Synthesis, characterization, and polarized luminescence properties of platinum(ii) complexes having a rod-like ligand

Article abstract of DOI:10.1039/c2dt30071k

We have synthesized a series of platinum (Pt) complexes having a rod-like ligand: 'Pt(F₂PPy)acac', 'Pt(12F₂PPy)acac', and 'Pt(12F₂PPyO4)acac'. The crystal form of Pt(12F₂PPy)acac was successfully determined to be triclinic by single-crystal X-ray structural analysis. The molecules were parallelly aligned in the unit cell. Monomer and excimer emissions of Pt(12F₂PPy)acac were observed in hexane solution, poly(methyl methacrylate) film, and various nematic LCs. Homogeneous LC cells with the Pt complex/LC mixtures exhibited polarized optical emission resulting from monomer and excimer states. The PL intensity perpendicular to the orientation direction was higher than the parallel one in the whole wavelength region of the Pt complex and the polarization ratio of the excimer was higher than that of the monomer. The polarization ratios of the excimers were estimated to be 1.4-2.5 in nematic LC at room temperature, and decreased gradually with increasing temperature. The polarization ratios of Pt(12F₂PPyO4)acac were higher than those of Pt(F₂PPy)acac and Pt(12F ₂PPy)acac in all the LCs. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt30071k

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PAPER
Synthesis, characterization, and polarized luminescence properties of
platinum(II) complexes having a rod-like ligand†
Takeshi Sato,* Hiroshi Awano, Osamu Haba, Hiroshi Katagiri, Yong-Jin Pu, Tatsuhiro Takahashi and
Koichiro Yonetake
Received 11th January 2012, Accepted 2nd May 2012
DOI: 10.1039/c2dt30071k
We have synthesized a series of platinum (Pt) complexes having a rod-like ligand: ‘Pt(F2PPy)acac’,
‘Pt(12F2PPy)acac’, and ‘Pt(12F2PPyO4)acac’. The crystal form of Pt(12F2PPy)acac was successfully
determined to be triclinic by single-crystal X-ray structural analysis. The molecules were parallelly
aligned in the unit cell. Monomer and excimer emissions of Pt(12F2PPy)acac were observed in hexane
solution, poly(methyl methacrylate) film, and various nematic LCs. Homogeneous LC cells with the Pt
complex/LC mixtures exhibited polarized optical emission resulting from monomer and excimer states.
The PL intensity perpendicular to the orientation direction was higher than the parallel one in the whole
wavelength region of the Pt complex and the polarization ratio of the excimer was higher than that of the
monomer. The polarization ratios of the excimers were estimated to be 1.4–2.5 in nematic LC at room
temperature, and decreased gradually with increasing temperature. The polarization ratios of
Pt(12F2PPyO4)acac were higher than those of Pt(F2PPy)acac and Pt(12F2PPy)acac in all the LCs.
they probably improve the light extraction efficiency of an
Introduction
OLED device due to an effect of their horizontal molecular
orientation.18 Moreover, if control of polarizing axis in each
Organometallic complexes, such as iridium, platinum (Pt), and
pixel is possible, it will be applicable to 3D displays.19,20
osmium complexes, have attracted great interest from the points
of both basic and applied research. For example, chemical
sensors and photocatalysts are well-known applications.1–4 Fur-
thermore, high internal quantum efficiency of the organometallic
complexes is much effective in organic light emitting diodes
(OLEDs), as the strong spin-orbital coupling allows efficient
phosphorescence at room temperature (RT).5–7 The emission
wavelength and luminescent efficiency depend on its ligands,
because the ligand molecular orbitals have an influence on the
metallic atom d-orbitals.8–10 Thus, it is important to examine the
crystal structures and photophysical properties of the organo-
metallic complexes. Particularly, square planar Pt(II) complexes
are well-known examples of revealing unique luminescence
affected by the structure or dispersion state, e.g., excimer or
metal–metal-to-ligand charge transfer (MMLCT) emission.11–13
Whereas many studies of highly efficient emitting materials
have been carried out to improve the performance of OLED
devices, the polarized optical emission properties of organic
materials have been also investigated.14–17 The polarized optical
emitting devices can reduce the light loss of a liquid crystal
display (LCD) backlight, by cutting out a polarizing plate, and
The linearly polarized emitting device requires uniaxial mol-
ecular orientation. This can be performed using liquid crystalline
materials. For example, an oriented liquid crystalline polyfluorene
derivative, a well-known blue-emitting conjugated polymer,
generated high polarized optical emission.21 In a luminescent
material and liquid crystal mixture, the temperature dependence
of the polarized emitting properties has been investigated.22 The
order parameter evaluated by polarized absorption spectroscopy
decreased steeply when the temperature approached the smectic/
isotropic phase transition temperatures.
The above devices used fluorescent materials. On the other
hand, polarized emission of lanthanide(III) complexes has been
obtained with the aid of the stretched polymer film technique,23
and lanthanide(III) complexes doped into a nematic LC also
showed polarized luminescence.24,25 Moreover, circularly polar-
ized light was measured in single crystals of lanthanide(III) com-
plexes.26 Recently, a discotic-like Pt complex exhibited polarized
phosphorescent emission perpendicular to the axis of the colum-
nar stack,27 and metallomesogens based on Pt complexes exhi-
biting smectic LC generated comparatively high polarized
emission using rubbed polymer layers.28,29 However, few studies
into the relationship between molecular orientation and polarized
emission of the organometallic complexes have been carried out.
In this study, we have synthesized a series of platinum (Pt)
complexes: platinum(II) (2-(5′,6′-difluorophenyl)pyridinato-N,
C2′) (2,4-pentanedionato-O,O) (Pt(F2PPy)acac), platinum(II)
Graduate School of Science and Engineering, Yamagata University,
4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan.
