Photochromism of Metal Complexes Composed of Diarylethene Ligands and Zn(II), Mn(II), and Cu(II) Hexafluoroacetylacetonates

Article abstract of DOI:10.1021/ic035020r

Metal complexes composed of bidentate 1,2-bis(2-methyl-5-(4-pyridyl)-3-thienyl)perfluorocyclopentene (1a) and monodentate 1-(2-methyl-5-phenyl-3-thienyl)-2-(2-methyl-5-(4-pyridyl)-3-thienyl) perfluorocyclopentene (2a) photochromic ligands and M(hfac)₂ (M = Zn(II), Mn(II), and Cu(Il)) were prepared, and their photoinduced coordination structural changes were studied. X-ray crystallographic analyses showed the formation of coordination polymers and discrete 1:2 complexes for bidentate and monodentate ligands, respectively. The complexes underwent reversible photochromic reactions by alternate irradiation with UV and visible lights in solution as well as in the single-crystalline phase. Upon photoirradiation with UV and visible light, the ESR spectra of the copper complexes of 1a reversibly changed. While the open-ring isomer gave an axial-type spectrum, the photogenerated closed-ring isomer showed a rhombic-type spectrum. This indicates that the photoisomerization induced the change in the coordination structure.

Full text of DOI:10.1021/ic035020r

Inorg. Chem. 2004, 43, 482−489
Photochromism of Metal Complexes Composed of Diarylethene Ligands
and Zn(II), Mn(II), and Cu(II) Hexafluoroacetylacetonates
,†
Kenji Matsuda,*,†,‡ Kohsuke Takayama,† and Masahiro Irie*
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu UniVersity,
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan, and PRESTO, JST, Japan
Received August 30, 2003
Metal complexes composed of bidentate 1,2-bis(2-methyl-5-(4-pyridyl)-3-thienyl)perfluorocyclopentene (1a) and
monodentate 1-(2-methyl-5-phenyl-3-thienyl)-2-(2-methyl-5-(4-pyridyl)-3-thienyl)perfluorocyclopentene (2a) photochromic
ligands and M(hfac)2 (M ) Zn(II), Mn(II), and Cu(II)) were prepared, and their photoinduced coordination structural
changes were studied. X-ray crystallographic analyses showed the formation of coordination polymers and discrete
1:2 complexes for bidentate and monodentate ligands, respectively. The complexes underwent reversible photochromic
reactions by alternate irradiation with UV and visible lights in solution as well as in the single-crystalline phase.
Upon photoirradiation with UV and visible light, the ESR spectra of the copper complexes of 1a reversibly changed.
While the open-ring isomer gave an axial-type spectrum, the photogenerated closed-ring isomer showed a rhombic-
type spectrum. This indicates that the photoisomerization induced the change in the coordination structure.
Introduction
property changes have been utilized to control functions of
photoresponsive molecules and polymers, such as host-guest
interactions,4 alignment of liquid crystals,5 and electronic
conductions.6 One of the novel examples is the photoswitch-
ing of magnetic interactions.7 When nitroxide radicals were
located at both ends of the aryl groups of the diarylethenes,
the magnetic interaction between nitroxide radicals can be
changed more than 150-fold by photoirradiation. The
Photochromic compounds have attracted remarkable at-
tention because of their potential ability for optical memory
media and optical switching devices.1 Among them, di-
arylethenes with heterocyclic aryl groups are the most
promising compounds for the applications because of their
fatigue resistant and thermally irreversible photochromic
properties.2 Moreover, it has recently been found that some
diarylperfluorocyclopentenes having thiophene or ben-
zothiophene rings undergo photochromic reactions even in
the single-crystalline phase.3
(3) (a) Irie, M.; Uchida, K.; Eriguchi, T.; Tsuzuki, H. Chem. Lett. 1995,
899-900. (b) Kobatake, S.; Yamada, T.; Uchida, K.; Kato, N.; Irie,
M. J. Am. Chem. Soc. 1999, 121, 2380-2386. (c) Kobatake, S.;
Yamada, M.; Yamada, T.; Irie, M. J. Am. Chem. Soc. 1999, 121,
8450-8456. (d) Yamada, T.; Kobatake, S.; Irie, M. J. Am. Chem.
Soc. 2000, 122, 1589-1592. (e) Irie, M.; Lifka, T.; Kobatake, S.; Kato,
N. J. Am. Chem. Soc. 2000, 122, 4871-4876. (f) Kodani, T.; Matsuda,
K.; Yamada, T.; Kobatake, S.; Irie, M. J. Am. Chem. Soc. 2000, 122,
9631-9637. (g) Yamada, T.; Kobatake, S.; Irie, M. Bull. Chem. Soc.
Jpn. 2000, 73, 2179-2184. (h) Shibata, K.; Muto, K.; Kobatake, S.;
Irie, M. J. Phys. Chem. A 2002, 106, 209-214. (i) Kobatake, S.;
Uchida, K.; Tsuchida, E.; Irie, M. J. Chem. Soc., Chem. Commun.
2002, 2804-2805.
Diarylethenes have been proven as excellent photoswitch-
ing devices as well. The open- and closed-ring isomers of
the diarylethenes differ from each other not only in their
absorption spectra but also in various physical and chemical
properties such as geometrical structures, refractive indices,
dielectric constants, and oxidation/reduction potentials. These
(4) (a) Takeshita, M.; Uchida, K.; Irie, M. J. Chem. Soc., Chem. Commun.
1996, 1807-1808. (b) Kawai, S. H. Tetrahedron Lett. 1998, 39, 4445-
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616. (d) Takeshita, M.; Irie, M. J. Org. Chem. 1998, 63, 6643-6649.
