Terminal Molecular Isomer-Effect on Supramolecular Self-Assembly

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Terminal Molecular Isomer-Effect on Supramolecular SelfAssembly System Based on Naphthalimide Derivative and Its Sensing Application for Mercury(#) and Iron (#) Ions Xinhua Cao, Na Zhao, Aiping Gao, qianqian Ding, Yiran Li, and Xueping Chang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00991 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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Langmuir

Terminal

Molecular

Isomer-Effect

on

Supramolecular

Self-Assembly System Based on Naphthalimide Derivative and Its Sensing Application for Mercury(Ⅱ) and Iron (Ⅲ) Ions Xinhua Caoa, b*, Na Zhaoa, Aiping Gaoa, Qianqian Dinga, Yiran Lia, Xueping Changa* aCollege

of Chemistry and Chemical Engineering＆Henan Province Key laboratory of Utilization of Non-metallic

Mineral in the South of Henan, Institute for Conservation and Utilization of Agro-bioresources in Dabie Mountains，Xinyang Normal University, Xinyang 464000, China bState

Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, P.R. China

Abstract: A series of naphthalimide derivatives gelators (G-o, G-m and G-p) with three molecular isomers as their terminal groups were designed and synthesized. Only G-m and G-p could form stable organogels in some solvents including methanol, acetonitrile, n-hexane, toluene, ethanol, DMSO, DMF and mixed solvents of acetonitrile / H2O (1/1, v/v). The different self-assembly structures were obtained from the self-assembly process of G-o, G-m and G-p such as structures like a Chinese chestnut formed by irregular micron pieces, microbelts and microbelts structures mingled with the bird's nest structures which exhibited different surface hydrophobicity with the water contact angle of 121-139° due to their different intermolecular noncovalent interaction. To our surprise, G-p acetonitrile solution emitted 492 nm light with a redshift of 72 nm compared with that emitted from G-o and G-m acetonitrile solution under 350 nm light excitation. Three gelators showed different detection abilities towards metal ions. G-o did not have any ability for sensitive and selective detection towards any ion. In contrast, G-m and G-p could sensitively and selectively detect Hg2+ and Fe3+. The detection limits for Fe3+ and Hg

2+

by

G-m were 4.76 × 10-5 M and 7.01 × 10-6 M with the corresponding association constant (K) of 1.64 × 104 M-1 and 3.79 × 104 M-1, respectively. The detection limits for Fe3+ and Hg2+ by G-p were 3.26 × 10-5 M and 1.77 × 10-6 M with the corresponding association constant (K) of 1.44 × 105 M-1 and 1.99 × 104 M-1, respectively. More interestingly, the back titration of SCN- could distinguish Hg2+ from Fe3+. At the same time, xerogels G-m and G-p also exhibited responsiveness toward Fe3+ and Hg2+ through fluorescence changes. The photophysical properties, gel formation, hierarchical structures, surface wettability and their function in this self-assembly system could be tuned through the molecular isomer effect. This work provided a new research paradigm for molecular isomer tuned supramolecular self-assembly materials from noncovalent interaction to molecular function.

Keywords: isomer-effect; self-assembly; naphthalimide; organogel; tune

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Introduction The mercury ion (Hg2+), one of the most toxic heavy metal ions has attracted extensive concerns because it is highly toxic, mobile and bio-accumulated in the ecosystem and due to its role in environmental pollution1, 2. Meanwhile, Hg2+ can be converted into highly hazardous methyl mercury by aquatic microorganisms, which can enter the food chain (especially in fish) accumulate in the human body, and finally result in damage to the central nervous system, liver, brain and kidneys ( which is especially harmful to a developing fetus)3. Therefore, development a rapid and sensitive detection method for Hg2+ is still a big challenge. Many methods such as the fluorescence approach4, ratiometric sensors5, electrochemiluminescence sensors6, and materials including Covalent Organic Frameworks7, Polymers8, MOFs9, upconversion nanomaterials10, soft materials11, carbon electrode materials12, semiconductor quantum dots13 and metal complexes14 have been designed and applied for detection Hg2+. However, the sensing process also suffers from interference by different ions，especially from iron (Ⅲ) Fe3+, as another ubiquitous ecological pollutant in our surroundings.15 Therefore, the improvement of sensors with high specific affinity toward metal ions has undoubtedly become an essential requirement for analytical applications. In many detection methods, fluorescent sensing arrays have been reported for the analysis of many analyte classes including various cations, anions and different neutral molecules because they provide fast, real-time responses, high sensitivity and require inexpensive equipment. Low-molecular-mass gels have been extensively developed due to their application in many fields including pollutant removal systems16, electrolyte materials17, sensors18,

19,

drug delivery systems20, Liquid Crystals21,

supramolecular chirality22, light-harvesting systems23, super-wettability. surfaces24, materials science25, and so on. Supramolecular gels can be obtained through the self-assembly of organic molecule in organic solvents or water under the driving force of noncovalent interactions including hydrogen bonding, π-stacking, hydrophobic and hydrophilic interactions, electrostatic interactions, coordination interactions, and van der Waals forces.26-30 These weak noncovalent interaction allow the gel system to respond to environmental stimuli such as light, temperature, magnetism, mechanical force and chemical stimulation29. Fluorescent organogels for such applications have therefore emerged as novel and promising materials, especially in application as sensors. Yi and coworkers have reported a sonication-triggered organogel based on a naphthalimide derivative with the ability of visual recognition of aliphatic and aromatic amines30. Lin and coworkers have developed an efficient method for the fabrication of a simple sensor array based on the competitive binding in supramolecular gels which can accurately identify fourteen important ions (F-, Cl-, I-, CN-, HSO4-, SCN-, S2-, OH-, Al3+, Fe3+, Zn2+, Hg2+, Pb2+ and H+) in water31. To control the self-assembly process and increase the function of a gel, the rational design of the gelator


