Rich-colour mechanochromism of a cyanostilbene derivative with chiral self-assembly

Article abstract of DOI:10.1039/d1nj01528a

Mechanochromic luminophores exhibiting rich-colour transitions among multiple states have important potential applications, but few such compounds are known. Herein, chiral self-assembly fabrication is proposed as a valid method to realize a new type of metastable state, which may bring enriched PL colours. A novel chiral fluorophore,ACNS, was synthesized and assembled into helices with specific handedness, showing a tricolour photoluminescence (PL) transition among yellow, green and blue under grinding and other stimuli. The crystallineACNSpowder (B-phase) emits bright sky-blue light. Interestingly, the luminophore assembles into helical nanofibres (Y-phase) with specific handedness, which present yellow fluorescence at 539 nm. Moreover, upon grinding, both Y-phase and B-phase powder change into green-emitting powder (G-Phase) with a PL peak at 498 nm. This strategy is conceptually simple and affords a promising approach to the development of mechanochromic luminophores with multicolour fluorescence switching.

Full text of DOI:10.1039/d1nj01528a

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Rich-colour mechanochromism of a cyanostilbene
derivative with chiral self-assembly†
Cite this: New J. Chem., 2021,
45, 11530
Li Wen,‡a Jingwei Sun,‡b Chengjian Li,a Chenfei Zhu,a Xi Zhang,b Zhenbo Wang,b
b
Qingbao Song,*a Chunyan Lv *b and Yujian Zhang
*
Mechanochromic luminophores exhibiting rich-colour transitions among multiple states have important
potential applications, but few such compounds are known. Herein, chiral self-assembly fabrication is
proposed as a valid method to realize a new type of metastable state, which may bring enriched PL
colours. A novel chiral fluorophore, ACNS, was synthesized and assembled into helices with specific
handedness, showing a tricolour photoluminescence (PL) transition among yellow, green and blue under
grinding and other stimuli. The crystalline ACNS powder (B-phase) emits bright sky-blue light. Interestingly,
the luminophore assembles into helical nanofibres (Y-phase) with specific handedness, which present yellow
fluorescence at 539 nm. Moreover, upon grinding, both Y-phase and B-phase powder change into green-
emitting powder (G-Phase) with a PL peak at 498 nm. This strategy is conceptually simple and affords a
promising approach to the development of mechanochromic luminophores with multicolour
fluorescence switching.
Received 29th March 2021,
Accepted 7th June 2021
DOI: 10.1039/d1nj01528a
rsc.li/njc
However, most of these examples depend heavily on serendipitous
discovery or random screening. Therefore, developing effective
1. Introduction
Mechanochromic fluorescent (MCF) luminophores, which exhibit strategies for constructing metastable states, which bring enriched
reversibly tunable photoluminescence (PL) through alteration of PL colours, has remained a great challenge.
the molecular arrangement upon mechanical triggering, are pro-
DNA and proteins with helical motifs are typical metastable
spective materials for applications in mechanical sensing, optical matter in a living system. With this inspiration, introducing a
recording and display devices.1 In particular, rich-colour switching helical assembly into a conventional MCF luminophore may
from a single material enables diversified microstructures to be provide more metastable states. Advantageously, helical self-
reported and facilitates the fields of multicolour imaging and high- assembly plays a unique role in regulating the electronic, vibrational
density optical data storage.2 Currently, most MCF materials show and rotational energy of molecules, as well as the exciton coupling
two different types of fluorescence in a stable crystal state and a of p-conjugates through control over the molecular orientation.7 As
metastable amorphous/crystal state. Thus, to accomplish rich- a result, such chiral aggregates generally exhibit optical properties
colour switching, the most substantial requirement is the presence that are distinct from those of the achiral aggregates based on the
of at least a second metastable state.3,4 However, metastable states same fluorophore. Moreover, helical self-assembly is apt to undergo
are generally difficult to capture, as they deviate from the favour- helicity changes under external stimuli such as light, heat and
able stable state with the lowest energy and prefer to dynamically ions, leading to variations in intermolecular interactions and thus
rearrange to the stable state in a finite lifetime. A few rich-colour emission colour.8 Duan et al. found that both chirality and
MCF behaviours have been reported, including crystal polymorphs energy can be transferred from a chiral p-donor to an achiral
or liquid crystals with multiple phases,4 donor–acceptor (D–A) p-acceptor, resulting in a helical assembly with prominently red-
molecules undergoing conformation twisting,5 and compounds shifted fluorescence.9 Zhang et al. prepared viologen carboxylic
combining mechanisms of assembly and chemical changes.6 acid-based helices, which undergo pH-triggered helical transfor-
mation accompanied by a PL change.10 Accordingly, importing
a College of Chemical Engineering, Zhejiang University of Technology,
additional metastable states with helical assembled structures
might be a simple and ideal candidate strategy to realize rich-
colour MCF behaviour. Furthermore, the knowledge of the optical
changes and structural evolution of chiral aggregates under
grinding, which are fundamentally important due to the inspira-
tions of helical assembly in biological entities and materials
science, still substantially lags behind that of achiral aggregates.11
Hangzhou 310014, P. R. China. E-mail: qbsong@zjut.edu.cn
b Department of Materials Chemistry, Huzhou University, East 2nd Ring Rd.
