Synthesis of novel thermally irreversible photochromic 1-aryl-1,3-butadiene derivatives

Article abstract of DOI:10.1246/bcsj.76.363

A new thermally irreversible photochromic compound, 3,3,4,4,5,5-hexafluoro-1-(1-isopropylidene-1-phenylmethyl)-2- (2,4,5-trimethyl-3-thienyl)cyclopentene 30, was synthesized and its photochromic properties were examined. It showed thermally irreversible photochromism. While 30 is colorless, it turned yellow (λ_max 445 nm) upon the irradiation of 313-nm light, caused by the generation of 3C as the result of photochemical 6π-electrocyclization. It returned to the initial colorless state by 405-nm light irradiation. The coloration and decoloration quantum yields with 313-nm light were 0.43 and 0.14, respectively, and the decoloration quantum yield with 405-nm light was 0.16. The colored form 3C in a PMMA film did not fade when stored at 80 °C for more than one month, and the fatigue resistivity in the PMMA film for iterative irradiation was excellent.

Full text of DOI:10.1246/bcsj.76.363

c. Jpn.
© 2003 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 76, 363–367 (2003)
363
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Synthesis of Novel Thermally Irreversible Photochromic
1-Aryl-1,3-butadiene Derivatives
Novel Thermally Irreversible Photochrome
S. M. Shrestha et al.
Sujen Man Shrestha, Hitoshi Nagashima, Yayoi Yokoyama,† and Yasushi Yokoyama*
03
3
7
Department of Advanced Materials Chemistry, Graduate School of Engineering, Yokohama National University,
79-5, Tokiwadai, Hodogaya-ku, Yokohama, 240-8501
SJA8
09-2673
205
†Department of Home Economics, Faculty of Home Economics, Tokyo Kasei Gakuin University,
2600, Aihara-cho, Machida, Tokyo 194-0292
(Received July 24, 2002)
02
02
A new thermally irreversible photochromic compound, 3,3,4,4,5,5-hexafluoro-1-(1-isopropylidene-1-phenylmeth-
yl)-2-(2,4,5-trimethyl-3-thienyl)cyclopentene 3O, was synthesized and its photochromic properties were examined. It
showed thermally irreversible photochromism. While 3O is colorless, it turned yellow (λmax 445 nm) upon the irradia-
tion of 313-nm light, caused by the generation of 3C as the result of photochemical 6π-electrocyclization. It returned to
the initial colorless state by 405-nm light irradiation. The coloration and decoloration quantum yields with 313-nm light
were 0.43 and 0.14, respectively, and the decoloration quantum yield with 405-nm light was 0.16. The colored form 3C
in a PMMA film did not fade when stored at 80 °C for more than one month, and the fatigue resistivity in the PMMA
film for iterative irradiation was excellent.
Although a number of families of thermally reversible pho-
tochromic compounds are known, few are known to be ther-
mally irreversible photochromic compounds.1–3 As far as we
know, only fulgides,4 diarylethenes,5 and phenoxynaph-
thacenequinones6 are known to be thermally irreversible to
date. Thermally reversible photochromic compounds have al-
ready been applied to the auto-regulator of sunlight as oph-
thalmic lenses. On the other hand, thermally irreversible pho-
tochromic compounds have long been said to be potential can-
didates of materials for rewritable optical recording media,
though they have not been realized. It is therefore invaluable
to develop new thermally irreversible photochromic com-
pounds.
Fulgides and diarylethenes undergo the 6π-electrocycliza-
tion. When fulgides have fully substituted carbon atoms at
both ends of the hexatriene moiety7 and diarylethenes have ar-
omatic rings with rather low aromaticity,8 they may show a
Chart 1.
thermally irreversible photochromism. With these precedents
in mind, we designed a 1-aryl-1,3-butadiene system A (Chart
1) as a part of our project to develop new thermally irreversible
photochromic compounds. The terminal aryl group and the
butadiene moiety consist of a hexatriene system, and the addi-
tional aryl group on C-3 will extend the conjugation after elec-
trocyclization.
thermal sigmatropic rearrangement of the hydrogen on the
ring-closing carbon atom gave a thermally reversible photo-
chromic compound.9 If the rearranging hydrogen atom would
be an alkyl substituent, a sigmatropic rearrangement would
hardly occur, and it would give a thermally irreversible photo-
chromic compound.
Consequently, the structure of the novel thermally irrevers-
ible photochromic compound to be prepared is depicted as 2.
