Syntheses, luminescence switching, and electrochemical studies of photochromic dithienyl-1,10-phenanthroline zinc(II) bis(thiolate) complexes

Article abstract of DOI:10.1021/ic061359c

A series of zinc(II) diimine bis(thiolate) complexes with photochromic diarylethene-containing phenanthroline ligands was synthesized, and their photophysical and photochromic properties were studied. The X-ray crystal structures of two of these complexes have been characterized. All complexes exhibit strong ³LLCT phosphorescence at 510-620 nm in the solid state at 77 and 298 K and in EtOH-MeOH glass at 77 K. Detailed studies revealed that the absorption, emission, and electrochemical properties of the complexes could be readily switched via the photochromic ring-closing and ring-opening reactions.

Full text of DOI:10.1021/ic061359c

Inorg. Chem. 2007, 46, 1144−1152
Syntheses, Luminescence Switching, and Electrochemical Studies of
Photochromic Dithienyl-1,10-phenanthroline Zinc(II) Bis(thiolate)
Complexes
Tung-Wan Ngan, Chi-Chiu Ko,† Nianyong Zhu, and Vivian Wing-Wah Yam*
Department of Chemistry and HKU-CAS Joint Laboratory on New Materials, The UniVersity of
Hong Kong, Pokfulam Road, Hong Kong, P. R. China
Received July 21, 2006
A series of zinc(II) diimine bis(thiolate) complexes with photochromic diarylethene-containing phenanthroline ligands
was synthesized, and their photophysical and photochromic properties were studied. The X-ray crystal structures
of two of these complexes have been characterized. All complexes exhibit strong 3LLCT phosphorescence at
510−620 nm in the solid state at 77 and 298 K and in EtOH−MeOH glass at 77 K. Detailed studies revealed that
the absorption, emission, and electrochemical properties of the complexes could be readily switched via the
photochromic ring-closing and ring-opening reactions.
Introduction
metal complex systems. The amalgamation of the photo-
chromic moieties, such as azo, stilbene,4 diarylethene,5
Photochromism refers to the reversible transformation of
a chemical species by absorption of electromagnetic radiation
between two forms having distinguishable absorption spec-
tra.1 Many families of photochromic compounds have been
discovered, as they are promising candidates for the manu-
facture of advanced materials.2 In addition to these applica-
tions, they are also ideal candidates for use in photofunctional
molecular switches, such as photoswitchable liquid crystals,
molecular receptors, luminescence materials, and biomate-
rials.3 One strategy employed in the design of photoswitches
is to incorporate these photochromic units as a part of the
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* To whom correspondence should be addressed. E-mail: wwyam@
hku.hk.
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† Current address: Department of Biology and Chemistry, City University
of Hong Kong, Tat Chee Ave, Kowloon, Hong Kong, P. R. China.
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1144 Inorganic Chemistry, Vol. 46, No. 4, 2007
10.1021/ic061359c CCC: $37.00
© 2007 American Chemical Society
Published on Web 01/26/2007

Photochromism of Dithienylphenanthroline Zinc(II) Complexes
sized according to a procedure previously reported by us.5p 5,6-
Bis(2-methyl-3-thienyl)-1,10-phenanthroline (L2) was synthesized
according to a procedure similar to that of L1 except 2-methylthien-
3-yl boronic acid was used in place of 2,5-dimethylthien-3-yl
boronic acid.5p Acetonitrile and benzene for physical measurements
were distilled over calcium hydride and sodium, respectively, before
use. Tetra-n-butylammonium hexafluorophosphate (nBu4NPF6) (Al-
drich, 98%) was purified by recrystallization from hot ethanol for
3 times and vacuum-dried for several days before use. All other
reagents were of analytical grade and were used as received.
spiropyran, and spirooxazine,6 with the metal complexes has
also been shown to afford materials that possess novel and
perturbed photochromic properties. We recently reported the
synthesis of the first diarylethene-containing phenanthroline
ligand and the photosensitization of its photochromism
through the incorporation into the rhenium(I) tricarbonyl
diimine complex system.5h,p Extension of the work to other
metal complex systems may represent an interesting approach
to tune the photochromic properties. We believe that through
the coordination to different metal complex systems, the
photochromic behavior of the ligands could be utilized as a
photoswitch for the tuning of the photophysical properties
of the metal complex systems; conversely, the excited-state
properties of the metal complex systems could be used to
fine-tune or control the photochromic behavior of the ligands.
In view of the well-known ligand-to-ligand charge transfer
(LLCT) excited-state properties and the associated intense
luminescence in the zinc(II) diimine bis(thiolate) system,7
the incorporation of photochromic phenanthroline ligands
into this metal complex system has been pursued. Herein,
we report the syntheses, crystal structures, photophysical,
and photochromic properties of a series of zinc(II) bis-
(thiolate) complexes with the photochromic phenanthroline
ligands.
Zn(L1)(SC6H4-CH3-p)2 (1). This was prepared by modification
of a literature method for the synthesis of the related Zn(II) diimine
bis(thiolate) complexes.7 To a stirred solution of zinc acetate
dihydrate (29.6 mg, 0.135 mmol) and p-thiocresol (33.5 mg, 0.270
mmol) in methanol (4 mL) was added L1 (54.3 mg, 0.135 mmol)
in methanol (20 mL) in a dropwise manner. The reaction mixture
was stirred overnight at room temperature to give a yellow solution.
The yellow solution was reduced in volume under reduced pressure,
during which time the crude product was precipitated out. This was
filtered and washed successively with cold methanol and hexane.
Subsequent recrystallization by vapor diffusion of n-hexane into a
concentrated chloroform solution of the complex gave 1, isolated
1
as yellow microcrystals. Yield: 56.1 mg, 0.079 mmol, 58%. H
NMR (400 MHz, CDCl3, 298 K): δ 8.92 (dd, 2H, J ) 4.7, 1.4 Hz,
phenanthrolinyl proton at 2,9-position), 8.20 (dd, 2H, J ) 8.4, 1.4
Hz, phenanthrolinyl proton at 4,7-position), 7.76 (dd, 2H, J ) 8.4,
4.7 Hz, phenanthrolinyl proton at 3,8-position), 7.12 (d, 4H, J )
7.2 Hz, phenyl H ortho to S), 6.60 (d, 4H, J ) 7.2 Hz, phenyl H
meta to S), 6.32 (s, 1H, thienyl proton at 4-position), 6.30 (s, 1H,
thienyl proton at 4-position), 2.42 (s, 3H, 5-Me on thiophene ring),
2.34 (s, 3H, 5-Me on thiophene ring), 2.09 (s, 6H, Me on phenyl
ring), 1.98 (s, 3H, 2-Me on thiophene ring), 1.94 (s, 3H, 2-Me on
thiophene ring). Positive-ion FAB (fast atom bombardment) mass
spectrum: m/z 587 [M - SC6H4-CH3-p]+. Anal. Calcd for
C38H34N2S4Zn‚1/2H2O: C 63.27, H 4.89, N 3.88. Found: 63.39, H
4.80, N 3.76.
