Photochirogenic nanosponges: phase-controlled enantiodifferentiating photoisomerization of (Z)-cyclooctene sensitized by pyromellitate-crosslinked linear maltodextrin

Article abstract of DOI:10.1039/C6RA28211C

Linear maltodextrin (LM) was cross-linked by pyromellitic dianhydride to afford LM polymers of different cross-linking degrees. When soaked in water, these cross-linked LM polymers (nanosponges (NSs)), evolved into several phases from sol to suspension, then to flowing gel, and finally to rigid gel with an increase in their content. Enantiodifferentiating photoisomerization of (Z)-cyclooctene (1Z) to chiral (E)-isomer (1E), which was employed as a benchmark reaction to quantitatively assess the environmental-to-molecular chirality transfer process, was performed in aqueous media containing these pyromellitate-crosslinked LM-NSs in different phases. The enantiomeric excess (ee) of 1E obtained was relatively insensitive to the phases at least up to the flowing gel phase, but became highly sensitive in the rigid gel phase, exhibiting an abrupt drop in the early rigid gel phase followed by a rapid recovery in the late rigid gel phase. A comparison with the phase-dependent ee profiles previously reported for similar pyromellitate-crosslinked cyclodextrin (CD)- and cyclic nigerosylnigerose (CNN)-NSs revealed that the chiral void space created around the pyromellitate linker in NS is responsible for the dramatic changes in ee in the rigid gel phase, whereas the inherent host cavity in CD/CNN plays only limited roles in the supramolecular photochirogenesis mediated by the sensitizer-crosslinked NSs. The latter insight allows us to further expand the applicable range of the present concept and methodology by employing a much wider variety of oligosaccharides as well as substrates and sensitizing cross-linkers.

Full text of DOI:10.1039/C6RA28211C

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PAPER
Photochirogenic nanosponges: phase-controlled
enantiodifferentiating photoisomerization of
(Z)-cyclooctene sensitized by pyromellitate-
crosslinked linear maltodextrin†
Cite this: RSC Adv., 2017, 7, 17184
Wenting Liang,a Meiling Zhao,a Xueqin Wei,b Zhiqiang Yan,b Wanhua Wu,b
c
c
d
Fabrizio Caldera, Francesco Trotta, Yoshihisa Inoue, Dan Su,b Zhihui Zhongb
and Cheng Yang
*
*
b
*
Linear maltodextrin (LM) was cross-linked by pyromellitic dianhydride to afford LM polymers of different
cross-linking degrees. When soaked in water, these cross-linked LM polymers (nanosponges (NSs)),
evolved into several phases from sol to suspension, then to flowing gel, and finally to rigid gel with an
increase in their content. Enantiodifferentiating photoisomerization of (Z)-cyclooctene (1Z) to chiral
(E)-isomer (1E), which was employed as
a benchmark reaction to quantitatively assess the
environmental-to-molecular chirality transfer process, was performed in aqueous media containing
these pyromellitate-crosslinked LM-NSs in different phases. The enantiomeric excess (ee) of 1E obtained
was relatively insensitive to the phases at least up to the flowing gel phase, but became highly sensitive
in the rigid gel phase, exhibiting an abrupt drop in the early rigid gel phase followed by a rapid recovery
in the late rigid gel phase. A comparison with the phase-dependent ee profiles previously reported for
similar pyromellitate-crosslinked cyclodextrin (CD)- and cyclic nigerosylnigerose (CNN)-NSs revealed
that the chiral void space created around the pyromellitate linker in NS is responsible for the dramatic
changes in ee in the rigid gel phase, whereas the inherent host cavity in CD/CNN plays only limited roles
in the supramolecular photochirogenesis mediated by the sensitizer-crosslinked NSs. The latter insight
allows us to further expand the applicable range of the present concept and methodology by employing
a much wider variety of oligosaccharides as well as substrates and sensitizing cross-linkers.
Received 14th December 2016
Accepted 24th February 2017
DOI: 10.1039/c6ra28211c
rsc.li/rsc-advances
consequence of a chiral photoreaction is not a simple task, in
particular, for enantiodifferentiating photosensitization reac-
Introduction
tion, where only short-lived weak intermolecular stereochem-
ical interactions are available and the electronically excited
states generated are highly reactive.7 Indeed, the enantiose-
lectivities reported for many of the enantiodifferentiating
photosensitization reactions are modest,1–6 which are not
inherent and are due to our lack of comprehensive under-
standing of the factors and mechanism that control the enan-
tiodifferentiating photosensitization process.
