Photochromic alkynes as versatile building blocks for metal alkynyl systems: Design, synthesis, and photochromic studies of diarylethene-containing platinum(II) phosphine alkynyl complexes

Article abstract of DOI:10.1021/ic101298c

The synthesis of newly designed photochromic dithienylethene-containing ethynylthiophene and ethynylthieno[3,2-b]thiophene has been described, and their incorporation as versatile ligands into the platinum(II) phosphine system was demonstrated. All platin

Full text of DOI:10.1021/ic101298c

Inorg. Chem. 2011, 50, 471–481 471
DOI: 10.1021/ic101298c
Photochromic Alkynes as Versatile Building Blocks for Metal Alkynyl Systems:
Design, Synthesis, and Photochromic Studies of Diarylethene-Containing
Platinum(II) Phosphine Alkynyl Complexes
Hok-Lai Wong, Chi-Hang Tao, Nianyong Zhu, and Vivian Wing-Wah Yam*
Institute of Molecular Functional Materials, Department of Chemistry,
and HKU-CAS Joint Laboratory of New Materials, The University of Hong Kong,
Pokfulam Road, Hong Kong, P.R. China.
Received June 29, 2010
The synthesis of newly designed photochromic dithienylethene-containing ethynylthiophene and ethynylthieno[3,2-b]-
thiophene has been described, and their incorporation as versatile ligands into the platinum(II) phosphine system
1
was demonstrated. All platinum(II) complexes have been successfully characterized by H and 31P NMR spectro-
scopies, positive fast atom bombardment (FAB) mass spectrometry, as well as elemental analysis. One of the
complexes has been characterized by X-ray crystallography. Their photophysical, photochromic, and electrochemical
properties have been studied. Upon photoexcitation, all the photochromic diarylethene-containing alkynes and
platinum(II) complexes exhibited reversible photochromism. The thermal bleaching kinetic of complex 6 was studied in
toluene at 298 and 313 K. Complexes 1, 3, and 4, which contained the labile chloro- ligand, represent a new class of
versatile building blocks for photoswitchable functional materials.
Introduction
Moreover, upon photoinduced isomerization from open form
to closed form, the relatively lower-lying excited state of the
closed form may alter the direction of energy transfer between
various chromophores, resulting in interesting photophysical
properties.15 With our recent interest in the design and synthesis
of various photochromic diarylethene-containing ligands and
their metal complexes12a,d,16 as well as various luminescence
switchable materials,12c,e we herein describe the design and
synthesis of a new class of photochromic diarylethene-contain-
ing alkynes and the photophysical and photochromic studies of
their platinum(II) phosphine complexes.
The research on molecular functional materials is un-
doubtedly one of the most active areas of topical interest
owing to their potential capability for applications as useful
devices from the bottom-up approach. Scientists have started
developing and searching for versatile building blocks for the
construction of materials with desired properties to facilitate
various applications. Among them, the platinum(II) phos-
phine bis-alkynyl system, with its simple square planar
geometry, has been quite widely explored1 as the connect-
ing unit because of its well developed synthetic procedures
and ease of synthesis of unsymmetrical complexes.2,3,4a-4c
Recently, utilization of the platinum(II) phosphine bis-alky-
nyl building block for the construction of polymeric light-
harvesting materials,5 optical power limiting materials,3
dendrimers,4 polymers,1p,3a,5a,5b,6 and others7 has been
achieved.
On the other hand, researches have shown that through the
incorporation of metal centers into the photochromic moieties,
such as azo,8 stilbene,9 diarylethene,10-12 and spirooxazine,13
various attractive photochromic and photophysical behaviors
might result. In addition, the presence of the photochromic
moiety introduces an additional photocontrolled switching func-
tionality which could be utilized to modulate the lumine-
scence,11,12b,12c magnetic,10c and non-linear optical properties.14
Experimental Section
Materials. n-Butyllithium in hexanes (1.6 M, 2.5 M), thiophene,
cesium carbonate, copper powder, quinoline, trans-[Pt(PEt3)2Cl2],
copper(I) iodide and 1-formylpiperidine were obtained from
Aldrich Chemical Co. Phenylacetylene, N,N-diisopropylamine, and
N,N-diethylamine were purchased from Lancaster Synthesis Ltd.
2,5-Dimethylthiophene, 2,3-dibromothiophene, and N-iodosucci-
nimide were obtained from Fluorochem, Inc. (Trimethylsilyl)-
acetylene was obtained from GFS Chemical, Inc. Potassium
tetrachloroplatinate(II) and palladium(II) chloride were purchased
from Strem Chemicals, Inc. Tri-n-butyl borate and triphenylpho-
sphine were obtained from Acros Organics, Inc. Tetrakis(tri-
phenylphosphine)palladium(0)17 as catalyst for Suzuki cross-
coupling, dichlorobis(triphenylphosphine)palladium(II)18 as cata-
lyst for Sonogashira coupling, trans-[Pt(PPh3)2Cl2],19 and 2,3,4,5-
tetrabromothiophene20 were prepared according to literature pro-
cedures with slight modifications. All amines were distilled over
*To whom correspondence should be addressed. E-mail: wwyam@hku.
hk. Fax: 852 28571586. Phone: 852 28592153.
r
2010 American Chemical Society
Published on Web 12/09/2010
pubs.acs.org/IC

472 Inorganic Chemistry, Vol. 50, No. 2, 2011
Wong et al.
potassium hydroxide and stored over potassium hydroxide prior to
use. Tetra-n-butylammonium hexafluorophosphate was purified
by recrystallization from ethanol for at least three times and dried
prior to use. Benzene (Lab Scan, AR), tetrahydrofuran (THF, Lab
Scan, AR) and diethyl ether (Scharlau, anhydrous) were distilled
over sodium before use. Acetonitrile was distilled over calcium
hydride before use. All other solvents and reagents were of
analytical grade and were used as received.
3,4,5-Tribromothiophene-2-carbaldehyde. The reaction was
performed under anhydrous condition using standard Schlenk
techniques. To a well stirred solution of 2,3,4,5-tetrabromothio-
phene (5 g, 12.5 mmol) in anhydrous diethyl ether (50 mL)
at -10 °C was added n-butyllithium (2.5 M, 5.5 mL, 13.8 mmol)
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Article
Inorganic Chemistry, Vol. 50, No. 2, 2011 473
in a dropwise manner. The reaction mixture was then stirred
at -10 °C for 2 h, after which 1-formylpiperidine (1.56 g, 1.53 mL,
13.8 mmol) was slowly added. The resulting mixture was gradually
raised to room temperature and stirred for another further 18 h.
The reaction was quenched with deionized water and poured into a
separating funnel. The organic layer was washed with deionized
water, dried over anhydrous magnesium sulfate, filtered, and
evaporated to dryness under reduced pressure. The crude product
was then purified by column chromatography on silica gel (70-
230 mesh) using hexane as eluent to give after solvent removal the
product as a pale yellow solid. Yield: 2.62 g, 7.5 mmol; 60%.
1H NMR (400 MHz, CDCl3, 298K): δ 9.85 (s, 1H, -CHO).
Positive-ion EI mass spectrum: m/z 348 {M}þ.
5,6-Bis(2,5-dimethylthiophen-3-yl)thieno[3,2-b]thiophene-2-car-
boxylic Acid. To a stirred solution of ethyl 5,6-bis(2,5-dimethylthio-
phen-3- yl)thieno[3,2-b]thiophene-2-carboxylate (1 g, 2.31 mmol) in
ethanol-water (40 mL, 1:1 v/v) was added potassium hydroxide (389
mg, 6.93 mmol). The resulting mixture was heated under reflux for
1.5 h, after which it was allowed to cool to room temperature and was
acidified with hydrochloric acid (2 M). This was followed by extrac-
tion using ethyl acetate (3 ꢀ 30 mL). The combined organic layers
were washed with water and then dried over anhydrous magnesium
sulfate, filtered, and evaporated to dryness under reduced pressure to
1
give a white solid. Yield: 890 mg, 2.2 mmol; 95%. H NMR (300
MHz, CDCl3, 298K): δ1.99 (s, 3H, -CH3), 2.09(s, 3H, -CH3), 2.41
(s, 3H, -CH3), 2.45 (s, 3H, -CH3), 6.56 (s, 1H, dimethylthienyl),
6.69 (s, 1H, dimethylthienyl), 8.08 (s, 1H, 3-thienothienyl). Positive-
ion EI mass spectrum: m/z 404 {M}þ.
