Near-infrared photochromic diarylethenes based on the changes in the π-conjugated system and the electronic properties of the heteroaryl moieties

Article abstract of DOI:10.1002/ejoc.201301132

It is very challenging to construct photochromic materials with absorption and reactivity in the near-infrared (NIR) spectral region. Herein, four unsymmetrical diarylethene derivatives 1o-4o, which contain both indole and thiophene moieties, have been successfully synthesized, and their photochromic behaviors have been shifted to the NIR region. These newly developed NIR photochromic materials are highly sensitive and responsive to photostimuli both in solution and in poly(methyl methacrylate) films. It has been shown that 1o-4o exhibit remarkable UV/Vis and fluorescence spectral changes, allowing them to behave as reversible optical molecular switches in the NIR region. Four unsymmetrical diarylethene derivatives, 1o-4o, which contain both indole and thiophene moieties, have been successfully synthesized, and their photochromic behaviors have been shifted to the NIR region. The photophysical and photochromic properties of 1o-4o were comprehensively investigated by NMR, UV/Vis, fluorescence spectral and X-ray crystallographic analysis. Copyright

Full text of DOI:10.1002/ejoc.201301132

FULL PAPER
DOI: 10.1002/ejoc.201301132
Near-Infrared Photochromic Diarylethenes Based on the Changes in the π-
Conjugated System and the Electronic Properties of the Heteroaryl Moieties
[a]
[a]
[a]
Hong-Bo Cheng, Xin Tan, and Mei-Li Pang*
Keywords: Photochromism / UV/Vis spectroscopy / Molecular electronics / Molecular devices / Diarylethenes
It is very challenging to construct photochromic materials
with absorption and reactivity in the near-infrared (NIR)
spectral region. Herein, four unsymmetrical diarylethene de-
rivatives 1o–4o, which contain both indole and thiophene
moieties, have been successfully synthesized, and their
photochromic behaviors have been shifted to the NIR region.
These newly developed NIR photochromic materials are
highly sensitive and responsive to photostimuli both in solu-
tion and in poly(methyl methacrylate) films. It has been
shown that 1o–4o exhibit remarkable UV/Vis and fluores-
cence spectral changes, allowing them to behave as revers-
ible optical molecular switches in the NIR region.
Introduction
generating closed and opened isomers by irradiation with
light in the UV and NIR regions. We introduced the indole
chromophore in 1o–4o to enhance electron transport prop-
erties in order to facilitate low-energy intramolecular charge
transfer (CT) from electron-donor indole units to the
powerful electron-accepting chromophores of the photo-
chromic switches. We also introduced the thermally stable
thiophene chromophore to our unsymmetrical DAEs using
perfluorocyclopentene as the central ethene linker because
of its resistance to fatigue.
Research on photochromic materials has received much
attention because of the materialsЈ potential application to
optical data storage and optoelectronic devices.[ Due to
excellent thermally irreversible photochromic behavior and
outstanding fatigue resistance properties, diarylethene de-
rivatives (DAEs) are the most promising optically respon-
1]
[
2]
sive compounds among photochromic compounds. To en-
hance the susceptibility of optical memory storage, photo-
chromic compounds with absorption and reactivity in the
near-infrared (NIR) region are highly desirable.[ Recently,
a few research groups have developed a series of NIR pho-
tochromic materials, including introducing transition metal
and borane complexes[ to the aromatic heterocyclic
chromophore of diarylethenes. When two naphthopyran
entities were joined through a dithienylethene bridge, a tri-
photochromic system resulted in an absorption behavior
reaching far into NIR region.[ However, to design and
synthesize NIR-responsive photochromic materials remains
a challenge. Extending the π-conjugation of DAEs and in-
troducing organic donor–acceptor (D–π–A) chromophores
are important and efficient in shifting the absorption max-
ima to the NIR region.[
3]
4]
5]
6]
In this work, we describe the design and synthesis of four
NIR-responsive unsymmetrical diarylperfluorocyclopent-
enes bearing both indole and thiophene moieties, 1o–4o,
and the reference compound 5o (Scheme 1). Their photo-
physical, X-ray crystallographic, and photochromic behav-
iors were investigated in detail. These photochromic com-
pounds 1o–4o underwent a reversible pericyclic reaction,
Scheme 1. Photoisomerization of 1o–4o and the reference com-
pound 5o.
