The Chameleonic Nature of Platinum(II) Imidazopyridine Complexes

Article abstract of DOI:10.1002/chem.201703908

The synthesis and characterization of cyclometalated C^C* platinum(II) complexes with unique photophysical properties, aggregation induced enhancement of the quantum yields with a simultaneous decrease of phosphorescence lifetimes, is reported. Additional

Full text of DOI:10.1002/chem.201703908

DOI: 10.1002/chem.201703908
Communication
&
Phosphorescence
The Chameleonic Nature of Platinum(II) Imidazopyridine
Complexes
Piermaria Pinter, Rebecca Pittkowski, Johannes Soellner, and Thomas Strassner*[a]
attracted interest as white OLED (WOLED) dopants.[9] Unfortu-
Abstract: The synthesis and characterization of cyclometa-
lated C^C* platinum(II) complexes with unique photo-
physical properties, aggregation induced enhancement of
the quantum yields with a simultaneous decrease of phos-
phorescence lifetimes, is reported. Additionally, a change
of emission color is induced by variation of the excitation
wavelength. The aggregation behavior of these complexes
is controlled by the steric demand of the substituents. The
photophysical properties of these complexes are investi-
gated through emission-excitation matrix analysis (EEM).
The monomeric complexes are excellent room tempera-
ture phosphorescent blue emitters with emission maxima
below 470 nm and quantum yields of up to 93%.
nately, the aggregation process is often associated with a set-
back of the photophysical properties, in particular, lower quan-
tum yields. Examples for an enhanced quantum yield by aggre-
gation are rare.[10] Here, we present an aggregation induced
enhancement of quantum yields with simultaneous shortening
of the phosphorescence decay time, modulation of the emis-
sive color through selective excitation, and control of the ag-
gregation process through rational molecular design.
The imidazolium salts 4 (R1 =methyl) and 8 (R1 =mesityl)
were prepared following a four-step procedure (Scheme 1).
N-R1-3-nitropyridin-2-amines (1,5) were obtained by reaction of
2-chloro-3-nitropyridine and aqueous methylamine in isopro-
panol (i) and by reaction of 2-chloro-3-nitropyridine with
2,4,6-trimethylphenylamine (Mes-NH2) in the presence of po-
The energy problem has prompted extensive research into
more efficient sources of illumination. In particular, organic
light emitting diodes (OLEDs) have been found to be a promis-
ing alternative to conventional lighting. The most widely used
phosphorescent dopants today are iridium organometallic
compounds, although platinum compounds have been proven
to be an alternative.[1] Additionally, the square planar geometry
of platinum(II) compounds has attracted a large interest in the
field of inorganic photochemistry as it allows for intermolecular
interactions that are not possible in octahedral and tetrahedral
complexes.[2] The tendency of platinum(II) to aggregate has
been known for a long time but was only recently explained.[3]
The photophysical properties of platinum complexes strongly
vary upon intermolecular interactions. Interestingly, the proper-
ties of an aggregated system largely differ from those of the
monomers.[4] The unique photophysical properties of platinum
aggregates have been employed for molecular sensing appli-
cations: efficient vapochromism[5] and solvatochromism ef-
fects[6] have been observed. The intermolecular interactions of
platinum(II) emitters are based on two possible scenarios: by
self-assembly or accurate molecular design of rigid scaffolds.
While the design of rigid scaffolds has intensively been investi-
gated by Thompson and co-workers[7] and other groups,[8] the
self-assembly has simply been classified as a phenomenologi-
cal effect. Recently, self-assembled platinum(II) complexes have
Scheme 1. Preparation of the imidazolium salts 4 and 8: (i) iPrOH, CH3NH2,
(ii) Mes-NH2, KF; (iii) Pd/C (10% Pd), MeOH, H2; (iv) TEOF, acid; (v) Ph2IOTf,
DMF, 10 mol% Cu(OAc)2.
tassium fluoride at elevated temperatures (ii), followed by re-
duction of the nitro groups over Pd/C (iii). Ring closure of com-
pounds 2 and 6 was achieved by reaction with triethylortho-
formate (TEOF, iv). The imidazolium salts 4 and 8 were ob-
tained through a copper catalyzed reaction of 3 and 7 with di-
phenyliodonium triflate (Ph2IOTf, v), respectively.
