Circularly polarized light modulated supramolecular self-assembly for an azobenzene-based chiral gel

Abstract

An unconventional supramolecular self-assembly triggered by left-handed circularly polarized light breaks the traditional knowledge of azobenzene photoisomerization.

Full text

Circularly polarized light modulated
supramolecular self-assembly for an azobenzene-
based chiral gel†
Kenan Shao,
ab
Ziyu Lv,
c
Yuting Xiong,
b
Guodong Li,
ab
Dongdong Wang,
b
Haining Zhang
a
and Guangyan Qing *
b
UV light-triggered trans-to-cis isomerization of azobenzene usually results in the collapse of a self-
assembly system owing to the breaking of molecular planarity. Interestingly, two totally opposite self-
assembly trends have been detected when a C
2v
-symmetric chiral gelator was irradiated by a circularly
polarized light (CPL) with specific handedness, indicating that CPL could become a powerful tool in
modulating the assembly behaviour of the photo-responsive system.
Modulating the self-assembling process and its nano-
architecture by external stimuli has long been a challenging
topic in supramolecular self-assembly.
1
Research ranging from
the fabrication of responsive supramolecules to the dynamic
control of self-assembly behaviors has promoted broad and
interesting applications in nanotechnology,
2
electronics,
3
tissue
engineering
4
and biomedical elds.
5
Numerous exterior factors
could affect self-assembly behaviors, such as temperature,
solvent, magnetic eld and light irradiation.
6
Among these
stimuli, light is considered to be remotely and accurately
controlled, quickly switched and easily focused
7
thus has
attracted great interest in the construction of photo-responsive
assemblies and devices.
Mainstreams of photo-responsive systems commonly focus
on natural light. With further studies, investigations have been
extended to polarized light, streams of photons with either
right- or le-handed spin, which can transfer integer photonic
spin to molecules. Such properties have endowed CPL as the
inherent chiral light and has been regarded as the possible
source of chiral information in living organisms.
8
As exciting
examples, CPL-driven absolute asymmetric (AAS) and mirror-
symmetry breaking (MSB) have been actively investigated in
recent years;
9
nanoparticles with chiral structures or helical
arrangements could be generated by the CPL irradiation.
10
By
comparison, the effect of CPL handedness on small molecule
self-assembly is still with less exploration, especially those with
photo-responsive capacities. In general, trans-azobenzene based
gelators have strong self-assembly capacities owing to their
favourable planar and symmetric structures. UV-light
irradiation-induced trans-to-cis transition normally will break
such molecular symmetry and lead to the collapse of self-
assembled structures. Interestingly, in this study, we nd that
the introduction of CPL with specic handedness breaks this
traditional idea (Fig. 1a). For an azobenzene-based chiral gel
with C
2v
-symmetric structure, right-CPL promotes the collapse
of gel; by contrast, le-CPL triggers the formation of a new self-
assembled structure, and macroscopic gel is well maintained.
This unconventional nding affords a new pathway to the
fabrications of photo-responsive devices.
Chemical structure of our chiral gelator, 4,40-azobenzene-
linked L-aspartate-L-phenylalanine methyl ester (abbreviated to
Azo-DF), is shown in Fig. 1b. The 4,40-di-substituted azobenzene,
which supplies a C
2v
-symmetric skeleton, functions as a photo-
responsive group. While two L,L-DF dipeptides are linked to the
two sides of azobenzene owing to their satisfactory self-assembly
capacities.
11
Synthesis process and characterization data of Azo-
DF are described in the ESI.†An additional control experiment
and the possible self-assembly modes calculated from quantum
chemical calculation indicated that p–pstacking between adja-
cent phenyl groups or between azo-benzene groups might be the
driving force for the supramolecular self-assembly of Azo-DF
(Fig. S3 and S4 in ESI†). Owning to the excellent gelation
capacity in chloroform/methanol (v/v ¼1:3),
12
our following
research mainly focused on this mixed solvent (Table S1 in ESI†).
