Reversible photochromism of novel silver(I) coordination complexes with 1,2-bis(2′-methyl-5′-(2″-pyridyl)-3′-thienyl)perfluorocyclopentene in crystalline phase

Article abstract of DOI:10.1016/j.ica.2006.06.010

Three novel silver(I) complexes with 1,2-bis(2-methyl-5′-(2″-pyridyl)-3′-thienyl)perfluorocyclopentene (BM-2-PTP) were synthesized by the reaction of Ag(CF₃SO₃) or Ag(CF₃COO) with BM-2-PTP in benzene at different temperatures. The structures of these metal complexes were revealed by X-ray crystallographic analyses and the correlation between crystal structures and photochromic performance was discussed. In complexes 1 and 2, silver(I) is three-coordinated to two nitrogens from distinct ligand molecules as well as one oxygen from anions to form a 1-D polymeric structure. On the other hand, complex 3 contains two crystallographic independent Ag(I) with different coordination environments, and the adjacent BM-2-PTP molecules are connected by Ag-CF₃CO₂-Ag chains to afford a 1-D double chain structure. The difference in structures of three complexes shows the interesting anionic effect on coordination and the subtleness of crystal engineering. It is noted that complex 3 underwent reversible photochromic reaction in crystalline state despite the unfavorable framework to the rotation of thiophene groups.

Full text of DOI:10.1016/j.ica.2006.06.010

Inorganica Chimica Acta 359 (2006) 4281–4288
www.elsevier.com/locate/ica
Reversible photochromism of novel silver(I) coordination complexes
with 1,2-bis(20-methyl-50-(200-pyridyl)-30-thienyl)perfluorocyclopentene
in crystalline phase
a,
a
a
a
Megumu Munakata *, Jing Han , Atsuhiro Nabei , Takayoshi Kuroda-Sowa ,
b
a
a
Masahiko Maekawa , Yusaku Suenaga , Nozomu Gunjima
a
Department of Chemistry, Kinki University, Kowakae, Higashi-Osaka 577-8502, Japan
Research Institute for Science and Technology, Kinki University, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
b
Received 8 April 2006; accepted 5 June 2006
Available online 16 June 2006
Abstract
Three novel silver(I) complexes with 1,2-bis(2-methyl-50-(200-pyridyl)-30-thienyl)perfluorocyclopentene (BM-2-PTP) were synthesized
by the reaction of Ag(CF3SO3) or Ag(CF3COO) with BM-2-PTP in benzene at different temperatures. The structures of these metal com-
plexes were revealed by X-ray crystallographic analyses and the correlation between crystal structures and photochromic performance
was discussed. In complexes 1 and 2, silver(I) is three-coordinated to two nitrogens from distinct ligand molecules as well as one oxygen
from anions to form a 1-D polymeric structure. On the other hand, complex 3 contains two crystallographic independent Ag(I) with
different coordination environments, and the adjacent BM-2-PTP molecules are connected by Ag–CF3CO2–Ag chains to afford a 1-D
double chain structure. The difference in structures of three complexes shows the interesting anionic effect on coordination and the sub-
tleness of crystal engineering. It is noted that complex 3 underwent reversible photochromic reaction in crystalline state despite the unfa-
vorable framework to the rotation of thiophene groups.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Diarylethene; Photochromism; Crystalline phase; Ag(I) complexes
1. Introduction
[5–9]. Furthermore, when metal ions are located at both
ends of the thienyl groups of the diarylethenes, the interac-
tion between metal ions can be switched by photoirradia-
tion because the p-conjugated bond structures between
the two thienyl groups are different in the open and closed
isomers [10]. It has also been found that some dithienyleth-
ene complexes undergo photochromic reaction even in
crystalline phase, showing its promising application value
[11–17].
