1,3-Bisarylbutadienes: Novel thermally irreversible photochromic system

Article abstract of DOI:10.1080/15421400590947018

Novel thermally irreversible photochromic compounds 1O, 2O and 3O based on 1,3-bisarylbutadiene system were synthesized. The closed ring-forms 1C and 2C showed absorption maximum at 445 nm, while the closed ring-form 3C, a methyl group of 2C on C-5 of thienyl ring was replaced with a formyl group, showed the absorption maximum at 500 nm.

Full text of DOI:10.1080/15421400590947018

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1,3-Bisarylbutadienes:
Novel Thermally Irreversible
Photochromic System
Yasushi Yokoyama a , Sujen Man Shrestha a & Yayoi
Yokoyama b
a Department of Advanced Materials Chemistry ,
Graduate School of Engineering, Yokohama National
University , Tokiwadai, Hodogaya, Yokohama, Japan
b Faculty of Home Economics , Tokyo Kasei Gakuin
University , Aiharacho, Machida, Tokyo, Japan
Published online: 31 Aug 2006.
To cite this article: Yasushi Yokoyama , Sujen Man Shrestha & Yayoi Yokoyama (2005)
1,3-Bisarylbutadienes: Novel Thermally Irreversible Photochromic System, Molecular
Crystals and Liquid Crystals, 431:1, 433-439
To link to this article: http://dx.doi.org/10.1080/15421400590947018
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Mol. Cryst. Liq. Cryst., Vol. 431, pp. 133=[433]–139=[439], 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590947018
1,3-Bisarylbutadienes: Novel Thermally Irreversible
Photochromic System
Yasushi Yokoyama
Sujen Man Shrestha
Department of Advanced Materials Chemistry, Graduate School of
Engineering, Yokohama National University, Tokiwadai, Hodogaya,
Yokohama, Japan
Yayoi Yokoyama
Faculty of Home Economics, Tokyo Kasei Gakuin University, Aiharacho,
Machida, Tokyo, Japan
Novel thermally irreversible photochromic compounds 1O, 2O and 3O based on
1,3-bisarylbutadiene system were synthesized. The closed ring-forms 1C and 2C
showed absorption maximum at 445 nm, while the closed ring-form 3C, a methyl
group of 2C on C-5 of thienyl ring was replaced with a formyl group, showed the
absorption maximum at 500 nm.
Keywords: bathochromic shift; bisarylbutadiene; fatigue resistivity; photochromism;
thermal irreversibility
INTRODUCTION
Although a great number of photochromic compounds have been
reported [1–3], very few are known to be thermally irreversible. They
are fulgides [4], diarylethenes [5], and phenoxynapthacenequinones
[6]. However, if the catalog of thermally irreversible photochromic
compounds is thicker, it would be more convenient to select an
appropriate one for the particular purpose. Therefore, we have
designed and synthesized thermally irreversible and fatigue resistant
photochromic 1,3-bisarylbutadiene system exemplified by 1, based on
Address correspondence to Yasushi Yokoyama, Department of Advanced Materials
Chemistry, Graduate School of Engineering, Yokohama National University, Tokiwadai,
Hodogaya, Yokohama, 240-8501, Japan. E-mail: yyokoyam@ynu.ac.jp
133=[433]

134=[434]
Y. Yokoyama et al.
SCHEME 1 Photoreactions of 1,3-arylbutadienes.
the photochemical 6p-eletrocyclization [7,8]. The photochromic reac-
tion of 1,3-arylbutadienes is shown in Scheme 1.
Thermal irreversibility and fatigue resistivity are the fundamental
and important properties for practical application to the rewritable
optical recording media. It is also desired to develop a series of photo-
chromic compounds that have sensitivity in several different wave-
length regions. We here report the synthesis of 2 and 3, the colored
forms of which were supposed to have absorption maxima at longer
wavelengths.
RESULTS AND DISCUSSION
1. Synthesis of 2O
Synthesis of 2O was carried out as shown in Scheme 2 in 53% yield,
according to the similar procedures described elsewhere [9].
2. Photoreaction of 1 and 2
Photoreaction of 2O was carried out as previously reported for 1O [7].
When a toluene solution of 2O was irradiated with 313-nm light, the
SCHEME 2 Synthesis of 2O.

Novel Thermally Irreversible Photochromic System
135=[435]
FIGURE 1 Change in absorption spectra of 2. Concentration=mol dmÀ3
:
2.16 Â 10Â 4 in toluene (a) Light intensity=mW cm À 2
: 0.37 (313 nm),
irradiation time=min: 0, 0.5, 1, 1.5, 2, 3, 4, 5, 6, 7, 8, 9. (b) Light intensity=mW
mW cmÀ2: 0.24 (405 nm), irradiation time=min: 0, 0.5, 1, 1.5, 2, 3, 4, 5, 6, 7.
color of the solution turned yellow, and a new absorption band in the
visible region. The absorption maximum of 2C was 445 nm which was
the same as that of 1C.
The change in absorption spectra of 2 according to the time of 313-
nm light irradiation and 405-nm light irradiation were shown in
Figure 1. The quantum yields of photoreactions and the spectroscopic
data of 2 in toluene is shown in Table 1, together with those of 1.
Apparently the introduction of an additional phenyl group was ineffec-
tive, because of the poor conjugation of those two non-coplanar phenyl
groups caused by steric congestion.
3. Synthesis of 3O
Unterhalt et al. [10] reported the oxidation of an alkyl group on a
benzothiophene ring by cerium sulfate oxidation. The oxidation
TABLE 1 Absorption Spectral Data and Quantum Yields of Photoreactions of
1C, 2C, and 3C in Toluene
Quantum yields of photoreactions
Absorption spectral data of colored forms
313 nm
UOC
405 nm
e
max=molÀ1 dm3
O: C at
Pss1
kmax=nm
cmÀ 1
UCO
UCO
1
2
3
445
445
500
9.9 Â 103
10.9 Â 103
10.2 Â 103
0.43
0.61
0.34
0.14
0.18
0.16
0.16
0.14
0.082
37 : 63
68 : 32
41 : 59
1Ratio of O-form and C-form at photostationary state of 313-nm light irradiation.
2505-nm light irradiation.

