Synthesis and photochromic reactivity of a new class of photochromic diarylethene dimer-containing dithieno[3,2-b:2′3′-d]thiophene

Article abstract of DOI:10.1016/j.tet.2012.08.026

Novel photochromic diarylethene dimers containing dithieno[3,2-b: 2′3′-d]thiophene were synthesized and their photochromic properties were studied in solution as well as in the crystalline phase. Only the isomer with the diarylethene units one in open-form and one in closed-form was produced upon irradiation with ultraviolet light because of the intramolecular excitation energy transfer in this isomer. Their electrochemical properties were also investigated associating with computational studies.

Full text of DOI:10.1016/j.tet.2012.08.026

Tetrahedron 68 (2012) 8719e8723
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Synthesis and photochromic reactivity of a new class of photochromic
diarylethene dimer-containing dithieno[3,2-b:2030-d]thiophene
,
,
a,
Hongke Wang a b, Wei Xu a , Daoben Zhu
*
*
a Beijing National Laboratory for Molecular Sciences, Organic Solid Laboratory, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
b Graduate School of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e i n f o
a b s t r a c t
Novel photochromic diarylethene dimers containing dithieno[3,2-b:2030-d]thiophene were synthesized
and their photochromic properties were studied in solution as well as in the crystalline phase. Only the
isomer with the diarylethene units one in open-form and one in closed-form was produced upon irra-
diation with ultraviolet light because of the intramolecular excitation energy transfer in this isomer.
Their electrochemical properties were also investigated associating with computational studies.
Ó 2012 Elsevier Ltd. All rights reserved.
Article history:
Received 2 June 2012
Received in revised form 20 July 2012
Accepted 10 August 2012
Available online 17 August 2012
Keywords:
Diarylethene
Photochromism
Single crystal
Intramolecular excitation energy transfer
1. Introduction
molecular system, which is critical for achieving photoswitchable
electrical transport induced by reversible photochromic reaction.
Fused oligothiophene derivatives have received increasing at-
tention in the last decade due to their fundamental optoelectronic
properties and their potential applications in the fabrication of
semiconductor devices.1e4 Amongst them, dithieno[3,2-b:20,30-d]
thiophene (DTT) is a fused oligomer of thiophene units with ex-
Herein, a new class of photochromic diarylethene dimer with a DTT
linkage, compounds 1a and 2a (Scheme 1) are designed and syn-
thesized. The two compounds show favorable photochromic per-
formance in solution as well as in the crystalline phase. To the best
of our knowledge, this is the first report on diarylethene dimers,
which display reversible photochromic reaction in the crystalline
state.
tended
p-conjugation, interesting electric and optical properties,
which have been widely employed as building block for the de-
velopment of organic semiconductors as active materials in or-
ganic field effect transistors (OFETs) and organic photovoltaics
(OPVs).5e10
Photochromic diarylethenes have attracted remarkable atten-
tion because of their potential ability for studies of optical memory
media and switching devices.11 Among them diarylper-
fluorocyclopentenes are the most promising candidate due to their
superior photochromic properties with high thermal stability, high
fatigue resistance, and fast response time.11,12 Aiming to develop
organic semiconductors with photoswitchable electrical transport
properties, we have tried to incorporate diarylethene units with
DTT.13 Recently, there is also a similar report on diarylethene
compound containing DTT moiety for the development of multi-
functional material.14,15 However, up to date, reversible photo-
chromic reaction in solid state is still unsuccessful in such
Scheme 1. Chemical structures of diarylethenes 1a and 2a.
2. Discussion and results
2.1. Synthesis
Diarylethenes 1a and 2a containing DTT units were synthesized
according to the route illustrated in Scheme 2. The key starting
material, 3,5-dimethyldithieno[3,2-b:2030-d]thiophene, was syn-
thesized with an efficient method developed in our group.13
* Corresponding authors. Tel.: þ86 10 6263 9355; fax: þ86 10 6256 9349; e-mail
addresses: wxu@iccas.ac.cn (W. Xu), zhudb@iccas.ac.cn (D. Zhu).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.08.026

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H. Wang et al. / Tetrahedron 68 (2012) 8719e8723
3,5-Dimethyldithieno[3,2-b:2030-d]thiophene has two reaction
sites in 2- and 6-positions, so coupling of lithiated
3,5-dimethyldithieno[3,2-b:2030-d]thiophene with compound
3a/3b afforded 1a and 2a with yields of 46% and 60% by controlling
the ratio of starting materials, respectively. The molecular struc-
The absorption characteristic of 2a was similar to that of 1a
(Fig. 1b). Upon irradiation with UV light, a new absorption band in
the visible region appeared as a result of the cyclization reactions,
which results in the formation of isomer 2b. In the photostationary
state, the conversion ratio from 2a to 2b was 87.91% under irra-
diation with a light of 380 nm as estimated by HPLC analysis
(Fig. S3).
tures of diarylethenes 1a and 2a were confirmed by 1H NMR, 13
C
NMR, EIMS, elemental analysis, and X-ray crystallography.
