Fluorescence property of photochromic diarylethenes with indole groups

Article abstract of DOI:10.1246/bcsj.76.1625

Diarylethenes having a fluorescent indole ring as the aryl group 1a and 2a were synthesized. Upon alternate irradiation with 366 nm and visible (λ > 480 nm) light, 1a underwent reversible photocyclization reactions to produce closed-ring isomer 1b. The fluorescence intensity also reversibly changed along with the reactions. The fluorescence quantum yields of the open-ring isomers 1a and 2a were 4.6 and 6.3% respectively, while the yields of the closed-ring isomers 1b and 2b were almost zero. The fluorescence quantum yields decreased with the increase in the photocyclization quantum yields.

Full text of DOI:10.1246/bcsj.76.1625

ꢀ
2003 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 76, 1625–1628 (2003) 1625
Fluorescence Property of Photochromic Diarylethenes with Indole
Groups
ꢀ
Kyoko Yagi and Masahiro Irie
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581
(
Received January 28, 2003)
Diarylethenes having a fluorescent indole ring as the aryl group 1a and 2a were synthesized. Upon alternate ir-
radiation with 366 nm and visible (ꢀ > 480 nm) light, 1a underwent reversible photocyclization reactions to produce
closed-ring isomer 1b. The fluorescence intensity also reversibly changed along with the reactions. The fluorescence
quantum yields of the open-ring isomers 1a and 2a were 4.6 and 6.3% respectively, while the yields of the closed-ring
isomers 1b and 2b were almost zero. The fluorescence quantum yields decreased with the increase in the photocycli-
zation quantum yields.
Photochromic compounds attract much attention because of
their potential for applications to photonics devices, such as
cal memory media as well as for fluorescent probes.
2-Substituted indoles are well-known to exhibit strong fluo-
1
–5
9–11
optical memory media and photo-optical switches.
Among
rescence.
In the present study, we have attempted to pre-
various types of photochromic compounds, diarylethene deriv-
atives are the most promising compounds for the applications
because of their thermally irreversible and fatigue resistant
pare fluorescent photochromic compounds by introducing a
fluorescent indole ring as the aryl groups of diarylethene deriv-
atives (Scheme 2).
6
–8
photochromic performance (Scheme 1).
Results and Discussion
So far, the most convenient method to follow the photochro-
mic reactions is to measure the absorption spectral changes.
To detect the spectral changes the concentration of the photo-
Synthesis. Diarylethene derivatives 1a and 2a with differ-
ent 2-substituted indoles were synthesized as shown in
Scheme 3. 3-Bromoindole derivatives 4 and 5 were obtained
ꢁ
6
ꢁ3
chromic compounds should be higher than 10 mol dm
even if the optical pass length is 1 cm. In polymer thin films
ꢂ
from 1,2-substituted indole by bromination with benzyltri-
12
ꢁ2
of 1 mm, the concentration should be higher than 10
mol dm . To detect the photochromic reactions at low con-
centrations, it is desired to develop photochromic compounds,
which change fluorescent properties upon photoirradiation.
The fluorescent photochromic compounds are useful for opti-
methylammonium tribromide. The coupling reaction of
monobenzothiophene-substituted perfluorocyclopentene with
4 and 5 produced diarylethenes 1a and 2a. 3a was synthesized
ꢁ3
ꢂ
8
as a reference compound. 1a–3a were purified by column
chromatography and recrystallization (hexane). The structures
1
of all compounds were confirmed by H NMR, mass spectro-
scopy, and elemental analysis.
Photochromic Reactions. Figure 1 shows the absorption
5
spectral change of
photoirradiation. Upon irradiation with 366-nm light, an ab-
sorption band around 300 nm decreased and a new band ap-
2
(3:2 ꢃ 10ꢁ M) in hexane by
Scheme 1.
Scheme 2.
Scheme 3.
Published on the web August 15, 2003; DOI 10.1246/bcsj.76.1625