Ethyl 5,6-Dibromothieno[3,2-b]thiophene-2-carboxylate. To a
stirred solution of 3,4,5-tribromothiophene-2-carbaldehyde (5.65 g,
16.2 mmol) and anhydrous potassium carbonate (2.95 g, 21.3 mmol)
in dimethylformamide (DMF, 60 mL) was added ethyl thioglycolate
(1.91 g, 1.74 mL, 15.9 mmol). The resulting mixture was stirred for
3 days, after which deionized water (300 mL) was added slowly to
commence precipitation. The solid was collected by filtration and
was dissolved in chloroform. The organic layer was washed with
water and then dried over anhydrous magnesium sulfate, filtered, and
evaporated to dryness under reduced pressure to give a brown oil. The
crude product was purified by column chromatography on silica gel
(70-230 mesh) using hexane-dichloromethane (4:1 v/v) as eluent to
give the product as a white solid. Yield: 2.52 g, 6.84 mmol; 43%. 1H
NMR (300 MHz, CDCl3, 298K): δ 1.40 (t, J = 7.1 Hz, 3H, -CH3),
4.39 (q, J = 7.1 Hz, 2H, -CH2-), 7.93 (s, 1H, 3-thienothienyl).
Positive-ion EI mass spectrum: m/z 369 {M}þ, 324 {M-OEt}þ.
Ethyl 5,6-Bis(2,5-dimethylthiophen-3-yl)thieno[3,2-b]thiophene-
2-carboxylate. The target compound was synthesized according
to standard Suzuki coupling reaction under a heterogeneous mixture
of water and THF. To a solution mixture of ethyl 5,6-dibro-
mothieno[3,2-b]thiophene-2-carboxylate (2 g, 5.42 mmol), 2,5-di-
methylthien-3-yl boronic acid12e (2.54 g, 16.3 mmol) and tetrakis-
(triphenylphosphine)palladium(0) (484 mg, 0.419 mmol) in THF
(80 mL) was added aqueous cesium carbonate solution (2 M, 10.6 g,
16.3 mL, 32.6 mmol). The reaction mixture was vigorously stirred
and refluxed in the dark, and the progress was monitored by thin
layer chromatography (TLC). This was then extracted with diethyl
ether. The combined extracts were washed with brine and water, and
finally dried over anhydrous magnesium sulfate. After filtration and
removal of the solvent, the crude product was purified by column
chromatography on silica gel (70-230 mesh) using hexane-dichlor-
omethane (4:1 v/v) as the eluent to give the product as a white solid.
2,3-Bis(2,5-dimethylthiophen-3-yl)thieno[3,2-b]thiophene. To
a stirred solution of 5,6-bis(2,5-dimethylthiophen-3-yl)thieno-
[3,2-b]thiophene-2-carboxylic acid (1 g, 2.47 mmol) in quinoline
(15 mL) was added copper powder (188 mg, 2.97 mmol). The
resulting mixture was heated to reflux for at least 2 h, and the
progress was monitored by TLC. After that, it was cooled to
room temperature, followed by the slow addition of hydrochlo-
ric acid (2 M, 50 mL) and extraction with diethyl ether (3 ꢀ
30 mL). The combined etheral layers were washed with hydro-
chloric acid (2 M), water and then dried over anhydrous
magnesium sulfate, filtered, and evaporated to dryness under
reduced pressure. The crude product was purified by column
chromatography on silica gel (70-230 mesh) using hexane as the
eluent. Further purification was achieved by recrystallization from a
minimal amount of hexane and stored at -18 °C to afford a white
1
crystalline solid. Yield: 653 mg, 1.81 mmol; 73%. H NMR (300
MHz, CDCl3, 298K): δ 2.01 (s, 3H, -CH3), 2.10 (s, 3H, -CH3),
2.43 (s, 3H, -CH3), 2.47 (s, 3H, -CH3), 6.58 (s, 1H, dimethyl-
thienyl), 6.73 (s, 1H, dimethylthienyl), 7.30 (d, J = 5.2 Hz, 1H,
4-thienothienyl), 7.40 (d, J = 5.2 Hz, 1H, 5-thienothienyl). Positive-
ion EI mass spectrum: m/z 360 {M}þ.
2,3-Bis(2,5-dimethylthiophen-3-yl)-5-iodothieno[3,2-b]thiophene.
To a stirred solution of 2,3-bis(2,5-dimethylthiophen-3-yl)thieno-
[3,2-b]thiophene (1 g, 2.77 mmol) in chloroform-acetic acid (40 mL,
1:1 v/v) was added N-iodosuccinimide (686 mg, 3.05 mmol). The
reaction mixture was stirred overnight. Water (70 mL) was added to
the mixture. The resulting solution was extracted with chloroform
(3 ꢀ 50 mL). The combined extracts were neutralized with aqueous
sodium carbonate solution, followed by washing with deionized
water. The organic layer was then dried over anhydrous magnesium
sulfate, filtered, and evaporated to dryness under reduced pressure.
The crude product was purified by column chromatography on silica
gel (70-230 mesh) using hexane as the eluent. Yield: 1.1 g, 2.26 mmol;
82%. 1H NMR (400 MHz, CDCl3, 298K): δ 1.96 (s, 3H, -CH3),
2.05 (s, 3H, -CH3), 2.39 (s, 3H, -CH3), 2.42 (s, 3H, -CH3), 6.52 (s,
1H, dimethylthienyl), 6.64 (s, 1H, dimethylthienyl), 7.40 (s, 1H,
4-thienothienyl). Positive-ion EI mass spectrum: m/z 486 {M}þ.
2,3-Bis(2,5-dimethylthiophen-3-yl)-5-iodothiophene. The target
compound was prepared according to a procedure similar to that
of 2,3-bis(2,5-dimethylthiophen-3-yl)-5-iodothieno[3,2-b]thiophene
except 2,3-bis(2,5-dimethylthiophen-3-yl)thiophene (1 g, 3.28 mmol)
was used instead of 2,3-bis(2,5-dimethylthiophen-3-yl)thieno[3,2-
b]thiophene. Yield: 1.07 g, 2.49 mmol; 76%. 1H NMR (300 MHz,
CDCl3, 298K): δ 2.00 (s, 6H, -CH3), 2.36 (s, 6H, -CH3), 6.38 (s,
1H, dimethylthienyl), 6.45 (s, 1H, dimethylthienyl), 7.15 (s, 1H,
4-thienyl). Positive-ion EI mass spectrum: m/z 430 {M}þ.
1
Yield: 1.9 g, 4.39 mmol; 81%. H NMR (300 MHz, CDCl3,
298K): δ 1.39 (t, J = 7.1 Hz, 3H, -CH3 in carboxylate group),
1.96 (s, 3H, -CH3), 2.06 (s, 3H, -CH3), 2.38 (s, 3H, -CH3), 2.43 (s,
3H, -CH3), 4.37 (q, J = 7.1 Hz, 2H, -CH2-), 6.53 (s, 1H,
dimethylthienyl), 6.66 (s, 1H, dimethylthienyl), 7.97 (s, 1H,
3-thienothienyl). Positive-ion EI mass spectrum: m/z 432 {M}þ.
