Synthesis, structures and optical spectroscopy of photoluminescent platinum-linked poly(silylacetylenes)

Article abstract of DOI:10.1039/b207575j

Synthetic routes to a series of novel oligomeric and polymeric platinum(II)-containing silylacetylenes have been developed. The CuI-catalyzed reaction of trans-[Pt(PEt₃)₂PhCl] with the alkynylsilane Ph₂Si(C≡CH)₂, in a basic medium, afforded the mononuclear trans-[Ph(Et₃P)₂PtC≡CSiPh₂C≡CH] (1) and the dinuclear trans[Ph(Et₃P)₂PtC≡CSiPh₂C≡CPt(PEt ₃)₂Ph] (2). Complex 1 can be utilized as a key starting material for access to the triplatinum(II) system trans-[Ph(Et₃P)₂PtC≡CSiPh₂C≡CPt(PBu ₃)₂C≡CSiPh₂C≡Pt(PEt₃) ₂Ph] (3). Attempted oxidative coupling of 1 only led to the isolation of the triacetylenic species trans-[Ph(Et₃P)₂PtC≡CC≡CC≡CPt(PEt₃ )₂Ph] (5), involving cleavage of Si-C(alkyne) bonds. Soluble and thermally stable platinum(II) poly-yne silane polymer trans-[-Pt(PBu₃)₂C≡CSiPh₂C≡C-] _n (4) was prepared in good yield by CuI-catalyzed polymerization of trans-[Pt(PBu₃)₂Cl₂] and Ph₂Si(C≡CH)₂. The single-crystal X-ray structural analyses and DFT calculations were performed on 1, 2 and 5. We report the optical absorption and photoluminescence spectra of these new metal-based oligomeric and polymeric silylacetylenes and the results are compared with platinum(II) poly-ynes with purely acetylenic and (hetero)aromatic conjugated units.

Full text of DOI:10.1039/b207575j

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Synthesis, structures and optical spectroscopy of photoluminescent
platinum-linked poly(silylacetylenes)
Wai-Yeung Wong,*a Chun-Kin Wong,a Guo-Liang Lu,a Kok-Wai Cheah,b Jian-Xin Shib
and Zhenyang Linc
a Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Kowloon Tong,
Hong Kong, P.R. China. E-mail: rwywong@hkbu.edu.hk
b Department of Physics, Hong Kong Baptist University, Waterloo Road, Kowloon Tong,
Hong Kong, P.R. China
c Department of Chemistry, The Hong Kong University of Science and Technology,
Clearwater Bay, Hong Kong, P.R. China
Received 2nd August 2002, Accepted 3rd October 2002
First published as an Advance Article on the web 6th November 2002
Synthetic routes to a series of novel oligomeric and polymeric platinum()-containing silylacetylenes have
᎐
been developed. The CuI-catalyzed reaction of trans-[Pt(PEt ) PhCl] with the alkynylsilane Ph Si(C᎐CH) ,
᎐
3
2
2
2
᎐
᎐
in a basic medium, afforded the mononuclear trans-[Ph(Et P) PtC᎐CSiPh C᎐CH] (1) and the dinuclear trans-
[Ph(Et P) PtC᎐CSiPh C᎐CPt(PEt ) Ph] (2). Complex 1 can be utilized as a key starting material for access to the
triplatinum() system trans-[Ph(Et P) PtC᎐CSiPh C᎐CPt(PBu ) C᎐CSiPh C᎐CPt(PEt ) Ph] (3). Attempted oxidative
coupling of 1 only led to the isolation of the triacetylenic species trans-[Ph(Et P) PtC᎐CC᎐CC᎐CPt(PEt ) Ph] (5),
᎐
᎐
3
2
2
᎐
᎐
᎐
᎐
3
2
2
3 2
᎐
᎐
᎐
᎐
᎐
2
᎐
᎐
᎐
3
2
2
3
2
3 2
᎐
᎐
᎐
᎐
᎐
᎐
3
2
3 2
involving cleavage of Si–C(alkyne) bonds. Soluble and thermally stable platinum() poly-yne silane polymer trans-
᎐
᎐
[–Pt(PBu ) C᎐CSiPh C᎐C–] (4) was prepared in good yield by CuI-catalyzed polymerization of trans-[Pt(PBu ) Cl ]
᎐
᎐
3
2
2
n
3
2
2
᎐
and Ph Si(C᎐CH) . The single-crystal X-ray structural analyses and DFT calculations were performed on 1, 2 and 5.
᎐
2
2
We report the optical absorption and photoluminescence spectra of these new metal-based oligomeric and polymeric
silylacetylenes and the results are compared with platinum() poly-ynes with purely acetylenic and (hetero)aromatic
conjugated units.
