New phosphorescent platinum(ii) complexes with tetradentate C*N^N*C ligands: liquid crystallinity and polarized emission

Article abstract of DOI:10.1039/c6dt04502b

New phosphorescent, liquid crystalline cyclometalated tetradentate platinum complexes (Pt-L¹⁶, Pt-L¹² and Pt-L⁶) based on the tetradentate C*N^N*C ligands (C*N^N*C = 6,6′-bis(4-(alkoxy)-phenoxy)-2,2′-bipyridine) are designed and synthesized. Their crystal structure, and photophysical, electrochemical and liquid crystal characteristics were investigated. The X-ray structure of Pt-L¹² shows a severe distortion of this complex towards a tetrahedral geometry. All complexes are emissive both in degassed solution and in the solid state at room temperature with emission maxima in the red region of the spectrum. Pt-L¹⁶ and Pt-L¹² show monotropic smectic liquid crystal characteristics. Moreover, these liquid crystal complexes can be aligned on a rubbed nylon-6 glass substrate and produce polarized emission with a dichroic ratio of 5.1.

Full text of DOI:10.1039/c6dt04502b

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New phosphorescent platinum(II) complexes with
tetradentate C*N^N*C ligands: liquid crystallinity
and polarized emission†
Cite this: DOI: 10.1039/c6dt04502b
Shilin Zhang, Kaijun Luo,* Hao Geng, Hailiang Ni, Haifeng Wang* and Quan Li*
New phosphorescent, liquid crystalline cyclometalated tetradentate platinum complexes (Pt-L16, Pt-L12
and Pt-L6) based on the tetradentate C*N^N*C ligands (C*N^N*C = 6,6’-bis(4-(alkoxy)-phenoxy)-2,2’-
bipyridine) are designed and synthesized. Their crystal structure, and photophysical, electrochemical and
liquid crystal characteristics were investigated. The X-ray structure of Pt-L12 shows a severe distortion of
this complex towards a tetrahedral geometry. All complexes are emissive both in degassed solution and in
the solid state at room temperature with emission maxima in the red region of the spectrum. Pt-L16 and
Pt-L12 show monotropic smectic liquid crystal characteristics. Moreover, these liquid crystal complexes
can be aligned on a rubbed nylon-6 glass substrate and produce polarized emission with a dichroic ratio
of 5.1.
Received 28th November 2016,
Accepted 15th December 2016
DOI: 10.1039/c6dt04502b
www.rsc.org/dalton
usually have higher device efficiency than those using bi-
Introduction
dentate or tridentate platinum(II) complexes as an emitter.5,8,10
Cyclometalated platinum(II) complexes (ionic or neutral) have Additionally, the intermolecular π⋯π and Pt⋯Pt interactions
received a great deal of attention in the past two decades in the square planar platinum(II) complexes facilitate the for-
because of their numerous potential applications in biological mation of metal-containing liquid crystals (metallomesogens)
sensing and imaging,1 photocatalysis,2 cation recognition,3 which probably are able to conjugate the luminescence pro-
as well as pH sensing.4 In particular, the cyclometalated perties with the mesomorphic behavior depending on the
complexes of platinum as highly efficient phosphorescent judicious combination of metals and ligands.6,7,11 Particularly,
materials in organic light-emitting diodes (OLEDs)5 have platinum-based luminescent metallomesogens with the polar-
always been extensively investigated because these materials ized phosphorescence property have been widely investigated
are capable of harvesting both singlet and triplet excitons due to their significant advantages and possible applications
arising from the strong spin–orbital coupling. In general, there in optoelectronics.12 However, all of them are generally based
are three main coordination patterns for such cyclometalated on bidentate or tridentate cyclometalated platinum(II) com-
platinum complexes: bidentate,6 tridentate7 and tetradentate.8 plexes. Kinoshita et al.13 designed a tetradentate cycloplati-
Very recently, rigid tetradentate platinum(II) complexes have nated complex, N,N′-bis(4-n-decyloxy-salicyl-aldehydo)ethylene-
drawn much attention.8 The tetradentate complexes have diaminateplatinum(II)complex(Pt(4-decyloxy)salen), which did
obvious merits compared with the bidentate and tridentate not show liquid crystal characteristics in spite of bearing two
platinum(II) complexes. The rigid cyclic configuration of tetra- peripheral alkoxy chains. Nevertheless, a polarized emission
dentate platinum(II) complexes in which the vibrational de- was obtained by doping it in a liquid crystal polymer with a
formation of the platinum(II) center and coordinated atoms is dichroic ratio (RPL) of 1.7. As we know, there are no reports
confined can improve the thermal stability and decrease the about metallomesogens based on tetradentate cyclometalated
non-radiative vibrational pathways, resulting in a high photo- platinum(II) complexes to date. In this context, here we firstly
luminescent quantum yield (PLQY).5,9 OLEDs fabricated by report a new class of phosphorescent, liquid crystalline cyclo-
using tetradentate plantinum(II) complexes as a triplet dopant metalated platinum complexes containing tetradentate
C*N^N*C ligands (Pt-L16, Pt-L12, Pt-L6), where C*N^N*C is
6,6′-bis(4-(alkoxy)-phenoxy)-2,2′-bipyridine. By introducing two
flexible alkoxy chains on the phenoxide moiety connected to
the 2,2′-bipyridine fragment, these complexes display thermo-
tropic liquid crystallinity and the range of the transition temp-
erature of metallomesogens can be affected by the length of the
College of Chemistry and Materials Science, Sichuan Normal University,
5 Jingan Road, Chengdu 610068, China. E-mail: Luo-k-j007@163.com
†Electronic supplementary information (ESI) available: 1H NMR, 13C NMR and
MS spectra. CCDC 1511011. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c6dt04502b
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peripheral alkoxy chains. In addition, these tetradentate plati-
num(II) complexes show a polarized emission in the aligned
nylon-6 film at room temperature.
