Enhanced head-to-head photodimers in the photocyclodimerization of anthracenecarboxylic acid with a cationic pillar[6]arene

Article abstract of DOI:10.1016/j.cclet.2016.04.021

The complexation behaviors of anthracenecarboxylic acid and water-soluble cationic pillararenes have been investigated by¹H NMR, UV–vis and ITC methods. The cationic pillar[6]arene was found to stepwise form 1:1 and 1:2 complexes, having a larg

Full text of DOI:10.1016/j.cclet.2016.04.021

G Model
CCLET 3686 1–5
Chinese Chemical Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
1
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Original article
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Enhanced head-to-head photodimers in the photocyclodimerization of
anthracenecarboxylic acid with a cationic pillar[6]arene
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Q1 Jian-Chang Gui a,1, Zhi-Qiang Yan a,1, Yuang Peng b, Ji-Gao Yi a, Da-Yang Zhou c, Dan Su a,
Zhi-Hui Zhong a, Guo-Wei Gao a, Wan-Hua Wu a, Cheng Yang a,
*
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a Key Laboratory of Green Chemistry & Technology, College of Chemistry, State Key Laboratory of Biotherapy, West China Medical Center and State Key
Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China
b Chengdu Environmental Monitoring Center, Chengdu 610042, China
c Comprehensive Analysis Center, ISIR, Osaka University, Osaka 5670047, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
The complexation behaviors of anthracenecarboxylic acid and water-soluble cationic pillararenes have
been investigated by 1H NMR, UV–vis and ITC methods. The cationic pillar[6]arene was found to
Received 21 March 2016
Received in revised form 7 April 2016
Accepted 14 April 2016
Available online xxx
stepwise form 1:1 and 1:2 complexes, having
a large K1 and a relatively small K2 values.
Photocyclodimerization of AC within the pillar[6]arene improved the yield of the head-to-head
photodimers. Up to 4.97 HH/HT ratio has been reached by optimizing the reaction conditions.
ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Keywords:
Pillar[6]arene
Photocyclodimerization
Anthracenecarboxylic acid
Supramolecular complexation
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12
1. Introduction
photosubstrates, such as g-cyclodextrins (CD) [4–7], crown ethers 30
[4], coordinated cages [8,9], cucurbiturils [10–12], templates [13] 31
and biomolecules [14], have been employed as host molecules for 32
conducting photodimerizations. Recently, pillar[n]arenes, a new 33
family of macrocyclic compounds composed by several 1,4- 34
disubstituted hydroquinone ethers, have attracted significant 35
attention from chemists [15]. Their cavities are possible to 36
accommodate organic guest mainly through electrostatic dipole 37
interactions in organic solvent [16–21]. Up to now, pillar[n]arenes 38
comprising 5–15 hydroquinone ether units have been explored [22– 39
24]. This makes pillar[n]arenes versatile hosts capable of binding 40
guest molecules of different sizes. On the other hand, water-soluble 41
pillar[n]arenes, synthesized by suitable chemical modification on 42
the rims of pillar[n]arenes, make the intriguing host molecules 43
possible to complex a wide range of organic guests through 44
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27
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29
Manipulating the chemo- and regio-selectivity of photochemical
reactions through supramolecular complexation is an intriguing
topic of current photochemistry. Photosubstrates at the electronic
excitedstatearefeaturedbyhighreactivityandshortlifetime, which
makes it difficult to control the selectivity of photoreactions
[1]. Supramolecular complexation could orientate substrates in
confined spaces, make reaction centre spatially close to the catalytic
site and stabilize their high-energy transition states. Photosub-
strates complexed in the cavity of molecular host often show
switched photophysical and photochemical properties [2]. Conse-
quentially, supramolecular complexation provides a promising
strategy to affect the rate and selectivity of photoreactions.