No. 759, Huzhou, 313000, People’s Republic of China. E-mail: lcy@zjhu.edu.cn,
yjzhang@zjhu.edu.cn
† Electronic supplementary information (ESI) available. CCDC 1863991. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/d1nj01528a
‡ The authors contributed equally to this manuscript.
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Y-phase became sky-blue (B-phase) upon sonication or recrystal-
lization. As depicted in Fig. S2 (ESI†), the B-phase displayed an
especially narrow and well-defined PL curve with resolved peaks
at 456 and 466 nm, similar to that of ACNS molecules in solution
(Fig. S3, ESI†). The results indicated that the blue fluorescence of
the B-phase might originate from monomer emission. Upon
grinding, both Y-phase and B-phase powders were further con-
verted into a green-emitting powder (G-Phase) with a PL peak at
498 nm (Fig. S2, ESI†). As illustrated in Fig. 1b and Fig. S4a (ESI†),
the conversions between the G- and B-phases were reversible
through mechanical grinding and fuming with common organic
vapours such as ethanol, dichloromethane and acetone. Both the
G- and B-phases readily returned to yellow fluorescence upon
reprecipitation in protic and poor solvents such as water and
ethanol (Fig. S4b, ESI†). Obviously, ACNS exhibited reversible
tricolour switching among blue, green and yellow luminescence
under different stimuli. To demonstrate the MCF behaviour more
clearly, B-phase powders were coated on filter paper, and then,
the letter ‘‘M’’ was written on the ‘‘paper’’ with a spatula, giving
rise to green PL letters (Fig. 1b). As a comparison, the analogous
molecule CCNS was prepared, in which the CSB moiety was linked
to an alkoxy chain in place of chiral cholesteryl (Scheme S1, ESI†).
The as-prepared CCNS powders exhibited blue fluorescence,
which became a green colour upon mechanical grinding
(Fig. S5, ESI†). Nevertheless, under the same conditions, the
yellow-emitting state could not be observed for CCNS, similar
to other congeneric CSBs with such short conjugation and weak
electron push–pull strength.14 Thus, the yellow emission of ACNS
(Y-phase) could be speculated to be related to its chirality.
Interestingly, G-phase powders presented clear photochromism
under UV light (Fig. 1b). Moreover, photochromism of ACNS
occurred in DMF solution. As shown in Fig. 2a, the PL colour of
the ACNS solution changed from bright green to sky-blue upon
UV light illumination. As previously reported,13c,15 isomerization
of the CSB moiety could be responsible for the photochromism.
The photoisomerization process was clearly monitored by 1H NMR
spectroscopy because the protons in vinylene and phenyl rings
(, and . in Fig. 2c, respectively) were in fairly different config-
urations. Distinguishable changes in the B-phase under UV light
were difficult to visualize since the PL colour was similar to that of
the isomerized molecules. Notably, the Y-phase did not exhibit
photochromic properties either. This result suggested that there
might be extremely close stacking and strong interactions
between the chiral molecules in the Y-phase, which efficiently
prohibited Z - E isomerization.
Fig. 1 (a) Chemical structure of ACNS, (b) PL colour changes of ACNS
solids under different stimuli.
