A Photoinduced Reversible Phase Transition in a Dipeptide Supramolecular Assembly

Article abstract of DOI:10.1002/anie.201711547

Tunable supramolecular assembly has found various applications in biomedicine, molecular catalysis, optoelectronics, and nanofabrication. Unlike traditional covalent conjugation, non-covalent introduction of a photoswitchable moiety enables reversible pho

Full text of DOI:10.1002/anie.201711547

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Accepted Article
Title: Photoinduced Reversible Phase Transition of Dipeptide
Supramolecular Assembly
Authors: Xianbao Li, Jinbo Fei, Youqian Xu, Dongxiang Li, Tingting
Yuan, Guangle Li, Chenlei Wang, and Junbai Li
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201711547
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10.1002/anie.201711547
Angewandte Chemie International Edition
ICATION
duced Reversible Phase Transition of Dipeptide
olecular Assembly
a, b Jinbo Fei, a Youqian Xu, a, b Dongxiang Li, c Tingting Yuan, a, b Guangle Li, a, b Chenlei
d Junbai Li*, a, b
variations of the system temperature and the solute
concentration lead to phase transitions. At present, for FF-based
gels, much attention has been paid to stimulus-responsive gel-
crystal transitions to construct various well-defined nano- or
microstructures, such as tubes, fibers, spheres, plates and
flowers. [10, 11] Nevertheless, in situ reversible gel-sol transitions
triggered by external non-thermo stimulations, especially light,
are fairly unexplored.
ble supramolecular assembly has found various
omedicine, molecular catalysis, optoelectronics and
Unlike traditional covalent conjugation, non-covalent
a
photoswitchable moiety enables reversible
of non-photosensitive dipeptide supramolecular
r light illumination, a long-lived photoacid generator
n and mediates the dissociation of dipeptide-based
by resulting in sol formation. Under darkness, the
In this communication, by non-covalently introducing a long-
lived photoacid generator, we develop a facile and universe
strategy to achieve the light-controlled reversible gel-sol
transition of FF supramolecular assembly. As shown in Figure 1,
upon exposed to visible light, the photoacid generator,
protonated merocyanine (MEH), undergoes a light-induced ring-
closing reaction to donate a proton to the environment and
changes into spiropyran (SP), while primary amine group in FF
accepts this proton. Consequently, the increasing electrostatic
interaction will break the previous π-π stacking of FF
supramolecular assembly, resulting in gel-to-sol transition.
Under dark, the opposite process (sol-to-gel transition) occurs,
following the SP-to-MEH transformation. Importantly, the
introduction of MEH has no obvious effect on the intrinsic
supramolecular assembly of FF. Outstandingly, there is no
formation of side product after photoreaction during the course
of phase transitions for several times. In addition, accompanying
the reversible gel-sol transition in a spatially controlled manner,
renewable patterns can be readily fabricated. This strategy can
be extended to reversibly tune phase transitions of other FF-
based organogels.
e
moiety entraps a proton, resulting in gel
rthermore, accompanying the isothermal recycled
in a spatially controlled manner, renewable patterns
sly fabricated. This new concept of light-controlled
of amino acid-based supramolecular assemblies will
sibility of wide applications.
assembly has achieved great success in the
of
functional
well-defined
nano-
or
es through non-covalent molecular interactions
p approach.
ely applied in many fields including molecular
edicine, energy conversion, optoelectronics and
[1, 2]
Such assembled architectures
[3-5]
To date, these supramolecular assemblies
ely designed and smartly controlled by external
ch as pH, temperature, electric or magnetic fields,
olecules. [6-8]Among them, light regulation is
to its favourable features of remote control and
on. However, reversible photocontrol of
[8a]
assembly still remains a great challenge.
certain class of photoswitchable moieties (such
, spiropyran and azobenzene) were covalently
non-photoresponsive building blocks to achieve
[8d]
molecular assemblies.
Inevitably, these
cations are always facing the issues including
ficiency and complicated purification. Therefore,
nt of simple and efficient approaches is in great
minimalist amphiphilic aromatic dipeptide,
(FF) has attracted increasing attention because
[9-12]
structure and tunable self-assembly.
In
xhibits excellent low molecular weight gelation
any pure or mixed organic solvents. [11] In nature,
[13]
astable systems.
