Structurally versatile novel photochromic bisarylindenone and its acetal: Achievement of large cyclization quantum yield

Article abstract of DOI:10.1021/ol901497t

Bisarylindenones and an acetal derivative, a novel thermally irreversible photochrome family based on 6π-electrocyclization with three easily modifiable functional groups, have been synthesized. 2,3-Bis(5-methyl-2-phenyl- 4-thiazolyl)indenone showed photo

Full text of DOI:10.1021/ol901497t

ORGANIC
LETTERS
2009
Vol. 11, No. 17
3890-3893
Structurally Versatile Novel
Photochromic Bisarylindenone and Its
Acetal: Achievement of Large
Cyclization Quantum Yield
Kaori Morinaka, Takashi Ubukata, and Yasushi Yokoyama*
Department of AdVanced Materials Chemistry, Graduate School of Engineering,
Yokohama National UniVersity, Tokiwadai, Hodogaya, Yokohama 240-8501, Japan
yyokoyam@ynu.ac.jp
Received July 1, 2009
ABSTRACT
Bisarylindenones and an acetal derivative, a novel thermally irreversible photochromic family based on 6π-electrocyclization with three easily
modifiable functional groups, have been synthesized. 2,3-Bis(5-methyl-2-phenyl-4-thiazolyl)indenone showed photochromic back-and-forth
reactions with two different visible lights. Its ethylene acetal recorded 0.81 as the photocyclization quantum yield in hexane, which is the
largest value known to date for 6π-electrocyclization in solution.
In order to meet the requirements of various applications
and research demands for photodriven switches,1 the devel-
opment of novel photochromic compounds classified as a
new structural family is vital. Although diarylethenes2 are
well-designed and, indeed, a superior class of thermally
irreversible photochromic compounds, one drawback is that
structural modifications are mainly carried out on the aryl
groups on the right and left ends of the molecule3 and a
modification of the central ethene moiety is sometimes
inconvenient. Nowadays, most diarylethenes are hexafluo-
rocyclopentene-based and its structural modification is almost
impossible. First-generation diarylethenes have maleic an-
hydride4 and maleimide5 structures; however, their extensive
structural modifications have been difficult. When the central
ethene is incorporated in an aryl group,6 it usually becomes
thermally reversible because of the large loss of aromatic
stabilization energy upon photocyclization. Other modifica-
tions have proved that the central ethene moiety can act as
a switchable functional group7 or the monomer for poly-
merization.8
We envision that if the central ethene moiety were a
functional group that could be modified structurally as well
as electronically in easily accessible ways, the diversity of
(4) Uchida, K.; Nakayama, Y.; Irie, M. Bull. Chem. Soc. Jpn. 1990, 63,
1311–1315.
(5) Yamaguchi, T.; Uchida, K.; Irie, M. J. Am. Chem. Soc. 1997, 119,
6066–6071.
(6) (a) Krayushkin, M. M.; Ivanov, S. N.; Martynkin, A. Yu.; Lichitsky,
B. V.; Dudinov, A. A.; Uzhinov, B. M. Russ. Chem. Bull. 2001, 50, 116–
121. (b) Krayushkin, M. M.; Lichitsky, B. V.; Mikhalev, A. P.; Nabatov,
B. V.; Dudinov, A. A.; Ivanov, S. N. Russ. J. Org. Chem. 2006, 42, 860–
864. (c) Nakashima, T.; Atsumi, K.; Kawai, S.; Nakagawa, T.; Hasegawa,
Y.; Kawai, T. Eur. J. Org. Chem. 2007, 3212–3218. (d) Kawai, S.;
Nakashima, T.; Atsumi, K.; Sakai, T.; Harigai, M.; Imamoto, Y.; Kamikubo,
H.; Kataoka, M.; Kawai, T. Chem. Mater. 2007, 19, 3479–3483.
(1) Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH: Weinheim,
2001.
(2) (a) Irie, M. Chem. ReV. 2000, 100, 1685–1716. (b) Tian, H.; Yang,
S. Chem. Soc. ReV. 2004, 33, 85–97.
(3) Wigglesworth, T. J.; Myles, A. J.; Branda, N. R. Eur. J. Org. Chem.
2005, 1233–1238.
10.1021/ol901497t CCC: $40.75
Published on Web 08/05/2009
 2009 American Chemical Society

