Thermal cycloreversion reaction of a photochromic dithienylperfluorocyclopentene with tert-Butoxy substituents at the reactive carbons

Article abstract of DOI:10.1246/cl.2002.572

tert-Butoxy groups were introduced at 2 and 2′-positions of the thiophene rings of a dithienylperfluorocyclopentene to prepare a photochromic compound with a thermal cycloreversion reactivity and a low photocycloreversion quantum yield. The cycloreversion

Full text of DOI:10.1246/cl.2002.572

572
Chemistry Letters 2002
Thermal Cycloreversion Reaction of a Photochromic Dithienylperfluorocyclopentene
with tert-Butoxy Substituents at the Reactive Carbons
Kentaro Morimitsu, Katsunori Shibata, Seiya Kobatake, and Masahiro Irieꢀ
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, and CREST,
Japan Science and Technology Corporation, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581
(Received February 20, 2002; CL-020162)
tert-Butoxy groups were introduced at 2 and 20-positions of
hexane solution (6.7 mmol) was added at À78 ꢁC. Then, to the
solution was added 0.45 mL (3.2 mmol) of octafluorocyclopen-
tene to yield 0.65 g (1.0 mmol) of 1a (Yield: 32%).9 1a was
characterized by 1H NMR spectrum, mass spectrum, and
elemental analysis.
the thiophene rings of a dithienylperfluorocyclopentene to
prepare a photochromic compound with a thermal cycloreversion
reactivity and a low photocycloreversion quantum yield. The
cycloreversion quantum yield was similar to the isopropyl-
substituted derivative and the half-life time of the colored isomer
at 100 ꢁC was 8 s.
Figure 1 shows the absorption spectral change of 1 upon
irradiation with 313-nm light. 1a has the absorption maxima at
278 and 306 nm in hexane. Upon irradiation with 313-nm light,
the colorless solution of 1a turned blue, in which a visible
absorption maximum was observed at 656 nm. The blue colored
isomer was isolated by HPLC (silica gel; hexane/ethyl acetate ¼
99:5=0:5 as the eluent). From the absorption spectrum of the
isolated colored isomer conversion ratio to the colored isomer in
the photostationary state under irradiation with 313-nm light was
determined to be 92%. The colored isomer was further examined
by 1H NMR spectroscopy. The methyl protons of the tert-butoxy
substituents and protons at 4 and 40-posotions of the thiophene
rings were observed at 1.52 ppm and 7.21 ppm for the colored
isomer, while those of 1a at 1.21 ppm and 6.48 ppm, respectively.
The peak shifts are similar to those observed for the methyl-
substituted derivative. Based on the shifts of the absorption band
and 1H NMR peaks the colored isomer was assigned to the closed-
ring form isomer 1b.2
Photochromism is defined as a reversible isomerization
between two isomers having different absorption spectra by
photoirradiation.1 Various types of photochromic compounds
have been so far developed in an attempt to apply the compounds
to optoelectronic devices, such as memories and switches.
Among these compounds, diarylethenes are the most promising
candidates for the application because of their thermally
irreversibility and fatigue-resistance.2 Recently, we have re-
ported that dithienylethenes having bulky substituents at the
reactive carbons of the thiophene rings decrease the thermal
stability of the colored closed-ring isomers.3;4 When isopropyl
groups are introduced at 2 and 20-positions of the thiophene rings
of 1,2-bis(5-phenyl-3-thienyl)perfluorocyclopentene, the half-
life time of the colored isomer decreased to 3.3 h at 80 ꢁC, while
the methyl-substituted derivative was stable even at 100 ꢁC.5 The
thermal reversible photochromic reactivity is applicable for
reusable imaging devices with thermal head. For the devices the
image should be readily bleached at high temperature and stable
under room light. To increase the thermal bleaching rate at higher
temperature and keep the photostability bulky tert-butoxy groups
were introduced to the reactive carbons, because methoxy
substitution at the reactive carbons is known to decrease the
cycloreversion quantum yield and provide the photostability.6
Figure 1. Absorption spectral change of 1 in
hexane (2:3 Â 10À5 mol dmÀ3) by photoirradiation:
