Photochromic oligothiophenes

Article abstract of DOI:10.1246/cl.2005.1580

New oligothiophenes having a photochromic switching unit were synthesized. Upon irradiation with UV and visible light the oligomers underwent photochromic reactions in solution. The chain length dependence on the photoreactivity was examined to reveal tha

Full text of DOI:10.1246/cl.2005.1580

1580
Chemistry Letters Vol.34, No.12 (2005)
Photochromic Oligothiophenes
Naoki Tanifuji,y Masahiro Irie,ꢀyy and Kenji Matsudaꢀy;yy
yPrecursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST)
yyDepartment of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Fukuoka 819-0395
(Received August 24, 2005; CL-051081)
New oligothiophenes having a photochromic switching unit
were synthesized. Upon irradiation with UV and visible light the
oligomers underwent photochromic reactions in solution. The
chain length dependence on the photoreactivity was examined
to reveal that the reactivity decreased as the chain length gets
longer. These molecules can be used as new photoswitching
units.
R = Me, MeO
F2
F2
F2
F2
F2
F2
UV
Vis. O
H
S
S
R
n
R
O
H
O
H
O
H
n
S
S
n
S
S
S
S
S
n
sp2
S
S
S
sp3
Scheme 1. Photochromism of thiophenes oligomer having a
diarylethene.
Oligothiophene, which is constructed by 2,5-thienylene unit,
is of great interest as a candidate for a potential molecular wire
and thereby various types of oligothiophenes have been reported
so far.1 The unique redox,2 photophysical,3 and conductive4
properties have been utilized for novel optoelectronic materials.
The ꢀ-conjugated olefinic electronic structure is the key feature
of this material. In this paper, we propose a new oligothiophene,
in which a photochromic diarylethene5 unit is incorporated. The
ꢀ-conjugated chain length of the oligothiophene can be altered
by the irradiation with appropriate wavelength of light due to
the change of the hybrid orbital at the 2-position of the thiophene
ring from sp2 to sp3.6 The photochromic reactivity of the new
diarylethene derivatives will be discussed.
F2
F2
F2
F2
F2
F2
a)
S
S
R
R
TMS
Br
TMS
Br
S
S
S
S
S
S
7a (R = Me), 8a (R = MeO)
9a (R = Me), 10a (R = MeO)
b)
b)
1a
2a
c)
c)
9a
10a
3a
4a
6a
d)
d)
5a
The pentathiophenes 1a, 2a, heptathiophenes 3a, 4a, and
nonathiophenes 5a, 6a are listed in Chart 1. The switching
unit is located in the middle of the ꢀ-conjugated chain. When
oligothiophene chromophores are placed at both sides of a
diarylethene, the photocyclization quantum yield is strongly
suppressed.7 To avoid the decrease of the quantum yield, the
switching unit was introduced in the middle of the oligothio-
phene. Formyl substituent was introduced at each end of the
ꢀ-conjugated chain for the further modification. The photochro-
mic reaction is shown in Scheme 1. Upon irradiation with UV
light, the photocyclization reaction of the diarylethene unit
would break the ꢀ-conjugation of the oligothiophene.
The syntheses of 1a–6a were performed according to
Scheme 2. Diarylethenes having TMS-substituted terthiophene
unit 7a and 8a were synthesized by two successive nucleophilic
substitutions into perfluorocyclopentene. Bromo-substituted 9a
and 10a were prepared by N-bromosuccimide in THF. Suzuki
Scheme 2. a) NBS, THF, 74–90%; b) 5-Formylthiophene-
2-boronic acid, Na2CO3, Pd(PPh3)4, THF, and H2O, 51–88%;
c) 5-(1,3-Dioxolane-2-yl)-[2,20-bithiophene]-50-boronic acid,
Pd(PPh3)4, Na2CO3, THF, H2O, and then Hþ, 45–50%; d) 5-
(1,3-Dioxolan-2-yl)-[2,20:50,200-terthiophene]-500-boronic acid,
Pd(PPh3)4, Na2CO3, THF, H2O, and then Hþ, 23–40%.
coupling with appropriate thienyl boronic acid derivatives gave
oligothiophenes 1a–6a. The structures of the compounds were
confirmed by the NMR, UV–vis, and high-resolution mass spec-
troscopy.8
The photochromic reactivity of 1a–6a was studied in ethyl
acetate solution. Before irradiation, the solution showed yellow,
yellow, pale orange, orange, reddish orange, and reddish orange
for 1a, 2a, 3a, 4a, 5a, and 6a, respectively. As the ꢀ-conjugation
chain gets longer, the absorption maximum of the open-ring iso-
mer shifts to longer wavelengths. These oligothiophenes under-
went photochromic reactions upon irradiation with UV light.