1
626 Bull. Chem. Soc. Jpn., 76, No. 8 (2003)
Fluorescence Property of Diarylethenes
Fig. 2. Absorption spectra of 2a (—) and 1-methyl-2-
phenylindole (- - -) in hexane.
Fig. 1. Absorption spectral changes of
(
tostationary state under irradiation with 366-nm light
2 in hexane
3:2 ꢃ 10ꢁ5 M) by photoirradiation: 2a (—), 2 in the pho-
(
– –), and 2b (- - -).
ꢂ
Table 1. Absorption Maxima of the Open- and Closed-Ring
aÞ
Isomers of 1–3
max/M 1 cm
ꢁ
ꢁ1
Compounds
ꢀ
max/nm
"
1
a
260
556
296
586
258
517
14200
10100
17400
12800
14000
9100
1b
2a
2b
3a
3b
a) In hexane.
peared in the visible region. The color of the solution changed
from colorless to purple, in which an absorption maximum was
observed at 586 nm. When the purple solution was irradiated
with visible light (ꢀ > 480 nm), the spectrum returned back to
the original one. The photogenerated colored product was sta-
ble and could be isolated by high performance liquid chroma-
tography (HPLC, hexane/ethyl acetate). NMR and mass spec-
tral measurement determined the structure of the colored
product, the closed-ring isomer 2b. 1 and 3 also underwent
similar photochromic reactions. Table 1 summarizes the main
absorption bands of the open and closed-ring form isomers 1a–
Fig. 3. Absorption spectra of 2b in several solvents; hexane
(—), ethyl acetate (- - -), and acetonitrile (– –). The con-
ꢂ
centration of 2b was 6:3 ꢃ 10ꢁ M.
5
closed-ring isomer 2b.
Table 2 summarizes the photocyclization and cyclorever-
sion quantum yields. Relatively high cyclization quantum
yields were observed for 1a–3a. 2b was stable in the dark
at room temperature. The thermal stability of the closed-ring
ꢄ
form isomer 2b was examined at 100–140 C in decalin. The
absorbance of the closed-ring isomers decreased slowly above
3
a and 1b–3b.
To know the details of the spectroscopic characteristics of
ꢄ ꢄ
1
00 C. Even at 120 C, the half-life time was 5.5 days.
the open- and closed-ring isomers 2a and 2b, we compared
the absorption spectra with the indole derivatives as shown
in Fig. 2. The edge of 1-methyl-2-phenylindole (ꢀmax ¼ 295
nm) is less than 360 nm, while the edge of 2a (ꢀmax ¼ 285
nm) extends to 400 nm. The donor–acceptor interaction be-
tween the electron-donating 1-methyl-2-phenylindole unit
and the electron-withdrawing perfluorocyclopentene unit is
considered to shift the absorption edge. The photocyclization
reaction of 2a was also observed by irradiation with light
longer than 350 nm. The result suggests that the charge-trans-
fer excited state can enter the photocyclization channel.
Figure 3 shows the solvent effects of closed-ring isomer 2b
in several solvents. Absorption maxima changed from 586 nm
to 613 nm by changing the solvent polarity from hexane to
acetonitrile. Such a shift suggests the polar structure of the
Table 2. Quantum Yields of Photochromic Reactions and
Fluorescence Emission
ꢁ
cÞ
cÞ
Compounds
ꢁf
ꢂ
0/ns
aÞ
bÞ
ꢁ
cyclization
0.44
0.26
ꢁcycloreversion
1
2
3
0.075
0.21
0.35
0.046
0.063
0.012
<1
<1
—
0.35
a) Irradiation at 366 nm light. Reference; furyl flugide
ꢁcyclization ¼ 0:21 in n-hexane). b) Irradiation at ꢀmax light
(ꢀ > 480 nm). Reference; furyl flugide (ꢁ
(
cycloreversion
¼
0:06 in toluene). c) Fluorescence quantum yields of the
open-ring isomers 1a–3a. Reference; anthracene in cyclohex-
ane (ꢁf ¼ 0:31, Ex; 366 nm).

