Supramolecular Fluorescent Polymers Containing α-Cyanostilbene-Based Stereoisomers: Z/ E-Isomerization Induced Multiple Reversible Switching

Article abstract of DOI:10.1021/acs.macromol.8b00347

Photoresponsive materials play an important role in smart sensors and readable optical devices. The α-cyanostilbene moiety gathers Z/E-isomerization, mesogenic, and aggregation-induced enhanced emission (AIEE) properties together and can be considered as

Full text of DOI:10.1021/acs.macromol.8b00347

Article
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Supramolecular Fluorescent Polymers Containing α‑Cyanostilbene-
Based Stereoisomers: Z/E‑Isomerization Induced Multiple Reversible
Switching
Yalan Zhu, Meiqing Zheng, Yuanyang Tu, and Xiao-Fang Chen*
State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, Jiangsu Key Laboratory of Advanced
Functional Polymer Design and Application, College of Chemistry Chemical Engineering and Materials Science, Soochow University,
Suzhou 215123, China
S
* Supporting Information
ABSTRACT: Photoresponsive materials play an important
role in smart sensors and readable optical devices. The α-
cyanostilbene moiety gathers Z/E-isomerization, mesogenic,
and aggregation-induced enhanced emission (AIEE) properties
together and can be considered as a multifunctional building
block bearing luminogen, AIEgen, mesogen, and chromphore
characteristics. Here, we report isomerization induced a series
of notable reversible switching based on α-cyanostilbene
containing hydrogen-bonded supramolecular polymers. Z-
and E-stereoisomers of dendritic α-cyanostilbene derivatives
(Z-CNBP and E-CNBP) were hydrogen-bonded with poly(4-vinylpyridine) (P4VP), giving rise to P4VP(Z-CNBP)x and
P4VP(E-CNBP)x. P4VP(Z-CNBP)x exhibit a hexagonal columnar (Φh) phase at 0.4 ≤ x ≤ 1.0, while the lamellar phase could be
formed for P4VP(E-CNBP)x at 0.3 ≤ x ≤ 0.7. P4VP(Z-CNBP)0.8 shows photothermal E/Z-isomerization induced reversible
switching, including phase structures, surface topographies, birefringence, and fluorescence properties, whereas P4VP(E-
CNBP)0.5 exhibits enhanced fluorescence emission upon UV irradiation.
isomer, such kind of isomerization could also simultaneously
induce fluorescence change. Compared to α-cyanostilbene,
most of photoactive chromophores are nonfluorescent, and
additional fluorescent groups should be incorporated to realize
isomerization-induced fluorescence switch. As such, it is
intriguing to see that α-cyanostilbene moiety gathers Z/E-
isomerization, mesogenic, and AIEE properties together and
can be considered as an all-round building block bearing
luminogen, AIEgen, mesogen, and chromophore characteristics.
Although a variety of responsive properties for α-cyanostilbene
derivatives have been explored so far, the reversible switching
induced via E/Z-isomerization only exists in a limit number of
small molecular systems.28,29,36,37 Concomitant multiple
reversible switching in polymeric system is still a challenge.
In recent years, supramolecular side-chain polymers prepared
via noncovalent interactions between polymers and molecular
additives have been extensively used in construction and
fabrication of various hierarchical nanostructures as well as
directed assembly of functional molecules within polymer
matrix.41−58 The supramolecular approach offers a facile and
efficient route for exploration of novel functional materials with
tailored properties, together with solution processability and
mechanical properties offered by the polymer chains. Motivated
INTRODUCTION
■
Stimuli-responsive materials, which adapt to reversibly change
their chemical and physical properties in response to an
external stimulus, have intensive applications in smart devices,
biotechnology, actuators, sensors, etc.1−5 Toward achieving
more complicated functional properties and multiple respon-
sive behaviors in one system in different dimensions and time
scale, one of the most used strategies is integrating different
functional molecular motives into a system and precisely
programing their responsive properties. While with increasing
the composition and complicity of system, the challenge to
control and manipulate various responses increases dramati-
cally. Single molecular motif exhibiting versatile responsive
properties can be considered as the candidate to reduce the
complicity of multicomponent integrated functional systems.6
α-Cyanostilbene derivatives,7−11 which exhibit aggregation-
induced enhanced emission (AIEE) properties,12−14 have
received significant attention recently, for their fluorescence
can be modulated under various stimuli, exhibiting such as
mechanochromic,15−22 thermochromic,23−25 and solvatochro-
mic26,27 responsive properties. Moreover, upon UV irradiation,
the α-cyanostilbene moiety can undergo Z/E-isomeriza-
tion28−40 that could subsequently induce self-assembly
structure changes similar to azobenzene and its derivatives,
such as LC-to-iso,29 gel-to-sol transition,31 or morphological
changes of solution assemblies.34 Moreover, since the Z-isomer
generally has a different fluorescence property from the E-
Received: February 15, 2018
Revised: April 4, 2018
© XXXX American Chemical Society
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Scheme 1. Synthesis and Schematic Representation of P4VP(Z-CNBP)x and P4VP(E-CNBP)x