E-mail: twn34755@st.yamagata-u.ac.jp; Tel: +81-238-26-3047
†Electronic supplementary information (ESI) available. CCDC 854595.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2dt30071k
(2-(4′-dodecyl-5′,6′-difluorophenyl)pyridinato-N,C2′)
(2,4-
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pentanedionato-O,O) (Pt(12F2PPy)acac), platinum(II) (2-(4′-
dodecyl-5′,6′-difluorophenyl)-5-butoxypyridinato-N,C2′) (2,4-
of Pt(F2PPy)acac, Pt(12F2PPy)acac, and Pt(12F2PPyO4)acac
were estimated to be 267, 289, and 289 °C, respectively, by ther-
mogravimetric analysis (TGA).
pentanedionato-O,O) (Pt(12F2PPyO4)acac), which have
a
β-diketonato ligand (O^O) and monoanionic cyclometalating
ligand (C^N). We have analyzed the crystal structure, thermal be-
havior, and photophysical properties of Pt(12F2PPy)acac mainly.
Furthermore, we have investigated the unique polarized optical
emission properties of the three Pt complexes in several nematic
LCs, and discussed the effect of the molecular structure of the
ligand on the polarization ratio.
Crystal structure and phase transition properties of
Pt(12F2PPy)acac
A single crystal of Pt(12F2PPy)acac was grown from a dilute
hexane solution in a refrigerator and characterized using X-ray
crystallography. Fig. 1 shows the molecular structure of
Pt(12F2PPy)acac in the single crystal. The ORTEP drawing is
shown in the supporting information (Fig. S1†). Pt(12F2PPy)-
acac has a planar geometry including a long alkyl chain moiety
which extends inside the molecule, as the dihedral angle
between the plane of the aromatic ring and carbon chain
(C11–C10–C12–C13) is 3.4(5)°. The platinum atom is located
just in the mean plane of both the aryl–pyridine and the
β-diketonato ligand (0.0255(9) Å). There is very little distortion
away from the square plane. The O–Pt–O and C–Pt–N bond
angles are 91.82(10) and 81.91(14)°. The Pt–C6, Pt–O1, Pt–O2,
and Pt–N bond lengths are 1.960(4), 2.004(3), 2.084(3), and
1.991(3) Å, respectively. They are typical bond angles and
lengths for cyclometalates and β-diketonate derivatives of
Pt.30,31
Results and discussion
Synthesis and thermal decomposition temperature of Pt
complexes
Pt(F2PPy)acac, Pt(12F2PPy)acac, and Pt(12F2PPyO4)acac were
prepared through 2, 4, and 5 steps, respectively, as shown in
Scheme 1. The starting material for synthesis of Pt(12F2PPy)-
acac and Pt(12F2PPyO4)acac is 1,2-difluorobenzene (1), and that
for Pt(F2PPy)acac is 2,3-difluorophenyl boronic acid (3). All
aryl–aryl coupling reactions were conducted by Suzuki–Miyaura
cross coupling reactions using aryl halide and boronic acid com-
pounds. A key reaction step is direct alkylation to 1 which is
ortho-lithiated by 2.6 M solution of butyllithium (n-BuLi). The
reaction proceeded under warming to RT after adding 1-iododo-
decane to the activated 1 in THF solution at −40 °C. The
unreacted 1-iodododecane in the residue was changed to alcohol
or alkene derivatives by reaction with base. The resulting
mixture was purified by column chromatography to give pure
alkylated compound 2. The target Pt complexes were prepared
by a complexation of the aryl–pyridyl ligands (5, 6, and 9) with
K2PtCl4 followed by a reaction with acetylacetone. The Pt com-
plexes were fully characterized by 1H-NMR, FT-IR spectral data,
and elemental analyses. The thermal decomposition temperatures
The pertinent details of X-ray crystallography are given in
Table 1. The crystal packing of Pt(12F2PPy)acac is shown in
ˉ
Fig. 2. This complex crystallizes in the triclinic P1 space group.
Fig. 1 The molecular structure of Pt(12F2PPy)acac.
Scheme 1 Synthesis of Pt(F2PPy)acac, Pt(12F2PPy)acac, and Pt(12F2PPyO4)acac. Conditions: (a) 1) n-BuLi, hexane, THF, −78 °C, 2) 1-iodo-
hexane −40 °C; (b) 1) n-BuLi, hexane, THF, −78 °C, 2) B(OMe)3, THF, −78 °C, 3) HCl, RT; (c) 2-bromopyridine, Pd(PPh3)4, 2M K2CO3aq., THF,
70 °C, (d) 1-iodobutane, K2CO3, 2-butanone, 60 °C, (e) 1) K2PtCl4, 2-ethoxyethanol, water, 80 °C, 2) acetylacetone, Na2CO3, 2-ethoxyethanol,
100 °C.
8380 | Dalton Trans., 2012, 41, 8379–8389
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Table 1 Crystal data for Pt(12F2PPy)acac
Compound
Chemical formula
Pt(12F2PPy)acac
C28H37F2NO2Pt
Color
Formula Weight
Yellow
652.68
Crystal system
a/Å
b/Å
c/Å
Triclinic
5.4867(13)
11.059(3)
21.565(5)
81.946(7)
84.414(8)
82.446(7)
1280.2(5)
103
α/°
β/°
γ/°
Unit cell volume/Å3
Temperature/K
ˉ
Space group
P1
No. of formula units per unit cell, Z
No. of reflections measured
No. of independent reflections
Rint
Final R1 values (I > 2σ(I))
Final wR(F2) values (I > 2σ(I))
Final R1 values (all data)a
Final wR(F2) values (all data)b
2
19 375
5834
0.0484
0.0315
0.0621
0.0378
0.0653
2 2
2 2
a R1 = Σ||Fo| − |Fc||/Σ|Fo|. b wR(F2) = {Σ[w(Fo2 − Fc ) ]/Σ[w(Fo ) ]}1/2
.
Fig. 3 POM images of Pt(12F2PPy)acac (a) cooled from the melt,
(b) after 5 days and (c) at 90 °C on subsequent heating.
Fig. 2 The crystal packing of Pt(12F2PPy)acac.
transitions were confirmed by DSC measurement and X-ray dif-
fraction analysis (see Fig. S3 and S4 in ESI†). Pt(12F2PPy)acac
single crystal exhibited two phase transitions. On the other hand,
the sample cooled from the melt showed only the isotropic phase
transition. The XRD pattern of the sample was not identical to
that of the single crystal (Fig. S4†), and the d-spacing values of
the reflections were not consistent with those of the single
crystal. The crystal forms after heating were definitely different
from that of the single crystal. Thus, Pt(12F2PPy)acac exhibits
metastable states, and the crystallization is very slow.