Previously, we reported a series of silver(I) coordination
polymers with photochromic cis-1,2-dicyano-1,2-bis(2,4,5-
trimethyl-3-thienyl)ethene (cis-dbe), which shows an inter-
esting anion effect on coordination modes and photore-
sponsive patterns [13]. In this work, we prepared a
derivative of dithienylethene family, BM-2-PTP, namely,
1,2-bis(20-methyl-50-(200-pyridyl)-30-thienyl)perfluorocycl-
Recently, photochromic compounds have attracted
remarkable interests because of their potential application
as photonic switch device, erasable-memory media, optical
data storage and photo-drive actuators, due to their out-
standing thermally irreversible and fatigue-resistant photo-
chromic abilities [1–4]. Spirobenzopyrans, azobenzenes,
fulgids and diarylethene moieties have been employed so
far. Among these known photochromic systems, diaryleth-
ene bearing two thienyl-derived groups is the best candi-
date because it is especially well suited as switching units
*
Corresponding author. Tel.: +81 6 6723 2332x4119; fax: +81 6 6723
2721.
E-mail address: munakata@chem.kindai.ac.jp (M. Munakata).
0020-1693/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.06.010

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M. Munakata et al. / Inorganica Chimica Acta 359 (2006) 4281–4288
opentene, which has sulfur and nitrogen protons and is
favorable to coordinate with silver(I) in the assembly of
various kinds of coordination polymers [13,18–23]. A sim-
ilar photochromic ligand, BM-4-BTP (1,2-bis(20-methyl-50-
(400-pyridyl)-30-thienyl)perfluorocyclopentene), was first
prepared by Lehn’s group in 1995 [7,17]. Recently, some
metal complexes with BM-4-PTP were synthesized and
their photoreactivities were studied both in solution and
solid state [3,10,24–26]. Herein, we present three novel sil-
ver(I) complexes with BM-2-PTP characterized by X-ray
crystallographic analysis. The phototogenerated behaviors
of free ligand (BM-2-PTP) and three metal complexes were
also investigated in crystalline state. All compounds dis-
played normal reversible photochromism in crystalline
state when irradiated with appropriate light. The photo-
chromism of complex 3 is especially striking, ascribed to
the rigid double chain structure.
57.57; H, 3.10; N, 5.34%. IR (KBr pellet): 3067 (w), 2854
(w), 1590 (m), 1564 (m), 1555 (m), 1494 (m), 1455 (m),
1433 (m), 1275 (s). EI-MS: m/z (%): 522 ([M]+), 507
1
([MꢀCH3]+). H NMR (CD2Cl2,270.5 MHz): d = 1.98 (s,
6H), 7.151–7.202 (m, 2H), 7.564 (m, 2H), 7.626–7.664 (m,
2H), 7.683–7.746 (m, 2H), 8.495–8.524 (m, 2H). UV–Vis
(solid state) kmax 340 nm: after irradiation with 340 nm
light (solid state) kmax 580 nm.
2.2.2. [Ag(BM-2-PTP)(CF3SO3)](benzene)1.5 (1)
To a solution of AgCF3SO3 (10.6 mg, 40 lmol) in ben-
zene (2 mL) was added a solution of BM-2-PTP (16.1 mg,
30 lmol) in benzene (2 mL). After mixing thoroughly, a
white precipitate was recrystallized from THF to afford
the colorless block single crystals suitable for X-ray analy-
sis. Anal. Calc. for C30H20AgF9N2O3S3: C, 43.33; H, 2.42;
N, 3.37. Found: C, 43.46; H, 2.65; N, 3.27%. IR (KBr pel-
let): 3071 (w), 2855 (w), 1595 (m), 1562 (m), 1494 (m), 1459
(m), 1434 (m), 1275 (s), 1060 (m), 1028 (m), 637 (m). UV–
Vis (solid state) kmax 340 nm: after irradiation with 340 nm
light (solid state) kmax 590 nm.
2. Experimental
2.1. General methods
Unless otherwise indicated, all starting materials were
obtained from commercial suppliers (Aldrich, Tokyo Kasei
Chemicals, and Kanto Chemicals) and used without fur-
ther purification. Infrared spectra were recorded as KBr
disk on JASCO FT-IR 8000 and FT/IR-430 spectrometers.
Absorption spectra in crystalline state were measured by
diffuse reflection using the Kubelka–Munk method on a
SHIMADZU UV-2450 spectrometer, and barium sulfate
was used as a reference. Photoirradiation was carried out
using a 150 W Xe lamp, and monochromatic light was
obtained by passing the light through a monochromator.