Intermolecular photochemical reactions demand suitable size and
reasonable driving force of binding site of the host to arrange two
photosubstrates together. In this context, controlling the reaction
selectivity ofphotodimerizationaremore challenging [3]. Molecular
hosts bearing a large cavity suitable for accommodating two
hydrophobic interaction [25,26].
We have comprehensively investigated the photocyclodimer- 46
ization of anthracenecarboxylic acid (AC) by using -cyclodextrin 47
45
g
(CD) [27–31], bio-macromolecules [32], chiral templates [33,34] as 48
well as coordinated cages [35] as host molecules. Photocyclodimer- 49
ization of AC affords anti- and syn-head-to-tail (HT) photodimers 1 50
and 2 (Scheme 1) accompanying by the anti- and syn-head-to-head 51
(HH) photodimers 3 and 4. Among different types of host molecules, 52
*
Corresponding author.
E-mail address: yangchengyc@scu.edu.cn (C. Yang).
zThese Authors contributed equally to this work.
1
g-CD derivatives have been most extensively employed, because 53
http://dx.doi.org/10.1016/j.cclet.2016.04.021
1001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: J.-C. Gui, et al., Enhanced head-to-head photodimers in the photocyclodimerization of
anthracenecarboxylic acid with a cationic pillar[6]arene, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.021

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J.-C. Gui et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
2.2. General preparation procedure and characterization for target
compounds
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Compound 5: Hydroquinone (10.0 g, 91 mmol), 1,2-dibro-
97
moethane (35 mL, 0.41 mol) and potassium carbonate (40 g,
0.29 mol) were added into acetone (150 mL), the mixture was
refluxed for 24 h under N2. After the reaction mixture was cooled
down to room temperature, precipitate was removed by filtration.
The solvent was removed under reduced pressure and the product
was purified by column chromatography (eluent: hexane:
dichloromethane = 1:1). A white solid was obtained (6.3 g, 21%).
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149
150
1H NMR (400 MHz, CDCl3):
d 6.86 (s, 4H), 4.24 (t, 4H, J = 6.3 Hz),
3.61 (t, 4H, J = 6.3 Hz). 13C NMR (101 MHz, CDCl3):
d 152.81,
116.07, 77.36, 77.04, 76.72, 68.69, 29.30.
Compound 6a:
A mixture of 5 (3.24 g, 10 mmol) and
paraformaldehyde (0.93 g, 30 mmol) in 1,2-dichloroethane
(20 mL) was stirred at room temperature for 30 min. BF3ꢀEt2O
(1.25 mL, 10 mmol) was added and the reaction mixture was
stirred for additional 30 min. The reaction mixture was washed
with water three times, and the organic phase was concentrated
and the product was purified by column chromatography (SiO2;
Petroleum ether/CH2Cl2/EA, 2:1:0.03) to give a white solid (1.18 g,
Scheme 1. Photocyclodimerization of AC with water-soluble pillararenes WP5 and
WP6.
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77
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79
80
g-CD has a large cavity that can simultaneously accommodate two
AC molecules. The photocyclodimerization of AC is thus accelerated
by a factor of 10 to show reaction selectivity significantly
35%). 1H NMR (400 MHz, CDCl3):
J = 5.6 Hz), 3.84 (s, 10H), 3.64 (t, 20H, J = 5.6 Hz). 13C NMR
(101 MHz, CDCl3): 149.66, 129.06, 116.09, 77.36, 77.05, 76.73,
d 6.92 (s, 10H), 4.23 (t, 20H,
different from those observed in homogeneous solution [5].
g
-CD
d
determines the photoreaction outcome through the formation of
1:2 complexes, and AC pairs of different stacking pattern in the
1:2 complexes lead to corresponding photodimers upon photo-
excitation [36–38]. Photocyclodimerization of AC has become a
model photochemical reaction for evaluating the supramolecu-
lar complexation between AC and host molecule, which provides
the detailed stacking model of AC pairs in the host cavity through
the analyses of the population of photodimers. Photocyclodi-
merization of AC in aqueous solution usually prefers the HT
photodimers 1 and 2 due to the electrostatic repulsion for HH
68.97, 53.43, 30.75, 29.40.
