Metallomesogens based on platinum(ii) complexes: Synthesis, luminescence and polarized emission

Article abstract of DOI:10.1039/c0dt01515f

Two series of heteroleptic cyclometalated platinum(ii) complexes [(C _nOppy)Pt(acac) and (C_nOFppy)Pt(acac)] have been prepared. Their liquid-crystal and optophysical properties were studied, in which C _nOppy is 2-(4-alkoxyp

Full text of DOI:10.1039/c0dt01515f

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PAPER
Metallomesogens based on platinum(II) complexes: synthesis, luminescence
and polarized emission†
Yafei Wang,a Yu Liu,*a Jian Luo,a Hongrui Qi,a Xiaoshuang Li,a Meijun Nin,a Ming Liu,a Danyan Shi,a
Weiguo Zhu*a and Yong Cao*b
Received 2nd November 2010, Accepted 2nd March 2011
DOI: 10.1039/c0dt01515f
Two series of heteroleptic cyclometalated platinum(II) complexes [(CnOppy)Pt(acac) and
(CnOFppy)Pt(acac)] have been prepared. Their liquid-crystal and optophysical properties were studied,
in which CnOppy is 2-(4-alkoxyphenyl)-5-(alkoxymethyl)pyridine and CnOFppy is
2-(3-fluoro-4-alkoxyphenyl)-5-(alkoxymethyl)pyridine. Only the heteroleptic cyclometalated
platinum(II) complexes (n = 12 and 16) exhibited enantiotropic mesophase transitions with smectic (Sm)
structure. Intense polarized luminescence with a maximum peak at 532 nm and a polarization ratio as
high as 10.5 were obtained in an aligned polyimide film under opto-excitation at room temperature.
This research work provides a simple approach to realize high-efficiency polarized emission by
heteroleptic cyclometalated platinum(II) complexes.
to emit high-efficiency phosphorescence due to strong spin–orbit
1. Introduction
coupling of platinum(II) ions at room temperature. Secondly,
With the maturation of the information display field, luminescent
platinum(II) complexes have a square-planar structure and can
liquid-crystals have been attractive for potential application in
facilitate metal–metal interactions. Furthermore, platinum(II)
optoelectronic devices because of their excellent luminescent
complexes have strong intermolecular interactions owing to
and carrier-transporting properties.1 In these luminescent liquid-
crystals, metallomesogens have a steadily growing interest recently
the p–p interactions of the ligand’s polyaromatic fragments.21
With this in mind, we tried to develop a new class of
owing to combining the optical, electronic and magnetic charac-
phosphorescent metallomesogens based on platinum(II) com-
teristics of transition metal complexes with anisotropic fluids.2–5
plexes with low transition temperature and highly polarized
To our best knowledge, most luminescent metallomesogens
emission.
contain lanthanide(III),6–14 palladium(II),15–18 platinum(II),2b–c,19–21
nickel(II),22 rhenium(I),6 gold(I),23 and gallium(III) ions.24 However,
these reported metallomesogens have high transition temperatures
(>100 ◦C) and low thermal stability, which is very difficult to
exhibit luminescence at high temperatures because of their strong
tendency of nonradiative deactivation.25 Moreover, few examples
of metallomesogens with polarized phosphorescence property
have been reported.
As 2-phenylpyridine is a class of good mesogen unit19,2b–c
and cyclometalated ligand,26 we designed two series of lumi-
nescent liquid-crystals about heteroleptic cyclometalated plat-
inum(II) complexes using the modified 2-phenylpyridine and 2-
(3-fluorophenyl)pyridine derivatives as ligands, which contain two
substituted alkoxy chains at the 5-position in the pyridine ring
and the 4-position in the phenyl ring, respectively. Their synthetic
route is shown in Scheme 1. Introduction of two flexible alkoxy
chains into the terminals of cyclometalated ligands was expected
to not only enhance the solubility and fluidity, but also facilitate
formation of the liquid-crystal mesophase for these platinum(II)
complexes. Here, the preparation, liquid-crystal behavior and
polarized luminescent property of these heteroleptic cyclometa-
lated platinum(II) complexes are described. While alkoxy chains
have enough carbon numbers (12 and 16), these two series of
heteroleptic cyclometalated platinum(II) complexes exhibited Sm
liquid-crystal phases and strong polarized luminescent behavior
with a polarization ratio as high as 10.5 in an aligned polyimide
film. This is the first example on the heteroleptic cyclometalated
platinum(II) complexes with highly polarized phosphorescence up
to date.
