Photochromic and fluorescent properties of bisfurylethene derivatives

Article abstract of DOI:10.1039/b611294c

Photochromic diarylethenes having furan units (1 and 3) were synthesized and their photochromic performances were compared with those having thiophene units. The cyclization quantum yields of both derivatives in hexane are similar. In contrast, the cyclor

Full text of DOI:10.1039/b611294c

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Photochromic and fluorescent properties of bisfurylethene derivatives{
Tadatsugu Yamaguchi{* and Masahiro Irie*
Received 4th August 2006, Accepted 5th October 2006
First published as an Advance Article on the web 23rd October 2006
DOI: 10.1039/b611294c
Photochromic diarylethenes having furan units (1 and 3) were synthesized and their photochromic
performances were compared with those having thiophene units. The cyclization quantum yields
of both derivatives in hexane are similar. In contrast, the cycloreversion quantum yield of the
derivative having furan units (1) is much larger than that having thiophene units (2) in hexane.
The difference is attributed to the conformation of the closed-ring isomers. Although 2a and 4a do
not show any fluorescence, 1a and 3a exhibit fluorescence. Photochromism in the single crystalline
phase was also observed for 1 and 3. Upon irradiation with 313 nm light, the colorless crystals 1
and 3 changed to violet and yellow, respectively.
photochromic and fluorescent performances with those of
diarylethenes having thiophene rings, 2 and 4.13,14
Introduction
Photochromism has attracted considerable attention because of
its potential ability for applications to molecular devices, such as
Results and discussion
optical memories and switches.1,2 Among various types of
photochromic compounds, diarylethene derivatives are the most
promising candidates for the applications because of their
fatigue resistant property.3 Although many diarylethene deriva-
tives have been so far synthesized, most of them are composed of
thiophene or benzothiophene heteroaryl groups.3–8 Theoretical
calculation predicts that 1,2-bis(3-furyl)ethene derivatives9 will
also undergo thermally irreversible photochromic reactions.
However, diarylethenes having furan units have not been
studied in detail yet. Recently, we have reported the synthesis
of 1,2-bis(2-alkyl-1-benzofuran-3-yl)ethene derivatives,10,11 and
showed that they undergo thermally irreversible and fatigue
resistant photochromic reactions.
Synthesis of bisfurylethenes 1a and 3a
Diarylethenes 1a and 3a were synthesized according to the
general procedure for diarylethene derivatives (Scheme 1). The
coupling reaction of 3-bromo-2-methyl-5-phenylfuran (5) with
octafluorocyclopentene in dry THF gave 1a in 28% yield. The
reaction of 3-bromo-2,5-dimethylfuran (6) with octafluorocy-
clopentene in dry THF gave 3a in 41%.
Photochromic reactions in hexane solution
Fig. 1(a) shows the absorption spectral change of 1 in hexane.
Upon irradiation with 313 nm light, the open-ring isomer 1a,
which shows a maximum at 285 nm, was transformed into the
closed-ring isomer 1b, which has an absorption maximum at
525 nm. The conversion from the open to the closed-ring
isomer was 92% under irradiation with 313 nm light. Upon
irradiation with light of wavelength longer than 480 nm, the
closed-ring isomer returned to the original open-ring isomer.
It is known that diarylethene derivatives having furan units,
such as 1-(9-anthryl)-2-(2-furyl)ethene,12 give efficient fluores-
cence in cyclohexane. It has been reported that the fluores-
cence intensity can be modulated by the photochromism of
diarylethene derivatives.13–17 In this work, we have synthesized
diarylethenes having furan rings, 1 and 3, and compared their
Department of Chemistry and Biochemistry, Graduate School of
Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku,
Fukuoka, 812-8581, Japan. E-mail: tyamagu@cstf.kyushu-u.ac.jp;
irie@cstf.kyushu-u.ac.jp; Fax: +81-92-642-3557; Tel: +81-92-642-3557
{ Electronic supplementary information (ESI) available: ORTEP
drawings of 1a and 3a, photographs of single crystals of 1 and 3,
and fluorescence spectra of 3. See DOI: 10.1039/b611294c
{ Present address: Hyogo University of Teacher Education, 942-1
Shimokume, Kato, Hyogo 673-1494, Japan.
