Reversible Quadruple Switching with Optical, Chiroptical, Helicity, and Macropattern in Self-Assembled Spiropyran Gels

Article abstract of DOI:10.1002/adfm.201701368

Enantiomeric glutamate gelators containing a spiropyran moiety are designed and found to self-assemble into a nanohelix through gelation. Upon alternating UV and visible light irradiation, the spiropyran experiences a reversible change between a blue zwitterionic merocyanine state and a colorless closed ring state spiropyran in supramolecular gels. This photochromic switch causes a series of subsequent changes in the optical, chiroptical, morphological properties from supramolecular to macroscopic levels. While the solution of the gelator molecules does not show any circular dichroism (CD) signal in the region of 250–700 nm due to the fact that the chromophore is far from the chiral center, the gel shows chiroptical signals such as CD and circularly polarized luminescence (CPL) because of the chirality transfer by the self-assembly. These signals are reversible upon alternating UV/vis irradiation. Therefore, a quadruple optical and chiroptical switch is developed successfully. During such process, the self-assembled nanostructures from the enantiomeric supramolecular gels also undergo a reversible change between helices and fibers under the alternating UV and visible light trigger. Furthermore, a rewritable material fabricated from their xerogels on a glass is developed. Such rewritable material can be efficiently printed over 30 cycles without significant loss in contrast and resolution using UV and visible light.

Full text of DOI:10.1002/adfm.201701368

FULL PAPER
Optical Switches
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Reversible Quadruple Switching with Optical, Chiroptical,
Helicity, and Macropattern in Self-Assembled Spiropyran Gels
Wangen Miao,* Sheng Wang, and Minghua Liu*
recoding materials, simultaneous reali-
zation of both the optical and chiroptical
Enantiomeric glutamate gelators containing a spiropyran moiety are designed
and found to self-assemble into a nanohelix through gelation. Upon alter-
nating UV and visible light irradiation, the spiropyran experiences a revers-
ible change between a blue zwitterionic merocyanine state and a colorless
closed ring state spiropyran in supramolecular gels. This photochromic
switch causes a series of subsequent changes in the optical, chiroptical,
morphological properties from supramolecular to macroscopic levels. While
the solution of the gelator molecules does not show any circular dichroism
(CD) signal in the region of 250–700 nm due to the fact that the chromophore
is far from the chiral center, the gel shows chiroptical signals such as CD
and circularly polarized luminescence (CPL) because of the chirality transfer
by the self-assembly. These signals are reversible upon alternating UV/vis
irradiation. Therefore, a quadruple optical and chiroptical switch is devel-
oped successfully. During such process, the self-assembled nanostructures
from the enantiomeric supramolecular gels also undergo a reversible change
between helices and fibers under the alternating UV and visible light trigger.
Furthermore, a rewritable material fabricated from their xerogels on a glass is
developed. Such rewritable material can be efficiently printed over 30 cycles
without significant loss in contrast and resolution using UV and visible light.
signals can add extra storage density. On
the other hand, the chiroptical materials
are found to exhibit many applications
such as molecular information processing,
data storage, or probes for the detection of
chirality,[2] In particular, developing mate-
rials exhibiting circularly polarized lumi-
nescence (CPL) has drawn more and more
attention, not only for understanding the
mysteries of homochirality, but also due
to the important potential applications of
CPL materials. For example, the devices
displaying CPL may be a powerful method
for addressing encoded information (cryp-
tography) or for preparing 3D displays.[3]
Generally, there are basically three
kinds of chiroptical information related
to the materials, i.e., the optical rotation
(OR), circular dichroism (CD) and CPL,
which are characterized by differences
between left- and right-handed circularly
polarized light transmission, absorp-
tion and emission, respectively. In devel-
oping the chiroptical materials, chiral molecules are generally
required. Two kinds of well-known molecules are widely used
as the chiroptical switches, which are sterically overcrowded
unsymmetrical thioxanthenes or analogue and the helicene.[4]
An alternative way in fabricating chiroptical materials is to cova-
lently attach the chiral moiety into the chromophore. Unfortu-
nately, in this latter case, the molecules do not always show the
chiral signals due to the deviation of the chiral moiety from the
chromophores. To address this challenge, the supramolecular
self-assembly provides an efficient way to transfer to the molec-
ular chirality to the supramolecular system and both chiral and
achiral molecules can be involved to construct the chiroptical
systems.[5] Thus, dynamic chiroptical switching is changing
from the molecular level to supramolecular and nanoscale
level. So far, a large number of chiroptical switches have been
developed.[1,6] However, many of them are limited to the switch
by using the individual CD or CPL signals. Here, we provided
the example showing quadruple reversible switches based on
the chiroptical signals in CD, CPL and even their chiral nano-
structures besides the optical reversibility. In addition, such chi-
roptical switch is stable and can be repeated many times.