Among these luminescent metallomesogens, platinum(II) com-
plexes have some significant advantages as follows. Firstly, plat-
inum(II) complexes can reap both singlet and triplet excitons
aCollege of Chemistry, Key Lab of Environment-Friendly Chemistry and
Application in Ministry of Education, Xiangtan University, Xiangtan,
411105, China. E-mail: zhuwg18@126.com, liuyu03b@126.com; Fax: +86-
731-58292251; Tel: +86-731-58298280
bInstitute of Polymer Optoelectronic Material and Devices, South China
University of Technology, Guangzhou, 510640, China. E-mail: poycao@
scut.edu.cn; Tel: +86-20-87114635
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c0dt01515f
5046 | Dalton Trans., 2011, 40, 5046–5051
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Scheme 1 Synthesis route of heteroleptic cyclometalated platinum(II) complexes.
Table 1 Thermal behaviors of (CnOppy)Pt(acac) and (CnOFppy)Pt(acac)
2. Results and discussion
Compounds
Transitiona Tg [◦C]b Td [◦C] DH [kJ mol-1]
2.1. Synthesis
(C4Oppy)Pt(acac)
(C8Oppy)Pt(acac)
(C12Oppy)Pt(acac)
Cr–Iso
Cr–Iso
Cr–Cr¢
Cr¢–Sm
Sm–Iso
Cr– Cr¢
Cr¢–Sm
Sm–Iso
192
171
31
87
128
62
306
252
292
—
21.0
1.8
31.3
9.0
2.2
37.1
8.1
19.2
1.9
11.4
18.3
5.1
14.1
16.9
The
intermediates
of
2-bromo-5-(alkoxymethyl)pyridine
(CnOpyBr), 1-bromo-4-alkoxybenzene (CnOpBr), 1-bromo-2-
fluoro-4-alkoxybenzene (CnOFpBr), 2-(4-alkoxyphenyl)-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (CnOpB) and 2-(4-alkoxy-2-flu-
orophenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (CnOFpB)
were synthesized based on the literature procedures.27,28
Cyclometalated ligands of 2-(4-(alkoxy)phenyl)-5-(alkoxy-
methyl)pyridine (CnOppyH) and 2-(3-fluoro-4-alkoxy)-phenyl)-
(C16Oppy)Pt(acac)
355
99
126
172
21
50
159
60
(C8OFppy)Pt(acac) Cr–Iso
(C12OFppy)Pt(acac) Cr– Cr¢
286
284
Cr¢–Sm
5-(alkoxy-methyl)pyridine
(CnOFppyH)
were
prepared
Sm–Iso
(C16OFppy)Pt(acac) Cr– Cr¢
Cr¢–Sm
via Suzuki cross-coupling reaction.29 The heteroleptic
cyclometalated platinum(II) complexes of (CnOppy)Pt(acac)
and (CnOFppy)Pt(acac) were obtained by two-steps which
contain a cyclometalation of K2PtCl4 with these cyclometalated
ligands and a chloride cleavage of the resulting chloro-bridged
dimmers with an ancillary ligand of pentane-2,4-dione (Hacac).30
All heteroleptic cyclometalated platinum(II) complexes have good
282
75
145
Sm–Iso
a Cr, Cr¢ = crystal; Sm = smectic phase; Iso = isotropic liquid. b Data from
the second DSC cycle starting from the crystal.
◦
atmosphere at a scanning rate of 10 C min-1, and their thermal
1
solubility in common solution and they were confirmed by H
NMR, 13C NMR spectroscopy and elemental analysis. For the
(CnOFppy)Pt(acac) complexes, there are two kinds of isomer
configurations and H NMR spectroscopic data are consistent
with their formulation as shown in Scheme 1. The protons in the
benzene ring appear as a singlet at 8.85–8.81 ppm.
data are summarized in Table 1. Obvi◦ously, the decomposition
temperature (Td) values were over 252 C for these two series of
platinum(II) complexes, which correspond to a 5% weight loss in
TGA. These results suggest the platinum(II) complexes have high
thermostability.
The mesomorphic phase behaviors of the (CnOppy)Pt(acac)
and (CnOFppy)Pt(acac) complexes were characterized accurately
by both differential scanning calorimetry (DSC) and polarized
optical microscopy (POM). The phase transition temperatures are
also summarized in Table 1. Some of these platinum(II) complexes
exhibited enantiotropic liquid-crystalline behavior (n = 12, 16).
