Structural, photophysical, and nonlinear absorption properties of trans-Di-arylalkynyl platinum(II) complexes with phenyl and thiophenyl groups

Article abstract of DOI:10.1021/jp066569p

Optical power limiting and luminescence properties of two Pt(II) complexes with thiophenyl and phenyl groups in the ligands, trans-Pt(P(n-Bu) ₃)₂(C≡C-Ar)₂, where Ar = -C ₄H₂S-C=C≡C-p-C₆H₄-n-C ₅H₁₁ (1) and -p-C₆H₄-C≡C- C₄H₃S (2), have been investigated. The fluorescence lifetimes were found to be on the sub-nanosecond time scale, and the quantum yields were low, in accord with fast intersystem crossing from the excited singlet to triplet manifold. The phosphorescence lifetimes of 1 and 2 were shorter than that of a Pt(II) complex having two phenyl groups in the ligands. In order to elucidate the C-Pt bonding nature in the ground state, the ¹³C NMR chemical shift of the carbon directly bonded to Pt, the coupling constants ¹J_PtC, ²J_PtC, and ¹J_PtP, and IR v_C≡C wavenumbers were obtained for 1, 2, and three other trans-diarylalkynyl Pt(II) complexes. X-ray diffraction data of 1 and 2 and density functional theory calculated geometries of models of 1, 2, and trans-Pt(P(n-Bu)₃)₂(C≡C-p- C₆H₄-C≡C-C₆H₅)₂ (3) show that 1 preferably exists in a different conformation from that of 2 and 3. The variations in photophysical, NMR, and IR data can be rationalized by differences in geometry and π-backbonding from Pt to the alkynyl ligand.

Full text of DOI:10.1021/jp066569p

1598
J. Phys. Chem. A 2007, 111, 1598-1609
Structural, Photophysical, and Nonlinear Absorption Properties of trans-Di-arylalkynyl
Platinum(II) Complexes with Phenyl and Thiophenyl Groups
Per Lind,† Dan Bostro1m,‡ Marcus Carlsson,† Anders Eriksson,| Eirik Glimsdal,§
Mikael Lindgren,§ and Bertil Eliasson*,†
Department of Chemistry, Organic Chemistry, Umeå UniVersity, SE-901 87 Umeå, Sweden, Energy Technology
and Thermal Process Chemistry, Umeå UniVersity, SE-901 87 Umeå, Sweden, Department of Physics,
Norwegian UniVersity of Science and Technology, NO-7491 Trondheim, Norway, and DiVision of Sensor
Technology, Swedish Defence Research Agency (FOI), SE-581 11 Linko¨ping, Sweden
ReceiVed: October 6, 2006; In Final Form: NoVember 22, 2006
Optical power limiting and luminescence properties of two Pt(II) complexes with thiophenyl and phenyl
groups in the ligands, trans-Pt(P(n-Bu)3)2(CtCsAr)2, where Ar ) sC4H2SsCtC-p-C6H4-n-C5H11 (1) and
sp-C6H4sCtCsC4H3S (2), have been investigated. The fluorescence lifetimes were found to be on the
sub-nanosecond time scale, and the quantum yields were low, in accord with fast intersystem crossing from
the excited singlet to triplet manifold. The phosphorescence lifetimes of 1 and 2 were shorter than that of a
Pt(II) complex having two phenyl groups in the ligands. In order to elucidate the C-Pt bonding nature in the
1
ground state, the 13C NMR chemical shift of the carbon directly bonded to Pt, the coupling constants JPtC
,
1
2JPtC, and JPtP, and IR νCtC wavenumbers were obtained for 1, 2, and three other trans-diarylalkynyl Pt(II)
complexes. X-ray diffraction data of 1 and 2 and density functional theory calculated geometries of models
of 1, 2, and trans-Pt(P(n-Bu)3)2(CtC-p-C6H4sCtCsC6H5)2 (3) show that 1 preferably exists in a different
conformation from that of 2 and 3. The variations in photophysical, NMR, and IR data can be rationalized
by differences in geometry and π-backbonding from Pt to the alkynyl ligand.
Introduction
It is likely that the developing photonics technology will take
advantage of optical nonlinearity in organic and organometallic
materials for manipulation of optical signals for various signal
processing tasks.1-3 A class of compounds that has attracted
attention for optical nonlinearity is transition metal acetylides,
where the rigidity and linear geometry of the alkynyl group has
made it an attractive unit in the design of interesting com-
plexes.4,5 These organometallic complexes can show extra-
ordinary photophysical properties due to the electronic interac-
tion between the transition metal and the organic molecular
framework.6,7 Among the compounds that show large cubic
hyperpolarizability, Ru, Ni, and Pt dialkynyl complexes are
fairly common.5 For applications where colorless materials are
required, Pt compounds can be of interest since they generally
have less absorbance than Ru and Ni compounds in the visible
region.5,8
Over the last several years, square-planar platinum(II) acetyl-
ides (Figure 1) is one group of the transition-metal complexes
that has been investigated for two-photon absorption (TPA),
excited-state absorption (ESA), and optical power limiting
(OPL) of high-intensity light.9-19
For OPL applications, a high transmittance of normal-intensity
light in the visible region may be desirable. Optical nonlinearity
of organic molecules with extended π-electron systems generally
increases with conjugation length. However, since this is
Figure 1. General structure of trans-Pt(II) acetylides, with numbering
of alkyne carbons as referred to in the text.
normally accompanied by decreased transmittance in the visible
region, it is crucial to find compounds that display a good
balance between the degree of π-electron delocalization and
transmittance at these wavelengths. Such conjugated compounds
may be comprised of aryl rings separated by other conjugated
groups. Because of the comparatively weak absorption of
diarylacetylenes in the visible region,20 the ethynyl group
appears to be a suitable bridging unit between aryl groups.
In this study, we focus on the molecular structure, optical
limiting ability, and luminescence of two new thiophenyl-
containing Pt(II) acetylide complexes, 1 and 2 (Figure 2).
Properties of 1 and 2 are compared with those of the
structurally related compounds 3 and 4 (Figure 2), both of which
have been studied for nonlinear absorption.9-19 The studies have
shown that 3 exhibits optical limiting that is effective for pulse
lengths ranging from picoseconds to hundreds of nanoseconds.
Good optical power limiting performance can result from
efficient resonant or nonresonant TPA and from ESA when the
absorbance is greater than that of the ground state. ESA may
involve absorption by a triplet state, which is reached by
intersystem crossing (ISC) from an initially populated singlet
state. The presence of a heavy metal atom (such as Pt)
interacting with a conjugated carbon system in the electronically
excited-state can favor ISC by spin-orbit coupling (SOC).21
* To whom correspondence should be addressed. E-mail:
Bertil.Eliasson@chem.umu.se.
† Energy Technology and Thermal Process Chemistry, Umeå University.
‡ Department of Chemistry, Inorganic Chemistry, Umeå University.
§ Norwegian University of Science and Technology.
| Swedish Defense Research Agency.
10.1021/jp066569p CCC: $37.00 © 2007 American Chemical Society
Published on Web 02/10/2007

trans-Di-arylalkynyl Platinum(II) Complexes
J. Phys. Chem. A, Vol. 111, No. 9, 2007 1599
CHART 1: (a) HOC(CH3)2CtCH, PdCl2(PPh3)2, PPh3,
CuI, THF/TEA. (b) n-C5H11sC6H4CtCH, PdCl2(PPh3)2,
PPh3, CuI, THF/TEA. (c) NaH, toluene. (d) PtCl2(PBu3)2,
PBu3, CuI, THF/DEA. (e) PtCl2(PBu3)2, CuI, DEA.