Therefore, exploring the differences between the chiral and achiral
alignments of fluorophores in regulating and altering emission
properties under mechanical forces is imperative.
In this study, we prepared the novel chiral fluorophore ACNS
(Fig. 1a) by integrating a cholesteryl group with a cyanostilbene
(CSB) derivative. The CSB building block was adopted owing to
its convenient synthesis, aggregation-induced emission and
mechanical-responsive behaviour.12 Cholesteryl was chosen
since it is a versatile chiral auxiliary to drive the p-moiety into
a helical stacking geometry.13 Furthermore, its bulky and rigid
planar skeleton is favourable for elaborately modulating the
p–p distance between fluorophores via rotation around a single
bond, resulting in different exciton coupling extents and
optical properties. As expected, helical assemblies of ACNS
with specific chirality were formulated, which showed a new
type of emission colour that has rarely been previously observed
from the same CSB skeleton, leading to tricolour fluorescence
switching among yellow (Y-phase), green (G-phase) and blue
(B-phase) under grinding and other stimuli.
2. Results and discussion
The molecule ACNS, featuring the CSB chromophore and chiral
To gain insight into how Y-phase aggregates emerged, we
cholesteryl linked by a carbamate bridge, was easily sequentially studied their reprecipitation process in THF/water mixed solvents
synthesized through aminolysis of chloride and Knoevenagel of various ratios via spectral analysis and SEM. The THF solution
condensation (Scheme S1, ESI†). The similar absorption peaks of ACNS exhibited a monomolecular PL peak at approximately
were observed at E355 nm in different polar solvents (Fig. S1a, 450 nm and a pronounced absorption band at 355 nm (Fig. 3a and b).
ESI†). In sharp contrast, the PL spectra of ACNS presented a When large fractions (450%) of poor solvent water were poured
bathochromic shift from 425 nm in n-hexane to 491 nm in DMF into the THF solution, the monomer emission at 450 nm was
(Fig. S1b, ESI†). Therefore, we speculated that ACNS exhibited gradually weakened, and a broad yellow PL band appeared at
a typical charge-transfer character. As shown in Fig. 1b, the 548 nm as a result of aggregation (Fig. 3b), which coincided with
as-prepared ACNS powder (Y-phase) emitted bright yellow light that of the as-prepared Y-phase. Meanwhile, new absorbance
that peaked at 539 nm. Interestingly, the PL colour of the peaks appeared at 326 and 297 nm, accompanied by a lifted
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Fig. 2 (a) Fluorescence images of ACNS dissolved in DMF solution before (left) and after (right) UV irradiation. (b) Fluorescent image of a G-phase solid
after UV irradiation at the central square patches. (c) 1H NMR spectra of ACNS in CDCl3 before and after UV irradiation. (d) Change in the ACNS molecular
structure upon photoisomerization.
Fig. 3 Steady-state absorption (a) and PL (b) spectra of ACNS in pure THF and THF/water mixtures with different volume fractions of water (c = 1 Â
10À4 mol LÀ1). (c) CD spectra of ACNS in solution and the Y-phase generated under different conditions.
level-off tail due to Mie scattering of nanoaggregates in suspensions
(Fig. 3a). Clearly, the reprecipitated Y-phase aggregates exhibited an
apparent blueshift of absorption and redshift of PL with respect to
the solution, which suggested that the CSB unit of ACNS adopted a
relatively ordered face-to-face H-aggregation.16 Interestingly, the
reprecipitated Y-phase aggregates were obtained in the form of
helical ribbons with left-handedness, which were further rolled
up into quite uniform tubular scrolls, as shown in Fig. 4a. In
Fig. 4 SEM images of the Y-phase (a) reprecipitated in water and (b) as-
prepared in ethanol; the inset of (a) shows a cross section of the tubular
scrolls.
comparison, the Y-phase as-prepared in ethanol assembled into
helical nanofibres with dominantly opposite handedness, which
further interlaced with each other and formed a network
structure (Fig. 4b). Circular dichroism (CD) spectra of the above
assemblies were obtained to clarify their chiroptical properties B-phase of ACNS did not generate a CD response at longer
(Fig. 3c). The dilute solution of ACNS in pure THF was CD silent wavelength region due to the achiral assembly. The Y-phase
at longer wavelength region (4200 nm), whereas both Y-phase showed clear reflections at angles distinct from those of the
assemblies obtained in water and ethanol exhibited strong B-phase, indicating their different crystalline structures (Fig. 5b).
