Luminescent PtII Complexes of Tridentate Cyclometalating 2,5-Bis(aryl)-pyridine Ligands

Article abstract of DOI:10.1002/ejic.201700792

Bis-cyclometalated Pt^II complexes of dianionic 2,5-bis(aryl)-pyridine ligands (L^1–6)^2–, carrying various cyclometalating or pending aryl groups, are synthesised in two steps. The reactions of H₂L protoligands with K₂[PtCl₄] in acetic acid give the mono-cyclometalated complexes [Pt(HL)Cl]₂. Heating these complexes in hot DMSO (dimethyl sulfoxide) yields the double-cyclometalated DMSO complexes [Pt(L^1–6)(DMSO)]. The reaction of [Pt(L⁴)(DMSO)] with N,N-dimethylimidazolium iodide in the presence of KOtBu as the base gives the carbene complex [Pt(L⁴)(Me₂Imd)]. Detailed photophysical studies reveal the intense orange luminescence of these complexes in CH₂Cl₂ solution, with quantum yields up to 0.22, and increased quantum yields of up to 1.00 in glassy frozen CH₂Cl₂/MeOH (1:1) and up to 0.44 in PMMA matrices. Detailed electrochemistry (including spectroelectrochemistry) reveals reversible ligand-based first reductions at –2.1 to –2.3 V, irreversible Pt-centred oxidations at around 0.8 V and electrochemical band gaps of 2.8–3.0 eV. Further reduction waves at very negative potentials interfere with the solvent (THF with traces of water) discharge and can be traced, with UV/Vis spectroelectrochemistry, to Pt-centred reductions for the DMSO complexes and to a second ligand-centred reduction for the Me₂Imd complex from. The photo/electrochemical properties can be roughly correlated with the ligand pattern and suggest their use in optoelectronic applications.

Full text of DOI:10.1002/ejic.201700792

A Journal of
Accepted Article
Title: Luminescent Pt(II) Complexes of Tridentate Cyclometalating 2,5-
Bis(aryl)-pyridine Ligands
Authors: Maren Krause, Dimitrios Kourkolos, Dario Gonzales-Abradelo,
Klaus Meerholz, Cristian A Strassert, and Axel Klein
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700792
Link to VoR: http://dx.doi.org/10.1002/ejic.201700792

10.1002/ejic.201700792
European Journal of Inorganic Chemistry
„Luminescent Pt(II) Complexes of Tridentate Cyclometalating 2,5-Bis(aryl)-pyridine
Ligands”
Maren Krause,[a] Dimitrios Kourkolos,[b] Dario Gonzalez-Abradelo,[c] Klaus Meerholz,[b] Cristian A.
Strassert,*[c] Axel Klein*[a]
[a] Department für Chemie, Institut für Anorganische Chemie, Universität zu Köln, Greinstraße 6,
D-50939 Köln, Germany; E-mail: axel.klein@uni-koeln.de, http://www.klein.uni-koeln.de/
[b]
Department für Chemie, Institut für Physikalische Chemie, Universität zu Köln, Luxemburger
Str. 116, D-50939 Köln, Germany
[c] Institute of Physics and Center for Nanotechnology, Westfälische Wilhelms-Universität Münster,
Heisenbergstr. 11, D-48149 Münster, Germany
Received Eur.J.Inorg.Chem.
Key Topic: Structure-Luminescence Correlations
Synopsis:
Platinum(II) complexes of bis-cyclometalated 2,5-bis(aryl)-pyridine ligands show orange
phosphorescence in solution and solid with moderate quantum yields up to 0.45 in PMMA matrix.
Structural modifications on the periphery of the 2-phenyl-6-(thiophen-2-yl)pyridine ligands have
remarkable impact on the quantum yields, while the emission energies vary only moderately.
Graphical abstract:
Keywords: Platinum / Heteroaromatic Ligands / Cyclometalation / Triplet Emission /
Electrochemistry
1
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10.1002/ejic.201700792
European Journal of Inorganic Chemistry
Abstract Biscyclometalated Pt(II) complexes of dianionic 2,5-bis(aryl)-pyridine ligands (L1-6)2−
carrying various cyclometalating or pending aryl groups have been synthesised in two steps.
Reaction of the H2L proto-ligands with K2[PtCl4] in acetic acid gave the mono-cyclometalated
complexes [Pt(HL)Cl]2. Heating of these complexes in hot DMSO (dimethyl sulfoxide) yielded the
double-cyclometalated DMSO complexes [Pt(L1-6)(DMSO)]. Reaction of [Pt(L4)(DMSO)] with
N,N-dimethylimidazolium iodide in the presence of KOtBu as base gave the carbene complex
[Pt(L4)(Me2Imd)]. Detailed photophysical studies reveal intense orange luminescence of these
complexes in CH2Cl2 solution with quantum yields up to 0.22, increased quantum yields up to 1.00
in glassy frozen CH2Cl2:MeOH (1:1) and up to 0.44 in PMMA matrices. Detailed electrochemistry
(including spectroelectrochemistry) reveals reversible ligand-based first reductions at ‒2.1 to ‒2.3
V, irreversible Pt-centred oxidations at around 0.8 V and electrochemical band gaps of 2.8 to 3.0
eV. Further reduction waves at very negative potentials interfere with solvent (THF with traces of
water) discharge and can be traced to Pt-centred reductions for the DMSO complexes but a second
ligand-centred reduction for the Me2Imd complex from UV-vis spectroelectrochemistry. The
photo/electrochemical properties can be roughly correlated with the ligand pattern and suggest their
application in optoelectronics.
–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
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10.1002/ejic.201700792
European Journal of Inorganic Chemistry
Introduction
Tridentate cyclometalating ligands such as the prototypical 6-(phen-2-yl)-2,2’-bipyridine (Phbpy)‒
(Scheme 1) have been used in organoplatinum(II) and -palladium(II) chemistry starting from the
1990s.[1-2] Having the same shape and electron configuration as 2,2’:6’,2’’-terpyridine (terpy)[3] the
anion (Phbpy)– provides very stable tridentate binding and the cyclometalation requiring C−H
activation of the phenyl core can be carried out at moderate conditions, usually heating suitable
Pt(II) or Pd(II) precursors and the C−H functionalised proto-ligands (ligand precursor prior to
metalation).[1-2, 4-14]
–
Scheme 1. Structural similarities of terpy, (Phbpy)– (HPhbpy = 6-phenyl-2,2’-bipyridine), and
(PhPyPh)– (HPhPyPhH = 2,6-diphenyl-pyridine) Pt complexes (from left to right). Note, that using
anionic X– coligands (halides, alkyl, aryl, …) leads to cationic (NNN), neutral (CNN) or anionic
(CNC) complexes, while the use of neutral ligands L such as DMSO, pyridines, phosphines,
isonitriles, or carbenes leads to neutral (CNC) complexes.
