Selective photoconversion of photochromic diarylethenes and their properties

Article abstract of DOI:10.1039/c2nj40377c

Selective photoconversions of photochromic diarylethene derivatives has been described using diarylethene derivative 1a as a model compound. Upon irradiation with 254 nm light, 1a undergoes photocyclization to yield the ring-closed isomer 1b in the absence of oxygen, or is transformed to the thiolactone derivative 1c in the presence of oxygen (in the air), respectively. It was found that 1b can be reversed back to 1a with visible light irradiation, the ring-opening and ring-closing photoswitch can be performed in the absence of oxygen. 1c is, however, photo-inactive and cannot be reversed back to 1a with UV or visible light irradiation. It was also found that 1c shows fluorescence whereas both 1a and 1b show no fluorescence. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/c2nj40377c

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PAPER
Selective photoconversion of photochromic diarylethenes and
their properties
Huan-huan Liu and Yi Chen*
Received (in Montpellier, France) 10th May 2012, Accepted 2nd August 2012
DOI: 10.1039/c2nj40377c
Selective photoconversions of photochromic diarylethene derivatives has been described using
diarylethene derivative 1a as a model compound. Upon irradiation with 254 nm light, 1a
undergoes photocyclization to yield the ring-closed isomer 1b in the absence of oxygen, or is
transformed to the thiolactone derivative 1c in the presence of oxygen (in the air), respectively. It
was found that 1b can be reversed back to 1a with visible light irradiation, the ring-opening and
ring-closing photoswitch can be performed in the absence of oxygen. 1c is, however, photo-
inactive and cannot be reversed back to 1a with UV or visible light irradiation. It was also found
that 1c shows fluorescence whereas both 1a and 1b show no fluorescence.
Introduction
Selective reaction plays a key role not only in biological
processes, such as the inhibition of enzymes,1 detection of
metal ions,2 functionalization of peptides,3 and activation of
proteins,4 but also in chemical processes, such as asymmetric
synthesis,5 specific recognition,6 and catalytic reactions.7 Currently,
Scheme 1 Structure of thiopyranothiopyran derivatives.
controllable and selective reactions have attracted considerable
attention in material science from both fundamental and
practical points of view.8
diarylethenes are photo-unstable and are photo-rearranged to
thiopyranothiopyran derivatives with UV light irradiation.
Although thiopyranothiopyran derivatives as the main by-product
was reported,22 the known diarylethene systems show good
fatigue resistance, and only a little by-product was obtained
after the photochromic interconversion reaction. In this paper,
we report a new finding, that photochromic diarylethenes with
a bis (5-chloro-2-methylthiophen-3-yl) system are selectively
converted to the ring-closed isomers and thiolactone derivatives
(not thiopyranothiopyran derivatives) in the absence and in the
presence of oxygen, respectively. Property studies show that the
formation of the thiolactone derivatives undergoes two steps:
first photocyclization, and then photooxidation (not photo-
rearrangement). The selective photo-conversion of diarylethene
1a to the ring-closed isomer 1b and thiolactone derivative 1c is
outlined in Scheme 2.
Photo-active molecules have generated considerable interest
due to wide potentials in chemistry,9 biochemistry,10 environ-
mental science11 and material science.12 Light is a particularly
effective stimulus to spatially and temporally trigger changes
in the structure and function of molecules and materials.13
Diarylethenes, notably the bis(thien-3-yl) system,14 are photo-
active molecules and have been widely used as promising
photoswitches to control various chemical and physical proper-
ties, including absorption and fluorescence,15 electrochemical
response,16 magnetic interactions,17 electronic conduction,18
chemical reactivity,19 and self-assembling behavior,20 because
of their fatigue resistant and thermally irreversible properties.21
Diarylethenes undergo not only ring-opening and ring-
closing photoisomerization but also photo-rearrangement
upon irradiation with UV light. It has been reported that
thiopyranothiopyran derivatives22 (Scheme 1) as the main
by-product was obtained during the photochromic inter-
conversion of diarylethenes under UV light irradiation. The
mechanism study demonstrated that the ring-closed isomers of
Experimental
General
1H NMR spectrum was recorded at 400 MHz with TMS as an
internal reference and DMSO-d6 as solvent. 13C NMR spectra
were recorded at 100 MHz with TMS as an internal reference
and DMSO-d6 as solvent. MS spectra were recorded with a
Trio-2000 GC-MS spectrometer. HRMS spectra were recorded
Key Laboratory of Photochemical Conversion and Optoelectronic
Materials, Technical Institute of Physics and Chemistry,
The Chinese Academy of Science, Beijing, 100190, China.