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molecules including their structure and function is generally considered to be of the utmost importance, and thus extensive effort has been devoted to this topic32, 33. Isomerism is an extremely common phenomenon in organic chemistry and increases the types of organic compounds. Up to now, the isomeric effect for the modulation of photophysical properties, gel ability and the function of organogels have been paid little attention. Liu and coworkers have reported an isomeric effect in the self-assembly of a pyridine-containing L-glutamic lipid which can control the morphology and supramolecular chirality of the self-assembly system34. Inspired by this concept, here, by linking 2-hydroxypyridine, 3-hydroxypyridine and 4-pyridinol to the 4-Br position of 1,8-naphthalimide and adding an octadecyl chain to the imide side of naphthalimide, we report molecular isomer-modulated photophysical properties, gel formation, hierarchical structures, and surface wettability properties of organogels as well as a sensing application for mercury (II) (G-o, G-m and G-p in Scheme 1). The gelation ability, hierarchical structure, and surface wettability properties of self-assembly system can be modulated via a molecular isomer. More interestingly, the photophysical properties and sensing ability for mercury (II) can change with the molecular isomer, and the G-m and G-p molecules can sensitively and selectively detect Hg2+ and Fe3+. To the best of our knowledge, this is the first example of molecular isomer modulation of fluorescence and function in organogel with tunable fluorescence properties and responsiveness to Hg2+ and Fe3+ with reversibility, which may be relevant for the design of Hg2+ and Fe3+ sensors. O

4 N 3 1 2O

N O

G-o N

O 4 N

3 1 O 2

O

G-m N

O

4

2

3 1 O

N O

G-p

Scheme 1 Molecule structures of compounds G-o, G-m and G-P.

Experimental Reagents and materials Octadecylamine, 4-bromo-1,8-naphthalimide, 4-pyridinol, 3-Hydroxypyridine and 2-Hydroxypyridine were all purchased from Sinopharm Chemical Reagent Co., Ltd. All inorganic metallic salts were provided by Zhengzhou Alfachem Co., Ltd. All other reagents were analytically pure. Water used throughout was deionized and then triply distilled. Gelation test


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The gelator and solvent were put in a septum-capped test tube and heated (>80 °C) until the solid was dissolved. The sample vial was then cooled to 25 °C (room temperature). Qualitatively, gelation was considered successful if no sample flow was observed upon inversion of the container at room temperature (the inverse flow method)34. Xerogels were obtained by evaporation of solvent from the gel via freeze drying. Instrumentation conditions 1H

NMR and

13C

NMR spectra were recorded on a Bruker-Avance (Bruker, Ltd., Switzerland), at 400 and

100 MHz, respectively. Proton chemical shifts were reported in parts per million downfield from tetramethylsilane. HRMS was recorded on a LTQ-Orbitrap mass spectrometer (ThermoFisher, San Jose, CA, USA). 1H NMR titration of G-p was performed upon dissolving G-P (21.8 mM) in acetonitrile-d3 at 75 °C Hg2+ or SCN- (0 to 5.0 equiv.) was added to the G-P solution and the spectra were recorded using identical parameters. Field emission scanning electron microscope (FESEM) images were obtained using a FE-SEM S-4800 instrument (Hitachi, Ltd., Tokyo, Japan). Samples were prepared by spinning the samples on glass slides and coating with Pt. Powder X-ray diffractions were generated by using a Philips PW3830 (Philips, Ltd., Eindhoven, Holland) with a power of 40 kV at 40 mA (Cu target, λ = 0.1542 nm). UV-vis absorption spectra were recorded on a UV-vis 2550 spectroscope (Shimadzu, Ltd., Tokyo, Japan). The structure of the HOMO and LUMO states of G-P and G-B with simplification were determined with the help of theoretical calculations, in the framework of density functional theory (DFT) calculations at the level of B3LYP/6-31G* in a suite of the Gaussian 09 programs.36 Results and discussion The gelation abilities of G-o, G-m and G-P were investigated in organic solvents varying from polar to non-polar. All these compounds were dissolved in the organic solvents at an elevated temperature in an inclosed test tube and formed organogels upon cooling to room temperature, which was then confirmed by the inverted test tube method. As shown in Table 1, the compounds G-o, G-m and G-P had obvious differences in their gelation abilities. G-m showed a relative stronger gelation ability and formed orgaogels in methanol, acetonitrile, n-hexane, toluene, ethanol and DMSO with critical gel concentration (CGCs) of 3.57, 4.17, 6.25, 6.25, 6.25 and 4.17 mg mL-1, respectively. For G-p, only DMF and DMSO could be gelled with the CGCs of 25.0 and 5.0 mg mL-1, respectively. Only a precipitate, solution or partial gel (PG) were obtained in various solvents for G-o. In view of the precipitate obtained from G-o, G-p in acetonitrile during the gelation experiment process, water was added to the acetonitrile to decrease the solubility and obtain organogel in mixed solvents. Fortunately, the organogel G-p was observed in mixed solvents of acetonitrile/ H2O (1/1, v/v) with the CGC of 12.5 mg mL-1. In order to


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understand the effect of molecule structures on the gelation ability, the dihedral angles of (G-o, G-m and G-P) were calculated. The dihedral angles (C1-C2-C3-C4) of the pyridine ring and naphthalimide group in molecules G-o, G-m and G-p were 17.7°, 92.2° and 36.6°, respectively. It was found that the gelation ability was increased with the dihedral angles increasing through comparing their gelation ability experimental results and dihedral angles. It is well known that gel formation need suitable non-covalent interactions which is not too strong or weak in the self-assembly system. If non-covalent interactions are too strong or weak in self-assembly system, precipitate or solution may be obtained. For G-o molecule with dihedral angle of 17.7°, the strong π-π stacking interaction may result in precipitate. For G-m and G-p molecules with 92.2° and 36.6°, the suitable non-covalent interactions including π-π stacking endow their gelation ability. Table 1 The gelation ability of compounds G-o, G-m and G-p in different solvents. Solvent