Figure 2. Structures of platinum acetylides 1-4, 9, and 10.
In the case of compound 3, three different wavelength regions
of linear and nonlinear absorption of were identified.11 In the
blue region (<500 nm) intersystem crossing followed single-
photon absorption from the ground state (S0) to the first excited
state (S1). In the green region (500-540 nm), a weak absorption
due to a direct single photon transition from S0 to T1 excited
state was found. At longer wavelengths (∼540-700 nm), the
excitation pathway was identified as two-photon absorption from
S0 to S1, followed by ISC to the triplet state.11
We have introduced a thiophene ring (thienyl) into this type
of structure in an attempt to enhance the optical limiting ability
over that displayed by 3. It has been demonstrated that
compounds with conjugated π-systems that incorporate thiophene
rings can exhibit large two-photon absorptions at, for instance,
60222 and 81023 nm. In addition, studies of thiophene and
oligothiophenes show that these structures have fast triplet level
formation originating from a TPA-populated singlet excited
state.24-27 This suggests that the presence of the S atom may
enhance the TPA, and/or increase the rate of ISC by additional
SOC,28 in Pt acetylides.
The structural data presented here for 1 and 2 include NMR
and crystallographic data. The latter are compared with results
from quantum chemistry calculations at the density functional
theory (DFT) level of theory for compounds 1′-3′ having
methyl groups in lieu of the butyl and pentyl groups in 1-3. In
addition, absorption and emission spectra, excitation energies
from time-dependent DFT calculations, time-resolved lumines-
cence data, and results from optical limiting measurements at
three wavelengths (532, 550, and 610 nm) are presented.
argon. To the solution was added CuI (40 mg, 0.2 mmol), PdCl2-
(PPh3)2 (95 mg, 0.13 mmol), and PPh3 (0.1 g, 0.4 mmol). The
2-methylbut-3-yn-2-ol (0.84 g, 10 mmol) was added dropwise,
and the reaction was allowed to proceed for 48 h at room
temperature and at 60 °C for 24 h. The solution was cooled
and poured into 100 mL of 1 M HCl. The organic layer was
separated, washed with water, and dried, and the solvent was
evaporated. Chromatography on silica using a heptane/EtOAc
5:1 mixture as eluent (Rf ) 0.3) gave the pure product (1.99 g,
1
68%) as an orange oil. H NMR (CDCl3): δ 7.07 (1H, d, J )
4 Hz), 6.8 (1H, d, J ) 4 Hz), 2.52 (1H, s), 1.57 (6H, s). 13C
NMR (CDCl3): δ 136.8, 133.2, 128.7, 99.3, 74.5, 74.3, 65.6,
31.2.
2-Methyl-4-(5-(4-pentylphenylethynyl)thiophen-2-yl)-3-bu-
tyn-2-ol (6). The alcohol 5 (1.0 g, 3.4 mmol) was added to a
mixture of 15 mL of THF and 10 mL of triethylamine under
argon, followed by CuI (20 mg, 0.1 mmol), PdCl2(PPh3)2 (50
mg, 0.07 mmol), and PPh3 (0.04 g, 0.14 mmol). 4-Pentyl-1-
ethynylbenzene (1.0 g, 5.8 mmol) was added dropwise to the
solution, which was then heated to reflux for 5 h. The solution
was allowed to cool and was poured into 50 mL of 1 M HCl.
The organic layer was separated, washed with water, and dried,
and the solvent was evaporated. Chromatography of the organic
residue on a silica column using heptane/EtOAc 5:1 as eluent
(Rf ) 0.4) yielded 0.98 g (86%) of pure product as a red oil.
1H NMR (CDCl3): δ 7.38 (2H, d, J ) 8 Hz), 7.12 (2H, d, J )
8 Hz), 7.05 (1H, d, J ) 4 Hz) 7.02 (1H, d, J ) 4 Hz), 2.57
(2H, t, J ) 8 Hz), 2.44 (1H, s), 1.56 (8H, m), 1.30 (4H, m),
0.88 (3H, t, J ) 7 Hz). 13C NMR (CDCl3): δ 143.9, 131.9,
131.4, 128.5, 124.7, 123.6, 119.5, 98.2, 94.1, 81.5, 75.2, 65.7,
35.9, 31.4, 31.2, 30.5, 22.5, 14.0.
Experimental Section
Solvents and Reagents. Solvents for synthesis were dried
and distilled prior to use. Tetrahydrofuran (THF) from Sigma-
Aldrich Co. for spectrometry was of spectrophotometric grade,
g99.5%. Reagents were obtained from Sigma-Aldrich Co. and
used as received.
Synthesis. The platinum(II) acetylides 1 and 2 were synthe-
sized following the approach described by Sonogashira,29 see
Chart 1. trans-PtCl2(P(n-Bu)3)2,30 3,9 and 419 were prepared as
earlier described. Compound 10 has been reported in the
literature,31 but more details of its synthesis are given here.
4-(5-Iodo-thiophen-2-yl)-2-methyl-3-butyn-2-ol (5). 2,5-
Diiodothiophene (5 g, 14.9 mmol) was dissolved in a mixture
of 40 mL of tetrahydrofuran and 20 mL of triethylamine under
2-Ethynyl-5-(4-pentylphenylethynyl)thiophene (7). The
alcohol 6 (0.91 g, 2.7 mmol) was dissolved in 100 mL of dry
toluene under argon. NaH (0.22 g, 60% dispersion, 5.4 mmol)
was added, and 60 mL of the solvent was distilled off. The
residue was carefully poured into 50 mL of cold 1 M HCl. The
organic phase was removed, evaporated, and filtered through a
short silica column. Chromatography of the residue on a silica
column using heptane as eluent gave pure product as white
crystals, 220 mg (29%). 1H NMR (CDCl3): δ 7.45 (2H, d, J )
8 Hz), 7.17 (3H, dd, J ) 8 and 4 Hz), 7.11 (1H, d, J ) 4 Hz),
3.38 (1H, s), 2.63 (2H, t, J ) 8 Hz), 1.65 (2H, m), 1.35 (4H,
m), 0.94 (3H, t, J ) 7 Hz). 13C NMR (CDCl3): δ 144.1, 133.0,

1600 J. Phys. Chem. A, Vol. 111, No. 9, 2007
Lind et al.
131.4, 131.2, 128.5, 125.4, 122.9, 119.5, 94.3, 82.0, 81.3, 76.6.
MS (EI, 70 eV): m/z (%) 221 (91), 278 (M+, 100), 279 (M +
1, 25).
