Photochromic alkynylplatinum(II) diimine complexes containing a versatile dithienylethene-functionalized 2-(2′-pyridyl)imidazole ligand

Article abstract of DOI:10.1016/j.jorganchem.2013.07.048

The synthesis and incorporation of a versatile photochromic dithienylethene-containing diimine ligand, 2-(2′-pyridyl)imidazole, into platinum(II) bis-alkynyl system have been described. All the platinum(II) complexes have been successfully characterized by ¹H NMR spectroscopy, ESI mass spectrometry as well as elemental analysis. One of the complexes has been characterized by X-ray crystallography. Their photochromic, photophysical and electrochemical properties have been studied. Upon photo-excitation, all the platinum(II) complexes exhibited reversible photochromism. Their photophysical and electrochemical behaviors were found to be tunable by various substituents on the ligands.

Full text of DOI:10.1016/j.jorganchem.2013.07.048

Journal of Organometallic Chemistry 751 (2014) 430e437
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Photochromic alkynylplatinum(II) diimine complexes containing a
versatile dithienylethene-functionalized 2-(20-pyridyl)imidazole
ligand
*
Hok-Lai Wong, Nianyong Zhu, Vivian Wing-Wah Yam
Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee (Hong Kong)) and Department of Chemistry,
The University of Hong Kong, Pokfulam Road, Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and incorporation of a versatile photochromic dithienylethene-containing diimine ligand,
2-(20-pyridyl)imidazole, into platinum(II) bis-alkynyl system have been described. All the platinum(II)
complexes have been successfully characterized by 1H NMR spectroscopy, ESI mass spectrometry as well
as elemental analysis. One of the complexes has been characterized by X-ray crystallography. Their
photochromic, photophysical and electrochemical properties have been studied. Upon photo-excitation,
all the platinum(II) complexes exhibited reversible photochromism. Their photophysical and electro-
chemical behaviors were found to be tunable by various substituents on the ligands.
Ó 2013 Elsevier B.V. All rights reserved.
Received 30 May 2013
Received in revised form
16 July 2013
Accepted 17 July 2013
Keywords:
Diarylethene photochromism
Pt(II) complexes
Metal alkynyls
Luminescence
1. Introduction
centers would enrich the resulting photophysical and photo-
chromic properties and enable the use of the relatively non-
The research on photochromic molecular functional materials
is currently one of the most active areas of topical interest owing
to their potential applications as molecular devices, such as
multifunctional switches and optical memory storage [1e6]. In
particular, diarylethene-based photochromic system has attracted
enormous attention due to its good stability, fatigue resistance and
fast response time [1e3]. Nevertheless, design and synthesis of
molecular materials with desirable photochromic and photo-
physical properties often involve tedious modification of the dia-
rylethene frameworks. Thus, researchers have started developing
versatile diarylethene-containing building blocks to facilitate the
tuning of the photochromic behaviors for various purposes [1e3].
One important approach employed by our group is to utilize
transition metal complex systems and to connect functional
moieties with the photochromic ligands so as to obtain photo-
switchable functional materials [3]. Recently, metal complexes
with photochromic groups, such as azobenzenes [4], stilbenes [5],
diarylethenes [1e3] and spirooxazines [6], have received much
attention. More importantly, participation of transition metal
destructive, lower-energy metal-to-ligand charge transfer (MLCT)
absorption bands to sensitize the photocyclization reaction in or-
der to improve the robustness of the materials. We anticipate that
incorporation of diarylethene moieties into the ligand framework
could introduce a more dramatic change after the photocyclization
reaction. This idea has been realized in a variety of metal complex
systems [3].
Platinum(II) diimine bis(alkynyl) complexes, with a general
formula of [Pt(N^N)(C^CR)2], have aroused enormous attention
due to their ease of design and synthesis, good solubility as well as
the ease of probing of the excited states [7]. Rich photophysical
properties with strong phosphorescence in the fluid state at room
temperature have been observed [7]. With our previous experi-
ence on the photophysical studies of platinum(II) alkynyl systems
and photochromic transition metal complexes [3,8], it is envisaged
that the combining of the platinum(II) diimine bis(alkynyl) system,
which possess highly tunable MLCT/LLCT excited state, with the
diarylethene-containing 2-(20-pyridyl)imidazole ligand could give
rise to photochromic materials with interesting photophysical and
sensitized photochromic properties. Herein, we describe the syn-
thesis, characterization, crystal structure, photophysical, photo-
chromic and electrochemical studies of this new class of
complexes.
* Corresponding author. Tel.: þ852 2859 2153; fax: þ852 2857 1586.