1
2.2. Thermal property and liquid-crystallinity behavior
The thermal stability of these platinum(II) complexes were ex-
amined via thermo-gravimetric analyses (TGA) under nitrogen
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In order to clarify transitions type, the DSC thermogram of
(C12Oppy)Pt(acac) was further measured as a typical example
and shown in Fig. 1. Five succes◦sive heating–cooling scans (at
different rates: 40, 20, 10, 5, 2.5 C min-1) were recorded. The
transition temperatures measured upon cooling and heating were
rather close to each other, which implies that (C12Oppy)Pt(acac)
exhibited a feature of the mesophase transition. As can be seen
from Table 1, the transition enthalpies from crystal to mesophase
increase with increasing alkoxy chain length. This is attributed
to the fact that the increasing alkoxy chain length makes its
complexes have a more symmetric shape and hence more favorable
for packing in their crystalline phase. In contrast, the transition
enthalpies from mesophase to isotropic transition decrease with
increasing alkoxy chain length, which results from the extra steric
crowding of the alkoxy chain. This phenomenon is according
with the suggestion reported by Kumar.31 Compared to the
(CnOppy)Pt(acac) complexes, the (CnOFppy)Pt(acac) complexes
displayed a broader mesophase range because of a lower melting
transition temperature and higher clearing point. Such a behavior
is suggested to be relative to the F ◊ ◊ ◊ F interactions of these
complexes based on Espinet and Dembinski’s reports.32,33
Fig. 1 DSC thermogram of (C12Oppy)Pt(acac) at different heating rates
(40 ◦C min-1, 20 ◦C min-1, 10 ◦C min-1, 5 ◦C min-1 and 2.5 ◦C min-1).
These two series of platinum(II) complexes (n = 12, 16) exhibited
a smectic phase clearly identified by a similar birefringence
texture under POM in both cooling and heating processes.
Their POM textures upon the cooling are shown in Fig. 2. We
take (C12Oppy)Pt(acac) as an example to describe the result of
these POM experiments. No fluidity and no typical texture were
◦
observed until the sample was heated to a temperature of 87 C
during the first heating. The cone-shaped texture indicates that
this liquid-crystal phase is a typical smectic mesophase.
To further confirm the smectic structures of these cyclometa-
lated platinum(II) complexes, we examined the (C12Oppy)Pt(acac)
complex by X-ray diffraction. Its structurally sensitive 1D WAXD
patterns at 60 ◦C, 80 ◦C, 120 ◦C and 160 ◦C are shown in
Fig. 3. The (C12Oppy)Pt(acac) complex exhibits a crystalline phase
at 80 ◦C as revealed by XRD, which is evident from a large
number of disorderly diffraction peaks in the X-ray pattern. When
the temperature rises to 120 ◦C, orderly diffractions from this
platinum(II) complex are displayed. In the low-angle region, the
XRD pattern shows five diffraction peaks at 28.9, 14.4, 9.80,
Fig. 2 Optical texture made by POM on a cooling process for (a)
(C12Oppy)Pt(acac) at 112 ◦C, (b) (C12OFppy)Pt(acac) at 136 ◦C, (c)
(C16Oppy)Pt(acac) at 108 ◦C, (d) (C16OFppy)Pt(acac) at 132 ◦C.
(d50) reflections, respectively. Their d-spacings are in the ratio of
1 : 2 : 3 : 4 : 5. This indicates that the (C12Oppy)Pt(acac) complex
˚
7.40 and 5.60 A, which correspond to (d10), (d20), (d30), (d40) and
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Fig. 3 X-ray diffraction pattern of (C12Oppy)Pt(acac) at 60 ◦C, 80 ◦C,
120 ◦C and 160 ◦C, respectively.