Very soon it was found that many Pt(II) derivatives of the Phbpy ligand show intense luminescence,
some even at ambient temperature.[11, 15-19] One reason lies in the destabilisation of dark metal-
centred excited states associated with the Pt(II) dx2-y2 orbital when using these very strong
carbanionic ligands,[20] the other reason lies in their rigidity. Consequently the idea came up using
tridentate dicarbanionic cyclometalating ligands of the 2,6-di(phen-2-yl)pyridine (–PhPyPh–, see
Scheme 1) type with Pt(II).[21-32] Recently, Che et al. employed several derivatives of this ligand
type with the aim of providing suitable excited states through variation of the 2,5-aryl groups at the
central pyridine unit and they achieved very efficient triplet emitters, which show phosphorescence
even at room temperature. One crucial move in their ligand design was the use of thiophene as one
of their pending metalated aryl groups, with the complex 4 in Scheme 2 being the most efficient
triplet emitter.[31]
3
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10.1002/ejic.201700792
European Journal of Inorganic Chemistry
Scheme 2. Chemical structures of the synthesised PtII complexes [Pt(L1-6)(coligand)].
[Pt(L1)(DMSO)] (1), [Pt(L2)(DMSO)] (2), [Pt(L3)(DMSO)] (3), [Pt(L4)(DMSO)] (4),
[Pt(L5)(DMSO)] (5), [Pt(L6)(DMSO)] (6), [Pt(L4)(Me2Imd)] (4a).
We became interested in these complexes and decided not only to re-synthesise and re-investigate
the prototypical complex 4 but also extend our study to further derivatives (Scheme 2) with the aim
of achieving structure-properties relationships for efficient luminescence in such complexes.
Therefore, we also replaced the DMSO coligand in one of the doubly cyclometalated complexes by
a strong carbene ligand and studied the outcome of this replacement.
Furthermore, we investigated in detail the electrochemistry of the prototype 4, which has not
been done yet, and its new derivatives in view of a possible application in OLEDs. In this
contribution, we will report on the synthesis and full characterisation of the ligands and complexes,
most of them were novel. We will also report on detailed absorption and emission spectroscopy, the
latter carried out at ambient temperature in CH2Cl2 solution, in glassy frozen CH2Cl2/MeOH
solution (v:v = 1:1) at 77 K as well as poly(methyl methacrylate) (PMMA) films. These results
were combined with detailed electrochemical (cyclic voltammetry and UV-vis
spectroelectrochemistry) and preliminary structure-properties relationships were drawn.
Results and Discussion
Preparations and analyses
The preparation of the 2,5-bis(aryl)-pyridine proto-ligands (H2L1-6) is outlined in the Supporting
Information (SI). The complexes were prepared according to a literature method.[23] In brief, the
proto-ligands H2L1-6 were heated with K2[PtCl4] in glacial acetic acid which resulted in the
1
formation of yellow-orange materials in excellent yields. The H NMR spectra exhibit typical
4
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10.1002/ejic.201700792
European Journal of Inorganic Chemistry
satellites of a 3JPt-H coupling of about 60 Hz (195Pt, I = ½, 33.8% nat. abundance) for the signals of
the ortho protons of the Pt bound phenyl group. The reaction products are either Cl-bridges dimers
[23]
[{Pt(HL1-6)}2(µ-Cl)2]
or monomers [Pt(HL1-6)Cl] with S-coordination through the thiophene
moiety, as has been suggested earlier for the reaction of HL1 with K2[PtCl4].[33] Our detailed 2D Pt-
x
H NMR spectroscopy searching evidence for JPt-H coupling of the thiophene protons through
extensive variation of the assumed coupling constant was not successful (for spectra, see Supporting
Information). Standard EI mass spectrometry shows monomers, HR-ESI measurements on a LTQ
Orbitrap XL also gave no evidence for the Cl-bridged dimers, but monomeric species. At the same
time, none of these methods can unambiguously exclude the presence of dimers and having no
evidence for monomers we infer dimeric structures from related complexes without a potentially
binding pendant group as e.g. for [{Pt(HPhPyPh)}2(µ-Cl)2].[23] For cryoscopy or ebullioscopy
measurements the solubility of the complexes turned out to be too low.
These intermediate complexes were reacted with dimethyl sulfoxide to yield the doubly
cyclometalated complexes [Pt(L1-6)(DMSO)] in high yields (69 to 95%). The DMSO coligand can
easily be exchanged by carbene ligand as has been shown for the conversion from complex 4 to
complex 4a.
Molecular structures in solution (NMR) and in the solid (XRD)
1
Detailed 1D and 2D H NMR spectroscopy proved the molecular entities of the seven complexes
1-6 and 4a. Especially, the metalation of the phenyl and the thiophenyl groups is obvious from the
platinum satellites at the signals of the corresponding ortho protons with 3JPt-H coupling constants of
about 25 Hz (Figure 1). From [Pt(L3)(DMSO)] (3) suitable single crystals were obtained from
evaporation of a CH2Cl2 solution and submitted to X-ray diffraction experiments. The compound
was found to crystallise in the monoclinic space group P21/c. Figure 2 shows the crystal structure.
The molecules are arranged in layers along the crystallographic b-axis. The intermolecular Pt…Pt
distance exceeds 5 Å, revealing the absence of such intermolecular interaction. In the molecular
structure (Figure 1) the platinum atom is distorted square planar surrounded (C‒Pt‒C angle of
157.6(9)°). The bond lengths Pt‒N (1.99(2) Ǻ), Pt‒SDMSO (2.21(1) Ǻ), Pt‒Cthiophene (2.16(2) Ǻ) and
Pt‒Cphenyl (2.02(3) Ǻ) accord with those of similar structures.[24, 31]
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10.1002/ejic.201700792
European Journal of Inorganic Chemistry
H3/H35
H42-H44
H41/H45
H23
H21
H2
H34
H33
H32
δ/ppm
8.00
7.90
7.80
7.70
7.60
7.50
7.40
7.30
7.20
7.10
1
Figure 1. Low field part of the H NMR spectrum of [Pt(L3)(DMSO)] (3) measured in CD2Cl2 at
300 MHz )left). Molecular structure (50% probability of ellipsoids, hydrogen atoms were omitted
for clarity, right).
Figure 2. Crystal structure of [Pt(L3)(DMSO)] (3) viewed along the crystallographic b axis.