E-mail address: wwyam@hku.hk (V.W.-W. Yam).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.07.048

H.-L. Wong et al. / Journal of Organometallic Chemistry 751 (2014) 430e437
431
2. Experimental
(400 MHz, CD2Cl2, 298 K):
d
1.95, 1.97 (s, 3H, eCH3) [10], 2.15, 2.54
(s, 3H, eCH3) [10], 2.24 (s, 3H, eCH3), 2.36, 2.4 (s, 3H, eCH3) [10],
6.16 (s, 1H, thienyl), 6.32, 6.69 (s, 1H, thienyl) [10], 6.49 (m, 1H,
pyridyl proton at 5-position), 7.40 (m, 2H, 4-(trifluoromethyl)
phenyl protons at 3-position), 7.76 (m, 2H, 4-(trifluoromethyl)
phenyl protons at 2-position), 7.81 (m, 1H, pyridyl proton at 4-
position), 7.90 (m, 1H, pyridyl proton at 3-position), 9.76 (d,
J ¼ 5.5 Hz, 1H, pyridyl proton at 6-position). Positive FAB mass
spectrum: m/z 776 {M}þ
2.1. Materials and reagents
1-Ethynylnaphthalene, 4-ethynylbenzonitrile and copper(I) io-
dide were obtained from Aldrich Chemical Company. Phenyl-
acetylene, 1-ethynyl-4-methylbenzene and N,N-diethylamine were
purchased from Lancaster Synthesis Ltd. Potassium tetra-
chloroplatinate(II) was purchased from Strem Chemicals, Inc. The
diarylethene-containing diimine ligands, 1-(4-methylphenyl)-4,5-
bis-(2,5-dimethyl-3-thienyl)-2-(20-pyridyl)imidazole (L1) [3d], 1-
(4-methoxyphenyl)-4,5-bis-(2,5-dimethyl-3-thienyl)-2-(20-pyr-
idyl)-imidazole (L2) [3d], and 1-(4-trifluoromethylphenyl)-4,5-bis-
2.2.4. [Pt(L1)(C^CeC6H5)2] (1)
The titled compound was prepared by modification of a litera-
ture method for the related bis-alkynylplatinum(II) bipyridine
complexes [3c]. To a mixture of [Pt(L1)Cl2] (100 mg, 0.138 mmol)
and catalytic amount of copper(I) iodide were added degassed
dichloromethane (20 mL) and dry diethylamine (2 mL). The
mixture was stirred under argon for 10 min, followed by addition of
an excess of phenylacetylene (141 mg, 0.152 mL, 1.38 mmol). The
resulting suspension was stirred for 24 h. The solution was evap-
orated to dryness, dissolved in dichloromethane, and purified by
column chromatography on neutral aluminum oxide using n-hex-
ane-dichloromethane (4:1 v/v) as eluent. Further purification was
achieved by slow diffusion of pentane vapor into a concentrated
dichloromethane solution of the complex or by layering of meth-
anol over a concentrated dichloromethane or chloroform solution
of the complex. Yield: 30 mg, 0.035 mmol; 26%. 1H NMR (400 MHz,
(2,5-dimethyl-3-thienyl)-2-(20-pyridyl)-imidazole
(L3)
[3d],
together with their respective dichloroplatinum(II) complexes,
[Pt(L1)Cl2] [9], [Pt(L2)Cl2] [9] and [Pt(L3)Cl2] [9], were prepared
according to literature procedures with modifications. Diethyl-
amine was distilled over potassium hydroxide and stored over
potassium hydroxide prior to use. Tetra-n-butylammonium hexa-
fluorophosphate was purified by at least three times of recrystal-
lization in absolute ethanol and was dried prior to use. Benzene was
distilled over sodium before use. Dichloromethane was purified
using Innovative Technology, Inc. model PureSolv MD 5 Solvent
Purification System before use. Acetonitrile was distilled over cal-
cium hydride before use. All other solvents and reagents were of
analytical grade and were used as received.
DMSO-d6, 353 K):
d 1.93 (s, 3H, eCH3), 2.06 (s, 3H, eCH3), 2.20 (s,
2.2. Synthesis
3H, eCH3), 2.36 (s, 3H, eCH3), 2.42 (s, 3H, eC6H4CH3), 6.40 (s, 1H,
thienyl), 6.50 (s, 1H, thienyl), 6.58 (d, J ¼ 8.2 Hz, 1H, pyridyl proton
at 5-position), 6.99 (m, 3H, phenyl protons at 2,4-positions), 7.11
(m, 3H, phenyl protons at 2,4-positions), 7.23 (m, 2H, phenyl pro-
tons at 3-position), 7.31 (d, J ¼ 7.5 Hz, 2H, tolyl protons at 3-
position), 7.40 (d, J ¼ 7.5 Hz, 2H, tolyl protons at 2-position), 7.50
(m, 2H, phenyl protons at 3-position), 7.64 (m, 1H, pyridyl proton at
4-position), 7.99 (m, 1H, pyridyl proton at 3-position), 9.66 (d,
J ¼ 5.4 Hz,1H, pyridyl proton at 6-position). ESI mass spectrum: m/z
852 {M}þ. Elemental analyses, Found (%): C 54.54, H 3.62, N 4.13;
Calcd (%) for C43H35N3PtS2$CHCl3: C 54.35, H 3.73, N 4.32.
2.2.1. [Pt(L1)Cl2]
The titled compound was synthesized by modification of a
literature procedure [9]. To a mixture of [Pt(DMSO)2Cl2] (93 mg,
0.220 mmol) and L1 (100 mg, 0.220 mmol) was added degassed
acetonitrile (20 mL). The reaction mixture was heated under reflux
for overnight under argon. The resulting yellow solution was
evaporated to dryness under reduced pressure. The crude product
was purified by column chromatography on silica gel (70e
230 mesh) using dichloromethane as eluent. Further purification
was done by slow diffusion of pentane vapor into its concentrated
dichloromethane solution. Yield: 105 mg, 0.146 mmol; 66%. 1H
2.2.5. [Pt(L1)(C^CeC6H4CH3-p)2] (2)
NMR (400 MHz, DMSO-d6, 353 K):
d
1.97 (s, 3H, eCH3), 2.19 (s, 3H, e
The target compound was prepared according to a procedure
similar to that of [Pt(L1)(C^CeC6H5)2] except 4-tolylacetylene
(160 mg, 1.38 mmol) was used instead of phenylacetylene. Yield:
52 mg, 0.059 mmol; 43%. 1H NMR (400 MHz, DMSO-d6, 353 K):
CH3), 2.29 (s, 6H, eCH3), 2.40 (s, 3H, eC6H4CH3), 6.43 (m, 3H,
thienyl protons, pyridyl proton at 5-position), 7.37 (d, J ¼ 7.3 Hz, 2H,
tolyl protons at 3-position), 7.49 (m, 3H, tolyl protons at 2-position,
pyridyl proton at 4-position), 7.98 (m, 1H, pyridyl proton at 3-
position), 9.55 (d, J ¼ 4.3 Hz, 1H, pyridyl proton at 6-position).
