no observable reduction wave on scan-
which can be assigned to the n˜(C
stretching vibration of
ꢁ
C)
(Figure S4 in the Supporting Information). Complex 7b was
found to exhibit similar behavior to that of 7a. The vibra-
Table 3. Electrochemical potentials for 4a–9a and 4b–9b.[a]
tional progressional spacings of complex 8b are 1106, 1613,
ꢀ1
ꢀ
and 2092 cm , which can be assigned to the C N stretching
Complex
Eox [V]
Complex
Eox [V]
vibration, the skeletal vibration of the phenyl ring, and the
4a
5a
6a
7a
8a
9a
+0.86
+0.84
+0.58
+0.86
+0.94
+0.85
4b
5b
6b
7b
8b
9b
+0.79
+0.77
+0.66
+0.75
+0.83
+0.78
n˜(C
stretching vibration, respectively (Figure S4 in the
ꢁ
C)
Supporting Information). Complex 8a exhibits a similar be-
havior. The rest of the complexes display features like those
of 4b (Figure S5 in the Supporting Information), comprising
two vibrational progressional spacings of 1100 cmꢀ1 (C N
ꢀ
[a] Scan rate=100 mVsꢀ1 in 0.1m [nBu4N]
/Fc; 208C; DMF).
[PF6] (Au electrode; E vs Fc+
ACHTUNGTRENNUNG
stretching vibration of benzimidazole) and 2100 cmꢀ1 (n˜(C
ꢁ
C)
stretching vibration). These vibronic emission peaks further
support the involvement of the alkynyl ligand in the triplet
emission. At 77 K, the emission maxima of the trans isomers
show moderate redshifts with respect to those of the corre-
sponding cis isomers, although for complex 6b the redshift
is only 2 nm relative to the maximum for 6a. This difference
is also reflected in the oxidation potentials of the complexes
derived from the CV measurements of the cis and trans
complexes (see below). All of the synthesized complexes 4–
9 show emission in the solid state. The absolute quantum
yields of all of the cis and trans complexes in pure crystalline
form were measured. Interestingly, the maxima of the triplet
emissions of all the complexes appear at different wave-
lengths in different media due to the effects of the solvent
environment and molecular aggregation (Figures S6–S8 in
the Supporting Information). These influences also extend
to the luminescent efficiencies of the complexes. The quan-
tum yields of most of the complexes in fluid solution were
found to lie in a low range from 0.1 to 2.6%, but increased
to 47% in the case of complex 5b. Moreover, when the
complexes were dispersed at 10 wt% in poly(methyl metha-
crylate) (PMMA) films, their quantum yields were found to
be significantly enhanced, to 80% in the case of complex
ning up to ꢀ2.57 V (Figure S9 in the Supporting Informa-
tion). This behavior is consistent with the stronger s-donat-
ing nature of the NHC ligand and quite closely resembles
ꢁ
that of analogous [Pt(phosphine) CHTUNGTERN(NUNG C CR)2] complexes.
2A
ꢁ
However, it differs from that of [Pt
A
plexes, which show one or more reversible reduction waves
associated with the diimine ligand. For complexes 4, 5, and
6, the oxidation potential decreases with increasing donor
ability of the alkynyl functional group (F<H<(OMe)3).
Comparing the oxidation potentials of the respective pairs
of cis and trans isomers, that of the cis isomer is about 50–
110 mV higher than that of the trans isomer, except in the
case of compounds 6a and 6b. This abnormal behavior of
6a and 6b is consistent with the emission maxima observed
at 77 K, with 6b emitting at a higher energy than 6a. The
oxidation waves observed for all of the complexes are tenta-
tively assigned to oxidation of the alkynyl ligand rather than
a PtII/PtIII process given the low oxidation potential (<+
1.0 V) and since the HOMO (93%) is mainly located on the
ꢁ
C CR p orbital, according to DFT calculations.
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Blue-Emitting N-Heterocyclic Carbene PtII Acetylide Complexes
FULL PAPER
DFT calculations: In order to study the luminescent proper-
ties of our cis and trans N-heterocyclic carbene PtII acetylide
complexes, we performed DFT and TD-DFT calculations on
5a and 5b with the Gaussian 03 program package.[34] The
hybrid functional PBE1PBE (also known as PBE0)[35] in
conjunction with the Stuttgart/Dresden effective core poten-
tials (SDD) basis set[36] for the Pt center, augmented with
one f-polarization function (exponent=0.993), and the stan-
dard 6-31+G(d) basis set[37] for the remaining atoms were
applied for all calculations. Full geometry optimizations
without symmetry constraints, but with a C2H5 model in-
stead of the long C12H25 chain of the carbene ligands in 5a
and 5b, were carried out in the gas phase for the singlet
ground states (S0) and the lowest triplet states (T1). The op-
timized geometries were confirmed to be potential energy
minima by vibrational frequency calculations at the same
level of theory, as no imaginary frequency was found. The
first ten singlet–singlet and singlet–triplet transition energies
were computed for the optimized S0 geometries, by using
the time-dependent DFT (TD-DFT) methodology. Solvent
effects were taken into account by using the conductor-like
polarizable continuum model (CPCM) with dichlorome-
thane (absorption/emission) or toluene (mechanistic study)
as solvent for single-point calculations on all of the opti-
mized gas-phase geometries.
are assigned to a mixture of metal-perturbed ligand-to-
ligand 1LLCT (palk!p*carb) and metal-to-ligand 1MLCT
(5d(Pt)!p*carb
) transitions. The high-energy absorption
bands, which show the characteristics of p!p* transitions,
are assigned to metal-perturbed intrali
A
R
!
