Self-assembled boronic ester cavitand capsule as a photosensitizer and a guard nanocontainer against photochemical reactions of 2,6-diacetoxyanthracene

Article abstract of DOI:10.1021/jo101255g

Figure presented. The optical properties of 2,6-diacetoxyanthracene 4 encapsulated in the self-assembled boronic ester cavitand capsule 3 in C ₆H₆ are described. Upon excitation at 285 nm, the encapsulated 4 showed strong fluorescence emission as a result of the energy transfer from the excited 3 to the encapsulated 4, while 4 alone in C ₆H₆ exhibited very weak emission. Upon photoirradiation at 365 nm, the encapsulated 4 also showed strong fluorescence emission and remained almost intact, whereas 4 alone in C₆H₆ gradually underwent photodimerization and photooxygenation to afford photodimers 5 and 5′ and 9,10-anthraquinone 6 via 9,10-endoperoxide 7, respectively. Thus, the capsule 3 serves as a photosensitizer for the encapsulated 4 as well as a guard nanocontainer to protect against the photochemical reactions of 4.

Full text of DOI:10.1021/jo101255g

pubs.acs.org/joc
Self-Assembled Boronic Ester Cavitand Capsule as a Photosensitizer
and a Guard Nanocontainer against Photochemical Reactions
of 2,6-Diacetoxyanthracene
Naoki Nishimura and Kenji Kobayashi*
Department of Chemistry, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku,
Shizuoka 422-8529, Japan
skkobay@ipc.shizuoka.ac.jp
Received July 1, 2010
The optical properties of 2,6-diacetoxyanthracene 4 encapsulated in the self-assembled boronic ester
cavitand capsule 3 in C6H6 are described. Upon excitation at 285 nm, the encapsulated 4 showed
strong fluorescence emission as a result of the energy transfer from the excited 3 to the encapsulated 4,
while 4 alone in C6H6 exhibited very weak emission. Upon photoirradiation at 365 nm, the
encapsulated 4 also showed strong fluorescence emission and remained almost intact, whereas 4
alone in C6H6 gradually underwent photodimerization and photooxygenation to afford photodimers
5 and 50 and 9,10-anthraquinone 6 via 9,10-endoperoxide 7, respectively. Thus, the capsule 3 serves as
a photosensitizer for the encapsulated 4 as well as a guard nanocontainer to protect against the
photochemical reactions of 4.
Introduction
cases.1 These reversible switching properties of anthracene
derivatives in response to external photochemical and thermal
stimuli have been applied to supramolecular architectures.2
Anthracene and its derivatives are also useful building blocks
for optoelectronics, such as organic electroluminescent devices,3,4
and for organic semiconductors such as organic field-effect
transistors5 and organic photoconductors.6 However, photo-
dimerization and photooxygenation of anthracene deriva-
tives are disadvantageous for uses related to optoelectronics.
Self-assembled molecular capsules have potential as reaction
nanovessels,7 as well as nanocontainers for the stabilization
It is well-known that anthracene and its derivatives react
under photoirradiation at 360-370 nm to afford [4πþ4π]
photodimerization products and [4πþ2π] photooxygenation
products (9,10-endoperoxides) and that these products return
to the monomer state under UV irradiation below 270 nm in
the former case and by the transfer of thermal energy in both
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DOI: 10.1021/jo101255g
r
Published on Web 08/17/2010
J. Org. Chem. 2010, 75, 6079–6085 6079
2010 American Chemical Society

Nishimura and Kobayashi
JOCFeatured Article
SCHEME 1. Encapsulation of 2,6-Diacetoxyanthracene 4 in
Boronic Ester Cavitand Capsule 3 Self-Assembled by 1 and 2
sulating capsule 4@3 serves as a photostable luminescence
material.
