Synthesis, characterization, luminescence and nonlinear optical (NLO) properties of truxene-containing platinum(II) alkynyl complexes

Article abstract of DOI:10.1016/j.jorganchem.2010.09.044

A series of luminescent trinuclear platinum(II) alkynyl complexes containing dihydro-5H-diindeno[1,2-a;1′,2′-c]fluorene (truxene) as the core and aryl alkynyl ligands with different electronic properties at the periphery has been successfully synthesized

Full text of DOI:10.1016/j.jorganchem.2010.09.044

Journal of Organometallic Chemistry 696 (2011) 1163e1173
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, characterization, luminescence and nonlinear optical (NLO) properties
of truxene-containing platinum(II) alkynyl complexes
Carmen Ka Man Chan a, Chi-Hang Tao a, King-Fai Li b, Keith Man-Chung Wong a, Nianyong Zhu a,
**
,
a,
*
Kok-Wai Cheah b , Vivian Wing-Wah Yam
a Institute of Molecular Functional Materials1, Department of Chemistry and HKU-CAS Joint Laboratory of New Materials, The University of Hong Kong, Pokfulam Road,
Hong Kong, PR China
b Centre for Advanced Luminescence Materials, Hong Kong Baptist University and Department of Physics, Hong Kong Baptist University, Kowloon, Hong Kong, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of luminescent trinuclear platinum(II) alkynyl complexes containing dihydro-5H-diindeno[1,2-
a;10,20-c]fluorene (truxene) as the core and aryl alkynyl ligands with different electronic properties at the
periphery has been successfully synthesized and characterized. The electronic absorption, emission,
nanosecond transient absorption and electrochemical properties of these complexes have been reported.
These complexes showed long-lived emissions in degassed benzene solution at room temperature, and
their emissions have been assigned to originate from triplet states of intraligand (IL) character with some
mixing of metal-to-ligand charge-transfer (MLCT) character. The luminescent platinum(II) alkynyl
complexes are found to show two-photon absorption (2PA) and two-photon induced luminescence
(TPIL) properties, and their two-photon absorption cross-sections have been determined to be 6e51 GM
upon excitation at 720 nm.
Received 15 July 2010
Received in revised form
14 September 2010
Accepted 17 September 2010
Keywords:
Luminescence
Platinum(II) complexes
Truxene
Ó 2010 Elsevier B.V. All rights reserved.
Metal alkynyls
Two-photon induced luminescence
Transient absorption
1. Introduction
Moreover, p-conjugated dendrimers with large building blocks can
exhibit two or three-dimensional architecture, which may over-
come luminescence quenching and improve the film formation
capability [15,16].
There has been a growing interest in the development of organic
-conjugated molecules in material chemistry due to their poten-
tial application in a wide range of electronic and optical devices
p
10,15-Dihydro-5H-diindeno[1,2-a;10,20-c]fluorene
(truxene)
[1e8]. One of the most challenging goals in material chemistry is to
develop new classes of p-conjugated materials with new branches
appears to be a promising building block for the construction of
functional materials due to its rigid structures, good thermal and
chemical stabilities and high quantum yields [9e13,17]. Truxene is
a heptacyclic aromatic system with C3 symmetry, which has been
recognized as a potential starting material for the construction of
larger polyarenes, bowl-shaped fragments of fullerenes, liquid
crystalline compounds and C3 tripodal compounds in asymmetric
catalysis and chiral recognition [9e13]. The structure of truxene
consists of three fluorene moieties sharing a central benzene ring,
which represents an excellent choice as a core for the construction
of star-shaped molecules [9e13]. It is believed that the truxene
and cores, in order to investigate their physical and chemical
properties, as well as their structureeproperty relationship within
the molecules. Star-shaped molecules are branched macromole-
cules that consist of linear conjugated chains, which are joined
together by a central core [9,10]. In contrast to polymers and small
molecules, rigid star-shaped molecules and extended p-conjugated
dendrimers possess interesting properties, such as electrical,
optical, nonlinear optical and electroluminescent properties, and
optoelectronic devices including organic light-emitting diodes
(OLEDs) based on these materials have been reported [9e14].
p-system would have similar properties as the fluorene p-system,
which has been demonstrated to be an efficient building block for
chromophores with high two-photon absorptivities [17]. Unlike the
one-dimensional quadrupolar fluorene chromophore, this attrac-
tive dendritic truxene building block can be readily functionalized
at C-2, C-7 and C-12 positions and C-5, C-10 and C-15 positions,
respectively, by virtue of its three-dimensional topology, to build
* Corresponding author. Tel.: þ852 2859 2153; fax: þ852 2857 1586.
** Corresponding author. Tel.: þ852 3411 7029.
E-mail addresses: kwcheah@hkbu.edu.hk (K.-W. Cheah), wwyam@hku.hk
(V.W.-W. Yam).
1
Areas of Excellence Scheme, University Grants Committee (Hong Kong).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.09.044

1164
C.K.M. Chan et al. / Journal of Organometallic Chemistry 696 (2011) 1163e1173
octupolar or dendritic chromophores [9,10,17,18]. Since unsub-
stituted truxene is sparingly soluble in common organic solvents
[18], introducing alkyl groups around the rim can enhance the
solubility of the compound, which might increase the yield and
facilitate the isolation of the desired products. This star-shaped
truxene molecule has received much attention in the field of
liquid crystals and self-association in solution [18e26]. Continua-
received. All amines were distilled over potassium hydroxide and
stored in the presence of potassium hydroxide prior to use.
2.2. Physical measurements and instrumentation
1H NMR spectra were recorded on a Bruker DPX 300 (300 MHz)
or a Bruker DPX 400 (400 MHz) Fourier-transform NMR spec-
trometer with chemical shifts in ppm (d) reported relative to tet-
tion of the development of these organic
p-conjugated molecules
or dendrimers has been pursued by several research groups,
who have reported the synthesis of a number of truxene-contain-
ing compounds and the study of their optical properties
[9e11,14,17,27e36].
ramethylsilane, Me4Si, while 31P{1H} spectra were recorded either
on a Bruker DPX 400 or a Bruker DPX 500 MHz Fourier-transform
NMR spectrometer with chemical shifts relative to 85% H3PO4. IR
spectra were obtained as KBr disks on a Bio-Rad FTS-7 Fourier-
transform infrared spectrophotometer (4000ꢀ400 cmꢀ1). Positive-
ion fast-atom bombardment (FAB) mass spectra were recorded on
a Finnigan MAT95 mass spectrometer or a Thermo Scientific DFS
High Resolution Magnetic Sector mass spectrometer. Elemental
analyses were performed on a Flash EA 1112 elemental analyzer at
the Institute of Chemistry, Chinese Academy of Sciences in Beijing.
