Photochromic thiophene oligomers based on bisthienylethene: Syntheses, photochromic and two-photon properties

Article abstract of DOI:10.1039/b608545h

A series of novel, functional symmetric and unsymmetric photochromic thiophene oligomers based on bisthienylethenes (BTEs) were synthesized via control of the reaction condition with a Suzuki coupling method. Their optical, photochromic quantum yields, tw

Full text of DOI:10.1039/b608545h

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Photochromic thiophene oligomers based on bisthienylethene: syntheses,
photochromic and two-photon properties
Yanli Feng,a Yongli Yan,b Sheng Wang,a Weihong Zhu,a Shixiong Qianb and He Tian*a
Received 16th June 2006, Accepted 25th July 2006
First published as an Advance Article on the web 8th August 2006
DOI: 10.1039/b608545h
A series of novel, functional symmetric and unsymmetric photochromic thiophene oligomers
based on bisthienylethenes (BTEs) were synthesized via control of the reaction condition with
a Suzuki coupling method. Their optical, photochromic quantum yields, two-photon and
electrochemical properties in THF as well as in poly(methylmethacrylate) (PMMA) films were
measured and discussed. The fluorescence emission can be tuned reversibly by alternating
irradiation with UV and visible light. These BTEs could be used as nanowires, fluorescent switch
and electrochemical switch.
and the substituents on the absorption peaks as well as two-
Introduction
photon properties are also discussed. We expect that the
Photochromism is a reversible transformation between two
thiophene oligomers could work as molecular nanowires
isomers with different absorption spectra caused by alternating
with a photoswitch (BTE), fluorescent switch, electrochemical
irradiation with UV and visible light. Among the various types
switch or potential application for three dimensional optical
of photochromic compounds, bisthienylethene derivatives
data storage.
(BTEs) are one of the most promising materials because of
their excellent fatigue resistance and thermal stability in both
Experimental
isomeric forms, picosecond switching times, and fairly high
photocyclization quantum yields.1–4 The open- and closed-
General procedure
ring isomers of BTEs differ from each other not only in their
1H NMR spectra were recorded on a Bruker AM 500
absorption but also in various physical and chemical pro-
spectrometer with tetramethyl silane (TMS) as internal
perties, such as luminescence, refractive indices, oxidation/
reference. MS were recorded on EI or ESI mass spectroscopy.
reduction potentials, chiral properties and magnetic interac-
Absorption spectra were measured on a Varian Cary500 UV-
tions.5–10 Up to now, intense research efforts have been carried
Vis spectrophotometer. Fluorescence spectra were measured
out in an attempt to enhance their optical data storage and
on a Varian Cary Eclipse Fluorescence spectrophotometer.
optical signal processing.11
Fluorescence lifetimes were measured on an Edinburgh
Recently, thiophene oligomers have generated considerable
Lifespec-Ps spectrofluorometer. The optical switch experi-
attention for their possible applications in light-emitting
ments were carried out using a photochemical reaction
diodes, lasers, field-effect transistors (FETs), photovoltaic
apparatus with a 200 W Hg lamp. Cyclic voltammetry
cells12 and molecular nanowires13 because of their excellent
measurements were carried out at a platinum electrode using
electron and energy transfer properties through the p-con-
millimolar solutions in CH2Cl2 containing 0.1 M of the
jugation. Specifically, how to optically control such molecular
support electrolyte, tetrabutylammonium perchloride, in a
wires of thiophene oligomers by external stimuli becomes an
three electrode cell and potentiostat assembly by VersaStarII
important theme. On the basis of our earlier work,14–16 we
electrochemical analyzer. HPLC analyses were determined by
Agilen 1100 eluted by methanol at a flow rate of 0.6 ml min21
herein incorporated the photochromic BTE unit into thio-
.
phene oligomers via a Suzuki coupling reaction in order to
develop molecular wires with photo-switching characteristics.
