Solvent dependent isomerization of photochromic dithienylethenes: Synthesis, photochromism, and self-assembly

Article abstract of DOI:10.1039/c6ra21348k

A series of photochromic dithienylethene incorporated with phenol (DTE1), catechol (DTE2), azophenol (DTE3) groups was synthesized and characterized by means of NMR spectroscopy, mass spectrometry and elemental analysis. The photophysical, photochromic and photoisomerization properties of molecules were studied using absorption and emission spectroscopies in different solvents. The cis-trans photoisomerizations of azophenols in DTE3 compete with photocyclization of dithienylethene, resulting in low photoconversion yields in DTE3. Thermal isomerization of cis-azophenols was found to be solvent-dependent-a fast thermal relaxation from cis- to trans-isomer in chloroform, and a slow process in THF. For example in chloroform, formation of a closed-ring dithienylethene with trans-azophenols (trans-DTE3_closed) was observed upon irradiation with UV light (365 nm). On the other hand, closed-ring dithienylethene with cis-azophenols (cis-DTE3_closed) was formed in THF under the same condition. After forming a complex with Fe³⁺ ions, DTE2 showed a red shift of 27 nm in the absorption maximum. Scanning electron microscopy (SEM) analysis revealed formation of circular nanostructures with diameters in the range of 300-600 nm from DTE2_closed film. Factors such as solvent, photoisomerization, and hydrogen bonding toward the formation of such supramolecular nanostructures and morphologies are discussed.

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ARTICLE
Solvent Dependent Isomerization of Photochromic
Dithienylethenes: Synthesis, Photochromism, and Self-assembly
Received 00th January 20xx,
Accepted 00th January 20xx
Choong Ping Sen, and Suresh Valiyaveettil*
DOI: 10.1039/x0xx00000x
A series of photochromic dithienylethene incorporated with phenol (DTE1), catechol (DTE2), azophenol (DTE3) groups was
synthesized and characterized by means of NMR spectroscopy, mass spectrometry and elemental analysis. The
photophysical, photochromic and photoisomerization properties of molecules were studied using absorption and emission
spectroscopies in different solvents. The cis-trans photoisomerizations of azophenols in DTE3 compete with
photocyclization of dithienylethene, resulting in low photoconversion yields in DTE3. Thermal isomerization of cis-
azophenols was found to be solvent-dependent – a fast thermal relaxation from cis- to trans- isomer in chloroform, and a
slow process in THF. For example in chloroform, formation of a closed-ring dithienylethene with trans- azophenols (trans-
DTE3closed) was observed upon irradiation with UV light (365 nm). On the other hand, closed-ring dithienylethene with cis-
azophenols (cis-DTE3closed) was formed in THF under the same condition. After forming a complex with Fe3+ ions, DTE2
showed a red shift of 27 nm in the absorption maximum. Scanning electron microscopy (SEM) analysis revealed formation
of circular nanostructures with diameters in the range of 300 – 600 nm from DTE2closed film. Factors such as solvent,
photoisomerization, and hydrogen bonding toward the formation of such supramolecular nanostructures and
morphologies are discussed.
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of open and closed isomeric forms.10-13 Therefore, it is
interesting to study the effect of azophenol group on the
overall photochromic properties of the system when the
azophenol group is conjugated to the dithienylethene. Studies
Introduction
Photoinduced changes in chemical and physical properties of
have shown that the azo group (non-conjugated to
dithienylethene) significantly accelerated the ring-opening
process of dithienylethene.14, 15
molecules have attracted great attention owing to potential
2
applications in sensor,1,
optical data processing3 and
molecular switches.4 For example, azobenzene derivatives
have been used extensively as photoresponsive system owing
to the easy switchability of azobenzene, where trans-isomer
can be switched to the cis-isomer by irradiation at around 340
nm and back to trans-isomer either by heating or by irradiation
at around 450 nm.5 The photoisomerization properties and
kinetics have been well-established by varying the electron-
withdrawing or electron-donating substituents on the
azobenzene moiety.6-8 The azophenolic derivatives with push-
pull effects were reported to undergo fast cis-to-trans thermal
relaxation within microseconds, which is useful for high speed
optical oscillators.9 On the other hand, considerable interests
have also been focused on dithienylethene derivatives, which
showed fast and stable photoinduced cyclization and
cycloreversion, fatigue resistance, and high thermal stabilities
F
F
F
F
F
F
S
S
HO
OH
DTE1
F
F
F
F
F
F
S
S
HO
OH
DTE2
HO
OH
F
F
F
F
F
F
S
S
N
N
N
N
DTE3
HO
OH
*
Figure 1. Chemical structures of dithienylethene with phenolic
DTE1 DTE2) and azophenolic (DTE3) end groups.
(
-
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Demethylations of compound
1
and
5
were carried out using
In addition, hydrogen bonding and π-π interaction are boron tribromide in anhydrous dichloromethane to afford
known for directing self-assembly of the molecules into well- compounds and in 90% and 84% yield, respectively. The
defined macrostructures. Recently, the self-assembly of DTE bromine substituent on biphenyl compounds ( and ) or
derivatives was investigated to show interesting azophenol ( ) was replaced with ethynylene group in two
2
6
2
6
8
supramolecular architectures such as helical fibrous steps (69 – 81% yields); a palladium-catalyzed Sonogashira
microstructures,16, 17 microspheres,18 and flakes.19, 20 In view of cross-coupling reaction22 with ethynyltrimethylsilane, followed
these, two photochromic dithienylethene molecules by desilylation using tetrabutylammonium fluoride. The
incorporated with end phenolic groups (DTE1 - 2) and another corresponding desilylated compounds (
3, 7, and 10) were
dithienylethene molecule incorporated with end azophenolic reacted with iodinated dithienylperfluorocyclopentene (DTE
)
group (DTE3) were synthesized and the properties of all three via Sonogashira coupling in presence of Pd(PPh3)4 and CuI as
molecules are compared (Figure 1). A detailed investigation of catalysts in tetrahydrofuran/diisopropylamine (2 : 1) as solvent
photoisomerization, electrochemistry, and self-assembly of all mixture to afford the DTEs (34%
- 79% yields). All
target molecules are carried out. Here, the roles of hydrogen intermediates and products were characterized fully using
bonding, azogroups, and photoisomerization toward formation NMR spectroscopy, mass spectrometry, and elemental
of final self-assembled architecture of molecules are studied.
analyses (ESI, Figure S1 – S33). All DTEs are soluble in common
organic solvents such as chloroform, toluene, acetonitrile,
methanol and tetrahydrofuran.
Scheme 2. Sonogashira reaction condition for DTE1 - 3; (i)
Pd(PPh3)4, CuI, THF/DIPA, reflux, overnight, 49% (DTE1), 34%
(
DTE2), and 79% (DTE3).