p*alk) transitions. TD-DFT calculations carried out on the
ground-state structures of 5a and 5b confirmed the experi-
mental analyses. In the ground-state geometry, the lowest
significant singlet excited states S1, S3, and S8 (f>0.05) of 5b
are calculated to give rise to discrete HOMO!LUMO,
HOMO!LUMO+1, and HOMOꢀ1!LUMO+3 excita-
tions at 327 (f=0.238), 301 (f=0.860), and 278 nm (f=
0.597), respectively (Table S2 in the Supporting Informa-
tion). The LUMO is of p* character with the electron densi-
ty delocalized over both carbene ligands (82%) in an out-
of-phase combination with a Pt 5d orbital (11%). On the
contrary, the HOMO is located mainly on one alkyne ligand
(77%) and on the metal center (23%) and has no contribu-
tion from the carbene ligands (Figure 5 and Table S3 in the
Supporting Information). The HOMOꢀ1 is almost degener-
ate with the HOMO, showing a main contribution from the
other alkyne ligand. The participation of the metal is slightly
greater in the HOMO than in the HOMOꢀ1 (23 vs. 16%),
which destabilizes the former orbital and accounts for the
energy gap between these two orbitals (DEHꢀ1/H =0.17 eV).
The electron densities in the LUMO+1 and LUMO+3 are
located mainly on the same alkyne ligand as the HOMO
and HOMOꢀ1, respectively. We can therefore surmise that
the intense absorption band observed experimentally at
288 nm arises from overlap of the TD-DFT-calculated S0!
S3 and S0!S8 singlet–singlet transitions and shows, in agree-
The experimental UV/Vis absorption spectra of com-
plexes 5a and 5b in solution (dichloromethane) at 298 K ex-
hibit similar absorption patterns with intense absorption
bands at 285 and 288 nm and shoulder absorption bands at
300 and 315 nm, respectively. Based on the spectroscopic
studies, the low-energy absorption bands in these complexes
Figure 5. Spatial plots of selected frontier orbitals of the DFT-optimized ground state of 5b.
Chem. Eur. J. 2013, 19, 15689 – 15701
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15695

K. Venkatesan et al.
ment with the experimental indications, intra
li
E
from excited states with a ligand-to-ligand 1LLCT (palk
!
(palk!p*alk) charge-transfer character. The low-energy ab-
sorption band, observed as a shoulder in the experimental
UV/Vis spectrum, corresponds to the single HOMO!
LUMO excitation assigned to an admixture of ligand-to-
ligand 1LLCT (palk!p*carb) and metal-to-ligand 1MLCT
(5d!p*carb) characteristics.
p*carb) charge transfer, as in 5b, but with sizable amounts of
1
1
metal-to-ligand MLCT (5d!p*carb) and intra
ACHUTGTNRENlNUG iACHTGNUTRENNUgG and ILCT
(palk!p*alk) character. At higher energies, four transitions
overlap to produce the intense experimental band at 285 nm
(S0!S5, 292 nm, f=0.204; S0!S8, 282 nm, f=0.298; S0!S9,
278 nm, f=0.326; S0!S10, 277 nm, f=0.292). All frontier or-
bitals in the range HOMOꢀ2 to LUMO+3 play a role in
the single excitations of these four transitions, leading to an
unclear assignment of the band.
For complex 5a, the assignment of the experimental
bands and lowest-energy transitions is not as trivial as in the
case of 5b. Indeed, TD-DFT calculations showed only three
singlet–singlet transitions (among the ten lowest transitions
calculated) with an oscillator strength f>0.1 for 5b, whereas
no fewer than six transitions with f>0.2 were computed for
5a. Furthermore, one transition exhibits a large oscillator
strength of 0.860 for 5b, whereas all transitions of 5a are
more or less equivalent (f=0.204–0.380). In the ground-
state geometry, the lowest significant singlet excited states
S1 and S2 of 5a are calculated to give rise to HOMO!
LUMO and HOMOꢀ1!LUMO excitations at 320 (f=
0.357) and 307 nm (f=0.380), respectively. In the HOMO
and HOMOꢀ1, the electron density is mainly located on
both alkyne ligands (81–85%) in an out-of-phase combina-
The lowest singlet–triplet vertical excitation (T1–S0) ener-
gies obtained by TD-DFT calculations on the ground-state
structures of 5a and 5b are 431 and 437 nm, which are con-
sistent with the experimental emissions at 425 and 430 nm,
respectively. The transitions responsible for the emission
bands comprise the HOMO$LUMO (ai =0.438) and HO-
MOꢀ1$LUMO+3 (ai =0.417) single excitations for 5a, and
the HOMO$LUMO+1 excitation for 5b. As already men-
tioned, the HOMO and HOMOꢀ1 are mainly located on
both alkyne ligands in an out-of-phase combination with
a Pt 5d orbital, whereas the LUMO is delocalized over the
whole molecule. The LUMO+3 is 98% localized on both
alkyne ligands. The promotion of one electron to an unoccu-
pied orbital leads to a singlet excited state, and then a spin-
orbit interaction between excited states operates to induce
S!T intersystem crossing to form the corresponding triplet
excited state. Geometry relaxation of the latter leads to the
lowest-energy triplet state, the optimized structure of which
can be determined by DFT calculations. Careful analysis of
the molecular orbital diagrams of the optimized triplet
states of 5a and 5b revealed that the highest singly occupied
molecular orbitals (SOMOs) are located almost exclusively
on the alkyne ligands (Table S2 in the Supporting Informa-
tion with
a Pt 5d orbital (15–16%). The bonding
(HOMOꢀ1) and antibonding orbitals (HOMO) of 5a and
5b differ in the through-space interactions between the
alkyne ligands (Figure 6). The main difference compared to
5b is that the unoccupied orbital involved in the one-elec-
tron excitation, here the LUMO, is delocalized over the
whole molecule: 46% on the carbene ligands, 40% on the
alkyne ligands, and 14% on the metal center. Considering
that the low-energy absorption band (observed as a shoulder
in the UV/Vis spectrum) is produced by the overlap of these
two transitions, this energy absorption predominately arises
Figure 6. Spatial plots of selected frontier orbitals of the DFT-optimized ground state of 5a.