The platinum(II) complexes (9–14) were prepared in a one-
pot multistep reaction, following our recently reported proce-
dure (Scheme 2):[11] synthesis of the silver(I) N-heterocyclic car-
bene (NHC) complex in DMF through deprotonation of the
imidazolium salts and formation of the carbene complex with
silver(I) oxide (vi), transmetalation of the silver(I) carbene to di-
chloro(1,5-cyclooctadiene)platinum(II) [Pt(COD)Cl2] (vii), cyclo-
metalation at elevated temperature followed by reaction with
acetylacetone ligand (HR2acac) in the presence of potassium
tert-butanolate as a base (viii). The complexes were isolated
after column chromatography in yields of 11–62%.
[a] P. Pinter, R. Pittkowski, J. Soellner, Prof. Dr. T. Strassner
Fachrichtung Chemie und Lebensmittelchemie, TU Dresden
Bergstrasse 66, 01069 Dresden (Germany)
All complexes were fully characterized by 1H, 13C, and
195Pt NMR spectroscopy, as well as elemental analysis and, in
the case of 14, by a solid-state structure. The formation of the
carbene complex was verified by the disappearance of the
E-mail: thomas.strassner@chemie.tu-dresden.de
Supporting information for this article can be found under:
https://doi.org/10.1002/chem.201703908.
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the blue region of the spectrum with excellent quantum yields
and relatively short phosphorescence decay times (Table 1).
Complexes 10–14 show
a structured emission profile
(Figure 2), characteristic for a metal perturbed 3ILCT emissive
state.[11] In the emission spectrum of complex 9 two bands are
observed: a structured band centered at 460 nm, assigned to a
Scheme 2. Synthetic route for the preparation of the platinum(II) complexes:
(vi) DMF, Ag2O; (vii) [Pt(COD)Cl2]; (viii) HR2acac, KOtBu.
3
metal perturbed ILCT emissive state, and a broad structureless
band centered at 543 nm.
characteristic NCHN proton signal of the imidazolium salt in
1
Table 1. Photophysical data for complexes 9–14 measured after excita-
tion at 290 nm, at 298 K, film 2.0 wt.% emitter in PMMA (lexc. =290 nm)
the H NMR experiment and the appearance of the characteris-
tic ipso carbon signal of the carbene at 163–164 ppm in the
13C NMR experiment. The 195Pt NMR spectra for all complexes
show chemical shifts from À3350 (9) to À3325 ppm (11),
which is typical for cyclometalated C^C* platinum(II) com-
plexes. Thermal stability of all complexes was analyzed by de-
termining their melting point, which are in the range of 2348C
(11) to more than 3508C (12). No decomposition was detected
during the measurements.
[a]
[e]
Comp.
lem
F[b]
t[c]
kr[d]
[103 sÀ1
knr
[nm]
[%]
[ms]
]
[103 sÀ1
]
9
462
464
467
464
467
467
76
77
90
87
65
93
3.6
6.0
3.5
3.6
6.6
3.8
280.2
165.3
287.4
275.6
150.8
262.7
67.3
38.0
28.7
35.8
52.8
18.4
10
11
12
13
14
Single crystals of 14, suitable for X-ray diffraction, were ob-
tained by slow evaporation of a solution of the compound in
acetone. The complex crystallizes in the triclinic space group
[a] maximum emission wavelength, [b] absolute quantum yield Æ5%,
[c] decay lifetimes (excited by laser pulses 360 nm, 20 kHz) given as t=
[d]
texp/F, kr =(F/texp), [e] knr =(1ÀF)/texp. CIE coordinates are given in the
¯
P1 with the unit cell containing two molecules. The two biden-
Supporting Information.
tate ligands form a slightly distorted square-planar coordina-
tion sphere around the metal center (Figure 1a).
