Enhanced photochromism of 1,2-dithienylcyclopentene complexes with metal ion

Article abstract of DOI:10.1039/b303028h

A new bis(5-pyridyl-2-methylthien-3-yl)cyclopentene, as a photochromic bridging ligand, has been synthesized and the photocyclization quantum yield was found to increase in the presence of a metal ion.

Full text of DOI:10.1039/b303028h

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Enhanced photochromism of 1,2-dithienylcyclopentene complexes
with metal ion
Bing Qin, Rongxia Yao, Xueli Zhao and He Tian*
Institute of Fine Chemicals, East China University of Science and Technology,
Shanghai 200237, P. R. China
Received 20th March 2003, Accepted 6th May 2003
First published as an Advance Article on the web 21st May 2003
A new bis(5-pyridyl-2-methylthien-3-yl)cyclopentene, as a photochromic bridging ligand, has been synthesized
and the photocyclization quantum yield was found to increase in the presence of a metal ion.
Diarylfluorocyclopentenes, which displayed excellent photo-
Introduction
chromic properties and marked resistance to photofatigue, have
been widely investigated. In particular, the closed form of the
perfluorocyclopentene switches have been shown to be stable up
to temperatures well above 100 ЊC.17–20 Although 1,2-bisaryl-
substituted perfluorocyclopentenes have been reported with
excellent photochromic behavior, there are disadvantages with
these compounds such as expense and the volatile (bp 26–28
ЊC)21 starting material octafluorocyclopentene. Comparatively,
the synthesis of dithienylcyclopentene could be performed on
a large scale and the starting materials are easily accessible.
Dithienylcyclopentene also showed excellent thermal stability.21
1,2-Bis (5-chloro-2-methyl-3-thienyl)cyclopentene is a highly
versatile starting material for a variety of dithienyl-cyclo-
pentene-based compounds. It can be synthesized from rather
cheap starting materials and the synthesis route is rather simple
as described previously.12
In the present work we report on the synthesis, characteriz-
ation, and photochromic properties of a pyridine-tethered
BTE, namely, bis(5-pyridyl-2-methyl-3-thienyl)cyclopentene
shown in Scheme 1. To enhance the cyclization quantum yield,
the optical properties of the pyridine-tethered BTE in solution
with metal ions have also been investigated. Improved ring-
closing quantum yields were observed in THF in the presence
of zinc ion compared with a solution without metal ions.
Nevertheless, it would be useful to design new photochromic
materials for information data processing.
Considerable attention has been paid to photochromic com-
pounds including diarylethenes, fulguides, spiropyrans, which
show reversible photoisomerization upon irradiation with light
of appropriate wavelength.1–4 In particular, studies on the 1,2-
bisthienylethene derivatives (BTEs) are of interest for potential
applications as erasable-memory media, photo-optical switch-
ing, display and photo-drive actuators owing to their excellent
thermal-stability and fatigue-resistance.5 Although 1,2-bis-
thienylethene derivatives are the most promising compounds
for applications in photonic devices such as erasable data stor-
age systems and optical switches, several additional properties
need to be improved, such as cyclization quantum yield and
non-destructive readout capability.1,2 The property changes of
BTEs during the photochromic process, such as the refractive
index, relative permittivity, oxidation/reduction potential,
photoluminescence are the basis of such applications.3–14
Among them, reversible changes of luminescence are interest-
ing from the viewpoint of applications for erasable memory
media, optical switches and fluorescence probes. Most recently,
Irie et al.15 have shown that digital switching of the fluorescence
of diarylethene molecules can be controlled by irradiation with
UV and visible light at the single-molecular level. Branda’s
group16 also described BTE-porphyrin system switches to
achieve non-destructive readout using the luminescent changes.
A novel family of photochromic BTE-based tetraazaporphyrin
or phthalocyanine hybrids was developed in our lab.2 The
changes of near infrared luminescence of this family of com-
pounds along with photochromism provided a wonderful non-
destructive readout method and optical switches.
