Mesomorphism, polymerization, and chirality induction in α-cyanostilbene-functionalized diacetylene-assembled films: Photo-triggered Z/E isomerization

Article abstract of DOI:10.1002/pola.28633

Engineering of molecular stacking arrangement via environmental stimuli is of particular interest in stimuli-responsive self-assembling architectures. A novel dual photo-functionalized diacetylene ((Z)-CNBE-DA) molecule was synthesized, in which photo-res

Full text of DOI:10.1002/pola.28633

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Mesomorphism, Polymerization, and Chirality Induction in
a-Cyanostilbene-Functionalized Diacetylene-Assembled Films:
Photo-Triggered Z/E Isomerization
Yan Zhang, Jingang Hu, Guang Yang, Hongli Zhang, Qijin Zhang, Feng Wang, Gang Zou
CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, iChEM, University of Science
and Technology of China, Hefei, Anhui 230026, People’s Republic of China
Correspondence to: G. Zou (E-mail: gangzou@ustc.edu.cn) or F. Wang (E-mail: drfwang@ustc.edu.cn)
Received 8 February 2017; accepted 7 April 2017; published online 00 Month 2017
DOI: 10.1002/pola.28633
ABSTRACT: Engineering of molecular stacking arrangement via
environmental stimuli is of particular interest in stimuli-
responsive self-assembling architectures. A novel dual photo-
functionalized diacetylene ((Z)-CNBE-DA) molecule was synthe-
sized, in which photo-responsive cyanostilbene moieties exhib-
ited interesting Z-E isomerization upon UV light irradiation and
could be utilized to modulate mesomorphism, molecular stack-
ing arrangement and their resulting polymerization behavior.
Rod-like (Z)-CNBE-DA could self-assemble into well-defined
lamellar structures and the helical polydiacetylene (PDA) chains
could be formed upon irradiation with circularly polarized
ultraviolet light (CPUL). However, the bent-shaped (E)-CNBE-
DA molecules only self-assembled into irregular loose packing,
inhibiting the formation of ordered helical PDA chains upon
CPUL irradiation. In this work, we established the links
between chemical structures, molecular packing engineering
and photophysical properties, which would be of great funda-
mental value for the rational design of smart soft materials.
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KEYWORDS: chiral; cyanostilbene; isomer/isomerization; liquid
crystal;
photoisomerization;
photopolymerization;
polydiacetylene
far, various types of functional groups have been incorpo-
rated into diacetylene monomer, and a variety of experimen-
tal and theoretical works have been carried out to
manipulate the well-defined molecular architectures.17–19
However, there were few reports concerning on the links
among the mesomorphism, polymerization and chirality
induction in DA assemblies.20 In particular, control of the
mesomorphic and crystalline organization in diacetylene
assemblies via photo-triggered isomerization of photo-
responsive a-cyanostilbene central core structure, and their
resulting polymerization and chiral induction behavior have
not been described to date.
INTRODUCTION Over the past few decades, stimuli-
responsive chiral self-assembling architectures continue to
be a hot topic, since chirality is a basic character of nature
and the chiral assemblies have emerged as the primary
building blocks for the design of chiral functional materials
at various hierarchical levels.1–4 Of particular interest is engi-
neering of molecular stacking arrangement via environmen-
tal stimuli rather than the variation of their primary
chemical structure. Therefore, fine-tuning the intermolecular
interaction and subsequently controlling of the molecular
stacking arrangement, have been regarded as a promising
approach to control the optoelectronic properties of the
functional conjugated organic materials.5–7 Polydiacetylene
(PDA) is one of the most interesting conjugated organic
materials due to its unique environmentally responsive opti-
cal characteristics and the promising application in optics,8
electronics,9,10 chemosensors,11,12 and biosensors.13–15 As is
well known, the topo-chemical polymerization of the diacety-
lene (DA) monomer is very sensitive to the molecular stack-
ing arrangement and orientation (the monomer diyne units
should be arranged with a repeat distance of 5 Å and an ori-
entation angle of 45 8C relative to the translation axis).16 So
Linear p-conjugated organic molecules consisting of a-cya-
nostilbene unit have been rapidly developed as an interest-
ing molecules toward innovative optoelectronics devices
applications, due to their peculiar photophysical and self-
assembling characteristics.21–24 The fluorescent behavior,
known as aggregation-induced emission,25,26 as well as their
structural modulation upon reversible trans–cis photo-
isomerization have attracted enormous attention.27–33 Exten-
sive studies in this field are mainly focused on the (Z)-1-
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Synthetic routes of (Z)-CNBE-DA.
