Photo/chemo dual-controlled reversible morphological conversion and chiral modulation of supramolecular nanohelixes with nanosquares and nanofibers

Article abstract of DOI:10.1039/C6CC07089B

A photo/chemo dually interconvertible system was constructed through the supramolecular assembly of azobenzene-diphenylalanine (Azo-FF) with α-cyclodextrin. The resultant chiral nanohelix was able to interconvert into a nanosquare upon irradiation at different wavelengths, but into a nanofiber upon changing solvent polarity, which provides a feasible way to achieve highly ordered nanostructures with various morphologies, dimensions and chiralities.

Full text of DOI:10.1039/C6CC07089B

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Photo/chemo dual-controlled reversible
morphological conversion and chiral modulation
of supramolecular nanohelixes with nanosquares
and nanofibers†
Cite this:DOI: 10.1039/c6cc07089b
Received 30th August 2016,
Accepted 16th November 2016
a
a
a
a
ab
Wen Zhang, Yong Chen, Jie Yu, Xu-Jie Zhang and Yu Liu*
DOI: 10.1039/c6cc07089b
www.rsc.org/chemcomm
1
1
A photo/chemo dually interconvertible system was constructed though the addition/extraction of supramolecular compartments.
through the supramolecular assembly of azobenzene-diphenylalanine Recently, we reported a photocontrolled reversible nanotube–
8
d
(Azo-FF) with a-cyclodextrin. The resultant chiral nanohelix was able to nanoparticle conversion mediated by b-cyclodextrin dimers.
interconvert into a nanosquare upon irradiation at different wave- More recently, when we were preparing this manuscript, Chi,
lengths, but into a nanofiber upon changing solvent polarity, which Tian and Zhu reported a photocontrolled reversible nanohelix-
provides a feasible way to achieve highly ordered nanostructures amorphous particle conversion accompanied by the dynamic
1
2
with various morphologies, dimensions and chiralities.
amplification of chirality with photoreversibility.
On the other hand, the diphenylalanine (L-Phe–L-Phe, FF)
Supramolecular assemblies with stimuli-responsivity have attracted peptide, a core recognition motif of Alzheimer’s b-amyloid poly-
considerable research interest because of their applications in peptide, was found to be able to self-assemble into stable nano-
13
various fields such as drug carriers, catalysts, sensors, protein tubes in 2003. From then on, lots of effort was contributed to
1
probes, and optoelectronic materials. Various external triggers, organize FF-based building blocks into various well-defined supra-
2
3
4
5
6
such as pH, temperature, solvent photoirradiation, enzyme,
molecular nanostructures such as nanotubes, spherical vesicles,
7
and (electro)chemical redox, have been used in these systems. nanofibers, nanowires, macroporous honeycomb scaffolds and
Among them, light is widely regarded as a very important peony-flower-like hierarchical nanostructures through the self-
approach to achieve the smart control of supramolecular assembly assembly of diphenylalanine and its derivatives such as Ac-FF,
5a,6a,14,15
because of its noninvasive, clean and remote-controlling proper- Boc-FF, Z-FF, Cbz-FF, Nap-FF, Fc-FF and Cys-FF.
He et al.
ties. In the last decade, many photocontrolled supramolecular introduced a ferrocene group to diphenylalanine and made it
8
16
assemblies based on azobenzene–cyclodextrin have been con- capable of being reversibly controlled by altering its redox state.
structed taking advantage of the reversible photoisomerization of They also created a range of rigid chiral nanostructures through
1
4c
azobenzene and the different binding behaviors of trans- and the modulation of counterions, temperature, and solvent.