Results and discussion
Synthesis
The synthesis of the ligands and the Pt(II) complexes is
depicted in Fig. 1. The ligand precursors, Pn, were obtained by
etherification of excess hydroquinone with 1-bromoalkane
under Williamson ether conditions. Another modified
Williamson reaction5 was used to give the ligands, Ln. The
final platinum(II) complexes, Pt-Ln, were prepared by the com-
plexation of Ln with K2PtCl4 in glacial acetic acid under reflux.
Fig. 2 Molecular structure and atomic labeling scheme (50% prob-
ability thermal ellipsoids) of Pt-L12 (top) and the crystal packing of Pt-L12
(bottom), viewed from perpendicular to the ac plane. The hydrogen
atoms are omitted for clarity.
Crystallographic analysis of Pt-L12
The crystal structure of Pt-L12 was determined by single-crystal
X-ray diffraction analysis. The detailed crystallographic data
and structure refinement parameters are summarized in
Table S1 (ESI†). The Pt(01)–C(008) (2.005(4) Å), Pt(01)–C(00A)
(1.999(4) Å), Pt(01)–N(003) (2.053(3) Å), and Pt(01)–N(002)
(2.058(3) Å) bond lengths are similar to the analogous tetra-
dentate platinum complexes.14 The bond angles of C(00A)–
Pt(01)–C(008) and N(003)–Pt(01)–N(002) are 97.95(15)° and
79.59(12)°, respectively. The angles between the plane of the
bipyridyl group and the planes of the two benzene rings are
32.43° and 18.37°, respectively, indicating that the molecular
structure deviates greatly from the square planar geometry
(Fig. 2 top). The molecules arrange as head-to-tail dimers,
which are approximately superimposed when viewed from per-
pendicular to the ac plane. The distance of Pt⋯Pt in each dimer
is 5.101 Å and 5.566 Å between adjacent dimers, which excludes
any intermolecular Pt⋯Pt interactions. Moreover, the distance
between the planes of the proximal molecules is about 3.625 Å,
suggesting weak intermolecular π⋯π interactions. Furthermore,
the molecules pack in the interdigitation mode of alkoxy chains
between two molecules and form a layered structure (Fig. 2
bottom). The interlayer spacing is 27.1 Å, which is equal to the
length of the c axis in a unit cell.15
tion and emission spectra of Pt-L16 in dichloromethane (DCM)
solution. The absorption of all complexes follows Beer’s
law with the complex concentrations ranging from 10−4 to
10−5 mol dm−3. The intense absorption bands below 350 nm
are assigned to spin-allowed ligand-centered (1LC) transitions,
which are similar to those from free ligands. In the visible
region (360–470 nm), there are weak absorption bands with
ε values of 2.74–5.08 × 103 dm3 mol−1 cm−1, which are absent in
the corresponding ligands. As depicted in Fig. 4 (left), Table S2
(ESI†), these absorption bands display a solvatochromic shift
on changing the solvent from DCM to toluene, THF, DMF and
benzonitrile, evidencing a charge transfer nature. Furthermore,
in terms of molecular orbital analysis based on time-dependent
density functional theory (TD-DFT) calculations of Pt-L16 (dis-
cussion below), we tentatively assign these weak bands to the
metal–ligand charge-transfer (1MLCT), to a certain extent, mixed
with a phenoxide moiety (π) to the bipyridyl group (π*) charge-
transfer transition, which is also similar to the analogous
tetradentate platinum complexes.5,16 As is the case for most
cyclometalated Pt(II) complexes that bear peripheral flexible
chains,6,12,14,16 the length of the alkoxy chains has little influ-
ence on the UV-vis absorption and photoluminescence spectra.
