Influence of CâHÂ·Â·Â·O Hydrogen Bonds on Macroscopic Properties of

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The influence of C-H…O hydrogen bonds on macroscopic properties of supramolecular assembly Wei Ji, Guofeng Liu, Zijian Li, and Chuanliang Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00580 • Publication Date (Web): 04 Feb 2016 Downloaded from http://pubs.acs.org on February 16, 2016

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ACS Applied Materials & Interfaces

The influence of C−H…O hydrogen bonds on macroscopic properties of supramolecular assembly Wei Ji,[a] Guofeng Liu,[a] Zijian Li,[b] and Chuanliang Feng*[a] [a]

State Key Lab of Metal Matrix Composites, School of Materials Science and Engineering, and

[b]

School of Chemistry and Chemical Technology, Shanghai Jiao Tong University 800

Dongchuan Road, Shanghai, 200240, China Corresponding author: Fax: +86 2154747651; E-mail: [email protected] Keywords: CH…O hydrogen bonds, Coumarin, Self-assembly, Hydrogels, Nanofibers

Abstract: For CH…O hydrogen bonds in assembled structures and the applications, one of the critical issues is how molecular spatial-structures affect their interaction modes, as well as how to translate the different modes into the macroscopic properties of materials. Herein, coumarinderived isomeric hydrogelators with different spatial structures are synthesized, where only nitrogen atoms locate at ortho-, meso-, or para-position in pyridine ring, respectively. The gelators can self-assemble into single crystals and nanofibrous networks through CH…O interactions, which are greatly influenced by nitrogen spatial-positions in pyridine ring, leading to the different self-assembly mechanism, packing modes, and properties of the nanofibrous networks. Typically, different cell proliferation rates are obtained on the different CH…O bonds driving nanofibrous structures, implying that tiny variation of stereo-position of nitrogen atoms can be sensitively detected by cells. The study paves a novel way to investigate the influence of isomeric molecular assembly on macroscopic properties and functions of materials.

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1. Introduction CH…O interaction is one of the weakest hydrogen bonds (0.3-0.8 kcal/mol)1-4 and usually “softer” than classical hydrogen bonds (3-7 kcal/mol), such as NH…O, NH…N, OH…N hydrogen bonds.5-13 However, CH…O hydrogen bonds often play very important roles in the fields of structural chemistry and biology,14-19 which have been successfully proved by designing different types of CH…O hydrogen bonds and the related research. So far, the research mainly focus on designing different types of CH…O hydrogen bonds and their application in protein recognition and a lot of questions remaining to be answered.20-22 Among these, how stereo-position of atoms (e.g. nitrogen) inside molecules to regulate CH…O interaction in self-assembled aggregates is especially important, since it is closely related with overall macroscopic properties and functions of the aggregates,23 which still is an unexplored research area so far. Usually, to have a CH…O hydrogen bond based aggregate, the molecule with well-designed spatial-structures is very necessarily needed. Thanks to the designable merit of chemical composition and structures of supramolecular gelators,24-41 self-assembled nanofibrous aggregates can be prepared by non-covalent interactions between gelators (e.g. hydrogen bonds42-45). Thus, with properly designing gelators structures, it should be feasible to construct the assembly driven by CH…O hydrogen bonds. Based on this, 7-substituted coumarin-derived gelators have been recently designed, which can self-assemble into biomimetic nanofibers just through CH…O hydrogen bonds as determined by Single crystal X-ray diffraction (SC-XRD).46 However, single crystals of the gelators are so difficult to be cultured if molecular spatialstructures are tuned, since they have strong tendency to form nanofibers. Hence, the influence of detailed CH…O interaction mechanism between isomeric gelators was difficult to be obtained by


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SC-XRD, which leaves behind the study of how molecular spatial-structures regulate CH…O hydrogen bonds and further designate the corresponding self-assembly.

