Synthesis and photophysical properties of thiolactone derivatives

Article abstract of DOI:10.1016/j.tet.2012.12.057

A class of bis-thiolactone derivatives is prepared by photo-conversion of photochromic diarylethenes with a bis (5-chloro-2-methylthiophen-3-yl) system in moderate yields (50-60%). Bis-thiolactone derivatives show fluorescence in solution with moderate fluorescent quantum yields (0.11-0.17). Reaction mechanism exhibits that the photo-conversion of diarylethenes to bis-thiolactone derivatives undergoes a two-step reaction: photocyclization and photo-oxidation.

Full text of DOI:10.1016/j.tet.2012.12.057

Tetrahedron 69 (2013) 1872e1876
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Synthesis and photophysical properties of thiolactone derivatives
*
Huan-Huan Liu, Yi Chen
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, The Chinese Academy of Science, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 August 2012
Received in revised form 12 December 2012
Accepted 21 December 2012
Available online 31 December 2012
A class of bis-thiolactone derivatives is prepared by photo-conversion of photochromic diarylethenes
with a bis (5-chloro-2-methylthiophen-3-yl) system in moderate yields (50e60%). Bis-thiolactone
derivatives show fluorescence in solution with moderate fluorescent quantum yields (0.11e0.17).
Reaction mechanism exhibits that the photo-conversion of diarylethenes to bis-thiolactone derivatives
undergoes a two-step reaction: photocyclization and photo-oxidation.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Thiolactone derivative
Diarylethene
Photo-conversion
Synthesis
Fluorescence
1. Introduction
photoisomerization of diarylethenes upon irradiation with UV/Vis
light (Fig. 1).13,14
Thiolactones and analogs are one of important class of hetero-
cyclic compounds, not only as building blocks in the synthesis of
natural products1 or as key structural units of compounds2 with
interesting biological activities but also in the field of materials
chemistry.3 For example, (5R)-thiolactomycin (TML) is a thiolactone
antibiotic,4 and it exhibits broad-spectrum antibiotic activity
in vitro against Gram þve and Gram eve bacteria,5 Mycobacterium
tuberculosis,6 the malarial parasite, Plasmodium falciparum,7 and
African trypanosomes.8
UV
Vis
R
R
S
S
S
S
R
R
ring-open isomer
ring-closed isomer
Fig. 1. Ring-opening and ring-closing photoisomerization of diarylethenes with UV/Vis
light irradiation.
In this paper, we report an unexpected result: upon irradiation
with UV light, photochromic diarylethenes, with a bis (5-chloro-2-
methylthiophen-3-yl) system, are converted to thiolactone de-
rivatives rather than ring-closed isomers in the air (Fig. 2). Further
investigation reveals that the formation of thiolactone derivatives
undergoes two steps: photocyclization and photooxidation.
Diarylethenes,9 notably the bis(thien-3-yl) system, are well-
known as the most popular class of photochromic compounds for
photoelectronic applications such as optical memory media10 and
photoswitching devices11 because of their fatigue resistant and
thermally irreversible properties. Many studies on photochromic
diarylethenes have been focused on the development of novel
molecular systems12 and the modulation of variously functional
properties of optical devices by ring-opening and ring-closing
* Corresponding author. Tel.: þ86 10 8254 3595; fax: þ86 10 6487 9375; e-mail
address: yichencas@yahoo.com.cn (Y. Chen).
Fig. 2. The photo-conversion of diarylethenes to thiolactone derivatives.