Experimental Section
Materials. 1,10-Phenanthroline, fuming sulfuric acid, and n-
butyllithium (1.6 M) in hexane were purchased from Aldrich
Chemical Co. and were used as received. 2,5-Dimethylthiophene,
2-methylthiophene, tri-n-butylborate, bromine, glacial acetic acid,
zinc dust, p-thiocresol, thiophenol, and p-chlorothiophenol were
obtained from Lancaster Synthesis Limited and were used as
received. Zinc acetate dihydrate was purchased from Merck
Chemicals and recrystallized from water prior to use. 3-Bromo-2-
methylthiophene was prepared according to a literature procedure.8
Tetrakis(triphenylphosphine)palladium(0) as a catalyst for Suzuki
coupling was synthesized according to a literature procedure.9
Tetrahydrofuran was distilled over sodium benzophenone ketyl
before use. 2-Methylthien-3-yl boronic acid was synthesized
according to modification of a previously reported procedure5p for
2,5-dimethylthien-3-yl boronic acid except 3-bromo-2-methylth-
iophene was used in place of 3-bromo-2,5-dimethylthiophene. 5,6-
Bis(2,5-dimethyl-3-thienyl)-1,10-phenanthroline (L1) was synthe-
Zn(L1)(SC6H5)2 (2). This was synthesized according to a
procedure similar to that of 1 except thiophenol (29.7 mg, 0.270
mmol) was used in place of p-thiocresol. Yield: 58.0 mg, 0.085
1
mmol, 63%. H NMR (300 MHz, CD3CN, 298 K): δ 9.01 (dd,
2H, J ) 4.7, 1.4 Hz, phenanthrolinyl proton at 2,9-position), 8.13
(dd, 2H, J ) 8.4, 1.4 Hz, phenanthrolinyl proton at 4,7-position),
7.90 (dd, 2H, J ) 8.4, 4.7 Hz, phenanthrolinyl proton at 3,8-
position), 6.94 (dd, 4H, J ) 7.5, 1.4 Hz, phenyl H ortho to S),
6.60 (m, 6H, phenyl H meta and para to S), 6.46 (s, 1H, thienyl
proton at 4-position), 6.44 (s, 1H, thienyl proton at 4-position), 2.39
(s, 3H, 5-Me), 2.37 (s, 3H, 5-Me), 1.98 (s, 3H, 2-Me), 1.95 (s, 3H,
2-Me). Positive-ion FAB mass spectrum: m/z 573 [M - SC6H5]+.
Anal. Calcd for C36H30N2S4Zn‚1/2H2O: C 62.37, H 4.51, N 4.04.
Found: C 62.31, H 4.51, N 3.91.
(6) (a) Ko, C. C.; Wu, L. X.; Wong, K. M. C.; Zhu, N. Y.; Yam, V.
W.-W. Chem.sEur. J. 2004, 10, 766-776. (b) Querol, M.; Bozic,
B.; Salluce, N.; Belser, P. Polyhedron 2003, 22, 655-664. (c) Bahr,
J. L.; Kodis, G.; Garza, L.; Lin, S.; Moore, A. L.; Moore, T. A.; Gust,
D. J. Am. Chem. Soc. 2001, 123, 7124-7133. (d) Yam, V. W.-W.;
Ko, C. C.; Wu, L. X.; Wong, K. M. C.; Cheung, K. K. Organometallics
2000, 19, 1820-1822. (e) Khairutdinov, R. F.; Giertz, K.; Hurst, J.
K.; Voloshina, E. N.; Voloshin, N. A.; Minkin, V. I. J. Am. Chem.
Soc. 1998, 120, 12707-12713.
Zn(L1)(SC6H4-Cl-p)2 (3). This was synthesized according to
a procedure similar to that of 1 except p-chlorothiophenol (39.0
mg, 0.270 mmol) was used in place of p-thiocresol. Yield: 57.1
1
mg, 0.076 mmol, 56%. H NMR (400 MHz, CDCl3, 298 K): δ
(7) (a) Highland, R. G.; Crosby, G. A. Chem. Phys. Lett. 1985, 119, 454-
458. (b) Truesdell, K. A.; Crosby, G. A. J. Am. Chem. Soc. 1985,
107, 1787-1788. (c) Jordan, K. J.; Wacholtz, W. F.; Crosby, G. A.
Inorg. Chem. 1991, 30, 4588-4593. (d) Highland, R. G.; Brummer,
J. G.; Crosby, G. A. J. Phys. Chem. 1986, 90, 1593-1598. (e) Burt,
J. A.; Crosby, G. A. Chem. Phys. Lett. 1994, 220, 493-496. (f) Yam,
V. W.-W.; Pui, Y. L.; Cheung, K. K.; Zhu, N. New J. Chem. 2002,
26, 536-542.
8.97 (dd, 2H, J ) 4.7, 1.4 Hz, phenanthrolinyl proton at 2,9-
position), 8.24 (dd, 2H, J ) 8.4, 1.4 Hz, phenanthrolinyl proton at
4,7-position), 7.83 (dd, 2H, J ) 8.4, 4.7 Hz, phenanthrolinyl proton
at 3,8-position), 7.09 (d, 4H, J ) 8.2 Hz, phenyl H ortho to S),
6.69 (d, 4H, J ) 8.2 Hz, phenyl H meta to S), 6.38 (s, 1H, thienyl
proton at 4-position), 6.36 (s, 1H, thienyl proton at 4-position), 2.42
(s, 3H, 5-Me), 2.41 (s, 3H, 5-Me), 2.01 (s, 3H, 2-Me), 1.96 (s, 3H,
2-Me). Positive-ion FAB mass spectrum: m/z 609 [M - SC6H4-
(8) Hallberg, A.; Liljefors, S.; Pedaja, P. Synth. Commun. 1981, 11 (1),
25-28.