Fig. 4 Normalized UV-vis absorption and emission spectra of the
(CnOppy)Pt(acac) and (CnOFppy)Pt(acac) complexes in dichloromethane
at room temperature.
forms a lamellar structure. In the high-angle region, the XRD
pattern reveals three diffuse scattering halos at 4.5, 4.0 and 3.5
˚
A, which correspond to the liquid-like order of the alkoxy chains,
the Pt ◊ ◊ ◊ Pt interaction and the loose lateral interactions between
rigid parts of the complex, respectively.2b,34 In addition, the XRD
pattern displays a sharp peak in the low angle region at 3.08 (2q),
tic platinum(II) complexes in DCM, in which the high-energy
*
absorption band is assigned to the spin-allowed 1p–p transitions
from the ligand centers of CnOppy and CnOFppy, the low-energy
absorption band corresponds to the mixing electronic excitations
resulting from the spin-allowed singlet metal-to-ligand charge
transfer (1MLCT), spin-forbidden triplet metal-to-ligand charge
˚
which corresponds to the d-spacing of 28.9 A. Obviously, the layer
˚
thickness d (28.9 A) is much smaller than the calculated length of
˚
the molecule (approximately 43 A Figure S1†). It implies that
the molten aliphatic chains interdigitate between intermolecular
dipole–dipole interactions in the smectic phase (Figure S1†). It is
in good agreement with a lamellar structure with interdigitation
of the molten aliphatic chains.35 Furthermore, according to these
diffuse scattering halos in the high-angle region, it confirms that
(C12Oppy)Pt(acac) forms a Sm phase, which is in good agreement
with the reported literature.36–38
*
transfer (3MLCT) and ligand-centered 3p–p transition states. On
the other hand, a PL peak at 510 nm with a shoulder at 529 nm for
the (CnOppy)Pt(acac) complexes and at 511 nm with a shoulder
at 544 nm for the (CnOFppy)Pt(acac) complexes are observed in
the PL spectra as shown in Fig. 4, respectively. Compared to the
(CnOppy)Pt(acac) complexes, the (CnOFppy)Pt(acac) complexes
present slightly red-shifted absorption and emission profiles due
to the effect of fluorine atom.39 The PL quantum yields (Uem
)
2.3. Optophysical properties
of these heteroleptic platinum(II) complexes were measured using
Ru(bpy)3(PF6)2 as a standard in DCM (1 ¥ 10-5 M) (Table 2).40,41
The Uem of (CnOppy)Pt(acac) and (CnOFppy)Pt(acac) are in the
range of 0.126 0.016, which implies that the fluorine atom has a
minor effect on PL quantum yields of its platinum(II) complex.
To further study the luminescence in different phase states and
polarized emission, we also took (C12Oppy)Pt(acac) as an example
and measured its PL spectra at 300 K and 373 K in its neat film
as shown in Fig. 5. Clearly, different PL profiles are observed at
300 K and 373 K. Only a maximum emission peak of 512 nm at
300 K, but a red-shifted maximum emission peak of 582 nm with
two shoulders of 500 nm and 534 nm at 373 K are presented in the
PL profiles. According to the XRD pattern of (C12Oppy)Pt(acac)
at 300 K as shown in Figure S2,† the (C12Oppy)Pt(acac) complex
should form a disorder amorphous structure and is very difficult
to form excimers as the luminescent groups are surrounded by
the side chains. As a consequence, the emission is attributed to
its monomer rather than excimer. While the temperature rises
to 373 K, the (C12Oppy)Pt(acac) complex is available to form
an ordered lamellar structure at liquid-crystal phase, which can
facilitate the excimer formation. In this situation, its emission is
resulted from both of monomer and excimer. Therefore, different
PL spectra between its amorphous state and liquid-crystal state
are exhibited.2b,42
Fig. 4 shows the UV-vis absorption and photoluminescence spec-
tra of the (CnOppy)Pt(acac) and (CnOFppy)Pt(acac) complexes in
dichloromethane (DCM) at room temperature, and their data are
summarized in Table 2. Similar UV-vis absorption and photolumi-
nescence spectra are exhibited for these heteroleptic platinum(II)
complexes due to their analogous molecular configuration. An
intense absorption band from 267 to 313 nm and a moderate ab-
sorption band from 355 to 392 nm are observed for these heterolep-
Table 2 Photophysical
properties
of
(CnOppy)Pt(acac)
and
(CnOFppy)Pt(acac) in dichloromethane at room temperature
a
Compounds
Absorption (nm)
Emission (nm) Uem
(C4Oppy)Pt(acac)
(C8Oppy)Pt(acac)
(C12Oppy)Pt(acac)
(C16Oppy)Pt(acac)
(C8OFppy)Pt(acac)
(C12OFppy)Pt(acac)
(C16OFppy)Pt(acac)
267, 293, 308, 356, 391 500, 529
268, 294, 312, 355, 391 500, 529
268, 293, 312, 357, 390 500, 529
268, 294, 311, 356, 389 500, 529
267, 288, 312, 356, 391 511, 544
268, 289, 313, 357, 392 511, 544
267, 289, 313, 355, 391 511, 544
0.142
0.110
0.147
0.118
0.113
0.132
0.125
a Luminescence quantum yields (Uem) of the unknown samples are
calculated according to the equation Uunk = Ustd(Iunk/Aunk) (Istd/Astd
)
(hunk/hstd)2 using Ru(bpy)3(PF6)2 as a standard sample.