UVeVis absorption spectra were recorded on a HewlettePack-
ard 8452A diode array spectrophotometer. Steady-state excitation
and emission spectra obtained at room temperature and at 77 K
were recorded on a Spex Fluorolog-2 Model F111 fluorescence
spectrofluorometer with a Hamamatsu R928 photomultipler tube
(PMT) detector. The solutions were rigorously degassed with
at least four successive freeze-pump-thaw cycles. Solid-state
photophysical studies were carried out with solid-samples
contained in a quartz tube inside a quartz-walled Dewar flask.
Measurements of the ethanol-methanol (4:1, v/v) glass or solid-
samples at 77 K were conducted by using liquid nitrogen filled in
the optical Dewar flask. Emission lifetime measurements were
performed by using a conventional nanosecond pulsed laser
system. The excitation source used was with a 355-nm output
(third harmonic, 8 ns) from a Spectra-Physics Quanta-Ray Q-
switched GCR-150 pulsed Nd-YAG laser (10 Hz). Luminescence
decay signals were detected by a Hamamatsu R928 photomultiplier
Although different metal centres could be incorporated into
the conjugated system, platinum atoms are preferred due to their
strong excited state absorptions in the visible region and their
relatively large spin-orbit coupling [37e44]. Because of their long-
lived 3IL and 3MLCT emissions, platinum(II) alkynyl-based conju-
gated polymers and oligomers have received much interest [45].
Various platinum complexes, oligomers and metallopolymers with
different spacers and chain lengths have been reported by Lewis
et al. [46,47], Raithby et al. [48e51], Schanze et al. [52e54], Gladysz
et al. [55e57], Wong et al. [58e61] and others [62e64], and some of
them have proven to be versatile and promising building blocks for
the development of multinuclear luminescent complexes
[52,64e68]. The incorporation of platinum(II) alkynyl complexes
and polymers has attracted attention in the fields of nonlinear
optics, such as optical limiting, two-photon absorption and two-
photon induced luminescence [37,39,42,58,69,70]. Therefore, the
study of nonlinear optical platinum(II) alkynyl polymers and
molecular materials has received much attention, but most of these
complexes are mononuclear and are structurally related to the (4-
phenylethynyl)phenylethynyl backbone [38,39,42,43].
Although organic chromophores with truxene have been
synthesized, their two-photon absorption properties have not
been much explored [14,17,71]. In contrast to truxene-containing
organic chromophores, the construction of truxene-core coordina-
tion compounds was also relatively unexplored [72e74]. Recent
works in the construction of linear and branched luminescent plat-
inum(II) alkynyl complexes by Yam and coworkers [69,70,75e80]
have also provided the motivations for the design and construction
of bimetallic and polymetallic alkynyl complexes with different
central cores. Herein, we report the design, synthesis, characteriza-
tion and luminescence properties of a series of branched platinum
(II) complexes with truxene as the core and aryl alkynyl ligands
containing groups with different electronic properties, such as
methoxybenzene, 2,5-diphenyl-[1,3,4]oxadiazole and naphthalene,
as the peripheral units. The nanosecond transient absorption spectra
of these complexes have been studied. The X-ray crystal structure
of [{(MeOC6H4C^C)(PPh3)2PtC^C}3truxene] (4) has also been
determined.
tube, recorded on a Tektronix Model TDS-620A (500 MHz, 2 GS se1
)
digital oscilloscope, and analyzed by using a program for expo-
nential fits.
Photoluminescence quantum yields were measured by the
optical dilute method reported by Demas and Crosby [84]. An
aqueous solution of quinine sulfate in 1.0 N H2SO4 has been used as
the reference and all samples were irradiated at 365 nm since all
the complexes absorb strongly in this region. The refractive index
of the samples and the reference solution were 1.501 and 1.333,
respectively. The luminescence quantum yield of the reference was
0.55 with excitation wavelength at 365 nm [84].
The two-photon absorption (2PA) cross-section (s2) could be
determined by two main methods [85,86]. Two-photon induced
luminescence (TPIL) is one of the methods to determine s2, in
which the luminescence intensity generated by 2PA is measured
and used to estimate the s2 values according to equation (1). An
unknown sample is compared to a reference with known s2 values.
Fluorescein (1 ꢁ10ꢀ4 M in 0.1 M NaOH aqueous solution) was used
as the reference in the present study [87].
2. Experimental
2.1. Materials and reagents
2-(4-Ethynylphenyl)-5-phenyl-1,3,4-oxadiazole[81] and [{Cl
(PEt3)2PtC^C}3-truxene[70] were synthesized according to litera-
ture procedures. Triethynylhexabutyltruxene[82] and trans-
[MeOC6H4C^C(PPh3)2PtCl][83] were prepared according to slight
modification of reported procedures. Trans-[Pt(PEt3)2Cl2]
(Aldrich, 98%), 1-ethynyl-4-methoxybenzene (Maybridge, 97%),
1-ethynylnaphthalene (Aldrich, 97%), and fluorescein (Acros, 99%
pure, laser grade) were purchased and used as received. All solvents
were purified and distilled using standard procedures before use.
All other reagents were of analytical grade and were used as
F
Crnr rSs
s2ðsÞ
¼
s2ðrÞ
(1)
CsnsFsSr
where s2 is the two-photon absorption cross-section, C is the
concentration, n is refractive index, is the luminescence quantum
F
yield and S is the integrated luminescence signal. The subscripts r
and s represent the reference and the sample material respectively.
The s2 and luminescence quantum yield of fluorescein in 0.1 M
NaOH solution (pH w 11) at 720 nm were taken to be 19 GM
(Gøppert-Mayer, 1 GM ¼ 1 ꢁ10ꢀ50 cm4 s photonꢀ1 moleculeꢀ1) and

C.K.M. Chan et al. / Journal of Organometallic Chemistry 696 (2011) 1163e1173
1165
0.90, respectively [87]. The time-averaged laser power at 720 nm
was kept constant for the sample and the reference. The laser
water, and dried over anhydrous MgSO4. The solution was then
filtered, and the solvent was removed under reduced pressure. The
yellow residue was subjected to column chromatography on basic
pulses were generated by
a
mode-locked Ti:Sapphire laser
repetition rate of
(Spectra-Physics, Tsunami) operating at
a
alumina oxide (50e200 mm) with dichloromethane as the eluent.
85 MHz. The pulses were amplified up to 1 mJ per pulse by the
Ti:Sapphire regenerative amplifier (Spectra-Physics, Spitfire) with
Subsequent recrystallization of the crude product with dichlor-
omethaneemethanol afforded 1 as a yellow solid. Yield: 0.09 g,
pulse duration of 120 fs and repetition rate of 1 kHz. The 720-nm
37%. 1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si):
d
0.46
e
laser output from the OPA (Coherent, TOPAS-C light conversion)
was used as the excitation source. The laser output was passed
through the sample cell and the luminescence intensity was
monitored by the spectrometer system with monochromator
(ACTON Research Corporation, SpectraPro 2300i) and photo-
multiplier tube (Hamamatsu, R636-10). For the power dependence
measurements, the laser beam power was altered by using two
polarizers, which was partially reflected to a photodiode for the
monitoring of the excitation power. The schematic diagram for
the experimental setup has been described elsewhere [70].