(t, JCP ) 6.5 Hz), 23.9 (t, JCP ) 17 Hz), 13.8. 31P NMR (CDCl3,
relative to ext. H3PO4, δ ) 0): δ 3.6 (s, d,1JPtP ) 2350 Hz).
trans-Pt(P(n-Bu)3)2(CtC-p-C6H4sCtC-p-C6H4sNO2)2
(10).31 trans-PtCl2(P(n-Bu)3)2 (239 mg, 0.356 mmol), (4-(4-
nitro-phenylethynyl)-phenyl)-ethyne34,35 (176 mg, 0.712 mmol),
and a catalytic amount of CuI were dissolved in TEA (7 mL)
and THF (7 mL). The reaction and the workup were done as
described for 9. Flash chromatography over silica (heptane/
EtOAc 14:1) gave 361 mg (yield 93%) as yellow crystals, 168
°C. IR ν(cm-1) ) 2208, 2094. 1H NMR (CDCl3): δ 8.21 (4 H,
d, J ) 8.9 Hz), 7.63 (4 H, d, J ) 8.9 Hz), 7.41 (4 H, d, J ) 8.4
Hz), 7.26 (4 H, d, J ) 8.4 Hz), 2.24-2.04 (12 H, m), 1.70-
1.56 (12 H, m), 1.52-1.37 (12 H, m), 0.93 (4 H, t, J ) 7.3
Hz). 13C NMR (CDCl3): δ 146.9, 132.2, 131.7, 131.0, 130.7,
130.3, 123.8, 118.1, 113.4 (tCsPt(P)2, d,1JCPt ) 980 Hz, t,2JCP
) 15.0 Hz), 109.6 (CtCsPt, d,2JCPt ) 271 Hz), 95.8, 88.4,
26.5, 24.5 (t, JCP ) 7 Hz), 24.1 (t, JCP ) 16 Hz), 13.9. 31P
NMR (CDCl3, relative to ext. H3PO4, δ ) 0): δ 3.8 (s, d,1JPtP
) 2340 Hz).
trans-Pt(P(n-Bu)3)2(CtCsC4H2SsCtC-p-C6H4-n-
C5H11)2 (1). The alkyne 7 (110 mg, 0.395 mmol) was added to
a stirred solution of deairated diethyl amine (10 mL) and THF
(10 mL), PtCl2(P(n-Bu)3)2 (130 mg, 0.194 mmol), CuI (5 mg,
0.026 mmol), and a few drops of P(n-Bu)3 at 25 °C under Ar
atmosphere. After 3 days the reaction mixture was filtered. The
filtrate was diluted with EtOAc and washed three times with 1
M HCl. The organic extract was dried with MgSO4, filtered,
and concentrated. Flash chromatography on silica with heptane/
ethyl acetate 20:1 gave 200 mg (yield 90%), mp 98 °C. IR:
1
ν(cm-1) ) 2088. H NMR (CDCl3): δ 7.38 (4H, d, J ) 8.0
Hz), 7.12 (4H, d, J ) 8.0 Hz), 7.00 (2H, d, J ) 3.7 Hz), 6.68
(2H, d, J ) 3.7 Hz), 2.58 (4H, t, J ) 7.7 Hz), 2.12-2.05 (12H,
m), 1.62-1.50 (16H, m), 1.50-1.39 (12H, m), 1.35-1.25 (8H,
m), 0.93 (18 H, t, J ) 7.2 Hz), 0.87 (6H, t, J ) 6.8 Hz). 13C
NMR (CDCl3): δ 143.3, 131.5, 131.4, 131.2, 128.4, 127.1,
120.2, 119.7, 116.3 (tCsPt(P)2, d,1JCPt ) 985 Hz, t,2JCP
)
Measurements and Instrumentation. 1H, 13C, and 31P NMR
14.3 Hz), 101.3, (CtCsPt, d,2JCPt ) 280 Hz), 92.2, 82.7, 45.7
(imp.), 35.8, 31.4, 30.8, 26.3 (br s), 24.3 (t, JCP ) 7.1 Hz),
23.9 (t, JCP ) 16.7 Hz), 22.5, 14.0, 13.8, 8.6 (imp.). 31P NMR
(CDCl3, relative to ext. H3PO4, δ ) 0): δ 4.1 (s, d,1JPtP ) 2319
Hz). MS (ESI): m/z (% relative int. of M+ peaks) 1153 (41),
1154 (96), 1155 (100), 1156 (76).
1
were carried out on a Bruker DRX 400 MHz. H and 13C
chemical shifts are reported relative to TMS as internal
reference, while 31P chemical shifts are given relative to external
0.1 M P(C6H5)3 in CDCl3 with δ ) -4.89, which corresponds
to δ ) 0 for 85% H3PO4. CDCl3 was used as solvent for the
NMR measurements. IR spectra were recorded on neat com-
pounds on a Mattson ATI 60AR FTIR equipped with a Golden
Gate Single Reflection Diamond ATR accessory. Mass spectra
were obtained from either of two instruments: (1) a JMS-SX/
SX102A double focusing magnetic sector mass spectrometer
(Jeol, Tokyo), using direct inlet and electron impact ionization
(EI+), with ionizing voltage 70 eV, acceleration voltage 10 kV,
and resolution 1000; or (2) a Waters Micromass ZQ quadrupole
MS using direct inlet, electrospray ionization (ESI), and a
sampling cone voltage of 40 V. Optical limiting spectra were
recorded with a f/5 focusing system using a frequency-doubled
Nd:YAG laser delivering 5-ns pulses at 532-610 nm with a
repetition rate of 10 Hz.36,37 OPL data were obtained for 0.03
M THF solutions in 2-mm quartz cuvets. UV-visible spectra
were recorded on a Shimadzu UV-3101PC spectrophotometer
in dual mode using 10-mm quartz cells and THF as solvent
and repeated on a Shimadzu UV-1601PC spectrometer for the
fluorescence quantum yield measurements.
trans-Pt(P(n-Bu)3)2(CtC-p-C6H4sCtCsC4H3S)2 (2). To
a solution of deairated diethyl amine (25 mL), PtCl2(P(n-Bu)3)2
(180 mg, 0.27 mmol), and CuI (8 mg, 0.042 mmol) was added
3 equiv of alkyne 832 (168 mg, 0.84 mmol) at 25 °C under Ar
atmosphere. After 3 h, the reaction mixture was worked up and
chromatographed on silica gel with toluene/heptane 1:2. An
orange powder, 46 mg (yield 16.8%) was obtained, mp 145
°C. TLC Rf ) 0.3 (1:2 toluene/heptane). IR: ν(cm-1) ) 2094.
1H NMR (CDCl3): δ 7.35 (4 H, d, J ) 8.4 Hz), 7.24-7.27 (4
H, m), 7.22 (4 H, d, J ) 8.4 Hz), 7.00 (2H, dd, J ) 3.7 and 5.1
Hz), 2.09-2.16 (12 H, m), 1.56-1.66 (12 H, m), 1.40-1.49
(12 H, m), 0.92 (18 H, t, J ) 7.3 Hz). 13C NMR (CDCl3): δ
131.5, 131.0, 130.6 (br), 129.2 (br), 127.0, 126.9, 123.7, 118.8,
112.1 (tCsPt(P)2, d,1JCPt ) 974 Hz, t,2JCP ) 14.5 Hz), 109.3
(CtCsPt, d,2JCPt ) 270 Hz), 93.7, 82.9, 26.3, 24.4, (t, JCP
)
7 Hz), 23.9 (t, JCP ) 17.3 Hz), 13.8. 31P NMR (CDCl3, relative
to ext. H3PO4, δ ) 0): δ 3.8 (s, d,1JPtP ) 2346 Hz). MS (ESI):
m/z (% relative int. of M+ peaks) 1014 (73), 1015 (100), 1016
(96), 1017 (46).