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Chem. Eur. J. 2013, 19, 15689 – 15701

Blue-Emitting N-Heterocyclic Carbene PtII Acetylide Complexes
FULL PAPER
tion). The highest SOMO, from which the emission is pro-
duced, is localized at 95 (5a) and 93% (5b) on one alkyne
ligand and 5 (5a) and 7% (5b) on the metal center, whereas
the second highest SOMO, called SOMOꢀ1, to which the
excited electron should return, is localized at 85% on the
other alkyne ligand and 13% on the metal center (both 5a
and 5b). The spin density surfaces of 5a and 5b, which take
into account the electron density on all singly occupied orbi-
Additional 1D-TOCSY NMR experiments further con-
firmed the presence of the intermediate (Figure S12 in the
Supporting Information). Based on this observation, a plausi-
ble mechanism involving stepwise substitution with the two
NHC ligands is proposed in Scheme 2. The trans isomer was
confirmed to be the thermodynamically more stable prod-
1
uct, since resonances in the H NMR spectrum correspond-
ing to this isomer started to appear when the reaction mix-
tals and not only the SOMOs, visually identify the emission
ture containing the cis complex was further heated to 758C.
3
from a transition with mainly intra
character and minor ligand-to-ligand LLCT character (Fig-
ure S10 in the Supporting Information).
li
E
)
3
Precursor-catalyzed isomerization: After carrying out sever-
al reactions in different reactant combinations,[42] we found
a notable cis to trans conversion only when 4a was treated
ꢁ
Mechanistic investigations: Investigations of the cis to trans
isomerization of square-planar PtII complexes have been car-
ried out since the 1950s.[9,38,39] The mechanisms of isomeriza-
tion in this class of complexes can be divided into two major
categories. The first involves consecutive displacement, con-
sisting of ligand dissociation resulting in the formation of
a three-coordinate intermediate, whereas the second in-
volves pseudorotation, consisting of an intramolecular rear-
rangement accompanied by formation of a five-coordinate
intermediate.[39d,40] In spite of extensive mechanistic studies
on this class of complexes, mechanisms for the photochemi-
cally and thermally induced isomerizations still remain elusi-
ve.[39e]
with a stoichiometric amount of [PtACHTUNGTRENNU(G cod)CAHTUNGTRENN(UGN C CC6H5)2] in
THF at 758C for 24 h. The yield of 4b was 71% (see the
Supporting Information). This finding is consistent with the
formation of a small amount of the trans isomer when only
2.0 equivalents of the NHC ligand is used, whereas little or
none of the trans isomer was observed when using 3.0 equiv-
alents of the NHC ligand. The conversion of 5a to 5b in
a similar yield of 64% was accomplished by treating only
ꢁ
10% of the precursor complex [PtACTHUNGTRNNEUG(cod)CAHTUNGTREN(NGUN C CC6H5)2], but
the reaction proceeded very slowly (see the Supporting In-
formation). This can be rationalized by the hypothesis that
the initially formed cis isomer reacts with an intermediate
ꢁ
COD-free solvent-stabilized PtII complex [(S)2Pt
ACHTUNGTRENNUNG(C
In order to determine the optimal conditions for obtaining
the trans complexes in high yields, we set out to elucidate
the possible intermediates involved in the cis to trans iso-
merization process. The dependence of product formation
on stoichiometry was first examined by varying the ratio of
NHC and precursor in [D8]THF at 758C (Figure S11 in the
Supporting Information). The results revealed initial forma-
tion of the cis isomer and the cis isomer was found to scale
up with increase in the stoichiometry of the NHC ligand.
CC6H5)2] (S=THF).[43] The interaction between the two
fragments takes place through the binding of the platinum
to the alkyne in an h2 fashion and additional support may
be provided through a Pt–Pt interaction. Binuclear com-
plexes showing similar kinds of interactions have been re-
ported previously.[22] Through this interaction, the COD-free
platinum(II) fragment acting as a fifth ligand on the cis plat-
inum complex 5a could then undergo pseudorotation fol-
lowed by expulsion, resulting in the formation of the trans
complex. The fact that only trans 4b and no trans 5b was
ꢁ
1
When the reaction was closely monitored by H NMR spec-
troscopy with a 1:2 stoichiometry of precursor to NHC
ligand, resonances attributable to the cis isomer were first
detected, along with signals of both COD bound to the PtII
center in a monodentate fashion and unbound COD.[41] The
proton resonances of the bound monodentate COD ap-
peared further downfield than those of the free COD.
obtained corroborates the participation of the [PtACHTUNGTRENUNG(cod)ACHTUNGTRENNUNG(C
CC6H5)2] precursor in the conversion of the cis to the trans
isomer. Although we were able to obtain the desired trans
compound by treating the cis isomer with the precursor, the
thermally induced isomerization reaction was pursued with
a view to obtaining the trans compound in good yield.
