Syntheses and photochromic properties of diarylethenes with a naphthalene and a thiophene ring

Article abstract of DOI:10.1002/ejoc.201301677

Two types of diarylethene, each of which contains a naphthalene and a thiophene ring, were synthesized, and their photochromic properties were studied. The photochromic properties are dependent on the bridge position of the thiophene ring. The cycloreversion of one of the closed forms of some of the diarylethenes occurred almost photon quantitatively, and the photochromic reactions were thermally irreversible, at least at room temperature. Thermally irreversible photochromic diarylethenes with a naphthalene and a thiophene ring are synthesized, and their photochromic properties are studied. The quantum yield for the photocycloreversion of one diarylethene is very high. Except for one example, both photoisomers of the diarylethenes are thermally stable at ambient temperature; hence, they are candidates for molecular photomemory. Copyright

Full text of DOI:10.1002/ejoc.201301677

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FULL PAPER
DOI: 10.1002/ejoc.201301677
Syntheses and Photochromic Properties of Diarylethenes with a Naphthalene
and a Thiophene Ring
Michinori Takeshita,*[a] Erika Mizukami,[a] Kazuto Murakami,[a] Yuta Wada,[a] and
Yuji Matsuda[a]
Keywords: Photochromism / Polycycles / Sulfur heterocycles / Density functional calculations
Two types of diarylethene, each of which contains a naphth-
alene and a thiophene ring, were synthesized, and their
photochromic properties were studied. The photochromic
properties are dependent on the bridge position of the
thiophene ring. The cycloreversion of one of the closed forms
of some of the diarylethenes occurred almost photon quanti-
tatively, and the photochromic reactions were thermally
irreversible, at least at room temperature.
been reported.[12] These facts prompted us to develop diary-
lethenes with both a thiophene and a naphthalene ring and
to investigate their photochromic properties.
Introduction
Photochromic compounds have been vigorously studied
because they are expected to be photomemory materials.
Diarylethene is the one of the most attractive photochromic
molecules because of its high fatigue resistance and the
thermal irreversibility of its photochromism.[1] A highly ef-
fective photochromic reaction is an essential feature for mo-
lecular photomemory, and an extremely high photocycliza-
tion reaction quantum yield has recently been found by fix-
ing the conformation of a diarylethene with a bridging moi-
ety (i.e., cyclophan-1-ene)[2] or through intramolecular hy-
drogen bonding.[3] However, to the best of our knowledge,
despite attempts to control the cycloreversion quantum
yields by changing the substituents,[4] photon-quantitative
reactions (when the quantum yield is almost 1.0 in solution)
have not yet been reported. A highly efficient photochromic
reaction and the thermal stability of both photoisomers are
essential for photomemory. High thermal irreversibility can
be achieved by using different diarylethene aromatic rings,[1]
and dithienylethenes, in which both aromatic rings are thio-
phene rings, were first developed by Irie and co-workers
and are promising candidates for photomemory and photo-
switching devices.[5–10] Thiophene has a low aromatic stabi-
lization energy compared with that of benzene; hence, the
energy difference between the aromatic dithienylethene
open form and the nonaromatic closed form is smaller than
that of stilbene.[1] Diarylethenes with two naphthalene rings
have also been synthesized and show thermally irreversible
photochromism.[11] However, only one basic diarylethene
structure with both a thiophene and a naphthalene ring has
Results and Discussion
The diarylethenes with a thiophene and a naphthalene
ring have been synthesized as shown in Scheme 1. We pre-
pared four diarylethenes of two types: two are bridged at
the 3-position of the thiophene ring (1a and 2a), and two
1
are bridged at the 2-position (3a and 4a). The H NMR
spectrum of 1a at room temperature revealed that there
were two conformations of the open ring form present in a
1:1 ratio. These conformations are assignable to the anti-
parallel and parallel conformations, according to a previous
study.[1] The antiparallel form is photoactive, and the paral-
lel form is photoinactive; therefore, the existence of the par-
allel conformation may decrease the photocyclization quan-
tum yield. Four stable conformations of 2a were observed
by 1H NMR spectroscopy. DFT calculations (B3LYP/6-
31G*) for 2a with the Spartan program[13] indicated that
two of the conformations were antiparallel and the others
were parallel (Figure 1). The DFT calculations revealed
that the ratio of antiparallel to parallel conformations was
1
4:6. Different conformations were not indicated by the H
NMR spectra of 3a and 4a; the rotational barrier may be
1
too low to show separate H NMR spectra.