cyano-1,2-diphenyl-ethylene ((Z)-CNBE) derivatives, since
they have strong p–p interactions between rigid rod-like aro-
matic segments and dipole interactions from polar cyano
functionality group substituted on the vinyl group, control-
ling the molecular stacking to generate various supramolecu-
lar, mesomorphic, and crystalline organizations.5,34–36 However,
very little is known for the characteristics of (E)-CNBE deriva-
tives, especially the modulation of their molecular self-
assembled structure and mesomorphism via photo-triggered Z-
E isomerization. Soo et al. reported the structural, photophysi-
cal, and thermal properties of the trans and cis isomers of (Z)-
CN-MBE, which exhibited a reversible photo/thermal Z-E isom-
erization process, accompanied by a remarkable fluorescence
on/off switching in the solid state.28 Zhao et al. achieved in situ
tuning luminescent color conversion along yellow, white, and
blue based on cyanostilbene functionalized quantum dots by Z-
E photo-isomerization.33 In this article, it is anticipated that the
introduction of a-cyanostilbene unit into DA system would fruit
dual photo-functionalized materials (abbreviated as (Z)-CNBE-
DA) with novel photo/thermal responsive properties, the meso-
morphism, the molecular stacking arrangement, and the result-
ing polymerization behavior in the diacetylene assemblies could
be easily controlled and modulated via photo-triggered Z-E
isomerization of the CNBE core. Further, we achieved the chiral
transference from the helical PDA backbone to the chiral
arrangement of the CNBE core upon irradiation with circularly
polarized ultraviolet light (CPUL). On the other hand, the
desired chain configuration could be permanently locked by the
polymerization of diacetylene groups to gain a deep insight into
the structure–property relationship. Herein, it is realized that
the construction of strong links between molecular packing
arrangement and photochemical properties of the resulting mol-
ecules would be the core research for the development of smart
and multifunctional soft materials.
hydroxyphenylboronic acid, 4-bromophenylace-tonitrile, 4-bro-
mobenzaldehyde, Pd(PPh3)4, 4-dimethylaminopyridine (DMAP),
and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochlo-
ride (EDC) were purchased from Aldrich and used as received.
Dichloromethane (DCM) was dried over anhydrous magnesium
sulfate and filtered before use. All other solvents and reagents
were of analytical grade and used as received. Milli-Q water (18.2
MX cm) was used in all cases.
Sample Preparation
4’-Hydroxy-biphenyl-4-carbaldehyde (1), (4’-hydroxy-biphe-
nyl-4-yl)-acetonitrile (2), and (Z)-2,3-bis(4’-hydroxy-biphe-
nyl-4-yl)-acrylonitrile (3) were synthesized according to the
methods described in the ref. 37.
1
Compound 3: H NMR (300 MHz, DMSO-d6, d, ppm): 8.02 (s,
1H), 7.98 (d, 2H, J 5 8.4 Hz), 7.82–7.66 (m, 6H), 7.53 (t, 4H,
J 5 9.0 Hz), 6.76 (t, 4H, J 5 9.2 Hz). FTIR (KBr, cm21): 3388
(OH), 3024 (CACH), 2217 (CN).