136=[436]
Y. Yokoyama et al.
occurred exclusively on the alkyl group on C-2 of the benzothiophene,
to produce a carbonyl group.
We carried out this oxidation reaction of 2O with the same reaction
conditions – with cerium (IV) sulfate in acetic acid. Two new photo-
chromic compounds 3O in 41% and 4O in 39% yields were obtained.
4. Structure Determination of 3O
1H NMR spectrum (CDCl3) of 2O showed five singlet methyl signals at
d 1.47, 1.66, 1.75, 1.92, and 2.19. The oxidation product 3O showed
singlet methyl signals at d 1.67 (3H, s), 1.85 (3H, s), and 1.95 (6H,
s). In addition, a new signal appeared at d 9.86 (1H, s), assigned to a
formyl hydrogen. Also, a strong carbonyl absorption at 1667 cmÀ1 in
IR spectrum was observed. Thus, an electron-withdrawing formyl
group was introduced by the oxidation of a methyl group.
Reduction of 3O with NaBH4 in methanol followed by acetylation
gave 4O. Thus, the structure of 4O was determined to be the acetox-
ymethyl derivative of 3O.
Photoreaction of 3O was carried out to examine the photochromic
properties of 3 and to elucidate the effect of the electron-withdrawing
substituent. Irradiation of 313-nm light to a toluene solution of 3O
produced 3C, possessing the absorption maximum at 500 nm. Thus
the formyl group worked to lengthen the absorption maximum of the
colored form. This also indicated that the formyl group takes part in
the conjugation in the structure of 3C. Therefore the oxidized methyl
group should have been located on C-4 or C-5 on the trimethyl thio-
phene ring. Other methyl groups of 2, after photoirradiation, are
located on the sp3 carbon atoms.
To identify the oxidized methyl group, ZINDO and PPP-CI calcula-
tions were carried out for the possible candidates of 3C. For PPP-CI
calculations, while 4-CHO derivative showed blue shift by 17 nm, 5-
CHO derivative showed red shift by 29 nm, based on the calculated
absorption maximum of 2C as 394 nm. Similarly, while ZINDO calcu-
lations on 4-CHO derivative showed red shift of only 11 nm, 5-CHO
derivative showed red shift by 27 nm, deviated from the calculated
absorption maximum of 339 nm for 2C. Therefore we concluded that
the oxidized methyl group of 2 was on C-5 of the trimethylthiophene
ring (Scheme 3).
5. Photochromic Reaction of 3O
Photoreaction of 3O was carried out similarly to that of 1O and 2O.
The absorption spectral changes of 3 in toluene upon alternate

Novel Thermally Irreversible Photochromic System
137=[437]
SCHEME 3 Synthesis of 3O and 4O.
irradiation of 313-nm and 505-nm lights are shown in Figure 2, and
spectroscopic data and quantum yields of photoreactions are summar-
ized in Table 1.
6. Thermal Stability and Fatigue Resistivity of 1, 2 and 3
We have examined the thermal stability and fatigue resistivity of com-
pounds 1, 2 and 3 in PMMA films. The films containing their photosta-
tionary states were kept at 80ꢀC in the dark in the air for 30 days, and
the absorption spectrum was measured at intervals. As shown in
Figure 3, all these compounds remained constant for 30 days. It is thus
proved that 1, 2 and 3 are thermally stable.
As shown in Figure 4, iterative irradiations with 313- and 405- or
505-nm lights were performed for ten times. Although a slight
decrease in the absorption of the colored form at photostationary state
was observed after the several iterations, it remained at the same level
for ten cycles. Thus 1, 2 and 3 showed an excellent fatigue resistivity.
FIGURE 2 Change in absorption spectra of 3. Concentration=mol dmÀ3
:
2.16 Â 10À4 in toluene (a) Light intensity=mW cmÀ2: 0.33 (313 nm), irradiation
time=min: 0, 0.5, 1, 1.5, 2, 3, 4, 5, 6, 7, 8, 9,10. (b) Light intensity=mW cmÀ2
0.3 (505 nm), irradiation time=min: 0, 0.5, 1, 1.5, 2, 3, 4, 5, 6, 7, 8.
:

138=[438]
Y. Yokoyama et al.
FIGURE 3 Thermal stability of 1C, 2C and 3C in a PMMA film at 80ꢀC.
FIGURE 4 Fatigue resistivity of 1, 2 and 3 by irradiating UV and vis light in
a PMMA film.
CONCLUSION
A novel thermally stable and fatigue resistive photochromic bisarylbu-
tadiene compounds 1, 2 and 3 based on 6p-electocyclization were
synthesized. The compound 3 with a formyl group at C-5 of the thienyl
ring extended the p-conjugation in closed form, and shifted the absorp-
tion maximum by 55-nm to the longer wavelength.
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