Interface Effect on the Hierarchical Assemblies of

Subscriber access provided by TULANE UNIVERSITY

Interface-Rich Materials and Assemblies

Solvent and Surface/Interface Effect on the Hierarchical Assemblies of Chiral Aggregation Induced Emiting Molecules Bing Shi Li, Xuejiao Huang, Hongkun Li, Wenjuan Xia, Shan Xue, Qing Xia, and Ben Zhong Tang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03358 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 17, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Solvent and Surface/Interface Effect on the Hierarchical Assemblies of Chiral Aggregation Induced Emitting Molecules Bing Shi Li,*a Xuejiao Huang, a Hongkun Li,*a,b,c Wenjuan Xia,a Shan Xue,a Qing Xia, a Ben Zhong Tang* b a.College

of Chemistry and Environmental Engineering, Shenzhen University, 1066 Xueyuan Avenue, Nanshan, Shenzhen 518055, China b The Hong Kong University of Science & Technology (HKUST)‐Shenzhen Research Institute, No. 9 Yuexing 1st RD, South Area, Hi‐tech Park, Nanshan, Shenzhen 518057, China. cLaboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 50 Donghuan Road, Suzhou 215123, China

_____________________________________________________________ Correspondence authors:

Prof. Bing Shi Li [email protected] Prof. Ben Zhong Tang [email protected] Prof. Hongkun Li [email protected]

ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: The core of aggregation induced emitting (AIE) molecules was their

aggregation behavior. It was in essence a self-assembly process driven by non-covalent interactions, which were governed not only by the chemical structures of the molecules, but also by the conditions where the self-assemblies were formed. The self-assemblies of two AIE molecules, tetraphenylethene (TPE) derivatives carrying one valine attachment (TPE-Val) and two valine attachments (TPE-2Val) were studied, respectively. Both kinds of molecules self-assembled in to supramolecular helical fibers with different handedness upon the addition of poor solvent to their solution. However, when deposited on air/water interface both kinds of molecules formed aligned elementary helical fibers instead of supramolecular fibers. The lateral solvophobic effect exerted by water molecules caused a shift of the original noncovalent balance between molecules and solvent, thus the supramolecular helical assemblies were unraveled into aligned helical elementary fibers. Similar elementary assemblies were formed on the surface of 3-aminopropyl triethoxysilane (APTES) modified mica, confirming that lateral solvophobic effect on the self-assemblies of the molecules.


Page 2 of 24

Page 3 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

INTRODUCTION AIE molecules are a kind of novel fluorescent molecules that are almost nonemissive in good solvent, but emit intensive fluorescence upon the addition of poor solvent.1-2 The addition of poor solvent leads to the aggregation of the AIE molecules and their dramatic fluorescence enhancement. Upon aggregation molecular rotation and vibration are inhibited, thus the non-radiative energy loss is blocked and fluorescence intensity increases. The aggregation of AIE molecules in essence is a self-assembly process driven by solvophobic effect. If AIE molecules are deliberately decorated with self-assembling directing components to guide their aggregation, they are ideal candidates to fabricate desirable fluorescent architectures.3,4,5 It is a simple and efficient way to fabricate regular architectures with novel optical properties. In nature, proteins self-assemble into helical structures via non-covalent interactions which are determined by their amino acids building blocks. Chirality and amphiphilicity of the amino acids are two important inducers for the induction of self-assembling of protein molecules. Hybridizing AIE molecules with chiral amino acid may also lead AIE scaffold to adopt a helical conformation and amplify it as helical assemblies in the higher order architectures. AIE molecules are then endowed with the ability to self-assemble into regular functional nano/micro architectures. Using this strategy, we have modified different AIE scaffolds with different amino acid attachments and synthesized a series of chiral AIE molecules. 4, 6 The self-assembly of molecules is critical for their fluorescence properties, which are not only determined by the building blocks of the molecules, but also influenced by the environment of the molecules, such as solvent, surface/interface, additives, pH and light. Variation of these factors might lead to the formation of diverse self-assembled structures and accordingly different optical properties.7-14 Solvent is also a crucial factor for the induction of the AIE properties of molecules, namely the formation of the selfassemblies and enhanced optical properties. The occurrence of the poor solvent in the solution generates a strong solvophobic repulsion of the AIE molecule and leads to the self-assembly of molecules.9 Besides solvent, the surface/interface also has considerable influence for the formation of assemblies.7, 10,15-19 The surface/interface


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 24

where the assemblies are deposited not only provides a physical support of them, but also has interactions with the supported molecules and breaks its original balance of the noncovalent interactions of the molecules.11,16 Different surfaces usually lead to divese changes to the arrangement of the molecules, thus leading to the formation of different assemblies.11,