Transient absorption measurements were performed on an
LP920-KS Laser Flash Photolysis Spectrometer (Edinburgh Instru-
ments Ltd, Livingston, UK) at room temperature. The excitation
source was the 355 nm output (third harmonic) of a Nd:YAG laser
(Spectra-Physics Quanta-Ray Lab-130 Pulsed Nd:YAG Laser) and
the probe light source was an Xe900 450 W xenon arc lamp. The
transient absorption spectra were detected by an image intensified
CCD camera (Andor) with PC plug-in controller, operated by L900
spectrometer software. The absorption kinetics were detected by
a Hamamatsu R928 photomultiplier tube and recorded on a Tek-
tronix Model TDS3012B (100 MHz, 1.25 GS/s) digital oscilloscope
and analyzed using the same software for exponential fit (tail-fit
data analysis). Samples were freshly prepared and degassed with
at least four freeze-pump-thaw cycles on a high-vacuum line in
a two-compartment cell consisting of a 10 mL Pyrex bulb and
a 1 cm path length quartz cuvette.
(t, 30H,
J
¼
7.3 Hz; eCH2CH2CH2CH3), 0.84e0.93 (m, 12H;
eCH2CH2CH2CH3), 1.24e1.31 (vq, 54H, J ¼ 8.0 Hz; eCH3), 1.98e2.04
(m, 6H; eCH2CH2CH2CH3), 2.24e2.27 (m, 36H; eCH2P), 2.84e2.91
(m, 6H; eCH2CH2CH2CH3), 3.79 (s, 9H; eC6H4OMe), 6.78 (d, 6H,
J ¼ 8.7 Hz; eC6H4e), 7.23 (d, 6H, J ¼ 8.7 Hz; eC6H4e), 7.29 (d, 3H,
J ¼ 8.3 Hz; protons at C-3, C-8, C-13), 7.33 (s, 3H; protons at C-1, C-6,
C-11), 8.16 (d, 3H, J ¼ 8.3 Hz; protons at C-4, C-9, C-14). 31P{1H} NMR
(162 MHz, CDCl3, 298 K, relative to 85% H3PO4):
d
11.00
(s, JPteP ¼ 2381 Hz). IR (KBr disk,
n n(C^C). Posi-
/cmꢀ1): 2099 (m)
tive-ion FAB-MS: m/z: 2438 [M þ 2]þ. Elemental analysis calcd (%)
for 1: C 59.17, H 7.20; found: C 58.91, H 7.00.
2.3.2. [{(C6H5-oxa-C6H4C^C)(PEt3)2PtC^C}3truxene] (2)
The procedure was similar to that for 1 except that 2-(4-ethy-
nylphenyl)-5-phenyl-1,3,4-oxadiazole (0.10 g, 0.40 mmol) was used
instead of 1-ethynyl-4-methoxybenzene. Subsequent recrystalli-
zation of the crude product with dichloromethaneemethanol
afforded 2 as a yellow solid. Yield: 0.22 g, 79%. 1H NMR (400 MHz,
CDCl3, 298 K, relative to Me4Si):
d
0.47 (t, 30H, J ¼ 7.3 Hz;
eCH2CH2CH2CH3), 0.85e0.94 (m,12H; eCH2CH2CH2CH3), 1.26e1.34
(vq, 54H, J ¼ 8.0 Hz; eCH3), 1.99e2.04 (m, 6H; eCH2CH2CH2CH3),
2.25e2.29 (m, 36H; eCH2P), 2.86e2.91 (m, 6H; eCH2CH2CH2CH3),
7.30 (d, 3H, J ¼ 8.1 Hz; protons at C-3, C-8, C-13), 7.35 (s, 3H; protons
at C-1, C-6, C-11), 7.41 (d, 6H, J ¼ 8.4 Hz; eC6H4e), 7.52e7.55 (m, 9H;
eC6H5e), 7.99 (d, 6H, J ¼ 8.4 Hz; eC6H4e), 8.13e8.15 (m, 6H;
eC6H4e), 8.18 (d, 3H, J ¼ 8.4 Hz; protons at C-4, C-9, C-13). 31P{1H}
Cyclic voltammetric measurements were performed using a CH
Instruments, Inc., model CHI 750A electrochemical analyzer. Elec-
trochemical measurements were performed in dichloromethane
solutions with 0.1 M nBu4NPF6 as the supporting electrolyte at room
temperature. The reference electrode was an Ag/AgNO3 (0.1 M in
acetonitrile) electrode and the working electrode was a glassy
carbon electrode (CH Instruments, Inc.) with a platinum wire as the
counter electrode. The working electrode surface with first pol-
NMR (162 MHz, CDCl3, 298 K, relative to 85% H3PO4):
d
11.24
(s, JPteP ¼ 2362 Hz). IR (KBr disk,
n n(C^C). Posi-
/cmꢀ1): 2097 (m)
tive-ion FAB-MS: m/z: 2510 [M]þ. Elemental analysis calcd (%) for 2:
C 60.96, H 6.53, N 3.03; found: C 60.88, H 6.42, N 3.46.
2.3.3. [{(NpC^C)(PEt3)2PtC^C}3truxene] (3)
The procedure was similar to that for 1 except that 1-ethy-
nylnaphthalene (0.06 g, 0.40 mmol) was used instead of 1-ethynyl-
4-methoxybenzene. Subsequent recrystallization of the crude
product with dichloromethaneemethanol afforded 3 as a pale
orange solid. Yield: 0.10 g, 40%. 1H NMR (400 MHz, CDCl3, 298 K,
ished with 1
mm alumina slurry (CH Instruments, Inc.) and then
with 0.3 m alumina slurry (CH Instruments, Inc.) on a microcloth
m
(Buehler Co.). It was rinsed with ultrapure deionized water and
sonicated in a beaker containing ultrapure deionized water for
5 min. The polishing and sonicating steps were repeated twice and
then the working electrode was rinsed under a stream of ultrapure
deionized water. The ferrocenium/ferrocene couple (FeCpþ2 /0) was
used as the internal reference [88]. All solutions for electrochemical
studies were deaerated with prepurified argon gas prior to
measurements.