Compound 6b: 5 (2 g, 6.17 mmol), paraformaldehyde (926 mg,
30.85 mmol) and FeCl3 (200 mg, 1.24 mmol) were added to CHCl3
(90 mL), and the mixture was heated to 45 8C for 72 h. The mixture
was cooled down to room temperature and then washed with
water three times, the organic phase was concentrated and
subjected to column chromatography (SiO2; Petroleum ether/
CH2Cl2/EA, 2:1:0.06). Finally, a white solid was obtained (520 mg,
25%). 1H NMR (400 MHz, CDCl3):
J = 5.8 Hz), 3.87 (s, 12H), 3.56 (t, 24H, J = 5.8 Hz). 13C NMR
(101 MHz, CDCl3): 150.19, 128.53, 115.84, 77.37, 77.05, 76.73,
68.97, 30.66, 30.35.
d 6.78 (s, 12H), 4.17 (t, 24H,
photodimers. Complexation with
g
-CD led to an enhancement of
d
the HT photodimers, and the HH photodimers were given in poor
yield of < 15%. Therefore, to improve the yield of HH photo-
dimers 3 and 4 are more challenging. It occurred to us that
introduction of cationic groups on the two rims of a pillararene
will make pillararenes water-soluble, and thus extend the ability
of pillararenes to complex a wide range organic guest through
hydrophobic interaction. More importantly, the presence of
cationic groups will improve the electrostatic attraction and
consequently reduce the electrostatic repulsion between car-
boxylate anions of HH-stacked AC pairs. In this study, we report
our efforts to improve the HH photodimers of AC by using the
water-soluble pillar[6]arene (WP6).
WP5: Trimethylamine (2 mL, 33% in water) was added to 15 mL
DMF solution containing 6a (300 mg, 0.18 mmol), and the resulting
mixture was heated to 80 8C for 24 h. After cooling down to room
temperature, the solvent was removed and the residue was
dissolved in water, and the solution was filtered and applied on a
reversed-phase column. After lyophilization, a white solid was
obtained (350 mg, 86%). 1H NMR (400 MHz, D2O):
d 6.93 (s, 10H),
4.44 (s, 20H), 3.91 (s, 10H), 3.80 (s, 20H), 3.23 (d, 90H, J = 15.7 Hz).
13C NMR (101 MHz, D2O):
59.55, 54.05, 29.51.
d 149.28, 129.84, 116.42, 64.89, 63.41,
WP6: Trimethylamine (0.5 mL, 33% in water) was added to a
solution of 6b (100 mg, 0.05 mmol) in DMF (5 mL), and the
resulting mixture was heated to 80 8C for 24 h. After cooling down
to room temperature, the solvent was removed under vacuum and
the solid was dissolved in water. The resulted solution was
membrane-filtered and applied on a reversed-phase column. After
lyophilization, a white solid was obtained (115 mg, 85%). 1H NMR
81
82
2. Experimental
2.1. Materials and instruments
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2-Anthracenecarboxylic acid (AC) was purchased from TCI
(China) and used as received. Doubly distilled water and HPLC
grade solvents were used for photoreactions and spectral measure-
ments. Other solvents were purchased from Wako Pure Chemical
Industries, Ltd. 1H NMR and 13C NMR spectra were measured at 400
and100 MHz, respectively, ona BrukerDRX-400instrument. HR-MS
were obtained by using the Shimadzu LCMS-IT TOF (ESI) spectrom-
eter. UV–visible spectra were recorded on a JASCO V650 spectro-
photometer. Fluorescence measurements were carried out by using
a JASCO-FP 8500 spectrofluorimeter. Photoproducts were analyzed
by using a Shimadzu LC Prominence 20 HPLC instrument equipped
with UV–vis and fluorescence detectors.