All three complexes are luminescent in degassed solution
and in the solid state with emission quantum yields in the
Photophysical properties
The primary photophysical data obtained for all platinum(II) range of 4.4%–6.5% (in DCM solution) and emission lifetimes
complexes are listed in Table 1. Fig. 3 shows UV-visible absorp- (τ) in the range of 0.9–1.4 μs (Table 1). The low quantum yield
may be attributed to the fact that the severe distortion of the
molecular structure described by the X-ray structure of Pt-L12,
to a certain extent, gives rise to the deformation of the geo-
metric configuration in the excited state, which could acceler-
ate non-radiative decay.17 The emission spectrum of Pt-L16 at
room temperature in DCM shows a broad and structureless
red emission band centered at 622 nm. Similar to the absorp-
tion data, the solvent effect on the emission of Pt-L16 was also
observed (e.g., λmax of Pt-L16 in benzonitrile at 618 nm is red-
Fig. 1 Synthesis of tetradentate Pt(II) complexes. Conditions: (i) 1-bromo-
alkane, anhydrous alcohol, K2CO3, 90 °C, 24 h. (ii) 6,6’-Dibromo-2,2’-
bipyridine, 1-methylimidazole, K2CO3, CuI, toluene, 120 °C, 2 days.
(iii) K2PtCl4, AcOH, reflux, 2 days.
shifted to 624 nm in toluene) (Fig. 4 right). We tentatively
assign the emissions to mixed MLCT (1MLCT + 3MLCT) and
ligand centered triplet (3LC) excited states.16
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Table 1 Photophysical data of the platinum(II) complexes
λabs/nm
λem d/nm
(ε/103 dm3 mol−1 cm−1 a
)
298 Kb
77 Kb
Solid
Filmc
τb/μs
ϕPL
e
Complex
Pt-L16
Pt-L12
Pt-L6
226 (38.7), 242 (39.0), 278 (29.2), 336 (7.90), 412 (4.34), 470 (2.74)
224 (29.1), 244 (28.1), 278 (19.9), 334 (6.94), 412 (3.92), 470 (2.63)
224 (36.3), 246 (35.0), 276 (25.2), 334 (8.56), 412 (4.92), 472 (3.27)
622
623
626
596
584
578
594
579
558
586
566
579
0.9
1.1
1.4
4.4%
4.8%
6.5%
a Measurements were carried out in dichloromethane solution. b Measured in degassed dichloromethane solution. c Prepared by 5% doped in
PMMA. d The excitation wavelength of 412 nm was used for all complexes. e Measured in degassed dichloromethane solution relative to [Ru
(bpy)3]Cl2 (excitation wavelength = 436 nm, ϕlum = 0.042).
28–68 nm blue shifts in the solid state and 36–57 nm in the
5% doped PMMA films, respectively. Obviously, the blue-
shifted emission in the concentrated state shows monomer
emission characteristics, indicating very weak intermolecular
interactions in the concentrated phases, which is coincident
with the crystal-packing characteristics of the complex Pt-L12
discussed above.
TD-DFT calculations
Time-dependent density functional theory (TD-DFT) calcu-
lations were performed on all complexes Pt-L16, Pt-L12 and Pt-
Fig. 3 UV-visible absorption and emission spectra of Pt-L16 in degassed
CH2Cl2 (≈1.2 × 10−5 mol dm−3) at 298 K and 77 K (λex = 412 nm).
L6. The quantum mechanical computer software program
BHANDHLYP and B3LYP exchange–correlation functional were
used to perform all calculations. The HOMO and LUMO orbi-
tals of Pt-L16 are shown in Fig. 5, and the rest of the selected
molecular orbitals are provided in Fig. S10 (ESI†). The results
clearly indicate that the HOMO has dominant contributions
from the platinum center and the phenoxide groups. The
LUMO is almost localized on the bipyridyl moiety. Moreover,
neither the HOMO nor the LOMO orbital density is localized
on the terminal alkoxy chains, implying that the two peri-
pheral alkoxy chains on phenoxide fragments do not partici-
pate in the composition of the frontier molecular orbitals. The
Fig. 4 UV-visible absorption (left) and emission spectra (right) of Pt-L16
calculated absorption spectra are shown in Fig. S2 (ESI†) and
are generally well consistent with the experimentally measured
spectra of three complexes.
in different air-equilibrated solvents at 298 K (≈10−5 mol dm−3, λex
=
412 nm).