cis-azobenzenes with cyclodextrin. However, most of these studies Li and co-workers used the photoswitchable sulfonicazobenzene
9
were focused on the photocontrolled assembly/disassembly to co-assemble with diphenylalanine to achieve the photo-induced
5
a
process, while the morphological conversion of supramolecular structural transition. Therefore, one can believe that the
assembly switched by light irradiation was rarely reported. association of azobenzene–cyclodextrin with FF may become a
Kim et al. reported a chemocontrolled one-cycle nanotube– significant approach to the construction of nano-scaled supra-
nanoparticle morphological conversion of pyrene-modified molecular assemblies with a smart stimuli-responsivity and struc-
dendritic self-assembly induced by the sequential addition of tural diversity. In this work, we wish to report a photo/chemo
10
cyclodextrin and poly(propylene glycol). Stupp et al. reported a dual-controlled nanotube based on azobenzene–diphenylalanine
chemocontrolled reversible nanohelicoidal–nanocylinder conver- (Azo-FF) and a-cyclodextrin (Scheme 1), which can be reversibly
sion of a simultaneous covalent and noncovalent hybrid polymer converted into a nanosquare using a photocontrol but into a
nanofiber using a chemocontrol.
Azo-FF was prepared via a four-stepped synthesis as described
in Scheme S1 (ESI†). Firstly, diphenylalanine was esterified
a
Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry,
Nankai University, Tianjin 300071, P. R. China. E-mail: yuliu@nankai.edu.cn
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300071, P. R. China
b
through a reaction with AcCl and CH OH, and the residue was
3
cast to the next step directly. After the reaction with bromoacetyl
bromide at a low temperature (À10 1C), N-terminus-modified
bromide was obtained and then undertook a substitution
†
Electronic supplementary information (ESI) available: Detailed synthesis,
characterization data, Job’s plot, 2D NMR spectra, DLS date, CD, and scanning
electron micrograph. See DOI: 10.1039/c6cc07089b
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Scheme 2 Self-assembly/disassembly based on the photo-responsive
interconversion of Azo-FF and a-cyclodextrin.
Scheme 1 Structural illustration of the host and guest and the dual-
controlled conversion of self-assembly.
light (Fig. S6, ESI†). A possible reason for these observations
may be that the photo-induced conversion from trans-Azo-FF to
cis-Azo-FF led to the dissociation of the Azo-FF/a-cyclodextrin
complex (Scheme 2), because a-cyclodextrin was reported as
reaction with hydroxyazobenzene to give compound 3. Through
a subsequent hydrolysis under basic conditions, Azo-FF was
obtained as an orange solid in 44% yield.
Generally, upon irradiation with UV light, the trans-isomer
of an azobenzene compound can transform to the cis-isomer,
which can reverse back to the trans-isomer through either visible
1
8
unfavorable to bind the cis-isomer of azobenzene. In addition,
the mass spectrum also gave evidence for the photo-responsive
host–guest association/dissociation (Fig. S5 and S7, ESI†).
1
light irradiation or heating. Herein, H NMR experiments were
1
Through a comparison of the integral area in the H NMR spectra,
performed in D O : DMSO (v : v/9 : 1) to investigate the binding
2
the conversion ratio of cis–trans isomerization was calculated
to be 90%.
behavior of the trans- (trans-Azo-FF) or cis-isomer (cis-Azo-FF) of
Azo-FF with a-cyclodextrin. With the addition of a-cyclodextrin to
1
The interconversion between the trans-Azo-FF/a-cyclodextrin
and cis-Azo-FF/a-cyclodextrin systems can also be tracked using
UV-vis spectroscopy. As seen in Fig. 2a, the absorption band
of the trans-Azo-FF/a-cyclodextrin complex around 344 nm
dramatically decreased under the irradiation at 365 nm, accom-
panied by the obvious hypsochromic shift of the absorption
maximum. In addition, the appearance of a new absorption
band around 431 nm and a clear isosbestic point at 410 nm
jointly indicated the one-step transformation of the trans-Azo-FF/
a-cyclodextrin complex to the simple mixture of a-cyclodextrin
with cis-Azo-FF, because the free cis-Azo-FF has a p–p* transition
that is less intense and blue-shifted relative to its trans isomer as
well as a weak n–p* transition that is more intense in the cis form
