Modulation of color change and photocyclization of diarylethene with metal complex

Article abstract of DOI:10.1002/poc.1798

Modulation of color change and photocyclization of diarylethene with metal complex is described. A diarylethene derivative with 4-(ethylideneamino) phenol group 1a is prepared and photochromic behavior is investigated. It is found that 1a exhibits ring-opening and ring-closing photoisomerization with UV/Vis light irradiation in solution. Addition of Cu(OAc)₂ to the solution of 1a produces a metal complex 1a-Cu, which generates not only marked color change but also significant promotion in photocyclization. Copyright 2010 John Wiley & Sons, Ltd. Modulation of color change and photocyclization of diarylethene with metal complex has been demonstrated. Diarylethene-metal complex generates not only marked color change but also significant promotion in photocyclization, which may provide a simple and convenient way to modulate the color and the photocyclization of diarylethene system. Copyright

Full text of DOI:10.1002/poc.1798

Research Article
Received: 5 May 2010,
Revised: 11 August 2010,
Accepted: 25 August 2010,
Published online in Wiley Online Library: 22 October 2010
(wileyonlinelibrary.com) DOI 10.1002/poc.1798
Modulation of color change and
photocyclization of diarylethene with
metal complex
Huan-Huan Liua and Yi Chena
*
Modulation of color change and photocyclization of diarylethene with metal complex is described. A diarylethene
derivative with 4-(ethylideneamino) phenol group 1a is prepared and photochromic behavior is investigated. It is
found that 1a exhibits ring-opening and ring-closing photoisomerization with UV/Vis light irradiation in solution.
Addition of Cu(OAc)2 to the solution of 1a produces a metal complex 1a–Cu, which generates not only marked color
change but also significant promotion in photocyclization. Copyright ß 2010 John Wiley & Sons, Ltd.
Keywords: color modulation; diarylethene; metal complex; photochromism; photocyclization regulation
to the thiophene rings or by modifying the bridge unit,[42–44] and
another is to incorporate the dithienylethenes into a polymer
INTRODUCTION
Molecules that undergo reversible photoreactions between two
isomers with different UV–Vis absorption characteristics are
termed photochromic and the process is referred to as
photochromism.[1,2] Photochromic diarylethenes are one of the
most promising candidates[3–12] for photoelectronic applications
because of their thermo-irreversible and fatigue-resistant
photoisomerization performances. A lot of applications exist
for photochromic diarylethenes that take advantage of their
inherent ability to produce switches between two isomers by
UV–Vis phototrigger.[13–26] Basic requirements for photoswitch
are bistability, suitable wavelength, high efficiency of photo-
reaction, and fatigue resistance.
backbone.[45,46]
In this paper, we report a simple and efficient approach to
modulation the color change and to improvement of photo-
cyclization efficiency by diarylethene–metal complex. It is found
that by binding with metal ion, the color of closed isomer was
changed from blue to purple. Meanwhile, it is also found that the
photocyclization of open isomer to closed isomer was promoted
with metal complex. The chemical structure of the diarylethene
derivative 1a and its ring-opening and ring-closing photoisome-
rization is presented in Scheme 1.
Modulation of color change is one of interesting subject for
photochromic materials due to the applications such as displays
and inks. Both open isomers and closed isomers of diarylethenes
have distinct absorption.[27–31] In general, the open isomers have
absorption bands at UV region and are colorless, and the closed
isomers have absorption bands at visible light region and are
colored. Most diarylethenes show very large spectral shifts upon
photocyclization of open isomers to closed isomers due to
p-electrons delocalization throughout the two condensed
thiophene rings and further extension of the substituents.
Modulation of the absorption bands (color) of closed isomers are
usually due to the modification of the electron properties of
substituents[32–35] and the extent of conjugation linked to
thiophene rings.[36–38]
Improvement of photochemical reaction efficiency is import-
ant and hard subject for photochromic materials. A diarylethene
with five-membered heterocyclic rings has two conformations
with the two rings in mirror symmetry (parallel conformation) and
in C2 symmetry (anti-parallel conformation). The photocyclization
reaction can proceed only from the antiparallel confor-
mation.[39–41] In general, the population ratio of the two
conformations is 1:1; therefore, the photocyclization quantum
yield cannot exceed 0.5. There are two main approaches to
increase the yield of diarylethene photocyclization. One way is to
change diarylethene structure by introducing bulky substituents
EXPERIMENTAL
General Information
1H NMR spectrum was recorded at 400 MHz with TMS as an
internal reference and CDCl3 as solvent. MS spectra were
recorded with TOF-MS spectrometer. Absorption spectra were
measured with an absorption spectrophotometer (Hitachi
U-3010). All chemicals for synthesis were purchased from
commercial suppliers, and solvents were purified according to
standard procedures. The reaction was monitored by TLC silica
gel plates (60F-254). Column chromatography was performed on
silica gel (Merck, 70–230 mesh). A 365 nm lamp (36 W) and a
Xenon light (500 W), with different wavelength filters, were used
as light sources for photocoloration and photobleaching,
respectively.
*
Correspondence to: Y. Chen, Key Laboratory of Photochemical Conversion and
Optoelectronic Materials, Technical Institute of Physics and Chemistry, The
Chinese Academy of Sciences, Beijing 100190, China.
E-mail: yichencas@yahoo.com.cn
a
H.-H. Liu, Y. Chen
Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, The Chinese Academy of
Sciences, Beijing 100190, China
J. Phys. Org. Chem. 2011, 24 517–521
Copyright ß 2010 John Wiley & Sons, Ltd.

