Cyclometalated platinum(II) complex with C^N^N tridentate ligand as sensitizer for lanthanide luminescence

Article abstract of DOI:10.1016/j.inoche.2009.06.004

The preparation, characterization and photophysical properties of heterobinuclear complexes {Pt(C^N^N)(C{triple bond, long}Cbpy)}Ln(hfac)₃ (C^N^N = 2-(6-(naphthalen-3-yl)-4-phenylpyridin-2-yl)pyridine; HC{triple bond, long}Cbpy = 5-ethynyl-2,2′

Full text of DOI:10.1016/j.inoche.2009.06.004

Inorganic Chemistry Communications 12 (2009) 744–746
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Cyclometalated platinum(II) complex with C^N^N tridentate ligand as sensitizer
for lanthanide luminescence
*
*
Kun-Jiao Chen, Hai-Bing Xu , Li-Yi Zhang, 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, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The preparation, characterization and photophysical properties of heterobinuclear complexes
{Pt(C^N^N)(C„Cbpy)}Ln(hfac)3 (C^N^N = 2-(6-(naphthalen-3-yl)-4-phenylpyridin-2-yl)pyridine; HC„
Cbpy = 5-ethynyl-2,20-bipyridine; Ln = Nd, Eu, Yb; hfac = hexafluoroacetylacetonate) are described. With
excitation at 390 6 kex 6 500 nm which is the MLCT/LLCT absorption region of the Pt(C^N^N)(C„Cbpy)
chromophore, lanthanide luminescence is successfully attained by Pt ? Ln energy transfer from the plat-
inum(II) antenna chromophore to the lanthanide center across the bridging C„Cbpy ligand.
Ó 2009 Elsevier B.V. All rights reserved.
Received 29 April 2009
Accepted 8 June 2009
Available online 10 June 2009
Keywords:
Energy transfer
Lanthanide
Luminescence
Platinum
Sensitizer
Lanthanide luminescence is currently attracting considerable
interests in many fields due to their extensive applications [1–3].
In order to overcome the weak intensities of the f–f electronic tran-
sitions for the lanthanide(III) complexes, energy transfer from tran-
sition metal based 3MLCT excited states of the light-harvesting
antenna chromophores to f–f levels of the lanthanide ions have
been widely applied to achieve long-lived lanthanide emission
[4–8]. Current efforts are focused to further optimize the energy
transfer process in d–f bimetallic complexes and extend the excita-
tion wavelength throughout the visible and even into the NIR re-
gion [9–12].
Nd 2, Eu 3, Yb 4; hfac = hexafluoroacetylacetonate), achieving suc-
cessfully sensitization of long-lived NIR lanthanide luminescence.
Synthetic routes to 1–4 are summarized in Scheme 1.
(C^N^N)PtCl was prepared by refluxing K2PtCl4 and C^N^N ligand
in glacial acetic acid, and the product washed with water, acetone
and ether. Compound 1 was synthesized by reaction of an excess of
bpyC„CSiMe3 with (C^N^N)PtCl, catalyzed by cuprous iodide via
fluoride-promoted desilylation in the presence of potassium fluo-
ride. Purification by silica gel column chromatography using
dichloromethane-methanol (v/v = 100:2) as eluent gave the pure
product 1. The Pt–Ln heterobinuclear complexes were isolated as
orange microcrystals by mixing 1 and Ln(hfac)3(H2O)2 in CH2Cl2,
followed by recrystallization in CH2Cl2/n-C6H6. These complexes
Utilizing bpyC„CH (5-ethynyl-2,20-bipyridine) as a conjugated
bridging ligand, a family of Pt–Ln multicomponent and heteronu-
clear complexes have been described, where platinum(II) ion is
bound to the acetylide via
unit is chelated by the 2,20-bipyridyl [8]. Because of the excellent
-conjugation and strong -donor character, tridentate C^N^N
r-coordination whereas lanthanide sub-
p
r
[C^N^N = 2-(6-(naphthalen-3-yl)-4-phenylpyridin-2-yl)pyridine]
ligand shows a strong preference for a planar geometry upon com-
plexation with the platinum(II) ion. The neutral Pt(C^N^N)(C„
Cbpy) complex (1) exhibits intense luminescence in visible to NIR
spectral region, making it as a promising organometallic sensitizer
for achieving efficient lanthanide (NIR) luminescence. We report
herein the use of 1 as a precursor for construction of neutral Pt–Ln
heterobinuclear complexes {Pt(C^N^N)(C„Cbpy)}Ln(hfac)3 (Ln =
N
N
N
N
N
N
Pt
Pt
Pt
Cl
O
O
O
N
N
O
O
N
N
O
Ln
= F3CC(O)CHC(O)CF3
O
O
1
Ln = Nd 2, Eu 3, Yb 4
* Corresponding authors. Fax: +86 591 8379 2346.
E-mail addresses: haibingxu@fjirsm.ac.cn (H.-B. Xu), czn@fjirsm.ac.cn (Z.-N.
Chen).
Scheme 1.
1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.06.004