ligand and higher quantum efficiencies may be achieved in
Introduction
the light-emitting devices by harvesting the energy of the non-
With the realization of polyacetylenes and other conjugated
emissive triplet excitons.9a–e These metal-containing poly-ynes
heteronuclear polymers as potentially important electronic and
also provide model systems for the study of some of the photo-
optically active materials, research on electron delocalized
physical processes that occur in conjugated organic polymers.10
organic and metalloorganic polymers has received increased
For commercial exploitation of these materials and for
controlled application-aimed synthesis, a fundamental and
thorough understanding of the structure–property relationship
is crucial such that suitable modifications of the chemical
attention in the scientific community.1 Among them, organo-
silicon polymers are becoming important in many aspects
of device technology and play an increasingly vital role in the
electronic industry today.2 In particular, the molecular design
structures and geometries may fine-tune the electronic and
and synthesis of linear, cyclic or dendritic systems composed of
optical properties of the polymers. Recent works by us and
silicon atoms and acetylene units have attracted enormous
others have investigated the dependence of the intersystem
research interest.2,3 The lower ionization energy of the silicon
crossing rate from the singlet excited state to the triplet excited
atom compared with carbon is expected to enhance the
through-bond interaction of ethynyl units along the back-
state as a function of the extent of π-conjugation along the
backbone.9a–e While considerable effort has been devoted to
bone.4,5 These organosilicon compounds can act as functional
the use of (hetero)aromatic or carbon-rich acetylenic spacer
materials such as semiconductors, hole-transporting and heat-
units, much less is known for the silicon-linked system and
related studies on platinum() alkynylsilanes remain relatively
unexplored. Here we report the first synthesis, structural char-
acterization and luminescence behaviour of a series of novel
platinum-containing oligo- and poly-(silylacetylenes). Their
optical and theoretical properties will be discussed in com-
parison to those systems having purely acetylenic and other
(hetero)aromatic spacers.
resistant materials and ceramic precursors.2 The electrical
and optical properties associated with these materials are
generally attributed to the delocalization of electrons through
the silylene linkages via π–dSi interaction6 and σ–π conjug-
ation.4,7 π-Conjugated organosilicon systems have been
extensively studied regarding their applications to electro- and
optical devices.8
There is a long-term interest in the design, synthesis and
photophysics of rigid-rod organometallic poly-ynes of the
general formula trans-[–Pt(PBu ) C᎐CRC᎐C–] (R = aromatic
᎐
᎐
Results and discussion
᎐
᎐
3
2
n
or heteroaromatic spacer groups) and these metal-based poly-
mers contribute a particularly important class of new materials
for basic and applied research.9 One of the merits in the present
system is that incorporation of a heavy platinum centre in
the polymer chain induces strong spin–orbit coupling to allow
light emission from the triplet excited state of the conjugated
Synthesis
The preparations of new complexes and polymers are shown in
Scheme 1. Using the well-established dehydrohalogenation
route under the CuI–iPr2NH catalytic system,9a–k trans-
᎐
[Pt(PEt ) PhCl] reacts with Ph Si(C᎐CH) (I) to produce the
᎐
3
2
2
2
DOI: 10.1039/b207575j
J. Chem. Soc., Dalton Trans., 2002, 4587–4594
This journal is © The Royal Society of Chemistry 2002
4587

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Scheme 1 Reagents and conditions: (i) trans-[Pt(PEt3)2PhCl], CuI, iPr2NH–CH2Cl2, rt; (ii) trans-[Pt(PBu3)2Cl2] (0.5 equiv.), CuI, iPr2NH–CH2Cl2,
rt; (iii) trans-[Pt(PBu3)2Cl2] (1 equiv), CuI, iPr2NH–CH2Cl2, rt; (iv) Cu(OAc)2 (1.5 equiv.), pyridine, O2, CH2Cl2.
᎐
᎐
᎐
mono-platinum complex trans-[Ph(Et P) PtC᎐CSiPh C᎐CH]
cmϪ1) and the peaks are lower than that for the free silyl ligand
precursor I (2041 cmϪ1), in line with π-conjugation through the
platinum group. The terminal acetylenic C–H absorption is also
located at 3260 cmϪ1 for 1. The triacetylenic species 5 shows
᎐
3
2
2
᎐
᎐
(1) and the dinuclear species trans-[Ph(Et P) PtC᎐CSiPh C᎐
᎐
᎐
3
2
2
CPt(PEt3)2Ph] (2), the product distribution of which depends
on the stoichiometry of the reactants used. Starting from 1
with a free alkyne unit, this can provide an easy entry to
a strong νC᎐C stretch at 2087 cmϪ1, which compares well with
᎐
the value (2096 cmϪ1) for the polymeric analogue trans-
᎐
᎐
the triplatinum compound trans-[Ph(Et P) PtC᎐CSiPh C᎐CPt-
᎐
᎐
3
2
2
13
᎐
᎐
᎐
᎐
᎐
(PBu ) C᎐CSiPh C᎐CPt(PEt ) Ph] (3) accompanied by
a
[–Pt(PBu ) C᎐CC᎐CC᎐C–] but the frequency is significantly
᎐
᎐
᎐
᎐
᎐
3
2
2
3
2
3
2
n
1
doubling of the chain length. However, attempted oxidative
homocoupling of 1 in Cu(OAc)2–O2–pyridine11 has met with
little success and only a complicated mixture was obtained from
which we were able to isolate the triacetylenic complex trans-
larger than that for 2. The H NMR spectra of all the com-
pounds exhibit the expected signals for the systems and clearly
demonstrate a well-defined structure for each compound.
Except for the trinuclear complex 3, the unique singlets in the
31P-{1H} NMR spectra flanked by platinum satellites for the
mono- and di-platinum complexes and the polymer confirm
the trans arrangement of the phosphine ligands in such square-
planar geometry. Two distinct 31P signals are observed for the
terminal and central phosphine groups in 3. The 1JP–Pt coupling
᎐
᎐
᎐
[Ph(Et P) PtC᎐CC᎐CC᎐CPt(PEt ) Ph] (5). The formation of
᎐
᎐
᎐
3
2
3 2
5 involves the cleavage of Si–C(alkyne) bonds and this kind of
bond breaking was also observed when [Au(PPh3)Cl] was
allowed to react with I in a NaOMe–MeOH mixture to furnish
12
᎐
the known complex [(Ph P)AuC᎐CAu(PPh )]. All these new
᎐
3
3
complexes were isolated as off-white to light yellow solids
constants are normal for related trans-PtP2 systems.9a–k The 13
C
from preparative TLC plates on silica. Organometallic polymer
NMR spectral data for polymer 4 also give precise information
about the regiochemical structure of the main-chain skeleton
and show a high degree of structural regularity in the polymer.