Scheme 2. Synthesis of Z-CNBP and E-CNBP
by these advantages, we propose to introduce α-cyanostilbene
and its derivatives into supramolecular polymers. The
molecular packing of α-cyanostilbene-based molecules could
be modulated via noncovalent interaction, which would further
influence their fluorescence and AIEE properties. Meanwhile,
the dynamic nature of noncovalent interaction may favor the
reversible change about molecular assembly and corresponding
physical properties of α-cyanostilbene derivatives within
polymeric system, giving rise to novel multiresponsive func-
tional materials. Last but not least, supramolecular polymers are
usually prepared under mild condition via molecular recog-
nition. It gives us an opportunity to precisely incorporate
stereoisomers of molecules into supramolecular polymers. For
example, the main-chain-type supramolecular polymers based
on Z- and E-isomers of a tetraphenylethene derivative have
recently been reported.59 In our work, both Z- and E-isomers of
α-cyanostilbene moiety could be complexed with polymer
matrix to achieve novel isomer-based supramolecular polymers,
which have the identical chemical structure but with different
spatial configurations in the side chains. It would help us to gain
more insight into the interplay between molecular structures
(especially the molecular configuration) and self-assembled
supramolecular structures.
Therefore, in this work, we design a dendritic molecule,
CNBP, which contains an α-cyanostilbene moiety and a
phenolic end group as shown in Scheme 1. Its Z- and E-
isomers are named as Z-CNBP and E-CNBP, respectively.
Through hydrogen-bonding interaction, the supramolecular
side-chain polymer with poly(4-vinylpyridine) (P4VP) as
backbone and CNBP as “side chain” could then be constructed.
The configuration effect on self-organized supramolecular
structure, as well as the E/Z-isomerization induced multi-
responsive behavior, was thoroughly investigated. The rever-
sible switching between Z and E stereoisomers of the α-
cyanostilbene moiety within Z-isomer-based supramolecular
polymers was successfully realized, which could simultaneously
trigger the self-organized structure transformation with
concurrent photochromic fluorescence, birefringence, and
surface topography switching in this system.
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Table 1. Photophysical Properties and Thermal Behavior of CNBP Isomers
a
b
c
d
compounds
λabs (nm) (ε)
λem (nm) (Φf)
λem (nm) (Φf)
phase transition T (°C)
Z-CNBP
E-CNBP
362 (21000)
336 (10700)
445 (0.005)
416 (0.003)
499 (0.28)
481 (0.03)
Cr 50 Sm 78 Iso
Cr1 55 Cr2 75 Iso
a
b
Absorption maximum wavelength (molar extinction coefficient) of CNBP in chloroform (1 × 10−4 mol/L). Emission maximum wavelength
c
(quantum yields obtained from absolute measurements in an integrating sphere) of CNBP in chloroform (2 × 10−5 mol/L) solution. Emission
maximum wavelength (quantum yields) of CNBP in solid state. Data determined by DSC, temperatures at the maximum of the phase transition
d
peaks in the second heating cycle at a scanning rate of 10 °C/min. Cr: crystalline phase; Sm: smectic phase; Iso: isotropic liquid.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of CNBP and Corre-
sponding Supramolecular Polymers. As shown in Scheme
2, Z-CNBP was first synthesized and purified. By irradiating Z-
CNBP solution under 365 nm UV irradiation, Z- to E-
isomerization was triggered instantly as detected via UV−vis
absorption (Figure S1) and 1HNMR spectra (Figure S2). After
2 h, the CDCl3 solution of Z-CNBP (10 mg/mL) in an NMR
tube reached their photostationary state with the molar ratio of
E- and Z-CNBP as 4:1. So E-CNBP was prepared via UV
irradiating Z-CNBP in CHCl3 solution for at least 2 h and then
separated and purified via column chromatography. As we
expected, due to their different molecular configurations, Z-
CNBP and E-CNBP have distinct photophysical properties and
thermal behavior as listed in Table 1. Z-CNBP shows
fluorescent liquid crystalline behavior with the AIEE effect
while E-CNBP does not (Figures S3−S7). Both Z- and E-
isomers were hydrogen-bonded with P4VP to form P4VP(Z-
CNBP)x and P4VP(E-CNBP)x complexes, respectively, via the
solution blending method. The x here refers to the molar ratio
of CNBP to 4VP repeating unit. The formation of hydrogen
bond in polymer complexes can be confirmed via FT-IR spectra
as shown in Figure 1. The absorption bands of pyridine ring in
pure P4VP appear at 1597, 1415, and 993 nm−1. For P4VP(Z-
CNBP)x complexes in Figure 1a, the absorption bands at 1597
and 1415 nm−1 gradually shift to 1604 and 1421 nm−1,
respectively. This result is consistent with the previously
reported hydrogen-bonded supramolecular polymers contain-
ing P4VP and molecular additives with phenolic end
groups.40,48,52,55,56 The absorption band at 993 nm−1 tends to
shift to 1008 nm−1, which overlaps with the absorption peaks
from Z-CNBP in this case. The hydrogen bond formation in
P4VP(E-CNBP)x complexes can also be confirmed according
Figure 1. FT-IR spectra of pure P4VP, Z-CNBP, E-CNBP, and P4VP
complexes containing (a) Z-CNBP and (b) E-CNBP of the different
molar ratios (x) to the pyridine units.