A mode-locked titanium:sapphire laser (Coherent Mira 900-
F) was used as excitation source for the luminescence measure-
ments. The pulse repetition frequency was reduced from 76 MHz
to the range 4.75 MHz-9.0 kHz using an acousto-optic
modulator (Coherent 9200 Pulse Picker). For the steady-state
luminescence and time-resolved measurements, the fundamental
laser beam (740 nm) was frequency doubled to 370 nm using
a SHG crystal (Inrad Ultrafast Harmonic Generation System,
Model 5-050). The emission spectra and emission decay traces
were taken on a Jobin Yvon IBH FluoroCube spectrometer using
the multichannel scaling and the time-correlated single-photon
counting modes. IBH DataStation v 2.1 software was used for
operation of the spectrometer.
trans-Pt(P(n-Bu)3)2(CtC-p-C6H4sCtC-p-C6H4s
OCH3)2 (9). trans-PtCl2(P(n-Bu)3)2 (309 mg, 0.460 mmol), (4-
(4-methoxy-phenylethynyl)-phenyl)-ethyne33 (214 mg, 0.920
mmol), and a catalytic amount of CuI were dissolved in TEA
(6 mL) and THF (6 mL). The reaction flask was placed in a
preheated 60 °C oil bath for 5 min. The mixture was allowed
to reach room temperature, diluted with CHCl3, and washed
three times with 1 M HCl(aq). The organic extract was dried
with MgSO4, filtered, and concentrated. Flash chromatography
over silica (heptane/EtOAc 20:1, 10:1, 0:1,) gave 446 mg (yield
91%) as light-yellow crystals, mp 177 °C. IR: ν(cm-1) ) 2096.
1H NMR (CDCl3): δ 7.45 (4 H, d, J ) 8.9 Hz), 7.35 (4 H, d,
J ) 8.4 Hz), 7.22(4 H, d, J ) 8.4 Hz), 6.88 (4 H, d, J ) 8.9
Hz), 3.82 (6 H, s), 2.18-2.08 (12 H, m), 1.66-1.55 (12 H, m),
1.50-1.40 (12 H, m), 0.93 (18 H, t, J ) 7.3 Hz). 13C NMR
(CDCl3): δ 159.4, 132.9, 131.1, 130.6, 128.8, 119.5, 115.7,
113.9, 111.4 (tCsPt(P)2, d,1JCPt ) 976 Hz, t,2JCP ) 14.1 Hz),
109.3 (CtCsPt, d,2JCPt ) 271 Hz), 89.7, 88.7, 55.3, 26.3, 24.4
The fluorescence quantum yields were determined relative
to the quantum yields of a reference system of Quinine Sulfate
(0.546) and Coumarin 110 (0.6).38 The data were analyzed
according to a procedure reported by Williams et al.39 and “A
Guide to Recording Fluorescence Quantum Yields” from Jobin
Yvon Inc.40 Further details on the experimental procedure for
photophysical characterization will be published elsewhere.41

trans-Di-arylalkynyl Platinum(II) Complexes
J. Phys. Chem. A, Vol. 111, No. 9, 2007 1601
TABLE 1: Crystallographic Data and Refinement Results of 1 and 2 (ESD Values Are Given in Parentheses)
molecular formula
cryst syst
space group
a (Å)
C62 H88 P2 Pt S2
triclinic
P-1
15.0379(4)
C52 H68 P2 Pt S2
triclinic
P-1
10.2292(5)
b (Å)
c (Å)
15.5099(5)
20.1090(5)
13.4827(6)
19.9805(10)
R (deg)
94.723(2)
107.609(2)
93.353(1)
4438.0(2)
72.402(3)
87.443(3)
68.678(3)
2440.3(2)
â (deg)
γ (deg)
volume (Å3)
cell formula units
crystal dimensions (mm3)
3
2
0.43 × 0.14 × 0.10
0.34 × 0.14 × 0.12
F
calc (g·cm-3
)
1.296
1.380
diffractometer
radiation
Bruker-Nonius KappaCCD
Bruker-Nonius KappaCCD
Mo KR
Mo KR
µ (Mo KR) (mm-1
)
2.53
3.059
absorption correction
min/max transmission
scan type
2θmax (deg)
temperature (K)
multiscan, SORTAV 45
multiscan, SORTAV 45
0.645/0.776
æ scans and ω with κ offsets
55.76
0.536/0.693
æ scans and ω with κ offsets
55.76
100
100
total data
21173
11277
observed data (I > 2σ(I))
method of structure solution
method of refinement
treatment of hydrogens
R1(Fo > 4σ(Fo))/R1 (all data)
wR2/Fo > 4σ(Fo))/wR2 (all data)
no. parameters
15164
9640
direct methods, SIR 9743
direct methods, SIR 9743
full least-squares refinement on F2, SHELXL-9744 full least-squares refinement on F2, SHELXL-9744
riding model
0.0380/0.0641
0.0800/0.0888
907
riding model
0.0285/0.0332
0.1111/0.1152
517
goodness-of-fit (for F2)
0.80
0.998
residual electron density (max/min)(eÅ3) 3.525/-1.957
1.614/-2.440
Cambridge Structural Database
data deposited
Cambridge Structural Database
Crystal Structure Determination. Single crystals of 1 and
2 were obtained from heptane at room temperature. The crystal
structures were determined from single-crystal X-ray diffraction
data at 100 K. Crystals of both 1 and 2 were mounted on the
top of glass fibers and data were collected on a Nonius Kappa
charge-coupled device (CCD) area detector diffractometer (Mo
KR radiation, graphite monochromator). Data reduction was
performed using HKL DENZO and SCALEPACK.42 The
structures were solved by direct methods using SIR97.43 Fourier
calculations and subsequent full-matrix least-squares refinements
were carried out using SHELXL97.44,45 The crystallographic
data and detailed structure refinement information are shown
in Table 1. All H atoms were positioned geometrically and
refined as riding atoms.
Molecular Orbital Calculations. Structure optimizations and
frequency calculations were performed with the B3LYP three-
parameter hybrid functional46 and the LANL2DZ effective core
potential (ECP) basis set,47 using GAUSSIAN 03 software.48
Excitation energies were calculated at the time-dependent (TD)
DFT level using the B3LYP functional and the LANL2DZ ECP
in GAUSSIAN 98W. Each structure was solved for eight singlet
and eight triplet excited states. No empirical corrections were
applied to the calculated energies.
Because of the mixing of Pt d and alkyne π* orbitals,
electronic singlet excitation may result in unpaired spin density
at Pt. If this takes place, spin-orbit coupling may enhance the
rate of ISC from the excited S state to a T state,21 which can
bring about additional excited-state absorption in the triplet
manifold and thus provide more efficient nonlinear absorption.
Hence, we are interested in details that can shed light upon
the bonding interaction between Pt and the ethynylaryl units.
In the following two sections, structure information is presented
from NMR, IR, crystallography, and quantum chemistry cal-
culations. After that, data from UV-visible, OPL, and lumi-
nescence (steady state and time-resolved) spectroscopy are
presented and discussed.
NMR and IR Spectroscopy. In an NMR study of dialkynyl
bisphosphine Ni(II), Pd(II), and Pt(II) complexes, Wrackmeyer
has argued that π-backbonding from the metal into the CtC
π* orbitals can be probed by 195Pt and 13C chemical shifts.50 It
was concluded that π-backbonding decreases with the metal ion
M in the order Ni > Pd > Pt, in for instance compounds trans-
M(P(n-Bu)3)2(CtC-C6H5)2, as indicated by δC1 values of
113.1, 112.3, 108.7 and δC2 values of 120.1, 111.0, 109.7,
respectively.50 (C1 is the alkyne carbon closest to Pt, Figure 1.)