The synthesized photochromic thiophene oligomers (BTEs)
with good solubility in common organic solvents could be
triggered between the colorless ring-open and colored ring-
closed forms by alternating irradiation with UV and visible
light in THF as well as in poly(methylmethacrylate) (PMMA)
films (Scheme 1). Moreover, the effect of thiophene numbers
Materials
Reagents and starting materials were used as received. Solvents
were distilled and dried before use. 1,2-Bis(5-chloro-2-
methylthien-3-yl)cyclopentene was synthesized and purified
according to the established procedures.16,17
Synthesis of BTE1, BTE2
aLaboratory for Advanced Materials and Institute of Fine Chemicals,
East China University of Science & Technology, Shanghai 200237,
P. R. China. E-mail: tianhe@ecust.edu.cn; Fax: +86 (0)21 64252288;
Tel: +86 (0)21 64252756
To the solution of 1,2-bis(5-chloro-2-methylthien-3-yl)cyclo-
pentene (0.50 g, 1.52 mmol) in anhydrous THF (10 mL) was
added n-BuLi (2.5 mL of 1.6 M solution in hexane, 4 mmol)
using a syringe in 2 portions under nitrogen at 278 uC. This
solution was stirred for 30 min at room temperature, then
bDepartment of Physics, Fudan University, Shanghai 200433,
P. R. China
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Scheme 1 Photochromic bisthienylethene–thiophene oligomers (BTEs).
B(OBu)3 (1.2 mL, 4.2 mmol) was added in one portion. This
reddish solution was stirred for 1 h at room temperature,
and was then used in the Suzuki cross coupling reaction
without any workup because the product is deboronized
during isolation.
mixture was poured into H2O and extracted with ether, and
dried with anhydrous Na2SO4. After concentration, the
compound was purified by column chromatography on silica
(petroleum ether–ethyl acetate = 25 : 2 v/v) to yield BTE3
(0.22 g), 19.6%, m.p. 136–138 uC.
A
mixture of 2-bromo-5-chlorothiophene (0.61 g,
1
BTE3. H NMR (500 MHz, CDCl3, ppm): d = 1.95 (s, 6H,
3.10 mmol), Pd(PPh3)4 (0.20 g) and THF (10 mL) was stirred
for 15 min at room temperature. Then aqueous Na2CO3
(10 mL, 2 M) was added. The reactive mixture was heated at a
temperature of 60 uC, and the solution of bis(boronic) esters
of BTE (compound 2) was added dropwise via a syringe.
Subsequently, the mixture was refluxed for 2 h and cooled to
room temperature. The reactive mixture was poured into H2O
and extracted with ether, and dried with anhydrous Na2SO4.
After concentration, the compound was purified by column
chromatography on silica (petroleum ether–ethyl acetate =
20 : 1 v/v) to yield BTE2 (0.33 g), yield 44.1%, m.p. 101–103 uC.
To a solution of 1,2-bis(5-chloro-2-methylthien-3-yl) cyclo-
pentene (0.50 g, 1.52 mmol) in anydrous THF (10 ml) was
added n-BuLi (1 ml of 1.6 M solution in hexane, 1.6 mmol) in 2
portions using a syringe under nitrogen at 278 uC. At the same
reaction conditions with BTE2, a solution of the unsymmetric
bis(boronic) esters of BTE (compound 1) was obtained. Then a
mixture of 2-bromo-5-chlorothiophene (0.30 g, 1.52 mmol),
Pd(PPh3)4 (0.10 g) was added, and BTE1 (0.21 g) was similarly
obtained (petroleum ether–ethyl acetate = 25 : 2 v/v) with a
yield of 34.2%, m.p. 95–96 uC.
–CH3), 2.09 (m, 2H, –CH2–), 2.81 (t, J = 7.4 Hz, 4H,
–CH2CLCCH2–), 6.88 (m, 4H, thiophene-H), 6.94–6.98 (m,
6 H, thiophene-H), HRMS (m/z) [M+] calcd for C31H22S6Br2:
743.8413, found: 743.8393.