UV-vis absorption and emission spectroscopy
Photophysical data of all complexes [Pt(L1-6)(DMSO)] (1-6) and [Pt(L4)(Me2Imd)] (4a) are
summarised in Table 1. In Figure 3 absorption and emission spectra of complexes 1-4 in CH2Cl2 at
room temperature are shown, Figure 4 includes the spectra of complexes 4-6 and 4a. Our data for
complex 4 matches with the results of Che et al.[31] with the exception of the quantum yield.
Different measurement methods are very probably responsible for this deviation.
All complexes show absorption bands between 250-300 (ε ≈ 20000-50000 L mol‒1 cm‒1)
which are assigned to transitions with predominantly ligand character (IL = intraligand). According
6
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10.1002/ejic.201700792
European Journal of Inorganic Chemistry
to literature[31-32] the moderate absorption bands at 350-400 nm (ε ≈ 10000-30000 L mol‒1 cm‒1) can
1
be assigned to mixed MLCT/3IL transitions of [(5d)Pt→π*(C^N^C)] and weak absorption bands
>440 nm are assigned to transitions with mixed 3MLCT/3IL character.
(2)
(1)
50000
40000
30000
20000
10000
0
2000
1500
1000
500
0
3
1
4
2
(4)
(3)
ε
450
475
500
525
λ
/ nm
3
1
4
ε
2
250 300 350 400 450 500 550 600 650 700 750 800
/ nm
λ
Figure 3. Absorption and emission spectra of complexes [Pt(L1-4)(DMSO)] (1-4) in CH2Cl2 at
298 K with the inset showing the long-wavelength absorption bands.
60000
4
4a
50000
(4)
4
4a
6
6
(5)
(6)
5
5
40000
30000
20000
10000
0
ε
(4a)
250 300 350 400 450 500 550 600 650 700 750
/ nm
λ
Figure 4. Absorption and emission spectra of complexes [Pt(L4-6)(DMSO)] (4-6) and
[Pt(L4)(Me2Imd)] (4a) in CH2Cl2 at 298 K.
7
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10.1002/ejic.201700792
European Journal of Inorganic Chemistry
Upon excitation at λex = 350-450 nm, complexes 1-6 and 4a in CH2Cl2 show orange
phosphorescence (λem = 579-595 nm; Φ = 0.05-0.22). Even complex 1, with the most simplified
ligand structure, is emissive in CH2Cl2 solution with a quantum yields of Φ = 0.06. These quantum
yields are remarkably high for such C^N^C complexes which are frequently non-emissive in
solution and we ascribe these improved luminescence properties to the presence of the thiophene
unit. The reason why the thiophenyl complexes are superior in quantum yields over phenyl,
[31]
naphthyl, carbazole, or fluorenyl containing derivatives as described in Che’s paper
is not yet
clear. Both absorption and emission energies are generally red-shifted for thiophenyl derivatives
compared with the other complexes,[31] and the more efficient luminescence is either the simple
consequence of the energy-gap law or might be due to a slightly different excited state. TD-DFT
calculations on a reasonable level (TD-B3LYP/6-31G(d)-LANL2DZ) were able to reproduce
qualitatively the observed red-shifts and suppose a slightly different character of the excited states
with varying contributions of 3MLCT and 3IL (π-π*).[32]
Exchange of a cyclometalating phenyl group by a naphthyl unit leads to slightly enhanced radiative
rate constants (kr) for complexes 2, 4 and 4a in solution. Recent calculations on [Pt(Phbpy)Cl]
showed that an extension of the π conjugation of the lateral aryl ring prevents distortion of the
excited triplet state, thus increasing the kr values.[36] The non-radiative constants (knr) are generally
lower with naphthyl units, and in the case of complexes 4 and 4a a significant enhancement of knr
can be attributed to the exchange of DMSO by carbene as the ancillary ligand. Che et al. observed
an enhanced Φ upon replacement of DMSO with an isocyanide ligand and suggested that the good
σ-donating ability of isocyanides leads to destabilisation of metal-centred quenching states.[31] The
same effect in complex 4a was expected due to the strong σ-donating capabilities of Me2Imd, but
the quantum yield of 4a is markedly lower in solution than that of 4. The increased knr value
observed in this case suggested that the radiationless decay favoured by a higher density of
vibrational states introduced by Me2Imd played a more significant role, even when compared to the
complexes bearing an ancillary DMSO. In contrast, the phenyl groups at the 4-position of the
pyridine on complexes 3, 4 and 4a did not markedly affect the values of the radiative and non-
radiative constants. Interestingly, the insertion of p-methoxy (5) and p-fluoro (6) substituents at the
phenyl ring lead to decreased kr values in solution, without significantly affecting the radiationless
deactivation rate constants. (TD)DFT studies are planned to explain their role.
Results of equimolar low complex concentrations (c = 16 µmol g-1) are shown in Table 1. We
observed that the emission maxima and quantum yields in PMMA matrices are only marginally
sensitive to complex concentration (spectra shown in the SI), even though platinum(II) complexes
can potentially interact through co-planar stacking. Consequently, the energy of the highest
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10.1002/ejic.201700792
European Journal of Inorganic Chemistry
occupied molecular orbital (HOMO) is raised and the energy of the lowest unoccupied molecular
orbital (LUMO) is reduced resulting in a bathochromic shift of emission bands.[34-35] Instead, we do
not observe such marked concentration-dependent excimer formation and ascribe the slight red-
shifts to changes in polarity of the environment. In PMMA matrices and glassy frozen CH2Cl2/
MeOH mixtures (v:v = 1:1) the quantum yields are increased due to reduced radiationless
deactivation by suppressed molecular vibrations and solvent coupling, whereas the emission
energies are similar in accordance with a predominant ligand centred character of the emissive
triplet state.