G-o

G-m

G-p

methanol

p

G (3.57)

P

DMF

P

S

G (25)

acetonitrile

P

G (4.17)

p

acetonitrile/H2O (1/1, v/v)

P

P

G (12.5)

1,4-dioxane

S

S

P

n-hexane

P

G (6.25)

NI

Acetone

P

S

PG

toluene

S

G (6.25)

P

ethyl acetate

S

S

P

ethanol

P

G (6.25)

P

petroleum ether

P

S

NI

DMSO

PG

G (4.17)

G (5.0)

THF

S

S

P

P = precipitate; NI = not insoluble; S = solution; PG = partial gel; G = gel; the values in the brackets denote the CGC.

Field emission scanning electron microscopy (FESEM) of the precipitate or xerogels from air-died organogels revealed the self-assembly structure of G-m, G-o and G-p in different state, as illustrated in Figure 1. As shown in Figure 1a, the irregular micron pieces structure was observed in the precipitate G-o, and the micron pieces were further resembled into a sphere like a Chinese chestnut. The diameter of the microsphere was approximately 30μm. For the organogel G-m, G-m molecules were self-assembled into microbelts structures with the width and length of 1.0-2.0 μm and tens of microns, respectively. A completely different morphology was obtained in the organogel G-p formed in mixed solvents of acetonitrile / H2O (1/1, v/v). A microbelts structure was obtained with the width and length of 1.0 μm and tens of microns, respectively. At the same time, some microbelts were rolled into the bird's nest structure in the self-assembly system. From the above results, it was shown that different structures


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could be obtained from different self-assembly behavior via changing the molecular isomer of the terminal pyridine.

Figure 1 SEM images of precipitate G-o (a) from acetonitrile, gel G-m (b) formed in acetonitrile and G-P (c) in mixed solvents of acetonitrile / H2O (1/1, v/v).

To investigate the self-assembly behavior, the UV-vis absorption spectra of G-o, G-m and G-p in solution, precipitate and organogel states were obtained and are shown in Figure 2. As shown in Figure 2a, the acetonitrile solution of G-o showed two absorption bands at 342 nm with ε of 3.39 ×104 dm3 mol-1 cm-1 and 355 nm with ε of 3.49×104 dm3 mol-1 cm-1 in its UV-vis absorption spectra which were related to the n–π* transition band37. For the precipitate G-o formed in acetonitrile, the above two UV-vis absorption bands were transformed into a broad absorption band at 363 nm with redshift of 21 and 8 nm, respectively. Only an absorption band at 355 nm with ε of 5.56 ×104 dm3 mol-1 cm-1 was observed in the UV-vis absorption spectra of the G-m acetonitrile solution, and the absorption band did not have any shift in its organogel state. For the compound G-p, the absorption band was located at 348 nm with ε of 5.31 ×104 dm3 mol-1 cm-1 for the G-p solution and gel state. From the above results, the absorption spectra properties of this system could be tuned by the terminal pyridine. G-o precipitate G-o solution

0.8 0.6 0.4 0.2 0.0

1.0

b

G-m gel G-m solution

0.8 0.6 0.4 0.2

340

360

380

400

Wavelength / nm

420

440

c

G-p gel G-p solution

0.8 0.6 0.4 0.2 0.0

0.0

320

1.0

Normalized absorbance

a


1.0


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320

340

360

380

400

420

440

Wavelength / nm

300

320

340

360

380

400

420

440

Wavelength / nm

Figure 2 UV-vis absorption spectra of G-o (a) in solution and precipitate state in acetonitrile, G-m (b) in acetonitrile solution and organogel state; G-p (c) in acetonitrile solution and organogel in mixed solvents of acetonitrile/H2O (1/1, v/v). The solution concentration was 5 ×10-5 M. The precipitate was formed from the hot solution of G-o with the concentration of 25 mg mL-1. The gel concentration was at their corresponding CGC.

Fluorescence emission behavior was the main important property for organic functional materials which decided or affected their application. The fluorescence properties of G-o, G-m and G-p in different states were


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investigated and are shown in Figure 3. At the same time, the fluorescence quantum yield and lifetime values of G-o, G-m and G-p in different state were also researched (Figure S1). As shown in Figure 3a, the G-o acetonitrile solution emitted 420 nm with an absolute quantum yield of 12.21% and a lifetime of 5.84 ns. When the hot G-o solution was cooled and formed the G-o precipitate, the fluorescence emission of the G-o precipitate had changed to 443 nm with a redshift of 23 nm which showed the occurrence of π-π stacking in the G-o precipitate38. Similar fluorescence emission behavior was observed in the fluorescence experiment on the compound G-m (Figure 3a). The G-m acetonitrile solution also emitted 420 nm light with an absolute quantum yield of 11.31% and a lifetime of 6.15 ns. A fluorescence emission peak of the G-m organogel formed in acetonitrile appeared at 427 nm with a redshift of 7.0 nm. Compared with the G-o and G-m acetonitrile solution, the fluorescence emission of the G-p acetonitrile solution had an obvious redshift, and changed from 420 nm to 492 nm with an absolute quantum yield of 27.12% and the lifetime of 1.88 ns. It was more significantly different that the fluorescence emission of the G-p organogel formed in the mixed solvent of acetonitrile/H2O (1/1, v/v) did not have red shift compared with that of the G-o and G-m solution. In contrast, the fluorescene emission of the G-p organogel was blue-shifted to 463 nm. It is well-known that naphthalimide derivatives were typical intramolecular charge transfer (ICT) molecules such as G-o, G-m and G-p39. For G-p, it was possible that the twisted intramolecular charge transfer (TICT) might

a

G-o precipitate G-o solution

0.8 0.6 0.4 0.2 0.0 400

450

500

Wavelength / nm

550

600

1.0 0.8

b

Fluorescence intensity(a.u)

1.0

Fluorescence intensity(a.u.)

cause color-tunable fluorescence emission due to the inhibition of the conformation rotation in the gel phase40-42 Fluorescence intensity(a.u.)