Positive FAB mass spectrum: m/z 722 {M}þ.
d
1.93 (s, 3H, eCH3), 2.08 (s, 3H, eCH3), 2.20 (s, 3H, eCH3), 2.24 (s,
3H, eCH3), 2.27 (s, 3H, eC6H4CH3), 2.35 (s, 3H, eC6H4CH3), 2.40 (s,
3H, eC6H4CH3), 6.39 (s, 1H, thienyl), 6.50 (s, 1H, thienyl), 6.57 (d,
J ¼ 8.2 Hz, 1H, pyridyl proton at 5-position), 6.87 (d, J ¼ 8.1 Hz, 2H,
N-tolyl protons at 3-position), 6.92 (d, J ¼ 8.1 Hz, 2H, N-tolyl protons
at 2-position), 7.05 (d, J ¼ 8.0 Hz, 2H, tolyl protons at 3-position),
7.20 (d, J ¼ 8.0 Hz, 2H, tolyl protons at 2-position), 7.39 (d, J ¼ 8.2 Hz,
2H, tolyl protons at 3-position), 7.50 (d, J ¼ 8.2 Hz, 2H, tolyl protons
at 2-position), 7.62 (m, 1H, pyridyl proton at 4-position), 7.97 (m,
1H, pyridyl proton at 3-position), 9.65 (d, J ¼ 5.4 Hz, 1H, pyridyl
proton at 6-position). ESI mass spectrum: m/z 880 {M}þ. Elemental
analyses, Found (%): C 60.72, H 4.47, N 4.84; Calcd (%) for
2.2.2. [Pt(L2)Cl2]
The target compound was prepared according to a procedure
similar to that of [Pt(L1)Cl2] except L2 (100 mg, 0.212 mmol) was
used instead of L1. Yield: 112 mg, 0.152 mmol; 72%. 1H NMR
(400 MHz, DMSO-d6, 353 K): d 1.98 (s, 3H, eCH3), 2.20 (s, 3H, eCH3),
2.27 (s, 6H, eCH3), 3.84 (s, 3H, eC6H4OCH3), 6.39 (s, 1H, thienyl),
6.43 (s, 1H, thienyl), 6.51 (d, J ¼ 8.2 Hz, 1H, pyridyl proton at 5-
position), 7.09 (d, J ¼ 8.1 Hz, 2H, p-methoxyphenyl protons at 3-
position), 7.52 (m, 3H, p-methoxyphenyl protons at 2-position,
pyridyl proton at 4-position), 8.00 (m, 1H, pyridyl proton at 3-
position), 9.55 (d, J ¼ 5.7 Hz, 1H, pyridyl proton at 6-position).
Positive FAB mass spectrum: m/z 738 {M}þ
C45H39N3PtS2$0.5CH3OH: C 60.92, H 4.61, N 4.68.
2.2.6. [Pt(L1)(C^CeC6H4CN-p)2] (3)
The target compound was prepared according to a procedure
similar to that of [Pt(L1)(C^CeC6H5)2] except 4-ethynylb
enzonitrile (175 mg, 1.38 mmol) was used instead of phenyl-
acetylene. Yield: 82 mg, 0.091 mmol; 66%. 1H NMR (400 MHz,
2.2.3. [Pt(L3)Cl2]
The target compound was prepared according to a procedure
similar to that of [Pt(L3)Cl2] except L3 (100 mg, 0.196 mmol) was
used instead of L1. Yield: 103 mg, 0.133 mmol; 68%. 1H NMR
DMSO-d6, 353 K):
d 1.94 (s, 3H, eCH3), 2.05 (s, 3H, eCH3), 2.20 (s,
3H, eCH3), 2.34 (s, 3H, eCH3), 2.42 (s, 3H, eC6H4CH3), 6.39 (s, 1H,

432
H.-L. Wong et al. / Journal of Organometallic Chemistry 751 (2014) 430e437
thienyl), 6.48 (s, 1H, thienyl), 6.60 (d, J ¼ 8.2 Hz, 1H, pyridyl proton
at 5-position), 7.10 (d, J ¼ 8.3 Hz, 2H, tolyl protons at 3-position),
7.39 (d, J ¼ 8.2 Hz, 2H, p-cyanophenyl protons at 3-position), 7.44
(d, J ¼ 8.3 Hz, 2H, tolyl protons at 2-positions), 7.51 (m, 4H, p-
cyanophenyl protons at 2,4-positions), 7.64 (m, 3H, p-cyanophenyl
protons at 2-position, pyridyl proton at 4-position), 8.00 (m, 1H,
pyridyl proton at 3-position), 9.52 (d, J ¼ 4.9 Hz, 1H, pyridyl proton
at 6-position). ESI mass spectrum: m/z 902 {M}þ. Elemental ana-
lyses, Found (%): C 59.71, H 3.69, N 7.59; Calcd (%) for C45H33N5PtS2:
C 59.85, H 3.68, N 7.76.