Table 1. Selected photophysical data of complexes [Pt(L1-6)(DMSO)] (1-6) and [Pt(L4)(Me2Imd)]
(4a).[a]
CH2Cl2
glassy,
PMMA[b]
[b]
77 K[d]
[b]
[b]
[b]
[c]
[c]
[d]
λabs [b] (ε
)
λem
Φ [b] kr
knr
λem [c] (τ) Φ [c]
kr
knr
λem
Φ [d]
(τ)
586
(2.9),
633,
693sh
579
(3.3),
625,
683sh
252 (2.5), 269 (2.6),
295sh (1.2), 335 (1.5),
350 (1.7), 464 (0.9),
492 (0.1)
270 (3.2), 294 (3.1),
328sh (1.1), 347sh
(1.1), 361 (1.5), 376sh
(1.0), 396 (0.9), 446
(0.1), 474sh (0.1)
252 (2.6), 274sh (3.7),
296 (4.2), 339 (2.1),
352 (2.1), 470 (1.0),
499 (0.1)
0.06
2.1
32.4 577
0.31
1.6
3.5
584, 633, 0.25
1
2
(±0. (±0. (20.0),
7)
(±0.3) (±0.3) 693sh
7)
598,
627, 687
24.8 568
0.18
5.5
0.34
1.5
(±0.1)
3.0
(±
0.1)
576, 623, 0.28
685sh
(±0. (±0. (22.3),
6)
6)
589,
617, 674
595
(3.0),
637,
696sh
585
(3.9),
629,
0.09
3.0
30.3 582
1.00
0.64
4.2
(±0.2)
< 0.2 589, 636, 0.34
694sh
3
4
(±0. (±0. (23.7),
3)
3)
601,
631, 687
20.0 572
273 (5.3), 293 (5.1),
304sh (4.6), 331sh
(1.9), 365 (1.8), 379sh
(1.2), 401 (1.1), 447
(0.2), 481 (0.1)
0.22
5.6
2.5
1.4
582, 625, 0.36
[e]
(±0. (±0. (25.6),
3)
(±0.1) (±0.1) 682sh
3)
591,
694sh
620, 677
273 (3.7), 296 (3.8),
310 (4.0), 330 (3.5),
355 (2.6), 377sh (1.2),
400 (1.0), 449 (0.2),
476 (0.1)
274 (4.5), 294 (4.4),
304sh (4.0), 331sh
(1.6), 365 (1.6), 379sh
(1.1), 402 (1.0), 452
(0.1), 478 (0.1)
579
(4.0),
625,
0.06
0.05
0.06
1.5
23.5 570
1.00
0.38
0.65
3.5
(±0.2)
< 0.2 577, 625, 0.40
5
(±0. (±1) (29.0),
3)
677sh
587,
616, 672
688sh
583
(3.4),
628,
1.5
27.9 572
1.3
2.1
583, 626, 0.44
6
(±0. (±0. (29.8),
3)
(±0.1) (±0.1) 685sh
6)
589,
692sh
620, 675
260 (5.1), 282 (5.8),
293sh (5.7), 353sh
(1.5), 372 (2.1), 401
(1.1), 468 (0.1)
593
(0.9),
638,
7
104 578
4.4
2.4
585, 632, 0.37
4a
(±1) (±3) (14.9),
601,
(±0.3) (±0.3) 696sh
696sh
627, 685
[a] Absorption maxima λabs in nm with absorption coefficient ε in 10000 L mol‒1 cm‒1; emission maxima λem in nm,
excitation wavelength with τ = triplet state lifetime in µs. Standard deviations of the fitted lifetime values were
negligible for the calculation of the unimolecular deactivation rate constants, excitation wavelength λex = 350 nm for all
measurements; Φ = photoluminescence quantum yield. Φ values carried a maximum 5% standard deviation intrinsic to
the measurement procedure; radiative kr and non-radiative knr rate constants in 104s–1. [b] Determined in deoxygenated
CH2Cl2 (c = 2.5×10‒5 mol L‒1). [c] Measured in PMMA matrices (c = 16 µmol g‒1). [d] Measured in glassy frozen (77
K) CH2Cl2/MeOH mixtures (v:v = 1:1). [e] Compare a Φ of 0.13 in ref [31].
9
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10.1002/ejic.201700792
European Journal of Inorganic Chemistry
At 77 K, in glassy frozen solutions, complexes 4 and 4a show identical Φ and complexes 3 and 5
possess unitary quantum yields. The latter represent a phenyl (3) or naphthyl (5) metalated core and
phenyl or p-methoxyphenyl substituents. For these two complexes, the kr is enhanced upon freezing,
whereas for the other examples a drop is observed. A reduced charge-transfer character in the
excited state in the absence of solvent reorganisation usually restricts the participation of the metal
centre in the excited state and therefore lowers the luminescence rate constant. TD-DFT
calculations on a higher level of sophistication than in the previous report by Zhang et al. (TD-
B3LYP/6-31G(d)-LANL2DZ)[32] will be needed to clarify the influence of the substituents on the
vertical transitions for complexes 3 and 5 and their relaxation pathway(s).
Electrochemistry
Cyclic voltammograms of complexes 1-6 and 4a show irreversible oxidations between 0.5-0.8 V,
selected plots were shown in Figure 5, data is collected in Table 2. A cathodic shift in the oxidation
potential upon substitution of phenyl (1) to naphthyl (2) is in line with DFT calculations showing
contributions of the naphthyl moiety to the HOMO.[32] First reduction waves occur between ‒2.06 V
and ‒2.30 V with markedly varying reversibility (Table 2).
1
2
3
3
4
1.0
0.5
-1.5
-2.0
-2.5
+
E / V vs. FeCp2/FeCp2
Figure 5. Cyclic voltammograms of oxidation and first reduction of complexes [Pt(L1-4)(DMSO)]
(1-4) measured in 0.1 M nBu4NPF6/THF vs. ferrocene/ferrocenium.
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10.1002/ejic.201700792
European Journal of Inorganic Chemistry
The potentials are slightly anodically shifted for the 4-phenyl-pyridine containing systems 3 to 6
compared to 1 and 2 and the reversibility is higher. Substitution of phenyl to naphthyl (compare 1-2
and 3-4) has no impact on the potentials but the reversibility of the first reduction increases
markedly. The reason is probably a superior stabilisation of negative charge through extension of
the π-conjugated system in both cases. According to DFT calculations, the LUMO is predominantly
located on the pyridine moiety and the peripheral phenyl,[32] in line with our results. Substitution at
the para position of peripheral 4-phenyl group does not affect markedly the reduction potentials
(compare 4-5 and 4-6 in Table 2). An exchange of DMSO against Me2Imd causes a cathodic shift of
both the oxidation and the reduction potentials. This is in line with the expected superior σ-donating
ability of this carbene ligand compared with DMSO.
Table 2. Electrochemical data of complexes [Pt(L1-6)(DMSO)] (1-6) and [Pt(L4)(Me2Imd)] (4a).[a]
complex Epa Ox1 E1/2 (Ipa/Ipc) Red1
E1/2 or Epc Red2
∆E Ox1-Red1 / eV
0.67
0.79
0.78
0.81
0.81
0.81
0.50
‒2.25 (0.00)
‒2.20 (0.63)
‒2.09 (0.77)
‒2.06 (0.86)
‒2.10 (0.77)
‒2.08 (0.64)
‒2.30 (0.85)
‒2.57 irr.
‒2.51 irr.
‒2.68 irr.