2,3-Bis(2,5-dimethylthiophen-3-yl)thiophene. The target com-
pound was prepared according to a procedure similar to that of
ethyl 5,6-bis(2,5-dimethylthiophen-3-yl)thieno[3,2-b]thiophene-2-
carboxylate except 2,3-dibromothiophene (2 g, 0.936 mL, 8.27
mmol) was used instead of ethyl 5,6-dibromothieno[3,2-b]thio-
phene-2-carboxylate. The crude product was purified by column
chromatography on silica gel (70-230 mesh) using hexane as the
eluent. Further purification was achieved by recrystallization from
a minimal amount of hexane and stored at -18 °C to afford a white
crystalline solid. Yield: 1.8 g, 5.91 mmol; 72%. 1H NMR (300 MHz,
CDCl3, 298K): δ 2.01 (s, 3H, -CH3), 2.03 (s, 3H, -CH3), 2.38 (s,
6H, -CH3), 6.43 (s, 1H, dimethylthienyl), 6.48 (s, 1H, dimethyl-
thienyl), 7.03 (d, J = 5.2 Hz, 1H, 4-thienyl), 7.28 (d, J = 5.2 Hz, 1H,
5-thienyl). Positive-ion EI mass spectrum: m/z 304 {M}þ.
((5,6-Bis(2,5-dimethylthiophen-3-yl)thieno[3,2-b]thiophen-2-yl)-
ethynyl)trimethylsilane (TMS;CtC;TTh-DTE). To a solution
of 2,3-bis(2,5-dimethylthiophen-3-yl)-5-iodothieno[3,2-b]thiophene
(1 g, 2.06 mmol), dichlorobis(triphenylphosphine)palladium(II)
(144 mg, 0.205 mmol), copper(I) iodide (39 mg, 0.205 mmol), and
triphenylphosphine (161 mg, 0.616 mmol) in degassed N,N-
diisopropylamine (60 mL) was added trimethylsilylacetylene
(302 mg, 0.431 mL, 3.08 mmol) under an inert atmosphere of
(20) Punidha, S.; Sinha, J.; Kumar, A.; Ravikanth, M. J. Org. Chem.
2008, 73, 323–326.

474 Inorganic Chemistry, Vol. 50, No. 2, 2011
Wong et al.
argon. The reaction mixture was maintained at 60 °C overnight.
After cooling to room temperature, the mixture was acidified
with hydrochloric acid (2 M) and followed by extraction with
diethyl ether (3 ꢀ 40 mL). The combined organic layers were
washed with deionized water and dried over anhydrous magne-
sium sulfate. After filtration and evaporation under reduced
pressure, the crude product was purified by column chromatog-
raphy on silica gel (70-230 mesh) using hexane as eluent to give
the product as a pale yellow solid. Yield: 645 mg, 1.41 mmol; 69%.
1H NMR (400 MHz, CDCl3, 298K): δ 0.26 (s, 9H, -SiMe3), 1.95
(s, 3H, -CH3), 2.03 (s, 3H, -CH3), 2.37 (s, 3H, -CH3), 2.40 (s,
3H, -CH3), 6.51 (s, 1H, dimethylthienyl), 6.63 (s, 1H, dimethyl-
thienyl), 7.35 (s, 1H, 3-thienothienyl). Positive-ion EI mass spec-
trum: m/z 456 {M}þ.
analyses, Found (%): C 55.64 H 3.84; Calcd (%) for C56H45ClP2-
PtS4 CH2Cl2: C 55.94, H 3.87.
3
[Pt(PPh3)2(CtC;TTh-DTE)(CtC-Ph)] (2). The reaction was
performed under argon. To a solution of [Pt(PPh3)2(CtC;TTh-
DTE)Cl] (150 mg, 0.132 mmol) and phenylacetylene (14.8 mg, 0.145
mmol) in chloroform (15 mL) was added diethylamine (0.5 mL) and
a catalytic amount of copper(I) iodide. The resulting mixture was
heated under reflux for 3 h, after which it was filtered and the solvent
was removed under reduced pressure. The residue was dissolved in
dichloromethane, washed with water, and dried over anhydrous
magnesium sulfate. After filtration and evaporation under reduced
pressure, the crude product was purified by column chromatography
on silica gel (70-230 mesh) using hexane-dichloromethane (4:1 v/v)
as eluent. Further purification was achieved by slow diffusion of
pentane vapor into its concentrated dichloromethane solution. Yield:
64 mg, 0.0532 mmol; 40%. 1H NMR (400 MHz, CDCl3, 298K): δ
1.92 (s, 3H, -CH3), 1.97 (s, 3H, -CH3), 2.35 (s, 3H, -CH3), 2.41 (s,
3H, -CH3), 6.10 (s, 1H, 3-thienothienyl), 6.26 (m, 2H, -CtCPh),
6.44 (s, 1H, dimethylthienyl), 6.51 (s, 1H, dimethylthienyl), 6.89 (m,
((5,6-Bis(2,5-dimethylthiophen-3-yl)thiophen-2-yl)ethynyl)tri-
methylsilane (TMS;CtC;Th-DTE). The target compound
was prepared according to a procedure similar to that of TMS;
CtC;TTh-DTE except 2,3-bis(2,5-dimethylthiophen-3-yl)-
5-iodothiophene (1 g, 2.33 mmol) was used instead of 2,3-bis(2,5-
dimethylthiophen-3-yl)-5-iodothieno[3,2-b]thiophene. Yield: 670
mg, 1.67 mmol; 72%. 1H NMR (400 MHz, CDCl3, 298K): δ 0.25
(s, 9H, -SiMe3), 2.02 (s, 6H, -CH3), 2.36 (s, 6H, -CH3), 6.36 (s,
1H, dimethylthienyl), 6.42 (s, 1H, dimethylthienyl), 7.16 (s, 1H,
3-thienyl). Positive-ion EI mass spectrum: m/z 400 {M}þ.
3H, -CtCPh), 7.39 (m, 18H, -PPh3), 7.80 (m, 12H, -PPh3); 31
P
NMR (162 MHz, CDCl3, 298K): δ 18.39 (s, J (31P-195Pt) = 2630
Hz). Positive FAB mass spectrum: m/z 1203 {M}þ, 941
{M-PPh3}þ. Elemental analyses, Found (%): C 64.09, H 4.34;
Calcd (%) for C64H50P2PtS4: C 63.82, H 4.18.
2,3-Bis(2,5-dimethylthiophen-3-yl)-5-ethynylthieno[3,2-b]thiophene
(HCtC;TTh-DTE). To a solution of TMS;CtC;TTh-DTE
(500 mg, 1.30 mmol) in THF-methanol (40 mL, 3:1 v/v) was added
an excess of anhydrous potassium carbonate. The mixture was stirred
overnight, after which the volume was reduced and water was added.
It was followed by extraction with diethyl ether (3 ꢀ 30 mL). The
combined organic layers were washed with deionized water and dried
over anhydrous magnesium sulfate. After filtration and evaporation
under reduced pressure, the crude product was purified by column
chromatography on silica gel (70-230 mesh) using hexane as eluent
to give the product as a pale yellow solid. Yield: 461 mg, 1.20 mmol;
92%. 1H NMR (300 MHz, CDCl3, 298K): δ 2.05 (s, 3H, -CH3),
2.13 (s, 3H, -CH3), 2.45 (s, 3H, -CH3), 2.49 (s, 3H, -CH3), 3.48 (s,
1H, -CtCH), 6.61 (s, 1H, dimethylthienyl), 6.73 (s, 1H, dimethyl-
thienyl), 7.48 (s, 1H, 4-thienothienyl). Positive-ion EI mass spectrum:
m/z 384 {M}þ.