In the unit cell, the closest aligned two Pt complexes are slightly
off in a parallel direction, and they are stacked along their long
alkyl chains. Also, the alkyl chains are face to face and interdigi-
tate with each other (Fig. 2). The average separation between
the aryl ring (C6–C11) and mean plane of the pyridine ring is
3.363(5) Å. On the other hand, this situation can be regarded as
each alkyl chain facing to the anti-parallel direction (see Fig. S2
in ESI†). In this case, the distance between two mean planes of
the pyridine moieties is 3.326(5) Å. The dimers in both parallel
and anti-parallel directions have a plane-to-plane separation of
3.3–3.4 Å, indicative of moderate π–π interaction. However,
there is no Pt–Pt bond because the Pt–Pt distances in the parallel
and anti-parallel directions are 5.487(1) and 6.703(1) Å,
respectively.
Fig. 3 shows optical textures of Pt(12F2PPy)acac observed
under a polarizing optical microscope (POM) with crossed polar-
izers. Cooling Pt(12F2PPy)acac from the isotropic phase, a
spherulitic texture started growing at 55 °C. The spherulites
shown in Fig. 3(a) were observed and stored at ambient tempera-
ture. After 5 days, the optical structure was changed by maintain-
ing at ambient temperature, as shown in Fig. 3(b). After
subsequent heating to 90 °C, the optical texture was changed
(Fig. 3(c)). These structural changes indicating solid–solid phase
Photophysical properties of Pt(12F2PPy)acac
Fig. 4 shows ultra violet–visible light (UV–vis) absorption
spectra of Pt(12F2PPy)acac in hexane and chloroform solutions.
They show two absorption maxima in the wavelength of
200–300 nm whose molar extinction coefficients (ε) were higher
than 10 000 M−1 cm−1, suggesting high energy transition. The
1
high energy absorption bands can be assigned to a π–π* ligand
centered (1LC) transition because the spectrum of Pt(12F2PPy)-
acac in this range was analogous to that of the ligand (12F2PPy),
whose peaks were observed at 246 and 278 nm in chloroform
solution.
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Fig. 4 UV–vis absorption spectra of Pt(12F2PPy)acac in chloroform
and hexane solutions.
Fig. 5 PL and excitation spectra of Pt(12F2PPy)acac in hexane
solution.
The molar absorbance of the bands from 350 to 440 nm, on
the other hand, was lower than 6000 M−1 cm−1, and the spec-
trum profile of the hexane solution was evidently different from
that of the chloroform solution in this range. Characteristics of
low ε value and solvatochromism are observed in MLCT tran-
sition.11 Thus, they are assigned to d–π* metal-to-ligand charge
transfer (1MLCT) transitions. In addition, a weak band at
487 nm was observed in the spectrum of 10−3 mol L−1 hexane
solution. The low extinction of this band (ε = 68 M−1 cm−1
)
suggests a transition to the triplet excited state enabled by spin–
orbit coupling (3MLCT).32
Fig. 5 shows PL and excitation spectra of Pt(12F2PPy)acac in
hexane solution. The PL spectra of the solutions with three
different concentrations (10−3, 10−4, and 10−5 mol L−1) are
approximately consistent, and their intensity maxima are at
around 490 and 520 nm. This indicates monomer emission of
3
Pt(12F2PPy)acac due to MLCT or mixed LC-MLCT states. The
Fig. 6 PL spectra of Pt(12F2PPy)acac neat film and doped PMMA
films.
spectrum of the high concentration solution (10−3 mol L−1
)
exhibited a broad emission band at longer wavelength region
(λmax = 648 nm). It possibly results from excimer state or
MMLCT transition due to a weak Pt–Pt bond. The excitation
spectrum of 10−3 mol L−1 hexane solution for 490 and 650 nm
was analogous. The result revealed that the two emissions were
originated from the same ground state species, i.e., the broad
emission at around 650 nm is generated from the excimer.30 No
broad emission was observed in the chloroform solution,
because the polar Pt complex can be well dissolved in the polar
chloroform compared to non-polar hexane.
Pt(12F2PPy)acac neat film and Pt(12F2PPy)acac/poly(methyl
methacrylate) (PMMA) films were prepared by spin coating to
explore further emission properties. The photoluminescence
quantum yield (PLQY) of 2 wt% Pt(12F2PPy)acac/PMMA film
was 0.52, and the lifetime was 9.0 μs at 5 K, which is compar-
able with that of other platinum complexes.11,12,32
PMMA film and the neat film were observed at 619 and 626 nm,
respectively. They were blue shifted by 20–30 nm compared
with the solution due to the difference in their polarities between
the polymer matrix and solvent.
Polarized optical emission properties of Pt complexes
Pt(F2PPy)acac (1.9 wt%), Pt(12F2PPy)acac (2.6 wt%), and
Pt(12F2PPyO4)acac (2.9 wt%) were completely dissolved into
various nematic LCs. All the mixtures exhibited a nematic phase
at ambient temperature and no crystallization of the Pt complexes
in LCs was confirmed by POM observation and DSC measure-
ment. The PL spectra revealed two sharp emission peaks at
490–510 and 520–550 nm, and broad emission at around
630–680 nm at ambient temperature, which were analogous
to those of the high concentration solution and film shown in
Figs 5 and 6, respectively. Thus, the excimer emission was also
observed in the Pt complex/LC mixtures.
PL spectra of all the films exhibited the monomer emission at
around 490 nm, as shown in Fig. 6. The excimer emission
appeared clearly in the neat film and 20 wt% Pt(12F2PPy)acac/
PMMA film. In brief, Pt(12F2PPy)acac easily came to form the
excimer with increasing the concentration. The PL intensity
maxima of the excimer emission for the Pt(12F2PPy)acac/
The Pt complex/LC mixtures were injected into LC cells as
described in the Experimental section. In the LC cells,
8382 | Dalton Trans., 2012, 41, 8379–8389
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homogeneous alignment parallel to the rubbing direction was
observed under the POM with crossed polarizers. Fig. 7 shows
polarized PL spectra of the Pt complexes/MLC-6608 cells which
were measured through a polarizer, when the direction of the
molecular alignment in the cells was in parallel and perpendicu-
lar positions with the transmission axis of the polarizer. In the
cells, the PL intensities in the perpendicular position were higher
than those in the parallel position in the whole emission wave-
length region of Pt complexes. Similar luminescence behaviors
were obtained in the other cells. Thus, they evidently showed
polarized optical emission.