‒2.60 rev.
‒2.69 irr.
‒2.74 irr.
‒2.98 irr.
2.92
2.99
2.87
2.87
2.91
2.89
2.80
1
2
3
4
5
6
4a
[a] Measured in 0.1 M nBu4NPF6/THF vs. ferrocene/ferrocenium at a scan rate of 100 mV/s. Half-wave potentials E1/2
in V with current ratio Ipa/Ipc (1 = reversible, 0 = irreversible), Epa = anionic peak potential in V, Epc = cathodic peak
potential in V.
For most of the complexes a second reduction wave can be observed at potentials cathodically
shifted to the first by approximately 0.3 V for the complexes 1 and 2 and about 0.6 V for the
systems containing the 4-phenyl-pyridine moieties, in line with a pyridine-centred LUMO. For
complex 4a the shift is largest with about 0.7 V. Also, the electrochemical band gaps can be derived
from comparison of the first (reversible) reductions and the irreversible oxidations (Table 2). For
complexes 1-6 values around 2.9 eV were found, complex 4a exhibits the smallest value with 2.8
eV. Unfortunately, the lacking reversibility devaluates a bit these very interesting numbers.
However, we are confident that the observed irreversibility in the CV measurements is only due to
overlay of the reduction waves of the complexes with solvent discharge currents at these very
negative potentials and that the reduced complexes were quite stable.
11
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10.1002/ejic.201700792
European Journal of Inorganic Chemistry
Spectroelectrochemical UV-vis absorption measurements were conducted for 4 and 4a (data
summarised in Table 3). Absorption spectra of oxidised species 4+ and 4a+ are very similar to 4 and
4a, indicating that oxidation occurs predominantly at the platinum centre (Figures in the SI).
Importantly, the initial spectrum cannot be recovered upon re-reduction in line with the irreversible
character of the CV waves. During first reduction of 4 and 4a new long-wavelength bands with
maxima at 816 and 870 nm arise which are typical for a pyridine based reduction[37-38] and the
corresponding species are best described as [Pt(II)(L•3–)(DMSO)]•–. These absorptions are only
slightly red-shifted upon second reduction from 4•– to 42‒ (Figure 6). In analogy to the complex
[Pt(bpy)(Py)2]2+ and related derivatives[37] the rather similar spectra of 4•– and 42‒ point to a Pt-
based reduction with the products best described as [Pt(I)(L•3–)(DMSO)]2– (both containing L•3– as
the essential chromophore).
Table 3. UV-vis absorption data of [Pt(L4)(DMSO)]+/n‒ (4+/n‒) and [Pt(L4)(Me2Imd)]+/n‒ (4a+/n‒).[a]
reduction
λabs / nm
oxidation
λabs / nm
4‒
268, 300sh, 350, 383, 431sh 738, 816
267, 335sh, 397, 469 843, 916
246, 276, 307, 392
4+
42‒
4a‒
4a2‒
4a+
240, 270, 299sh, 319, 360, 407, 771, 870
267, 307sh, 357, 395, 502, 532, 575, 664, 919
238, 272, 294sh, 390, 438sh
[a] Measured in 0.1 M nBu4NPF6/THF, absorption maxima λabs in nm.
1.0
0.8
0.6
0.4
0.2
0.0
0 V
1st Red (4-)
2nd Red (42-)
200
400
600
800
1000
λ
/ nm
Figure 6. UV-vis absorption spectra recorded during electrochemical reduction of [Pt(L4)(DMSO)]
(4) (green). First reduction from ‒1.9 V to ‒2.4 V leading to 4‒ (blue), second reduction from to ‒
2.5 V to ‒2.8 V leading to 42‒ (pink). Measured in 0.1 M nBu4NPF6/THF solution.
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European Journal of Inorganic Chemistry
0.6
0.4
0.2
0.0
0 V
1st Red (4a−)
2nd Red (4a2−)
400
600
800
1000
λ
/ nm
Figure 7. UV-vis absorption spectra recorded during electrochemical reduction of [Pt(L4)(Me2Imd)]
(4a) (green). First reduction from ‒2.2 V to ‒2.4 V leading to 4a‒ (blue), second reduction from ‒
2.7 V to ‒3.0 V leading to 4a2‒ (pink). Measured in 0.1 M nBu4NPF6/THF solution.
A different behaviour is observed during the second reduction of 4a containing the N-heterocyclic
carbene ligand. A massive spectral change including the blue-shift of the long-wavelength band
maxima to 502 and 664 nm (Figure 7) indicate a second ligand-based reduction [Pt(II)(L4–
)(MeImd)]2– in analogy to observations for complexes [Pt(bpy)(R)2] with strong carbanionic R‒
ligands (R = mesityl, phenyl or methyl).[38]
Conclusions and Outlook
Bis-cyclometalated Pt(II) complexes of dianionic (CNC)2‒ 2,5-bis(aryl)-pyridine ligands (L1-6)2−
carrying various aryl substituents have been synthesised in two steps (H2L1 = 2-phenyl-6-(thiophen-
2-yl)pyridine; H2L2 = 2-(naphthalen-2-yl)-6-(thiophen-2-yl)pyridine; H2L3 = 2,4-diphenyl-6-
(thiophen-2-yl)pyridine; H2L4 = 2-(naphthalen-2-yl)-4-phenyl-6-(thiophen-2-yl)pyridine; H2L5 = 4-
(4ˈ-fluorophenyl)-2-(naphthalen-2-yl)-6-(thiophen-2-yl)pyridine; H2L6 = 4-(4ˈ-methoxyphenyl)-2-
(naphthalen-2-yl)-6-(thiophen-2-yl)pyridine). Reaction of the H2L proto-ligands with K2[PtCl4] in
acetic acid gave the mono-cyclometalated complexes of the composition [Pt(HL)Cl]n. The
previously assumed monomeric character (n = 1) including S binding of the thiophene group was
studied using sophisticated NMR and MS methods. Unfortunately, neither a monomeric (n = 1) nor
a dimeric (n = 2) character could be concluded and a dimeric structure can only be inferred from
related compounds. Heating these complexes in hot DMSO (dimethyl sulfoxide) yielded the
double-cyclometalated DMSO complexes [Pt(L1-6)(DMSO)]. Reaction of [Pt(L4)(DMSO)] with
N,N-dimethylimidazolium iodide in the presence of KOtBu as base gave the carbene complex
[Pt(L4)(Me2Imd)]. Detailed photophysical studies reveal intense orange luminescence of these
