A pH-responsive molecular capsule with an acridine shell: Catch and release of large hydrophobic compounds

Article abstract of DOI:10.1039/c6cc09094j

Unlike common polyaromatic hydrocarbons, acridine is a characteristic compound bearing both π-stackable large surfaces and a protonable nitrogen atom. Here we report the first synthesis of a supramolecular capsule with multiple acridine panels. In water, the assembly and disassembly of the capsule reversibly occur under neutral and acidic conditions, respectively (≥10 cycles). Notably, the pH-responsive capsule encapsulates a variety of large hydrophobic compounds (up to 1.6 nm in diameter) such as coumarins, metallophthalocyanines and subphthalocyanine in neutral water and subsequently releases them by simple addition of acid.

Full text of DOI:10.1039/c6cc09094j

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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DOI: 10.1039/C6CC09094J
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A pH-Responsive Molecular Capsule with an Acridine Shell:
Catch and Release of Large Hydrophobic Compounds
Mai Kishimoto, Kei Kondo, Munetaka Akita, and Michito Yoshizawa*
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Unlike common polyaromatic hydrocarbons, acridine is a
characteristic compound bearing both !-stackable large
surfaces and a protonable nitrogen atom. Here we report the
first synthesis of a supramolecular capsule with multiple
acridine panels. In water, the assembly and disassembly of
the capsule reversibly occur under neutral and acidic
conditions, respectively (!10 cycles). Notably, the pH-
responsive capsule encapsulates
a
variety of large
hydrophobic compounds (up to 1.6 nm in diameter) such as
coumarins, phthalocyanines, and subphthalocyanine in
neutral water and subsequently releases them by simple
addition of acid.
452(-/)P2Q)65I50,+R35)G+<50+S2:+.1).;)21:*029515,)P35;:),+G5Q)21G)K0.:.12:+.1).;)290+G+15,)
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!-Stacking interactions are superior tools to construct well-
05,K.1,+I5)92K,/35)U+:*)</3:+K35)290+G+15)0+1H,):.U20G)320H5)*7G0.K*.R+9)9.<K./1G,)
PVQ@)
controlled supramolecular assemblies with characteristic photo-
properties.1,2
and
electrochemical
Various
columnar
nanostructures composed of polyaromatic rings (e.g.,
triphenylene, hexabenzocoronene, and phthalocyanine rings)
have been prepared through infinite !-stacking interactions.3
However, as compared with other non-covalent interactions
such as hydrogen-bonding interactions and hydrophobic effects,
!-stacking interactions themselves are less sensitive to changes
in external environment (e.g., temperature, light, and pH).
Anthracene is an exceptional polyaromatic compound capable
of changing its stereostructure upon light irradiation, reversibly
(Fig. 1a, left)4 but insensitive to pH change like most of other
polyaromatic rings. On the other hand, acridine is an
anthracene-shaped polyaromatic compound but provides both
!-stackable large surfaces and a reversibly protonable nitrogen
atom (Fig. 1a, right).5 We thus envisioned that the incorporation
of multiple acridine switches into supramolecular systems,
which has not been accomplished in contrast to that of
anthracene and other polyaromatic panels,6,7 could develop
novel pH-responsive polyaromatic nanostructures (Fig. 1b).
Although there are copious reports on stimuli-responsive
supramolecular cages and capsules consisting of aliphatic and
small aromatic frameworks, their host capabilies toward large
hydrophobic compounds (>1 nm) have been so far less
explored.8
pH-Responsive compound 1 designed here is composed of
two acridine rings and two trimethylammonium groups
connected by a meta-phenylene spacer (Fig. 2a). The simple
and rigid V-shaped framework is characteristic of this new
amphiphilic compound (Fig. 2b). Herein we report that bent
bisacridine amphiphiles 1 assemble into spherical capsule 2 in
water through !-stacking interactions and the hydrophobic
effect under neutral conditions, in a manner similar to
bisanthracene-based analogues.9 In marked contrast, under
acidic conditions, the neutral polyaromatic panels of 2 convert
to cationic ones due to protonation of the terminal nitrogen
atoms. Accordingly, the !-stacked polyaromatic capsule can
disassemble into monomeric species (1’) by electrostatic
repulsion (Fig. 1b). In addition, we demonstrate, by the
combination of the dynamic behavior and host capability, the
catch and release of various hydrophobic compounds with
diameters of 0.6-1.6 nm (e.g., coumarins, phthalocyanines, and
subphthalocyanine) by pH-responsive capsule 2 in water at
ambient temperature.
