Diaryl-1,2,3-Triazolylidene Platinum(II) Complexes

Article abstract of DOI:10.1002/chem.201705738

Control of the excited state geometry by rational ligand design leads to a new class of phosphorescent emitters with extraordinary photophysical properties. Extension of the π-system in the triplet state leading to a significant bathochromic shift of the

Full text of DOI:10.1002/chem.201705738

DOI: 10.1002/chem.201705738
Full Paper
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Conjugated Systems
Diaryl-1,2,3-Triazolylidene Platinum(II) Complexes
Johannes Soellner and Thomas Strassner*[a]
Abstract: Control of the excited state geometry by rational
ligand design leads to a new class of phosphorescent emit-
ters with extraordinary photophysical properties. Extension
of the p-system in the triplet state leading to a significant
bathochromic shift of the emission was avoided by introduc-
tion of additional steric demand. We report the synthesis,
characterization and photophysical properties of novel plati-
num(II) complexes bearing C^C* cyclometalated mesoionic
carbene (MIC) with different b-diketonate ligands. The MIC
ligand precursors were prepared from 1-phenyl-1,2,3-triazole
using arylation protocols, introducing phenyl or mesityl
functionalities. A solid state structure confirming the NMR
assignments is presented. The emission properties were in-
vestigated in detail at room temperature and 77 K and are
supported by DFT calculations and cyclic voltammetry. All
complexes, with emission maxima between 502–534 nm,
emit with quantum efficiencies ranging from 70–84% in
PMMA films.
donating abnormal NHCs (aNHCs),[8] where the carbene did
not form in the C2-position, but rather in the backbone of the
heterocycle, resulting in a mesoionic structure. These aNHCs
have inspired other motifs, such as 1,2,3-triazolylidenes, which
also are mesoionic in nature.[9] While these mesoionic carbenes
(MICs) have been studied regarding their performance in ho-
mogeneous catalysis,[9c] exploration of their prospect as ligands
in luminescent transition metal compounds has only recently
commenced.[6b,10] Here, we present four novel phosphorescent
diaryl-1,2,3-triazole-based MIC platinum(II) complexes with re-
markable photophysical properties due to the strongly donat-
ing heterocycle and their rigid geometry enforced by cyclome-
talation. We compare systems concerning their ability to in-
clude the second aryl ring into the p-system. By nuclear mag-
netic resonance studies and a solid state structure we con-
firmed the presence of a mesoionic carbene in the target
compounds. We also studied the effect of aryl substituents on
the b-diketonate which led to highly interesting phosphores-
cent emitter molecules.
Introduction
Photoluminescent organometallic compounds[1] are an impor-
tant pillar of applied chemistry. The interaction of organic com-
ponents and metal center results in valuable material proper-
ties which allow for application in various fields, such as photo-
catalysis,[2] dye-sensitized solar cells[3] or organic light emitting
diodes (OLEDs).[4] The development of the first OLED setup[5]
ultimately required optimizations also for phosphorescent
emitter molecules.
Phosphorescent organometallic complexes allow for quan-
tum yields of up to 100%, but defined electronic states and
molecular stability are essential to obtain a high-quality
dopant. This can be achieved by electronic and steric modifica-
tions, which change the properties of the complexes. Especially
platinum(II)[6] and iridium(III)[6q,r,7] complexes bearing cyclome-
talating ligands based on phenylpyridines and aryl-substituted
N-heterocyclic carbene (NHC) systems are promising candi-
dates for these applications. Recent results indicated that a
rigid geometry in the excited state is crucial for high quantum
yields.[6a]
Results and Discussion
A number of different N-heterocyclic carbene systems have
been tested as ligands in platinum(II) complexes in recent
years.[6a,r] The majority of these motifs were derived from imi-
dazolium-based ligands by deprotonation of the C2-hydrogen
atom. In 2001, Crabtree and co-workers discovered strongly
Syntheses of the ligands starts from 1-phenyl-1,2,3-triazole 1
which was prepared according to a literature procedure from
aniline (Scheme 1).[6b,11] The aryl substituents in 2 and 3 were
introduced using hypervalent diaryliodonium tetrafluorobo-
rates. In copper(II) acetate catalyzed reactions,[12] diphenyliodo-
nium tetrafluoroborate[13] and bis(2,4,6-trimethylphenyl)iodoni-
um tetrafluoroborate[14] were reacted with the triazole sub-
strate 1 to yield the quarternized products 2 and 3, respective-
ly. The platinum(II) complexes were synthesized following a
multi-step procedure.[6b,15] The ligand precursors 2 and 3 were
reacted with silver(I)-oxide to give the respective silver(I) car-
bene complexes. Upon addition of Pt(COD)Cl2 (COD=1,5-cy-
clooctadiene), the NHC ligand is transmetalated to the plati-
[a] J. Soellner, Prof. Dr. T. Strassner
Physikalische Organische Chemie
Technische Universitꢀt Dresden
01069 Dresden (Germany)
E-mail: thomas.strassner@chemie.tu-dresden.de
Homepage: http://www.chm.tu-dresden.de/oc3
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/chem.201705738.