The closed-ring isomers 1b and 2b were isolated by HPLC from
the photogenerated colored solution and characterized by 1H NMR.
For diarylethene 1a in the photostationary state upon irradiation
with 380 nm, the photogenerated colored solution gave two peaks
with retention time of 31 and 39 min in HPLC. The first one is
assigned to 1a, and the second peak is due to 1b according to their
absorption spectra and 1H NMR spectroscopy. Fig. 2a shows 1H
NMR spectra of the isomer 1b in the range of 2.6e1.6 ppm. Six
different protons with the same integration corresponding to six
methyls could be observed for protons at 2.44, 2.31, 1.93, 1.89, 1.72,
and 1.64 ppm. The assignment of these signals was illustrated in
inset molecular structure in Fig. 2a. It is expected that if the isomer
1c with two closed-ring form of dithienylethene was generated
upon UV irradiation, three methyl protons should be observed due
to the symmetric molecular structure of 1c.
Scheme 2. Synthetic route of diarylethenes 1a and 2a.
2.2. Photochromic reactions in solution
The photochromic behavior of diarylethenes 1a and 2a induced
by photoirradiation at room temperature was measured in hexane.
The absorption spectral changes of 1a and 2a induced by alter-
nating irradiation with UV light and visible light at appropriate
wavelength are shown in Fig. 1. As shown in Fig. 1a, the open-ring
isomer 1a exhibited an absorption maximum at 386 nm as a result
of a
pep
*
transition.16 Upon irradiation with 380 nm, the absorp-
tion band at 386 nm decreased along with the appearance of a new
absorption band at 468 nm, indicating the formation of a closed-
ring isomer. The original absorption spectrum was recovered by
irradiation with visible light. Two well-defined isosbestic points at
about 338 and 426 nm were observed, suggesting that the photo-
cyclization could only take place at one of the diarylethene units
(Scheme 3) due to the intramolecular excitation energy transfer
from the excited open-ring unit to the closed-ring unit prohibits the
formation of the two closed-ring form dimer 1c.14,15 This could be
further verified by 1H NMR spectra analysis. The absence of the
isomers having two closed-ring forms 1c in the photoirradiated
dimer agrees to the result observed in the dimers reported by
Branda.17 Similar inactivity of the second photocyclization has also
been reported for molecules containing two diarylethene uni-
ts.14,15,18e20 In the photostationary state, the conversion ratio from
1a to 1b was 49.01% under irradiation with a light of 380 nm as
estimated by HPLC analysis (Fig. S1).
Fig. 2. 1H NMR spectra of 1b (a) and 2b (b) in the range of 2.6e1.6 ppm.
Similar to that of 1b, four methyl protons could be observed in
the 1H NMR spectra of isolated 2b with chemical shifts of 2.05, 1.96,
1.79, and 1.72 ppm (Fig. 2b) corresponding to the formation of the
isomer with one closed-ring form dithienylethene. The signals at
1.96 ppm (b0) and 1.72 ppm (b00) are assigned to the 2-methyl
protons of the open-ring form unit and the closed-ring form unit,
respectively. The signals at 2.05 ppm (a0) and 1.79 ppm (a00) are
assigned to the methyl protons of the DTT. The ratio of the signals at
1.96, 1.72, 2.05, and 1.79 ppm is 1:1:1:1. So, the formation 2b can be
verified by this 1H NMR data.
Fig. 3 shows absorption spectra of 1a, 1b, 2a, and 2b. As diary-
lethenes 1a and 2a possess similar molecular structures, it is in-
teresting to compare their absorption bands. The absorption
maxima of the open isomers 1a and 2a are at 386 and 388 nm. The
absorption maximum of closed-ring isomer 2b (508 nm) showed
red shift about 40 nm in comparison with 1b (468 nm) because of
the conjugation extending induced by the introduction of the
phenyl group.
Fig. 1. Absorption spectral changes of 1a and 2a in hexane upon irradiation with 380
and 500 nm (C¼1.5ꢀ10ꢁ5 mol/L): (a) 1a, (b) 2a.
Fig. 3. Absorption spectrum of 1a (solid line), 1b (dash line), 2a (dot line), 2b (dash dot
Scheme 3. Photochromic reaction of 1 and 2.
line).