1. Introduction
Currently, light-triggered dynamic chiroptical switching
between different states in molecules and materials is attracting
great interest.[1] On one hand, from the view of photoresponsive
Dr. W. Miao, Dr. S. Wang
School of Chemistry and Chemical Engineering
Institute of Physical Chemistry
Lingnan Normal University
Development Centre for New Materials Engineering &
Technology in Universities of Guangdong
Zhanjiang 524048, P. R. China
E-mail: miaowangen@iccas.ac.cn
Prof. M. Liu
Beijing National Laboratory for Molecular Science
CAS Key Laboratory of Colloid
Institute of Chemistry
Chinese Academy of Sciences
National Center for Nanoscience and Technology
Beijing 100190, P. R. China
E-mail: liumh@iccas.ac.cn
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201701368.
Figure 1 shows the spiropyran based gelators, which are
composed of the spiropyran moiety and the dialkyl glutamides.
Both of these compounds form a pair of enantiomers and are
DOI: 10.1002/adfm.201701368
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Figure 1. A) Molecular structure of the enantiomeric glutamate derivatives containing spiropyran moiety. And the schematic reversible transformation
between a blue zwitterionic MC form and colorless closed ring state in supramolecular gels upon alternating UV and visible light irradiation. The SEM
images of SP-LG xerogels before (left) and after (right) UV irradiation. B) The schematic reversible transform from the P- and M-helices assembled in
the SP-LG and SP-DG gels to nanofibers in the MC-LG and MC-DG gels upon alternating UV/vis irradiation.
abbreviated as SP-LG or SP-DG, where L and D refer to the
compounds derived from the l- and d-glutamic acid, respec-
tively. Spiropyran and its chemical derivatives are known as
the most widely studied photochromic compounds besides the
well-known fulgides, diarylethenes, and azobenzenes.[7] The
closed form of spiropyran would be transformed into the open
(merocyanine, MC) form under UV light irradiation.[8] Back-
conversion from the MC form to the spiro mode is accelerated
by excitation in the visible band or heating.[9] By attaching the
chiral groups covalently to the spiropyran moiety, it is expected
that the chiroptical properties can be endowed into the system.
However, the phototriggered chiroptical switch based on spiro-
pyran moiety was scarcely presented although the photochromic
switches have been reported extensively. Only a few efforts have
been contributed for such investigation. For example, Angiolini
et al. tried to bond the spiropyran chromophore covalently to
a polymer bearing the optically active l-lactic acid in the side
chain or to binaphthalene moiety.[10] In such instances, they
found that the photoisomerization of the spiropyran group
caused changes in the chiral signals, which allowed the system
to behave as a chiroptical switch. We reported a co-gel system
containing an amphiphilic achiral spiropyran and a chiral
gelator, which could be served as a chiroptical logic circuit
using UV irradiation and acid stimuli.[11] In order to develop
a complete light-driven multichiroptical switches, herein, we
designed two enantiomeric spiropyran-conjugated l(d)-gluta-
mate gelators (SP-LG and SP-DG) and investigated their self-
assembly as well as the chiroptical properties.