Scheme 2. Proposed reaction path for the formation of cis complex 5a.
Chem. Eur. J. 2013, 19, 15689 – 15701
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15697

K. Venkatesan et al.
Thermally induced isomerization: In order to test the pro-
pensity of our complexes to undergo thermally induced iso-
merization,[39e,44] thermogravimetric analysis (TGA) and dif-
ferential scanning calorimetry (DSC) measurements were
carried out on complexes 5a and 5b (Figures S14–S16 in the
Supporting Information). The data from these measure-
ments revealed the isomerization to progress at high temper-
atures around 2008C and, based on these experiments, we
were also able to obtain an isomerization enthalpy DHisom of
ꢀ4.5 kcalmolꢀ1 by using RbNO3 as a calibration standard
(Figure S17 in the Supporting Information). This value is in
good accordance with the relative thermodynamic stability
of the trans isomer 5b over the cis isomer 5a, estimated as
ꢀ6.0 kcalmolꢀ1 by DFT calculations (see the Supporting In-
formation).
The results from the DSC studies were further verified by
heating the neat complex 5a in a J. Young NMR tube at
2008C for 6 h under an inert atmosphere. The black-brown
residue obtained after the reaction was chromatographed on
silica gel to obtain a colorless product, which was confirmed
as the trans complex 5b based on 1H NMR studies. The
yield of 5b was found to be 64%. Investigation of one of
the fractions collected during chromatography showed it to
contain a trace amount of diphenylbutadiyne (confirmed on
the basis of GC-MS analysis). Although one could attribute
the formation of this molecule to reductive elimination from
the cis complex, it still does not entirely clarify the conver-
sion process. Previous mechanistic studies on platinum com-
plexes bearing phosphine or pyridine ligands have suggested
that cis–trans isomerization either proceeds by a dissociative
mechanism that involves loss of the phosphine, pyridine, or
halide ligands leading to a three-coordinate T-shaped com-
plex intermediate, or by an associative mechanism that in-
volves coordination of the phosphine or pyridine ligand
leading to a five-coordinate intermediate.[45] We found that
the conversion of complex 5a to 5b proceeded efficiently
when 5a was heated in [D8]toluene at 2008C for 2 days in
a J. Young NMR tube to give an isolated yield of 73%. The
reaction was found to proceed more slowly than that carried
out in the neat solid. Performing the reaction in the pres-
ence of the free carbene ligand and tBuOK neither acceler-
ated nor retarded formation of the product.
of the four-coordinate cis complex to form a pentacoordinate
complex stabilized through a Pt0–PtII interaction. This could
then undergo pseudorotation, resulting in expulsion of the
Pt0 complex and leading to formation of the desired trans
PtII complex. Acting like a catalyst, the Pt0 complex could
coordinate to another cis complex and could repeat the
same process over and over again to transform all of the cis
complex into the corresponding trans complex. In this way,
the Pt0 complex would only be required in catalytic
amounts, which would be consistent with the small amount
of diphenylbutadiyne expelled during the entire reaction
and the obtained high yield of the trans complex. The need
for such elevated temperatures for the isomerization to pro-
ceed is further corroborated by the DFT-calculated energy
of activation of 34.9 kcalmolꢀ1 (see the optimized transition
state in the Supporting Information) required for reductive
elimination of the alkyne ligands. It is important to note
that, although rare, well-defined Pt0 species bearing carbene
ligands have recently been isolated and characterized by
Braunschweig and co-workers.[46] In order to further confirm
our hypothesis, we performed a scrambling experiment be-
tween two cis complexes bearing different carbene ligands.
To this end, we synthesized complex 9a bearing the IBIM
ligand instead of the DBIM ligand as in the case of complex
5a. For comparison, complex 9b was also synthesized and
characterized separately. A 1:1 mixture of 5a and 9a in tolu-
ene was heated at 2008C for 2 days, and subsequent work-
up gave complexes 5b and 9b as the major products. We
were also able to isolate and characterize the new complex
10, in which both IBIM and DBIM are ligated to the PtII
center. The structure of 10 was confirmed by various NMR
and MS studies. The formation of 10 is supportive of scram-
bling of the NHC ligand during the cis to trans conversion,
although our earlier experiments suggested no dissociation
of the free carbene ligand. We presume that transfer of the
ligand proceeds through bridging of the carbene ligand be-
tween the two platinum centers, which could well facilitate
the scrambling without the need for complete dissociation
of the carbene ligand. Based on the above experimental ob-
servations,
Scheme 3.