Synthesis of BTE4 and BTE5
To the solution of BTE2 (0.52 g, 1.05 mmol) in anhydrous
THF (10 mL) was added n-BuLi (0.78 mL of 1.6 M solution in
hexane, 1.25 mol) using a syringe in 2 portions under nitrogen
at 278 uC. This solution was stirred for 30 min at room
temperature, then B(OBu)3 (0.40 mL, 1.26 mmol) was added
in one portion. This reddish solution was stirred for 1 h at
room temperature, and was then used in the Suzuki cross
coupling reaction without any workup because the product is
deboronized during isolation.
The bis(boronic) esters of BTE2 Suzuki coupling with
4-iodoanisole and 4-iodobenzaldehyde gave the corresponding
compounds BTE4 and BTE5, respectively. The only difference
was that the reactant was purified by column chromatography
on silica (petroleum ether–CH2Cl2 = 25 : 4 v/v) to yield a dark-
green solid BTE4 (0.22 g, yield 38.1%, m.p. 123–125 uC), and
purified by column chromatography on silica (petroleum
ether–CH2Cl2 = 25 : 2 v/v) to yield a green solid BTE5
(0.24 g, yield 41.2%, m.p. 108–109 uC).
1
BTE1. H NMR (500 MHz, CDCl3, ppm): d = 1.87 (s, 3H,
–CH3), 1.94 (s, 3H, –CH3), 2.01–2.07 (m, 2H, –CH2–), 2.71–
2.77 (m, 4H, –CH2CLCCH2–), 6.59 (s, 1H, thiophene-H), 6.76
(d, J = 1.7 Hz, 1H, thiophene-H), 6.77 (s, 1H, thiophene-H),
6.79 (d, J = 3.9 Hz, 1H, thiophene-H). HRMS (m/z) [M+] calcd
for C19H16S3Cl2: 409.9791, found: 409.9755.
1
BTE4. H NMR (500 MHz, CDCl3, ppm): d = 1.95 (s, 3H,
–CH3), 1.96 (s, 3H, –CH3), 2.09 (m, 2H, –CH2–), 2.82 (m, 4H,
–CH2CLCCH2–), 3.84 (s, 3H, –OCH3), 6.80 (d, J = 3.7 Hz,
1H, thiophene-H), 6.81 (s, 1H, thiophene-H), 6.88 (s, 1H,
thiophene-H), 6.91 (m, 3H, thiophene-H, benzene-H), 6.99 (d,
J = 3.8 Hz, 1H, thiophene-H), 7.07 (d, 1H, thiophene-H, J =
3.6 Hz), 7.51 (d, J = 8.7 Hz, 2H, benzene-H). HRMS (m/z)
[M++H] calcd for C30H26S4OCl: 565.0555, found: 565.0525.
BTE2. 1H NMR (500 MHz, CDCl3, ppm): d = 1.99 (s,
6H, CH3), 2.07 (m, 2H, –CH2), 2.81 (t, J = 7.4 Hz, 4H,
–CH2CLCCH2–), 6.78–6.80 (m, 6H, thiophene-H). HRMS
(m/z) [M+] calcd for C23H18S4Cl2: 491.9668, found: 491.9670.