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View Article Online
DOI: 10.1039/C6CC09094J
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92:+.1+9)K20:).K:+<+S5G)R7)LX()9239/32:+.1)PWCYZFO[DCBV\)35I53Q@)
Six-step syntheses including Negishi cross-coupling
reaction afforded amphiphilic compound 1 in ~30% overall
yield and the structure of the obtained product was
characterized by NMR and ESI-TOF MS analyses (Fig. S1-
18).10 In neutral water (2.0 mL), compounds 1 (1.4 mg, 2.0
µmol) were quantitatively assembled into spherical capsule 2 at
1
room temperature within 1 min. The H NMR spectrum of 2
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$`DI+,+R35),K59:02)PK3.:).;):*5)+1:51,+:7).;):*5)2R,.0K:+.1)R21G)2:)]Bb)1<Q@)
showed the upfield shifts of the aromatic signals Ha-f (!"max = –
1.65 ppm for Hb) as compared with that of 1 in CD3OD (Fig.
S13), which indicates efficient !-stacking interactions between
the bisacridine moieties (Fig. 3a). Whereas micellar product 2
is not stable enough under ESI-TOF MS conditions, the
dynamic light scattering (DLS) analysis and atomic force
microscopy (AFM) of 2 confirmed the quantitative formation of
spherical assemblies (1)n (n = ~5) with a core diameter of
approximately 1 nm (Fig. S23,24).10
)
Next we revealed that acridine-shelled capsule 2 can
encapsulate hydrophobic compounds, which are subsequently
released by pH-stimuli in water. When hydrophobic coumarin
3a (1.6 µmol) was stirred in a D2O solution (2.0 mL) of capsule
2 (2.0 mM based on 1) for 30 min at room temperature, 1:2
host-guest complex 2"(3a)2 was quantitatively obtained (Fig.
4a, left side) after separation of excess 3a suspended in the
pH-Responsive assembly and disassembly behavior of
supramolecular capsule 2 was observed in water at room
temperature. When an aqueous HCl solution (8.0 equiv. based
on 1) was added to a D2O solution of 2 (2.0 mM based on 1),
the colorless solution quickly turned yellow and the proton
NMR signals were dramatically changed (Fig. 3a,b). All of the
aromatic signals (Ha-f) were shifted downfield, indicating
disassembly of 2 due to protonation of the acridine panels. The
aromatic signal Hb was shifted downfield by +1.55 ppm.
Notably, protonated compounds 1’ (= 1•2HCl) were re-
assembled into capsule 2 through deprotonation by addition of
1
resultant solution by centrifugation and filtration.10 In the H
NMR spectrum (Fig. 4b), the signals derived from the
polycyclic framework (HC-J) of 3a were shifted significantly
upfield (!"max = –3.24 ppm) due to the encapsulation in the
polyaromatic shell of 2. On the other hand, the ethyl signals
(HA,B
) of 3a were almost unchanged, indicating their
penetration through the capsule shell. The NMR integrals of the
product indicated the 1:3a molar ratio to be 3:1 and the DLS
analysis elucidated the diameter of the product being 1.9 nm
(Fig. 4e, right side), which agrees with the formation of a
spherical (1)6"(3a)2 structure in water (Fig. 4f). Next, an
aqueous HCl solution (8.0 eq. based on 1) was added to the
resultant solution to release the encapsulated 3a from the cavity
of capsule 2 (Fig. 4a, right side). The 1H NMR signals of both 2
and 3a were replaced by those of 1’ within 1 min at room
temperature (Fig. 4c). Released molecules 3a were suspended
in the aqueous solution due to the hydrophobic feature. It
should be noted that the released 3a was re-encapsulated into
capsule 2 after (i) addition of an aqueous NaOH solution (8.0
eq. based on 1) and (ii) sonication (42 kHz, 70 W, 30 min),
through the deprotonation of 1’ and the formation of 2. The
resultant 1H NMR spectrum revealed the quantitative
regeneration of the original host-guest complex, 2"(3a)2 (Fig.