H. Wang et al. / Tetrahedron 68 (2012) 8719e8723
8721
2.3. X-ray crystallographic analysis
2.4. Electrochemical properties of diarylethenes 1a/1b and 2a/
2b
Crystals suitable for single-crystal X-ray diffraction analysis
were obtained by slow evaporation of the solution of corresponding
compounds in hexane at room temperature. The X-ray crystallo-
graphic analysis data are collected in Table S1. The ORTEP drawing
of the molecule structures in single crystals of 1a and 2a are shown
in Fig. 4 and their packing diagrams are shown in Fig. S5.
Switching behaviors triggered by electrochemical reactions
have been employed for the development of molecular switches. It
has already been demonstrated that ring opening and closing re-
action of diarylethenes could also be induced by electrochemically
redox reactions. Therefore, it should be interesting to investigate
the electrochemical behaviors of diarylethenes synthesized here.
Fig. 6 shows the cyclic voltammetry (CV) curves of diarylethenes in
acetonitrile. The oxidation onsets of the open-ring isomers 1a and
2a were at 1.61 and 1.58 V, and those of the closed-ring isomers 1b
and 2b were at 1.13 and 1.10 V, respectively. The result showed that
the oxidation process for the open-ring isomers 1a and 2a occurs at
higher potentials than the corresponding closed-ring isomers 1b
and 2b, indicating the extending of the p-conjugation in the closed-
ring isomers. This is in accordance with the theory that the longer
conjugation length of the closed-ring isomer generally leads to
a less positive oxidation potential. Because the
localize throughout the two connected aryl moieties and extend to
the substituents after cyclization reaction, the -conjugation length
p-electrons de-
Fig. 4. ORTEP drawing of crystals 1a (a) and 2a (b), showing 50% probability dis-
placement ellipsoids.
p
of the closed-ring isomers 1b and 2b was much longer than those of
their open-ring isomers 1a and 2a, resulting in the larger oxidation
onsets. However, no ring closing or opening reaction was observed
during the electrochemical characterizations.
It can be observed that compounds 1a and 2a adopted photoactive
antiparallel conformation in the crystalline phase. For 1a, there were
two independent molecules in the nonsymmetrical unit and all of
them were in photoactive antiparallel conformation. The distance
ꢀ
ꢀ
betweenthephotoactivecarbonswere3.464 A (C5/C13)and3.593 A
ꢀ
ꢀ
(C19/C29) in molecule 1a-I, and 3.550 A (C37/C45) and 3.535 A
(C51/C59) in molecule 1a-II. For diarylethene 2a, the distance be-
ꢀ
ꢀ
tween the photoactive carbons were 3.616 A (C10/C18) and 3.616 A
ꢀ
(C10A/C18A), respectively. These values are all less than 4.2 A the
distances required for a photochromic reaction in the solid state.21
In fact, the crystals 1a and 2a showed good photochromism in
accordance with the expected analysis. Their color changes upon
photoirradiation are shown in Fig. 5. For diarylethene dimer, in
most cases the intramolecular excited energy transfer from the
open-form unit to the closed-form one prohibits the formation of
the two closed-ring form dimer. Crystals 1a and 2a could only take
place at one of the diarylethene units even upon prolonged irra-
diation and this could be confirmed by the HPLC chromatograms
(Figs. S2 and S4). Upon irradiation with 380 nm light, the pale-
yellow crystals of 1a and 2a turned orange quickly, as a result of
the formation of the closed-ring isomers 1b and 2b. Alternatively,
the orange crystals returned to pale-yellow and reproduced 1a and
2a upon irradiation with visible light (>500 nm). The photochromic
conversion ratios of 1a and 2a crystals in the photostationary state
under UV irradiation were 8.08% for 1a to 1b conversion and 15.01%
for 2a to 2b as evaluated by HPLC (Figs. S2 and S4). Those crystals
exhibited coloration/decoloration cycles by alternate irradiation
with UV and visible light and their closed-ring forms 1b/2b
remained stable for more than four months in the dark at room
temperature. This indicated that they could be potentially used for
the construction of certain optoelectronic devices.22,23
Fig. 6. Cyclic voltammetry of 1a and 2a (solid line) and 1b and 2b (dash line): (a) 1a/
1b, (b) 2a/2b. Measurements were performed in acetonitrile (1.0ꢀ10ꢁ3 M) on a plati-
num electrode, scan rates were 50 mV/s.