to the similar absorption band shifts of pyridine ring as shown
1
in Figure 1b. HNMR spectra of polymer complexes confirm
that Z- and E-isomers are quite stable in polymer complexes
during complexation (Figure S8) and thermal annealing below
90 °C (Figure S9).
molecular length in the most extended form is 3.2 nm for Z-
CNBP with all-trans conformation of the alkoxy chains, the
schematic inset in Figure 2a illustrates that the hydrogen-
bonded dendritic Z-CNBP molecules are wrapping around the
P4VP chains, giving rise to cylindrical assemblies. The
supramolecular cylinders are parallel to each other, forming a
hexagonal packing.45−56 Upon increasing the content of Z-
CNBP, the polymer backbones within the cylinder become
more stretched to accept more grafted Z-CNBP molecules,
which finally shrink the lateral size of each cylinder. Z-CNBP
itself possesses lamellar structure in liquid crystalline phase as
shown in Figure S7 and tends to crystallize when cooling to
room temperature. After being hydrogen-bonded with P4VP,
the Φh structure appears at 0.4 ≤ x ≤ 1.0, and no phase
separation is detected via SAXS and DSC, indicating the
formation of homogeneous complex with highly ordered
supramolecular structure.
Phase Behavior of Isomer-Based Supramolecular
Polymer. After complexation, the self-organized structures of
obtained polymer complexes and their thermal behaviors were
studied via small-angle X-ray scattering (SAXS) and differential
scanning calorimetry (DSC). As shown in Figure 2a, the SAXS
profiles of P4VP(Z-CNBP)x show characteristic x-dependent
phase behavior (Figure 2a). When x ≤ 0.3, only a weak and
broad scattering peak in the low angle region is detected,
corresponding to amorphous state. When x ≥ 0.4, the SAXS
profiles show three peaks at q*, √3q*, and 2q* respectively
corresponding to (10), (11), and (20) reflections. This suggests
clearly the formation of hexagonal columnar (Φh) structure.
The lattice parameter a of Φh structure, which is associated
with the diameter of each cylinder, reduces gradually from 6.66
nm at x = 0.4 to 5.92 nm at x = 1.0. Reminding the calculated
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Figure 2. SAXS profiles of P4VP(Z-CNBP)x (a) and P4VP(E-CNBP)x (b) at room temperature. (c) The d-spacing of the first scattering peak as a
function of x. DSC curves of P4VP(Z-CNBP)x (d) and P4VP(E-CNBP)x (e) during the second heating cycle at the rate of 10 °C/min under a N2
atmosphere. (f) Tg and Ti of P4VP(Z-CNBP)x and P4VP(E-CNBP)x as a function of x.
When the complexed CNBP adopts E configuration, the
resulting P4VP(E-CNBP)x samples possess different self-
organized structures as shown in Figure 2b. Only two scattering
peaks with a q-ratio of 1:2 are observed at 0.3 ≤ x ≤ 0.7,
indicating the existence of lamellar structure as shown in the
schematic inset in Figure 2b. The layer thickness (L) slightly
decreases from 4.7 to 4.3 nm as increasing x. When further
increasing the content of E-CNBP, additional scattering peaks
could be detected, which is due to the crystalline E-CNBP that
is macroscopically separated from P4VP(E-CNBP)x. The plots
of the d-spacing of the first scattering peak as a function of x in
Figure 2c clearly show that P4VP(Z-CNBP)x have relatively
larger d-spacing than that of P4VP(E-CNBP)x due to the
different molecular size between Z- and E-isomers.