(Z)-2,3-Bis(4’-(10,12-pentacosadiynoate)-biphenyl-4-yl)-acrylo-
nitrile (abbreviated as (Z)-CNBE-DA) was synthesized in anal-
ogy to the following procedure and the molecular structure was
shown in Scheme 1. DA (100 mg, 0.27 mmol), compound 3
(47 mg, 0.12 mmol), EDC (93mg, 0.48 mmol), and DMAP
(13 mg, 0.11 mmol) were dissolved in 15 mL dry DCM. The
reaction mixture was stirred at room temperature (RT) for 24 h
under the protection of nitrogen. After removing the solvent,
the residue was successively extracted with saline/DCM for
three times, and the collected organic phase was dried with
anhydrous magnesium sulfate. After solvent evaporation, the
crude product was purified by column chromatography (silica
gel, petroleum ether:DCM, 1:3) to obtain a white solid.
Yield: 70.5 mg, 48%. mp. 109 8C. 1H NMR (300 MHz, CDCl3,
d, ppm): 8.01 (d, 2H, J 5 8.3 Hz), 7.79 (d, 2H, J 5 8.3 Hz),
7.72 (s, 1H), 7.71–7.59 (m, 8H), 7.20 (dd, 4H, J 5 8.5, 2.0
Hz), 2.60 (t, 4H, J 5 7.5 Hz), 2.26 (q, 8H, J 5 6.5 Hz), 1.87–
1.71 (m, 4H), 1.26 (m, 60H), 0.95–0.83 (m, 6H). 13C NMR
(300 MHz, CDCl3, d, ppm): 172.24, 150.74, 141.27, 137.57,
133.46, 132.72, 129.89, 128.06, 126.42, 122.12, 118.04,
110.95, 65.35, 34.40, 31.92, 29.63, 24.91, 22.69, 19.21,
EXPERIMENTAL
Materials
10,12-Pentacosadiynoic acid (DA) purchased from Tokyo Chem-
ical Industry was purified by dissolving in dichloromethane and
filtrating to remove the polymer before use. 4-
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14.13. FTIR (KBr, cm21): 3061 (C@CH); 2921, 2850 (CH2);
2223 (CN); 2175, 2139 (CBC); 1747 (C@O); 1600, 1497,
1467. MALDI-MS (m/z): Calcd for C27H18NO2, 388.13; Found,
388.97. Calcd for C16H26, 218.20; Found, 217.99.
details were shown in the Experimental section. The termi-
nal diacetylene fragments were directly attached to the
CNBE central core by esterification reactions to facilitate a
mesomorphic organization.5 The molecular structure of (Z)-
CNBE-DA was confirmed by 1H NMR, 13C NMR, FTIR, and
mass spectroscopy (Supporting Information Figs. S1–S4).
Compound (Z)-CNBE-DA was dissolved in cyclopentanone
with a concentration of 5 mg mL21. Then, the monomer
films were prepared by the solution casting method. A few
drops of the solution were added to the solid substrates.
After solvent evaporation, the transparent monomer films
were dried under vacuum (5 mmHg) at 50 8C for 12 h. All
the preparation procedures descried above were performed
in darkness to avoid polymerization in advance. A 365-nm
UV light irradiation was employed to realize the Z–E photoi-
somerization of a-cyanostilbene mesogen within CNBE-DA
monomer in the solution. A hand-held UV lamp (k5254 nm)
and circularly polarized UV light (CPUL, k5313 nm) were
used for normal UV polymerization or to prepare chiral poly-
diacetylene films, respectively. The crystalline colorless thin
films of (Z)-CNBE-DA and the pale yellow thin films of (E)-
CNBE-DA were both converted into blue PDA films. The films
thickness were about 200 nm for (Z)-CNBE-DA and 225 nm
for (E)-CNBE-DA by AFM (Supporting Information Fig. S5).