the thermally irreversible photochromic molecules could be
widely extended. We chose a carbonyl group and a phenyl
group as such easily modifiable functional groups. However,
as a number of 2,3-bisaryl-1,4-naphtho- and benzoquinones9
and 3,4-bis(2-methyl-5-phenyl-3-thienyl)-3-cyclobutene-1,2-
dione10 were reported to be nonphotochromic, we decided
to synthesize 2,3-bisarylindenones. These compounds have
three easily modifiable functional groups: two aryl groups
and the indenone group possessing a carbonyl and a phenyl
group.11
organic materials (a mixture of starting ethyne and the
product) by fast silica gel column chromatography were
subjected to the same reaction two more times. The yield of
1O in this manner was 73%.
In acetonitrile, 1O showed an absorption maximum at 433
nm, in addition to the maxima in the UV region at 262 and
311 nm (Figure 1). The color of the solution was yellow.13 The
We employed 5-methyl-2-phenyl-4-thiazolyl and 2,5-
dimethyl-3-thienyl groups as the aryl groups and synthesized
2,3-bis(5-methyl-2-phenyl-4-thiazolyl)indenone (1O) and 2,3-
bis(2,5-dimethyl-3-thienyl)indenone (2O). When they cyclize
by photoirradiation, the closed forms (1C and 2C) as the
result of photoinduced 6π-electrocyclization possess large
conjugate systems, which may show the absorption bands
at longer wavelengths in the visible region.
The synthesis of 2,3-bisarylindenones 1O and 2O was
carried out, as shown in Scheme 1, by employing Tsuka-
moto’s Pd-catalyzed [2 + 3] ring-closing reaction12 as the
Figure 1. Absorption spectral change of 1 during irradiation of
436 nm light in acetonitrile. Concentration: 0.558 × 10-4 mol dm-3
.
Light intensity: 2.4 mW cm-2. Irradiation time: 0-12 min.
Scheme 1
.
Synthetic Outline and Photochromic Reactions of 1,
2, and 4
absorption band in the visible region tailed to over 550 nm.
Since we confirmed that the longest absorption maximum
wavelength of 2,3-diphenylindenone was 433 nm in toluene,
the absorption in the visible light region is characteristic of 2,3-
bisarylindenones. Upon irradiation of 436 nm light, new
absorption maxima appeared at 542, 429, 363, 307, and 253
nm, and the color of the solution turned brown. This is
attributable to the formation of the cyclic form 1C. HPLC
analysis revealed that the conversion ratio to 1C at the
photostationary state (pss) of 436-nm light irradiation was 58%.
The cyclized form was thermally so stable that it showed no
change after 1.5 months storage in the dark at room temperature.
When the 436-nm pss solution was irradiated with 579
nm light, the color turned yellow, and HPLC analysis proved
that 1O was recovered again. However, due to the long
absorption tail of 1O in the visible region, the ratio of 1O/
1C at the pss of 579-nm light irradiation was 99/1.
The absorption spectral properties of 1 are listed in Table
1, and the quantum yields of the photochromic reactions of
1 are listed in Table 2.
Thus, a novel photochromic system, in which back-and-
forth reactions can be driven by visible light of two different
wavelengths, was created.
(7) (a) Lemieux, V.; Branda, N. R. Org. Lett. 2005, 7, 2969–2972. (b)
Lemieux, V.; Gauthier, S.; Branda, N. R. Angew. Chem., Int. Ed. 2006, 45,
6820–6824. (c) Sud, D.; Wigglesworth, T. J.; Branda, N. R. Angew. Chem.,
Int. Ed. 2007, 46, 8017–8019. (d) Lemieux, V.; Spantulescu, M. D.;
Baldridge, K. K.; Branda, N. R. Angew. Chem., Int. Ed. 2008, 47, 5034–
5037.
key step using 2-methoxycarbonylphenylboronic acid 3 and
bisarylethyne.13
We prepared two corresponding symmetric ethynes ac-
cording to known procedures and reacted them with boronic
acid 3 using Pd(OAc)2 as the catalyst. The yields were 25
and 27% for 1O and 2O, respectively. Because the low yield
is due to the deactivation of the catalyst, the recovered
(8) Wigglesworth, T. J.; Branda, N. R. AdV. Mater. 2004, 16, 123–125.
(9) Yoshida, S.; Kubo, H.; Saika, T.; Katsumura, S. Chem. Lett. 1996,
25, 139–140.
(10) Ku¨hni, J.; Belser, P. Org. Lett. 2007, 9, 1915–1918.
(11) Bergmann, E. D. J. Org. Chem. 1956, 21, 461–464.
(12) Tsukamoto, H.; Kondo, Y. Org. Lett. 2007, 9, 4227–4230.
Org. Lett., Vol. 11, No. 17, 2009
3891