2.2.3. [Ag(BM-2-PTP)(CF3COO)](benzene)(H2O) (2)
To a solution of AgCF3COO (8.2 mg, 37 lmol) in ben-
zene (5 mL) was added BM-2-PTP (18.7 mg, 35 lmol),
and was then layered with a hexane solution. After stand-
ing at ꢀ30° C for 1 week, colorless block crystals were
obtained at the interface between two solutions. IR (KBr
pellet): 3051 (w), 2949 (w), 1590 (m), 1555 (m), 1494 (w),
1455 (w), 1433 (s), 1273 (s), 1191 (s), 1108 (s), 1059 (m),
985 (m), 777 (s). UV–Vis (solid state) kmax 340 nm: after
irradiation with 340 nm light (solid state) kmax 575 nm.
2.2.4. [Ag2(BM-2-PTP)(CF3COO)2](benzene) (3)
The colorless needles of complex 3 were obtained similar
to those of complex 2 at room temperature. Anal. Calc. for
C35H22Ag2F12N2O4S2: C, 40.33; H, 2.13; N, 2.69. Found:
C, 39.80; H, 2.15; N, 2.71%. IR (KBr pellet): 3051 (w),
2949 (w), 1682 (s), 1591 (m), 1554 (m), 1494 (m), 1433
(m), 1273 (s), 1191 (s), 1109(s), 1060 (m), 985 (m), 777
(m). UV–Vis (solid state) kmax 340 nm: after irradiation
with 340 nm light (solid state) kmax 570 nm.
2.2. Syntheses
2.2.1. 1,2-Bis(20-methyl-50-(200-pyridyl)-30-thienyl)-
perfluorocyclopentene (BM-2-PTP)
BM-2-PTP was synthesized using a modified literature
method by employing 2-bromopyridine in the place of 4-
bromopyridine hydrochloride [7], see Scheme 1. Anal. Calc.
for C25H16F6N2S2: C, 57.46; H, 3.09; N, 5.36. Found: C,
Br
Br
1) n - BuLi
Br2
2) B(OBu)3
ether, -780C
CH3COOH
Br
S
B(OH)2
S
S
F2
Br
F2
F2
F2
F2
F2
1) 2-Bromo pyridyne
2)
1) n - BuLi
2) [Pd(PPh3)4], Na2CO3
THF, refluxed
F
F
S
THF, -780C
N
S
S
N
N
Scheme 1. Synthesis of BM-2-PTP.

M. Munakata et al. / Inorganica Chimica Acta 359 (2006) 4281–4288
4283
Table 2
2.3. X-ray data collection and structure solutions and
refinements
˚
Selected bond lengths (A) and angles (°) for complexes 1–3
Complex 1
Ag(1)–O(1)
2.1640(2)
2.171(3)
Ag(1)–N(1)
2.184(3)
Diffraction data for free ligand and complexes 1–3 were
Ag(1)–N(2#)
collected on a Quantum CCD area detector coupled with a
Rigaku MSC Mercury CCD diffractometer with graphite-
O(1)–Ag(1)–N(1)
O(1)–Ag(1)–N(2#)
Ag(1)–N(1)–C(15)
Ag(1*)–N(2)–C(25)
88.63(9)
105.86(9)
119.0(2)
115.6(2)
N(1)–Ag(1)–N(2#)
Ag(1)–N(1)–C(11)
Ag(1*)–N(2)–C(21)
164.46(10)
122.6(2)
126.1(2)
˚
monochromated Mo Ka radiation (k = 0.71070 A). The
intensity data were collected at 150 K using the multi-scan
technique and a total of 5183, 8230, 7544 and 8808 reflec-
tions were collected for ligand and complexes 1–3, respec-
tively. The linear absorption coefficient l for Mo Ka
radiation is 2.99, 8.19, 8.22, and 12.17 cmꢀ1, respectively.