The CPL pumping platform was consisted of an LED-UV (365 nm)
light torch (SCOUT UVFLUXS-3W, 20 mW cm
2
), a Glan–
Thompson prism (200–900 nm) and a quarter-wave plate (Fig. 2a).
Details on the CPL pumping platform are described in Part S4 in
ESI.†Because of subtle changes under 3 mW cm
2
(distance (D):
a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,
Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, P. R. China
b
Key Laboratory of Separation Science for Analytical Chemistry, Dalian Institute of
Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian
116023, P. R. China. E-mail: qinggy@dicp.ac.cn
c
College of Electronic Science and Technology, Shenzhen University, Shenzhen 518060,
P. R. China
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra01974j
Cite this: RSC Adv.,2019,9,10360
Received 14th March 2019
Accepted 29th March 2019
DOI: 10.1039/c9ra01974j
rsc.li/rsc-advances
10360 |RSC Adv.,2019,9,10360–10363 This journal is © The Royal Society of Chemistry 2019
RSC Advances
PAPER

45 cm) light irradiation and too swiresponse to catch under 10
mW cm
2
(D:10cm)irradiation,5mWcm
2
(D: 32.5 cm) was
regarded as the optimal light intensity in the following experi-
ments. Meanwhile, the light intensities of le-andright-CPLwere
identical (Fig. S5b and c in ESI†).
First, the photo-responsiveness of Azo-DF triggered by right-
or le-CPL was detected by UV-vis absorption spectra. Azo-DF
gelator was dissolved in chloroform at a low concentration of
0.05 mg ml
1
, in which Azo-DF was in isolated state and supra-
molecular self-assembly would not happen. Initially, the UV-vis
spectrum of Azo-DF consisted of a strong UV band with
a maximum absorption peak at 326 nm, which could be ascribed
to the p–p*transition and corresponds to the vibrational struc-
ture of the typical trans-azobenzene (Fig. 2b). Aer irradiation of
right- or le-CPL, two well separated bands in the UV region (l
max
290 nm) and visible region (l
max
440 nm) increased, repre-
senting the p–p*and n–ptransition of cis-azobenzene structure,
respectively.
13
The photo-stationary state was reached aer
30 min right- or le-CPL irradiation, revealing that the typical
trans-tocis-transformation of azobenzene in Azo-DF had indeed
happened. It is worth noting that the amount of range caused by
CPL handedness were almost equal within the error range. A
consistent tendency was observed when the individual azo-
benzene was irradiated by the le- or right-CPL (Fig. S6 in ESI†).
Then the gelator concentration was increased to 2 mg ml
1
in chloroform/methanol (v/v ¼1 : 3), allowing the gel forma-
tion. The distinct gel–sol transition degree caused by the CPL
handedness on macroscopic gel was observed. As shown in
Fig. 2c, when the gel was exposed to the right-CPL irradiation,
the gel showed an obvious tendency to loose and collapse aer
3 h irradiation, remaining some uffy aggregates in the 30 mL
solvents. By comparison, Azo-DF gel maintained stable in the
gel state aer 3 h of le-CPL irradiation, only a few solvents (12
mL) were squeezed out. A similar solvent squeezed out situation
of azobenzene gel, where a stable layered superstructure aer
UV light irradiation was claimed to be responsible, was reported
by Jeong.
14
Scanning electronic microscopy (SEM) were utilized
to observe the gel morphology before and aer 3 h of right- or
le-CPL irradiation, respectively (Fig. 2d–f). Many long and
regular ribbon-like bres were observed. These bres inter-
twined with each other and formed a dense three-dimensional
(3D) network, revealing strong self-assembly capacities and
linear packing pattern of Azo-DF. Large-scaled bre distribution
and enlarged view of a single bre are shown in Fig. S7 in ESI.†
Aer 3 h of right-CPL irradiation, the ribbon-like bres were
divided into numerous short stubs that scattered and piled up
randomly, corresponding to the collapse of the gels. While aer
3hle-CPL irradiation, the intertwined ribbon-bres were well-
preserved. The arrangement of bres became slightly loose,
which explained why there some solvents were squeezed out.
