Photoconversion of a protonated diarylethene derivative

Article abstract of DOI:10.1021/jp3070163

A photochromic diarylethene with N,N-dimethyl substituted group 1a was prepared. 1a showed ring-opening and ring-closing photoisomerization with UV/vis light irradiation. Treatment of 1a with CF₃COOH produced a protonated diarylethene 1aH. With UV light irradiation, 1aH photoconverted to protonated ring-closed isomer 1c, which is photoinactive. It is found that 1c is hydrophilic and 1aH is hydrophobic.

Full text of DOI:10.1021/jp3070163

Article
pubs.acs.org/JPCA
Photoconversion of a Protonated Diarylethene Derivative
Huan-huan Liu, Xu Zhang, Zeng Gao, and Yi Chen*
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese
Academy of Sciences, Beijing, 100190, China
ABSTRACT: A photochromic diarylethene with N,N-dimethyl substituted
group 1a was prepared. 1a showed ring-opening and ring-closing photo-
isomerization with UV/vis light irradiation. Treatment of 1a with CF3COOH
produced a protonated diarylethene 1aH. With UV light irradiation, 1aH
photoconverted to protonated ring-closed isomer 1c, which is photoinactive.
It is found that 1c is hydrophilic and 1aH is hydrophobic.
Scheme 1. Chemical Structures of 1a and 1aH
1. INTRODUCTION
Photoactive molecules have currently attracted considerable
attention because of wide potentials in chemistry,1−5
biochemistry,6−10 environmental science,11−14 and material
science.15−19 Integrating an external stimulus to regulate targets
affords a means to control chemical processes and modulate the
behavior of functional materials containing them. Light is a
particularly effective stimulus to spatially and temporally trigger
changes in structure and function of molecules and
materials.20−23
Diarylethenes display excellent photochromic properties and
are one of the most promising photochromic compounds for
photoelectronic applications such as optical memory media and
photoswitching devices because of their fatigue resistant and
thermally irreversible properties.24−29 Photochromic diary-
lethenes act as photoswitching and have been extensively
studied for controlling various chemical and physical properties,
including absorption spectral and fluorescence spectral,30−35
molecular chirality,36,37 magnetic interactions,38,39 electronic
conduction,40−43 chemical reactivity,44,45 and self-assembling
behavior.46,47
Diarylethenes that can be protonated have been attracting
interest48,49 because their photochromic reactivity can be
modulated by simple pH stimulus. Protonated diarylethenes
usually exhibit different photoreactivity and physical and
chemical properties from their original diarylethenes such as
inhibiting photochromism of ring-opened isomers or ring-
closed isomers or both, which provides a stragety for
controlling or regulating photoreactivity. Controllability and
selectivity in photochromism would endow such molecules
with the ability to act as controllable photoactive molecules and
be used for selective functionalization. In this paper, a
photochromic diarylethene with an N,N-dimethyl substitutent
group 1a is empolyed as a model compound (Scheme 1); by
simple protonation with CF3COOH, a protonated diarylethene
derivative 1aH (Scheme 1) is prepared. As compared to
reported protonated diarylethenes, the presented protonated
diarylethene 1aH herein shows some novel and meritorious
properties including: (1) 1aH is photoactive and photo-
converted to a protonated ring-closed isomer 1c upon
irradiation with UV light, and (2) 1c is photoinactive and
differs from its precursor 1aH in solubility. The finding may be
beneficial to insight into the relationship between molecular
structure and properties and provides information of designing
artificial molecules for controlling or regulating reactivity and
function of molecules.
2. EXPERIMENTAL SECTION
1
2.1. General Methods. The H NMR spectrum and 13C
NMR spectrum were recorded at 400 and 100 MHz with
tetramethylsilane (TMS) as an internal reference, respectively.
Both CDCl3 and DMSO-d6 are used as solvents. MS spectra
were recorded with a GC-TOF MS spectrometer. Absorption
spectra were measured with an absorption spectrophotometer
(Hitachi U-3010). Coloration was carried out with a UV light
(λ = 254 nm, intensity: 4.3 mW/cm−2). All chemicals for
synthesis were purchased from commercial suppliers, and
solvents were purified according to standard procedures.
Reaction was monitored by TLC silica gel plates (60F-254).
Column chromatography was performed on silica gel (Merck,
70−230 mesh). A 360 nm lamp (36W) and a Xenon light (500
W), with different wavelength filters, were used as light sources
for photocoloration and photobleaching, respectively.
2.2. Material. Diarylethene 1a was prepared according to
synthetic route shown in Scheme 2, and the detailed procedures
and spectra data were as follows: To a solution of 3,4-bis(5-
iodo-2-methylthien-3-yl)-2,5-dihydrothiophene50 (270 mg, 0.5
mmol) in anhydrous THF (15 mL) were added Pd(PPh3)4 (40
mg, 3 mol %) and a solution of K2CO3 solution (2.5 M, 2 mL).