(5) (a) Denekamp, C.; Feringa, B. L. AdV. Mater. 1998, 10, 1080-1082.
(b) Yamaguchi, T.; Inagawa, T.; Nakazumi, H.; Irie, S.; Irie, M. Chem.
Mater. 2000, 12, 869-871. (c) Uchida, K.; Kawai, Y.; Shimizu, Y.;
Vill, V.; Irie, M. Chem. Lett. 2000, 654-655.
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Commun. 1993, 1439-1442. (b) Gilat, S. L.; Kawai, S. H.; Lehn,
J.-M. Chem. Eur. J. 1995, 1, 275-284. (c) Kawai, T.; Kunitake, T.;
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* To whom correspondence should be addressed. E-mail: kmatsuda@
cstf.kyushu-u.ac.jp (K.M.); irie@cstf.kyushu-u.ac.jp (M.I.).
† Kyushu University.
‡ PRESTO, JST.
(1) (a) Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH: Weinheim,
2001. (b) Brown, G. H. Photochromism; Wiley-Interscience: New
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482 Inorganic Chemistry, Vol. 43, No. 2, 2004
10.1021/ic035020r CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/18/2003

Photochromism of Metal Complexes
Scheme 1
switching is due to the changes in π-conjugated chain length
between the two isomers.
Coordination-driven self-assembly is an important strategy
to construct supramolecular architecture.8 Predesigned ge-
ometry of the metal core and organic ligands determines the
final assembled structure, which can be two- or three-
dimensional. An assembled structure can have unique
physical properties, such as magnetism9 or gas adsorption.10
Precisely controlled magnetic exchange interactions between
the metal centers in the complex afford many unique
molecule-based magnetic materials. Highly porous robust
metal-organic frameworks afford the ideal platform for the
gas adsorption.
Although there are several reports on the synthesis of metal
complexes of photochromic diarylethenes,11 no report on the
photoreactivity in the single-crystalline phase nor on the
photoswitching of the coordination structure has been
reported.12 We have prepared complexes composed of
monodentate or bidentate diarylethene pyridyl ligands and
transition metals (Zn(II), Cu(II), and Mn(II)) and examined
the reactivity in solution as well as in the crystalline phase.
When the photochromic reaction takes place efficiently, the
physical properties of the metal complexes are expected to
be reversibly changed. To prove the photoswitch, the
coordination structural change with photochromism has been
studied using ESR.
tene (1a) and monodentate 1-(2-methyl-5-phenyl-3-thienyl)-
2-(2-methyl-5-(4-pyridyl)-3-thienyl)perfluorocyclopentene (2a)
photochromic ligands by incorporating one or two pyridyl
groups to the diarylethene backbone (Scheme 1). Phenyl
derivative of this diarylethene, 1,2-bis(2-methyl-5-phenyl-
3-thienyl)perfluorocyclopentene, has been reported to show
photochromism in the crystalline phase. Zinc, copper, or
manganese hexafluoroacetylacetonate was chosen for the
metal source because of its high affinity for pyridyl ligands.
Zn2+ complexes have no absorption in the visible region
because of fully occupied d10 electronic structure. On the
other hand, Cu2+ and Mn2+ complexes have unpaired
electrons and show pale green and pale yellow colors,
respectively.
The colorless ethyl acetate solutions of 1a or 2a turned
blue upon irradiation with UV light, indicating the formation
of the closed-ring isomer 1b or 2b. The absorption maxima
of closed-ring isomers 1b and 2b in ethyl acetate were 589
and 586 nm, respectively. The conversion from the open-
ring to the closed-ring isomers under irradiation with 313
nm light was 98% for 1a. The conversion of 2a under
irradiation with 333 nm light was 95%. The high conversion
ratios are ascribable to low cycloreversion quantum yields.13
Results and Discussion
Design of Ligands and Synthesis of Complexes. Pyridyl
groups are widely used for constructing extended structures
by means of metal coordination. We have prepared bidentate
1,2-bis(2-methyl-5-(4-pyridyl)-3-thienyl)perfluorocyclopen-
The complexation with transition metals was carried out
by mixing dehydrated M(hfac)2 (M ) Zn, Cu, Mn) and
diarylethene pyridyl ligand 1a or 2a. Successive recrystal-
lization from appropriate solvents (see Experimental Section)
gave 1a‚Zn(hfac)2, 1a‚Mn(hfac)2, 1a‚Cu(hfac)2, 2a2‚Cu-
(hfac)2, 2a2‚Cu(hfac)2‚(hexane), and 2a2‚Zn(hfac)2‚(hexane).
The colors of the complex crystals were colorless, yellow,
and pale green for Zn, Mn, and Cu complexes, respectively.
IR spectra of the crystal powders showed superposition of
ligands and M(hfac)2, indicating the formation of the
complex. NMR spectra for zinc compounds and ESR spectra
for manganese and copper compounds were consistent with
the structures. Confirmation of the structure was finally
performed by elemental analysis and X-ray crystallography.
(7) (a) Matsuda, K.; Irie, M. Chem. Lett. 2000, 16-17. (b) Matsuda, K.;
Irie, M. J. Am. Chem. Soc. 2000, 122, 7195-7201. (c) Matsuda, K.;
Irie, M. J. Am. Chem. Soc. 2000, 122, 8309-8310. (d) Matsuda, K.;
Matsuo, M.; Irie, M. Chem. Lett. 2001, 436-437. (e) Matsuda, K.;
Irie, M. Chem. Eur. J. 2001, 7, 3466-3473. (f) Matsuda, K.; Irie, M.