Photochromic Properties of DTE1 and DTE2
The photochromic properties of DTEs were investigated using
the absorption and emission spectroscopies (Figure 2). The
absorption maxima of DTE1open and DTE2open (open-ring
isomers) were observed at 328 nm in chloroform and showed
red-shifts in absorption maximum (λmax = 332 - 336 nm) when
dissolved in THF (Figure 3).23 DTE2open with catechol group
Scheme 1. Synthetic approach to obtain compound
10; (i) PdCl2(PPh3)2, THF/Aliquat® 336/aqueous 2M K2CO3
solution, reflux, overnight, 83% ( ); 79% ( ); (ii) BBr3, DCM, -50
°C to r.t., 3 hrs, 90% ( ); 84% ( ) (iii) (a) ethynyltrimethylsilane,
Pd(PPh3)4, CuI, THF/TEA, 80 °C, 24 hrs, (b) TBAF, THF, °0 C, 30
mins, 80% ), 69% ), 81% 10); (iv) PdCl2(dppf),
3, 7, and
1
5
2
6
exhibits an absorption maximum at longer wavelength (λmax
336 nm) than that of DTE1open (λmax = 332 nm) in THF. Similarly,
red-shifts in emission peaks were observed for DTE1open (λems
=
(
3
(
7
(
bis(pinacolatodiboron), KOAc, 1,4-dioxane, 80 °C, overnight,
85%.
=
390 nm and 412 nm) and DTE2open (λems = 416 nm) in THF in
comparison to that in chloroform (λems for DTE1open= 382 nm
and 406 nm; for DTE2open = 388 nm and 410 nm).
Formation of DTE1closed and DTE2closed isomers was
observed with appearance of a new absorption maximum at
around 620 nm, and two low-energy absorption maxima
(within the ranges of 368 – 372 nm; and 287 - 300 nm) upon
irradiation with UV-light (at 365 nm, 5 minutes) in chloroform
or THF. The colorless DTE1 - 2open solutions changed to dark-
Results and Discussion
Synthesis of Target Molecules
Synthetic routes for the target molecules, DTE1
, DTE2, and
DTE3 are given in Schemes 1 and 2. The biphenyl molecule
with methoxy (compound 1) group was obtained from Suzuki-
Miyaura coupling reaction21 between 4-iodoanisole and 4-
bromophenylboronic acid in 83% yield. Similarly, the coupling
blue upon reaching
a photostationary state. The three
absorption peaks are assigned to the S0 – S1 (620 nm), S0 – S2
(325 – 372 nm), and S0 – S3 (287 – 300 nm) transitions of DTE1
24
between 1-bromo-4-iodobenzene and compound
4
afforded
- 2closed
.
dimethoxy biphenyl bromide (compound
5) in 79% yield.
2 | J. Name., 2012, 00, 1-3
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1.0
cycloreversion quantum yields were observed for DTE1closed
(φc-o = 0.0031) and DTE2closed (φc-o = 0.0025).
DTE1
DTE1
160
120
80
40
0
0.8
0.6
0.4
0.2
0.0
The DTE1open and DTE2open thin films were prepared from
chloroform solutions (ESI, Figure S38). Photoisomerizations of
DTE1open and DTE2open were carried out by exposing the thin
films under UV-light for 5 minutes. In general, the absorption
maxima of thin films of DTE1
– 2open and DTE1 – 2closed showed
400
500
600
300 400 500 600 700 800
Wavelength (nm)
Wavelength (nm)
significant red-shift than those in chloroform solutions, and
were similar to the values observed in THF solutions. For
example, thin films of DTE2open showed absorption maxima at
336 nm, as compared to that in chloroform solution (λmax = 328
1.0
0.5
0.0
DTE2
320
240
160
80
DTE2
250
0
DTE1
DTE1
1.0
0.5
0.0
200
150
100
50
300 400 500 600 700 800
350 400 450 500 550 600
Wavelength (nm)
Wavelength (nm)
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
DTE3
DTE3
0
200
0
300
400
500
600
700
300 400 500 600 700 800
DTE2Wavelength (nm)
Wavelength (nm)
DTE2
0.6
600
400
200
0
0.4
0.2
0.0
300 400 500 600 700 800
450
500
550
Wavelength (nm)
Wavelength (nm)
Figure 2. Absorption (left) and emission spectra (right) of DTEs
in chloroform; open-ring isomer (- -) and closed-ring isomer (-
■
300 400 500 600 700 800
300
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
●-) at photostationary state upon irradiation at 365 nm for 5
minutes.
DTE3
DTE3
0.3
0.2
0.1
0.0
200
100
0
DTE1closed showed a broadened absorption peak with
insignificant changes in absorption maximum from THF to
chloroform, which indicated a minimal solvent-dependent
effect on its electronic property. On the other hand, DTE2closed
exhibited a significant blue-shift in S0 – S2 transition (λmax = 325
nm) in THF as compared to that in chloroform (λmax = 366 nm),
suggesting a stronger solvent effect on the electronic property
300 400 500 600 700 800
400
450
500
Wavelength (nm)
Wavelength (nm)
Figure 3. Absorption (left) and emission spectra (right) of DTEs
in THF for open-ring isomer (ꢀ■ꢀ) and closed-ring isomer (-●-)
at photostationary state upon irradiation at 365 nm for 5
minutes; for DTE3, the absorption and emission spectra of
open-ring isomer (ꢀ■ꢀ) after irradiation at 365 nm for 1 minute
(-●-), 3 minutes (ꢀ▲ꢀ), and 5 minutes (ꢀ▼ꢀ) are shown.
of DTE2closed. The fluorescence quantum yield of DTE1
– 2
decreased with changes in solvent from chloroform to MeOH,
as a result of the photoinduced electron transfer in polar
solvent.12
1H NMR analyses were carried out to calculate the
photoconversion yields of DTE1 - 2open at photostationary
states (ESI, Figure S34 – 35). For example, the chemical shift of
methyl groups at the bridge carbon for DTE1closed was shifted
downfield to 2.22 ppm compared to the one found in DTE1open
(2.00 ppm). Moreover, the thienyl proton of ring-closed isomer
had shifted upfield to 6.53 ppm from 7.28 ppm. Based on the
NMR integration, DTE1open and DTE2open showed similar
photoconversion yields of 70% and 74%, respectively, at
nm) and in THF solution (λmax = 332 nm). Similarly, DTE2closed
thin film showed two absorption maxima at 323 nm and 624
nm, as compared to that in chloroform solution (λmax = 296
nm, 620 nm) and in THF solution (λmax = 325 nm, 624 nm).
Photochromic Properties of DTE3 in chloroform
The photochromic properties of DTE3 incorporated with
dithienylethene and azophenol units were studied in
chloroform (Figure 2). The orange colored solutions of
DTE3open in chloroform gradually turned into green upon
irradiation with UV-light for 15 minutes. The appearance of
new absorption peak at 630 nm indicates formation of
photostationary states. The cyclization (φo-c
)
and
cycloreversion (φc-o) quantum yields of DTEs were estimated
by comparing the initial reaction yields with the known yield of
the
thienyl)perfluorocyclopentene.25,
quantum yields were obtained for DTE1open (φo-c = 0.28) and
DTE2open (φo-c 0.26). On the other hand, smaller
compound,
1,2-bis(2-methyl-5-phenyl-3-
26
Similar
cyclization
DTE3closed
.
The photophysical properties of DTEs are
summarized in Table 1.
=
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Table 1. Summary of photophysical properties of DTEs in different solvents.