The two 13C NMR peaks for the sp carbons in 4 are shifted
downfield with respect to those for I (δ = 83.37 and 97.36). For
1–4, the 29Si NMR chemical shifts appear at more downfield
positions in comparison to I (δ = Ϫ48.42) and do not differ
significantly from those for related polymers consisting of
silylacetylene and π-conjugated organic groups.14
᎐
᎐
trans-[–Pt(PBu ) C᎐CSiPh C᎐C–] (4) was synthesized by CuI-
᎐
᎐
3
2
2
n
catalyzed polycondensation polymerization between equimolar
quantities of trans-[Pt(PBu3)2Cl2] and I.9a–k Purification by silica
column chromatography using CH2Cl2 as eluent gives 4 in high
purity and yield (78%) and the product appears oily at room
temperature.
All these platinum complexes are air-stable and can be stored
without any special precautions. They have good solubility
in common organic solvents except hydrocarbons. The Mw
value determined by gel permeation chromatography (GPC)
in THF indicates a high degree of polymerization for 4
and the number of repeat units per chain calculated from
The electrochemical nature of our compounds was investi-
gated in CH2Cl2 by cyclic voltammetry (Table 1). Within the
measured scan range, each only showed a single irreversible
wave in the range of ca. 0.53–0.90 V due to the one-electron
Mn is 63, which is lower than that for the totally linear
Pt2ϩ
Pt3ϩ oxidation but we observe no reduction prior to the
13
᎐
᎐
᎐
trans-[–Pt(PBu ) C᎐CC᎐CC᎐C–] . The polydispersity index
solvent limit. Ligand I is electrochemically silent in our
measurements. There is no particular trend in the electro-
chemical data of these complexes apart from a slight anodic
shift of the oxidation potential when going from the polymer to
the monomeric, dinuclear and trinuclear species. Similar shifts
were also observed in some fluorene- and carbazole-based
platinum poly-ynes.9e
᎐
᎐
᎐
3
2
n
(Mw/Mn) of 1.51 for 4 is a common value for polycondensation
polymers. It should be noted that GPC does not give absolute
values of molecular weights but provides a measure of hydro-
dynamic volume. The hydrodynamic properties of rigid-rod
type polymers in solution are quite different from those for
flexible polymers. In view of this, calibration of the GPC with
polystyrene standards is likely to inflate the molecular weight
values of our metal poly-yne. The lack of discernible NMR
resonances attributable to the end groups confirms a high
degree of polymerization in 4.
Structural and theoretical studies
The regiochemical structure of the polymer has been ascer-
tained by single-crystal X-ray analysis on the model complexes
1 and 2 and the solid-state structure of 5 was also determined in
comparison to that of 2. Perspective views of 1, 2 and 5 are
shown in Figs. 1–3, respectively, and important bond distances
and angles are given in Tables 2–4. There are two structurally
independent but similar molecules per asymmetric unit in the
unit cell of 2. In each case, the coordination geometry at the Pt
Spectroscopic and redox properties
The IR, NMR, MS and elemental analysis data shown in the
Experimental section are consistent with the chemical struc-
tures of our new compounds 1–5. The IR spectra of 1–4 are
each dominated by a sharp νC᎐C absorption band (ca. 2025–2031
4588
J. Chem. Soc., Dalton Trans., 2002, 4587–4594

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Table 1 Absorption, emission and electrochemical data for 1–5
λmax a/nm
λmax/nm
λem/nm
CH2Cl2
Film
Eg b/eV
Film (290 K)
Film (11 K)
Eox c/V
ϩ0.90
1
2
3
266 (1.0), 308 (0.5)
265
307
264
309
262
275
311
261
278
314
273
289
319
345
371
3.87
3.81
3.78
268 (1.5), 308 (0.8)
Very weak
421,d 504d
401,d 443, 484, 505, 535,d 561d
421,d 462,d 504, 542d
ϩ0.88
ϩ0.90
261 (1.6), 275 (1.8), 310 (1.8)
4
5
261 (1.4), 277 (1.7), 312 (2.0)
3.70
3.25
418,d 510d
419,d 508
ϩ0.53
272 (0.24), 288 (0.22), 317 (0.16), 344 (0.05), 370 (0.02)
500, 561, 634d
500, 561, 634
a Extinction coefficients (104 MϪ1 cmϪ1) are shown in parentheses. b Estimated from the onset wavelength of the solid-state optical absorption.
c Irreversible waves. d Emission peaks appear as shoulders or weak bands.
Table 2 Selected bond lengths (Å) and angles (Њ) for 1
Pt(1)–P(1)
2.2895(12)
2.009(4)
1.792(5)
1.181(7)
Pt(1)–P(2)
C(19)–C(20)
Si(1)–C(21)
2.3019(12)
1.217(6)
1.838(5)
Pt(1)–C(19)
Si(1)–C(20)
C(21)–C(22)
P(1)–Pt(1)–C(19)
Pt(1)–C(19)–C(20)
Si(1)–C(21)–C(22)
92.06(12)
173.7(4)
179.3(5)
P(2)–Pt(1)–C(19)
Si(1)–C(20)–C(19)
C(20)–Si(1)–C(21)
88.93(13)
171.5(4)
109.6(2)
Fig. 1 A perspective view of complex 1. Thermal ellipsoids are drawn
at the 25% probability level.