(9) Coulson, D. R. Inorg. Synth. 1990, 28, 107-108.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1145

Ngan et al.
Cl-p]+. Anal. Calcd for C36H28Cl2N2S4Zn‚1/2H2O: C 56.73 H 3.84,
N 3.68. Found: C 56.86, H 3.74, N 3.55.
eter. For solution emission and excitation spectra, samples were
degassed on a high-vacuum line in a degassing cell with a 10 cm3
Pyrex round-bottom 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. Solutions were rigor-
ously 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 EtOH-MeOH (4:1, v/v)
glass at 77 K were recorded similarly, with liquid nitrogen inside
the optical Dewar flask. Closed forms in 77 K glasses were prepared
by irradiation at 365 nm at 298 K to reach the photostationary state,
at which time the amount of the closed form, monitored by the
UV-vis absorption spectroscopy, remained unchanged on continu-
ous irradiation before immersion of the samples into the liquid
nitrogen bath for emission measurements. Emission lifetimes of
the solution samples were determined by the time-correlated single-
photon counting (TCSPC) technique using an IBH Fluorocube
system with a NanoLED-11 excitation source, which has its
excitation peak wavelength at 371 nm and a pulse width shorter
than 200 ps.
Chemical actinometry was employed for the photochemical
quantum yield determination.10 Incident light intensities were taken
from the average values measured just before and after each
photolysis experiment using ferrioxalate actinometry.10 In the
determination of the photochemical quantum yield, the sample
solutions were prepared at concentrations with absorbance slightly
greater than 2.0 at the excitation wavelength. The quantum yield
was determined at a small percentage of conversion by monitoring
the initial rate of change of absorbance (∆A/∆t) 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 measurements 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 piece of
platinum gauze as a counter electrode in a compartment separated
from the working electrode by a sintered-glass frit. The ferrocenium/
ferrocene couple (FeCp2+/0) was used as the internal reference.11
All solutions for electrochemical studies were deaerated with
prepurified argon gas before measurement. The UV-vis spectral
changes for the photocyclization process of 3 were monitored using
an Ocean Optics USB2000 with DT-1000CE deuterium-tungsten
halogen light source and transmission T400-RT dip probe during
the electrochemical measurements.
Zn(L2)(SC6H4-CH3-p)2 (4). This was synthesized according
to a procedure similar to that of 1 except L2 (50.5 mg, 0.135 mmol)
1
was used in place of L1. Yield: 0.054 g, 0.079 mmol, 59%. H
NMR (400 MHz, CDCl3, 298 K): δ 8.95 (dd, 2H, J ) 4.7, 1.4 Hz,
phenanthrolinyl proton at 2,9-position), 8.12 (dd, 2H, J ) 8.5, 1.4
Hz, phenanthrolinyl proton at 4,7-position), 7.78 (dd, 2H, J ) 8.5,
4.7 Hz, phenanthrolinyl proton at 3,8-position), 7.12 (m, 6H, phenyl
H ortho to S and thienyl proton at 5-position), 6.69 (d, 2H, J ) 5.2
Hz, thienyl proton at 4-position), 6.60 (d, 4H, J ) 7.2 Hz, phenyl
H meta to S), 2.17 (s, 3H, 2-Me on thiophene ring), 2.11 (s, 3H,
2-Me on thiophene ring), 2.03 (s, 6H, Me on phenyl ring). Positive-
ion FAB mass spectrum: m/z 559 [M - SC6H4-CH3-p]+. Anal.
Calcd for C36H30N2S4Zn‚1/2H2O: C 61.79, H 4.47, N 3.95. Found:
C 61.93, H 4.40, N 4.06.
Zn(L2)(SC6H5)2 (5). This was synthesized according to a
procedure similar to that of 1 except L2 (50.5 mg, 0.135 mmol)
and thiophenol (29.7 mg, 0.270 mmol) were used in place of L1
and p-thiocresol, respectively. Yield: 0.056 g, 0.085 mmol, 63%.
1H NMR (400 MHz, CDCl3, 298 K): δ 8.94 (dd, 2H, J ) 4.7, 1.4
Hz, phenanthrolinyl proton at 2,9-position), 8.18 (dd, 2H, J ) 8.5,
1.4 Hz, phenanthrolinyl proton at 4,7-position), 7.84 (dd, 2H, J )
8.5, 4.7 Hz, phenanthrolinyl proton at 3,8-position), 7.21 (dd, 4H,
J ) 7.5, 1.4 Hz, phenyl H ortho to S), 7.12 (d, 2H, J ) 5.2 Hz,
thienyl proton at 5-position), 6.76 (m, 6H, phenyl H meta and para
to S), 6.68 (d, 2H, J ) 5.2 Hz, thienyl proton at 4-position), 2.09
(s, 3H, 2-Me), 2.06 (s, 3H, 2-Me). Positive-ion FAB mass
spectrum: m/z 545 [M - SC6H5]+. Anal. Calcd for C34H26N2S4-
Zn‚1.5H2O: C 61.13 H 4.13, N 3.96. Found: 61.37, H 3.98,
N 4.14.
Zn(L2)(SC6H4-Cl-p)2 (6). This was synthesized according to
a procedure similar to that of 1 except L2 (50.5 mg, 0.135 mmol)
and p-chlorothiophenol (39.0 mg, 0.270 mmol) were used in place
of L1 and p-thiocresol, respectively. Yield: 0.054 g, 0.075 mmol,
1
55%. H NMR (300 MHz, CDCl3, 298 K): δ 9.00 (dd, 2H, J )
4.7, 1.4 Hz, phenanthrolinyl proton at 2,9-position), 8.18 (dd, 2H,
J ) 8.5, 1.4 Hz, phenanthrolinyl proton at 4,7-position), 7.84 (dd,
2H, J ) 8.5, 4.7 Hz, phenanthrolinyl proton at 3,8-position), 7.14
(d, 2H, J ) 5.2 Hz, thienyl proton at 5-position), 7.07 (d, 4H, J )
8.2 Hz, phenyl H ortho to S), 6.75 (d, 2H, J ) 5.2 Hz, thienyl
proton at 4-position), 6.68 (d, 4H, J ) 8.2 Hz, phenyl H meta to
S), 2.12 (s, 3H, 2-Me), 2.04 (s, 3H, 2-Me). Positive-ion FAB mass
spectrum: m/z 581 [M - SC6H4-Cl-p]+. Anal. Calcd for
C34H24Cl2N2S4Zn‚H2O: C 54.95, H 3.53, N 3.77. Found: C 54.92,
H 3.37, N 3.70.
