Preparation, characterization, and photophysical properties of cis-or trans-PtLn2 (Ln = Nd, Eu, Yb) arrays with 5-ethynyl-2,2′- bipyridine

Article abstract of DOI:10.1021/om8005883

Reaction of cis- or trans-Pt(PPh₃)₂Cl₂ (PPh₃ = triphenylphosphine) with 5-[2-(trimethylsilyl)-1-ethynyl]-2, 2′-bipyridine (bpyC≡CSiMe₃) gave cis-Pt(PPh ₃)₂(C≡Cbpy)₂ (1

Full text of DOI:10.1021/om8005883

Organometallics 2008, 27, 5665–5671
5665
Preparation, Characterization, and Photophysical Properties of cis-
or trans-PtLn2 (Ln ) Nd, Eu, Yb) Arrays with
5-Ethynyl-2,2′-bipyridine
Hai-Bing Xu,† Jun Ni,† Kun-Jiao Chen,† Li-Yi Zhang,† and Zhong-Ning Chen*,†,‡
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter,
Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China, and State Key
Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, Shanghai 200032, People’s Republic of China
ReceiVed June 25, 2008
Reaction of cis- or trans-Pt(PPh3)2Cl2 (PPh3 ) triphenylphosphine) with 5-[2-(trimethylsilyl)-1-ethynyl]-
2,2′-bipyridine (bpyCt CSiMe3) gave cis-Pt(PPh3)2(Ct Cbpy)2 (1) or trans-Pt(PPh3)2(Ct bpy)2 (5).
Incorporating 1 or 5 with Ln(hfac)3(H2O)2 (hfac ) hexafluoroacetylacetonate) induced formation of the
corresponding cis- or trans-PtLn2 heterotrinuclear complexes cis-Pt(PPh3)2{(Ct Cbpy)Ln(hfac)3}2 (Ln
) Nd 2, Eu 3, Yb 4) or geometrical isomers trans-Pt(PPh3)2{(Ct Cbpy)Ln(hfac)3}2 (Ln ) Nd 6, Eu 7,
Yb 8). As verified through 31P NMR characterization as well as determination of the structures of 1, 4,
and 8 by X-ray crystallography, the cis- or trans-arranged forms around PtII centers remain unchanged
in both solutions and solid states without thermal or light-induced isomerization during the reactions.
With excitation at 360 < λex < 450 nm, which is the absorption region of metal-perturbed πfπ* (Ct C)
transitions and dπ(Pt)fπ*(Ct Cbpy) MLCT transition, sensitized lanthanide luminescence occurs with
a microsecond range of lifetimes in both cis- and trans-arranged PtLn2 complexes, revealing that efficient
PtfLn energy transfer is indeed operating from the platinum(II) acetylide chromophore to lanthanide
centers.
metal ions, whereas the “hard” polypyridyl N donors are
frequently chelated to lanthanide centers. This affords a feasible
approach for the design of d-f heterometallic arrays containing
both transition metal and lanthanide ions,17-23 where effective
energy transfer usually occurs from d-block organometallic
chromophores to lanthanide centers, thus inducing sensitized
lanthanide luminescence by excitation of d-block organometallic
chromophores in the near-UV-vis region. As a typical bifunc-
tional alkynyl ligand, 5-ethnyyl-2,2′-bipyridine is feasible for
the design of heterometallic and/or multicomponent arrays
containing a prefabricated platinum(II) bis(σ-acetylide) module
with square-planar geometry, in which subsequently introducing
d-block metal or lanthanide subunits through 2,2′-chelation
Introduction
The chemistry of heterometallic and/or multicomponent
alkynyl complexes has recently received more and more
attention in view of their extensive applications as nonlinear
optical materials, organic light emitting diodes (OLED), lumi-
nescence sensors, molecular devices, etc.1-5 Among various
classes of organic ligands, polypyridyl-functionalized alkynyl
ligands are particularly useful for the design of heterometallic
and/or multicomponent arrays as optoelectronic materials at the
molecular level.6,7 Their bifunctional character makes them
favorable bridging ligands capable of connecting with two types
of organometallic chromophores or emitters through both
metal-alkynyl coordination and metal-polypyridyl chelation.8-16
The “soft” ethynyl C donors are usually σ-bonded to d-block
* To whom correspondence should be addressed. E-mail: czn@
fjirsm.ac.cn.
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Onaka, S. Chem. Commun. 2002, 590.
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Chem. Soc., Dalton Trans. 1995, 4067.
† Fujian Institute of Research on the Structure of Matter.
‡ Shanghai Institute of Organic Chemistry.