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10.1002/ejic.201700792
European Journal of Inorganic Chemistry
complexes in CH2Cl2 solution with quantum yields up to 0.22, increased quantum yields up to 1.00
in glassy frozen CH2Cl2:MeOH (1:1) and up to 0.44 in PMMA matrices and also largely varied
excited states lifetimes from 0.9-4.0 µs in CH2Cl2 solution and from 14.9-29.8 µs in in glassy frozen
CH2Cl2:MeOH (1:1). The quite similar emission maxima however, point to the very same excited
states regardless of the conditions. Preliminary structure-properties relations include the increase of
the radiative constant by extension of the π conjugated system at the metalated ring (phenyl to
naphthyl), whereas the insertion of a phenyl substituent at the luminophore only slightly affects the
properties. Detailed electrochemistry (cyclic voltammetry and square-wave) reveals ligand-based
first reductions ranging from ‒2.1 to ‒2.3 with markedly increased reversibility for the 4-phenyl-
pyridine substituted systems, which points to essentially pyridine-centred LUMOs and a description
of the reduced complexes as Pt(II) complexes of reduced ligand L•3‒ [Pt(II)(L•3–)(DMSO)]•–. In
contrast to the degree of reversibility, the potentials do not vary markedly upon substitution; the 4-
phenyl substituted derivatives reveal a slight anodic shift. UV-vis spectroelectrochemistry strongly
supports this assignment. Irreversible Pt-centred oxidations at around 0.8 V can be assigned to
essentially Pt(II/III) oxidations also in line with the spectroelectrochemistry. Electrochemical band
gaps of 2.8 to 3.0 eV can be calculated from the electrochemical data, although the irreversibility of
the oxidation waves devaluates the data. Second reduction waves at very negative potentials
interfere with solvent (THF with traces of water) discharge and can be traced from UV-vis
spectroelectrochemistry to Pt-centred reductions for the DMSO complexes [Pt(I)(L•3–)(DMSO)]2–
but a second ligand-centred reduction for the Me2Imd complex [Pt(II)(L4–)(MeImd)]2–. These
electronic descriptions of the reduced complexes with a high negative charge and/or radical
character at the ligands are in line with the irreversibility of many reduction waves in these
complexes. However, in the absence of protons and hampered possibilities of radical reactions e.g.
in solid matrices we are confident that these systems provide high stability in combination with very
suitable photo/electrochemical properties suggesting their application in optoelectronics. In future
work we will further characterise the reduced and excited states of these complexes by experimental
(EPR, transient absorption) and theoretical (highly sophisticated TD-DFT) approaches and will
further vary the ligand pattern to refine our preliminary structure-properties relations.
Experimental Section
General Information: Commercially available reagents (solvents and ligands) were used without
further purification. Solvents (CH2Cl2, THF, toluene, diethyl ether and CH3CN) were dried using a
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10.1002/ejic.201700792
European Journal of Inorganic Chemistry
MBRAUN MB SPS-800 solvent purification system. For electrochemical measurements THF was
distilled over a Na/K alloy prior to use.
The syntheses and analyses of the proto-ligands H2L1-6 is described in the Supporting Information.
Synthesis of monocyclometalated complexes [Pt(HL1-6)Cl]2 – general procedure
Following a literature procedure[23-24], K2[PtCl4] (208 mg, 0.5 mmol) was added to a solution of
H2L1-6 (0.7 mmol, 1.4 eq) in glacial acetic acid (250 mL). The reaction was stirred at 110 °C for
72 h. The orange precipitates were filtered off and washed with glacial acetic acid, water and dried
in vacuo.
[Pt(HL1)Cl]2: Yield: 208 mg (0.45 mmol, 90%). C15H10ClNPtS (466.84): calcd. C 38.59, H 2.16, N
3.00, S 6.87; found C 38.56, H 2.18, N 3.01, S 6.87%. 1H NMR (600 MHz, CD2Cl2): δ = 8.00 (t, J =
7.9 Hz, 1H), 7.76 (dd, J = 7.6, 1.3 Hz, JPt,H = 58 Hz, 1H), 7.65-7.58 (m, 3H), 7.51-7.44 (m, 2H),
7.34 (dt, J = 10.8, 5.4 Hz, 1H), 7.23-7.16 (m, 2H) ppm. EI-MS(+): m/z = 467 [monomer]+, 430
[monomer‒HCl]+, 237 [H2L1]+.
[Pt(HL2)Cl]2: Yield: 253 mg (0.49 mmol, 98%). C19H12ClNPtS (516.94): calcd. C 44.15, H 2.34, N
1
2.71, S 6.20; found C 44.22, H 2.38, N 2.73, S 6.22%. H NMR (300 MHz, CD2Cl2): δ = 8.10 (s,
JPt,H = 62 Hz, 1H), 8.04 (t, J = 8.0 Hz, 1H), 7.98 (s, 1H), 7.86-7.76 (m, 3H), 7.69-7.61 (m, 2H),
7.54-7.39 (m, 3H), 7.35 (dd, J = 5.2, 3.7 Hz, 1H) ppm. EI-MS(+): m/z = 517 [monomer]+, 480
[monomer‒HCl]+, 287 [H2L2]+.
[Pt(HL3)Cl]2: Yield: 262 mg (0.48 mmol, 97%). C21H14ClNPtS (542.94): calcd. C 46.46, H 2.60, N
2.58, S 5.90; found C 46.42, H 2.66, N 2.61, S 5.91%. 1H NMR (300 MHz, CD2Cl2): δ = 7.87-7.81
(m, 5H), 7.67 (d, J = 5.2 Hz, 1H), 7.65-7.56 (m, 5H), 7.40 (dd, J = 5.2, 3.7 Hz, 1H), 7.30-7.20 (m,
2H) ppm. EI-MS(+): m/z = 543 [monomer]+, 506 [monomer‒HCl]+, 313 [H2L3]+.
[Pt(HL4)Cl]2: Yield: 579 mg (0.98 mmol, 98%). C25H16ClNPtS (593.00): calcd. C 50.64, H 2.72, N
2.36, S 5.41; found C 50.68, H 2.75, N 2.36, S 5.41%. 1H NMR: (300 MHz, CD2Cl2): δ = 8.05-7.81
(m, JPt,H = 63 Hz, 2H), 7.78-7.68 (m, 5H), 7.61-7.55 (m, 1H), 7.54-7.49 (m, 5H), 7.38-7.28 (m, 2H),
7.25 (dd, J = 5.1, 3.8 Hz, 1H) ppm. EI-MS(+): m/z = 593 [monomer]+, 556 [monomer‒HCl]+, 363
[H2L4]+.