Scheme 1 Synthesis of the diarylethenes 1a and 3a. Reagents and
conditions: i) n-butyllithium (1.1 eq), THF at 278 uC, ii) octafluoro-
cyclopentene (0.5 eq) at 78 uC.
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closed-ring isomer returned to the open-ring isomer. Under
argon atmosphere the decomposition was not observed at 80 uC
for 3h in toluene. However the closed-ring isomer 3b readily
decomposed in the presence of air. We could not exchange the
mixed (hexane–ethyl acetate) solution for hexane solution.
Table 1 shows the absorption maxima and their coefficients
of the open- and closed-ring isomers, and the cyclization and
cycloreversion quantum yields of 1–4. The absorption max-
imum wavelength of 1b is 50 nm shorter than that of 2b in
hexane. The absorption maximum of 3b is 56 nm shorter than
that of 4b. A similar bathochromic shift is also observed for
the closed-ring isomer of 1,2-bis(2-methyl-1-benzofuran-3-
yl)perfluorocyclopentene.10
Enhancement of absorption coefficient was observed for 1b
(e =16500 for 1b and e = 15600 for 2b). Theoretical calculation
for the absorption spectra of the closed-ring isomer 1b and 2b
was carried out with Gaussian 03.21 The calculated wave-
lengths of 1b and 2b are 557.59 nm (f = 0.4604) and 594.83 nm
(f = 0.4306), respectively. The calculated absorption wave-
length maxima correlate well with the experimental ones.
Similar enhancement of absorption coefficient was observed
for 3b (e = 8700 for 3b and e = 8000 for 4b).
The cyclization quantum yields for 1 and 2 are similar (0.53
and 0.59). In contrast, the cycloreversion quantum yield for 1
(0.077) is much larger than that of 2 (0.013). The cyclorever-
sion quantum yield is known to depend on the conformations
of the closed-ring isomer. When the conformation is coplanar
and p-conjugation extends throughout the molecule, the
quantum yield decreases. To know the origin of the relatively
large quantum yield, X-ray crystallographic analysis of the
closed-ring form 1b was carried out as shown in Fig. 2.22 The
torsion angle of 1b (C2a–C12–C16–C18a: 23.8u) is much larger
than that of 2b (C6b–C6a–C3b–C3a: 23.4u).18 The large
torsion angle of 1b suggests that the lack of coplanarity of the
closed-ring isomer 1b. This may contribute to the higher
cycloreversion quantum yield of 1.
Fig. 1 (a) Absorption spectra of 1a (solid line), 1b (dashed line) and
in the photostationary state (dotted line) under irradiation with 313 nm
light in hexane (c = 2.2 6 1025 mol L21). (b) Absorption spectra of 3a
(solid line), 3b (dashed line) and in the photostationary state (dotted
line) under irradiation with 313 nm light in hexane–ethyl acetate 9 : 1
solution (c = 4.6 6 1025 mol L21).
The coloration/decoloration cycles of 1 could be repeated
more than 150 times in hexane under argon atmosphere. It is
reported that 2 decomposes in less than 200 cycles under the
same conditions and produces a by-product.3,20 Similar by-
product (lmax = 470 nm) formation was observed for 1.
Although the closed-ring isomer was stable at 60 uC for 3 h
in toluene solution in the presence of air, the closed-ring
isomer started to decompose at 80 uC. The decomposition was
not observed under argon atmosphere.