Results and Discussion
UV-vis Absorption and Fluorescence Emission Spectra of
4@3. The self-assembled boronic ester cavitand capsule 3
encapsulates one molecule of 2,6-diacetoxyanthracene 4 with
an association constant (Ka) of 1.83 ꢀ 106 M-1 in C6D6 at
313 K.11b The UV-vis absorption and fluorescence emission
spectra of guest-free 3, free-4, and the guest-encapsulating
capsule 4@3 were measured at a concentration of 1.0 ꢀ 10-5
M in C6H6 at 296 K. Under these conditions, 3 maintains
100% encapsulation of 4 upon addition of 1 equiv of 4, and
4@3 does not release 4. The UV-vis absorption spectra of
free-3, free-4, and 4@3 showed the absorption maxima
(λmax(abs)) at 277 nm for free-3; at 349, 361, and 382 nm
for free-4; and at 277, 348, 361, and 382 nm for 4@3 (Figure 2
and Table 1).14,15 The λmax(abs) of 4 encapsulated in 3 was
almost the same as that of free-4 in C6H6.
The fluorescence emission spectra and spectral data of
free-3, free-4, and 4@3 are shown in Figure 3 and Table 2.15
Upon excitation at 360 nm, free-4 and 4@3 showed fluores-
cence emission maxima (λmax(em)) at 394 nm (peak A) and
415 nm (peak B) for free-4 and at 393 nm (peak A) and 413
nm (peak B) for 4@3, whereas free-3 alone was not emissive
(Figure 3a). The λmax(em) of 4 encapsulated in 3 was slightly
blue-shifted by 1-2 nm relative to that of free-4, and the
fluorescence emission intensities of 4@3 decreased by 8%
relative to the intensities of free-4. In marked contrast, upon
excitation at 285 nm, the fluorescence emission intensities of
peaks A and B of 4@3 increased by 15% relative to those
upon excitation at 360 nm, whereas the fluorescence emis-
sion intensities of free-4 upon excitation at 285 nm decreased
by 96% relative to those upon excitation at 360 nm (Figure 3b
vs 3a). As a result, upon excitation at 285 nm the fluorescence
emission intensities of peaks A and B of 4@3 were 30 times
stronger than those of free-4 in C6H6 (Figure 3b). Upon
excitation at 285 nm the λmax(em) of 4@3 also appeared at
341 nm (peak C), in addition to peak A and peak B, because
the free-3 alone showed the fluorescence emission peak C
upon excitation at 285 nm (Figure 3b), wherein the fluores-
cence emission intensity of free-3 was six times stronger than
that of 4@3.
Capsule 3 as a Photosensitizer for the Encapsulated Guest 4.
The fluorescence excitation spectra and spectral data of free-
4 and 4@3 upon fluorescence emission at 410 nm in C6H6 at
296 K are shown in Figure 4 and Table 3, respectively. For
free-4, the fluorescence emission at 410 nm mainly arises
from the excitation maxima (λmax(ex)) at 348, 359, and 381 nm.
For 4@3, the fluorescence emission at 410 nm mainly arises
from λmax(ex) at 285 nm, as well as at 348, 360, and 381 nm.
The fluorescence excitation spectral pattern of 4@3 upon
fluorescence emission at 410 nm is similar to the UV-vis
absorption spectral pattern of 4@3. These results clearly
indicate that the strong fluorescence emission of 4@3 upon
excitation at 285 nm shown in Figure 3b is caused by the
of reactive intermediates,7,8 and for the control of photo-
chemical properties and reactions of encapsulated guests.9,10
Recently, we reported on the encapsulation of 2,6-diacetox-
yanthracene 4 in the self-assembled boronic ester cavitand
capsule 3 (Scheme 1).11 The latter is synthesized by the
dynamic boronic ester formation of cavitand tetraboronic
acid 1 with the bis-catechol linker 2.11 We now describe the
use of the capsule 3 as a photosensitizer for the encapsulated
4,12 as well as a guard nanocontainer to protect against the
photochemical reactions of 4 (Figure 1).13 The guest-encap-
(7) (a) Rebek, J., Jr. Angew. Chem., Int. Ed. 2005, 44, 2068–2078.
(b) Yoshizawa, M.; Klosterman, J. K.; Fujita, M. Angew. Chem., Int. Ed.
2009, 48, 3418–3438. (c) Pluth, M. D.; Bergman, R. G.; Raymond, K. N. Acc.
Chem. Res. 2009, 42, 1650–1659.