Cotton effects at wavelengths of 230, 260, 297 and 330 nm, The G-phase from the ground Y-phase exhibited several weak
despite the opposite signals. The CD peaks at 297 and 330 nm, diffraction peaks in the XRD profile at the small-angle region
in accordance with the absorption bands (Fig. 3a), were attrib- (Fig. S6a, ESI†), which was consistent with that of Y-phase. The
uted to the chiral assembly. As shown in Fig. S6b (ESI†), the result demonstrated that a small quantity of achiral assembly
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Fig. 5 (a) PL decay time profiles and (b) XRD patterns of B-, G- and Y-phase solids; the inset of (b) shows the XRD pattern of the Y-phase in the
low-angle region. (c) Schematic representation of the chiral assembly behaviour of the Y-phase.
aggregate still existed in the G-phase. Thus, the CD peaks were weaker PLQY of 43.4%. The double-exponential decay process had
also observed in the G-phase (Fig. S6b, ESI†). For the as-prepared lifetimes of 2.9 ns (53.6%) and 13.4 ns (46.4%). The results, along
assembly in ethanol, the positive signals indicated the P-helicity with the broad PL spectra (Fig. S2, ESI†) indicated that the green
(right-handed), and for the reprecipitated aggregates in water, the fluorescence resulted from the synergistic effect of monomers and
negative signals implied their M-helicity (left-handed), which intermolecular p–p interactions. Distinctly, the Y-phase exhibited a
fully coincided with the SEM results. In addition, the positive tav of 37.1 ns with the predominant component (accounting for
Cotton effect at wavelengths longer than 350 nm, where the 84.1%) as long as 42.9 ns. Such an extremely long lifetime, in
optical absorption was negligible, was most likely due to the combination with the clear redshifted and unstructured PL,
scattering of aggregates.17 In addition, the CD intensity gradually supported that the fluorescence of the Y-phase originated
weakened as the water fraction increased from 70% to 90%, mainly from the ‘‘sandwich’’ type excimers as a result of
suggesting that a less ordered chiral assembly formed in the mixed optimized H-aggregation in the helical assembly.18 Moreover,
solvent with a larger content of water due to lower solubility the very slow radiative rate constant (kr) of 0.015 nsÀ1, which
and more rapid precipitation. These results revealed that the corresponded to a strictly forbidden transition from the excimer
H-aggregation CSB moieties in the Y-phase readily assembled to the ground state, implied a centrosymmetry in this kind of
into helical structures owing to the chiral cholesteryl attach- excimer. That is, spatially isolated and antiparallel pairwise
ment. Moreover, assemblies with favoured single handedness packing dimers were formed in the Y-phase,19 which was further
could be generated depending on the kind of solvent.
confirmed by the XRD results (vide infra). The isolated excimers
High-quality single crystals of ACNS, which showed bright also lead to moderately high PLQY (55.0%) owing to the effective
blue PL, were obtained by slow diffusion of ethanol vapor into a prevention of generation of larger-size and lower-energy excimer
DMF solution of ACNS. The calculated PXRD pattern from the clusters. Additionally, the even lower nonradiative rate constant
single crystal was almost consistent with that of the B-phase. As (knr) respect to kr in Y-phase also contributed to its high PLQY.
is shown in Fig. S7 (ESI†), the single-crystal structure of the These results indicated that there were substantially strong
B-phase containing two molecules inÀa unit cell was found to intermolecular interactions in pairwise dimers, which effectively
be triclinic with the space group P1 The twisted molecular restricted intramolecular motions and thus deactivated the non-
conformation, along with the loose triclinic structure, indicated radiative channels. The shorter lifetime component (6.8 ns, 16.0%)
the absence of effective p–p interactions in the B-phase, which in the Y-phase could be ascribed to small ill-defined structures.