A lower δ value was accounted for by a smaller paramagnetic
nuclear shielding.
Compounds trans-Pt(P(n-Bu)3)2(CtC-p-C6H4sX)2, with Xd
H, NO2, and OCH3, were also included in the study by
Wrackmeyer. δC of the two alkyne carbons were found to
decrease with X in the order NO2 > H > OCH3.
In the present study, we included NMR data for two additional
Pt complexes, 9 and 10 (Figure 2), to be able to make a
comparison with data from the study by Wrackmeyer. Selected
NMR data is given in Table 2. We were more interested in δ
of C1 than of C2, because C1 is directly connected to Pt, and
because C2 is bonded to the 2,5-thiophenylene group in 1, but
to a 1,4-phenylene unit in 2, 3, 9, and 10.
Results and Discussion
Structural Considerations. Although the exact nature of the
bonding between C and Pt in Pt(II) alkynyl complexes may be
difficult to pinpoint, it is often described by σ-type overlap of
a C sp hybrid orbital and a Pt hybrid mainly constructed from
2
2
2
the 6s, 5dx -y , and 5dz atomic orbitals, where the C-Pt-C
and the P-Pt-P bonds lie along the x and y axes, respectively.49
Mixing of alkyne π and 5d and 6p orbitals also influences the
MO levels. Finally, additional bonding of π-type (π-backbond-
ing) may result from overlap of dxy and dxz orbitals with alkyne
πy* and πz* orbitals, respectively.49

1602 J. Phys. Chem. A, Vol. 111, No. 9, 2007
Lind et al.
TABLE 2: 13C NMR Chemical Shifts, 13C-195Pt and
31P-195Pt Coupling Constants and Infrared Wavenumbers
for Compounds 1-3, 9, and 10
for the magnitude of 195Pt-31P and 195Pt-13C coupling
constants.54-57 An inverse relationship between Pt-P bond
1
length (lPtP) and JPtP was found for some cis and trans Pt(II)
c,d
J(C1-Pt)a J(C2-Pt)a J(P-Pt)a
ν(C1tC2)b
complexes but for a rather large range of l and J.56 Correlations
between the magnitude of the coupling constants and a
qualitative estimate of s-electron density was also considered
in the study by Wrackmeyer, but no simple relationships were
found.50 It was concluded that other factors in the theoretical
expression by Pople and Santry for spin-spin coupling need
to be considered, such as energies for ground-to-excited-state
transitions.58
compd δC1
1
2
3
9
10
116.3
112.1
111.8c,d
111.4
113.4
985
974
975c
976
980
280
270
270c
271
271
2319
2346
2088
2094
2347c, 2344d 2094c, 2096d
2350
2340
2096
2094
a In Hz. Estimated uncertainty is (2 Hz for J(C1-Pt) and (0.5 Hz
for J(C2-Pt) and J(P-Pt). b In cm-1. Estimated uncertainty is (1 cm-1
.
c This work. d Reference 31.
In spite of these results, we recorded 1JPtC, 2JPtC, and1JPtP for
1-3, 9, and 10 in an attempt to obtain information about the
electronic coupling between Pt and the alkyne ligand. The values
For all compounds, the C1 signal appears as a triplet due to
13C-31P coupling to two magnetically equivalent 31P nuclei.
This is consistent with a trans configuration of the alkynyl
groups. Both carbons in the PtsC1tC2 fragment also show
coupling to 195Pt, (195Pt has I ) 1/2 and a natural abundance of
33.8%). For 3, 9, and 10, δC1 decreases with the substituent in
the order NO2 > H > OCH3 (Table 2), that is, in the same
order as for the related compounds in the study by Wrackmeyer.
The order 10 > 3 > 9, together with the fact that the values of
δC1 are rather similar for 3 and 9, indicates that the π-acceptor
property of the substituent has more influence than π-donor or
inductive effects on δC1. In the comparison of all five
compounds, δC1 decreases in the order 1 > 10 > 2 > 3 > 9.
The order 1 > 2 (and 1 > 3) seems reasonable using a Hammett
substituents constant scale such as R-, which describes the
ability for π-electron withdrawal from an electron-rich center.
The R- values are +0.06 and -0.10 for 2-thiophenyl and
phenyl,51 respectively, which means that 2-thiophenyl is the
better π-electron acceptor. (Note: 2-Thiophenyl is also a better
π-donor than phenyl according to the R+ scale, and its electron-
releasing conjugative effect is stronger than the electron-
withdrawing effect.52) Further, 2-thiophenyl and phenyl have
very similar inductive/field properties according to the F scale
of Swain and Lupton.53 Of course, also the more remote groups
(including n-pentyl in 1 with a weak π-electron-donating
property, R- ) -0.18), contribute to the π-accepting ability of
the alkyne ligand and need to be taken into account in a
comparison between 1 and 2. In other words, it is difficult to
draw conclusions from the differences in δC1 of 1 and 2 (and
3, 9, and 10) based on Hammet substituent constants when
several structural factors vary. Possibly, the different geometry
of 1 compared to that of 2 and 3 (see the section on molecular
geometry below) can provide an explanation: Given that the
P-Pt-P bond axis is close to the plane of the thiophene rings
in 1, but nearly perpendicular to the corresponding plane in 2
and 3, more efficient back-donation from Pt to the extended
π* orbital of the alkyne ligand should occur in 1 since the dxz-
type orbital should not be involved in bonding to P.
1
of JPtC decrease in the order 1 > 10 > 9 ≈ 3 ≈ 2 (Table 2),
and a similar trend is found for 2JPtC, (the Pt-C2 coupling) with
1 > 9 ) 10 g 2 ) 3. Thus, JPtC and δC1 display essentially the
same order of decreasing values for the five compounds. If the
s-electron density factors were determining the difference in
JPtC in this series, 10 would be expected to have the smallest
1
value of JPtC because the NO2 group has a strong inductive
withdrawing effect (F ) +0.6551). But as noted by Yoshida,57
increased Pt-C π-backbonding might result in a greater value
of JPtC in two ways: (1) the shortened Pt-C1 bond length may
cause enhanced s-character of the Pt σ hybrid orbital and an
increase of its electron density; (2) removal of charge from the
Pt d orbitals may result in an increase of the s electron density
in the Pt σ hybrid orbital due to an increased effective nuclear
1
charge. In view of these possibilities, the decrease of JPtC in
the order 1 > 10 > 9 ≈ 3 ≈ 2 may at least partly be explained
by a different geometry of 1 than that of the other four
compounds and an associated decrease of the π-backbonding.
However, it may be noted that the OCH3 group in 9 normally
functions as a π-donor (R- ) -0.5551) and a σ-acceptor (F )
+0.2951), and one might therefore expect 9 to have the smallest
1
2
values of JPtC and JPtC
.
1
The magnitude of JPtP decreases in the order 9 > 3 ≈ 2 >
10 > 1, which is almost opposite to that of 1JPtC and 2JPtC. This
is similar to the findings by Wrackmeyer where 1JPtP of trans-
Pt(P(n-Bu)3)2(CtC-p-C6H4sX)2 decreased from XdOCH3
(2395 Hz) to H (2371 Hz) to NO2 (2326 Hz).50 Since similar
1
and relatively small values of JPtP would be expected for 9
and 10 if the magnitude of the coupling constant was solely an
effect of withdrawal of s-electron density in the PtsP bond due
to the substituent, one can conclude that an explanation based
on inductive effects does not suffice. Again, other effects and/
or bond contributions than the σ-type need to be considered. It
has been shown that JPtP varies with the PsPtsP bond angle
for cis-Pt(II) phosphines having the same PtsP bond length,59
which indicates that molecular geometry and Pt electron back-
donation to P can be important for the magnitude of JPtP
.