It is clearly to see that the molecular configuration
dramatically influences the self-organized structures. In our
work, the Z-CNBP molecule possesses hemiphasmidic
structure that is the combination of rod-like segment and
Percec-type dendron. It is well-known that Percec-type
dendrons are frequently used to realize the columnar phase
formation, while in this case the rod-like segment here also
plays an important role to stabilize the columnar structure. It
can be found from the published data that the minimum
content of dendritic side chains to form Φh phase structure
decreases from x = 0.7 to x = 0.3 with increasing the length of
rigid part in hydrogen-bonded supramolecular dendronized
polymers.51−56 In our work, Z- and E-CNBP have the same
dendron but the different configurations in rigid part. Based on
SAXS results, the bent-shaped E-isomer could not favor the
columnar packing. The lamellar structure appearance is
attributed to the microphase separation between aromatic
polar and aliphatic nonpolar part in polymer complexes.
Meanwhile, the bulky shape of E-isomer hinders the hydrogen-
bonding interaction to some extent. Only when x < 0.8 could
homogeneous complexes be prepared.
The isomer-based supramolecular polymers also show
different thermal behavior as revealed by DSC. As shown in
Figure 2d, the glass transition temperature (Tg) of P4VP(Z-
CNBP)x decreases from 110 °C at x = 0.1 to 58 °C at x = 1.0.
When x ≥ 0.5, an endothermic peak originating from Φh to
isotropic phase transition (Ti) becomes detectable, and the Ti
gradually decreases from 148 °C at x = 0.5 to 128 °C at x = 1.0.
While for P4VP(E-CNBP)x, only Tg is detected as shown in
Figure 2e, which is also decreased with increasing the content
of E-CNBP. No phase transition could be observed when x <
1.0. Until x = 1.0, macrophase separation could be detected in
Figure 2e. The decline of Tg with increasing x is due to the
plasticization effect of molecules. Meanwhile, the Tg of
P4VP(E-CNBP)x is also lower than that of P4VP(Z-CNBP)x
as shown in Figure 2f, indicating E-CNBP has larger
plasticization effect than Z-CNBP. It is attributed to the
different configurations of E/Z-stereoisomers. The rigid
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conjugated segment offers Z-CNBP stronger intermolecular
interaction than E-CNBP, which finally influenced the Tg of
corresponding polymer complexes.
structure change is attributed to the E/Z-isomerization of
CNBP (Figure S10). Upon UV irradiation, around 37% E-
CNBP appeared in polymer complex. Then, after subsequent
heating process, most of E-CNBP molecules convert to Z-
CNBP, and the content of E-CNBP decreased to 5%. It should
be noted that in some reported cases cyanostilbene can also
take [2 + 2] cycloaddition in the bulk phase.60−62 It could be
The E/Z-stereoisomer-based supramolecular polymers show
distinctly different self-assembly behavior, including complex-
ation ability, phases transition, and phase structure. It is the first
time to directly study the effect of molecular configuration on
the self-organized structures in supramolecular polymers. After
complexation, homogeneous polymer complexes could be
obtained, which could be processed into fibers, films, or other
forms. A more intriguing thing is the photoresponsive property
of CNBP still exists in a supramolecular system. The
isomerization-induced physical properties switching in polymer
complexes were thus systematically studied.
1
identified as the appearance of a singlet at 5.39 ppm in H
NMR, which is due to the cyano-substituted cyclobutane ring.
In the polymer complexes systems, only Z- and E-isomers are
detected and no other reactions take place during UV
irradiation and thermal treatment (Figure S10). So it is the
photothermal isomerization that induced the reversible phase
transition in the film. Meanwhile, the appearance of E-CNBP
during UV irradiation disturbed the columnar structure
formation.
E/Z-Isomerization Induced Multiple Reversible
Switching. Figure 3a shows polarized optical microscopy
The self-organized ordered structure and orientation of
P4VP(Z-CNBP)0.8 in thin film could also be detected via
atomic force microscopy (AFM) and grazing-incidence SAXS
(GISAXS). A thin film of P4VP(Z-CNBP)0.8 with the thickness
around 90 nm was prepared via spin-coating of its chloroform
solution (10 mg/mL) and subsequent thermal annealing for 12
h at 80 °C. The GISAXS pattern of annealed film in Figure 4b
confirms the Φh structure with parallel orientation in the film.