[Pt(PEt3)2(CtC;TTh-DTE)Cl] (3). The target com-
pound was prepared according to a procedure similar to that of
[Pt(PPh3)2(CtC;TTh-DTE)Cl] except trans-[Pt(PEt3)2Cl2] (131
mg, 0.26 mmol) was used instead of trans-[Pt(PPh3)2Cl2]. Yield:
95 mg, 0.112 mmol; 43%. 1H NMR (400 MHz, CDCl3, 298K): δ
1.22 (m, 18H, P(CH2CH3)3), 1.96 (s, 3H, -CH3), 2.03 (s, 3H,
-CH3), 2.15 (m, 12H, P(CH2CH3)3), 2.37 (s, 3H, -CH3), 2.42 (s,
3H, -CH3), 6.50 (s, 1H, dimethylthienyl), 6.64 (s, 1H, dimethyl-
thienyl), 7.01 (s, 1H, 3-thienothienyl); 31P NMR (162 MHz, CDCl3,
298K): δ 11.52 (s, J (31P-195Pt) = 2342 Hz). Positive FAB mass
spectrum: m/z 850 {M}þ. Elemental analyses, Found (%): C 45.49,
H 5.22; Calcd (%) for C32H45ClP2PtS4: C 45.19, H 5.33.
[Pt(PPh3)2(CtC-Th-DTE)Cl] (4). The target compound was
prepared according to a procedure similar to that of [Pt(PPh3)2-
(CtC;TTh-DTE)Cl] except HCtC-Th-DTE (100 mg, 0.304
mmol) was used instead of HCtC;TTh-DTE. Yield: 175 mg,
0.162 mmol; 53%. 1H NMR (400 MHz, CDCl3, 298K): δ 1.89 (s,
3H, -CH3), 1.90 (s, 3H, -CH3), 2.31 (s, 3H, -CH3), 2.32 (s, 3H,
-CH3), 5.78 (s, 1H, 3-thienyl), 6.17 (s, 1H, dimethylthienyl),
6.26 (s, 1H, dimethylthienyl), 7.40 (m, 18H, -PPh3), 7.76 (m,
12H, -PPh3); 31P NMR (162 MHz, CDCl3, 298K): δ 21.20 (t,
J (31P-195Pt) = 2640 Hz). Positive FAB mass spectrum: m/z 1083
{M}þ, 1046{M-Cl}þ. Elemental analyses, Found (%): C 53.96, H
2,3-Bis(2,5-dimethylthiophen-3-yl)-5-ethynylthiophene (HCtC-
Th-DTE). The target compound was prepared according to a
procedure similar to that of HCtC;TTh-DTE except TMS;
CtC;Th-DTE(500 mg, 1.52mmol) was usedinstead ofTMS;
1
CtC;TTh-DTE. Yield: 450 mg, 90%. H NMR (400 MHz,
CDCl3, 298K): δ 2.01 (s, 3H, -CH3), 2.03 (s, 3H, -CH3), 2.36 (s,
6H, -CH3), 3.38 (s, 1H, -CtCH), 6.38 (s, 1H, dimethylthienyl),
6.45 (s, 1H, dimethylthienyl), 7.20 (s, 1H, 3-thienyl). Positive-ion
EI mass spectrum: m/z 328 {M}þ.
3.74; Calcd (%) for C54H45ClP2PtS3 2CH2Cl2: C 53.70, H 3.94.
3
[Pt(PPh3)2(CtC-Th-DTE)(CtC-Ph)] (5). The target com-
pound was prepared according to a procedure similar to that of
[Pt(PPh3)2(CtC;TTh-DTE)(CtC-Ph)] except [Pt(PPh3)2(CtC-
Th-DTE)Cl] (150 mg, 0.139 mmol) was used instead of [Pt(PPh3)2-
(CtC;TTh-DTE)Cl]. Yield: 77 mg, 0.0671 mmol; 48%. 1H NMR
(300 MHz, CDCl3, 298K): δ 1.94 (s, 6H, -CH3), 2.34 (s, 6H,
-CH3), 5.93 (s, 1H, 3-thienyl), 6.22 (s, 1H, dimethylthienyl), 6.30
(m, 3H, dimethylthienyl, -CtCPh), 6.91 (m, 3H, -CtCPh),
7.40 (m, 18H, -PPh3), 7.81 (m, 12H, -PPh3); 31P NMR (162
MHz, CDCl3, 298K): δ 18.29 (s, J (31P-195Pt) = 2636 Hz). Posi-
tive FAB mass spectrum: m/z 1147 {M}þ. Elemental analyses,
[Pt(PPh3)2(CtC;TTh-DTE)Cl] (1). The reaction was per-
formed under argon. To a solution of HCtC;TTh-DTE (100
mg, 0.260 mmol) and trans-[Pt(PPh3)2Cl2] (205 mg, 0.260 mmol) in
chloroform (15 mL) was added diethylamine (0.5 mL). The resulting
mixture was heated under reflux for 3 h, after which it was filtered
and the solvent was removed under reduced pressure. The residue
was dissolved in dichloromethane, washed with water, and dried over
anhydrous magnesium sulfate. After filtration and evaporation
under reduced pressure, the crude product was purified by column
chromatography on silica gel (70-230 mesh) using hexane-dichlor-
omethane (4:1 v/v) as eluent. Further purification was achieved by
slow diffusion of pentane vapor into its concentrated dichloro-
methane solution to yield 1 as a yellow solid. Yield: 145 mg, 0.127
mmol; 49%. 1H NMR (400 MHz, CDCl3, 298K): δ 1.89 (s, 3H,
-CH3), 1.95 (s, 3H, -CH3), 2.34 (s, 3H, -CH3), 2.40 (s, 3H,
-CH3), 5.97 (s, 1H, 3-thienothienyl), 6.42 (s, 1H, dimethylthienyl),
6.48 (s, 1H, dimethylthienyl), 7.38 (m, 18H, -PPh3), 7.75 (m, 12H,
-PPh3); 31P NMR (162 MHz, CDCl3): δ 21.54 (s, J (31P-195Pt) =
2634 Hz). Positive fast atom bombardment (FAB) mass spectrum:
m/z 1138 {M}þ, 1103 {M-Cl}þ, 840 {M-Cl-PPh3}þ. Elemental
Found (%): C 63.21, H 4.39; Calcd (%) for C62H50P2PtS3 0.5
3
CH2Cl2: C 63.04, H 4.32.
[Pt(PEt3)2(CtC-Th-DTE)2] (6). The target compound was
prepared according to a procedure similar to that of [Pt(PPh3)2-
(CtC;TTh-DTE)(CtC-Ph)] except trans-[Pt(PEt3)2Cl2] (150
mg, 0.299 mmol) was used instead of [Pt(PPh3)2(CtC;TTh-
DTE)Cl], together with the use of HCtC-Th-DTE (215 mg,
0.657 mmol) instead of phenylacetylene. Yield: 173 mg, 0.159
mmol; 53%. 1H NMR (400 MHz, CDCl3, 298K): δ 1.22 (m, 18H,
P(CH2CH3)3), 1.99 (s, 12H, -CH3), 2.17 (m, 12H, P(CH2CH3)3),

Article
Inorganic Chemistry, Vol. 50, No. 2, 2011 475
2.35 (s, 6H, -CH3), 2.36 (s, 6H, -CH3), 6.42 (s, 2H, dimethyl-
Table 1. Crystal and Structure Determination Data for Complex 2
thienyl), 6.45 (s, 2H, dimethylthienyl), 6.79 (s, 2H, 3-thienyl); 31
P
empirical formula
formula weight
temperature
C64H50P2PtS4 (CHCl3)2.5
3
NMR (162 MHz, CDCl3, 298K):δ 11.39 (s, J (31P-195Pt) = 2350
Hz). Positive FAB mass spectrum: m/z 1086 {M}þ. Elemental
analyses, Found (%): C 50.67, H 5.39; Calcd (%) for C48H60-
1502.73
301(2) K
˚
0.71073 A
wavelength
crystal system
space group
unit cell dimensions
triclinic
P1 (No. 2)
P2PtS6 0.5 CHCl3: C 50.83, H 5.32.