E-mail: yichencas@yahoo.com.cn; Fax: +86 10 6487 9375;
Tel: +86 10 8254 3595
c
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phases were washed with H2O (30 ml) and dried over MgSO4.
After evaporation of the solvent, the crude product was
purified by flash column chromatography (petroleum ether
(60–90 1C)–ethyl acetate = 10 : 1, v/v) to give diarylethene 1a
1
in 75% yield. H NMR (400 MHz, DMSO-d6): 12.54 (s, 1H),
7.95 (d, 2H, J = 8.6 Hz), 7.10 (s, 1H), 7.05 (d, 2H, J = 8.7 Hz),
6.81 (s, 1H), 3.81 (s, 3H), 2.19 (s, 3H), 2.01 (s, 3H). HRMS
(GC-TOF) (m/z) calcd for C20H16Cl2N2OS2: 435.9602, found:
435.9602 (100%).
Thiolactone derivative 1c was prepared as follows: 1a (50 mg,
0.15 mmol) was dissolved in CH3CN (50 ml). The resulting
solution was then irradiated for 30 min under a high pressure Hg
lamp (500 W). After the conversion was complete (detection by
TLC), the solvent was evaporated under reduce pressure, the
crude product was purified by flash column chromatography
(neutral Al2O3, 100–200 mesh, elute: petroleum ether/acetone =
1 : 3, v/v) to afford 1c in 63% yield. IR (KBr) n: 3318 (NH), 3251
(arom, CH), 2879 (alipha, CH), 1687, 1786 (CQO), 1615 (arom,
1
CH). H NMR (400 MHz, DMSO-d6): 13.69 (s, 1H), 8.10 (d,
2H, J = 8.8 Hz), 7.14 (d, 2H, J = 8.9 Hz), 6.74 (s, 2H), 3.84 (s,
3H), 1.58 (s, 6H). 13C NMR (100 MHz, DMSO-d6: 194.3, 161.2,
152.8, 148.1, 130.2, 128.3, 121.1, 120.7, 114.6, 67.4, 55.4, 26.9.
HRMS (GC-TOF) (m/z) calcd. for C20H16N2O3S2: 396.0602,
found: 396.0608 (100%). Anal. calcd. for C20H16N2O3S2: C,
60.59, H, 4.07; N, 7.07. Found: C, 60.71; H, 3.98; N, 6.96.
Scheme 2 Selective photoconversion of diarylethene 1a to ring-closed
isomer 1b and thiolactone derivative 1c with 254 nm light irradiation.
with a GC-TOF MS spectrometer. Absorption and fluorescence
spectra were measured with an absorption spectrophotometer
(Hitachi U-3010) and a fluorescence spectrophotometer (Hitachi
F-2500), respectively. All chemicals for synthesis were purchased
from commercial suppliers, and solvents were purified according
to standard procedures. Reactions were monitored by TLC silica
gel plates (60F-254). Column chromatography was performed on
silica gel (Merck, 70–230 mesh) and neutral Al2O3 (100–200 mesh),
respectively. A 360 nm lamp (36 W) and a Xenon light (500 W),
with different wavelength filters, were used as light sources for
photocoloration and photobleaching, respectively.
