Synthesis, photophysics and pyrolytic ceramization of a platinum(II)-containing poly(germylacetylene) polymer

Article abstract of DOI:10.1016/j.jorganchem.2013.06.027

A synthetic route to a new polymeric platinum(II)-containing germylacetylene has been developed. Soluble and thermally stable platinum(II) polyyne polymer trans-[ePt(PBu₃)₂C=CGePh₂C=Ce]n (P1) was prepared in good yield by CuI-catalyzed polymerization of trans-[Pt(PBu3)2Cl2] and Ph2Ge(C=CH)₂. We report the optical absorption and photoluminescence spectra of P1 and the results are compared with platinum(II) polyynes with purely acetylenic and other (hetero)aromatic conjugated units. Harvesting of organic triplet emission harnessed through the strong heavy-atom effect of Pt and Ge was examined. Polymer P1 was found to have a high optical bandgap (ca. 3.84 eV). The influence of metal and sp3-hybridized germyl-based conjugation interrupter on the intersystem crossing rate and the spatial extent of the lowest singlet and triplet excitons were fully elucidated. Our investigations indicate that a high-bandgap polymer P intrinsically gives rise to a very efficient phosphorescence with fast radiative decay. Remarkably, a simple single-step method was also established for the synthesis of platinum egermanium (PtGe) alloy nanoparticles using P1 as the polymer precursor and the resulting ceramic metal alloys have been characterized by powder X-ray diffraction, energy dispersive X-ray analysis and transmission electron microscopy.

Full text of DOI:10.1016/j.jorganchem.2013.06.027

Journal of Organometallic Chemistry 744 (2013) 165e171
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, photophysics and pyrolytic ceramization of a
platinum(II)-containing poly(germylacetylene) polymer
,
a
b
,
,d
Cheuk-Lam Ho a , Suk-Yue Poon , Kun Liu c, Chun-Kin Wong a, Guo-Liang Lu a
**
,
***
Srebri Petrov b, Ian Manners e , Wai-Yeung Wong
,
a,
*
a Institute of Molecular Functional Materials, Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road,
Kowloon Tong, Hong Kong, China
b Department of Chemistry, University of Toronto, Toronto M5S 3H6, Canada
c State Key Laboratory of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin Street, Changchun 130012, China
d Auckland Cancer Society Research Centre, Faculty of Medical and Health Sciences, The University of Auckland, Private Bag 92019, Auckland 1143,
New Zealand
e School of Chemistry, University of Bristol, Bristol BS8 1TS, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
A synthetic route to a new polymeric platinum(II)-containing germylacetylene has been developed.
Soluble and thermally stable platinum(II) polyyne polymer trans-[ePt(PBu3)2C^CGePh2C^Ce]n (P1)
was prepared in good yield by CuI-catalyzed polymerization of trans-[Pt(PBu3)2Cl2] and Ph2Ge(C^CH)2.
We report the optical absorption and photoluminescence spectra of P1 and the results are compared
with platinum(II) polyynes with purely acetylenic and other (hetero)aromatic conjugated units. Har-
vesting of organic triplet emission harnessed through the strong heavy-atom effect of Pt and Ge was
examined. Polymer P1 was found to have a high optical bandgap (ca. 3.84 eV). The influence of metal and
sp3-hybridized germyl-based conjugation interrupter on the intersystem crossing rate and the spatial
extent of the lowest singlet and triplet excitons were fully elucidated. Our investigations indicate that a
high-bandgap polymer P intrinsically gives rise to a very efficient phosphorescence with fast radiative
decay. Remarkably, a simple single-step method was also established for the synthesis of platinum
egermanium (PtGe) alloy nanoparticles using P1 as the polymer precursor and the resulting ceramic
metal alloys have been characterized by powder X-ray diffraction, energy dispersive X-ray analysis and
transmission electron microscopy.
