Synthesis and Photochromism of Diarylethenes with Isopropyl Groups at the Reactive Carbons and Long π-Conjugated Heteroaryl Groups

Article abstract of DOI:10.1246/cl.2003.1078

Diarylethenes with long π-conjugated heteroaryl groups were synthesized and their absorption spectra, photocyclization and cycloreversion quantum yields, and thermal stability were examined. The diarylethene with isopropyl groups at the reactive carbons was found to have a low decoloration quantum yield and thermal reversibility at high temperature above 100°C.

Full text of DOI:10.1246/cl.2003.1078

1078Chemistry Letters Vol.32, No.11 (2003)
Synthesis and Photochromism of Diarylethenes with Isopropyl Groups at the Reactive
Carbons and Long ꢀ-Conjugated Heteroaryl Groups
Seiya Kobatakeꢀ and Masahiro Irieꢀ
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581
(Received September 3, 2003; CL-030809)
Diarylethenes with long ꢀ-conjugated heteroaryl groups
were synthesized and their absorption spectra, photocyclization
and cycloreversion quantum yields, and thermal stability were
examined. The diarylethene with isopropyl groups at the reactive
carbons was found to have a low decoloration quantum yield and
thermal reversibility at high temperature above 100 ꢁC.
Photochromic diarylethenes undergo reversible photoiso-
merization between two isomers with different absorption spec-
tra upon irradiation with light of appropriate wavelength.1–3 Di-
arylethenes with heteroaryl groups are the most promising
candidates for practical applications to optical memory media
and switching devices because of their thermal stability and fa-
tigue resistance.4,5 In order to apply the photochromic com-
Scheme 2. Synthetic route of diarylethene derivatives.
1
according to Scheme 2 and were identified by H NMR spec-
trum, mass spectrum, and elemental analysis.9,10
pounds to recording materials, it is desired to decrease the pho-
todecoloration quantum yields to avoid the bleaching under
room light. If the colored isomers return to the initial colorless
isomers thermally at high temperature above 100 ꢁC, it can be
potentially used for reusable recording materials.
One of the approaches is to introduce alkoxy groups at the
reactive carbons of diarylethenes.6 The introduction of cyclo-
hexyloxy groups decreased the photodecoloration quantum
yields and increased the thermal decoloration rate at high tem-
perature above 100 ꢁC. The colored isomers can return to the col-
orless isomers thermally at the high temperature. Another strat-
egy to decrease the decoloration quantum yields of diarylethenes
is to introduce long ꢀ-conjugation to the diarylethenes. Here we
report on synthesis of diarylethene derivatives (Scheme 1) which
have both low decoloration quantum yields and thermal reversi-
bility at high temperature.
Figure 1a shows the absorption spectrum of 3a in toluene at
room temperature. Compound 3a has an absorption maximum at
350 nm in toluene. Upon irradiation with 350-nm light, the col-
orless solution of 3a turned blue, in which the visible absorption
maximum was observed at 640 nm. The color is due to the pho-
togenerated closed-ring isomer 3b. The colored product was iso-
lated by HPLC (silica gel; hexane as the eluent). It was con-
firmed that the spectrum of 3b was the same as that in the
photostationary state upon irradiation with 350-nm light, indicat-
ing that the conversion from 3a to 3b is 100%. The blue color
Synthetic procedures of diarylethenes 1a and 2a were
described before.7,8 Diarylethenes 3a and 4a were synthesized
Figure 1. Absorption spectra of 3a (—) and 3b (- - - -) (a) and
4a (—) and 4b (- - - -) (b) in toluene at room temperature.
Scheme 1. Photochromic reactions of diarylethene derivatives.
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.11 (2003)
Table 1. Absorption data and photochromic property of 1a–4aa
1079
ꢁmax
"/molꢂ1 dm3 cmꢂ1
ꢂa!b
ꢁmax
"/molꢂ1 dm3 cmꢂ1
ꢂb!a
0.013
0.026
0.00014
0.0024
1a7
2a
3a
4a
280
290
350
350
35600
34900
51600
43800
0.59
0.50
0.47
0.47
1b7
2b
3b
4b
575
600
640
655
15600
14800
29800
22600
a1 and 2 were determined in hexane, and 3 and 4 were in toluene.
disappeared by irradiation with visible light (ꢁ > 500 nm), and