The structures were solved by direct methods followed
by subsequent Fourier calculations [27]. The non-hydrogen
atoms were refined anisotropically for all complexes. The
final cycle of the full-matrix least-squares refinement was
based on 4538, 6914, 5064, 6998 observed reflections and
364, 478, 488, and 533 variable parameters for ligand and
Complex 2
Ag(1)–O(1)
Ag(1*)–N(2)
2.396(5)
2.237(4)
Ag(1*)–N(1*)
2.244(4)
O(1)–Ag(1)–N(1)
O(1*)–Ag(1*)–N(2)
Ag(1)–N(1)–C(15)
Ag(1*)–N(2)–C(25)
105.0(2)
103.6(1)
116.0(3)
117.0(4)
N(1)–Ag(1)–N(2#)
Ag(1)–N(1)–C(11)
Ag(1*)–N(2)–C(21)
150.0(2)
126.0(3)
126.1(2)
Complex 3
Ag(1)–N(2)
Ag(1)–O(1)
Ag(1)–O(3)
Ag(1)–O(1*)
2.247(4)
2.432(3)
2.232(3)
2.494(4)
Ag(2)–N(1*)
Ag(2)–O(2)
Ag(2)–O(4)
2.298(4)
2.243(4)
2.344(4)
complexes 1-3, respectively, converged with the unweighted
P
and weighted agreement factors of R = iFoj ꢀ jFci/
P
P
P
2
jFoj and Rw ¼ ½ wðF 2o ꢀ F c2Þ = wðF 2oÞ2ꢁ1=2. The atomic
N(2)–Ag(1)–O(1)
O(3)–Ag(1)–O(1*)
O(2)–Ag(2)–O(4)
O(4)–Ag(2)–N(1*)
99.6(1)
96.0(1)
127.4(1)
93.4(1)
N(2)–Ag(1)–O(3)
O(1)–Ag(1)–O(1*)
O(2)–Ag(2)–N(1*)
164.9(1)
77.1(1)
126.3(1)
scattering factors and anomalous dispersion terms were
taken from the International Tables for X-ray Crystallog-
raphy, vol. IV [28]. All calculations were performed using
the teXsan crystallographic software package [29]. Details
of the X-ray experiments and crystal data are summarized
in Table 1. Selected bond lengths and bond angles are given
in Table 2.
Symmetry codes: complex 1 ( ) 1/2 + x, ꢀ1/2 + y, z; (#) ꢀ1/2 + x, 1/2 +
*
y, z; complex 2 ( ) x, 1 + y, z; (#) x, ꢀ1 + y, z; complex 3 ( ) x, 1 + y, z.
*
*
Fig. 1. The two thienyl rings of BM-2-PTP adopt antipar-
3. Results and discussion
allel conformations with the dihedral angles of 59.6(2)°.
˚
The distance of 3.49 A between two reactive carbons C(6)
3.1. Structural characterizations
and C(16) is short enough for the occurrence of cycliza-
tion/cycloreversion in crystalline phase [30]. The mean
dihedral angle between thiophene and pyridyl rings is
14.16°, indicating that the two groups slightly deviate from
coplanar conformations.
3.1.1. BM-2-PTP
The crystal structure of metal free ligand was deter-
mined by X-ray crystallographic analysis, as depicted in
Table 1
Crystallographic data for complexes 1–3
BM-2-PTP
25H16F6N2S2
Complex 1
Complex 2
Complex 3
Formula
C
C35H25AgF9N2O3S3
896.63
C33H24AgF9N2O3S2
839.54
C35H22Ag2F12N2O4S2
1042.41
monoclinic
P21/n (#14)
13.886(5)
9.193(3)
30.06(1)
102.930(4)
3864(2)
4
Formula weight
Crystal system
Space group
522.53
monoclinic
P21/n (#14)
13.104(5)
9.330(4)
18.906(8)
98.614(5)
2285(1)
4
monoclinic
C2/c (#15)
15.831(5)
17.299(5)
26.573(8)
98.956(3)
7188(3)
8
monoclinic
P21/n (#14)
16.534(9)
11.223(5)
18.761(10)
108.067(6)
3309(2)
4
˚
a (A)
˚
b (A)
˚
c (A)
b (°)
3
˚
V (A )
Z
Dcalc (g/cm3)
1.519
2.99
1.657
8.19
1.685
8.22
1.792
12.17
l (mmꢀ1
)
Observed reflections with [I > 2r(I)]
4538
0.041
0.089
6914
0.049
0.101
5064
0.072
0.146
6998
0.059
0.111
Ra
Rw
b
Goodness-of-fit
1.08
1.11
1.09
1.10
P
P
a
R = iFoj ꢀ jFci/ jFoj.