[Pt(HL5)Cl]2: Yield: 309 mg (0.49 mmol, 99%). C26H18ClNOPtS (623.02): calcd. C 50.12, H 2.91,
N 2.25, S 5.15; found C 50.13, H 2.95, N 2.26, S 5.14%. 1H NMR (300 MHz, CD2Cl2): δ = 8.24-
7.98 (m, JPt,H = 63 Hz, 2H), 7,92 (d, J = 1.8 Hz, 1H), 7.85-7.76 (m, 5H), 7.63 (d, J = 5.1 Hz, 1H),
7.58 (d, J = 3.2 Hz, 1H), 7.52-7.39 (m, 2H), 7.36 (dd, J = 5.2, 3.7 Hz, 1H), 7.12 (d, J = 8.8 Hz, 2H),
3.92 (s, 3H) ppm. EI-MS(+): m/z = 623 [monomer]+, 586 [monomer‒HCl]+, 393 [H2L5]+.
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[Pt(HL6)Cl]2: Yield: 298 mg (0.49 mmol, 98%). C25H15ClFNPtS (610.99): calcd. C 49.15, H 2.47,
1
N 2.29, S 5.25; found C 49.17, H 2.48, N 2.30, S 5.24%. H NMR (300 MHz, CD2Cl2): δ = 8.20-
7.96 (m, JPt,H = 63 Hz, 2H), 7.91-7.73 (m, 6H), 7.64 (d, J = 5.2 Hz, 1H), 7.59 (d, J = 3.7 Hz, 1H),
7.45 (dq, J = 6.9, 5.5 Hz, 2H), 7.39-7.26 (m, 3H) ppm. EI-MS(+): m/z = 611 [monomer]+, 574
[monomer‒HCl]+, 381 [H2L6]+.
Syntheses of doubly cyclometalated complexes [Pt(L1-6)(DMSO)]
Following a literature procedure,[23-24] the monocyclometalated complexes [Pt(HL1-6)Cl]2 were
dissolved in a small amount of hot dimethyl sulfoxide and small amounts of sodium carbonate were
added. The reaction mixture was stirred at 110 °C for 6 h. The raw product was precipitated through
addition of several small portions of water, filtered off and washed with water. The material was
purified by filtration over aluminium oxide using CH2Cl2 as eluent and the products were obtained
as yellow-orange solids.
[Pt(L1)(DMSO)] (1): Yield: 127 mg (0.25 mmol, 75%). C17H15NOPtS2 (508.51): calcd. C 40.15, H
2.97, N 2.75, S 12.61; found C 39.98, H 2.96, N 2.74, S 13.36%. 1H NMR (300 MHz, CD2Cl2): δ =
7.96 (d, J = 7.3 Hz, JPt,H = 25 Hz, 1H, Hc), 7.61-7.48 (m, 2H, H4/H5ˈ), 7.45 (d, J = 7.6 Hz, 1H, Hf),
7.25 (t, J = 6.8 Hz, 1H, Hd), 7.20-6.96 (m, 4H, He/H3/H5/H4ˈ), 3.66 (s, JPt,H = 27 Hz, 6H, S-CH3)
ppm. EI-MS(+): m/z = 508 [M]+, 430 [M–DMSO]+, 237 [L1]+.
[Pt(L2)(DMSO)] (2): Yield: 150 mg (0.27 mmol, 69%). C21H17NOPtS2 (558.57): calcd. C 45.16, H
3.07, N 2.51, S 11.48; found C 44.99, H 3.23, N 2.47, S 11.35%. 1H NMR (300 MHz, CD2Cl2): δ =
8.39 (s, JPt,H = 28 Hz, 1H, Hc), 7.98 (s, 1H, Hj), 7.76 (t, J = 7.8 Hz, 2H, He/Hh), 7.66 (t, J = 7.9 Hz,
1H, H4), 7.54 (d, J = 4.6 Hz, 1H, H5ˈ), 7.48-7.33 (m, 3H, Hf/Hg/H3), 7.17-7.07 (m, 2H, H4ˈ/H5),
3.73 (s, JPt,H = 27 Hz, 6H, S-CH3) ppm. HR-ESI-MS(+): m/z = 580.02724 [M+Na]+, 559.04784
[M+H]+ (calcd. 559.04778).
[Pt(L3)(DMSO)] (3):Yield: 133 mg (0.23 mmol, 77%). C23H19NOPtS2 (584.61): calcd. C 47.25, H
3.28, N 2.40, S 10.97; found C 46.98, H 3.29, N 2.45, S 11.10%. 1H NMR (300 MHz, CD2Cl2): δ =
7.95 (d, J = 7.3 Hz, JPt,H = 26 Hz, 1H, Hc), 7.75-7.69 (m, 2H, H2ˈˈ/H6ˈˈ), 7.57-7.48 (m, 5H,
Hf/H5ˈ/H3ˈˈ-H5ˈˈ), 7.32 (s, 1H, H3), 7.29-7.16 (m, 2H, Hd, H5), 7.16-7.03 (m, 2H, He/H4ˈ), 3.64
(s, JPt,H = 27 Hz, 6H, S-CH3) ppm. EI-MS(+): m/z = 584 [M]+, 506 [M–DMSO]+, 313 [H2L3]+.
[Pt(L4)(DMSO)] (4): Yield: 255 mg (0.40 mmol, 79%). C27H21NOPtS2 (634.67): calcd. C 51.10, H
3.34, N 2.21, S 10.10; found C 51.35, H 3.39, N 2.28, S 10.10%. 1H NMR (300 MHz, CD2Cl2) δ =
8.38 (s, JPt,H = 29 Hz, 1H, Hc), 8.03 (s, 1H, Hj), 7.75 (dd, J = 16.3, 7.5 Hz, 4H, He/Hh/H2ˈˈ/H6ˈˈ),
7.62-7.50 (m, 5H, H5/H5ˈ/ H3ˈˈ-H5ˈˈ), 7.43-7.30 (m, 3H, Hf/Hg/H3), 7.10 (d, J = 4.6 Hz, JPt,H = 10
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Hz, 1H, H4ˈ), 3.70 (s, JPt,H = 27 Hz, 6H, S-CH3) ppm. HR-ESI-MS(+): m/z = 635.07971 [M+H]+
(calcd. 635.07907).