Meanwhile, the corresponding AFM height and phase images
in Figure 4c clearly show fingerprint-like morphology on the
surface, which is a characteristic topography associated with the
parallel alignment of cylinders in the thin film as confirmed via
GISAXS. Since the surface topography of the film is related to
the self-organized structure, the photoinduced structure change
would further induce the surface topographical change in the
film. After UV irradiation for 10 min, the fingerprint pattern
disappears, instead of irregular morphology as shown in Figure
4c. Meanwhile, the diffraction pattern from Φh structure also
disappears in GISAXS, instead of diffused and weak halo from
lamellar structure. Correspondingly, the static contact angle of a
water droplet (θw) on the P4VP(Z-CNBP)0.8 film decreased
from 104° to 100° after UV irradiation, since the exposure of
the hydrophobic alkyl tail of CNBP unit to the topmost surface
changed because of the E/Z-isomerization. By heating up to
140 °C and then slowly cooling to its LC state, both hexagonal
diffraction pattern in GISAXS (Figure 4b) and the fingerprint
topographical pattern in AFM (Figure 4c) appear again.
After knowing the relationship between molecular config-
uration and self-organized structures, the stimuli-responsive
fluorescent properties of P4VP(Z-CNBP)x are readily to be
revealed. Figure 5a shows the photoluminescence (PL) spectra
of P4VP(Z-CNBP)0.8 film upon UV irradiation at different
times. The emission peak of initial state is at 493 nm. During
UV irradiation at room temperature, the intensity of emission
peak increases gradually and becomes constant after 90 min.
Meanwhile, the peak position shifts to 471 nm. The
hypsochromic shift of the emission peak means the conjugated
length decreases after irradiation, when Z-CNBP converts to E-
CNBP. As a comparison, fluorescence change of pure Z-CNBP
via UV irradiation stimuli at room temperature is presented in
the inset to Figure 5a. During UV irradiation the intensity and
peak position did not change much even after 2 h, which is due
to the crystalline structure that prevents the isomerization from
Z- to E-isomers. Different from pure Z-CNBP, the complexed
Z-CNBP in polymer matrix is well-dispersed with the aid of
hydrogen-bonding interaction. No crystalline structure formed
at room temperature based on SAXS, DSC, and POM studies.
Figure 3. POM photographs of P4VP(Z-CNBP)0.8 film taken at 125
°C (a) and then UV irradiated for 10 min through a mask (b). (c)
POM photograph of fully irradiated film. (d) The irradiated film is
heated to isotropic state at 140 °C and then cooled to 125 °C.
(POM) pictures of P4VP(Z-CNBP)0.8 taken at 125 °C.
Characteristic fan-like textures from columnar phase were
obtained by slowly cooling P4VP(Z-CNBP)0.8 from the
isotropic state. Upon UV irradiation through a mask for 10
min, the birefringence of irradiated zone decreases dramatically,
whereas other region still keeps its textures (Figure 3b). By
heating the fully irradiated film (Figure 3c) to its isotropic state
at 140 °C for 10 min and then slowly cooling down to 125 °C,
characteristic fan-like LC texture appears again (Figure 3d).
The distinct birefringence contrast is associated with the phase
structure change upon UV irradiation. SAXS experiments
directly revealed the structure evolution of P4VP(Z-CNBP)0.8
before and after UV irradiation. As shown in Figure 4a, the as-
prepared P4VP(Z-CNBP)0.8 sample exhibits Φh structure with
a parameter of 6.09 nm. After UV irradiation, the intensities of
scattering peaks decrease and become broad, together with the
disappearing of (11) scattering peak, indicating the phase
structure transforms to lamellar structure (L = 5.11 nm).
Meanwhile, q value of the first (10) peak increased from 1.19 to
1.23 nm−1. Then, after thermal treatment, the corresponding
SAXS profile confirms the formation of Φh structure with a
parameter of 6.05 nm, which is similar to the initial state
(Figure 4a). It is clearly demonstrated a reversible responsive
1
behavior. H NMR measurement confirms that the reversible
E
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Figure 4. (a) SAXS profiles of P4VP(Z-CNBP)0.8 recorded at initial state, after irradiation and after heating. GISAXS patterns (b) and AFM height
and phase images (c) of P4VP(Z-CNBP)0.8 thin film upon UV irradiation and thermal treatment.
Figure 5. (a) Photoluminescence (PL) spectra of P4VP(Z-CNBP)0.8 film upon UV irradiation for different times at room temperature. Inset: PL
spectra of Z-CNBP film upon UV irradiation for different times at room temperature. (b) PL spectra of P4VP(Z-CNBP)0.8 film before (pink
squares) and after (blue squares) UV irradiation and subsequent thermal treatment (green circulars). (c) Fluorescence images of P4VP(Z-CNBP)0.8
film during UV irradiation and thermal treatment processes, taken under 365 nm UV light. Both the diameter of each circular hole and the side
length of each square hole are 2 mm.