It is noteworthy that the polarized optical emission of the Pt
complexes was generated in the direction perpendicular to the
molecular-orientation direction. The polarized monomer and
3
excimer emissions are assigned to MLCT or mixed LC-MLCT
emission. It suggests that the polarized optical emission results
from intramolecular and intermolecular electronic transitions
along the ligand to Pt direction, i.e., the transition dipole
moment is approximately perpendicular to the molecular axis of
the ligand. The polarization ratio (P) and order parameter (S)
were estimated to explore the polarized optical emission by the
following equations;
PLperp
P ¼
PLpara
PLperp ꢀ PLpara
S ¼
PLperp þ 2PLpara
where PLpara and PLperp denote maximum intensities of the par-
allel and perpendicular spectra, respectively. The P and S values
of the cells measured at 20 °C were summarized in Table 2.
The P and S values of the three Pt complexes in the same LC
host compound were notably different; Pt(F2PPy)acac < Pt
(12F2PPy)acac < Pt(12F2PPyO4)acac. The result indicates that
the orientation ability of a Pt complex in a nematic LC has a sig-
nificant effect on the polarized luminescence properties. That is,
Fig. 7 Linearly polarized PL spectra of (a) Pt(F2PPy)acac,
(b) Pt(12F2PPy)acac, and (c) Pt(12F2PPyO4)acac in MLC-6608 at
20 °C. The solid line and dotted line are for the polarizer aligned parallel
and perpendicular to the rubbing direction, respectively.
Table 2 Nematic/isotropic phase transition temperatures (TN/I), polarization ratios (P), and order parameters (S) of the Pt complex/LC mixtures. The
P and S values were taken at 20 °C
P
S
Sample
Nematic LC mixtures
TN/I (°C)
Monomer
Excimer
Monomer
Excimer
Pt(F2PPy)acac/5CB
Pt(12F2PPy)acac/5CB
Pt(12F2PPyO4)acac/5CB
Pt(F2PPy)acac/E7
Pt(12F2PPy)acac/E7
N 35.4
N 35.2
N 35.2
N 58.8
N 58.2
N 58.3
N 69.8
N 70.2
N 69.4
N 75.4
N 75.0
N 75.5
N 89.1
N 91.7
N 89.7
N 90.3
N 91.8
N 92.7
1.18
1.08
1.5
0.97
1.17
1.6
1.13
1.15
1.5
1.24
1.21
1.59
1.24
1.23
1.63
1.09
1.14
1.5
1.45
1.66
1.97
1.7
0.058
0.027
0.14
−0.011
0.054
0.17
0.041
0.047
0.14
0.074
0.066
0.17
0.074
0.07
0.17
0.031
0.044
0.14
0.13
0.18
0.24
0.19
0.27
0.3
0.19
0.29
0.34
0.19
0.29
0.31
0.19
0.31
0.34
0.19
0.32
0.34
2.09
2.31
1.71
2.22
2.53
1.72
2.23
2.32
1.72
2.35
2.53
1.69
2.44
2.54
Pt(12F2PPyO4)acac/E7
Pt(F2PPy)acac/JD-7000XX
Pt(12F2PPy)acac/JD-7000XX
Pt(12F2PPyO4)acac/JD-7000XX
Pt(F2PPy)acac/JD-5023XX
Pt(12F2PPy)acac/JD-5023XX
Pt(12F2PPyO4)acac/JD-5023XX
Pt(F2PPy)acac/ZLI-4792
Pt(12F2PPy)acac/ZLI-4792
Pt(12F2PPyO4)acac/ZLI-4792
Pt(F2PPy)acac/MLC-6608
Pt(12F2PPy)acac/MLC-6608
Pt(12F2PPyO4)acac/MLC-6608
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long alkyl and alkoxy chains facilitate the molecular orientation
for the Pt complex along the long axis of LC molecules.
Pt(12F2PPyO4)acac/MLC-6608 cell showed the highest P and S
values in this study, which were estimated to be 2.54 and 0.34 in
the excimer.
The P and S values of the excimer are distinctly higher than
those of the monomer. The thermal agitation of the excimer is
possibly lower in the LC state because of the bulky and rod-like
shape of the excimeric Pt complex. The transition moment in the
monomeric Pt complex, on the other hand, probably has a direc-
tional bias between Pt atom and aryl–pyridyl ligand. It is hard to
explain the polarized optical emission in detail in terms of above
limited data, and further experimental evidence and theoretical
examination are needed to ensure our conjecture.
the polarized luminescent properties by focusing attention on the
excimer emission. The P values of all the samples gradually
decreased with increasing temperature, and ended up at 1.0 at the
isotropic phase, as shown in Fig. 8. Much the same is true for
the monomers (see Fig. S5 in ESI†). Thus, the polarized lumi-
nescence depends on the order parameter of the nematic LC and
molecular orientation of each Pt complex.
In the Pt(F2PPy)acac/LC cells, the same P values were
obtained in the lower temperature range except for the case of
5CB. In the Pt(12F2PPy)acac/LC and Pt(12F2PPyO4)acac/LC
cells, on the other hand, the P values were not the same even in
the low temperature range. The values of Pt complex/5CB cells
were comparatively lower than the other LC mixtures in the
temperature range. In general, the order parameter of the LC
decreases drastically at around the nematic/isotropic phase tran-
sition temperature (TN/I). Hence, the low P and S values in the
case of 5CB are probably due to the comparatively large thermal
agitation caused by the low TN/I close to ambient temperature.
These results suggest that the polarized optical emission depends
on the matrix LC and molecular structures of the ligand.
The PL spectrum of a Pt complex is affected by temperature
change.13,33 We have investigated the temperature dependence of
Fig. 9 presents changes in the maximum PL emission wave-
lengths (PL–λmax) of the excimer emission in Pt(12F2PPy)acac/
LC cells. They increased with elevating temperature, suggesting
a red-shift. For example, the PL–λmax of the excimer emission
for Pt(12F2PPy)acac/MLC-6608 red-shifted from 650 nm at
20 °C to 694 nm at 105 °C. The other Pt complex/LC cells
showed the same tendency. Increasing the molecular movement
allowed closer excimer formation and rearrangement in the
nematic LC, giving stronger π–π interactions.
