The light-driven macroscopic directional motion of a water droplet on an azobenzene-calix[4]arene modified surface

Article abstract of DOI:10.1039/d0cc00519c

A novel photo-responsive surface was constructedviamodifying azobenzene-calix[4]arene (ABC4) on a microstructured silicon surface. Asymmetric UV light irradiation could drive the macroscopic directional motion of a water droplet on this photo-responsive surface.

Full text of DOI:10.1039/d0cc00519c

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DOI: 10.1039/D0CC00519C
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Light-Driven Macroscopic Directional Motion of Water Droplet on
an Azobenzene–Calix[4]arene Modified Surface
Received 00th January 20xx,
Accepted 00th January 20xx
Fei Zhu, a Shiliang Tan, a Manivannan Kalavathi Dhinakaran, a Jing Cheng* and Haibing Li*
a
a
DOI: 10.1039/x0xx00000x
A novel photo-responsive surface was constructed by modifing rationally designed to contain three azobenzene units on the
azobenzene-calix[4]arene (ABC4) on a microstructured silicon upper rim of calix[4]arene and one allyl group on the lower rim
surface. Asymmetric UV light irradiation can drive the macroscopic to anchor it on the silica surface (Scheme 1), to form a photo-
directional motion of water droplet on this photo-responsive responsive monolayer on the surface (ABC4-SAMs). The
surface.
photo-responsive ABC4 on surface expose or conceal
hydrophilic carboxyl group with respect to UV and Vis light
irradiation due to the photoisomerization of azobenzene units
and thereby modify the surface wettability. Asymmetric UV
light irradiation on ABC4-SAMs caused a surface free energy
Biological molecular motors can be capable of realizing
macroscopic regular motion through their tiny movement on
molecular level,1 such as F
Their excellent performance in this aspect makes it possible to
drive the movement of objects that are many times more
massive than themselves. However, utilization of artificial
,2
3
0 1
F -ATPase which synthesize ATP.
26
gradient, leading to the directional motion of the water
droplet.
4
Azobenzene-calix[4]arene (ABC4) was synthesized according
to Scheme 1. Compound 2 and compound 3 were readily
synthetic molecular systems to achieve directional motion of a
cargo on macroscopic scale was found to be difficult task.
5
-7
27
synthesized in high yields via reported methods. Then, the
Reversible photo-responsive nature of azobenzene
derivatives have enlightened them to be the key component of
aqueous solution of p-aminobenzoic acid, hydrochloric acid
2
and NaNO was added to a solution of compound 3 and
8
-10
11-13
many photoswitches both in solution
and on interface
.
anhydrous sodium acetate in DMF/methanol, the reaction
Calixarene, the third-generation supramolecular host, has
been proven to be a wonderful platform for the construction
of self-assembled monolayer on surface (SAMs) due to its
28
system was stirred at -5℃ for about six hours to afford ABC4,
in yield of 88%. The structure of ABC4 was fully characterized
1
13
by H NMR, C NMR and ESI-MS (Figure S2-S4, ESI).
1
4-22
“cup-like” conformation.
For instance, our group has
fabricated a photo-responsive wettability switch on a
functional micro/nanostructured silicon surface via introducing
a photo-responsive host-guest system including azobenzoic
2
3
acid and calix[4]arene. Interstingly, Ichimura and co-workers
had realized light-driven directional movement of olive oil
droplets on surface with the aid of photochromic azobenzene
2
4,25
functionalized calix[4]resorcinarene system.
Nevertheless,
macroscopic directional motion of water droplet driven by
synthetic molecular systems, have not yet been reported to
the best of our knowledge.
Herein, we report the macroscopic directional transport of
water droplet on a photo-responsive surface modified with
azobenzene-calix[4]arene (ABC4). The molecule ABC4 was
Scheme 1. Synthetic route of azobenzene-calix[4]arene (ABC4).
a. Key Laboratory of Pesticide and Chemical Biology (CCNU), Ministry of Education,
College of Chemistry, Central China Normal University, Wuhan 430079, P. R.
China. E-mail: chengjingok@mail.ccnu.edu.cn; lhbing@mail.ccnu.edu.cn
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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DOI: 10.1039/D0CC00519C
Figure 2. (A) The fabrication of trans-ABC4-mofified surface (trans-
ABC4-SAMs) and the change on water contact angle; (B) The X-ray
photoelectron spectroscopy of the trans-ABC4-SAMs.
Figure 1. (A) UV-Vis absorption spectra of a trans-ABC4 solution (1×
1 -
5
0 M in ethanol); (B) Changes of the absorbance at 378 nm of a trans-
1
50.2±2.1° of Si-H exposed silicon surface into 33.8±3.5°,
-5
ABC4 solution (1×10 M in ethanol) upon alternating irradiation with
UV and Vis light; (C) The optimized structures of trans-ABC4 and cis-
ABC4.
which indicated that the trans-ABC4-modified silica surface is
hydrophilic in nature. This ABC4-modified surface (ABC4-
SAMs) was further characterized by X-ray photoelectron
spectroscopy (XPS) to identify its elemental composition
(
Figure S9 and Table S2, ESI). The appearance of a N1s peak
Ultraviolet and visible (UV-Vis) absorption spectroscopy was
employed to confirm the photo-controllable switching of
ABC4. As shown in Figure 1A and Figure S5 (ESI), the
absorption band of the trans-ABC4 solution was observed
around 378 nm, which shows a clearly decrement upon
irradiation with 365 nm UV light for 10 min, while the band at
around 266 nm increased slightly. Similarly reversibility in
configuration of ABC4 was achieved through the irradiation of