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Figure 1. Relevant segments of the H (CDCl3, 600 MHz) and 195Pt (CDCl3,
64 MHz) NMR spectra for compound 4a showing the heteroatom coupling
between hydrogen and platinum nuclei.
Scheme 1. Synthetic approach towards 4a–c and 5. (i) [Ph2I][BF4], Cu(OA-
c)2·H2O, DMF, 1008C. (ii) [Mes2I][BF4], Cu(OAc)2·H2O, DMF, 1008C. (iii) Ag2O,
DMF, 758C. (iv) Pt(COD)Cl2, RT to 1258C. (v) b-diketone, base, RT to 1008C.
num center and subsequently cyclometalated at elevated tem-
peratures, followed by addition of the respective b-diketone
ligand and base. Compounds 4a–c and 5 were isolated by
flash column chromatography.
All synthesized compounds were characterized by means of
standard NMR techniques, mass spectrometry and elemental
analysis. Due to its symmetry, compound 2 shows only one sin-
glet resonance for its backbone protons at a chemical shift of
d=9.79 ppm. The mesitylated salt 3 on the other hand shows
the expected two doublet resonances for these protons at 9.49
and 9.90 ppm, with the latter belonging to the proton adja-
cent to the phenyl ring (based on the NOESY assignment).
These backbone hydrogen resonances are a useful probe for
the determination of the binding situation. Changes in integra-
tion and chemical shift of these resonances are an indication
for the formation of the respective mesoionic carbene struc-
ture. Interestingly, cyclometalation into the neighboring phenyl
ring can also be confirmed by means of NMR spectroscopy as
described below for complex 4a.
Figure 2. ORTEP3 drawings of 4a. Ellipsoids are given at 50% probability
level. a) Molecular structure. b) Packing diagram; view normal to (100).
c) Packing diagram; view normal to (010). Selected bond lengths [ꢁ] and
angles [deg]: Pt(1)ꢀC(1) 1.943(2); Pt(1)ꢀC(4) 1.992(2); Pt(1)ꢀO(1) 2.0802(16);
Pt(1)ꢀO(2) 2.0539(16); C(1)-Pt(1)-C(4) 81.07(9); O(1)-Pt(1)-O(2) 91.15(7); O(1)-
Pt(1)-C(1)-C(2) 2.5(3); C(2)-N(1)-C(9)-C(14) ꢀ142.1(3).
Pt(1)-C(4) of the MIC ligand in the five-membered metallacycle
is significantly smaller with 81.07(9)8. All platinum–carbon and
platinum–oxygen bond lengths are within the expected range
and compare to those of structurally related compounds.[6b,15b]
The non-coordinating phenyl ring rotates out of the coordina-
tion plane by ꢀ142.1(3)8, which also impacts the packing in
the solid state (Figure 2b). The molecules stack roughly 1808
rotated against one another (Figure 2c), which allows the
aforementioned phenyl group to use a gap between the MIC
and the b-diketonate ligand. In this orientation, the closest Ptꢀ
Pt distance is observed to be 4.53 ꢁ, which rules out a direct
metal–metal interaction in this structure.
The signal shape of the proton resonance at d=7.9 ppm
(and the coupling constant for its shoulders in the range of
50 Hz) is indicative for
a proton–heteroatom coupling
(Figure 1). The hetero nucleus responsible for this coupling is
the platinum center, which is confirmed by the shape of the
signal in the 195Pt spectrum. Here, a doublet with a coupling
constant of 53 Hz is found in the NMR experiment. The vicinal
nuclear interaction was confirmed by 2D NMR techniques
which allowed us to assign the proton resonance in question
to the proton adjacent to the cyclometalating position in the
phenyl ring of the metallacycle. Additionally, we were able to
obtain a solid state structure for complex 4a which confirms
the mesoionic binding mode of the N-heterocyclic ligand. The
platinum(II) complex crystallizes in the monoclinic space group
C2/c, with eight molecules per unit cell. The metal center
forms a square planar coordination sphere, which is slightly
twisted out of plane (Figure 2a). While the O(1)-Pt(1)-O(2)
angle is close to the ideal value of 908, the bite angle C(1)-
The photophysical properties of all complexes were investi-
gated at room temperature and for 4a and 5 additionally at
low temperature. Absorption spectra were recorded in di-
chloromethane (DCM) solution and are given in Figure 3. Com-
plexes 4a–c show an absorption maximum at around 280 nm.