a
preliminary mechanism is proposed in
Furthermore, when complex 5a was heated in the pres-
ence of the carbene ligand IBIM and tBuOK, no incorpora-
tion of IBIM in the final trans product was found. This ob-
servation disfavors dissociation of the carbene ligand during
the isomerization process and is consistent with the theoreti-
cal study since the calculated carbene dissociation enthalpy
was as high as 54.6 kcalmolꢀ1 (see the Supporting Informa-
tion). Considering the temperature at which the reaction
was performed, such dissociation can be ruled out. The ob-
servation of a small amount of diphenylbutadiyne, as men-
tioned above, may be attributed to reductive elimination of
the alkyne ligands from the cis complex. This would gener-
ate the corresponding biscarbene Pt0 complex, which is iso-
lobal with a carbene ligand. The in situ generated small
amount of Pt0 complex could then associate with a molecule
Conclusion
We have presented the synthesis and characterization of
a new class of triplet emitter molecules based on NHC-ligat-
ed platinum(II) acetylide complexes. A viable synthetic
route for a series of cis and trans complexes of the type [Pt-
ꢁ
ACHUTNRGEN(NUG dbim)2ACHTUNTGREN(NUNG C CR)2] is reported. Although the yields of trans
isomers obtained by a direct approach were very low, we
have demonstrated that they can be significantly improved
by employing different reaction conditions starting from the
readily available cis NHC PtII acetylide complexes. Experi-
mental and theoretical studies of the photophysical proper-
ties of the complexes have revealed very interesting lumi-
nescent properties, with two of the complexes displaying the
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Chem. Eur. J. 2013, 19, 15689 – 15701

Blue-Emitting N-Heterocyclic Carbene PtII Acetylide Complexes
FULL PAPER
Scheme 3. Proposed isomerization path from 5a to 5b.
highest quantum yields hitherto reported for deep-blue
emitters in the solid state. The properties of the NHC-bear-
ing PtII acetylide complexes are very different to those of
ꢁ
trans isomer. DSC measurements have further confirmed
the trans isomer as the thermodynamically stable product
and the isomerization enthalpy (DHisom) was found to be
4.5 kcalmolꢀ1. The novel structural and electronic features
of the presented complexes, coupled with their interesting
luminescent properties in solution, at 10 wt% in PMMA
films, and in the solid state, opens up new opportunities for
studying a wide range of optical and electronic (or electro-
chemical) properties.
the non-emissive complex [PtACHTNUTRGENNUG(PBu3)2ACHTUNTGNERN(UGN C CC6H5)2] and the
ꢁ
mostly green-emitting [PtACTHNUTRGENNUG(diimine)2ACHTUNGTRENN(UNG C CC6H5)2] complexes.
Experimental results coupled with DFT and TD-DFT calcu-
lations have indicated that the emission is intrinsically a mix-
3
ture of metal-perturbed IL with a small contribution from
3MLCT. Close examination of the synthetic reaction re-
vealed the cis complexes to be the kinetically favored iso-
mers and the trans complexes to be the more thermodynam-
ically stable isomers. Detailed investigations have been car-
ried out with a view to improving the yield of the trans
products. The cis to trans isomerization was found to be
strongly influenced by temperature, light, and the precursor
ꢁ
Acknowledgements
The authors wish to thank Prof. Heinz Berke for scientific discussions,
Dr. Thomas Fox for help with NMR studies, and Dr. Ferdinand Wild for
DSC and TGA measurements. Financial support from the Swiss National
Science Foundation (Grant no. 200021-135488) and the University of
Zꢁrich is gratefully acknowledged.
complex [PtACHTUNGTRENNUNG(cod)ACHTUNGTRENNUNG(C CR)2]. Based on preliminary mechanis-
tic studies, a solvent-stabilized PtII dialkyne complex and
a bis-NHC Pt0 species serving as a catalyst are invoked as
being involved in the key transformation of the cis to the
Chem. Eur. J. 2013, 19, 15689 – 15701
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15699

K. Venkatesan et al.
[12] C.-H. Tao, N. Zhu, V. W.-W. Yam, Chem. Eur. J. 2005, 11, 1647.
[13] a) W.-Y. Wong, C.-K. Wong, G.-L. Lu, A. W.-M. Lee, K.-W. Cheah,
J.-X. Shi, Macromolecules 2003, 36, 983; b) K. Sonogashira, S. Taka-
hashi, N. Hagihara, Macromolecules 1977, 10, 879; c) K. A. Williams,
A. J. Boydston, C. W. Bielawski, Chem. Soc. Rev. 2007, 36, 729.
[14] C.-W. Chan, L.-K. Cheng, C.-M. Che, Coord. Chem. Rev. 1994, 132,
87.
[15] a) M. Hissler, W. B. Connick, D. K. Geiger, J. E. McGarrah, D. Lipa,
R. J. Lachicotte, R. Eisenberg, Inorg. Chem. 2000, 39, 447; b) J. E.
McGarrah, R. Eisenberg, Inorg. Chem. 2003, 42, 4355; c) T. J.
Wadas, S. Chakraborty, R. J. Lachicotte, Q.-M. Wang, R. Eisenberg,
Inorg. Chem. 2005, 44, 2628; d) J. Schneider, P. Du, P. Jarosz, T. Laz-
arides, X. Wang, W. W. Brennessel, R. Eisenberg, Inorg. Chem.
2009, 48, 4306; e) P. Jarosz, K. Lotito, J. Schneider, D. Kumaresan,
R. Schmehl, R. Eisenberg, Inorg. Chem. 2009, 48, 2420; f) M. Hiss-
ler, J. E. McGarrah, W. B. Connick, D. K. Geiger, S. D. Cummings,
R. Eisenberg, Coord. Chem. Rev. 2000, 208, 115.
[1] a) M. E. Thompson, MRS Bull. 2007, 32, 694; b) W.-Y. Wong, P. D.
Parvey, Macromol. Rapid Commun. 2010, 31, 671; c) H. Yersin,
A. F. Rausch, R. Czerwieniec, T. Hofbeck, T. Fischer, Coord. Chem.
Rev. 2011, 255, 2622; d) Y. Unger, D. Meyer, O. Molt, C. Schild-
knecht, I. Mꢁnster, G. Wagenblast, T. Strassner, Angew. Chem. 2010,
122, 10412; Angew. Chem. Int. Ed. 2010, 49, 10214; e) K. Li, G.