(8) For covalent-bound molecular capsules for stabilization of reactive
chemical species, see: (a) Warmuth, R.; Yoon, J. Acc. Chem. Res. 2001, 34,
95–105. (b) Warmuth, R.; Makowiec, S. J. Am. Chem. Soc. 2007, 129, 1233–
1241 and references cited therein
(9) (a) Yoshizawa, M.; Takeyama, Y.; Okano, T.; Fujita, M. J. Am.
Chem. Soc. 2003, 125, 3243–3247. (b) Kaanumalle, L. S.; Gibb, C. L. D.;
Gibb, B. C.; Ramamurthy, V. J. Am. Chem. Soc. 2005, 127, 3674–3675.
(c) Kaanumalle, L. S.; Ramamurthy, V. Chem. Commun. 2007, 1062–1064.
(d) Parthasarathy, A.; Kaanumalle, L. S.; Ramamurthy, V. Org. Lett. 2007,
9, 5059–5062. (e) Gibb, C. L. D.; Sundaresan, A. K.; Ramamurthy, V.; Gibb,
B. C. J. Am. Chem. Soc. 2008, 130, 4069–4080. (f) Ams, M. R.; Ajami, D.;
Craig, S. L.; Yang, J.-S.; Rebek, J., Jr. J. Am. Chem. Soc. 2009, 131, 13190–
13191.
(10) For control of product selectivity in organic phototransformation in
supramolecular systems, see: Tung, C.-H.; Wu, L.-Z.; Zhang, L.-P.; Chen, B.
Acc. Chem. Res. 2003, 36, 39–47.
(11) (a) Nishimura, N.; Kobayashi, K. Angew. Chem., Int. Ed. 2008, 47,
6255–6258. (b) Nishimura, N.; Yoza, K.; Kobayashi, K. J. Am. Chem. Soc.
2010, 132, 777–790.
(12) For enhancement in the fluorescence emission of chromophores by
energy transfer in supramolecular systems, see: (a) Slone, R. V.; Hupp, J. T.
Inorg. Chem. 1997, 36, 5422–5423. (b) Choi, M.-S.; Yamazaki, T.; Yamazaki,
I.; Aida, T. Angew. Chem., Int. Ed. 2004, 43, 150–158. (c) Inagaki, S.; Ohtani,
O.; Goto, Y.; Okamoto, K.; Ikai, M.; Yamanaka, K.; Tani, T.; Okada, T.
Angew. Chem., Int. Ed. 2009, 48, 4042–4046.
(13) For stabilization of photoreactive organic dyes by rotaxane encap-
sulations, see: (a) Frampton, M. J.; Anderson, H. L. Angew. Chem., Int. Ed.
2007, 46, 1028–1064. (b) Yau, C. M. S.; Pascu, S. I.; Odom, S. A.; Warren,
J. E.; Klotz, E. J. F.; Frampton, M. J.; Williams, C. C.; Coropceanu, V.;
ꢀ
Kuimova, M. K.; Phillips, D.; Barlow, S.; Bredas, J.-L.; Marder, S. R.;
Millar, V.; Anderson, H. L. Chem. Commun. 2008, 2897–2899.
(14) The λmax(abs) of 4 at around 255 nm is canceled by that of benzene as
a reference solvent.
(15) For UV-vis absorption and fluorescence spectra of 1, 2, cavitand
without boryl groups 9 as a reference of 1, and p-tolylboronic acid bis-
(catechol)ethane diester 10 as a reference of 3, see Figures S1 and S2,
Supporting Information.
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Nishimura and Kobayashi
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FIGURE 1. Use of capsule 3 as (a) a photosensitizer for the encapsulated 4 and (b) a guard nanocontainer to protect against the photochemical
reactions of 4. (c, d) Photochemical properties of 4 alone.
FIGURE 2. UV-vis absorption spectra of free-3 (green line), free-4
(blue line), and 4@3 (red line) (1.0 ꢀ 10-5 M in C6H6 at 296 K).