CHCl3 THF
MeOH
Thin Film
a
a
a
b
λmax
,
λems a, nm
(φf , %)
λmax
,
λems a, nm
(φf , %)
λmax
,
λems a, nm
(φf , %)
λmax
,
c
c
c
nm
nm
nm
nm
328
382, 406 (15.3)
382, 410 (16.1)
388, 410 (12.4)
388, 412 (13.9)
437 (8.7)
332
390, 412 (13.2)
390, 415 (8.6)
416 (9.8)
332
410 (10.4)
410 (6.3)
453 (7.1)
455 (5.6)
434 (5.4)
441 (3.9)
330
DTE1open
DTE1closed
DTE2open
DTE2closed
DTE3open
287, 368, 620
328
300, 372, 620
336
296, 360, 615
335
295, 374, 624
336
296, 366, 620
382
325, 624
384
419 (6.7)
311, 610
381
323, 624
379
437 (6.5)
387, 630
437 (8.5)
393, 630
437 (5.9)
386, 615
642
DTE3closed
a
b
Absorption maxima and emission peak were measured in dilute solution. Thin films were prepared by dropcasting the
solution of molecules onto quartz plates, followed by slow evaporation at room temperature. c Fluorescence quantum yields
were estimated by using quinine sulfate in 0.5 M H2SO4 as reference.27
F F
F
F
F
F
S
S
N
N
N
N
cis-DTE3closed
OH
HO
Scheme 3. DTE3 isomers in THF and chloroform at photostationary state (PSS).
The small decrease in intensity of absorption at 382 nm for isomerization of DTE3 are ~48% and ~62%, respectively, at
DTE3closed after irradiation suggests that the azophenol groups photostationary state (ESI, Figure S37). The presence of
1
in DTE3closed molecules are in trans- form. In addition, the H additional NMR peaks suggests the formation of DTE3 isomer
NMR analysis showed no changes in chemical shifts for trans- with open-ring dithienylethene and cis-azophenol groups. A
azophenol groups at photostationary state. Moreover, the sharp hydroxyl proton peak (9.03 ppm) was found for DTE3 in
integration of 1H NMR signals showed low photoconversion deuterated THF, which indicates a slow proton exchange
yields of 32% for DTE3 (ESI, Figure S36), which is explained as owing to the strong hydrogen bonding. This demonstrates that
the energy loss from fast thermal relaxation in azophenols. The the stabilization of azophenols improves the photoconversion
cyclization and cycloreversion quantum yields of DTE3 in yield of dithienylethene. In addition, the cyclization and
chloroform were estimated to be 0.09 and 0.0011, cycloreversion quantum yields of DTE3 in THF were estimated
respectively, which are smaller than that observed for DTE1 to be 0.15 and 0.0021, respectively, which are higher than that
and DTE2
.
in chloroform.
Thin films of DTE3open were prepared from solutions of
compounds in chloroform and THF and the photochromic
Photochromic Properties of DTE3 in THF and in Thin Film
The THF solution of DTE3open showed a fast decrease in properties were investigated by spectroscopic techniques
absorbance at 384 nm and appearance of absorption peak at (Figure 4). Similar photochromic changes were observed for
630 nm (Figure 3) upon irradiation with UV-light for one DTE3open upon irradiation with UV-light, where ring-closed
minute, which is different from DTE3open in chloroform. This dithienylethene and cis-azophenols were formed inside the
indicates that DTE3closed has both ring-closed dithienylethene films prepared from THF solution (Figure 4b). However,
and cis-azophenols in THF (Scheme 3).
thermal relaxation of cis-azophenols happened after 30
The photoisomerization can be switched reversibly and the minutes due to complete evaporation of THF from DTE3open
increase in absorbance (Abs) at 630 nm was observed for film.
DTE3closed in THF (Abs = 0.09) as compared to that obtained in
chloroform (Abs = 0.05), indicating a higher photoconversion
yield at the photostationary state in THF. The yield of
photoinduced ring closing of the dithienylethene and trans-cis
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0.5
a)
0.4
compound 10 in different solvents were examined. A dilute
chloroform solution of 10 was monitored by absorption
spectroscopy before and after irradiation at 365 nm (Figure
5a).
0.6
0.5
0.4
0.3
0.2
0.1
b)
0.3
0.2
0.1
0.4
300 400 500 600 700 800
Wavelength (nm)
300 400 500 600 700 800
b)
a)
0.4
0.2
0.0
Wavelength (nm)
CHCl3
THF
0.2
0.0
Figure 4. Normalized absorption spectra of DTE3 films
prepared from chloroform ( ) and THF ( ); open-ring isomer (-
-) and closed-ring isomer at photostationary state (-●-) upon
a
b
■
irradiation at 365 nm for 5 minutes. The films were prepared
by dropcasting the solution of DTE3 onto a quartz plate,
followed by evaporation at room temperature.
300 400 500 600 700 800
300 400 500 600 700 800
Wavelength (nm)
Wavelength (nm)
Figure 5. Absorption spectra of azophenolic compound, 10 in
chloroform ( ) and THF ( ); trans- (- -) and cis-isomer at the
a
b
■
photostationary state (-●-) upon irradiation at 365 nm until no
further changes in absorption profile.
Comparison of Photochromic Properties of DTE3 with DTE1 –
2
DTE3 absorbs at longer wavelength (i.e. λmax = 382 nm in
chloroform) due to incorporation of azo- group with more
extended conjugation as compared to DTE1 (i.e. λmax = 328 nm
in chloroform) and DTE2 (i.e. λmax = 328 nm in chloroform).
The observed strong absorption maximum at 360 nm (π -
π*) is consistent with the trans- form of 10, and no changes in
absorption profile was observed after irradiation (365 nm) for
10 minutes in chloroform. It was reported that the
azobenzenes incorporated with phenol substituent showed
fast thermal relaxation in milliseconds scale which is difficult to
be detected at room temperature.28 This is explained by the
formation of hydrazine intermediate via intermolecular proton
transfer between phenol and nitrogen atom of the azo group,
which lowers the energy barrier of switching from cis-isomer
to trans-isomer.8 In contrast, the THF solution of 10 showed
trans-cis conversion with a decrease in intensity of absorption
maximum at 360 nm and appearance of a new absorption
peak at 450 nm after irradiation (365 nm, 1 minute, Figure
5b).29 The trans-isomer can be recovered by irradiating the
solution of cis-10 with visible light (ESI, Figure S41). It is
conceivable that a polar solvent, such as THF had weakened
the intramolecular hydrogen bonding, which led to slow
thermal relaxation. The cis- and trans- isomers of 10 can be
switched reversibly by exposing the THF solution to UV- and
visible light, respectively. Similarly, fast thermal relaxation was
observed for 10 in MeOH owing to the intermolecular proton
transfer between methanol and azo group (data not given).
Similarly, DTE3 showed red-shift in emission maximum (λems
=
437 nm) in chloroform and THF. However, DTE2 showed a
longer emission maximum (λems = 455 nm) as compared to that
of DTE3 (λems = 441 nm) in methanol, which is attributed to the
strong hydrogen bond between methanol and DTE2
. In
comparison, DTE3open showed a relatively slower
photoinduced ring closing reaction due to the energy loss from
fast thermal relaxation in azophenolic groups.9 Also, DTE3
showed smaller fluorescence quantum yield (3.9% - 8.7%) as
compared to DTE1 – 2 in different solvents.