J. Am. Chem. Soc. 2001, 123, 9896-9897. (g) Matsuda, K.; Matsuo,
M.; Irie, M. J. Org. Chem. 2001, 66, 8799-8803. (h) Matsuda, K.;
Matsuo, M.; Mizoguti, S.; Higashiguchi, K.; Irie, M. J. Phys. Chem.
B 2002, 106, 11218-11225.
(8) (a) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100,
853-908. (b) Fujita, M.; Umemoto, K.; Yoshizawa, M.; Fujita, N.;
Kusukawa, T.; Biradha, K. J. Chem. Soc., Chem. Commun. 2001, 509-
518.
(9) (a) Kahn, O. Molecular Magnetism; VCH: Weinheim, 1993. (b)
Gatteschi, D. Curr. Opin. Solid State Mater. 1996, 1, 192. (c) Sano,
Y.; Tanaka, M.; Koga, N.; Matsuda, K.; Iwamura, H.; Rabu, P.;
Drillon, M. J. Am. Chem. Soc. 1997, 119, 8246-8252.
(10) (a) Kitaura, R.; Kitagawa, S.; Kubota, Y.; Kobayashi, T. C.; Kindo,
K.; Mita, Y.; Matsuo, A.; Kobayashi, M.; Chang, H.-C.; Ozawa, T.
C.; Suzuki, M.; Sakata, M.; Takata, M. Science 2002, 298, 2358-
2361. (b) Eddaoudi, M.; Moler, D. B.; Li, H.; Chen, B.; Reineke, T.
M.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319-
330.
Crystal Structure. The structures of the complexes were
determined by X-ray crystallographic analysis. The com-
plexes of bidentate ligand 1a were chainlike structures. The
crystal structures of the repeating unit were shown in Figure
1, and the chain structures were shown in Figure 2. The
crystallographic parameters are summarized in Table 1. Two
pyridyl nitrogens of two different molecules are coordinated
with M(hfac)2 in a trans configuration to form linear
coordination polymers. All diarylethenes in the complex
(11) (a) Munakata, M.; Wu, L. P.; Kuroda-Sowa, T.; Maekawa, M.; Suenaga
Y.; Furuichi, K. J. Am. Chem. Soc. 1996, 118, 3305-3306. (b)
Ferna´ndez-Acebes, A.; Lehn, J.-M. Chem. Eur. J. 1999, 5, 3285-
3292. (c) Collins, G. E.; Choi, L.-S.; Ewing, K. J.; Michelet, V.;
Bowen, C. M.; Winkler, J. D. J. Chem. Soc., Chem. Commun. 1999,
321. (d) Fraysse, S.; Coudret, C.; Launay, J.-P. Eur. J. Inorg. Chem.
2000, 1581-1590. (e) Murguly, E.; Norsten, T. B.; Branda, N. R.
Angew. Chem., Int. Ed. 2001, 40, 1752-1755. (f) Konaka, H.; Wu,
L. P.; Munakata, M.; Kuroda-Sowa, T.; Maekawa, M.; Suenaga, Y.
Inorg. Chem. 2003, 42, 1928-1934.
(13) (a) Irie, M.; Sakemura, K.; Okinaka, M.; Uchida, K. J. Org. Chem.
1995, 60, 8305-8309. (b) Matsuda, K.; Irie, M. Tetrahedron Lett.
2000, 41, 2577-2580.
(12) Preliminary communication has been reported. Matsuda, K.; Takayama,
K.; Irie, M. J. Chem. Soc., Chem. Commun. 2001, 363-364.
Inorganic Chemistry, Vol. 43, No. 2, 2004 483

Matsuda et al.
Table 2. Crystallographic Parameters of Metal Complexes of the
Open-Ring Isomer 2a
2a2‚Cu(hfac)2‚ 2a2‚Zn(hfac)2‚
2a2‚Cu(hfac)2
C62H36F24N2- C62H36F24N2-
O4S4Cu O4S4Cu
1520.71 1520.71
(hexane)a
(hexane)a
formula
C62H36F24N2-
O4S4Zn
1522.54
293
fw
T/K
111
136
cryst syst
space group
a/Å
b/Å
c/Å
R/deg
â/deg
γ/deg
V/Å3
Z
triclinic
P1h
monoclinic
C2/c
27.547(4)
17.256(3)
14.021(2)
monoclinic
C2/c
27.867(6)
17.212(4)
13.922(3)
8.900(2)
10.774(3)
17.648(5)
86.483(4)
75.633(4)
72.277(4)
1561.4(7)
2
90.181(3)
90.732(4)
6664.8(17)
4
6677(2)
4
data/restraints/params 6308/32/507
7160/0/438
0.0626
0.1397
6976/0/438
0.0787
0.1865
R1 (I > 2σ)
0.0476
0.1384
wR2 (all data)
a Hexane molecule incorporated in the crystal was eliminated by the
SQUEEZE method.
(hfac)2, 1a‚Mn(hfac)2, and 1a‚Cu(hfac)2, respectively, which
are short enough to react in the single-crystalline phase.3i
A discrete 2:1 complex was formed from monodentate
ligand 2a. Figure 3 shows the discrete structures of 2a2‚Cu-
(hfac)2, 2a2‚Cu(hfac)2‚(hexane), and 2a2‚Zn(hfac)2‚(hexane).
Crystallographic parameters were summarized in Table 2.