Electrochemical properties
For complex Pt-L16, independent of the applied concen-
tration (≈1.2 × 10−6–10−3 mol dm−3), a unique phosphor-
escence band at about 622 nm was observed in DCM solution
(Fig. S3, ESI†), indicating that the emissions of Pt-L16 in solu-
tion clearly originate from the monomer. This is likely owing
to the considerable distortion from the planarity induced by
the phenoxide fragment, which prevents the intermolecular
interactions necessary for excimer formation.5 Furthermore,
these tetradentate complexes show a rigidochromic blue shift
of 26–48 nm when a solution sample was cooled to 77 K
(Fig. S4, ESI†). We note that unlike most bidentate platinum(II)
complexes with a square planar geometry, these tetradentate
complexes exhibit the rigidochromic effect in the concentrated
phases, such as a solid film and a 5% doped PMMA film
(Fig. S5, ESI†). Comparing their emission peak at about
As was just discussed above, the two peripheral alkoxy chains
could hardly influence the energy gap of the frontier molecular
orbitals. Therefore, here we just take Pt-L16 as a representative
to analyse the electrochemical properties of this kind of
Fig. 5 Time-dependent density functional theory (TD-DFT) calculation
622 nm in solution, the maximum emission bands show of the orbital density for HOMO (left) and LUMO (right) of Pt-L16
.
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Fig. 6 Cyclic voltammograms of Pt-L16 in DCM solution containing
0.1 mol dm−3 Bu4NPF6 at a scan rate of 50 mV s−1
.
Fig. 7 TGA thermograms of platinum complexes (top left) and polar-
ized light microphotographs of the smectic mesophases observed on
cooling from the isotropic phase at 95 °C (top right) for Pt-L16. Bottom
left: Heating and cooling DSC curves of Pt-L16. Bottom right: Variable-
temperature XRD patterns of Pt-L16 obtained from the cooling process
at 140 °C, 95 °C and 25 °C, respectively. Inset shows the magnified XRD
patterns at high-angle regions.
complex. The cyclic voltammogram of Pt-L16 in degassed DCM
is shown in Fig. 6. Pt-L16 shows an irreversible, oxidation peak
with the observed peak potential at 0.52 V and this can be
attributed to the oxidation of Pt(II) to Pt(III). The oxidation of
tetradentate Pt(II) complexes is typically irreversible because of
the rapid solvolysis of the resultant platinum(III) species.14,18
The reduction process of Pt-L16 also exhibited an irreversible
behaviour with the peak potential at −1.52 V, which is
presumably assigned to molecular reduction localized on the
cyclometalated tetradentate ligand.2 As the HOMO and LUMO
energy levels (EHOMO and ELUMO) can be estimated from
that Pt-L16 and Pt-L12 possess monotropic liquid crystal pro-
perties. DSC data of complexes Pt-L16, Pt-L12 and Pt-L6 are
summarized in Table S5 (ESI†). Like most thermotropic
metallomesogens,12,15,20 the melting and clearing points
decrease and the temperature range of the mesophase widens
on increasing the length of the aliphatic chain. The range of
the mesophase temperature of Pt-L16 and Pt-L12 is 57–116 °C
(Fig. 7, bottom left) and 127–140 °C (Fig. S6, ESI†),
respectively.
In order to further confirm the nature of the mesophase of
these platinum(II) complexes, powder X-ray diffraction analysis
(XRD) at variable temperatures was explored both in heating
and in cooling processes. In the heating process, Pt-L16 only
displays crystalline patterns in regions of the phase transition
indicated by the DSC thermogram (Fig. S7, ESI†). When
cooling down from the isotropic state to 95 °C, the XRD pat-
terns of Pt-L16 exhibit sharp inner reflections and diffuse outer
halos (Fig. 7, bottom right). In the low-angle region, the XRD
the Eopx/Erped values based on the empirical equations EHOMO
=
[−(Eopx − 0.14) − 4.8] and ELUMO = [−(Erped − 0.14) − 4.8] eV, in
which Eopx/Erped is the peak potential of chemically irreversible
oxidation/reduction,19 0.14 V is the potential of ferrocene vs.
Ag/Ag+ and 4.8 eV is the energy level of ferrocene versus the
vacuum energy level. Thus the EHOMO/ELUMO values of
−5.17/−3.13 eV for Pt-L16 were obtained. As a result, Pt-L16 dis-
plays an electrochemical band gap of 2.04 eV, which is in
agreement with the optical band gap (2.17 eV) estimated from
the absorption spectra.