Figure 2 shows the absorption spectral changes of the
diarylethenes 1a/1b, 2a/2b, 3a/3b, and 4a/4b in n-hexane.
The colorless solutions turned red or yellow upon UV irra-
diation, and new absorption maxima appeared in the visible
region. The colors and spectra returned to their initial states
after irradiation in the visible region. The spectral changes
of 3a/3b and 4a/4b were much smaller than those of 1a/1b
and 2a/2b, as shown in Figure 2. The conversions at the
photostationary state under irradiation with 313 nm light
[a] Department of Chemistry and Applied Chemistry, Faculty of
Science and Engineering, Saga University,
Honjo 1, Saga 840-8502, Japan
E-mail: michi@cc.saga-u.ac.jp
http://extwww.cc.saga-u.ac.jp/~michi/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301677.
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M. Takeshita, E. Mizukami, K. Murakami, Y. Wada, Y. Matsuda
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3b was only 12.6%, which implies that the photostationary
state of 3a/3b at 313 nm consisted of 87.4% of the open
form and 12.6% of the closed form. The extinction coeffi-
cients of the open form 3a and the closed form 3b were
almost the same at 313 nm; therefore, one can consider that
the cyclization quantum yield is quite low compared to the
cycloreversion quantum yield or vice versa. The quantum
yields of the diarylethene cyclization and cycloreversion re-
actions were measured and are shown in Table 1.[15] The
quantum yields for the photocyclization reactions were
measured at 313 nm, and those for the cycloreversions were
obtained by means of visible light (517 or 435 nm).
Scheme 1. Synthesis of diarylethenes composed of a thiophene and
a naphthalene ring.
Figure 2. Absorption spectral changes of 1, 2, 3, and 4 in n-hexane
upon irradiation with 313 nm light.
Table 1. Conversions[a] at 366 nm and quantum yields for photocy-
clization and cycloreversion reactions.
Conversion [%][a]
Φcyclization
Φcycloreversion
1
30.8
34.5
12.6
21.7
–
77
0.34 (313 nm)
0.32 (313 nm)
0.25 (313 nm)
0.23 (313 nm)
0.21 (280 nm)
0.40 (366 nm)
0.62 (517 nm)
0.29 (517 nm)
0.29 (435 nm)
0.95 (435 nm)
0.13 (492 nm)
0.58 (425 nm)
2
3
4
9[16]
10[16]
[b]
[a] Determined by HPLC. [b] Not reported.
As expected, the quantum yield of the cycloreversion re-
action of the closed form of 4b is quite high, 0.95, and is
almost quantitative (Figure 2). On the other hand, that of
3b is 0.29. In spite of the very low conversion, such a small
quantum yield could be responsible for the wavelength-de-
pendence of the quantum yield. The reasons for these re-
Figure 1. Four stable conformations of 2a obtained by DFT calcu-
lations (B3LYP/6-31G*) with Spartan ’08: (a) and (b) have anti-
parallel conformations, and (c) and (d) have parallel conformations. markable quantum yields are still obscure, but it has been
The calculated ratio was (a)/(b)/(c)/(d) = 0.16/0.24/0.33/0.27.
reported that dithienylethenes with thiophene rings con-
nected at the 2-position tend to show high cycloreversion
were estimated by HPLC analyses.[14] The ratios of the pho- quantum yields.[16] For example, Table 1 shows the quan-
toisomers were determined by measuring the corresponding tum yields of the reported dithienylethenes 9 and 10. In-
absorption areas at the isosbestic points. The results are deed, the quantum yield for the cycloreversion of 10 was
summarized in Table 1. For instance, the conversion of 3a/ much larger than that of 9. Compounds 3a and 4a are
2
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Diarylethenes with a Naphthalene and a Thiophene Ring
bridged at the 2-position of the thiophene rings, and the Our future research will be the development of further cy-
additional 10-π-electron naphthalene rings may enhance the clophan-1-ene systems with a thiophene and a naphthalene
cycloreversion quantum yields.