2.3. Physical measurements and instrumentation
1H NMR spectra were recorded either on a Bruker DPX-300
(300 MHz) or a Bruker AV400 (400 MHz) at 353 K. Chemical
shifts (d
, ppm) for 1H NMR and 31P NMR were recorded relative to
tetramethylsilane (Me4Si) and 85% phosphoric acid respectively.
Positive-ion electrospray ionization (ESI) mass spectra were
recorded on a Finnigan LCQ mass spectrometer. Elemental analyses
of the new compounds were performed on a Carlo Erba 1106
elemental analyzer at the Institute of Chemistry, Chinese Academy
of Sciences, Beijing.
2.2.7. [Pt(L2)(C^CeC6H5)2] (4)
UVeVis absorption spectra were recorded on a HewlettePack-
ard 8452A diode array spectrophotometer. Photoirradiation was
carried out using a 300 W Oriel Corporation Model 66011 Xe
(ozone-free) lamp, and monochromic light was obtained by passing
the light through an Applied Photophysics F 3.4 monochromator.
All measurements were conducted at room temperature.
The target compound was prepared according to a procedure
similar to that of [Pt(L1)(C^CeC6H5)2] except [Pt(L2)Cl2] (100 mg,
0.136 mmol) was used instead of [Pt(L1)Cl2]. Yield: 58 mg,
0.067 mmol; 49%.1H NMR (400 MHz, DMSO-d6, 353 K):
d 1.94 (s, 3H, e
CH3), 2.06 (s, 3H, eCH3), 2.21 (s, 3H, eCH3), 2.32 (s, 3H, eCH3), 3.85 (s,
3H, eOCH3), 6.40 (s,1H, thienyl), 6.51 (s,1H, thienyl), 6.62 (d, J ¼ 8.2 Hz,
1H, pyridyl proton at 5-position), 6.98 (m, 3H, phenyl protons at 2,4-
positions), 7.11 (m, 5H, p-methoxyphenyl protons at 3-position,
phenyl protons at 2,4-positions), 7.22 (m, 2H, phenyl protons at 3-
position), 7.30 (m, 2H, phenyl protons at 3-position), 7.54 (d,
J ¼ 7.7 Hz, 2H, p-methoxyphenyl protons at 2-position), 7.64 (m, 1H,
pyridyl proton at 4-position), 8.01 (m,1H, pyridyl proton at 3-position),
9.66(d,J¼ 5.6 Hz,1H, pyridyl proton at 6-position). ESI mass spectrum:
m/z 868 {M}þ. Elemental analyses, Found (%): C 58.60, H 4.11, N 4.76;
Calcd (%) for C43H35N3OPtS2$CH3OH: C 58.65, H 4.36, N 4.66.
Steady-state emission and excitation spectra at room tempera-
ture 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 cm [3] Pyrex round-bottomed flask connected by a sidearm
to a 1-cm quartz fluorescence cuvette and were sealed from the
atmosphere by a Rotaflo HP6/6 quick-release Teflon stopper. Solu-
tions were rigorously degassed with no fewer than four freezee
pumpethaw 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 EtOHeMeOH (4:1 v/
v) 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 photomultiplier tube con-
2.2.8. [Pt(L2)(C^C-1-Np)2] (5)
The target compound was prepared according to a procedure
similar to that of [Pt(L1)(C^CeC6H5)2] except [Pt(L2)Cl2] (200 mg,
0.272 mmol) and 1-ethynylnaphthalene (414 mg, 2.72 mmol) were
used instead of [Pt(L1)Cl2] and phenylacetylene, respectively. Yield:
56 mg, 0.058 mmol; 21%. 1H NMR (400 MHz, DMSO, 353 K):
d 1.86
(s, 3H, eCH3), 1.95 (s, 3H, eCH3), 2.21 (s, 3H, eCH3), 2.37 (s, 3H, e
CH3), 3.86 (s, 3H, eOCH3), 6.43 (s, 1H, thienyl), 6.54 (s, 1H, thienyl),
6.68 (d, J ¼ 8.2 Hz, 1H, pyridyl proton at 5-position), 7.11 (m, 3H, p-
methoxyphenyl protons at 3-position, naphthyl), 7.20 (m, 2H,
naphthyl), 7.35 (m, 4H, naphthyl), 7.57 (m, 4H, naphthyl, p-
methoxyphenyl protons at 3-position, pyridyl proton at 4-position),
7.66 (d, 2H, J ¼ 7.8 Hz, p-methoxyphenyl protons at 2-position), 7.75
(d, J ¼ 8.1 Hz, 1H, naphthyl), 7.80 (d, J ¼ 8.1 Hz, 1H, naphthyl), 8.04
(m, 1H, pyridyl proton at 3-position), 8.55 (d, J ¼ 8.4 Hz, 1H,
naphthyl), 8.66 (d, J ¼ 8.4 Hz, 1H, naphthyl), 9.79 (d, J ¼ 5.6 Hz, 1H,
pyridyl proton at 6-position). ESI mass spectrum: m/z 968 {M}þ.