Physical Measurements and Instrumentation. 1H NMR spectra
were recorded either on a Bruker DPX-300 (300 MHz) or a Bruker
AV400 (400 MHz) at 298 K. Chemical shifts (δ, ppm) were
recorded relative to tetramethylsilane (Me4Si). Positive-ion fast atom
bombardment (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
Academy of Sciences, Beijing.
Crystal Structure Determination. All the experimental details
are given in Table 1. Single crystals of 4 suitable for X-ray studies
were obtained by vapor diffusion of n-hexane into a concentrated
dichloromethane solution of 4. A yellow crystal of dimensions 0.7
mm × 0.2 mm × 0.1 mm mounted in a glass capillary was used
UV-vis absorption spectra were recorded on a Hewlett-Packard
8452A diode array spectrophotometer. Photoirradiation was carried
out using a 300 W 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.
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Braslavsky, S. E.; Schmidt, R. Pure Appl. Chem. 2004, 76 (12), 2105-
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Steady-state emission and excitation spectra at room temperature
and 77 K were recorded on a Spex Fluorolog 111 spectrofluorom-
1146 Inorganic Chemistry, Vol. 46, No. 4, 2007

Photochromism of Dithienylphenanthroline Zinc(II) Complexes
Scheme 1
Table 1. Crystal and Structure Determination Data for Complexes 4
and 5
4
5
formula
fw
C37H32Cl2N2S4Zn
769.16
C34H26N2S4Zn
656.18
T, K
253
301
a, Å
b, Å
c, Å
15.205(3)
8.952(2)
26.552(5)
90
91.49(3)
90
3612.9(12)
pale yellow
monoclinic
P21/c
25.168(5)
14.457(3)
17.048(3)
90
91.28(3)
90
6201(2)
pale yellow
monoclinic
P21/c
R, deg
â, deg
γ, deg
V, Å3
cryst color
cryst syst
space group
Z
4
8
F(000)
1 584
1.414
2 704
1.406
calculation of final R-indices. The final difference Fourier map
Dc, g cm-3
cryst dimens, mm
λ, Å (graphite,
monochromated, Mo KR)
collection range, deg
h
showed maximum rest peaks and holes of 0.435 and -0.236 e Å-3
,
0.7 × 0.2 × 0.1
0.6 × 0.3 × 0.2
respectively.
0.71073
0.71073
Single crystals of 5 suitable for X-ray studies were obtained by
vapor diffusion of n-hexane into a concentrated dichloromethane
solution of 5. A yellow crystal of dimensions 0.6 mm × 0.3 mm
× 0.2 mm mounted in a glass capillary was used for data collection
at 28 °C on a MAR diffractometer with a 300 mm image plate
detector using graphite monochromatized Mo KR radiation (λ )
0.71073 Å). Data collection was made with 2° oscillation step of
æ, 600 s exposure time, and scanner distance at 120 mm. The
images were collected and interpreted, and intensities were
integrated using the program DENZO.12 The structure was solved
by direct methods employing the SHELXS-97 program13 on a PC.
The Zn, S, and most non-hydrogen atoms were located according
to direct methods. The positions of the other non-hydrogen atoms
were found after successful refinement by the full-matrix least-
squares method using the program SHELXL-9713 on a PC. One
crystallographic asymmetric unit consists of two formula units. In
the final stage of least-squares refinement, all non-hydrogen atoms
were anisotropically refined. Hydrogen atoms were generated by
the program SHELXL-97.13 The positions of hydrogen atoms were
calculated on the basis of the riding mode with thermal parameters
equal to 1.2 times those of the associated carbon atoms and
participated in the calculation of final R-indices. The final difference
Fourier map showed maximum rest peaks and holes of 0.334 and
-0.293 e Å-3, respectively.
2θmax ) 50.34
-15 to 15
-10 to 9
-31 to 31
2
100
120
600
13 484
4 319
2θmax ) 50.68
-29 to 29
-17 to 16
-18 to 17
2
100
120
600
29 036
8 288
k
l
oscillation, deg
no. of images collected
distance, mm
exposure time, s
no. of data collected
no. of unique data
no. of data used in
refinement, m
no. of parameters
refined, p
2 489
4 646
419
743
Ra
0.0376
0.0765
0.847
0.001
0.0422
0.0956
0.883
0.001
wRa
GOF, S
maximum shift, (∆/σ)max
residual extrema in final
difference map, e Å-3
0.435, -0.236
0.334, -0.293
2
2
2
a w ) 1/[σ2(F0 ) + (aP)2 + bP], where P ) [2Fc + max(F0 , 0)]/3
for data collection at -20 °C on a MAR diffractometer with a 300
mm image plate detector using graphite monochromatized Mo KR
radiation (λ ) 0.71073 Å). Data collection was made with 2°
oscillation step of æ, 600 s exposure time, and scanner distance at
120 mm. The images were collected and interpreted, and intensities
were integrated using the program DENZO.12 The structure was
solved by direct methods employing the SHELXS-97 program13
on a PC. The Zn, S, and most non-hydrogen atoms were located
according to the direct methods. The positions of the other non-
hydrogen atoms were found after successful refinement by the full-
matrix least-squares method using the program SHELXL-9713 on
a PC. One dichloromethane solvent molecule was also located. One
crystallographic asymmetric unit consists of one formula unit,
including one dichloromethane molecule. In the final stage of least-
squares refinement, all non-hydrogen atoms were anisotropically
refined. Hydrogen atoms were generated by the program SHELXL-
97.13 The positions of hydrogen atoms were calculated on the basis
of the riding mode with thermal parameters equal to 1.2 times
those of the associated carbon atoms and participated in the
Results and Discussion
Phenanthroline-based photochromic ligands L1 and L2
were synthesized by Suzuki cross-coupling reactions14 of 5,6-
dibromo-1,10-phenanthroline and the substituted 3-thienyl
boronic acid using a previously reported procedure.5p Through
the functionalization of thienyl boronic acid, functionalized
photochromic dithienyl-1,10-phenanthroline ligand could be
readily synthesized. Reaction of these ligands with Zn(OAc)2‚
2H2O in the presence of various substituted thiophenols in
methanol in a mole ratio of 1:1:2 afforded a series of
photochromic zinc(II) bis(thiolate) dithienylphenanthroline
complexes (Scheme 1). All of them gave satisfactory
1
elemental analyses and were characterized by H NMR
spectroscopy and positive-ion FAB mass spectrometry. Due
(12) Gewirth, D; DENZO, version 1.3.0; “The HKL Manual-A description
of programs DENZO, XDISPLAYF, and SCALEPACK”, with the
cooperation of the program authors Otwinowski, Z. and Minor, W.;
Yale University, New Haven, CT, 1995.