Fig. 10 shows the PL spectra in the perpendicular direction for
Pt(12F2PPy)acac/MLC-6608 cell. The PL intensities in the
entire wavelength region decreased with increasing temperature
as the PL intensities of the monomer became relatively low com-
pared to those of the excimer. The same behavior was also
observed in the parallel direction. The intensity ratio of the
monomer/excimer at PL–λmax were plotted against temperature
in Fig. 11. They decreased with increasing temperature. This is
due to a diffusion-controlled process and a difference in the
thermal stability between the monomer and the excimer.34 In the
Fig. 8 The temperature dependence of polarization ratios of the exci-
mers in (a) Pt(F2PPy)acac/LC, (b) Pt(12F2PPy)acac/LC, and (c)
Pt(12F2PPyO4)acac/LC cells.
Fig. 9 The temperature dependence of maximum wavelengths of the
excimers in Pt(12F2PPy)acac/LC cells.
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nematic LC state, the molecular mobility of the Pt complex is
comparatively high, and it probably enhances the encounter of
the Pt complexes to form the excimer. On the other hand,
Pt(12F2PPyO4)acac/LC cells show an intriguing behavior.
The intensity ratios in the parallel position decreased definitely
around TN/I (Fig. 11(e)). This was maybe caused by decreased
viscosity and thermal agitation. The ratio in the perpendicular
one showed convex curves suggesting the existence of a
dissociation-controlled process (Fig. 11(f)). This behavior was
possibly determined by the correlation between diffusion- and
dissociation-controlled processes.
We have examined the temperature dependence of the PL
intensities for the Pt complexes doped PMMA film to clarify the
influence of the molecular movement of Pt(12F2PPy)acac on the
excimer formation. Fig. 12 shows the PL intensities of the
monomer and excimer for 20 wt% Pt(12F2PPy)acac doped
PMMA film comparably decreased with increasing temperature.
The PL intensity ratio of the monomer/excimer remained almost
unchanged under 80 °C close to the melting temperature of
Pt(12F2PPy)acac and the glass transition of PMMA, as shown in
Fig. S6.† This suggests the molecular movement of Pt(12F2PPy)-
acac was inhibited in the PMMA film by its high viscosity. The
decreasing PL intensity was chiefly caused by the temperature
dependence of their PLQY.33 Therefore, the excimer formation
and subsequent emission are influenced by the mobility and dif-
fusion of the Pt complexes in the nematic LC state.
Conclusions
Fig. 10 Temperature-dependent emission spectra of Pt(12F2PPy)acac
(2.6 wt%)/MLC-6608 cell in the perpendicular direction.
We have synthesized a series of Pt complexes; ‘Pt(F2PPy)acac’,
‘Pt(12F2PPy)acac’, and ‘Pt(12F2PPyO4)acac’ having a rod-like
Fig. 11 Changes in PL intensity ratios of the monomer/the excimer of (a) Pt(F2PPy)acac/LC, (b) Pt(12F2PPy)acac/LC, and (c) Pt(12F2PPyO4)acac/
LC cells in the parallel direction, (d) Pt(F2PPy)acac/LC, (e) Pt(12F2PPy)acac/LC, and (f) Pt(12F2PPyO4)acac/LC cells in the perpendicular direction.
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excitation spectra were recorded on a HORIBA Jobin Yvon
Fluoromax-4 spectrofluorometer. PLQY was measured by a
PLQY measurement system C9920-01 (Hamamatsu Photonics).
Pt(12F2PPy)acac was doped into PMMA, and the thin film was
cast by spincoating. The PLQY of the film was measured using
an integrating sphere under a nitrogen purge. Transient photo-
luminescence decay was measured by a synchronous delay
generator C4792-02 equipped with a streak scope C4334
(Hamamatsu Photonics) under excitation by irradiating a nitro-
gen laser (λ = 337 nm, 50 Hz, 800 ps pulses). The thermal
decomposition temperature was determined on the basis of 5%
decrease in weight by a thermogravimetric analyzer Q-50
(TA instruments). Thermal properties were examined using a
differential scanning calorimeter Q-200 (TA instruments)
at heating–cooling rates of 10 °C min−1 under a nitrogen purge.
Fig. 12 Temperature-dependent emission spectra of Pt(12F2PPy)acac
(20 wt%)/PMMA thin film.
ligand. The crystal form of Pt(12F2PPy)acac was successfully
determined to be triclinic: a = 5.4867(13) Å, b = 11.059(3) Å,
c = 21.565(5) Å, α = 81.946(7)°, β = 84.414(8)°, γ = 82.446(7)°
by single-crystal X-ray structural analysis. In the unit cell, the
closest two Pt complexes are stacked slightly off in a parallel
direction, and other Pt complex interdigitates on the part of the
long alkyl group. However, there is no Pt–Pt bond in the dimer
states. The DSC, POM, and X-ray diffraction analysis revealed
the presence of a metastable structure. The UV–vis absorption
spectra showed an intense transition (1LC) state, and solvato-
chromic band (1MLCT). Moreover, a weak absorption peak at
487 nm was also observed in high concentration hexane solution,
implying a 3MLCT band. The hexane solution displayed
monomer and excimer emission at around 490 and 650 nm,
respectively. The Pt complexes dissolved in nematic LCs showed
homogeneous alignment between two glass substrates with
rubbed polyimide. The uniaxially-oriented samples exhibited a
linearly polarized luminescence perpendicular to the orientation
direction. Particularly, the excimer emission showed compara-
tively high polarization ratio. The highest polarization ratio
amounted to 2.54 in the Pt(12F2PPyO4)acac/MLC-6608 cell at
RT. The polarization ratios of Pt(12F2PPyO4)acac were higher
than those of Pt(F2PPy)acac and Pt(12F2PPy)acac in all the LCs.