16

Thus, revealing the influence of solvent and surface/interfaces is

critical for characterizing the assemblies of AIE molecules. In our previous paper, we have reported the synthesis of AIE molecules bearing TPE core and valine peripheries (TPE-Val and TPE-2Val) and the occurrence of chiral center of the amino acid attachments exerted an asymmetric force to the TPE core and led the molecules to assume a helical conformation and helical assemblies.3-5 Hereby, with atomic force microscope (AFM), scanning electron microscope (SEM), transmission electron microscope (TEM) and Langmuir Blodget (LB) technique, we focus on the influence of solvents and surface/interfaces on the self-assembly of the molecules. We systematically characterized the assemblies of molecules formed by TPE-Val and TPE-2Val upon the evaporation from different ratios of mixed solvents and on air/water interface. The assemblies of TPE-Val and TPE-2Val were determined not only by the chemical structures of the molecules, but also by the solvent and the surface/interface that the molecules were deposited on. An alteration of the ratios of the mixed solvent of the molecules can lead to an abrupt change of the morphology of the molecules. Meanwhile, the change of the polarity of the surface also had considerable influence on the morphologies of the molecules.

EXPERIMENTAL SECTION Materials.

TPE-Val and TPE-2Val were synthesized according to the procedure

described in our previous publication. 3,4 Circular dichoism (CD) and photoluminescence (PL) measurement

A stock

solution of TPE derivative in DCE with a concentration of 0.1 mM was first prepared. Aliquots (1 mL) of the stock solution were then transferred to 10 mL volumetric flasks. After adding appropriate amounts of DCE, hexane was added dropwise under vigorous stirring to afford solutions with different hexane fractions (0-95 vol%) and with a


Page 5 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

concentration of 100 M for CD measurement, and 10 M for PL measurement. CD spectra were obtained using a JASCO J-810 spectropolarimeter in a 1 mm quartz cuvette using a step resolution of 0.1 nm, a scan speed of 100 nm min-1, a sensitivity of 0.1 nm, and a response time of 0.5 s. PL measurements of the resulting solutions were then conducted immediately on an Edinburgh Instrument FLS 980 fluorospectrometer. AFM Measurements.

The AFM sample of the polymer was prepared by allowing a

droplet (4 μL) of a diluted solution of the desired concentration to evaporate on the surface of mica under ambient conditions. AFM measurements were performed on a Multimode VIII atomic force microscope (Bruke, USA) operating in tapping mode using hard silicon cantilever tips with a spring constant of 40 N/m. Infrared (IR) spectra

IR spectra were taken on a Bruker VERTEX 70v FT-IR

spectrophotometer by allowing solution with desired concentration to evaporate on KBr disks. LB Film. The π-A isotherm of molecules on the subphase of H2O was measured at 23±1 °C, using a Sigma 70Cam 200 LB 5000 trough, equipped with a Wilhelmy platinum plate. Ultrapure water was used as the subphase. After ensuring that the water surface was clean, 50 μL of a THF solution of TPE-Val (2.5μM) was spread on the water surface using a microsyringe. The π-A isotherms were measured after allowing the solvent to evaporate for 15 min by closing the barriers at a constant rate of 10 mm/min. LB films of molecules were transferred to a newly cleaved mica at 5 mm/min, at desired surface pressure. SEM and TEM Imaging.

A stock DCE solution of TPE-Val (1 ×10-3 M) was first

prepared. The stock solution was then transferred to a 5 mL glass vial and diluted to the suspension with a final concentration of 1 ×10-4 M by adding hexane dropwise with vigorous stirring. 4mL of this suspension was immediately dropped onto the surface of silicon wafer and carbon-coated copper grid. After evaporation of the solvent under ambient conditions, the samples were characterized by SEM (S-3400N(II), Hitachi, Japan) and TEM (Tecnai G2Spirit120KV, FEI, Netherland), respectively. Fluorescence imaging.

A drop (40 μL) of dilute solution of TPE–Val (1 ×10-4 M)

was put on the surface of the glass slide and the droplet was sealed with a glass cover


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 24

slip. After incubation for about 30 min, images were taken with a fluorescence microscope (DHG-9070A, Olympus, Japan). Surface modification and contact angel measurement.

Modification of mica was

achieved by placing the newly cleaved mica in the vapor of APTES for a proper time.17 Contact angel of the modified mica was measured by dropping a droplet of ~1μL ultrapure water on the surface and then measured with a contact angel goniometer (JGW-360, China) using an optical subsystem to capture the profile of a pure liquid droplet on a desired substrate.

Scheme 1 Chemical structure of TPE-Val and TPE-2Val.