a solution of trans-Azo-FF, the H NMR signals of azobenzene
protons (marked by orange dots) of trans-Azo-FF shifted to the
1
low field, while the H NMR signals of aromatic protons of
the diphenylalanine moiety (marked by green dots) showed no
significant shifts (Fig. 1a and b). Moreover, the NOESY spectrum of
an equimolar trans-Azo-FF + a-cyclodextrin mixture showed the
clear NOE correlations between the inner protons of a-cyclodextrin
and the azobenzene protons of Azo-FF (Fig. S4, ESI†). These jointly
indicate that the azobenzene moiety of trans-Azo-FF was included
in the a-cyclodextrin cavity to form the trans-Azo-FF/a-cyclodextrin
17
complex. On the other hand, after irradiating the trans-Azo-FF/
a-cyclodextrin complex using UV light (365 nm) for 10 min, all of
the aromatic protons in the Azo-FF/a-cyclodextrin complex gave
1
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1
than in the trans one in the visible region. However, the
decreased intensity at 344 nm was recovered to its original level
upon the subsequent irradiation at 450 nm. A possible reason
may be that, owing to the reverse photoisomerization from cis
to trans form of Azo-FF, the simple mixture of cis-Azo-FF with
a-cyclodextrin could revert to the trans-Azo-FF/a-cyclodextrin
H NMR signals very similar to those of free cis-Azo-FF (Fig. 1c
and d). Moreover, no NOE correlations between the inner protons
of a-cyclodextrin and the protons of Azo-FF could be observed
after expositing the trans-Azo-FF/a-cyclodextrin complex to UV
Fig. 2 (a) UV-vis spectra of Azo-FF/a-cyclodextrin (1 :1) (0.03 mM in H
to determine the photoisomerization rate constant (k ) in water. The duration
of UV light was set at 0 - 121 s at 365 nm. Inset: Determination of the
value of Azo-FF/a-cyclodextrin (1 :1) upon exposure to UV light at 365 nm.
2
O)
t
1
Fig. 1 Partial H NMR spectra (400 MHz, D
2
O : DMSO-d
6
/(v/v) 9 : 1, 25 1C)
k
t
of (a) trans-Azo-FF, (b) trans-Azo-FF + a-cyclodextrin (1 : 1), and (c) after
65 nm irradiation of (a) and (d) after 365 nm irradiation of (b), Azo-FF =
a-cyclodextrin] = 1 mM.
(b) Cyclic responses of the absorbance values at 344 nm of trans-Azo-FF/
a-cyclodextrin solution (0.03 mM) on alternating irradiation between UV
and visible light.
3
[
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system (Scheme 2). Significantly, this assembly/disassembly side length of 100–500 nm (Fig. 3c). In the control experiment,
cycle could be repeated several times (Fig. 2b). In addition to similar nanosquares could also be obtained by irradiating the
the light irradiation, the continuous heating of a cis-Azo-FF/ nanofibers of free trans-Azo-FF at 365 nm (Fig. S11, ESI†).
a-cyclodextrin mixture at 80 1C for 2 h could also result in the Besides the SEM images, the morphological conversion of the
conversion of the cis-Azo-FF + a-cyclodextrin mixture to the assembly could also be distinguished by the naked eye through the
trans-Azo-FF/a-cyclodextrin system (Fig. S8, ESI†). By utilizing turbidity experiments. As shown in Fig. 3d, the subtransparent
first-order kinetics, the trans - cis photoisomerization rate trans-Azo-FF/a-cyclodextrin solution became completely transparent
constant (k
t
À1
) of trans-Azo-FF in solution was measured to be after the irradiation of UV light for 10 min due to the conversion of
À1
0.03628 s (Fig. S9, ESI†), but increased to 0.04096 s in the the nanohelix to the nanosquare, and the resultant transparent
presence of 1.0 equiv. of a-cyclodextrin (Fig. 2a, inset). This result solution of the nanosquare would revert to being subtransparent
suggested that the association of Azo-FF with a-cyclodextrin could after the irradiation of visible light owing to the photoreversible
20
accelerate the isomerization of Azo-FF.
nanosquare - nanohelix transformation. DLS data (Fig. S12, ESI†)
Moreover, the circular dichroism spectrum of trans-Azo-FF showed that the assembly of the trans-Azo-FF/a-cyclodextrin
3
À1
À1
showed a positive Cotton effect peak (De = 2.63 dm mol cm
at 325 nm and negative Cotton effect peak (De
)
complex and the trans-Azo-FF monomer possess a hydrodynamic
diameter of up to thousands of nanometers, which is dramatically
a
=
3
À1
À1
À3.17 dm mol cm ) at 367 nm, but these peaks changed larger than the diameter observed for their cis-Azo-FF system.