K. Yagi et al.
Bull. Chem. Soc. Jpn., 76, No. 8 (2003) 1627
reactions. The fluorescence quantum yield of 2a was as high
as 6.3%.
Experimental
General. 1H NMR spectra were recorded on a GSX-400 NMR
spectrometer. Tetramethylsilane was used as an internal standard.
Mass spectra were taken with a Shimadzu GCMS-QO5050A gas
chromatography–mass spectrometer. Melting points were not
corrected. Absorption and fluorescence spectra were measured
with a Hitachi U-3410 absorption spectrophotometer and a Hitachi
U-3010 fluorescence spectrophotometer, respectively. Photoirra-
diation was carried out using an USHIO 500W high-pressure mer-
cury lamp or an USHIO-500W xenon lamp as the light source.
Monochromic light was obtained by passing the light through a
monochromator (Ritsu MV-10N) or
a
band-pass filter
(
time-resolved spectrofluorometer (Hamamatsu Photonics.
ꢀꢀ1=2 ¼ 15 nm). Fluorescence lifetime was measured with a
Fig. 4. Fluorescence spectral changes of 2 in hexane by
photoirradiation: 2a (—), 2 in the photostationary state un-
der irradiation with 366-nm light (- - -), and 2b (– –).
C4334 and C4792) excited with a 337-nm N2 laser (ILEE. NN-
1
ꢂ
00).
Materials. Solvents of spectroscopic grade were purified by
Fluorescent Properties. Figure 4 shows the fluorescence
spectral change of 2a along with the photochromic reaction in
hexane by UV irradiation. The fluorescence maximum was
observed at 433 nm. Upon irradiation with 366-nm light,
the fluorescence intensity decreased and the emission was re-
duced to a half in the photostationary state. The closed-ring
isomer 2b was non-fluorescent. Similar fluorescence intensity
changes upon irradiation with 366-nm light were observed for
distillation before use. All reactions were performed under dry ar-
gon atmosphere unless otherwise specified. The reactions were
monitored by thin-layer chromatography. Column chromatogra-
phy was performed on silica gel.
1-(1,2-Dimethylindol-3-yl)-2-(2-methyl-1-benzothiophen-3-
yl)hexafluorocyclopentene (1a). To the solution of 3-bromo-
1,2-dimethylindole (493 mg, 2.2 mmol) in anhydrous THF (40
mL) was added dropwise n-BuLi (1.6 M in hexane, 1.5 mL, 2.3
ꢄ
1
a and 3a. The fluorescence quantum yields (ꢁf) of 1a–3a by
mmol) at ꢁ78 C under argon atmosphere. The reaction mixture
ꢄ
irradiation with 366-nm light were measured; values are sum-
marized in Table 2. The highest quantum yield of 6.3% was
observed for 2a.
Indole derivatives are known to exhibit strong fluorescence.
The fluorescence quantum yield of 1-methyl-2-phenylindole
9
was reported to be 0.85 in hexane. Although diarylethenes
was stirred for 1 h at ꢁ78 C. The solution was cooled down at
ꢄ
ꢁ95 C, and then 1-(2-methyl-1-benzothiophen-3-yl)heptafluoro-
cyclopentene (820 mg, 2.4 mmol) in anhydrous THF (20 mL)
was added. The reaction mixture was warmed up to room
temperature and then saturated aqueous NH4Cl was added. The
product was extracted with ether, dried with magnesium sulfate,
and concentrated. Purification was performed by column chroma-
tography (SiO2, hexane/ethyl acetate = 90/10) and recrystalliza-
tion (hexane). 1-(1,2-Dimethylindol-3-yl)-2-(2-methyl-1-benzo-
thiophen-3-yl)hexafluorocyclopentene was obtained as a pale-
yellow solid (665 mg, 65%). The closed-ring isomer (1b) was iso-
lated from UV photoirradiated hexane solution of 1a by HPLC
1
a and 2a having an indole ring showed a higher fluorescent
emission in comparison with the reference compound 3a, the
fluorescence quantum yields were much less than the value