Mesomorphic properties
The mesomorphic properties of the complexes Pt-L16, Pt-L12 diffractograms reveal four well-defined peaks (33.45, 16.70,
and Pt-L6 were determined via thermogravimetric analysis 11.12, 8.34 Å) in a 1 : 1/2 : 1/3 : 1/4 d-spacing ratio that can be
(TGA), polarized optical microscopy (POM), differential scan- indexed to the (d10), (d20), (d30), and (d40) reflections of a lamel-
ning calorimetry (DSC) as well as variable-temperature X-ray lar structure. In the high-angle region, the XRD pattern reveals
diffraction (XRD). The TGA profiles reveal that these complexes a diffuse scattering halo centered at 3.80 Å, which could be
are thermally stable close to 350 °C (Fig. 7, top left). Complex attributed to the liquid-like order of the alkoxy chains. As is
Pt-L6 with the shortest alkoxy chain length melts directly into discussed above, it is further proved that Pt-L16 forms a mono-
the isotropic fluid at 190 °C on heating and no liquid crystal- tropic smectic liquid crystal. Pt-L12 reveals similar mesophase
line textures are found in the succeeding cooling process. In properties (Fig. S8 ESI†). The XRD pattern of Pt-L12 at 25 °C
contrast, for Pt-L16 and Pt-L12, the typical thermotropic (before heating) indicated that the crystalline phase is a lamel-
smectic phase with the focal-conic fan textures is observed by lar structure as well, and the d-spacing value of the reflection
POM on slow cooling from the isotropic liquid (Fig. 7, top is consistent with that of the single crystal. Furthermore, for
right). We note that for Pt-L16 and Pt-L12 no liquid crystal tex- Pt-L12, the layer spacing calculated from the XRD patterns in
tures emerge on the repeating heating processes, indicating the mesomorphic phase is 31.8 Å, which is larger than the
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peripheral alkoxy chains on the phenoxide fragments of the
tetradentate ligand were obtained. All complexes are emissive
in degassed DCM solutions and the solid state at room temp-
erature. Furthermore, the new complexes Pt-L16 and Pt-L12
show monotropic smectic liquid crystal characteristics.
Moreover, these liquid crystal complexes can be aligned on a
rubbed nylon-6 glass substrate and produce polarized emis-
sion with a dichroic ratio of 5.1. This kind of complex may be
a promising lead structure for the future development of
luminescent liquid crystal materials based on tetradentate
cyclometalated complexes, and optimization studies to
further improve the photoluminescence quantum yield are in
progress.
Fig. 8 Polarized PL spectra from Pt-L16 on an aligned nylon-6 film at
room temperature (||: parallel emission light; ⊥: perpendicular emission
light; λex = 412 nm).
Experimental section
Measurements and materials
All commercially available starting materials were used directly
without further purification. The solvents were carefully dried
molecule length estimated from the single crystal molecule
structure (L ≈ 24 Å), indicating that the layer spacing in the
smectic phase is formed through the interdigitation arrange-
ment of molten alkoxy chains between molecules by the
dipole–dipole interactions.12,15 Remarkably, layer spacing in
the mesomorphic phase is larger than that in the crystalline
phase, which is ascribed to the stretched structure of the
molten alkoxy chains induced by thermal motion in the meso-
morphic phase at higher temperatures.
1
prior to use. H NMR and 13C NMR spectra were recorded on
Varian INOVA-400 MHz and 100 MHz spectrometers. Mass
spectra were recorded on a Finnigan-LCQDECA Mass spectro-
meter. Elemental analyses were performed with a Finnigan-
LCQDECA microanalyzer. UV-Vis absorption spectra were
recorded at room temperature on a Perkin Elmer Lambda 950
spectrophotometer. Time-dependent density functional theory
(TD-DFT) calculations based on BHANDHLYP and B3LYP were
performed on all three platinum(II) complexes. The photo-
luminescence (PL) spectra at 298 K and 77 K were recorded on
a Horiba Fluorolog-4 spectrophotometer. The samples were
degassed by more than three freeze–pump–thaw cycles prior to
measurement. Quantum efficiency measurements were carried
out at room temperature in a solution of dichloromethane;
[Ru(bpy)3]Cl2 in degassed aqueous solution (excitation wave-
length = 436 nm, ϕlum = 0.042)23 was used as a reference. The
equation Φs = ϕr (Ar/As)(ns/nr)2(Is/Ir) was used to calculate the
quantum yields, where s is the quantum yield of the sample,
r is the quantum yield of the reference, n is the refractive index
of the solvent, As and Ar are the absorbance of the sample and
the reference at the wavelength of excitation and Is and Ir are
the integrated areas of emission bands.24 Phosphorescence
lifetime measurements were performed on a Horiba Fluorolog-4
spectrophotometer. The excitation source is NANOLED 370
(λ = 370 nm) and the photo collection adopts the TCSPC (time-
correlated single photo counts) method.