relative to Me4Si):
d
0.47 (t, 30H, J ¼ 7.2 Hz; eCH2CH2CH2CH3),
0.84e0.95 (m, 12H; eCH2CH2CH2CH3), 1.25e1.34 (vq, 54H,
J ¼ 8.0 Hz; eCH3), 1.99e2.04 (m, 6H; eCH2CH2CH2CH3), 2.25e2.32
(m, 36H; eCH2P), 2.86e2.92 (m, 6H; eCH2CH2CH2CH3), 7.31 (d, 3H,
J ¼ 8.1 Hz; protons at C-3, C-8, C-13), 7.33 (s, 3H; protons at C-1, C-6,
C-11), 7.34e7.37 (m, 3H; Np), 7.42e7.49 (m, 9H; Np), 7.62 (d, 3H,
J ¼ 8.2 Hz; Np); 7.79 (d, 3H, J ¼ 9.0 Hz; Np), 8.18 (d, 3H, J ¼ 8.1 Hz;
protons at C-4, C-9, C-14), 8.51 (d, 3H, J ¼ 9.0 Hz; Np). 31P{1H} NMR
2.3. Synthesis of truxene-containing platinum(II) alkynyl complexes
(162 MHz, CDCl3, 298 K, relative to 85% H3PO4):
d
11.55
(s, JPteP ¼ 2365 Hz). IR (KBr disk,
n n(C^C). Posi-
/cmꢀ1): 2089 (m)
2.3.1. [{(MeOC6H4C^C)(PEt3)2PtC^C}3truxene] (1)
tive-ion FAB-MS: m/z: 2496 [M]þ. Elemental analysis calcd (%) for 3:
This was synthesized according to modification of a literature
procedure for the related platinum(II) phosphine alkynyl
complexes [70,77]. [{Cl(PEt3)2PtC^C}3truxene] (0.12 g, 0.10 mmol)
and 1-ethynyl-4-methoxybenzene (0.05 g, 0.40 mmol) were dis-
solved in a mixture of THF (20 mL) and triethylamine (10 mL). CuI
(5 mg, 0.03 mmol) was added to this reaction mixture as a catalyst.
The yellow mixture was then stirred under nitrogen overnight at
room temperature, after which the solvent was removed under
reduced pressure. The yellow residue was then dissolved in
dichloromethane, washed successively with brine and deionized
C 62.08, H 7.03; found: C 62.33, H 7.24.
2.3.4. [{(MeOC6H4C^C)(PPh3)2PtC^C}3truxene (4)
Trans-[(MeOC6H4C^C)(PPh3)2PtCl] (0.16 g, 0.22 mmol) and
triethynylhexabutyltruxene (0.78 g, 0.88 mmol) were dissolved in
a mixture of THF (40 mL) and triethylamine (10 mL). CuI (5 mg,
0.03 mmol) was added to this reaction mixture as a catalyst. The
yellow suspension was then refluxed under nitrogen overnight,
after which the solvent was removed under reduced pressure. The
yellow residue was then dissolved in dichloromethane, washed

1166
C.K.M. Chan et al. / Journal of Organometallic Chemistry 696 (2011) 1163e1173
successively with brine and deionized water, and dried over
anhydrous Na2SO4. The solution was then filtered, and the solvent
was removed under reduced pressure. Further purification was
accomplished by column chromatography on basic aluminum
values larger than
4
s
(F0) was also given (corresponding to
P
P
Intensity
ꢄ
2
s
(I)). wR2 ¼ f ½wðFo2 ꢀ Fo2Þ2ꢅ= ½wðF02Þ2ꢅg1=2
,
P
P
R1
¼
jjFoj ꢀ jFcjj= jFoj. The goodness of fit (GoF) was always
P
based on F2: GoF ¼ S ¼ f ½wðF02 ꢀ Fc2Þ2ꢅ=ðn ꢀ pÞg1=2, where n was
oxide (50e200
mm), in which the excess trans-[(MeOC6H4C^C)
the number of reflections and p was the total number of parameters
(PPh3)2PtCl] was first eluted with dichloromethane–n-hexane (1:1,
v/v) followed by the elution of 4 with dichloromethane. Subsequent
recrystallization of the crude product with dichlor-
omethaneemethanol afforded 4 as a light yellow crystal. Yield:
0.62 g, 85%. 1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si):
refined. The weighting scheme was: w ¼ 1=½
s
ðF02Þ þ ðaPÞ2 þ bPꢅ,
where P was ½2Fc2 þ MaxðFo2; 0Þꢅ=3. Convergence ((
D/s
)
¼ 0.001,
max
av. 0.001) for 182 variable parameters by full-matrix least-squares
refinement on F2 reached to R1 ¼ 0.0852 and wR2 ¼ 0.2272 with
a goodness-of-fit of 1.033; the parameters a and b for weighting
scheme were 0.1522 and 0.00. The final difference Fourier map
showed maximum rest peaks and holes of 2.056 (near Pt) and
ꢀ1.111 eÅꢀ3 respectively.
d
0.38 (t, 30H, J ¼ 7.3 Hz; eCH2CH2CH2CH3), 0.74e0.88 (m, 12H;
eCH2CH2CH2CH3), 1.67e1.74 (m, 6H; eCH2CH2CH2CH3), 2.64e2.72
(m, 6H; eCH2CH2CH2CH3), 3.67 (s, 9H; eC6H4OMe), 6.18 (d, 6H,
J ¼ 8.8 Hz; eC6H4e), 6.35 (s, 3H; protons at C-1, C-6, C-11), 6.36
(d, 3H, J ¼ 7.4 Hz; protons at C-3, C-8, C-13), 6.46 (d, 6H, J ¼ 8.8 Hz;
eC6H4e), 7.35e7.44 (m, 54H; ePPh3), 7.81 (d, 3H, J ¼ 7.4 Hz;
protons at C-4, C-9, C-14), 7.82e7.88 (m, 36H; ePPh3). 31P{1H} NMR
3. Results and discussion
3.1. Synthesis and characterization of the metal complexes
(162 MHz, CDCl3, 298 K, relative to 85% H3PO4):
d
18.59
(s, JPteP ¼ 2639 Hz). IR (KBr disk,
n
/cmꢀ1): 2109 (w)
n
(C^C). Posi-
The synthetic routes for the trinuclear truxene-containing
platinum(II) complexes were outlined in Scheme 1. The branched
platinum(II) bisealkynyl complexes 1e3 were synthesized using
a copper-catalyzed dehydrohalogenation approach with the cor-
responding precursor complex, [{Cl(PEt3)2PtC^C}3truxene]. It
has been demonstrated that addition of a catalytic amount of
copper(I) halide in the preparation of these platinum(II) alkynyl
complexes could increase the rate of reaction through trans-
metallation processes [91]. Triphenylphosphine-containing plat-
inum(II) complex 4 was prepared in a different way from that of its
triethylphosphine analogues, in which triethynylhexabutyltruxene
and trans-[(MeOC6H4C^C)(PPh3)2PtCl] were heated to reflux at
elevated temperature so as to enhance the solubility of the starting
materials. Complexes 1e4 were obtained in reasonable yields with
different alkynyl ligands, which demonstrated that the precursor
complex, [{Cl(PEt3)2PtC^C}3truxene], is a versatile starting mate-
rial for the synthesis of luminescent branched carbon-rich plat-
inum-containing materials.
tive-ion FAB-MS: m/z: 3301 [M]þ. Elemental analysis calcd (%) for
4$H2O: C 69.49, H 5.35; found: C 69.18, H 5.40.