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.12.057

H.-H. Liu, Y. Chen / Tetrahedron 69 (2013) 1872e1876
1873
2. Results and discussion
2.3. X-ray crystallography of thiolactone derivative 1b
2.1. Synthesis of diarylethenes
The chemical structures of all thiolactone derivatives were
determined by 1H and 13C NMR spectroscopy, MS spectroscopy,
and elemental analysis. To confirm the chemical structure of
thiolactone derivatives, crystallized 1b, obtained from crystal
grown in the mixed solvent petroleum ether/dichloromethane
(2:1, v/v), was analyzed by X-ray (Fig. 3) (CCDC number: 905319)
and the crystallographic parameters was summarized in refer-
ence16. It revealed that 1b contains ring-closed structure: the
open form of diarylethene occurred cyclization to closed form, the
five-numbered rings containing thiophene atom still exist but the
polyene structure occurred isomerization. In addition, it is found
that two Cl atoms in thiophene rings were replaced by two O atoms
when 1a photo-converted to 1b. No change in imidazole bridge unit
was observed when 1a transformed to 1b.
Diarylethenes (1ae9a) were synthesized starting from
2-methylthiophene, which was chlorinated at the 5-position
with NCS in AcOH, followed by acylation with oxalyl chloride in
1,2-dichloroethane.15 1,2-Bis (5-chloro-2-methylthiophen-3-yl)eth-
ane-1,2-dione was used in the condensation reaction with corre-
sponding aromatic aldehydes in the presence of NH4Ac to afford
targets in good yields (60e78%). The synthesis of diarylethenes
1ae9a is outlined in Scheme 1.
Scheme 1. Reagents and conditions: (i) NCS, AcOH, reflex, 82% (ii) (COCl)2, ClCH2CH2Cl,
Pyridine, 30% (iii) ArCHO, NH4Ac, AcOH, reflex, (1a: 60%, 2a: 61%, 3a: 72%, 4a: 60%, 5a:
73%, 6a: 70%, 7a: 70%, 8a: 75%, 9a: 78%).
2.2. Photo-conversion of diarylethenes to thiolactone
derivatives
Fig. 3. X-ray crystal structure of 1b.
The photo-conversion of diarylethenes 1ae9a to thiolactone
derivatives 1be9b is carried out in THF with a high-pressure Hg
lamp. Irradiating a solution of diarylethene (1ae9a) (5ꢀ10ꢁ3 mol/L)
with UV light for 50 min yielded thiolactone derivatives 1be9b with
isolated yields of 51e65%. The generality of the photo-conversion of
diarylethenes to thiolactones is demonstrated by diarylethenes
containing electron-donating and electron-withdrawing groups in
bridge unit as substrates. It is found that all substrates gave their
corresponding thiolactone derivatives in moderate yields, the dia-
rylethenes containing reactive substituents for further potential
synthetic modification showed no negative influence on the effi-
ciency of the photoreaction, and electronic properties of substituents
(donor or acceptor) had also no distinct influence over the photo-
conversion of diarylethenes to thiolactone derivatives. It is worth
noting that strong electron-withdrawing group in bridge unit
inhibits the photo-conversion reaction, and no thiolactone derivative
was obtained when diarylethene with NO2 group in bridge unit was
irradiated with UV light. The photo-conversion of diarylethenes to
thiolactone derivatives is presented in Scheme 2.