3
Physical Measurements and Instrumentation. 1H NMR spec-
tra were recorded either on a Bruker DPX-300 (300 MHz) or a
Bruker AV400 (400 MHz) at 298 K. Chemical shifts (δ, ppm) for
1H NMR and 31P NMR were recorded relative to tetramethyl-
silane (Me4Si) and 85% phosphoric acid, respectively. Positive-
ion FAB and electron impact (EI) mass spectra were recorded
on a Finnigan MAT95 mass spectrometer. Elemental analyses
of the new compounds were performed on a Carlo Erba 1106
elemental analyzer at the Institute of Chemistry, Chinese Acad-
emy of Sciences, Beijing.
˚
a = 8.196(1) A
˚
b = 11.971(1) A
˚
c = 36.875(5) A
R = 97.64(2)°
β = 95.55(2)°
γ = 97.61(2)°
3
˚
volume
Z
density (calculated)
absorption coefficient
F(000)
3530.1(7) A
2
1.414 g cm-3
2.472 mm-1
1502
UV-vis absorption spectra were recorded on a Hewlett-
Packard 8452A diode array spectrophotometer. Photoirradia-
tion was carried out using a 300 W Oriel Corporation Model
66011 Xe (ozone-free) lamp, and monochromic light was ob-
tained by passing the light through an Applied Photophysics F
3.4 monochromator. All measurements were conducted at room
temperature.
crystal size
θ range for data collection
index ranges
0.6 mm ꢀ 0.25 mm ꢀ 0.15 mm
1.68 to 25.68°
-9 e h e 9
-14 e k e 14
-44 e l e 41
20702
reflections collected
independent reflections
completeness to θ = 25.68°
absorption correction
refinement method
13071 [R(int) = 0.0182]
97.60%
empirical
1.000000 and 0.330019
full-matrix least-squares on F2
13071/43/744
Steady-state emission and excitation spectra at room tem-
perature and 77 K were recorded on a Spex Fluorolog-2 Modal
F111 spectrofluorometer. For solution emission and excitation
spectra, samples were degassed on a high-vacuum line in a
degassing cell with a 10 cm3 Pyrex round-bottomed flask con-
nected by a side arm to a 1 cm quartz fluorescence cuvette and
were sealed from the atmosphere by a Rotaflo HP6/6 quick-
release Teflon stopper. Solutions were rigorously degassed with
no fewer than four freeze-pump-thaw cycles prior to the
measurements. Solid-state emission and excitation spectra at
room temperature were recorded with solid samples loaded in a
quartz tube inside a quartz-walled Dewar flask. Solid samples at
low temperature (77 K) and in butyronitrile glass at 77 K were
recorded similarly, with liquid nitrogen inside the optical Dewar
flask. Excited state lifetimes of solution, solid, and glass samples
were measured using a conventional laser system. The excitation
source was the 355 nm output (third harmonic, 8 ns) of a
Spectra-Physics Quanta-Ray Q-switched GCR-150 pulsed Nd:
YAG laser (10 Hz). Luminescence decay traces at a selected
wavelength were detected by a Hamamatsu R928 photomulti-
plier tube connected to a 50 Ω load resistor and the voltage
signal recorded on a Tekronix Modal TDS 620A digital oscillo-
scope (500 MHz, 2 GS/s). The lifetime (τ) determination was
achieved by the single exponential fittings of the luminescence
decay traces with the model equation, I(t) = Io exp(-t/τ), where
I(t) and Io refer to the luminescence intensity at the time = t and
time = 0, respectively. Luminescence quantum yield was mea-
sured by the optical dilute method developed by Demas and
Crosby.21a A degassed aqueous solution of [Ru(bpy)3]Cl2 was
used as standard21b,c at 298 K.
refinement method
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
1.179
R1 = 0.0424, wR2 = 0.1058
R1 = 0.0463, wR2 = 0.1074
-3
˚
largest diff. peak and hole
1.307 and -2.848 e A
Δt) in the absorption maximum of the closed forms in the visible
region. Cyclic voltammetric measurements were performed by
using a CH Instruments, Inc., model CHI 620 electrochemical
analyzer interfaced to a personal computer. The electrolytic cell
used was a conventional two-compartment cell. The salt bridge
of the reference electrode was separated from the working
electrode compartment by a vycor glass. Electrochemical mea-
surements were performed in acetonitrile solutions with 0.1 mol
dm-3 nBu4NPF6 as supporting electrolyte at room temperature.
The reference electrode was a Ag/AgNO3 (0.1 M in acetonitrile)
electrode, and the working electrode was a glassy carbon (CH
Instrument) electrode with a platinum wire as a counter elec-
trode in a compartment separated from the working electrode
by a sintered-glass frit. The ferrocenium/ferrocene couple
(FeCp2þ/0) was used as the internal reference.23 All solutions
for electrochemical studies were deaerated with prepurified
argon gas before measurement.
Crystal Structure Determination. All the experimental details
are given in Table 1. Single crystals of complex 2 suitable for
X-ray studies were obtained by vapor diffusion of pentane into a
concentrated chloroform solution of complex 2. A crystal of
dimensions 0.6 mm ꢀ 0.25 mm ꢀ 0.15 mm mounted in a glass
capillary was used for data collection at 28 °C on a Bruker Smart
CCD 1000 using graphite monochromatized Mo-KR radiation
Chemical actinometry was employed for the photochemical
quantum yield determination.22 Incident light intensities were
taken from the average values measured just before and after
each photolysis experiment using ferrioxalate actinometry.22 In
the determination of the photochemical quantum yield, the
sample solutions were prepared at concentrations with absor-
bance slightly greater than 2.0 at the excitation wavelength. The
quantum yield was determined at a small percentage of conver-
sion by monitoring the initial rate of change of absorbance (ΔA/
˚
(λ = 0.71073 A). Raw frame data were integrated with the
SAINT24 program. Semiempirical absorption correction with
SADABS25 was applied. The structure was solved by direct
methods employing the SHELXS-97 program26 on a PC. Pt, S,
(23) (a) Gagne, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980,
19, 2854–2855. (b) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877–
910.
(21) (a) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991–1024.
(b) Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 98, 4853–4858.
(c) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583–5590.
(22) (a) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956,
235, 518. (b) Murov, S. L. Handbook of Photochemistry; Marcel Dekker:
New York, 1973. (c) Kuhn, H. J.; Braslavsky, S. E.; Schmidt, R. Pure Appl.
Chem. 2004, 76, 2105–2146.
(24) SAINTþ; SAX area detector integration program, version 7.34A;
Bruker AXS, Inc.: Madison, WI.
(25) Sheldrick, G. M. SADABS, Empirical Absorption Correction Pro-
€
€
gram; University of Gottingen: Gottingen, Germany, 2004.
(26) Sheldrick, G. M. SHELX97, Programs for Crystal Structure Anal-
€
€
ysis, release 97-2; University of Gottingen: Gottingen, Germany, 1997.

476 Inorganic Chemistry, Vol. 50, No. 2, 2011
Scheme 1. Synthetic Pathway of Complexes 1-6
Wong et al.
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
the program SHELXL-9726 on a PC. Detailed experimental
procedures and other results including atomic coordinates, equiva-
lent isotropic displacement parameters, bond lengths, bond angles,
anisotropic displacement parameters, hydrogen coordinates, and
isotropic displacement parameters for complex 2 were included in
the Supporting Information and were tabulated in Supporting
Information, Tables S1-S4.
After successive cross-coupling reaction, de-esterification
and decarboxylation, 2,3-bis(2,5-dimethylthiophen-3-
yl)thieno[3,2-b]thiophene was obtained in moderate yield.
The alkyne, HCtC;TTh-DTE, was then synthesized
using the similar synthetic pathway as HCtC-Th-DTE.
Complexes 1-6 were characterized by 1H NMR, 31P
NMR, FAB mass spectroscopy, and elemental analysis.