TABLE 1. UV-vis Absorption Spectral Data of Free-3, Free-4, and
4@3 in C6H6 (1.0 ꢀ 10-5 M) at 296 K
free-3
free-4
4@3
λ
max(abs)
λ
max(abs)
(nm)
λmax(abs)
(nm)
(nm)
277
abs
abs
abs
0.454
274
349
361
382
0.041
0.042
0.042
0.041
277
348
361
382
0.521
0.031
0.031
0.033
energy transfer from the excited capsule 3 to the encapsu-
lated 4; i.e., the capsule 3 serves as a photosensitizer for the
encapsulated 4.12
FIGURE 3. Fluorescence emission spectra of free-3 (green line),
free-4 (blue line), and 4@3 (red line) upon excitation at (a) 360 nm
and (b) 285 nm (1.0 ꢀ 10-5 M in C6H6 at 296 K).
To further confirm that the capsule 3 serves as a photo-
sensitizer for the encapsulated 4, competitive encapsulation
experiments between 4 and other guests were carried out
(Scheme 2). It is known that 2,6-diacetoxy-9,10-anthraqui-
none 6 and 4,40-diacetoxybiphenyl 8 are good guests for 3
(Ka = 4.25 ꢀ 106 and 1.26 ꢀ 106 M-1, respectively, in C6D6
at 313 K).11b Thus, 6 and 8 are good competitors with respect
to 4(Ka = 1.83 ꢀ 106 M-1) for the encapsulation in 3. FigureS4
(Supporting Information) shows the fluorescence emission
spectra of free-3, free-6, free-8, 6@3, and 8@3 (1.0 ꢀ 10-5 M
in C6H6 at 296 K) upon excitation at 285 nm. Free-6 and free-
8 alone were almost nonemissive. Upon excitation at 285 nm,
the fluorescence emission intensity at 341 nm of 6@3 de-
creased by 83% relative to that of free-3, indicating that the
J. Org. Chem. Vol. 75, No. 18, 2010 6081

Nishimura and Kobayashi
JOCFeatured Article
TABLE 2. Fluorescence Emission Spectral Data of Free-3, Free-4, and
4@3 in C6H6 (1.0 ꢀ 10-5 M) at 296 K upon Excitation at 360 nm and
at 285 nm
free-3
free-4
4@3
λ (ex)
(nm)
λ
max(em)
λ
max(em)
(nm)
λmax(em)
(nm)
(nm)
341
int
int
int
360
285
394
415
209.5
240.5
393
413
341
392
413
191.0
223.1
39.0
222.6
253.1
233.1
395
416
7.4
8.5
FIGURE 4. Fluorescence excitation spectra of free-4 (blue line) and
M
in C6H6 at 296 K).
4@3 (red line) upon fluorescence emission at 410 nm (1.0 ꢀ 10-5
FIGURE 5. (a) Fluorescence emission spectra of free-4 (blue line),
4@3 (red line), a 1:1:1 mixture of 3, 4, and 6 (orange line), and a 1:1:1
mixture of 3, 4, and 8 (purple line) upon excitation at 285 nm (1.0 ꢀ
10-5 M each in C6H6 at 296 K). (b) Fluorescence excitation spectra
upon fluorescence emission at 410 nm (1.0 ꢀ 10-5 M each in C6H6 at
296 K).
TABLE 3. Fluorescence Excitation Spectral Data of Free-4 and 4@3 in
C6H6 (1.0 ꢀ 10-5 M) at 296 K upon Fluorescence Emission at 410 nm
free-4
4@3
λ (ex) (nm)
int
λ (ex) (nm)
int
285
348
360
381
253.3
200.4
196.9
176.3
escence emission spectra and spectral data of a 1:1:1 mixture
of 3, 4, and 6 or 8 (1.0 ꢀ 10-5 M each) in C6H6 at 296 K upon
excitation at 285 nm are shown in Figure 5a and Table S1
(Supporting Information), respectively. Their fluorescence
excitation spectra and spectral data upon fluorescence emis-
sion at 410 nm are also shown in Figure 5b and Table S2
(Supporting Information), respectively. Upon excitation at
285 nm, the fluorescence emission intensity at 413 nm from 4
in a 1:1:1 mixture of 3, 4, and 6 decreased by 57% relative to
that of 4@3 (Figure 5a). In contrast, the fluorescence emis-
sion intensity at 413 nm from 4 in a 1:1:1 mixture of 3, 4, and
8 decreased by only 19% relative to that of 4@3. This trend is
related to the order of the Ka values of the capsule 3 with
guests 8 < 4 < 6. These results clearly indicate that the
fluorescence emission from 4 in a 1:1 mixture of 3 and 4 upon
excitation at 285 nm occurs as a result of the intramolecular
energy transfer from the excited 3 to the encapsulated 4.