Fig. 3 A perspective view of complex 5. Thermal ellipsoids are drawn
at the 25% probability level.
appear to be slightly shorter than those found in other sily-
lacetylene compounds in the literature.3c,16 These data, together
᎐
with the observed C᎐C bond lengths, are suggestive of partial
᎐
through-Si conjugation along the molecule. For 5, the four-
coordinate platinum atoms are bridged by a virtually linear
2Ϫ
C6 chain with the crystallographic symmetry located at
the centre of the hexatriynediyl unit. The structural data for
bridging acetylenic units in 5 compare well with those reported
by Gladysz, Bruce, Lapinte, Akita and Che.11,17 The bond
᎐
angles close to 180Њ for the fragment Pt–C᎐C confirm the
᎐
rigid-rod nature of all complexes. All of the remaining struc-
tural parameters are comparable to those found for other
crystal structures of platinum() acetylide compounds in the
literature.9e–g
Fig. 2 A perspective view of complex 2. Thermal ellipsoids are drawn
at the 25% probability level.
centre is square-planar and the two PEt3 groups are positioned
trans to each other. To our knowledge, complex 2 is the first
To study the electronic structures of platinum diynyl com-
plexes which differ in the central unit that bridges the two metal
fragments, we have carried out molecular orbital calculations
structurally characterized example of a silylacetylene-linked
᎐
᎐
᎐
diplatinum complex. The internal C᎐C bond lengths lying
on 1, 2, 5 and trans-[Ph(Et P) PtC᎐CRC᎐CPt(PEt ) Ph] (R =
᎐
3 2 3 2
᎐
᎐
within the narrow range 1.196(9)–1.226(10) Å are typical of
2,5-thienyl)18 at the B3LYP level of density functional theory
based on their experimental geometries obtained from X-ray
data. Examining the characteristics of the molecular orbitals in
the frontier region, we find that the highest occupied (HOMO)
᎐
metal-acetylide σ bonding and the C᎐CH bond [1.181(7) Å] in
᎐
1 shows a slight shortening due to the libration effect.15 The
Si–C(sp) single bond lengths are 1.792(5)–1.838(5) Å, which
J. Chem. Soc., Dalton Trans., 2002, 4587–4594
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Table 3 Selected bond lengths (Å) and angles (Њ) for 2
Molecule 1
Molecule 2
Molecule 1
Molecule 2
Pt(1)–P(1)
Pt(2)–P(3)
Pt(1)–C(19)
C(19)–C(20)
Si(1)–C(33)
2.295(2)
2.275(2)
2.019(7)
1.210(10)
1.821(7)
2.297(2)
2.268(2)
2.011(8)
1.221(11)
1.814(8)
Pt(1)–P(2)
Pt(2)–P(4)
Pt(2)–C(34)
Si(1)–C(20)
C(33)–C(34)
2.276(2)
2.276(2)
2.027(7)
1.807(8)
1.196(9)
2.282(2)
2.278(3)
2.000(8)
1.810(8)
1.226(10)
P(1)–Pt(1)–C(19)
P(3)–Pt(2)–C(34)
Pt(1)–C(19)–C(20)
C(20)–Si(1)–C(33)
91.66(19)
89.8(2)
177.6(6)
114.7(3)
91.6(2)
90.5(3)
177.5(8)
112.2(4)
P(2)–Pt(1)–C(19)
P(4)–Pt(2)–C(34)
Pt(2)–C(34)–C(33)
90.93(19)
87.7(2)
177.3(7)
87.7(2)
86.5(3)
177.7(8)
Table 4 Selected bond lengths (Å) and angles (Њ) for 5
Thermal analysis
Polymer 4 showed a very good thermal stability as confirmed by
thermogravimetric analysis (heating rate 20 ЊC minϪ1 under N2)
and no decomposition took place up to 381 ЊC. Such an onset
temperature is higher than those in related polymers trans-
Pt(1)–P(1)
Pt(1)–C(19)
C(20)–C(21)
2.295(2)
2.002(7)
1.370(10)
Pt(1)–P(2)
C(19)–C(20)
C(21)–C(21A)
2.292(2)
1.217(10)
1.208(14)
P(1)–Pt(1)–C(19)
Pt(1)–C(19)–C(20)
C(20)–C(21)–C(21A)
87.0(2)
174.7(8)
179.3(9)
P(2)–Pt(1)–C(19)
C(19)–C(20)–C(21)
93.2(2)
178.3(11)
᎐
᎐
[–Pt(PBu ) C᎐CRC᎐C–] (R = phenylene, thienylene, pyridyl,
᎐
᎐
3
2
n
substituted fluorenylene),9b,e,f,i,k suggesting that use of the SiPh2
unit can increase the thermal stability of platinum() poly-ynes.
The mass loss in 4 was quite sharp and 48% of the sample
mass was lost as the temperature rose from 381 to 423 ЊC. The
decomposition step is assigned to the removal of two PBu3
groups from the polymer. Although glass transition temper-
atures can be located for platinum() poly-ynes containing
phenylene and thienylene spacers,9b polymer 4 did not exhibit
discernable glass transition, reminiscent of the recently
reported oligopyridyl analogues.9k In addition, this polymeric
material decomposes without melting.