2.4. Photophysical properties of thiolactone derivatives
The photophysical properties of thiolactone derivatives 1be9b
were determined by examining both absorption and fluorescence
of thiolactone derivatives 1be9b in CH3CN. As presented in Fig. 4,
the
absorption
of
1b
has
3
three
peaks
at
318
(
(
3
¼4.4ꢀ104 L molꢁ1 cmꢁ1), 355 (
¼2.0ꢀ104 L molꢁ1 cmꢁ1) and 475
3
¼8.0ꢀ103 L molꢁ1 cmꢁ1) nm, respectively, they corresponded to
the transition of n /
p* and p / p*, respectively. With 365 nm
light as excitation wavelength, the fluorescence at lem¼500 nm was
observed (Fig. 4), and a moderate fluorescence quantum yield
(ff¼0.11) was obtained by using quinine sulfate (ff¼0.577, in
H2SO4) as a reference. Similar results were obtained with other
thiolactone derivatives and their UV and fluorescence data are lis-
ted in Table 1. It was found that all thiolactone derivatives showed
similar fluorescence properties, the quantum yields are between
0.11 and 0.17, and emission wavelength located in the range of
500e535 nm, which suggested that thiolactone derivatives
containing different electronic properties of substituents (donator
or acceptor) had no distinct influence over the fluorescence except
for 7b in which both very weak fluorescence (0.01) and short
emission wavelength (407 nm) were obtained as compared to other
thiolactone derivatives. The weak fluorescence and short emission
wavelength probably resulted from the intramolecular hydrogen
bond between OH and N atom, which was confirmed by the
following results: (1) when OH group is attached at para-position
(6b), a strong fluorescence (0.16) and long emission wavelength
(536 nm) were obtained in consequence of no intramolecular
hydrogen bond; and (2) when OH group is replaced by OCH3 group
Scheme 2. Reagent and condition: high-pressure Hg lamp (500 W, exposure:
39.5 mW/cm2), THF, rt, 50 min (1b: 54%, 2b: 65%, 3b: 56%, 4b: 57%, 5b: 63%, 6b: 57%,
7b: 65%, 8b: 51%, 9b: 55%).

1874
H.-H. Liu, Y. Chen / Tetrahedron 69 (2013) 1872e1876
was photo-converted to thiolactone derivatives. Careful examination
found that the color change from colorless to yellow contained two
steps: colorless to red first, red to yellow followed. Comparing to the
solution color of ring-closed isomer (red solution, lmax¼520 nm), it
could be supposed that the red solution attributed to ring-closed
isomer. It was worth noting that the color change from red to
yellow was quickly in solution. Further control experiment demon-
strated that ring-closed isomer, resulted from 1a with UV light
irradiation in absence of oxygen, was converted to 1b upon irradia-
tion with UV light in the air. The results indicated that diarylethenes
photo-converted to thiolactone derivatives via ring-closed isomer
and containing a two-step reaction: photo-cyclization and photo-
oxidation. A possible mechanism is outlined in Scheme 3.
0.6
0.4
0.2
0.0
300
350
400
450
500
wavelength (nm)
800
600
400
200
0
Scheme 3. Photo-conversion of diarylethenes to thiolactone derivatives via ring-
closed isomer.
The photostability of thiolactone derivatives was explored pre-
liminarily. No significant change (detected by UV/Vis absorption
spectroscopy and 1H NMR spectroscopy) was obtained when the
solution of 1b was irradiated with UV light or visible light for more
than 10 min, which suggested that 1b is photostable. Similar results
were obtained with other thiolactone derivatives 2be9b, and no
significant changes were detected when they were irradiated
with UV light or visible light for 10 min. Results demonstrated that
thiolactone derivatives 1be9b were photoinactive. Besides, it was
found that 1be9bwere also stable in the air at ambient temperature.
400
450
500
550
600
wavelength (nm)
Fig. 4. Absorption and fluorescence of 1b (25
mM, CH3CN). lex¼360 nm.
Table 1
UV and fluorescence data of 1be9b
Compd
lab (nm)
lem (nm)
ff
3. Conclusion
1b
2b
3b
4b
5b
6b
7b
8b
9b
318, 355, 475
325, 375, 460
315, 360, 455
323, 363, 451
274, 330, 392
325, 402, 470
340, 395, 455
275, 330, 390
280, 336, 397
500
513
520
509
533
536
407
505
520
0.11
0.16
0.14
0.12
0.17
0.16
0.01
0.11
0.12
A
convenient protocol for the preparation of thiolactone
derivatives has been described. It has demonstrated that photo-
chromic diarylethenes with a bis (5-chloro-2-methylthiophen-3-
yl) system are photo-converted to thiolactone derivatives via
photo-cyclization and photo-oxidation. Thiolactone derivatives
containing different electronic properties of substituents (donator
or acceptor) show similar photophysical properties.
lab: absorption wavelength of the maximum; lem: fluorescence wavelength of the
maximum; ff: fluorescence quantum yield (using quinine sulfate [ff¼0.577, in
H2SO4] as a reference and 360 nm light as excitation wavelength).