Photochemical Stability of 4@3. Figure 6 shows the
UV-vis absorption spectral changes of 4@3 and free-4 in
C6H6 (2.0 ꢀ 10-4 M) at 296 K as a function of irradiation
time, upon exposure to air and light (500 W ultra-high-
pressure mercury lamp), between 310 and 400 nm centering
at 365 nm. The 4 encapsulated in 3 almost remained intact
under photoirradiation after 2 h (Figure 6a). In marked
contrast, the absorption band of free-4 in C6H6 dramatically
decreased under the same photoirradiation conditions
(Figure 6b). Figure 7 shows the plots of absorption maxima
changes (Abs-t/Abs-0) of 4@3 and free-4 at 382 nm as a
348
359
381
208.6
206.2
181.7
SCHEME 2. Competitive Encapsulation between 4 and Other
Guests upon Encapsulation in 3
fluorescence quenching of 3 is probably due to energy
transfer from the excited 3 to the encapsulated 6. On the
other hand, the fluorescence emission intensity at 341 nm of
8@3 increased slightly relative to that of free-3. The fluor-
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FIGURE 8. Photochemical changes of 4@3 and free-4 (1.0 ꢀ 10-3
M in C6D6 at 296 K) upon exposure to air and light (500 W ultra-
high-pressure mercury lamp), monitored by H NMR: photoirra-
1
diation of 4@3 containing a small amount of free-3 for (a) 0 h, (b)
0.5 h, (c) 1 h, and (d) 2 h and photoirradiation of free-4 for (e) 0 h, (f)
0.5 h, (g) 1 h, and (h) 2 h. The signals marked “e” and “f” indicate
peaks representing 4 encapsulated in 3 and free-3, respectively. The
representative signals of free-4, photodimers 5 (50), and anthraqui-
none 6 are marked with open circles, solid circles, and open squares,
respectively. The signals marked “s” are the spinning sidebands of
the residual solvent (C6D5H).
FIGURE 6. UV-vis absorption spectral changes of (a) 4@3 and
(b) free-4 (2.0 ꢀ 10-4 M in C6H6 at 296 K) as a function of irradia-
tion time (10 min each) during the period of 2 h upon exposure to air
and light between 310 and 400 nm centering at 365 nm (500 W ultra-
high-pressure mercury lamp).
(videinfra). Itisnotedthat5, 50, and6werenotproduced from
4@3 even after 4 h photoirradiation (Figure S5, Supporting
Information). Under fluorescent light (room light) and in air
the 4@3 in C6D6 was also stable and remained fully intact for
at least 10 days (Figure S6, Supporting Information). These
results clearly indicate that the capsule 3 serves as a guard
nanocontainer, by itsencapsulation of4, to protect against the
photochemical reactions of 4 (Figure 1b).13
At the concentrations used in our experiments, 3 maintains
100% encapsulation of 4 upon addition of 1 equiv of 4, and
4@3 does not release 4. The capsule 3 can encapsulate only one
molecule of 4, but not two molecules of 4. Therefore, 3 protects
against the formation of photodimers 5 and 50 by the encapsu-
lation of 4. The anthraquinone 6 is formed via 9,10-endoper-
oxide 7 from 4. The 4@3 possesses four equatorial portals
through which singlet oxygen, as the source of the formation of
7, can come into contact with 4 encapsulated in 3. However, 7
has a bent molecular shape due to sp3 carbon atoms at the 9,10-
positions. Therefore, 7 and a transition state to reach 7 would
not fit the inner cavity of 3 for encapsulation. Thus, 3 protects
against the formation of 6 via 7 by the encapsulation of 4.