Symmetry transformations used to generate equivalent atoms: Ϫx ϩ 2,
Ϫy ϩ 2, z.
and lowest unoccupied molecular orbitals (LUMO) are mainly
related to orbitals from the metal centres and π/π* from the
conjugate organic structural units. The HOMOs correspond to
the linear combination of π orbitals from the conjugate organic
units and also mix slightly with the metal d orbitals in an anti-
bonding fashion. The LUMOs contain mainly the vacant p⊥
orbitals of the Pt centre mixed with π* orbitals of the organic
bridge. Here, p⊥ stands for the p orbitals perpendicular to the
square planes around Pt, i.e., pz when each coordination plane
is specifically defined as the xy plane. From the contour plots,19
shown in Fig. 4, the calculated HOMO–LUMO gap follows the
Optical spectroscopy
Optical absorption spectra of our new platinum()-containing
silylacetylenes were taken at room temperature in CH2Cl2
solutions and as thin films. Relevant data are shown in Table 1.
For 1–4, they display similar structured absorption bands (ca.
266–312 nm) in the near UV region in solution. In each case,
the first absorption band arises mainly from the π–π* transition
in the organic system, possibly with some admixture of metal
d orbitals which may change the overall energy of the
transition.9a–c We associate the lowest energy peak with the S0
S1 transition from the HOMO to the LUMO, which are
mainly delocalized π and π* orbitals. Well-resolved structure in
the absorption spectrum is apparent in 5, which is a result of
᎐
᎐
order 5 (4.18 eV) ≈ trans-[Ph(Et P) PtC᎐CRC᎐CPt(PEt ) Ph]
᎐
᎐
3
2
3 2
(R = 2,5-thienyl) (4.30 eV) Ӷ 2 (5.22 eV) < 1 (5.38 eV), in line
with the extent of π-delocalization in the conjugated chain.
Apparently, the triple bond and thiophene units are able to
extend π-conjugation effectively while the SiPh2 group does not
since the two metallic moieties are separated by two Si–C single
bonds. The small difference in the optical gaps between 1 and
2 agrees with the almost identical lowest energy band in their
absorption spectra (vide infra).
᎐
᎐
Fig. 4 Contour plots of the frontier molecular orbitals for 1, 2, 5 and trans-[Ph(Et3P)2PtC᎐CRC᎐CPt(PEt3)2Ph] (R = 2,5-thienyl). (1 au = 27.2114
᎐
᎐
eV).
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J. Chem. Soc., Dalton Trans., 2002, 4587–4594

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Table 5 Optical gaps (in eV) for platinum di-ynes and poly-ynes with a
series of spacer groups
Decreasing delocalization of electrons
᎐
᎐
᎐
Spacer
SiPh2
Di-yne
Poly-yne
3.22
2.82
3.22
2.85
3.34
2.98
3.38
3.05
3.81
3.70
transitions from ground to excited electronic states coupled
with transitions to the various vibronic levels of the excited
state. The spectral pattern is similar to that of its polymeric
counterpart trans-[–Pt(PBu ) C᎐CC᎐CC᎐C–] .13 With reference
᎐
᎐
᎐
᎐
᎐
᎐
3
2
n
to the value at 272 nm for I, we note a red shift of the position
of the lowest energy absorption band in 1–4 after the inclusion
of platinum fragment. This reveals that π-conjugation of the
ligand is preserved through the metal site. The apparent lack of
Fig. 6 Temperature dependence of the photoluminescence of 4.
to dissociation sites at room temperature. At 11 K, the principal
emissive peak occurs at ca. 484, 504 and 508 nm for 2, 3 and 4,
respectively. Lifetime (τ) data suggest that the main emission
peak for 3 and 4 is rather long-lived. The τ values are found to
be 37.7 0.2 (3) and 11.0 0.1 (4) µs at 290 K and 45.0 0.3 (3)
and 40.2 0.1 (4) µs at 11 K, which can be compared to τ = 30
an energy shift in the S0
S1 transition among 1–4 shows that
the lowest singlet excited state is confined to a single repeat
᎐
᎐
᎐
unit, which is in contrast to trans-[–Pt(PBu ) C᎐CRC᎐C–]
᎐
3
2
n
(R = 1,4-phenylene, 2,5-thienylene, 2,5-pyridyl) with a well-
extended singlet excited state.9a–c The experimentally deter-
mined HOMO–LUMO energy gaps (Eg) as measured from the
onset wavelength in the solid film are tabulated in Table 5 for a
series of spacer groups in platinum di-ynes and poly-ynes. The
degree of electronic conjugation roughly follows the order:
µs at 10 K for trans-[–Pt(PBu ) C᎐C(p-C H )C᎐C–] .20 Vibronic
᎐
᎐
᎐
᎐
3
2
6
4
n
substructure is seen for the emission bands of 2 and 3 as the
temperature is lowered but the emission pattern for 4 appears
broad and unstructured even at low temperatures. The substan-
tial Stokes’ shift between the strongest bands in the absorption
and the emission spectra in the narrow range 1.50 0.05 eV
supports the π–π* character of the phosphorescent triplet state
for the emission bands (Fig. 7).9a–e,20 The hindered conjugation
9b–d,13
᎐
C᎐C ≈ 2,5-thienyl > 1,4-phenylene > 2,5-pyridyl > SiPh .
᎐
2
In other words, the energy of the S1 singlet state is highest for
the silicon-linked derivatives among them. It is obvious from
Table 1 that the Eg values show a gradual decrease in the order
1 > 2 > 3 > 4 with increasing chain length.
The thin film photoluminescence (PL) spectra of 2–5 were
measured at various temperatures and the results are gathered
in Table 1. For these silicon-linked compounds, we observe no
fluorescence band but only the phosphorescence bands associ-
ated with the mixed ligand-metal orbitals. We consider that the
emission peaks arise from a triplet excited state (i.e. T1
S0)
based on the observed temperature dependence of emission
data and the lifetime measurements, in accordance with earlier
work on platinum() poly-ynes.9a–e,k The temperature depend-
encies of the PL spectra of 3 and 4 are shown in Figs. 5 and 6,
respectively.