P
P
2
2 1=2
b
Rw ¼ ½ wðF 2o ꢀ F c2Þ = wðF 2oÞ ꢁ
.

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M. Munakata et al. / Inorganica Chimica Acta 359 (2006) 4281–4288
bond angle of 164.46(1)° for complex 1 and 150.0(2)° for
complex 2, respectively (Fig. 3). The Ag–N bond lengths
in both complexes fall within the expected values [31–39]
and are shorter than those of silver(I) complexes with cis-
dbe [13]. For comparison, the Ag–N distances of complex
˚
1 [Ag(1)–N = 2.171(3)–2.184(3) A] are shorter than those
˚
of complex 2 [Ag(1)–N = 2.237(4)–2.244(4) A]. On the
˚
other hand, the Ag–O distance of complex 1 [2.640(2) A]
˚
is longer than that of complex 2 [2.396(5) A]. The difference
of bond length between complexes 1 and 2 is well explained
by the fact that the three-coordination complexes deviated
from regular trigonal planar, and are compensated for by
regulating their bond distances [40]. In addition, the length
˚
between Ag(1) and S(1) is 2.96–3.06, and 2.88 A in com-
Fig. 1. ORTEP view with an atomic labeling scheme of BM-2-PTP,
showing 50% thermal ellipsoids.
plexes 1 and 2, respectively, which is longer than the com-
mon values of Ag–S bond [18,23,41–45] indicating the
weak Agꢂ ꢂ ꢂS interaction. Further the two thienyl rings in
both complexes 1 and 2 adopt an antiparallel fashion with
the dihedral angles of 64.1(2)° and 62.0(1)°, and the dis-
tance between the two reactive carbons C(6) and C(16) is
3.63 and 3.57 A in complexes 1 and 2, respectively, which
is slightly longer than that of metal free ligand [3.49 A]
and Ag(I) complexes with cis-dbe [3.45–3.58 A], but still
short enough to undergo photocyclization [30]. The mean
torsion angles between thiophene and perfluorocyclopen-
tene rings [C(2)–C(1)–C(7)–C(6) and C(1)–C(2)–C(17)–
C(16)] are 53.35° and 51.36° for complexes 1 and 2,
respectively, which are responsible for the differences in
the distance between reactive carbons. The mean rotation
angles between thiophene and pyridyl ring [C(8)–C(9)–
C(11)–C(12) and C(18)–C(19)–C(21)–C(22)] are 33.01°
and 28.26° for complexes 1 and 2, respectively.
3.1.2. Complexes 1 and 2
According to X-ray crystallographic analysis, complex 1
is a one-dimensional polymer as shown in Fig. 2. Selected
bond distances and bond angles were listed in Table 2.
The basic coordination structure of complex 2 is similar
to complex 1, except the different anions. In one unit,
silver(I) is three-coordinated by two pyridine nitrogens
from two distinct BM-2-PTP and one oxygen atom from
triflate or trifluoroacetate, respectively, showing a distorted
T-shaped coordination geometry with the N–Ag(1)–N
˚
˚
˚
3.1.3. Complex 3
Complex 3 was obtained using the same procedure as
complex 2 at 25 °C. Interestingly, the structure of complex
3 is significantly different from complex 2 as determined by
X-ray crystallographic analysis (Fig. 4). The asymmetric
unit contains two crystallographically unique Ag(I) centers:
the first one is coordinated by one nitrogen from one ligand
˚
[Ag(1)–N(2) = 2.247(4) A] and three oxygen atoms from
three different triflate anions [Ag(1)–O = 2.232(3)–
˚
2.464(4) A], showing a distorted tetrahedral structure of
[O3N1] donor set. The second one has a three-coordinate
Fig. 2. Crystal structure of complex 1. (a) ORTEP view with an atomic
labeling scheme, showing 50% thermal ellipsoids (b) Schematic view of 1-
D chain structures.
Fig. 3. Coordination environment of complex 1 (a) and complex 2 (b).

M. Munakata et al. / Inorganica Chimica Acta 359 (2006) 4281–4288
4285
˚
3.55 A, which is the shortest of the three complexes corre-
sponding to the smallest torsion angles of 48.43° between
thiophene and perfluorocyclopentene rings, implying the
effective photochromism in crystals.