For 2a2‚Cu(hfac)2‚(hexane) and 2a2‚Zn(hfac)2‚(hexane),
n-hexane molecule contained in the lattice were severely
disordered and could not be resolved. The program
SQUEEZE14 in PLATON15 was used to remove the n-hexane
solvent density. The SQUEEZE method found a total electron
count of 141 and 175 e in a volume of 807 and 840 Å3 in
the solvent regions of the unit cell for 2a2‚Cu(hfac)2‚
(hexane) and 2a2‚Zn(hfac)2‚(hexane), respectively. The
calculated electron count for four hexanes is 200, which is
substantially larger than observed. The elemental analysis
data of vacuum-dried powdered crystals revealed 20% of
hexane occupancy. These results suggest that the hexane
molecules incorporated in the crystal lattice can easily
evaporate. For three 1:2 complexes, 2a2‚Cu(hfac)2, 2a2‚Cu-
(hfac)2‚(hexane), and 2a2‚Zn(hfac)2‚(hexane), the distances
between reactive carbons were 3.59, 3.53, and 3.53 Å,
respectively, suggesting the photocyclization in the crystal
lattice is possible. The 2a2‚Cu(hfac)2‚(hexane) crystal and
2a2‚Zn(hfac)2‚(hexane) crystal were isostructural except the
metal center. Therefore, they were ideal for studying the
effect of the different metal ions on the physical properties
of the complexes.
Figure 1. ORTEP drawings of the linear chain complexes of the open-
ring isomer 1a (50% probability): (a) 1a‚Zn(hfac)2, (b) 1a‚Mn(hfac)2, and
(c) 1a‚Cu(hfac)2. Hydrogen atoms are omitted for clarity. Only one
repeating unit is shown.
Table 1. Crystallographic Parameters of Metal Complexes of the
Open-Ring Isomer 1a
a
1a‚Zn(hfac)2
1a‚Mn(hfac)2 1a‚Cu(hfac)2
formula
C35H18F18N2-
O4S2Zn
1002.01
C35H18F18N2-
O4S2Mn
991.57
C35H18F18N2-
O4S2Cu
1000.17
293
fw
T/K
111
95
cryst syst
space group
a/Å
b/Å
c/Å
R/deg
â/deg
γ/deg
V/Å3
Z
monoclinic
P21/c
12.3592(9)
18.3531(14)
17.9411(13)
triclinic
P1h
monoclinic
P2/n
11.436(4)
10.719(3)
17.046(5)
10.0408(7)
11.6439(8)
17.7049(13)
90.0680(10)
92.0120(10)
112.1240(10)
1916.1(2)
2
109.4780(10)
104.726(5)
3836.7(5)
4
2021.0(11)
2
data/restraints/params 8418/0/561
9169/0/556
0.0409
0.1042
4440/0/352
0.0435
0.1338
The complexation of Cu(hfac)2 and the isolated closed-
ring isomers 1b and 2b afforded 1b2‚{Cu(hfac)2}3 and 2b2‚
Cu(hfac)2. Crystallographic parameters were summarized in
Table 3, and the molecular structures of 1b2‚{Cu(hfac)2}3
and 2b2‚Cu(hfac)2 are shown in Figure 4. Closed-ring isomer
1b, though this is a bidentate ligand, gave a discrete 2:3
complex. This may be attributed to the reduced coordination
ability of the pyridyl ligand upon photocyclization.
R1 (I > 2σ)
wR2 (all data)
0.0309
0.0777
a Taken from ref 9.
adopted antiparallel conformation, in which photocyclization
reactions take place by a conrotatory fashion.
The chain structure was dependent on the central metal
ions. While the chain was zigzag shaped in the case of 1a‚
Zn(hfac)2 and 1a‚Cu(hfac)2, an almost straight chain was
obtained in the case of 1a‚Mn(hfac)2. The distances between
the reactive carbons were 3.56, 3.58, and 3.77 Å for 1a‚Zn-
(14) Van der Sluis, P.; Spek, A. L. Acta Crystallogr., Sect. A 1990, 46,
194-201.
(15) Spek, A. L. Acta Crystallogra., Sect. A 1990, 46, C34.
484 Inorganic Chemistry, Vol. 43, No. 2, 2004

Photochromism of Metal Complexes
Figure 2. Linear chain structures of coordination polymers (a) 1a‚Zn(hfac)2, (b) 1a‚Mn(hfac)2, and (c) 1a‚Cu(hfac)2.
Table 3. Crystallographic Parameters of Metal Complexes of the
Closed-Ring Isomers 1b and 2b
1b2‚{Cu(hfac)2}3
2b2‚Cu(hfac)2
formula
C80H38F48N4-
O12S4Cu3
2478.02
C62H36F24N2-
O4S4Cu
1520.71
293
fw
T/K
283
cryst syst
space group
monoclinic
P21/c
monoclinic
P21/c
a/Å
b/Å
c/Å
10.546(6)
20.176(10)
23.434(13)
9.796(3)
12.692(3)
24.549(6)
R/deg
â/deg
91.76(2)
90.605(5)
γ/deg
V/Å3
4984(5)
2
3051.8(13)
2
Z
data/restraints/params
R1 (I > 2σ)
wR2 (all data)
9638/0/703
0.0616
0.1803
6533/0/441
0.0510
0.1353
In copper complexes, Cu2+ ions adopted an elongated
tetragonal octahedral structure.16 The bond lengths around
copper atoms of the copper complexes are listed in Table 4.