Chiral photochemistry (photochirogenesis), which is employed
to induce molecular chirality in an excited-state reaction, is an
attractive yet challenging area of chemistry, which provides
environmentally benign direct access to optically active mole-
cules with unconventional, oen constrained, skeletons that
are difficult or tedious to construct via thermal asymmetric
synthesis.1–6 However, controlling the stereochemical
The supramolecular approach to photochirogenesis provides
a convenient promising tool for facilitating the excited-state
chirality transfer from an enantiomeric host to prochiral
substrate through the long-lasting intimate supramolecular
contacts of guest substrate(s) with a chiral host in the ground as
well as excited state.2–15 A variety of chiral supramolecular hosts
including cyclodextrins,16–23 chirally modied zeolite superc-
aInstitute of Environmental Sciences, Department of Chemistry, Shanxi University,
Taiyuan 030006, China
bState Key Laboratory of Biotherapy, West China Medical School, Key Laboratory of
Green Chemistry, Technology of Ministry of Education, College of Chemistry,
Sichuan University, 29 Wangjiang Road, Chengdu 610064, China. E-mail:
yangchengyc@scu.edu.cn
cDepartment of Chemistry, University of Torino, Via P. Giuria 7, 10125 Torino, Italy.
E-mail: francesco.trotta@unito.it
ages,24–26
biomolecules,27,28
chiral
hydrogen-bonding
dDepartment of Applied Chemistry, Osaka University, 2-1 Yamada-oka, Suita 565-
0871, Japan. E-mail: inoue@chem.eng.osaka-u.ac.jp
templates,29–31 and rotaxanes32 have hitherto been examined to
achieve modest to high enantioselectivities. More recently,
chiral supramolecular assemblies such as gels33,34 and liquid
† Electronic supplementary information (ESI) available: Thermogravimetric
analyses, dynamic light scattering data for LM-NS 7, optical microscope images
of LM-NS 7, and SEM images of LM-NSs 7 and 8. See DOI: 10.1039/c6ra28211c
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rigid gel via suspension and owing gel upon a gradual increase
in the NS content in water and more crucially served as effective
chiral sensitizers for the enantiodifferentiating photo-
isomerization of 1Z to chiral (E)-isomer (1Z).36,37 Interestingly,
the enantiomeric excess (ee) of 1E obtained upon the photo-
sensitization of 1Z with NS was critically affected by the phase
condition. This phase-dependent photochirogenic behaviour
had never been observed for the non-polymeric cyclic
oligosaccharides-based sensitizers reported earlier.42–45 We
proposed a progressive aggregation mechanism to rationalize
this phase-dependent behaviour of the ee product. For CD-
based NS, the interparticle complexation between the pyro-
mellitate linker and the CD host exposed on the surfaces of
neighbouring CD-NS particles is deemed to drive the progres-
sive aggregations, leading to phase transitions upon the gradual
increase in the CD-NS concentration. As a cyclic tetrasaccharide,
CNN is much smaller in ring size than CDs and bears only
a shallow concave. Apparently, this concave does not effectively
function as a binding site and instead intra- and/or inter-
particle voids were thought to function as the binding and
sensitizing sites to inuence the stereochemical outcomes. The
size, shape, and exibility of these polymer voids are highly
sensitive to the degree of aggregation or the phase of the
medium and hence are considered to be responsible for the
phase-dependent ee values. These considerations led us to two
further questions concerning the general validity of the
phenomenon and the roles of the molecular host (CD and CNN)
incorporated in NSs: (1) how widely the phase-control concept is
applicable in supramolecular photochirogenesis and (2) to what
extent the molecular host used as a building block contributes
to the photochirogenesis in the cross-linked oligosaccharide
polymer.
Scheme 1 Enantiodifferentiating photoisomerization of 1Z to chiral
1E.
crystals35 have also been employed as chiral environments for
supramolecular photochirogenesis.