Received 21 April 2013
Received in revised form
15 June 2013
Accepted 20 June 2013
Dedicated to Professor W.A. Herrmann on
the occasion of his 65th birthday
Keywords:
Acetylide
Germanium
Nanoparticles
Phosphorescence
Platinum
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
disorder to the conjugated system [18,19], we envisioned that the
sp3-germanium unit can be a good choice of spacer group in metal
In contrast to the large body of work on luminescent poly-
metallaynes with -electron units [1e5], similar systems having
germylene units in the backbone have been reported, and little
research on rigiderod transition metal -alkynylgermane polymers
has been carried out [6,7]. In light of the findings in some conju-
gated polymers (e.g. poly(fluorenes)) that the light-emitting per-
formance can be improved by the deliberate inclusion of
conjugation-interrupting units [8e17] and introduction of
polyynes. In addition, inclusion of the sp3-Ge linker can limit the
effective conjugation length and trigger the triplet light emission
by taking advantage of the heavy-atom effect of Ge atoms [6,7].
Therefore, the key feature of this study is to evaluate how the metal
center and the germyl moiety would influence the electronic and
optical properties of this class of materials.
On the other hand, metallic nanoparticles (NPs) have attracted
great interest over the years due to their unique optical, electrical,
magnetic and catalytic properties [20]. Properties of the NPs differ
from those of the bulk since the quantum size effects dominate
their unique characteristics [21]. In particular, platinum NPs have
been widely used in many applications, for example, catalysts
[22,23], fuel cells [22e24], electro-optical devices [25] and mag-
netic devices [26], etc. The properties of Pt NPs depend strongly
on the particle size, shape, composition, and structure [27,28].
p
s
* Corresponding author. Tel.: þ852 3411 7074; fax: þ852 3411 7348.
** Corresponding author. Tel.: þ44 117 928 7649; fax: þ44 117 929 0509.
*** Corresponding author.
E-mail addresses: clamho@hkbu.edu.hk (C.-L. Ho), ian.manners@bristol.ac.uk
(I. Manners), rwywong@hkbu.edu.hk (W.-Y. Wong).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.06.027

166
C.-L. Ho et al. / Journal of Organometallic Chemistry 744 (2013) 165e171
Platinum is also known to have high catalytic activity for hydro-
genation and dehydrogenation [29]. However, the very high ac-
tivity of Pt would generate a large amount of undesired methane
during the process of hydrogenolysis [30]. To reduce the activity of
Pt NPs, intermetallic compounds such as Pt3Ge and PtGe have been
used [30,31]. The lower activity of PteGe NPs reduces the genera-
tion of undesired methane for the hydrogenation than Pt NPs but is
associated with their higher selectivity to specific alkenes.
There are several reports on the synthesis of PteGe type NPs
[30e34]. Most of them require an arc melting of the mixture of
pure Pt and Ge granular metals under extreme conditions (high
pressure and high temperature) or vaporization of Ge(acac)2Cl2
(acac ¼ acetylacetonate) on Pt loaded substrate prepared by calci-
nation with Pt(NH3)4Cl2 at high temperature. The Pt/Ge molar ratio
was varied by changing the relative amount of metals or ionic metal
precursors charged into the system. Although PteGe NPs are still
predominantly prepared by physical methods, there is a desire to
prepare the materials by chemical techniques. Chemical routes are
generally simpler and less expensive and have the advantages of
improved stoichiometric control and intimate mixing [35].
In this paper, we report full details of our studies on the syn-
thesis, characterization and photophysical properties of a Pt(II)-
containing poly(germylacetylene) polymer P1. Their optical and
emission properties will be discussed in comparison to those sys-
tems having purely acetylenic and other (hetero)aromatic spacers.
The effect of adding the heavy metal and the germanium residue on
the phosphorescence decay rate of P1 is also investigated.
Furthermore, we have developed a new approach for a ‘one-pot’
synthesis of PtGe NPs by the pyrolysis of P. A study of such ceramic
composites made from P1 was also described.