Circular dichroism (CD) spectroscopy was used to monitor
this self-assembly process (Fig. 2g and h). Initially, there are two
weak peaks at 260 and 330 nm in the CD spectrum, which could
be attributed to the molar chirality of two dipeptide arms
carried by Azo-DF gelator itself. Such weak peaks were consis-
tent with the morphology observed by SEM, where no evidential
helical structure was detected. When the Azo-DF gel was irra-
diated with right-CPL, uctuations around the baseline were
observed in the CD spectra. But with the le-CPL irradiation,
the CD spectrum (Fig. 2h) shows negative and positive cotton
effect bands at 314 and 385 nm, respectively. These bands
enhanced remarkably aer 3 hours of the le-CPL irradiation,
indicating the appearance of new helical structure during self-
assembly process under the le-CPL irradiation,
15
although
such helical structure was not a common scenario.
X-ray powder diffraction (XRD) was implemented to further
reveal the stacking structures of the xerogel. Fig. 3a shows XRD
spectrum of Azo-DF xerogel before CPL irradiation, multiple
sharp peaks indicated long-range molecular packing and high
crystallinity of the gel structure.
16
Aer r-CPL irradiation for 3 h,
the measured XRD signals sharply diminished to bottom line
(Fig. 3b), which indicated that the trans-to-cis transition of the
azobenzene obviously decreased the planar symmetry of the
gelators, destroying the highly ordered stacking of the gelators.
This phenomenon were well consistent with the morphological
changes observed by SEM. By comparison, for the gel sample
aer le-CPL illumination for 3 h, the crystal order was well
maintained except some slight decreases in the XRD intensity.
In addition, several new diffraction peaks are observed in
Fig. 3c. Specically, the peak A (2q¼7.81) splits into two
bands, while two new peaks (B0and C0) are found at 2q¼1.66,
Fig. 1 (a) Illustration of different supramolecular self-assembly
modulated by CPL handedness, right-CPL promotes the collapse of
ordered structure, but left-CPL triggers the formation of new helical
structure. (b) Chemical structure of 4,40-azobenzene-linked dipeptide
gelator, abbreviated to Azo-DF.
This journal is © The Royal Society of Chemistry 2019 RSC Adv.,2019,9,10360–10363 | 10361
Paper RSC Advances

7.90. In wide-angle region (2q¼18–25) the position of
diffraction peak appears a shi,
16
while the peak D splits into
two peaks and the peak E decreases remarkably. These data
clearly suggested that new crystalline state came into being aer
le-CPL illumination, moreover, the decomposition of the self-
assembled structure might be largely postponed by le-CPL.