Results and discussion
Also the use of a diarylethene as a photoswitching ligand has
attracted much attention because of its potential in photonic
switching devices. The interaction between metal ions located at
both ends of pyridyl groups of the diarylethene can be switched
by irradiation, because the π-conjugated bond structures
between the two aryl groups in the two isomers are different.6,7
A linear chain polymer complex, bridged by pyridyl groups, of
1,2-bis [2-methyl-5-(4-pyridyl)-3-thienyl]perfluorocyclopentene
and Zn(hfac)2 has been reported.3 The complex underwent a
photochromic reaction in the single-crystalline state and the
absorption maximum showed a 10 nm bathochromic shift. Fur-
thermore, the opened ring form of diarylethene has parallel and
anti-parallel conformations. The balance between the two con-
formations limits the maximum cyclization quantum yield to
0.5.1 The existence of the parallel conformation limits the
quantum yield for the ring-closure reaction.17 If the ratio of
anti-parallel conformation can be increased, the cyclization
quantum yield is expected to increase accordingly.7,8 Takeshita
et al.10 has reported the enhancement of photocyclization
yield by the inclusion of a photochromic diarylethene in a
cyclodextrin cavity.
Absorption spectra of compound 2 upon UV irradiation and
the addition of various amounts of Zn2ϩ in THF are shown in
Fig. 1. In THF, there is almost no difference between the com-
pound 2 (free ligand) and the zinc complex. In both cases the
open isomer exhibits a UV absorption that has a maximum
around 295 nm. Upon irradiation with 254 nm light the color-
less solution of the open-ring form 2a turns purple and an
immediate increase in the absorption intensity is observed in the
visible spectral region (λ = 558 nm). The purple color is due to
the transformation of 2a to 2b by photocyclization. The open
isomer 2a can be photochemically regenerated by irradiation
with light of 570 nm wavelength and its characteristic purple
color disappeared again. Similar spectral changes are observed
for 2a upon the addition of various amounts of Zn2ϩ in THF
under identical conditions as shown in Fig. 1.
After several minutes of continuous irradiation on com-
pound 2 (4 × 10Ϫ4 M in CDCl3), the photostationary state was
reached with the isosbestic point in the absorption spectra. A
second set of peaks appeared in its proton NMR spectra which
are slightly shifted (upfield shifts for the thiophene C–H signals
and downfield shifts for the methyl thiophene signals) with
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 1 8 7 – 2 1 9 1
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Table 1 Selected 1H NMR data (500 MHz) of open and closed forms of compound 2 in CDCl3 and in CDCl3 with Zn2ϩ
δ for protons(ppm)
Compound
Thiophene C–H
Pyridine α-H
–CH2
Thiophene CH3
2 only (open)a
7.22
6.59
7.23
6.60
8.52(doublet)
8.59
8.53(doublet)
8.60
2.85
2.52
2.85
2.53
2.01
2.02
2.02
2.06
2 only (closed)b
2 ϩ 4 eq. Zn2ϩ(open)a
2 ϩ 4 eq. Zn2ϩ(closed)b
a 4 × 10Ϫ4 M in CDCl3. b The closed form refers to the photostationary state.
Scheme 1 Photochromism and metal-complex of compound 2.
The two conformations with the two rings in mirror sym-
metry and in C2 symmetry of bisthienylethene determine that
only the anti-parallel conformation is allowed in the photo-
cyclization reaction. The two conformations expected are in
dynamic equilibrium. In general, the population ratio of the
two conformations is 1 : 1. Therefore, when the ratio of the anti-
parallel conformation is increased, the photocyclization quan-
tum yield is also expected to increase.1 The absorption maxima
of 2b upon the addition of various amounts of Zn2ϩ in THF
are all found to be in the range 554–558 nm and are shown
in Table 2. It is worthwhile to note the different absorption
maxima values in the photostationary state (Aps) of compound
2 in THF and in the presence of Zn2ϩ and the indication of the
conversion between the two states. A higher Aps indicates a
preferred switching process to the closed form, i.e. enhanced
photochromism.23 The Aps values are shown in Table 2. The
results for compound 2 upon the addition of various amounts
of Zn2ϩ in THF was surprising and indicate that the quantum
efficiency of interconversion into the photostationary state is
nearly three times higher for compound 2 in the presence of
Zn2ϩ than without zinc ion. The photocyclization quantum
yield calculated by the comparable method24 is also almost
three times higher for compound 2 in the presence of Zn2ϩ as
shown in Table 2. The ring-closure coloration quantum yield
increases from 15.5% to 44.6% upon the addition of varying
amounts of Zn2ϩ. It may be ascribed to the influence of the zinc
ion. It plays an important role in directing the steric course of
the photocyclization reaction. Although it is difficult to predict
the mechanism of how the pyridyl units orient themselves
around a metal ion in solution, it clearly increases the photo-
cyclization quantum yield as shown by the values in Table 2.