ring to obtain much more efficient photochromic com-
pounds such as those we have already reported.[2,17]
Experimental Section
General Remarks: Absorption spectra were measured with an ab-
sorption spectrophotometer (Hitachi, U-3310). 1H NMR spectra
were recorded with a JEOL-AL300 FT-NMR spectrometer at
300 MHz. All chemical shifts are given in ppm relative to that of
tetramethylsilane (TMS), and coupling constants are given in Hz.
Mass spectra were recorded with a JEOL JMS-GCMATE II spec-
trometer (EI, 70ev). Photoirradiation was performed with a 500 W
super-high-pressure mercury lamp, and monochromic light was ob-
tained by passing it through a monochromator (JOBIN YVON).
The quantum yields were determined by a similar manner to that
described previously.[15]
Another important feature for organic photomemory is
thermal irreversibility; hence, the thermal cycloreversion re-
actions of the closed forms 1b, 2b, 3b, and 4b were studied.
Sealed toluene solutions of 1b, 2b, 3b, and 4b were stored
at 100 °C in the dark. After 3 h of heating, no changes were
observed in the absorption spectra of the photostationary
states of 2a/2b, 3a/3b, and 4a/4b, but the absorption spec-
trum of 1a/1b changed slightly. Next, the thermal cyclore-
version reaction rates of 1b were measured at various tem-
peratures, and the activation energy was estimated from the
Arrhenius plot (Figure 3). The activation energy and the
pre-exponential factor of the thermal cycloreversion of 1b
were 134 kJmol–1 and 6.3ϫ1013 s–1, respectively, and the
calculated half-life at 20 °C was ca. 250 years. Therefore,
the photocyclization of 1b is thermally reversible, but it still
has a long lifetime at ambient temperatures.
1-(2,3,3,4,4,5,5-Heptafluoro-1-cyclopenten-1-yl)-2-methylnaphth-
alene (6):[2] To a flame-dried 1000 mL round-bottomed flask, 1-
bromo-2-methylnaphthalene (5; 25.0 g 114 mmol) and dry tetra-
hydrofuran (THF, 220 mL) were added, and the solution was co-
oled to –70 °C in a dry-ice–acetone bath under an Ar atmosphere.
To this solution, a 1.6 n nBuLi in hexane solution (89.0 mL) was
added dropwise, and the mixture was stirred for 30 min at this tem-
perature. A dry THF (30 mL) solution of 1,2,3,3,4,4,5,5-octafluoro-
cyclopentene (15.2 g, 114 mmol) was added dropwise, and the mix-
ture was stirred for 2 h at the same temperature; the bath was then
removed, and the mixture was stirred at room temperature over-
night. Methanol and brine were added to the solution, and the
organic layer was separated and washed with brine twice. The mix-
ture was dried with MgSO4, and the solvent was evaporated in
vacuo. Recrystallization (MeOH) of the residue afforded 6 (19.8 g,
59.1 mmol) as colorless prisms in 52.1% yield; m.p. 61.8–63.2 °C.
1H NMR (300 MHz, CDCl3): δ = 2.43 (s, 3 H), 7.48 (d, J = 8 Hz,
1 H), 7.47–7.55 (m, 3 H), 7.86–7.92 (m, 2 H) ppm. HRMS (EI):
calcd. for C16H9F7 [M]+ 334.0592; found 334.0592.