The photophysical properties of complex 9 are found to be
concentration dependent (Table 2). The formation of the lower
energetic band is concomitant with the increase of the con-
centration (Figure 3). In accordance with the concentration de-
Figure 1. Solid state structure representations of 14. Ellipsoids are drawn at
50% probability. a) ORTEP3 representation of the molecular structure. Select-
ed bond lengths [ꢁ] and angles [8]: Pt(1)ÀC(1) 1.924(2), Pt(1)ÀC(8) 1.983(2),
Pt(1)ÀO(1) 2.0884(16), Pt(1)ÀO(2) 2.0471(15), C(1)-Pt(1)-C(8) 80.42(9), O(1)-
Pt(1)-O(2) 90.62(6). b) Mercury plot of the packing in the solid state. H atoms
omitted for clarity.
The carbene–metal bond (1.924(2) ꢁ) is noticeably shorter
than the Pt(1)ÀC(8) bond of 1.983(2) ꢁ. Due to the formation
of a five-membered metallacycle, the bite angle of the cyclo-
metalating ligand is contracted to 80.42(9)8, while the O(1)-
Pt(1)-O(2) angle is close to the ideal value of 908. The mole-
cules form slightly offset stacks in the single crystal, where the
planar N-heterocyclic fragments face one another and the ster-
ically highly demanding b-diketonates point outwards, orthog-
onal to the coordination plane (Figure 1b). The UV/Vis absorp-
tion spectra of complexes 9–14 were measured at 298 K in
0.1 mm CH2Cl2 solution (Figure S4 in Supporting Information).
All complexes show a strong absorption at 280 nm and a
second band centered at 330 nm. The very weak low energy
3MLCT absorption is observed at 430 nm (Figure S5). All com-
plexes show strong phosphorescence at room temperature in
Figure 2. Normalized emission spectra of complexes 9–14 at 298 K (2.0 wt.%
in PMMA); excitation at 290 nm.
pendency, associated with a red shift of the emission and a
structureless band shape, we suggest that the emission cen-
tered at 550 nm could be assigned to an excimeric emissive
species. This assignment is confirmed by concentration depen-
dent measurements at 77 K in a glassy 2Me-THF matrix (Fig-
ure S7). By lowering the temperature to 77 K, the emission cen-
tered in the blue region of the spectrum becomes vibronically
resolved (as expected for a metal perturbed 3LC emissive
state). However, the emission at lower energies maintains an
unstructured profile.
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(EEM) was performed. The previously investigated concentra-
tions (0.1 to 10.0 wt.%) of complex 9, were excited at wave-
lengths ranging from 250 to 400 nm in increments of 5 nm
while the resulting emission spectra were recorded. The con-
centration dependent EEM spectrum of complex 9 is shown in
Figure 4. As previously discussed, the beginning of the
self-aggregation is now clearly observed at 0.5 wt.%. At a con-
centration of 2.0 wt.%, nearly equal emission intensities of the
monomer and excimer are detected. The EEM analysis compre-
hensibly shows the unique aggregation dependent photophys-
Table 2. Photophysical data for complex 9 measured at different concen-
trations (0.1–10.0 wt.%), at 298 K (lexc. =290 nm).
[a]
[e]
Conc.
lem
F[b]
t[c]
kr[d]
[103 sÀ1
knr
[nm]
[%]
[ms]
]
[103 sÀ1
]
0.1%
0.2%
0.5%
1.0%
2.0%
10.0%
464
462
461
464
462
566
67
61
63
71
76
78
6.2
6.9
6.3
5.7
3.6
1.6
160.8
144.4
158.1
176.9
280.2
630.9
53.1
56.3
58.5
51.3
67.3
138.8
[a] maximum emission wavelength, [b] absolute quantum yield Æ5%
[c] decay lifetimes (excited by laser pulses 360 nm, 20 kHz) given as t=
texp/F, [d] kr =(F/texp), [e] knr =(1ÀF)/texp
.
Figure 3. Normalized emission spectra of complex 9 at 298 K at different
concentrations in PMMA; excitation at 290 nm.
Therefore, we studied the self-aggregation of complex 9.