The polarization ratios of all the samples gradually decreased
with increasing temperature, finally coming to 1.0 in the isotro-
pic phase. Additionally, the emission wavelengths were red-
shifted about 40 nm at most with increasing temperature. The
molecular movement of the Pt complex is important factor to
form an excimer in nematic LC. We presently prepare novel
smectic LCs to dissolve the Pt complex and investigate the polar-
ization properties in the smectic phase.
X-ray diffraction was carried out by
a MicroMax007
R-AXIS IV++ (Rigaku Denki) using Cu-Kα irradiation. The
optical textures of the Pt complex and the LC samples were
examined using a POM BX-50P (Olympus) equipped with a hot
stage TH-600RMS (Linkam). Polarized emitting cells were set
on the hot stage, and irradiated by a UV-LED irradiator Aicure
UJ30/35 (Panasonic). Polarized photoluminescence spectra were
measured through a polarizer with a photonic multichannel
analyzer PMA-11 (Hamamatsu Photonics). The polarized PL
measurements were taken from the isotropic phase to 20 °C at
intervals of 5 °C.
Reagents
1,2-Difluorobenzene (Tokyo Kasei), 1-iodododecane (Kanto
Chemicals), 2.6 M solution of n-BuLi in n-hexane (Kanto
Chemicals), trimethyl borate (Alfa Aesar), 2,3-difluorophenyl
boronic acid (Tokyo Kasei), tetrakistriphenylphosphine palladium
(0) (Pd(PPh3)4) (Tokyo Kasei), 2-bromopyridine (Tokyo
Kasei), 2-chloro-5-hydroxypyridine (Tokyo Kasei), 1-iodobutane
(Kanto Chemicals), potassium tetrachloroplatinate(II) (K2PtCl4)
(Aldrich), acetylacetone (Kanto Chemicals), 6 mol% aqueous
hydrochloric acid (Kanto Chemicals), sodium carbonate
(Na2CO3) (Kanto Chemicals), potassium carbonate (K2CO3)
(Kanto Chemicals), anhydrous magnesium sulfate (MgSO4)
(Kanto Chemicals), dry THF (Kanto Chemicals), n-hexane
(Kanto Chemicals), ethanol (Kanto Chemicals), 2-butanone
(Kanto Chemicals), 2-ethoxyethanol (Kanto Chemicals), ethyl-
acetate (Kanto Chemicals), chloroform (Kanto Chemicals), and
potassium hydroxide (KOH) (Kanto Chemicals) were purchased
and used as received. Unless otherwise noted, the remaining
chemicals were commercially available and used without further
purification.
Experimental
Measurements
Synthesis of ligands
Infrared spectra were recorded on a HORIBA FT-210 spec-
trometer. NMR spectra were obtained on a JEOL JNM-ECX 400
(400 MHz for 1H and 100 MHz for 13C) spectrometer. Elemental
analysis was carried out by a CHNS/O 2400II analyzer (Perkin
Elmer). Single crystal X-ray diffraction was carried out by a
Saturn 724+ (Rigaku Denki) using Mo-Kα irradiation. Similar-
ADP restraint SIMU was applied to the part of alkyl disorder for
the refinement. UV–vis absorption spectra were recorded on a
Shimadzu UV3150 spectrometer. Photoluminescence and
1,2-Difluoro-3-dodecylbenzene (2). To a solution of 1,2-
difluorobenzene (1) (7.32 g, 64.2 mmol) in dry THF (110 ml), a
2.6 M solution of n-BuLi in n-hexane (24.9 ml, 64.2 mmol) was
added slowly at −78 °C under nitrogen atmosphere. The solution
was stirred at −78 °C for 2 h. After 1-iodododecane (19.0 g,
64.2 mmol) was added to the solution at −40 °C, the mixture
was allowed to warm to RT and stirred overnight. The solvents
were removed by a rotary evaporator, and ethanol (100 ml) and
8386 | Dalton Trans., 2012, 41, 8379–8389
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KOH (2.0 g) was added to the residue. The mixture was stirred
at 70 °C for 20 h to remove the unreacted 1-iodododecane.
The reaction mixture was diluted with water, and extracted with
n-hexane. The extract was washed with brine, dried over
anhydrous MgSO4 and concentrated under reduced pressure. The
residue was purified by column chromatography with n-hexane
to give a colorless liquid (11.8 g, 41.8 mmol, 65%).
CH
–CH
719.
̲
2–), 1.40–1.15 (m, 18H, –CH
̲
̲
2–), 0.87 (t, J = 7.0 Hz, 3H,
3). IR: ν (cm−1) = 2960, 2910, 2850, 1590, 1470, 843, and
5-Butoxy-2-chloropyridine (8). A solution of 2-chloro-5-
hydroxypyridine (7) (4.0 g, 30.8 mmol), 1-iodobutane (5.67 g,
30.8 mmol), and K2CO3 (0.48 g, 30.8 mmol) in 2-butanone
(150 ml) was stirred under reflux for 24 h at 60 °C. The resulting
suspension was concentrated under reduced pressure, extracted
with diethylether, and the extract then washed with aqueous
K2CO3 solution and finally water. The organic layer was dried
over MgSO4 and concentrated under reduced pressure. The
residue was purified by column chromatography with chloroform
to give a colorless liquid (5.18 g, 27.9 mmol, 91%).
1H-NMR (400 MHz, CDCl3): δ (ppm) = 7.00–6.90 (m, 3H,
ArH
Ar–CH2–CH
Hz, 3H, –CH
1600, 1490, 1210, 827, 773, and 719.
̲
̲
2–), 1.63–1.55 (m, 2H,
̲
2̲ –), 0.93 (t, J = 7.0
̲
2,3-Difluoro-4-dodecylphenylboronic acid (4). To a solution of
2 (11.8 g, 41.8 mmol) in dry THF (90 ml), 2.6 M solution of
n-BuLi in n-hexane (17.8 ml, 46.0 mmol) was added dropwise
at −78 °C under nitrogen atmosphere. After the reaction mixture
was stirred for 2 h, previously cooled trimethyl borate (9.1 ml,
58.4 mmol) was added slowly. The reaction mixture was allowed
to warm to RT and stirred overnight, and then stirred for 1 h with
6 mol% (50 ml) aqueous hydrochloric acid. The resulting
mixture was extracted with diethyl ether, and the organic layer
was washed with water several times and dried over anhydrous
MgSO4. The solvents were removed under reduced pressure. The
resulting solid was recrystallized from n-hexane twice to give
white needles (12.2 g, 37.2 mmol, 89%).