(13) Sheldrick, G. M.; SHELX-97: Programs for Crystal Structure Analysis,
Release 97-2; University of Gottingen: Gottingen, Germany, 1997.
(14) (a) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457-2483. (b)
Suzuki, A. J. Organomet. Chem. 1999, 576, 147-168. (c) Suzuki, A.
In Metal-Catalyzed Cross-Coupling Reactions; Diederich, F., Stang,
P. J., Eds.; Wiley-VCH: New York, 1998; Chapter 2. (d) Stanforth,
S. P. Tetrahedron 1998, 54, 263-303.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1147

Ngan et al.
Table 2. Selected Bond Distances (Å) and Angles (deg) with
Estimated Standard Deviations (esds) in Parentheses for 4 and Both
Conformations of 5
4
Zn(1)-N(2)
Zn(1)-S(1)
S(1)-C(23)
2.082(3)
2.272(13)
1.772(4)
Zn(1)-N(1)
Zn(1)-S(2)
S(2)-C(30)
2.086(3)
2.246(14)
1.760(4)
S(1)-Zn(1)-S(2)
121.42(5)
C(23)-S(1)-Zn(1) 98.97(13)
N(1)-Zn(1)-N(2) 79.18(13)
C(30)-S(2)-Zn(1) 102.78(15)
5a Zn(1)-N(2)
Zn(1)-S(1)
2.096(3)
2.248(13)
1.751(4)
Zn(1)-N(1)
Zn(1)-S(2)
S(2)-C(29)
2.010(3)
2.266(13)
1.751(4)
S(1)-C(23)
Figure 1. Perspective drawing of 4 with the atomic numbering scheme.
Hydrogen atoms have been omitted for clarity; thermal ellipsoids are drawn
at the 30% probability level.
S(1)-Zn(1)-S(2)
126.69(5)
N(1)-Zn(1)-N(2) 78.49(12)
C(23)-S(1)-Zn(1) 107.85(17) C(29)-S(2)-Zn(1) 97.29(13)
5b Zn(2)-N(3)
Zn(2)-S(5)
2.105(3)
2.262(16)
1.757(6)
Zn(2)-N(4)
Zn(2)-S(6)
S(6)-C(63)
2.105(3)
2.238(13)
1.770(5)
S(5)-C(57)
S(5)-Zn(2)-S(6)
124.17(5)
C(57)-S(5)-Zn(2) 96.98(17)
N(3)-Zn(1)-N(4) 78.42(13)
C(63)-S(6)-Zn(2) 105.78(19)
average N-Zn-N angle of 78.8°. The deviation of both the
N-Zn-N and S-Zn-S bond angles from the ideal angle
of 109.5° for tetrahedral geometry is ascribed to the steric
requirement of the chelating diimine ligands. The average
bond distances of Zn-S and Zn-N were 2.26 Å for 4 and
2.25 Å for 5 and 2.08 Å for 4 and 2.10 Å for 5, respectively.
These observations are typical of the related Zn(II) diimine
bis(thiolate) systems.7,15 The distances between the phenan-
throline and the phenyl ring of one of the thiolate ligands
were in the range of 3.4-4.2 Å for both complexes,
indicative of the possible presence of π-π interactions
between the phenanthroline and the phenyl rings. Such
intramolecular π-stacking of aromatic rings has also been
observed in other related systems.7c,15d For the L2 moiety,
both crystal structures adopted a parallel conformation with
the two methyl groups pointing in the same direction. Due
to the steric hindrance exhibited by the two thiophene
moieties, they tend to orient themselves perpendicular to the
plane of the phenanthroline moiety. The interplanar angles
between the thiophene rings and 1,10-phenanthroline in the
crystal structures were determined to be 84.5° and 63.0° for
4 and 83.0° and 70.3° and 69.7 and 63.9° for the two
conformations of 5, all of which are essentially orthogonal
and typical for the dithienylphenanthroline unit.5h As a result,
the π-conjugation between the thiophene rings and the
phenanthroline moiety can be assumed to be almost negli-
gible in the open form.
Figure 2. Perspective drawings of the two conformations of 5 with their
atomic numbering. Hydrogen atoms have been omitted for clarity; thermal
ellipsoids are drawn at the 30% probability level.
to the sterically demanding nature of the 1,10-phenanthroline
ring, with the protons at the 4- and 7-positions hindering
the rotation of the thiophene moieties, two sets of 1H NMR
signals, corresponding to the resonances of the parallel
(photochromic inactive) and the antiparallel (photochromic
active) conformers, were observed. Similar observations have
also been reported in the related systems.3d,5p Complexes 4
and 5 were also structurally characterized by X-ray crystal-
lography.
X-ray Crystal Structures. Figures 1 and 2 depict the
perspective drawings of the complexes 4 and 5 with their
atomic numbering. The crystal and structure determination
data are collected in Table 1 and selected bond distances
and angles are summarized in Table 2. For complex 5, two
conformations with the thiolate ligands in different orienta-
tions were also determined. Except for the orientations in
the thiolate ligands, both conformations show very similar
bond lengths, bond angles, and geometries for the metal
center and the dithienylphenanthroline ligand. The Zn(II)
center in both crystal structures adopted a distorted tetrahedral
geometry with an average S-Zn-S angle of 124.0° and an
Electronic Absorption and Emission Properties. All the
complexes dissolve in benzene to give clear yellow solutions.
Their electronic absorption spectra showed an intense
absorption at ca. 320 nm, which is ascribed to the intraligand
π f π* transitions of the phenanthroline, probably with some
mixing of π f π* and n f π* transitions of the thiophene
rings and the benzenethiolate ligands, as ligands L1, L2, and
the benzenethiolate ligands were found to absorb in a similar
(15) (a) Watson, A. D.; Rao, C. P.; Dorfman, J. R.; Holm, R. H. Inorg.