Later, some modifications were made on 2 to afford the pho-
tochemical fatigue resistivity. As shown in 3, the thiophene
group was fully methylated, and the adamantylidene group
was replaced by a sterically less-demanding isopropylidene
group.
In a preceding paper,9 we reported on a new thermally re-
versible photochromic compound. We show here new struc-
turally closely related, but thermally irreversible photochromic
compounds.
Results and Discussion
Design of the Molecules. In
a preceding paper, we
showed that the 6π-electrocyclization of 1 followed by the

364 Bull. Chem. Soc. Jpn., 76, No. 2 (2003)
Novel Thermally Irreversible Photochrome
Synthesis of 2. The synthesis of 2 was carried out as
shown in Scheme 1. The synthesis of 4 was described in a pre-
ceding paper.9 The reaction of 3-lithio-2,5-dimethylthiophene,
generated from 3-bromo-2,5-dimethylthiophene and butyllithi-
um, with 4 afforded 2 in 28% yield
Photoreaction of 2. When 2O in toluene was irradiated
with 313-nm light, the color of the solution turned yellow, and
a new absorption band in the visible light region appeared on
the UV-vis spectroscopy. The absorption maximum was
450 nm. It was attributed to the generation of the colored form
2C. The color was retained when the colored solution was
kept in the dark.
When a PMMA film doped with 2O was irradiated with 313
nm light, the colored species also appeared. In order to assess
the thermal stability of the colored form, the film was kept at
80 °C under an atmospheric environment in the dark. The ab-
sorption band showed almost no change after 30 days. Thus, 2
proved to be thermally irreversible.
However, when the irradiation of 313-nm light to the tolu-
ene solution of 2 was continued for a prolonged time, the new
band in the visible region began to diminish. As can be seen in
Fig. 1, the absorption in the visible region increased for 18
minutes of irradiation, and then turned to decrease. Apparently
a photochemical decomposition of the compound occurred.
Recently, Irie et al. reported on the formation of photochem-
ical by-products of diarylethene 5.10 They reported that the
photochemical cyclization of 5O occurred not only from the
s-cis-s-cis conformation around the hexatriene moiety to afford
5C, but also from the s-trans-s-cis conformation. They ex-
Scheme 2. Formation of by-product from 5O.
plained, as depicted in Scheme 2, that the latter may have oc-
curred from the electrocyclization of 5Oꢀ containing the iso-
electronic structure of the thiophene moiety, followed by dehy-
drogenation from 5Cꢀ to form the light-insensitive colorless fi-
nal product 5D. In the case of 2O, a similar reaction is possi-
ble. In addition to the regular electrocyclization to lead 2C
(route A in Scheme 3), an abnormal electrocyclization (route
B) to give 2Cꢀ could be possible from 2Oꢀ that is isoelectronic
with 2O. If the thermal 1,5-hydrogen shift, as we described in
the preceding paper,9 may have followed on 2Cꢀ, the resulting
compound 2R would restore the aromaticity of the thiophene
ring. To confirm this hypothesis, we took 1H NMR after pho-
toirradiation. After 2O in CDCl3 was irradiated with light
longer than 300 nm (Pyrex filter) from a high-pressure Hg
lamp for 40 min, the solution was colorless. All of 2O was
consumed, and no signals attributable to 2C were observed by
1H NMR. Instead, a new set of signals attributed to the new
compound appeared. The most characteristic signal was the
doublet at δ 4.58, with J = 2.97 Hz (probably a coupling with
one of the adamantane proton), attributed to the hydrogen adja-
cent to the phenyl group. As we showed in the preceding pa-
per, the hydrogen under a similar situation of 1R appeared as a
doublet at δ 4.57, with J = 8.91 Hz. The difference of the cou-
pling constants may reflect the slight difference in the confor-
mation of the compounds. In addition, because the mass spec-
Scheme 1. Synthesis of 2O.
Fig. 1. Absorption spectral change of 2 during 313-nm light
irradiation in toluene. Irradiation time/min; 0, 1, 3, 5, 8, 18
(largest absorbance at 450 nm), 21, 30.
Scheme 3. Formation of by-product from 2O.

S. M. Shrestha et al.
Bull. Chem. Soc. Jpn., 76, No. 2 (2003) 365
trum of 2R showed the same molecular ion peak (508) as 2O,
neither dehydrofluorination nor dehydrogenation occurred.
We therefore concluded that an irreversible side reaction oc-
curred by way of the abnormal electrocyclization of 2O, fol-
lowed by a thermal sigmatropic 1,5-hydrogen rearrangement
to give 2R.