Elemental analyses, Found (%): C 63.16, H 4.21, N 4.31; Calcd (%) for
nected to a 50
Tekronix Modal TDS 620A digital oscilloscope (500 MHz, 2 GS/s). The
lifetime ( ) determination was achieved by the single exponential
fittings of the luminescence decay traces with the model equation,
), where I(t) and Io refer to the luminescence in-
U load resistor and the voltage signal recorded on a
s
I(t) ¼ Io exp(ꢀt/
s
tensity at the time ¼ t and time ¼ 0, respectively. Luminescence
quantum yield was measured by the optical dilute method devel-
oped by Demas and Crosby [11]. A degassed aqueous solution of
[Ru(bpy)3]Cl2 was used as the standard [12] at 298 K.
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 refer-
ence electrode was separated from the working electrode
compartment by a vycor glass. Electrochemical measurements
were performed in acetonitrile solutions with 0.1 mol dmꢀ3
nBu4NPF6 as supporting electrolyte at room temperature. The
reference electrode was a Ag/AgNO3 (0.1 M in acetonitrile) elec-
trode, and the working electrode was a glassy carbon (CH Instru-
ment) electrode with a platinum wire as a counter electrode in a
compartment separated from the working electrode by a sintered-
glass frit. The ferrocenium/ferrocene couple (FeCpþ2 /0) was used as
the internal reference [13]. All solutions for electrochemical studies
were deaerated with prepurified argon gas before measurement.
C
51H39N3OPtS2: C 63.21, H 4.06, N 4.34.
2.2.9. [Pt(L3)(C^CeC6H5)2] (6)
The target compound was prepared according to a procedure
similar to that of [Pt(L1)(C^CeC6H5)2] except [Pt(L3)Cl2] (100 mg,
0.129 mmol) was used instead of [Pt(L1)Cl2]. Yield: 55 mg,
0.061 mmol; 47%. 1H NMR (400 MHz, DMSO-d6, 353 K):
d 1.94 (s,
3H, eCH3), 2.06 (s, 3H, eCH3), 2.18 (s, 3H, eCH3), 2.37 (s, 3H, eCH3),
6.36 (s, 1H, thienyl), 6.52 (s, 1H, thienyl), 6.62 (d, J ¼ 8.2 Hz, 1H,
pyridyl proton at 5-position), 6.96 (m, 3H, 4-(trifluoromethyl)
phenyl protons at 3-position, phenyl proton at 4-position), 7.11 (m,
3H, phenyl protons at 2,4-positions), 7.23 (m, 2H, phenyl protons at
2-position), 7.31 (d, J ¼ 7.3 Hz, 2H, 4-(trifluoromethyl)phenyl pro-
tons at 2-position), 7.68 (m, 1H, pyridyl proton at 4-position), 7.93
(m, 5H, pyridyl proton at 3-position, phenyl protons at 3-position),
9.67 (d, J ¼ 5.4 Hz, 1H, pyridyl proton at 6-position). ESI mass
spectrum: m/z 906 {M}þ. Elemental analyses, Found (%): C 53.21, H
3.44, N 4.05; Calcd (%) for C43H32F3N3PtS2$CH2Cl2: C 53.28, H 3.46,
N 4.24.
2.4. Crystal structure determination
All the experimental details are given in Table 1. Single crystals of
complex 1 suitable for X-ray studies were obtained by vapor diffusion

H.-L. Wong et al. / Journal of Organometallic Chemistry 751 (2014) 430e437
433
Table 1
At 353 K in DMSO-d6, the signals were sharpened and became well-
resolved. The presence of a signal at
9.52e9.79 ppm in the 1H
Crystal and structure determination data for complex 1.
d
NMR spectra, which was downfield shifted from that observed in the
free ligand and characteristic of the proton at the 5-position of pyri-
dine, was indicative of the successful coordination of the ligand to the
platinum(II) center. The identities of the compounds have also been
established by ESI-MS and satisfactory elemental analysis.
Empirical formula
C44H36Cl3N3PtS2
Formula weight
Temperature
Wavelength
Crystal system
Space group
Unit cell dimensions
972.32
301 (2) K
ꢀ
0.71073 A
Triclinic
P1(No. 2)
a ¼ 12.948 (1) A
a
b
g
¼ 109.74 (2)ꢂ
¼ 96.05 (2)ꢂ
¼ 95.13 (2)ꢂ
ꢀ
3.2. X-ray crystal structure
ꢀ
b ¼ 15.968 (1) A
ꢀ
c ¼ 22.538 (2) A
3
ꢀ
ꢀ
Volume
Z
Density (calculated)
Absorption coefficient
F (000)
Crystal size
Theta range for data collection
Index ranges
4322.4 (6) A
4
The perspective drawing and selected bond distances (A) and
angles (ꢂ) of complex 1 are depicted and tabulated in Fig. 1 and
Table 2 respectively. The crystal structure showed the antiparallel
conformation. The interplanar angles between the dimethylth-
iophene rings and the imidazole ring were both ca. 54ꢂ. The
interplanar angle between the N-aryl ring and the imidazole ring
was 74ꢂ, indicating a deviation from coplanarity. The d [8] plati-
num(II) metal center adopted a distorted square planar geometry,
which has been commonly observed in other platinum(II) diimine
systems [7c]. This could probably be due to the steric demand
required for the N,N-chelating diimine ligand. In addition, the PteN
1.494 g cmꢀ3
3.561 mmꢀ1
1928
0.35 mm ꢁ 0.15 mm ꢁ 0.1 mm
1.93e25.68ꢂ
ꢀ15 & h & 15, ꢀ18 & k & 19,
ꢀ27 & l & 25
Reflections collected
25,582
Independent reflections
Completeness to theta ¼ 25.68ꢂ
Absorption correction
Refinement method
16,029 [R(int) ¼ 0.0176]
97.60%
None
Full-matrix least-squares on F2
16,029/0/955
ꢀ
bond distances were found to be around 2.08 A, slightly longer than
Data/restraints/parameters
those of [Pt(5,50-dmbpy)Cl2] [15] and [Pt(phen)Cl2] [16]. This could
Goodness-of-fit on F2
1.066
Final R indices [I > 2
R indices (all data)
Largest diff. peak and hole
s
(I)]
R1 ¼ 0.0400, wR2 ¼ 0.1109
R1 ¼ 0.0553, wR2 ¼ 0.1173
2.409 and ꢀ0.893 e Åꢀ3
be ascribed to the trans effect imposed by the s-bonded alkynyl
groups, whose bond distances with the platinum center were ca.