Results and Discussion
[
a] Department of Chemistry, Nankai University,
Tianjin 300071, P. R. China
Synthesis
E-mail: pangmeili@nankai.edu.cn
http://www.nankai.edu.cn/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301132.
The syntheses of 1o–4o are shown in Scheme 2. The tar-
get unsymmetrical DAEs 1o and 3o were prepared in 55–
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H.-B. Cheng, X. Tan, M.-L. Pang
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Scheme 2. Synthesis of compounds 1o–4o.
60% yield by condensation between the 3-bromothiophene X-ray Crystallographic Study
derivatives and the perfluorocyclopentene-substituted in-
[
7]
dole. Compound 2o was synthesized from 1o by reaction
Single crystals of 1o, 3o and 7 were obtained by slow
with hydroxylamine hydrochloride (yield, 60%). Compound evaporation from an ethyl acetate/hexane solution. Final
4o was prepared in 63% yield by the reaction of 3o with structural conformations of 1o, 3o and 7 were provided by
malononitrile under basic conditions. The structures were X-ray crystallographic analysis. Their structural features are
identified by NMR spectroscopy, HRMS spectra, and X- shown in Figure 1, and packing diagrams are shown in Fig-
ray crystallographic analysis (see Supporting Information ure 2. As shown in Figure 2, they exclusively adopt the anti-
Figures S1–S8).
parallel conformations, which are in sharp contrast to the
Figure 1. Crystal structures of (a) 7, (b) 1o, and (c) 3o.
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Near-IR Photochromic Diarylethenes
Figure 2. Molecular packing diagrams of (a) 1o and (b) 3o.
solution-phase behavior, where the antiparallel and parallel peared and the absorption bands of the open-ring forms
conformers coexist. Moreover, the 3D supermolecular were restored. On the other hand, the differences in their
framework and 2D layers are formed by the hydrogen- spectral properties are attributed to the heteroaryl substitu-
bonding interactions in 1o and 3o. The distance between ents, which alter the electronic properties of the switches.
the photocyclizing carbon atoms (C8···C21 for 1o, C2···C19 Compound 3c exhibited its maximum at 630 nm, whereas
for 3o) is 4.369 Å and 3.597 Å, respectively. The crystals of the maximum of 1c is located at 675 nm. The bathochromic
3o show photochromic effect upon irradiation with UV shift of 1c is due to a relative increase of electron density in
light because photochromic reactivity usually appears when the indole moiety caused by the electron-donating methoxy
the distance between the reactive C atoms is less than 4.2 Å substituents. Moreover, compared with compound 3o, com-
and the molecule is fixed in an antiparallel mode.[
8]
pound 4o showed a strong bathochromic shift (ca. 92 nm)
upon UV irradiation. This observation confirmed that ex-
Photochromic Behaviour
Table 1. The UV/Vis absorption maxima of compounds 1o–4o in
The spectral changes of 1o–5o in hexane exposed to UV hexane, toluene and poly(methyl metharcrylate) (PMMA) before
light are shown in Figures S1 (in the Supporting Infor-
and after irradiation with 365 nm light at 25 °C (0–2 min).
mation) and Table 1. Upon irradiation with UV light, the
initially colorless solutions of compounds 1o–4o display a
strong coloration (Figure S9, Figure S10). The photo-
generated closed-ring isomers have their absorption max- 3o
ima at 675, 650, 630 and 720 nm, respectively. Upon NIR
Hexane λmax [nm]
Toluene λmax [nm] PMMA λmax [nm]
1
2
o
o
675
650
630
722
605
702
672
653
748
608
728
680
660
754
612
4o
o
5
light irradiation (λ Ͼ 600 nm) the dark blue color disap-
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H.-B. Cheng, X. Tan, M.-L. Pang
FULL PAPER
tending the π-conjugation of DAEs and introducing organic by alternate UV and NIR-light irradiation without any ap-
D–π–A chromophores are efficient approaches to shifting parent degradation (Figure S11). Moreover, 1o–4o dis-
the absorption maxima to longer wavelengths.