The coloration quantum yield by 313-nm light is as large as
0.43, and the decoloration quantum yields for 313- and 405-
nm lights are practically the same, 0.14 and 0.16, respectively.
The largeness of the molar absorption coefficient of 3C at
313 nm suppressed the conversion ratio of 3O to 3C by 313-
nm light irradiation to 63%.
Synthesis of Improved Compound 3. Because of the hy-
drogen atom on C-4 of the thiophene group of 2O, that can mi-
grate after abnormal photocyclization, a thermally as well as
photochemically inert by-product 2R was generated. In order
to avoid the formation of 2R, we decided to introduce a methyl
group at C-4. The trimethylthiophene has often been used for
diarylethenes as aromatic rings.5 However, the sterically more
demanding 3-lithio-2,4,5-trimethylthiophene than the 3-lithio-
2,5-dimethylthiophene did not react with 4. The introduction
of trimethylthiophene to the octafluorocyclopentene, followed
by the introduction of the right half (1-adamantylidene-1-phe-
nylmethyl group), was also not successful. We then decided to
reduce the steric bulkiness of the adamantylidene group by re-
placing it with the isopropylidene group, which has been fre-
quently used in fulgides.4
Although the isolation of 3C has not yet been carried out,
the change in 1H NMR spectrum during the photoirradiation in
CDCl3 was followed, and the assignment of the methyl signals
of 3C was done. Before 313-nm light irradiation, 3O showed
five singlet methyl signals (δ = 1.43, 1.59, 1.73, 1.90, 2.19),
along with two (δ = 6.69 (2H, m)) and three (δ = 7.13–7.16
(3H, m)) protons on the phenyl ring. After UV irradiation for
315 min, four small new singlet peaks attributable to 3C ap-
peared (δ = 0.95, 1.46, 1.69, 1.99: one peak may be overlap-
ping with the peaks of 3O) as well as the down-field shifted ar-
omatic protons (δ = 7.35, m) appeared, while the peaks of 3O
remained almost unchanged (3O/3C = 86/14). These new
peaks disappeared upon the irradiation of visible light
(405 nm) for 270 minutes. Therefore, the new peaks are as-
signed to the signals of 3C.
A synthesis of the right half 6 was carried out using similar
procedures to obtain 4, which was described in the preceding
article.9 Starting with 2-methyl-1-phenyl-1-propene, bromina-
tion, dehydrobromination, and lithiation followed by a reaction
with octafluorocyclopentene, afforded 6 in 66% yield in three
steps. The reaction of 3-lithio-2,4,5-trimethylthiophene with 6
afforded 3O in 64% yield (Scheme 4).
Thermal Stability. The thermal stability is important for
“thermally irreversible photochromic compounds.” Therefore,
we next evaluated the thermal stability of 3C. A PMMA film
containing 0.36 weight% of 3O was irradiated with 313-nm
light to make a photostationary state. This film was kept at
80 °C in the dark in the air for 30 days, and the absorption
Photoreaction of 3. When the thus-prepared arylbutadi-
ene 3O was dissolved in toluene and the solution was irradiat-
ed with 313-nm light, a new absorption band appeared in the
visible-light region. Its absorption maximum was 445 nm.
Apparently 3C was produced by photochemical 6π-electrocy-
clization. The colored species 3C was stable, and was detected
by HPLC. It returned to colorless 3O upon the irradiation of
visible light. The changes in the absorption spectra during
photoirradiation experiments are shown in Figs. 2(a) and 2(b),
and the photoreaction processes are depicted in Scheme 5.
The quantum yields of photoreactions, shown in Table 1
along with the absorption spectroscopic data, were determined
by analyzing the change in the concentration of 3O and 3C,
determined by HPLC as the function of the irradiation time.11
Scheme 4. Synthesis of 3O.
Fig. 2. Change in absorption spectra in toluene. (a) Irradiation of 313-nm light. Irradiation time/min; 0, 0.5, 1.5, 3, 5, 8, 12, 17, 23,
30, 38, 47, 57, 68, 80, 95, 110, 125, 140, 155, 170, 185, 200. (b) Irradiation of 405-nm light. Irradiation time/min; 0, 0.5, 1.5, 3, 5,
8, 12, 17, 23, 30, 38.