The HOMO, LUMO energy levels derived from the CV data, and
HOMOeLUMO energy gap (HLG) of each compound were sum-
marized in Table 1. The data showed that the HLG of close-ring
isomers 1b and 2b (2.02 and 2.03 eV) was smaller than those of
their corresponding open-ring isomers 1a and 2a (2.82 and
2.83 eV). This was in well agreement with the bathochromic shift
observed by UVevis spectroscopy and also can be mainly ascribed
to the extending of conjugation length for the closed-ring isomers
1b and 2b than that of the open-ring isomers 1a and 2a.
Table 1
Electrochemical properties of diarylethenes 1a/1b and 2a/2b in acetonitrile
Compounds Oxidation
Reduction
HLG Optical band
(eV) gap (eV)
Eonset (V) HOMO (eV) Eonset (V) LUMO (eV)
1a
1b
2a
2b
1.61
1.13
1.58
1.10
ꢁ6.01
ꢁ5.53
ꢁ5.98
ꢁ5.50
ꢁ1.21
ꢁ0.90
ꢁ1.25
ꢁ0.92
ꢁ3.19
ꢁ3.50
ꢁ3.15
ꢁ3.48
2.82 2.88
2.03 2.34
2.83 2.86
2.02 2.11
2.5. Theoretical investigations
To gain insight into nonsymmetrical structure and electronic
absorption properties of diarylethenes 1a/1b and 2a/2b, we carried
Fig. 5. Photographs of photochromic processes for diarylethenes 1a and 2a in the
crystalline phase.

8722
H. Wang et al. / Tetrahedron 68 (2012) 8719e8723
out molecular orbital calculation on the open- and closed-ring
isomers of diarylethenes. The geometry optimization was calcu-
lated at the density functional theory (DFT) level using the B3LYP
spectra were performed in CDCl3 with TMS as internal standard on
Bruker Advance 400 MHz spectrometers. MS (EI) measurements
were performed on SHIMADZU GCMSQP2010 or UK GCT-
Micromass spectrometers. MALDI-TOF mass spectrometric mea-
surements were performed with a Bruker Biflex III 45 MALDI-TOF
spectrometer. Elemental analyses were measured on a Carlo Erba
1106 elemental analyzer. HRMS measurements were performed on
a Bruker APEXIIFT-ICRMS spectrometer. UVevis spectra were
recorded on a JASCO V-570 spectrometer. Photoirradiation was
carried out using a PLS-SXE300UV xenon arc lamp (Beijing Trust-
tech Co. Ltd). Appropriate wavelength was isolated by passing
through a bandfilter.
functional with 6-31G basis sets. To reduce the computational cost,
*
only the photoactive antiparallel conformation in the open-form
was considered.
The HOMOeLUMO gaps (HLG) of these isomers were summa-
rized in Table S2. The HLG of the open-ring isomers 1a and 2a was
3.37 and 3.30 eV, while a significant decrease in energy gap is found
in the corresponding closed-ring isomers 1b and 2b (3.01 and
2.77 eV). This was in agreement with data extracted from the
electrochemical studies.
Fig. 7 illustrates the calculated HOMO and LUMO of the geom-
etry optimized structures in the ground states for the diaryelthenes
1a/1b and 2a/2b. The electron densities of both HOMO and LUMO
with ring-opening isomers are different from those with closed-
ring isomers. For open-ring isomers 1a and 2a, the HOMO orbital
4.2. (2,5-Dimethyl-3-thienyl)perfluorocyclopentene (3a)
n-BuLi (4.2 mL, 2.5 M solution in hexanes) was added dropwise
to a stirred solution of 3-iodo-2,5-dimethylthiophene (2.38 g,
10.0 mmol) in THF at ꢁ78 ꢂC under nitrogen atmosphere. After
stirring at this low temperature for 2 h, perfluorocyclopentene
(1.43 mL, 10.5 mmol) was added to the reaction mixture at ꢁ78 ꢂC,
and the mixture was stirred for another 2 h at this temperature.
The reaction mixture was warmed to room temperature sponta-
neously and then stopped by the addition of water. The product
was extracted with ether. The organic layer was washed with
water and dried over magnesium sulfate, filtrated, and evapo-
rated. Compound 3a was obtained after purification by column
chromatography using silica (hexane) (2.23 g, 73.4%) as colorless
distribution of the electron density is mainly composed of the
p
orbital of DTTcore and spread partially on the peripheral thiophene
units. For closed-ring isomers 1b and 2b, the electron densities of
HOMO and LUMO both localize on the one side of the closed dia-
rylethene units.
oil. 1H NMR (400 MHz, CDCl3):
d (ppm) 6.73 (s, 1H), 2.43 (s, 3H),
2.39(s, 3H). HRMS (m/z): calcd for C11H7F7S: 304.0157, found:
304.0162.