ꢀ
1.95 A. In addition, the shortest Pt/Pt distance between two
ꢀ
adjacent molecules was found to be 3.8162(7) A, indicating the
absence of any significant Pt/Pt interaction.
of pentane vapor into a concentrated chloroform solution of complex
1. A crystal of dimensions 0.35 mm ꢁ 0.15 mm ꢁ 0.10 mm mounted in
a glass capillary was used for data collection at 28 ꢂC on a Bruker
3.3. Electronic absorption and emission properties
Smart CCD 1000 using graphite monochromatized Mo-Ka radiation
All complexes were dissolved in benzene to give yellow to orange
solutions. The electronic absorption spectra showed high-energy
absorption bands at ca. 310 and 360 nm, with molar extinction co-
efficients in the order of 104 dm3 molꢀ1 cmꢀ1 and a long absorption
tail at ca. 420 nm with molar extinction coefficient in the order of
103 dm3 molꢀ1 cmꢀ1. The high-energyabsorption bands are ascribed
ꢀ
(l
¼ 0.71073 A). The structure was solved by direct methods
employing SHELXS-97 [14] program on PC. Pt, S and many non-H
atoms were located according to the direct methods. The positions
of the other non-hydrogen atoms were found after successful
refinement by full-matrix least-squares using program SHELXL-97
[14] on PC. Detailed experimental procedures and other results
including atomic coordinates, equivalent isotropic displacement pa-
rameters, bond lengths, bond angles, anisotropic displacement pa-
rameters, hydrogen coordinates and isotropic displacement
parameters for complex 1 were included in Supplementary material
and were tabulated in Tables S1eS4.
to the intraligand (IL)
p / p* transitions of the diimine and alkynyl
ligands. The low-energy absorption tails at ca. 400e500 nm were
found to be shifted to the lower energy when the platinum center
was coordinated to the more electron-rich alkynyl ligand with the
absorption energies in the order of 3 (405 nm) > 1 (421 nm) > 2
(423 nm) or to the more
p-accepting diimine ligand with the ab-
sorption energies in the order of 1 (421 nm) w 4 (422 nm) > 6
3. Results and discussion
(429 nm). Thus, the low-energy absorption tails were assigned as
the metal-to-ligand charge transfer MLCT [d
transitions, probably with some mixing of ligand-to-ligand charge
transfer LLCT [ (alkynyl) / *(diimine)] character. This assignment
is consistent with those reported in other related platinum(II) dii-
mine bis-alkynyl systems [3c,7a,c,o]. The electronic absorption data
are summarized in Table 3.
p(Pt) / p*(diimine)]
3.1. Synthesis and characterization
p
p
The diarylethene-functionalized 2-(20-pyridyl)imidazole ligands
have been designed and reported by Yam and co-workers previ-
ously [3d]. The incorporation of diimine ligands into the plati-
num(II) center followed a general synthetic procedure, which was
achieved by the reaction of the diimine ligand with [Pt(DMSO)2Cl2]
in acetonitrile [9]. The resulting [Pt(diimine)Cl2] complex was
readily functionalized with various terminal alkynes in dichloro-
methane in the presence of diethylamine and a catalytic amount of
copper(I) iodide. The target complexes were purified by column
chromatography on aluminum oxide and several times of recrys-
tallization by slow diffusion of pentane vapor into their chloroform
solutions. The synthetic pathway for this type of platinum(II) dii-
mine complexes is outlined in Scheme 1.
Upon excitation into either the IL or MLCT/LLCT bands of the
complexes in degassed benzene solution at 298 K, luminescence at
ca. 540e580 nm was observed, with lifetimes in the range of micro-
seconds. These, together with the large Stokes shift, are suggestive
of a triplet emission origin. As shown in Fig. 2, the emission was
found to be slightly blue-shifted in energy (4 > 1 > 6) upon
increasing the electron-richness of the diimine ligand, while a red
shift in energy (3 > 1 > 2) was observed upon increasing the
electron-richness of the alkynyl ligand. This was suggestive of an
emission origin of 3MLCT [d
(alkynyl) / *(diimine)] character. Similar assignments have
p
(Pt)
/
p*(diimine)]/LLCT
All the complexes gave broad signals in the 1H NMR spectra at
room temperature, suggestive of isomer interconversion at a time-
scale similar to that of NMR. This is most likely due to the conver-
sion between the antiparallel and the parallel forms of the complexes.