RESULTS AND DISCUSSION CD and PL spectra of TPE-Val and TPE-2Val. We investigated the optical activity using CD spectroscopy. As can be seen from Figure 1A, in DCE solution and the DCE/hexane mixture with low contents of hexane, TPE-Val exhibited almost no CD signal at the wavelength longer than 300 nm; obvious CD absorption was observed at the wavelength of 330 nm in DCE/hexane (1/9, v/v) mixture, corresponding to the absorption of the TPE moieties. Because TPE-Val is insoluble in hexane, the higher hexane content in the DCE/hexane mixture thus leads to the aggregation of the molecules. Therefore, the CD absorption at the wavelength of 330 nm is an indication of aggregation-induced circular dichroism (AICD),


Page 7 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

suggesting a successful chirality transference from the valine-containing pendants to the TPE moieties in the aggregate state. TPE-Val at film state exhibited significantly enhanced CD absorption compared with that in suspension. Powder of TPE-Val was also measured (The figure is not shown) for comparison, but it is CD silent at the wavelength longer than 300 nm. Thus the CD enhancement of the molecules at film state was also ascribed to the AICD effect due to the self-assembly of TPE-Val. The spectra of TPE-2Val is similar with that of TPE-Val that the molecule barely shows signals in DCE solution and the DCE/hexane mixture with hexane content lower than 60%. In the DCE/hexane mixture with the hexane content higher than 80%, it exhibits an CD absorption at the wavelength of 350 nm, which is ~20 nm red-shifted due to the incorporation of one more valine attachment. While in the suspension with 90% hexane content in it, the CD absorption turns weak again due to the cancellation of comparable different handedness of the molecules. At film state, the CD absorption of TPE-2Val shows a complete reversion and great enhancement at the wavelength of 315 nm, whereas the powder of TPE-2Val was CD silent at the wavelength longer than 300 nm (The figure is not provided). It suggests that the CD absorption is due to the self-assembly of molecules and one handedness of chirality becomes dominant in the self-assembly process at film state. Though TPE-2Val demonstrates AICD effect both in the suspension and at film state, their CD absorption is different in term of the different absorption wavelength. Such difference is closely related with the formation of different aggregates of molecules in different self-assembling processes and it exerts a critical effect to the chirality transference from the valine attachments to the TPE core.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The PL spectra of TPE-Val and TPE-2Val in DCE, DCE/hexane mixtures and at film states were measured to check if the TPE derivatives still kept AIE property. As shown in Figure 1C, the PL curve of TPE-Val in the DCE and DCE/hexane mixtures with hexane fractions lower than 95% are almost flat lines parallel to the abscissa. The PL intensity of TPE-Val increases significantly when the hexane fraction is 95%, which is 700 times higher than that in DCE solution. TPE-Val is insoluble in hexane, the higher hexane content thus leads to the aggregation of the molecules in the DCE/hexane mixture. At film state, TPE-Val showed similar fluorescence emission with that in the suspension with 95% content of hexane. Thus, TPE-Val still preserves the AIE feature. The PL spectra of TPE-2Val also showed a typical AIE property (Figure 1D) that the compound barely showed emission in DCE and DCE/hexane mixtures with hexane content lower than 80%, whereas the PL intensity increases dramatically at the hexane content higher than 80% and reaches the highest value at the hexane content of 90%, which is 28 times higher than that in its DCE solution. Hexane is the poor solvent for TPE-2Val, the addition of hexane into its DCE solution at higher hexane content would induce the aggregation of the molecules. Similar emission is observed in the film of the molecules. But compared with that of TPE-Val, the PL increase of TPE-2Val was not so significant as that of the former due to the incorporation of additional valine attachment in the molecular structure.


Page 8 of 24

Page 9 of 24

20

10

A

B

Film

15

6

10 5

2

0

0 -5

Suspensions (fH)

-2

Suspensions (fH)

0 20% 40% 60% 80% 90%

-4 -6 -8 -10 220

280

340

400

Wavelength (nm)

460

520

220

280

340

-10

0 20% 40% 60% 80% 90%

Film

400

Wavelength (nm)

-15 -20 -25 520

460

100

100

C

Film

80

D

Film

fH (vol %)

fH (vol %)

90 80 70 60 50 40 30 20 10 0

95 90 80 70 60 50 40 30 20 10 0

60

40

20

0 360

CD (mdeg)

CD (mdeg)

4

430

500

Wavelength (nm)

570

80

60

40

PL intensity (a. u.)

8

PL intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

20

640

360

430

500

Wavelength (nm)

570

0 640

Figure 1. CD spectra of TPE-Val (A) and TPE-2Val (B) in DCE-hexane mixtures with different fractions of hexane (fH) and film prepared by natural evaporation of its DCE solution. Concentration: 100 M. PL spectra of TPE-Val (C) and TPE-2Val (D) in DCE-hexane mixtures with different hexane fractions (concentration: 10 M) and film prepared by natural evaporation of its DCE solution (concentration: 100 M). λex: 315 nm.