2.4. Crystal structure determination
Single crystals of [{(MeOC6H4C^C)(PPh3)2PtC^C}3truxene] (4)
suitable for X-ray diffraction studies were grown by layering
methanol onto a concentrated dichloromethane solution of the
complex.
Crystal data for [{(MeOC6H4C^C)(PPh3)2PtC^C}3truxene] (4):
[C192H174O3P6Pt3]; formula weight ¼ 3300.40, trigonal, space group
P321 (No. 150), a ¼ 26.425(6) Å, b ¼ 26.425(6) Å, c ¼ 14.535(3) Å,
a
m
¼ 120ꢂ,
b
¼ 90ꢂ,
g
¼ 90ꢂ, V ¼ 8790(3) Å3, Z ¼ 2, Dc ¼ 1.247 mg mꢀ3
,
(Mo-Ka) ¼ 2.486 mmꢀ1, F(000) ¼ 3348, T ¼ 173 K. A light yellow
crystal of dimensions 0.20 mm ꢁ 0.30 mm ꢁ 0.60 mm was used for
data collection at 173 K on an Oxford Diffraction Gemini S Ultra X-
Ray single crystal diffractometer using graphite monochromatized
Mo-Ka radiation (
l
¼ 0.71073 Å). The structure was solved by direct
All the complexes have been characterized by 1H and 31P{1H}
NMR spectroscopy, IR spectroscopy, positive FAB mass spectrom-
etry, and gave satisfactory elemental analyses. The IR spectra of
these trinuclear truxene-containing platinum(II) alkynyl complexes
showed a moderate band at ca. 2100 cme1, which is assigned as the
methods by employing the SHELXS-97 [89] program on PC. Pt, P
and many non-H atoms were located according to the direct
methods. The positions of the other non-hydrogen atoms were
found after successful refinement by full-matrix least-squares
using program SHELXL-97 [90] on PC. There was one third of
formula unit in the asymmetric unit. Restraints were applied to
the benzene rings, assuming them to be regular hexagon rings
with edges of 1.39 Å. Restraints were also applied to the n-butyl
groups, assuming bond lengths of CeC to be around 1.55(2) Å. The
OeC bond lengths were also assumed to be around 1.50(2) Å. The
absolute structure was assisted by the calculated Flack absolute
structure parameter, which was equal to 0.09(2). According to the
SHELXL-97 program [90], all 9198 independent reflections (Rint
n
(C^C) stretch due to the alkynyl group attached to the platinum
(II) centre. The 31P{1H} NMR spectra of complexes 1e4 all showed
a singlet signal, which is indicative of the highly symmetrical
structure of the molecules, in the range of
triethylphosphine-containing complexes 1e3 and
d
11.0e11.6 ppm for the
18.6 ppm for
d
the triphenylphosphine-containing complex 4. Platinum satellites
with JPteP w2360 Hz were observed for complexes 1e3, which are
characteristic of a trans-PePteP configuration [92].
equal to 0.0578, 6321 reflections larger than
4
s
(Fo), where
3.2. Crystal structure analysis of complex 4
P
P
Rint
¼
jFo2 ꢀ Fo2ðmeanÞj= jFo2j) from a total 22393 reflections
were participated in the full-matrix least-squares refinement
against F2. These reflections were in the range ꢀ32 ꢃ h ꢃ 31,
ꢀ31 ꢃ k ꢃ 28, ꢀ17 ꢃ l ꢃ 14 with 2qmax equal to 51.36ꢂ. In the final
stage of least-squares refinement, only Pt and P atoms were refined
anisotropically, other non-H atoms were refined isotropically. H
atoms were generated by program SHELXL-97 [90]. The positions
of H atoms were calculated based on riding mode with thermal
parameters equal to 1.2 times that of the associated C atoms
were refined isotropically, and participated in the calculation of
final R-indices. Since the structure refinements were against F2, R-
indices based on F2 were larger than (more than double) those
based on F. For comparison with older refinements based on F and
an OMIT threshold, a conventional index R1 based on observed F
Fig. 1 shows the perspective drawing of complex 4. The crystal
structure determination data is summarized in Table 1, and the
selected bond lengths and angles are given in Table 2. Only the ipso
carbons on the butyl chains and the phenyl rings in the triphe-
nylphosphines are shown in the figure. The platinum atoms in
complex 4 adopted essentially a trans-square planar arrangement
with PePteC angles in the range of 84.7(5) and 95.0(5)ꢂ. Such slight
distortion around the coordination plane was likely due to the
coordination constraints imposed by the bulky triphenylphosphine
ligands [93,94]. The Pt–C bond distances (1.958(19)e2.049(19) Å),
as well as the C^C bond lengths (1.16(3)e1.29(3) Å) are found to be
within the expected ranges for platinum(II) alkynyl complexes
[62,64,66,76e80]. The distances of the PteP bond in 4 were in the

C.K.M. Chan et al. / Journal of Organometallic Chemistry 696 (2011) 1163e1173
1167
Scheme 1. Synthesis of truxene-containing trinuclear platinum(II) complexes 1e4.
range of 2.268(6) and 2.298(4) Å, which were slightly shorter than
those reported previously [83,95]. The bond angles about the
alkynyl ligand and the platinum metal centres were in the range of
168.8(15)e170.9(18)ꢂ, which were close to the ideal sp-hybridized
characteristic
of
platinum
s
-aryl
alkynyl
complexes
[62,64,66,76e80]. The truxene backbone was nearly coplanar with
six butyl chains lying out of the aromatic plane, as was expected for
the sp3-hybridized carbons. No
p/p stacking was found due to the
carbon bond angle and further confirmed the
s
-bonded nature of
presence of these butyl chains sticking out of the planes from the
backbone.
the alkynyl groups in the complex. Out-of-plane twisting of the
coordination planes around the platinum atoms in complex 4 has
also been observed, similar to that previously reported in other
platinum(II) complexes [62,64,66,76e80]. The coordination planes
about the platinum atoms were not coplanar with the aromatic
rings and gave interplanar angles of around 26.3ꢂ. This feature was
3.3. Electronic absorption spectra of the metal complexes
The electronic absorption spectra of these trinuclear platinum
(II) complexes showed intense absorption bands at ca.

1168
C.K.M. Chan et al. / Journal of Organometallic Chemistry 696 (2011) 1163e1173
Table 2
Selected bond distances (Å) and bond angles (ꢂ) with estimated standard deviations
(e.s.d.s.) in parentheses for complex 4.