For all Cu complexes two of the axial Cu-O bonds in trans
configuration were appreciably longer than four other
equatorial bonds due to the Jahn-Teller distortion (Chart
1). The structures of the open-ring isomer and the closed-
ring isomer were noticeably different for both bidentate and
monodentate complexes. In the case of the bidentate ligand,
Figure 3. ORTEP drawings of X-ray crystallographic structures of discrete
complexes of the open-ring isomer 2a (50% probability): (a) 2a2‚Cu(hfac)2,
(b) 2a2‚Cu(hfac)2‚(hexane), and (c) 2a2‚Zn(hfac)2‚(hexane). Hydrogen
atoms are omitted for clarity.
the open-ring isomer complex 1a‚Cu(hfac)2 had the longer
axial bond than closed-ring isomer complex 1b2‚{Cu-
(hfac)2}3. As for the monodentate ligand, the closed-ring
isomer complex 2b2‚Cu(hfac)2 had the longer axial bond
(16) Hathaway, B. J.; Billing, D. E. Coord. Chem. ReV. 1970, 5, 143-
207.
Inorganic Chemistry, Vol. 43, No. 2, 2004 485

Matsuda et al.
Figure 4. ORTEP drawings of X-ray crystallographic structures of discrete
complexes of the closed-ring isomer 1b and 2b (50% probability): (a) 1b2‚
{Cu(hfac)2}3 and (b) 2b2‚Cu(hfac)2. Hydrogen atoms are omitted for clarity.
Table 4. Bond Lengths (Å) around Metal Centers in Cu Complexes
1a‚Cu-
(hfac)2
1b2‚{Cu-
2a2‚Cu-
(hfac)2
2b2‚Cu-
(hfac)2
a
(hfac)2}3
Figure 5. Absorption spectra of diarylethene complex after irradiation
with 313 nm light in ethyl acetate: (a) 1a‚Cu(hfac)2; (b) 1a‚Mn(hfac)2;
(c) 1a‚Zn(hfac)2; and (d) 1a.
Cu-N
Cu-O(equatorial)
Cu-O(axial)
2.007(2)
2.000(2)
2.274(2)
1.995(7)
2.022(7)
2.194(8)
2.018(2)
2.086(2)
2.162(2)
2.008(3)
1.999(2)
2.288(3)
a Geometries around the central copper atoms.
Chart 1
Figure 6. (a) Polarized absorption spectra of 1a‚Zn(hfac)2 (blue line),
1a‚Mn(hfac)2 (orange line), and 1a‚Cu(hfac)2 (green line) single crystals.
For manganese and copper complexes, the direction of the maximum
intensity was defined as 0°. For zinc complex, [201h] direction, the direction
of the linear polymer chain was defined as 0°. (b) Polar plot of polarized
absorption of 1a‚Zn(hfac)2 measured at 620 nm.
than open-ring isomer complex 2a2‚Cu(hfac)2. This differ-
ence is ascribed to the change in diarylethene pyridyl ligand,
because the hexafluoroacetylacetone is the same. The dif-
ference will be discussed along with the ESR spectra.
Photochromic Reactions in Solution and in the Single-
Crystalline Phase. The ethyl acetate solutions of the
complexes underwent photochromic reactions by alternate
irradiation with 313 and 578 nm light. The original solutions
of the open-ring isomer complexes were slightly colored
yellow for Mn complexes and green for Cu complexes due
to the d-d transition of the metal ions. Upon irradiation with
UV light, the solutions turned strongly blue. The blue color
is due to the closed-ring isomers of diarylethenes. The
absorption maxima of the colored isomers 1b‚Zn(hfac)2, 1b‚
Mn(hfac)2, 1b‚Cu(hfac)2 were 590, 588, and 589 nm,
respectively, which are the same as the maxima of 1b itself
in ethyl acetate solution (Figure 5).
prohibit the photochromic reactions of the diarylethene units
in the single-crystalline phase.
The color of the crystals was observed under polarized
light. Figure 6 shows the polarized absorption spectra of 1a‚
Zn(hfac)2, 1a‚Mn(hfac)2, and 1a‚Cu(hfac)2 crystals and the
polar plot of the 1a‚Zn(hfac)2 crystal. Upon rotation, the
absorption maxima did not change, but the intensities
dramatically changed. The blue color intensity changes by
rotating the crystals, indicating that the closed-ring isomers
are regularly oriented in the crystals. In other words, the
photochromic reactions proceed in the crystal lattice. The
absorption maxima were 620, 580, and 600 nm for 1a‚Zn-
(hfac)2, 1a‚Mn(hfac)2, and 1a‚Cu(hfac)2 crystals, respec-
tively. The absorption maxima in crystals were different
among the complexes, while they are similar in solutions.
The relation between the transition moment and the crystal
structure was examined on the face-indexed crystal of 1a‚
Zn(hfac)2. The polar plot measured normal to (010) plane
is shown in Figure 6b. In this plot, [201h] direction, the
direction of the linear polymer chain is defined as 0°. The
The complex crystals also showed photochromic reactivity.
Upon irradiation with 313 nm light the single crystals turned
blue. Upon irradiation with 578 nm light the blue color
disappeared. The complexation with metal ions did not
486 Inorganic Chemistry, Vol. 43, No. 2, 2004

Photochromism of Metal Complexes
Figure 7. (a) Polarized absorption spectra of single crystal of 2a2‚Cu-
(hfac)2 (gray line), 2a2‚Cu(hfac)2‚(hexane) (green line), 2a2‚Zn(hfac)2‚
(hexane) (blue line). The direction of the maximum intensity was defined
as 0°. (b) Polar plot of polarized absorption of 2a2‚Cu(hfac)2 measured at
580 nm.
maximum of the absorption was identical to the direction of
the linear polymer chain.