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G-m gel G-m solution

0.6 0.4 0.2 0.0 400

450

500

550

600

Wavelength / nm

1.0

c

G-p gel G-p solution

0.8 0.6 0.4 0.2 0.0 400

450

500

550

600

650

Wavelength / nm

Figure 3 Fluorescence emission spectra of G-o (a) in solution and precipitate state in acetonitrile, G-m (b) and G-p (c) in solution and organogel state (λex = 350 nm). The solution concentration was 5 ×10-5 M. The precipitate was formed from the hot solution of G-o with the concentration of 25 mg mL-1. The gel concentration was at their corresponding CGC.

To understand the huge difference in the fluorescence emission properties between G-o, G-m and G-p, the theoretical investigation of the difference in the geometry optimizations, harmonic vibration and excited state emission was carried out using the hybrid B3LYP density functional theory method (DFT) at the level of 6-31G* (Table 2). The long alkyl chains in the G-o, G-m and G-B molecules were simplified as methyl group. The calculated HOMOs and LUMOs energies of G-o, G-m and G-B molecules were -6.100, -2.259, -6.226, -2.307, -6.375 and -2.498 eV, respectively which had corresponding theoretical absorption bands at 355.17 345.87 and 350.75 nm. This result was line with the experiment value in the vicinity of 350 nm. The HOMO primarily resided


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on the pyridine and naphthalene imide ring for G-o and G-p. Nevertheless, the HOMO primarily resided on only naphthalene imide ring for G-m. Similarly, the LUMO states all primarily resided on the naphthalene imide ring for G-o, G-m and G-p. The absorption and emission spectra of G-o, G-m and G-p were also simulated via theoretical calculations, and the absorption spectrum is shown in Figure S5. The absorption band of G-o, G-m and G-p mainly cover 250-425 nm with the maximum absorption band at approximately 350 nm which was completely in agreement with the experimental results. The maximum fluorescence emission peaks of G-o, G-m and G-p through theoretical calculations were 436, 429 and 458 nm which basically showed a similar fluorescence change trend. The dihedral angles (C1-C2-C3-C4) of the pyridine ring and naphthalimide group in molecules G-o, G-m and G-p were 17.7°, 92.2° and 36.6°, respectively. It was found that the gelation ability was increased with the dihedral angles increasing through comparing their gelation ability experimental results and dihedral angles. It was possible there were stronger non-covalent interactions in the self-assembly system of G-m than that of G-o or G-P systems. Table 2 HOMOs and LUMOs of compounds G-o, G-m and G-p.

Simplified molecule

HOMO (eV)

LUMO (eV)

-6.100

-2.259

-6.226

-2.307

-6.375

-2.498

O N N O O

G-o O

N

N O O

G-m O

N

N O O

G-p

The variable-temperature 1H MNR experiments of precipitate G-o in d3-acetonitrile, gel G-m in d3-acetonitrile and gel G-p in mixed solvents of d3-acetonitrile / D2O (1/1, v/v) were carried out In order to understand the self-assembly process and are shown in Figure S2. As shown in Figure S2a, the proton signals (H1-H9) of the pyridine and naphthalimide were gradually strengthened with the temperature increasing from


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25 °C to 75 °C. At the same time, the proton signals of H2, H4, H7 and H8 were obviously shifted upfield from 8.72, 7.90, 7.52 and 7.65 ppm to 8.69, 7.89, 7.50 and 7.62 ppm, respectively with the increased temperature. The 1H

NMR spectra strongly suggested the presence of π−π stacking in the precipitate state, and this experiment result

was highly consistent with the fluorescence emission spectra43, 44. For gel G-m in d3-acetonitrile, the proton signals of H1, H2 and H4 were also shifted upfield from 7.28, 7.96 and 7.83 ppm to 7.26, 7.95 and 7.82 ppm, respectively in the gel-to-sol transition process which proved that the π−π stacking also occurred in gel G-m (Figure s2b). When the gel G-p in mixed solvents of d3-acetonitrile / D2O (1/1, v/v) was under the variable-temperature, the proton signals of the aromatic ring did not had any obvious shift in the process of increasing temperature from 45-75 °C except for the proton H6 assigned to the pyridine which showed that no obvious π−π stacking in gel G-p in mixed solvents of d3-acetonitrile/D2O (Figure s2c). From the above experimental results, the intermolecular noncovalent interaction could be tuned via a molecular isomer in this self-assembly system. X-ray diffraction measurements were used to evaluate the different aggregation structures of the G-o precipitate, G-m organogel and G-p organogel (Figure s3). The XRD pattern of the G-o precipitate from acetonitrile exhibited a series of peaks at θ = 1.72°, 3.48° and 5.29° with d-space values of 2.58 nm, 1.28 nm and 0.84 nm, respectively which was just with the ratio of 1:1/2:1/3, indicating the lamellar ordered structure of molecules45 (Figure s3a). The d-space value of 2.58 nm was close to the length of a single molecule, suggesting a single molecule structure in the precipitate state. The patterns of the G-m xerogel from acetonitrile gave a similar result. A series of peaks at θ = 1.72°, 3.40°, 5.10° and 6.92° with the d-space value of 2.64, 1.31, 0.86 and 0.64 nm, respectively which were in line with the ratio of 1:1/2:1/3:1/4, suggest the presence of the lamellar ordered structure in the G-m organogel (Figure s3b). At the same time, the d-space value of 2.64 nm also approached the molecule length of G-m which showed that the single molecule self-assembly mode was employed in the G-m organogel. For the G-p xerogel, the XRD pattern exhibited the completely different experiment results. The XRD pattern exhibited a series of peaks at θ = 1.16°, 1.89°, 2.83, 3.55, 4.71, 4.88, 5.30, 5.56, 5.59° and so on with the corresponding d-space values of 3.81, 2.35, 1.56, 1.25, 0.94, 0.91, 0.84, 0.79 and 0.75 nm, respectively, which indicated a more complicated structure (Figure s3c). The d-space value of 3.81 nm was greater than the single molecule length of G-p and less than the length of two molecules of G-p, which showed the screwy-like two molecule self-assembly behavior in the G-p organogel formed in mixed solvents of acetonitrile / H2O (1/1, v/v). The packing modes of three molecules in assemblies were speculated according to the XRD data (Figure s4). G-o and G-m molecules were packed in single molecule self-assembly. G-p molecule was packed in two molecules self-assembly through the winding of the alkyl chain of two molecules.