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good as that of SP-LG gels. Similar results were obtained from
the investigation of SP-DG gels. Based on these observations,
we speculated the formation of chiral helices are mainly due to
the tetrahedron configuration of closed form from spiropyran,
in which two aromatic rings were distorted and not situated
in the same plane. In gels, the π–π stacking of above aromatic
rings, together with other noncovalent bond interactions, would
lead the gelator molecules to distortion easily. As a result, chiral
helices were observed. However, after UV irradiation, two aro-
matic rings of MC forms are located in the same plane due to
the large conjugated π-bond interaction. Therefore, the distor-
tion might become weaker. As a result, the gelator molecules
self-assembled into nanofibers. The schematic illustration on
the formation of the nanohelices was presented in Figure 1B.
In addition, the similar mirror-imaged helical structure were
also observed in the SP-L(D)G organogels form some other sol-
vents with medium polarity, such as tetrahydrofuran, n-butanol,
and 1,4-dioxane (Figure S3, Supporting Information). This con-
firmed the self-assembly mechanism in the gels from these sol-
vents with medium polarity is similar.
2. Results and Discussion
2.1. Gel Formation and Reversible Switching
of Micromorphology
It was found that both SP-LG and SP-DG could immobilize
many kinds of organic solvents in a relative low critical gela-
tion concentration (CGC), as shown in Table S1 (Supporting
Information), suggesting the good capacity for the formation
of organogels from these compounds. Interestingly, all the gels
were found to be photochromic. For example, in ethyl acetate,
the off-white supramolecular gels from closed SP-LG or SP-DG
compounds were readily responsive to form the blue open
merocyanine (MC-LG or MC-DG) upon UV light irradiation
(Figure 1A). When the UV light was removed, the open mero-
cyanine structure would recover its original closed spiropyran
gradually. In this process, if the additional visible light was
employed, the recovery would be accelerated.
Interestingly, the nano/microstructures of SP-LG or SP-DG
gels were also reversible by the stimuli from UV and visible light
alternately. In ethyl acetate, the SP-LG gels always self-assembled
into P- helices as shown in Figure 1A. Accordingly, the M-helices
could always be observed in SP-DG gels (Figure S1, Supporting
Information). The similar width and pitch in both enantiomeric
gels were ≈300 and 800 nm, respectively. Under UV irradiation,
the P- or M-helices changed into disordered nano fibers without
clear expression of helicities. Subsequent irradiation of visible
light would recover the original helices. Thus, through irradia-
tion of alternating UV and visible light, a regulation of reversible
switch of nano structures was achieved.
In order to understand the packing in the organogels, we
have measured the X-ray diffraction (XRD) patterns for the xero-
gels, as shown in Figure S2 (Supporting Information). In the
case of SP-LG xerogels (Figure S2a, Supporting Information),
the XRD patterns revealed that three peaks appeared at 2θ
values of 1.84°, 3.68°, and 5.52°, respectively. According to Bragg
equation 2dsin θ = nλ, these three peaks suggested 4.80, 2.40,
and 1.60 nm, which corresponded to the 001, 002, and 003 dif-
fractions. Thus, the d spacing was about 4.80 nm. This value was
larger than the extended length of an SP-LG molecule (3.1 nm
from CPK space-filling model), but smaller than twice that of
the molecular length. Thus, it was suggested that the SP-LG
molecules self-assembled into a bilayer structure with the alkyl
chains tilted, while the amide moieties organized into a well-
defined arrangement through strong hydrogen-bonding inter-
actions and the aromatic chromophore exhibited π–π stacking.
As a result, the gelators formed a bilayer structure as the basic
unit and then the multilayered bilayers formed the helices and
nanofibers, as shown in Figure 1B.[12] The π–π stacking of aro-
matic chromophore could be confirmed by the UV–vis spectral
measurements, as will be discussed in the following section.
For the MC-LG xerogels, which were obtained from SP-LG
gels after successive irradiation of UV light (365 nm) for 8 min
(Figure S2b, Supporting Information), a peak at 2θ value of
1.84° was observed, which corresponded to a d space of 4.80 nm.