2,4-Dimethyl-3-[3,3,4,4,5,5-hexafluoro-2-(2-methyl-1-naphthalen-
yl)-1-cyclopenten-1-yl]thiophene (1a): To a flame-dried 200 mL
round-bottomed flask, 3-bromo-2,4-dimethylthiophene (7a; 1.0 g
5.24 mmol)[16] and dry THF (50 mL) were added, and the solution
was cooled to –70 °C in a dry-ice–acetone bath under an Ar atmo-
sphere. To this solution, a 1.6 n nBuLi in hexane solution (3.9 mL)
was added dropwise, and the mixture was stirred for 30 min at this
temperature. A dry THF (30 mL) solution of 6 (1.59 g, 4.80 mmol)
was added dropwise, and the mixture was stirred for 2 h at the
same temperature; the bath was then removed, and the mixture was
stirred at room temperature overnight. Methanol and brine were
added to the solution, and the organic layer was separated and
washed with brine twice. The mixture was dried with MgSO4, and
the solvent was evaporated in vacuo. The residue was subjected
to silica gel column chromatography, and the hexane eluate was
evaporated in vacuo. Recrystallization (MeOH) of the residue af-
forded 1a (0.65 g, 1.54 mmol) as colorless prisms in 32% yield; m.p.
Figure 3. Arrhenius plot for the thermal cycloreversion of 1b in
toluene.
Conclusions
Diarylethenes with both a thiophene and a naphthalene
ring showed almost thermally irreversible photochromic
properties. The photocycloreversion quantum yield for one
diarylethene was almost photon quantitative, and the pho-
tocyclization reaction quantum yields were relatively low. If
an extra bridge was formed, the conformation could be
fixed in the photoactive antiparallel conformation, and the
1
113.0–115.0 °C. H NMR (300 MHz, CDCl3): δ = 1.91 (s, 1.5 H),
2.15 (s, 1.5 H), 2.16 (s, 1.5 H), 2.42 (s, 1.5 H), 2.43 (s, 3 H), 6.50
(s, 0.5 H), 6.60 (s, 0.5 H), 7.28 (d, J = 9 Hz, 1 H), 7.42–7.51 (m, 2
H), 7.75–7.80 (m, 3 H) ppm (parallel/antiparallel 1:1). HRMS (EI):
calcd. for C22H16F6S [M]+ 426.0877; found 426.0878.
2,4-Dimethyl-3-[3,3,4,4,5,5-hexafluoro-2-(2-methyl-1-naphthalen-
photocyclization reaction efficiency would be increased. yl)-1-cyclopenten-1-yl]-5-phenylthiophene (2a): To a flame-dried
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M. Takeshita, E. Mizukami, K. Murakami, Y. Wada, Y. Matsuda
FULL PAPER
200 mL round-bottomed flask, 3-bromo-2,4-dimethyl-5-phenyl-
thiophene (7b; 0.80 g, 3.0 mmol)[18] and dry THF (65 mL) were
added, and the solution was cooled to –70 °C in a dry-ice–acetone
bath under an Ar atmosphere. To this solution, a 1.6 n nBuLi in
hexane solution (2.2 mL) was added dropwise, and the mixture was
stirred for 30 min at this temperature. A dry THF (30 mL) solution
ate was evaporated in vacuo. Recrystallization (n-hexane) of the
residue afforded 4a (0.56 g, 1.15 mmol) as colorless prisms in 20%
yield; m.p. 74.4–76.5 °C. H NMR (300 MHz CDCl3): δ = 2.21 (s,
3 H), 2.41 (s, 3 H), 6.92 (s, 1 H), 7.21–7.28 (m, 4 H), 7.38 (d, J =
8 Hz, 1 H), 7.43–7.51 (m, 2 H), 7.64 (d, J = 8 Hz, 1 H), 7.81–7.87
(m, 2 H) ppm. HRMS (EI): calcd. for C27H18F6S [M]+ 488.1033;
1
of 6 (1.19 g, 3.55 mmol) was added dropwise, and the mixture was found 488.1041.
stirred for 2 h at the same temperature; the bath was then removed,
Supporting Information (see footnote on the first page of this arti-
and the mixture was stirred at room temperature overnight. Meth-
anol and brine were added to the solution, and the organic layer
was separated and washed with brine twice. The mixture was dried
with MgSO4, and the solvent was evaporated in vacuo. The residue
was subjected to silica gel column chromatography, and the hexane
eluate was evaporated in vacuo. Recrystallization (MeOH) of the
residue afforded 2a (0.65 g, 1.29 mmol) as colorless prisms in 43%
cle): NMR spectra of 1a–4a and HPLC charts for the open forms
and the photostationary states of diarylethenes 1–4.