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Surface wettability is an important physical property for a material which also reflects the self-assembly behavior, the molecular structure and the properties of the surface layer46, 47. The surface wettability of a film coating of the precipicate G-o, organogels G-m and G-p were investigated one by one and are shown in Figure 4. The hydrophobic nature of the precipitate film surface was confirmed by finding that a water droplet on a glass surface spin-coated with precipitate G-o formed in acetonitrile showed a much larger contact angle (139°) than those of the surfaces coated with organogels G-m and G-p from the diluted organogel solutions (121° and 124.5°, respectively). Except for the film surface roughness, to some extent the molecular structure could decide the surface wettability. The obvious difference was that the hydrophilic N atom of pyridine was on the interior of G-o comparing with that of G-m and G-p. In order to eliminate the effect of water on the surface wettability, the water contact angles of film coating with the three molecular isomers self-assemble in acetonitrile were carried out (Figure s5). The hot solution of G-o, G-m and G-p with the concentration of 25 mg mL-1 were cooled to room temperature and formed their corresponding self-assembly systems. The above self-assembly systems were coated onto glass. The film coating of G-o, G-m and G-p self-assembly systems still exhibited hydrophobicity with the contact angles of 150°, 136.5° and 121.5°. Especially for G-p, the addition of water did not obviously affect the surface wettability of self-assembly materials. Even if the self-assembly behavior of organogels G-m and G-p was different, the terminal hydrophilic groups of the molecules G-m and G-p onto the self-assembly material surface were outside and exhibited lower hydrophobicity.

Figure 4 Water contact angle experiments results of the film coating with precipicate G-o, organogels G-m and G-p. The precipitate was formed from the hot solution of G-o with the concentration of 25 mg mL-1. The gel concentration was at their corresponding CGC.

In fact, the effect of isomer was on the function of surpamolecular self-assembly system except for their photophysical properties, gel formation, hierarchical structures, surface wettability. G-o, G-m and G-p were tested for sensing metal ions on the base of the coordinating ability of pyridine48 (Figure 5). As shown in Figure 5a, when 10.0 eq. of different metal ions including Ag+, Cd2+, Co2+, Cu2+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+, Zn2+, Fe3+ and Fe2+ were added to the G-o solution, the maximum fluorescence intensity decreasing was only approximately 45.1% for Fe2+. In other words, the G-o solution did not exhibted high selectively for any metal ion. The fluorescence emission changes were well verified by visual color changes of the G-o solution under addition of different metal


Page 10 of 25

Page 11 of 25

ions (Figure s4). When the solution was under a 365 nm UV lamp, the fluorescence emission intensity also did not exhibit obvious changes. When the G-m and G-p acetonitrile solutions were used to try to detect metal ions, they exhibited high selectively towards Hg2+ and Fe3+. As shown in Figure 5b, the fluorescence emission intensity of the G-m solution obviously decreased to 11.8% and 24.0% of its original state after the addition of 10.0 eq. of Hg2+ and Fe3+, respectively. For the addition of the other metal ions, the fluorescence emission intensity change was within the scope of 62.1-115.6%. Compared with the G-m acetonitrile solution, the fluorescence emission intensity of the G-p solution had greater reduction and decreased to 3.8% and 8.4% of its original state after the addition of 10.0 eq. of Hg2+ and Fe3+ (Figure 5c), respectively. Under the addition of the other metal ions, the fluorescence emission intensity change of the G-p solution was in the range of 74.9% - 101.6% of its original state. The visual changes of the G-m and G-p solutions under the addition of metal ions were well proven by the fluorescence emission change (Figure s6). The blue light emitted from the G-m solution was almost quenched under the addition of Hg2+ and Fe3+. Compared with the G-o and G-m solutions, the green light emitted from the G-p solution was almost quenched by Hg2+ and Fe3+. G-o solution + 10eq Ag 2+ Cd 2+ Co 2+ Cu 2+ Hg 2+ Mg 2+ Mn 2+ Ni 2+ Pb 2+ Zn 3+ Fe 2+ Fe

900 600 300

1500 1200

G-m solution + 10eq Ag 2+ Cd 2+ Co 2+ Cu 2+ Hg 2+ Mg 2+ Mn 2+ Ni 2+ Pb 2+ Zn 3+ Fe 2+ Fe

b

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Langmuir

1000

c

G-p solution + 10eq Ag 2+ Cd 2+ Co 2+ Cu 2+ Hg 2+ Mg 2+ Mn 2+ Ni 2+ Pb 2+ Zn 3+ Fe 2+ Fe

800 600 400 200 0 400

450

500

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Wavelength / nm

Figure 5 Fluorescence emission change of G-o (a), G-m (b) and G-p (c) acetonitrile solution (10-5 M, λex = 350 nm) toward different metal ions (10.0 equiv.).