Chem. 1985, 24, 2820-2826. (b) Abrahams, I. L.; Garner, C. D. J.
Chem. Soc., Dalton Trans. 1987, 1577-1580. (c) Halvorsen, K.;
Crosby, G. A.; Wacholtz, W. F. Inorg. Chim. Acta 1995, 228, 81-
88. (d) Anjali, K. S.; Sampanthar, J. T.; Vittal, J. J. Inorg. Chim. Acta
1999, 295, 9-17.
1148 Inorganic Chemistry, Vol. 46, No. 4, 2007

Photochromism of Dithienylphenanthroline Zinc(II) Complexes
Scheme 2
Table 3. Photophysical Data for the Open and Closed Forms of the
Ligands and Their Zinc(II) Bis(thiolate) Complexes
medium
(T/K)
emission λem
nm (τo/ns)
/
absorptiona λabs
/
compound
nm (ꢀ/dm3 mol-1 cm-1
)
L1 (open form)b
benzene (298)
solid (298)
solid (77)
383
--d
304 (8670)
--d
glass (77)c
--d
577
L1 (closed form)b glass (77)c
366 (31730), 510 (3950),
540 (3730)
L2 (open form)
benzene (298)
391
--d
298 (9730)
solid (298)
solid (77)
glass (77)c
--d
at ca. 520-620 nm upon excitation at λ g 310 nm. The
energies of these emissions are found to be sensitive to the
nature of the thiolate ligand. With the same diimine ligand,
the emission energies are in the order of 3 > 2 > 1 and 6 >
5 > 4, which is in the reverse order as the electron-donating
ability of their thiolate ligands, p-Cl-C6H4S- < C6H5S- <
p-Me-C6H4S-. With reference to previous spectroscopic
works on the related monomeric [Zn(phen)(SR)2] system7
and the observed emission energy dependence on the thiolate
ligands, these broad low-energy emission bands are ascribed
to be derived from the LLCT [pπ(SR-) f π*(phen)] origin.
In general, the presence of electron-donating substituents on
the thiolate ligands would raise the π-orbital energies of the
ligands, leading to a lower LLCT [pπ(SR-) f π*(phen)]
energy, whereas with electron-withdrawing substituents on
the thiolate ligands, the converse is true. The 77 K EtOH-
MeOH glass emissions showed well-resolved vibronic
structures, which are suggestive of the emission origin
--d
584
L2 (closed form) glass (77)c
368 (21830), 510 (3610),
540 (3400)
1 (open form)
benzene (298)
390 (3.4)
610
619
513
604
320 (6540), 382 (1470)
solid (298)
solid (77)
glass (77)c
glass (77)c
1 (closed form)
2 (open form)
382 (31780), 534 (4700),
566 (4270)
320 (6480), 380 (1720)
benzene (298)
solid (298)
solid (77)
389 (2.7)
567
551
513
607
glass (77)c
glass (77)c
2 (closed form)
3 (open form)
382 (31800), 538 (4600),
566 (4070)
328 (6070), 382 (1680)
benzene (298)
solid (298)
solid (77)
398 (2.2)
538
524
513
605
glass (77)c
glass (77)c
3 (closed form)
4 (open form)
386 (30680), 540 (4540),
572 (3960)
322 (7000), 382 (1300)
benzene (298)
solid (298)
solid (77)
383 (5.6)
618
624
glass (77)c
glass (77)c
benzene (298)
solid (298)
solid (77)
508
3
3
derived from admixtures of LLCT and IL states, similar
to those observed in related systems.7b,g
4 (closed form)
5 (open form)
--e
380, 512, 558e
320 (7500), 380 (1620)
383 (5.5)
580
584
Photochromic Properties. On excitation at λ ) 313 nm,
complexes 1-3 undergo photocyclization reactions (Scheme
2), leading to the formation of an intense absorption band at
ca. 382 nm with molar extinction coefficients on the order
of 104 dm3 mol-1 cm-1 and two moderately intense absorp-
tions at ca. 534 and 566 nm with molar extinction coefficients
on the order of 103 dm3 mol-1 cm-1. The significant
bathochromic shift in absorption maxima of the closed forms
compared with their open forms is mainly due to the increase
in the extended π-conjugation across the condensed thiophene
rings, resulting in a reduced HOMO-LUMO energy gap for
π f π* and n f π* transitions. The close resemblance of
these new absorption bands in different complexes to the
closed form absorptions of the free ligands is suggestive of
an assignment as metal-perturbed π f π* and n f π*
intraligand transitions of the condensed thiophene rings. The
slight red shift of the absorption bands of the closed form in
the complexes (382, 534, and 566 nm) relative to those in
the closed form of free ligands (366, 510, and 540 nm) is
attributed to perturbation of the transition by the metal center
in the metal complexes. On the other hand, excitation into
the absorption bands of the closed form results in photocy-
cloreversion as reflected by the decrease in the intensity of
the absorption bands of the closed form. A slight amount of
precipitate was observed after one cycle in aerated solution,
indicative of photodecomposition, whereas under anaerobic
conditions, these photoprocesses could be recycled a number
of times without significant decomposition (Figure 3).
Representative UV-vis absorption spectral changes of
glass (77)c
glass (77)c
benzene (298)
solid (298)
solid (77)
508
5 (closed form)
6 (open form)
--e
382, 514, 560e
322 (6400), 374 (1770)
392 (6.0)
543
558
glass (77)c
glass (77)c
507
6 (closed form)
--e
383, 510, 558e
a In benzene solution, at 298 K. b From ref 5p,h. c EtOH/MeOH ) 4:1
(v/v). d Emission not detected (indicated by --). e Due to low percentage of
conversion, the λem or ꢀ values could not be obtained with certainty
(indicated by --).
region.7 In addition to this intense absorption, all complexes
show a moderately intense absorption shoulder at ca. 380
nm, which is assigned as the ligand-to-ligand charge transfer
[LLCT, pπ(SR-) f π*(phen)] transition, typical of the zinc-
(II) diimine bis(thiolate) system.7,15 The photophysical data
for the ligands and complexes 1-6 are summarized in
Table 3.