Compound 4a further displays
a broad band around
375 nm, while 4b and 4c absorb without distinct bands at
higher wavelengths. The spectrum of 5 resembles that of 4a
in terms of structuring but is blue-shifted by approximately
15 nm. TD-DFT calculations using Gaussian 09[16] were em-
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Figure 3. Absorption (solid) and emission (dotted) spectra of 4a–c and 5.
Absorption spectra were measured in 5ꢂ10ꢀ5 molLꢀ1 DCM solution. Emis-
sion spectra were measured using PMMA films with a 2 wt% emitter load.
ployed to assign the visible bands to electronic transitions (see
Supporting Information, Figures S1–S4 and Tables S1–S6). The
high energy bands at 280 nm (4a–c) and 265 nm (5) represent
a complex mixture of transitions involving both ligands as well
as the metal center. The bands at 375 nm (4a–c) and 355 nm
(5) on the other hand can be attributed to transitions from the
HOMOꢀ1/HOMO to the NHC-centered LUMO, to which the
MIC ligand, the platinum center and the b-diketonate contrib-
ute equally.
Figure 4. DFT (B3LYP/6-31G(d)) optimized structures for complex 4a (a and
b) as well as 5 (d and e) with dihedral angles (red bonds). Spin density simu-
lations for the excited states of 4a (c) and 5 (f), plotted for an isovalue of
0.0008.
of 4b (533 nm) and 4c (534 nm) is small, since the intra-
molecular distance between the b-diketonate and the non-co-
ordinating substituent of the MIC ligand is too large to hinder
the rotation of the N-aryl moiety. Notably, in these complexes,
the quantum yield is increased to 81% (4b) and 82% (4c),
when compared to 4a, which achieved a quantum efficiency
of 70% (Table 1). The luminescence lifetimes changed only
marginally and remained in the range of 12–15 ms upon intro-
duction of the b-diketonate. To test our assumption concern-
ing the planarization in the excited state, we synthesized the
N-mesitylated compound 5 with a higher steric demand in the
Emission properties for all complexes were investigated by
preparing poly(methyl methacrylate) (PMMA) films with an
emitter concentration of 2 wt%. Additionally, spectra in 2-
MeTHF solution were measured (see Experimental Section, for
PL data see Supporting Information Table S7). Figure 3 shows
the emission spectra for all platinum compounds at room tem-
perature. The emission maxima are flanked by weak shoulders
on each side. The emission maximum of complex 4a is in the
green region of the spectrum at a wavelength of 532 nm,
which corresponds to a bathochromic shift of 39 nm compared
to a recently published N-methylated analogous complex.[6b]
The introduction of the N-aryl functionality in 4a exhibits a
profound effect on the emission process. To investigate this
bathochromic shift we employed DFT calculations which show
a distortion of the molecule for 4a when comparing the triplet
with the singlet geometry. In the optimized ground state struc-
ture the non-coordinating phenyl ring is twisted out of plane
with a dihedral angle of ꢀ149.88 to the triazole (Figure 4a).
Upon excitation from S0 into the triplet state T1 the phenyl
ring of 4a rotates into the molecular plane, effectively enlarg-
ing the conjugated p-system (Figure 4b). This results in a stabi-
lization of the excited state, which corresponds to the ob-
served bathochromic shift (see Supporting Information, Fig-
ure S5). A simulation of the spin density distribution in the ex-
cited state visualizes the contribution of the phenyl moiety
(Figure 4c). As expected, this effect persists when introducing
more sterically demanding b-diketonate systems like the mesi-
tyl and duryl substituents. The impact on the emission maxima
Table 1. Photoluminescence (2 wt% emitter load in PMMA) and electro-
chemical data for 4a–c and 5.
Photoluminescence data
CV
lexc
[nm][a]
CIE
x;y[b]
lem
[nm][c]
F
[%][d]
tv
[ms][e]
t0
[ms][f]
E1/2(red)
[V][g]
4a
4b
4c
5
360
360
360
360
0.349;
0.569
0.340;
0.569
0.337;
0.568
0.239;
0.474
532
(527)
533
(526)
534
(524)
502
70
81
82
84
10.6
10.5
10.2
9.2
15.1
12.9
12.4
11.0
ꢀ2.09
ꢀ2.07
ꢀ2.07
ꢀ2.37
(492)
[a] Excitation wavelength. [b] CIE coordinates at RT. [c] Maximum emission
wavelength at RT. lem in degassed 2-MeTHF (c=4.5ꢂ10ꢀ5 molLꢀ1) at 77 K
given in parentheses. [d] Quantum yield at lexc =360 nm and RT. [e] Ex-
perimental phosphorescence lifetime at RT. [f] Phosphorescence lifetime
given as t0 =tv/F. [g] Cyclic voltammetry experiments referenced to Fc/
Fc+.