played excellent photochromic performance in poly(methyl
The spectral and the color changes upon UV light irradi- methacrylate) (PMMA), as well as in solution (Figure 5).
ation of 1o–4o in toluene are shown in Figure 3 and Fig- Irradiation with UV light triggered the emergence of a
ure 4. The absorption spectra showed a strong batho- broad absorption band in the NIR range, with 1c and 2c
chromic shift with the increase in the polarity of solvents. having their maxima at 728 and 680 nm, whereas the max-
The photogenerated closed-ring isomers have their absorp- ima of 3c, 4c were located at 660 and 754 nm.
tion maxima at 702, 672, 653 and 748 nm, respectively. As
The absorption maxima of closed-ring isomers 1c–4c
discerned from Table 1, the absorption maxima of closed- underwent an obvious redshift of 26, 8, 7, and 6 ppm in
ring isomers 1c–4c underwent an obvious solvent-induced PMMA films compared with solutions of these compounds
shift of 27, 22, 23, and 28 ppm, respectively. Of particular in toluene. The bathochromic shift of 4c is due to a relative
note, NIR photochromic materials 1–4 could be repeatedly increase of the π-conjugation and a relative decrease of elec-
transformed between the ring-open and ring-closed forms tron density in the thiophene moiety caused by the electron-
withdrawing malononitrile substituents. This illustrates the
possibility of fine-tuning the spectral profile of these mo-
lecular switches by changing the electronic properties of the
[
9]
heteroaryl moieties. Significantly, 1o–4o underwent an ob-
vious redshift compared with the reference compound 5o
both in solution and in PMMA under UV irradiation.
Moreover, upon irradiation with NIR light (Ͼ600 nm), the
absorption bands of the open-ring forms of 1o–4o were re-
stored (Figure S12, S13). These observations indicate these
compounds can revert to the colorless open form even in
PMMA film.
It is well documented that distinct differences in NMR
signals between the opened isomers and the closed isomers
of DAEs at both high-and low-field could be observed.[
As shown in Figure 6, for example, the methyl hydrogens
9c]
Figure 3. Photographic images of 1o–4o upon alternating UV and
NIR-light irradiation in toluene at 25 °C.
Figure 4. UV/Vis spectral changes of (a) 1o, (b) 2o, (c) 3o and (d) 4o upon UV-light irradiation in toluene at 25 °C (0–2 min).
936 Eur. J. Org. Chem. 2013, 7933–7940
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Near-IR Photochromic Diarylethenes
Figure 5. UV/Vis spectral changes of a poly(methyl methacrylate) (PMMA) film loaded at 2 wt.-% with (a) 1o, (b) 2o, (c) 3o and (d) 4o
at 25 °C (0–2 min) after irradiation with 365 nm light.
1
2
3
4
(
H and H ) at the reactive carbon in 2o give two single the other hand, in contrast with the H and H signals (3.80
peaks at δ = 1.99 and 1.86 ppm, respectively. Upon irradia- and 3.63 ppm) in 2o, the signals of H3 and H in 2c are
Ј
4Ј
tion with UV light, two new peaks appear at δ = 1.68 and shifted upfield to 3.78 and 2.71 ppm, respectively. Addition-
5
1
.64 ppm, corresponding to the methyl protons of 2c. On ally, the protons H located on the thiophene moiety un-
dergo a significant upfield shift by 0.62 ppm.
The photocyclization process of 3o was also examined by
1
H NMR spectroscopy. Upon irradiation at 365 nm (Fig-
1
2
ure 7), the methyl hydrogens H and H at the reactive car-
bon in 3o were shifted upfield (Δδ = –0.20 and –0.32 ppm,
4
respectively), while the protons H located on the thiophene
moiety in 3o was shifted downfield (Δδ = 0.04 ppm). In ad-
3
dition, the methyl hydrogens H located on the indole por-
tion of 3o shifted downfield after photocyclization. These
changes in NMR signals might be ascribed to the great
changes in the chemical environment upon photochromic
reaction. Moreover, the conversion yields from opened-
form to closed-form were determined to be 86.1% for 2 and
1
8
5.9% for 3 by H NMR (Table S2). These observations
jointly indicate that the unsymmetrical DAEs 2o and 3o
exhibited excellent photochromic performance.