Acknowledgments
This work is supported by the Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan through a Grant-
in-Aid for Science Research in a Priority Area New Frontiers in
Photochromism (471) (21021021) and by the Japan Society for the
Promotion of Science (JSPS) through a Grant-in-Aid for Scientific
Research (C) (21550119). The authors are also grateful to Zeon Co.
Ltd. for providing perfluorocyclopentene.
1
yield; m.p. 113.0–114.5 °C. H NMR (300 MHz CDCl3): δ = 1.93
(s, 1.5 H), 2.17 (s, 0.75 H), 2.17 (s, 0.75 H), 2.21 (s, 1.5 H), 2.44 (s,
0.75 H), 2.44 (s, 0.75 H), 2.46 (s, 1.5 H), 2.46 (s, 1.5 H), 7.14–7.16
(m, 1 H), 7.26–7.35 (m, 4.5 H), 7.43–7.52 (m, 2 H), 7.78–7.81 (m,
3.5 H) ppm (a mixture of several conformations). HRMS (EI):
calcd. for C28H20F6S [M]+ 502.1190; found 502.1190.
3,5-Dimethyl-2-[3,3,4,4,5,5-hexafluoro-2-(2-methyl-1-naphthalen-
yl)-1-cyclopenten-1-yl]thiophene (3a): To a flame-dried 500 mL
round-bottomed flask, 2,4-dimethylthiophene (8a; 1.12 g,
10.0 mmol),[19] N,N,NЈ,NЈ-tetramethylethylenediamine (TMEDA;
1.8 mL 11.7 mmol), and dry diethyl ether (130 mL) were added,
and the solution was cooled to 0 °C in an ice bath under an Ar
atmosphere. To this solution, a 1.6 n nBuLi in hexane solution
(7.6 mL) was added dropwise, and the mixture was heated to reflux
for 45 min. The mixture was cooled to –70 °C, and a dry diethyl
ether (70 mL) solution of 1 (3.3 g, 9.0 mmol) was added dropwise
at this temperature, and the mixture was stirred for 2 h at the same
temperature; the bath was then removed, and the mixture was
stirred at room temperature overnight. Methanol and brine were
added to the solution, and the organic layer was separated and
washed with brine twice. The mixture was dried with MgSO4, and
the solvent was evaporated in vacuo. The residue was subjected
to silica gel column chromatography, and the hexane eluate was
evaporated in vacuo. Recrystallization (n-hexane) of the residue af-
forded 3a (0.65 g, 1.29 mmol) as colorless prisms in 43% yield; m.p.
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1
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the mixture was stirred at room temperature overnight. Methanol
and brine were added to the solution, and the organic layer was
separated and washed with brine twice. The mixture was dried with
MgSO4, and the solvent was evaporated in vacuo. The residue was
subjected to silica gel column chromatography, and the hexane elu-
[14] Details are shown in the Supporting Information.
[15] The quantum yields for the photocyclization reaction were ob-
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Received: November 8, 2013
Published Online: ᭿
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Diarylethenes with a Naphthalene and a Thiophene Ring
Photochromism
M. Takeshita,* E. Mizukami,
K. Murakami, Y. Wada,
Y. Matsuda ....................................... 1–5
Syntheses and Photochromic Properties of
Diarylethenes with a Naphthalene and a
Thiophene Ring
Thermally irreversible photochromic di-
arylethenes with naphthalene and
thiophene ring are synthesized, and their
photochromic properties are studied. The
quantum yield for the photocycloreversion
of one diarylethene is very high. Except for
one example, both photoisomers of the di-
arylethenes are thermally stable at ambient
temperature; hence, they are candidates for
molecular photomemory.
a
a
Keywords: Photochromism / Polycycles /
Sulfur heterocycles / Density functional
calculations
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