Fluorescent properties in hexane solution
Fig. 3(a) shows the fluorescence spectra of 1 in hexane (c =
2.0 6 105 M). The excitation wavelength is 313 nm. The solid
line represents the fluorescence in the open isomer. The dashed
line represents the fluorescence at the photostationary state
under irradiation with 313 nm light. Although 2a is non-
fluorescent, 1a exhibits fluorescence (lem = 438 nm). The
fluorescence quantum yield23 and the fluorescence life time are
0.03 and 1.19 ns, respectively. The fluorescence intensity
reversibly changed upon irradiation with 313 nm and visible
(.480 nm) light.
Fig. 1(b) shows the absorption spectral change of 3 by
photoirradiation in a mixed solution (hexane–ethyl acetate =
9 : 1). The mixed solvent was used for the purification of the
product. At the photostationary state under irradiation with
313 nm light, 90% of the open-ring isomer 3a converted to the
closed-ring isomer 3b. Upon irradiation with 440 nm light, the
Table 1 Absorption characteristics and photoreactivity of diarylethene derivatives 1, 2, and 4 in hexane and 3 in hexane–ethyl acetate 9 : 1
solution
e/104 dm3 mol21 cm21
Quantum yield
Cyclization
Compound
a
b
Cycloreversion
1
2a
3
4
a
3.87 (285 nm)
3.56 (280 nm)
0.61 (302 nm)
0.53b (303 nm)
1.65 (525 nm)
1.56 (575 nm)
0.87 (449 nm)
0.80b (505 nm)
0.53 (313 nm)
0.59 (313 nm)
0.35 (313 nm)
0.40 (313 nm)
0.077 (517 nm)
0.013 (492 nm)
0.32 (436 nm)
0.12 (505 nm)
b
Ref. 18 data. Ref. 19 data.
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Fig. 4 (a) Absorption spectra of a single crystal 1 after irradiation
with 365 nm light. (b) Polar absorption plot at 546 nm of a single
crystal of 1 after irradiation with 365 nm light.
crystalline phase. The ORTEP drawing of 1a indicates that 1a
is packed in a photoactive anti-parallel conformation in the
crystal24 and the distance between the reactive carbon atoms is
0.354 nm. The distance is shorter than the value of 0.42 nm
which allows a photochromic reaction in the crystal. Upon
irradiation with 365 nm for 3 s, the crystal turned violet, which
absorbs at 546 nm. Fig. 4(a) shows the polarized absorption
spectra of the colored crystal 1. By rotating the sample under
polarized light, the absorption intensity ratio at 546 nm
changed. The order parameter of 0.84 (546 nm) indicates that
the closed-ring isomers are regularly orientated in the crystal.
Fig. 4(b) showed the polar plot at 546 nm.
Fig. 2 ORTEP drawings of 1b: (a) top view and (b) side view. The
ellipsoid represents 50% displacement of atoms.
The fluorescence quantum yield of a diarylethene derivative
having a furan unit (0.47) is reported to be larger than that of
the corresponding thienyl derivative (0.28) in cyclohexane.12
The quantum yield of the furyl derivative 1a (0.03) is larger
than that of the thienyl derivative 2a (non-fluorescent) in
hexane. The thiophene ring is more flexible than the furan
ring. The flexibility difference is considered to affect the non-
radiative process and the quantum yield. The fluorescence
intensity reversibly changed upon irradiation with 313 nm and
visible (.480 nm) light. Although 4a is non-fluorescent, 3a
also exhibits fluorescence. The fluorescence maximum is
382 nm in hexane solution (excitation wavelength: 313 nm).
The fluorescence quantum yield is 0.03 (313 nm).23
3 also undergoes photochromism in the single crystalline
phase. Upon irradiation with 365 nm for 3 s, the crystal turned
yellow, which absorbs at 450 nm. Fig. 5(a) and (b) show the
polarized absorption spectra of 3 and the polar plots at 450 nm.