The salts of transition metals group (II) are well known as good
complex agents. Zinc complexes with the coordination number
varying from 2 to 6, and especially 4, are most favored.25,26 In
our work ZnCl2 was chosen as the metal source, the two pyridyl
groups of two molecules of compound 2 possibly coordinate
with the zinc ion with a coordination number of four in a crab-
like configuration in solution as shown in Scheme 1. Because of
the high binding affinity of pyridyl with metal ions,27 the inter-
conversion efficiency from the parallel to the anti-parallel con-
figuration is strongly enhanced by Zn2ϩ complex and this allows
the thiophene ring to rotate more freely. The possible crablike
Fig. 1 UV–Vis absorption spectra of compound 2 (2 × 10Ϫ5 M) upon
the addition of various amounts of Zn2ϩ in THF and the changes in
absorption under different irradiation time by light of 254 nm. Insert
figure: relationship between the changes in absorbance at the peak of
556 nm and time of photocyclization by irradiation with 254 nm.
respect to those of the open form and this indicates that a
photochemical reaction has occurred.20,21 The proton NMR
spectral trends of compound 2 are listed Table 1. For the
α-protons of the pyridine, the NMR signal feature remains the
sharp doublet after complexation with Zn2ϩ. This means that
the conformation of the zinc complex is as shown in Scheme 1.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 1 8 7 – 2 1 9 1
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Table 2 Absorption behavior of 2a upon the addition of various amounts of Zn2ϩ in THF irradiated with 254 nm lighta
24
Compoundb
λmax/nm (open form)
λmax/nm (closed form)
Aps 23 (ΦO–C
)
2 only
290
551
554
555
556
558
0.114 (15.5%)
0.296 (36.2%)
0.333 (39.2%)
0.355 (44.6%)
0.332 (42.4%)
2 ϩ 1 eq. Zn2ϩ
2 ϩ 2 eq. Zn2ϩ
2 ϩ 4 eq. Zn2ϩ
2 ϩ 5 eq. Zn2ϩ
292, 263
295, 255
296, 256
295, 257
a Absorption maximum obtained at the photostationary state under irradiation with 254 nm light.23 ΦO–C is the quantum yield for the photocycliz-
ation from the open form to the closed form calculated by the comparable method.24 b 2 × 10Ϫ5 M upon the addition of various amounts of Zn2ϩ
in THF.
Fig. 2 Fluorescence emission spectra of compound 2 (2 × 10Ϫ5M, excited at 370 nm) upon the addition of various amounts of Zn2ϩ in THF and
their changes when 2a was transformed to the closed form with different irradiation time by 254 nm light.
configuration shown in Scheme 1 may favor the anti-parallel
conformation and this may explain why the interconversion
efficiency to the photostationary state of compound 2 in the
presence of Zn2ϩ is obviously higher than that in the solution
without zinc ion.
fluorescence. Initial addition of Zn2ϩ ion to the solution of
compound 2 in THF leads to an increase in the fluorescence
intensity and a new peak at 463 nm appears in the fluorescence
spectra, which is characteristic of the zinc complex. Further
addition of various amounts of Zn2ϩ increases the fluorescence
In fact, the zinc coordination number in the complexes in this
study can be determined by the experimental data listed in
Table 2. After normalization of absorbance at 295 nm (the
characteristic absorption of the free ligand 2a), the Aps values
for various amounts of Zn2ϩ in THF at the photostationary
state under irradiation with 254 nm are almost the same as that
1 : 1 molar ratio between compound 2 and zinc ion. This means
that the coordination in the complexes dominates the configur-
ation shown in Scheme 1, despite the variation of zinc ion
concentration in THF.
Recording the change of luminescence provides another
method to assess the process of storage of information, which
can minimize the extent of erased information during the signal
detection. An important condition must be met additionally.