Scheme 1. Schematic demonstration of the formation of nanofibrous networks self-assembled from hydrogelators G1, G2, and G3 through CH…O hydrogen bonds. The spatial arrangement of pyridine rings can be judged by O1 stereo-position, which faces to outside, down, and inside if nitrogen atoms locate at ortho-, meso-, and para-position in pyridine ring, respectively. Inspired from this, herein, three 6-substituted coumarin-derived hydrogelators with easily cultured single crystals (G1, G2, G3) are designed and synthesized in order to deeply explore CH…O interactions mechanism regulated by molecular spatial-structures. The spatial structures are controlled by locating nitrogen atoms at ortho-, meso-, or para-position in pyridine ring, respectively (Scheme 1). SC-XRD analysis proves that the self-assembled crystal networks are stabilized by C−H…O hydrogen bonds and π-π stacking interactions, but the packing modes are greatly influenced by the spatial-position of nitrogen atoms in isomeric gelators G1-G3. Powder


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X-ray diffraction (PXRD) suggests that their assembled nanofibrous structures have the same CH…O interaction as the corresponding single crystals, which are usually different in two states for other gelators.47 The packing modes ensure to not only understand the influence mechanism of atomic stereo-position on CH…O hydrogen bonds, but also be possible to explore their critical roles on properties and functions of (bio)nanofibrous structures, such as absorption, emission, and mechanics properties. Typically, different cell proliferation rates are obtained on the CH…O hydrogen bonds driven nanofibrous structures, implying that tiny variation of stereo-position of nitrogen atoms can be transferred into macroscopic properties of materials.

2. Experimental Section 2.1 Materials Chemical reagents and solvents were purchased from Aladdin and used without further purification. 1H NMR and

13

C NMR were obtained on a Bruker Advance III 400 Instrument

operating at 400 MHz. HRMS were recorded on a Water Q-Tof Mass Instrument. 2.2 Synthesis and Characterizations of hydrogelators G1-G3 G1: To a solution of 6-hydroxy-4-methylcoumarin (1.76 g, 10 mmol), 2-Picolinic Acid (1.23 g, 10 mmol) and DMAP (60 mg, 0.5 mmol) in CH2Cl2 (20ml) was added EDCI (2.10 g, 11 mmol) (Scheme 2). The mixture was stirred for 2 hours at room temperature. The resulting solution was washed three times with water (5 ml), saturated NaHCO3 solution (5 ml). Then the organic layer was collected and dried over anhydrous Na2SO4. The solvent was removed under vacuum to give the product G1 as white powder (2.50 g, 88.96%). 1H NMR (400 MHz, CDCl3) δ = 8.84(dd, J1 = 4.8Hz J2 = 0.8Hz, 1H), 8.27(dd,


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J1 = 8.0Hz J2 = 0.8Hz, 1H), 7.93(td, J1 = 7.6Hz J2 = 1.2Hz, 1H), 7.58(m, 1H), 7.50(d, J = 2.4Hz, 1H), 7.43(dd, J1 = 8.8Hz J2 = 2.0Hz, 1H), 7.38(d, J = 8.8Hz, 1H), 6.32(s, 1H), 2.40(s, 3H). 13C NMR (400 MHz, CDCl3) δ = 163.9, 160.4, 151.8, 151.3, 150.2, 147.0, 146.9, 137.4, 127.8, 126.1, 125.4, 120.8, 118.2, 117.4, 115.9, 18.7. HRMS (ESI) calcd for C16H11NO4 [M+H]+ 282.0766; found 282.0739.