Synthesis of BTE3
1
BTE5. H NMR (500 MHz, CDCl3, ppm): d = 1.97 (s, 3H,
A
mixture of 5,59-dibromo-2,29-bisthiophene (0.99 g,
3.10 mmol), Pd(PPh3)4 (0.20 g) and THF (10 mL) was stirred
for 15 min at room temperature. Then aqueous Na2CO3
(10 mL, 2 M) was added. The reactive mixture was heated at a
temperature of 60 uC and the solution of compound 2 was
added dropwise via a syringe. Subsequently, the mixture was
refluxed for 2 h and cooled to room temperature. The reactive
–CH3), 1.99 (s, 3H, –CH3), 2.06–2.09 (m, 2H, –CH2–), 2.82–
2.97 (m, 4H, –CH2CLCCH2–), 6.78 (d, 1H, thiophene-H,
J = 3.6 Hz), 6.8 (s, 1H, thiophene-H), 6.81 (d, J = 3.8, 1H,
thiophene-H), 6.84 (s, 1H, thiophene-H), 7.01 (s, 1H,
thiophene-H), 7.1 (s, 1H, thiophene-H), 7.74–7.76 (d, J =
8.2 Hz, 2H, benzene-H,), 7.96–7.94 (d, 2H, benzene-H,
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J = 8.1 Hz), 10.05 (s, 1H, –CHO). HRMS (m/z) [M+] calcd for
–CH2CLCCH2–), 4.86 (b, 1H, –OH), 6.8 (d, J = 4.5 Hz, 1H,
thiophene-H), 6.81 (s, 1H, thiophene-H), 6.84 (d, J = 8.3 Hz,
2H, benzene-H), 6.87 (s, 1H, thiophene -H), 6.90 (d, 1H,
thiophene-H, J = 4.4 Hz), 6.99 (d, 1H, thiophene-H, J =
3.5 Hz), 7.06 (d, J = 3.4 Hz, 1H, thiophene-H), 7.46 (d, J =
8.5 Hz, 2H, benzene-H). HRMS (m/z) [M+] calcd for
C29H23S4OCl: 550.0320, found: 550.0322.
C30H25S4OCl: 562.0320, found: 562.0325.
Synthesis of BTE6
To a solution of BTE4 (83.5 mg, 0.15 mmol) in anhydrous
CH2Cl2 (20 mL) was added BBr3 (1 mL) using a syringe under
nitrogen at 278 uC. This solution was stirred for 2 h, then
poured into H2O and extracted with ether. The organic layer
was separated and dried with Na2SO4. After concentration, the
compound was purified by column chromatography on silica
(CH2Cl2–n-hexane = 1 : 1 v/v) to yield a purple-green solid
BTE6 (0.054 g, 65.1% yield, m.p. 111–113 uC).
Synthesis of BTE7 (shown in Scheme 2)
A mixture of BTE5 (0.1038 g, 0.18 mmol), 2-aminophenyl
disulfide (0.0223 g, 0.09 mol) in anhydrous CH2Cl2
(20 mL) was stirred for 20 h at room temperature. After
concentration, the compound was purified by column
chromatography on silica (CCl4–CH2Cl2 = 1 : 1 v/v) to
1H NMR (500 MHz, CDCl3, ppm): d = 1.95 (s, 3H, –CH3),
1.97 (s, 3H, –CH3), 2.04–2.08 (m, 2H, –CH2–), 2.82 (m, 4H,
Scheme 2 Routes for synthesizing BTEs. Reagents: (i) n-BuLi (1 eq.), B(OBu)3, THF, 278 uC; (ii) n-BuLi (2 eq.), B(OBu3), THF, 278 uC; (iii) 2 M
Na2CO3, Pd(PPh3)4, 2-bromo-5-chlorothiophene, THF, 60 uC; (iv) 2 M Na2CO3, Pd(PPh3)4, 5,59-dibromo-2,29-bisthiophene, THF, 60 uC; (v) 2 M
Na2CO3, Pd(PPh3)4, 4-iodoanisole, THF, 60 uC; (vi) 2 M Na2CO3, Pd(PPh3)4, 4-iodobenzaldehyde, THF, 60 uC; (vii) BBr3, CH2Cl2, 50 uC. (viii)
CH2Cl2, 2-aminophenyl disulfide, r. t.
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yield a purple-green solid BTE7 (0.013 g), yield 11.3%, m.p.
151–153 uC.