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platinum(II) complex. The emission intensity at parallel direction
is 2.7 times to that at the perpendicular direction, i.e. polarization
ratio (R) equals 2.7. In contrast, while this aligned film is subjected
to mechanical rub, its polarized emission significantly decreases
and even disappears. This indicates that polarized emission is
related to array order of molecules.
To further improve the polarized luminescence properties, we
made another aligned film of (C12Oppy)Pt(acac) on the surface
of an aligned polyimide thin film by heating into the liquid-
crystal temperature region and annealing. This aligned film with
polyimide exhibited a substantially different PL spectrum from
the aligned film without polyimide. Highly polarized phospho-
rescence with a maximum peak at 532 nm is observed for this
aligned film with polyimide under photo-irradiation as shown in
Fig. 7. The emission intensity at the parallel direction is about
10.5 times to that at the perpendicular direction (R = 10.5). Our
results demonstrate that highly polarized PL can be achieved by
luminescent liquid-crystal platinum(II) complexes in their aligned
films.
Fig. 5 PL spectra of (C12Oppy)Pt(acac) in DCM, amorphous film at
273 K and in liquid-crystal state at 373 K, respectively.
2.4. Polarized luminescence properties
Intrinsic polarized luminescence plays an important role in
liquid-crystal displays without polarizers and color filters.43
Therefore, various methodologies to induce linearly polarized
luminescence of emitters have been reported, such as mechan-
ical stretching of polymer films,44,45 Langmuir–Blodgett (LB)
technique,46 friction-transfer technique,47–51 and nematic liquid-
crystal self-organization.52–55 Here, a simple approach is pre-
sented to obtain polarized luminescence by (CnOppy)Pt(acac) and
(CnOFppy)Pt(acac). Firstly, we make this platinum(II) complex
form an aligned liquid-crystalline thin film at elevated temperature
within a thermotropic mesophase and subsequently quench it into
room temperature to keep liquid-crystalline state.56 Secondly, it
gives polarized emission at room temperature under polarized
photo-irradiation owing to the intrinsic anisotropy of its elec-
tronic structure. Fig. 6 shows the polarized PL spectra of the
(C12Oppy)Pt(acac) complex by this aligned method. Similar PL
profiles with a wide band from 500 to 650 nm are observed under
excitation of parallel and perpendicular light. Intense polarized
luminescence is presented in this heteroleptic cyclometalated
Fig. 7 Polarized PL spectra from (C12Oppy)Pt(acac) on a aligned
polyimide film at room temperature (H: parallel emission light, V:
perpendicular emission light).
3. Conclusions
In summary, two series of the heteroleptic cyclometalated plat-
inum(II) complexes of (CnOppy)Pt(acac) and (CnOFppy)Pt(acac)
were successfully obtained. Some of the heteroleptic cyclomet-
alated platinum(II) complexes (n = 12, 16) exhibited a lamellar
smectic-type mesophase and different emission spectra in different
phase states. Highly polarized luminescence peaked at 532 nm with
a polarization ratio of 10.5 was achieved in the aligned platinum(II)
complex film with aligned polyimide. These luminescent liquid-
crystal materials present a promising application in the display
field with polarized luminescence.
Acknowledgements
Fig. 6 Polarized PL spectra of (C12Oppy)Pt(acac) in the aligned film at
room temperature (H: parallel emission light, V: perpendicular emission
light; Hr: parallel emission light in the aligned film after rubbing, Vr:
perpendicular emission light in the aligned film after rubbing).
Financial support from the National Natural Science Founda-
tion of China (50973093, 20772101 and 20872124), the Science
Foundation of Hunan Province (2009FJ2002), the Specialized
5050 | Dalton Trans., 2011, 40, 5046–5051
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Research Fund for the Doctoral Program of Higher Education
(2009 4301110004), the New Teachers Fund for the Doctoral
Program of Higher Education of China (200805301013) and
the Postgraduate Science Foundation for Innovation in Hunan
Province (CX2009B124 and S2008 yjscx09).
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