The solutions of 1a and 2a turned blue green by irradiation with
365 nm light. The blue green color is due to the formation of the
closed-ring isomer 1b and 2b. The formation of the closed-ring
isomer was supported by the ca. 0.1 ppm downfield shift of
methyl or methoxy proton on the reactive carbon. These colors
were bleached by the irradiation with visible light. The solution
of 4a turned dark brown. The color changes of solutions of 3a,
5a, and 6a were much less than the solutions of 1a, 2a, and 4a
(Figure 1). The conversion from the open- to the closed-ring
isomer decreased as the ꢀ-conjugation chain becomes longer.
The low conversion is ascribed to the low cyclization quantum
F2
F2
S
F2
R
O
H
O
H
S
S
S
n
S
S
n
1a R = Me , n = 1 4a R = MeO, n = 2
2a R = MeO, n = 1 5a R = Me , n = 3
3a R = Me , n = 2 6a R = MeO, n = 3
Chart 1. Oligothiophenes having a diarylethene.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.12 (2005)
1581
Table 1. Absorption maxima and quantum yields of 1–6 in
ethyl acetate
a) 0.1
b) 0.5
0.4
0.3
0.2
0.1
0
0.08
0.06
0.04
ꢁ
max/nm
Quantum yield
Conversion/%
a
b
a ! b (365 nm) b ! a (575 nm)
1
2
3
4
5
6
408 619
411 640
432 625
434 645
445 629
447 650
0.019
0.016
N/A
0.010
0.00060
N/A
49
87
0.02
0
300
400
500
600
700
800
300
400
500
600
700
800
ꢁ5
35
Wavelength / nm
Wavelength / nm
c)
d)
0.00079
N/A
N/A
0.00069
N/A
N/A
0.4
0.3
0.2
0.1
0
0.08
ꢁ1
ꢁ10
0.06
0.04
0.02
0
gates, cyclization quantum yield is suppressed. Energy dissipa-
tion can be the main cause of the suppression of cyclization re-
action.
As indicated in Scheme 1, the photochromic reaction can
switch the pathway of the conjugation. In the open-ring isomer
the conjugation extends along red line while the conjugation ex-
tends through blue line in the closed-ring isomer. The molecules
can be used as new photoswitching units.
300
400
500
600
700
800
300
500
600
700
800
400Wavelength / nm
Wavelength / nm
e)
f)
0.4
0.3
0.2
0.3
0.2
0.1
0
In conclusion, new oligothiophenes having a photochromic
diarylethene unit were synthesized and their photoreactivity was
studied to reveal that the reactivity decreased as the chain length
gets longer.
0.1
0
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength / nm
Wavelength / nm
Figure 1. Absorption spectra of photochromic oligothiophenes.
Solid line: the open-ring isomer; dotted line: the closed-ring
isomer; dashed line: in the photostationary state. a) 1a, b) 2a,
c) 3a, d) 4a, e) 5a, and f) 6a.
This work was supported by PRESTO, JST.
References and Notes
1
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yields and relatively high cycloreversion quantum yield. The
color of the closed-ring isomer was also dependent on the ꢀ-con-
jugated chain length. Bathochromic shift was observed for the
longer oligothiophene derivatives. The methoxy-substituted oli-
gothiophenes also showed significant bathochromic shift.
Cyclization and cycloreversion quantum yields were meas-
ured in ethyl acetate using 1,2-bis(2-methyl-5-phenyl-3-thienyl)-
perfluorocyclopentene as a reference.9 The quantum yields and
the absorption maxima were summarized in Table 1. The cycli-
zation quantum yields of photochromic oligothiophenes were
smaller than those of regular diarylethenes. Compound 2a has
methoxy substituent at reactive carbon, which is known to sup-
press the cycloreversion reaction.10 The cycloreversion reaction
of 2b is much smaller than that of 1b and cyclization quantum
yield of 1a and 2a were almost the same. Therefore, the conver-
sion of 2a (87%) is higher than the value of 1a (49%). Heptathio-
phene 4a has very small cyclization and cycloreversion quantum
yield. But, cyclization quantum yield is larger than cyclorever-
sion reaction by the effect of methoxy substituent. Therefore,
the closed-ring isomer was accumulated by the irradiation with
UV light. Cyclization quantum yield depended on ꢀ-conjugated
chain length. As the ꢀ-conjugated chain of oligothiophene elon-
2
3
4
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For the physical data of the compounds, see Supporting
Information.
5
6
7
8
9
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Published on the web (Advance View) October 27, 2005; DOI 10.1246/cl.2005.1580