1
BTE7. H NMR (500 MHz, CDCl3, ppm): d = 1.97 (s, 6H,
CH3), 1.98 (s, 6H, CH3), 2.04–2.08 (m, 4H, CH2), 2.82 (m,
8H, –CH2CLCCH2–), 6.77 (m, 8H, thiophene-H), 6.81 (s, 2H,
thiophene-H), 7.05 (s, 2H, thiophene -H), 7.26 (s, 2H,
–NLCH–), 7.41 (m, 2H, benzene-H), 7.51 (m, 2H, benzene-
H), 7.72 (m, 4H, benzene-H), 7.92 (2H, benzene-H), 8.10 (m,
2H, benzene-H), 8.17 (d, J = 8.4 Hz, 4H, benzene-H). HRMS
(m/z) [M+] calcd for C72H54S10N2Cl2: 1336.0871, found:
1336.0869.
Results and discussion
Fig. 1 Absorption changes of BTE4 in THF (1.0 6 1025 mol L21
under different irradiation times at 365 nm.
)
Molecular design and synthesis
Usually, Suzuki coupling method is used to synthesize
symmetric bisthienylethene derivatives. However, here BTE1,
BTE4, BTE5 and BTE6 are structurally unsymmetric and are
obtained by controlling the ratio of starting materials and
reagents.18–20
state (PSS) after about 20 min. Especially, the colored isomers
(closed forms) of these compounds are very stable in the dark
at room temperature. Irradiation of them in their photo-
stationary state with visible (.540 nm) light could lead to a
complete recovery of the initial absorption signal, and the
purple or blue solution turning colorless. Photochromism of
BTE2 in poly(methylmethacrylate) (PMMA) films was also
investigated, which was almost the same with spectral changes
of BTE2 in THF (Fig. 2). When irradiated by 365 nm, the
PMMA film turned gradually purple and new absorption
bands at 381 and 548 nm appeared due to the large
p-conjugation, and attained the photo-stationary state at
40 min, which is twice as long as that of BTE2 in THF.
The absorption data of BTEs in THF for the open and
closed forms are summarized in Table 1. Compared with the
known bisthienylethene derivatives,19 all oligothiophenes with
a BTE unit showed the characteristic photochromic properties
of bisthienylethene. From the viewpoint of application to
optical memory media, it is desirable to develop photochromic
compounds that have sensitivity in the wavelength region of
530–650 nm (DVD).20
1,2-Bis(5-chloro-2-methylthien-3-yl)cyclopentene
(com-
pound 1) was converted into the bis(boronic) esters 2 via
n-BuLi–B(OBu)3, and then directly used in the next Suzuki
reaction without any workup because during isolation the
bis(boronic) esters are easily deboronized. Suzuki coupling
with 2-bromo-5-chlorothiophene gave the corresponding sym-
metric BTE2 and unsymmetric BTE1 by controlling the ratio
of starting materials and reagents (Scheme 2). And treatment
of bis(boronic) esters 2 with 5,59-dibromo-2,29bithiophene got
BTE3. BTE2 was converted into the bis(boronic) esters 3 via
n-BuLi–B(OBu)3, then Suzuki coupling with 4-iodoanisole
and 4-iodobenzaldehyde gave the corresponding compounds
BTE4 and BTE5. BTE6 was obtained from BTE4 in yield up
to 89% and BTE7 was got from BTE5.
Absorption spectra
All synthesized bisthienylethene substituted with oligothio-
phene units could readily dissolve in common organic
solvents, such as chloroform, methylene chloride, THF and
toluene, which overcame the insolubility of most unsubstituted
thiophene oligomers.18
The effect of the thiophene number on the absorption
maximum was examined. When increasing the number of
thiophene units, the absorption peak in THF was red shifted
Such thiophene oligomers show typical photochromism.