In the UV-vis absorption spectra of 4@3 under photoirra-
diation (Figure 6a), the absorption in the region between 400
and 550 nm gradually increased, especially after photoirradia-
tion for 1.5 h. This arises from the decomposition of the linker
unit 2 of the capsule 3, probably due to photooxygenation of
the catechol (or catecholate) moiety of the 2 unit to the o-
semiquinone radical (or o-semiquinonate) or o-quinone.16
FIGURE 7. Plots of absorption maxima changes (Abs-t/Abs-0) of
4@3 (red) and free-4 (blue) at 382 nm (2.0 ꢀ 10-4 M in C6H6 at
296 K) as a function of irradiation time upon exposure to air and
light (500 W ultra-high-pressure mercury lamp).
function of irradiation time under the same photoirradiation
conditions. Figure 8 shows the 1H NMR spectral changes of
4@3 and free-4 in C6D6 (1.0 ꢀ 10-3 M) at 296 K under the
same photoirradiation conditions. After photoirradiation
for 1 h, 4@3 remained almost intact (Figure 8c), whereas
free-4 dramatically decreased by 87% and new signals
appeared (Figure 8g), indicating the formation of photodimers
5 and 50 as a result of [4πþ4π] photocycloaddition of 4 and
2,6-diacetoxy-9,10-anthraquinone 6 via 9,10-endoperoxide 7
as a result of [4πþ2π] photooxygenation of 4 (Figure 1d)
(16) (a) Hong, B. H.; Bae, S. C.; Lee, C.-W.; Jeong, S.; Kim, K. S. Science
2001, 294, 348–351. (b) Sato, O.; Cui, A.; Matsuda, R.; Tao, J.; Hayami, S.
Acc. Chem. Res. 2007, 40, 361–369.
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JOCFeatured Article
FIGURE 10. 1H NMR spectra for the photochemical changes of
free-4 (1.0 ꢀ 10-3 M in C6D6 at 296 K) under fluorescent light (room
light) and in air: (a) 0 d, (b) 1 d, (c) 3 d, (d) 5 d, (e) 8 d, and (f) 10 d.
The representative signals of free-4, 5 (50), 6, and 9,10-endoperoxide
7 are marked with open circles, solid circles, open squares, and solid
squares, respectively. The signals marked “s” are the spinning
sidebands of the residual solvent (C6D5H).
and in air, a mixture of 5 and 50 changed to a mixture of 4, 5
(50), and 6 in a 5:89:6 ratio and a 14:43:43 ratio after 1 and 2 h
of photoirradiation, respectively (Figure S7, Supporting Infor-
mation). After 4 h of photoirradiation, 4 and 5 (50) were fully
converted to 6. This result indicated that 5 and 50 reversibly
returned to the monomer 4 under this condition and that 6
was formed not directly from 5 but from 4 and 7. Under
fluorescent light and in air, 5 and 50 were stable and remained
fully intact for at least 10 days (Figure S8, Supporting
Information).
FIGURE 9. UV-vis absorption spectral changes of (a) guest-free 3
and (b) 2 alone (2.0 ꢀ 10-4 M in C6H6 at 296 K) as a function of
irradiation time (10 min each for 2 h in total) upon exposure to air
and light (500 W ultra-high-pressure mercury lamp). The concen-
tration of the linker unit 2 of 3 corresponds to 8.0 ꢀ 10-4 M because
3 has four molecules of 2 unit.
Figures 9a and 9b show the UV-vis absorption changes
of guest-free 3 and 2 alone, respectively, in C6H6 at 296 K,
as a function of irradiation time upon exposure to air and
light (500 W ultra-high-pressure mercury lamp); the colorless
solutions gradually became yellow. Photooxygenation of 2
alone was more noticeable than that of free-3.