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non-coordinating N-aryl unit. DFT calculations suggest that the
mesityl ring is oriented perpendicular to the coordination
plane in the ground state (Figure 4d). In the triplet state ge-
ometry, the mesityl system and the triazole span a dihedral
angle of ꢀ116.28 as the methyl groups in the ortho-position do
not allow further rotation (Figure 4e). An extension of conjuga-
tion into the N-aryl unit in the excited state can thereby be at-
tenuated, which is visible in the calculated spin density (Fig-
ure 4 f). Compared to 4a, the sterically demanding aryl group
in 5 partakes in the emissive process to a significantly lower
extent. Consequently, this is also reflected in the position of
the emission maximum at 502 nm (Figure 3, Table 1); a hypso-
chromic shift of 30 nm compared to complex 4a. Additionally,
the quantum yield is increased by 14% while the emission life-
times are slightly shortened to 11 ms. Complexes 4a–c with the
phenyl group and 5 carrying a mesityl ring differ significantly
in emission color, while the emission band shapes are quite
similar, pointing towards a related phosphorescence mecha-
nism. In order to gain more resolved spectra, measurements at
low temperature (77 K) were conducted for complexes 4a and
5 in degassed solutions of the emitters in 2-methyltetrahydro-
furan. The results given in Figure 5 show resolved emissions
the time-dependent and excited state calculations (Figure 4
and Supporting Information Figure S5) we can characterize the
emission as an ILCT/MLCT process.
Cyclic voltammetry experiments were performed on de-
gassed DMF solutions of all complexes (see Supporting Infor-
mation Figures S6–S9). Compounds 4a–c and 5 show a reversi-
ble reduction at ꢀ2.07–2.09 V and ꢀ2.37 V versus ferrocene
(Table 1 and Figure 6), respectively, while the onset of oxidative
Figure 6. Cyclic voltammetry spectra of complexes 4a–c and 5 in degassed
DMF solutions at a scan rate of 50 mVsꢀ1. Referenced against Fc/Fc+.
processes is between 0.0–0.25 V. The unaltered potential of the
reductive event for 4a–c leads to the conclusion that the b-di-
ketonate moiety of the complexes does not partake in the re-
ductive process and the involved molecular orbitals. Moreover,
electron-accepting, N-heterocyclic ligands are known to enable
reversible reductive processes[17] which hints towards the re-
duction taking place at the MIC ligand. This is also consistent
with the LUMO plots obtained in the DFT calculations and the
higher potential necessary to reduce 5. DFT calculations of the
radical anions of 4a and 5 display a similar behavior compared
to the neutral molecules in the excitation process (see Sup-
porting Information, Figure S10). Unlike complex 5, the pheny-
lated compound 4a is able to planarize, which aids in stabiliz-
ing the surplus electron and results in a significantly lower re-
duction potential.
Figure 5. Emission spectra at RT and 77 K for compounds 4a and 5. RT
measurements were carried out in 2 wt% PMMA films and at 77 K in 2Me-
THF (4.5ꢂ10ꢀ5 molLꢀ1) solutions.
which are blue-shifted by approximately 10 nm compared to
the measurements at room temperature. The spectra at 77 K
display three well defined bands and a weak fourth band at
higher wavelengths. The first and the second band differ ener-
getically by approximately 1297 cmꢀ1 (4a) or 1290 cmꢀ1 (5).