[Pt(L5)(DMSO)] (5): Yield: 207 mg (0.31 mmol, 95%). C28H23NO2PtS2 (664.70): calcd. C 50.60, H
3.49, N 2.11, S 9.65; found C 50.20, H 3.38, N 2.10, S 9.79%.1H NMR (300 MHz, CD2Cl2): δ =
8.42 (s, JPt,H = 29 Hz, 1H, Hc), 8.09 (s, 1H, Hj), 7.78 (dd, J = 10.3, 8.3 Hz, 4H, He/Hh/H2ˈˈ/H6ˈˈ),
7.63 (d, J = 1.5 Hz, 1H, H5), 7.56 (d, J = 4,7 Hz, 1H, H5ˈ), 7.49-7.30 (m, 3H, Hf/Hg/H3), 7.18-7.07
(m, 3H, H4ˈ/H3ˈˈ/H5ˈˈ), 3.94 (s, 3H, O-CH3), 3.75 (s, JPt,H = 27 Hz, 6H) ppm. EI-MS(+): m/z = 664
[M]+, 586 [M–DMSO]+, 393 [H2L5]+.
[Pt(L6)(DMSO)] (6): Yield: 190 mg (0.29 mmol, 89%). C27H20FNOPtS2 (652.66): calcd. C 49.69,
H 3.09, N 2.15, S 9.82; found C 49.54, H 2.99, N 2.12, S 9.91%.1H NMR (300 MHz, CD2Cl2): δ =
8.37 (s, JPt,H = 28 Hz, 1H, Hc), 8.01 (s, 1H, Hj), 7.82-7.68 (m, 4H, He/Hh/H2ˈˈ/H6ˈˈ), 7.53-7.51 (m,
2H, H5/H5ˈ), 7.37 (dtd, J = 14.6, 6.9, 1.3 Hz, 2H, Hf/Hg), 7.29-7.21 (m, 3H, H3/H3ˈˈ/H5ˈˈ), 7.09
(d, J = 4.7 Hz, JPt,H = 10 Hz, 1H, H4ˈ), 3.69 (s, JPt,H = 27 Hz, 6H, S-CH3) ppm.
Synthesis of [Pt(L4)(Me2Imd)] (4a)
Following a literature procedure[39] a solution of [Pt(L4)(DMSO)] (4) (100 mg, 0.16 mmol, 1 eq) in
acetonitrile (40 mL) was treated with N,N-dimethylimidazolium iodide (38 mg, 0.17 mmol, 1.05 eq)
and potassium tert-butoxide (21 mg, 0.19 mmol, 1.18 eq). The reaction mixture was stirred at 90 °C
for 16 h. The solvent was reduced and the orange product was precipitated with water. Yield: 91 mg
(0.14 mmol, 88%). C30H24N3PtS (653.68): calcd. C 55.12, H 3.70, N 6.43, S 4.90; found C 53.75, H
3.72, N 6.11, S 4.95%. 1H NMR (300 MHz, CD2Cl2): δ = 8.11 (s, 1H, Hj), 7.84 (d, J = 6.9 Hz, 2H,
H2ˈˈ/H6ˈˈ), 7.78 (d, J = 7.7 Hz, 1H, Hh), 7.68 (s, 1H, H4), 7.64-7.28 (m, 9H, Hc/He-
Hg/H5/H5ˈ/H3ˈˈ-H5ˈˈ), 7.13 (s, JPt,H = 12 Hz, 2H, N-HC=CH-N), 6.84 (d, , J = 4.5 Hz, JPt,H = 13
Hz, 1H, H4ˈ), 3.84 (s, 6H, CH3) ppm. HR-ESI-MS (+): m/z = 652.12584 [M]+ (calcd. 652.12601).
Instrumentation: Elemental analysis was obtained using a Hekatech CHNS EuroEA 3000
analyzer. NMR spectra were recorded on a Bruker Avance II 300 MHz spectrometer (1H: 300.13
MHz) equipped with a BBO ATM 5 mm probe head with z-gradient (1H/19F X 2H), or on a Bruker
13
Avance II+ 600 spectrometer (1H: 400.13 MHz, C: 100.61 MHz) using a TBI 5 mm probe head
31
2
1
1
1
with z-gradient (1H P X H). The assignment of the H was obtained from H NOESY and H
COSY experiments. All 2D NMR experiments were performed using standard pulse sequences
from the Bruker pulse program library. Chemical shifts were relative to TMS. EI-MS in positive
mode were determined using a Finnigan MAT 95 spectrometer. If not indicated otherwise,
ionisation energy of 20 eV was applied. HR-ESI-MS high resolution measurements were recorded
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European Journal of Inorganic Chemistry
with a LTQ-Orbitrap XL spectrometer. UV-vis absorption spectra were recorded on Varian Cary 50
Scan spectrophotometers. Emission spectra at room temperature were recorded on a Spex
FluoroMax-3 spectrometer. For emission spectra at 77 K and life time measurements (77 K and
293 K) a PicoQuant FluoTime 300 spectrometer was used. Lifetime analysis was performed using
the commercial FluoFit software. The quality of the fit was assessed by minimizing the reduced chi
squared function (χ2). Luminescence quantum yields were determined using a Hamamatsu
Photonics absolute PL quantum yield measurement system (C9920-02) equipped with a L9799-01
CW Xenon light source, monochromator, photonic multi-channel analyser and integrating sphere
(error of maximum ± 5% for Φ is estimated). All solvents used were of spectroscopic grade and
degassed prior to use. Matrices of complexes in PMMA were prepared by solving complex and
PMMA in toluene at different amounts and evaporation of the solvent on a glass substrate
(5x5 mm). A PG STAT 30 potentiostat from Metrohm was used for cyclovoltammetric
measurements using a three-electrode arrangement consisting of a glassy-carbon working electrode,
a platinum counter electrode and a Ag/AgCl reference electrode.
Single crystal structure analysis: The measurement was performed using graphite-
monochromatised Mo-Kα radiation (λ = 7.1073 Å) on a STOE IPDS IIT diffractometer (at 293 K).
The structure was solved by direct methods (SIR2011)[40] and refined by full-matrix least-squares
techniques against F2 (SHELXL-2014).[41] Non-hydrogen atoms were refined with anisotropic
displacement parameters without any constraints. The hydrogen atoms were included by using
appropriate riding models. Numeric absorption correction was carried out with X-RED[42] and
X-SHAPE.[43] Full structural information has been deposited with the Cambridge Crystallographic
Data Centre, No. CCDC-1528069 for [Pt(L3)(DMSO)]. Copies of the data can be obtained, free of
charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK.
Supporting Information (see footnote on the first page of this article): More experimental details,
tables and additional figures with NMR spectra, emission spectra, cyclic voltammograms, and
spectroelectrochemical UV-vis absorption spectra were provided.
Acknowledgements
We thank Dr. Ingo Pantenburg (University of Cologne) for crystal data collection. We also thank
Michael Neihs and Dr. Mathias Schäfer (University of Cologne) for ESI-MS high resolution
measurements recorded with an LTQ-Orbitrap XL.
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