Although these xerogels showed only one peak, from the struc-
tural similarity, it can be suggested that the MC-LG also formed
a bilayer as the basic unit. The disappearance of the 002 and 003
peaks suggested that the packing of the MC-LG gel was not as
2.2. Optical Switch by UV–Vis Absorption
The optical or photochromic properties were investigated by
UV–vis spectra. Figure 2A showed the UV–vis absorption of
both organogel and solution of SP-LG from ethyl acetate before
and after UV irradiation of 365 nm. As shown in Figure 2A-a,
the colorless SP-LG solution displayed an absorption band
at 332 nm before UV irradiation, which was ascribed to the
internal charge-transfer transition of the closed spiropyran
system.[10] However, in SP-LG organogel, this absorption would
take a red shift to 345 nm (Figure 2A-c). This suggested that
J- like aggregates were formed in SP-LG organogel, where
the chromophores stacked in a head-to-tail way. Following
UV irradiation of 365 nm for 8 min, both the SP-LG solution
and organogel became blue and accompanied by a remark-
able increase of the absorption in the region of 500–650 nm,
implying that the closed-ring state SP was photoconverted to
the blue MC open form. In the case of SP-LG solution after UV
irradiation (Figure 2A-b), the absorption peak appeared around
553 nm, which was attributed to the absorption from the
free MC forms.[11] However, upon UV irradiation, the SP-LG
organogel exhibited absorption band at 577 nm (Figure 2A-d),
which showed a red shift of about 24 nm to that in solution. This
also indicated that the π–π stacking of chromophores formed
in the organogel.[13] In addition, the intensity of the absorption
at 577 nm in the organogel would reach the maximum as the
irradiation time exceed 8 min. Thereafter, after removing UV
irradiation, the blue gels were exposed to the natural condition,
this broad absorption disappeared gradually due to the transfor-
mation of MC into the closed SP form as expected. We detected
such variation and plotted in Figure 2B. It could be seen that
the MC form would change into closed form completely after
2 h. The visible light could accelerate the conversion. From
these observations, it could be concluded that the closed form
of spiropyran is more stable while the open MC form can be
negligible in the natural condition. Thus, in the gels, the open
MC-L(D)G obtained upon UV irradiation could be converted to
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Figure 2. A) Absorption spectra of SP-LG in ethyl acetate solution a) before and b) after UV irradiation and in ethyl acetate organogels c) before and
d) after UV irradiation for 8 min. B) Absorption spectra of blue MC-LG gels under the natural condition for a certain time. C) The reversible switch
of the absorption intensity of SP-LG gels at 577 nm by alternating UV and visible light irradiation. The reversible cycles could be repeated as many
as 50 times. D) Fluorescence spectra of SP-LG gels a) before and b) after UV irradiation for 8 min, the excitation wavelength was 560 nm. The insets
showed the photo of SP-LG gels under UV (365 nm) light illumination. E) Confocal laser scanning microscopy (CLSM) images of SP-LG xerogels after
irradiation by UV light (365 nm) for 8 min, the excitation wavelength was 560 nm. F) The reversible switch of the fluorescence intensity of SP-LG gels
at 662 nm by alternating UV and visible light irradiation.
the closed SP-L(D)G completely. Here, a traditional light-trig-
gered photochromic switch was obtained both in solution and
gels. In the case of SP-DG, similar phenomena were observed,
which was displayed in Figures S4 and S5 (Supporting
Information).
obtained from SP-LG gels after UV irradiation (365 nm) for
8 min, emitted strong red light near 662 nm by the same exci-
tation wavelength. Similar phenomena were obtained in the
observation of luminescence from its xerogels. After UV irradi-
ation, the xerogel also showed strong red fluorescence as indi-
cated by the confocal laser scanning microscopy (CLSM) image
(Figure 2E), where red fibers were observed. Therefore, the
strong red fluorescence of MC-LG gels can be switched off by
additional visible light irradiation due to the conversion of the
MC form into the closed form of SP. In this way, the reversible
regulation of fluorescence from supramolecular gels by alter-
nating UV and visible light irradiation was obtained as shown
2.3. Optical Switch by Fluorescence
Interestingly, when excited by 560 nm laser light, the closed off-
white SP-LG gels showed almost no luminescence as shown in
Figure 2D. However, the open blue MC-LG gels, which were
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in Figure 2F, where the changes of the fluorescence intensity at
662 nm were investigated. As a result, a reversible fluorescence
switch was developed by elicitation of alternating UV and vis-
ible light. The SP-DG gels and their xerogels showed the same
results as SP-LG, which were displayed in Figures S6 and S7
(Supporting Information).