The detection abilities of G-m and G-p towards Fe3+ and Hg2+ were studied by the absorption and fluorescence changes and are shown in Figure 6, Figure 7, Figure s7 and Figure s8. Without the presence of Fe3+, the G-m solution showed an intense absorption band at 239 nm and a relatively weak absorption band at 355 nm in Figure s7. With the addition of Fe3+, the absorbance of the two bands gradually increased, accompanied by a blue-shift of approximately 4-5 nm (Figure s7a). The fluorescence titration of the G-m acetonitrile solution showed a decrease in the fluorescence emission intensity at 420 nm with the progressive addition of Fe 3+ (Figure 6a). Upon addition of 9.0 eq. Fe3+, the fluorescence intensity was decreased by 80%. Furthermore, the plot of the fluorescence intensity ratio of F0 /F (F0=the original fluorescence intensity; F=the real-time fluorescence intensity) at 420 nm with the addition of Fe3+ showed a linear correlation with R = 0.9984 (Figure 6b). The binding constant was determined using the Benesi-Hildebrand equation for 1:1 stoichiometry (Figure s9)49. The association constant (K) was estimated as 1.64 × 104 M-1. The detection limit was 4.76 × 10-5 M (Table s1). The UV-vis absorption


Langmuir

spectra of the G-m solution exhibited a different change trend in the process of the addition of Hg2+ in Figure s7b. With the addition of Hg2+, the absorbance at 239 nm gradually increased along with some blue-shift, and the absorbance at 355 nm did not obviously change except for some blue-shift. With the addition of Hg2+ to the G-m acetonitrile solution, the fluorescence emission intensity of the G-m solution gradually decreased. When the addition of Hg2+ was up to 1.5 eq., the fluorescence emission peak began to blue-shift until the addition amount of Hg2+ reached its destination. The fluorescence emission intensity of the G-m solution was decreased by approximately 88.7%, and the maximum emission peak changed from 420 nm to 409 nm. The plot of the fluorescence intensity ratio of F0 /F at 420 nm with the addition of Hg2+ showed a linear correlation with R = 0.99011 (Figure 6d). The association constant (K) was estimated as 3.79 × 104 M-1. The detection limit was 7.01 × 10-6 M (Figure s10 and Table s1).

a

G-m solution 3+ 0.5 eq. Fe 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0

1400 1200

F0 / F


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5

b y = 0.57375 + 47859.52528 x R = 0.9984

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3+

Wavelength / nm 1500

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c G-m solution 2+ 0.5 eq Hg 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

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F0 / F


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

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600 300 0 375

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y = -7.06937 + 325088 x R = 0.99011

8 6 4 2 0 0.000025 0.000030 0.000035 0.000040 0.000045 0.000050 2+

[Hg ] / M

Figure 6 (a) Fluorescence titration of G-m (10-5 M) in acetonitrile (λex. = 350 nm) with the gradual addition of Fe3+. (b) Plot of the fluorescence emission intensity ratio (F0 / F) at 419 nm with added Fe3+. (c) Fluorescence titration of G-m (10-5 M) in acetonitrile (λex. = 350 nm) with the gradual addition of Hg2+. (d) Plot of the fluorescence emission intensity ratio (F0 / F) at 419 nm with added Hg2+.

The free G-p acetonitrile solution showed a series of absorption bands at 231, 237, 273 and 348 nm in its UV-vis absorption spectrum (Figure S8a). With the addition of Fe3+, the absorption band at 273 nm disappeared, and a new absorption band at 330 nm appeared. The absorbance of the two absorption bands at 231 and 237 nm gradually increased without any shift. In contrast, the absorption band at 350 nm was blue-shifted to 344 nm with


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the exception of an absorbance increase. The fluorescence intensity of the G-p acetonitrile solution at 492 nm gradually decreased with the simultaneous appearance of the blue-shifted new band at 383 nm in the titration process by Fe3+ (Figure 7a). A well-defined isosbestic point at 416 nm was observed. The fluorescence intensity at 492 nm and 380 nm reached the plateau point after the addition of 9.0 eq. Fe3+. In addition, a linear correlation between the ratio of the emission intensities (F0/F) and the Fe3+ concentration was observed with R = 0.99106 (Figure 7b). The association constant (K) for G-p and Fe3+was estimated as 1.44 × 105 M-1 (Figure s11). The detection limit toward Fe3+ was 3.26 × 10-5 M (Table s2). When the G-p solution was used to detection Hg2+, it also exhibited excellent performance (Figure 7c). The fluorescence intensity of G-p solution at 492 nm began to decrease, and the new fluorescence emission peak at 383 nm appeared and increased with the addition of Hg2+. A well-defined isosbestic point also appeared at 416 nm. When the added amount of Hg2+ was increased up to 4.5 eq., the fluorescence intensity change reached the plateau point, which showed a linear correlation with R = 0.9905 (Figure 7d). The association constant (K) between G-p and Hg2+ was estimated as 1.99× 104 M-1 (Figure s12). The detection limit toward Hg2+ was 1.77 × 10-6 M (Table s2).

a

10 G-p solution

0.5 eq. Fe 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 6.0 7.0 8.0 9.0 10.0

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3+

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c

45 G-p solution 2+ 0.5 eq. Hg 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

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y = 0.64768 + 133311.8892 x R = 0.99106

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Langmuir

40

[Fe ] / M

d

y = -78.45638 + 2450240 x R = 0.9905

35 30 25 20 15 10

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0.000040

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0.000045

0.000050

2+

[Hg ] / M

Figure 7 (a) Fluorescence titration of G-p (10-5 M) in acetonitrile (λex. = 350 nm) with the gradual addition of Fe3+. (b) Plot of the fluorescence emission intensity ratio (F0 / F) at 492 nm with added Fe3+. (c) Fluorescence titration of G-p (10-5 M) in acetonitrile (λex. = 350 nm) with the gradual addition of Hg2+. (b) Plot of the fluorescence emission intensity ratio (F0 / F) at 492 nm with added Hg2+.