The thermal stabilities of closed-isomers of DTEs in
chlorobenzene were monitored by measuring their
absorbance decay at 620 – 630 nm at 100 °C under darkness
(ESI, Figure S39). The cycloreversion process of closed-isomers
of DTEs becomes faster at elevated temperature. It is found
that DTE1closed has higher thermal stability as compared to
DTE2closed and DTE3closed. The half-life of DTE1closed, DTE2closed,
and DTE3closed are estimated to be around 60 hours, 39 hours,
and 46 hours, respectively, at 100 °C. On the other hand, the
THF solution of DTE3closed showed cis-trans isomerization after
15 minutes at 50 °C with reappearance of absorption peak at
384 nm and negligible absorbance changes at 630 nm region
(ESI, Figure S40). This indicates the cis-azophenol is less
thermally stable than closed-dithienylethene for DTE3. In
addition, the changes in absorbance of the chloroform solution
of DTEs at 620 – 630 nm were monitored after alternate
irradiation with UV- and visible light. All DTEs showed no
significant absorbance decay up to 15 irradiation cycles, which
pH-dependent Photochromic Properties of DTE1 and DTE2
As DTEs incorporated with phenolic group are sensitive to pH,
it is interesting to study their absorption changes upon
deprotonation in basic medium. In this study, the
photochromic properties of DTEs in MeOH were monitored
using absorption spectroscopy before and after addition of
aqueous NaOH solution (Figure 6). DTE1open and DTE2open
showed red-shifts in absorption maxima (10 nm) with decrease
in absorption intensities after the addition of aqueous NaOH
solution (Figure 6a and 6c). The colorless solutions of DTE1open
and DTE2open have immediately turned into yellowish after
deprotonation of phenolic group in presence of NaOH (ESI,
Figure S42). On the other hand, addition of aqueous NaOH
solution to the methanol solutions of DTE1closed and DTE2closed
suggests a good fatigue resistance for DTEs
.
Photochromic Properties of Azophenolic Compound 10
In order to prove that the observed changes in absorption
intensities of DTE3 upon irradiation with UV-light was caused
by the formation of cis-azophenol, photochromic properties of
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resulted in red-shifts in absorption maxima (10 nm, Figure 6b resembles to that of DTE1closed in the absence of Fe3+ ions,
and 6d). The color of DTE1closed has changed from blue to green which suggested that Fe3+ has negligible effect on DTE1open
upon addition of aqueous NaOH solution, whereas DTE2closed photoisomerization (Figure 7b).
showed a dark blue color after deprotonation. The color exhibited large red-shift in absorption maxima (27 nm) after
contrast of DTE1closed before and after deprotonation addition of Fe3+ ions in solution, indicating
strong
demonstrates potential applications for developing a pH complexation between Fe3+ ions and catechol groups of
In Figure 7c, DTE2open
a
sensor.
DTE2open. On the other hand, smaller photoconversion was
observed in DTE2closed complex as evident from the lower
absorbance at 620 nm (Figure 7d). DTE3open showed a decrease
in intensities of absorption maximum at 382 nm and
appearance of a shoulder peak at 460 nm upon addition of
0.9
0.6
0.3
0.0
0.9
0.6
0.3
0.0
a)
DTE1open
DTE1
b)
closed
Fe3+ ions in solution (Figure 7e). In comparison to DTE1
, DTE3
showed a strong interaction with Fe3+ ions (Figure 7f). It is
conceivable that the presence of azo- group influences the
interaction with Fe3+ ions, which gave large red-shift in
absorption maximum.30
300 400 500 600 700 800
300 400 500 600 700 800
Wavelength (nm)
Wavelength (nm)
0.6
0.4
0.2
0.0
0.6
0.4
0.2
0.0
DTE2
c)
open
DTE2
d)
closed
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
DTE1open
DTE1closed
a)
b)
300 400 500 600 700 800
300 400 500 600 700 800
Wavelength (nm)
Wavelength (nm)
300 400 500 600 700 800
300 400 500 600 700 800
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
Wavelength (nm)
Wavelength (nm)
e)
DTE3open
DTE3
f)
closed
0.6
0.4
0.2
0.0
0.6
0.4
0.2
0.0
DTE2
d)
c) DTE2open
closed
300 400 500 600 700 800
300 400 500 600 700 800
Wavelength (nm)
Wavelength (nm)
300 400 500 600 700 800
300 400 500 600 700 800
Wavelength (nm)
Wavelength (nm)
Figure 6. Absorption spectra of DTEopen solution in MeOH
before (ꢀ■ꢀ) and after (-●-) addition of aqueous NaOH solution;
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
DTE3
f) DTE3closed
e)
open
and DTEclosed solution in MeOH before (ꢀ▲ꢀ) and after (ꢀ▼ꢀ
addition of aqueous NaOH solution at room temperature.
)
300 400 500 600 700 800
300 400 500 600 700 800
Wavelength (nm)
Wavelength (nm)
pH-dependent Photochromic Properties of DTE3
DTE3 with azophenolic group showed a more significant red-
shift in absorption maximum after deprotonation. Appearance
of a new absorption maximum at 452 nm for DTE3open was
observed upon addition of aqueous NaOH solution (Figure 6e),
which gave deep orange solution. Similar changes in
absorption spectra (red shift of 64 nm) were also observed for
DTE3closed after addition of aqueous NaOH solution (Figure 6f).
However, the photoisomerization of deprotonated DTE3open
was observed to be slower and required an irradiation time of
up to 10 minutes to reach photostationary state in comparison
to neutral DTE3open (5 minutes).
Figure 7. Absorption spectra of DTEopen solution in MeOH
before (ꢀ■ꢀ) and after (-●-) addition of aqueous Fe3+ solution;
and DTEclosed solution in MeOH before (ꢀ▲ꢀ) and after (ꢀ▼ꢀ
addition of aqueous Fe3+ solution; the ratio of DTE : Fe3+ used
was 1,
and Fe3+ ions were added prior to
)
1
:
photoisomerization of DTEs in presence of K2CO3.
Electrochemical Properties of DTEs
The electrochemical properties of DTEs were evaluated using
cyclic voltammetry (CV) with a platinum working electrode and
Ag/AgCl reference electrode, using a solution of 0.1 M
tetrabutylammonium hexafluorophosphate in anhydrous
dichloromethane and at a scan rate of 100 mVs-1 before and
after irradiation with UV-light (365 nm, 10 minutes). As shown
in Figure 8a, DTE1open showed an irreversible oxidation peak at
+1.52 V, which corresponds to the oxidation of dithienylethene
Photochromic properties of DTEs in presence of Fe3+ ions
It is understood that the phenolic compounds have strong
affinity towards Fe3+ ions. DTE1open showed no spectral
changes upon addition of Fe3+ ions (1 eq) in MeOH (Figure 7a).
The absorption peak of DTE1closed in presence of Fe3+ ions
6 | J. Name., 2012, 00, 1-3
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group. The electrochemical oxidation has led to formation of
DTE1closed in solution. The repetitive CV scans resulted in a
smaller oxidation peak at + 1.52 V and appearance of a new
oxidation peak at + 1.20 V, where the latter was caused by the
formation of DTE1closed. This is confirmed from the CV trace of
DTE1closed obtained from photoisomerization. Similar
electrochemically induced ring-closing reactions have been
observed for other dithienylethene derivatives.31 As shown in
Figure 8b, DTE2open underwent an irreversible oxidation at
around + 1.50 V in the first scan, followed by formation of
DTE2closed with the appearance of a new oxidation peak at
around + 1.17 V in the second scan. This suggests both
DTE1open and DTE2open can be switched electrochemically to
DTE1closed and DTE2closed
.
Figure 9. FESEM images of DTE1 and DTE2 films from THF
solution (6.0 x 10-4 M) on precleaned glass substrates. THF was
allowed to evaporate slowly inside a desiccator at room
temperature under darkness. Films from DTE1closed and
DTE2closed were prepared by irradiating the solution with UV
light (365 nm, 10 minutes) prior to drop casting.