Monodentate complexes also showed photochromic reac-
tivity in the single-crystalline phase. Figure 7 shows the
polarized absorption spectra of 2a2‚Cu(hfac)2, 2a2‚Cu-
(hfac)2‚(hexane), and 2a2‚Zn(hfac)2‚(hexane) crystals and
the polar plot of the 2a2‚Cu(hfac)2 crystal. The absorption
maximum of 2a2‚Cu(hfac)2 was observed at 580 nm, while
the absorption maxima of 2a2‚Cu(hfac)2‚(hexane) and 2a2‚
Zn(hfac)2‚(hexane) were observed at 620 nm. As mentioned
in the crystallography section, 2a2‚Cu(hfac)2‚(hexane) and
2a2‚Zn(hfac)2‚(hexane) were isostructural. The isostructural
crystals 2a2‚Cu(hfac)2‚(hexane) and 2a2‚Zn(hfac)2‚(hexane)
showed the same absorption maxima regardless of the
difference in the metal ions, while absorption maximum of
2a2‚Cu(hfac)2 showed hypsochromic shift. This strongly
suggests that the absorption spectrum of the closed-ring
isomers is determined by the conformation of the di-
arylethene units, not by the metal ions. The absorption spectra
of the closed-ring forms of diarylethenes are dependent on
the torsion angle between the thiophene ring and the terminal
phenyl or pyridyl ring.17
ESR Study. X-band ESR spectra of copper complexes
were measured in a toluene matrix at 60-70 K for 1a‚Cu-
(hfac)2 and 2a2‚Cu(hfac)2 (Figure 8). The complex of open-
ring ligand 1a (Figure 8a, black) showed a typical axial-
type Cu2+ spectrum in which gx and gy are almost the same.
The simulation of the spectrum gave g ) 2.08, g| )2.34,
and A| ) 160 G.18 The values suggest the elongated
tetragonal octahedral structure in which two coordination
bonds in trans configuration are longer than the others as
determined in X-ray crystallographic analysis. Upon irradia-
tion with UV light, the spectrum changed along with the
photochromism. The irradiation was performed in solution
and in the frozen matrix. In both cases, the spectral change
was observed. After irradiation with visible light, the original
spectrum was regenerated. The conversion from the open-
ring to the closed-ring isomers in the photostationary state
was estimated to be more than 90%. The spectrum of the
photogenerated closed-ring isomer could be reproduced by
Figure 8. (a) X-band ESR spectra of 1a‚Cu(hfac)2 in toluene matrix (1
mM) measured at 70 K (9.36 GHz). (b) X-band ESR spectra of 2a2‚Cu-
(hfac)2 in toluene matrix (1 mM) measured at 60 K (9.36 GHz).
Measurement was performed before (black) and after (gray) irradiation with
UV light. Irradiation with visible light reproduced the original spectra.
using three different g values, g1 ) 2.07, g2) 2.17, and g3)
2.34 (Figure 8a, red). This is a typical rhombic spectrum,
which is observed in elongated rhombic octahedral stereo-
chemistry.16 The photoisomerization reaction of the di-
arylethene unit caused the distortion in the copper center.
In the case of 2a2‚Cu(hfac)2, the spectral change before
and after irradiation of UV light was not so significant
(Figure 8b). Both the open-ring isomer and the closed-ring
isomer had axial-type spectra, with g ) 2.08, g| )2.31,
and A| ) 150 G.
These results of the ESR measurement suggest that the
coordination structures around Cu2+ ion changed from
elongated tetragonal octahedral in 1a to elongated rhombic
octahedral in 1b, while the structures of the complexes of
2a and 2b were similar each other. Because the exchange
coupling between two Cu2+ ions through more than 17 Å of
distance is negligibly small, the difference should originate
from the difference in the ligands.
Some possibilities for the origin of this spectral change
are taken into consideration. Since the hexafluoroacetylac-
etone ligand is the same, the difference between open-ring
and closed-ring isomers of the diarylethene pyridyl ligand
had induced spectral change. As shown in the X-ray
crystallography section, geometry around Cu2+ ion changed
along with photochromism. The shortening of the bond
length of the axial ligand in 1 upon photocyclization was
(17) Morimoto, M.; Kobatake, S.; Irie, M. Chem. Eur. J. 2003, 9, 621-
627.
(18) Simulation was performed using EPRSim XOP program by John
Boswell, Oregon Graduate Institute, an adaptation of QPOW program
by Mark J. Nilges and R. Linn Belford.
Inorganic Chemistry, Vol. 43, No. 2, 2004 487

Matsuda et al.
The solvent was removed. Column chromatography (silica gel, CH2-
Cl2/AcOEt ) 4:1) afforded dithienylethene 2a (3.6 g, 81%) as a
slightly green powder: mp 154.5-155.0 °C; 1H NMR (200 MHz,
CDCl3) δ 1.96 (s, 3 H), 2.00 (s, 3 H), 7.2-7.6 (m, 9 H), 8.60 (d,
J ) 6.2 Hz, 2 H); 13C NMR (100 MHz, CDCl3) δ 14.6, 14.7, 119.5,
122.3, 124.9, 125.6, 126.4, 128.0, 129.1, 133.2, 139.1, 140.3, 141.3,
142.6, 143.7, 150.6; 19F NMR (376 MHz, CDCl3, CF3COOH as
external reference) δ -132.4, -110.5, -110.7; MS m/z 521 ([M]+),
506 ([M - CH3]+), 491([M - 2CH3]+); UV-vis (AcOEt) λmax
285 nm; after irradiation with 313 nm light (AcOEt) λmax 308, 586
nm; Anal. Calcd for C26H17F6NS2: C, 59.88; H, 3.29; N, 2.69.