The trans- configuration of the complexes was confirmed
by the sharp singlet signals at about δ 11.39-21.54 ppm in
the 31P NMR spectra, accompanied by a platinum satellite
with J(P-Pt) values of2342-2640 Hz, typicalofthe trans-
platinum(II) bis-phosphine systems.28 The synthetic
scheme is outlined in Scheme 1.
Results and Discussion
Synthesis. Complexes 1-6 were prepared by standard
dehydrohalogenation of trans-[Pt(PR3)2Cl2] with different
alkynes in the presence of copper(I) iodide and diethylamine
according to literature procedures.2,3,5 The intermediate, 2,3-
bis(2,5-dimethylthiophen-3-yl)-thiophene, was prepared by
modification of a procedure previously reported by us12a,c-e
through the bis-Suzuki cross-coupling reaction between 2,3-
dibromothiophene and 2,5-dimethylthien-3-yl boronic acid.
The resulting 2,3-bis(2,5-dimethylthiophen-3-yl)-thiophene
was subjected to iodination, standard Sonogashira reaction
with trimethylsilylacetylene, and finally deprotection by
potassium carbonate to yield the alkyne. On the other hand,
the preparation of 2,3-bis(2,5-dimethylthiophen-3-yl)-5-
ethynylthieno[3,2-b]thiophene was accomplished by first
synthesizing ethyl 5,6-dibromothieno[3,2-b]thiophene- 2-car-
boxylate as the precursor for the bis-Suzuki cross-coupling
reaction using a method similar to that described previously.27
X-ray Crystal Structure. The perspective drawing and
˚
selected bond distances (A) and angles (deg) of complex 2
are depicted and tabulated in Figure 1 and Table 2,
respectively. The crystal structure of complex 2 adopted
an antiparallel conformation, in which two dimethylthio-
phene rings were pointing in the opposite directions. The
interplanar angles between the dimethylthiophene rings
and the thienothiophene core are 46.817° and 47.834°,
which are not orthogonal to each other. The platinum(II)
center adopts a square planar geometry that is slightly
distorted from its ideal geometry with the C-Pt-P bond
angle in the range of 87-94°. The Pt-C bond distances
˚
˚
(ca. 1.99 A) and the Pt-P bond distances (ca. 2.30 A) are
comparable to the literature values reported in typical
platinum(II) phosphine bis-alkynyl systems.3a,4b,29 The
(28) Clark, H. C.; Milne, C. R. Can. J. Chem. 1979, 57, 958–960.
(29) Yam, V. W.-W.; Yu, K.-L.; Wong, K. M.-C.; Cheung, K.-K.
Organometallics 2001, 20, 721–726.
(27) Fuller, L. S.; Iddon, B.; Smith, K. A. J. Chem. Soc., Perkin Trans.
1997, 3465–3470.

Article
Inorganic Chemistry, Vol. 50, No. 2, 2011 477
Figure 1. Perspective view of complex 2 with atomic numbering scheme. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids were shown at
the 30% probability level.
˚
Table 2. Selected Bond Distances (A) and Angles (deg) with Estimated Standard
Deviations (e.s.d.s.) in Parentheses for Complex 2
Pt-Pt separation between the two closest complexes is
˚
8.196(1) A, which indicates the absence of Pt-Pt interac-
tions in the crystalline form.
Electronic Absorption and Emission Properties. All com-
˚
Bond Distances/A
Pt(1)-P(1)
Pt(1)-P(2)
Pt(1)-C(1)
Pt(1)-C(9)
2.3016(13)
2.3002(13)
1.999(5)
C(1)-C(2)
C(2)-C(3)
C(9)-C(10)
C(10)-C(11)
1.204(7)
1.434(7)
1.199(7)
1.420(7)
plexes are found to dissolve in benzene to give clear yellow
solutions. The intense absorptions at about 300-350 nm
with extinction coefficients in the order of 104 dm3 mol-1
cm-1 are ascribed to intraligand transitions of the thiophene-
containing alkynyl and phosphine ligands. The low-energy
absorption bands at about 370-390 nm are found to be
slightly red-shifted when the platinum center is coordinated
to the more electron-rich triethylphosphine ligand (1 vs 3) or
is coordinated to the more electron-donating phenyl alkynyl
ligand (1 vs 2 and 4 vs 5). This suggests the possible
involvement of a metal character in the highest occupied
molecular orbital (HOMO). In addition, the absorption
band is also found to be sensitive toward the nature of the
thiophene-containing alkynyl ligands. Upon increasing the
π-conjugation from thiophene in 4 and 5 to thienothiophene
in 1 and 2, respectively, the lower energy absorption bands at
about 370 nm are also found to be red-shifted, suggesting the
participation of the thiophene-containing alkynyl ligands in
the transition. Thus, the low-energy absorption is tentatively
assigned as the metal-to-ligand charge transfer MLCT
[dπ(Pt) fπ*(thiophene-containing alkynyl)] transition, with
substantial mixing of an intraligand IL [π f π*(thiophene-
containing alkynyl)] character, given the large extinction
1.993(5)
Bond Angles/deg
C(1)-Pt(1)-P(1)
P(1)-Pt(1)-C(9)
C(9)-Pt(1)-P(2)
P(2)-Pt(1)-C(1)
86.80(14)
92.96(14)
86.38(14)
93.85(14)
Pt(1)-C(1)-C(2)
Pt(1)-C(9)-C(10)
C(1)-C(2)-C(3)
C(9)-C(10)-C(11)
176.3(5)
178.0(4)
177.1(6)
175.7(6)
670 nm, with lifetimes in the range of microseconds. The
long-lived emission and the large Stokes shift observed are
suggestive of a triplet emissive origin. In other related
platinum(II) diphosphine bis-alkynyl systems,4a,b the
3
phosphorescence was believed to originate from the IL
[π f π*(alkynyl)]/3MLCT [dπ(Pt) f π*(alkynyl)] state. It
was believed that the emissions of complexes 1-6 were
attributed to a similar origin. However, in view of the lack
of an obvious trend in complexes 1-6, it was suggested
that the emissive origin was dominated by an IL character.
As shown in Figure 3, it was found that the emission bands
of complexes 1-6 were broad and of similar band shape
3
and position, supportive of the predominant IL assign-
ment. The luminescence quantum yields of these com-
plexes were found to vary from 0.0027 to 0.028. In
general, complexes 1 and 3, which contain the chloro
ligand, showed a lower luminescence quantum yield. This
is probably due to the presence of the low-lying ligand field
excited state, which quenches the emission through non-
radiative deactivation pathways.
Most complexes showed photoluminescence in the solid
state at 77 K, 298 K, and glass at 77 K, with a blue shift in
the emission maxima and well-defined vibrational progres-
sional spacings, indicative of substantial involvement of the
ligand orbitals in the emissive state. Thus, the emission is
coefficients observed in the order of 104 dm3 mol-1 cm-1
.
This is consistent with the literature assignment and the
computational studies on the related platinum(II) dipho-
sphine bis-alkynyl system.30 Thus, the low-energy intense
absorptions of these complexes are best described as an
admixture of IL [π f π*(alkynyl)]/MLCT [dπ(Pt) f π*-
(alkynyl)] transition, or alternatively, as a metal-perturbed IL
transition. The absorption data are summarized in Table 3.
Figure 2 depicts the representative absorption spectra of
complexes 1, 2, 4, and 5.
Upon excitation at λ = 360 nm, the open form of
complexes 1-6 exhibited red emission at about 630-
3
assigned as predominantly IL [π f π*(alkynyl)] origin
with mixing of 3MLCT character, as a subtle trend of the
emissive energy dependence on the electronic properties of
the ancillary ligands is observable. The blue shift in emission
maxima relative to the solution state is commonly observed
(30) (a) Sacksteder, L.; Baralt, E.; DeGraff, B. A.; Lukehart, C. M.; Demas,
J. N. Inorg. Chem. 1991, 30, 2468–2476. (b) Choi, C. L.; Cheng, Y. F.; Yip, C.;
Philips, D. L.; Yam, V. W.-W. Organometallics 2000, 19, 3192–3196. (c) Pan, Q.-J.;
Fu, H.-G.; Yu, H.-T.; Zhang, H.-X. Inorg. Chim. Acta 2006, 359, 3306–3314.