Polarized emission
The polarized emission based on luminescent metallomesogens
has attracted a great deal of attention from both academic and
industrial research communities.21 Therefore, various methodo-
logies12,22 have been developed to prepare the aligned films
which are, generally speaking, crucial to achieve polarized emis-
sion with a high dichroic ratio. Herein, referring to the reported
approach,22 we prepared the pre-aligned films by spin-coating
nylon-6 onto clean glass substrates, then drying at 110 °C, and
finally rubbing glass substrates uniaxially with a dust-free cloth.
The powder of Pt-L16 was deposited on the surface of the
aligned nylon-6 film, and the film was heated in an isotropic
liquid, then subsequently cooled in the liquid-crystal tempera-
ture region, and finally quenched to room temperature. As a
result, when the polarizer is parallel (||) to the rubbing direc-
tion, the emission intensity is much higher than that when the
polarizer is perpendicular (⊥). The aligned film of Pt-L16 on a
rubbed nylon-6 film exhibits polarized emission at 581 nm
under polarized photo-irradiation with a dichroic ratio (I||/I⊥) of
5.1 (Fig. 8). PL spectra of the aligned sample at different
degrees of polarization are given in Fig. S9 (ESI†).
Electrochemical measurements were carried out on a vol-
tammetry analyzer (Model LK2003 from Tianjin Lanlike
Instrument Co.) in a 0.1 mol dm−3 tetrabutylammonium hexa-
fluorophosphate (Bu4NPF6) DCM solution at a 50 mV s−1 scan
rate under nitrogen protection. Cyclic voltammograms were
measured using a three-compartment three electrode cell.
A Pt wire was used as the counter-electrode and an Ag/AgNO3
(0.1 M in acetonitrile) electrode functioned as the reference
Conclusions
In summary, a new series of room temperature phosphores- electrode. The working electrode was glassy carbon. All of the
cent tetradentate platinum(II) complexes with two attached electrochemical data reported here were measured relative to
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an external ferrocenium/ferrocene reference (Fc+/Fc). TGA m, CH2), 1.48 (4H, m, γ-CH2), 1.80 (4H, m, β-CH2), 3.97 (4H, t,
was carried out on a TA Discovery TGA. Polarized optical J = 6.6 Hz, α-CH2), 6.77 (2H, d, J = 8.1 Hz, pyridyl), 6.93 (4H, d,
microscopy (POM) was performed on an OPLYMUS BX41. J = 9.2 Hz, phenyl), 7.11 (4H, d, J = 9.2 Hz, phenyl), 7.67 (2H, t,
Differential scanning calorimetry (DSC) experiments were per- J = 7.8 Hz, pyridyl), 7.86 (2H, d, J = 7.5 Hz, pyridyl).
formed on a TA Discovery DSC. X-ray diffractions (XRD) were
measured on a Rigaku SmartLab (Cu Kα).
6,6′-Bis(4-(dodecyloxy)phenoxy)-2,2′-bipyridine (L12). It was
synthesized following the preparation procedure of L6. A light-
grey solid of L12 was obtained in 78.8% yield. 1H NMR
(400 MHz, CDCl3) δH = 0.88 (6H, t, J = 6.8 Hz, CH3), 1.32 (32H,
Synthesis and analytical data
Ligands (L), ligand precursors (P) and cyclometalated platinum m, CH2), 1.47 (4H, m, γ-CH2), 1.80 (4H, m, β-CH2), 3.97 (4H, t,
complexes (Pt-Ln) are mentioned in this paper.
J = 6.6 Hz, α-CH2), 6.77 (2H, d, J = 8.1 Hz, pyridyl), 6.93 (4H, d,
4-(Hexyloxy)phenol (P6). 1-Bromohexane (2.00 g, 0.012 mol) J = 9.2 Hz, phenyl), 7.11 (4H, d, J = 8.8 Hz, phenyl), 7.67 (2H, t,
was added to a solution of hydroquinone (1.6 g, 0.015 mol) J = 7.8 Hz, pyridyl), 7.86 (2H, d, J = 7.5 Hz, pyridyl).
and potassium carbonate (5.02 g, 0.036 mol) in EtOH (80 ml).