Selected bond distances (Å)
Selected bond angles (ꢂ)
Pt(1)eC(1)
Pt(1)eC(20)
Pt(1)eP(1)
Pt(1)eP(2)
C(1)eC(2)
C(20)eC(21)
C(25)eO(1)
C(28)eO(1)
1.958(19)
2.049(19)
2.298(4)
2.268(6)
1.29(3)
1.16(3)
1.513(18)
1.542(19)
C(1)ePt(1)eC(20)
C(20)ePt(1)eP(1)
C(1)ePt(1)eP(2)
C(20)ePt(1)eP(2)
C(1)ePt(1)eP(1)
C(20)ePt(1)eP(1)
C(2)eC(1)ePt(1)
C(20)eC(21)ePt(1)
C(1)eC(2)eC(3)
C(20)eC(21)eC(22)
C(7)eC(11)eC(16)
C(10)eC(11)eC(12)
C(7)eC(11)eC(12)
C(12)eC(11)eC(16)
C(10)eC(11)eC(16)
C(12)eC(11)eC(6)
176.5(7)
178.5(3)
92.2(6)
84.7(5)
88.2(6)
95.0(5)
168.8(15)
170.9(18)
161.9(17)
175(2)
106.2(15)
117.2(15)
109.6(14)
108.1(15)
113.0(16)
108.1(15)
Fig. 1. Perspective drawing of complex 4 with atomic numbering scheme. Only the
ipso-carbons of the butyl chains and the phenyl groups in triphenylphosphine were
shown. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids were shown
at the 30% probability level.
spectra of these complexes in dichloromethane and benzene
were found to be nearly identical. These absorption bands were
found to be relatively insensitive towards the polarity of the
solvents, in accordance with the largely intraligand
p/
p* char-
acter of these transitions. A red shift in the absorption energy of
354e382 nm with extinction coefficients in the order of
104ꢀ105 dm3 molꢀ1 cmꢀ1. Fig. 2 shows the electronic absorption
spectra of [{Cl(PEt3)2PtC^C}3truxene], and complexes 2 and 3 in
benzene at room temperature. The photophysical data of
complexes 1e4 are summarized in Table 3.
In the view of rich vibronic structure and the very large
extinction coefficient observed for the chloroplatinum(II) precursor
complex, [{Cl(PEt3)2PtC^C}3truxene], that are comparable to the
corresponding truxene alkynyl ligand, a substantial mixing of an
intraligand (IL) [p/
p*(C^C)3truxene] character of the central
alkynyl core unit is likely. In addition, the electronic absorption
triethynylhexabutyltruxene (314e328 nm) to [{Cl(PEt3)2PtC^C}3
-
truxene] (316e352 nm) to complexes 1e4 (354e382 nm) is
observed, which suggests an involvement of the platinum(II)
metal centre in such transition. With reference to previous spec-
troscopic studies on trans-[Pt(PEt3)2(ChCR)2] [96e100], in which
the absorption bands at ca. 300e360 nm were assigned to contain
a platinum-to-alkynyl metal-to-ligand charge transfer (MLCT)
transitions, the low-energy transitions in these branched
complexes might also involve a platinum-to-alkynyl MLCT char-
acter. Although absorptions due to the IL [p/
p*] transitions of
the peripheral alkynyl ligands may also occur at similar energies
in the related systems [96e100], the insensitive nature of the
absorption energies in 1e3 to the peripheral alkynyl ligands
suggests a minor contribution of the peripheral aryl alkynyl
ligands in these low-energy absorptions. Therefore, the low-
energy absorptions in these branched complexes are described
Table 1
Crystal and data collection parameters for complex 4.
Empirical formula
C192H174O3P6Pt3
Formula weight
Crystal system
Space group
a (Å)
3300.40
Trigonal
P 321(No. 150)
26.425(6)
as an admixture of IL [p/ p(Pt)/
p*(C^C)3truxene] and MLCT [d
p*(C^C)3truxene] transitions with predominantly IL character.
b (Å)
26.425(6)
c (Å)
14.535(3)
90.00
90.00
120.00
a
b
g
(ꢂ)
(ꢂ)
(ꢂ)
V (Å3)
8790(3)
Z
2
Dcal (gcmꢀ3
)
1.217
2.486
m
(mmꢀ1
)
F(0 0 0)
T (K)
3348
173(2)
Crystal colour
Crystal size
Light yellow
0.30 mm ꢁ 0.20 mm ꢁ 0.15 mm
q
range for data collection
3.20e25.68ꢂ
Index ranges
Total reflections collected
Independent reflections
ꢀ32 ꢃ h ꢃ 31, ꢀ31 ꢃ k ꢃ 28, ꢀ17 ꢃ l ꢃ 14
22393
9198 [Rint ¼ 0.0578]
Completeness to
q
¼ 25.68ꢂ
98.1%
Data/restraints/parameters
9198/10/182
1.033
Goodness-of-fit on F2
Final R-indices [I > 2
R indices (all data)
Largest diff. peak and hole
s(I)]
R1 ¼ 0.0852, wR2 ¼ 0.2272a
R1 ¼ 0.1142, wR2 ¼ 0.2399a
2.056 and ꢀ1.111 eÅꢀ3
a
Fig. 2. Electronic absorption (left) and normalized emission spectra (right) of [{Cl
(PEt3)2PtC^C}3truxene] (/), 2 (d) and 3 (- - -) in benzene at room temperature.
w ¼ 1/[
s
2(Fo2) þ (aP)2 þ bP], where P is [2Fc2 þ Max(F2o,0)]/3.

C.K.M. Chan et al. / Journal of Organometallic Chemistry 696 (2011) 1163e1173
1169
Table 3
Electronic absorption and emission data of complexes 1e4.
b
Complex
labs [nm]
[dm3molꢀ1 cmꢀ1])a
Medium (T [K])
lem [nm]
so s])
Flum
(e
(
[m
1
320 (68200, sh),
354 (188900),
366 (187400)
C6H6 (298)
solid (298)
solid (77)
glass (77)c
C6H6 (298)
solid (298)
solid (77)
glass (77)c
C6H6 (298)
solid (298)
solid (77)
glass (77)c
C6H6 (298)
solid (298)
solid (77)
glass (77)c
515 (21)
517 (4.9)
520 (29)
509 (584)
516 (42)
ed
0.05
2
3
4
308 (103400),
366 (305700)
0.07
0.06
ee
535 (80)
516 (243)
547 (60)
ed
308 (60500),
356 (203700, sh),
364 (208500)
553 (53)
540 (513)
517
342 (80000, sh),
356 (101400, sh),
382 (207900)
ed
514 (166)
506 (930)
a
Measured in C6H6 at 298 K.