For the low temperature experiments, samples were mounted in a
closed-cycle cryostat (Oxford CC1104) in which the temperature
can be adjusted from 10 K to 330 K. For lifetime measurements, the
third harmonics, 355 nm line of a Q-switched Nd:YAG laser was
used as the excitation light source. The emission was recorded by
using a PMT and a HP54522A 500 MHz oscilloscope. The molecular
weights of the polymers were determined by GPC (HP 1050 series
HPLC with visible wavelength and fluorescent detectors) using
polystyrene standards and thermal analyses were performed with a
PerkineElmer TGA6 thermal analyzer [2,3].
2.3. Preparation of compounds
2.3.1. Synthesis of L1-TMS
To a chilled solution of Me3SiC^CH (104 mg, 1.05 mmol) in
dried THF (20 ml) at ꢀ78 ꢁC, nBuLi (0.8 ml, 1.28 mmol) was added
dropwisely under N2. The mixture was stirred for 0.5 h at this
temperature and then 0.5 h at room temperature. The resulting
solution was again cooled to ꢀ78 ꢁC and a solution of Ph2GeCl2
(142 mg, 0.4 mmol) was added dropwise over 20 min and the re-
action mixture was stirred for a further 0.5 h before stirring was
continued for another 1 h at room temperature. The volatile com-
ponents were evaporated and the residue was extracted with
CH2Cl2. The filtrate was concentrated and subjected to preparative
TLC isolation on silica plates using n-hexane/CH2Cl2 (1:2, v/v) as the
eluent. From the first band, compound L1-TMS was obtained as a
white solid with 60% yield (121 mg). Spectral data: IR (CH2Cl2):
2038 cmꢀ1
(
n
C^C); 1H NMR (CDCl3):
d
7.71e7.74 (m, 4H, Ar), 7.43e
134.34,
7.46 (m, 6H, Ar), 0.28 (s, 18H, TMS); 13C{1H} NMR (CDCl3):
d
133.70, 129.76, 128.41 (Ar), 116.37, 104.91 (C^C), ꢀ0.16 (TMS); FAB-
MS: m/z ¼ 345 (MePh)þ. Anal. Calc. for C22H28Si2Ge: C, 62.73; H,
6.70. Found: C, 62.55; H, 6.79%.
2. Experimental
2.1. General information
2.3.2. Synthesis of L1
A mixture of compound L1-TMS (121 mg, 0.3 mmol) and K2CO3
(55 mg, 0.4 mmol) in Et2O/MeOH (30 ml 2:1, v/v) was stirred at
room temperature overnight. Infrared spectroscopy showed that all
the starting materials had been consumed. Solvent was removed
under reduced pressure to a leave colorless residue. This residue
was dissolved in the minimum amount of CH2Cl2 and subjected to
TLC on silica using n-hexane/CH2Cl2 (4:1 v/v) as eluent to afford a
major colorless but UVevisible product identified as L1 (68.8 mg,
All reactions were performed under nitrogen. Solvents were
carefully dried and distilled from appropriate drying agents prior to
use. Commercially available reagents were used without further
purification unless otherwise stated. All reagents for the chemical
syntheses were purchased from Aldrich or Acros Organics. The
Pt(II) precursor trans-[PtCl2(PBu3)2] was prepared according to the
literature method [36]. All reactions were monitored by thin-layer
chromatography (TLC) with Merck pre-coated glass plates. Flash
column chromatography and preparative TLC were carried out
using silica gel from Merck (230e400 mesh). Fast atom bombard-
ment (FAB) mass spectra were recorded on a Finnigan MAT SSQ710
system. NMR spectra were measured in CDCl3 on a Varian Inova
400 MHz FT-NMR spectrometer and chemical shifts are quoted
relative to tetramethylsilane for 1H and 13C nuclei and an 85% H3PO4
external standard for 31P nucleus. Infrared spectra were recorded as
CH2Cl2 solutions on the Nicolet Magna 550 Series II FTIR spec-
trometer, using CaF2 cells with a 0.05 mm path length.