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10.1021/om8005883 CCC: $40.75
2008 American Chemical Society
Publication on Web 10/02/2008

5666 Organometallics, Vol. 27, No. 21, 2008
Xu et al.
provides an excellent approach to access a series of multicom-
ponent complexes with desired spatial orientations.7-9,18,21
Experimental Section
General Procedures. All manipulations were performed under
a dry argon atmosphere using Schlenk techniques and a vacuum-
line system. The solvents were dried, distilled, and degassed prior
to use except that those for spectroscopic measurements were of
spectroscopic grade. The reagents potassium tetrachloroplatinum
(K2[PtCl4]), 2,2′-bipyridine (bpy), triphenylphosphine (PPh3), and
hexafluoroacetylacetone (Hhfac) were commercially available. 5-[2-
(Trimethylsilyl)-1-ethynyl]-2,2′-bipyridine (bpyCt CSiMe3),35 trans-
Platinum(II) acetylide complexes have been extensively
investigated owing to their multifold applications in various
optical materials,6,7,24 single-molecule insulators,25,26 molecular
electronics, etc.27 The platinum(II) centers usually exhibit
square-planar geometry with bis(σ-acetylide) oriented in cis or
trans form. Taking judicious advantage of a prefabricated cis
or trans arrangement, stereochemical structures can be elabo-
rately envisaged.28-34 These cis- or trans-platinum(II) bis(σ-
acetylide) species are kinetically stable in solutions without
thermal or light-induced isomerization, thus facilitating exact
37
Pt(PPh3)2Cl2,36 and cis-Pt(PPh3)2Cl2 were prepared by literature
procedures.
Ln(hfac)3(H2O)2 (Ln ) Nd, Eu, Yb) were prepared by modifica-
tion of the literature procedures38 as follows. To an aqueous solution
of lanthanide(III) acetate (pH ) 5-7) was added dropwise 3.3 equiv
of hfac with stirring at room temperature for 3 h. The precipitate
was filtered, washed with water, and dried under vacuum to afford
the quantitative product.
positioning prefabricated modules into ordered arrays.14,15,28
A
series of Pt-Ln heteronuclear species with polypyridyl alkynyl
ligands have been prepared in our laboratory using cis- or trans-
arranged square-planar platinum(II) bis(acetylde) complexes as
precursors to incorporate Ln(hfac)3 units,21-23 in which sensi-
tized lanthanide(III) luminescence is successfully achieved by
effective PtfLn energy transfer from a platinum(II) bis(acetyl-
ide) antenna chromophore. Since the photons collected by the
central platinum(II) site can be quantitatively transferred to the
peripheral lanthanide(III) emitters, the central platinum(II) bis(σ-
acetylide) chromophore serves as an efficient near-UV light
harvesting antenna. In order to explore the influence of
geometric orientations in the central platinum(II) bis-acetylide
antenna chromophore on PtfLn energy transfer and subse-
quently sensitized lanthanide(III) luminescence in Pt-Ln com-
plexes, we are interested in the construction of a series of
cis- and trans-PtLn2 heterotrinuclear isomers with 5-ethynyl-
2,2′-bipyridine. We describe herein the use of cis- and trans-
platinum(II) bis-σ-acetylide isomers as prefabricated modules
for the preparation of a series of cis- and trans-PtLn2 heterot-
rinuclear isomeric complexes by incorporating Ln(hfac)3 (Ln
) Nd, Eu, Yb; hafc ) hexafluoroacetylacetonate) units through
2,2′-bipyridyl chelation. For both cis- and trans-PtLn2 isomers,
sensitized lanthanide luminescence is indeed achieved with
excitation at 360 nm < λex < 450 nm, which is the absorption
region of platinum(II) bis-acetylide chromophores, revealing
unambiguously that substantial PtfLn energy transfer occurs
from Pt-based organometallic antenna chromophores in these
cis- and trans-PtLn2 arrays.
cis-Pt(PPh3)2(Ct Cbpy)2 (1). To a THF (30 mL) solution of
bpyCt CSiMe3 (160 mg, 0.64 mmol) and cis-PtCl2(PPh3)2 (221 mg,
0.28 mmol) were added a solution of CuI (5 mg) in acetonitrile (2
mL) and a methanol (5 mL) solution of KF (58 mg, 1.0 mmol)
with stirring at room temperature for five days. The solution was
concentrated by rotary evaporation to give the crude product, which
was purified by silica gel column chromatography. Elution with
dichloromethane-methanol (v/v ) 100:2) gave a pale yellow
product. Yield: 66% (200 mg). Anal. Calcd for C60H44N4P2Pt: C,
66.85; H, 4.11; N, 5.20. Found: C, 66.43; H, 4.18; N, 5.34. ESI-
MS (CH3OH-CH2Cl2): m/z (%) 1079 (100) ([M + H]+), 816 (5)
([M - PPh3 + H]+). IR (KBr, cm-1): 2115s (Ct C). 1H NMR
(CDCl3, ppm): 8.61 (s, 2H, bpyCt C), 8.22 (d, 2H, J ) 7.92 Hz,
bpyCt C) 7.93 (d, 2H, J ) 8.25 Hz, bpyCt C), 7.81 (d, 14H, J )
5.58 Hz, C6H5 and bpyCt C), 7.72 (d, 2H, J ) 7.8 Hz, bpyCt C),
7.65 (s, 2H, bpyCt C), 7.4 (d, 18H, J ) 7.56 Hz, C6H5 and
bpyCt C), 6.61 (d, 2H, J ) 8.25 Hz, bpyCt C). 31P NMR (CDCl3,
ppm): 21.5 (t, JPt-P ) 2648 Hz).
cis-Pt(PPh3)2{(Ct Cbpy)Ln(hfac)3}2 (Ln ) Nd, Eu, Yb). These
PtLn2 complexes were prepared by addition of 2.2 equiv of
Ln(hfac)3(H2O)2 to a dichloromethane solution of 1 with stirring
for 1 h. After filtering, the concentrated dichloromethane solutions
were layered with hexane to afford the products as pale yellow
crystals. Yield: 68-75%.