It was well known that a three-dimension network was formed in the self-assembly process, and further


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

gelated the solvent molecule. The three-dimension network provided an excellent platform for contacting and detecting the analyte due to the large specific surface area and weak noncovalent interactions. For example, Xue and coworkers reported a fluorescent galunamide derivative self-assembly system with the ability of sensing acid vapor and mechanical force stimuli.50 Herein, the organogels G-m and G-p were used to detect Hg2+ and Fe3+ (Figure s13). When 5.0 eq. of Hg2+ solution was added to the organogel surfaces of G-m and G-p, the fluorescence intensity of the organogels G-m and G-p showed a certain degree of quenching with an efficiency of about 48.9% and 39.2%, accompanied by fluorescence emission blue-shifts of 12 and 30 nm, respectively (Figure s13b and 13c). This occurred possibly because the binding force between G-m or G-p and Hg2+ was smaller than that between the gelator moleculse, and Hg2+ could not completely permeate into the network. The color and state of the organogels G-m and G-p was still maintained after the addition of Hg2+ (Figure s13a). When 5.0 eq. of Fe3+ was added to the organogel surfaces of G-m and G-p, the fluorescence emission intensity of the organogels G-m and G-p was absolutely quenched with efficiencies of 92% and 96%, accompanied by fluorescence emission blue-shifts of 27 and 14 nm, respectively (Figure s11b and c). The color of the organogels G-m and G-p changed to yellow, and the organogel state of G-m and G-p was partially destroyed under the addition of Fe3+ (Figure s13a). At the same time, the xerogels G-m and G-p were tested to detect Hg2+ and Fe3+. As shown in Figure s14, when Hg2+ and Fe3+ solution was dropped onto the surfaces of xerogels G-m and G-p, the fluorescence of the xeorgels G-m and G-p was almost quenched, which showed that compounds G-m and G-p in the xerogel state were able to detect Hg2+ and Fe3+. The morphology changes of the gel of G-m and G-p after addition of Hg2+ and Fe3+ were investigated (Figure s15). The addition amount of Hg2+ and Fe3+was 5.0 eq.. For the organogel G-m, the microbelts structures were changed into the interwinded microfibers structure and broader microbelts structure after addition of Fe3+ and Hg2+, respectively. For organogel G-p, the microbelts structures were changed into left handed helical nanofibers structure and interwinded nanofibers after addition of Fe3+ and Hg2+, respectively. At the same time, some microbelts were rolled into the bird's nest structure in the self-assembly system. In order to resolve the mutual interference between Hg2+ and Fe3+, we tried to regenerate the fluorescence via the addition of a masking agent. According to the previous report in the literature16, SCN- was used as a masking agent to recover the fluorescence of the G-m or G-p solutions with the addition of Hg2+ or Fe3+. Fortunately, the fluorescence of the G-m and G-p solutions with the addition of Hg2+ could be recovered, whereas that of the G-m and G-p solutions with the addition of Fe3+ could not be regenerated. As shown in Figure 8a and 8b, the fluorescence of G-m/5.0 eq. Fe3+ and G-p/10.0 eq. Fe3+ did not exhibit obvious recovery even if the addition amount of SCN- was up to 50.0 and 20 eq., respectively. For the G-m and G-p with addition of 5.0 eq.


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Langmuir

Hg2+, the fluorescence intensities of the G-m and G-p solutions were gradually recovered with the addition of SCN- (Figure 8c and 8d). When the added amount of SCN- reached 15.0 and 28.0 eq., the regeneration efficiencies of G-m /Hg2+ and G-p/Hg2+ were as high as 94.2% and 80.4%, respectively. This result indicated that compounds G-m and G-p could sensitively and selectively detect Hg2+ and Fe3+. To exclude the effect of SCN- on the fluorescence of G-m and G-p, the fluorescence titration experiments of G-m and G-p via SCN- were carried out (Figure 8e and 8f). With addition of SCN- to G-m and G-p solution, the fluorescence intensity of G-m and G-P solution did not show obvious quenching, and the quenching efficiency was only approximately 4.1% for G-m solution and 15.5% for G-p solution, respectively after the addition of 20.0 eq. SCN-. This result effectively explained why the regeneration efficiency of G-p solution was lower than that of G-m solution after the addition of SCN-. The fluorescence change was also well-verified by the images of the G-m and G-p solutions before and after the addition of SCN- (Figure s16). The fluorescence of the G-m and G-p solutions with the addition of Hg2+ and Fe3+was almost quenched. The fluorescence of the G-m and G-p solutions with the addition of Hg2+ could be recovered after the addition of SCN-. However, the fluorescence of the G-m and G-p solutions with the addition of Fe3+ could not be recovered, regardless how much SCN- was added.