Self-assembly of DTE1 and DTE2 in THF
Self-assembly of DTE1 – 2 was examined using scanning
electron microscopy (SEM). Thin films were prepared by drop
casting dilute solutions of DTEs in THF (6.0 x 10-4 M) on glass
substrates. DTE1 – 2closed films were prepared by irradiating the
dilute solution with UV-light (365 nm, 10 minutes) prior to
drop casting. Solvent was allowed to evaporate slowly in a
desiccator at room temperature under darkness prior to SEM
examination. DTE1open and DTE2open films showed formation of
long micrometer-length needles with diameters of 120 to 240
nm (Figure 9) upon slow evaporation of THF at room
temperature.
In comparison, DTE1closed showed formation of plate-type
structures with sizes at around 150 nm. It is conceivable that
the increase in rigidity of closed-ring DTE1 disrupted the self-
assembly of molecules into one-dimensional needles. On the
other hand, the higher rotational freedoms for DTE1open and
DTE2open and intermolecular interactions such as hydrogen
bonding and π - π stacking showed an important role towards
the formation of large needles. Nano-sized spheres and donut-
like structures with various sizes (300 nm to 2 µm) were
formed from DTE2closed under identical conditions. The rings
were only formed from DTE2closed with end catechol groups but
not from other molecules, highlighting the importance of
multiple intra- and inter- molecular hydrogen bonds towards
the formation of circular structures.
Figure 8. Cyclic voltammograms of DTE1
DTE3 ) recorded in a 0.1 M solution of tetrabutylammonium
hexafluorophosphate in dichloromethane at a scan rate of 100
mVs-1 with
Ag/AgCl as reference electrode. The
(a), DTE2 (b), and
(
c
a
a
photoisomerizations of DTEs were carried out by irradiating
the dichloromethane solutions with UV-light (365 nm, 10
minutes).
In contrast, the electrochemical switching of DTE3open is
relatively weaker, which showed irreversible oxidations at
around + 1.58 V in the first scan. After a few scans, small
oxidation peaks were observed for DTE3closed at + 1.32 V
(Figure 8c), which suggested a weak electrochemical switching
of DTE3 molecules. The CV trace of photochemically switched
DTE3closed showed no significant difference from the
electrochemically switched DTE3closed
.
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DTE2closed formed on a silicon substrate (Figure 11). The DTE2
solution in MeOH was irradiated with UV light (365 nm, 10
minutes) prior to dropcasting. The thickness and height of ring-
shaped nanostructures were determined to be in the range of
160 – 250 nm, and 55 – 90 nm, respectively. Formation of
nanorings with a diameter of 150 nm were also reported from
polyfluorene by using water droplet as template.32
It is also known that coffee ring formations are commonly
observed from slow evaporation of liquid containing dispersed
particles on the surface.33 However, smallest coffee rings
reported were in the size range of 8 – 10 µm,34, 35 which are
much larger than the observed circular nanostructures from
this work. Moreover, the capillary flow that causes the non-
uniform deposition of particles in the coffee rings was
suppressed by organic solvents such as ethanol, methanol,
octane and THF with strong Marangoni effect,36 which induced
convection flow and resulted in uniform depositions.37 In
addition, the self-assembly of DTE2 are controllable from one
dimensional micro-sized needle in open-ring isomer to circular
nanostructures in closed-ring isomer. Therefore, the self-
assembly mechanism for DTE2 is different from that of the
coffee ring formation.
Figure 10. FESEM images of DTE1 and DTE2 films drop-casted
from MeOH solution (6.0 x 10-4 M) on a precleaned glass
substrate. MeOH was allowed to evaporate slowly inside a
desiccator at room temperature under darkness. DTE1closed and
DTE2closed films were prepared by first irradiating the solution
with UV light (365 nm, 10 minutes) prior to drop casting.
In an effort to further understand the mechanism of the
formation of circular nanostructures in DTE2closed
,
deprotonation of DTE2closed (6.0 x 10-4 M) was carried out by
using NaOH (4 eq) in MeOH. It is found that the deprotonated
DTE2closed film showed random shaped aggregates with no
observable circular or needle-like structures (ESI, Figure S43).
It is proposed that the formation of circular nanostructures in
DTE2closed involved an interplay of both inter/intramolecular
hydrogen bonding and rigidity of the molecular structure of
closed dithienylethene group (Figure 12). The DTE2closed
molecules are connected via intermolecular hydrogen
bonding, while the rigid dithienylethene backbone of DTE2closed
could provide a “bent” conformation that leads to circular
Figure 11. AFM image (tapping mode) and the height profiles
of DTE2closed rings on silicon substrate. The DTE2closed solution
(365 nm, 10 minutes) in MeOH (6.0 x 10-4 M) was drop-casted
on a pre-cleaned silicon substrate and the solvent was allowed
to evaporate slowly at room temperature under darkness.
Inset: the thickness (x-axis) and height (y-axis) of the DTE2closed
rings were determined to be in the range of 160 – 250 nm, and
55 – 90 nm, respectively.
nanostructures.
The
self-assembly
of
macrocyclic
triazolophanes into nano-sized toroids and donuts, which
involved both hydrogen bonds and rigid macromolecules was
reported recently.38
Self-assembly of DTE1 and DTE2 in MeOH
Thin films of DTE1
– 2 prepared from methanol solutions were
also examined in order to understand the solvent effects on
the self-assembly of DTEs (Figure 10). Long needles (length
~100 µm) of DTE1open and DTE2open were obtained from MeOH.
Similarly, DTE1closed film showed micron-sized crystalline
structures and DTE2closed gave smaller circular structures with
diameters in the range of 300 – 600 nm. DTE2open needles
were irradiated at 365 nm prior to SEM examination. However,
no transformation of assembled structures was observed. It is
conceivable that the structural changes of DTE2 in solid state
are difficult as compared to that in solution state.
Figure 12. The photoinduced ring-closing of DTE2 happens
when it is in antiparallel conformation; Schematic
representation of possible mechanism for DTE2closed self-
assembly via inter/intra-molecular hydrogen bonding between
end catechol groups.
Atomic force microscopy (AFM) was used to measure the
thickness and the height of ring-shaped nanostructures of
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Self-assembly of DTE3
spectrofluorophotometer. The cyclic voltammograms were
recorded with a computer controlled CHI electrochemical
analyzer at a constant scan rate of 100 mV/s. Scanning
Electron micrograph was recorded on a JEOL JSM-6701F Field
Emission Scanning Electron Microscope (SEM). DTE1-3 were
dissolved in respective solvents (THF and MeOH), followed by
drop-casting on a precleaned glass substrate, and solvent was
allowed to evaporate at 25 ºC under ambient conditions.
Atomic Force Microscope (AFM) measurements were recorded
using a Agilent Technologies AFM Picoscan 5 instrument.
DTE3open film cast from MeOH (6.0 x 10-4 M) showed random-
shaped aggregates in the micron size range (ESI, Figure S44),
which is morphologically different from the DTE1open without
azo group. DTE3open film cast from THF (6.0 x 10-4 M) showed
random-shaped structures with smooth and featureless
surfaces. In contrast, DTE3closed film showed well-defined
needle structures, which indicate that the assembly of DTE3 is
dependent on the structural changes by photocyclization.