of 1-methyl-2-phenylindole. The rotation of the substituted
thiophene or indole along the single bond connecting the per-
fluorocyclopentene unit and the contribution of the intramolec-
ular charge transfer interaction are considered to increase radi-
(
hexane/ethyl acetate = 97/3).
ꢄ
1
1
.93 (s, 3H), 2.23 (s, 3H), 3.52 (s, 3H), 7.14–7.66 (s, 8H); MS m=z
a: mp. 134–135 C (decomp.); H NMR (400 MHz, CDCl3) ꢃ
1
3
ationless transition to the ground state and reduce the yield.
1
4
3
Comparison of Cyclization and Fluorescence Quantum
Yields. The fluorescence quantum yield of 1a was smaller
than the value of 2a, while the photocyclization quantum yield
of 1a was larger than that of 2a, as shown in Table 2. The dif-
ference can be explained as follows. A part of the excited
molecules, which can not enter a cyclization reaction channel,
deactivate to the relaxed fluorescence state and emit the
þ
67 (M ); Anal. Calcd for C24H17F6NS: C, 61.93; H, 3.68; N,
.01. Found: C, 61.84; H, 3.82; N, 3.05. UV-vis (hexane) ꢀmax
(
") 264 (14200), 285 (11300), 292 (10600), 300 (6700), 342
(6100). Selected data for 1b: 1H NMR (400 MHz, CDCl ) ꢃ
1.77 (s, 3H), 2,16 (s, 3H), 3.68 (s, 3H), 6.53–7.20 (s, 8H); MS
3
þ
m=z 467 (M ). UV-vis (hexane) ꢀmax (") 272 (15000), 347
(10600), 425 (3800), 556 (10100).
1
3
fluorescence.
When the cyclization quantum yield is high,
Synthesis of 1-(1-Methyl-2-phenylindol-3-yl)-2-(2-methyl-1-
benzothiophen-3-yl)hexafluorocyclopentene (2a). To the solu-
tion of 3-bromo-1-methyl-2-phenylindole (630 mg, 2.2 mmol) in
anhydrous THF (40 mL) was added dropwise n-BuLi (1.6 M in
the number of unreacted excited molecules becomes small
and the fluorescence intensity decreases.
ꢄ
hexane, 1.5 mL, 2.3 mmol) at ꢁ78 C under argon atmosphere.
Conclusions
ꢄ
The reaction mixture was stirred for 1 h at ꢁ78 C. The solution
Diarylethenes having an indole ring as the aryl groups 1a
and 2a were synthesized. 1a–3a underwent reversible photo-
cyclization reactions by alternate irradiation with UV
(ꢀ ¼ 366 nm) and visible (ꢀ > 480 nm) light. The fluores-
cence intensity also reversibly changed with the photochromic
ꢄ
was cooled down at ꢁ95 C, and then 1-(2-methyl-1-benzothio-
phen-3-yl)heptafluorocyclopentene (820 mg, 2.4 mmol) in anhy-
drous THF (20 mL) was added. The reaction mixture was warmed
up to room temperature and saturated aqueous NH Cl was added.
4
The product was extracted with ether, dried with magnesium sul-

1
628 Bull. Chem. Soc. Jpn., 76, No. 8 (2003)
Fluorescence Property of Diarylethenes
fate, and concentrated. Purification was performed by column
chromatography (SiO2, hexane/ethyl acetate = 90/10) and re-
crystallization (hexane). 1-(1-Methyl-2-phenylindol-3-yl)-2-(2-
methyl-1-benzothiophen-3-yl)hexafluorocyclopentene was ob-
tained as pale-yellow crystals (695 mg, 60%). The closed-ring
isomer (2b) was isolated from UV photoirradiated hexane solution
of 2a by HPLC (hexane/ethyl acetate = 97/3).
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(
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0
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1
1
9
(
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2
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The present work was supported by Grants-in-Aid for Sci-
entific Research on Priority Areas (Optomechatronics for Gen-
erating Nanostructures, 12131211) and for the 21st Century
COE Program (Functional Innovation of Molecular Informa-
tics) from the Ministry of Education, Culture, Sports, Science
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