That is, the emission wavelengths being measured in readout
process must reside far enough outside the spectral regions
where the photochromic reactions are induced.11,21 Fig. 2 shows
the fluorescence intensity changes of 2a in THF upon the addi-
tion of various amounts of Zn2ϩ. In the case of 2a in THF, it
fluoresces with a peak at 415 nm when excited by 370 nm, where
the BTE photochromic unit has no absorption. The fluor-
escence intensity change is observed under different irradiation
times. The open form 2a in THF has relatively low fluorescence,
whereas the mixture solution of 2a and Zn2ϩ shows medium
intensity only at 463 nm, as shown in Fig. 2. The ratios (I463 nm /
I415 nm) of the initial fluorescent intensity between the peak at
463 nm and that at 415 nm, which is characteristic of com-
pound 2a alone, increase with the increase in concentration of
zinc ion in the system. That is, (I463
/ I415 nm) is 1.09 for one
nm
equal molar of Zn2ϩ, 1.21 for two equal molars of Zn2ϩ, 1.71
for four equal molars of Zn2ϩ, 1.79 for five equal molars of
Zn2ϩ, respectively. This trend implies that the conformation of
the zinc complex shown in Scheme 1 is reasonable. Since the
crablike conformation shown in Scheme 1 for the metal com-
plex is a rigid structure, it would enhance fluorescence. In addi-
tion, an increase of zinc concentration in the system would
convert the parallel conformation of thiophene ring of com-
pound 2 into the anti-parallel conformation, which has been
confirmed by the data listed in Table 2 and the three times
enhancement in Aps values. The new and strong fluorescence
intensity at 463 nm is ascribed to the fluorescence of the Zn2ϩ
complex. When the photocyclization reaction was carried out
by irradiation on the Zn2ϩ complex with UV-light (254 nm), the
intensity of the new emission at 463 nm was strongly quenched.
Several minutes of irradiation later the emission at 463 nm was
quenched completely and the fluorescent spectra of the sys-
tems with different concentrations of zinc ion became similar to
each other. The intensity of the residual emission after UV
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 1 8 7 – 2 1 9 1
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Scheme 2 Synthetic route of the compound 2 in the study.
irradiation for compound 2 in the presence of Zn2ϩ became
1,2-Bis(5-boronic acid-2-methyl-3-thienyl)cyclopentene (1)
similar to the fluorescence spectra of compound 2 in THF
without zinc ion. The results indicate that the zinc complex
favors the interconversion to the anti-parallel conformation,
however, the crablike configuration of the complex shown in
Scheme 1 is destroyed by 254 nm light irradiation and the zinc
ion is cleaved. The residual emission observed in the solution of
2a upon the addition of Zn2ϩ is considered to be the emission
from compound 2b (shown in Fig. 2). It should be pointed out
that the zinc complex is photostable following excitation by 370
nm light, as shown in Fig. 2.
To a stirred solution of compound 3 (2 g, 6 mmol) in THF
(20 ml) at Ϫ78 ЊC under Ar in the absence of light was added
dropwise 1.6 M n-BuLi in hexane (0.78 g, 12 mmol), and the
reaction mixture was stirred at Ϫ70 ЊC for a further 30 min. To
the reaction mixture was quickly added tributyl borate (2.76 g,
12 mmol) by syringe, and the reaction mixture was stirred at
room temperature for 15 h. To the reaction mixture was added
CH2Cl2 (30 ml) and HCl (15 ml, 3 M). The phases were separ-
ated and the organic phase was extracted with 15 ml of 10%
NaOH aqueous solution three times. The combined aqueous
phase was acidified with 10% HCl. The gray precipitate was
collected by filtration and washed with water. Drying of the
white powder in vacuum gave 1.34 g of compound 1 in yield of
In the presence of Zn2ϩ, the complex emitted at 463 nm,
which is an inactive wavelength for photo-recyclization despite
the slight overlap with the UV–Vis absorption spectrum of 2a.
The photochromic compound 2 in the presence of metal ion as
optical switch exhibits a light-triggered fluorescence on excit-
ation of absorption bands that have only a minimal effect on
the switching process itself. In the presence of Zn2ϩ the emission
intensity of the complex can be modulated by photochemically
switching between 2a and 2b, which provided a non-destructive
readout method. Irradiation with 570 nm regenerated the open
ring form and restored the luminescence to near its original
value. A very important property of the dithienylcyclopentene
switches is the thermal irreversibility of the photochemical
cyclization.1,21 For compound 2 in this work it was found that
the closed form shows excellent thermal stability up to 80 ЊC for
several hours.
1
63.8%. H NMR (500 MHz, CDCl3, ppm): 8.02(s, 4H, –OH),
7.42(s, 2H, thienyl C–H), 2.75(t, 4H, –CH2–), 2.02(m, 2H,
–CH2–), 1.76(s, 6H, –CH3).