Synthesis Procedures
Conclusions
Compound 1 : A solution of 4-iodoanisole (1.08 g, 4.6 mmol),
4-bromophenylboronic acid (1.2 g, 5.98 mmol) and
bis(triphenylphosphine)palladium(II) (0.16 g, 0.22 mmol) in
tetrahydrofuran/Aliquat® 336/2M aqueous K2CO3 solution (12
: 2 : 5 mL) was stirred at reflux temperature for overnight,
cooled, diluted with ethyl acetate (50 mL), washed with water
(2 x 15 mL) and brine solution (10 mL). The organic layer was
dried over anhydrous sodium sulfate and the excess solvent
was removed under reduced pressure. The crude product was
purified on a silica gel column using 5% dichloromethane in
hexane as eluent to afford a white solid (1.01 g, yield 83%). 1H
NMR (300 MHz, CDCl3) δ 7.53 (d, J = 8.6 Hz, 2H), 7.49 (d, J = 8.9
Hz, 2H), 7.41 (d, J = 8.6 Hz, 2H), 6.97 (d, J = 8.9 Hz, 2H). 13C
NMR (126 MHz, CDCl3) δ 159.46, 139.78, 132.54, 131.79,
128.30, 127.98, 120.79, 114.36, 55.36. HR-MS (EI): M+
(C13H11BrO) Calculated m/z =261.9993, Found m/z = 261.9992.
Elemental analysis calculated (%) for C13H11BrO: C, 59.34; H,
4.21; Found: C, 59.14; H, 4.28.
In this work, dithienylethene based photochromic compounds
with phenols (DTE1), catechols (DTE2), azophenols (DTE3) as
end groups were synthesized. All target molecules showed
solvent dependent photochromic properties owing to the
photoinduced transition between closed-ring and open-ring
isomers of dithienylethene. The photoconversion yields of
DTE3 (32%) are smaller than that of DTE1 - 2 (70 – 74%) due to
a fast thermal relaxation of cis-azophenols to trans-azophenols
upon photoisomerization in chloroform. In contrast, both
closed-ring dithienylethene and cis-azophenols (cis-DTE3closed
)
were observed in THF owing to a better thermal stability of cis-
azophenols in THF. The transition from needle microstructures
to circular nanostructures with diameters in the range of 300 –
600 nm was observed for DTE2 film upon ring-closing of
dithienylethene unit, which could be used for developing
interesting optical materials.
Experimental
Compound 2: Compound
1 (0.42 g, 1.59 mmol) was dissolved
Materials
in anhydrous dichloromethane (14 mL) under nitrogen
atmosphere. The reaction mixture was cooled to -50 °C
followed by dropwise addition of BBr3 (0.31 mL, 3.26 mmol).
The reaction mixture was warmed up to room temperature,
stirred for 3 hours, and quenched with water (10 mL). The
organic layer was washed with water (2 x 5 mL) and brine
solution (3 mL), dried over anhydrous sodium sulfate, filtered,
and the excess solvent was removed under reduced pressure.
The crude product was purified on a silica gel column using a
mixture of 10% ethyl acetate in hexane as eluent to afford a
white solid (0.36 g, yield 90%). 1H NMR (300 MHz, CDCl3) δ
7.53 (d, J = 8.6 Hz, 2H), 7.43 (d, J = 8.7 Hz, 2H), 7.39 (d, J = 8.6
Hz, 2H), 6.90 (d, J = 8.7 Hz, 1H), 4.84 (br, 1H). 13C NMR (75
MHz, CDCl3) δ 155.39, 139.67, 132.75, 131.78, 128.28, 128.22,
120.83, 115.77. HR-MS (EI): M+ (C12H9BrO) Calculated m/z
The dithienylperfluorocyclopentene
according to the previously reported procedures.39,
(DTE) was prepared
40
All
reagents and solvents were purchased from commercial
sources (Sigma Aldrich, Alfa Aesar and Merck) and used
without further purification. Preparative separations were
performed by column chromatography on a silica gel grade 60
(0.040 – 0.063 mm) from Silicycle.
Instrumentation
1H NMR and 13C NMR spectra were recorded on a Bruker
Avance AV300 (300 MHz), or a Bruker Avance AV500 (500
MHz) NMR instruments using appropriate deuterated solvents
purchased from Cambridge Isotope Laboratories. The chemical
shifts were reported in part per million or ppm and referenced
to the residual solvent peak: s = singlet, d = doublet, t = triplet,
=247.9836, Found m/z
= 247.9838. Elemental analysis
calculated (%) for C12H9BrO: C, 57.86; H, 3.64; Found: C, 57.91;
H, 3.63.
m
= multiplet, and b = broad. Electron Impact mass
spectroscopy (EI–MS) mass spectra were obtained on a
Finnigan TSQ7000. Atmospheric pressure chemical ionization
(APCI) mass spectra were obtained on a Bruker AmaZonX.
Elemental analysis was carried out on Elementar Vario Micro
Cube. Absorption spectra were measured on a UV-1800
Shimadzu UV-VIS spectrophotometer with an optical filter
calibrated at a bandwidth of 1 nm. The emission spectra were
Compound 3: A solution of Compound
tetrakis(triphenylphosphine)palladium(0) (0.074 g, 0.06 mmol)
and copper(I) iodide (0.02g, 0.10 mmol) in
tetrahydrofuran/triethylamine (6 : 6 mL) was stirred at room
temperature under nitrogen atmosphere.
2 (0.32 g, 1.28 mmol),
Ethynyltrimethylsilane (0.55 mL, 3.89 mmol) was added via
measured
on
a
RF-5301PC
Shimadzu
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syringe and the reaction mixture was stirred at 80 °C for 24 calculated (%) for C14H13BrO2: C, 57.36; H, 4.47; Found: C,
hours, cooled, and the excess solvent was removed under 58.01; H, 4.53.
reduced pressure. The residue solid was purified on a silica
column using a mixture of 10% ethyl acetate in hexane to Compound 6: The synthesis and work-up of
afford a white solid, which was dissolved in tetrahydrofuran (5 described for Compound
mL) and cooled to 0 °C. Tetrabutylammonium fluoride (2.5 mL, Compound (0.74 g, 2.52 mmol) and BBr3 (0.72 mL, 7.59
6 were same as
2.
5
2.5 mmol, 1 M in tetrahydrofuran) was added dropwise and mmol) were reacted in anhydrous dichloromethane (20 mL) as
the reaction mixture was stirred at 0 °C for 30 minutes. The solvent. The crude solid was purified on a silica gel column
reaction mixture was diluted with ethyl acetate (20 mL), using a mixture of 30% ethyl acetate in hexane as eluent to
washed with water (2 x 10 mL) and brine solution (5 mL), dried afford a white solid (0.56 g, yield 84%). 1H NMR (300 MHz,
over anhydrous sodium sulfate, and the excess solvent was CDCl3) δ 7.52 (d, J = 8.6 Hz, 2H), 7.38 (d, J = 8.6 Hz, 2H), 7.10 –
removed under reduced pressure. The crude product was 6.98 (m, 2H), 6.92 (d, J = 8.2 Hz, 1H), 5.26 (s, 2H). 13C NMR (126
purified on a silica gel column using a mixture of 10% ethyl MHz, CDCl3) δ 143.88, 143.39, 139.59, 133.51, 131.79, 128.32,
acetate in hexane as eluent to afford a white solid (0.20 g, 120.98, 119.70, 115.80, 114.08. HR-MS (EI): M+ (C12H9BrO2)
yield 80%). 1H NMR (300 MHz, CDCl3) δ 7.50 (m, 4H), 7.48 (d, J Calculated m/z =263.9786, Found m/z = 263.9776. Elemental
= 8.6 Hz, 2H), 6.91 (d, J = 8.6 Hz, 2H), 4.88 (br, 1H), 3.11 (s, 1H). analysis calculated (%) for C12H9BrO2: C, 54.37; H, 3.42; Found:
13C NMR (126 MHz, CDCl3) δ 155.48, 141.10, 132.97, 132.52, C, 54.45; H, 3.33.