The THF solution of BTE2 gradually turned purple upon light
irradiation at 365 nm, with a new absorption band at 548 nm
arising from the ring-closed form of bisthienylethene. From
the absorption spectrum, two isobestic points could be found
at 358 and 315 nm, indicative of two existing isomers. A
similar result was also obtained from Fig. 1 when unsymmetric
BTE4 was irradiated at 365 nm, and the two new absorption
bands appeared at 398 and 558 nm, respectively, which were
ascribed to the closed form isomer.
The absorption spectral changes of BTE1, BTE3, BTE4,
BTE5 and BTE6 are almost the same as the spectral changes of
BTE2 and BTE4 no matter whether the compounds are
symmetric or unsymmetric. Upon irradiation with 365 nm
light, the solution of thiophene oligomers with a BTE unit in
THF gradually coloured and attained the photo-stationary
Fig. 2 Absorption changes of PMMA film containing 1 wt% BTE2
under different irradiation times at 365 nm.
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Table 1 Absorption data, the photochromic quantum yields (W) of thiophene oligomers and ratios of ring-open and closed forms at the
photostationary state (PSS).
Photochromic quantum yields (THF)
lAmbax/nm (e/104, THF)
(open)
lAmbax/nm (e/104, THF)
(PSS)
Ratios (PSS, methanol)
(open : closed)
Compounds
Wo-c (lmax/nm)
Wc-o (lmax/nm)
BTE1
BTE2
BTE3
BTE4
BTE5
BTE6
BTE7
278 (9.5)
333 (7.7)
381 (12.7)
350 (11.5)
283 (12.4), 342 (9.5)
349 (11.8)
330 (9.0)
503 (6.7) 313 (10.5)
548 (5.5) 379 (3.0)
593 (7.9)
558 (9.1) 398 (7.2)
553 (7.1) 384 (4.3)
557 (7.7) 396 (7.5)
553 (4.3) 380 (2.9)
0.27 (278)
0.29 (333)
0.37 (381)
0.34 (334)
0.25 (342)
0.27 (349)
0.11 (330)
0.45 (503)
0.28 (548)
—
0.42 (558)
—
1 : 0.80
1 : 0.91
1 : 1.20
1 : 1.10
1 : 0.75
1 : 0.83
1 : 0.33
0.31 (557)
0.23 (553)
because of the larger p-conjugation system. For example, the
absorption of BTE1 is red shifted by as much as 38 nm for
ring-open isomer and 59 nm for ring-closed isomer in
comparison with that reported for bisthienylethene.19,21
Further red shifts of the absorption of BTE2 (containing four
thiophene units) and BTE3 (containing six thiophene units)
were similarly observed (Table 1). It is also interesting to
compare the absorption peaks of the ring-open isomer and
ring-closed isomer of symmetric and unsymmetric compounds
such as BTE1 and BTE2 (Table 1). The thiophene ring is
rich-electronic and can contribute electron to the p-system for
the inductive effect of S atom.22 The absorption maximum of
BTE2 was red shifted by as much as 55 nm for ring-open
isomer and 45 nm for ring-closed isomer compared with that
of BTE1 in THF.
eluted by methanol at a flow rate of 0.6 ml min21 are 10.5 and
11.0 min for the ring-open and closed forms of BTE1, respec-
tively. Thus the ratio between the ring-open and closed forms
can be obtained as 1 : 0.80 from the ratio of the integrated area
of HPLC peaks. Similarly, the ratios of other ring-open and
closed forms in the photo-stationary state were listed in
Table 1. Notably, HPLC is a convenient way to measure the
ratio since the ring-open and closed forms of the BTEs are
difficult to be separated by silica column chromatography.
Fluorescence spectra
Typical fluorescence spectral changes of the novel thiophene
oligomers were shown in the photochromic reaction. The open
form of BTE2 (Fig. 4) shows significant fluorescence intensity
located at 398 nm upon excitation at 333 nm. A similar result
was obtained from Fig. 5 when BTE4 was excited at 350 nm,
and the significant fluorescence intensity appeared at 409 and
430 nm. After irradiation at 365 nm to reach the photosta-
tionary state, the fluorescence intensities of BTE2 and BTE4 in
THF decreased by 82 and 90%, respectively. Moreover, in the
case of BTE2 doped in the matrix of PMMA, irradiated at
365 nm to reach the photostationary state, the fluorescence
intensity was decreased by 86% (Fig. 6), a very similar
fluorescence quenching as that of BTE2 in THF. This is
attributed to the close form as a fluorescence quencher.4,8,23
Clearly, the fluorescence intensity can be regulated by the
photochromic reaction.