Silver(I) salts with counterions such as perchlorate
ðClO4ꢀÞ, triflate ðCF3SO3ꢀÞ, and trifluoroacetate
(CF3COOꢀ) exhibit labile coordination modes [19,20]. In
this work, the counteranions were chosen as triflate and tri-
fluoroacetate and they are coordinated to Ag(I) metal cen-
ters in three complexes to generate different structures. For
photochromic Ag(I) complexes with cis-dbe, the structures
of Ag(I) complexes with triflate and trifluoroacetate are
isomorphous with near-isostructural unit. Complexes 1
and 3, which were prepared using different counteranions
at room temperature, have quite distinct structure showing
that the anions virtually have direct influence on the molec-
ular structures. Complexes 2 and 3 were synthesized at dif-
ferent temperatures to give different structures,
respectively, indicative of the subtleness of crystal engineer-
ing. All complexes have indispensable antiparallel confor-
mations and short reactive distances, which govern the
photochromic reaction in crystals suggesting the possible
photoreactivities in crystalline phase.
3.2. Photochromism in crystalline phase
The photoirradiation process of BM-2-PTP and three
silver(I) complexes were followed by electronic spectros-
copy. The absorption spectra in the crystalline state were
measured by diffuse reflection using barium sulfate as the
reference, and are illustrated in Fig 5. All complexes are
insoluble in common solvent due to polymer structures.
3.2.1. Photochromism of BM-2-PTP
The photochromic reaction of free ligand (BM-2-PTP)
was examined in solid state. Colorless powders of BM-2-
PTP have a strong absorption at 340 nm before irradiation.
Upon irradiation with 340 nm light, the sample changed
color to blue and a new strong peak at 580 nm was
observed, indicating that the open form was transferred
to closed form, and the blue color originated from the
closed form (Fig. 5(a)). After photostationary state is
reached, the blue color of closed isomer converted to the
initial color with irradiation of 580 nm light. The absorp-
tion of 580 nm gradually flattened, suggesting that revers-
ible photochromism occured in crystalline phase (Scheme
2). The kmax of 580 nm of the closed isomer in solid state
is much longer than that of cis-dbe, which was observed
at 513 nm in CCl4 and 520 nm in solid state, respectively
[13,46].
Fig. 4. Crystal structure of 3. (a) ORTEP view with an atomic labeling
scheme, showing 50% thermal ellipsoids (b) Schematic view of 1-D double
chain structures. Symmetry codes: (0) 1 ꢀ x, 2 ꢀ y, ꢀz; (#) x, ꢀ1 + y, z;
(A) 1 ꢀ x, 1 ꢀ y, ꢀz; (B) 1 ꢀ x, 3 ꢀ y, ꢀz.
structure of [O2N1] donor set and is defined by one nitro-
gen from another ligand [Ag(2)–N(1*) = 2.298(4) A] and
˚
two oxygen atoms from triflate anions [Ag(2)–
˚
˚
O(2) = 2.243(4) A, Ag(2)–O(4) = 2.344(4) A]. The two
crystallographically independent Ag(I) ions are coupled
in pairs through Ag–O–C–O–Ag bonds to form a
[Ag2O4C2] unit, which are bridged by Ag–O bonds to give
two unit chains. In turn, each BM-2-PTP molecule links
two Ag–CF3CO2–Ag chains through N atoms to afford a
1-D double chain structure. The bond lengths of Ag–N
and Ag–O in complex 3 are well within the ranges of those
of photochromic Ag(I) complexes with cis-dbe [13]. The
˚
Ag–S distance of 2.860(1) and 3.006(1) A is a little longer
3.2.2. Photochromism of complexes 1–3
than the common distances observed for an Ag–S bond,
indicating weaker interaction.
In complex 3, the two thiophene rings of ligand run anti-
parallel to each other with the dihedral angles of 61.0(1)°.
The length between the reactive carbons is as short as
As described above, both thienyl groups of ligand in
complexes 1–3 are packed into antiparallel conformations.
The distances between the two carbon atoms, C(6) and
C(16), where a r bond is formed during cyclization, are
˚
3.63, 3.57, and 3.55 A for complexes 1-3 respectively, and

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M. Munakata et al. / Inorganica Chimica Acta 359 (2006) 4281–4288
Fig. 5. Absorption spectral changes of BM-2-PTP (a), complex 1 (b), complex 2 (c), and complex 3 (d). Open form (solid line), closed form (broken line).