The Tg of P4VP(Z-CNBP)0.8 is 58 °C, which is above the room
temperature, so the E/Z-isomerization induced fluorescence
change could take place even below the Tg. A more exciting
thing is after heating the irradiated film at 140 °C for 10 min,
the emission peak returns to 493 nm, which is the same as the
initial state as shown in Figure 5b. The reversible fluorescence
switching is also realized in the polymer complex.
Since the PL intensity and wavelength are quite different
before and after UV irradiation, fluorescence patterns with dual
emission colors would be easily obtained by UV irradiating
P4VP(Z-CNBP)0.8 film through a mask. Figure 5c depicts the
fluorescence images of P4VP(Z-CNBP)0.8 film during the
writing−erasing cycles. The pristine film exhibits green
fluorescence under UV light. After UV irradiation for 60 min,
the irradiated zone became blue. It is worthwhile to point out
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Figure 6. (a) PL spectra of P4VP(E-CNBP)0.5 film before (blue squares) and after (purple circles) UV irradiation and subsequent thermal treatment
(green rhombuses). (b) Florescence image of irradiated P4VP(E-CNBP)0.5 film after UV through mask taken under 365 nm UV irradiation. (c)
SAXS profiles of P4VP(E-CNBP)0.5 after first UV irradiation and subsequent first heating treatment. (d) SAXS profiles of P4VP(Z-CNBP)x(E-
CNBP)y (x + y = 0.5). Inset: q as a function of x for P4VP(Z-CNBP)x(E-CNBP)y with x + y = 0.5.
that fluorescence change is more sensitive than other switching
behaviors. The dual color pattern could be identified by naked
eyes only after 1 min. The best contrast pattern could be
obtained after 60 min. Then, the fluorescence pattern could be
erased, and the whole film appears green fluorescence again by
heating to 140 °C for 10 min. The reversible switching is
repeatable. Different fluorescence patterns could be written
again in the same film by using different masks as shown in
Figure 5c. As a comparison, we also dispersed Z-CNBP into
polystyrene (PS) matrix with the same molar ratio (nZ‑CNBP:nSt
= 0.8:1) and with the same preparation condition. In this case,
Z-CNBP is dispersed into PS without hydrogen-bonding
interaction. However, only the PL intensity decreased during
UV irradiation (Figure S11). Meanwhile, the light-induced PL
change is irreversible. It is hard to return to its initial state by
heating to 140 °C or even higher temperatures (Figure S12).
So the fluorescence pattern written into the Z-CNBP/PS film is
not erasable, and the film even becomes heterogeneous during
heating/irradiation cycles (Figure S13). Comparing to Z-
CNBP/PS film, the hydrogen-bonded P4VP(Z-CNBP)x film is
quite uniform and stable during UV irradiation and thermal
treatment. At the same time, the hydrogen-bonding interaction
could keep ordered arrangement of CNBP molecules within
the polymer matrix. The fluorescent property of polymer
complex is associated with both molecular structure and
supramolecular self-organized structure, exhibiting a typical
reversible responsive characteristic.
Contrary to P4VP(Z-CNBP)x complex, the corresponding
P4VP(E-CNBP)x show irreversible responsive behavior. On the
basis of UV−vis spectra (Figure S1) results, in pure E-CNBP
state, only a small amount of E-CNBP could convert to Z-
CNBP under UV irradiation according to E/Z-isomerization
equilibrium. The interesting thing is the appearance of Z-CNBP
could “turn on” the fluorescence in polymer complexes. For
example, the as-prepared film of P4VP(E-CNBP)0.5 shows PL
emission peak at 472 nm as shown in Figure 6a. After
irradiation upon 365 nm UV light at room temperature, the PL
intensity increases dramatically and then reaches a constant at
469 nm after 1 h. The high contrast fluorescence pattern was
easily obtained by UV irradiating P4VP(E-CNBP)0.5 film
through a mask. The irradiated zone shows enhancement of
fluorescence, as shown in Figure 6b. Then, by heating at 140
°C, the intensity of PL peak decreases, and the peak position
1
red-shifts to 485 nm. H NMR detected the content of Z-
CNBP was 22% after first UV irradiation and 35% after first
heating (Figure S14), indicating an irreversible switching.
SAXS also shows that the intensity of scattering peaks
decreases significantly after irradiation. Then by thermal
treatment, the intensity of scattering peak increases (Figure
6c). The scattering vector q of the first peak at low angle region
is 1.31 nm−1 (after irradiation) and 1.28 nm−1 (after heating).