1a (- - - -), 1 in the photostationary state under
irradiation with 313-nm light (— - —), and 1b
(—––).
1,2-Bis(2-tert-butoxy-5-phenyl-3-thienyl)perfluorocyclo-
pentene (1a) was synthesized as follows. To dry THF containing
3,5-dibromo-2-tert-butoxythiophene7 (6.0 g, 19 mmol) was
added a 15% n-BuLi hexane solution (12 mL, 20 mmol) at
À78 ꢁC. Tributylborate (6.4 mL, 24 mmol) was slowly added to
the reaction mixtureat À78 ꢁC, andthe mixture was stirred for1h.
Iodobenzene (3.9 g, 19 mmol), 20 wt% Na2CO3 (aq) (40 mL), and
Pd(PPh3Þ4 (0.97 g, 0.82 mmol) were added to the mixture, and the
mixture was refluxed for 5 h at 70 ꢁC to form 3.9 g of 3-bromo-2-
tert-butoxy- 5-phenylthiophene (2) (Yield: 65%).8 To the mixture
of 2 (2.0 g, 6.4 mmol) and 20 mL of dry THF, 4.1mL of a n-BuLi
1b in hexane was stored in the dark at room temperature.
After one day the blue color solution turned to colorless. To prove
that the decoloration is not due to decomposition but due to
thermal cycloreversion HPLC measurement was carried out after
one-day storage at room temperature. In the chart only one peak
due to the initial open-ring isomer was observed. This indicates
that the colored closed-ring isomer thermally returned to the
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
573
open-ring isomer 1a.
The thermal decoloration process of 1b was examined in
detail between 30 and 70 ꢁC in decalin. Figure 2 shows the decay
curves of the absorbance of the closed-ring isomer. The decay
curves followed the first-order kinetics. Figure 3 shows the
temperature dependence of the rate (k). The activation energy
(Ea) and frequency factor (A) were 91.7 kJ molÀ1 and 5:7Â
1011 sÀ1, respectively. The half-life time of the thermal
cycloreversion reaction for 1b at 70 ꢁC was 110 s and decreased
to 8 s at 100 ꢁC. The thermal bleaching rate was remarkably
increased by the introduction of the bulky tert-butoxy substituents
in comparison to the isopropyl substituents. The bulky sub-
stituents are considered to destabilize the cyclohexadiene ring
structure.
Figure 3. Temperature dependence of the thermal
fading rates of 1b.
This work was supported by CREST (Core Research for
Evolutional Science and Technology) of Japan Science and
Technology Corporation (JST). We also thank NIPPON ZEON
CO., Ltd. for their supply of octafluorocyclopentene.
References and Notes
1H. Du rr and H. Bouas-Laurent, ‘‘Photochromism: Molecules
¨
and Systems,’’ Elsevier, Amsterdam (1990).
2
3
M. Irie, Chem. Rev., 100, 1685 (2000).
S. Kobatake, K. Shibata, K. Uchida, and M. Irie, J. Am. Chem.
Soc., 122, 12135 (2000).
4
5
S. Kobatake, K. Uchida, E. Tsuchida, and M. Irie, Chem. Lett.,
2000, 1340.
M. Irie, T. Lifka, S. Kobatake, and N. Kato, J. Am. Chem. Soc.,
122, 4871(2000).
K. Shibata, S. Kobatake, and M. Irie, Chem. Lett., 2001, 618.
J. Z. Mortensen, B. Hedegaard, and S.-O. Lawesson,
Tetrahedron, 27, 3839 (1971).
Figure 2. Thermal fading curves of 1b in decalin.
The cyclization and cycloreversion quantum yields of 1 were
measured at room temperature in hexane. The values were
determined to be 0.48 (305 nm) and 0.031(656 nm), respectively.
The thermal cycloreversion reaction was negligibly small during
the measurement. Both cyclization and cycloreversion quantum
yields were similar to the isopropyl-substituted derivative. The
tert-butoxy substituents strongly enhanced the thermal cyclor-
eversion rate, while the photochemical reactivity was scarcely
affected by the substitution.
6
7
8
2: pale yellowish oil: 1H NMR (200 MHz, CDCl3) ꢀ 1.47 (s,
9H), 7.00 (s, 1H), 7.2–7.5 (m, 5H); MS m=z 254, 256
(Mþ À CH2 ¼ C(CH3Þ2). Anal. Calcd. for C14H15BrOS: C,
54.03; H, 4.86%. Found: C, 54.39; H, 4.83%.
1
9
1a: pale yellow colored crystals: mp 194–195 ꢁC; H NMR
The absorption maximum of 1b was observed at longer
wavelength than that of isopropyl-substituted derivative.4;5 This
suggests that electron-donating property of tert-butoxy groups
caused spectral shift to the longer wavelength.
(200 MHz, CDCl3) ꢀ 1.21 (s, 18H), 7.21 (s, 2H), 7.2–7.5 (m,
10H); MS m=z 524 (Mþ À 2CH2 ¼ C(CH3Þ2). Anal. Calcd.
for C33H30F6O2S2: C, 62.25; H, 4.75%. Found: C, 61.90; H,
4.72%.