The optical characteristics of (Z)-CNBE-DA in solution were
investigated by UV–vis absorption spectroscopy. The revers-
ible photoisomerization of CNBE-DA was shown in Figure
1(a) upon irradiation with 365- and 254-nm UV light,
respectively. The (Z)-CNBE-DA in THF solution exhibited the
absorption maximum peak (kmax) at 355 nm [black bold line
in Fig. 1(c)], which was attributed to p–p* transition absorp-
tion of the chromophore CNBE core. Upon irradiation with
365-nm UV light (1.5 mWꢀcm22), the absorption maximum
peak (kmax) was gradually shifted from 355 to 340 nm,
accompanied by a decrease of absorption intensity, and the
photo-stationary state (PSS) was quickly achieved after
approximately 15 s of 365-nm UV irradiation [Fig. 1(c)]. This
blue-shift in the absorption should be ascribed to the Z!E
isomerization, due to the reduction of effective conjugation
length in the bent E-isomer conformation. Then upon irradia-
tion with 254-nm UV light (0.07 mWꢀcm22), the opposite
red-shift for the absorption kmax and the partial recovery of
the absorption intensity should be ascribed to the E!Z
isomerization, and its absorbance increased until another
equilibrium was reached after 50-s irradiation time [Fig.
1(d)]. We must note here that two absorbing species (Z- and
E-isomer) were both present in this case since there was the
isosbestic point appearing in the same position of 323 nm.28
In addition, the Z!E and E!Z photoisomerization process
Characterization
1H NMR and 13C NMR spectra experiments were carried out
with a JEOL FX-90Q NMR 300 spectrometer. FTIR experiments
were performed on a MAGNA 750 FTIR spectrometer. Matrix-
assisted laser desorption/ionization time-of-flight mass spec-
trum (MALDI-TOF-MS) was recorded on a Bruker BIFLEXe III
mass spectrometer using a nitrogen laser (337 nm). The
UV2vis absorption spectra were recorded on a SHIMADZU
UV-2550 PC spectrophotometer. A Hamamatsu LC6 Hg/Xe
lamp combined with a 313-nm interface filter and a 365-nm
filter were used as the CPUL irradiation source and the UV
irradiation source, respectively. Polarized optical microscopy
(POM) images were obtained in an Olympus BX-51 fluores-
cence microscope equipped a heating stage with an INSTEC
mK1000 high precision temperature controller. Differential
scanning calorimetry (DSC) measurements were performed on
a TA Instruments Q2000 DSC at a heating and cooling rate of
10 8C min21 under nitrogen atmosphere. X-Ray diffraction
(XRD) measurements were performed with a Rigaku AX-G by
using a CuKa (k50.154 nm) beam. The Raman spectra were
performed on a LABRAM-HR Confocal Laser MicroRaman
Spectrometer with 514.5 nm radiation. The circular dichroism
(CD) spectra were measured by using JASCO CD spectrometer
J-810. Transmission electron microscopy (TEM) images were
obtained in a JEOL-2000 microscope (operated at 200 kV).
Atomic force microscope (AFM) images were recorded on
Veeco diInnova microscope with a DAC controller.
1
could also be monitored by H NMR characterization. At ini-
tial stage, CNBE-DA molecules were all in the Z-form [Fig.
1(b) (i)]. After irradiation with 365-nm UV light for 20 s,
partial Z-form CNBE-DA molecules were transferred to the E-
isomer, and the Z/E ratio could be estimated to be approxi-
1
mately 33/67 according to the H NMR integral studies [Fig.
1(b) (ii)]. Then irradiation with 254-nm UV light for 60 s,
only partial E-isomer could be transformed back to the Z-iso-
mer, and the Z/E ratio was estimated to be about 55/45
[Fig. 1(b) (iii)]. Similar results could be obtained when moni-
tored Z/E ratio by proton peak split of alkyl area connected
with ester group (located in 2.6 ppm) according as the 1H
NMR integral studies.