Scheme 2. Synthetic route to G1 in one-step reaction. G2-G3 were prepared using the same procedure for G1. G2: 6-Hydroxy-4-methylcoumarin (1.76 g, 10 mmol), Nicotinic acid (1.23 g, 10 mmol), EDCI (1.23 g, 10mmol) and DMAP (60 mg, 0.5 mmol) in CH2Cl2 (20ml) yielded G2 as white powder (2.43 g, 86.47%). 1H NMR (400 MHz, CDCl3) δ = 9.40(d, J = 2.0Hz, 1H), 8.88(dd, J1 = 4.8Hz J2 = 1.6Hz, 1H), 8.45(dt, J1 = 8.0Hz J2 = 1.6Hz, 1H), 7.50(m, 1H), 7.47(t, J = 1.6Hz, 1H), 7.40(d, J = 2.4Hz, 2H), 6.35(d, J = 1.2Hz, 1H), 2.43(d, J = 1.2Hz, 3H). 13C NMR (400 MHz, CDCl3) δ = 164.0, 160.3, 154.3, 151.6, 151.4, 151.3, 146.5, 137.8, 125.3, 125.2, 123.7, 120.8, 118.3, 117.3, 116.0, 18.7. HRMS (ESI) calcd for C16H11NO4 [M+H]+ 282.0766; found 282.0757. G3: 6-Hydroxy-4-methylcoumarin (1.76 g, 10 mmol), Isonicotinic acid (1.23 g, 10 mmol), EDCI (1.23 g, 10mmol) and DMAP (60 mg, 0.5 mmol) in CH2Cl2 (20ml) yielded G3 as white powder (2.35 g, 83.62%). 1H NMR (400 MHz, CDCl3) δ = 8.88(dd, J1 = 4.4Hz J2 = 1.6Hz, 2H), 8.01(dd, J1 = 4.4Hz J2 = 1.6Hz, 2H), 7.47(d, J = 2.4Hz, 1H), 7.40(m, 2H), 6.36(d, J = 1.2Hz, 1H), 2.44(d, J = 1.2Hz, 3H). 13C NMR (400 MHz, CDCl3) δ = 163.9,


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160.3, 151.6, 151.4, 151.0, 146.4, 136.3, 125.1, 123.2, 120.8, 118.2, 117.2, 116.1, 18.7. HRMS (ESI) calcd for C16H11NO4 [M+H]+ 282.0766; found 282.0758. 2.3 Hydrogels characterization 2.3.1 Gel test for G1-G3: 10 mg powder of G1, G2, or G3 were dissolved in 100 µL DMSO with gentle heating, respectively. 1 mL deionized water was slowly added into above solution. A stable colourless opaque gel was formed after several minutes and they could be turned upside down safely. Gel formation was judged by the “invert-vial” method. 2.3.2 Scanning electron microscopic (SEM): Samples were prepared by depositing dilute solutions (approximately 0.3 mg/ml) of G1-G3 on silicon wafer, freeze dried overnight, and sprayed with a thin gold layer. SEM images were taken on a FEI QUANTA 250 microscope. 2.3.3 Transmission electron microscope (TEM): Samples were prepared by placing three drops of dilute solution (approximately 0.3 mg/ml) of G1-G3 onto the copper omentum and dried overnight in the air. TEM images were taken on an analytical transmission electron microscope (JEM-2010). 2.3.4 Fluorescence microscope: Samples were prepared by placing G1, G2, or G3 hydrogels on a glass slide, washed with deionized water for three times, and then freeze dried overnight. Fluorescence images were taken on a Olympus IX73 microscope. 2.3.5 Rheology measurements: The rheological properties of hydrogels G1-G3 were measured with a Rotary Rheometer (Gemini HRnano). The dynamic strain sweep measurements were performed using a constant frequency (1 rads-1) over a range of sinusoidal shear strain (0.01%-100%) at 25 ℃ . The dynamic frequency sweep


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measurements were performed using a sinusoidal shear strain of constant peak amplitude (0.01%) over a range of frequencies (0.1-100 rads-1) at 25℃. 2.4 X-ray diffraction (XRD) 2.4.1 Preparation of single crystals G1-G3: Crystals suitable for X-ray diffraction was obtained