(400 MHz, D2O): d 6.84 (s, 12H), 4.43 (s, 24H), 3.88 (s, 14H), 3.69 (s,
24H), 3.04 (s, 108H). 13C NMR (101 MHz, D2O):
116.17, 65.07, 63.36, 59.60, 54.28, 30.13.
d 149.73, 129.02,
3. Results and discussion
151
The synthesis of the cationic pillararenes WP5 and WP6 was
represented in Scheme 2, which were synthesized following a
modified procedure given in the previous reports [39–41]. The
chemical structures of WP5 and WP6 were identified by HR mass
and 1H NMR and 13C NMR spectroscopic examinations. Both WP5
and WP6 are well soluble in aqueous solution due to the presence
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157
Please cite this article in press as: J.-C. Gui, et al., Enhanced head-to-head photodimers in the photocyclodimerization of
anthracenecarboxylic acid with a cationic pillar[6]arene, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.021

G Model
CCLET 3686 1–5
J.-C. Gui et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
3
Scheme 2. The synthesis of the cationic pillararenes WP5 and WP6.
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of a large amount of ammonium groups. In view of the
hydrophobicity of hydroquinone ether units, we deduced that
WP5 and WP6 can accommodate AC molecules in aqueous
solution mainly through the hydrophobic interactions. The
framework of pillararenes, with benzene rings linked by methylene
group, are relatively rigid. The tethers grafted on the rims form a
flexible hydrophobic wall, which should jointly function in
complexation. On the other hand, the electrostatic interaction
between ammonium cations and carboxylate anion of AC should
also play an important role for the complexation. To understand the
binding behavior between cationic pillararenes and AC, UV–vis, 1H
NMR spectroscopies and Isothermal titration calorimetry (ITC) were
carried out (Scheme 3).
The 1H NMR titration of AC with WP6 clearly demonstrates the
formation of the host–guest complex between AC and WP6. As
shown in Fig. 1, adding WP6 into the aqueous solution of AC led to
an evident upfield shift of the proton signals of AC. The largest
change was seen for the protons at the 90-H and 100-H of the central
nucleus of AC, exhibiting a shift larger than 0.7 ppm (Fig. 1),
accompanied by a broadening of 90-H and 100-H protons. This could
be reasonably rationalized by the shielding effect from the
aromatic rings of WP6. On the other hand, the singlet aromatic
proton of WP6 presents a downfield shift with the concentration of
WP6, demonstrating that the wall of WP6 is suffering the shielding
effect from AC rings.
Fig. 1. Partial 1H NMR of a) 0.2 mmol/L AC, b) 0.2 mmol/L AC + 0.2 mmol/L WP6, c)
0.2 mmol/L AC + 1.0 mmol/L WP6 and d) WP6 in pD = 9.0 D2O solutions.
1.0
[WP6]/[AC]
0.8
0.6
0.4
0.2
0.0
4.0
0
325
350
375
400
425
450
Wavelength (nm)
Fig. 2. UV–vis spectral changes of AC upon increasing the concentration of WP6
measured in aqueous solution at 25 8C.
The complexation between cationic pillararenes and AC was also
confirmed by the UV–vis spectral studies. As exemplified in Fig. 2,
UV–vis spectral change in the wavelength range of 310–450 nm
were carried out by keeping the concentration of AC constant and
varying the concentration of WP6 in aqueous solution at 25 8C.
Increasing the concentration of WP6 resulted in an apparent
bathochromic shift and the band broadening of the 1La transition.