UV light, strong CD signals in visible region appeared. As
shown in Figure 3A-c, two new positive signals from SP-LG
gels appeared at 616 and 661 nm, respectively. Accordingly,
the mirror-imaged CD signals exhibited in SP-DG gels
(Figure 3A-d). It seemed that the CD band at 662 nm was
derived from the UV-absorption. However, to our curiosity,
the absorption of the same gels detected in CD measure-
ment was a little different from that of the UV–vis spectra
investigation. As shown in Figure S9 (Supporting Informa-
tion), not a single peak at 577 nm (Figure 2A) but two absorp-
tion at 577 and 626 nm occurred when irradiated by UV
light. In addition, such phenomena were investigated in all
gels from various solvents, such as cyclohexane and DMF in
Figures S10 and S11 (Supporting Information). To sum up,
the CD detection was always in good accordance with the
observation of UV spectra no matter before and after UV irra-
diation. Moreover, the CD signals from MC-LG and MC-DG
gels in the region of 550–750 nm would disappear completely
in the natural condition or upon visible light irradiation for a
certain time, which further confirmed that the closed form of
spiropyran is more stable in the natural condition. Therefore,
like the UV absorption and fluorescent emission, the CD sig-
nals from SP-LG and SP-DG gels could also be switched as
on-off by alternating UV and visible light. Thus, as shown in
Figure 3B, the chiroptical switch was obtained.
2.4. Chiroptical Switch by CD
The enantiomeric SP-LG and SP-DG gelator molecules have
the chiral center localized in the glutamic moiety. While in
the molecular state, for example in solution, such localized
chirality cannot be transferred to the chromophore. As a
result, no CD signal was detected in the absorption region
of 250–700 nm. However, this molecular chirality could be
transferred to the chromophore of spiropyran as supramo-
lecular chirality upon gelation. Figure 3A showed the CD
spectra of SP-LG and SP-DG gels from ethyl acetate before
and after UV irradiation. Before UV irradiation, a positive
signal centered at 410 nm in SP-LG gels was observed due
to the self-assembly (Figure 3A-a). In the case of SP-DG gels,
the mirror-imaged CD spectrum (Figure 3A-b) could always
be obtained. This confirmed that the supramolecular chi-
rality was due to the chirality transfer. Under exposure to
Figure 3. A) CD spectra of a) SP-LG and b) SP-DG gels before UV irradiation and those of c) MC-LG and d) MC-DG gels after UV irradiation for 8 min.
B) Corresponding reversible switch of the G value at 662 nm of enantiomeric gels by the stimuli of alternating UV and visible light. C) CPL of a) SP-LG
and b) SP-DG gels before UV irradiation and those of c) MC-LG and d) MC-DG gels after UV irradiation for 8 min. D) Corresponding reversible switch
of the CPL intensity at 675 nm of enantiomeric gels by the stimuli of alternating UV and visible light irradiation.
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2.5. Chiroptical Switch by CPL
The supramolecular gels having CPL prop-
erty are attracting current interest.[14] Gen-
erally, it is necessary the gel possesses both
chirality and luminescence for obtaining
CPL gels. Since the solution of these com-
pounds showed no CD signals, they showed
no CPL. On the other hand, the closed
SP-LG and SP-DG showed no lumines-
cence. Thus, no CPL in gels was detected
as shown in Figure 3C-a,b. However, the
MC-LG and MC-DG gels obtained from
SP-LG and SP-DG gels under exposure
to UV light exhibited both CD and fluo-
rescenece, which gave rise to the obvious
CPL properties (Figure 3C-c,d). Under irra-
diation of visible light, such CPL signals
disappeared along with the transforma-
tion from MC forms to original closed SP
modes. Consequently, the chiroptical CPL
switches were obtained from the supramo-
lecular gels by alternating UV and visible
light, which were illustrated in Figure 3D.
Such cycle can also be repeated many times.
In addition, both MC-LG and MC-DG gels
showed the mirror-CPL.