4. Experimental section
4.1. General
(8b), intramolecular hydrogen bond was disappeared, and both
quantum yield (0.11) and emission wavelength (505 nm) were
recovered.
1H NMR and 13C NMR spectra were recorded at 400 MHz and
100 MHz, respectively, with TMS as an internal reference and
DMSO-d6 as solvent. HRMS spectra were recorded with GC-TOF
MS spectrometer. UV absorption spectral was measured on an
absorption spectrophotometer (Hitachi U-3010). All chemicals for
synthesis were purchased from commercial suppliers. Reactions
were monitored by TLC silica gel plate (60F-254). Column chro-
matography was performed on silica gel (Merck, 70e230 mesh).
A high-pressure mercury lamp (500 W) was used as light sources
for photo-conversion of diarylethenes to thiolactone derivatives.
2.5. Mechanism of photo-conversion
Control experiments showed that oxygen (in the air) plays
a key role in the photo-conversion of diarylethenes to thiolactone
derivatives. First, it is found that ring-closed isomer (Fig. 2) was
only detected (TLC plate, Rf¼0.82, elute: petroleum ether/ether
acetate¼2:1, v/v) when the solution of 1a was irradiated with UV
light in absence of oxygen (saturated with N2). By contraries, thio-
lactone derivative 1b was only detected (TLC plate, Rf¼0.14, elute:
petroleum ether/ether acetate¼2:1, v/v) when the solution of 1a was
irradiated with UV light in the air. Second, examining the color
change of solution during the photo-conversion found that the color
of solution was changed from colorless to yellow when diarylethenes
4.2. General procedure for the preparation of diarylethenes
1ae9a
Diarylethenes 1ae9a were prepared according to the synthetic
route described in Scheme
1 and the detailed procedures

H.-H. Liu, Y. Chen / Tetrahedron 69 (2013) 1872e1876
1875
are according to the literature,16 which summarized as follows:
Diketone was obtained starting from commercial available
2-methylthiophene, which was chlorinated at the 5-position
with NCS in AcOH,17 followed by acylation with oxalyl chloride in
1,2-dicholoethane.15 Treatment diketone with benzenealdehydes
in the presence of NH4Ac afforded target compounds. The details of
procedure are as follows: To a solution of diketone (100 mg,
0.31 mmol) in acetic acid (10 ml) was added corresponding
aromatic aldehydes (0.37 mmol) and NH4Ac (143 mg, 1.86 mmol),
and the mixture was heated at reflux until the starting material
disappeared (TLC detection). The mixture solution was then slowly
poured into NaHCO3 solution (10%, 50 ml), the product was
extracted with CHCl3 (3ꢀ20 ml). The combined organic phase was
washed with water and dried over MgSO4. After evaporation of the
solvent, the crude product was purified by flash column chroma-
tography (elute: petroleum ether/ethyl acetate¼10:1, v/v) to afford
diarylethenes 1ae9a.
13C NMR (100 MHz, DMSO):
d
194.3 (C]O), 161.2 (C), 160.9
(CeOCH3), 152.8 (C]N), 128.3 (CeH), 127.5 (C), 121.1 (CeH), 120.7
(CeH), 114.6 (CeN), 67.4 (C), 55.4 (OCH3), 26.9 (CH3). MS (EI) m/z
(%): 396 (100%). HRMS-EIS: calcd for (C20H16N2O3S2) 396.0602
found 396.0598.