The relationship between q and the content of isomers could
be obtained by complexing both E-CNBP and Z-CNBP with
P4VP, and the SAXS profiles of P4VP(Z-CNBP)x(E-CNBP)y
(x + y = 0.5) are presented in Figure 6d. When x ≥ 0.4, Φh
structure appears, below which only the lamellar structure
exists. The plot of q vs x is shown in the inset of Figure 6d as a
linear fit of the experimental data with q = −0.594x + 1.381.
Thus, the content of Z-CNBP in the polymer complex could be
estimated via a q−x plot, which is 24% (after irradiation) and
34% (after heating). The result is also close to that obtained
1
from H NMR results.
CONCLUSION
■
In summary, we have successfully incorporated Z- and E-
stereoisomers of α-cyanostilbene derivatives into supramolec-
ular polymers via hydrogen-bonding interaction in an effective
and facile way. The corresponding P4VP(Z-CNBP)x and
P4VP(E-CNBP)x complexes have different self-assembly
behavior and stimuli-responsive properties. P4VP(Z-CNBP)x
possess Φh structure at 0.4 ≤ x ≤ 1.0, while lamellar structure
could be formed for P4VP(E-CNBP)x at 0.3 ≤ x ≤ 0.7. The
P4VP(Z-CNBP)0.8 film exhibited photothermal E/Z-isomer-
ization induced concomitant multiple reversible switching,
including the self-organized structures, surface topographies
and properties, birefringence, and fluorescence intensity and
color. The P4VP(E-CNBP)0.5 film shows irreversible switching
and enhanced fluorescence emission upon UV irradiation.
G
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Article
−(CH2)9−), 0.88 (t, 9H, −CH3). MS: calcd for C57H87NO4, 849.7 Da;
found m/z 850.7 [M + H]+.
Thus, by controlling over the molecular configuration in
supramolecular polymer system, we can precisely tune the
phase structure and functional responsive properties at the
same time. On the basis of those results, it would be intriguing
to further incorporate various functional moieties into
cyanostilbene-based supramolecular system to develop new
kind of smart photoactive materials.
Polymer Complex Preparation. The supramolecular complexes
were prepared as the following procedure. P4VP and CNBP (Z- or E-
isomers) were separately dissolved into chloroform to get 5 wt %
solutions. In order to obtain the P4VP(CNBP)x complex, P4VP and
CNBP solutions were mixed together with calculated proportion
related to x. A homogeneous solution was obtained. After mechanical
stirring for 24 h at room temperature, the solvent was slowly
evaporated. The product was then placed in a vacuum oven at 35 °C
to remove residual solvent. The supramolecular complex was thus
obtained.
Thin Film Preparation. P4VP and CNBP were separately
dissolved in chloroform to yield 10 mg/mL solutions and then
mixed in proportion to give homogeneous solutions. The solution was
filtered with 0.2 μm PTFE filters and spin-coated onto a freshly
cleaned silicon wafer at the rotating speed of 2000 rpm for 30 s to get
the film around 80−90 nm as judged from spectroscopic ellipsometer.
Isomerization Experiment. Photoinduced isomerization was
performed under UV irradiation from a hand-held UV light (1.0
mW/cm2) at a distance of 5 cm. Solutions for UV−vis tests were
irradiated in quartz cells at room temperature. Solutions for NMR
experiments were irradiated in NMR tube at room temperature. Films
were irradiated at the mesophase (125 °C) or at room temperature as
required.
EXPERIMENTAL SECTION
■
Materials. Z-CNBP was synthesized according to the synthetic
route as shown in Scheme 2 and purified through column
chromatography. E-CNBP was obtained by irradiating concentrated
solution of Z-CNBP in CHCl3 by 365 nm UV light for at least 2 h to
get a mixture of Z-CNBP and E-CNBP. E-CNBP was then separated
and purified by column chromatography. Poly(4-vinylpyridine)
(P4VP, weight-average molecular weight, Mw = 6.0 × 104, PDI =
1.3) was purchased from Polymer Source Inc. Other materials were
used as received. The detailed information is listed in the Supporting
Information.
Synthesis of 3,4,5-Tris(dodecyloxy)benzaldehyde (Com-
pound A). K2CO3 (2.81 g, 20.33 mmol) and KI (catalytic amount)
were added to a solution of 3,4,5-trihydroxybenzaldehyde mono-
hydrate (1.00 g, 5.81 mmol) in dry DMF (20 mL). The mixture was
stirred at 80 °C. 1-Bromododecane (5.62 mL, 23.24 mmol) was slowly
dropped into the mixture. The reaction lasted overnight. After cooling
to RT, the mixture was poured into brine and extracted with
dichloromethane. The organic layer was dried by anhydrous
magnesium sulfate, and the solvent was evaporated by a rotor vapor.
The product (3.26 g, 85%) was obtained by column chromatography
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.8b00347.