2.6. Soft Recoding and Rewritable Printing
Figure 4. A) Illustration of the recording of ‘‘GEL’’ characters on SP-LG gels by exposure to UV
The reversible process of the gels can be fur- light (365 nm) for 2 min, and erasing the recorded pattern by exposure to the natural condition
for 2 h or under the irradiation of visible light for 5 min. B) The printing letters and complex
ther extended to the soft recoding and rewrit-
able printing by light. The MC form in gels is
more stable than that in solution, which facil-
patterns from the rewritable SP-LG xerogels on a glass (15 × 20 cm2) using photomask upon
UV light irradiation. The quotation in panel (B) is from the Bible.
itates the development of soft materials for
information storage. As illustrated in Figure 4A, the off-white
gel from spiropyran derivatives in quartz cell was covered with
3. Conclusion
a photomask containing the characters ‘‘GEL’’ and these charac-
ters became apparent after exposure to UV light for 2 min. This
pattern is relatively stable and could be exhibited for ≈2 h in the
natural condition as above-mentioned. To erase the written data
and regenerate the original gel state rapidly, the gels should be
either illuminated by visible light for 5 min or heated at 50 °C
for 3 min.
Moreover, it was found that the xerogels on a glass
(15 × 20 cm2) could also be rewritten by letters and all kinds of
patterns upon UV light irradiation for 8 min through a photo-
mask, which was preproduced by ink-jet printing on a plastic
transparency, as shown in Figure 4B. Such written patterns
on xerogels were much more stable than those on gels, which
could be investigated even for four days under the natural con-
dition. To erase the written data, the written patterns should
be exposed to visible light over 40 min, which was much
longer than that for gels. Or else, the prints could be erased
completely at 80 °C for 30 min. In addition, such rewritable
material can be efficiently printed over 30 cycles without sig-
nificant loss in contrast and resolution using ultraviolet and
visible light.
We designed and prepared two enantiomeric super gluta-
mate gelators containing spiropyran moiety. It was found that
these two enantiomeric gelators could immobilize all tested
solvent. Moreover, by the stimuli of alternating UV and vis-
ible light, the UV–vis absorption, fluorescence, CD signal, and
CPL of the enantiomeric supramolecular gels exhibited on–
off properties. Therefore, a reversible quadruple optical and
chiroptical switch was developed successfully. During such
process, the self-assembled nanostructures from the enantio-
meric supramolecular gels also underwent a reversible change
between helices and fibers under the alternating UV and vis-
ible light trigger. Furthermore, a rewritable material fabricated
from their xerogels on a glass was developed. Such rewritable
material can be efficiently printed over 30 cycles without sig-
nificant loss in contrast and resolution using ultraviolet and vis-
ible light.
4. Experimental Section
Materials: All starting materials were obtained from commercial
suppliers and used as received. The solvents used were dried and
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and 0.20 g of 1-hydroxybenzotrizole (HOBT, 1.5 mmol) were added.
The mixture was stirred at room temperature for 72 h. After removal
of the solvents, the remained substance was dissolved in 100 mL of
tetrahydrofuran and then poured into the 1000 mL of 0.5% Na2CO3
aqueous solution. The solid was collected by filtration, dried in vacuum,
and then recrystallized from the mixed solvent of CH2Cl2 and EtOH
(1:4, v/v, 3 × 100 mL) to afford 0.86 g of pure purplish product, SP-LG
(yield: 85%). mp: 116–117 °C; 1H NMR (400 MHz, CDCl3, δ): 0.90
(t, J = 6.0 Hz, 6H), 1.15–1.38 (m, 66H), 1.42–1.50 (m, 4H), 1.87–2.00
(m, 2H), 2.17–2.36 (m, 2H), 2.45–2.59 (m, 2H), 3.11–3.20 (m, 4H),
3.51–3.69 (m, 2H), 4.25–4.29 (t, J = 6.8 Hz, 1H), 5.95 (m, 2H), 6.68–6.72
(t, J = 6.8 Hz, 1H), 6.77–6.80 (m, 1H), 6.85–6.95 (m, 2H), 7.09 (d,
J = 6.8 Hz, 1H), 7.18–7.22 (m, 2H), 8.01–8.04 (m, 2H). MALDI-TOF-MS:
calcd for C62H101N5O6: 1012.5. Found: 1034.9 [M+ + Na]. Anal. calcd for
C
62H101N5O6: C 73.55, H 10.05, N 6.92; found: C 73.45, H 10.16, N 7.07.