We recently reported the results of a series of studies on the
phase-dependent enantiodifferentiating photoisomerization of
(Z)-cyclooctene (1Z) (Scheme 1) mediated by pyromellitate-
crosslinked polymers of cyclic oligosaccharides with different
cavity diameters and depths, i.e. b- and g-cyclodextrins (CDs)36
and cyclic nigerosylnigerose (CNN),37,38 as shown in Scheme 2.
These cross-linked polymers showed intriguing swelling
behaviour upon soaking in water and hence were named
nanosponges (NSs).39–41 Both the CD-based 2–4 and CNN-based
NSs 5 and 6 (Scheme 2) evolved into several phases from sol to
In the present study, to address these issues and also to
further expand our understanding of the relationship between
the phase evolution and the stereochemical outcome of the
photosensitized enantiodifferentiation, we employed linear
maltodextrin (LM) as a catenulate oligosaccharide building
block, which has no inherent cavity or concave and hence is
inclusion-free, and pyromellitic dianhydride (PDA) as a cross-
linking agent to obtain a pyromellitate-crosslinked linear mal-
todextrin polymer (LM-NS). The photochirogenic performance
of LM-NSs 7 and 8 upon enantiodifferentiating photosensiti-
zation of 1Z was examined in different phases evolved in water,
and the results have been comparatively discussed with those
obtained using similar pyromellitate-crosslinked CD- and CNN-
NSs (2–6) (Scheme 2).
Results and discussion
Syntheses
Two LM-NS sensitizers with different degrees of cross-linking (7
and 8) were prepared by reacting LM with PDA in the presence
of trimethylamine in DMSO at room temperature and puried
by extensive Soxhlet extraction of the resulting polymer with
acetone (Scheme 2); note that the formal molar ratios indicated
herein were calculated as the ratios of seven glucose units
(equivalent to b-CD) to PDA. The LM-NSs thus obtained were
Scheme 2 Chemical structures of CD-NSs 2–4, CN-NNSs 5, 6, and
LM-NS 7, 8 synthesized using different oligosaccharide : linker ratios.
CD: cyclodextrin; CNN: cyclic nigerosylnigerose; LM: linear malto-
dextrin; and PDA: pyromellitic dianhydride.
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characterized by thermal analyses, FT-IR, elemental analyses, solutions of 7 and 8 on their chiroptical properties were exam-
and swelling properties. In this reaction, two carboxylic acid ined via UV-vis and circular dichroism spectroscopies. As shown
moieties remain on the pyromellitate linker, a signicant frac- in Fig. 1, the addition of 10–50% methanol led to appreciable
tion of which forms a salt with trimethylammonium ion and is decreases in absorbance in UV-vis and increases in anisotropy
partially ionized in aqueous media.
in the circular dichroism spectra, but the original peak position
and shape of the main band were maintained, thus implying
that the cross-linked polymers have a relatively rigid framework
and the addition of an organic solvent does not signicantly
alter the orientation and conformation of the pyromellitate
moieties. Subsequent addition of 1Z (3 mM) to the isotropic LM-
NS solutions did not cause any appreciable changes in the
circular dichroism spectra, for which, the lack of dened
hydrophobic binding sites and relatively rigid framework of the
cross-linked LM-NS would be responsible. In summary, the
microenvironmental chirality created around the pyromellitate
sensitizer in LM-NS is manipulable to certain extent through the
orientational and/or conformational changes caused upon the
addition of methanol, but is not altered by substrate 1Z.
Chiroptical properties
b-CD possesses a hydrophobic cavity that can include one 1Z
molecule with a relatively high binding affinity.40,41 In contrast, LM
has no dened binding site for accommodating 1Z. However, the
combined use of the conformationally exible hydrophilic LM and
rigid hydrophobic linker is expected to leave some void space,
particularly around the pyromellitate moiety, in the resulting
loosely packed cross-linking LM polymer. In this LM-NS, the
pyromellitate moiety plays triple roles of a cross-linker, pillar, and
photosensitizer, whereas the inherently chiral LM segments
nearby provide the chiral environment for effecting the enantio-
differentiating photoisomerization of prochiral 1Z to chiral 1E
sensitized by pyromellitate.