The fluorescence properties of unsymmetrical DAEs can
Figure 6. Changes in the H NMR signals of 2o upon UV irradia- also be modulated by their photochromism. As discerned
1
tion in CDCl solution.
3
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H.-B. Cheng, X. Tan, M.-L. Pang
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of 350 nm, 3o exhibited an intense fluorescence at ca.
4
61 nm. Moreover, when 3o was exposed to 365 nm UV
light, the fluorescence intensity maximum decreased ca.
0% (Figure 8, b). However, after NIR-light (λ Ͼ 600 nm)
irradiation for 10 min, the fluorescence intensity of 1o and
o were totally recovered.
8
3
Conclusions
In summary, we have designed and synthesized four un-
symmetrical diarylethene derivatives, 1o–4o, which contain
both indole and thiophene moieties, and their photochro-
mic behaviors have been shifted to the NIR region. These
compounds form molecular switches that undergo light-in-
duced cyclization and decylization reversion reactions.
Moreover, their photophysical and photochromic proper-
ties were comprehensively investigated by NMR, UV/Vis,
fluorescence spectral and X-ray crystallographic analysis.
These newly developed NIR photochromic materials
showed good photochromism both in solution and in
PMMA films. Compared with the reference compound 5o,
the absorbance maxima of 1o–4o were dramatically red-
shifted both in solution and in PMMA under UV irradia-
tion. Significantly, 4o underwent an obvious redshift com-
pared with 1o–3o both in solution and in PMMA under
UV irradiation. The bathochromic shift may be due to a
relative increase of the π-conjugation and a relative decrease
of the electron density in the thiophene moiety. We have
shown that this can be exploited to generate switches with
distinct spectral properties by changing the electronic prop-
erties of the heteroaryl moieties. Further exploration of
such switches could be useful for the development of more
sophisticated and functional NIR-sensitive optoelectronic
devices.
1
Figure 7. Changes in the H NMR signals of 3o upon UV irradia-
tion in CDCl solution.
3
from Figure 8a, upon excitation at the isosbestic point of
75 nm, 1o exhibits an intense fluorescence at ca. 470 nm.
Upon irradiation at UV (365 nm), an obvious fluorescence
quenching was observed on account of the resulting closed
form (1c). When reaching the photostationary state (PSS),
the fluorescence intensity of 1o decreased by about 88%.
On the other hand, upon excitation at the isosbestic point
3
Experimental Section
Instrumentation: All chemicals were commercially available unless
stated otherwise. 4,4Ј-(Perfluorocyclopent-1-ene-1,2-diyl)bis(5-
methylthiophene-2-carbaldehyde) (5o),[
10]
3-bromo-5-(diethoxy-
methyl)-2-methylthiophene (6),[ 5-methoxy-1,2-dimethyl-3-(per-
11]
[
1d]
fluorocyclopent-1-enyl)-1H-indole (7)
and 1,2-dimethyl-3-(per-
fluorocyclopent-1-enyl)-1H-indole (8)[ were prepared according to
the literature procedures. NMR spectra were recorded with a Var-
ian Mercury VX400 instrument. The optical switch experiments
were carried out using a photochemical reaction apparatus with a
7]
500 W Hg lamp. UV/Vis spectra were recorded with a Shimadzu
UV-3600 spectrophotometer equipped with a PTC-348WI tempera-
ture controller. Steady-state fluorescence spectra were recorded in
a conventional quartz cell (light path 10 mm) with a Varian Cary
Eclipse instrument equipped with a Varian Cary single-cell Peltier
accessory to control the temperature.
4
-[3,3,4,4,5,5-Hexafluoro-2-(5-methoxy-1,2-dimethyl-1H-indol-3-
yl)cyclopent-1-enyl]-5-methylthiophene-2-carbaldehyde (1o): n-
Butyllithium (2.4 m in hexane, 15.6 mmol, 6.5 mL) was added drop-
wise to a stirred solution of compound 6 (3.35 g, 12 mmol) in THF
(100 mL) at –78 °C under a nitrogen atmosphere. After 60 min, the
reaction mixture was transferred to a Schlenk tube containing a
Figure 8. Fluorescence spectral changes of (a) 1o and (b) 3o (2.0ϫ
–5
10
m) upon 365 nm light irradiation in toluene at 25 °C (0–2 min).