4d).
1
NaOH aq. (8.0 equiv.) within 1 min, as confirmed by the H
NMR spectrum (Fig. 3c). The UV-visible absorption and
fluorescence studies of capsule 2 also supported the reversible
assembly-disassembly process. The new absorption band
derived from 1’ was observed at #max = 418 nm upon addition
of HCl aq. (8.0 equiv.) to capsule 2 in water (Fig. 3d, step i).
The original absorption bands of 2 (280-430 nm) were
recovered by addition of an equivalent molar of NaOH aq. to 1’
(Fig. 3d, step ii). The emission band of 2 (#max = 473 nm) also
reversibly converted to that of protonated species 1’ (#max = 650
nm) under the similar acidic conditions (Fig. 3e, steps i-ii).
Importantly, the assembly-disassembly cycle could be repeated
more than 10 times (Fig. 3f and S29), without decomposition of
the component of capsule 2 and notable influence of the
increase in ionic strength.
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in water upon encapsulation within capsule 2 by a sequence of
View Article Online
DOI: 10.1039/C6CC09094J
grinding a 2:1 mixture of amphiphile 1 and 3d for 1 min,
addition of H2O, and filtration of the resultant suspended
solution.10 Then, to the obtained blue solution of host-guest
complex 2"(3d)2, an aqueous HCl solution (8.0 equiv. based on
1) was added at ambient temperature to release the encapsulated
3d from 2. The UV-visible spectrum of 2"(3d)2 showed new
broad absorption bands derived from encapsulated 3d around
480-820 nm.12 The absorption bands vanished upon addition of
the acid to the aqueous solution owing to precipitation of the
released 3d (Fig. 5a). By the same way, two molecules of
perfluorinated Cu(II)-phthalocyanine 3e accommodated in the
cavity of capsule 2 were also released by the acid stimulus in
water.
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The encapsulation and release processes were also
monitored by UV-visible spectroscopy. A new absorption band
derived from encapsulated 3a was observed around 415-500 nm
by mixing an aqueous solution of 2 with 3a (Fig. 4g). Relative
intensity of the host-guest absorption bands also supports the
formation of 1:2 host-guest complex 2"(3a)2. The absorption
band of 3a disappeared under acidic conditions (Fig. 4g, step i)
and subsequently reappeared upon neutralization (Fig. 4g, step
ii).11 Fluorescent emission of 3a ($F = 76% in CH2Cl2) was
suppressed within 2 ($F = 7% in H2O), indicating the relatively
strong interactions between the acridine frameworks of 2 and
3a in neutral water (Fig. S34). In a manner similar to 3a, the
catch and release of medium-sized hydrophobic compounds
such as DCM (3b) and Nile Red (3c) were carried out using
pH-responsive capsule 2 in water (Fig. S35). In sharp contrast,
the analogous host-guest complexes composed of the
anthracene-shelled capsule9 and 3a-c displayed no releasing
ability under acidic conditions (Fig. S39).10 Therefore,
protonation of the polyaromatic panels of capsule 2 is essential
for the guest release.
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:*5+0)<.359/320)<.G53+1H@)
Non-planar
and
bulky
large
compounds,
i.e.,
subphthalocyanine (3f)13 and pigment 3g,14 were also caught
and released by capsule 2 in water. Whereas many water-
soluble 3f derivatives have been synthesized as building blocks
in biological applications, reversible switching the water-
solubility of 3f by non-covalent functionalizations has been
undemonstrated to date.13,15 In a manner similar to the
preparation of 2"(3d)2, a violet aqueous solution of 2"3f was
obtained through the grinding of a mixed solid of 1 and
hydrophobic 3f (1.1 nm in diameter). The UV-visible spectrum
displayed new absorption bands corresponding to the
encapsulated 3f in the range of 430-660 nm (Fig. 5b). The
broadening and red-shifts (+22 nm) of the bands, as compared
with absorption bands of free 3f in CH2Cl2 (Fig. S37), indicate
the strong interactions between 3f and the acridine panels of 2.