It is also intricate to rationalize the δC1 values in terms of
greater π-backbonding in 1 compared to that in 10 only on the
basis of Hammet substituent constants. The remote NO2
substituent (R- ) +0.6251) undoubtedly makes the -C6H4-
CC-C6H4-NO2 group in 10 a stronger π-electron acceptor than
the -SC4H2-CC-C6H4-C5H11 unit is in 1. But again, a
different orientation of the extended π-system in 1 compared
to that in 10 may provide an explanation in terms of a better
π-donation from Pt when the dxz electrons are not engaged in
bonding to P. However, a refined quantum chemical analysis
using accurate geometries of the compounds seems required to
obtain more insight into the question of π-backbonding.
In studies of various phosphine Pt complexes, it has been
suggested that the s-orbital electron density is of importance
Moreover, several studies have shown that PtsP back-bonding
in phosphine Pt complexes involves significant d-σ* interac-
tion.60,61
The decrease of 1JPtP in the order 9 > 3 ≈ 2 > 10 > 1 may
therefore be rationalized in terms of weaker Pt-P bonding when
Pt-C π-backbonding is more pronounced. This appears to be
an example of cis influence in complexes of heavier transition
metal ions, meaning that strengthening of one bond causes a
weakening of another bond in cis position to the former. Cis
influence is generally weaker than trans influence but has been
observed in NMR studies of Pt complexes.62,63
Delocalization of π-electrons is generally regarded as an
important factor for optical nonlinearity in organic substances.

trans-Di-arylalkynyl Platinum(II) Complexes
J. Phys. Chem. A, Vol. 111, No. 9, 2007 1603
Figure 3. Crystal structure of one of the two symmetry-independent molecules in the unit cell of 1 (a) and 2 (b). Displacement ellipsoids are
shown at the 50% probability level.
The infrared stretching frequencies for unsaturated groups in,
for instance, dialkynyl bisphosphine Pt complexes can provide
information on the degree of delocalization or bond length
alternation.64 We therefore sought information about the PtsC
bond strength in 1-3, 9, and 10 from CtC stretching frequen-
cies (νCtC). A decreased value of ν, related to the C1tC2 bond,
was expected to reflect a shortening of the PtsC1 bond.64 The
IR spectra of 1-3 and 9 showed only one CtC absorption, in
the range of 2088-2096 cm-1 (see Table 2). Two CtC
absorptions were found for 10, at 2094 and 2208 cm-1, and the
former was assigned to the PtsCtC fragment on the basis of
work by Butler.64 The wavenumbers decrease in the order 3 )
9 > 2 ) 10 > 1 and thus imply weaker C1tC2 bonds and
reduced bond alternation in that order and that the PtsC1 bond
strength is greatest for 1. The value for 2 is more similar to
that of 3 than of 1, as also found for the NMR chemical shifts
and coupling constants.
lie ca. 50° out of the plane that contains the inner phenylene
rings. Crystallographic data of 3 show that this structure is
similar to that of 2, with only a slight deviation from a common
plane for the six-membered rings and with the Pt-P bonds
aligned ca. 62° out of that plane.31 The Pt-C1 bond lengths in
1-3 do not differ significantly (Table 3), which is also true for
the C1-C2 bond lengths in 1 and 3, while those in 2 appears to
be slightly longer. The lengths of the Pt-P bonds are slightly
shorter in 1 than in 2 and 3. Hence, these bond lengths do neither
support nor oppose the idea from IR νCtC and NMR data that
there are differences in the Pt-C1 interaction.
It may also be noted that the C1-Pt-P bond angles deviate
3-4° from right angles in the X-ray structures for all three
compounds. Such deviations appear to be common in crystal
structures of Pt(II) acetylides15 and are also found in the DFT-
optimized geometries of the models we employed for 1-3
(Table 3 and the following section).
Although data for νCtC and J show small differences between
the compounds, only slightly greater than experimental uncer-
tainty (Table 2), it is apparent that the IR and NMR data have
a consistent trend in that 1 appears to have a stronger Pt- C1
π-type interaction.
Molecular Geometry from Crystallography and DFT
Calculations. The X-ray analyses show that both 1 and 2 have
crystal unit cells with two symmetry-independent molecules of
similar geometry. All aromatic rings are planar within experi-
mental uncertainty. In 1 (Figure 3a), the aryl rings deviate
slightly from coplanarity (maximum of 21°, as measured by
angles between least-squares planes), but good conjugation is
nevertheless indicated. The Pt-P bonds are aligned along the
plane containing the two thiophene rings (the P-Pt-P bond
axis is only ca. 1° out of the thiophene planes when sighting
along the C1-C2 axis). The four rings are more coplanar in 2
(deviation of max 11°, Figure 3b) than in 1, and the Pt-P bonds
Geometry optimizations of three structures similar to 1-3
were performed with the B3LYP functional and the LANL2DZ
basis set. To shorten the computational time, the phosphine butyl
groups in 1-3 and the pentyl groups in 1 were replaced by
methyl groups. These three model structures are here denoted
1′-3′. The optimizations were performed starting from three
different geometries for 1′ and from two different geometries
for each of the other two. When the start structure of 1′ was
based on the X-ray structure of 1, the energy minimum (as
established by subsequent frequency calculations) was found
to have rather coplanar aromatic rings (a maximum deviation
of 15°) and the P-Pt-P axis close to the plane of the rings (9°
out of the plane of the thiophene rings).
In the second optimization, the start structure of 1′ had the
two phosphine groups relocated so that the P-Pt-P bond axis
was close to perpendicular to the plane of the thiophene rings.
The geometry optimization resulted in a transition state structure

1604 J. Phys. Chem. A, Vol. 111, No. 9, 2007
Lind et al.
Figure 4. Absorption spectra of 10 µM solutions of 1 (circles), 2
(triangles), and 4 (squares) in THF.
with coplanar aromatic rings (maximum deviation of 4°) and a
dihedral angle of ca. 77° between the P-Pt-P axis and the
plane of the thiophene rings. Notably, this unstable conformation
was found to have only 0.2 kcal/mol higher energy that that of
the true energy minimum. The third start structure of 1′ was
generated by an approximate 180° rotation of one alkyne ligand
around the long axis of the molecule, and thus had the S atoms
of the thiophene rings on the same side (a syn conformation),
instead of on the opposite side as in the X-ray diffraction
structure. Again, the optimization found a transition-state
structure, with an energy of only 0.05 kcal/mol above that of
the true energy minimum.
The optimization of both 2′ and 3′ resulted in only one energy
minimum for each structure, irrespective of if the start structures
were taken from the X-ray coordinates or were structures having
the P-Pt-P axis in the plane of the rings. The optimized
structures of 2′ and 3′ have coplanar aromatic rings and ring-
plane to Pt-P axis dihedral angles of 89 and 90°, respectively.
The DFT calculations show slightly shorter Pt-C1 and longer
C1-C2 bonds for 1′, in comparison with 2′ and 3′ (Table 3), as
also indicated by the IR (and NMR) parameters. In contrast to
the crystallographic Pt-P bond lengths, which increase slightly
from 1 to 2 and 3, the calculated bond lengths of 1′-3′ decrease
in the same order. The differences are nevertheless very small.