These energy gaps correspond to infrared (IR) active 0!1 tran-
sitions, which were assigned to be centered on the MIC ligand
by DFT simulations of the respective IR spectra. With these vi-
brations being represented in the emission spectra, it can be
concluded that the heterocyclic ligand is strongly involved in
the emissive process, which is consistent with the data pre-
sented earlier. Along with the theoretical results obtained from
Conclusion
Four new platinum(II) complexes bearing C^C* cyclometalated
mesoionic carbene ligands based on 1,2,3-triazole systems are
presented. The phenyl and mesityl groups were introduced to
the 1-phenyl-1,2,3-triazole system using the corresponding io-
donium reagents. Cyclometalation of the MIC ligand was con-
1
firmed by a proton–heteroatom coupling found in the H and
195Pt spectra, as well as 2D NMR experiments and a solid state
structure. All complexes proved to be strongly emissive at
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room temperature in PMMA films with quantum yields ranging
from 70 to 84%. Depending on the aryl fragment a significant
change of the emission color was noticed. Theoretical calcula-
tions confirmed that this effect is due to a distortion of the ge-
ometry in the excited state. In the case of the phenyl ligand,
the aryl unit rotates into the molecular plane and enlarges the
conjugated system. This causes a stabilization of the excited
state, resulting in an emission maximum at lem =536 nm, while
for the mesityl ligand a planarization is hindered due to steric
demand. This significantly reduces its contribution to the con-
jugated system and blue-shifts the emission by 33 nm.
Electrochemistry
Cyclic voltammetry experiments were carried out using a Biologic
SP-150 potentiostat in degassed, dry DMF solutions, employing a
platinum wire counter electrode, a glassy carbon working elec-
trode and a Ag/Ag+ pseudo reference electrode. All complexes
were measured as 0.5 mm solutions along with 0.1m supporting
electrolyte ((nBu)4ClO4) at a sweep rate of 50 mVsꢀ1. All measure-
ments were internally referenced against Fc/Fc+.
Computational details
The Gaussian 09[16] package was used to perform all quantum
chemical calculations employing the hybrid functional B3LYP[19] to-
gether with a 6-31G(d)[20] and 6–31+G(d)[20g,21] basis set. Platinum
was described by a decontracted Hay–Wadt(n+1) ECP and basis
set.[22] All given structures were verified as true minima by vibra-
tional frequency analysis and the absence of negative eigenvalues.
Calculated geometries were visualized with CYLview and Gauss-
View.
Experimental Section
General considerations
All compounds were synthesized in flame dried Schlenk tubes and
under argon atmosphere unless otherwise noted. Solvents of at
least 99.0% purity were used in all reactions in this study. Dime-
thylformamide (DMF) was dried using standard techniques and
stored over molecular sieve (3 ꢁ). Dichloro(cycloocta-1,5-diene)pla-
tinum(ii) was prepared according to a modified literature proce-
dure.[15b] Bis-1,3-(2,4,6-trimethyl-phenyl)propan-1,3-dione and bis-
1,3-(2,3,5,6-tetramethylphenyl)-propan-1,3-dione were prepared ac-
cording to a modified literature procedure.[18] All other chemicals
were obtained from common suppliers and used without further
Synthesis and Characterization
1,3-Diphenyl-1,2,3-triazolium tetrafluoroborate (2): 0.726 g (5 mmol)
1-Phenyl-1,2,3-triazole 1, 2.759 g (7.5 mmol, 1.5 equiv) diphenylio-
donium tetrafluoroborate and 0.050 g (0.25 mmol, 0.05 equiv) cop-
per(II)-acetate monohydrate are placed in a Schlenk tube and are
dissolved in 20 mL DMF. The mixture is heated to 1008C and
stirred for 17 h. The reaction is cooled to room temperature and all
volatiles are removed under reduced pressure. The product (1.41 g,
91%) is isolated by crystallization from hot ethanol. M.p. 1658C.
1H NMR (300 MHz, [D6]DMSO): d=9.79 (s, 2H, CHarom), 8.21–8.08 (m,
4H, CHarom), 7.86–7.71 ppm (m, 6H, CHarom). 13C NMR (75 MHz,
[D6]DMSO): d=135.0 (Ci), 132.2 (CHarom), 130.5 (CHarom), 130.0
(CHarom), 121.9 ppm (CHarom). 19F NMR (282 MHz, [D6]DMSO): d=
ꢀ148.8 (s, BF4ꢀ), ꢀ148.9 ppm (s, BF4ꢀ). MS (ESI, m/z): 222.0 (M-
BF4)+. Anal Calcd for C14H12BF4N3: C 54.41, H 3.92, N 13.60, found:
C 54.79, H 4.00, N 13.69.
1
purification. H, 13C, 19F and 195Pt spectra were acquired on Bruker
NMR Avance 300, Bruker DRX 500 and Bruker Avance 600 NMR
spectrometers. 1H and 13C spectra were referenced internally (1H:
7.26, 13C 77.0 for CDCl3; 1H: 2.50, 13C 39.43 for [D6]DMSO). 195Pt
spectra were referenced externally by using potassium tetrachloro-
platinate(ii) in D2O (PtCl4 : ꢀ1617.2). 19F NMR spectra were also ref-
2
erenced externally against trifluoromethylbenzene (ꢀ63.72 ppm vs.