478 Inorganic Chemistry, Vol. 50, No. 2, 2011
Wong et al.
Table 3. Electronic Absorption Data for TMS;CtC;TTh-DTE, TMS;CtC;Th-DTE, and Complexes 1-6 in the Open Form and Closed Form
absorptiona
compound
configuration
λ )
abs/ nm (ε/dm3 mol-1 cm-1
TMS;CtC;TTh-DTE
open
close
open
close
open
close
open
close
open
close
open
close
open
close
open
close
336 (28070)
326 (14480), 362 sh (30310), 378 (46210), 560 (8090)
322 (16350)
348 sh (26200), 362 (32920), 572 (11820)
302 (16630), 356 (40360), 378 (35600)
358 sh (34740), 384 (44400), 402 sh (38960), 534 sh (7140), 566 (7830)
340 sh (31490), 386 (54830)
338 sh (29260), 388 (56570), 410 (52960), 532 sh (7530), 564 (8350)
314 (21940), 380 (66060)
340 (35730), 356 (32680), 414 (103740), 536 sh (13730), 570 (15780)
346 (20010), 368 sh (15570)
326 sh (18370), 362 (32240), 376 sh (29150), 534 sh (7326), 562 (8400)
336 sh (22870), 376 (44500)
352 sh (39230), 368 (51280), 392 sh (37880), 530 sh (8820), 562 (10400)
366 (53040)
306 sh (24730), 368 (59490) 382 sh (55650), 526 sh (10020), 556 (11770)
TMS;CtC;Th-DTE
1
2
3
4
5
6
a Data obtained in benzene at 298 K.
Figure 2. Electronic absorption spectra of complexes 1, 2, 4, and 5 in
benzene at 298 K.
Figure 3. Normalized corrected emission spectra of the open forms of
1-3 in degassed benzene at 298 K.
in related systems31 because of the presence of luminescence
rigidochromism.32 The energies of the vibrational progres-
sional spacings have been determined and are listed in
Table 4. The vibrational progressional spacings of about
1080 cm-1 are assigned as C-C vibrational modes.4b In
additional, other vibrational progressional spacings of
about 1370 and 1500 cm-1 are typical of CH3 bending
and aromatic ring CdC stretching modes.4b,30a,33
Photochromic Properties. Upon photoexcitation at λ =
320 nm for ligands and λ = 370 nm for complexes 1-6,
the solutions turned purple, with the emergence of low-
energy absorption bands at about 560 nm, attributed to
the ring closed form of the 8a,8b-dimethyl-1,8-thia-as-
indacene moiety. The absorption pattern of complexes
1-6 is comparable to that of TMS-protected ethynylthio-
phene, simple dithienylethene-containing thiophene,
thienothiophene,12c and dithienopyrrole,12e suggesting
that the absorption is only slightly perturbed by the
incorporation of the platinum center. For complex 6,
which contains two pairs of photochromic dithieny-
lethene moieties, it did not undergo dual cyclization as
evidenced by the existence of well-defined isosbestic
points at about 349 and 357 nm as shown in Figure 4.
The lack of photocyclization of the second dithieny-
lethene moiety is probably due to the rapid energy
transfer to the low-lying excited state resulting from the
closed form that quenches the second photocyclization
step.34 Upon photoexcitation into the absorption bands
at about 470-600 nm in the closed form, the UV-vis
absorption spectral changes were reversed, resulting in
the regeneration of the open form. The cyclo-reversibility
has been demonstrated in complex 6 as depicted in
Figure 5.
The photocyclization quantum yields are larger than that
of the photocycloreversion quantum yields, commonly ob-
served in photochromic diarylethene systems.12a,c-e It was
(31) Chan, S.-C.; Chan, M. C. W.; Wang, Y.; Che, C.-M.; Cheung, K.-K.;
Zhu, N. Chem.;Eur. J. 2001, 7, 4180–4190.
(32) (a) Ernst, S.; Kaim, W. Angew. Chem. 1985, 95, 431–433. (b) Staal,
L. H.; Stufkens, D. J.; Oskam, A. Inorg. Chim. Acta 1978, 26, 255–262. (c) Balk,
R. W.; Stufkens, D. J.; Oskam, A. Inorg. Chim. Acta 1979, 34, 267–274.
(33) McGarrah, J. E.; Eisenberg, R. Inorg. Chem. 2003, 42, 4355–4365.
(34) (a) Peters, A.; Branda, N. R. Adv. Mater. Opt. Electron. 2000, 10,
245–249. (b) Kaieda, Y.; Kobatake, S.; Miyasaka, H.; Murakami, M.; Iwai, N.;
Nagata, Y.; Itaya, A.; Irie, M. J. Am. Chem. Soc. 2002, 124, 2015–2024.

Article
Inorganic Chemistry, Vol. 50, No. 2, 2011 479
Table 4. Emission Data for TMS;CtC;TTh-DTE, TMS;CtC;Th-DTE, and Complexes 1-6 in Various Media
medium
emission
vibrational progressional
spacing ν/cm-1
b
compound
(T/K)
λema/nm (τo / μs)
φlum
e
TMS;CtC;TTh-DTE
TMS;CtC;Th-DTE
1
benzene (298)
benzene (298)
benzene (298)
solid (298)
solid (77)
402
403
640 (2.0)
646 (9.8)
641 (59.1)
574, 625 (135.3)
630 (16.6)
573, 612 (21.8)
573, 614 (65.4)
572, 620 (190.2)
632 (17.8)
d
-
e
-
0.0095
0.0222
0.0162
0.0027
0.0276
0.0077
glassc (77)
benzene (298)
solid (298)
solid (77)
1420
2
3
4
5
6
1110
1170
1350
glassc (77)
benzene (298)
solid (298)
solid (77)
635, 670 (6.4)
578, 632 (145.3)
647 (0.1)
glassc (77)
benzene (298)
solid (298)
solid (77)
1480
570 (8.5)
568 (23.8)
577 (99.2)
640 (2.4)
glassc (77)
benzene (298)
solid (298)
solid (77)
567 (5.3)
562 (93.9)
567 (122.4)
677 (2.0)
glassc (77)
benzene (298)
solid (298)
solid (77)
d
562 (58.2)
547, 581 (226.2)
glassc (77)
1070
a Emission maxima are corrected values. b Luminescence quantum yields are reported using a degassed aqueous solution of [Ru(bpy)3]Cl2 as standard
at 298 K. c Butyronitrile. d Non-emissive. e Not determined.
Figure 4. Electronic absorption spectral change of complex 6 in benzene
Figure 5. UV-vis absorbance changes of complex 6 at 560 nm on
alternate excitation at 365 and 560 nm over three cycles in degassed
upon irradiation at λ = 370 nm at 298 K. Inset shows the expanded
spectral change in the region of 340-365 nm.
benzene solution at 293 K.
found that complexes 2 and 6 had similar photocyclization
complex 5. The percentage conversion of the TMS-
protected ethynylthiophene, ethynylthienothiophene and
complexes 1-6 was determined. It was found that the
percentage conversion to the closed form at photostation-
ary state of complexes 2, 5, and 6, which do not contain the
chloro ligand, was higher, and was enhanced when com-
pared with the chloro-containing complexes (1, 3, and 4),
probably because of the enhanced photocyclization quan-
tum yield. The results are summarized in Table 5.
quantum yield with their corresponding TMS-protected
ligands. On the other hand, complexes 1, 3, and 4, which
contain the chloro ligand, showed lower photocyclization
quantum yields than their corresponding alkynyl analogues
(2 and 5). This is in line with the results from the studies on
the luminescence quantum yield. The presence of the weak
field chloro ligand gives rise to a low-lying ligand field
excited state that undergoes facile non-radiative deactiva-
tion, competing over luminescence and photochromic pro-
cesses. Similar observation was also found in complexes 5
and 6, with complex 6 showing higher photocyclization
quantum yield but lower luminescence quantum yield than
A study on the thermal stability of the closed form was
carried out on a representative sample of complex 6. At
room temperature, the half-life was found to be about

480 Inorganic Chemistry, Vol. 50, No. 2, 2011
Wong et al.