To evaluate the chiral environment induced around the
pyromellitate linker, UV-vis and circular dichroism spectral
examinations of LM-NSs (7 and 8) were performed in aqueous
solutions. As shown in Fig. 1b and d, both 7 and 8 exhibit
Phase evolution
We demonstrated that both CD-NSs 2–4 (ref. 36) and CNN-NSs
5, 6 (ref. 37) evolve into a series of phases changing from sol
to rigid gel upon a gradual increase in the polymer content in
water. Interestingly, the enantiomeric excess (ee) of 1E
produced upon photosensitization by 2–6 critically varies with
the polymer content, i.e. the phase evolved, exhibiting the most
remarkable changes around the border of owing gel to rigid
gel. This result led us to the idea that the NS void space for
photochirogenesis is manipulable via phase evolution.
To assess the aggregation and phase-evolution behaviour of
LM-NS in water at room temperature, the contents of 7 and 8
were gradually increased and the appearance of the resulting
solutions was observed. As shown in Fig. 2a, both 7 and 8
exhibit a stepwise phase-evolution from transparent sol to
suspension, then to owing gel, and nally to rigid gel, as was
1
negative Cotton effect for the La band of pyromellitate (at 265
and 268 nm, respectively) in pure water. The anisotropy factors
(g ¼ D3/3, where D3 denotes the molar circular dichroism and 3
denotes the molar extinction coefficient) observed for LM-NSs
are smaller than those reported for CD-NSs,36 whereas slightly
larger than those reported for CNN-NSs37 under comparable
measuring conditions. These induced circular dichroism (ICD)
signals clearly indicate that the pyromellitate chromophore is
located in a chiral environment and hence is applicable to
chirality transfer through photosensitization.
Chiral environment
To assess the conformational exibility of the cross-linked
polymers, the effects of methanol added to the aqueous
Fig. 2 (a) Images of aqueous solutions of LM-NS 8 at 0.1 (sol), 10
(suspension), 125 (flowing gel), and 245 mg mLꢀ1 (rigid gel) (from left to
Fig. 1 UV-vis (a and c) and anisotropy (g ¼ D3/3) factor (b and d) right). Inset: schematic of the phase evolution of 8. (b) Dynamic light
spectra of LM-NS 7 (0.10 mg mLꢀ1) and LM-NS 8 (0.10 mg mLꢀ1) in scattering (DLS) intensities measured at 0.05 (black), 0.1 (red), and 0.2
aqueous solutions containing 0% (black), 10% (red), and 50% (blue) (blue) mg mLꢀ1 concentrations of LM-NS 8. (c) Optical microscopy
methanol at ambient temperature.
image of LM-NS 8 at 245 mg mLꢀ1
.
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Table 1 Enantiodifferentiating photoisomerization of 1Z to chiral 1E sensitized by LM-NSs 7 and 8 in various phases developed in aqueous media
containing 0–50% methanola
[LM-NS]/mg
mLꢀ1
Irradiation
time/min
E/Z
ratio
LM-NS
Phase
Sol
MeOH content/%
0
Temperature/ꢁC
ee/%
7
25
0.5
0.1
0.1
0.15
0.1
0.1
0.1
0.25
0.5
1
10
1
10
0.5
1
10
50
55
70
90
60
30
30
30
30
30
60
30
30
30
30
30
60
30
30
60
60
60
60
60
60
60
60
60
60
60
60
60
60
180
300
60
60
60
60
60
60
60
30
30
30
30
60
30
30
30
30
30
60
30
60
60
60
60
60
60
60
60
60
60
60
60
0.05
0.03
0.01
0.16
0.10
0.02
0.07
0.03
0.06
0.16
0.27
0.05
0.12
0.47
0.13
0.05
0.01
0.01
0.02
0.01
0.01
0.14
0.13
0.03
0.01
0.01
0.01
0.02
0.02
0.02
0.02
0.02
0.01
0.01
0.10
0.06
0.05
0.03
0.03
0.31
0.18
0.10
0.02
0.01
0.07
0.06
0.22
0.07
0.21
0.14
0.05
0.02
0.03
0.01
0.01
0.06
0.08
0.01
0.02
0.03
0.02
4.9
10.5
10.1
4.9
5.2
7.7
10.9
10.0
9.7
7.7
4.8
5.2
3.3
2.8
1.8
3.0
9.7
10.4
12.5
7.3
7.7
3.0
2.5
5.9
ꢀ1.4
4.2
6.6
8.4
7.5
7.3
7.8
8.5
1.7
ꢀ5.4
2.7
2.9
3.7
8.9
9.2
2.5
2.0
3.0
7.5
7.6
7.1
5.4
2.5
3.3
1.7
1.3
2.3
5.3
5.3
7.1
4.8
2.2
1.6
4.2
4.6
ꢀ1.4
ꢀ5.0
10
50
0.5
0.5
ꢀ30
0.5
Suspension
0
10
50
0.5
0.5
Flowing gel
0
0.5
10
50
0
0.5
0.5
0.5
100
100
125
100
110
120
130
160
Rigid gel
10
0.5
0.5
150
200
250
300
150
200
250
0.1
0.1
0.1
0.1
0.1
0.25
0.5
1
10
1
10
1
10
50
125
200
230
130
130
200
245
250
280
300
50
0
8
Sol
25
0.5
0.5
0.5
ꢀ30
0.5
10
50
Suspension
0
10
50
0.5
0.5
Flowing gel
Rigid gel
0
0.5
10
50
0.5
0.5
0
0.5
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Table 1 (Contd.)