The luminescence quantum yield, measured at room temperature using quinine
b
sulfate in 1.0
N
H2SO4 as the reference (excitation wavelength
¼
365 nm,
Flum ¼ 0.55).
c
Measured in EtOH-MeOH (4:1, v/v) glass.
Non-emissive.
Not determined.
d
e
3.4. Emission properties of the metal complexes
Upon excitation at
ꢄ 365 nm, vibronically structured emission
l
bands were observed for complexes 1 (515 nm), 2 (516 nm), and
3 (547 nm) in degassed benzene solutions while the triphenyl-
phosphine-containing complex 4 was weakly emissive (517 nm)
at room temperature. All complexes exhibited luminescence at 77 K
and showed large Stokes shift and lifetimes in the microsecond
range, which are indicative of their triplet parentage. The strong
spin-orbit coupling introduced by the heavy platinum(II) centre
would enhance the accessibility of the 3IL [ p*] excited states.
p/
The emission data of the complexes are summarized in Table 3. In
contrast to the non-emissive behaviour of the precursor complex,
[{Cl(PEt3)2PtC^C}3truxene], at room temperature, the bis-alkynyl
complexes 1e3 showed intense yellowish green to yellowish-
orange vibronically structured emission bands at room tempera-
ture, with emission maxima at about 515e547 nm. The emission
of 1 and 4 were found to occur at nearly identical wavelengths with
similar vibronic structures as the emission bands of their corre-
sponding chloroplatinum(II) complexes at 77 K, either in the solid
state or in alcoholic glass. The emission energy of 1 and 4 appeared
to be insensitive to the peripheral aryl alkynyl ligands. This finding
is supportive of a triplet IL emission derived mainly from the
central triethynylhexabutyltruxene moiety. However, the emission
energies observed for the branched complexes were found to show
a red shift on going from palladium complexes to platinum
analogues, which suggested the involvement of some triplet MLCT
character in the emissive state [76,77,79]. Therefore, the emission of
these complexes was assigned as derived from a mixed 3IL
Fig. 3. Emission spectra of 1 (top), 2 (middle) and 3 (bottom) in ethanol–methanol
(4:1, v/v) glass at 77 K. Excitation wavelength at 370 nm.
emission that displayed vibronically structured emission bands at
room temperature, with the emission maximum at 516 nm. Its
emission maximum was found to be similar to that of complexes 1
and 4. However, the band shapes of this complex either in benzene
solution at room temperature or in alcoholic glass at 77 K were
different from those observed for 1 and the other analogues (Fig. 3)
[70]. With reference to previous spectroscopic studies on [{ClPt
(PEt3)2(C^CC6H4)}2-oxa-2,5] in alcoholic glass at 77 K (518 nm)
[70], the emission of 2 is likely to be originated from similar
intraligand triplet excited states that are originated from the
peripheral oxadiazole moieties of lower-lying energy, with some
mixing of dp
(Pt)/p*(ChC-oxa-2,5) 3MLCT character. Similarly,
complex 3 with polyaromatic peripheral naphthalene-containing
ligands was found to emit in the yellowish-orange region at
547 nm, which is red-shifted from its analogues. With reference
to previous spectroscopic studies on cis-[Pt(dppe)(C^CNp)2] [101],
in which the 3IL emissions of these platinum complexes with
polyaromatic naphthyl alkynyl ligands have been observed at
similar energies with comparable vibronic structures, the
emission of 3 has been assigned as derived from states of 3IL
[p/ p
p*(C^C)3truxene]/3MLCT [d (Pt)/p*(C^C)3truxene] state
with predominantly IL character. The emission from the 3IL state of
the peripheral alkynyl ligands was not observed since they are
higher-lying in energy. The energy absorbed by the peripheral
ligands would be transferred to the central truxene emitting core
through directional energy transfer. Similar findings have also been
observed in other related branched platinum(II) alkynyl complexes
[70,77].
[p/
p*(C^CNp)] character. The lack of 3IL emission from the
truxene core could be ascribed to the presence of the lower-lying
3IL excited state associated with the peripheral NpC^C units, i.e.
the energy absorbed by the central truxene core would be
Degassed benzene solution of the branched platinum(II) alkynyl
complex 2 with oxadiazole ligand showed intense yellowish green

1170
C.K.M. Chan et al. / Journal of Organometallic Chemistry 696 (2011) 1163e1173
Table 4
emission induced by 720-nm excitation could not be attributed to
a linear process but rather to a nonlinear process. The upconverted
emissions of these complexes were nearly identical to that
observed in their corresponding single-photon excited emission.
The dependence of the upconverted luminescence intensities
on the incident laser power was measured and a typical power
dependence curve of complex 2 in benzene solution at room
temperature is shown in Fig. 5 as an example. The inset shows the
plot of log(emission intensity) vs. log(laser power) that gave
a straight line with a slope of 1.96. Since the 2PA has a quadratic
dependence on intensity and the TPIL would be expected to show
intensity dependence on the excitation power, theoretically, the log
(emission intensity) vs. log(laser power) should give a straight
line with a slope of 2 [43]. Therefore, the observed quadratic
dependence of the emission intensity on the incident laser power
further confirmed the two-photon nature of the process. The s2
values of these branched complexes with excitation wavelength
at 720 nm have been determined using the TPIL method and
were found to be in the range of 6e51 GM, which are comparable
to the values of 5e10 GM observed in other platinum(II) alkynyl
complexes [38,40,43]. Through a systematic comparison study of
these complexes, the s2 values were found to be slightly dependent
on the electronic properties of the peripheral substituents. In
general, higher s2 values could be observed for complexes with
relatively more electron-rich or electron-deficient peripheral
substituents. Complex 2 gave the greatest s2 values among these
truxene-containing complexes. It was interesting to note that
incorporation of oxadiazole, which is a well-known electron-defi-
cient moiety, would create a large change in the dipole moment
within the molecules, which led to larger s2 values. However,
complex 3 did not give rise to an increase in s2 values as compared
to its analogues [70], because the naphthyl moieties are less
electron-rich than the tolyl groups, which has been reported
previously by our group [69]. These findings have demonstrated
Nonlinear photophysical data for complexes 1e3 at 298 K.
Complex
lema/nm at lex ¼ 720 nm
s2/GM
Power dependence
1
2
3
516
517
549
12
51
6
2.04
1.96
1.73
a
Measured in C6H6.
transferred to the lowest energy emissive state of the peripheral
ligands. Therefore, the emission of 3 is assigned as derived
predominantly from the 3IL states of the naphthyl alkynyl moieties
with some mixing of dp
(Pt)/p*(C^CNp) 3MLCT character [70,77].
The emission spectrum of the complexes 1 and 4 in 77 K glass
showed rich vibronic structures with vibrational progressional
spacings of ca. 1220 and 2100 cmꢀ1, typical of the aromatic ring
deformation and the
n(C^C) vibrational modes, respectively.