1,2-Bis[2-methyl-5-(4-pyridyl)-3-thienyl]cyclopentene (2)
To solid 4-bromopyridinium hydrochloride (0.5 g, 1.45 mmol)
in dioxane under Ar was added a 1 M aqueous Na2CO3 solu-
tion (8 ml). The mixture was stirred and heated to 55 ЊC. The
catalyst Pd(PPh3)4 was added followed by the addition of a
solution of compound 1 (0.62 g, 3.19 mmol) in dioxane by
cannula. After being stirred at 60 ЊC for 6 h, the mixture was
hydrolyzed with water and extracted with CH2Cl2. The com-
bined extracts were washed with water twice, dried with MgSO4
and concentrated. The residue was purified by chromatography
on silica gel with hexane–EtOAc (1 : 1) to yield 140 mg (23%) of
Conclusion
1
In summary, the photochromic processes of a typical pyridine-
tethered 1,2-bisthienylethene (BTE) with or without zinc ion
were observed. The photochromism was strongly enhanced by
the addition of Zn2ϩ and the photochromic reaction proceeded
effectively from the open to the closed-ring form. The change of
luminescence of the pyridine-tethered BTE upon the addition
of metal ion holds promise for the application to non-destruc-
tive readout of optical memory media and optical switching.
compound 2 as a white powder. H NMR (500 MHz, CDCl3,
ppm): 8.52(d, 4H, pyridyl α-H), 7.33(d, 4H, pyridyl β-H),
7.22(s, 2H, thienyl C–H), 2.85(t, 4H, –CH2–), 2.12(m, 2H,
–CH2–), 2.02(s, 6H, –CH3). HR-MS: calculated for (C25H22-
N2S2) 414.58, found: 414.1. Element analysis: Calc. for C25
H22N2S2: C 72.46, H 5.31, N 6.76; Found: C 72.38, H 5.28,
N 6.72%.
Acknowledgements
Experimental
This work is supported by NSFC/China and Shanghai Educa-
tion Committee.
The synthesis of 1,2-bis(5-chloro-2-methyl-3-thienyl)cyclo-
pentene (compound 3 shown in Scheme 2) was based on the
literature method.21 The synthesis of the 1,2-bis[2-methyl-5-(4-
pydidyl)-3-thienyl]cyclopentene is shown in Scheme 2, the start-
ing material 1,2-bis(5-chloro-2-methyl-3-thienyl)cyclopentene
was lithiated with n-BuLi in THF at Ϫ78ЊC, then treated with
B(OBu)3 to provide the bis(boronic acid) 1. The bis(boronic
acid) 1 was used in Suzuki coupling reaction22 with 4-bromo-
pyridinium hydrochloride to provide compound 2 shown in
Scheme 2. 1H NMR spectra were recorded on a Bruker AM-500
spectrometer. UV–Vis spectra were recorded on Varian Cary
500. IR spectra were recorded on a Nicolet FT-IR20SX. photo-
luminescent spectra were recorded on a Varian Cary Eclipse.
Elemental analysis data were obtained on a Perkin Elmer 240c
instrument. Mass spectra and TOF-mass spectra were obtained
at 70 eV on a VG 12–250 (VG Mass lab) and Mariner API TOF
spectrometers (time of flight, TIS ion source, PE Corp.). All
solvents were purified and dried. All reactions were performed
in flame-dried glassware under Ar.
Notes and references
1 (a) M. Irie, Chem. Rev., 2000, 100, 1685; (b) B. L. Feringa, Molecular
Switches, Wiley-VCH, Weinheim, 2001.
2 (a) B. Z. Chen, M. Wang, Y. Wu and H. Tian, Chem. Commun.,
2002, 1060; (b) H. Tian, B. Z. Chen, H. Tu and K. Müllen,
Adv. Mater., 2002, 14, 918; (c) Q. F. Luo, B. Z. Chen, M. Z. Wang
and H. Tian, Adv. Fun. Mater., 2003, 13, 233.
3 K. Matsuda, K. Takayama and M. Irie, Chem. Commun., 2001,
363.
4 J. H. K. K. Hirschberg, L. Brunsveld, A. Ramzi, J. A. Vekemans,
R. P. Sijbesma and E. W. Meijer, Nature, 2000, 407, 167.
5 M. Irie, S. Kobatake and M. Horichi, Science, 2001, 291,
1769.