Synthesis of Target Compound SP-DG: 0.38 g of SP-COOH (1 mmol)
and 0.65 g of N,N′-Bisoctadecyl-D-aminoglutamicdiamide (DG) were
dissolved in 150 mL of dichloromethane. Then, 0.29 g of EDC·HCl
(1.5 mmol) and 0.20 g of HOBT (1.5 mmol) were added. The mixture
was stirred at room temperature for 72 h. After removal of the solvents,
the remaining substance was dissolved in 100 mL of tetrahydrofuran
and then poured into the 1000 mL of 0.5% Na2CO3 aqueous solution.
The solid was collected by filtration, dried in vacuum, an then
recrystallized from the mixed solvent of CH2Cl2 and EtOH (1:4, v/v,
3 × 100 mL) to afford 0.77 g of pure purplish product, SP-DG (yield:
76%). mp: 116–117 °C; 1H NMR (400 MHz, CDCl3, δ): 0.90 (t, J =
6.0 Hz, 6H), 1.15–1.38 (m, 66H), 1.42–1.50 (m, 4H), 1.87–2.00 (m, 2H),
2.17–2.36 (m, 2H), 2.45–2.59 (m, 2H), 3.11–3.20 (m, 4H), 3.51–3.69
(m, 2H), 4.25–4.29 (t, J = 6.8 Hz, 1H), 5.95 (m, 2H), 6.68–6.72 (t, J =
6.8 Hz, 1H), 6.77–6.80 (m, 1H), 6.85–6.95 (m, 2H), 7.09 (d, J = 6.8 Hz,
1H), 7.18–7.22 (m, 2H), 8.01–8.04 (m, 2H). MALDI-TOF-MS: calcd
for C62H101N5O6: 1012.5. Found: 1034.9 [M+ + Na]. Anal. calcd for
Scheme 1. Synthetic route for enantiomeric SP-LG and SP-DG gelator
molecules.
C
62H101N5O6: C 73.55, H 10.05, N 6.92; found: C 73.29, H 10.02, N 7.15.
Gel Formation: The preparation of supramolecular gels was under
purified by standard methods prior to use. 1H NMR spectra were
obtained on an ARX400 (Bruker) NMR spectrometer in DMSO-d6 or
CDCl3 with TMS as an internal standard. MS spectra were determined
with BEFLEX III for MALDI-TOF mass spectrometer. Elemental analyses
were performed on a Carlo-Erba-1106 instrument. The synthetic route
for SP-LG and SP-DG was exhibited in Scheme 1 as following.
Synthesis of Carboxyl-Containing Spiropyran (SP-COOH): The synthetic
method was modified from previous report.[15] The main procedures
were as follows: All the reactions should be performed in the dark. 3.18 g
of 2,3,3-trimethylindolenine (20 mmol) and 4.0 g of 3-iodopropanoic acid
(20 mmol) were mixed in a sealed round bottom flask. Then, the reaction
mixture was stirred vigorously under nitrogen at the temperature of
85 °C for 2 h. After cooling to room temperature, the residue was
dissolved in 500 mL of water and washed with chloroform (3 × 100 mL).
The organic layer was removed, while the aqueous phase was evaporated
to give 5.4 g of pure l-(b-carboxyethyl)-2,3,3-trimethylindolenine iodide
(yield: 75%), which was not further purified for next step.
