Thermal bleaching reactions of photochromic diarylethenes with thiophene-S,S-dioxide for a light-starting irreversible thermosensor

Article abstract of DOI:10.1039/c3cc00053b

Thiophene-S,S-dioxidized diarylethenes introducing bulky substituents at the reactive positions were newly synthesized. The diarylethenes showed reversible photochromism, whereas the photocycloreversion reaction was suppressed by thiophene-oxidation. The diarylethene closed-ring isomers having secondary alkyl groups at the reactive positions were found to undergo thermal bleaching reactions which produce at least three types of byproducts. Such materials could find application as light-starting irreversible thermosensors. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc00053b

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Thermal bleaching reactions of photochromic
diarylethenes with thiophene-S,S-dioxide for a
light-starting irreversible thermosensor†
Cite this: Chem. Commun., 2013,
49, 2362
Received 4th January 2013,
Accepted 2nd February 2013
Hiroaki Shoji and Seiya Kobatake*
DOI: 10.1039/c3cc00053b
www.rsc.org/chemcomm
Thiophene-S,S-dioxidized diarylethenes introducing bulky substituents trimethylsilyl or methoxymethyl group was introduced at the reactive
at the reactive positions were newly synthesized. The diarylethenes positions, thermal bleaching reactions which produce colorless
showed reversible photochromism, whereas the photocycloreversion byproducts were observed.12,13 If diarylethenes have a photostability
reaction was suppressed by thiophene-oxidation. The diarylethene of the closed-ring isomers in addition to the formation of colorless
closed-ring isomers having secondary alkyl groups at the reactive byproducts, a light-starting irreversible thermosensor can be created
positions were found to undergo thermal bleaching reactions which by combining the following conditions: (1) fast coloration upon
produce at least three types of byproducts. Such materials could find irradiation with UV light, (2) a photostability of the colored state, (3)
application as light-starting irreversible thermosensors.
thermal bleaching at an appropriate temperature, and (4) a photo-
stability of the thermal bleaching state. The light-starting irreversible
Photochromic compounds can exhibit interconversion between two thermosensor can be used as a sensor which can detect a rise in heat at
isomers upon irradiation at an appropriate wavelength of light.1,2 low temperature below room temperature. Such a thermosensor has
The photoisomerizations of these compounds produce changes in an advantage that it can be stored at room temperature before use.
chemical and physical properties, such as molecular structures,
absorption spectra, refractive indices, dielectric constants, and
oxidation–reduction potentials. Photochromic compounds are
categorized into two types, T-type and P-type.1,3 T-type photochromic
compounds provide thermally unstable isomers by photoirradiation.
Azobenzene, spiropyran, and spirooxazine are well known as repre-
sentative T-type photochromic compounds. In contrast, P-type
photochromic compounds are thermally stable for both isomers at
room temperature. Furylfulgide and diarylethene are well known as
representative P-type photochromic compounds. Among these com-
pounds, diarylethenes have attracted much attention due to their
excellent fatigue resistance.4–8
We have recently designed and synthesized diarylethenes with
novel functions. Photochromic diarylethenes can change their prop-
It has been recently reported that thiophene-S,S-dioxidized
erties such as thermal and photochemical reactivities by introducing diarylethenes can suppress the photocycloreversion reaction.14–16
substituents at the aryl groups and the reactive positions. The This finding can encourage us to make the light-starting irreversible
thermal cycloreversion reactivity can be promoted by introduction thermosensor. Here, we have designed and synthesized thiophene-
of bulky substituents at the reactive positions.9–11 However, when a S,S-dioxidized diarylethenes (1a–3a) which can suppress the photo-
cycloreversion reaction and produce colorless byproducts upon
heating.
Department of Applied Chemistry, Graduate School of Engineering, Osaka City
Thiophene-S,S-dioxidized diarylethenes 1a–3a were prepared
by oxidation of the precursor compounds, 1,2-bis(2-alkyl-5-phenyl-
3-thienyl)perfluorocyclopentenes (4a–6a) using 3-chloroperoxy-
benzoic acid (m-CPBA) (see Table S1, ESI†). The oxidation reaction
proceeded at only one side of the thiophene rings.
University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan.