by

slow

evaporation

in

n-hexane/tetrahydrofuran

(v/v

1:1),

n-

hexane/dichloromethane (v/v 2:1), and n-hexane/tetra-hydrofuran (v/v 4:1) solution at room temperature for G1, G2, and G3, respectively. 2.4.2 Single crystal X-ray diffraction (SC-XRD): Single-crystal data were collected on a Bruker SMART Apex II CCD-based X-ray diffractometer with Mo-Kαradiation (λ = 0.71073 Å) at 293 K. The empirical absorption correction was applied by SADABS program (G. M. Sheldrick, SADABS, program for empirical absorption correction of area detector data; University of Göttingen, Göttingen, Germany, 1996). The structure was solved using direct method, and refined by full-matrix least-squares on F2 (G. M. Sheldrick, SHELXTL97, program for crystal structure refinement, University of Göttingen, Germany, 1997). 2.4.3 X-ray powder diffraction (PXRD): Xerogels and crystals of G1-G3 were tested using a D8 Advance instrument from Bruker-AXS Company. 2.5 Spectroscopic properties of nanofibers G1-G3 2.5.1 UV-Vis absorption: The solution (0.3 mg/ml) of G1, G2, or G3 was tested using instrument of Lambda 20 from Perkin Elmer, Inc., USA, respectively. 2.5.2 Fluorescence spectra: The solution (0.3 mg/ml) of G1, G2, or G3 was tested by using LS 50B from Perkin Elmer, Inc., USA.


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2.5 Cell viability The G1, G2, or G3 hydrogels were added into a well of 96 well plates, respectively. The plates were dried in a vacuum oven under 37℃. Cells in 200µl culture medium were seeded in a 96 well plate and cultured for 24 h. 20µl of cell counting kit-8 (CCK-8) was added into each well. The plated cells were incubated for 2 h and the absorbance was then determined and the entire process was repeated three times. 3. Results and discussion Hydrogelators G1, G2, and G3 were synthesized in only one-step reaction each from commercially available reagents, by reacting the corresponding 2-, 3- or 4-pyridyl acids with 6hydroxy-4-methylcoumarin through EDCI/DMAP condensation at room temperature in dichloromethane (Scheme 2). Chemical structures and purities of all these new compounds were fully characterised and confirmed by 1H NMR,

13

C NMR and HRMS (Figure S1-S9). The

gelation properties of G1-G3 were tested by “solvent-mediated” and “stable to inversion of a test tube” methods.48 Powder of G1-G3 were initially dissolved at relatively high concentration in dimethyl sulfoxide (DMSO), water was then added to trigger the gelation and give a final DMSO concentration of approximately 10% at room temperature. A stable colourless opaque gel was formed after several minutes, as it could be turned upside down safely (Scheme 1 and Figure 1a, 1c, 1e). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to explore more structural information of hydrogels assembled from hydrogelators G1-G3. The morphologies of xerogels G1-G3 were firstly examined by SEM and they could form well-defined nanofibrous structures (Figure 1g, 1h, 1i). TEM images indicated that the thickness of these nanofibers was about 200 nm (Figure S10)


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and length was in the range of hundred of micrometers (Figure 1), which was in the size scale of natural extracellular matrix (ECM) components (10–500 nm). In addition, the blue emission of hydrogels G1-G3 can be visualized using the naked eye under UV light due to the existence of fluorescent coumarin ring (Figure 1b, 1d, 1f).49-50 The entangled nanofibrous structures of the xerogels with blue emission were further proved by fluorescence optical microscopy (under 360–370 nm excitation) (Figure 1j, 1k, 1l).

Figure 1. Photographs of the hydrogels from (a) G1; (c) G2; (e) G3 before UV irradiation and (b) G1; (d) G2; (f) G3 after UV irradiation (365 nm). SEM images of (g) G1; (h) G2; (i) G3 nanofibers. Scale bar = 10 µm. Fluorescent images of the freeze dried (j) G1; (k) G2; (l) G3 hydrogels. Scale bar = 50 µm. To gain insight into self-assembly mechanism of the hydrogelators and investigate the influence on C−H…O hydrogen bonds in packing mode caused by changing the position of nitrogen atom in pyridine ring, single crystals for G1, G2, and G3 were cultured in specific organic solvents, respectively.51 PXRD was used to detect the packing modes in both