H.-H. LIU AND Y. CHEN
Scheme 1. Structure of diarylethene 1a and its ring-opening and ring-closing photoisomerization with UV/Vis light irradiation
Scheme 2. Synthesis of diarylethene 1a
Synthesis of diarylethene 1a
RESULTS AND DISCUSSION
Diarylethene 1a was prepared according to synthetic route
shown in Scheme 2, and the detailed procedures and spectra
data were as follows: a mixture of 3,4-bis (5-formyl-2 -methylthien
-3-yl)-2,5-dihydrothiophene[47] (100 mg, 0.3 mmol) and
4-aminophenol (72 mg, 0.66 mmol) in anhydrous ethanol
(10 ml) was refluxed. After no starting material was detected
by TLC plate, the mixture was cooled, and the yellow product was
filtered off, washed with EtOH. Pure diarylethene 1a was obtained
Upon irradiation with UV light, the absorption band
(lmax ¼ 353 nm, e ¼ 4.6 ꢀ 104, in CH3CN), which attributes to
the open-ring isomer 1a, decreased in intensity, and a new band
(lmax ¼ 590 nm), which corresponds to the closed-ring isomer 1b,
appeared at the same time (Fig. 1). The new band increased in
intensity with the irradiation time till the photostationary state
(PSS) was reached. This process was accompanied by color
change of solution from colorless to pale blue. The pale blue
solution was bleached back to colorless solution with visible light
1
after vacuum drying. Yield: 45% (70 mg). M.p. ¼ 210–2118C. H
NMR (400 MHz, DMSO-d6): 9.48 (s, 2H), 8.58 (s, 2H), 7.43 (s, 2H),
7.14 (d, 4H, J ¼ 8.7 Hz), 6.76 (d, 4H, J ¼ 8.7 Hz), 4.15 (s, 4H), 1.98 (s,
6H). 13C NMR: 156.2, 149.8, 141.9, 139.9, 139.5, 134.3, 133.0, 132.7,
122.5, 115.7, 42.1, 14.6. TOF-MS EI (m/z) calcd. For C28H24N2O2S3:
516.1000, found: 516.1000 (100%).
Figure 2. Absorption changes of 1a (25 mM, CH3CN) with the addition of
Cu(OAc)2 (50 mM). Addition of Cu (OAc)2 to the solution of 1a produced a
complex of Cu–1a with a new absorption band at 430 nm as shoulder. The
inset is tritration curve of 1a (25 mM, CH3CN) with the addition of
Cu(OAc)2. A 1:2 complex formation was determined from the absorption
spectra changes of 1a, where the absorption decreased linearly with
addition of Cu2þ up to 2 equiv
Figure 1. Absorption changes of 1a (25 mM, CH3CN) with 365 nm light
irradiation (irradiation time: 0, 2, 4, 6 min)
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 517–521