Found: C, 60.03; H, 3.41; N, 2.68.
observed in molecular structure. This means the relative
increase of the axial ligand field of the Cu2+ ion, namely
the relative decrease of the equatorial ligand field of the
diarylethene pyridyl ligand.19 This decrease of the equatorial
ligand field can be ascribed to the decrease in the π-acceptor
character of the ligand along with photocyclization. The
change in the photochromic ligand may induce the coordina-
tion structure and the ESR spectrum. In the case of the X-ray
structure of 2, the bond length of the axial ligand got even
longer, so that the ESR spectrum did not change.
We have also reported that diarylethenes shrink along with
photocyclization.20 The shrinkage in size in 1 cannot be
relieved by the movement of the diarylethene molecules
because both ends of the molecule are coordinating to
different metal ions. The shrinkage of the diarylethene units
is accumulated in the coordination polymer, the coordination
structure is distorted, and the ESR spectra are changed
significantly. On the contrary, as 2 is monodentate, the
complex is composed of two diarylethene molecules and one
metal ion, so that the ESR spectra changed only slightly.
Both possibilities suggest that the ESR spectral change is
due to the distortion around the Cu2+ center upon photo-
isomerization. Photoinduced structural change in the organic
ligand caused the structural change in the metal complex.
1a‚Zn(hfac)2. To a solution of 1a (50 mg, 96 µmol) in
dichloromethane (2 mL) was added a solution of Zn(hfac)2‚2H2O
(49 mg, 95 µmol) in dichloromethane/methanol (1:1, v/v) (2 mL).
Pale green precipitates were collected and recrystallized from
dichloromethane/methanol (1:1, v/v) to afford colorless block
crystals: mp 274 °C (decomp); 1H NMR (400 MHz, DMSO-d6) δ
2.01 (s, 6 H), 5.55 (s, 2 H), 7.65 (d, J ) 3.5 Hz, 4 H), 7.83 (s, 2
H), 8.59 (brs, 4 H); IR (KBr) ν 1130, 1250, 1500, 1600 cm-1
;
UV-vis (AcOEt) λmax 300 nm; after irradiation with 313 nm light
(AcOEt) λmax 590 nm; Anal. Calcd for C35H18F18N2O4S2Zn: C,
41.95; H, 1.81; N, 2.80. Found: C, 42.13; H, 1.82; N, 2.95.
1a‚Mn(hfac)2. To a solution of 1a (200 mg, 380 µmol) in
dichloromethane (2 mL) was added a solution of Mn(hfac)2‚xH2O
(179 mg) in dichloromethane/methanol (1:1, v/v) (2 mL). After
solvents are evaporated, precipitates were recrystallized from
dichloromethane/methanol (1:1, v/v) to afford yellow block crys-
tals: mp >200 °C (decomp); IR (KBr) ν 1130, 1240, 1480, 1600
cm-1; UV-vis (AcOEt) λmax 301 nm; after irradiation with 313
nm light (AcOEt) λmax 588 nm. Anal. Calcd for C35H18F18N2O4S2-
Mn: C, 42.40; H, 1.83; N, 2.83. Found: C, 42.40; H, 1.83; N,
2.77.
Conclusions
We have synthesized metal complexes composed of
bidentate and monodentate diarylethene pyridyl ligands and
transition metal ions. The complexes underwent reversible
photochromic reactions in the single-crystalline phase as well
as in solution. The complexation with metal ions does not
prohibit the photochromic reactions in the single-crystalline
phase. The ESR spectra of the copper complexes of 1a in
toluene matrices at 70 K were reversibly interconverted by
irradiation with UV and visible light. It was concluded that
photochromic reaction of the ligand caused the change of
the coordination structure.
1a‚Cu(hfac)2. To a solution of 1a (200 mg, 380 µmol) in
dichloromethane (2 mL) was added a solution of Cu(hfac)2‚xH2O
(182 mg) in dichloromethane/methanol (1:1, v/v) (2 mL). Precipi-
tates were collected and recrystallized from THF/methanol (1:1,
v/v) to afford pale green block crystals: mp >200 °C (decomp);
IR (KBr) ν 1130, 1250, 1500, 1610 cm-1; UV-vis (AcOEt) λmax
300 nm; after irradiation with 313 nm light (AcOEt) λmax 589 nm.
Anal. Calcd for C35H18F18N2O4S2Cu: C, 42.03; H, 1.81; N, 2.80.
Found: C, 41.89; H, 1.82; N, 2.90.
Experimental Section
(A) Materials. 1H NMR spectra were recorded on Varian Gemini
200 and JEOL JNM-ECP400 instruments. UV-vis spectra were
recorded on a Hitachi U-3500 spectrophotometer. Mass spectra were
obtained by a Shimadzu QP-5050A spectrometer. Zn(II), Mn(II),
and Cu(II) hexafluoroacetylacetonates were purchased from TCI.
Melting points are not corrected. Compound 1a was synthesized
according to the literature.6b
2a2‚Cu(hfac)2. To a solution of 2a (45 mg, 86 µmol) in toluene
(1 mL) was added a solution of Cu(hfac)2‚xH2O (21 mg) in toluene
(0.5 mL). Precipitates were collected and recrystallized from
2-propanol to afford pale green block crystals: mp 182.6-183.0
°C; UV-vis (AcOEt) λmax 297 nm; after irradiation with 333 nm
light (AcOEt) λmax 589 nm. Anal. Calcd for C62H36F24N2O4S4Cu:
C, 48.97; H, 2.39; N, 1.84. Found: C, 49.08; H, 2.40; N, 1.96.