For all three structures, the DFT-calculated Pt-C1, C1-C2,
and Pt-P bond lengths are greater than those from crystal-
lography by <1, ca. 3, and ca. 4%, respectively. Thus, the
calculated bond lengths are in good to fair agreement with the
experimental data. A good agreement is also found for the angles
listed in Table 3. Hence, the B3LYP/LANL2DZ model seems
to perform well in describing the ground state geometries of
Pt(II) arylalkynes such as 1-3, although one should pay
attention to the orientation of the Pt-P bonds relative to the
plane of the aryl rings and the possibility of more than one
coexisting conformation.
The calculations of harmonic vibration frequencies for 1′-
3′, undertaken to verify the energy minima, showed only one
high-intensity CtC stretch vibration for each structure. This
vibration involves the two inner ethynyl groups (closer to Pt).
The unscaled values are 2162, 2174, and 2174 cm-1 for 1′, 2′,
and 3′, respectively, in fair agreement with the experimental
data (Table 2).
UV-Vis Absorption Data and Excitation Energies from
TD-DFT Calculations. The absorbance spectra of 1, 2, and 4
(Figure 4) show that the compounds are quite transparent in

trans-Di-arylalkynyl Platinum(II) Complexes
J. Phys. Chem. A, Vol. 111, No. 9, 2007 1605
TABLE 4: Absorption Spectral Data of Compounds 1-4 and Excitation Data from TD-B3LYP/LANL2DZ Calculations of
Compounds 1′-3′
exptl
ꢀ/104 at λmax (M-1 cm-1
8.9
calcd
compd
λ
max (nm)
)
λ
exc (nm)a
f b
MOs, coefficientsc
1 or 1′
378
454.1
352.0
2.25
0.72
HfL, 0.66
H-1fL+1, 0.67
HfL+2, -0.10
HfL, 0.67
H-1fL+1, 0.67
HfL, 0.12
HfL, 0.67
H-1fL+1, 0.67
HfL, 0.13
2 or 2′
3 or 3′
362
11
388.6
350.8
2.30
0.98
353d,e
350f
355g
357
9.5d
368.4
330.0
2.49
1.09
7.71e
8.52 ( 1.80g
4
10.2
9
10
354
395
a Wavelength corresponding to DFT calculated vertical excitation energy for optimized ground state geometry, with oscillator strength above
0.05. b Oscillator strength. c Molecular orbitals (H ) HOMO, L ) LUMO) involved in promotion of a single electron, with corresponding wavefunction
coefficient above 0.1. d In THF, ref 67 and this work. e In THF, ref 13. f In dichloroethane, refs 9,10. g In benzene, ref 14.
the visible region. The absorbance spectra of 3 (not displayed)
and 4 are very similar and in accord with those published
earlier.9,13 The spectra of all compounds have similar featureless
band shapes, but the absorption maximum of 1 and 10 is red-
shifted from that of 4 by ca. 20 and 40 nm, respectively, while
2 and 9 have approximately the same values as 4 (Table 4).
Although the value of λmax for 10 is larger than for 1, instead
of the opposite, the order of increasing λmax (3 ≈ 9 ≈ 4 e 2 <
1 < 10) indicates that an increase in π-backbonding, as
suggested by our NMR and IR data, has some relationship to
the red-shift of the absorption band. Because a π*-orbital in a
square-planar metal π-ligand complex normally is higher in
energy than the metal d orbitals,65 a d-π* interaction (greater
π-backbonding) should result in stabilization of the d orbital
and destabilization of the π* orbital. Therefore, the explanation
of a relationship between wavelength of absorption and degree
of π-backbonding has to include also the occupied π orbital(s)
involved in the transition.
shows such an excitation, but at ca. 100 nm shorter wavelength
and with f ≈ 0.7. The low-energy excitation is dominated by
the highest-occupied molecular orbital (HOMO) f lowest-
unoccupied molecular orbital (LUMO) transition for all struc-
tures, while the second one is described mainly by the
HOMO-1 f LUMO+1 transition but with a smaller contribu-
tion also from HOMO f LUMO. No other excitations with f
g 0.05 were found. Hence, the featureless absorption bands of
1-4 are very likely to originate from the HOMO f LUMO
type of excitation. The small absorption at ca. 300-320 nm
can then be associated with the second singlet excitation at ca.
330-360 nm found for 2′ and 3′. It can also be noted that one
should not, from the calculations in this work, exclude the
possibility that 1-4 also can exist in other stable conformations,
for instance, with the alternative orientation of the Pt-P bond
relative to the ring planes. Even minor populations in such
conformations will then contribute to the absorption bands in
the 300-400-nm region.
In earlier studies of 3, the main absorption band has been
associated with the S0 f S1 transition,9,10,16 which is in
agreement with a theoretical study of other Pt(II) acetylides by
Norman.66 In that study, one-photon excitations were calculated
for structures similar to those of 1-3, but the C5H11 groups in
1 were replaced by hydrogens and the PBu3 ligands in all three
compounds were replaced by PH3 groups.66 It was found that
the geometry-optimized structures had the Pt-P bonds in the
common plane of the aromatic rings, which seems to be
preferred when the size of the phosphine ligands are sufficiently
small, as also pointed out by others.15 A TD-DFT method using
the B3LYP functional and the Stuttgart/Dresden (SDD) ECP
was employed by Norman to calculate the excitation wave-
lengths for the 1′-3′ type of structure, which resulted in values
of ca. 450, 419, and 403 nm, respectively. Although the
excitation wavelengths were longer than our experimental values
of λmax, they decreased in the order 1′ > 2′ > 3′, with a greater
difference between 1′ and 2′ (31 nm) than between 2′ and 3′
(15 nm), which is in good agreement with the measured data.
In the present work, TD-B3LYP calculations were performed
with the LANL2DZ potential on the geometry-optimized
structures. The calculated value for the main excitation of 1′ is
454 nm, in accord with Norman’s results, while 2′ and 3′ (with
the different orientation of the Pt-P bonds) have the values of
389 and 368 nm, respectively (Table 4). These excitations all
have oscillator strengths (f) of 2-2.5. In addition, the calcula-
tions revealed a second singlet excitation for 2′ and 3′ at ca.
40-50 nm shorter wavelength, with f ≈ 1. Structure 1′ also
Inspection of the HOMO isodensity surfaces of 2′, 3′, and
the TS structure of 1′ with the P-Pt-P bond axis nearly
perpendicular to the plane of the rings displays a rather uniform
π-distribution along the arylalkynyl framework, including the
dxy Pt atomic orbital (Figure 5a). The LUMO (also of π-type)
has a similar distribution but with a node at Pt. The HOMO of
the stable conformation of 1′ is very similar to those of the other
structures, but the LUMO surface is somewhat different (Figure
5b). The latter has a significant coefficient at Pt, due to positive
mixing of a p orbital at Pt and a π-orbital of the alkyne ligands.