2 (Ln ) Nd). Anal. Calcd for C90H50Nd2F36N4O12P2Pt: C, 41.44;
H, 1.93; N, 2.15. Found: C, 41.54; H, 2.05; N, 2.21. IR (KBr,
cm-1): 2120m (Ct C), 1651s (CdO).
3 (Ln ) Eu). Anal. Calcd for C90H50Eu2F36N4O12P2Pt: C, 41.19;
H, 1.92; N, 2.13. Found: C, 41.44; H, 1.85; N, 2.11. IR (KBr,
cm-1): 2120m (Ct C), 1651s (CdO).
4 (Ln ) Yb). Anal. Calcd for C90H50F36N4O12P2PtYb2: C, 40.54;
H, 1.89; N, 2.10. Found: C, 40.94; H, 1.87; N, 2.01. IR (KBr,
cm-1): 2120m (Ct C), 1651s (CdO).
trans-Pt(PPh3)2(Ct Cbpy)2 (5). This compound was prepared
by the same procedure as that of 1 except using trans-Pt(PPh3)2Cl2
instead of cis-Pt(PPh3)2Cl2. The product was purified by silica gel
column chromatography using dichloromethane as an eluent.
Recrystallization of the product by slow diffusion of hexane into a
dichloromethane solution afforded 5 as pale yellow crystals. Yield:
79%. Anal. Calcd for C60H44N4P2Pt2: C, 66.85; H, 4.11; N, 5.20.
Found: C, 66.63; H, 4.08; N, 5.14. ESI-MS (CH3OH-CH2Cl2):
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cis- or trans-PtLn2 (Ln ) Nd, Eu, Yb) Arrays
Organometallics, Vol. 27, No. 21, 2008 5667
Table 1. Crystallographic Data of 1, 4 · CH2Cl2 · H2O, and 8
1
4 · CH2Cl2 · H2O
8
empirical formula
fw
space group
a, Å
b, Å
C60H44N4P2Pt
1078.02
P1
C91H54Cl2F36N4O13P2PtYb2
2769.39
P21/n
20.594(12)
18.854(10)
C45H25F18N2O6PPt0.5Yb
1333.23
P21/n
11.779(4)
27.718(9)
j
11.1503(2)
15.0906(1)
15.5381(3)
74.182(6)
83.504(7)
72.694(4)
2400.17(7)
2
1.492
3.034
0.71073
293(2)
0.0344
c, Å
27.594(17)
15.891(5)
R, deg
ꢀ, deg
γ, deg
V, Å3
Z
92.108(5)
94.628(4)
10707(11)
4
1.718
5171(3)
4
F
calc, g/cm-3
1.712
3.295
0.71073
293(2)
0.0653
0.1105
1.149
µ, mm-1
3.236
radiation (λ), Å
temp, K
0.71073
293(2)
0.0679
0.1811
1.079
R1(Fo)a
wR2(Fo
2 b
)
0.0741
1.041
GOF
2
2 2
a R1 ) ∑|Fo - Fc|/∑Fo. b wR2 ) ∑[w(Fo - Fc ) ]/∑[w(Fo2)]1/2
.
m/z (%) 1079 (100) ([M + H]+), 898 (16) ([M - bpyCt C]+),
816 (10) ([M - PPh3 + H]+), 719 (30) ([M - 2bpyCt C + H]+).
were calculated by Φs
)
Φr(Br/Bs)(ns/nr)2(Ds/Dr) using
[Ru(bpy)3](PF6)2 in acetonitrile as the standard (Φem ) 0.062),
where the subscripts r and s denote reference standard and the
sample solutions, respectively, and n, D, and Φ are the refractive
index of the solvents, the integrated intensity, and the luminescence
quantum yield, respectively.39 The quantity B is calculated by B )
1 - 10-AL, where A is the absorbance at the excitation wavelength
and L is the optical path length. All the solutions used for
determination of emission lifetimes and quantum yields were
prepared under vacuum in a 10 cm3 round-bottom flask equipped
with a sidearm 1 cm fluorescence cuvette and sealed from the
atmosphere by a quick-release Teflon stopper. Solutions used for
luminescence determination were prepared after rigorous removal
of oxygen by three successive freeze-pump-thaw cycles.
Crystal Structural Determination. Single crystals of 1,
4 · CH2Cl2 · H2O, and 8 suitable for X-ray diffraction were grown
by layering n-hexane onto the corresponding dichloromethane
solutions. Crystals coated with epoxy resin or sealed in capillaries
with mother liquors were measured on a SIEMENS SMART CCD
diffractometer by the ω scan technique at room temperature using
graphite-monochromated Mo KR radiation (λ ) 0.71073 Å). Lp
corrections were carried out in the reflection reduction process. The
structures were solved by direct methods, and the heavy atoms
were located from an E-map. The remaining non-hydrogen atoms
were determined from the successive difference Fourier syntheses.