87%). Spectral data: IR (CH2Cl2): 2039 cmꢀ1
nC 7.73e7.75 (m, 4H, Ar), 7.43e7.50 (m,
CH); 1H NMR (CDCl3):
6H, Ar), 2.64 (s, 2H, C^CH); 13C{1H} NMR (CDCl3):
133.60, 132.78,
(n
C^C), 3282 cmꢀ1
(
^
d
d
130.18,128.62 (Ar), 82.41 (C^C), 98.67 (C^CH); FAB-MS: m/z ¼ 277
(M)þ. Anal. Calc. for C16H12Ge: C, 69.41; H, 4.37. Found: C, 69.65; H,
4.54%.
2.3.3. Synthesis of P1
CuI (1.2 mg) was added to a mixture of L1 (250 mg, 0.90 mmol)
i
and trans-[PtCl2(PBu3)2] (606 mg, 0.90 mmol) in Pr2NH/CH2Cl2
(30 ml, 1:1, v/v) [2,3]. The pale yellow solution was stirred at room
temperature over a period of 15 h, after which all solvents were
evaporated off. The residue was redissolved in CH2Cl2, and filtered
through a silica column using the same eluent. After removal of
solvent, a pale yellow oily solid was obtained, and it was then
washed with MeOH. Further purification can be accomplished by
precipitating the polymer solution in toluene from MeOH to afford
P1 as a pale yellow solid in 76% yield (600 mg). Spectral data: IR
2.2. Physical measurements
UVevis spectra were obtained on a HP-8453 spectrophotom-
eter. The photoluminescent properties and lifetimes of the com-
pounds were probed on the Photon Technology International (PTI)
Fluorescence Master Series QM1 system. The quantum yields were
determined in CH2Cl2 solutions at 293 K against quinine sulfate in
1.0 N H2SO4 (V ¼ 0.40) as a reference [37]. For solid-state emission
spectral measurements, the 325 nm line of a HeeCd laser was used
as an excitation source. The luminescence spectra were analyzed by
a 0.25 m focal length double monochromator with a Peltier cooled
photomultiplier tube (PMT) and processed with a lock-in-amplifier.
(CH2Cl2): 2026 cmꢀ1 C^C); 1H NMR (CDCl3):
(n d 7.72 (m, 4H, Ar),
7.23 (m, 6H, Ar), 1.99 (m, 12H, PCH2 of Bu), 1.39 (m, 12H, PCH2CH2 of
Bu), 1.26 (m, 12H, CH2CH3 of Bu), 0.80 (m, 18H, CH3 of Bu); 13C{1H}
NMR (CDCl3):
d
139.88, 135.03, 128.19, 126.32 (Ar), 107.27, 93.70
3.86
(C^C), 26.36, 24.17, 23.32, 13.94 (Bu); 31P{1H} NMR (CDCl3):
d

C.-L. Ho et al. / Journal of Organometallic Chemistry 744 (2013) 165e171
167
Table 1
(JPePt ¼ 2391 Hz). Anal. Calc. for (C40H64GeP2Pt)n: C, 54.93; H, 7.38.
Structural and thermal properties of P1.
Polymer Mn Mw Mw/Mn
P1 8600 15900 1.9
Found: C, 55.21; H, 7.60%.
n
Tdecomp(onset) (ꢁC) Weight loss (%)
2.4. X-ray crystallography
10 377 ꢃ 5 36
X-Ray diffraction data for L1-TMS were collected at 293 K using
ꢀ
graphite-monochromated MoeK
a
radiation (
l
¼ 0.71073 A) on a
up and thin layer chromatography can afford the compound L1-
TMS in moderate yield. Then, the TMS-protected germane L1-TMS
was placed under mild desilylation conditions (K2CO3/MeOH) to
remove the TMS group. The resulting mixture was subjected to
another flash short column on silica to give dieth-
ynyldiphenylgermane L1 in 87% yield. Dehydrohalogenating
coupling of trans-[PtCl2(PBu3)2] and L1 was used to produce poly-
mer P1 with a feed mole ratio of 1:1. Purification was effected by
silica column chromatography using CH2Cl2 as the eluent. The Pt(II)
polyyne P1 was obtained as a pale yellow solid in good yield (76%).