[LM-NS]/mg
mLꢀ1
Irradiation
time/min
E/Z
ratio
LM-NS
Phase
MeOH content/%
Temperature/ꢁC
ee/%
350
425
500
575
250
300
400
500
250
300
500
60
60
60
60
60
60
60
60
60
60
60
0.02
0.01
0.01
0.01
0.02
0.01
0.01
0.01
0.07
0.06
0.04
ꢀ11.1
ꢀ3.8
5.8
5.9
4.2
4.2
5.0
4.7
1.5
10
50
0.5
0.5
2.2
2.4
a
Irradiation performed at 254 nm in quartz cell under N2; [1Z] ¼ 3 mM.
exactly the case with the previously reported CD- and CNN-NSs
The concentration-dependent stepwise phase-transitions of
2–6. For the less cross-linked LM-NS 7, the critical concentra- LM-NS also occurred in aqueous solutions containing 10–50%
tions for sol-suspension, suspension-owing gel, and owing methanol. The phase transition concentrations in aqueous
gel-rigid gel phase-transitions were 0.15, 55, and 100 mg mLꢀ1
,
methanol were higher than the corresponding values observed
respectively. These critical phase-transition concentrations in pure water. LM-NS 7 exhibited the phase transition concen-
were lower than the corresponding values (0.25, 125, and trations for suspension, owing gel and rigid gel at 0.25, 60 and
245 mg mLꢀ1, respectively) determined for the more cross- 150 mg mLꢀ1, respectively, in 1 : 9 and 1 : 1 methanol–water.
linked LM-NS 8. This is presumably due to the smaller More crosslinked LM-NS 8 also showed comparable transition
amount of LM segments exposed on the surface of 8, which concentrations at 0.5, 130 and 250 mg mLꢀ1, respectively, in
causes the aggregation of the cross-linked polymers via 1 : 9 and 1 : 1 methanol–water.
hydrogen-bonding interactions.
Apart from the critical phase-transition concentrations, the
overall phase-evolution behaviours of 7 and 8 resemble each
Photochirogenesis in LM-NS
other. Typically, LM-NS 8 smoothly dissolved in water to afford
an isotropic solution up to 0.25 mg mLꢀ1 concentration, aer
which, transparent precipitates started to appear (Fig. 2a).
Even in the apparently homogeneous solution, the polymer
started to aggregate at relatively low concentrations; thus, the
mean hydrodynamic diameter evaluated by dynamic light
scattering (DLS) gradually increased from 250 to 350 and then
to 600 nm with an increase in the concentration from 0.05 to
0.1 and then to 0.2 mg mLꢀ1 (Fig. 2b). The solution obviously
became turbid at 0.25 mg mLꢀ1 to afford a suspended solution.