Fig. 7 The room temperature optical absorption spectrum and the
photoluminescence spectrum (11 K) of 4 in the solid state.
in the poly(silylacetylene) derivative 4 shifts the phosphor-
escence (at 2.44 eV) to the blue by 0.06 and 0.38 eV, respectively,
compared to the 1,4-phenylene and 2,5-thienylene containing
counterparts.9c
In the solid state, complex 5 is emissive both at 290 and 11 K
and sharp vibronically structured emissions are observed with
λ0–0 lines appearing at 500 nm. The large Stokes’ shift from the
absorption spectrum suggests triplet emission, reminiscent of
its polymeric analogue (Fig. 8).21 With reference to the recent
work by Che and Friend,9d,m,17f such triplet emissions are
Fig. 5 Temperature dependence of the photoluminescence of 3.
3
In each case, the triplet photoluminescence band shows a
strong temperature dependence and there is a strong increase in
intensity with decreasing temperature. From 290 to 11 K, the
intensity of the triplet emission increases by a factor of 38.0
and 58.3 for 3 and 4, respectively, typical of this class of metal
poly-ynes, and such an increase in emission intensity indicates a
triplet exciton that is quenched by thermally activated diffusion
ascribed to the lowest-energy acetylenic (ππ*) excited states
associated with the (C᎐C) 2Ϫ chain. The single vibronic progres-
᎐
᎐
3
sion of 2087 cmϪ1 in the emission spectrum of 5 is attributed to
the νC᎐C mode.13,17f,21 While trans-[–Pt(PBu ) C᎐CC᎐CC᎐C–] is
᎐
᎐
᎐
᎐
᎐
᎐
3
2
n
᎐
phosphorescent at 1.88, 2.09 and 2.37 eV,21 we observe a blue
shift of the phosphorescence band in 4 after replacement of the
᎐
C᎐C unit by the SiPh group.
᎐
2
J. Chem. Soc., Dalton Trans., 2002, 4587–4594
4591

View Article Online
a closed-cycle cryostat (Oxford CC1104) in which the temper-
ature can be adjusted from 10 to 330 K. For the lifetime
measurements, the third harmonic, 355 nm line of a Q-switched
Nd:YAG laser was used as the excitation light source. The emis-
sion was recorded using a PMT and a HP54522A 500 MHz
oscilloscope. Cyclic voltammetry experiments were done with
a Princeton Applied Research (PAR) model 273A potentiostat.
A conventional three-electrode configuration consisting of a
glassy-carbon working electrode and a Pt-wire as the counter
and reference electrodes was used at a scan rate of 100 mV sϪ1
.
The solvent in all measurements was deoxygenated CH2Cl2 and
the supporting electrolyte was 0.1 M [Bu4N]PF6. Ferrocene
was added as a calibrant after each set of measurements and
all potentials reported were quoted with reference to the
ferrocene–ferrocenium couple. The molecular weight of the
polymer was determined by GPC (HP 1050 series HPLC with
visible wavelength and fluorescent detectors) using polystyrene
standards and thermal analyses were performed with a
Shimadzu DSC-50 and Perkin-Elmer TGA6 thermal analyzers.
For density functional calculations at the B3LYP level, the basis
set used for C and H atoms was 6-31G25 while effective core
potentials with a LanL2DZ basis set26 were employed for P, S,
Si and Pt atoms. The Gaussian 98 program was used for the
calculations.27 Polarization functions were added for P, S and Si
atoms [ζd(P) = 0.340, ζd(S) = 0.421, ζd(Si) = 0.262].
Fig. 8 The room temperature optical absorption spectrum and the
photoluminescence spectrum (11 K) of 5 in the solid state.
Concluding remarks
Our present work reports the synthesis and characteriz-
ation of a novel class of platinum()-containing oligo- and
poly-(silylacetylenes) with interesting electronic and phos-
phorescence properties. Optical spectroscopy and theoretical
calculations suggest that while the silyl group is weak in elec-
tronic conjugation, the resulting organometallic materials
exhibit strong triplet emission with negligible fluorescence
intensity at low temperatures. The reduced conjugation in
the presence of the SiPh2 unit shifts the optical gap and the
phosphorescence band to the blue compared to other platinum
poly-ynes with aromatic and heteroaromatic spacers. These new
polymer systems enable the study of the relationship between
chemical structure and triplet energy levels in conjugated poly-
mers and such an investigation is desirable for optoelectronic
applications that harvest the triplet excited state for light
emission. Further investigations of light-emitting devices
and photocells fabricated from these materials are planned and
work is currently in progress to examine similar systems based
on other Group 14 elements.
Synthetic procedures
᎐
᎐
᎐
trans-[Ph(Et P) PtC᎐CSiPh C᎐CH] (1) and trans-[Ph(Et P) -
᎐
3
᎐
2
2
3
2
᎐
PtC᎐CSiPh C᎐CPt(PEt ) Ph] (2). To a stirred mixture of
Ph Si(C᎐CH) (I, 63.0 mg, 0.27 mmol) and trans-[Pt(PEt ) -
᎐
᎐
2
3 2
᎐
᎐
2
2
3 2
i
PhCl] (130.0 mg, 0.24 mmol) in Pr2NH (5 cm3) and CH2Cl2
(10 cm3) was added CuI (6 mg). The solution was stirred at
room temperature over a period of 15 h, after which all volatile
components were removed under vacuum. The crude product
was taken up in CH2Cl2 and purified on preparative silica TLC
plates with hexane–CH2Cl2 (1 : 1, v/v) as eluent. From the first
colorless band (Rf = 0.66) 1 was obtained as a white solid (103.0
mg, 58%) while 2 was isolated as a white powder from another
band at Rf = 0.43 with a yield of 20% (30.0 mg). An improved
yield of 1 was achieved at the expense of 2 when a larger
sample size of the ligand was used. However, when the reaction
was carried out using two molar equivalents of trans-
[Pt(PEt3)2PhCl], the dimeric species 2 was obtained as the sole
product in 75% yield.