the absorption spectrum returned to that of 3a. Diarylethene 4a
showed the similar behavior (Figure 1b). The open-ring isomers,
3a and 4a, have the same absorption maxima, but the absorption
maxima in the closed-ring isomers showed a bathochromic shift
by the introduction of the isopropyl groups. This is ascribed to
the bulky substituents at 2- and 20-positions of the thiophene
rings.6,11
Table 2. Activation energy (Ea) and frequency factor (A) of the
thermal cycloreversion reactions of 1b–4b
Ea/kJ molꢂ1
A/sꢂ1
1b7
2b8
3b
139
118
131
112
1:0 ꢃ 1013
1:9 ꢃ 1013
2:0 ꢃ 1012
4:4 ꢃ 1012
4b
The quantum yields of cyclization (ꢂa!b) and cyclorever-
sion reactions (ꢂb!a) were measured in toluene at room temper-
ature. The results are summarized in Table 1. The photocycliza-
tion quantum yields of 3a and 4a were similar to those of 1a and
2a. The ꢀ-conjugation length and the introduction of isopropyl
groups scarcely affected the quantum yields. However, the pho-
tocycloreversion quantum yields were found to depend on the ꢀ-
conjugation length of the aryl groups. The cycloreversion quan-
tum yield of 3b was much smaller than that of 1b by a factor of
100. The quantum yield of 4b was also smaller than that of 2b by
a factor of 11. The quantum yield of 4b was larger than that of 3b
by a factor of 17. The bulky isopropyl substituents increased the
photocycloreversion quantum yield.
slope of the first-order kinetic plots. The temperature depend-
ence of k is plotted in Figure 2b. The activation energy (Ea)
and frequency factor (A) of the cycloreversion were determined
from the linear relation. The values are summarized in Table 2.
Extrapolation of the temperature dependence indicates that the
half-life time of the closed-ring isomer 4b is 36 days at 30 ꢁC
and 5 sec at 160 ꢁC. The bulky substituents at 2- and 20-positions
of the thiophene rings are considered to destabilize the cyclohex-
adiene ring structure. It is concluded that introduction of the bul-
ky isopropyl substituents and long ꢀ-conjugated heteroaryl
groups resulted in the low decoloration quantum yield and the
fast thermal reversion at high temperature above 100 ꢁC.
Diarylethenes 1a–4a showed thermally irreversible photo-
chromism at room temperature.7,8 However, 2b returned to the
open-ring isomer 2a upon heating above 100 ꢁC.8 The thermal
cycloreversion reaction of 4b at high temperature was examined
in decalin. Figure 2a shows the decay curves of the absorbance
of the closed-ring isomer 4b at several temperatures. The decay
curves followed the first-order kinetics. The rate constants (k) of
the thermal cycloreversion reaction were estimated from the
This work was partly supported by the Grant-in-Aids for
Scientific Research (S) (No. 15105006), Scientific Research on
Priority Areas (Nos. 15033252 and 12131211), and the 21st cen-
tury COE program from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan.
References and Notes
1
2
3
4
M. Irie and M. Mohri, J. Org. Chem., 53, 803 (1988).
M. Irie and K. Uchida, Bull. Chem. Soc. Jpn., 71, 985 (1998).
M. Irie, Chem. Rev., 100, 1685 (2000).
M. Irie, S. Kobatake, and M. Horichi, Science, 291, 1769
(2001).
5
6
7
M. Irie, T. Fukaminato, T. Sasaki, N. Tamai, and T. Kawai,
Nature, 420, 759 (2002).
K. Morimitsu, K. Shibata, S. Kobatake, and M. Irie, J. Org.
Chem., 67, 4574 (2002).
M. Irie, T. Lifka, S. Kobatake, and N. Kato, J. Am. Chem. Soc.,
122, 4871 (2000).
8S. Kobatake, K. Uchida, E. Tsuchida, and M. Irie, Chem. Lett.,
2000, 1340.
9
3a: mp = 160–161 ꢁC. 1H NMR (CDCl3, 200 MHz): ꢃ ¼ 1:98
(s, 6H), 7.11 (d, 2H), 7.15 (s, 2H), 7.23 (d, 2H), 7.25–7.45 (m,
6H), 7.60 (d, 4H). MS m=z ¼ 684 (Mþ). Anal. Calcd for
C35H22F6S4: C, 61.39%; H, 3.24%. Found: C, 61.75; H, 3.43%.
10 4a: mp = 205–206 ꢁC. 1H NMR (CDCl3, 200 MHz): ꢃ ¼ 0:98
(d, 12H), 2.80 (sept, 2H), 7.06 (s, 2H), 7.11 (d, 2H), 7.23 (d,
2H), 7.25–7.45 (m, 6H), 7.60 (d, 4H). MS m=z ¼ 740 (Mþ).
Anal. Calcd for C39H30F6S4: C, 63.22; H, 4.08%. Found: C,
63.29; H, 4.12%.
11 K. Morimitsu, K. Shibata, S. Kobatake, and M. Irie, Chem.
Lett., 2002, 572.
Figure 2. Decay curves (a) and temperature dependence (b) of
thermal cycloreversion reactions of 4b in decalin.
Published on the web (Advance View) October 27, 2003; DOI 10.1246/cl.2003.1078