The non-hydrogen atoms were refined anisotropically except for
the F atoms in 4 and 8, and the hydrogen atoms were generated
geometrically with isotropic thermal parameters. The structures were
refined on F2 by full-matrix least-squares methods using the
SHELXTL-97 program package.40 For 4 and 8, the refinements
were carried out by fixing the C-F distances (1.32 ( 0.01 Å) with
the occupancy factors of F1-F36 and F1′-F36′ being 0.50,
respectively. Crystallographic data of 1, 4 · CH2Cl2 · H2O, and 8 are
summarized in Table 1.
1
IR (KBr, cm-1): 2109s (Ct C). H NMR (CDCl3, ppm): 8.61 (s,
2H, bpyCt C), 8.22 (d, 2H, J ) 7.05 Hz, bpyCt C), 7.94 (d, 2H,
J ) 7.56 Hz, bpyCt C), 7.81 (d, 14H, J ) 5.82 Hz, bpyCt C and
C6H5), 7.72 (d, 2H, J ) 7.6 Hz, bpyCt C), 7.66 (s, 2H, bpyCt C),
7.40 (d, 18H, J ) 7.38 Hz, C6H5 and bpyCt C), 6.62 (d, 2H, J )
8.07 Hz, bpyCt C). 31P NMR (CDCl3, ppm,): 18.1 (t, JPt-P ) 2606
Hz).
trans-Pt(PPh3)2{(Ct Cbpy)Ln(hfac)3}2 (Ln ) Nd, Eu, Yb).
These trans-PtLn2 complexes were prepared by the same synthetic
procedure as that of cis-PtLn2 except using trans-Pt(PPh3)2-
{(Ct Cbpy) (5) instead of cis-Pt(PPh3)2{(Ct Cbpy) (1). Yield:
62-78%.
6 (Ln ) Nd). Anal. Calcd for C90H50Nd2F36N4O12P2Pt: C, 41.44;
H, 1.93; N, 2.15. Found: C, 41.34; H, 2.15; N, 2.01. IR (KBr,
cm-1): 2111m (Ct C), 1654s (CdO).
7 (Ln ) Eu). Anal. Calcd for C90H50Eu2F36N4O12P2Pt: C, 41.19;
H, 1.92; N, 2.13. Found: C, 41.64; H, 1.95; N, 2.21. IR (KBr,
cm-1): 2108m (Ct C), 1651s (CdO).
8 (Ln ) Yb). Anal. Calcd for C90H50Yb2F36N4O12P2Pt: C, 40.54;
H, 1.89; N, 2.10. Found: C, 40.54. H, 1.81; N, 2.07. IR (KBr, cm-1):
2110m (Ct C), 1654s (CdO).
Physical Measurements. Elemental analyses (C, H, N) were
carried out on a Perkin-Elmer model 240C elemental analyzer.
Electrospray ion mass spectra (ESI-MS) were performed on a
Finnigan LCQ mass spectrometer using dichloromethane-methanol
mixture as mobile phases. UV-vis absorption spectra were
measured on a Perkin-Elmer Lambda 25 UV-vis spectrophotom-
eter. Infrared (IR) spectra were recorded on a Magna750 FT-IR
spectrophotometer with a KBr pellet. Emission and excitation
spectra in the UV-vis region were recorded on a Perkin-Elmer
LS 55 luminescence spectrometer with a red-sensitive photomul-
tiplier type R928. Emission lifetimes in solid states and degassed
solutions were determined on an Edinburgh Analytical Instrument
(F900 fluorescence spectrometer) using a LED laser at 397 nm
excitation, and the resulting emission was detected by a thermo-
electrically cooled Hamamatsu R3809 photomultiplier tube. The
instrument response function at the excitation wavelength was
deconvolved from the luminescence decay. Near-infrared (NIR)
emission spectra were measured on an Edinburgh FLS920 fluo-
rescence spectrometer equipped with a Hamamatsu R5509-72
supercooled photomultiplier tube at 193 K and a TM300 emission
monochromator with NIR grating blazed at 1000 nm. The NIR
emission spectra were corrected via a calibration curve supplied
with the instrument. The emission quantum yields (Φ) of 1, 3, 5,
and 7 in degassed dichloromethane solutions at room temperature
Results and Discussion
Syntheses and Characterization. Synthetic routes to 1-8
are summarized in Scheme 1. The cis- (1) and trans-arranged
(5) mononuclear platinum(II) bis(σ-acetylide) complexes
Pt(PPh3)2(Ct Cbpy)2 were synthesized by reaction of 2.3 equiv
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(b) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991. (c) Chan,
S. C.; Chan, M. C. W.; Wang, Y.; Che, C. M.; Cheung, K. K.; Zhu, N.
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(40) Sheldrick, G. M. SHELXS-97 and SHELXL-97; University of
Go¨ttingen: Germany, 1997.