Table 1. Absorption Spectral Data of 1a and 4b
compd
λmax/nm (ε/103 dm3 mol-1 cm-1
)
1O
1C
4O
4C
262 (40.0) 311 (22.4) 433 (2.51)
253 (18.9) 307 (34.5) 363 (28.3) 429 (11.1) 542 (7.37)
229 (29.9) 319 (22.2)
290 (27.2) 353 (14.7) 511 (10.0)
a CH3CN. b Hexane.
In contrast, 2O showed poor photochromism13 and was
decomposed gradually upon 436-nm light irradiation.
Figure 2. Absorption spectral changes of 4 during irradiation of
313 nm light in hexane. Concentration: 0.628 × 10-4 mol dm-3
.
Light intensity: 0.24 mW cm-2. Irradition time: 0-20 min.
Table 2. Quantum Yields of the Photoreactions of 1a and 4b
reported by Kawai and co-workers16 and 0.79 reported by
our group.17
shorter
wavelength
lighta
longer
wavelength
lightb
The reason the cyclization quantum yield of 4O is so large
is not yet known and is currently under investigation.
However, we assume that the two sets of intramolecular
nitrogen-hydrogen interactions (a thiazole nitrogen atom and
the proximate hydrogen atom on the indenone phenyl group,
and a thiazole nitrogen atom and a hydrogen atom on the
carbon atom adjacent to an oxygen atom in the acetal group)
may play an important role in constraining the conformation
of 4O in favor of the cyclization.18 The solvent effect on
the quantum yield of ring closure of 4O can be explained
by the polar N-H interactions. It recorded the highest value
in hexane while it decreased as the solvent polarity became
larger (Table 2). As the polar solvent molecules prevent
effective N-H interactions, the distribution of the conformers
becomes less biased, thus decreasing the quantum yield.
In conclusion, we have synthesized a novel thermally
irreversible photochromic system based on 6π-electrocy-
clization with three easily modifiable functional groups.
Bisarylindenone 1 showed photochromic back-and-forth
reactions with visible light of different wavelength. The
photochromic and absorption spectral properties can be
switched by its acetalization. Significantly, the photocy-
clization quantum yield of acetal 4O, i.e., 0.81 in hexane,
is the largest value known to date for 6π-electrocyclization
in solution. Achievement of the large quantum yield may
be explained by the intramolecular interactions of nitrogen
and hydrogen atoms to fix the conformation in favor of
photochemical cyclization.
ET(30)/kcal
compd solvent
mol-1
ΦOC
ΦCO
ΦCO
CRc/%
1
4
CH3CN
hexane
toluene
AcOEt
CH3CN
EtOH
45.6
31.0
33.9
38.1
45.6
51.9
0.10
0.81
0.72
0.70
0.63
0.68
0.016
0.011
0.024
0.037
0.063
0.017
0.0051
0.0089
0.0091
0.012
58
99
97
96
91
97
0.023
0.012
a 436 nm for 1, 313 nm for 4. b 579 nm for 1, 512 nm for 4. c CR:
Conversion ratio to the colored form at the photostationary state of the
cyclization reaction.
As described above, the carbonyl and phenyl groups can
be easily modified. The electronic influence of the carbonyl
group was changed by acetalization in a manner similar to
that of Belser and Ku¨hni for 3,4-bis(2-methyl-5-phenyl-3-
thienyl)-3-cyclobutene-1,2-dione.10 In their case, acetalization
of both carbonyl groups could realize photochromic proper-
ties.
We tried to synthesize acetals of 1,2-ethanediol and 1,3-
propanediol. While 1,3-propanediol did not give the corre-
sponding 1,3-dioxane due to steric congestion, 1,2-ethanediol
reacted with 1O to give the corresponding 1,3-dioxolane 4O
in 76% yield based on the consumed 1O.
Photochromism of 4O was investigated in various solvents
with different polarities, and the photocyclization quantum
yield in hexane was proved to be the largest. The changes
observed in the absorption spectra by photochromic reactions
in hexane are shown in Figure 2. The conversion ratio to
4C at the pss of 313-nm light irradiation was 99%. The
absorption spectral data and quantum yields of the photo-
reactions are summarized in Tables 1 and 2, respectively.
Acetalization induced a blue shift of the absorption spectra
as well as an increase in the photocyclization quantum yield.
The photocyclization quantum yield value of 0.81 recorded
in hexane is, to the best of our knowledge, the largest one
based on 6π-electrocyclization in solution,14,15 which has
exceeded previous large quantum yield values, such as 0.77
(14) In order to secure the quantum yield values, we used Parker’s
K3Fe(C2O4)3 chemical actinometer to determine the intensity of the lights
used for photochromic reactions.
(15) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. 1956, A235, 518–
536.
(16) Kawai, S.; Nakashima, T.; Kutsunugi, Y.; Nakagawa, H.; Nakano,
H.; Kawai, T. J. Mater. Chem. 2009, 19, 3606–3611.
(17) Kiji, J.; Okano, T.; Kitamura, H.; Yokoyama, Y.; Kubota, S.; Kurita,
Y. Bull. Chem. Soc. Jpn. 1995, 68, 616–619.
(18) A preliminary result obtained by DFT calculations suggests that
two antiparallel (i.e., photocyclizable) conformations are particularly stable
(more than 98% population at 25 °C), in which the two sets of N-H
distances are around 0.26 nm, almost the same as the sum of van der Waals
radii of N and H. Theoretical aspects of this problem will be reported
separately.
(13) See the Supporting Information.
3892
Org. Lett., Vol. 11, No. 17, 2009

Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research in Priority Areas “New
Frontiers in Photochromism (No. 471)” from the Ministry
of Education, Culture, Sports, Science and Technology
(MEXT), Japan.
2O, and 4O by photoirradiation, HPLC chromatograms
during photoreactions of 1O and 4O, pictures of solution of
1O/1C(pss) and 4O/4C(pss), and H NMR spectra of new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
Supporting Information Available: Details of the syn-
thesis of 1O, 2O, and 4O, UV-vis spectral change of 1O,
OL901497T
Org. Lett., Vol. 11, No. 17, 2009
3893