Results and discussion
(1) Photoconversion of 1a to 1b and its photoswitching
properties
The ring-opening and ring-closing photoisomerization of 1a is
described in Scheme 1. Upon irradiation with UV light, the
absorption band (lmax = 300 nm, e = 2.5 Â 104 L molÀ1 cmÀ1),
in CH3CN solution in the atmosphere of N2, which is attributed
to the ring-open isomer 1a, decreased in intensity, and two new
bands (lmax = 520 nm and 330 nm, respectively), which
correspond to the ring-closed isomer 1b (whose structure was
identified by 1H NMR spectroscopy), appeared at the same time
(Fig. 1). The band at 520 nm increased in intensity with the
increase of irradiation time until the photostationary state (PSS)
was reached (quantum yield: f1a-1b = 0.364, conversion yield:
90%). This process was accompanied by a colour change of the
solution from colourless to red. The red solution could be
bleached back to colourless with visible light ( Z400 nm)
irradiation, and the coloration and decoloration could be
recycled. As presented in Fig. 1, the figure shows the typical
absorption spectra changes of the photochromic diarylethene
derivatives in solution, and the clear isosbestic point (l =
318 nm) indicates that only two isomers (1a and 1b) exist when
1a undergoes photo-conversion to 1b.
Material
Diarylethene 1a was prepared according to the synthetic route
shown in Scheme 3. Diketone was obtained starting from
commercially available 2-methlthiophene, which was chlorinated
at the 5-position with NCS in AcOH,23 followed by acylation
with oxalyl chloride in DCM.24 Treatment of the diketone with
4-methoxybenzaldehyde in the presence of NH4Ac afforded the
target compound 1a. The detailed procedures and spectra data
were as follows: To a solution of diketone (100 mg, 0.31 mmol) in
acetic acid (10 ml) was added 4-methoxybenzaldehyde
(0.37 mmol) and NH4Ac (143 mg, 1.86 mmol). The resulting
solution was stirred at reflux until all the starting material had
reacted (TLC detection). The resulting solution was then slowly
poured into NaHCO3 solution (10%, 50 ml), and the product
was extracted with CHCl3 (3 Â 20 ml). The combined organic
(2) Photoconversion of 1a to 1c and its formation mechanism
The photoconversion of 1a to 1c was performed in the
presence of oxygen with 254 nm light irradiation, and the
spectral changes of 1a were examined in acetonitrile solution
in air. As presented in Fig. 2, it was found that the absorption
spectral changes of 1a, that are a function of irradiating time,
involved two stages: before 45 s (irradiation time), the absorption
Scheme 3 Synthesis of diarylethene 1a.
c
2224 New J. Chem., 2012, 36, 2223–2227
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NMR, HRMS, IR, and elemental analysis (see experimental).
The different structure of 1c from the fused 6-membered rings
is probably due to chlorination of the thiophene ring. The
control experiment showed that no thiolactone derivative was
formed when the chloro-substitutent was replaced by methyl-
substituents. Secondly, after careful irradiation of the solution
of 1a with a controlled irradiation time of within 30 s, we
found that the colourless solution was changed to the red
solution, and the red solution was completely bleached back to
colourless solution with visible light ( Z 400 nm) irradiation,
both coloration and decoloration could be repeated with
UV/Vis light irradiation, which indicated that 1a underwent
a photochromic reaction to yield the ring-closed isomer 1b
within 30 s of irradiation. Thirdly, no 1c, but only 1b, was detected
when the solution of 1a was irradiated in a N2atmosphere
(O2-free). Finally, 1b was converted to 1c upon irradiation
with UV light in air. The colour change experiments demon-
strated that the colour of the solution changed from colourless
to yellow via two steps: colourless - red - yellow during the
photoconversion of 1a to 1c. All results indicated that the
photoconversion of 1a to 1c underwent two steps: first 1a was
photo-cyclized to 1b, and then 1b was photo-oxidised to 1c
(Scheme 4). Further investigation found that 1c could not be
reversed back to 1a or 1b with UV light or visible light
irradiation. Moreover, 1c was found to be photo-inactive
and no significant change was detected (colour and absorption
spectral) after irradiation for more than 10 min with UV light.