Superhelical assemblies of TPE-Val. As both TPE derivatives showed AICD absorptions upon the addition of hexane to their DCE solution, we then studied their corresponding aggregates at different contents of hexane. AFM images showed that TPE-Val self-assembled into helically twisted ribbons, upon the evaporation of its DCE solution, as shown in Figure 2. The helical ribbons twisted around a same stereogenic center with a typical left-handedness to give the appearance of a dendrimer. The thickness of the ribbon was measured to be 1.7-2.3 nm. Together with the helically


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

twisted ribbons were patches of films, as shown in Figure 2B. The film was jointed with the twisted ribbons, with the thickness ~1.7 nm. The coexistence of the film and helical ribbons showed an importance clue that the precursors of the helically twisted ribbon were the flat film or ribbons and they gave the morphology of helical ribbons with a further helical twist. When the poor solvent of hexane was added to the solution, it had considerable influence on the self-assembled structure of TPE-Val. At the content of 60% hexane in the mixture, TPE-Val still formed helical ribbons. However, some of the ribbons kept the left-handedness, as shown from the TEM image in Figure 2C and some has converted into right-handed helical ribbons, as shown in Figure 2D; when the content of hexane was 90%, most of the helically twisted ribbons exhibited the righthandedness, as shown in Figure 2E and 2F. Thus, the addition of poor solvent led to a converse of the handedness of the helical assemblies. Both AFM and TEM showed the high resolution images of the helical assemblies of TPE-Val at nanoscale when the solution was evaporated from diluted solution. We then use fluorescence microscope (FM) to study the fluorescent assembly of the molecule which was formed with both the concentration and the volume 10 times higher than that used for AFM imaging. Because of the limitation of the resolution of FM, the self-assembled structures at nano scale were too small to be observed under FM. As shown from Figure 2G, blue fluorescent broom-like assemblies were observed. Each broom exhibited a stereogenic center because of the helically twisting of the ribbons. Helical pitches of the twisted ribbons were most clearly shown in the area labeled with arrows. The broom-like assemblies were consistent with that in AFM and TEM images, but they were several microns long and much bigger than that revealed by AFM and TEM. It also suggested that the self-assemblies of TPE-Val were hierarchical and increase of concentration of the molecule can lead to the formation of higher order of assemblies at macro scale.


Page 10 of 24

Page 11 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 2. AFM image of helical fibers formed by TPE-Val upon the evaporation of DCE. (A and B) Concentration: 10 μM. TEM image of helical fibers formed by TPE-Val upon the evaporation of DCE/hexane (4:6, C and D) and (1:9, E and F). Concentration: 10 μΜ. Fluorescence image of the self-assembled helically twisted brooms formed by TPE-Val on the evaporation of its DCE/hexane mixture (1:9, G). Helical pitches are most clearly shown in the area labeled with arrows. Concentration: 100 μM.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Superhelical assemblies of TPE-2Val.

TPE-2Val with two valine attachments, is

also anticipated to form helical assemblies. We thus investigated the self-assembly of TPE-2Val using SEM, TEM and AFM techniques. As shown in Figure 3A, TPE-2Val readily assembled into helical nanofibers with a predominantly right-handedness upon the evaporation of DCE. The helical nanofibers interlaced each other. Their average width and helical pitch were ~30-70 and ~100-200 nm, respectively. Their length can be up to several microns. TEM showed combined structures with both the morphology of helical ribbons and fibers as shown in Figure 3B. It showed an important clue that the helical nanofibers likely evolved from the wrapping up of the nanoribbons. We then check the influence of the poor solvent of hexane (40%) on the morphology of the selfassembled structure of TPE-2Val with TEM, SEM and AFM, as shown in Figure 3C, 3E and 3G. The TEM images in Figure 3C revealed that TPE-2Val formed helically twisted fibers and still kept right-handedness. The SEM image in Figure 3E revealed that the formation of helical fibers was hierarchical and thin helical fibers further twisted together to form thicker ones. It also showed a consistent result with TEM image that the helical fibers all exhibited right-handedness. The AFM image in Figure 3G showed a consistent handedness with that in SEM and TEM. At the hexane content of 80%, the TEM image in Figure 3D showed that TPE-2Val formed helical fibers which twisted together to exhibit the morphology of dendrimers while still keeping the right handedness. The SEM image in Figure 3F clearly showed a more clear picture of the helical contour of the fibers which were entangled to give thick helical fibers with the increase of hexane content in the solution. The AFM image in Figure 3H showed that the helical fibers entangled with each other more tightly than that at lower content of hexane, but the helical fibers still kept the right-handedness, which agrees well with that in SEM and TEM. We also carried out FM imaging to study the fluorescent assembly of TPE-2Val, as shown in Figure 3I. The molecules self-assembled into blue fluorescent helical assemblies, which were constructed by coiled fibers. There are also films connecting to the fibers, indicating that the fibers were formed by the wrapping up of the films. Due to the limited resolution, helical pitches look more like knots along the fibers.


Page 12 of 24

Page 13 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

A

1 m

B

1 m

Figure 3. SEM (A) and TEM image (B) of TPE-2Val formed by natural evaporation of its DCE solution, concentration: 100 μM. TEM images of TPE-2Val formed by natural evaporation of its DCE/hexane mixture (6:4, C) and (1:9, D) respectively. Concentration: 10 μM. SEM images of


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TPE-2Val formed by natural evaporation of its DCE/hexane mixture (6:4, E) and (2:8, F). Concentration: 10 μM. AFM images of TPE-2Val formed by natural evaporation of its DCE/hexane mixture ((6:4, G) and (2:8, H). Concentration: 10 μM. Fluorescence image (I) of the helically twisted fibers formed by TPE-2Val upon the evaporation of its DCE/hexane mixture (1:9). Concentration: 100 μM.