1-(2-Methyl-5-phenyl-3-thienyl)-2-(2-methyl-5-(4-pyridyl)-3-
thienyl)-3,3,4,4,5,5-hexafluorocyclopentene (2a). To a well stirred
solution of 2-methyl-3-bromo-5-(4-pyridyl)-thiophene (2.2 g, 8.5
mmol) in anhydrous THF (100 mL) under Ar at -78 °C was added
dropwise 1.6 M n-BuLi in hexane (5.6 mL, 9.0 mmol), and stirring
was continued for 2 h at -78 °C. Then, a solution of 3-(2,3,3,4,4,5,5-
heptafluorocyclopent-1-en-1-yl)-2-methyl-5-phenylthiophene (3.3 g,
9.0 mmol) was added dropwise. The mixture was stirred for 4 h
and allowed to warm to room temperature, and aqueous NH4Cl
was added. The resultant mixture was then extracted with Et2O,
and the organic extract was washed with brine and dried (MgSO4).
2a2‚Cu(hfac)2‚(hexane). To a solution of 2a (41 mg, 79 µmol)
in dichloromethane was added Cu(hfac)2‚xH2O (19 mg) in dichlo-
romethane. After the solvent was evaporated, the residue was
recrystallized from dichloromethane/hexane (1:1, v/v) to give pale
green block crystals. Anal. Calcd for C62H36F24N2O4S4Cu-
(C6H14)0.2: C, 49.36; H, 2.54, N, 1.82. Found: C, 49.32; H, 2.64;
N, 2.09.
2a2‚Zn(hfac)2‚(hexane). To a solution of 2a (22 mg, 42 µmol)
in dichloromethane was added Zn(hfac)2‚2H2O (10 mg, 19 µmol)
in dichloromethane. After the solvent was evaporated, the residue
was recrystallized from hexane to give colorless block crystals: 1H
NMR (200 MHz, CDCl3) δ 1.94 (s, 6 H), 2.01 (s, 6 H), 5.97 (s, 2
H), 7.26-7.55 (m, 18 H), 8.57 (brs, 4 H). Anal. Calcd for
(19) Lever, B. P.; Mantovani, E. Inorg. Chem. 1971, 10, 817.
(20) Irie, M.; Kobatake, S.; Horichi, M. Science, 2001, 291, 1769-1772.
488 Inorganic Chemistry, Vol. 43, No. 2, 2004

Photochromism of Metal Complexes
C62H36F24N2O4S4Zn(C6H14)0.2: C, 49.30; H, 2.54, N, 1.82. Found:
C, 49.36; H, 2.79; N, 1.78.
disorder originating from ring-flipping, and the disordered part was
refined isotropically. For 2a2‚Cu(hfac)2‚(hexane) and 2a2‚Zn-
(hfac)2‚(hexane), n-hexane molecule contained in the lattice were
severely disordered and could not be resolved. The program
SQUEEZE14 in PLATON15 was used to remove the n-hexane
solvent density.
(B) Photochemical Measurement. Solvents used for physical
measurements were spectroscopic grade and purified by distillation
before use. Absorption spectra were measured on a spectropho-
tometer (Hitachi U-3500). Absorption spectra in single-crystalline
phases were measured by using a Leica DMLP polarizing micro-
scope connected with Hamamatsu PMA-11 detector. The polarizer
and analyzer were set parallel to each other.
Photoirradiation was carried out by using a USHIO 500 W super
high-pressure mercury lamp and a USHIO 500-W xenon lamp.
Mercury lines of 313, 333, 578 nm were isolated by passing the
light through a combination of a Toshiba band-pass filter or a cutoff
filter and a monochromator (Ritsu MC-20L).
(D) ESR Spectroscopy. A Bruker ESP 300E spectrometer was
used to obtain X-band ESR spectra. Temperatures were controlled
by an RMC CT-470-ESR cryogenic temperature controller. The
cryostat was maintained at high vacuum by a turbo molecular/rotary
pump set. The sample was dissolved in solvent (ca. 1 mM) and
degassed with argon bubbling for 5 min.
Acknowledgment. This work was partly supported by a
Grant-in-Aid for the 21st Century COE Program, “Functional
Innovation of Molecular Informatics”, from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
The measurement of 13C and 19F NMR was made using JEOL
JNM-ECP400 spectrometer at the Center of Advanced
Instrumental Analysis, Kyushu University
(C) Crystallography. The data collection was performed on a
Bruker SMART1000 CCD-based diffractometer (50 kV, 40 mA)
with Mo KR radiation. The data were collected as a series of ω-scan
frames, each with a width of 0.3°/frame. The crystal-to-detector
distance was 5.118 cm. Crystal decay was monitored by repeating
the 50 initial frames at the end data collection and analyzing the
duplicate reflections. Data reduction was performed using SAINT-
PLUS software,21 which corrects for Lorentz and polarization
effects, and decay. The cell constants were calculated by the global
refinement. The structure solved by direct methods and refined by
full least-squares on F2 using SHELXL software.22 The positions
of all hydrogen atoms were calculated geometrically and refined
by the riding model. Several hexafluorocyclopentene ring had
Supporting Information Available: The photochromic reaction
of 2a in ethyl acetate and the result of X-ray crystallographic
analysis of 2a. X-ray crystallographic data of 1a‚Mn(hfac)2, 1a‚
Cu(hfac)2, 2a2‚Cu(hfac)2, 2a2‚Cu(hfac)2‚(hexane), 2a2‚Zn(hfac)2‚
(hexane), 1b2‚{Cu(hfac)2}3, 2b2‚Cu(hfac)2, and 2a (CIF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
(21) SAINTPLUS Software Package, Version 6; Bruker AXS: Madison,
WI, 2001.
(22) SHELXTL Software Package, Version 5; Bruker AXS: Madison, WI,
2001.
IC035020R
Inorganic Chemistry, Vol. 43, No. 2, 2004 489