The regiochemical structures of the ligand precursors and
polymer P1 were studied by various spectroscopic analyses. A full
account of these data is given in the Experimental section (vide
Bruker Axs SMART 1000 CCD diffractometer. The collected frames
were processed with the software SAINTþ [38] and an absorption
correction (SADABS) [39] was applied to the collected reflections.
The structure was solved by the Direct methods (SHELXTL) [40] in
conjunction with standard difference Fourier techniques and sub-
sequently refined by full-matrix least-squares analyses on F2.
Hydrogen atoms were generated in their idealized positions and all
non-hydrogen atoms were refined anisotropically. Crystal data for
L1-TMS: C22H28Si2Ge, Mw ¼ 421.21, monoclinic, space group P21/c,
¼ 90.799(1)ꢁ,
ꢀ
a ¼ 18.768(2), b ¼ 10.1078(8), c ¼ 12.484(1) A,
b
3
ꢀ
V
m
¼
2368.0(3) A ,
Z
¼
4, rcalcd
¼
1.181 mg mꢀ3
,
(MoKa) ¼ 1.397 mmꢀ1, F(000) ¼ 880. 13198 reflections measured,
of which 5272 were unique (Rint ¼ 0.0193). Final R1 ¼ 0.0273 and
infra). All the compounds exhibited one characteristic
vibration (ca. 2026e2039 cmꢀ1).
characteristic absorption
(^CH) at 3282 cmꢀ1 was shown for L1 which disappeared after
the incorporation of Pt(II) fragment in P1, confirming the formation
of PteC bond in the polymer. The (C^C) stretching frequency for
n(C^C) IR
wR2 ¼ 0.0767 for 4480 observed reflections with I > 2
s(I).
A
n
2.5. Nanoparticle synthesis and characterization
s
n
Pyrolysis of P1 was performed on a SDT Q600 TGA under ni-
trogen at a flow rate of 100 mL minꢀ1 (99.999% prepurified from
Matheson). Powder X-ray diffraction (PXRD) analysis was per-
P1 is lower than that for the free germyl ligand precursor L1, in line
with a higher degree of conjugation in the former [44,45]. The
single 31P-{1H} NMR signal flanked by platinum satellites for P1 is
consistent with a trans geometry of the Pt(PBu3)2 unit in such
square-planar geometry. The 1JPePt value is 2391 Hz, typical of those
for related trans-PtP2 systems [46e49]. 1H NMR analyses clearly
demonstrate a well-defined structure for each compound. In all
cases, 1H NMR resonances stemming from the protons of the
organic moieties were observed. The 13C NMR spectral data for the
ligand precursors and polymer give precise information about the
regiochemical structure of the main chain skeleton and show a high
degree of structural regularity in P1.
formed through a Siemens D5000 X-ray diffractometer using Ni-
ꢀ
filtered Cu K
a
radiation (
l
¼ 1.54178 A). Transmission electron
microscopy (TEM) image was obtained with a Hatachi HD-2000
microscope operating at 200 kV. The sample was prepared by
placing the P1 ceramic powder onto the TEM Cu grid covered by a
thin layer of carbon film. Energy dispersive X-ray (EDX) analysis
was performed on the P1 ceramic powder with an operation area of
ca. 10
operating at 200 kV.