The mesomorphism behavior of (Z)-CNBE-DA were studied
by differential scanning calorimetry (DSC), polarized optical
microscopy (POM) and X-ray diffraction (XRD) characteriza-
tion, which revealed the presence of crystalline, nematic, and
isotropic phases. As shown in Figure 2(a), (Z)-CNBE-DA mol-
ecules melted to the liquid crystal (LC) phase at about
109 8C (with a typical schlieren texture of nematic phase
observed by POM, inset in Fig. 2(a)] and switched to isotro-
pic phase at about 183 8C with a heating rate of 10 8C/min.
During the cooling process with the same rate of 10 8C/min,
two exothermic peaks were observed at approximately 178
and 91 8C with a wide LC phase temperature range,
RESULTS AND DISCUSSION
Morphology and Structural Characterization of CNBE-DA
Films
The molecular structure and the synthetic routes of (Z)-
CNBE-DA were presented in Scheme 1, detailed experimental
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FIGURE 1 (a) Reversible photo-isomerization of CNBE-DA upon irradiation with 365- and 254-nm UV light, respectively. (b) 1H
NMR spectra changes of (Z)-CNBE-DA in CDCl3: (i) before irradiation, (ii) upon 365-nm UV irradiation for 20 s and (iii) then 254-nm
UV irradiation for 60 s. (protons in the vinylene of Z-isomer form ( ) and E-isomer form ( ). UV–vis absorption spectra changes
of (Z)-CNBE-DA in THF solution (2 3 1025 M) upon irradiation with (c) 365-nm UV light (1.5 mWꢀcm22) and (d) then 254-nm UV
light (0.07 mWꢀcm22). [Color figure can be viewed at wileyonlinelibrary.com]
ascribable to the isotropic to nematic phase transition, and
the crystallization transition [with spherulites formation,
inset in Fig. 2(a)], respectively. The XRD profile of (Z)-CNBE-
DA in nematic phase at 120 8C was demonstrated in Figure
2(b), with a sharp and intense reflection at 1.758, corre-
sponding to layer spacing of 5.05 nm. The broad hump in
the wide-angle region suggested the absence of in-plane
positional order in LC phase.38 Transmission electron
microscopy (TEM) images for the (Z)-CNBE-DA assemblies in
crystal state showed highly ordered lamellar structure (Sup-
porting Information Fig. S6a). The incorporating of CNBE chro-
mophore would reinforce the intermolecular interactions,
resulting in the formation of regular lamellar aggregates struc-
ture. The formation of highly ordered crystal structure with an
intense (001) reflection at 6.0 nm was demonstrated by small-
angle XRD analysis [Fig. 2(c)], and layer spacing (d) with ratios
of 1, 1/3, 1/4, and 1/6 characteristic for a lamellar phase,
which were indexed as diffractions from the (001), (003),
(004), and (006) planes, respectively.39 In the wide angle
region, a p–p stacking peak at the value of 19.28 could be
observed, corresponding to a mean lateral spacing of 0.46 nm
between the molecules [inset in Fig. 2(c)]. The extended
molecular length (L) for (Z)-CNBE-DA was evaluated using
Materials Studio calculation to be 7.5 nm, longer than d-
spacing of 6.0 nm, suggesting the molecular arrangement in
the layer must be tilted by an angle of 378 relative to the layer
normal.40 A schematic diagram of molecular packing arrange-
ment of (Z)-CNBE-DA assemblies in crystal state was pre-
sented in Figure 2(d).
As we know, CNBE-based core could undergo Z!E isomeri-
zation upon 365-nm UV light irradiation. Therefore, it is
anticipated that the Z!E isomerization of the CNBE-based
core may influence the molecular packing arrangement as
well as their mesomorphic behavior. The (Z)-CNBE-DA solu-
tion were irradiated with 365-nm UV light for 20 min and
partial (Z)-CNBE-DA transformed into (E)-CNBE-DA. As indi-
cated by DSC and POM characterization (Fig. 3a), this sample
(pretreated with 365-nm light irradiation) melted to isotro-
pic phase at about 108 8C with a heating rate of 10 8C/min.