It can be seen that both TPE-Val and TPE-2Val self-assembled into helical fibers or ribbons, which were stabilized by the noncovalent interactions between molecules. Both TPE derivatives contain amino acid attachments and hydrogen bonds are easily formed between the amino acid attachments. The patterns of their hydrogen bonds might be different due to the different numbers of amino acid attachments they carried, and thus may lead to the formation of the helical assemblies with different characteristics upon the evaporation of DCE. Besides chemical structure, the formation of helical assemblies were also influenced by the addition of poor solvent in the solution. The

presence of poor solvent will enhance solvophobic effect of the molecules and

cooperate with hydrogen bonds between amino acid attachments to stabilize the helical assemblies. To confirm the formation of hydrogen bonds in DCE and the DCE/hexane mixture, we thus carried out a series of IR measurements. As shown in Figure S1A in SI, TPEVal has strong absorption at the wavenumber of 3340 cm-1 and 1620 cm-1, corresponding to the absorption of associated NH-groups and associated C=O groups, respectively. It suggests that NH- and C=O groups are both involved in the formation of hydrogen bonds. Upon the addition of hexane, the absorption of NH-groups and C=O showed a slight red-shift in their wavenumbers, but is still corresponding to the associated state. The red-shift is likely related with the enhanced solvophobic effect upon the addition of hexane to the solution, which makes some originally formed hydrogen bonds unavailable, thus causing a slight red shift of the NH- and C=O in the absorption. Similar trend was also observed for TPE-2val, as given in Figure S2 in SI. Thus, hydrogen bonding plays an important role in stabilizing the helical assemblies. Besides it, the addition of nonpolar solvent hexane to the solution would generate strong


Page 14 of 24

Page 15 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

solvophobic repulsion between the polar amino acid attachments and nonpolar hexane. The higher the hexane content was, the stronger the solvophobic repulsion would be. The solvophobic repulsion would drive the amino acid attachments to adjust their arrangements to minimize their contacts with the nonpolar solvent. The adjustment was followed by breaking the previously existent balance and shifting to a new balance of the noncovalent interactions. The two kinds of molecules had diverse responses to the addition of poor solvent hexane to the solution: TPE-Val, showed a converse from lefthandedness to right-handedness; while TPE-2Val did not and the addition of hexane only caused enhanced entanglement of the helical fibers. Their different responses to the poor solvent were related with their different chemical structures. For TPE-Val, solvophobic repulsion caused a significant shift of the original noncovalent balance of the molecules, which was stabilized mainly by hydrogen bonds. Accordingly, the helical fibers showed a converse of the handedness of the helical rotation. With respect to TPE-2Val, each molecule had two amino acid attachments and there were more hydrogen bonds between the molecules. The previously existent noncovalent balance of the molecules was balanced by more hydrogen bonds and the solvophobic repulsion only caused a mild shift of it and led to the more entanglement of the helical fibers, instead of changing the handedness of the helical fibers; It thus suggested that both the chemical structures and the environment of the molecules can influence the assembling behaviours of molecules. LB film of TPE-Val and TPE-2Val. To further explore the influence of environment on the self-assembly of TPE-Val and TPE-2Val, we also prepared monolayers of the two kinds of molecules on air/water interface by LB technique.18,19-20 The surface pressure-molecular area (π-A) isotherm of TPE-Val and TPE-2Val were given in Figure 4A. The isotherm of TPE-Val started from a gradually increased compression stage of ab’ and showed a turning point b’ at the surface pressure of 5 mN/m; when the surface pressure higher than 5 mN/m there was an abrupt compression stage. The limiting area of TPE-Val was 0.92 nm2. For TPE-2Val, the isotherm exhibited three regions and it started from the region ab, where the surface pressure gradually increased with compression; while in the middle region bc of the isotherm there was a short platform,


Langmuir

where the surface pressure kept almost unchanged; In the third region cd, the surface pressure increased again upon compression. The limiting area of TPE-2Val was 1.15 nm2. The isotherms of the two kinds of molecules reflected their different compression behaviors of the molecules on air/water interface because of their different molecular shapes and rigidness, as illustrated in the scheme in Figure 4B. TPE-Val had a polar amino acid end and a hydrophobic TPE scaffold and it can be considered a conventional linear amphiphilic molecule, which showed a characteristic isotherm of linear amphiphilic molecules upon compression on air/water interface. While TPE-2Val had two polar amino acid ends which were connected to the TPE core like two arms extending in a V shape. When TPE-2Val solution was deposited on air/water interface, the two hydrophilic amino acid ends were both expected to lie on air/water interface; with the increase of the surface pressure, one of the ends of TPE-2Val would be curved up from water surface and became tilted. With further compression the tilting angle of the molecule changed, but it did not lead to the obvious increase of the surface pressure, corresponding to the short platform of the isotherm; upon further compression, TPE2Val molecules stood with a single amino acid attachment contacting water surface and became more compactly packed with a significant increase of surface pressure.21,16 Thus, the limiting area of TPE-2Val then became close to that of TPE-Val. B