Table 5. Photochemical Quantum Yields and the Percentage Conversion at
Photostationary State (PSS) for TMS;CtC;TTh-DTE, TMS;CtC;Th-
DTE, and Complexes 1-6 in Degassed Benzene Solution at 298 K
Table 6. Electrochemical Data for TMS;CtC;TTh-DTE, TMS;CtC;Th-
DTE, and Complexes 1-6 in Acetonitrile Solution (0.1 mol dm-3 nBu4NPF6)
at 298 Ka
photochemical quantum yield/φa
E1/2b/V vs SCE (ΔEp / mV)c
oxidation
reduction
photo-
cyclization
photo- conversion
cycloreversion at PSS (%)
compound
compound
[Epad/V vs SCE]
[Epce/V vs SCE]
TMS;CtC;TTh-DTE
0.204b
0.226b
0.118c
0.243c
0.117c
0.063c
0.170c
0.242c
0.009d
0.009d
0.026d
0.015d
0.010d
0.061d
0.025d
0.029d
60b
TMS;CtC;TTh-DTE [þ1.25], [þ1.74]
TMS;CtC;Th-DTE [þ1.37], [þ1.96]
[-2.20], [-2.32]
- f
[-1.69]
TMS;CtC;Th-DTE
47b
1
2
3
þ0.80 (80), [þ1.20]
þ0.78 (57), [þ1.17, þ1.36] [-1.68]
1
2
3
4
5
6
48d
þ0.75 (75), [þ0.96,
þ1.20, þ1.44]
[-1.70]
>95d
44d
4
5
6
þ0.86 (95), [þ1.28]
[-1.68]
þ0.82 (75), [þ1.24, þ1.44] [-1.66]
þ0.80 (113), [þ1.07,
[-1.67]
22d
þ1.46, þ1.59]
a Working electrode, glassy carbon; scan rate, 100 mV s-1 b E1/2
. =
94d
(Epa þ Epc)/2; Epa and Epc are anodic and cathodic peak potentials
d
respectively. c ΔEp = (Epa - Epc). Epa is reported for irreversible
>95d,e
oxidation wave. Epc is reported for irreversible reduction wave. f No
reduction wave was observed.
e
a Data obtained with estimated (10% error. b Data obtained using
320 nm as the excitation source. c Data obtained using 370 nm as the
excitation source. d Data obtained using 500 nm as the excitation source.
e Based on singly cyclized product.
and -2.3 V versus SCE. No reduction waves were found
in TMS;CtC;Th-DTE within the potential of the
solvent window though one could not completely exclude
the possibility that similar reductive waves might occur
beyond the solvent windows. Thus, the oxidative and
reductive waves were tentatively assigned as the ethy-
nylthiophene- or ethynylthienothiophene-centered oxi-
dations and reductions, respectively. On the other hand,
complexes 1-6 showed several oxidation waves at about
þ0.75 to þ0.86 V, þ0.96 to þ1.28 V and þ1.36 to þ1.59 V
versus SCE. The first oxidation wave is quasi-reversible
and is sensitive toward the nature of the thiophene-
containing alkynyl ligands. For instance, upon going
from the chloro ligand in 1 and 4 to the stronger elec-
tron-donating phenyl alkynyl ligand in 2 and 5, the
platinum center becomes more electron-rich and the first
oxidation wave is shifted to less positive potential, in-
dicating that the species is easier to be oxidized. Similar
trend could be observed on different alkynyl ligands, in
which complexes with more electron-rich 2-ethynylthie-
nothiophene ligand in 1 and 2 are easier to be oxidized
than complexes with 2-ethynylthiophene ligand as in 4
and 5. Thus, it is likely that the HOMO contains both the
alkynyl ligand and the metal character, and the first
oxidation wave should be assigned as the ethynylthio-
phene-centered oxidation with mixing of Pt(II) to Pt(III)
oxidation, which is commonly observed in other related
systems.4a,b,35 The second irreversible oxidation wave is
tentatively assigned as Pt(III) to Pt(IV) oxidation, which also
appeared in other related systems.35 Other irreversible oxi-
dative waves might probably be attributed to the thiophene
alkynyl-centered oxidations, which also appeared in the
TMS-protected ligands. In addition, complexes 1-6 showed
one irreversible reduction wave at about -1.68 V, which is
slightly sensitive to the nature of the ethynylthiophene
ligands, with the more electron-rich ethynylthienothiophene
shown to be less easy to be reduced. Moreover, this reduction
Figure 6. A plot of ln(A/Ao) versus time for the absorbance decay of
complex 6 at 560 nm at various temperatures in argon-flushed toluene
solution; A denotes absorbance at time t, and Ao denotes the initial
absorbance; solid lines represent the theoretical linear fits.
21 h while at 313 K, the half-life was found to be less than
5 h. A plot of the ln(A/Ao) versus time at various
temperatures is depicted in Figure 6.
Electrochemical Studies. The electrochemical data for
TMS;CtC;TTh-DTE, TMS;CtC;Th-DTE, and
complexes 1-6 in acetonitrile (0.1 mol dm-3 nBu4NPF6)
are tabulated in Table 6. Representative cyclic voltam-
mograms of complex 2 are depicted in Supporting In-
formation, Figure S1. The cyclic voltammograms of
TMS;CtC;TTh-DTE (þ1.25, þ1.74 V vs SCE) and
TMS;CtC;Th-DTE (þ1.37, þ1.96 V vs SCE) showed
two irreversible oxidation waves, which were found to be
sensitive toward the different extent of π-conjugation of
the ethynylthiophene or ethynylthienothiophene core. In
addition, it was found that only TMS;CtC;TTh-DTE
possessed two irreversible reduction waves at about -2.2
(35) Younus, M.; Kohler, A.; Cron, S.; Chawdhury, N.; Al-Mandhary,
M. R. A.; Khan, M. S.; Lewis, J.; Long, N. J.; Friend, R. H.; Raithby, P. R.
Angew. Chem., Int. Ed. 1998, 37, 3036–3039.

Article
Inorganic Chemistry, Vol. 50, No. 2, 2011 481
wave is found to be absent in other platinum(II) bis-phos-
phine systems that do not contain the thiophene moiety.4b
Thus, the reduction wave is tentatively assigned as thio-
phene-centered reduction.
Acknowledgment. V.W.-W.Y. acknowledges support
from The University of Hong Kong under the Distinguished
Research Achievement Award Scheme and the URC Strate-
gic Research Theme on Molecular Materials. This work has
been supported by the University Grants Committee Areas of
Excellence Scheme (AoE/P-03/08) and the Research Grants
Council General Research Fund (HKU7057/07P). H.-L.W.
acknowledges the receipt of a postgraduate studentship from
The University of Hong Kong.
Conclusion
A new class of photochromic dithienylethene alkynes has
been successfully synthesized. The photophysical, photo-
chromic and electrochemical studied have been carried out
on their corresponding platinum(II) phosphine complexes.
They show red phosphorescence and reversible photochro-
mism. The existence of labile chloro ligands in complexes 1, 3,
and 4 demonstrated their capability to function as building
blocks for further functionalization.
Supporting Information Available: Experimental description
for crystal structure determination, tables of crystal data, atom-
ic coordinates, thermal parameters, and a full list of bond
distances and angles for complex 2. This material is available
free of charge via the Internet at http://pubs.acs.org.