Fig. 7. Frontier molecular orbital densities for diarylethenes 1a/1b and 2a/2b at the
B3LYP/6-31G level of theory.
*
4.3. (2-Methyl-5-phenyl-3-thienyl)perfluorocyclopentene
(3b)
3. Conclusion
Compound 3b was prepared by a method similar to that used of
3a. The crude product was purified by column chromatography
using silica (hexane) to afford 4.91 g (71.5%) of compound 3a as
In conclusion, a new class of photochromic diarylethene dimer
with a DTT linkage 1a and 2a were designed and synthesized.
Their electronic structures and computational studies have been
carried out. Their photochromic behavior was examined in solu-
tion as well as in the crystalline phase. However, only the isomer
with the diarylethene units one in open-form and one in closed-
form 1b/2b was produced upon irradiation with ultraviolet light
because of the intramolecular excitation energy transfer between
the closed-ring and open-ring dithienylethene unit. The molec-
ular structures of isomers 1b and 2b were confirmed by absorp-
tion spectra and 1H NMR spectroscopy. In the photostationary
state, the conversion ratios for 1a to 1b and 2a to 2b were 49.01%
and 87.91% in solution under irradiation with a light of 380 nm,
which are higher than that (8.08% for 1a to 1b and 15.01% for 2a
to 2b) in the crystalline phase. Although similar compounds have
been reported previously, no photochromic reaction in the solid
state has been studied for such molecules. It should be useful for
the development of molecular materials based on DTT units.
Further investigations on their possible applications are in
progress.
white powder. 1H NMR (400 MHz, CDCl3):
d
(ppm) 7.51 (d, J¼7.2 Hz,
2H), 7.38 (t, J¼7.3 Hz, 2H), 7.27 (t, J¼8.0 Hz, 1H), 7.10(s, 1H), 2.42(s,
3H). Anal. Calcd for C16H9F7S: C, 52.46; H, 2.48. Found: C, 52.58; H,
2.60.
4.4. 2,6-Bis[1-(2,5-dimethyl-3-thienyl)-2-perfluorocyclo-
pentene]-3,5-dimethyldithieno[3,2-b:2030-d]thiophene (1a)
n-BuLi (2.5 M solution in hexanes, 0.176 mL) was added
dropwise to
a stirred solution of 3,5-dimethyldithieno[3,2-
b:20,30-d]thiophene (0.0472 g; 0.21 mmol) in THF at ꢁ78 ꢂC un-
der nitrogen atmosphere. Stirring was continued for 2 h at the
low temperature. Compound 3a (0.134 g; 0.44 mmol) in THF was
slowly added to the reaction mixture at ꢁ78 ꢂC and the mixture
was stirred for 2 h at this low temperature. Then, the reaction
mixture was quenched with water. The product was extracted
with ether. The organic layer was washed with water, dried with
anhydrous magnesium sulfate, and concentrated under reduced
pressure. The crude product was purified by column chroma-
tography using silica (hexane) to afford 0.101 g (60 %) compound
1a as yellow powder. Mp: 138e139 ꢂC; 1H NMR (400 MHz,
4. Experiment section
4.1. General
CDCl3):
d
(ppm) 6.75(s, 2H), 2.43 (s, 6H), 1.89 (d, J¼4.2 Hz, 12H);
13C NMR (100 MHz, CDCl3):
d
(ppm) 144.1, 140.5, 138.4, 133.8,
THF was distilled over sodium benzophenone ketyl before use.
All other reagents and solvents were purchased from commercial
suppliers and used directly as received. 1H NMR and 13C NMR
132.2, 124.9, 124.5, 15.31, 14.6, 14.5; MS (m/z): calcd 792.8, found:
792.7. Anal. Calcd for C32H20F12S5: C, 48.48; H, 2.54. Found: C,
48.65; H, 2.81.

H. Wang et al. / Tetrahedron 68 (2012) 8719e8723
8723
4.5. 2,6-Bis[1-(2-methyl-5-phenyl-3-thienyl)-2-perfluo-
rocyclopentenyl]-3,5-dimethyldithieno[3,2-b:2030-d]thio-
phene (2a)
http://dx.doi.org/10.1016/j.tet.2012.08.026. These data include MOL
files and InChiKeys of the most important compounds described in
this article.
Compound 2a was prepared by a method similar to that used for
1a. The crude product was purified by column chromatography
using silica (hexane) to afford 0.089 g (63%) of compound 2a as
yellow powder. Mp: 191e192 ꢂC; 1H NMR (400 MHz, CDCl3):
References and notes
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Supplementary data
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