The ORTEP drawing of 3a indicates that the crystal has two
molecules in the asymmetric unit.25 The molecules have furan
rings in a photoactive anti-parallel conformation, and the
distances between the reactive carbon atoms are determined to
be 0.354 and 0.374 nm, which are short enough for the
molecule to undergo photochromism in the single crystalline
phase. The coloration/decoloration cycles of the crystals 1 and
3 could be repeated more than 100 times without destruction.
Single crystalline photochromism
Conclusions
2 is known to show photochromism in a single-crystalline
phase.18 Just like 2, 1 undergoes photochromism in the single
Diarylethenes having furan units (1a and 3a) were synthesized
and their photochromic and fluorescent performances were
examined in solution as well as in the single crystalline phase.
Fig. 3 Fluorescence spectra of 1a (solid line) and in the photosta-
tionary state under irradiation with 313 nm light (dashed line) in
hexane (c = 2.0 6 1025 M).
Fig. 5 (a) Absorption spectra of a single crystal 3 after irradiation
with 365 nm light. (b) Polar absorption plot at 450 nm of a single
crystal of 3 after irradiation with 365 nm light.
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Although 2a and 4a did not show any fluorescence, 1a and 3a
exhibit fluorescence. 1 and 3 undergo photochromism in the
single crystalline phase. The cycloreversion quantum yield for
1 is much larger than that of 2 in hexane. The closed-ring
isomer 1b has a less coplanar conformation, and this may
contribute to the higher cycloreversion quantum yield of 1.
sodium ammonium chloride solution and extracted with
diethyl ether. The organic layer was dried over anhydrous
magnesium sulfate and evaporated in vacuo. The crude
product was purified by column chromatography on silica
gel (hexane) to give 0.224 g of 1a in 28% yield as colorless
1
crystals; mp. 116–117 uC; H NMR (200 MHz) d 2.05 (6H, s),
6.71 (2H, s), 7.26–7.30 (2H, m), 7.37–7.41 (4H, m), 7.62–7.64
(4H, m). MS (FAB) 488.1231 (M+, C27H18F6O2 requires
488.1211). Anal. Calcd for C27H18F6O2: C 66.40, H 3.71%.
Found: C 66.21, H 3.72%.
Experimental
General
1
1b: violet crystals; mp. 165–166 uC; H NMR (200 MHz) d
1.68 (6H, s), 6.30 (2H, s), 7.45–7.47 (6H, m), 7.78–7.80 (4H,
m). MS m/z (M+) 488.
Solvents used were spectrograde and were purified by
distillation before use. Absorption spectra were measured with
a spectrophotometer (Hitachi, U-3410). The quantum yields
were determined by comparing the reaction rates of the
diarylethene derivatives in hexane against that of furylfulgide
in toluene. The samples were not degassed. For solution
measurements, a super-high pressure mercury lamp (Ushio,
500 W) was used as a light source. Light of appropriate
wavelength was isolated by passing light through a mono-
chromator (RITSU MC-10N) or through L-29 and Y-48
filters. Fluorescence spectra were measured with a Hitachi
F-3010 fluorescence spectrophotometer. Fluorescence life
times were measured with a time-resolved spectrofluorometer
(Hamamatsu C4334-01, C5094, and C4792) excited with a
pulsed N2 laser. The pulse width was 600 ps and the
wavelength was 337.1 nm. The decay curve was analyzed with
the single-exponential fitting after deconvolution of the
excitation light pulse profile. The goodness of the fit was
judged with the reduced x2 value (0.68). Absorption spectra in
the single crystalline phase were measured using an OPTI-POL
1,2-Bis(3-(2,5-dimethylfuryl))perfluorocyclopentene (3a)
To a stirred THF solution (30 ml) containing 3-bromo-2,5-
dimethyl-5-phenylfuran (3)27 (3.6 g, 20.6 mmol) was slowly
added 14.1 ml of 1.6
M butyllithium hexane solution
(22.6 mmol) at 278 uC, and the solution was stirred for
15 min at 278 uC. Then octafluorocyclopentene (1.36 ml,
10.3 mmol) was added slowly to the reaction mixture at
278 uC, and left to stand with stirring at 278 uC to 30 uC for
12 h. The reaction mixture was poured into concentrated
sodium ammonium chloride solution and extracted with
diethyl ether. The organic layer was dried over anhydrous
magnesium sulfate and evaporated in vacuo. The crude
product was purified by column chromatography on silica
gel (hexane) to give 1.53 g of 3a in 41% yield as colorless
1
crystals; mp. 39–40 uC; H NMR (200 MHz) d 1.90 (3H, s),
2.24 (3H, s), 6.00 (2H, s). MS m/z (M+) 364. Anal. Calcd for
C17H14F6O2: C 56.05, H 3.87%. Found: C 56.07, H 3.87%.