5668 Organometallics, Vol. 27, No. 21, 2008
Scheme 1. Synthetic Routes to the PtLn2 Heterotrinuclear Complexes 1-8
Xu et al.
of bpyCt CSiMe3 with cis-Pt(PPh3)2Cl2 and trans-Pt(PPh3)2Cl2,
respectively, catalyzed by cuprous iodide via fluoride-promoted
desilylation in the presence of potassium fluoride. Both the cis
and trans isomers were isolated as pale yellow products, which
were purified by silica gel column chromatography using
dichloromethane-methanol (v/v ) 100: 2) and dichloromethane
as the eluents, respectively. The cis- or trans-oriented PtLn2
heterotrinuclear arrays were prepared by reactions of the
corresponding cis- or trans-Pt(PPh3)2(Ct Cbpy)2 with 2.2 equiv
of Ln(hfac)3(H2O)2 in dichloromethane solutions, respectively.
Crystallization by layering n-hexane onto the corresponding
dichloromethane solutions afforded PtLn2 complexes as pale
yellow crystals or microcrystals. Both 1 (cis) and 5 (trans) are
insoluble in common solvents and sparingly soluble in dichlo-
romethane and chloroform, but the latter (trans) is more soluble
than the former (cis). Upon formation of the PtLn2 complexes,
however, the cis-PtLn2 species is much more soluble in
dichloromethane and chloroform than the corresponding trans
isomers. It is noteworthy that the solubility of both cis- and
trans-PtLn2 complexes improves significantly compared with
that of the corresponding precursor complexes 1 (cis) and 5
(trans). As revealed by X-ray crystallography and 31P NMR
spectral characterization (vide infra), both cis and trans isomers
are kinetically stable in solutions without thermal- or light-
induced isomerization during the reactions and spectroscopic
measurements. This favors exact positioning of the prefabricated
modules into the desired cis- and trans-PtLn2 arrays.
observation of a strong band at ca. 1650 cm-1 ascribed to
ν(CdO) vibrational frequency of Ln(hfac)3 units. In the 31P
NMR spectra of mononuclear platinum(II) bis(σ-acetylide)
complexes, the cis complex 1 showed a set of satellite peaks
centered at 21.5 ppm with JPt-P ) 1324 Hz, whereas the
corresponding signals for trans species 5 occurred at 18.1 ppm
with JPt-P ) 1303 Hz.
The structures of cis-Pt(PPh3)2(C&#x2bc;Cbpy) complex 1,
cis-PtYb2 species 4, and its trans isomer 8 were determined by
single-crystal X-ray diffraction. Selected atomic distances and
angles for 1, 4, and 8 are presented in Table 1. ORTEP drawings
of 1, 4, and 8 are depicted in Figures 1, 2, and 3, respectively.
1, 4, and 8 are all characteristic of platinum(II) square-planar
structures composed of C2P2 donors from σ-bonded bis(acetyl-
ide) and two P donors from PPh3. The Pt-P and Pt-C distances
are comparable to those of previously described
Pt(PPh3)2(Ct CR)2 (R ) alkyl or aryl) complexes, and the C-C
lengths of acetylides are in the range 1.17-1.21 Å,28-34
suggesting a typical character of a Ct C bond. The Pt-acetylide
σ-coordination is quasi-linear, as indicated by the angles of
C2-C1-Pt ) 171.7(3)° and C22-C21-Pt ) 179.3(4)° for 1,
1-8 were characterized by elemental analysis (C, H, and N),
ESI-MS spectrometry, IR, 1H and 31P NMR spectroscopy, and
X-ray crystallography for 1, 4, and 8. The microanalyses
indicated that the measured C, H, and N contents coincide well
with the calculated values. In the IR spectra of mononuclear
platinum(II) bis(σ-acetylide) complexes, a strong ν(Ct C)
frequency is observed at 2115 cm-1 for 1 (cis) and 2109 cm-1
for 5 (trans). Upon formation of PtLn2 arrays by incorporating
Ln(hfac)3 moieties, vibrational frequency due to the acetylide
occurs at ca. 2120 cm-1 for cis-PtLn2 complexes and ca. 2110
cm-1 for the corresponding trans isomers, together with
Figure 1. ORTEP drawing of 1 with atom-labeling scheme showing
30% thermal ellipsoids. Phenyl rings on the phosphorus atoms are
omitted for clarity.