CCl3F). Chemical shifts are given in ppm, coupling constants J in
Hz. Elemental analyses were performed by the microanalytical lab-
oratory of our institute on a Hekatech EA 3000 Euro Vector ele-
mental analyzer. Melting points were determined by using a
Wagner and Munz PolyTherm A system and are not corrected.
1-Phenyl-3-(2,4,6-trimethylphenyl)-1,2,3-triazolium
tetrafluorobo-
rate (3): 0.363 g (2.5 mmol) 1-Phenyl-1,2,3-triazole 1, 1.356 g
(3 mmol, 1.2 equiv) bis(2,4,6-trimethylphenyl)iodonium tetrafluoro-
borate and 0.025 g (0.125 mmol, 0.05 equiv) copper(II)-acetate
monohydrate are placed in a Schlenk tube and are dissolved in
20 mL DMF. The mixture is heated to 1008C and stirred for 17 h.
The reaction is cooled to room temperature and all volatiles are re-
moved under reduced pressure. The raw product is crystallized by
addition of diethyl ether to a methanol solution of the reaction
mixture. The analytically pure product (0.41 g, 47%) is obtained by
extraction of the raw product with DCM and subsequent drying in
vacuo. M.p. 2038C. 1H NMR (600 MHz, [D6]DMSO): d=9.90 (d, J=
1.6 Hz, 1H, CHarom), 9.49 (d, J=1.6 Hz, 1H, CHarom), 8.17–8.10 (m,
2H, CHarom), 7.83–7.73 (m, 3H, CHarom), 7.25 (d, J=0.8 Hz, 2H,
CHarom), 2.39 (s, 3H, CCH3), 2.13 ppm (s, 6H, CCH3). 13C NMR
(151 MHz, [D6]DMSO): d=142.0, (Ci), 134.9 (Ci), 134.3 (Ci), 133.7
(CHarom), 131.9 (CHarom), 131.2 (Ci), 130.2 (CHarom), 130.0 (CHarom),
129.5 (CHarom), 121.8 (CHarom), 20.63 (CCH3), 16.75 ppm (CCH3).
19F NMR (282 MHz, [D6]DMSO): d=ꢀ148.8 (s, BF4ꢀ), ꢀ148.9 ppm (s,
BF4ꢀ). MS (ESI, m/z): 264.1 (M-BF4)+; 615.2 (2M-BF4)+. Anal Calcd for
C17H18BF4N3: C 58.15, H 5.17, N 11.97; found: C 57.90, H 5.24, N
12.13.
Photoluminescence measurements
The 2 wt% emitter films were prepared by doctor blading a solu-
tion of an emitter in a 10 wt% PMMA solution in dichloromethane
on a quartz substrate with a 60 mm doctor blade. The film was
dried and the emission was measured under nitrogen atmosphere.
Room temperature experiments in degassed 2-methyltetrahydro-
furan (2-MeTHF) solution were carried out at a concentration of 1ꢂ
10ꢀ5 molLꢀ1. Solutions for the low temperature measurements at
77 K were prepared by dissolving the emitter in degassed 2-MeTHF
to give a concentration of 4.5ꢂ10ꢀ5 molLꢀ1. The frozen samples in
quartz cuvettes were inserted in a quartz finger dewar containing
liquid nitrogen. Excitation was conducted in a wavelength range of
250–400 nm (Xe lamp with a monochromator), and the emission
was detected with a calibrated quantum yield detection system
(Hamamatsu, model C9920-02). The uncertainty of the quantum
yield is ꢁ2% for quantum yields of >10%. The phosphorescence
decay was measured with an Edinburgh Instruments mini-t by ex-
citation with pulses of an EPLED (360 nm, 20 kHz) and time-re-
solved photon counting (TCSPC). Absorption spectra were mea-
sured on a PerkinElmer Lambda 25 UV/Vis spectrometer.
Acetylacetonato-kO,kO’-[1-phenyl-3-(phenyl-kC2)-triazol-4-ylidene-
kC4] platinum(II) (4a): In a flame dried Schlenk tube, 0.247 g
(0.8 mmol) 1,3-diphenyl-1,2,3-triazolium tetrafluoroborate (2) and
0.093 g (0.4 mmol, 0.5 equiv) silver(i)-oxide are dissolved in 20 mL
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dry DMF and heated to 858C for 24 h. After adding 299 mg
(0.8 mmol, 1 equiv) Pt(COD)Cl2, the mixture is stirred at room tem-
perature for three hours and subsequently heated to 1308C for an-
other 21 hours. The solution is cooled to room temperature,
0.160 mg (1.6 mmol, 2 equiv) acetylacetone and 221 mg (1.6 mmol,
2 equiv) potassium carbonate are added and stirred at room tem-
perature for 21 hours and afterwards at 1008C for six hours. All vol-
atiles are removed in vacuo and the residue is washed with
ca. 40 mL of distilled water. The remaining solid is filtered, dried at
608C overnight and extracted with DCM. The product is purified
by flash column chromatography with DCM as eluent. The product
is obtained after washing with iso-hexanes and diethyl ether (3ꢂ
5 mL each) and drying in vacuo. (185 mg, 45%). M.p. 2208C.