K.-J. Chen et al. / Inorganic Chemistry Communications 12 (2009) 744–746
745
were all characterized by elemental analyses, IR spectra, and ESI-
MS spectrometry, and by X-ray crystallography for 3.
A view of 3 is depicted in the Fig. 1. The platinum(II) center
exhibits a distorted square-planar environment built by a r-coor-
dinated C donor from the acetylide and CN2 donors from the tri-
dentate C^N^N ligand. The Pt atom and C2N2 donors are almost
coplanar with a torsion angle of ca. 12.8° between the phenyl ring
and this plane. The Pt–N and Pt–C distances (2.0–2.1 Å) are compa-
rable to those of analogous platinum(II) complexes in the literature
[13]. The Pt–C„C–C array is quasi-linear with C37–C38–
Pt = 177.64° and C37–C38–C35 = 178.76°. The EuIII center is
eight-coordinated with six oxygen atoms from three hfac and
two N atoms from bpy. The Ptꢀ ꢀ ꢀEu separation through the bridging
C„Cbpy is ca. 8.5 Å [8].
0.3
0.2
0.1
0.0
300
400
500
λ (nm)
UV–vis absorption spectrum of 1 in dichloromethane solution
(Fig. 2) displays intense bands at 290–370 nm, assignable to the
intraligand transitions since similar absorptions are found in the
free RC^N^NH ligand [13]. The low-energy absorption at 390–
Fig. 2. UV–vis is absorption spectra of 1 (solid), and 4 (dot) in dichloromethane
solutions.
500 nm arises most likely from Pt-based MLCT transitions, mixed
probably with some character from p(C„Cbpy) ? p (C^N^N) LLCT
*
quenched, but does not vanish completely. This was further veri-
fied by titration of 1 with Yb(hfac)3(H2O)2 in dichloromethane,
where rapid attenuation of the Pt-based emission was detected
when 1 equiv. of Yb(hfac)3(H2O)2 was added (fig. s3, Supplemen-
tary material).
(ligand-to-ligand charge-transfer) state [13]. Upon formation of
the Pt–Ln complexes, apart form the occurrence of a new absorp-
tion band at ca 300 nm from hfac [14,15], the low-energy band
due to MLCT/LLCT states showed ca. 10 nm blue shift. Titration of
1 with Yb(hfac)3(H2O)2 in dichloromethane solution induced the
MLCT/LLCT maximum shift from ca. 453 nm to 445 nm when
1 equiv of Yb(hfac)3(H2O)2 was added (fig. s1, Supplementary
material).
Photophysical data of 1–4 are summarized in Table 1. The emis-
sive quantum yield in degassed dichloromethane at 298 K is 0.17
for 1. Upon excitation at 390–500 nm, which is the absorption re-
gion of Pt-based antenna chromophore, the Pt–Ln (2–4) complexes
exhibit characteristic line-like emission from the corresponding
LnIII ions in both solid states and dichloromethane solutions
(Fig. 3), demonstrating unambiguously that sensitized lanthanide
luminescence is indeed achieved by Pt ? Ln energy transfer from
the Pt-based 3MLCT excited triplet state. As the precursor Ln(haf-
c)3(H2O)2 is non-emissive at excitation kex > 350 nm [15], the lan-
thanide emission in Pt–Ln complexes should be sensitized by
energy transfer from PtII-based light-harvesting chromophores
with near-UV irradiation at 390 nm < kex < 500 nm. On the other
hand, the Pt-based emission from 3[MLCT] triplet state is mostly
For Pt–Nd species 2, the residual PtII chromophore-based emis-
sion is too weak so that the lifetime could not be detected (<10 ns),
suggesting that Pt ? Nd energy transfer rates is very fast
(>108 sꢁ1). For Pt–Eu species 3, severely spectral overlapping
(Fig. 3) between unquenched Pt-based 3MLCT emission and Eu-
centered emission excluded the possibility to estimate the radia-
tive lifetime sr, the Eu-centered luminescence quantum yield UEu,
and the efficiency and rate of the Pt ? Eu energy transfer. For
Pt–Yb complex 4, the Pt ? Yb energy transfer rates kET from the
Pt(C^N^N)(acetylide) chromophore to the YbIII centers can be esti-
mated by the equation kET = 1/
s
ꢁ1/
s0, where s (47 ns) is the life-
time of residual Pt-based emission, and s0 (251 ns) is the lifetime
in the reference Pt–Gd complex which lacks energy transfer from
the PtII-based antenna triplet states. The energy transfer rates
(kET) can thus be calculated as kET = 1/sPtYbꢁ1/
s
PtGd = 1.7 ꢂ 107 sꢁ1
for PtYb (4) species.
As the NdIII complex displays transitions at 880 and 1055 nm, it
has three ff levels lying within 15,400–20,000 cmꢁ1 (650–500 nm)
Fig. 1. ORTEP drawing of 3 with atom labeling scheme showing 30% thermal ellipsoids. The F atoms on the trifluoromethyl are omitted for clarity.

746
K.-J. Chen et al. / Inorganic Chemistry Communications 12 (2009) 744–746
Table 1
Luminescence data for compounds 1–4 at 298 K.
Compound
1
Medium
kabs/nm (
e
/Mꢁ1 cmꢁ1
)
kem/nm (sem/l
s)a at 298 K
Solid
CH2Cl2
Solid
CH2Cl2
Solid
CH2Cl2
Solid
746 (0.34)
584 (0.28)
282 (8200), 305 (8700), 388 (2400), 453 (1600)
298 (23,600), 370 (9000), 382 (8300), 445 (5200)
298 (23,600), 370 (8700), 382 (8300), 445 (5000)
295 (26,500), 370 (10,000), 382 (9600), 445 (5800)
2
3
4
1061 (weak)
1061 (weak)
613 (449.9)
613 (172.7)
980 (12.4)
980 (13)
CH2Cl2
a
In degassed dichloromethane at 298 K.
Appendix A. Supplementary material
CCDC 729706 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.inoche.2009.06.004.
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*
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This work was financially supported by the NSFC (20625101,
20773128 and 20821061), the 973 Project (2007CB815304)
from MSTC, and NSF of Fujian Province (2008I0027 and
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