Experimental
General procedures
All reactions were carried out under a nitrogen atmosphere
with the use of standard Schlenk techniques, but no special
precautions were taken to exclude oxygen during workup.
Solvents were predried and distilled from appropriate drying
agents. All reagents and chemicals, unless otherwise stated,
were purchased from commercial sources and used without
further purification. Preparative TLC was performed on
0.7 mm silica plates (Merck Kieselgel 60 GF254) prepared in our
For 1. (Found: C, 55.02; H, 6.10. C34H46P2PtSi requires C,
55.20; H, 6.27%). IR (CH2Cl2): 3260 (ν᎐CH) and 2031 cmϪ1 (νC᎐C).
᎐
1H NMR (CDCl3): δ 7.84 (m, 4H, ᎐Ph), 7.41–7.26 (m, 8H,
Ph ϩ Hortho of Ph–Pt), 6.98 (t, J = 7.2 Hz, 2H, Hmeta of Ph–Pt),
᎐
6.82 (t, J = 7.2 Hz, 1H, Hpara of Ph–Pt), 2.60 (s, 1H, C᎐CH), 1.72
᎐
(m, 12H, CH2) and 1.06 (m, 18H, CH3). 29Si NMR (CDCl3):
δ Ϫ54.27. 31P{1H} NMR (CDCl3): δ 11.28 (1JP–Pt = 2623 Hz).
MS (FAB): m/z 740 (calc. Mϩ 740).
laboratory. The compounds trans-[Pt(PEt3)2PhCl],22 trans-
24
[Pt(PBu3)2Cl2]23 and Ph Si(C᎐CH) (I) were prepared by lit-
For 2. (Found: C, 49.90; H, 6.14. C52H80P4Pt2Si requires C,
᎐
᎐
2
2
1
50.07; H, 6.46%). IR (CH2Cl2): 2027 cmϪ1 (νC᎐C). H NMR
erature methods. Infrared spectra were recorded as CH2Cl2
solutions using a Perkin-Elmer Paragon 1000 PC or Nicolet
Magna 550 Series II FTIR spectrometer. NMR spectra were
measured in appropriate solvents on a Jeol EX270 or a Varian
Inova 400 MHz FT-NMR spectrometer, with 1H NMR
chemical shifts quoted relative to SiMe4 and 31P chemical shifts
relative to an 85% H3PO4 external standard. Electron impact
(EI) and fast atom bombardment (FAB) mass spectra were
recorded on a Finnigan MAT SSQ710 mass spectrometer.
Electronic absorption spectra were obtained with a HP 8453
UV-VIS spectrometer. For emission spectral measurements, the
325 nm line of a He–Cd 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 photo-
multiplier tube (PMT) and processed with a lock-in-amplifier.
For the low temperature experiments, samples were mounted in
᎐
(CDCl3): δ 7.84 (m, 4H, Ph), 7.27 (m, 10H, Ph ϩ Hortho of
Ph–Pt), 6.93 (t, J = 7.2 Hz, 4H, Hmeta of Ph–Pt), 6.77 (t, J =
7.2 Hz, 2H, Hpara of Ph–Pt), 1.69 (m, 24H, CH2) and 1.03 (m,
36H, CH3). 29Si NMR (CDCl3): δ Ϫ58.10. 31P{1H} NMR
(CDCl3): δ 11.94 (1JP–Pt = 2630 Hz). MS (FAB): m/z 1247 (calc.
Mϩ 1247).
᎐
᎐
᎐
᎐
᎐
2
trans-[Ph(Et P) PtC᎐CSiPh C᎐CPt(PBu ) C᎐CSiPh C᎐
᎐
᎐
᎐
3
2
2
3
2
CPt(PEt3)2Ph] (3). A mixture of 1 (148.0 mg, 0.20 mmol) and
trans-[Pt(PBu3)2Cl2] (67.0 mg, 0.10 mmol) in a 2 : 1 molar ratio
i
in Pr2NH–CH2Cl2 (20 cm3, 1 : 1, v/v) was allowed to react in
the presence of CuI (3 mg) at room temperature for 15 h. The
resulting solution was evaporated to dryness. The residue was
redissolved in CH2Cl2 and the filtrate was subjected to prepar-
ative TLC on silica eluting with hexane–CH2Cl2 (3 : 2, v/v) to
4592
J. Chem. Soc., Dalton Trans., 2002, 4587–4594

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Table 6 Crystal data for complexes 1, 2 and 5
1
2
5
Empirical formula
M
C34H46P2PtSi
739.83
C52H80P4Pt2Si
1247.31
C42H70P4Pt2
1089.04
Crystal system
Space group
Monoclinic
P21/c
Triclinic
P1
Orthorhombic
Fdd2
¯
a/Å
b/Å
c/Å
α/Њ
12.0760(7)
15.3158(9)
18.5935(11)
9.9773(6)
13.7256(8)
43.156(2)
85.6250(10)
84.4180(10)
76.4340(10)
5709.1(6)
4
16.1148(13)
65.387(5)
9.1219(8)
β/Њ
92.5910(10)
γ/Њ
U/Å3
3435.4(3)
4
4.232
9611.8(14)
8
5.973
Z
µ(Mo-Kα)/mmϪ1
5.058
No. reflections collected
19872
7710 (0.0359)
5328
R1 = 0.0399
wR2 = 0.1029
R1 = 0.0648
wR2 = 0.1151
33832
24553 (0.0295)
16782
R1 = 0.0494
wR2 = 0.1158
R1 = 0.0770
wR2 = 0.1284
13649
4852 (0.0386)
4642
R1 = 0.0346
wR2 = 0.0891
R1 = 0.0365
wR2 = 0.0902
Unique reflections (Rint
)
Observed reflections [I > 2σ(I )]
R indices [I > 2σ(I )]
R indices (all data)
2
R1 = Σ||Fo| Ϫ |Fc||/Σ|Fo|. wR2 = {Σ[w(Fo2 Ϫ Fc2)2]/Σ[w(Fo )2]}1/2
.