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single crystal and xerogel states. Colorless single crystals of G1 were firstly characterized by X-ray crystallography (Figure 2a, b). Detailed crystallographic data were summarized in Table S1A, S1B, and Table 1. G1 crystal belonged to a monoclinic space group P21/n. No solvent molecules and classic hydrogen bonds were found in the crystal frameworks. There is only one type of intermolecular hydrogen bond (C12−H12…O4) in the molecular array (Figure 2a, b). The bonds came from a hydrogen of coumarin ring and the C=O of lactone with the bond length of 2.58 Å (H12…O4). In addition, C4−H4…O2 could also form intramolecular hydrogen bonds to further increase the stability of self-assembly, which came from a hydrogen of pyridine ring (H4) and C−O of ester bond (O2) (H4…O2: 2.35 Å). The formation of C4−H4…O2 bonds should ascribe to N atom in ortho- position, which could have p-π conjugation with O atom and lead to high electronagtivity for O2, enabling to form C4−H4…O2 intramolecular hydrogen bonds. The largest bond length for N1−C1 (1.35Å) and N1−C5 (1.36Å) compared with those from G2 and G3 further proved the existence of the p-π conjugation in G1 assembly. Pyridine ring only provide a hydrogen atom to form intramolecular hydrogen bonds instead of intermolecular hydrogen bonds in the process of aggregation. Followed by the frequently occurred C−H…O hydrogen bonds and π-π stacking interactions between the hydrogelators in aqueous solution, they could self-assemble into three dimensional (3D) nanofibrous networks (Figure S11, Table S2). Colorless single crystals of G2 belonged to a triclinic space group P-1 and no classic hydrogen bonds or solvent molecules were found in self-assembly (Table S3A, S3B, and Table 1, for crystallographic details). There were two types of intermolecular C−H…O interactions between adjacent molecules (Figure 2c, d). One intermolecular bond is formed between C=O of ester bonds and a hydrogen from pyridine ring (C2−H2…O1)


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with bond length of 2.44 Å (H2…O1). The formed intermolecular hydrogen bonds should also ascribe to N atom in meso- position, which still had weak p-π conjugation with O atom. It led to relatively high electronagtivity for O2, enabling to form C2−H2…O1 intermolecular hydrogen bonds with neighbor molecules. The larger length of N1−C1 (1.33Å) and N1−C5 (1.34Å) bonds than those in G3 proved the existence of p-π conjugation in G2 fibers. Thus, another intermolecular hydrogen bonds could only be formed between two neighbor coumarin rings, from a hydrogen of coumarin ring and oxygen atom of lactone (C11−H11…O3) with bond length of 2.54 Å (H11…O3). The molecules self-assembled in the way of head to head and tail to tail as shown in Figure S12. Propagation of such hydrogen bonding interactions and π-π stacking interactions leads to the formation of crystal networks. The two intermolecular hydrogen bonds are obviously different as the intermolecular hydrogen bond in G1 crystal (H12…O4).

Figure 2. (a, c, e) Molecular structures of G1, G2, and G3 in unit cell. (b, d, f) C−H…O bonds (pink dashed lines) in the packing mode of G1, G2, and G3 crystal states. (Gray C, black H, red O, blue N).


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Table 1 Geometrical parameters of hydrogen bonds in crystals G1-G3. Hydrogelators

D-H…A

D-H(Å)

H…A(Å)

D…A(Å)

G1

Intra C4-H4…O2

0.93

2.35

2.678

C12-H12…O4

0.93

2.58

3.378

C2-H2…O1

0.97

2.44

3.37

C11-H11…O3

0.95

2.54

3.46

C12-H12…O4

0.93

2.57

3.35

G2 G3

The crystallographic data of G3 were summarized in Table S4A, S4B, and Table 1. The G3 crystal belonged to a monoclinic space group P21/c and there were also no classic hydrogen bonds and solvent molecules in the crystal networks. The G3 crystal structure and packing diagram (Figure 2e, f) suggested that zigzag chains were formed in the process of self-assembly through one type of intermolecular hydrogen bonds (C12−H12…O4), which come from the C=O of lactone and a hydrogen of coumarin ring with bond length of 2.57 Å (H12…O4). Although G3 belongs to the same monoclinic crystal system and the same sources to form intermolecular hydrogen bonds with G1, but different space group indicated their different packing modes in the self-assembly process (Figure S13). Pyridine ring in G3 had no any contribution on the formation of intermolecular or intramolecular hydrogen bonds (Figure 2f). This was attributed to para- position of N atom, which kept the pyridine ring structure exactly symmetric. Thus, the electronagtivity of O2 should had no any changes, which did not favourite the formation of hydrogen bonds with other neighbor molecules. The smallest bond length for N1−C1 (1.32Å) and N1−C5 (1.32 Å) compared with those from G2 and G3 fibers further proved N had very less interaction with O atom from esters. Such weak interaction let pyridine ring almost perpendicular to coumarin rings as show in Figure S13. The changes of spatial arrangement for pyridine ring made it


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impossible

to

form

intermolecular

or

intramolecular hydrogen

bonds.