Basically, gels can be
thermo-dynamically stable solutions (gel-to-sol
crystals (gel-to-crystal transition). Conventionally,
Figure 1. Schematic representation of light-controlled reversible gel-sol
transition of non-photosensitive diphenylalanine supramolecular assembly.
[14]
MEH was synthesized according to the previous reports.
[a]
X. Li, Dr. J. Fei, Y. Xu, T. Yuan, G. Li, C. Wang, Prof. Dr. J. Li.
Beijing National Laboratory for Molecular Sciences (BNLMS), CAS
Key Lab of Colloid, Interface and Chemical Thermodynamics,
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190, China.
Its chemical structure is confirmed by 1H nuclear magnetic
resonance (NMR) spectrum and mass spectroscopy,
respectively (Figure S1). It has been found MEH is a long-lived
photoacid generator and the half-life of the proton-dissociation
state is about 70 s, longer than that of other photoacid
E-mail: jbli@iccas.ac.cn
[14a,
X. Li and J. Fei contributed equally to this work.
X. Li, Y. Xu, T. Yuan, G. Li, C. Wang, Prof. Dr. J. Li.
University of Chinese Academy of Sciences, Beijing 100049, China.
Prof. D. Li
College of Chemistry and Molecular Engineering, Qingdao
University of Science and Technology, Qingdao 260042, China
generators such as triphenylsulfonium triflate and pyranine.
14c]
[b]
[c]
This feature would allow the interaction of released proton
and FF supramolecular assembly.
o
In a typical experiment, at room temperature (about 25 C),
MEH and FF were dissolved in 1,1,1,3,3,3-hexafluoro-2-
propanol (HFIP) spontaneously, and then toluene was added to
achieve gelation. Their final concentrations were 4 and 16 mM,
respectively. Instantaneously, a homogeneous, transparent and
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201711547
Angewandte Chemie International Edition
COMMUNICATION
eep orange gel forms. The detailed morphologies of FF and
F/MEH gels were obtained by carrying out scanning electronic
icroscopy (SEM). Clearly, Figure 2A shows similar ultrafine
bers in FF and FF/MEH xerogels. This result indicates that the
troduction of MEH has no obvious effect on the intrinsic
upramolecular assembly of FF. Changing the molar ratios of
EH and FF in the systems allow us to obtain organogels with
fferent shades (Table S1 and Figure S2). Notably, when MEH
as overloaded by FF organogel, MEH self-assemble into
icrospheres with the diameter of 2 μm (Figure S3). It could be
xplained by the limited solubility of MEH in toluene. In addition,
e viscoelastic properties of the FF and FF/MEH gels were
easured using oscillatory rheology. Figure 2B shows their
echanical spectra at room temperature. It can be observed
at the storage modulus (G’) of two gels are found to be an
rder of magnitude than their loss modulus (G’’), implying that
e gels are elastic rather than viscous material. Meanwhile,
eir G’ and G’’ were essentially independent of frequency,
uggesting that these gels are solid-like gels. [15] The mechanical
rength of FF/MEH gel was 18.3% higher than FF gel. This
ight result from MEH-mediated enhancement of molecular
teractions.
Importantly, under irradiation, FF/MEH gel transforms into a
light orange sol (shown in Figure 3A). The progress is shown in
the real-time movie (see the Supporting Information) and Figure
S4. In detail, FF and FF/MEH gels are placed in a centrifuge
tube upside down (Figure S4a), respectively. After 30 s
irradiation, there is no significant change for self-standing FF
and FF/MEH gels (Figure S4b). When these gels are illuminated
for 60 s, a part of FF/MEH gel turns into sol while FF gel remains
silent (Figure S4c). When the irradiation time is extended to 65 s,
the rest FF/MEH gel drop into the bottom, indicating it transits
into flowing sol (Figure S4d). As a contrast, there is still no
change for FF gel in the whole process, indicating the
supramolecular assembly is not photosensitive. Notably, the
temperatures in two gels keep constantly during the whole
course (Figure S5). That is, the gel-to-sol transition is almost
isothermal. Thus, thermo effect induced phase transition can be
[10b ]
excluded, although FF organogel is thermo-responsive.
In
addition, after FF/MEH sol is put in the dark for 20 min, self-
standing gel reforms. This phenomenon can be repeated many
times (Figure 3A), which can be monitored by the UV-Vis
absorbance at 424 nm of the gels (Figure 3B, left). It can be
observed that the gel-sol transition was accompanied by the
reversible colour change (deep orange-light orange), which is
similar to the case of MEH aqueous solution (Figure S6).