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Near-IR Photochromic Diarylethenes
solution of compound 7 (4.38 g, 12 mmol) in THF (20 mL). The
reaction mixture was stirred for 1 h at –78 °C, then warmed to
room temperature, and stirred for an additional 16 h. After the
addition of water (200 mL), the THF was removed under reduced
CH
MgSO
2
Cl
2
(3ϫ 30 mL). The combined organic phases were dried with
, filtered and the solvents were evaporated in vacuo. The
4
residue was purified by chromatography over silica gel. Elution
with a 1:12 ethyl acetate/pentane mixture afforded 3o as yellow
crystals (2.92 g, 55 %); m.p. 162–163 °C. H NMR (400 MHz
1
pressure, and the residue was extracted with CH
2
Cl
2
(3ϫ 30 mL).
The combined organic phases were dried with MgSO
4
, filtered and
3
CDCl ): δ = 9.84 (s, 1 H), 7.82 (s, 1 H), 7.55–7.53 (d, J = 8.0 Hz,
the solvents were evaporated in vacuo. The residue was extracted
with 50 mL of THF. Then p-toluenesulfonic acid (PTSA) (0.23 g,
1 H), 7.29–7.28 (d, J = 8.1 Hz, 1 H), 7.24–7.22 (d, J = 8.3 Hz, 1
H), 7.16–7.12 (t, J = 7.5 Hz, 1 H), 3.64 (s, 3 H), 2.00 (s, 3 H), 1.87
(s, 3 H) ppm. C NMR (100 MHz CDCl ): δ = 182.2, 151.6, 141.3,
3
1
3
1.2 mmol) and a few drops of water were added. The reaction mix-
ture was stirred for 24 h at 45 °C. The THF was removed under
reduced pressure, and then water (100 mL) was added, and the resi-
137.9, 137.1, 136.9, 127.5, 125.3, 122.3, 121.3, 119.3, 109.4, 100.6,
77.4, 77.0, 76.8, 30.0, 15.6, 11.4 ppm. HRMS (ESI): calcd. for
2
Cl
2
(3ϫ 30 mL). The combined organic
, filtered and the solvents evaporated
21 15 6
C H F
NOS+ [M + H]+ 444.0851; found 444.0855.
due was extracted with CH
phases were dried with MgSO
4
2
-({4-[2-(1,2-Dimethyl-1H-indol-3-yl)-3,3,4,4,5,5-hexafluorocyclo-
in vacuo. The residue was purified by chromatography over silica
pent-1-enyl]-5-methylthiophen-2-yl}methylene)malononitrile (4o):
gel. Elution with an ethyl acetate/pentane (1:12) mixture afforded
Compound 3o (0.91 g, 2.0 mmol) was added to a mixture of tetra-
hydrofuran (100 mL), malononitrile (0.14 g, 2.1 mmol), and piperi-
dine (0.5 mL), and the resulting mixture was heated for 24 h at
1
1
o as yellow crystals (2.83 g, 50 %); m.p. 158–159 °C. H NMR
(400 MHz CDCl
3
): δ = 9.77 (s, 1 H), 7.77 (s, 1 H), 7.11–7.09 (d, J
=
8.9 Hz, 1 H), 6.90 (s, 1 H), 6.82–6.80 (d, J = 9.8 Hz, 1 H), 3.73
6
5 °C. The solvent was removed in vacuo, and then water (100 mL)
was added. The residue was extracted with CH Cl (3ϫ 30 mL),
and the combined organic phases were dried with MgSO , filtered
13
(
(
1
1
s, 3 H), 3.54 (s, 3 H), 1.91 (s, 3 H), 1.80 (s, 3 H) ppm. C NMR
100 MHz CDCl ): δ = 181.2, 154.2, 150.7, 140.2, 137.2, 135.8,
31.2, 126.6, 124.7, 111.0, 109.0, 100.6, 100.5, 99.4, 54.7, 29.2, 28.7,
2
2
3
4
and the solvents evaporated in vacuo. The crude product was puri-
fied by chromatography over silica gel. Elution with a 1:9 ethyl
+
4.7, 10. 6 ppm. HRMS (ESI) m/z [M + H] calcd. for
+
22 17 6 2
C H F NO S 474.0948, found 474.0957.
acetate/pentane mixture afforded 4o as grey crystals (0.63 g, 63%),
1
4
-[3,3,4,4,5,5-Hexafluoro-2-(5-methoxy-1,2-dimethyl-1H-indol-3- m.p. 194–195 °C. H NMR (400 MHz CDCl ): δ = 7.80 (s, 1 H).