The photophysical properties as a function of concentration
(from 0.1 to 10.0 wt.% in PMMA film) are shown in detail in
Figure 3. In the range from 0.1 to 0.2 wt.%, only the emission
from the monomer is observed. The formation of the excimer
starts at 0.5 wt.%, and at 10.0 wt.%, only emission from the ex-
cimer is detectable. The aggregation process has a profound
effect on the photophysical properties. We observe a shorter
decay time of the excited state and, remarkably, an increase of
the phosphorescent quantum yield (see Table 2). The aggrega-
tion enhanced photophysical process seems to be characteris-
tic of the imidazopyridine system.[10c] Strikingly, this phenome-
non is not observed in the case of the analogous benzimida-
zole platinum(II) complex[12] (Figure S13 and Table S3).
Figure 4. EEM analysis of complex 9; an enlarged version is given in the
Supporting Information.
ical behavior of complex 9. In addition, the two emissive spe-
cies possess different excitation spectra. It is thus possible to
selectively excite one or the other of the two emissive species.
This aspect is clearly shown in a concentration-dependent EEM
experiment at 77 K (Figure S11). We further observe modu-
lation of the dominant emissive color, from blue to orange,
whilst only changing the excitation wavelength from 320 to
390 nm as shown in Figure 5 (measurements at 77 K are re-
ported in Figure S7b).
We also observed a change in the aggregation behavior: the
imidazopyridine complex 9 begins to aggregate at lower con-
centrations compared to the analogous benzimidazole com-
plex. A possible explanation could be the lower electron densi-
ty in the pyridine ring due to the electron withdrawing nitro-
gen atom. In the case of an electrostatically induced aggrega-
tion process, the reduced electron density on the system
favors the p–p stacking process.[13] As a result, the formation
of aggregates is preferred in the case of the imidazopyridine
with respect to the analogous benzimidazole complex.
The tendency to form aggregates can be prevented by ra-
tional molecular design. The introduction of bulkier
substituents in the acetylacetonate fragment (complexes 10–
12) or in the NHC moiety (complexes 13–14) efficiently pre-
vents the aggregation process. To confirm our theory, we de-
signed complex 13 where the methyl group in the NHC frag-
The two emissive species of complex 9, namely the mono-
mer and the aggregate, possess different photophysical prop-
erties. To understand the photophysical self-aggregation be-
havior of complex 9, an emission-excitation matrix analysis
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Communication
Conflict of interest
The authors declare no conflict of interest.
Keywords: cyclometalated C^C*
photophysics · platinum
·
NHC
·
OLED
·
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Figure 5. Normalized emission spectra of complex 9 at 298 K (2.0 wt.% in
PMMA); excitation at different wavelengths.
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hindrance of the mesitylene moiety does not allow a planar
structure (see DFT calculations in the Supporting Information).
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complexes and their unique excimeric enhanced photophysical
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excellent candidates for blue emitting phosphorescent OLED
(PhOLEDs) with very high quantum yields (up to 93%) and
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gate leads to a red shift of approx. 100 nm, an enhanced quan-
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sion-excitation matrix analysis (EEM). Herein we present the
possibility to tune the emissive wavelength upon variation of
the concentration and, even more remarkably, the possibility
to obtain different emission colors from a single complex at a
constant concentration only by selective excitation of the
emissive species.
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Experimental Section
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Experimental Details are reported in the supporting information.
CCDC 1560929 contains the supplementary crystallographic data
for this paper. These data are provided free of charge by The Cam-
bridge Crystallographic Data Centre.
Acknowledgements
Manuscript received: August 29, 2017
Accepted manuscript online: August 30, 2017
Version of record online: && &&, 0000
We are grateful to the ZIH at the TU Dresden for computation
time at their high-performance computing facility.
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Communication
COMMUNICATION
Imidazopyridine C^C* platinum(II) com-
plexes show unique photophysical
properties: aggregation induced en-
hancement of the quantum yields with
simultaneous decrease of phosphores-
cence lifetimes. Additionally, a change
of emission color is induced by variation
of the excitation wavelength. The ag-
gregation behavior is controlled by the
steric demand of the substituents. The
complexes are excellent room tempera-
ture phosphorescent blue emitters with
emission maxima below 470 nm and
quantum yields of up to 93%.
&
Phosphorescence
P. Pinter, R. Pittkowski, J. Soellner,
T. Strassner*
&& – &&
The Chameleonic Nature of
Platinum(II) Imidazopyridine
Complexes
Chem. Eur. J. 2017, 23, 1 – 5
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These are not the final page numbers! ÞÞ