The substituent effect on the absorption maximum in THF
was also studied. When the substituents at the 5- and
59- position of the thiophene ring are strong electron-donating
or electron-withdrawing groups, the corresponding absorption
of the ring-open isomer and the ring-closed isomer are
obviously different. The absorption peaks of BTE5 is red
shifted both for ring-open isomer and ring-closed isomer with
respect to that of BTE2 in THF. Larger absorption differences
of BTE4 and BTE6 were also observed (Table 1).
When we set the isosbestic absorption point as the detecting
wavelength, the ratio between the ring-open and closed forms
is equal to the ratio of the integrated area of HPLC peaks. As
an example (illustrated in Fig. 3) the HPLC retention times
The fluorescence lifetimes of all oligothiophene BTEs
(open forms) were obtained by an interactive non-linear
Fig. 3 HPLC traces of BTE1 (in methanol, 2.0 6 1025 mol L21
)
eluted by methanol at a flow rate of 0.6 ml min21 detected at the
isosbestic wavelength of 332 nm: (a) before UV light irradiation, (b)
under UV light irradiation of 365 nm until the photostationary state
is reached.
Fig. 4 Fluorescence changes of BTE2 in THF (1.0 6 1025 mol L21
lex = 350 nm) under different irradiation times at 365 nm.
,
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Fig. 7 Modulated emission intensity at a peak of 432 nm (excited at
350 nm) of BTE6 in THF during alternating irradiation at the
wavelengths of 365 nm and .540 nm, respectively.
Fig. 5 Fluorescence changes of BTE4 in THF (1.061025 mol L21
lex = 350 nm) under different irradiation times at 365 nm.
,
Fig. 8 Linear scan of BTE2 in dichloromethane containing tetra-
butylammonium tetrafluorobrate (0.1 mol L21) at the scan rate of
100 mV s21
.
Electrochemical properties
Fig. 6 Fluorescence changes (lex = 333 nm) of BTE2 in PMMA film
(1 wt%) under different irradiation times at 365 nm.
The electrochemical studies were performed on BTE2 and
BTE3 by VersaStar II electrochemical analyzer. Fig. 8 shows
linear scan of BTE2 in dichloromethane containing tetra-
butylammonium tetrafluorobrate (0.1 M). The open-ring
isomer of BTE2 shows two oxidation potentials at 1.46 and
1.30 V. After 1 h irradiation with 365 nm to reach the
photostationary state, the oxidation potentials were shifted to
0.84 and 0.60 V, indicating that there exist the oxidation
potential shifts by 0.62 and 0.7 V from the open form to the
closed form for BTE2. Similarly (Fig. 9), the oxidation
potential of BTE3 was shifted by 0.32 V from the open form
(0.98 V) to the closed form (0.66 V). It clearly indicates that the
communication extending throughout the thiophene unit of
deconvolution fitting procedure and the fluorescent lifetimes
were well fitted with a single exponential (Table 2). For
example, the fluorescence of open form of BTE2 in THF
decays with a fluorescence lifetime of 8.69 ns (excited at
372 nm).
Notably, irradiation of all compounds with visible light
(.540 nm) could lead to a complete recovery of the initial
fluorescence signal. Fig. 7 shows that the fluorescence intensity
of BTE6 at 432 nm changed reversibly upon a excitation of
330 nm by alternating irradiation with UV (365 nm) and
visible (.540 nm) light. This cycle could be repeated more
than 20 times, indicative of being potentially utilized as good
photochromic flurorescence switches.