6,6′-Bis(4-(hexadecyloxy)phenoxy)-2,2′-bipyridine (L16). It was
The solution was stirred for 24 h at 90 °C. After cooling to synthesized following the preparation procedure of L6. A light-
room temperature, the mixture was filtered and washed with grey solid of L16 was obtained in 81.3% yield. 1H NMR
DCM (2 × 20 ml). The filtrate was rotary evaporated to dryness (400 MHz, CDCl3) δH = 0.88 (6H, t, J = 6.4 Hz, CH3), 1.26 (48H,
and the residue was purified by column chromatography on m, CH2), 1.47 (4H, m, γ-CH2), 1.80 (4H, m, β-CH2), 3.97 (4H, t,
silica using petroleum ether/ethyl acetate (5 : 1) as an eluent to J = 6.4 Hz, α-CH2), 6.77 (2H, d, J = 8.1 Hz, pyridyl), 6.93 (4H, d,
1
obtain P6 as a white solid (1.94 g, 82.6%). H NMR (400 MHz, J = 7.2 Hz, phenyl), 7.11 (4H, d, J = 7.1 Hz, phenyl), 7.67 (2H, t,
CDCl3) δH = 0.94 (3H, t, J = 8.9 Hz, CH3), 1.35 (4H, m, CH2), J = 7.8 Hz, pyridyl), 7.867 (2H, d, J = 7.5 Hz, pyridyl).
1.46 (2H, m, γ-CH2), 1.78 (2H, m, β-CH2), 3.93 (2H, t, J =
6.7 Hz, α-CH2), 6.80 (4H, m, phenyl).
Preparation of (C6OphObpy)Pt (Pt-L6). A mixture of 6,6′-bis
(4-(hexyloxy)phenoxy)-2,2′-bipyridine (L6) (0.15 g, 0.277 mmol),
4-(Dodecyloxy)phenol (P12). It was synthesized following the K2PtCl4 (0.12 g, 0.289 mmol), and glacial acetic acid (10 mL)
preparation procedure of P6. A white solid of P12 was obtained was degassed and then refluxed under nitrogen for 2 days.
1
in 80.6% yield. H NMR (400 MHz, CDCl3) δH = 0.88 (3H, t, J = After cooling to room temperature, the solvent was removed
6.8 Hz, CH3), 1.29 (18H, m, CH2), 1.43 (2H, m, γ-CH2), 1.75 under reduced pressure and the residue was purified by
(2H, m, β-CH2), 3.89 (2H, t, J = 6.6 Hz, α-CH2), 6.77 (4H, m, column chromatography on silica gel using DCM/PE (1 : 1.2) as
phenyl).
an eluent to gain Pt-L6 as a yellow solid (0.167 g, 82.3%).
4-(Hexadecyloxy)phenol (P16). It was synthesized following 1H NMR (400 MHz, CDCl3) δH = 0.89 (6H, t, J = 6.4 Hz, CH3),
the preparation procedure of P6. A white solid of P16 was 1.32 (8H, m, CH2), 1.42 (4H, m, γ-CH2), 1.74 (4H, m, β-CH2),
obtained in 82.3% yield. H NMR (400 MHz, CDCl3) δH = 0.88 3.96 (4H, t, J = 6.5 Hz, α-CH2), 6.70 (2H, dd, J = 8.7, 2.9 Hz,
1
(3H, t, J = 6.8 Hz, CH3), 1.29 (24H, m, CH2), 1.43 (2H, m, phenyl), 7.17 (2H, d, J = 8.7 Hz, phenyl), 7.29 (2H, d, J =
γ-CH2), 1.75 (2H, m, β-CH2), 3.89 (2H, t, J = 6.6 Hz, α-CH2), 8.4 Hz, phenyl), 7.45 (2H, d, J = 2.9 Hz, pyridyl), 7.57 (2H, d,
6.77 (4H, m, phenyl).
J = 7.6 Hz, pyridyl), 7.90 (2H, t, J = 7.9 Hz, pyridyl). 13C NMR
6,6′-Bis(4-(hexyloxy)phenoxy)-2,2′-bipyridine (L6). This com- (100 MHz, CDCl3) δ (ppm): 156.0, 155.0, 153.4, 147.4, 138.2,
pound was synthesized according to the preparation procedure 126.5, 125.8, 116.5, 115.8, 115.7, 111.5, 77.3, 77.0, 76.7, 68.4,
described by J. Li.5 Under a nitrogen atmosphere, a dry 31.7, 29.6, 25.9, 22.6, 14.1. MS(ESI+) m/z: 756.2371 for
Schlenk tube was charged with 6,6′-dibromo-2,2′-bipyridine [M + Na]+. Calcd for C34H38N2O4Pt (733.2479): C 55.65, H 5.22,
(0.20 g, 0.64 mmol), 4-(hexyl-oxy)phenol (P6) (0.50 g, N 3.82; found C 55.56, H 5.16, N 3.73.