E-mail: kobatake@a-chem.eng.osaka-cu.ac.jp; Fax: +81 6 6605 2797;
Tel: +81 6 6605 2797
† Electronic supplementary information (ESI) available: Synthesis of 1a–3a, HPLC
chart after the thermal reaction at 100 1C, 1H NMR spectra, mass spectra, and
X-ray crystallographic data of 2c–2e, and the reaction mechanism of the for-
mation of the byproducts. CCDC 917414–917416. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3cc00053b
Diarylethenes 1a–3a showed photochromism upon alternating
irradiation with UV and visible light. Fig. 1 shows absorption
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Fig. 1 Absorption spectra of 2a (1.9 Â 10À5 M) (---), 2b (TT), and the photo-
stationary solution (-Á-) upon irradiation with 313 nm light in hexane.
Table 1 Optical properties of diarylethenes having thiophene or thiophene-S,S-
dioxide substituent in hexane
Open-ring
isomer
Closed-ring
isomer
Quantum yields
l
max/ e/MÀ1 max/ e/MÀ1
l
Diarylethene R
nm cmÀ1 nm cmÀ1 Fo-c Fc-o
1a
2a
CH2CH3
284 29 200 556 20 600 0.49 2.1 Â 10À4
CH(CH3)2 282 24 100 550 17 300 0.36 2.8 Â 10À5
a
a
a
3a
CH(C3H7)2 286 29 500 580
CH2CH3
CH(CH3)2 290 34 900 600 14 800 0.50 0.026
CH(C3H7)2 296 37 600 602 12 600 0.48 0.074
—
—
—
4ab
5ac
6a
286 40 000 600 17 000 0.52 0.0081
Fig. 2 Decay curves of 1b (a), 2b (b), and 3b (c) in toluene.
a
Not determined due to less thermal stability for the closed-ring
b
c
isomer. Ref. 9. Ref. 17.
The reaction obeyed the first-order kinetics; thus, the rate constant
(k) was determined from the slope of the linear relationship of
the first-order plots. Fig. 3 shows the k value of the bleaching
reaction depending on the temperature. Activation energy (Ea) and
frequency factor (A) can be estimated from Arrhenius equation
using the plots in Fig. 3. The Ea and A values for 2b were estimated
spectral changes of 2a upon irradiation with UV light. Upon
irradiation with 313 nm light, the new absorption band of the
closed-ring isomer 2b appeared at 550 nm. The violet-colored
solution gradually returned to colorless at a considerably slow rate
upon irradiation with visible light (l > 500 nm).
to be 121 kJ molÀ1 and 9.7 Â 1013
s
À1, respectively. Those of 3b
Photocyclization and photocycloreversion quantum yields were
examined in hexane. Table 1 shows the optical properties of the
diarylethenes before and after the oxidation reaction. The molar
absorption coefficient (e) and the photocyclization quantum yield
(Fo-c) of each compound are similar before and after the oxidation
reaction. However, the absorption maximum wavelength (lmax) of the
closed-ring isomers 1b–3b in the visible range is shifted to 20–50 nm
shorter than those of the closed-ring isomers 4b–6b. The photocyclo-
reversion quantum yields of 1b–2b are lower than those of the closed-
ring isomers 4b–6b by a factor of 102–103. It is concluded that shorter
wavelength shift of lmax and the suppression of the photocyclorever-
sion reaction are due to the influence of the thiophene-S,S-dioxide
ring. The energy level of the closed-ring isomer at the ground state is
considered to become lower with the thiophene-S,S-oxidation.
Thermal stability of the closed-ring isomers 1b–3b was examined
in toluene. Fig. 2 shows decay curves of absorbance at lmax in the
closed-ring isomers at various temperatures. The closed-ring isomer
having ethyl substituents (1b) is thermally stable at 120 1C; whereas 4b
returns to 4a in 68% yield for 10 h at 120 1C.9 In contrast, the thermal
bleaching reactivities of the closed-ring isomers having CH(CH3)2
and CH(CH2CH2CH3)2 substituents (2b and 3b) are quite different
from 1b. The reaction rates were accelerated by an increase of tempera-
ture. The thermal bleaching reaction of 3b proceeded even at 20 1C,
and the half-life time was estimated to be 4.4 h at that temperature.
were 101 kJ molÀ1 and 5.3 Â 1013
s
À1. The k value and half-life time
(t1/2) of 2b at 100 1C were calculated to be 1.2 Â 10À3 sÀ1 and
10 min, respectively. Those of 3b were 3.6 Â 10À1 sÀ1 and 1.9 s,
respectively. It is revealed that the more bulky substituents are
introduced, the more the thermal bleaching reaction is promoted.