A

60

Surface pressure(mN/m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 24

c'

——TPE-2val

50

TPE-val

40

d 30 20

c b

10

b'

a

0 25

50

75

100

125

150

175 o

200

225

250

275

2

Molecular Area(A /molecule)

Figure 4. π-A isotherm of TPE-Val and TPE-2Val on the subphase of water; concentration of its THF solution of TPE-Val (A). concentration 2.5 μM, volume:50 μL，speed of barrier:10 mm/min. The arrangement TPE-Val and TPE-2Val on air/water interface at increasing surface pressure ( B).


Page 17 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

We then check the corresponding assemblies of the two kinds of molecules on air/water interface by transferring the monolayer of the molecules to the surface of newly cleaved mica at desired surface pressure. The assemblies were then imaged with AFM (Figure 5) and the thickness of the fibers obtained at different surface pressure was accurately measured and provided in Figure S3 in SI. For TPE-Val, at the surface pressure of 0.6 mN/m (Figure 5A and 5D), short rods were formed on the surface of mica with the length varying from 100 nm to more than 1 μm. The height of the rods was around 0.37 nm and the width of the rods is 12 nm (Figure S3A in SI) at the surface pressure of 0.6 mN/m. At the surface pressure of 5 mN/m (Figure 5B and 5E), domains of rods were formed. The domains had different orientations, so there was vacancy between jointed neighbouring ones due to the different orientation of the rods. The rods were well aligned within each domain and they had the length around 1-3 m and the height around 1.18 nm.(Figure S3B in SI )

A

B

C

D

E

F

Figure 5. AFM image of the aligned TPE-Val assemblies on air/ water interface, transferred at the surface pressure of 0.6 mN/m (A and D), 5 mN/m (B，E) and 20 mN/m (C and F), respectively. Concentration: 2.5 μM.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

At the surface pressure of 20 mN/m ( Figure 5C and 5F), domains of rods were arranged more compactly with the height of the rods increasing to ~1.50 nm. So the rods had the thickness from 0.37 nm at a random state to 1.5 nm in a compact associated state (Figure S3C in SI ). The change of the thickness of the fibers reflected that the fibers was from a nearly flat arrangement to almost a standing state with the hydrophilic end contacting water surface. For TPE-2Val, at the surface pressure of 5 mN/m, micelles were formed (Figure 6A); at the surface pressure of 7mN/m, clusters of micelles were formed (Figure 6B); Enlarged image (Figure 6C) showed that the micelles were coalesced with each other to give the morphology of random coils; at the surface pressure of 20 mN/m, bundles of fibers were formed with the length of more than 4 μm. Different bundles of fibers were intercrossed with each other in the middle to give the morphology of stars (Figure 6D-6F). Close scrutinizing the rods suggested that they were well oriented and closely packed.

Figure 6. AFM images of the self-assembled formed by TPE-2Val at the surface pressure of 5 mN/m(A), 7 mN/m (B) and (C), 20 mN/m (D, E and F)，concentration: 2.5 μM.

Thus, both TPE-Val and TPE-2Val formed rods or fibers which were highly


Page 18 of 24

Page 19 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

oriented at the surface pressure of 20 mN/m. High resolution images of the rods and fibers formed by the two kinds of molecules ( Figure 7) revealed that both kinds of fibers were still helically twisted. The fibers formed by TPE-Val showed the helical pitch ~15 nm and the fibers formed by TPE-2Val were actually composed of many short rigid rods with the length of 200-300 nm, which were most clearly observed in the area labeled with arrows (Figure 7B). The short rods were well aligned locally and were joined in a head to tail manner to get elongated with the appearance of flexible fibers and reached the length of several microns. High resolution images of the aligned rods showed that the aligned rods had helically assembled elementary units with the helical pitch ~11.5 nm. We also carried out experiments to measure the IR and CD spectra of the LB films of these two molecules, but did not obtain strong enough signal due to the low transferring ratio of the molecules to the surface of quartz. Both molecules have the propeller-shaped TPE core and bulky valine attachments, once the first molecular layer is transferred to the surface of quartz, it makes the following molecules become more difficult to transfer to the covered surface.

Figure 7. High resolution AFM images of the aligned rods formed by TPE-Val (A) and TPE-2Val (B) at the surface pressure of 20 mN/m.