Importantly, there is no obvious damage during the transitions,
indicating the reversible system is robust.
gure 2. (A) SEM images of FF and FF/MEH xerogels; (B) the rheology data
FF and FF/MEH gels; (C) FTIR spectra of FF/MEH, FF and MEH,
spectively; (D) photoluminescence emission spectra of FF and FF/MEH gels
xcited at 250 nm).
Fourier transform infrared (FTIR) spectroscopy was further
erformed to investigate the molecular interactions before and
fter introducing MEH (Figure 2C) into FF organogel. Overall,
e spectrum for FF/MEH is the sum of those of MEH and FF.
he main peaks do not exhibit obvious redshifts or blueshifts
nd no new peaks appear. This finding may illustrate that there
no change of FF-based molecular assembly, in consistence
ith the SEM results above. It may be because MEH is just
bsorbed on the surface of FF nanofibers rather than inserted
[12b]
to FF assembly.
To further understand the diversity of the
olecular interactions, fluorescence (FL) spectroscopy was
tilized to measure the emission spectra of two gels above
Figure 2D). When excited at 250 nm, the photoluminescence
mission peak of FF gel appears at 285 nm, which can be
Figure 3. (A) Photos of reversible light-ccontrolled phase transition of FF/MEH
gel and sol; (B) the UV-Vis spectra oof FF/MEH gel before and after light
illumination (left) and the relevant reversiible changes of the absorbance at 424
nm; (C) a serials of stripes created in thee same FF/MEH gel loaded in a glass
tube.
ttributed to π-π* transition of FF assembly, having good
[11a]
onsistence with the previous report.
After adding MEH into
F gel, the emission peak appears still at 285 nm but the
mission intensity decreases about 50%. This might be ascribed
partial quench induced by MEH.
Based on the results above, we envision self-erasing systems
can be constructed. Preliminarily, renewable well-defined
patterns were fabricated using the corresponding photomasks.
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10.1002/anie.201711547
Angewandte Chemie International Edition
COMMUNICATION
gure 3C shows a series of strips in the same glass tube
ontaining FF/MEH gel. When FF/MEH gel with the colour of
eep orange is put in a glass tube with a black mask, the colour
the exposed region is turned into light orange, revealing the
el-to-sol transition after irradiation for 2 min. Next, under
arkness, the pattern vanishes within 20 min and the original gel
regenerated. New patterns are created after the same
perations using the corresponding photomasks, which will hold
otential application in nanofabrication.
introducing
transition of dipeptide supramolecular assembly is achieved
under light illumination. Meanwhile, in dark, sol-to-gel
transition occurs. Noticeably, the photoacid generator has no
obvious influence on the intrinsic supramolecular assembly of
dipeptide. Following the robustly isothermal reversible gel-sol
transition, renewable patterns can be fabricated in a spatially
controlled manner. This work may readily open a new way to
remotely tune amino acid-based supramolecular assembly
systems, holding great potential in wide applications.
a long-lived photoacid generator, a gel-to-sol
a
This strategy above can be extended for photoinduced phase
ansitions of other FF-based supramolecular assembly systems.
gure 4 shows the similar reversible gel-sol transitions are
bserved in the FF-CH2Cl2/DMF (3%), FF-styrene, FF-CCl4/DMF
2%) and FF-xylene gel systems, respectively. During the phase
ansitions in these cases, the colour shadow was reversibly
hanged. These findings demonstrate the versatility of our
pproach. In addition, the reverse reaction depends on the type
f hosts and the recovery times are different (Table S2).
Experimental Section
Materials. FF (L-Phe-L-Phe) and HFIP were purchased from Sigma-
Aldrich Company. 2,3,3-trimethyl-indolenine, propane sultone and 2-
hydroxybenzaldehyde were obtained from Alfa Aesar. Toluene, styrene,
xylene, dichloromethane (CH2Cl2), tetrachloromethane (CCl4), N, N-
Dimethylformamide (DMF) and CH3COOH were purchased from Beijing
Chemical Reagents Company. All reagents are commercially available
and were used without further purification or modification. Deionized
water (specific resistance of 18.2 MΩ) was used in all the experiments.