1H NMR (600 MHz, CDCl3): d=8.05 (s, 1H, NCH), 7.97–7.84 (m, 1H,
PtCCH), 7.84–7.78 (m, 2H, CHortho of N3-Ph), 7.65–7.59 (m, 2H,
CHmeta of N3-Ph), 7.59–7.55 (m, 1H, CHpara of N3-Ph), 7.54 (dd, J=
7.8, 1.4 Hz, 1H, CHortho of N1-Ph), 7.18 (t, J=7.4 Hz, 1H, CHpara of
N1-Ph), 7.12 (t, J=7.5 Hz, 1H, CHmeta of N1-Ph), 5.50 (s, 1H),
2.06 ppm (s, 3H), 2.00 (s, 3H). 13C NMR (151 MHz, CDCl3): d=185.1
(CO), 184.8 (CO), 145.4 (PtCCN), 139.4 (PtCN), 136.1 (Ci of N3-Ph),
132.4 (PtCCH), 130.3 (CHpara of N3-Ph), 130.1 (CHmeta of N3-Ph),
128.0 (CHpara of N1-Ph), 127.2 (PtCCN), 124.7 (NCH), 123.5 (CHmeta
of N1-Ph), 120.8 (CHortho of N3-Ph), 113.6 (CHortho of N1-Ph), 102.2
(CHarom), 27.9 (COCH3), 27.8 ppm (COCH3). 195Pt NMR (64 MHz,
CDCl3): d=ꢀ3410.3 ppm (d, J=53 Hz). MS (ESI, m/z): 515.2
(M+H)+; 929.3 (2M-acac)+. Anal Calcd for C19H17N3O2Pt 0.08 CH2Cl2:
C 43.73, H 3.29, N 8.02; found: C 43.69, H 3.25, N 8.05.
(M+H)+. Anal Calcd for C37H37N3O2Pt 0.27 CH2Cl2: C 57.86, H 4.89,
N 5.43; found: C 57.84, H 4.88, N 5.41.
Acetylacetonat-kO,kO’-[3-(phenyl-kC2)-1-(2,4,6-trimethylphenyl)-tria-
zol-4-ylidene-kC5]platinum(ii) (5): In a flame dried Schlenk tube,
0.351 g (1 mmol) 1-phenyl-3-(2,4,6-trimethylphenyl)-1,2,3-triazolium
tetrafluoroborate (3) and 0.116 g (0.5 mmol, 0.5 equiv) silver(I)-
oxide are dissolved in 24 mL dry DMF and heated to 808C for 24 h.
After adding 374 mg (1 mmol, 1 equiv) Pt(COD)Cl2, the mixture was
stirred at room temperature for three hours and subsequently
heated to 1308C for another 21 hours. The solution is cooled to
room temperature, 0.400 mg (4 mmol, 4 equiv) acetylacetone and
553 mg (4 mmol, 4 equiv) potassium carbonate are added and
stirred at room temperature for 21 hours and afterwards at 1008C
for six hours. All volatiles are removed in vacuo and the residue is
washed with ca. 40 mL of distilled water. The remaining solid is fil-
tered, dried at 608C overnight and extracted with DCM. The prod-
uct is purified by flash column chromatography with an eluent
mixture of DCM/methanol (1% methanol). The product is obtained
after drying in vacuo. (129 mg, 23%). M.p.: 2558C. 1H NMR
(300 MHz, CDCl3): d=8.07–7.74 (m, 1H, CHarom), 7.63 (s, 1H, CHarom),
7.47 (dd, J=7.7, 1.5 Hz, 1H, CHarom), 7.18 (td, J=7.4, 1.5 Hz, 1H,
CHarom), 7.10 (td, J=7.5, 1.5 Hz, 1H, CHarom), 7.04 (s, 2H, CHarom), 5.49
(s, 1H, COCH), 2.38 (s, 3H, CCH3), 2.09 (s, 6H, CCH3), 2.06 (s, 3H,
CCH3), 1.96 ppm (s, 3H, CCH3). 13C NMR (75 MHz, CDCl3): d=185.2
(CO), 185.0 (CO), 145.9 (Ci), 141.4 (Ci), 138.8 (Ci), 135.1 (Ci), 132.8 (Ci),
132.6 (CHarom), 129.5 (CHarom), 129.2 (CHarom), 128.0 (CHarom), 127.0
(Ci), 123.6 (CHarom), 113.7 (CHarom), 102.3 (CHarom), 28.1 (CCH3), 28.0
(CCH3), 21.3 (CCH3), 17.3 ppm (CCH3). 195Pt NMR (64 MHz, CDCl3)
d=ꢀ3414.6 ppm (d, J=56.9 Hz). MS (ESI, m/z): 557.4 (M+H)+. Anal
Calcd for C22H23N3O2Pt: C 47.48, H 4.17, N 7.55; found: C 47.15, H
4.05, N 7.54.