Thus, we have studied the self-assemblies of TPE-Val and TPE-2Val on air/water interfaces. Both of them formed aligned rods on air/water interfaces, which was in sharp contrast to the superhelical fibers formed on the surface of mica. The superhelical


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

assemblies were unraveled into aligned elementary helical fibers. DCE is a good solvent for both the TPE scaffold and amino acids. While on the air/water interface, water is a poor solvent for the TPE scaffold but a good solvent for amino acid attachments, so there existed great repulsions between the nonpolar TPE scaffold and water molecules, and attractions between amino acid attachments and water. Water molecules exerted lateral solvophobic repulsion to the molecules and shifted the noncovalent interactions balance of the molecules. The balanced effect of the above two kinds of interactions thus brought an additional asymmetric force field to the amphiphilic TPE derivatives and broke the noncovalent balance of the molecules. Accordingly the TPE derivatives would reorganize to achieve a new balance of the interaction between TPE derivatives and with water molecules. Amino acid attachments were hydrophilic and expected to anchor on the surface of water, while the hydrophobic TPE scaffold tends to avoid their contacts with water molecules. The cooperative effect of these noncovalent interactions would unravel the superhelical aggregates formed in DCE and drive them into aligned rods. Upon a proper surface pressure and pressing angle, different domains of aligned rods were formed. TPE-Val can be considered a linear amphiphilic molecules and it was easier to get compressed and aligned; while TPE-2Val contained two amino acid attachments and formed more hydrogen bonds than that of TPE-Val, so it was much harder to get TPE-2Val compactly compressed. The alteration of the surface from mica to a more polar surface of water led to the obvious unraveling of the superhelical fibers, suggesting that the interactions of the self-assembling molecules as well as the surface/interfaces where the assemblies were deposited determined their assemblies as well. It is expected that if the surface of mica is replaced with a hydrophobic one, diverse assemblies will be also formed. We then modified mica with APTES to increase the hydrophobicity of it and then deposited the TPE derivatives to APTES modified mica. The self-assemblies of TPE derivatives were then studied with AFM. As shown in Figure S4 in SI, for TPE-Val, elementary fibers were well-aligned and arranged in a dendrimer morphology; for TPE-2Val, elongated fibers were formed. Similarly with that on the surface of mica, both kinds of molecules did not form supramolecular helical assemblies on the surface of mica with enhanced


Page 20 of 24

Page 21 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

hydrophobicity. The hydrophobic surface caused the unraveling of the suprahelical assemblies into elementary fibers, confirming that the surface/interface was critical for the formation of the assemblies. Conclusion We systematically studied the self-assemblies of chiral AIE molecules TPE-Val and TPE-2Val upon the addition of poor solvent and on different surface/ interface. Both kinds of molecules self-assembled in to helical fibers upon the evaporation of DCE, with TPE-Val showing left-handedness and TPE-2Val showing right-handedness. The presence of poor solvent has considerable effect on the helical assemblies of the molecules causing the converse of the handedness of the helical fibers of TPE-Val and the entanglement of that of TPE-2Val without changing its handedness. When deposited on air/water interface both kinds of molecules did not form suprahelical assemblies, instead they formed aligned elementary helical fibers due to the shift of noncovalent balance caused by the strong lateral solvophobic effect exerted by water molecules. Both kinds of molecules did not form suprahelical assemblies on the surface of APTES modified mica, the less polar surface. TPE-Val formed aligned fibers and TPE-2Val formed elongated fibers, both binds of fibers exhibiting no suprahelical characteristics.

ASSOCIATED CONTENT Supporting Information The Supporting Information includes FT-IR, sectional analysis of the assemblies on air/interface and the assemblies on APTES modified mica The Supporting Information is available free of charge on the ACS Publications website at DOI AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; * E-mail: [email protected] * E-mail: [email protected]


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 24

ORCID Bing Shi Li : 0000-0002-0530-0294 Ben Zhong Tang: 0000-0002-0293-964X Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21574085), the National Natural Science Foundation of Guangdong province (2016A030312002, 2017A030313067). The work was also supported by Shenzhen Scientific Technology Research Program (JCYJ20170302143846672). References (1)

Hong, Y., Aggregation-induced emission—fluorophores and applications. Methods Appl.

Fluoresc. (2)

2016, 4, 022003.

Hong, Y.; Lam, J. W.; Tang, B. Z., Aggregation-induced emission: phenomenon, mechanism and

applications. Chem. Commun. 2009, 4332-4353. (3)

Li, H.; Zheng, X.; Su, H.; Lam, J. W.; Sing Wong, K.; Xue, S.; Huang, X.; Huang, X.; Li, B. S.; Tang,

B.Z., Synthesis, optical properties, and helical self-assembly of a bivaline-containing tetraphenylethene. Sci. Rep. 2016, 6, 19277. (4)

Li, H.; Cheng, J.; Zhao, Y.; Lam, J. W. Y.; Wong, K. S.; Wu, H.; Li, B. S.; Tang, B. Z., l-Valine methyl

ester-containing tetraphenylethene: aggregation-induced emission, aggregation-induced circular dichroism, circularly polarized luminescence, and helical self-assembly. Mater. Horiz. 2014, 1, 518-521. (5)

Li, H.; Cheng, J.; Deng, H.; Zhao, E.; Shen, B.; Lam, J. W. Y.; Wong, K. S.; Wu, H.; Li, B. S.; Tang, B.