When the concentration was further increased to 125 mg
mLꢀ1, the turbid solution started to gelate to provide a gel-like
binary phase composed of liquid and gel, or the ‘owing gel’
state, which was eventually converted to rigid gel at a critical
gelation concentration of 245 mg mLꢀ1 (Fig. 2c). Optical
microscopic (Fig. 2c and S6†) and SEM (Fig. S7 and S8†)
investigations of the rigid gel phase revealed the adhesive
nature of the gel with the particle sizes in the range of 10–
100 nm. These results indicate that the phase evolution is
a continuous growth process, in which the size of the aggre-
gates gradually increases to form precipitates, which further
adhere to afford gels (as illustrated in the inset of Fig. 2a). The
abundant hydroxyl groups of the LM moieties in LM-NS are
likely to dominate the progressive interparticle adhesion via
the hydrogen-bonding interaction.
Enantiodifferentiating photoisomerization of 1Z sensitized by 7
and 8 was carried out under different phase conditions in
aqueous media containing 0%, 10%, and 50% methanol. Since
the photosensitizations with 7 and 8 gave higher or comparable
ee in solution phase at 0.5 ꢁC than at 25 ꢁC (Table 1), practically,
ꢁ
all the experiments were performed at 0.5 C.
In isotropic pure water solution, both 7 and 8 afforded (S)-1E
in comparable 10.5% and 9.2% ee. In 10–50% methanolic
water, the ee of 1E decreased to 4.9–5.2% ee for 7 and to 2.0–
2.5% ee for 8, which were however appreciably improved to
7.7% and 3.0% by lowering the temperature to ꢀ30 ꢁC (Table 1,
top lines). These results indicate that despite the fact that the
same building monomer and linker were employed, the
different cross-linking degrees and framework rigidity afforded
diverse chiral environments for the enantiodifferentiating
photoisomerization of 1Z, which is readily tunable by adjusting
the external factors such as solvent composition and
temperature.
Intriguingly, the enantioselectivity was a critical function of
the phase condition. As shown in Fig. 3f, the ee value of 1E
obtained upon sensitization with 7 in pure water was practically
kept constant at 10 ꢂ 2% with some uctuations through the
solution to the late owing gel phase, but suddenly reduced and
sign-inverted to reach a minimum (ꢀ1.4%) in the early rigid gel
phase, and nally recovered to +8.4% (or 7.3–7.5% upon pro-
longed irradiations) in the late rigid gel phase, presumably due
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to the structural evolution of rigid gel. Moreover, upon sensiti- Obviously, no dened binding site exists in the catenulate
zation with 8 (Fig. 3g), the ee behaved very similarly, remaining oligosaccharide LM-based NSs 7 and 8, revealing that our
at 7.0 ꢂ 1.5% up to the late owing gel phase, suddenly reduced previously proposed switching mechanism is not valid and
and sign-inverted to attain ꢀ11.1% in the early rigid gel phase, further implying that the phase-dependent chirality is a more
and then recovered to +5.9% in the late rigid gel phase.
universal phenomenon for photosensitization in hydrogel,
These ee proles resemble those observed for CD-NSs (except regardless of the building block employed.
g-CD-based NS 4) and CNN-NSs (Fig. 3a–e), which also show
To better understand the phase-dependent photochiro-
a sudden drop (sometimes with accompanying inversion of genesis behaviour, we examined the chiroptical properties of
product chirality) near the owing-rigid gel border. This is LM-NS in two extreme phases. As can be seen from Fig. 4, the
rather unexpected because in contrast to the CD- and CNN- circular dichroism spectra of LM-NS 7 in all the phases (from
analogues, LM-NS does not bear any inherent cavity or solution to rigid gel) exhibit simple negative Cotton effects for
concave in the cross-linked polymer structure. Previously, we the main band of the pyromellitate linker, the anisotropy (g)
proposed that the sudden change in ee observed at the phase factors of which are ꢃ10ꢀ4 at the extremum (260–270 nm),
border in the photosensitization of 1Z with CD- and CNN-NSs 2– a typical value for the allowed p–p* transition. The lack of
6 is driven by the switching of the photosensitizing site from the exciton couplets at the main band implies that the pyromellitate
cavity or concave of the cyclic oligosaccharide CD or CNN to the chromophores are spatially separated by the LM moieties in the
chiral void space surrounded by the outside walls of CD or CNN. cross-linked polymer LM-NS. However, the g factor at 271 nm
gradually decreases in the sol and suspension phases and more
rapidly in the gel phases (Fig. 4 inset). This means that the
chiral environment created around the pyromellitate linker by
the surrounding LM moieties differs in each phase. Since the
enantiodifferentiating photoisomerization of 1Z occurs upon
sensitization by pyromellitate, the ee of 1E thus produced
should be sensitive to the chiral environment created in the
photosensitization site, which is likely to be a polymer void
space encircled by a pyromellitate and several LM moieties. This
mechanism based on the chiroptical property change of the LM-
NS well explains the observed sudden change in ee in the rigid
gel.