On excitation at λ g 310 nm, complexes 1-6 in their open
forms in benzene solution displayed luminescence at ca. 390
nm. These emissions are tentatively assigned as fluorescence
and of metal-perturbed intraligand (IL) π f π* origin,
probably from the phenanthroline moiety, since the emission
bands are found to be very similar in energy, band shape,
and lifetime to those of the corresponding free ligands L1
and L2, which exhibit fluorescence at ca. 383 and 391 nm,
respectively, upon excitation at λ g 310 nm.5h,p
On the other hand, the open forms of complexes 1-6 in
the solid state at 298 and 77 K exhibited strong luminescence
Inorganic Chemistry, Vol. 46, No. 4, 2007 1149

Ngan et al.
Table 4. A Summary of the Photochemical Quantum Yields of
Complexes 1-6 in Benzene Solution at 298 K
photochemical quantum yield/φ
photocyclization,a photocycloreversion,
compound
φ313
φ510
conversionb/%
L1c
L2
1
0.33
0.49
0.012
0.029
0.013
0.011
0.008
0.016
0.011
0.011
83
92
41
47
43
<5
<5
<5
0.033
0.028
0.025
0.014
0.012
0.011
2
3
4d
5d
6d
a Values reported are corrected to the ratio of the photochromic active
conformation, i.e., with respect to the antiparallel configuration. b Conversion
percentage (corrected to the active conformation) at photostationary state
with 313 nm excitation. c From ref 5p,h. d Estimated using the average
extinction coefficients obtained for complexes 1-3.
Figure 3. UV-vis absorption spectral changes of 1 (3.4 × 10-4 M) upon
photoirradiation at 313 nm in benzene solution. The inset show absorbance
changes at 540 nm on alternating excitation at 313 and 540 nm over three
cycles.
complex 1 upon irradiation at 313 nm in benzene solution
are shown in Figure 3.
Unlike in the rhenium(I) tricarbonyl diimine complex
system, in which the photochromism could be sensitized by
the excitation into the metal-to-ligand charge transfer absorp-
tion band,5h,p excitation into the LLCT absorption band of
the complexes did not produce photocyclization. This is
probably attributed to the very inefficient internal conversion
3
3
process from the LLCT to the IL state due to the poor
coupling of these two states with an activation barrier, which
is typically observed in zinc(II) diimine bis(thiolate) complex
systems.7a,c-e The photocyclization conversions from the open
form to the closed form on 313-nm excitation corrected for
the antiparallel configuration for complexes 1-3 were found
to be ca. 40-50% at the photostationary state, whereas the
conversion percentage for complexes 4-6 at the photosta-
tionary state was too low to be precisely measured by NMR
spectroscopy (<5%). Thus the extinction coefficients for the
corresponding closed form absorptions could not be deter-
mined with certainty, even though similar absorption spectral
changes were also observed upon UV excitation. The
exceptionally low photocyclization conversion for complexes
4-6 may be attributed to the low quantum efficiency for
the photocyclization (Table 4), with comparable quantum
yield for the photocycloreversion, resulting in a very low
ratio of the closed form to the open form at the photosta-
tionary state. In contrast to the free ligands and most organic
diarylethene systems,2d-f in which the quantum yield for
photocyclization is much higher than that for photocyclor-
eversion, complexes 1-6 showed very similar quantum
yields for both photocyclization and photocycloreversion.
The relatively low quantum yield for the photocyclization
in the complexes may be attributed to the presence of the
lower-lying 1LLCT excited state, which quenches the reactive
1IL excited-state of the ligand. Subsequent intersystem
Figure 4. Proposed qualitative energy state diagram of the zinc(II)
complexes.
reactivity of the zinc(II) complexes is shown in Figure 4.
The relative energy ordering of the various states can be
substantiated by the higher energy of the 1IL transition than
that of the 1LLCT transition observed in the UV-vis
spectrum and the lower energy of the 3IL emission of L1 (77
K MeCN5p) than that of the 3LLCT/3IL emission (508-513
nm in 77 K glass) (Table 3). The lower photocyclization
quantum yields in the chlorobenzenethiolate complexes of
3 and 6 than the unsubstituted benzenethiolate complexes
of 2 and 5 and the methyl substituted benzenethiolate
complexes of 1 and 4, with φ313 in the order of 3 < 2 < 1
and 6 < 5 < 4, though subtle, are consistent with the
proposed mechanism, since this is in line with the LLCT
energy trend, in which 3 > 2 > 1 and 6 > 5 > 4. The higher
1LLCT energies of 3 and 6 are expected to result in a better
1
spectral energy overlap between the LLCT acceptor state
and the 1IL donor state, giving rise to a better intramolecular
energy transfer efficiency, causing a more efficient quenching
1
of the photoreactive IL state.
On conversion of the open form to the closed form in the
photostationary state, the emissions of all the complexes in
77 K EtOH-MeOH glass were found to shift to the red,
consistent with the reduced HOMO-LUMO energy gap as
a result of the extended π-conjugation in the condensed
thiophene units upon occurrence of the photocyclization
reaction. In view of the close resemblance of these emissions
to the closed form emissions of the free ligands in the same
medium as well as the insensitivity of these emissions to
changes in the nature of the thiolate ligands, the emissions
3
crossing into the LLCT state did not efficiently populate
3
the IL state due to the poor coupling between these two
states. Thus the 1LLCT state serves as a quenching state that
1
competes with the IL state that gives rise to the photocy-
clization reaction. A proposed qualitative energy state
diagram to account for the relatively low photochromic
1150 Inorganic Chemistry, Vol. 46, No. 4, 2007

Photochromism of Dithienylphenanthroline Zinc(II) Complexes
Figure 5. Overlaid emission spectra of the open (s) and closed (---) form
of 1 in EtOH-MeOH (4:1 v/v) glass at 77 K.