4.3.6. Compound 6b. 1H NMR (400 MHz, DMSO):
d 13.60 (1H, s,
NH), 10.10 (1H, s, OH), 7.92 (2H, d, J¼8.7 Hz, aromatic-H), 6.94
(2H, d, J¼8.6 Hz, aromatic-H), 6.63 (2H, s, CH), 1.57 (6H, s, CH3). 13
C
NMR (100 MHz, DMSO):
d 194.8 (C]O), 162.4 (C), 160.2 (CeOH),
154.1 (C]N), 128.7 (CeH), 127.7 (C), 121.0 (CeH), 120.1 (CeH), 116.5
(CeN), 68.0 (C), 27.6 (CH3). MS (EI) m/z (%): 382 (100%). HRMS-EIS:
calcd for (C19H14N2O3S2) : 382.0446 found 382.0439.
4.3.7. Compound 7b. 1H NMR (400 MHz, DMSO):
d 13.60 (1H, s,
NH),11.54 (1H, s, OH), 8.03 (2H, d, J¼9.2 Hz, aromatic-H), 7.39 (2H, t,
J¼7.8 Hz, aromatic-H), 7.04 (2H, d, J¼8.1 Hz, aromatic-H), 6.87
(2H, s, CH), 1.59 (6H, s, CH3). 13C NMR (100 MHz, DMSO):
d 194.4
4.3. General procedure for the photo-conversion of diary-
lethenes to thiolactone derivatives 1be9b
(C]O), 162.8 (CeOH), 156.7 (C]N), 152.0 (CeH), 132.2 (CeH), 128.9
(CeH), 127.3 (CeH), 121.9 (CeH), 119.6 (C), 117.1 (CeN), 112.8 (CeN),
67.5 (C), 26.8 (CH3). MS (EI) m/z (%): 382 (100%). HRMS-EIS: calcd
for (C19H14N2O3S2) 382.0446 found 382.0451.
Thiolactone derivatives 1be9b were prepared by the photo-
converison of diarylethenes 1ae9a with a 500 W high-pressure
mercury lamp (exposure: 39.5 mW/cm2) in a glass beaker of
500 ml. The details are as follows: the solution of 1ae9a (100 mg)
in THF (100 ml) was stirred with the light irradiation till no starting
material was detected by TLC plate (ca. 50 min). After evaporation
of the solvent, the product was purified by flash column (neutral
Al2O3, 100e200 mesh, elute: petroleum ether/acetone¼1:3, v/v).
4.3.8. Compound 8b. 1H NMR (400 MHz, DMSO):
d 12.92 (1H, s,
NH), 8.10 (1H, d, J¼7.8 Hz, aromatic-H), 7.54e7.47 (1H, m, aromatic-
H), 7.25 (1H, d, J¼8.3 Hz, aromatic-H), 7.11 (1H, t, J¼7.8 Hz,
aromatic-H), 6.81 (2H, s, CH), 3.97 (3H, s, OCH3), 1.57 (6H, s, CH3).
13C NMR (100 MHz, DMSO):
d 194.9 (C]O), 160.2 (C), 157.1
(CeOCH3), 151.0 (C]N), 132.7 (CeH), 130.4 (CeH), 127.1 (CeH),
121.9 (CeH), 121.5 (C), 117.3 (CeN), 112.6 (CeN), 68.1 (C), 56.5
(OCH3), 27.4 (CH3). MS (EI) m/z (%): 396 (100%). HRMS-EIS: calcd for
(C20H16N2O3S2) 396.0602 found 396.0605.
4.3.1. Compound 1b. 1H NMR (400 MHz, DMSO):
d 13.98 (1H, s,
NH), 7.94 (1H, d, J¼8.4 Hz, aromatic-H), 7.77 (1H, d, J¼1.8 Hz,
aromatic-H), 7.55 (1H, d, J¼8.2 Hz, aromatic-H), 6.54 (2H, s, CH),
1.57 (6H, s, CH3). 13C NMR (100 MHz, DMSO)
d: 194.9 (C]O), 163.9
4.3.9. Compound 9b. 1H NMR (400 MHz, DMSO):
d 13.71 (1H, s,
(C]N), 153.0 (C), 135.2 (C), 133.6 (CeCl), 133.0 (CeCl), 130.6 (CeH),
129.7 (CeH), 128.2 (CeH), 127.4 (CeH), 120.3 (CeN), 68.2 (C), 27.8
(CH3). MS (EI) m/z (%): 435 (100%). HRMS-ESI: calcd for
(C19H12Cl2N2O2S2) 433.9717 found 433.9713.