128.33, 126.47, 120.31, 115.76, 83.63, 77.51. HR-MS (EI): M+
(C14H10O) Calculated m/z =194.0731, Found m/z = 194.0736. Compound 7: The synthesis and work-up of
Elemental analysis calculated (%) for C14H10O: C, 86.57; H, 5.19; described for
Found: C, 85.99; H, 5.15. Compound
tetrakis(triphenylphosphine)palladium(0) (0.11 g, 0.095 mmol),
Compound 4: 4-Bromo-1,2-dimethoxybenzene (1.00 g, 4.6 copper(I) iodide (0.026 g, 0.136 mmol), and
7 were same as
3
.
6
(0.46
g,
1.74
mmol),
mmol), bis(pinacolato)diboron (1.51 g, 5.95 mmol), PdCl2(dppf) ethynyltrimethylsilane (1.2 mL, 8.68 mmol) were reacted in
(0.22 g, 0.27 mmol) and potassium acetate (0.45 g, 4.6 mmol) tetrahydrofuran/triethylamine (6 : 6 mL). The crude solid was
were dissolved in 1,4-dioxane (30 mL) under nitrogen purified on a silica gel column using a mixture of 20% ethyl
atmosphere. The reaction mixture was stirred at 80 °C for acetate in hexane as eluent to afford the pale-yellow solid,
overnight, cooled, added with water (20 mL), and the product which was dissolved in tetrahydrofuran (10 mL) followed by
was extracted with ethyl acetate (2 x 30 mL). The combined addition of tetrabutylammonium fluoride (3.5 mL, 3.47 mmol).
organic layer was washed with water (2 x 15 mL) and brine The crude product was purified on a silica gel column using a
solution (10 mL), dried over anhydrous sodium sulfate, and the mixture of 20% ethyl acetate in hexane as eluent to afford a
excess solvent was removed under reduced pressure. The white solid (0.25 g, yield 69%). 1H NMR (300 MHz, CDCl3) δ
crude product was purified on a silica gel column using a 7.56 – 7.34 (m, 4H), 7.14 – 6.98 (m, 2H), 6.93 (d, J = 8.2 Hz, 1H),
mixture of 5% ethyl acetate in hexane as eluent to afford a 5.23 (s, 2H), 3.11 (s, 1H). 13C NMR (75 MHz, CDCl3) δ 143.85,
1
sticky colorless liquid (1.03 g, yield 85%). H NMR (300 MHz, 133.76, 132.52, 131.79, 128.32, 126.54, 119.89, 115.81,
CDCl3) δ 7.43 (d, J = 8.0 Hz, 1H), 7.29 (s, 1H), 6.88 (d, J = 8.0 Hz, 114.19. HR-MS (EI): M+ (C14H10O2) Calculated m/z =210.0680,
1H), 3.92 (s, 3H), 3.90 (s, 3H), 1.34 (s, 12H). 13C NMR (75 MHz, Found m/z = 210.0676. Elemental analysis calculated (%) for
CDCl3) δ 151.65, 148.35, 128.55, 116.59, 110.50, 83.64, 55.84, C14H10O2: C,79.98; H, 4.79; Found: C, 80.79; H, 4.75.
55.73, 25.00, 24.84. HR-MS (EI): M+ (C14H21BO4) Calculated m/z
=264.1533, Found m/z
= 264.1523. Elemental analysis Compound 10: The synthesis and work-up of 10 were same as
calculated (%) for C14H21BO4: C, 63.66; H, 8.01; Found: C, 63.89; described for
3
.
H, 8.01.
Compound
tetrakis(triphenylphosphine)palladium(0) (0.48 g, 0.42 mmol),
5 were same as copper(I) iodide (0.08 g, 0.42 mmol), and
8
(1.2
g,
4.33
mmol),
Compound 5: The synthesis and work-up of
described for Compound
bromo-4-iodobenzene
1
. Compound
4
(1.2 g, 4.54 mmol), 1- ethynyltrimethylsilane (0.92 mL, 6.5 mmol) were reacted in
4.95 mmol), tetrahydrofuran/triethylamine (10 : 10 mL). The crude solid
(1.4 g,
bis(triphenylphosphine)palladium(II) (0.16 g, 0.22 mmol) were was purified on a silica gel column using a mixture of 15% ethyl
reacted in tetrahydrofuran/Aliquat® 336/2M aqueous K2CO3 acetate in hexane as eluent to afford the brown solid, which
solution (12 : 2 : 5 mL) as solvent. The crude solid was purified was dissolved in tetrahydrofuran (10 mL) followed by addition
on silica gel column using a mixture of 5% ethyl acetate in of tetrabutylammonium fluoride (8.6 mL, 8.64 mmol). The
hexane as eluent to afford a white solid (1.05 g, yield 79%). 1H crude product was purified on a silica gel column using a
NMR (300 MHz, CDCl3) δ 7.54 (d, J = 8.6 Hz, 2H), 7.42 (d, J = 8.6 mixture of 15 % ethyl acetate in hexane as eluent to afford a
Hz, 2H), 7.14 – 7.03 (m, 2H), 6.94 (d, J = 8.3 Hz, 1H), 3.95 (s, brown solid (0.78 g, 81%). 1H NMR (300 MHz, CDCl3) δ 7.89 (d,
3H), 3.92 (s, 3H). 13C NMR (75 MHz, CDCl3) δ 149.24, 148.89, J = 8.8 Hz, 2H), 7.84 (d, J = 8.4 Hz, 2H), 7.62 (d, J = 8.4 Hz, 2H),
139.94, 132.94, 131.78, 128.41, 120.94, 119.28, 111.51, 6.96 (d, J = 8.8 Hz, 2H), 3.21 (s, 1H). 13C NMR (75 MHz, CDCl3) δ
110.16, 55.98. HR-MS (EI): M+ (C14H13BrO2) Calculated m/z 158.66, 152.22, 147.10, 132.94, 125.19, 123.98, 122.53,
=292.0099, Found m/z
=
292.0094. Elemental analysis 115.88, 83.32, 79.12. Elemental analysis calculated (%) for
10 | J. Name., 2012, 00, 1-3
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C14H10N2O: C,75.66; H, 4.54; N, 12.60; Found: C, 74.99; H, 4.45; 14.53. HRMS (APCI): calculated for C43H26F6N4O2S2 (M)+ m/z
N, 11.90.
808.1396, found m/z 808.1397. Elemental analysis calculated
(%) for C43H26F6N4O2S2: C, 63.86; H, 3.24; N, 6.93; S: 7.93;
Found: C, 64.53; H, 3.31; N, 7.01; S, 7.69.