In addition to the sudden ee change near the owing-rigid
gel border as a common feature shared by all the NSs
employed, the ee variation prole reveals an interesting differ-
ence between the CD/CNN- and LM-based NSs. As can be seen
from Fig. 3, the ee of 1E obtained upon sensitization with cyclic
oligosaccharide-based 2–6 is not constant and appreciably var-
ies across the sol to owing gel region. This ee prole has been
ascribed36,37 to the gradual switching of the sensitization site
Fig. 3 ee profiles of 1E obtained upon enantiodifferentiating photo-
isomerization of 1Z sensitized by (a) b-CD-NSs 2,36 (b) 3,36 (c) g-CD-NS
4,36 (d) CNN-NSs 5,37 (e) 6,37 (f) LM-NSs 7 and (g) 8 at various Fig. 4 Circular dichroism (anisotropy factor) spectra of LM-NS 7 in
concentrations and in various phases. The violet, green, yellow, and water at concentrations from 0.1 to 160 mg mLꢀ1 measured in a quartz
pink regions represent the sol, suspension, flowing gel, and rigid gel cell or by sandwiching the gel with two quartz plates. Inset: the g-
phases, respectively.
factor at 271 nm as a function of concentration/phase of 7.
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from inside to outside of the host cavity/concave in the cross- was lowered to some degree in the sol to owing gel region and
linked polymer at higher NS concentrations, where the cavity/ no signicant ee change occurred in the rigid gel or any other
concave is progressively occupied by the pyromellitate moiety. phase (Fig. 5b). Similarly, appreciably lower ee and at ee
In contrast, the ee prole for catenulate oligosaccharide-based 7 proles were obtained for the photosensitization with 8 in 10%
and 8 is practically at in the same region, indicating that the or 50% methanolic water (Fig. 5c and d). These results reveal the
chiral environment of the sensitization site is not greatly altered critical roles of the hydrophobicity of the solvent and degree of
in LM-NS at least up to the owing gel phase. We deduced that cross-linking in this phase-sensitive photosensitization. The
the hydrophobic binding/sensitizing site formed around the addition of methanol may loosen the hydrogel to make the
pyromellitate moiety is loose and exile until the owing gel hydrophobic sensitizing site more exible, eventually leading to
phase, but starts to continuously change its shape and rigidity a global decrease in ee and phase-insensitive ee prole.
in the rigid gel phase. Without considering the continued
shape/rigidity change, we cannot rationalize the sudden
decrease and the subsequent recovery of ee observed upon
Conclusions
sensitization with 7 (Fig. 3f).
In this study, we prepared new linear maltodextrin-based
nanosponges (LM-NSs) cross-linked by pyromellitate as photo-
sensitizing media for the enantiodifferentiating photo-
isomerization of cyclooctene (1Z). By comparing the phase-
evolution and photochirogenic behaviours of LM-NSs with
those of cyclodextrin- and cyclic nigerosylnigerose-based CD/
CNN-NSs, we elucidated that the inherent host cavity/concave
existing in CD/CNN-NSs does not affect the overall phase-
evolution behaviour, but causes an appreciable dri in ee in
the sol to owing gel region through the inclusion of guest
substrate 1Z. However, the chiral void space created around the
mellitate linker in both LM-NSs and CD/CNN-NSs plays the
major role in determining the stereochemical consequences of
photoisomerization to 1E, the ee of which shows a rather at
prole in the sol to owing gel region but abrupt change(s) in
the rigid gel phase, i.e. a sudden drop. These ee proles disclose
the nominal contribution of the host cavity/concave to the
photochirogenesis in NS and also highlight the more crucial
roles played by the phase-responsive chiral void space created in
the cross-linked oligosaccharide polymers.