Z., Aggregation-induced chirality, circularly polarized luminescence, and helical self-assembly of a leucine-containing AIE luminogen. J Mater. Chem. C 2015, 3, 2399-2404. (6)

Li, H.; Xue, S.; Su, H.; Shen, B.; Cheng, Z.; Lam, J. W.; Wong, K. S.; Wu, H.; Li, B. S.; Tang, B. Z.,

Click Synthesis, Aggregation-Induced Emission and Chirality, Circularly Polarized Luminescence, and Helical Self-Assembly of a Leucine-Containing Silole. Small (7)

2016, 12, 6593-6601.

Wen, T.; Ho, R.-M., Effects of the Chiral Interface and Orientation-Dependent Segmental

Interactions on Twisting of Self-Assembled Block Copolymers. ACS Macro Letters 2017, 6, 370-374. (8)

Hu, Q.; Wang, Y.; Jia, J.; Wang, C.; Feng, L.; Dong, R.; Sun, X.; Hao, J., Photoresponsive chiral

nanotubes of achiral amphiphilic azobenzene. Soft Matter 2012, 8, 11492-11498. (9)

Rong, Y.; Chen, P.; Liu, M., Self-assembly of water-soluble TPPS in organic solvents: from

nanofibers to mirror imaged chiral nanorods. Chem. Commun. 2013, 49, 10498-500.


Page 23 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(10)

Jonkheijm, P.; Fransen, M.; Schenning, A. P. H. J.; Meijer, E. W., Supramolecular organisation of

oligo(p-phenylenevinylene) at the air–water interface and in water. J. Chem. Soc. Perkin Trans. 2001, 8, 1280-1286. (11). Raval, R., Molecular assembly at surfaces: progress and challenges. Faraday Discuss 2017, 204, 9-33. (12). Guo, P.; Liu, M., In Situ Coordination and Supramolecular Chirality of Some Achiral Benzothiazole-derived Schiff Bases Fabricated at Air/Water Interface Chem. Lett. 2012, 41, 1199-1200. (13). Sun, Y.; Li, S.; Zhou, Z.; Saha, M. L.; Datta, S.; Zhang, M.; Yan, X.; Tian, D.; Wang, H.; Wang, L.; Li, X.; Liu, M.; Li, H.; Stang, P. J., Alanine-Based Chiral Metallogels via Supramolecular Coordination Complex Platforms: Metallogelation Induced Chirality Transfer. J. Am. Chem. Soc. 2018, 140, 3257-3263. (14). Krafft, M. P., Controlling phospholipid self-assembly and film properties using highly fluorinated components--fluorinated monolayers, vesicles, emulsions and microbubbles. Biochimie 2012, 94, 11-25. (15). Virgil Percec, J. G. R., † Martin Wagner,† Makoto Obata,† Catherine M. Mitchell,† Wook-Dong Cho,† and Sergei N. Magonov‡, AFM Visualization of Individual and Periodic Assemblies of a Helical Dendronized Polyphenylacetylene on Graphite. Macromolecules 2006, 39, 7342-7351. (16). Zhai Xiaodong , Z., Li

Liu Minghua, Supramolecular Assemblies between a New Series of

Gemini-Type Amphiphiles and TPPS at the Air/Water Interface: Aggregation, Chirality, and Spacer Effect. J. Phys. Chem. B 2004, 108, 7180-7185. (17). Li, B. S.; Berni, S. D.; Goh, C. M. Direct and Real-Time Visualization of the Disassembly of a Single RecA-DNA-ATPγS Complex Using AFM Imaging in Fluid. Nano Lett. 2006, 6, 1474-1478. (18). Karras, M.; Holec, J.; Bednarova, L.; Pohl, R.; Schmidt, B.; Stara, I. G.; Stary, I., Asymmetric Synthesis of Nonracemic 2-Amino[6]helicenes and Their Self-Assembly into Langmuir Films. J. Org. Chem. 2018, 83, 5523-5538. (19). Lv, K.; Lin, L.; Wang, X.; Zhang, L.; Guo, Y.; Lu, Z.; Liu, M., Significant Chiral Signal Amplification of Langmuir Monolayers Probed by Second Harmonic Generation. J. Phys. Chem. Lett. 2015, 6 (9), 171923. (20)

Li, S.; Zhang, L.; Jiang, J.; Meng, Y.; Liu, M., Self-Assembled Polydiacetylene Vesicle and Helix

with Chiral Interface for Visualized Enantioselective Recognition of Sulfinamide. ACS Appl. Mater. Interfaces 2017, 9, 37386-37394. (21)

Yao, P.; Wang, H.; Chen, P.; Zhan, X.; Kuang, X.; Zhu, D.; Liu, M., Hierarchical assembly of an

achiral pi-conjugated molecule into a chiral nanotube through the air/water interface. Langmuir 2009, 25, 6633-6636.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 24

TOC suprahelical

aligned

Surface/ interface changes


Interface Effect on the Hierarchical Assemblies of

Recommend Documents