6 (a) S. L. Gilat, S. H. Kawai and J. M. Lehn, Chem. Eur. J., 1995, 1,
275; (b) J. C. Owrutsky, H. H. Nelson, A. P. Baronavski, O.-K. Kim,
G. H. Tsivgoulis, S. L. Gilat and J. M. Lehn, Chem. Phys. Lett.,
1998, 293, 555; (c) A. F. Acebes and J. M. Lehn, Chem. Eur. J., 1999,
5, 3285.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 1 8 7 – 2 1 9 1
2190

View Article Online
7 (a) M. Munakata, L. P. Wu, T. Kuroda-Sowa, M. Maekawa,
Y. Suenaga and K. Furuichi, J. Am. Chem. Soc., 1996, 118, 3305;
(b) M. Irie, K. Uchida and T. Eriguchi, Bull. Chem. Soc. Jpn., 1998,
71, 985.
8 (a) M. Irie and K. Uchida, Chem. Lett., 1995, 899; (b) T. Yamada
and S. Kobatake, J. Am. Chem. Soc., 2000, 122, 1589; (c) T. Yamada
and S. Kobatake, Bull. Chem. Soc. Jpn., 2000, 73, 2179.
9 K. Uchida, M. Saito, A. Murakami, S. Nakamura and M. Irie,
Adv. Mater., 2003, 15, 121.
10 M. Takeshita, C. N. Choi and M. Irie, Chem. Commun., 1997, 2265.
11 A. Osuka, D. Fujikane and M. Irie, J. Org. Chem., 2001, 66, 3913.
12 (a) L. N. Lucas, J. H. van Esch, R. M. Kellogg and B. L. Feringa,
Chem. Commun., 1998, 2313; (b) L. N. Lucas, J. V. Esch, R. M.
Kellogg and B. L. Feringa, Chem. Commun., 2001, 759.
13 G. Benkovic, V. Krongauz and V. Weiss, Chem. Rev., 2000, 100,
1741.
14 T. Mrozek, H. Görner and J. Daub, Chem. Eur. J., 2001, 7, 1028.
15 M. Irie, T. Fukaminato, T. Sasaki, N. Tamai and T. Kawai, Nature,
2002, 420, 759.
17 M. Irie, O. Miyatake, K. Uchida and T. Eriguchi, J. Am. Chem. Soc.,
1994, 116, 9894.
18 M. Hanazawa, R. Sumiya, Y. Horikawa and M. Irie, J. Chem. Soc.,
Chem. Commun., 1992, 206.
19 M. Irie, Mol. Cryst. Liq. Cryst., 1993, 227, 263.
20 (a) M. Irie and M. Mohri, J. Org. Chem., 1988, 53, 803;
(b) S. Nakamura and M. Irie, J. Org. Chem., 1988, 53, 6136.
21 L. N. Lucas, J. J. D. Jong, J. H. van Esch, R. M. Kellogg and
B. L. Feringa, Eur. J. Org. Chem., 2003, 155.
22 M. Miyaura, T. Yanagi and A. Suzuki, Synth. Commun., 1981, 11,
513.
23 (a) K. Uchida, E. Tsuchida, Y. Aoi, S. Nakamura and M. Irie,
Chem. Lett., 1999, 63; (b) M. Frigoli and G. H. Mehl,
CHEMPHYSCHEM, 2003, 1, 101.
24 (a) Y. Yokohama, T. Goto, T. Inoue, M. Yokoyama and Y. Kurita,
Chem. Lett., 1988, 1049; (b) K. Yoshida, T. Koujiri, T. Horii and
Y. Kubo, Chem. Lett., 1987, 2057.
25 P. Tomasik and Z. Ratajewicz, Pyridine-Metal Complexes, John
Wiley & Sons Inc., 1985.
16 (a) T. B. Norsten and N. R. Branda, Adv. Mater., 2001, 13, 347;
(b) A. J. Myles and N. R. Branda, Adv. Funct. Mater., 2002, 12, 167;
(c) E. Murguly, T. B. Norsten and N. R. Branda, Angew. Chem., Int.
Ed., 2001, 40, 1752.
26 R. D. Chambers and R. Edwards, J. Chem. Soc., Perkin Trans. 1,
1997, 24, 3623.
27 Y.-B. Dong, M. D. Smith, R. C. Layland and H.-C. Loye,
Inorg. Chem., 1999, 38, 5027.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 1 8 7 – 2 1 9 1
2191