m
m ꢂ 10
mm using the same Hatachi HD-2000 microscope
3. Results and discussion
3.2. Structural and thermal properties
3.1. Synthetic methodologies and chemical characterization
The molecular weight of the polymer P1 was determined in THF
by GPC (Table 1). The Mw value indicates a high degree of poly-
merization and suggests a macromolecular nature for P1. Index of
polydispersity (Mw/Mn) of 1.9 has been calculated, which is a
common value for polycondensates. It should be noted that GPC
does not give absolute values of molecular weights but provides a
measure of hydrodynamic volume. Rod-like polymers in solution
possess different hydrodynamic properties than flexible polymers
The chemical structures and synthetic procedures of the
new diethynyldiphenylgermane ligand L1 and its Pt(II) polyyne
polymer 1 are shown in Scheme 1. When trimethylsilylacetylene
(Me3C^CH) was treated with 1 equiv. of nBuLi followed by
0.5 equiv. of dichlorodiphenylgermane (Ph2GeCl2) at ꢀ78 ꢁC, a
silylated bis-acetylene germane L1-TMS was formed [41e43]. Work
Ph
Ge
Ph
i) nBuLi, THF, 78 oC
−
K2CO3
MeOH
Me3SiC CH
Me3Si
Ge
SiMe3
ii) Ph2GeCl2
Ph
Ph
L1-TMS
L1
trans-[PtCl2(PBu3)2]
CuI, iPr2NH, CH2Cl2
PBu3
Pt
Ph
Ge
Ph
n
PBu3
P1
Scheme 1. Synthesis of diethynyldiphenylgermane ligand L1 and its platinum(II) polyyne P1.

168
C.-L. Ho et al. / Journal of Organometallic Chemistry 744 (2013) 165e171
be ascribed to the elimination of one PBu3 group and one phenyl
ring from the polymer.
Colorless crystals of L1-TMS suitable for X-ray diffraction studies
were grown by slow evaporation of its solution in n-hexane/CH2Cl2
at room temperature and the molecular structure is depicted in
Fig. 1. There are two C^C units in the molecule, and the bond
lengths are typical of other known C^C triple bond [54]. Based on
the bond angle of GeeC(sp)eC(sp), the acetylenic units in the
molecule adopt a linear geometry. The selected bond lengths and
bond angles are listed in Table 2.
3.3. Photophysical and electrochemical characterization
Table 3 contains the photophysical data of P1. The absorption
spectrum of P1 was taken at room temperature in CH2Cl2 solution
and as thin film. Polymer P1 shows the absorption peaks in the near
UV region due to
pep* transitions of the bridging ligand. With
reference to the value at 271 nm for L1, we find that the position of
the lowest energy absorption band of P1 is red-shifted after the
inclusion of Pt fragment. This reveals that the presence of Pt does
Fig. 1. A perspective drawing of the X-ray crystal structure of L1-TMS.
not disrupt the p-conjugation. In energy terms, the energy gap (Eg)
of P1 as measured from the onset absorption wavelength in the
solid film state is 3.84 eV and this high Eg value indicates the
ineffective electronic communication along the polymer chain with
Ph2Ge group as the bridging ligand relative to other conjugated
aromatic spacers (P2eP6 in Fig. 2) [55]. The energy difference in Eg
can be as large as over 2 eV between P1 and P6. From Fig. 2, we note
that the degree of electronic conjugation decreases in the order:
P6 > P5 > P4 > P3 > P2 > P1, in which the energy of the S1 singlet
state is the highest for the germanium-bridged derivative among
them.
Table 2
ꢁ
ꢀ
Selected bond lengths (A) and angles ( ) for compound L1-TMS.
C(13)eC(14)
Ge(1)eC(1)
Ge(1)eC(13)
Ge(1)eC(13)eC(14)
Si(1)eC(14)eC(13)
C(13)eGe(1)eC(18)
C(7)eGe(1)eC(13)
C(1)eGe(1)eC(13)
1.190(2)
1.942(2)
1.913(2)
177.8(2)
177.9(2)
107.42(7)
112.27(7)
107.16(7)
C(18)eC(19)
Ge(1)eC(7)
Ge(1)eC(18)
Ge(1)eC(18)eC(19)
Si(2)eC(19)eC(18)
C(7)eGe(1)eC(18)
C(1)eGe(1)eC(18)
C(1)eGe(1)eC(7)
1.198(2)
1.938(2)
1.909(2)
176.3(2)
177.2(2)
106.65(7)
110.95(7)
112.33(7)
The thin film photoluminescence (PL) spectrum of P1 was
measured at various temperatures. Two characteristic emission
bands were observed at 11 K. The higher energy emission with a
relatively low intensity near 400 nm is due to fluorescence from the
singlet excited state (S1 / S0), which is in line with the small Stokes
shift observed between the strongest band in the absorption and
the emission spectra. The lower-lying emission beyond 500 nm is
attributed to that of a triplet excited state (spin-forbidden phos-
phorescence T1 / S0), accompanied by the substantial Stokes shift
observed. In addition, its emission does not change in dilute solu-
tions, which excludes an excimer origin. Vibronic structure with
most weight in the 0e0 vibrational peak can be seen in the triplet
emission at low temperatures and the spectral shape is similar to
the phosphorescence in related compounds [56]. The assignment
can also be rationalized in terms of the observed temperature
dependence of the emission data, in accordance with earlier work
on platinum complexes [57]. The temperature dependence of the
PL spectrum for P1 is displayed in Fig. 3. The S1eT1 energy gap (ca.