[p
p
also been reported for other platinum(II) diimine bis-alkynyl sys-
tems [3c,7a,c,o]. The rather small effect observed on the emission
maxima upon a variation of the electron-richness of the diimine

434
H.-L. Wong et al. / Journal of Organometallic Chemistry 751 (2014) 430e437
Scheme 1. Synthetic pathway of complexes 1e6.
ligand could be ascribed to the long distance between the sub-
stituent and the central diimine core. In addition, the aryl ring was
not coplanar to the imidazole ring (74ꢂ), which minimized
communication between them.
Most complexes showed photoluminescence in the solid state
at 77 K, 298 K and in glass at 77 K, with a blue shift in the emission
maxima relative to the solution state with well-resolved vibra-
tional progressional spacings, indicative of substantial involve-
ment of ligand orbitals in the emissive state. Representative
spectra have been shown in Figs. S1,S2. Thus, for complexes 1e6,
the emissions were assigned as derived from an 3IL [
p / p*
(diimine)] origin. The emission data for all the complexes are
summarized in Table 4.
3.4. Photochromic properties
Upon photo-excitation into either the intraligand or the metal-to-
ligand charge transfer transitions, the degassed benzene solutions of
complexes 1e6 turned green, suggesting the formation of a new
absorption band in the visible region, commonly observed in the
formation of the closed forms [1e3]. The newly emerged bands were
located at ca. 720 nm, with slight variations between different
complexes and were comparable to that observed in the rhenium(I)
analogs [3d]. The large bathochromic shift with respect to the open
form could be attributed to the increase in the extent of
p-conju-
gation in the two dimethylthiophene rings (8a,8b-dimethyl-1,8-thia-
as-indacene) upon ring-closing, suggesting its origin as a predomi-
nantly
pep* intraligand transition. When compared to the free li-
Fig. 1. Perspective view of complex 1 with atomic numbering scheme. Hydrogen
atoms have been omitted for clarity. Thermal ellipsoids were shown at the 30%
probability level.
gands [3d], the lowest new energy absorption band of the closed
form showed a red shift from ca. 580 nm to ca. 720 nm. This could

H.-L. Wong et al. / Journal of Organometallic Chemistry 751 (2014) 430e437
435
Table 2
ꢂ
ꢀ
Selected bond distances (A) and angles ( ) with estimated standard deviations
1
4
6
(e.s.d.s.) in parentheses for complex 1.
ꢀ
Bond distances/A
Pt(1)eN(1)
Pt(1)eN(3)
Pt(1)eC(28)
Pt(1)eC(36)
C(3)eC(4)
C(28)eC(29)
C(29)eC(30)
C(36)eC(37)
C(37)eC(38)
2.076 (4)
2.083 (5)
1.949 (6)
1.959 (7)
1.451 (7)
1.190 (8)
1.442 (8)
1.207 (9)
1.419 (9)
Bond angles/ꢂ
N(1)ePt(1)eN(3)
C(28)eC(29)eC(30)
178.4 (8)
177.0 (7)
112.8 (3)
140.8 (3)
117.0 (4)
124.9 (5)
78.07 (17)
500
550
600
650
700
750
800
N(1)ePt(1)eC(36)
C(36)ePt(1)eC(28)
C(28)ePt(1)eN(3)
Pt(1)eC(28)eC(29)
Pt(1)eC(36)eC(37)
C(36)eC(37)eC(38)
Pt(1)eN(1)eC(3)
Pt(1)eN(1)eC(1)
Pt(1)eN(3)eC(4)
Pt(1)eN(3)eC(8)
Wavelength / nm
101.5 (2)
86.1 (3)
94.5 (2)
175.9 (6)
175.4 (6)
Fig. 2. Normalized corrected emission spectra of the open forms of complexes 1, 4 and
6 in degassed benzene solutions at 298 K.
imidazole ring. Some representative spectral changes were illus-
trated in Fig. 4 and Fig. S3. In addition, the emission spectral changes
upon photo-irradiation have been studied for complex 3. Upon
excitation at the isosbestic point at ca. 363 nm, reduction in lumi-
nescence intensity was observed (Fig. S4), indicative of emission
quenching upon photocyclization, which is commonly found in
photochromic diarylethene systems [3].
most likely be attributed to the difference in conjugation, either due
to the extended conjugation through the central metal center or the
effect of planarization triggered by metal coordination. The insig-
nificant difference in absorption wavelengths between the present
platinum(II) system and a previously reported rhenium(I) system
[3d] suggested that the participation of the metal center in the
conjugation might not be the predominant factor. This could also be
supported by the smaller amount of red shift observed in the plati-
num(II) complexes containing photochromic 1,10-phenanthroline
ligand where planarization was not possible [3d]. Therefore, the
large red shift observed would best be ascribed to the metal
coordination-assisted planarization of the diimine ligand (Fig. 3),
leading to good conjugation between the pyridine ring and the
3.5. Electrochemical studies
The electrochemical behavior of the compounds in acetonitrile
was studied and representative cyclic voltammograms of complex
3 are shown in Fig. S5. Complexes 1e6 showed an irreversible
oxidation wave at ca. þ1.05 to þ1.32 V vs. SCE, which is commonly
observed in platinum(II) diimine complexes [17] and platinum(II)
Table 4
Emission data for complexes 1e6 in various media.
b
Compound
Medium (T/K)
Emission lema/nm (so
/m
s)
flum
Table 3
1
Benzene (298)
Solid (298)
Solid (77)
577 (0.2)
573 (1.2)
0.04
Electronic absorption data for L1, L2, L3 and complexes 1e6 in the open form and
closed form.