1H-NMR (400 MHz, CDCl3): δ (ppm) = 7.44 (d, J = 3.2 Hz,
1H, ArH
O–CH2–), 1.77 (quint, J = 7.0 Hz, 2H, Ar–O–CH2–CH
(sext, J = 7.4 Hz, 2H, –CH2–CH3), 9.69 (t, J = 7.5 Hz, 2H,
–CH
3). IR: ν (cm−1) = 2960, 2930, 2870, 1590, 1570, 1450,
̲
̲
̲
̲
̲
1380, 1270, 1220, 1140, 1130, 1020, and 966.
2-(4-Dodecyl-2,3-difluorophenyl)-5-butoxypyridine (9). Quan-
tities: 4 (3.0 g, 9.21 mmol), 8 (1.71 g, 9.21 mmol), Na2CO3
(3.27 g, 29.6 mmol) and Pd(PPh3)4 (0.26 g, 0.23 mmol), THF
(40 ml).
The experimental procedure of Suzuki coupling was as
described previously and yielded a white solid (2.10 g,
4.87 mmol, 53%).
1H-NMR (400 MHz, CDCl3): δ (ppm) = 7.44 (t, J = 7.7 Hz,
1H, ArH
B–OH), 2.67 (t, J = 7.7 Hz, 2H, Ar–CH
Ar–CH2–CH2–), 1.40–1.18 (m, 18H, –CH
Hz, 3H, –CH
3). IR: ν (cm−1) = 3500–3000, 2960, 2930, 2860,
̲
̲
̲
̲
1H-NMR (400 MHz, CDCl3): δ (ppm) = 8.38 (d, J = 3.1 Hz,
̲
1H, ArH
7.6 Hz, 1H, ArH
1H, ArH), 4.05 (t, J = 6.4 Hz, 2H, Ar–O–CH
Hz, 2H, Ar–O–CH2–), 1.81 (quint, J = 7.0 Hz, 2H, Ar–O–CH2–
CH2–), 1.62 (quint, J = 7.0 Hz, 2H, –CH2–), 1.51 (sext, J = 7.4
Hz, 2H, –CH2–CH3), 9.89 (t, J = 7.5 Hz, 2H, –CH3), 8.68 (t, J =
7.0 Hz, 2H, –CH
3). IR: ν cm−1) = 2910, 2850, 1590, 1580,
̲
̲
̲
̲
1640, 1500, 1450, 1340, 1130, 951, and 897.
̲
̲
̲
2-(2,3-Difluorophenyl)pyridine (5). A solution of 2-bromo-
pyridine (1.94 g, 16.7 mmol), 3 (4.0 g, 16.7 mmol), 2 M
aqueous solution of K2CO3 (5.93 g, 53.8 mmol) and Pd(PPh3)4
(0.47 g, 0.41 mmol) in THF (60 ml) was stirred under reflux
24 h at 70 °C. The resulting mixture was concentrated under
reduced pressure, extracted with diethylether, and poured into
water. The organic layer was washed with water three times and
dried over anhydrous MgSO4. The solvents were concentrated
under reduced pressure, and the residue was purified by column
chromatography with ethylacetate–n-hexane (1 : 3, v/v) to give a
colorless liquid (2.6 g, 13.6 mmol, 81.4%).
̲
̲
̲
̲
̲
1480, 1450, 1300, 1270, 1030, and 895.
Synthesis of Pt complexes
Pt(F2PPy)acac. A solution of K2PtCl4 (2.75 g, 6.63 mmol), 5
(2.60 g, 13.6 mmol) in 2-ethoxyethanol (40 ml) and water
(13 ml) was heated to 80 °C for 18 h under nitrogen atmosphere.
Afterwards, the reaction mixture was cooled to RT and concen-
trated under reduced pressure. The resulting red brown precipi-
tate was extracted with chloroform and washed with water three
times. The organic layer was dried over anhydrous MgSO4, and
concentrated under reduced pressure to give a yellow solid
(4.4 g).
1H-NMR (400 MHz, CDCl3): δ (ppm) = 8.74–8.71 (td,
J = 1.4 and 4.7 Hz, 1H, ArH
7.75–7.69 (m, 1H, ArH), 7.28 (sext, J = 4.5 Hz, 1H, ArH
7.23–7.15 (m, 2H, ArH
). IR: ν (cm−1) = 1580, 1570, 1490,
̲
̲
̲
̲
̲
1460, 1430, 1270, and 901.
2-(4-Dodecyl-2,3-difluorophenyl)pyridine (6). Quantities:
4
(4.0 g, 12.3 mmol), 2-bromopyridine (1.94 g, 12.3 mmol),
Pd(PPh3)4 (0.47 g, 0.41 mmol), THF (60 ml), and 2 M aqueous
solution of K2CO3 (5.93 g, 43.0 mmol).
A solution of the resulting yellow solid (4.40 g), acetylacetone
(1.31 g, 13.1 mmol), and Na2CO3 (5.54 g, 52.3 mmol) in
2-ethoxyethanol (100 ml) was stirred at 100 °C for 16 h under
nitrogen atmosphere. The reaction mixture was diluted with
water, and extracted with chloroform. The organic layer was
washed with water, dried over anhydrous MgSO4, and concen-
trated under reduced pressure. The residue was purified by
column chromatography with chloroform–n-hexane (1 : 1, v/v)
to give a yellow solid (1.94 g, 4.00 mmol, 60.4%).
The experimental procedure was as described previously and
yielded a white solid (2.1 g, 5.27 mmol, 78.9%).