Cheng, C. Ma, X. Guan, W.-M. Kwok, Y. Chen, W. Lu, C.-M. Che,
Chem. Sci. 2013, 4, 2630; f) Z. M. Hudson, C. Sun, M. G. Helander,
Y.-L. Chang, Z.-H. Lu, S. Wang, J. Am. Chem. Soc. 2012, 134,
13930; g) H.-F. Xiang, S.-W. Lai, P. T. Lai, C.-M. Che in Highly Effi-
cient OLEDs with Phosphorescent Materials (Ed.: H. Yersin),
Wiley-VCH, Weinheim, 2008, pp. 259; h) J. A. G. Williams, Topics in
Current Chemistry, Vol. 281, Photochemistry and Photophysics of
Coordination Compounds: Platinum, Springer, Heidelberg, 2007;
i) Y. Chi, P.-T. Chou, Chem. Soc. Rev. 2010, 39, 638; j) S.-Y. Chang,
J.-L. Chen, Y. Chi, Y.-M. Cheng, G.-H. Lee, C.-M. Jiang, P.-T. Chou,
Inorg. Chem. 2007, 46, 11202; k) H. Yersin, W. Finkenzeller in
Highly Efficient OLEDs with Phosphorescent Materials (Ed.: H.
Yersin), Wiley-VCH, Weinheim, 2008, p. 15.
[2] a) S. W. Thomas, K. Venkatesan, P. Mꢁller, T. M. Swager, J. Am.
Chem. Soc. 2006, 128, 16641; b) Z. M. Hudson, C. Sun, K. J. Harris,
B. E. G. Lucier, R. W. Schurko, S. Wang, Inorg. Chem. 2011, 50,
3447; c) C.-S. Lee, R. R. Zhuang, S. Sabiah, J.-C. Wang, W.-S.
Hwang, I. J. B. Lin, Organometallics 2011, 30, 3897; d) Q.-Z. Yang,
L.-Z. Wu, H. Zhang, B. Chen, Z.-X. Wu, L.-P. Zhang, C.-H. Tung,
Inorg. Chem. 2004, 43, 5195; e) L.-Y. Zhang, L.-J. Xu, X. Zhang, J.-
Y. Wang, J. Li, Z.-N. Chen, Inorg. Chem. 2013, 52, 5167.
[3] a) R. Zieba, C. Desroches, F. Chaput, M. Carlsson, B. Eliasson, C.
Lopes, M. Lindgren, S. Parola, Adv. Funct. Mater. 2009, 19, 235;
b) C. K. M. Chan, C.-H. Tao, H.-L. Tam, N. Zhu, V. W.-W. Yam, K.-
W. Cheah, Inorg. Chem. 2009, 48, 2855; c) C. K. M. Chan, C.-H. Tao,
K.-F. Li, K. M.-C. Wong, N. Zhu, K.-W. Cheah, V. W.-W. Yam,
Dalton Trans. 2011, 40, 10670; d) R. Liu, A. Azenkeng, D. Zhou, Y.
Li, K. D. Glusac, W. Sun, J. Phys. Chem. A 2013, 117, 1907.
[4] a) P. Du, J. Schneider, P. Jarosz, R. Eisenberg, J. Am. Chem. Soc.
2006, 128, 7726; b) X. Wang, S. B. Goeb, Z. Ji, N. A. Pogulaichenko,
F. N. Castellano, Inorg. Chem. 2011, 50, 705; c) M. N. Roberts, J. K.
Nagle, M. B. Majewski, J. G. Finden, N. R. Branda, M. O. Wolf,
Inorg. Chem. 2011, 50, 4956.
[16] C. E. Whittle, J. A. Weinstein, M. W. George, K. S. Schanze, Inorg.
Chem. 2001, 40, 4053.
[17] a) I. E. Pomestchenko, C. R. Luman, M. Hissler, R. Ziessel, F. N.
Castellano, Inorg. Chem. 2003, 42, 1394; b) F. Hua, S. Kinayyigit,
J. R. Cable, F. N. Castellano, Inorg. Chem. 2005, 44, 471; c) F. Hua,
S. Kinayyigit, J. R. Cable, F. N. Castellano, Inorg. Chem. 2006, 45,
4304.
[18] a) W.-S. Tang, X.-X. Lu, K. M.-C. Wong, V. W.-W. Yam, J. Mater.
Chem. 2005, 15, 2714; b) A. Y.-Y. Tam, W. H. Lam, K. M.-C. Wong,
N. Zhu, V. W.-W. Yam, Chem. Eur. J. 2008, 14, 4562.
[19] a) V. W.-W. Yam, K. M.-C. Wong, N. Zhu, J. Am. Chem. Soc. 2002,
124, 6506; b) S. C. F. Kui, Y.-C. Law, G. S. M. Tong, W. Lu, M.-Y.
Yuen, C.-M. Che, Chem. Sci. 2011, 2, 221; c) Q.-Z. Yang, L.-Z. Wu,
Z.-X. Wu, L.-P. Zhang, C.-H. Tung, Inorg. Chem. 2002, 41, 5653;
d) R. McGuire, Jr., M. C. McGuire, D. R. McMillin, Coord. Chem.