366 Bull. Chem. Soc. Jpn., 76, No. 2 (2003)
Novel Thermally Irreversible Photochrome
high-pressure mercury lamp (Ushio Electric) was separated by fil-
ters (5-cm water filter, a Pyrex glass filter, Toshiba UV-35, V-44,
and KL-40 glass filters). The irradiation-light intensity was deter-
mined by a photometer (Newport Co., Optical Laser Power Meter,
Model 1830-C). Measurements of the concentration of compo-
nents during the photoreactions were made using a high-pressure
liquid chromatograph (Shimadzu LC-6A) and a detector (Shimad-
zu SPD-6AV) using a silica-gel column (Wakosil 5-sil, 4.6 mm ×
150 mm, Wako) with a mixture of ethyl acetate and hexane as an
eluent. The silica-gel column chromatographic separation was
carried out with a Merck Kieselgel 60 (230–400 mesh) with a
mixture of ethyl acetate and hexane as an eluent. Analytical thin-
layer chromatography was performed on Merck pre-coated silica
gel 60 F-254, 0.25-mm thick TLC plates. All of the synthetic re-
actions were carried out under a dry nitrogen atmosphere. Tet-
rahydrofuran (THF) was freshly distilled from benzophenone
ketyl, and diethyl ether and dichloromethane were distilled from
CaH2 immediately before use.
Synthesis of 1-(1-Adamantylidene-1-phenylmethyl)-2-(2,5-
dimethyl-3-thienyl)-3,3,4,4,5,5-hexafluorocyclopentene (2O).
To a solution of 3-bromo-2,5-dimethylthiophene (274 mg,
1.43 mmol) in 10 mL THF at −78 °C was added a hexane solution
of butyllithium (1.57 mol dm−3, 1.0 mL, 1.57 mmol). This mix-
ture was added dropwise to a THF (15 mL) solution of 1-(1-ada-
mantylidene-1-phenylmethyl)perfluorocyclopentene (4) (560 mg,
1.34 mmol) at −78 °C, and the resulting mixture was kept stirring
for 24 h. After the reaction was quenched with water, the mixture
was extracted with ethyl acetate three times. The organic layer
was washed with saturated brine, and dried with anhydrous sodi-
um sulfate. After the drying agent was removed, the solvent was
removed in vacuo. The residue was purified by silica-gel column
chromatography to give 2O (194 mg, 28%). Mp 103–105 °C. IR
(KBr) ν/cm−1 3056, 3033, 2965, 2916, 2851, 1614, 1493, 1446,
1340, 1272, 1187, 1133, 1101, 1074, 1038, 1020, 1001, 973, 956,
885, 858, 822, 749, 724, 700, 578, 566, 528. 1H NMR (270 MHz,
CDCl3) δ 1.62–1.92 (12H, m), 2.05 (3H, s), 2.34 (3H, s), 2.62
(2H, br), 6.33 (1H, s), 6.95–6.98 (2H, m), 7.21–7.24 (3H, m). MS
(EI, 70 eV) m/z 508 (M+, 41), 507 (100), 492 (3), 488 (3), 446 (3),
386 (8), 372 (16). Found: m/z 508.1692. Calcd for C28H26F6S: M,
508.1659.
Scheme 5. Photoreaction of 3O.
spectrum was measured at intervals. As shown in Fig. 3, the
absorbance at 445 nm decreased only slightly. It is thus proved
that 3C is thermally quite stable.
Fatigue Resistivity. In order to apply to photochemical
switches, the photochromic compound should be robust. Pho-
tochemical coloration and decoloration cycles should be re-
peated without any, or at least very few, side reactions. Some
of the diarylethenes are known to show excellent fatigue resis-
tance. Because the structure of 3 is similar to that of dia-
rylethenes, we anticipated a high fatigue resistivity of 3. A
similar PMMA film used for thermal stability was used for the
repeated irradiation of 313- and 405-nm lights. As shown in
Fig. 4, after iterative irradiation was performed ten times, little
decrease in the absorption of the colored form at photostation-
ary state was observed.
Conclusion
A new thermally irreversible photochromic 1-aryl-1,3-buta-
diene system was established. While compound 2 with a 2,5-
dimethyl-3-thienyl group exhibited a serious side reaction
upon 313-nm light irradiation, compound 3 with a 2,4,5-tri-
methyl-3-thienyl group showed thermally irreversible photo-
chromism. Its thermal stability and fatigue resistivity in
PMMA films were shown to be excellent. Further modifica-
tion of its structure is now in progress in our laboratory.
Experimental
General. 1H NMR spectra were recorded with a JEOL JNM-
EX-270 (270 MHz) spectrometer in CDCl3. The signals are ex-
pressed as parts per million down field from tetramethylsilane,
used as an internal standard (δ value). Splitting patterns are indi-
cated as s, singlet; d, doublet; t, triplet; m, multiplet. IR spectra
were measured using a Perkin-Elmer 1650 FT-IR spectrometer.