Table 2 Emission wavelengths (lemmax/nm) and fluorescent lifetimes
(t/ns) of the open forms of BTEs
Compounds
lemmax/nm
lemmax/nm (PSS)
t/ns (chi-squared)
BTE1
BTE2
BTE3
BTE4
BTE5
BTE6
BTE7
395
398
432, 453
409, 431
467
394.6
411
423, 459
410, 433
453.5
—
8.69 (1.054)
7.13 (1.033)
5.09 (1.029)
7.34 (1.039)
5.05 (1.068)
—
Fig. 9 Linear scan of BTE3 in dichloromethane containing tetra-
butylammonium tetrafluorobrate (0.1 mol L21) at the scan rate of
410, 432
459
411, 432
462
100 mV s21
.
3690 | J. Mater. Chem., 2006, 16, 3685–3692
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Fig. 10 Open-aperture Z-scan trace of BTE3. Note: the scattered
circles are experimental data, the line curve is a theoretical fit.
Fig. 12 Two-photon fluorescent spectra of BTE7 in THF. Inset: the
fluorescence intensity is linearly dependent on the square of excitation
intensity.
the ring-closed isomer is more than that of the ring-open
isomer, resulting in an easier oxidation to the closed form.
Moreover, such an electrochemical switch via light stimuli can
be reversibly controlled. After irradiation (.540 nm), the
colored solution turned colourless, and the electrochemical
properties changed to those characteristic of the ring-open
isomers.
Conclusions
In summary, a series of novel photochromic thiophene
oligomers with a BTE unit were successfully synthesized via
a Suzuki coupling method. Some structural symmetric or
unsymmetric bisthienylethene derivatives with a BTE unit
could be obtained by controlling the ratios of the starting
materials and reagents. All thiophene oligomers with a BTE
unit show good photochromic properties, excellent fatigue
resistance, excellent solubility in the pristine state and
potentially good charge carrier mobilities. Some of the
compounds have switchable electrochemical and two-photon
properties, which could be used as molecular wires with a
photoswitch (BTE), fluorescent switch, electrochemical switch
or potential application for three-dimensional optical data
storage. The further investigation of the applications is
currently in progress.
Two-photon properties
Synthesis of highly active organic two-photon materials is of
considerable interest due to their potential applications in
three-dimensional optical storage, two-photon laser scanning
fluorescence microscopy, two-photon upconverted lasing, and
two-photon optical power limiting.24–27 As an example, a two-
photon absorption (TPA) cross section of BTE3 was
determined by the femtosecond open-aperture Z-scan techni-
que, according to a previously described method.28 Fig. 10
shows the open-aperture Z-scan data of BTE3 with a TPA
coefficient (b) of 9.23 6 10211 cm W21 and a TPA cross
sections of 181GM at 800 nm.
Upon excitation at 800 nm with a 140 fs pulse (Fig. 11),
BTE2 and BTE6 showed yellow–green fluorescence, while
BTE3, BTE5 and BTE7 showed blue fluorescence. BTE7 in
THF exhibited intense frequency up-converted fluorescence
with the peak located at 475 nm, which could be observed
under excitation with an unfocused pulse with and energy of
several mJ. As illustrated in Fig. 12, the two-photon excited
fluorescence (TPEF) spectra of BTE7 under a different laser
intensity is linearly dependent on the square of the excitation
intensity. Notably, the two-photon fluorescence peak of BET7
in THF is red-shifted by 16 nm with respect to that of single-
photon fluorescence in THF (Table 2), which might be
explained by a reabsorption effect.29
Acknowledgements
This work was supported by the National Foundation of
China (90401026 and 20476027) and the Scientific Committee
of Shanghai. W. H. Zhu thanks the foundation for National
Excellent Dissertation of P. R. China (Project No.: 200143)
and the Laboratory of Organic Solids, Institute of Chemistry
(CAS/China).
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