afford a pale yellow band (Rf = 0.26). Compound 3 was isolated
as a light yellow solid in 27% yield (54 mg). (Found: C, 53.01;
H, 6.69. C92H144P6Pt3Si2 requires C, 53.19; H, 6.99%). IR
(m, 24H, CH2) and 1.05 (m, 36H, CH3). 31P{1H} NMR
(CDCl3): δ 11.87 (1JP–Pt = 2619 Hz). MS (FAB): m/z 1089
(calc. Mϩ 1089).
1
(CH2Cl2): 2026 cmϪ1 (νC᎐C). H NMR (CDCl3): δ 7.81 (m, 8H,
Ph), 7.27–7.20 (m, 16H,᎐Ph ϩ Hortho of Ph–Pt), 6.92 (t, J = 7.2
Hz, 4H, Hmeta of Ph–Pt), 6.76 (t, J = 7.2 Hz, 2H, Hpara of Ph–Pt),
2.03 (m, 12H, CH2 of Bu), 1.69 (m, 24H, CH2 of Et), 1.48 (m,
12H, CH2 of Bu), 1.32 (m, 12H, CH2 of Bu), 1.01 (m, 36H, CH3
of Et) and 0.82 (m, 18H, CH3 of Bu). 29Si NMR (CDCl3):
X-Ray crystallography
Colorless crystals of 1, 2 and 5 suitable for X-ray diffraction
studies were grown by slow evaporation of their respective
solutions in hexane–CH2Cl2 mixtures at room temperature.
Geometric and intensity data were collected at 293 K using
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) on
a Bruker AXS SMART 1000 CCD area-detector. The collected
frames were processed with the software SAINT28a and
an absorption correction was applied (SADABS)28b to the
collected reflections. The structure was solved by direct
methods (SHELXTL)29 in conjunction with standard differ-
ence Fourier techniques and subsequently refined by full-matrix
least-squares analyses on F 2. All non-hydrogen atoms were
assigned with anisotropic displacement parameters. The hydro-
gen atoms were generated in their idealized positions and
allowed to ride on the respective carbon atoms. Crystal data
and other experimental details are summarized in Table 6.
CCDC reference numbers 191134–191136.
1
δ Ϫ57.75. 31P{1H} NMR (CDCl3): δ 11.55 (PEt3, JP–Pt = 2640
1
Hz) and 4.06 (PBu3, JP–Pt = 2373 Hz). MS (FAB): m/z 2077
(calc. Mϩ 2077).
᎐
᎐
᎐
trans-[–Pt(PBu ) C᎐CSiPh C᎐C–] (4). Polymerization was
᎐
3
2
2
n
carried out by mixing the di-yne I (23.2 mg, 0.10 mmol), trans-
[Pt(PBu3)2Cl2] (67.0 mg, 0.10 mmol) and CuI (3 mg) in iPr2NH–
CH2Cl2 (30 cm3, 1 : 1, v/v). After stirring at room temperature
for 15 h, the solution mixture was evaporated to dryness. The
residue was redissolved in CH2Cl2, and filtered through a silica
column using the same eluent to remove ionic impurities and
catalyst residues. After removal of the solvent, the crude
product was purified by precipitation in toluene from MeOH.
Subsequent washing with hexane and drying in vacuo gave an
oily solid of 4 (65.0 mg, 78%). (Found: C, 57.55; H, 7.80.
(C40H64P2PtSi)n requires C, 57.88; H, 7.77%). IR (CH2Cl2): 2025
cmϪ1 (νC᎐C). 1H NMR (CDCl3): δ 7.75 (m, 4H, Ph), 7.20 (m, 6H,
Ph), 1.9᎐7 (m, 12H, PCH2), 1.43 (m, 12H, PCH2CH2), 1.27
[m, 12H, P(CH2)2CH2CH3] and 0.80 (m, 18H, CH3). 13C NMR
(CDCl3): δ 139.95, 134.87, 127.89, 126.81 (arom C), 107.38,
See http://www.rsc.org/suppdata/dt/b2/b207575j/ for crystal-
lographic data in CIF or other electronic format.
Acknowledgements
We are grateful to the Hong Kong Research Grants Council
(Grant no: HKBU 2054/02P) and the Hong Kong Baptist
University for financial support.
94.40 (C᎐C), 26.36, 24.14, 23.33 and 13.96 (Bu). 29Si NMR
᎐
᎐
(CDCl3): δ Ϫ57.50. 31P{1H} NMR (CDCl3): δ 4.14 (1JP–Pt
=
2377 Hz). GPC (THF as eluent): Mw = 78700, Mn = 52000,
polydispersity = 1.51.
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