Such a UV–vis spectral variation is similar to that observed in the
ITC titration of AC with WP5 based on 1:1 complexation model 197
at 25 8C showed a K1 value of 5.47 ꢁ 103 Lꢀmolꢂ1 (Fig. S13 in 198
Supporting information). This is
a typical binding affinity 199
commonly observed by artificial host–guest complexation driven 200
mainly by hydrophobic interaction. The binding is an enthalpy- 201
and entropy-favored process, showing
a
D
H
value of 202
ꢂ13.3 kJꢀLꢀmolꢂ1 and
D
S of 27.0 JꢀKꢂ1ꢀLꢀmolꢂ1. On the other hand, 203
complexation between ACand
1La band is ascribed to the formation of the 1:2 complex between
-CD andAC[5]. Relativelysmallerbathochromicshiftwithoutband
g-CD, where the significant change of
ITC titration of AC with WP6 on the basis of 1:1 and 1:2 204
complexation model offered stepwise binding constants 205
K1 = 1.21 ꢁ 104 Lꢀmolꢂ1 and K2 = 94 Lꢀmolꢂ1 (Fig. 3). This result 206
g
broadening was seen in the UV–vis titration with WP5, which has a
smaller cavity and is expected to accommodate only one AC
molecule.
remarkably differs from the complexation of AC with g-CD, which 207
shows a much smaller K1 with an overwhelmingly higher K2 value 208
[5]. While the
the
higher than the
D
S1 (27.1 JꢀKꢂ1ꢀLꢀmolꢂ1) value is comparable with 209
D
S value of WP5, The
D
H1 (ꢂ15.2 kJꢀLꢀmolꢂ1) is relatively 210
D
H value of WP5, for which more active water 211
molecules released from the cavity of WP6 is possibly responsible. 212
We deduce that the small K2 value observed with WP6 is due to 213
that the cavity of WP6 is relatively crowd for accommodating the 214
second AC molecule, and the secondly-coming AC may position 215
mainly in the hydrophobic environment formed by flexible tethers. 216
The inclusion of the second AC molecule in the cavity of WP6 217
should cause significant loss of rotational and motional freedom of 218
Scheme 3. Stepwise 1:1 and 1:2 complexation between WP6 and AC.
Please cite this article in press as: J.-C. Gui, et al., Enhanced head-to-head photodimers in the photocyclodimerization of
anthracenecarboxylic acid with a cationic pillar[6]arene, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.021

G Model
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J.-C. Gui et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
Time (min)
of AC, showing a combined yield of 75.4% (entry 1). Photo-
cyclodimerization of AC with WP5 offers similar product
distribution to that in the absence of any host molecule. This is
reasonable because the cavity of WP5 is too small to include two
AC molecules and photocyclodimerization of AC occurs mainly
with free AC. The yield ratio of HH photodimers versus HT
photodimers (HH/HT ratio) in the absence of host is 0.30 (entry 1).
With WP6, the yield of HH photodimers was greatly improved,
showing a HH/HT ratio of 0.75, for which the reduced electrostatic
repulsion due to the interaction from the electrostatic attraction
from cationic ammonium should be responsible.
In order to improve the HH photodimers by reducing the
electrostatic repulsion between carboxylate of AC, we attempted
the strategy of adding salt additive. Indeed, with all salts tried
(entries 4, 6 and 10), the yields of HH photodimers are much higher
than that obtained in aqueous buffer solution without salt additive.
Addition of WP6 further enhanced the yield of HH photodimers,
demonstrating the importance of supramolecular complexation on
the photoreaction selectivity. In 1.0 mol/L NH4Cl aqueous solution
at 0.5 8C, the HH/HT ratio was improved to 2.09 (entry 7) by
WP6. Moreover, raising the temperature lead to further increase of
the yield of HH photodimers, and a HH/HT ratio of 4.97 (entry 9)
was obtained at 40 8C in 1.0 mol/L NH4Cl in the presence of WP6.
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-10 0 10 20 30 40 50 60 70 80 90 100110
0.0
-0.5
-1.0
-1.5
-2.0
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
Data: WP6AC_NDH
Model: TwoSites
Chi^2 = 336.1
N1 0.970
0
K1 1.21E4 1.2E3
ΔH1-3641 184
ΔS1 6.47
N2 0.490
K2 94.4
ΔH2-1.949E4 6.12E3
ΔS2 -56.3
0
0
0
1
2
3
4
5
6
Molar ratio
4. Conclusion
260
Fig. 3. ITC titration of WP6 into the aqueous solution of AC.