3.6 g of l-(b-carboxyethyl)-2,3,3-trimethylindolenine iodide (10 mmol),
1.67 g of 5-nitrosalicylaldehyde (10 mmol), and 1 mL of piperidine
were dissolved in 50 mL of 2-butanone and then heated to reflux under
nitrogen for 4 h. After cooling to room temperature, the yellowish crude
product was filtered, dried under vacuum, and then recrystallized from
acetone (2 × 100 mL) to harvest 2.67 g of pure SP-COOH (yield: 70%).
mp: 206 °C; 1H NMR (400 MHz, DMSO-d6, δ): 1.08 (s, 3H), 1.19 (s,
3H), 2.45–2.55 (m, 2H), 3.45–3.55 (m, 2H), 5.95 (d, J = 10.4 Hz, 1H),
6.66 (d, J = 8.0 Hz, 1H), 7.11–7.15 (m, 2H), 7.20–7.23 (d, J = 10.4 Hz,
2H), 7.99–8.02 (m, 1H), 8.22 (d, J = 2.8 Hz, 2H), 12.2 (s, 1H). Anal.
calcd for C21H20N2O5: C 66.31, H 5.30, N 7.36; found: C 66.52, H 5.35,
N 7.32.
ambient temperature and visible light conditions (λ > 500 nm). A
weighed sample of SP-LG or SP-DG gelator was mixed with a solvent
(1 mL) in a septum-capped vial and heated until the solid was dissolved.
Then the sample vial was cooled to room temperature. If no flow was
observed when inverting the vial, a stable gel was formed. The CGC of the
gelators was determined by measuring the minimum amount of gelator
required for the formation of a stable gel at room temperature. For the
experimental measurement of all kinds of optical properties, the gels
containing 4 mg of SP-LG or SP-DG from ethyl acetate were employed
all the while. The obtained supramolecular gels from SP-LG or SP-DG
gelators are off-white, which would become blue after irradiation with
UV light (365 nm) for 8 min. The light source used for the UV-induced
opening process (SP→ MC) was UV lamp (λ = 365 nm, 40 W) and
the distance of UV source–sample used in the irradiation experiments
was 10 cm with a light intensity of 280 mW cm−2 in the dark. Natural
condition (ambient temperature and visible light conditions) or a visible
lamp with intensity of 520 mW cm−2 was employed for the recovery of
the gels. After exposure to natural environment for 2 h or irradiation
with visible light for 5 min, the blue gels faded to original off-white color.
Characterization: The supramolecular gels of SP-LG and SP-DG
compounds were cast onto single-crystal silica plates, the solvent was
evaporated under the ambient conditions and visible light, and then
vacuum-dried, while the silica plates fabricated with MC-LG and MC-DG
gels were dried under UV irradiation all the while. The sample surface
was coated with Pt, which were recorded on a Hitachi S-4800 FE-SEM
microscope, operating at accelerating voltages of 10 kV. Glass-sustained
xerogel films were used for XRD measurements on a Rigaku D/Max-2500
X-ray diffractometer (Japan) with CuKa radiation (λ = 1.5406 Å), which
was operated at 45 kV, 100 mA. UV and CD spectra were obtained on
JASCO UV-550 and JASCO J-810 CD spectrophotometers, respectively.
The CLSM system, FV-1000-IX81 Olympus (Japan), was employed.
Synthesis of Target Compound SP-LG: 0.38 g of SP-COOH (1 mmol)
and 0.65
g
of N,N′-Bisoctadecyl-l-aminoglutamicdiamide (LG),
which were synthesized according to our previous works,[16] were
dissolved in 150 mL of dichloromethane. Then, 0.29 g of 1-ethyl-3-(3-
dimethyllaminopropyl)carbodiimide hydrochloride (EDC·HCl, 1.5 mmol)
Fluorescence spectra were measured with
a
Hitachi F-4500
spectrophotometer. CPL measurements were performed with a JASCO
CPL-200 spectrometer.
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Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work was supported by the Basic Research Development Program
(2013CB834504), the National Natural Science Foundation of China
(Nos. 91027042, 21321063, 21227802, 21474118, and 21372194),
“Strategic Priority Research Program” of the Chinese Academy of
Sciences (XDA09020102 and XDB12020200), the Natural Science
Foundation of Guangdong Province, China (No. S2013010011844), and
the Fund of the Chinese Academy of Sciences.
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Conflict of Interest
The authors declare no conflict of interest.
Keywords
chiroptical switches, helices, optical switches, rewritable materials,
supramolecular chirality
Received: March 15, 2017
Revised: April 11, 2017
Published online:
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