3
À1
À1
to De = 4.06 dm mol
cm
at 329 nm and De = Consequently, UV-vis, circular dichroism, turbidity, DLS and
3
À1
À1
À1.39 dm mol cm at 382 nm, respectively, in the circular SEM experiments jointly confirmed the good reversibility and
dichroism spectrum of the trans-Azo-FF/a-cyclodextrin system, repetitiveness of the photocontrolled nanohelix–nanosquare
probably due to the chirality transfer of FF and a-cyclodextrin to morphological conversion as well as the chiroptical property
the azobenzene moiety. With the gradual irradiation of UV change. A similar photo-switchable nanofiber–nanosquare
light, the circular dichroism signal intensity of either the trans- morphological conversion was also observed in the case of free
Azo-FF/a-cyclodextrin system or free trans-Azo-FF gradually Azo-FF (Fig. S11, ESI†).
decreased and nearly vanished after 80–100 s (Fig. S10, ESI†).
Combining the results described above, we propose a possible
The circular dichroism experiment showed that the trans-Azo- mechanism for the assembly and transformation process. As
FF/a-cyclodextrin or trans-Azo-FF system would lose most of its illustrated in Fig. 4, mainly through the p–p stacking among
chirality when irradiated by UV light.
aromatic units, the free trans-Azo-FF, which adopted a linear
The photo-controlled morphological interconversion of the conformation, tended to self-assemble along the one-dimensional
Azo-FF/a-cyclodextrin assembly was characterized using a scanning direction to form nanofibers. In contrast, the cis-Azo-FF, which
electron microscope (SEM). As shown in Fig. 3a, the free trans- adopted an L-type conformation, firstly formed a quadrate dimer
Azo-FF exists as one-dimensional ambiguous spiral nanofibers through hydrogen bonding between the carboxylic groups of FF
with a uniform width of ca. 25 nm and a length of over 10 mm, fragments, and then the resultant dimers further self-assembled
although a very slight helix could not be rigorously ruled out. along the two-dimensional direction mainly though p–p stacking
However, the SEM image of the trans-Azo-FF/a-cyclodextrin system among aromatic units to form nanosquares. With the addition of
showed a number of right-handed nanohelixes (Fig. 3b) with a a-cyclodextrin, the azobenzene moiety of trans-Azo-FF would be
length of over 10 mm, a width of ca. 40 nm and a helical pitch of included into the cavity of a-cyclodextrin. Therefore, the large
ca. 100 nm, which were conformationally uniform in size and steric hindrance between the adjacent a-cyclodextrin cavities
shape. Significantly, after the irradiation of the trans-Azo-FF/ would lead to twisting of the trans-Azo-FF units in the nanofiber
a-cyclodextrin solution at 365 nm for 3 min, the original nano- and consequently the conversion of the nanofiber into a
helixes turned to a number of nonhelical nanosquares with a nanohelix. Therein, the D-configuration of glucose moieties in
a-cyclodextrin was deduced to play an important role in the
formation of a right-hand twist, although there was no direct
evidence. On the other hand, once the trans-Azo-FF/a-cyclodextrin
nanohelix was exposed to UV light, trans-Azo-FF photo-isomerized
Fig. 3 SEM images of (a) nanofibers formed by trans-Azo-FF and (b) nano-
helixes formed by trans-Azo-FF/a-cyclodextrin. (c) trans-Azo-FF/a-cyclodextrin
after being irradiated by UV light (365 nm) for 3 min. (d) Photograph of
Azo-FF/a-cyclodextrin solution under UV and visible light.
Fig. 4 Possible assembly mode of Azo-FF and trans-Azo-FF/a-cyclodextrin.
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