0.75 eV) for P1 is found to lie within the constant range of
0.7 ꢃ 0.l eV for similar conjugated metal polyynes [58]. We attri-
bute such a constant S1eT1 energy gap to the exchange energy and
[50]. In view of this, calibration of the GPC with polystyrene stan-
dards is likely to inflate the values of the molecular weights of the
polyyne to some extent. However, the lack of discernable reso-
nances that could be attributed to end groups in the NMR spectra
provides support for the view that there is a moderate extent of
polymerization in P1. We also found that P1 can cast tough, free-
standing thin film of good quality from appropriate solvents for
optical characterization.
The thermal properties of P1 were examined by thermogravi-
metric analysis (TGA). Analysis of the TGA trace (heating rate of
20 ꢁC minꢀ1) for P1 shows that it has a decomposition temperature
in excess of 370 ꢁC and thus exhibits excellent thermal stability. The
onset decomposition temperature of P1 appears higher than those
in trans-[ePt(PBu3)2C^CRC^Ce]n (R ¼ phenylene, thienylene,
pyridyl, substituted fluorenylene) [51e53]. Introduction of GePh2
unit into the arylacetylene segment has the added effect of
enhancing the thermal stability of platinum(II) polyyne. We
observe a sharp weight loss for P1 and the decomposition step can
Table 3
Absorption and emission data for the platinum(II) polymer P1.
Compound
lmax (nm)a
CH2Cl2
Eg (eV)b
lem (nm)c
Film
CH2Cl2d (290 K)
Film (11 K)
Frozen CH2Cl2 (77 K)e
P1
260, 277, 311
259, 276, 310
3.84
368*, 428 (0.15, 1.32)
411, 431, 548, 595*
415, 535 (14.3), 572*, 585*
a
Extinction coefficient (104 Mꢀ1 cmꢀ1) is shown in parentheses.
Estimated from the onset wavelength of the solid-state optical absorption.
Asterisks indicate emission peaks appear as shoulders or weak bands.
Fluorescence quantum yield (%) and lifetime (ns) shown in parentheses (VF,
b
c
d
e
sF) are measured in CH2Cl2 relative to quinine sulfate in 1.0 N H2SO4.
Phosphorescence lifetime sP (ms) at 77 K for the peak maximum is shown in parentheses.

C.-L. Ho et al. / Journal of Organometallic Chemistry 744 (2013) 165e171
169
Table 4
5
4
3
2
3.84
The measured phosphorescence quantum yield, lifetime, and radiative and non-
radiative decay rates of P1 and P3 at 20 K.
Polymer
VP (%)
sP
(
m
s)
(kr)P (sꢀ1
)
(knr) )
P (sꢀ1
P1
P3
53.7
30.0
40.0
51
1.34 ꢂ 104
1.16 ꢂ 104
6 ꢂ 103
1.4 ꢂ 104
2.25
P5
2.98
P3
3.00
P2
P1
2.80
P4
polymers and complexes may be quantitatively described by the
energy gap law [58]. The results of P, VP, (kr)P and (knr)P for P1 and
1.77
P6
s
its parent polymer with 1,4-phenylene ring (P3) are listed in
Table 4. Polymer P1 has a high FP of 53.7% and this may be caused
by the internal heavy-atom effect of both the Ge and Pt atoms. The
(kr)p values at 20 K is 1.34 ꢂ 104 sꢀ1 for P1. Thus, it is desirable to
design polymer with a high energy triplet (and concurrently high
optical gap) to avoid competition with nonradiative decay [22].