1H-NMR (400 MHz, CDCl3): δ (ppm) = 8.71 (d, J = 4.4 Hz,
1H, ArH
ArH), 7.25 (m, 1H, ArH
J = 7.7 Hz, 2H, Ar–CH2), 1.63 (quint, J = 7.7 Hz, 2H, Ar–CH2–
̲
̲
̲
̲
̲
̲
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View Article Online
1H-NMR (400 MHz, CDCl3): δ (ppm) = 9.06 (td, J = 0.8 and
5.9 Hz, 1H, ArH), 8.02 (d, J = 8.2 Hz, 1H, ArH), 7.86 (t, J =
7.9 Hz, 1H, ArH), 7.33–7.28 (m, 1H, ArH), 7.17 (t, J = 6.7 Hz,
1H, ArH), 7.11–7.03 (m, 1H, ArH), 5.47 (s, 1H, CH3–CvCH
C–CH3), 2.00 (d, J = 4.7 Hz, 6H, CH
n-octyloxy-1,1′-biphenyl, and 4-cyano-4′′-n-pentyl-1,1′,1′′-ter-
phenyl: their mass fractions are 51, 25, 16, and 8%, respectively.
All the samples exhibit a nematic phase at ambient temperature:
the TN/I of 5CB is 35 °C; E7: 59 °C; JD-5023XX: 76 °C;
ZLI-4792: 95 °C; MLC-6608: 90 °C. Pt(12F2PPy)acac
(1.9 wt%) or Pt(12F2PPy)acac (2.6 wt%) or Pt(12F2PPy)acac
(2.9 wt%)/LC mixtures were prepared. In the each case of Pt
complex/5CB mixture, the concentrations of 1,9 2.6, and 2.9 wt%
are nearly equal to 1.0 mol%, respectively.
The LC mixture was injected into a glass cell. The glass cell
was assembled with two glass plates: the gap between them was
5 μm, provided by a polyester film. On each glass substrate, a
rubbed polyimide alignment layer was coated. The rubbing
directions of the upper and bottom glass substrates were arranged
in anti-parallel orientations.
̲
̲
̲
̲
̲
̲
̲
̲
3–CvCH–C–CH3
̲
ν (cm−1) = 1580, 1530, 1490, 1260, 1110, 1020, 933, 908, and
876. Anal. Calcd for C16H13F2NO2Pt: C, 39.68.; H, 2.71; N,
2.89. Found: C, 39.61; H, 2.52; N, 2.81%.
Pt(12F2PPy)acac. Quantities:
(i)
K2PtCl4
(1.69
g,
4.07 mmol), 6 (3.0 g, 8.35 mmol), 2-ethoxyethanol (30 ml), and
water (10 ml). (ii) Yellow solid (3.8 g), acetylacetone (0.81 g,
8.08 mmol), Na2CO3 (3.42 g, 32.3 mmol), and 2-ethoxyethanol
(60 ml).
The experimental procedure for synthesis of the Pt complex
was as described previously and yielded a yellow solid (1.64 g,
2.51 mmol, 61.7%).
1H-NMR (400 MHz, CDCl3): δ (ppm) = 9.01 (d, J = 6.0 Hz,
Notes and references
1H, ArH
1H, ArH), 7.15–7.09 (m, 2H, ArH
C–CH3), 2.67 (t, J = 7.9 Hz, 2H, Ar–CH
6H, CH3–CvCH–C–CH3), 1.65 (quint, J = 7.9 Hz, 2H, Ar–
CH2–CH2–), 1.40–1.20 (m, 18H, –CH2–), 0.87 (t, J = 7.2 Hz,
3H, –CH2–CH3
). IR: ν (cm−1) = 2950, 2920, 2850, 1610, 1570,
̲
̲
̲
̲
1 C. S. Peyratout, T. K. Aldridge, D. K. Crites and D. R. McMillin, Inorg.
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̲
̲
̲
̲
̲
̲
1490, 1460, 1200, 1020, and 719. Anal. Calcd for
C28H37F2NO2Pt: C, 51.53.; H, 5.71; N, 2.15. Found: C, 51.61;
H, 5.71; N, 2.11%.
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Pt(12F2PPyO4)acac. Quantities: (i) K2PtCl4 (1.45 g,
3.48 mmol), 9 (3.0 g, 6.95 mmol), 2-ethoxyethanol (30 ml), and
water (10 ml). (ii) Yellow solid (3.5 g), acetylacetone (0.66 g,
6.63 mmol), Na2CO3 (2.81 g, 26.5 mmol), and 2-ethoxyethanol
(60 ml).
The experimental procedure for synthesis of the Pt complex
was as described previously and yielded a yellow solid (1.23 g,
1.70 mmol, 64%).
1H-NMR (400 MHz, CDCl3): δ (ppm) = 8.72 (d, J = 2.7 Hz,
1H, ArH
2.8 and 9.1 Hz, 1H, ArH
(s, 1H, CH3–CvCH–C–CH3), 4.06 (t, J = 6.6 Hz, 2H, Ar–O–
CH2–), 2.67 (t, J = 7.5 Hz, 2H, Ar–CH2–), 2.00 (d, J = 4.0 Hz,
6H, CH3–CvCH–C–CH3), 1.81 (quint, J = 7.0 Hz, 2H, Ar–O–
CH2–CH2–), 1.64 (quint, J = 7.5 Hz, 2H, Ar–CH2–CH2–),
1.57–1.46 (m, 2H, –CH2–), 1.4–1.2 (m, 18H, –CH2–), 0.99 (t, J
= 7.5 Hz, 3H, –CH3), 0.87 (t, J = 6.8 Hz, 3H, –CH3). IR:
̲
̲
̲
̲
̲
̲
̲
̲
̲
̲
̲
̲
̲
̲
ν (cm−1) = 2920, 2850, 1570, 1520, 1490, 1460, 1390, 1260,
831, and 810. Anal. Calcd for C32H45F2NO3Pt: C, 53.03; H,
6.26; N, 1.93. Found: C, 53.24; H, 6.29; N, 1.87%.
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Pt(F2PPy)acac, Pt(12F2PPy)acac, and Pt(12F2PPyO4)acac were
dissolved in six kinds of nematic LC compounds with different
T
N/I: 4-cyano-4′-n-pentyl-1,1′-biphenyl (5CB) (Tokyo Kasei) and
five kinds of commercial nematic LCs for LCD. The commercial
LCs were E7 (Merck), JD-7000XX (JNC), JD-5023XX (JNC),
ZLI-4792 (Merck), and MLC-6608 (Merck), which were mix-
tures of various LC compounds. E7 consists of an eutectic
mixture of 5CB, 4-cyano-4′-n-heptyl-1,1′-biphenyl, 4-cyano-4′-
8388 | Dalton Trans., 2012, 41, 8379–8389
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