In conclusion, we have demonstrated that water-soluble WP6
can form 1:2 complex with AC. The photocyclodimerization of AC
with the water-soluble WP6 significantly improves the inherently
unfavorable HH photodimers. By optimizing the reaction condi-
tion, up to 4.97 HH/HT ratio has been reached by using WP6 as a
host. This study opens a window to investigate intermolecular
photoreaction using the new host molecule of pillararenes. The
mechanism and the detailed effect of salt additive and temperature
in this supramolecular photoreaction system are under study.
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234
235
236
AC and WP6, and therefore results in a large entropic loss
(
D
S2 = ꢂ235 JꢀKꢂ1ꢀLꢀmolꢂ1).
Photolyses of AC in the absence and presence of a pillararene
have been carried out in aqueous buffer solution (pH 9.0) with an
LED lamp at 365 nm. The reaction was monitored by tracing the
UV–vis absorption change of AC. Unexpectedly, the photodimer-
ization of AC in the presence of WP6 is slower than that in the
absence of any host molecule. The observed reaction rate
constants, by regarding the reaction system as a simple second
order reaction, were calculated to be 95 Lꢀmolꢂ1ꢀsꢂ1 and
24 Lꢀmolꢂ1ꢀsꢂ1, respectively, corresponding to the photoreactions
in the absence and presence of WP6. This result implies that the
1:2 complex does not accelerate but rather inhibit the reaction.
Although the reason for this is not yet clear, it is possibly due to the
bad matching of the AC’s photoreactive 9 and 10 positions in the
cavity of WP6.
Acknowledgment
270
This work was supported by the grants from National Natural
Science Foundation of China (Nos. 21372165, 21321061 and
21572142 for CY, No. 21402129 for WW) and State Key Laboratory
of Polymer Materials Engineering (No. sklpme2014-2-06). We
thank Prof. Ye Tao and Dr. Yan Huang of BSRF for assistance during
the use of UV light sources.
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272
273
274
275
276
As shown in Table 1, in the absence of any host molecule at 0.5 8C,
the HT photodimers 1 and 2 dominate the photocyclodimerization
Table 1
Photocyclodimerization of AC mediated by water-soluble pillar[n]arenes.a
Appendix A. Supplementary data
277
Entry Solution
Host Temp. Relative yield (%)
HH/HTb anti/syn
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cclet.2016.04.021.
278
279
/8C
1
2*
3*
4
1/2 3/4
1
2
3
4
Aqu. bufferc No
0.5
WP5 0.5
WP6 0.5
37.9 39.3 13.4 9.5 0.30
36.2 33.9 17.0 12.9 0.43
28.2 29.0 21.5 21.5 0.75
32.8 36.1 15.2 16.0 0.45
0.96 1.41
1.07 1.31
0.97 1.00
0.91 0.95
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280
1.0 mol/L
NaCl
No
0.5
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WP6 0.5
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17.6 20.2 28.5 33.7 1.65
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WP6 0.5
WP6 25
WP6 40
15.8 16.5 31.0 36.7 2.09
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CsCl
No
0.5
11
WP6 0.5
21.1 20.0 26.6 32.4 1.44
1.06 0.82
a
[AC] = 0.2 mmol/L, [WP5] = [WP6] = 2.0 mmol/L. Irradiation at 365 nm using a
LED lamp for 30 min.
b
[3 + 4]/[1 + 2].
c
Q2
pH 9.0 phosphate buffer solution (25 mmol/L).
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anthracenecarboxylic acid with a cationic pillar[6]arene, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.021

G Model
CCLET 3686 1–5
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anthracenecarboxylic acid with a cationic pillar[6]arene, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.021