Continued slowly cooling with a same rate of 10 8C/min, the
crystallization transition occurred at 92 8C with spherulites
formation. No LC phase texture could be observed during
the heating or cooling process. And that, highly ordered
lamellar structures were broken for the sample treated with
365-nm UV light irradiation (Supporting Information Fig.
S6b). During the assembly process, partial CNBE based core
tended to adopt bent-shape structure, and the steric hin-
drance inhibited the formation of highly ordered molecular
close-packing. Thus, no distinct morphology could be
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FIGURE 2 (a) DSC curve of (Z)-CNBE-DA on heating/cooling rate of 10 8C/min. The insets showed POM images of nematic phase
and crystalline phase of (Z)-CNBE-DA during cooling (scale bar: 40 and 200 mm, respectively). (b) XRD pattern of (Z)-CNBE-DA in
LC phase at 120 8C during cooling. (c) XRD pattern of (Z)-CNBE-DA crystal powder at 30 8C. (d) The packing model of a highly
ordered lamellar structure of (Z)-CNBE-DA assemblies in crystal state. [Color figure can be viewed at wileyonlinelibrary.com]
observed in this case. XRD profile of the sample in the crys-
tal state was shown in Figure 3(b), with a sharp and intense
reflection at 1.528 and 1.778, corresponding to layer spacing
of 6.0 and 5.0 nm, indicating both of Z- and E- isomer were
present in the crystal state. The energy-optimized structure
of (E)-CNBE-DA was shown in Supporting Information Figure
S7 by Materials Studio calculation, and the molecular length
was calculated to be 5.1 nm close to the d-spacing of
5.0 nm. The simplified schematic illustration of photo-
induced molecular conformation transformation was demon-
strated in Figure 3(c). The proposed packing arrangement of
a disordered structure of (E)-CNBE-DA assemblies deduced
from XRD study was presented in Figure 3(d). Moreover, the
photo-triggered phase transition for the (Z)-CNBE-DA sample
from nematic phase to isotropic phase occurred within 5
min at 120 8C upon 365-nm UV irradiation (Supporting
Information Fig. S8), which should be ascribed to the Z-E
photo-isomerization of CNBE-based core. All above results
indicated the molecular packing arrangement as well as the
mesomorphic behavior of CNBE-DA assemblies could be eas-
ily controlled and modulated by 365-nm UV irradiation via
Z-E photo-isomerization of CNBE-based core.
characterization. (Z)-CNBE-DA films were prepared by the
solution casting method. As expected, (Z)-CNBE-DA films
could be polymerized upon irradiation with 254-nm UV light.
Typically intense absorption maximum at about 645 and
590 nm, corresponding to the electronic transition of the
conjugated
p
orbital of polydiacetylene (PDA) chain,
improved with the increasing of the irradiation time [Fig.
4(a)], tending to be saturated after UV irradiation for 4 min.