The half-life time of diarylethenes 5b and 6b before oxidation was
estimated to be 20 min and 72 s at 100 1C, respectively. Thus, the
thermal bleaching reaction was accelerated by oxidation of the
thiophene ring. The Ea values did not change before and after
oxidation. However, the A values of 2b and 3b are much larger than
Fig. 3 Temperature dependence of the rate constant for the thermal bleaching
reaction of 2b (J) and 3b (K) in toluene.
c
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Fig. 4 shows ORTEP drawings of 2c–2e. Byproducts 2c–2e
confirmed that the sulfonyl group is eliminated from the
diarylethenes. 2c and 2d were produced by rearrangement
and elimination of the isopropyl group, respectively. 2e was
produced by inversion of the thienyl group. Thus, 2c–2e were
confirmed to be produced by cleavage of the C–S bond. Bulky
secondary alkyl substituents have a critical role in generation of
the thermal reaction.
Thiophene-S,S-dioxidized diarylethenes introducing primary
alkyl substituents at the reactive positions do not exhibit any
thermal reaction; thus, 1b is thermally more stable than 4b.9
This indicates that the thermal cycloreversion reaction is
suppressed by oxidation of thiophene. In contrast, thiophene-
S,S-dioxidized diarylethenes introducing secondary alkyl groups
at the reactive positions show no thermal cycloreversion reaction
but show the formation of the colorless byproducts by the
thermal process. The thermal cycloreversion reaction and the
thermal formation of the byproducts are considered to take place
through different reaction paths. The reaction mechanism
proposed is depicted in Fig. S4 (ESI†). First, a sulfonyl radical
is formed by homolysis of a C–S bond, and SO2 is eliminated.
Second, intermediates are produced by radical migration, and
three byproducts are produced.
Fig. 4 ORTEP drawings of byproducts 2c–2e showing 50% probability displace-
ment ellipsoids.
In conclusion, we succeeded in designing diarylethenes
with suppression of photocycloreversion reactivity and thermal
formation of colorless byproducts by introduction of thiophene-
S,S-dioxide and secondary alkyl substituents at the reactive
positions. The structures of thermal bleaching byproducts
were confirmed by X-ray crystallographic analysis. 3b showed
the half-life time of 1 h at 30 1C. These compounds have the
potential for applications such as the light-starting irreversible
thermosensor.
those of 5b and 6b. The large A value contributes to a bond cleavage
in the bleaching reaction.
In order to confirm the structure of thermal bleaching products
of thiophene-S,S-dioxidized diarylethenes, thermal reaction pro-
ducts of 2a and 2b were analyzed by high performance liquid
chromatography (HPLC). The open-ring isomer 2a was confirmed
to have thermal stability even at 100 1C. Fig. S1 (ESI†) exhibits
chromatographs (hexane : ethyl acetate = 95 : 5) after thermal reac-
tion of 2b for 15 min at 100 1C. Judging from the HPLC chart, the
closed-ring isomer does not return to the open-ring isomer in the
thermal reaction, but other products were formed. At least five
byproducts can be seen from the HPLC profile. Among them, we
could isolate three byproducts. Byproducts 2c, 2d, and 2e were
eluted at 13, 15, and 16 min, respectively, and isolated by HPLC
(hexane : ethyl acetate = 99 : 1). Their products were purified and
recrystallized from hexane. The structures of 2c–2e were identified
by 1H NMR, mass spectroscopy, and X-ray crystallographic analysis.
The presence of isopropyl groups was confirmed by 1H NMR
spectroscopy (see Fig. S2, ESI†). Byproducts 2c and 2e have two
different types of isopropyl groups. In contrast, byproduct 2d has
only one type of isopropyl group; thus, one of the two isopropyl
groups was eliminated by the formation of the byproduct. The
molecular weights of the byproducts were determined by mass
spectroscopy (see Fig. S3, ESI†). The molecular mass of 2c and 2e
was 544 g molÀ1, which is 64 g molÀ1 smaller than that of 2b; thus,
it indicates that the sulfonyl group is eliminated from 2b. The
molecular mass of 2d was 502 g molÀ1, which is 106 g molÀ1
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c
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