2POL (Nikon) polarizing microscope connected to
a
Hamamatsu PMA-11 detector. Photoirradiation for single
crystal measurements was carried out using a 100 W mercury
lamp (Nikon, C-SHG1 and LH-M100CB-1) as a light source.
1H NMR spectra were recorded on a Gemini 200 spectrometer
(200 MHz) at room temperature with CDCl3 (chloroform-d) as
solvent and tetramethylsilane as an internal standard. Mass
spectra were measured with mass spectrometers (Shimadzu
GCMS-QP5050A and JEOL GC-mate II). Good quality
crystals (1a: 0.2 6 0.2 6 0.1 mm, 1b: 0.1 6 0.1 6 0.05 mm,
and 3a: 0.1 6 0.05 6 0.05 mm) were selected for the X-ray
diffraction study. The data collection was performed on a
Bruker SMART 1000 CCD-based diffractometer (55 kV,
35 mA) with MoKa irradiation. HPLC was carried out on a
Shimadzu LC-10AD liquid chromatograph coupled with a
Shimadzu SPD-10AV spectrophotomeric detector. A silica gel
column (Wako Wakosil-5SIL) was used to analyze diaryl-
ethene isomers.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research (S) (No.15105006) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
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added 2.29 ml of 1.6
M butyllithium hexane solution
(3.66 mmol) at 278 uC, and the solution was stirred for
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˚
group P21/n, a = 7.1697(3) A, b = 29.4835(11) A, c = 10.9137(5) A,
˚
˚
3
˚
a = 90u, b = 104.206(3)u, c = 90u, V = 2236.47(16) A , Z = 4, Dc =
1.451 g cm23, m(Mo-Ka) = 0.124 mm21, T = 203(2) K, R1 = 0.0534
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reflections. CCDC deposition number: 293125.
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24 Crystal data for 1a: C27H18F6O2, MW = 488.41, triclinic, space
¯
˚
˚
˚
group P1, a = 9.5522(17) A, b = 10.8168(19) A, c = 11.1672(19) A,
˚
3
a = 88.607(3)u, b = 86.743(3)u, c = 86.796(3)u, V = 1149.9(3) A , Z =
2, Dc = 1.411 g cm23, m(Mo-Ka) = 0.121 mm21, T = 298(2) K, R1 =
0.0405 for 2736 observed reflections with I . 2s(I) from
3811 unique reflections. CCDC deposition number: 293126.
25 Crystal data for 3a: C17H14F6O2, MW = 364.28, triclinic, space
¯
˚
˚
group P1, a = 11.196(4) A, b = 12.226(4) A, c = 13.502(5) A,
˚
3
˚
a = 75.000(5)u, b = 77.462(5)u, c = 68.203(5)u, V = 1642.5(10) A ,
Z = 4, Dc = 1.473 g cm23, m(Mo-Ka) = 0.141 mm21, T = 298(2) K,
R1 = 0.0783 for 2331 observed reflections with I . 2s(I) from 6573
unique reflections. CCDC deposition number: 615568.
26 D. Obrecht, Helv. Chim. Acta, 1989, 72, 447.
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4694 | J. Mater. Chem., 2006, 16, 4690–4694
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