Received: July 16, 2012
Revised: August 27, 2012
Published: September 18, 2012
© 2012 American Chemical Society
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Scheme 2. Synthesis of Diarylethene 1a
The resulting solution was stirred for 10 min at ambient
temperature. To the resulting solution was added (4-
(dimethyamino)phenyl)boronic acid (248 mg, 1.5 mmol)
under nitrogen, and the reaction was monitored by TLC.
After the starting material disappeared, H2O (20 mL) was
added to the residue, and the product was extracted with Et2O
(3 × 20 mL). The combined organic phase was washed with
H2O (30 mL) and a saturated solution of NaCl (30 mL),
respectively, and dried over MgSO4. After evaporation of the
solvent, the crude product was purified by flash column
chromatography (petroleum ether−ethyl acetate = 1:1, v/v) to
give diarylethene 1a in 33% yield (170 mg). Mp = 196−197 °C.
1H NMR (400 MHz, CDCl3): 7.37 (d, 4H, J = 7.8 Hz), 6.86 (s,
2H), 6.71 (d, 4H, J = 8.8 Hz), 4.18 (s, 4H), 2.95 (s, 12H), 1.96
(s, 6H). 13C NMR: 150.0, 141.1, 134.2, 133.7, 133.1, 126.5,
123.0, 121.4, 112.8, 43.1, 40.7, 14.4. TOF-MS EI (m/z) calcd
for C30H32N2S3: 516. 1728. Found: 516.1728.
Figure 1. Absorption change of 1a (20 μM, CH3CN) with 254 nm
light irradiation until the photosteady state (periods: 0, 20, 40, 60, 80,
100 s).
Protonated diarylethene 1aH was prepared as follows: To
solution of 1a (10 mmol) in CH2Cl2 was added CF3COOH
(21 mmol), and the mixture was stirred at room temperature
for 10 min. The target compound 1aH was obtained
quantitatively after evaporation of the solvent under reduced
1
pressure without further purification. Mp ≥ 300 °C. H NMR
Figure 2. Absorption change of 1aH (20 μM, CH3CN) with 254 nm
light irradiation (periods: 0, 30, 60, 90, 100 s).
(400 MHz, CDCl3): 7.90 (d, 4H, J = 9.8 Hz), 7.73 (d, 4H, J =
8.7 Hz), 7.08 (s, 2H), 4.12 (s, 4H), 3.06 (s, 12H), 1.98 (s, 6H).
Dye 1c was prepared as follows: 1aH (10 mg, 0.02 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 completed (detection by TLC,
starting material disappeared), the solvent was evaporated
under reduce pressure, and the target compound 1c was
1
obtained without further purification. Mp ≥ 300 °C. H NMR
(400 MHz, DMSO-d6): 7.87 (d, 4H, J = 9.1 Hz), 7.22 (d, 4H, J
= 8.9 Hz), 6.86 (s, 2H), 4.14 (s, 4H), 3.15 (s, 12H), 2.26 (s,
6H). 13C NMR (100 MHz, DMSO-d6): 159.6, 146.8, 146.5,
133.6, 131.5, 125.8, 119.8, 117.4, 60.9, 42.7, 40.9, 14.6. IR
(KBr) ν: 3452, 3213, 1687, 1412, 1216, 1108 cm−1.
3. RESULTS AND DISCUSSION
3.1. Photochromism of 1a with 254 nm Light
Irradiation. Spectral photochromic properties of 1a were
studied in acetonitrile solution. 1a showed ring-opening and
ring-closing photoisomerization with UV/vis light irradiation.
Figure 3. Absorption spectral of 1b with addition of CF3COOH in
CH3CN (before, black; after, red).
Scheme 3. Ring-Opening and Ring-Closing Photoisomerization of 1a with UV/Vis Light Irradiation
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Scheme 4. Conversion of 1c from 1aH and 1b
irradiation time and completed after irradiation of 100 s. This
process was accompanied by a color change of solution from
colorless to dark blue-green. The conversion of 1aH to 1c was
1
ca. 80% (determined by H NMR). As presented in Figure 2,
there is an isosbestic point at 345 nm, the clear isosbestic point
indicated that only two isomers existed when protonated
diarylethene 1aH underwent the photoconversion reaction.
Further study found that the dark green-blue solution could not
be bleached back to colorless solution with visible light (≥400
nm) irradiation, which suggested that 1c did not exhibit
photochromism.
Figure 4. Photographs of 1aH, 1b, and 1c in a mixture of solution
(top, toluene; bottom, water).
3.3. Structure of 1c. To find out the species whose
absorption corresponded to the new absorption in the range of
400−750 nm, the photoproduct 1c resulted from the
photoconversion of 1aH was obtained, and its structure was
The photoiosmerization of 1a is described in Scheme 3, and
changes in the absorption spectra are presented in Figure 1.