Vis light at 435 nm for 30 min, the absorbance at 378 nm and
and a C1s peak (Figure 2B) confirmed that the ABC4 were
successfully conjugated to the surface of the silicon substrate.
These findings confirmed the successful construction of photo-
responsive ABC4-SAMs.
To explore the photo-responsive property of ABC4-SAMs, it
was exposed to 365 nm UV light for 10 min and subjected to
CA analysis. As shown in Figure 3A, the water contact angle of
ABC4-SAMs increases from 33.8±3.5° to 110.5±3.9°, attributing
to the existence of ABC4 in the cis configuration, with the
switching of the hydrophilic surface into hydrophobic.
Reversible photo-responsive nature of the ABC4-SAMs surface
was confirmed by the repeated exposure of UV and Vis light
irradiation over the ABC4-SAMs surface for the period of five
cycles. The photo-induced switching of ABC4-SAMs was
confirmed by the repeated changes in the contact angle of the
2
66 nm was retained to their original state. The absorption
bands at 378 nm and 266 nm should be ascribed to π-π* and
n-π* transition in the trans configuration, while n-π* transition
8
alone taken place in cis configuration of ABC4.
Photoisomerization from trans to cis of ABC4 was also
reversible in nature, which was proved by the repeated UV and
Vis light irradiation on ABC4 molecules for the period of five
cycles (Figure 1B).
surface from
̴
̴3 3° (Figure S10, ESI). These
observations suggested that ABC4-SAMs possess good photo-
reversibility nature.
Further, photo-controlled isomerization of ABC4 was also
1
confirmed by H NMR analysis. Generally, azobenzene prefers
6
to exist in trans form in DMSO-d at room temperature (Figure
S6, ESI). Upon irradiated under 365 nm UV light for 20 min, the
trans-cis isomerization leads to appearance of four new signals
at ppm respectively. Moreover, from the integral area of
proton signals, the conversion rate was calculated to be about
2
0%~30%. Subsequently, Guassian stimulations were
conducted to obtain the optimized structures of trans-ABC4
and cis-ABC4 (Figure 1C), and the energy change (ΔE)
accompanying the conversion. The result showed that the
total energy of cis-ABC4 is 53.55 kcal/mol higher than that of
trans-ABC4 (Table S1, ESI).
Fabrication of silica surface with ABC4 surface was attained
2
9
through the hydrosilylation addition reaction between ABC4
with the silicon substrate as given in Figure 2A (Seeing more
details in Figure S8, ESI). Introduction of trans-ABC4 was
confirmed by the decrement in water contact angle (CA) from
Figure 3. (A) Schematic of the photo-responsive property of the ABC4-
SAMs and water contact angle characterization; (B) The calculated
surface free energy of trans- and cis- ABC4-SAMs.
2
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asymmetric UV irradiation, all ABC4 molecules on the surface
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DOI: 10.1039/D0CC00519C
were considered to exist in the trans form, and there was no
free energy gradient. Upon asymmetric UV irradiation, portion
of the trans-ABC4 SAMs switched into its respective cis
conformation, which ultimately decreased its surface energy,
in the meantime the UV unexposed region remained
unchanged. The asymmetric change of molecular structure
generated a surface energy gradient on the ABC4-SAMs, which
act as the driving force to move the water droplet
2
6,31,32
directionally.
In conclusion, novel azobenzene-calix[4]arene (ABC4) was
successfully synthesized and it exhibited pretty photo-
responsive behaviour, which was evidenced by UV-Vis
1
absorption spectra and H NMR spectrum. The ABC4 was
anchored over the silica substrate surface to result in
reversible photo-responsive surface (ABC4-SAMs), such a
reversible photo-responsive nature was confirmed by the UV
and Vis light controlled reversible contact angle shifting. A
surface free energy gradient was generated by the asymmetric
UV irradiation, due to partial photoisomerization of
Figure 4. (A) Schematic illustration of the free energy gradient on azobenzene units, resulting in the macroscopic directional
ABC4-SAMs under asymmetric UV light irradiation; (B) The UV light- motion of the water droplet. Thus photo-responsive surface
driven directional measurable movement of water droplet (1 μL) on can be potentially applied to construct microfluidic devices for
ABC4-SAMs.
water-soluble substance transport.
This work was financially supported by the National Natural
Science Foundation of China (21677058, 21772055,
Further, the surface free energy (γs-g) of trans-ABC4-SAMs 21911530178), the National Key Research and Development
and cis-ABC4-SAMs were calculated with the aid of water Program of China (2018YFD0200102), the 111 Project (B17019)
contact angle according to the reported formulas (See and Self-determined research funds of CCNU from the
3
0
calculating details in ESI). The value of γs-g represents the colleges’ basic research and operation of MOE.
ability of water droplets to spread out (Figure S11A, ESI). As
shown in Figure 3B and Table S3 (ESI), the surface free energy
of cis-ABC4-SAMs was calculated to be much lower than trans-
ABC4-SAMs. Therefore, it can be assumed that when only the
Conflicts of interest
left side of the water droplet is exposed to UV irradiation, the
decrease of surface free energy can make it possible to
generate an energy gradient on the solid-liquid interface, the
There are no conflicts of interest to declare.
value of γs-g on the left side of the water droplet will be lower Notes and references
than that on the right side (Figure S11B, ESI), which might be
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