Conclusion
We have found that the self-assembled boronic ester cavit-
and capsule 3 serves as a photosensitizer for the encapsulated
2,6-diacetoxyanthracene 4 upon excitation at 285 nm, as a
result of the energy transfer from the excited 3 to the encap-
sulated 4. Upon excitation at 285 nm, the 4 encapsulated in 3
in C6H6 showed strong fluorescence emission, while 4 alone
in C6H6 exhibited very weak emission. We also demonstrated
that 3 serves as a guard nanocontainer, by encapsulation of
4, to protect against the [4πþ4π] photodimerization and
[4πþ2π] photooxygenation of 4. Compound 4 encapsulated
in the capsule 3 in C6D6 at 296 K in air is stable upon
exposure to an ultra-high-pressure mercury lamp for 4 h and
fluorescent light (room light) for 10 days. In marked con-
trast, free-4 in C6D6 at 296 K in air is unstable under the same
photoirradiation conditions; it affords the photodimers 5
and 50 and the anthraquinone 6 via 9,10-endoperoxide 7. The
capsule 3 alters the excited-state chemistry of 4 by encapsu-
lating 4 within 3 and suppressing its most favored solution
pathway.9b,d Thus, the guest-encapsulating capsule 4@3
serves as a photostable luminescence material.
Photochemical Reactions of Free-4. As mentioned above,
free-4 in C6D6 (1.0 ꢀ 10-3 M) at 296 K is unstable under the
photoirradiation conditions used (light and air) (Figure 8e-
h). Compound 4 alone in C6D6 dramatically changed to a
mixture of 4, 5 (50), and 6 in a 31:35:34 ratio and a 13:28:59
ratio after 0.5 and 1 h of photoirradiation, respectively. After
2 h of photoirradiation, 4 and 5 (50) were fully converted to 6.
During the course of the photoirradiation, 9,10-endoperox-
ide 7 was not observed. Figure 10 shows the 1H NMR spectra
of the photochemical changes of 4 alone under fluorescent
light and in air, wherein 7 was observed in addition to 5 (50)
and 6. Free-4 changed to a mixture of 4, 5 (50), 6, and 7 in a
51:19:3:27 ratio and a 36:32:7:25 ratio after 3 days and 5 days
of photoirradiation, respectively. Additional photoirradia-
tion led to a decrease in 4 and 7 and an increase in 5 (50) and 6.
The ratio of 4, 5 (50), 6, and 7 changed to 11:47:33:9 after
10 days of photoirradiation. The photodimers 5 and 50, as
regioisomers, were formed in a 51:49 ratio (Figures S9 and
S10, Supporting Information).
A study of the encapsulation and protection of other
photoluminescence materials in 3, such as 2,6-diacetoxy-
9,10-bis(phenylethynyl)anthracene, is currently underway
in our laboratory.
The stability of the isolated mixture of 5and 50 in C6D6 (1.0 ꢀ
10-3 M) at 296 K under photoirradiation was also investi-
gated. Under light (500 W ultra-high-pressure mercury lamp)
6084 J. Org. Chem. Vol. 75, No. 18, 2010

Nishimura and Kobayashi
JOCFeatured Article
Experimental Section
128.02 (127.80), 120.43 (120.24), 118.44, 52.80 (52.78), 21.12
(21.09).