rationalized its monomer emission feature. The B-phase exhibited
The inset of Fig. 5b displays the XRD pattern of the Y-phase
monomer emission, presented a relatively short PL lifetime (tav) of in the low-angle region. Numerous diffractions were found at
2.4 ns (Table S2, ESI†) and a high PL quantum yield (PLQY) of d-spacing of 45.4, 22.9, 15.3, 13.9 and 9.7 Å, corresponding
68.1%. In comparison, the G-phase had a longer tav of 7.7 ns and a to a spacing ratio of 1 : 1/2 : 1/3 : 1/4 : 1/5, which indicated a
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long-range ordered lamellar structure. The much larger layer PL that has never been observed before from the same chromo-
spacing of 45.4 Å with respect to the molecular length of 33.6 Å phore skeleton. The fabrication of a helical assembly was found
revealed that an antiparallel bilayer orientation with a certain to be a candidate strategy for attaining an additional metastable
degree of molecular tilt and interdigitation was formed (Fig. S8, state showing intense and largely redshifted emission from
ESI†). In the wide-angle region, the two diffractions at d-spacings pairwise excimers. As a result, tricolour fluorescence switching
of 3.23 and 3.38 Å maybe corresponded to two types of p–p among yellow, green and blue was displayed under grinding and
interplanar distances, verifying the isolated and pairwise dimeric other stimuli. To our knowledge, this is the first time that chiral
packing structure in the Y-phase. The shorter value was ascribed to self-assembly has been employed to realize rich-colour MC by
the distance between molecules in a dimer, and the longer value affording a new type of metastable state.
was attributed to the distance between adjacent dimers. FTIR was
implemented to provide information about the interactions in the
4. Experimental section
Synthesis of the intermediate compound BCN
three phases at the molecular level. As shown in Fig. S9 (ESI†), the
IR absorption peaks at 1731 cmÀ1 and 1207 cmÀ1 were attributed
Cholesteryl chloroformate (2.69 g, 6.0 mmol) was dissolved in dry
THF (20 mL), and the solution was added dropwise to the dry
THF (30 mL) solution consisting of 2-(4-aminophenyl)acetonitrile
(0.66 g, 5.0 mmol) and triethylamine (2.10 mL, 15 mmol) at 0 1C.
After stirring at room temperature for 12 h, the solution was
poured into water and the product was extracted with CHCl3/
C2H5OH at volume ratio of 1 : 3, and dried in a vacuum. 1H NMR
(500 MHz, CDCl3): d (ppm) 7.41 (d, J = 8.35 Hz, 2H); 7.26 (d, J =
8.65 Hz, 2H); 6.63 (s, 1H); 5.42 (t, J = 2.55 Hz, 1H); 4.64–4.60 (m, 1H);
3.72 (s, 2H); 2.43–2.36 (m, 2H); 2.02–1.89 (m, 5H); 1.60–1.53 (m, 7H);
1.36–1.35 (m, 4H); 1.18–1.10 (m, 7H); 1.04 (s, 3H); 1.03–0.99 (m, 3H);
0.93 (d, J = 6.50 Hz, 3H); 0.88 (d, J = 4.45 Hz, 6H); 0.70 (s, 3H).
13C NMR (125 MHz, CDCl3): d (ppm) 152.90; 139.50; 138.02; 128.62;
124.41; 122.85; 119.04; 117.88; 75.18; 56.69; 56.16; 50.03; 42.33; 39.74;
39.52; 38.43; 36.96; 36.58; 36.19; 35.79; 31.91; 31.88; 28.22; 28.08;
28.00; 24.28; 23.84; 23.00; 22.80; 22.55; 21.05; 19.32; 18.72; 11.86.
MS (ESI) m/z: [M + NH4]+ calcd for C36H52N2O2, 544.4; found, 562.4.
to the stretching vibrations of aromatic CQO and C–O bonds,
respectively. Interestingly, the sharp vibrations of amide N–H at
3336 cmÀ1 and an ester group (–COO–) in B-phase were gradually
broadened and weakened when the solid changed to G-phase and
then Y-phase. The results indicated increased amounts of hydro-
gen bonded N–H and –COO– at the carbamate bridge, along with a
growing excimer component.