Rev. 2010, 254, 2574; e) X. Han, L.-Z. Wu, G. Si, J. Pan, Q.-Z. Yang,
L.-P. Zhang, C.-H. Tung, Chem. Eur. J. 2007, 13, 1231; f) H.-M. Wen,
Y.-H. Wu, Y. Fan, L.-Y. Zhang, C.-N. Chen, Z.-N. Chen, Inorg.
Chem. 2010, 49, 2210.
[20] a) R. Liu, Y. Li, Y. Li, H. Zhu, W. Sun, J. Phys. Chem. A 2010, 114,
12639; b) W. Lu, B.-X. Mi, M. C. W. Chan, Z. Hui, C.-M. Che, N.
Zhu, S.-T. Lee, J. Am. Chem. Soc. 2004, 126, 4958; c) Z. Wang, E.
Turner, V. Mahoney, S. Madakuni, T. Groy, J. Li, Inorg. Chem. 2010,
49, 11276; d) D. L. Rochester, S. Develay, S. Zalis, J. A. G. Williams,
Dalton Trans. 2009, 1728; e) P. Shao, Y. Li, A. Azenkeng, M. R.
Hoffmann, W. Sun, Inorg. Chem. 2009, 48, 2407.
[21] S.-C. Chan, M. C. W. Chan, Y. Wang, C.-M. Che, K.-K. Cheung, N.
Zhu, Chem. Eur. J. 2001, 7, 4180.
[22] S. Fernꢅndez, J. Fornies, B. Gil, J. Gomez, E. Lalinde, Dalton Trans.
2003, 822.
[23] a) G. C. Vougioukalakis, R. H. Grubbs, Chem. Rev. 2010, 110, 1746;
b) W. A. Herrmann, C. Kçcher, Angew. Chem. 1997, 109, 2256;
Angew. Chem. Int. Ed. Engl. 1997, 36, 2162; c) T. Fleetham, Z.
Wang, J. Li, Org. Electron. 2012, 13, 1430.
[24] D. Bourissou, O. Guerret, F. P. Gabbaꢆ, G. Bertrand, Chem. Rev.
2000, 100, 39.
[5] a) G.-J. Zhou, W.-Y. Wong, Chem. Soc. Rev. 2011, 40, 2541; b) F.
Guo, W. Sun, Y. Liu, K. Schanze, Inorg. Chem. 2005, 44, 4055.
[6] a) M. J. Currie, J. K. Mapel, T. D. Heidel, S. Goffri, M. A. Baldo, Sci-
ence 2008, 321, 226; b) W. Wu, J. Zhang, H. Yang, B. Jin, Y. Hu, J.
Hua, C. Jing, Y. Long, H. Tian, J. Mater. Chem. 2012, 22, 5382;
c) W.-Y. Wong, C.-L. Ho, Acc. Chem. Res. 2010, 43, 1246.
[7] M. Montalti, A. Credi, L. Prodi, M. T. Gandolfi, Handbook of Pho-
tochemistry, 3rd ed., Taylor & Francis Group, Boca Raton, 2006.
[8] a) M. L. Muro, A. A. Rachfor, X. Wang, F. N. Castellano in Topics
in Organometallic Chemistry, Vol. 29, Photophysics of Organometal-
lics (Ed.: A. J. Lees), Springer Verlag, Berlin, 2010, p.159; b) V. W.-
W. Yam, Acc. Chem. Res. 2002, 35, 555; c) T. Sajoto, P. I. Djurovich,
A. Tamayo, M. Yousufuddin, R. Bau, M. E. Thompson, R. J.
Holmes, S. R. Forrest, Inorg. Chem. 2005, 44, 7992.
[25] A. J. Boydston, K. A. Williams, C. W. Bielawski, J. Am. Chem. Soc.
2005, 127, 12496.
[9] J. Chatt, B. L. Shaw, J. Chem. Soc. 1959, 4020.
[10] a) M. N. Roberts, C.-J. Carling, J. K. Nagle, N. R. Branda, M. O.
Wolf, J. Am. Chem. Soc. 2009, 131, 16644; b) G. Ramakrishna, T.
Goodson, J. E. Rogers-Haley, T. M. Cooper, D. G. McLean, A.
Urbas, J. Phys. Chem. C 2009, 113, 1060; c) K. Haskins-Glusac, I.
Ghiviriga, K. A. Abboud, K. S. Schanze, J. Phys. Chem. B 2004, 108,
4969.
[26] a) Y. Unger, A. Zeller, S. Ahrens, T. Strassner, Chem. Commun.
2008, 3263; b) Y. Unger, A. Zeller, M. A. Taige, T. Strassner, Dalton
Trans. 2009, 4786; c) Y. Unger, D. Meyer, T. Strassner, Dalton Trans.
2010, 39, 4295.
[27] M. Koch, J. A. Garg, O. Blacque, K. Venkatesan, J. Organomet.
Chem. 2012, 700, 154.
[11] a) K. Glusac, M. E. Kçse, H. Jiang, K. S. Schanze, J. Phys. Chem. B
2007, 111, 929; b) T. Cardolaccia, Y. Li, K. S. Schanze, J. Am. Chem.
Soc. 2008, 130, 2535; c) J. Stahl, J. C. Bohling, T. B. Peters, L. D.
Quadras, J. A. Gladysz, Pure Appl. Chem. 2008, 80, 459; d) Q.
Zheng, J. C. Bohling, T. B. Peters, A. C. Frisch, F. Hampel, J. A. Gla-
dysz, Chem. Eur. J. 2006, 12, 6486; e) Y. Liu, S. Jiang, K. Glusac,
D. H. Powell, D. F. Anderson, K. S. Schanze, J. Am. Chem. Soc.
2002, 124, 12412.