547, 587 (8.3)
522, 568 (19.3)
581 (1.5)
537, 574 (1.2)
538, 578 (11.8)
520, 556, 571 (15.7)
574 (0.3)
541, 572, 659 (1.4)
533, 573 (16.8)
520, 555 (27.3)
573 (3.1)
573 (1.3)
570 (11.5)
520, 555 (21.2)
581 (0.1)
626 (0.2)
Absorptiona
Glassc (77)
Benzene (298)
Solid (298)
Solid (77)
Compound
Configuration
2
3
4
5
6
0.02
0.02
0.04
0.05
0.03
labs/nm (ε/dm3 molꢀ1 cmꢀ1
)
L1
L2
L3
Open
Closed
Open
Closed
Open
Closed
320 (9080)
342 (16,550), 406 sh (4100), 566 (4930)
320 (9540)
344 (19,840), 422 sh (3810), 568 (5790)
324 (13,110)
304 (21,580), 354 (25,760),
418 sh (6400), 582 (9240)
Glassc (77)
Benzene (298)
Solid (298)
Solid (77)
Glassc (77)
Benzene (298)
Solid (298)
Solid (77)
1
2
3
4
5
6
Open
Closed
Open
Closed
Open
Closed
Open
Closed
Open
Closed
Open
361 (11,410), 421 sh (6060)
719b
360 (14,720), 423 sh (7350)
Glassc (77)
Benzene (298)
Solid (298)
Solid (77)
717b
318 (41,930), 372 (12,900), 405 sh (9270)
730b
579, 625 (3.1)
549, 597 (167.8)
579 (0.2)
571 (1.5)
573 (6.06)
302 sh (18,230), 364 (10,120), 422 sh (5320)
Glassc (77)
Benzene (298)
Solid (298)
Solid (77)
717b
326 (38,990), 344 (42,800), 384 sh (15,130)
714b
301 sh (24,150), 366 (12,030), 429 sh (6060)
Glassc (77)
526, 568(20.5)
Closed
715b
a
Emission maxima are corrected values.
Luminescence quantum yields were reported relative to a degassed aqueous
a
b
Data obtained in benzene at 298 K.
Only the lowest energy absorption band was shown. Extinction coefficients
b
solution of [Ru(bpy)3]Cl2 as standard at 298 K.
c
could not be measured with certainty due to small conversion.
EtOHeMeOH (4:1 v/v).

436
H.-L. Wong et al. / Journal of Organometallic Chemistry 751 (2014) 430e437
4. Conclusion
A new class of photochromic dithienylethene-containing plati-
num(II) diimine bis(alkynyl) complexes has been successfully
synthesized and characterized. The photophysical, photochromic
and electrochemical studies have been carried out. They show or-
ange phosphorescence and photochromism in the red to near-
infrared region. The large extent of red shift in absorption of the
closed form is possibly due to the metal-co-ordination assisted
planarization, which enhanced the electronic communication be-
tween the pyridine and the imidazole rings. This study demon-
strates the versatility of the photochromic diarylethene-containing
diimine ligand design and opens up further exploration in other
transition metal complex systems.
Fig. 3. Planarization of pyridine and imidazole rings by incorporation into platinum(II)
center.
Acknowledgments
V.W.-W.Y. acknowledges support from The University of Hong
Kong under the URC Strategic Research Theme on New Materials.
This work has been supported by the University Grants Committee
Areas of Excellence Scheme (AoE/P-03/08), and a General Research
Fund (GRF) (HKU 7060/12P) grant from the Research Grants Council
of Hong Kong Special Administrative Region, PR China. H.-L. W.
acknowledges the receipt of a University Postdoctoral Fellowship.
Appendix A. Supplementary material
Fig. 4. Electronic absorption spectral changes of complex 1 in benzene upon irradia-
tion at 395 nm at 298 K.
CCDC 938706 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
2-(2-pyridyl)benzimidazole bis-alkynyl complexes [9]. With
different diimine ligands (1 vs 4 vs 6), there was insignificant effect
on the potential, but with more electron-donating alkynyl ligands,
the potential became less positive (3 (þ1.32 V) > 1 (þ1.12 V) w 2
(þ1.05 V)). Thus, the oxidation was assigned as metal-centered
oxidation [Pt(II) / Pt(III)], probably mixed with alkynyl-centered
oxidation. There was no improvement in the reversibility even
upon increasing the scan rate up to 1 V sꢀ1. On the other hand, two
reduction waves at ca. ꢀ1.54 to ꢀ1.62 V vs. SCE and ꢀ2.06
to ꢀ2.25 V vs. SCE were observed. The first quasi-reversible couple
was assigned as diimine-centered reduction, commonly observed
in platinum(II) bipyridine and phenanthroline systems [17] and
related diimine complexes [9]. The second irreversible reduction
wave, which was found to be similar to the thiophene-based
reduction reported in the literature [3] and absent in the cyclic
voltammograms of the related platinum(II) 2-(2-pyridyl)benz-
imidazole bis-alkynyl complexes [9], was assigned as the reduction
of the thiophene moiety. The results are summarized in Table 5.
Appendix B. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2013.07.048.
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