In contrast to planar compounds such as 3a and 3d, one
molecule of bowl-shaped 3f was encapsulated in the cavity of 2,
which was confirmed by UV-visible and DLS analyses (Fig.
S36,38).16 Similarly, a magenta aqueous solution of 1:1 host-
Flexibility and pH-responsiveness of the polyaromatic shell
of capsule 2 make it possible to encapsulate and release large
hydrophobic compounds. Planar blue pigment, Cu(II)-
phthalocyanine (3d), with a diameter of 1.6 nm was solubilized
(*+,)-./0123)+,)4)(*5)6.723)8.9+5:7).;)!*5<+,:07)=>??!
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Mhejabeen, P. Haridas, Phys. Chem. Chem. Phys., 2015, 17, 9519–
guest complex 2"3g was formed from 1 and bulky hydrophobic
pigment 3g (1.5 nm in diameter) (Fig. 5c). The encapsulated
compounds 3f and 3g were quantitatively released from the
capsule cavities under acidic conditions at room temperature.
The UV-visible absorption bands of 3f and 3g completely
disappeared by addition of HCl solutions to the aqueous
solutions of 2"3f and 2"3g, respectively (Fig. 5b,c).17 It is
worthy of note that the released water-insoluble dyes could be
fully recovered by simple filtration.
View Article Online
9532.
DOI: 10.1039/C6CC09094J
6
Covalent hosts with two acridine panels: (a) S. C. Zimmerman, C. M.
Vanzyl, J. Am. Chem. Soc., 1987, 109, 7894–7896. (b) S. C.
Zimmerman, C. M. Vanzyl, G. S. Hamilton, J. Am. Chem. Soc., 1989,
111, 1373–1381. (c) A. Petitjean, R. G. Khoury, N. Kyritsakas, J.-M.
Lehn, J. Am. Chem. Soc., 2004, 126, 6637–6647.
7
8
Coordination hosts with several acridinium panels: K. Yazaki, Y. Sei,
M. Akita, M. Yoshizawa, Chem. Eur. J., 2016, 22, 17557–17561.
Recent reviews on stimuli-responsive molecular cages: (a) Molecules
at Work: Self-assembly, Nanomaterials, Molecular Machinery (Ed. B.
Pignataro), Chapter 2 (G. H. Clever), John Wiley & Sons, 2012. (b)
W. Wang, Y.-X. Wang, H.-B. Yang, Chem. Soc. Rev., 2016, 45,
2656–2693.
In conclusion, we have designed and constructed a pH-
responsive host-guest system using multiple acridine panels,
which provide both !-stackable large surfaces and a protonable
nitrogen atom. Simple bisacridine amphiphiles assemble into a
spherical capsule with an acridine shell in neutral water. In
contrast, under acidic conditions, the capsule reversibly
disassembles into monomeric species due to the protonation of
the acridine panels. The assembly-disassembly cycle can be
repeated more than 10 times without decomposition of the
capsule component. Moreover, the pH-responsive capsule
catches a variety of large hydrophobic compounds (up to 1.6
nm) such as planar metallophthalocyanines and bowl-shaped
subphthalocyanine in neutral water and subsequently releases
them by simple acidification. The present strategy for the
reversible control of !-stacking interactions by pH change
would prompt further development of stimuli-responsive
supramolecular compounds and materials.
9
Anthracene-based micellar capsules without pH-responsive
properties: (a) K. Kondo, A. Suzuki, M. Akita, M. Yoshizawa,
Angew. Chem. Int. Ed., 2013, 52, 2308–2312. (b) A. Suzuki, K.