cis- or trans-PtLn2 (Ln ) Nd, Eu, Yb) Arrays
Organometallics, Vol. 27, No. 21, 2008 5669
Table 2. Selected Interatomic Distances (Å) and Bond Angles (deg)
for 1, 4 · CH2Cl2 · H2O, and 8
1
4 · CH2Cl2 · H2O
8
Pt-C1
2.002 (3)
2.020 (4)
2.3279 (9)
2.3143 (9)
1.982 (10)
2.003 (8)
2.327 (3)
2.311 (2)
2.465 (7)
2.292 (8)
1.179 (13)
1.196 (12)
174.6 (9)
175.1 (8)
85.3 (3)
171.5 (2)
87.9 (2)
87.6 (3)
172.9 (3)
99.03 (9)
73.1 (3)
1.993 (8)
Pt-C21
Pt-P1
2.305 (2)
Pt-P2
Yb1-N1
2.459 (7)
2.304 (7)
1.207 (10)
Yb1-O1
C1-C2
1.182 (5)
1.179 (5)
171.7 (3)
179.3 (4)
88.80 (14)
170.94 (10)
86.65 (10)
86.14 (10)
174.46 (10)
98.78 (3)
C21-C22
C2-C1-Pt
C22-C21-Pt
C1-Pt-C21
C1-Pt-P1
C1-Pt-P2
C21-Pt-P1
C21-Pt-P2
P1-Pt-P2
O1-Yb1-O2
O1-Yb1-N1
175.4 (8)
90.5 (2)
72.4 (3)
115.5 (3)
72.7 (3)
shorter in cis-PtYb2 complex 4 (11.29 Å) than that in trans-
isomer 8 (16.94 Å). For 4 (cis-PtYb2) and 8 (trans-PtYb2), the
2,2′-bipyridyls in neighboring molecules exhibit parallel ar-
rangements with bpy · · · bpy distances in the range 3.4-3.6 Å
(Figures S1 and S2, Supporting Information). This implies that
intermolecular π · · · π stacking is likely operative between the
2,2′-bipyridyls in 4 and 8.
Figure 2. ORTEP drawing of 4 with atom-labeling scheme showing
30% thermal ellipsoids. Phenyl rings on the phosphorus atoms and
the F atoms on the trifluoromethyl are omitted for clarity.
UV-Vis Absorption Properties. UV-vis absorption and
luminescence data of 1-8 are summarized in Table 2. The
electronic absorption spectra of 1 and 5 (Figure S3, Supporting
Information) in dichloromethane solutions display intense
absorption bands at ca. 308 nm and strong broad bands at ca.
370 nm. The former with high energy is most likely ligand-
centered absorptions, whereas the latter with low energy arises
from metal-perturbed πfπ* (Ct C) transitions in Ct Cbpy,
mixed probably with some d(Pt)fπ*(Ct Cbpy) MLCT char-
acter.23,41,42
Upon formation of the PtLn2 arrays by combining 1 or 5 with
Ln(hfac)3 units through 2,2′-bipyridyl chelation, apart from
occurrence of a new intense absorption at ca. 295 nm from the
Ln(hfac)3 moiety, the low-energy absorption bands due to metal-
perturbed πfπ* (Ct C) and/or MLCT transitions are red-shifted
ca. 30 nm to lower energy (Figures S4 and S5, Supporting
Information). The 2,2′-bipyridyl chelating to Ln(hfac)3 would
lower the π* orbital energy in the Ct Cbpy and reduce the
energy gap between HOMO (d) and LUMO (π*), thus inducing
an obvious red-shift of the metal-perturbed πfπ* (Ct C) and/
or MLCT absorption. As shown in Figure 4, titration of 1 or 5
(Figure S6, Supporting Information) with Yb(hfac)3(H2O)2 in
dichloromethane induced the MLCT maximum shifts from ca.
370 to 400 nm when 2 equiv of Yb(hfac)3(H2O)2 was added.
Luminescence Properties. Upon irradiation at 320 nm <
λex < 420 nm, 1 and 5 emit low-energy luminescence at
490-560 nm in both solid state (Figure S7, Supporting
Information) and dichloromethane solutions (Figure 5) with the
lifetimes in the microsecond range at 298 K, indicating a triplet
state parentage. As indicated in Figure 5, a high-energy
fluorescent emission also occurs at ca. 440 nm in the dichlo-
romethane solutions of 1 or 5 with the lifetime < 10 ns at 298
K, originating most likely from an intraligand singlet excited
Figure 3. ORTEP drawing of 8 with atom-labeling scheme showing
30% thermal ellipsoids. Phenyl rings on the phosphorus atoms and
the F atoms on the trifluoromethyl are omitted for clarity.
C2-C1-Pt ) 174.6(9)° and C22-C21-Pt ) 175.1(8)° for 4,
and C2-C1-Pt ) 175.4(8)° for 8.
For 1, 2,2′-bipyridiyl-5-acetylide is bound to the platinum(II)
center through Pt-acetylide bonding, whereas the 2,2′-bipyridyl
lacks coordination. Two cis-arranged Ct Cbpy ligands are
oriented to form a dihedral angle of 79.4° in 1. Upon formation
of cis-PtYb2 complex 4 (Figure 2), the dihedral angle formed
by two Ct Cbpy ligands becomes 81.1° as a result of incorpo-
rating Yb(hfac)3 moieties through 2,2′-bipyridyl chelation. In
contrast, two trans-arranged Ct Cbpy ligands in trans-PtYb2
complex 8 (Figure 3) are coplanar. Because of steric require-
ments, the two Yb(hafc)3 units in both 4 (cis) and 8 (trans) are
oriented in opposite directions. The YbIII centers are eight-
coordinated with N2O6 donors to form a distorted square
antiprism in both 4 and 8. The Pt · · · Yb separations through
bridging 2,2′-bipyridiyl-5-acetylides are 8.17 and 8.67 Å in 4
and 8.47 Å in 8. The Yb · · · Yb distance, however, is much
(41) Li, Q.-S.; Xu, F.-B.; Cui, D.-J.; Yu, K.; Zeng, X.-S.; Leng, X.-B.;
Song, H.-B.; Zhang, Z.-Z. Dalton Trans. 2003, 1551.