[28] Y. Zhang, J. A. Garg, C. Michelin, T. Fox, O. Blacque, K. Venkate-
san, Inorg. Chem. 2011, 50, 1220.
[29] Y. Han, H. V. Huynh, G. K. Tan, Organometallics 2007, 26, 4612.
[30] a) M. Herberhold, T. Schmalz, W. Milius, B. Wrackmeyer, J. Orga-
nomet. Chem. 2002, 641, 173; b) A. Garcꢇa, E. Lalinde, M. T.
Moreno, Eur. J. Inorg. Chem. 2007, 3553.
[31] Q.-X. Liu, F.-B. Xu, Q.-S. Li, H.-B. Song, Z.-Z. Zhang, J. Mol.
Struct. 2004, 697, 131.
15700
www.chemeurj.org
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15689 – 15701

Blue-Emitting N-Heterocyclic Carbene PtII Acetylide Complexes
FULL PAPER
[32] S. D. Adhikary, D. Bose, P. Mitra, K. D. Saha, V. Bertolasi, J. Dinda,
New J. Chem. 2012, 36, 759.
[33] M. S. Khan, R. K. M. Al-Saadi, L. Male, P. R. Raithby, J. K. Bjerne-
mose, Acta Crystallogr. Sect. E 2003, 59, m774.
B. B. Wayland, Inorg. Chem. 1978, 17, 2245; f) J. Burgess, M. E.
Howden, R. D. W. Kemmitt, N. S. Sridhara, J. Chem. Soc. Dalton
Trans. 1978, 1577; g) J. Vicente, A. Arcas, M.-D. Galvez-Lopez, P. G.
Jones, Organometallics 2006, 25, 4247.
[34] Gaussian 03, Revision D.01, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgom-
ery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S.
Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani,
N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J.
Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C.
Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cio-
slowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaro-
mi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Wall-
ingford CT, 2004.
[35] C. Adamo, V. Barone, J. Chem. Phys. 1999, 110, 6158.
[36] T. H. Dunning, P. J. Hay in Modern Theoretical Chemistry, Vol. 3
(Ed.: H. F. Schaefer), Plenum, New York, 1976.
[37] R. Ditchfield, W. J. Hehre, J. A. Pople, J. Chem. Phys. 1971, 54, 724.
[38] a) J. Chatt, R. G. Wilkins, J. Chem. Soc. 1952, 273; b) J. Chatt, R. G.
Wilkins, J. Chem. Soc. 1952, 4300.
[39] a) D. T. Rosevear, F. G. A. Stone, J. Chem. Soc. 1965, 5275; b) D. A.
Redfield, J. H. Nelson, J. Am. Chem. Soc. 1974, 96, 6219; c) W. J.
Louw, Inorg. Chem. 1977, 16, 2147; d) R. Romeo, M. R. Plutino,
L. I. Elding, Inorg. Chem. 1997, 36, 5909; e) J. H. Price, J. P. Birk,
[40] G. K. Anderson, R. J. Cross, Chem. Soc. Rev. 1980, 9, 185, and refer-
ences therein.
[41] N. Oberbeckmann, K. Merz, R. A. Fischer, Organometallics 2001,
20, 3265.
[42] The reactions of cis isomer 5a with different combinations were
probed by 1H NMR studies involving: 1) THF, 2) [dbimH]Br,
3) [dbimH]Br and tBuOK, 4) KBr, 5) tBuOK, 6) tBuOH, and 7) cy-
clooctadiene (COD). No isomerization was detected.
[43] a) R. J. Puddephatt, P. J. Thompson, J. Chem. Soc. Dalton Trans.
1977, 1219; b) C. Eaborn, K. J. Odell, A. Pidcock, J. Chem. Soc.
Dalton Trans. 1978, 1288.
[44] a) P. Haake, T. A. Hylton, J. Am. Chem. Soc. 1962, 84, 3774; b) R.
Ellis, T. A. Weil, M. Orchin, J. Am. Chem. Soc. 1970, 92, 1078;
c) J. R. Perumareddi, A. W. Adamson, J. Phys. Chem. 1968, 72, 414;
d) B. Cetinkaya, E. Cetinkaya, M. F. Lappert, J. Chem. Soc. Dalton
Trans. 1973, 906.
[45] a) F. Yamashita, H. Kuniyasu, J. Terao, N. Kambe, Inorg. Chem.
2006, 45, 1399; b) W. Baratta, S. Stoccoro, A. Doppiu, E. Herdt-
weck, A. Zucca, P. Rigo, Angew. Chem. 2003, 115, 109; Angew.
Chem. Int. Ed. 2003, 42, 105; c) E. Guido, G. DꢀAmico, N. Russo, E.
Sicilia, S. Rizzato, A. Albinati, A. Romeo, M. R. Plutino, R. Romeo,
Inorg. Chem. 2011, 50, 2224; d) S. Fantasia, H. Jacobsen, L. Cavallo,
S. P. Nolan, Organometallics 2007, 26, 3286.
[46] J. Bauer, H. Braunschweig, P. Brenner, K. Kraft, K. Radacki, K.
Schwab, Chem. Eur. J. 2010, 16, 11985.
Received: June 10, 2013
Revised: July 25, 2013
Published online: September 25, 2013
Chem. Eur. J. 2013, 19, 15689 – 15701
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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15701