Table 5. Electrochemical Data for the Ligands and Complexes in
Acetonitrile Solution (0.1 mol dm-3 nBu4NPF6) at 298 Ka
oxidation,b
pa /V vs SCE
reduction,b Epc /V vs
SCE (E1/2/V vs SCE)
compound
E
1 (open form)
1 (closed form)
2 (open form)
2 (closed form)
3 (open form)
3 (closed form)
4 (open form)
4 (closed form)
5 (open form)
5 (closed form)
6 (open form)
6 (closed form)
+1.80
+1.80
+1.79
+1.80
+1.80
+1.80
+1.76
+1.76
+1.72
+1.72
+1.74
+1.74
(-1.47), -2.08
-1.51, -1.78
(-1.48), -2.04
-1.51, -1.77
(-1.45), -2.06
-1.35, -1.72
(-1.48), -2.08
-1.53, -1.74
(-1.51), -2.04
-1.51, -1.72
(-1.45), -2.04
-1.34, -1.70
Figure 6. Cyclic voltammograms of (a) the oxidative scan of 3 and (b)
the repetitive reductive scans of 3 during the course of photochromic
conversion from open form to closed form in MeCN (0.1 mol dm-3 nBu4-
NPF6). Scan rate: 100 mV s-1. The inset shows the plot of current at
-1.35 V vs SCE against the change in absorbance at 540 nm.
a Working electrode, glassy carbon; scan rate, 100 mV s-1 b E1/2 is (Epa
.
+ Epc)/2; Epa and Epc are peak anodic and peak cathodic potentials,
which is typically observed in zinc(II) diimine bis(thiolate)
complexes, is attributed to the thiolate ligand-centered
oxidation. The reversible reduction couples at ca. -1.45 to
-1.51 V are assigned to the phenanthroline ligand-centered
reduction. The irreversible reduction waves at ca. -2.04 to
-2.08 V, which are similar to the thiophene-based irrevers-
ible reduction of the free ligand5p and absent in the related
zinc(II) diimine bis(thiolate) complexes,7d,16 are assigned to
the reduction of the thiophene moiety.
The electrochemical changes upon the conversion of the
open form to the closed form have also been studied using
cyclic voltammetric measurements (Figure 6) containing a
mixture of the open and closed forms in their photostationary
state. The lack of observable changes in the irreversible
oxidation wave at +1.72 to +1.80 V is in accord with the
assignment of a thiolate ligand-centered oxidation. The
insignificant perturbation of the π-accepting ability of the
phenanthroline moiety upon condensation of the thiophene
units could be reflected by a slight potential shift (e0.1 V)
in the reversible phenanthroline-based reduction couple. A
drop in the electrochemical signal of the irreversible reduc-
tion wave at ca. -2.04 V, with the concomitant formation
of a new irreversible reduction wave at less cathodic potential
(-1.72 to -1.78 V), was observed upon conversion of the
open form to the closed form. This supports the assignment
of a thiophene-based reduction, as photocyclization of the
thiophene rings in the open form to the condensed thiophene
units in the closed form would result in a better π-conjugation
and hence a better π-accepting ability, causing a shift of the
respectively.
are tentatively assigned as derived from states of a metal-
perturbed IL origin, probably originating from the condensed
thiophene moiety. However, a mixing of the 3LLCT emission
could not be completely excluded as similar red shift in
3
emission energy for the LLCT state and poor coupling of
3
3
the LLCT and IL are also anticipated in the closed form.
It is believed that the 3LLCT state is the predominant
emissive state in the open form, whereas on conversion to
the closed form, the extended π-conjugation renders the 3IL
excited state to an even lower lying energy level and becomes
the predominant emissive state in the closed form. The
representative overlaid emission spectra of the open and
closed forms of complex 1 in 77 K EtOH-MeOH glass are
revealed in Figure 5.
Electrochemical Studies. The electrochemical data for the
complexes in acetonitrile (0.1 mol dm-3 nBu4NPF6) are
tabulated in Table 5. The open form of all the complexes in
acetonitrile (0.1 mol dm-3 nBu4NPF6) displayed similar cyclic
voltammograms with an irreversible oxidation wave at ca.
+1.72 to +1.80 V vs SCE, a reversible reduction couple at
ca. -1.45 to -1.51 V vs SCE, and an irreversible reduction
wave at ca. -2.04 to -2.08 V vs SCE. On the basis of
previous electrochemical studies on other related zinc(II)
diimine bis(thiolate) systems,16 the first irreversible oxidation,
(16) Yam, V. W.-W.; Pui, Y. L.; Cheung, K. K. Inorg. Chem. 2000, 39,
5741-5746.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1151

Ngan et al.
reduction wave to a less negative potential. To further
confirm the assignment, a correlation of the growth of the
newly formed reduction wave with the amount of the closed
form generated in 3 upon irradiation was attempted. A linear
relationship between the current of the new reduction wave
and the absorbance of the closed form in the visible region
was established, giving further support to the assignment
(Figure 6). Although electrochemically induced cyclization
and cycloreversion have been recently17 reported for some
diarylethenes, no electrocyclization or electrocycloreversion
products were observed during the course of the electro-
chemical measurement.
complexes exhibit strong 3LLCT phosphorescence at 510-
620 nm in the solid state at 77 and 298 K. The switching of
absorption, emission, and electrochemical properties of the
complexes via photochromic reactions has been demon-
strated.
Acknowledgment. V.W.-W.Y. acknowledges financial
support from the University Development Fund, the Faculty
Development Fund, the URC Seed Funding for Strategic
Research Theme on Organic Optoelectronics, and URC Seed
funding for Basic Research of The University of Hong Kong.
T.-W.N. acknowledges the receipt of a postgraduate stu-
dentship, administered by The University of Hong Kong.
C.-C.K. acknowledges financial support from the Small
Project Funding of The University of Hong Kong and the
receipt of a University Postdoctoral Fellowship. The work
described in this paper has been supported by the Research
Grants Council of Hong Kong Special Administration
Region, China (Project No. HKU7050/04P).
Conclusion
A series of zinc(II) diimine bis(thiolate) complexes with
photochromic diarylethene-containing phenanthroline ligands
was synthesized and structurally characterized, and their
photophysical and photochromic properties were studied. All
(17) (a) Peters, A.; Branda, N. R. Chem. Commun. 2003, 954-955. (b)
Peters, A.; Branda, N. R. J. Am. Chem. Soc. 2003, 125, 3404-3405.
(c) Moriyama, Y.; Matsuda, K.; Tanifuji, N.; Irie, S.; Irie, M. Org.
Lett. 2005, 7, 3315-3318. (d) Yokojima, S.; Matsuda, K.; Irie, M.;
Murakami, A.; Kobayashi, T.; Nakamura, S. J. Phys. Chem. A 2006,
110, 8137-8143.
Supporting Information Available: Crystallographic data for
complexes 4 and 5. This material is available free of charge via
the Internet at http://pubs.acs.org.
IC061359C
1152 Inorganic Chemistry, Vol. 46, No. 4, 2007