After polymerization, the fractional conversion value for the
sample was about 0.37. The films exhibited two characteris-
tic Raman vibrational bands at 2098 and 1496 cm21, corre-
sponding to the CBC and C@C stretching vibrations of PDA
chain (Supporting Information Fig. S9). Time-depended
development of absorption maximum at 645 nm was used to
determine the polymerization kinetics of (Z)-CNBE-DA, which
described as a first-order reaction with the polymerization
rate constant k was 1.8 3 1022 s21 [Fig. 4(b)]. We found
that (Z)-CNBE-DA could not be polymerized in nematic LC
state since the (Z)-CNBE-DA monomers had a mobility and
they could not be arranged in-plane positional order in LC
phase, which disturbed the well-ordered stacking arrange-
ment and orientation of diacetylene groups (Supporting
Information Fig. S10). For the sample pretreated with 365-
nm light irradiation, partial (Z)-CNBE-DA molecules were
transformed into the E-isomer, and the steric hindrance
would inhibit the highly ordered molecular packing, resulting
Enantio-Selective Polymerization of CNBE-DA Films
The diacetylene monomer could be polymerized under UV
irradiation as detected optically by UV–vis absorption spectra
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FIGURE 3 (a) DSC curve of (E)-CNBE-DA on heating/cooling rate of 10 8C/min. The inset showed POM image of crystalline phase
of (E)-CNBE-DA during cooling (scale bar: 200 mm). (b) XRD pattern of (E)-CNBE-DA in crystal state at 30 8C. (c) Schematic illustra-
tion of photo-induced molecular conformation transformation. (d) The packing model of a disordered structure of (E)-CNBE-DA
assemblies in crystal state. [Color figure can be viewed at wileyonlinelibrary.com]
in lower monomer conversion degree (0.13) and lower
absorption intensity [Fig. 4(c)]. A slight blue-shift of the
absorption maximum at 640 and 580 nm for PDA chain
could be observed, due to the reduction of effective conjuga-
tion length for PDA chain. The polymerization rate constant
k was calculated to be about 2.0 3 1022 s21 [Fig. 4(d)].
via 1,4-addition reaction, as a result of specific rotations of
the monomeric units relative to the positions of their centers
of mass. In the p-conjugated PDA backbone, the p-orbital
overlap is altered by rotation of the CAC bonds in the back-
bone, causing the change of the planar configuration of PDA
backbone.41 In the case of normal UV irradiation, the rota-
tion direction of CAC bonds within PDA backbone were ran-
dom; thus, the film exhibited no macroscopical CD signals.
Upon irradiation with CPUL, the rotation direction of CAC
bonds within PDA backbone would follow the rotational
direction of CPUL due to the special interaction between
CPUL and the diacetylene dimer. Therefore, one kind of heli-
cal PDA backbone were predominantly formed and the film
exhibited obvious CD signals. Based on above results, a pos-
sible mechanism for the formation of helical assemblies in
the crystal state upon CPUL irradiation could be proposed to
explain this phenomenon [Fig. 5(b)].
Moreover, we demonstrated that left- and right-handed circu-
larly polarized UV light (CPUL) could be utilized to induce
the enantioselective photo-polymerization of DA assemblies,
which showed great dependence on the packing structure of
DA monomers.20 As shown in Figure 5(a), CD spectra for the
(Z)-CNBE-PDA films polymerized with left- and right-handed
CPUL clearly showed comparable but opposite responses for
PDA chains as well as for CNBE units, with strong CD activ-
ity at 298, 413, 625, and 673 nm, assigned to the absorption
of spiral CNBE cores and helical PDA backbone, respectively.