[Bis-1,3-(2,4,6-trimethylphenyl)propane-1,3-dionato-kO,kO’][1-phenyl-
3-(phenyl-kC2)-triazol-4-ylidene-kC4]platinum(II) (4b): Prepared anal-
ogously to 4a (see Supporting Information). 0.493 mg (1.6 mmol,
2 equiv) bis-1,3-(2,4,6-trimethylphenyl)propane-1,3-dione were
used instead of acetylacetone and 180 mg (1.6 mmol, 2 equiv) po-
tassium tert-butoxide instead of potassium carbonate (146 mg,
Acknowledgements
1
25%). M.p. 2698C. H NMR (300 MHz, CDCl3): d=8.02 (s, 1H), 7.98–
7.70 (m, 3H), 7.66–7.48 (m, 4H), 7.17–7.04 (m, 2H), 6.86 (d, J=
3.7 Hz, 4H), 5.67 (s, 1H), 2.38 (s, 6H), 2.35 (s, 6H), 2.31 (s, 3H),
2.30 ppm (s, 3H). 13C NMR (75 MHz, CDCl3): d=184.9 (CO), 184.3
(CO), 145.6 (Ci), 139.8 (Ci), 139.6 (2 Ci), 137.7 (Ci), 137.6 (Ci), 136.3
(Ci), 134.3 (Ci), 134.1 (Ci), 133.1 (CHarom), 130.4 (CHarom), 130.3
(CHarom), 128.6 (CHarom), 128.3 (CHarom), 128.2 (CHarom), 127.1 (Ci),
125.0 (CHarom), 123.8 (CHarom), 120.8 (CHarom), 113.7 (CHarom), 107.1
(CHarom), 21.2 (2 CCH3), 20.0 (CCH3), 19.8 ppm (CCH3). 195Pt NMR
(64 MHz, CDCl3): d=ꢀ3350.4 ppm. MS (ESI, m/z): 723.4 (M+H)+.
Anal Calcd for C35H33N3O2Pt: C 58.16, H 4.60, N 5.81; found: C
58.34, H 4.57, N 5.42.
We would like to thank the ZIH Dresden for allocating compu-
tational time on their high performance computing facility. J.S.
would also like to thank the Studienstiftung des Deutschen
Volkes for funding.
Conflict of interest
The authors declare no conflict of interest.
Keywords: excited state
phosphorescence · platinum
·
mesoionic
·
OLEDs
·
[Bis-1,3-(2,3,5,6-tetramethylphenyl)-propane-1,3-dionato-kO,kO’][1-
phenyl-3-(phenyl-kC2)-triazol-4-ylidene-kC4]platinum(II) (4c): Pre-
pared analogously to 4a (see Supporting Information). 0.538 mg
(1.6 mmol, 2 equiv) bis-1,3-(2,3,5,6-tetramethylphenyl)propane-1,3-
dione were used instead of acetylacetone and 180 mg (1.6 mmol,
2 equiv) potassium tert-butoxide instead of potassium carbonate
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19.6 (CCH3), 16.5 (CCH3), 16.3 ppm (CCH3). MS (ESI, m/z): 751.4
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Manuscript received: December 4, 2017
Version of record online: && &&, 0000
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FULL PAPER
Control of excited state geometry by
rational ligand design leads to a new
class of phosphorescent emitters with
extraordinary photophysical properties.
Extension of the p-system in the triplet
state leading to a significant bathochro-
mic shift of the emission was avoided
by introduction of additional steric
demand.
&
Conjugated Systems
J. Soellner, T. Strassner*
&& – &&
Diaryl-1,2,3-Triazolylidene Platinum(II)
Complexes
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Chem. Eur. J. 2018, 24, 1 – 8
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ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!