Fig. 1 Absorption changes of diarylethene 1a (25 mM, CH3CN) in a
N2 atmosphere with 254 nm light irradiation until the photostationary
state (periods: 0, 15, 30, 45 s).
of 520 nm increased in intensity with the increase of irradiation
time, but after 45 s, the absorption of 520 nm, however,
decreased in intensity with the increase of irradiation time,
and the absorption at 520 nm almost disappeared when the
solution was irradiated for 105 s. It was also found that
accompanying the decrease of 520 nm, another absorption at
380 nm appeared. Careful examination of Fig. 2 reveals that
there are two clear isosbestic points that have appeared at
318 nm and 420 nm, respectively. The first isosbestic point
appears at 318 nm, which was produced before 45 s, and the
second one at 420 nm was formed after 45 s. Besides, the
experiments also revealed that the colour of the solution was
changed from colourless to red when the irradiating time is not
more than 45 s, and with continuous irradiating of the
solution, the colour of solution changed from red to yellow.
All results suggested that a new chemical species (1c), rather
than 1b, was formed when the solution of 1a was irradiated in
the air.
(3) Absorption and fluorescence of 1c
The absorption and fluorescence of 1c were studied in aceto-
nitrile solution using pure 1c as the target. Fig. 3 and 4 represent
the absorption and fluorescence of 1c, respectively. There are
three absorption bands at 274 nm (e = 2.5 Â 104 L molÀ1 cmÀ1),
300 nm (e = 4.9 Â 104 L molÀ1 cmÀ1) with a shoulder at 335 nm,
and 388 nm (e = 2.1 Â 104 L molÀ1 cmÀ1), respectively, which
corresponded to the n - p* and p - p* transition, respectively.
With 365 nm light as excitation wavelength, the fluorescence at
lem = 490 nm was observed (Fig. 4), and a moderate fluores-
cence quantum yield (Pf = 0.17) was obtained by using quinine
sulfate (Pf=0.577, in H2SO4) as a reference.
To investigate the chemical structure of 1c and to explore its
formation mechanism, the following control experiments were
carried out. First, the photo-product 1c (Scheme 1) was
obtained by the photoconversion of 1a in solution. After
purification, the chemical structure of 1c was confirmed by
The fluorescence changes of the photoconversion of 1a to 1c
in acetonitrile solution with UV light irradiation was investigated. A
very weak fluorescence emission of 1a (lem = 375 nm, Pf = 0.002)
was detected when the solution of 1a was excited with an
excitation wavelength of 300 nm before irradiation. Upon
irradiation with 254 nm light, a new fluorescence at 490 nm
was detected with a 365 nm excitation wavelength, which was
Fig. 2 Absorption changes of diarylethene 1a (25 mM, CH3CN) in air
with 254 nm light irradiation (periods: 0, 15, 30, 45, 60, 75, 90, 105 s)
(lmax = 520 nm, 0 - 45 s, solid, increase with irradiation time;
lmax = 520 nm, 60 - 105 s, dot, decrease with irradiation time).
Scheme 4 Photoconversion of diarylethene 1a to the thiolactone
derivative 1c via the ring-closed isomer 1b.
c
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Conclusions
In summary, a photochromic diarylethene derivative, which
shows selective photoconversion, has been developed. It
has been demonstrated that the diarylethene derivative photo-
converts to the ring-closed isomer in the absence of oxygen,
and both the ring-open and ring-closed isomers show photo-
switching properties with UV/Vis light irradiation. It has also
been demonstrated that, in the presence of oxygen (in the air),
the diarylethene derivative photoconverts to the thiolactone
derivative via two-step photo-cyclization and photo-oxidation
reaction. The thiolactone derivative exhibits fluorescence, and
the turn-on fluorescence is also observed during the photo-
conversion of the diarylethene derivative to the thiolactone
derivative.
Fig. 3 Absorption spectrum of thiolactone derivative 1c (25 mM,
CH3CN).
Acknowledgements
This work was supported by the National Basic Research
Program of China (2010CB934103) and the National Nature
Science Foundation of China (91123032 and 21073214).
Notes and references
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Fig. 4 Fluorescence spectrum of thiolactone derivative 1c (25 mM,
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