Namely, the chiral information of CPUL could be transferred
to CNBE central core via the spiral PDA backbone. The heli-
cal morphologies of self-assembled (Z)-CNBE-DA after irradi-
ation with CPUL were demonstrated by the TEM images
(Supporting Information Fig. S11). However, upon irradiation
with nonpolarized UV light, no CD signal for PDA chains or
for CNBE units could be detected [Fig. 5(a)iii)]. The (Z)-
CNBE-DA had a planar molecular configuration, conducive to
ordered closely molecular packing arrangement. The topo-
chemical polymerization of the diacetylene units occurred
Meanwhile, the (Z)-CNBE-PDA films exhibited completely
reversible thermochromic phase transition behavior with a
recoverable blue-to-red color change when the heating treat-
ment temperature was below 80 8C [Fig. 6(a)].42 Interest-
ingly, optically chirality was maintained during the
thermochromics phase transition. As illustrated in Figure
6(b), CD signal for PDA chains exhibited corresponding blue-
shift, similar as those in the UV absorption band, while CD
signal for CNBE units remained almost unchanged. When
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FIGURE 4 Time-resolved development of (a) UV–vis absorption spectra and (b) absorption maximum at 645 nm for the (Z)-CNBE-
DA films upon 254-nm UV irradiation. Time-resolved development of (c) UV–vis absorption spectra and (d) absorbance at 645 nm
for the (E)-CNBE-DA films upon 254-nm UV irradiation. The irradiation intensity was about 0.07 mWꢀcm22. [Color figure can be
viewed at wileyonlinelibrary.com]
FIGURE 5 (a) UV–vis absorption spectra and CD spectra of (Z)-CNBE-DA films after irradiation with (i) left-handed CPUL, (ii) right-
handed CPUL, and (iii) nonpolarized UV light. (b) The formation mechanism of the helical (Z)-CNBE-PDA assemblies in the crystal
state upon CPUL irradiation. The irradiation intensity was about 14 mWꢀcm22. [Color figure can be viewed at wileyonlinelibrary.
com]
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FIGURE 6 (a) UV–vis absorption spectra and (b) CD spectra of (Z)-CNBE-PDA films at 25 8C (i), heating to 80 8C (ii), and then cool-
ing to 25 8C (iii). The inset in (a) showed the reversible color change for (Z)-CNBE-PDA films upon heating treatment. [Color figure
can be viewed at wileyonlinelibrary.com]
cooling to room temperature, CD signal for PDA chains could
return to the original position. All above results indicated
that this enantioselective polymerization of (Z)-CNBE-DA
films should be a deterministic process, and the handedness
of applied CPUL directed the helical direction of PDA chain
and the helical packing arrangement of CNBE cores.
exhibited nematic LC phase with a wide temperature range
between 178 and 91 8C, while no LC phase could be observed
for the bent-shaped (E)-CNBE-DA assemblies. (Z)-CNBE-DA
had a planar molecular configuration, favoring the topo-
polymerization and formation of helical PDA chains upon CPUL
irradiation. However, the intermolecular steric hindrance and
irregular loose packing within (E)-CNBE-DA assemblies inhib-
ited the topo-polymerization and formation of helical PDA
chains. This work not only provides an extended effort toward
molecular stacking engineering but also is of great fundamental
value for the rational design of novel PDA assemblies toward
practically smart photonic devices and sensors.
As for the sample pretreated with 365-nm UV light irradia-
tion, partial (Z)-CNBE-DA molecules were turned into the E-
isomer. Although the sample could be polymerized upon
CPUL irradiation, no obvious CD signal for PDA chains or for
CNBE units could be detected, indicating that the ordered
helical PDA chains or helical packing arrangement of CNBE
cores could not be formed in this case (Supporting Informa-
tion Fig. S12). The disordered loose molecular packing
arrangement could be obtained due to the complicated inter-
molecular steric hindrance within the bent-shaped E-iso-
mers. Therefore, the sample exhibited CD silence in this case.
ACKNOWLEDGMENTS
This research was carried out with funds from the National Nat-
ural Science Foundation of China (21574120), and Science and
Technological Fund of Anhui Province for Outstanding Youth
(1608085J01).
Furthermore, it should be noted here that no obvious change
in the structure, morphology, and chirality for the CNBE-DA
sample after polymerization could be observed when treated
with 365-nm or 254-nm light. The polymerization ensured
“locking” of the chain configuration, and inhibited the photo-
isomerization of the CNBE units in this case. Namely, the
molecular stacking arrangement and the resulting optical
properties (e.g., absorption, chirality) could firstly be easily
controlled through the Z/E isomerization by the external
light stimuli, and then be permanently locked through poly-
merization of adjacent DA moieties by light. Therefore, the
entire process of the control of molecular stacking arrange-
ment and the locking of resulting optical properties could be
solely manipulated by light.
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