1
determined by H NMR and 13C NMR spectroscopy. It was
Upon irradiation with UV light, the absorption band (λmax
=
found that the proton of CH3 group linked in the thiophene
ring shifted downfield from 1.98 ppm (1aH) or 1.96 ppm (1a)
to 2.26 ppm. It is known that the protons of CH3 linked in
thiophene rings usually shift downfield when ring-opened
isomers of diarylethenes photoconvert to their ring-closed
isomers. The result suggested that 1c possessed a ring-closed
structrue. Comparing both 13C NMR data (1a and 1c) found
that the signal arising from the CS in thiophene ring
shifted upfield from 112.8 (1a) to 60.9 (1c) ppm, which
confirmed that the double bond (CC−S) in thiophene ring
was changed to single bond (C−C−S) when 1aH was
photoconverted to 1c. Moreover, it was found that 1c was
also obtained from 1b by addition of acid. Addition of
CF3COOH to the solution of 1b produced the color of
solution changed immediately from purple to dark green-blue,
and maximum absorption shifted from 546 to 610 nm (Figure
3). As compared to Figure 2, Figure 3 showed very similar
spectral profile, which implied that 1b was converted to 1c in
the presence of acid. Besides, control experiments demon-
strated the following points: (1) the photoconversion of 1aH to
1c could also be performed in the atmosphere of N2, and no
significant change was observed as compared in the air, which
indicated that no oxidation occurred when 1aH photo-
converted to 1c. (2) 1c was transformed to 1b by the addition
of NaOH solution. It was found that the color of solution
changed from dark green-blue to purple when NaOH solution
330 nm, ε = 5.4 × 104 L mol−1 cm−1), which is attributed to the
ring-opened isomer 1a, decreased in intensity, and a new band
(λmax = 546 nm), which corresponds to the ring-closed isomer
1b, appeared at the same time. The band at 546 nm increased
in intensity with the increase of irradiation time until the
photostationary state (PSS) was reached. This process was
accompanied by a color change of solution from colorless to
purple. The conversion of 1a to 1b was ca. 90% (determined by
1H NMR) in PSS. As presented in Figure 1, the figure showed
the typical absorption spectra changes of photochromic
diarylethene derivatives in solution, and the clear isosbestic
point indicated that only two isomers existed when diarylethene
1a underwent the photoisomerization reaction. The purple
solution was bleached completely back to colorless solution
with visible light (≥400 nm) irradiation, and the coloration and
decoloration can be repeated.
3.2. Photoconversion of 1aH with 254 nm Light
Irradiation. Spectral examination of the photoconversion of
1aH was studied in acetonitrile solution with UV light
irradiation, and changes in the absorption spectra are presented
in Figure 2. Upon irradiation with UV light, the absorption
band (λmax = 320 nm, ε = 3.3 × 104 L mol−1 cm−1), which is
attributed to 1aH, decreased in intensity, and a new absorption
covering the full visible light wavelength range (400−750 nm),
which corresponds to 1c (Scheme 4), appeared at the same
time. The new band increased in intensity with the increase of
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was added to the solution of 1b. Further study found that the
recovered 1b could be bleached back to 1a with visible light
irradiation. In light of the above results, the structure of 1c was
proposed to be a protonated ring-closed isomer (it should be
emphasized that the structure is only a prediction based on
limited evidence). The conversion of 1c from both 1aH and 1b
was outlined in Scheme 4.
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3.4. Stability and Solubility of 1c. Both stability and
solubility of 1c were investigated. Preliminary experiments
showed that 1c is photoinactive and no significant change
(detection by absorption spectral) was detected when the
solution of 1c was irradiated with UV light or visible light for
more than 10 min. 1c is also thermal stable at ambient
temperature and no decompose was observed when the
solution of 1c was kept at ambient temperature for several
weeks. In addition, it was found that 1c is hydrophilic whereas
both 1aH and 1b are hydrophobic (1a is also insoluble in
water). As presented in Figure 4, 1aH or 1b dissolved in
toluene (up), after irradiation with UV light (for the solution of
1aH) or addition of acid (for the solution of 1b), both 1aH and
1b converted to 1c, which was soluble in water (bottom). The
change of solubility from hydrophilic to hydrophobic with light
irradiation probably provides a simple way to transfer
hydrophobic dye to hydrophilic dye by phototrigger, which is
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4. CONCLUSIONS
In summary, a protonated diarylethene derivative has been
synthesized. It has been demonstrated that the protonated
diarylethene derivative is photoactive and converted to a
photoinactive protonated ring-closed isomer upon irradiation
with UV light. It has also been demonstrated that the
protonated ring-closed isomer is hydrophilic whereas the
protonated ring-opened isomer is hydrophobic.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: yichencas@yahoo.com.cn.
Notes
Commun. 2012, 48, 2095−2097.
■
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The authors declare no competing financial interest.
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M. Chem. Rev. 2000, 100, 1789−1816.
ACKNOWLEDGMENTS
■
(38) Tanifuji, N.; Irie, M.; Matsuda, K. J. Am. Chem. Soc. 2005, 127,
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This work was supported the National Nature Science
Foundation of China (91123032) and Beijing Natural Science
Foundation (Synthesis and properties of recording material for
multilevel memory).
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