MODULATION OF COLOR CHANGE
Figure 3. (a) Absorption changes of 1b with addition of Cu2þ (concentration: 0, 13, 25, 38, 50 mM). (b) Photochromic bleaching of Cu–1b upon
irradiation at ꢁ450 nm (irradiation time: 0, 11, 22, 36, 54 min)
solution of 1a, only a small absorption changes were detected
(Fig. 2). The color change resulted from the complexation of 1b
with Cu2þ, and further study found that the absorption at 590 nm
was gradually blue-shifted to 560 and 495 nm, which corresponds
to complex of Cu–1b, with increasing concentrations of Cu2þ
(Fig. 3a). With visible light (ꢁ400 nm) irradiation, the purple
solution was bleached to colorless solution (Fig. 3b). The
coloration and bleaching could be recycled with UV/Vis light
irradiation.
Control experiment showed that the complex of Cu–1a
exhibited photochromic properties: Cu–1a converted to Cu–1b
upon irradiation with 365 nm light till photostationary state was
reached, and Cu–1b bleached back to Cu–1a with visible light
(l ꢁ 400 nm) irradiation. As presented in Fig. 4, the absorption at
353 nm, which corresponded to Cu–1a, decreased in intensity,
and two bands (lmax ¼ 560, 495 nm), attributed to Cu–1b,
appeared upon irradiation with 365 nm light. The absorption
Figure 4. Absorption changes of Cu–1a (25 mM, CH3CN) with 365 nm
light irradiation (irradiation time: 0, 2, 4, 6, 8, 10, 12, 14 min)
figure showed the same profile as Fig. 3(a), which confirmed that
the complex of Cu–1a was converted to Cu–1b with 365 nm light
irradiation. It is worth noting that the optical density of Cu–1b
(ꢁ400 nm) irradiation, and the coloration and decoloration can
be recycled with UV/Vis light irradiation.
Addition of Cu(OAc)2 (50 mM) to the solution of 1PSS (1a/1b)
produced a distinct color of solution: from pale blue to purple.
Control experiment showed that no significant color change was
observed when a solution of Cu(OAc)2 (50 mM) was added to a
(0.45) at 560 nm in Fig. 4 increased by more than 30% in
comparison with that of Cu–1b (0.34) in Fig. 3(a), which
suggested that the photocyclization of Cu–1a to Cu–1b was
probably promoted by the complexation of 1a with Cu2þ
.
To confirm the photocyclization of Cu–1a to Cu–1b was
promoted by the complexation of 1a with Cu2þ, the following
experiments were performed. First, upon irradiation with 365 nm
light, the solution color of Cu–1b resulted from Fig. 3(a), obtained
by addition of 50 mM of Cu2þ to 1PPS, darkened. Further
investigation found that the optical density of Cu–1PPS at 560
and 495 nm increased accompanying the decrease of optical
density at 353 nm (Fig. 5), and the largest optical density (0.43) at
560 nm was obtained at photostationary state, which is
agreement with the result (0.45) in Fig. 4. Second, the conversion
yield of Cu–1a to Cu–1b (58%) is larger than that of 1a to 1b
(26%) by calculating the ratio of proton in both open form and
closed form in 1H-NMR spectroscopy. All suggested that the
complexation of 1a with Cu2þ promoted the photocyclization of
Cu–1a to Cu–1b.
The binding site of 1a with Cu2þ was estimated by both NMR
1
spectroscopy and analogue. H NMR spectroscopy showed that
the signals arising from the aromatic protons (thiophen ring) shift
—
downfield by 0.05 ppm, and from imine protons (N CH) shift
—
upfield by 0.02 ppm, respectively. The signal of OH (d ¼ 9.5 ppm)
Figure 5. Absorption changes of Cu–1PPS (obtained from Figure 3a)
with 365 nm light irradiation (irradiation time: 0, 2, 4, 6, 8 min)
in phenol group, however, disappeared. These suggested that the
J. Phys. Org. Chem. 2011, 24 517–521
Copyright ß 2010 John Wiley & Sons, Ltd.
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H.-H. LIU AND Y. CHEN
Scheme 3. The structural of analogue 2a and its ring-opening and ring-closing photoisomerization with UV/Vis light irradiation
Figure 6. Absorption change of 1b with addition of Co2þ (a) and Mn2þ (b) (white: 1b, black: 1b-metal)
binding site may be at O atom. To further confirm the binding
site, an analogue 2a (Scheme 3) was prepared and investigated. It
was found that 2a showed ring-opening and ring-closing
photoisomerization with UV/Vis light irradiation, and accompa-
nying the photocyclization, the colorless solution of 2a was
changed to blue solution of 2b. Addition of Cu2þ to the solution
of 2PPS, no color change was observed. Further study found that
no change in both absorption spectra of 2a and 2b was detected
upon addition of Cu2þ. Results suggested that no complex was
formed between 2a or 2b with Cu2þ, and the binding site
occurred at O atom instead of at S and N atoms. The complex
Cu–1a is probably formed[48] by binding two Cu2þ with two
phenol units, which prefers to the photocyclization of diary-
lethene due to the increase of the anti-parallel confor-
mation.[49,50]
ion but also the photocyclization reaction is significantly
promoted with diarylethene-metal complex. This may provide
a simple and convenient way to modulate the color and the
photocyclization of diarylethene system.
Acknowledgements
This work was supported by the National Nature Science Founda-
tion of China (No. 60337020) and Beijing Natural Science Founda-
tion (No. 4082033).
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