NH), 7.82e7.73 (2H, m, aromatic-H), 7.14 (1H, d, J¼8.1 Hz, aromatic-
H), 6.78 (2H, s, CH), 3.86 (3H, s, OCH3), 3.82 (3H, s, OCH3),1.58 (6H, s,
CH3). 13C NMR (100 MHz, DMSO):
d 194.8 (C]O), 162.1 (C), 153.5
(CeOCH3), 151.4 (CeOCH3), 149.5 (C]N), 127.7 (CeH), 121.7 (CeH),
121.3 (CeH), 120.3 (C), 112.4 (CeN), 110.4 (CeN), 68.0 (C), 56.5
(OCH3), 56.2 (OCH3), 27.5 (CH3). MS (EI) m/z (%): 426 (100%). HRMS-
EIS: calcd for (C21H18N2O4S2) 426.0707 found 426.0703.
4.3.2. Compound 2b. 1H NMR (400 MHz, DMSO):
d 13.88 (1H, s,
NH), 8.08 (2H, d, J¼7.0 Hz, aromatic-H), 7.60e7.52 (3H, m, aromatic-
H), 6.72 (2H, s, CH), 1.59 (6H, s, CH3). 13C NMR (100 MHz, DMSO):
d
194.3 (C]O), 162.3 (C), 152.8 (C]N), 130.6 (C), 129.2 (CeH), 128.5
Acknowledgements
(CeH), 127.5 (CeH), 126.3 (CeH), 121.0 (CeN), 67.5 (C), 26.9 (CH3).
MS (EI) m/z (%): 366 (100%). HRMS-ESI: calcd for (C19H14N2O2S2)
366.0497 found 366.0491.
This work was supported by the National Nature Science
Foundation of China (91123032 and 21073214) and the Beijing
Natural Science Foundation (No. 2112041, Synthesis and Properties
of Recording Material for Multi-level Memory).
4.3.3. Compound 3b. 1H NMR (400 MHz, DMSO):
NH), 7.98 (2H, d, J¼7.7 Hz, aromatic-H), 7.40 (2H, d, J¼7.7 Hz,
aromatic-H), 6.66 (2H, s, CH), 2.38 (3H, s, CH3), 1.58 (6H, s, CH3). 13
NMR (100 MHz, DMSO): 194.3 (C]O), 161.8 (C), 153.1 (C]N),
d 13.79 (1H, s,
C
References and notes
d
140.5 (C), 129.8 (CeH), 127.6 (CeH), 126.3 (CeH), 125.9 (CeH), 120.8
(CeN), 67.5 (C), 26.9 (CH3), 21.0 (CH3). MS (EI) m/z (%): 380 (100%).
HRMS-EIS: calcd for (C20H16N2O2S2) 380.0653 found 380.0648.
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16. Crystallographic parameters of 1b (CCDC 905319). Formula: C19H12Cl2N2O2S2;
Formula weight: 435.33; Crystal color: yellow; crystal system: monoclinic; space
group: P21/n; a/A: 9.455(2); b/A: 20.369(5); c/A: 10.026(3); a b
/ꢂ: 90; /ꢂ: 109.
ꢀ
ꢀ
ꢀ
/ : 90; V/A : 1822.7(8); Z: 4; Dcalcd/g cmꢁ3: 1.586; R1: 0.0578; wR2: 0.1275.
ꢂ
274(3); g
3
ꢀ
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