Further, to preliminarily prove pH-dependent transition of the
el, CH3COOH was directly added into FF gel. The volumes of
el and sol varied with the amount of CH3COOH (Figure S7). In
ddition, we observed that when the molar ratio of MEH/FF is
creased up to 1:4, after irradiation, the gel transforms into sol
ompletely(Figure S8). Also, as an alternative, we investigated
H change in MEH aqueous solution under light irradiation and
arkness, and found pH switch in the system (Figure S9). As a
atter of fact, most of amino acid-based supramolecular
ssemblies are intrinsically pH-responsive. Taken together, we
xpect this strategy can be used for the photomodulation of their
ssemblies.
Synthesis of MEH. The photoacid generator MEH was synthesized
[14]
according to the previously reported studies.
In brief, the mixture of
2,3,3-trimethylindolenine (3.30 g, 0.02 mmol) and propane sultone (2.52
g, 0.02 mmol) was stirred at 90 oC for 5 h under a nitrogen atmosphere.
The purple precipitate 2,3,3-trimethyl-1-(3-sulfonatepropyl)-3H-indolium
was filtered, washed with cold diethyl ether and finally dried under
vacuum. Then, the obtain solid (200 mg, 0.72 mmol) and 2-
hydroxybenzaldehyde (96 mg, 0.78 mmol) in anhydrous ethanol (4 mL)
was refluxed for 12 h. After cooling, the orange product was collected by
filtration, washed with ethanol and dried in vacuum.
Preparation of FF and FF/MEH Gels. 500 μL of toluene was added into
the solution which was formed by FF (2.5 mg, 0.008 mmol) with different
amounts of MEH dissolved in 20 μL of HFIP at room temperature. The
gelation state of the sample could appear after several minutes. For
preparation of the other FF-based organogels, toluene was replaced by
other solvents with the same volume, such as styrene, xylene, CH2Cl2
contained 3% DMF in volume fraction, or CCl4 contained 2% DMF in
volume fraction respectively, and the other materials were unchanged.
Characterization. 1H NMR spectrum was recorded on a Bruker Advance
300 spectrometer using d6-DMSO as solvent and tetramethylsilane
(TMS) as a reference. Mass spectroscopy was carried out by using a
Bruker BIFLEX III mass spectrometer in reflector mode. α-cyano-4-
hydroxycinnamic acid was used as matrix for ionization. SEM images
were performed by a HITACHI S-4800 after the samples dried under
vacuum and sputtered a thin layer of platinum particles onto their surface.
FTIR spectra were collected on a TENSOR 27 FTIR spectrometer after
the dry samples grinded with spectroscopic grade potassium bromide
and compressed into disks. Fluorescence spectra were recorded on a
Hitachi Model F-4500 spectrofluorometer. The rheological properties of
the gels were measured with a Discovery DHR-1 rheometer equipped
with a parallel plate (20 mm diameter) and performed at 25 oC with a
frequency of 0.1-100 rad s-1 and a strain of 2 %. UV-Vis spectroscopy
was performed on a UV2600 in 0.1 cm quartz at high scanning speed.
The light source with a density of 0.19 W/cm2 for irradiation was a CEL-
S150 Xe lamp (, CEAULIGHT, China).
gure 4. Reversible phase transition of different FF-based organogels
ontaining MEH. (A) FF-CH2Cl2/DMF (3%); (B) FF-styrene; (C) FF-CCl4/DMF
%); (D) FF-xylene.
In summary, we have demonstrated a universal and efficient
pproach for reversible photomodulation of phase transition in
on-photoresponsive dipeptide organogels. By non-covalently
Acknowledgements
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ICATION
tefully acknowledge the financial support for this
Zhang, L.Yang, Q. Jiang, Y. Liu, Org. Lettt. 2008, 10, 5557-5560.
he National Nature Science Foundation of China
21320102004, 21573248, and 21273250) and
Basic Research Program of China (No.
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(No. Y6290512B1).
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10.1002/anie.201711547
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ICATION
NICATION
introducing
a long-
Xianbao Li, Jinbo Fei, Youqian Xu,
Dongxiang Li, Tingting Yuan, Guangle
Li, Chenlei Wang, Junbai Li*
generator enables
modulation of phase
eptide supramolecular
ccompanying
the
Page No. – Page No.
Photoinduced Reversible Phase
Transition of Dipeptide
Supramolecular Assembly
cled gel-sol transition
controlled manner,
ns are spontaneously
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