DTE1
A solution of DTE (0.20 g, 0.32 mmol),
3 (0.13 g, 0.67 mmol),
tetrakis(triphenylphosphine)palladium(0) (0.014 g, 0.012 Sample preparation for photophysical studies of DTEs
mmol), and copper(I) iodide (4.5 mg, 0.023 mmol) in a mixture Dilute solutions of DTEs were prepared by dissolving
of tetrahydrofuran/diisopropylamine (8 : 4 mL) was stirred at appropriate amount of DTEs in analytical grade solvents. The
80 °C for overnight under nitrogen atmosphere, cooled, and concentrations for DTE1,
DTE2, and DTE3 were 4.8 x 10-5 M,
the excess solvent was removed under reduced pressure. The 2.5 x 10-5 M, and 0.8 x 10-5 M, respectively. The quartz cuvette
residue was purified on a silica gel column using a mixture of with optical path length of 1 cm was used. For thin films
50% ethyl acetate in hexane as eluent to afford a pale-yellow studies, the DTEs solutions in analytical grade solvent were
1
solid (0.12 g, yield 49%). H NMR (300 MHz, CD2Cl2) δ 7.60 (s, drop-casted on quartz plates and the solvent was allowed to
8H), 7.55 (d, J = 8.7 Hz, 4H), 7.32 (s, 2H), 6.96 (d, J = 8.7 Hz, evaporate under ambient conditions.
4H), 5.17 (s, 2H), 2.01 (s, 6H). 13C NMR (126 MHz, CD2Cl2) δ
156.15, 144.12, 141.33, 132.94, 132.10, 131.64, 128.56, Sample preparation for photophysical studies of DTEs at
126.86, 125.09, 122.30, 120.90, 116.08, 94.17, 82.12, 14.67. different pHs
HRMS (APCl, +ve, (M +H)+) : (C43H27F6O2S2) Calculated m/z = Dilute solutions of DTEs were prepared by dissolving
753.1345, Found m/z
=
753.1351. Elemental analysis appropriate amount of DTEs in analytical grade solvents. The
DTE2, and DTE3 were 5.0 x 10-5 M,
2.5 x 10-5 M, and 2.2 x 10-5 M, respectively. The quartz cuvette
calculated (%) for C43H26F6O2S2: C, 68.61; H, 3.48; S, 8.52; concentrations for DTE1
,
Found: C, 68.72; H, 3.55; S, 8.67.
with optical path length of 1 cm was used. Aqueous NaOH
solution (1.0 x 10-5 M) was added to the DTEs solutions before
or after irradiation with UV light at 365 nm, prior to
DTE2
The synthesis of DTE2 was same as described for DTE1
.
A solution of DTE (0.21 g, 0.34 mmol),
7 (0.17 g, 0.80 mmol), spectroscopic analyses. Next, dilute aqueous HCl solution was
tetrakis(triphenylphosphine)palladium(0) (0.019 g, 0.016 added to neutralize the DTEs solutions to test the reversibility
mmol), and copper(I) iodide (3.9 mg, 0.020 mmol) in a mixture of the optical spectra.
of tetrahydrofuran/diisopropylamine (8 : 4 mL) was stirred at
80 °C for overnight under nitrogen atmosphere, cooled, and Sample preparation for photophysical studies of DTEs in
the excess solvent was removed under reduced pressure. The presence of Fe3+ ions
residue was purified on a silica gel column using a mixture of Dilute solutions of DTEs were prepared by dissolving
60% ethyl acetate in hexane as eluent to afford a pale-yellow appropriate amount of DTEs in analytical grade solvents. The
1
solid (0.09 g, yield 34%). H NMR (300 MHz, CDCl3) δ 7.53 (s, concentrations for DTE1
,
DTE2, and DTE3 were 5.0 x 10-5 M,
8H), 7.14 (s, 4H), 7.08 (d, J = 8.2 Hz, 2H), 6.94 (d, J = 8.2 Hz, 2.5 x 10-5 M, and 2.2 x 10-5 M, respectively. The quartz cuvette
2H), 5.36 (s, 4H), 1.97 (s, 6H). 13C NMR (75 MHz, MeOD) δ with optical path length of 1 cm was used. Aqueous Fe(NO3)3
146.82, 145.01, 143.17, 133.84, 133.28, 132.84, 132.24, solution (1 equivalent) was added to the DTEs solutions before
129.92, 127.51, 126.04, 123.64, 121.45, 119.59, 116.87, or after irradiation with UV light at 365 nm in presence of
114.93, 95.21, 82.14, 14.49. HRMS (APCl, +ve, (M +H)+) : K2CO3, prior to spectroscopic analyses.
(C43H27F6O4S2) Calculated m/z = 785.1249, Found m/z =
785.1242. Elemental analysis calculated (%) for C43H26F6O4S2: Sample preparation for electrochemical studies
C, 65.81; H, 3.34; S, 8.17; Found: C, 66.11; H, 3.34; S, 7.99.
Dilute solutions of DTEs were prepared by dissolving
appropriate amount of DTEs in anhydrous dichloromethane
(0.5 mg/mL), which was distilled from calcium hydride under
nitrogen atmosphere. The cyclic voltammograms of DTEs were
DTE3
The synthesis of DTE3 was same as described for DTE1
.
A solution of DTE (0.1 g, 0.16 mmol), 10 (0.29 g, 0.35 mmol), recorded in
a
0.1 M solution of tetrabutylammonium
tetrakis(triphenylphosphine)palladium(0) (0.037 g, 0.032 hexafluorophosphate in dichloromethane at a scan rate of 100
mmol), and copper(I) iodide (0.006 g, 0.032 mmol) in a mixture mV s-1, with a Ag/AgCl as reference electrode, platinum wire as
of tetrahydrofuran/diisopropylamine (8 : 4 mL) was stirred at counter electrode, and platinum disk as working electrode.
80 °C for overnight under nitrogen atmosphere, cooled, and
the excess solvent was removed under reduced pressure. The Sample preparation for morphology studies
residue was purified on a silica gel column using a mixture of Dilute solutions of DTEs were prepared by dissolving
40% ethyl acetate in hexane as eluent to afford a red solid appropriate amount of DTEs in analytical grade solvents. The
(0.10 g, yield 79%). 1H NMR (300 MHz, CDCl3) δ 7.90 (d, J = 6.0 concentration for all DTEs solutions was 6.0 x 10-4 M. 20 µL of
Hz, 4H), 7.87 (d, J = 6.0 Hz, 4H), 7.63 (d, J = 8.8 Hz, 4H), 7.30 (s, the DTEs solution was pipetted and drop-casted on glass
2H), 6.96 (d, J = 8.8 Hz, 4H), 1.97 (s, 6H). 13C NMR (75 MHz, substrates and the solvent was allowed to evaporate slowly in
CDCl3) δ 158.59, 152.13, 147.23, 143.76, 132.20, 131.66, a desiccator under ambient conditions under darkness for 24
125.21, 124.93, 124.37, 122.73, 121.66, 115.88, 93.95, 83.58, hours. DTE1closed and DTE2closed films were prepared by first
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12 | J. Name., 2012, 00, 1-3
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Solvent Dependent Isomerization of Photochromic Dithienylethenes: Synthesis,
Photochromism, and Self-assembly.
Choong Ping Sen and Suresh Valiyaveettil*
Department of Chemistry, National University of Singapore.
3 Science Drive 3, Singapore 117543.
Email: chmsv@nus.edu.sg
Dithienylethene molecules incorporated with phenolic or azophenolic groups are synthesized and characterized. Selfꢀassembly of
circular nanostructures are observed from dithienylethene with catechol end groups. The formation of nanostructures is dependent on
factors such as photoisomerization and hydrogen bonding.
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