3
yl)cyclopent-1-enyl]-5-methylthiophene-2-carbonitrile (2o): Com-
pound 1o (1.2 g, 2.5 mmol) was added to a mixture of ethanol 8.1 Hz, 1 H), 7.28–7.26 (d, J = 8.1 Hz, 1 H), 7.19–7.15 (t, J =
7.76 (s, 1 H), 7.52–7.49 (d, J = 7.9 Hz, 1 H), 7.35–7.33 (d, J =
1
3
(
100 mL), NH
2
OH·HCl (0.35 g, 5.06 mmol), and aqueous 7.4 Hz, 3 H), 3.69 (s, 3 H), 2.06 (s, 3 H), 1.97 (s, 3 H) ppm.
C
NaHCO
3
(10%, 3.3 mL), and the resulting mixture was stirred for
NMR (100 MHz CDCl ): δ = 152.4, 149.3, 138.3, 136.9, 136.2,
3
2
h at room temperature. The solvent was removed in vacuo, acetic
131.8, 127.2, 124.1, 121.5, 120.4, 118.2, 111.8, 108.5, 99.4, 29.1,
14.6, 10.5 ppm. HRMS (ESI): calcd. for C H F N S [M + H]
24 15 6 3
492.0964; found 492.0962.
+
+
anhydride (20 mL) was added to the residue, and the solution was
heated for 24 h at 100 °C. The reaction mixture was cooled to room
temperature, and water (50 mL) was added, followed by aqueous
NaHCO (10%, 5.0 mL). To the resulting aqueous mixture were
3
added ether (40 mL) and saturated aqueous sodium hydrogen car-
bonate (100 mL), and the organic layer was separated. The organic
Supporting Information (see footnote on the first page of this arti-
cle): NMR spectra, absorption spectra and crystallographic data of
NIR-sensitive diarylethene derivatives.
4
phase was dried with MgSO , filtered and the solvents evaporated
in vacuo. The residue was purified by chromatography over silica
Acknowledgments
gel. Elution with a 1:15 ethyl acetate/pentane mixture afforded 2o
1
as yellow crystals (0.7 g, 60 %); m.p. 150–151 °C. H NMR
This work was supported by the National Natural Science Founda-
tion of China (NSFC) (grant numbers 20971071 and 20602020).
(400 MHz CDCl
3
): δ = 7.68 (s, 1 H), 7.18–7.16 (d, J = 8.7 Hz, 1
H), 6.89 (s, 1 H), 6.87–6.86 (d, J = 8.9 Hz, 1 H), 3.79 (s, 3 H), 3.62
1
3
(
s, 3 H),1.99 (s, 3 H), 1.86 (s, 3 H) ppm. C NMR (100 MHz
CDCl ): δ = 155.3, 148.5, 138.3, 137.8, 132.3, 127.1, 125.6, 113.4,
12.1, 110.2, 107.6, 101.4, 100.2, 55.7, 30.2, 14.9, 11.6 ppm. HRMS
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(
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2.4 m in hexane, 15.6 mmol, 6.5 mL) was added dropwise to a
stirred solution of compound 6 (3.35 g, 12 mmol) in THF (100 mL)
at –78 °C under a nitrogen atmosphere. After 60 min, the reaction
mixture was transferred to a Schlenk tube containing a solution of
compound 7 (4.04 g, 12 mmol) in THF (20 mL). The reaction mix-
ture was stirred for 1 h at –78 °C, then warmed to room tempera-
ture, and stirred for an additional 16 h. THF was removed under
reduced pressure, then water (200 mL) was added, and the residue
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was extracted with CH
phases were dried with MgSO
2
Cl
2
(3ϫ 30 mL). The combined organic
, filtered, and the solvents were evap-
4
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