Based on all the above, a reasonable assembly model of the
Y-phase was proposed, as shown in Fig. 5c. When precipitated
from solution by adding ethanol or water, the chiral molecule ACNS
packed in antiparallel pairwise dimers that extended into a bilayer
under the driving force of the hydrogen bonds and p–p interactions.
To expose less hydrophobic regions to aqueous surroundings,
several bilayers then assembled to form nano/microribbons that
further hierarchically curled into well-ordered twisted morphologies
or helices as a result of the chiral nature of cholesteryl.20 Due to the
chiral torsion of the nano/microribbons, the H-aggregates were
further compactly packed, leading to specifically strong interactions
and short p–p distances (3.23 and 3.38 Å) between adjacent
Synthesis of ACNS
molecules. As a result, these aggregates successfully endowed the 4-Formylbenzonitrile (0.27 g, 2.05 mmol), and 1 (1.09 g,
chiral assemblies with bright yellow PL from pairwise excimers.21 2.00 mmol) were dissolved in anhydrous ethanol (50 mL) in
Considering all the experimental data, the origin of the tricolor-MCF
the presence of sodium methoxide (0.14 g, 2.50 mmol). The
behavior was proposed as follows: It is generally believed that a mixture was stirred at room temperature for 6 hours. The
mechanical stimulus converts a metastable state to a more thermo- resulting precipitate was filtered and washed with ethanol for
dynamically stable solid state with new PL properties. Thus, several times to afford 1.10 g of ACNS with a yield of 83.6%.
multicolor-fluorescence switching requires two or more kinds of 1H NMR (500 MHz, CDCl3): d (ppm) 7.96 (d, J = 8.50 Hz, 2H); 7.76
metastable self-assembled phases. In our case, The ACNS formed a (d, J = 8.45 Hz, 2H); 7.66 (d, J = 8.80 Hz, 2H); 7.52 (d, J = 8.70 Hz,
metastable, unimolecule emissive aggregates (B-phase). Upon repre- 2H); 7.48 (s, 1H); 6.74 (s, 1H); 5.43 (t, J = 2.60 Hz, 1H); 4.67–4.62
cipitation, the H-aggregation CSB moieties assembled into the (m, 1H); 2.47–2.38 (m, 2H); 2.02–1.89 (m, 5H); 1.60–1.51 (m, 8H);
helical structures (B-phase) because of the chiral cholesteryl attach-
1.36–1.35 (m, 3H); 1.19–1.10 (m, 7H); 1.05 (s, 3H); 1.02–0.99 (m, 3H);
ment. Interestingly, the helical self-assembly is also metastable 0.93 (d, J = 6.65 Hz, 3H); 0.88 (d, J = 4.30 Hz, 6H); 0.70 (s, 3H).
state. The two metastable states were converted into a more stable 13C NMR (125 MHz, CDCl3): d 152.70; 144.88; 139.96; 139.41; 138.01;
packing state (B-phase) under mechanical stimuli. As a conse- 137.68; 132.61; 129.46; 128.17; 127.10; 122.98; 118.65; 117.16; 114.74;
quence, the new chiral fluorophore ACNS presented tricolored 113.26; 75.46; 56.68; 56.14; 50.01; 42.32; 39.72; 39.51; 38.40; 36.94;
fluorescence switching. Clearly, the importing of a helical assembly 36.58; 36.18; 35.79; 31.91; 31.87; 28.22; 28.06; 28.01; 24.28; 23.83;
based on chiral fluorophores is an effective strategy for attaining an 22.81; 22.56; 21.05; 19.32; 18.71; 11.86. ESI-HRMS m/z: [M À H]+
additional metastable state exhibiting an intense and largely red- calcd for C44H54N3O2, 656.4216; found, 656.4207. The melt point of
shifted excimeric PL and resultant rich-colour MCF behaviour.
the Y, B and G phase is 268.5 1C, 248.2 1C, and 265.9 1C respectively.
3. Conclusion
Author contributions
In summary, the novel chiral fluorophore ACNS was synthe- C. Lv and Y. Zhang: designing of research, and writing-editing.
sized and assembled into helices, which showed a bright yellow Q. Song: designing of research, and data collection-analysis.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This work was supported by the Natural Science Foundation of
Zhejiang Province (LZ20E030001 and LY17F050001) and the
National Natural Science Foundation of China (52073089 and
21875219).
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