Unambiguous evidence for CPL handedness-dependent
supramolecular self-assembly was provided by atomic force
spectroscopy (AFM). Optimization experiments indicated that
the Azo-DF could form highly ordered self-assembled pattern at
a concentration of 0.5 mg ml
1
in chloroform. So we prepared
AFM sample by dripping one drop of Azo-DF solution (0.5 mg
ml
1
, chloroform) before and aer right- or le-CPL irradiation
onto freshly cleaved mica, respectively. As shown in Fig. 4a, the
initially self-assembled pattern of Azo-DF presents as numerous
short nanobers evenly distributed in three directions, the
average length, width and height of these nanobers are 2.4 mm,
80 nm and 25 nm, respectively. Upon the irradiation of right-
CPL for 2 h, the three-direction short nanobers are replaced
by some long and attened ribbons with average length of more
than 10 mm and height of 30 nm (Fig. 4b). Aer 3 h right-CPL
irradiation, the self-assembled structure was hard to be detec-
ted (Fig. 4c). Such obvious change could be reasonably attrib-
uted to the weak self-assembled capacity of the gelator in a cis-
azobenzene form, which dominated the conguration of the
gelator. In comparison, when Azo-DF was irradiated by le-CPL
for 2 h (Fig. 4d), rather than decomposition, those short
nanobers showed a strong tendency to aggregate, the ber
length and width were estimated to be 3.8 mm and 40 nm. A
detailed enlarged view (Fig. 4e) of 1 mm shows that these
nanobers intertwisted with each other to form a rope like
structure. More interestingly, when the Azo-DF solution was
exposed to le-CPL for 3 h (Fig. 4f), a dendritic structure was
observed featured with an aggregate center over 150 nm height
and numerous branches of nearly 50 nm height. The aggrega-
tion difference resulted from the CPL handedness was also
monitored by dynamic light scattering (DLS) measurement
(Fig. S8 in ESI†). The distinct self-assembly difference was well
consistent with the above gel morphological changes. At the
same time, the subsequent ber growth upon le-CPL irradia-
tion further demonstrated a special staking model of Azo-DF
gelators where the supramolecular self-assembly was success-
fully inverted by exchanging the handedness of CPL. A nearly
reversed CPL-modulated self-assembly trend was detected by
using Azo-D,D-DF (Fig. S9 in ESI†). Considering Azo-L,L-DF and
Azo-D,D-DF were not completely mirror symmetric owing to the
four chiral centers, this result provided an auxiliary proof that
the distinct self-assembly behaviors of the gelators were caused
by CPL handedness. Compared to natural light, the introduc-
tion of CPL signicantly emphasized the quantum character-
istic of light orientation,
8a
where the delicate balance between
Fig. 2 (a) Photo of the CPL pumping system. F1: Glan–Thompson
prism; F2: quarter-wave plate; s: gelator sample. (b) UV-vis spectra of
Azo-DF in chloroform (0.05 mg ml
1
) before (dotted line) and after (solid
lines) CPL illumination for 30 min. (c) Gel–sol transition of Azo-DF in
chloroform/methanol (v/v ¼1 : 3, 2 mg ml
1
), the gel collapsed under
right-CPL irradiation, compared to only few solvent squeezing out from
the gel under left-CPL irradiation. (d–f) Scanning electronic microscopy
(SEM) images of Azo-DF xerogel before (d) and after 3 h irradiation of
right-CPL (e) or left-CPL (f). (g and h) Circular dichroism (CD) spectra of
Azo-DF gel in chloroform/methanol (v/v ¼1 : 3, 2 mg ml
1
)beforeand
after the irradiation of right-(g) or left-CPL (h). (l:365nm,5mWcm
2
).
Fig. 3 XRD spectra of Azo-DF xerogels before and after 3 h irradiation
of right- or left-CPL, light intensity: 5 mW cm
2
.
10362 |RSC Adv.,2019,9,10360–10363 This journal is © The Royal Society of Chemistry 2019
RSC Advances Paper

the isomerization of azobenzene and the chiral preference of
the dipeptide played an important role in the dissociation or
supramolecular self-assembly of Azo-DF gelators.
13b,17
The
possible mechanism is described in Part S6 in ESI.†
In conclusion, we observed an unconventional chiral effect:
azobenzene-based chiral gelator showed a collapse tendency
under the right-CPL irradiation, by contrast, intensive self-
assembly emerged when the gelator was irradiated with le-
CPL. Such CPL handedness-triggered self-assembly difference
might arise great interest in the potential of CPL sources in
controlling the supramolecular self-assembly of chiral mole-
cules or polymers as well as for exploring their roles in the
fabrication of function materials, moreover, inspiring
rethought of some vital chemical and biological processes from
the unique perspective of “dynamic molecular chirality”.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (51473131, 51533007 and 21775116), DICP
Innovation Funding (DICP-RC201801). G. Qing acknowledges
Wuhan Morning Light Plan of Youth Science and Technology.