1
using ethyl acetate and n-hexane (1:10 v/v). H NMR (CDCl3) δ
[ppm]: 9.83 (s, 1H, −CHO), 7.08 (s, 2H, Ar−H), 4.04 (m, 6H,
−OCH2), 1.79 (m, 6H, −CH2), 1.48 (m, 6H, −CH2), 1.26 (m, 48H,
−CH2), 0.88 (t, 9H, −CH3).
Materials, instruments, and measurements; Figure S1
(UV−vis spectra of CNBP); Figure S2 (1H NMR of
CNBP); Figure S3 (POM of CNBP); Figure S4 (PL
spectra of CNBP); Figure S5 (PL spectra of Z-CNBP-
AIEE effect); Figure S6 (DSC of CNBP); Figure S7
(XRD of CNBP); Figure S8 (1H NMR of P4VP(Z-
CNBP)x); Figure S9 (1H NMR of P4VP(Z-CNBP)0.8
and P4VP(E-CNBP)0.5); Figure S10 (1H NMR of
P4VP(Z-CNBP)0.8); Figures S11 and S12 (PL spectra
of PS/Z-CNBP); Figure S13 (fluorescence images of PS/
Z-CNBP); Figure S14 (1H NMR of P4VP(E-CNBP)0.5)
(PDF)
Synthesis of (4′-Hydroxybiphenyl-4-yl)acetonitrile (Com-
pound B). 4-Bromophenylacetonitrile (1.7 g, 8.6 mmol) and 4-
hydroxybenzeneboronic acid (1.2 g, 8.7 mmol) were dissolved in 60
mL of THF. Then, 30 mL of an aqueous sodium carbonate (6.4 g, 60
mmol) solution was added. Under a nitrogen atmosphere, Pd(PPh3)4
(0.1 g, 0.08 mmol) was added, and the solution was refluxed for 12 h.
After cooling to room temperature, the mixture was neutralized by
hydrochloric acid and then extracted with brine/ethyl acetate for three
times. The collected organic phase was dried by anhydrous magnesium
sulfate. The crude product was purified by column chromatography
using petroleum ether and ethyl acetate (2:1 v/v) to obtain a white
solid. Yield: 1.08 g, 60%.1H NMR (CDCl3) δ [ppm]: 9.58 (s, 1H,
−OH), 7.54 (d, 2H, Ar−H), 7.46 (d, 2H, Ar−H), 7.36 (d, 2H, Ar−H),
6.90 (d, 2H, Ar−H), 3.78 (s, 2H, −CH2).
AUTHOR INFORMATION
Corresponding Author
*E-mail xfchen75@suda.edu.cn (X.-F.C.).
■
Synthesis of (Z)-2-(4′-Hydroxybiphenyl-4-yl)-3-(3,4,5-tris-
(dodecyloxy)phenyl)acrylonitrile (Z-CNBP). Compound A (540
mg, 0.82 mmol), compound B (171.6 mg, 0.82 mmol), and sodium
hydroxide (200 mg, 5 mmol) in 10 mL of anhydrous methanol were
stirred under a nitrogen atmosphere at 50 °C for 12 h. After cooling to
room temperature, the mixture was neutralized by hydrochloric acid.
The yellow precipitation was collected by filtration and extracted with
water and ethyl acetate. Yield: 0.507g, 80%. 1H NMR (CDCl3) δ
[ppm]: 7.76 (d, 2H, Ar−H), 7.67 (d, 2H, Ar−H), 7.57 (d, 2H, Ar−H),
7.50 (s, 1H, −CH−), 7.21 (s, 2H, Ar−H), 6.98 (d, 2H, Ar−H) 4.10
(m, 6H, −OCH2−), 1.73−1.87 (m, 6H, −OCH2CH2−), 1.26−1.54
(m, 54H, −(CH2)9−), 0.88 (t, 9H, −CH3). MS: calcd for C57H87NO4,
849.7 Da; found m/z 850.7 [M + H]+.
ORCID
Xiao-Fang Chen: 0000-0002-2973-0432
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Grants 21474073 and 21174003) and A
Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD) for financial support. We thank
Beamline BL16B1 at SSRF (Shanghai Synchrotron Radiation
Facility, China) for providing the beamtimes.
Synthesis of (E)-2-(4′-Hydroxybiphenyl-4-yl)-3-(3,4,5-tris-
(dodecyloxy)phenyl)acrylonitrile (E-CNBP). A concentrated sol-
ution of Z-CNBP in CHCl3 was irradiated under 365 nm UV light for
3 h to get a mixture of Z-CNBP and E-CNBP. E-CNBP was separated
from the mixture and purified by column chromatography as a powder
1
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