(42) Saha, R.; Qaium, M. A.; Debnath, D.; Younus, M.; Chawdhury,
N.; Sultana, N.; Kociok-Kohn, G.; Ooi, L.; Raithby, P. R.; Kijima, M. Dalton
Trans. 2005, 2760.

5670 Organometallics, Vol. 27, No. 21, 2008
Xu et al.
Figure 4. Titration of 1 with Yb(hfac)3(H2O)2 in aerated dichlo-
romethane solution, showing a red shift of the MLCT absorption
band from the PtII acetylide chromophore.
Figure 6. Emission spectra of 1 (dashes), 2 (dots), 3 (solid), and 4
(dash-dot) in dichloromethane solutions at 298 K.
4
4
4
4
and 1335 nm due to F3/2 f I9/2, I11/2, I13/2 transitions, four
5
for PtEu2 complexes at 595, 614, 651, and 696 nm due to D0
7
7
7
7
f F1, F2, F3, and F4, and one for PtYb2 complexes at ca.
2
2
980 nm due to a F5/2 f F7/2 transition. In contrast, the low-
3
3
energy ILCT and/or MLCT emission from the PtII alkynyl
chromophore in the visible region disappeared entirely, indicat-
ing that energy transfer from the Pt-based chromophore to
lanthanide centers is quite complete and effective. As indicated
in Table 3, lanthanide(III) luminescence including lifetimes and
quantum yields in cis-PtLn2 complexes is comparable to that
in the corresponding trans-PtLn2 isomers. Consequently, it
appears that the geometrical orientation in the central platinu-
m(II) bis(σ-acetylide) antenna chromophore exerts insignificant
influence on the PtfLn energy transfer as well as sensitized
lanthanide(III) luminescence in these cis- and trans-PtLn2
isomeric complexes.
Figure 5. Emission spectra of 1 (dashes), 5 (solid line), and
bpyCt CSiMe3 (dots) in degassed dichloromethane at 298 K.
For both cis- and trans-PtLn2 complexes, the high-energy
fluorescence due to intraligand singlet excited states, however,
is not completely quenched at 420-460 nm (Table 3). Titration
of 1 or 5 with Eu(hfac)3(H2O)2 (Figure S9, Supporting Informa-
tion) in aerated dichloromethane induced rapid attenuation of
the Pt(PPh3)2(acetylide)2-based emission so as to complete
quenching of the phosphorescence in the visible region upon
addition of 2 equiv of Eu(hfac)3(H2O)2. The high-energy
fluorescence from the ligand-centered singlet state is partially
quenched but still observed even if excess Eu(hfac)3(H2O)2 is
added to 1 or 5, suggesting lower efficiency in transferring
singlet state energy to the lanthanide centers, as found in a series
of cis-PtLn2 and Pt2Ln4 complexes containing the extended
Pt(P-P)(Ct CPhtpy)2 antenna chromophores (P-P ) η2-
chelating diphosphine, HCt CPhtpy ) 4′-(4-ethynylphenyl)-
2,2′:6′,2′′-terpyridine).22,23
state in 5-ethynyl-2,2′-bipyridine. Vibronic-structured emission
bands with vibrational progressional spacings of 1200-1400
cm-1 are observed, which is typical for the ν(Ct C) and ν(Ct N)
aromatic vibrational modes for the 2,2′-bipyridyl alkynyl ligands.
The appearance of vibronic progressions suggests the involve-
ment of the 2,2′-bipyridyl alkynyl ligand in the excited states.
The room-temperature phosphorescence of 1 and 5 is thus
ascribed to the extended and conjugated chromophore of the
2,2′-bipyridyl alkynyl ligand as well as the strong spin-orbital
coupling induced by PtII coordination, involved probably in
some 3MLCT character.43-45 Consequently, the low-energy
emissions in both 1 and 5 originate probably from an admixture
of intraligand [πfπ*(Ct Cbpy)] 3ILCT and [dπ(Pt)fπ*
3
(Ct Cbpy)] MLCT triplet excited states.23,41-45 It is worthy
to note that the quantum yield of the low-energy emission in
cis complex 1 (Φem ) 0.57%) is ca. 3.5 times that in trans
complex 5 (Φem ) 0.16%) in degassed dichloromethane
solutions.
Conclusions
Upon excitation at 360 nm < λex < 450 nm, which is the
platinum(II) bis(acetylide) chromophore-based ILCT/MLCT
absorption region, both cis- and trans-PtLn2 complexes exhibit
luminescence that is characteristic of the corresponding lan-
thanide ions. The emissive lifetimes (Table 2) of lanthanide
luminescence are in microsecond ranges in both the solid state
and dichloromethane solutions at room temperature. As depicted
in Figure 6 and Figure S8 (Supporting Information), three
emission bands occur for PtNd2 complexes at ca. 870, 1060,
cis- and trans-Pt(PPh3)2(Ct Cbpy)2 were prepared by reaction
of cis- and trans-Pt(PPh3)2Cl2 with bpyCt CSiMe3, respectively.