Kondo, M. Akita, M. Yoshizawa, Angew. Chem. Int. Ed., 2013, 52,
8120–8123. (c) Y. Okazawa, K. Kondo, M. Akita, M. Yoshizawa, J.
Am. Chem. Soc., 2015, 137, 98–101. (d) K. Kondo, M. Akita, T.
Nakagawa, Y. Matsuo, M. Yoshizawa, Chem. Eur. J., 2015, 21,
12741–12746. (e) T. Omagari, A. Suzuki, M. Akita, M. Yoshizawa, J.
Am. Chem. Soc., 2016, 138, 499–502. (f) K. Kondo, M. Akita, M.
Yoshizawa, Chem. Eur. J., 2016, 22, 1937–1940. (g) A. Suzuki, K.
Kondo, Y. Sei, M. Akita, M. Yoshizawa, Chem. Commun., 2016, 52,
3151–3154. (h) A. Suzuki, M. Akita, M. Yoshizawa, Chem.
Commun., 2016, 52, 10024–10027.
10 See the Supporting Information.
11 Fig. 4g (after step i) was obtained after (i) the addition of HCl aq.
(8.0 eq. based on 1) to the 2"(3a)2 solution and (ii) the filtration of
the resultant solution to remove suspended 3a. Fig. 4g (after step ii)
was obtained after (i) the addition of NaOH aq. (8.0 eq. based on 1)
and 3a (0.5 mg) to the acidic 1’ solution, (ii) the stirring and
sonication of the resultant mixture for 30 min, and (iii) the removal of
the excess 3a by filtration.
This study was supported by JSPS KAKENHI Grant Numbers
JP25104011/JP26288033 and “Support for Tokyotech
Advanced Researchers (STAR)”. We thank Dr. Ryuji Higashi
and Dr. Masanori Seki (Canon Inc.) for their supports in host-
guest studies. K.K. thanks the JSPS for a Research Fellowship
for Young Scientists.
12 The relative intensity of the host and guest absorption bands indicated
the formation of 1:2 host-guest complexes 2"(3d)2 and 2"(3e)2.10 In
addition, the DLS analysis of 2"(3d or 3e)2 elucidated the presence
of small particles with an average diameter of 1.9-2.0 nm (Fig. S38).
13 (a) F. Dumoulin, M. Durmus, V. Ahsen, T. Nyokong, Coordin. Chem.
Rev., 2010, 254, 2792–2847. (b) C. G. Claessens, D. González-
Rodríguez, M. S. Rodríguez-Morgade, A. Medina, T. Torres, Chem.
Rev., 2014, 114, 2192!2277.
)"C+0(&%A($+B+$+%1+0
Laboratory for Chemistry and Life Science, Institute of Innovative
Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku,
Yokohama 226-8503, Japan. E-mail: yoshizawa.m.ac@m.titech.ac.jp
†
Electronic Supplementary Information (ESI) available: Experimental
procedures and physical properties of amphiphilic compound 1,
molecular capsule 2, and its host-guest complexes. See
DOI: 10.1039/x0xx00000x
14 A magenta ink for Inkjet Printers: T. Okubo, K. Iida, H. Nitta, A.
Kitao, Patent US20150232678 A1.
1
2
J. W. Steed, J. L. Atwood, Supramolecular Chemistry, 2nd ed., Wiley,
Chichester, 2009.
15 To the best of our knowledge, there have been no reports on the
encapsulation of subphthalocyanine and its derivatives by
supramolecular cages even with large cavities surrounded by multiple
polyaromatic panels: M. Yoshizawa, M. Yamashina, Chem. Lett.,
2017, DOI:10.1246/cl.160852.
16 In a manner similar to previous systems9 using anthracene panels, 1H
NMR signals derived from the encapsulated, highly hydrophobic
guests (e.g., 3d and 3f) were significantly broadened within 2 owing
to restriction of the molecular motion by the limited cavity (Fig. S43)
17 As control experiments, bulky large compounds 3f and 3g were
poorly dissolved in water even by using excess SDS (10 mM; Fig.
S40).10 In addition, the host-guest composites showed no pH-
responsive properties at ambient temperature (Fig. S42).
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5
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