As shown in Table 1 and Fig. 5a, the ee product obtained
upon sensitization with 7 in 10% methanolic water is also
signicantly phase-dependent, exhibiting a slow increase from
4.9% to 8.5% in the sol to early rigid gel region, but a subse-
quent sudden decrease down to ꢀ5.4% in the late rigid phase.
When the methanol content was increased up to 50%, the ee
Experimental
Materials
Cyclooctene was purchased from Tokyo Chemical Industries
and used aer fractional distillation. LM-NSs 7 and 8 were
prepared as follows. Typically, LM (4.89 g) of average molecular
weight of ca. 12 000 Da was dissolved in DMSO (20 mL), to
which trimethylamine (5 mL) and a proper amount of pyro-
mellitic dianhydride (1.88 g for 7 or 3.76 g for 8) were added.
Then, the mixture was stirred for 24 h until the solution gelated.
The monolith block thus obtained was crushed in a mortar,
washed with an excess amount of deionized water, and then
ltered under vacuum. Aer air-drying, the whitish powder was
collected and puried by Soxhlet extraction with acetone for
20 h to afford 7 in 91.7% yield and 8 in 98.6% yield. The other
chemicals were commercially available and used as received.
Fig. 5 ee profiles of 1E obtained upon enantiodifferentiating photo-
isomerization of 1Z sensitized by LM-NSs 7 (a and b) and 8 (c and d) in
aqueous solutions containing 10% (a and c) and 50% (b and d) meth-
anol. The violet, green, yellow, and pink regions represent the sol,
suspension, flowing gel, and rigid gel phases, respectively, where the
phase transition concentrations in 1 : 1 MeOH–H2O were essentially
the same as in 1 : 9 MeOH–H2O.
Instruments and methods
The particle size distribution of LM-NS was determined by DLS
in an aqueous solution at 25 ꢁC by monitoring the scattered
light at an angle of 173ꢁ using a Zetasizer Nano instrument
(Malvern Instrument, UK). The size and shape of the
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precipitates in the suspension were observed using an optical 10 Y. Inoue, H. Ikeda, M. Kaneda, T. Sumimura, S. R. L. Everitt
microscope (DN-107T digital microscope). UV-vis and circular and T. Wada, J. Am. Chem. Soc., 2000, 122, 406.
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Photolysis
In a typical run, 3 mL aqueous sample containing LM-NS and 1Z
was placed in a quartz cell, purged with nitrogen gas, and then
irradiated at 254 nm with two 11 W low-pressure mercury lamps
(Philips TUV T5 4P-SE) in a Unisoku cryostat kept at a given
temperature. No appreciable phase change or transition was
observed upon irradiation. The photolyzed sample was poured
into an aqueous 10% KOH solution (3 mL), and the resulting
mixture was extracted with pentane (1 mL). Aer the gas chro-
matographic (GC) analysis of an aliquot to determine the E/Z
ratio using a Shimadzu GC-2014 (CBP-20 column, 0.25 mmf ꢄ
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Y. Inoue, Chem. Commun., 2011, 47, 6849.
ꢁ
25 m, 60 C), the organic layer was extracted with an aqueous
ꢁ
silver nitrate solution (2 mL) at 0 C. The aqueous phase was
washed twice with pentane (1 mL) and then added to an
aqueous ammonia (28%) solution at 0 ꢁC, and the liberated
cyclooctene was extracted with pentane (0.5 mL). The ee was
determined using a chiral GC equipped with a Supelco b-DEX
225 column (0.25 mmf ꢄ 30 m).
22 G.-H. Liao, L. Luo, H.-X. Xu, X.-L. Wu, L. Lei, C.-H. Tung and
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Y. Inoue, J. Am. Chem. Soc., 2007, 129, 3478.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21402110, 21572142, 21372165,
21402129 and 21321061), Research Project Supported by Shanxi
Scholarship Council of China (No. 2014-011), Scientic and
Technological Innovation Programs of Higher Education Insti-
tutions in Shanxi (No. 2014108), State Key Laboratory of Poly-
mer Materials Engineering, Grant No. sklpme 2014-2-06,
Comprehensive Training Platform of Specialized Laboratory,
College of Chemistry, Sichuan University and Japan Society for
the Promotion of Science (No. 21245011 and 26620030).
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