Additional vibronic structures with progressional spacings of ca.
1098 and 1555 cmꢀ1 from the highest energy emission band in
complex 2 were also observed, with the former corresponding to
the
n(CeO) stretch and the latter to the n(C]C) and n(C]N)
vibrational modes. Vibrational progressional spacings of ca.
1380 cme1 were observed in the emission spectrum of complex 3,
indicative of the n(C]C) stretching mode of aromatic rings. The
emission lifetimes in the solid state at room temperature are
considerably shorter than that observed in dilute solutions, which
may be ascribed to originate from tripletetriplet annihilation or
self-quenching in the solid state.
3.5. Two-photon induced luminescence (TPIL) properties of the
metal complexes
Complexes 1e3 are found to emit in the yellowish green and
yellowish orange regions in benzene solutions upon excitation with
a mode-locked femtosecond Ti:Sapphire laser at 720 nm. Table 4
shows the emission maxima and two-photon absorption cross-
section s2 at 720 nm determined using TPIL study. Fig. 4 shows the
TPIL spectrum of complex 2 in benzene at room temperature. No
linear absorption in the wavelength range of 500e820 nm was
observed for these platinum(II) complexes, indicating that the
that truxene serves as
a promising building block for the
construction of two-photon induced luminescent materials.
Through the incorporation of [{Cl(PEt3)2PtC^C}3truxene] and
alkynyl ligands with different electronic properties, a versatile
handle for the tuning of the s2 values and the emission energies
of this class of complexes could be readily achieved.
Fig. 5. Power dependence of the upconverted luminescence intensity of complex 2 in
benzene at 298 K. The line (-) shows the theoretical curve for the quadratic function.
The inset shows the log(emission intensity) (:) vs. log(laser power) and its linear
regression.
Fig. 4. Two-photon induced luminescence spectrum of complex 2 in benzene at room
temperature upon excitation with a mode-locked femtosecond Ti:Sapphire laser at
720 nm.

C.K.M. Chan et al. / Journal of Organometallic Chemistry 696 (2011) 1163e1173
1171
Table 5
Electrochemical data for truxene-containing trinuclear branched plat-
a b
,
inum(II) alkynyl complexes at 298 K.
Complex
Oxidation Epac [V versus S.C.E.]
1
2
3
4
þ0.99, þ1.29, þ1.49
þ0.96, þ1.19, þ1.65
þ0.94, þ1.09, þ1.45
þ0.99, þ1.30
a
Working electrode, glassy carbon; scan; scan rate ¼ 100 mV sꢀ1
Measured in dichloromethane (0.1 M nBu4NPF6).
Epa is the peak anodic potential.
.
b
c
These experiments were conducted with 355 nm pulses from
a Nd:YAG laser. All the complexes exhibited strong bleaching bands
at ca. 350e400 nm in the near-UV to blue region, with a broad and
moderately intense excited state absorption feature extending
throughout the whole visible region with a maximum between 400
and 700 nm. These findings are comparable to those observed in
other related platinum(II) diphosphine alkynyl monomers and
polymers [42,44,53,102e109]. The transient absorption spectra for
complexes 1e3 are shown in Fig. 6.
The negative difference absorption bands found at ca. 370 nm
for complexes 1e3 are characteristic of the
p,
p* (S0/S1) absorp-
tion [44,103,106,109]. The bands of the ground-state bleaching
represented the ground-state absorption of the central triethy-
nylhexabutyltruxene and the peripheral alkynyl moieties. The
positive bands that occurred at 400e700 nm for these complexes
could be assigned as derived from the 3p p* absorption of the
,
triplet excited state. The transient absorption decay lifetimes in
each case are found to be comparable to that of their emission
decay lifetimes, which supported the assignment of a transient
absorption due to the tripletetriplet transition from the lowest
energy triplet excited state [109]. The transient absorption spec-
trum of 3 showed a broad excited state absorption at lmax w550 nm
with a lifetime of 62
ms. This absorption is believed to arise from
the 3p p* excited-state localized on the peripheral naphthalene-
,
containing ligands. Similar findings have been observed for
a mononuclear platinum(II) bis(pyrenylethynyl) complex reported
by Ziessel and Castellano [102,104].
3.7. Cyclic voltammetry of the metal complexes
The electrochemical properties of the trinuclear platinum(II)
alkynyl complexes containing truxene as the core have been inves-
tigated in dichloromethane (0.1 M nBu4PF6) by cyclic voltammetry
and the data are summarized in Table 5. No observable reductive
wave for all the complexes upon scanning up to ꢀ2.0 V vs. S.C.E.
was found, which is similar to that reported in other structurally
related platinum(II) alkynyl complexes [70,77,79,80,110,111].
The oxidative scans of complexes 1e4 show two to three irre-
versible waves at þ0.94 to þ0.99, þ1.09 to þ1.30 and þ1.45 to þ1.65
vs. S.C.E. In the view of the observation of a similar first oxidation
of its chloroplatinum(II) precursor complex [{Cl(PEt3)2PtC^C}3-]
truxene] (þ1.06 V vs. S.C.E.) [70], the first irreversible oxidation
waves of complexes 1e4 are probably due to the central triethy-
nylhexabutyltruxene moiety. The irreversible oxidation waves of
these complexes might also involve a certain degree of metal-
centred character. Therefore, the irreversible oxidation waves of
this class of complexes could best be described as ligand-centred
oxidation with some mixing of a metal-centred character.
Fig. 6. Transient absorption difference spectra of 1 (top), 2 (middle) and 3 (bottom) in
degassed benzene solution at 298 K following 355 nm pulsed excitation. The inset
shows the transient absorption decays.
3.6. Transient absorption properties of complexes 1e3
In order to provide additional information on the properties of
the triplet state of the platinum(II) phosphine complexes produced
by photoexcitation, nanosecond transient absorption spectra were
measured at room temperature in degassed benzene solution.
4. Conclusions
Several trinuclear luminescent platinum(II) alkynyl complexes
of truxene were successfully synthesized and characterized. The

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C.K.M. Chan et al. / Journal of Organometallic Chemistry 696 (2011) 1163e1173
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Acknowledgements
V.W.-W.Y. acknowledges support from The University of Hong
Kong under the Distinguished Research Achievement Award
Scheme, and the UGC Strategic Research Theme on Molecular
Materials. The work described in this paper has been supported by
the University Grants Committee Areas of Excellence Scheme (AoE/
P-03/08) and the Special Equipment Grant (SEG_HKU07). C.K.M.C.
acknowledges support from The University of Hong Kong and the
receipt of a University Postgraduate Studentship. C.-H.T. acknowl-
edges support from The University of Hong Kong and the receipt of
a University Postdoctoral Fellowship. Dr. C.C. Ko is gratefully
acknowledged for his kind assistance in the crystallographic data
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