F2
F2
F2
F2
F2
F2
340 nm
580 nm
S
S
S
S
N
N
N
N
Scheme 2. Photochromic reaction of BM-2-PTP in solid state.
are close enough for the ring-closure. These essential condi-
tions are favorable for the occurrence of photochromic
reaction in solid state. Fig. 5(b)–(d) shows the absorption
spectral changes of complexes 1–3, respectively. Before
irradiation, complexes 1–3 have the same shoulder at ca.
340 nm, which is agreeable with the shoulder of free ligand.
Upon irradiation with 340 nm light, the colorless crystals
of three complexes turned blue and a new intense absorp-
tion at 590, 575, and 570 nm was observed for complexes
1–3, respectively, indicative of the formation of the
closed-form conformers. The blue species reverted to their
initial color by further irradiation with 590, 575, and
570 nm light, respectively, indicative of the reversible pho-
tochromic properties in crystalline phase. Compared with
the free ligand, bathochromic shift of 10 nm was observed
on the kmax value of closed form of complex 1. While the
kmax values of closed form of complexes 2 and 3 moved
to shorter wavelength due to an increase in the strain upon
complexation [10,11]. The kmax values of all Ag(I) com-
plexes with BM-2-PTP are longer than those of complexes
with cis-dbe (513–560 nm), which correspond to the longer
kmax of BM-2-PTP (580 nm) than cis-dbe (520 nm).
The thiophene rings are required to approach each other
by rotating along the C–C bond between thiophene and
ethylene moieties during the cyclization process upon irra-
diation [12–15]. In crystals, the molecules are packed in
fixed antiparallel conformation, which increases the photo-
chromic reactivity greatly, but meanwhile, a marked rota-

M. Munakata et al. / Inorganica Chimica Acta 359 (2006) 4281–4288
4287
tion is prohibited. Among the metal complexes with photo-
chromic cis-dbe, some Cu(I), Mo(II) and Rh(II) complexes
fail to undergo the photochromic reactions due to various
reasons, such as parallel conformation, steric hindrance of
the counteranions and/or thienyl rings being coordinated
to the metal center [12–15]. In the present study, all com-
plexes underwent the effective reversible photochromism
in crystalline state. In complex 3, BM-2-PTP is incorpo-
rated into double chain framework and thus the rotation
of thiophene group from the twisted ring-opening form
to coplanar ring-closed form will be hindered by the rigid-
ity of the framework and packing. Moreover, the neighbor-
ing chains in complex 3 were connected by sharing two
Ag(I) atoms of the [Ag–CF3CO2–Ag]2 chain, which further
suppressed the free rotation of thiophene rings. In spite of
these adverse features of complex 3, the reversible photo-
chromic reaction proceeded in a crystalline state and the
circles of coloration/decoloration can be repeated many
times, which demonstrate the excellent fatigue resistance
and the reasonable stabilities of complex 3. Although we
also found that some 1-D chain and 2-D sheet Ag(I) com-
plexes of cis-dbe underwent photochromism even with S
atoms bonded to metal atoms in the previous studies, no
double chain structure as complex 3 had been witnessed
[13]. This finding is of special interest as it indicates the pos-
sibility of combining the photochromic properties with ver-
satile functions arising from the construction of MOFs.
This strategy may give rise to new multi-functional materi-
als and will be applied in the rational design of novel pho-
tochromic compounds in a later program.
This finding has inspired us to combine the construction
of MOFs with photochromic ligand and the strategy will
be addressed in further studies.
Acknowledgment
This work was partially supported by a Grant-in-Aid for
Scientific Research (Nos. 14340211 and 16350037) from the
Ministry of Education, Science, Sports and Culture in
Japan.
Appendix A. Supplementary material
Crystallographic data for the X-ray crystal structural
analysis have been deposited with the Cambridge Crystal-
lographic Data Center, Nos. (0155) 602805–602808 for
BM-2-PTP and complexes 1–3, respectively. Copies of this
information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (fax: +44 1233 336 033; e-mail: deposit@ccdc.cam.
ac.uk or www: http://www.ccdc.cam.ac.uk).
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