Combining cis- or trans-Pt(PPh3)2(Ct Cbpy)2 with Ln(hfac)3
induced isolation of the corresponding cis- or trans-PtLn2
isomeric complexes. These cis- and trans-oriented isomers are
kinetically stable to resist thermal and photoinduced isomer-
ization. Both cis- and trans-Pt(PPh3)2(Ct Cbpy)2 exhibit low-
3
energy phosphorescence from [πfπ*(Ct Cbpy)] ILCT and
dπ(Pt)fπ*(Ct Cbpy) 3MLCT triplet excited states together with
high-energy fluorescence from a ligand-centered singlet state.
Upon irradiation of cis- or trans-PtLn2 complexes at the
absorption region of a Pt-based chromophore, sensitized lan-
thanide luminescence is successfully attained by effective
(43) Rogers, J. E.; Cooper, T. M.; Fleitz, P. A.; Glass, D. J.; McLean,
D. G. J. Phys. Chem. A 2002, 106, 10108.
(44) Haskins-Glusac, K.; Ghiviriga, I.; Abboud, K. A.; Schanze, K. S.
J. Phys. Chem. B 2004, 108, 4969.

cis- or trans-PtLn2 (Ln ) Nd, Eu, Yb) Arrays
Organometallics, Vol. 27, No. 21, 2008 5671
Table 3. Absorption and Emission Data of 1-8 at 298 K
compound
medium
λ
abs/nm (ꢀ/M-1 cm-1
)
λ
em/nm (τem/µs)a at 298 K
Φem × 102b,c
1
solid
CH2Cl2
522 (3.3) 561(sh)
454 (<0.01)
560 (0.1)
460 (<0.01)
1061 (weak)
423 (<0.01)
1061 (weak)
440 (<0.01)
613 (69.5)
420 (<0.01)
613 (109.5)
462 (<0.01)
980 (13.6)
428 (<0.01)
980 (12.6)
454 (<0.01)
518(1.6)
450 (<0.01)
558 (0.7)
444 (<0.01)
1060 (weak)
421 (<0.01)
1060 (weak)
440 (<0.01)
613 (103.8)
415 (<0.01)
613 (216.3)
448 (<0.01)
980 (13.8)
229(54 940), 308(23 700), 369(51 300)
228(49 270), 297(69 600), 371(41 290)
228 (48 390), 303(59 770), 375(36 600)
228 (47 990), 297 (68 300), 371(40 500)
229(71 900), 308(34 240), 370(78 770)
229(68 860), 305(61 140), 372(62 100)
230(68 290), 303(60 840), 375(23 180)
228(48 020), 293(67 040), 384(26 870)
3.8
0.57
2
3
4
5
6
7
8
solid
CH2Cl2
solid
CH2Cl2
solid
1.4
CH2Cl2
solid
0.63
CH2Cl2
solid
4.3
0.16
CH2Cl2
solid
CH2Cl2
solid
1.0
CH2Cl2
448 (<0.01)
980 (12.7)
0.64
a The excitation wavelength in the lifetime measurement is 397 nm. b The quantum yields of 1, 3, 5, and 7 in degassed dichloromethane were
determined relative to that of Ru(bpy)3(PF6)2 (Φ ) 0.062) in degassed acetonitrile. c The quantum yields of Yb complexes in dichloromethane solutions
are estimated by the equation ΦLn ) τobs/τ0, in which τobs is the observed emission lifetime and τ0 is the radiative or “natural” lifetime with τ0 ) 2 ms
for YbIII 5,8b,19,22
These values refer to the lanthanide-based emission process only and take no account of the efficiency of intersystem crossing and
.
energy transfer processes.
PtfLn energy transfer from the Pt(PPh3)2(Ct Cbpy)2 antenna
chromophore across bridging 5-ethynyl-2,2′-bipyridine with the
Pt · · · Ln distance being 8.2-8.7 Å. While low-energy phos-
phorescence from the Pt-based chromophore is entirely quenched
because of quite effective PtfLn energy transfer, high-energy
fluorescence from the ligand-centered singlet state still occurs
in these PtLn2 complexes. The PtfLn energy transfer as well
as sensitized lanthanide(III) luminescence is inappreciably
influenced by the geometrical orientation of the central plati-
num(II) bis(σ-acetylide) antenna chromophore in these cis- and
trans-PtLn2 isomeric complexes.
of Fujian Province (grants 2008I0027 and 2006F3131), and
the fund from the Chinese Academy of Sciences (grant
KJCX2-YW-H01).
Supporting Information Available: Figures giving additional
UV-vis absorption and emission spectra, and X-ray crystal-
lographic files in CIF format for the structure determination of
compounds 1, 4 · CH2Cl2 · H2O, and 8. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM8005883
Acknowledgment. This work was financially supported
by the NSFC (grants 20521101, 20625101, and 20773128),
the 973 project (grant 2007CB815304) from MSTC, the NSF
(45) Pomestchenko, I. E.; Castellano, F. N. J. Phys. Chem. A 2004, 108,
3485.