General Methods. 1H and 13C NMR spectra were recorded at
400 and 100 MHz, respectively. Photoirradiation was conducted
with an ultra-high-pressure mercury lamp (500 W) through a
combination of a color filter and a heat-absorbing filter for light
between 310 and 400 nm centering at 365 nm. The syntheses of
tetrakis(dihydroxyboryl) cavitand 1, 1,2-bis(3,4-dihydroxyphenyl)-
ethane 2, boronic ester cavitand capsule 3, and guests 4, 6, and 8
have previously been reported.11 trans-1,2-Bis(3,4-dimethoxyp-
henyl)ethene as a precursor of 2 was also synthesized by the
McMurry coupling of 3,4-dimethoxybenzaldehyde according to
the literature.17
Photodimer 5 and Its Regioisomer 50. A solution of 2,6-
diacetoxyanthracene 4 (40.0 mg, 0.136 mmol) in benzene (136
mL) was stirred at room temperature for 1 h under light (500 W
ultra-high-pressure mercury lamp) and in air. After evaporation
of solvent, the residue was purified by recycle preparative GPC
to give a 51:49 mixture of 5 and 50 (17.6 mg, 44% yield) as an off-
white solid (Figures S9 and S10, Supporting Information) and 6
(20.5 mg, 46% yield) (Figure S11, Supporting Information). 5:
1H NMR (C6D6) δ 6.74 (d, J = 2.0 Hz, 4H), 6.71 (dd, J = 2.0
and 8.3 Hz, 4H), 6.70 (d, J = 8.3 Hz, 4H), 4.02 (s, 4H), 1.69 (s,
12H); 1H NMR (CDCl3) δ 6.89 (d, J = 8.3 Hz, 4H), 6.72 (d, J =
2.4 Hz, 4H), 6.59 (dd, J = 2.4 and 8.3 Hz, 4H), 4.51 (s, 4H), 2.22
(s, 12H). 50: 1H NMR (C6D6) δ 6.83 (d, J = 2.0 Hz, 4H), 6.68
(dd, J = 2.0 and 8.3 Hz, 4H), 6.59 (d, J = 8.3 Hz, 4H), 4.01 (s,
4H), 1.72 (s, 12H); 1H NMR (CDCl3) δ 6.91 (d, J = 8.3 Hz, 4H),
6.69 (d, J = 2.4 Hz, 4H), 6.58 (dd, J = 2.4 and 8.3 Hz, 4H), 4.51
(s, 4H), 2.21 (s, 12H). 5 and 50: 13C NMR (CDCl3) δ 169.28
(169.26), 148.60 (148.52), 144.32 (144.29), 140.14 (140.07),
9,10-Endoperoxide 7. The transformation of 2,6-diacetoxyan-
thracene 4 (1.0 ꢀ 10-3 M) to its 9,10-endoperoxide 7 was carried
out in CDCl3 instead of benzene at room temperature under
fluorescent light (room light) and in air because this reaction in
CDCl3 gave a mixture of 4 and 7 in a 47:53 ratio after 5 days of
photoirradiation (Figure S12a, Supporting Information),
whereas the reaction in C6D6 gave a mixture of 4, 5 (50), 6, and
7 even after 3 days of photoirradiation (Figure 10c).
A solution of 4 (30.0 mg, 0.102 mmol) in CDCl3 (10 mL) was
stirred at room temperature for 5 days under fluorescent light
and in air. At this higher reaction concentration (10 ꢀ 10-3 M),
the reaction gave a mixture of 4, 5 (50), and 7 in a 27:2:71 ratio
(Figure S13, Supporting Information). After evaporation of
solvent, the residue was subjected to recycle preparative GPC
to separate 7, 5 (50), and 4. However, 7 was decomposed to 6
during the course of this treatment: 1H NMR (CDCl3) δ 7.42 (d,
J = 7.8 Hz, 2H), 7.21 (d, J = 2.0 Hz, 2H), 7.02 (dd, J = 2.0 and
7.8 Hz, 2H), 6.02 (s, 2H), 2.31 (s, 6H); 1H NMR (CD2Cl2) δ 7.45
(d, J = 7.8 Hz, 2H), 7.21 (d, J = 2.4 Hz, 2H), 7.02 (dd, J = 2.4
and 7.8 Hz, 2H), 6.04 (s, 2H), 2.28 (s, 6H); 1H NMR (C6D6) δ
6.90 (d, J = 2.0 Hz, 2H), 6.81 (dd, J = 2.0 and 8.3 Hz, 2H), 6.77
(d, J = 2.0 Hz, 2H), 5.44 (s, 2H), 1.74 (s, 6H); 13C NMR (CDCl3)
δ 169.20, 150.11, 139.23, 134.91, 124.83, 120.77, 117.54, 78.61,
21.09.
Acknowledgment. This work was supported in part by a
Grant-in-Aid from JSPS (No. 22350060).
Supporting Information Available: Synthetic procedures of
9 and 10 and additional spectral data. This material is available
free of charge via the Internet at http://pubs.acs.org.
(17) Shadakshari, U.; Rele, S.; Nayak, S. K.; Chattopadhyay, S. Indian J.
Chem. 2004, 43B, 1934–1938.
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