Notes and references
1(a) L. E. Buerklea and S. J. Rowan, Chem. Soc. Rev., 2012, 41,
6089; (b) E. R. Draper, E. G. B. Eden, T. O. McDonald and
D. J. Adams, Nat. Chem., 2015, 7, 848.
2 D. M. Raymond and B. L. Nilsson, Chem. Soc. Rev., 2018, 47,
3659.
3 X. Feng, V. Marcon, W. Pisula, M. R. Hansen, J. Kirkpatrick,
F. Grozema, D. Andrienko, K. Kremer and K. M¨
ullen, Nat.
Mater., 2009, 8, 421.
4 C. J. Bruns and J. F. Stoddart, Acc. Chem. Res., 2014, 47, 2186.
5 I. V. Kolesnichenko and E. V. Anslyn, Chem. Soc. Rev., 2017,
46, 2385.
6(a) S. Dai, P. Ravi and K. C. Tam, SoMatter, 2009, 5, 2513; (b)
N. Ousaka, Y. Takeyama and E. Yashima, Chem. Sci., 2012, 3,
466.
7 F. Ercole, T. P. Davis and R. A. Evans, Polym. Chem., 2010, 1, 37.
8(a) M. Padgett and R. Bowman, Nat. Photonics, 2011, 5, 343;
(b) C. Giri, F. Goesmann, C. Meinert, A. C. Evans and
U. J. Meierhenrich, Top. Curr. Chem., 2013, 333, 41.
9 L. Wang, L. Yin, W. Zhang, X. Zhu and M. Fujiki, J. Am. Chem.
Soc., 2017, 139, 13218.
10 Y. Tang and A. E. Cohen, Science, 2011, 332, 333.
11 (a) Y. Lin, Y. Qiao, P. Tang, Z. Lib and J. Huang, SoMatter,
2011, 7, 2762; (b) T. M. Doran, D. M. Ryan and B. L. Nilsson,
Polym. Chem., 2014, 5, 241.
12 Z. Chen, Z. Lv, G. Qing and T. Sun, J. Mater. Chem. B, 2017, 5,
3163.
13 (a) C. L. Forber, E. C. Kelusky, N. J. Bunce and M. C. Zerner, J.
Am. Chem. Soc., 1985, 107, 5884; (b) T. Schultz,
J. Quenneville, B. Levine, A. Toniolo, T. J. Mart´
ınez,
S. Lochbrunner, M. Schmitt, J. P. Shaffer, M. Z. Zgierski
and A. Stolow, J. Am. Chem. Soc., 2003, 125, 8098.
14 Y. J. Choi, J. T. Kim, W. J. Yoon, D. G. Kang, M. Park,
D. Y. Kim, M. H. Lee, S. k. Ahn and K. U. Jeong, ACS Macro
Lett., 2018, 7, 576.
15 G. Gottarelli, S. Lena, S. Masiero, S. Pieraccini and
G. P. Spada, Chirality, 2008, 20, 471.
16 J. Cui, Y. Zheng, Z. Shen and X. Wan, Langmuir, 2010, 26,
15508.
17 (a) L. Nikolova, L. Nedelchev, T. Todorov, T. Petrova,
N. Tomova and V. Dragostinova, Appl. Phys. Lett., 2000, 77,
657; (b) D. K. Hore and A. L. Natansohn, J. Phys. Chem. B,
2003, 107, 2197; (c) S. W. Choi, S. Kawauchi, N. Y. Ha and
H. Takezo, Phys. Chem. Chem. Phys., 2007, 9, 3671.
Fig. 4 AFM images of the self-assembled morphologies and the
corresponding section profiles of Azo-DF in chloroform (0.5 mg ml
1
)
before (a) and after 2 h (b) or 3 h (c) of right-CPL irradiation; (d–f) Azo-
DF after 2 h (d and e) or 3 h (f) of left-CPL irradiation.
This journal is © The Royal Society of Chemistry 2019 RSC Adv.,2019,9,10360–10363 | 10363
Paper RSC Advances