2.54 mmol), 1-methylimidazole (0.052 g, 0.64 mmol), potas-
Preparation of (C12OphObpy)Pt (Pt-L12). It was synthesized
sium carbonate (0.35 g, 2.54 mmol), and anhydrous toluene following the preparation procedure of Pt-L6. A yellow solid of
(10 mL). The solution was degassed by the method of freeze– Pt-L12 was obtained in 79.5% yield. 1H NMR (400 MHz, CDCl3)
pump–thaw cycles for three cycles. Copper(I) iodide (0.025 g, δH = 0.88 (6H, t, J = 6.8 Hz, CH3), 1.26 (32H, m, CH2), 1.41 (4H,
0.13 mmol) was then added, and the solution was degassed by m, γ-CH2), 1.74 (4H, m, β-CH2), 3.95 (4H, t, J = 6.5 Hz, α-CH2),
the method mentioned above for another 3 times, and the 6.70 (2H, dd, J = 8.7, 3.0 Hz, phenyl), 7.17 (2H, d, J = 8.7 Hz,
vessel was sealed. The mixture was stirred and heated at phenyl), 7.30 (2H, d, J = 8.3 Hz, phenyl), 7.45 (2H, d, J =
120 °C under a nitrogen atmosphere for 2 days. On cooling to 3.0 Hz, pyridyl), 7.58 (2H, d, J = 7.5 Hz, pyridyl), 7.91 (2H, t, J =
room temperature, the mixture was filtered and washed with 8.0 Hz, pyridyl). 13C NMR (100 MHz, CDCl3) δ (ppm): 156.0,
DCM (2 × 15 ml). The filtrate was then poured into 100 ml of 155.0, 153.4, 147.4, 138.2, 126.5, 125.8, 116.4, 115.8, 115.7,
water and was extracted with DCM (3 × 10 ml). The mixed 111.5, 77.3, 77.0, 76.7, 68.4, 31.9, 29.7, 29.7, 29.6, 29.5, 29.4,
organic layer was washed with water and brine, dried with 26.2, 22.7, 14.1. MS(ESI+) m/z: 924.4250 for [M + Na]+. Calcd
magnesium sulfate and filtered. The filtrate was rotary evapo- for C46H62N2O4Pt (901.4357): C 61.25, H 6.93, N 3.11; found
rated to dryness. The residue was then purified by column C 61.16, H 6.85, N 3.04.
chromatography on silica using DCM/PE (4 : 5) as an eluent to
Preparation of (C16OphObpy)Pt (Pt-L16). It was synthesized
gain L6 as a light-grey solid (0.277 g, 80.6%). 1H NMR following the preparation procedure of Pt-L6. A yellow solid of
(400 MHz, CDCl3) δH = 0.92 (6H, t, J = 7.0 Hz, CH3), 1.36 (8H, Pt-L16 was obtained in 81.7% yield. 1H NMR (400 MHz, CDCl3)
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δH = 0.88 (6H, t, J = 6.8 Hz, CH3), 1.25 (56H, m, CH2), 1.41 (4H,
m, γ-CH2), 1.74 (4H, m, β-CH2), 3.95 (4H, t, J = 6.5 Hz, α-CH2),
6.70 (2H, dd, J = 8.7, 3.0 Hz, phenyl), 7.17 (2H, d, J = 8.7 Hz,
phenyl), 7.31 (2H, d, J = 8.3 Hz, phenyl), 7.45 (2H, d, J =
3.0 Hz, pyridyl), 7.60 (2H, d, J = 7.6 Hz, pyridyl), 7.93 (2H, t, J =
8.0 Hz, pyridyl). 13C NMR (100 MHz, CDCl3) δ (ppm): 156.0,
155.0, 153.4, 147.4, 138.2, 126.5, 125.8, 116.4, 115.8, 115.7,
111.5, 77.3, 77.0, 76.7, 68.4, 31.9, 29.7, 29.69, 29.7, 29.5, 29.4,
26.2, 22.7, 14.1. MS(ESI+) m/z: 1036.5503 for [M + Na]+. Calcd
for C54H78N2O4Pt (1013.5609): C 63.94, H 7.75, N 2.76; found
C 63.82, H 7.64, N 2.70.
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We are grateful for financial support from the National Natural
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