Low- and high-resolution mass spectra were taken with a JEOL
JMS AX-500 mass spectrometer. UV-vis spectra were recorded
on a JASCO Ubest-50 UV-vis spectrophotometer. The emission
line of 313 nm of a 500 W high-pressure mercury lamp (Ushio
Electric) was separated by filters (5-cm water filter, Toshiba UV-
D35 glass filter, 5 cm aqueous NiSO4•6H2O solution, 1 cm aque-
ous K2CrO4–NaOH solution, and 1 cm aqueous potassium hydro-
gen phthalate solution). The emission line of 405 nm of a 500 W
Prolonged UV Irradiation on 2O. To a solution of 2O in
CDCl3 in an NMR sample tube was irradiated light longer than
300 nm from a high-pressure Hg lamp (only a Pyrex glass filter
was used) for 40 min, and the NMR spectrum was measured.
Only the signals of the by-product were observed. After evaporat-
ing the solvent, the mass spectrum of the by-product was taken. 1H
NMR (270 MHz, CDCl3) δ 1.50–2.53 (14H, m), 2.09, 2.57 (each
3H, s), 4.58 (1H, d, J = 2.97 Hz), 6.72 (2H, d, J = 7.26 Hz), 7.13–
7.19 (3H, m). MS (EI, 70 eV) m/z 509 (38), 508 (M+, 93), 507
(100), 492 (26), 416 (59), 388 (34), 387 (79), 386 (24).
Synthesis of 4-Bromo-2,3,5-trimethylthiophene. To a solu-
tion of 2,3,5-trimethylthiophene (1.92 g, 15.21 mmol) in 40 mL
Table 1. Absorption Spectral Data and Quantum Yields of Photoreactions of 3 in Toluene
λ
max/nm (εmax/mol−1 dm3 cm−1
)
Quantum Yield
313 nm 405 nm
3O
3C
ΦOC
ΦCO
ΦCO
No absorption peaks
longer than 280 nm
445 (9.90 × 103)
0.43
0.14
0.16

S. M. Shrestha et al.
Bull. Chem. Soc. Jpn., 76, No. 2 (2003) 367
dried with anhydrous sodium sulfate. The drying agent was fil-
tered off and the solvent was removed in vacuo. The residue was
purified by silica-gel column chromatography to give 3O (830 mg,
64%) as a colorless liquid. IR (neat) ν/cm−1 2996, 2943, 2864,
1443, 1338, 1274, 1189, 1145, 1120, 971, 744, 700, 581. 1H NMR
(270 MHz, CDCl3) δ 1.43, 1.59, 1.73, 1.90, 2.19 (each 3H, s), 6.69
(2H, m), 7.13–7.16 (3H, m). MS (EI, 70 eV) m/z 430 ((M+), 100),
415 (46), 387 (30). Found: m/z 430.1154. Calcd for C22H20F6S: M,
430.1190.
Photoreactions of 2 and 3. To the toluene solution (1.04 ×
10−4 mol dm−3 for 2O and 2.04 × 10−4 mol dm−3 for 3O) was ir-
radiated with 313-nm light, and the UV-vis spectra were recorded
with designated time intervals until they reached the photostation-
ary states. Also, the concentrations of the isomers were deter-
mined by HPLC. To the photostationary state solutions, 405-nm
light was irradiated until they returned to the initial O-forms. The
UV-vis spectra were recorded with designated time intervals to de-
termine the concentrations of the isomers.
Fig. 3. Thermal stability of 3C in a PMMA film at 80 °C.
Preparation of a PMMA Film Containing 3. A mixture of
PMMA pellets (Wako Chemicals Co., d 1.19, n = 1000–1500)
(320.8 mg) dissolved in 2 mL of dichloromethane and 3O
(1.16 mg) dissolved in 2 mL of toluene was cast into a Petri dish
(5.8 cm diameter). The cast solution was kept in the dark at room
temperature for 3 days to let the solvent evaporate. The thus-ob-
tained PMMA film was further dried under a vacuum at room tem-
perature for 24 h. The film was cut into two, and each piece was
used for a thermal-stability experiment and for a fatigue-resistivity
experiment, respectively.
The authors thank Shorai Foundation for Science and Tech-
nology for financial support.
References
Fig. 4. Fatigue resistivity of 3 in a PMMA film by iterative
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