A single-point DFT calculation was also performed on 1′ having
the X-ray geometry of 1, where hydrogens were added to the
carbons using C(sp3)-H and C(sp2)-H bond lengths of 1.113
and 1.100 Å, respectively. As expected, this calculation revealed
virtually identical HOMO/LUMO surfaces as those of the energy
minimum of 1′. The consequence of the LUMO spin density at
Pt is that 1 is likely to experience a stronger SOC in the first
excited singlet state than for 2-4, since the SOC should manifest
itself more effectively when the spin density at the heavy atom
is greater.21 This may cause a faster ISC for 1 compared to that
of 2-4 (from S1 to a T state), provided that the S1-T energy
difference is similar for the compounds and that the states have
correct symmetry for the transition.28
OPL. Previous work on 3 and 4 in THF solution has shown
similar OPL properties for these two compounds,19 and therefore
only 4 was utilized for a comparison with 1 and 2. OPL of
0.030 M solutions of 1, 2, and 4 in THF was measured using
5-ns 10-Hz pulses at three wavelengths (532, 550, and 610 nm).

1606 J. Phys. Chem. A, Vol. 111, No. 9, 2007
Lind et al.
Figure 5. Isodensity HOMO and LUMO surfaces (0.01 a.u.) for B3LYP/LANL2DZ-optimized geometries (slightly tilted and with hydrogens
omitted for clarity). (a) Compound 3′ (having the Pt-P bonds out of the plane of the aromatic rings). (b) Compound 1′ (with Pt-P bonds almost
coplanar with the aromatic rings).
Plots of transmitted laser energy (Eout) vs input energy (Ein)
show a significant OPL effect for all three compounds at 532
nm (Figure 6a and Table 5). However, the OPL of 2 is somewhat
The Stokes shifts are relatively small (40, 30, and 33 nm for
1, 2, and 4, respectively), which together with the shape of the
fluorescence bands are as expected for decay from S1 states
having geometries similar to those of the ground states.
less than that of 1 and 4; for instance with the values of Eout
≈
7.6 µJ for 2 compared to ∼5 µJ for 1 and 4 at Ein ) 150 µJ. At
550 nm, the OPL of the three compounds is weaker, and that
of 1 is intermediate between 2 and 4 (Figure 6b). At 610 nm,
the OPL is further diminished but less so for 4 than for 1 and
2, which have similar values of Eout ≈ 25-26 µJ at Ein ) 150
µJ (Figure 6c). Taken together, we find that the OPL is very
dependent on the wavelengths for 1 and 2 in comparison with
4 and that the introduction of the thiophene ring does not
improve the optical limiting from that of 4 under these
measurement conditions.
Fluorescence quantum yields were found to be very low, in
the range of 0.7-4.5 × 10-3 41
. Time-resolved fluorescence was
measured with the excitation wavelength of 370 nm for samples
with concentrations of approximately 10 µM. The fit of the
fluorescence decay of 1, 2, and 4 showed a dominant component
(g90%) with a decay time of e2.8 ps, and for 2 and 4 a second
component (e10%) with a time constant (τfl) of a few hundred
picoseconds (Table 5). These even shorter fluorescence lifetimes
compared to that of 350 ps reported previously for 3 in CH2-
9
Cl2, together with the low quantum yields, supports the
Steady State and Time-Resolved Luminescence. The
steady-state luminescence spectra of 1, 2, and 4 show maximum
fluorescence emission at 420, 392, and 390 nm, respectively
(Figure 7 and Table 5).41 Also weak phosphorescence peaks
are found at 550 and 526 nm for 2 and 4, and a very weak peak
around 610 nm is observed for 1.41
conclusion of a very efficient ISC to a triplet state. An additional
explanation for the low quantum yield and fast fluorescence
decay has recently been suggested from theoretical work using
TD-DFT calculations in our group.67 A conformation of 3 with
PH3 instead of PBu3 ligands and with the P-Pt bonds in the
plane of the aromatic rings was examined. In addition to the

trans-Di-arylalkynyl Platinum(II) Complexes
J. Phys. Chem. A, Vol. 111, No. 9, 2007 1607
Figure 6. OPL of 0.030 M THF solutions of 1 (circles), 2 (triangles), and 4 (squares) at (a) 532 nm, (b) 550 nm, and (c) 610 nm.
TABLE 5: OPL and Luminescence Spectral Data of Compounds 1-4
E
out (µJ) at Ein ) 150 µJa
compd
532 nm
5.0
550 nm
15
610 nm
25
λfl (nm)b
Qfl/10-3
Stokes shift (nm)
40
τfl (ps)
λ
phos (nm)c
610
τ
phos (ns)d
190
1
420
4.5
<2, 94%
∼450, 6%
2.8, 100%
330e
<2, 90%
∼350, 10%
2
3
4
7.6
5.2
20
26
11
392
410e
390
0.7
2.0
1.1
30
33
550
520e
525
330
∼500e
520
7.5
a In THF, 30 mM. b In THF, e0.2 × 10-5 M. c In THF, 0.2-0.3 × 10-4 M. d Excitation wavelength 380 nm, with 100-kHz pulse repetition
frequency. τphos was measured at λphos (maximum). Decay times change with concentration and with the amount of oxygen in solution. e In
dichloroethane at 300 K, ref 9.
dominant πfπ* excitation mentioned above, an excited singlet
state at slightly lower energy was found, very weakly coupled
to the ground state. By fast internal conversion from the initially
populated S2 state to the dark S1 state, a pathway using ISC to
a T2 and subsequent T1 state could be devised on the basis of
group theory.67
Luminescence spectra were also recorded via two-photon
absorption using the mode-locked Coherent Mira 900 set to 740
nm and 1-MHz pulse repetition frequency. The emission
spectrum was found to be very similar to the spectrum obtained
from single photon excitation (370 nm), which shows that the
relaxation proceeds through the same (S1) state. Weak phos-
phorescence was found in the region of 500-600 nm, as earlier
shown for 39,10 and 419, which is consistent with emission from
a triplet state. The phosphorescence decay time was found to
decrease in the order 4 > 2 > 1, with τ values of 520, 330, and
190 ns, respectively. These values are of the same order of
magnitude as the decay of approximately 500 ns reported for
3.9 The decay rate of the T state of 314,17 and 467 is largely
determined by quenching due to oxygen in the samples and is
likely to be similar for all compounds 1-4. Hence, it is
reasonable that the faster decay of the T state for 1, as measured
by its phosphorescence, is caused by a more efficient non-
radiative TfS0 ISC. This can be an effect of contributions to
the decay rate from Pt-induced SOC in conformations with
significant spin distribution close to the Pt nucleus and from
SOC due to the sulfur nucleus and aromatic ring twists.28
In light of several studies that have shown support for
localization of the long-lived triplet to one of the two arylalkynyl
ligands in Pt(II) complexes,15,17,18,68 it appears necessary to also
perform quantum chemical calculations of the relevant triplet
states to be able to clarify the path and dynamics of the
molecules on the excited-state potential surfaces. Collaborative
work along these lines has been initiated in our group.

1608 J. Phys. Chem. A, Vol. 111, No. 9, 2007
Lind et al.
for 1 and 2 in comparison with 4 at 550 and 610 nm, several
other factors that have not been investigated here remain to be
studied in order to delineate the relevant processes for OPL in
these compounds.
Acknowledgment. This work was supported (B.E., A.E.,
M.L.) by a Swedish Defence Nanotechnology Programme run
jointly by the Swedish Defence Research Agency (FOI) and
Defence Material Administration (FMV). M.L. acknowledges
a grant from The Research Council of Norway within the
NanoMat program, Contract No. 163529.
Supporting Information Available: Crystallographic in-
formation files (in CIF format). This material is available free
of charge via the Internet at http://pubs.acs.org.
References and Notes
Figure 7. One-photon excited luminescence of 1 (circles), 2 (triangles),
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