Relative to P3, replacement of p-phenylene ring by the germylene
component can increase (kr)P by about 1 order of magnitude in P1.
For P1, we are able to get comparable orders of magnitude for (kr)P
and (knr)P. For phosphorescence in aromatic hydrocarbons, (kr)P lies
typically between 0.1 and 1 sꢀ1 [63]. So, heavy-atom derivatization
using Pt and Ge atoms together with conjugation interruption
by the latter can greatly boost (kr)P values by ca. 5 orders of
magnitude. Therefore, the use of conjugation-interrupting sp3-Ge
unit in our metal polyyne is effective in limiting the effective
conjugation length and gives rise large (kr)P value for more efficient
phosphorescence.
Decreasing extent of conjugation
spacer
Fig. 2. Variation of the optical bandgap energy with the nature of spacer in Pt(II)
polyynes.
possibly some additional constant contribution due to the admix-
ture of the platinum orbitals [59].
In contrast to the singlet emission, the triplet emission peak is
strongly temperature dependent. The prominent phosphorescence
band of P1 was observed at 548 nm at 11 K (Table 3 and Fig. 3). From
290 to 11 K, the intensity of the triplet emission increases by a factor
of w30 and such an increase in intensity indicates a triplet exciton
that is quenched by thermally activated diffusion to dissociation
sites at room temperature. For the low-lying emission, the long
emission lifetime in the microsecond range is suggestive of its
triplet parentage as well, and it can be assigned to the 3(pp*) state
of the organic chromophore.
Since the intersystem crossing (ISC) efficiency (VISC) can be
roughly taken to be unity for third row metal chromophores [60e
62], the radiative (kr)P and nonradiative (knr P
related to the measured lifetime of triplet emission (
phosphorescence quantum yield (VP) by the following equations:
3.4. Pyrolysis of polymer P1 and analysis of PtGe nanoparticles
We have previously reported that the pyrolysis of Pt(II) polyyne
containing a pendant ferrocenyl ligand can afford ceramics con-
taining FePt alloy nanoparticles [64,65]. By pyrolyzing P1, PteGe
nanostructures can be expected. A sample of P1 was heated to
600 ꢁC at a heating rate of 10 ꢁC minꢀ1 and the temperature was
held constant for 5 h under N2 atmosphere. The pale yellow powder
of P1 was transformed into a black shiny metallic fine powder.
)
decay rates are
P) and the
s
Fig. 4 shows the PXRD pattern of P1 composite after pyrolysis
using Ni-filtered Cu K
ꢀ
a
radiation (
l
¼ 1.54178 A). The PXRD pattern
ðknrÞP ¼ ð1 ꢀ FPÞ=sP
of the pyrolytic ceramics of P1 shows a PtGe crystalline phase with
an orthorhombic, Pbnm structure and the lattice parameters of
a ¼ 6.095 A b ¼ 5.698 A and c ¼ 3.626 A. No other phases were
detected. The sample may contain some amorphous material as
well. The Scherrer equation was employed to calculate the domain
ðkrÞP
¼
=
FP sP
ꢀ
ꢀ
ꢀ
A number of literature reports have shown that the nonradiative
decay of triplet states in a series of Pt-containing conjugated
0.05
0.04
11 K
0.03
0.02
0.01
0.00
77 K
150 K
200 K
290 K
400
500
600
700
Wavelength (nm)
Fig. 3. Temperature dependence of the PL spectrum of P1.
Fig. 4. PXRD patterns of (a) P1 composite after pyrolysis and (b) simulated PtGe NPs.

170
C.-L. Ho et al. / Journal of Organometallic Chemistry 744 (2013) 165e171
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