Langmuir

a

1400

G-m solution 3+ G-m solution + 50 eq. Fe

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5.0 eq SCN 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0

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G-m solution 2+ G-m solution + 5.0 eq. Hg 1.0 eq SCN 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0

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650 G-p solution 2+ 5.0 eq. Hg 1.0 eq. SCN 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0

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Page 16 of 25

1200

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Figure 8 The fluorescence intensity regeneration of (a) G-m /Fe3+ solution recovered by SCN- when the addition amount increased from 5.0 to 50.0 eq.; (b) G-p /Fe3+ solution recovered by SCN- when the addition amount increased from 1.0 to 20.0 eq.; (c) G-m/Hg2+ solution recovered by SCN- when the addition amount increased from 1.0 to 15.0 eq.; (d) G-p/Hg2+ solution recovered by SCN- when the addition amount increased from 1.0 to 28.0 eq.; (e) the fluorescence titration of G-m solution via SCN-1 in the range of 2.0-20.0 eq,; (f) the fluorescence titration of G-p solution via SCN-1 in the range of 2.0-20.0 eq.. The concentration of G-m and G-p solutions was all 10-5 M.

To investigate the detection mechanism, the 1H NMR titration of G-p was performed in the presence of Hg2+ (Figure 9a). The 1H NMR experiment temperature was 75 °C due to the poor solubility of G-p in acetonitrile. Hg2+ was gradually added to the G-p solution, and the spectra were recorded immediately after addition. With the addition of Hg2+, the proton signals of H6 and H7 resulted in downfield shifts from 6.34 and 6.98 ppm to 8.07 and 8.54 ppm, respectively in the process of the addition of Hg2+ in the range of 0.0 - 1.0 eq. At the same time, the proton signals of H1 and H5 belonging to naphthalimides were also shifted downfield from 8.59 ppm to 8.68 ppm. After the addition amount of Hg2+ was increased up to 1.0 eq., none of the proton signals of the aromatic nucleus shifted, which exhibited a 1:1 stoichiometry for G-P and Hg2+. This indicated that the Hg2+ ion interacts directly with the pyridine’s nitrogen end of G-P49. It is well known that fluorescence naphthalimide derivatives G-m and G-p have the characteristic of intramolecular charge transfer (ICT). The fluorescence changes observed in G-m


Page 17 of 25 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

and G-p solution after the addition of Hg2+ and Fe3+ are due to the change in the intramolecular charge transfer (ICT) band of G-m and G-p. The 1H NMR titration done in the organic medium clearly indicated that the metal ion interacts with the pyridine’s nitrogen end which influences the ICT and results in the change in emission spectra of G-m and G-p.49, 51 The 1HNMR experiment on the G-p solution with addition of 2.0 eq. Hg2+ underwent back titration by SCN- in order to verify the mechanism of the fluorescence reversibility through the addition of Hg2+ and SCN- (Figure 9b). With the addition of SCN- to the G-p solution again, the proton signals of H6 and H7 was again back to upfield from 7.50 and 8.54 ppm to 6.69 and 7.99 ppm, repectively in the process of the addition of 5.0 eq. SCN-. The proton signals of H1 and H5 also returned from 8.69 ppm to 8.63 ppm. From these experimental results, the coordination interaction between the pyridine’s nitrogen and Hg2+ was destroyed due to the formation of the Hg(SCN)2 precipitate, and the fluorescence of the G-p solution was recovered. N 6

a

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1.0 eq. 0.8 eq.

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Figure 9 1H NMR spectra of G-p in the presence of 0–1.2 equivalents of Hg2+ (400 MHz, acetonitrile-d3) (a) and the G-p solution with addition of 2.0 eq. in the presence of 0-5.0 eq. SCN- (expanded spectrum in the aromatic region) (b). The testing temperature was at 75℃ due to the poor solubility of G-p in acetonitrile. The positions of the labeled protons are marked in the figure. The 1H NMR spectra of G-p solution in acetonitrile-d3 was inserted.

Conclusion

In conclusion, we designed and synthesized a series of naphthalimide derivatives (G-o, G-m and G-p) with different molecular isomers as the terminal group including 2-Hydroxypyridine, 3-Hydroxypyridine and 4-Hydroxypyridine. The photophysical properties, gel formation, hierarchical structures, surface wettability and the functional effects on this system could be tuned via the molecular isomer effect. The G-m and G-p could form stable organogels in different solvent. G-o solution, precipitate or partial gel could be obtained in the solvent used in the tests. In the self-assembly process of G-o, G-m and G-p, different hierarchical structures could be observed


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including the structure resembling a Chinese chestnut formed by irregular micron pieces, microbelts and microbelts structures mingled with bird's nest structures, which had corresponding surface wettability properties. More interestingly, the photophysical properties of the three compounds were entirely different. The maximum fluorescence emission of G-o and G-m in acetonitrile was at 420 nm under the excitation of 350 nm light, while G-p acetonitrile solution emitted 492 nm with a redshift of 72 nm. The compounds G-o, G-m and G-p exhibited different detection abilities with their different terminal group. G-m and G-p could sensitively and selectively detect Hg2+ and Fe3+, while G-o was without this ability. Hg2+ and Fe3+ could be distinguished via SCN- titration. The xerogels G-m and G-p also exhibited responsiveness towards Hg2+ and Fe3+. This research provide a new way for tuning and constructing functional supramolecular gel.

Associated content

Supplementary information is available: synthesis details and additional spectra, XRD pattern, some images of solution and gel state.

Author information

Corresponding authors

E-mail: [email protected]; [email protected]

Acknowledgment

The authors thanks for the financial support by the National Natural Science Foundation of China (U1704164 and 21401159), Institute for Conservation and Utilization of Agro-bioresources in Dabie Mountains, Funding scheme for the young backbone teachers of higher education institutions in Henan Province (2015GGJS-141), the Science & Technology Innovation Talents in Universities of Henan Province (No. 17HASTIT005) and the Nanhu Scholars Program for Young Scholars of XYNU.

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Toc graphic


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Graphical abstract


Terminal Molecular Isomer-Effect on Supramolecular Self-Assembly

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