Thus,

intermolecular hydrogen bonds (C12−H12…O4) from neighbour coumarin rings and π-π stacking interactions became the main driving forces to stabilize the nanofibrous networks and the molecules self-assembled in the way of head to tail.

Figure 3. Power XRD patterns of needle-shaped crystal and xerogel assembled from hydrogelators (a) G1, (b) G2 and (c) G3, respectively. PXRD patterns of two states could be essentially same with each other, implying the same packing modes in both single crystal and xerogel state. Therefore, the location of nitrogen atoms in pyridine ring plays crucial role on the formation of p-π conjugation, which has direct influence on the spatial structures of gelators. If coumarin rings are fixed in plane, the spatial arrangement of pyridine rings can be judged by O1 stereo-position, which face to outside, down, and inside if nitrogen atoms locate at ortho-, meso-, and para-position in pyridine ring, respectively (Figure 2a, c, e). The variation of spatial-structures has obvious influence on the self-assembly mechanism between isomeric gelators G1-G3 as proved by single crystals. Typically, different as other reported hydrogelators, both single crystal and xerogel state for G1, G2 and G3 had the same packing modes as characterized by PXRD (Figure 3). The similar PXRD patterns in two states for G1-G3 were observed, implying the same packing modes


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of self-assembly in both states.52-53 This facilitates to further investigate the influence of CH…O hydrogen bonds and π-π stacking interactions on overall properties of selfassembled structures.

Figure 4. (a) UV-Vis absorption spectra of G1, G2, and G3 nanofibers in clear aqueous solution. (b) Fluorescent emission spectra of G1, G2, and G3 nanofibrous solution under the excitation of 312nm. To explore the influence of isomeric gelators on the properties of the assembled nanofibrous networks, UV-Vis absorption spectra and fluorescent emission spectra of G1, G2, G3 nanofibrous solution were firstly recorded. Two absorption peaks at the wavelength of 270 and 312 nm were observed, which were from the absorption of pyridine and coumarin ring, respectively (Figure 4a). Here, the intensity of G1 nanofibers at the wavelength of 270 nm is the strongest compared with those of G2 and G3 nanofibers. It may be ascribed to the high π-π* electronic transition probability of π-electron on pyridine ring for ortho-substituted pyridine monocarboxylic acid (G1), which is due to the existence of strong p-π conjugation if N atom located in ortho- position in pyridine ring.54 However, if N atom located in meso-- position in pyridine ring, the relative weak p-π conjugation with O atom existed and lower π-π* electronic transition probability of πelectron could be obtained. Therefore, the relative low absorption intensity at the wavelength of


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270 nm could be expected in G2 nanofibers. Finally, it is reasonable to have the lowest absorption intensity in G3 nanofibers due to lack of p-π conjugation in pyridine rings. The maximum emission wavelength for G1, G2, and G3 nanofibers is found at the wavelength of 395 nm. The strongest emission intensity was from G2 assembly (Figure 4b), which should be closely related to the different strength of intermolecular hydrogen bonds and π-π stacking distance between the gelators. It is known that coumarin derivatives could form intramolecular charge transfer (ICT) states upon UV excitation and some of them may transform to a twisted intramolecular charge transfer (TICT) state due to less stability in polar solvent, resulting in the decrease of the fluorescent efficiency for the coumarin.55 Since CH...O bond distances and bond angles (2.44 Å and 160º for C2−H2…O1, 2.54 Å and 164º for C11−H11…O3) in G2 nanofibers are shortest and highest compared with those in G1 (2.58 Å and 144 º for C12−H12…O4) and G3 (2.57 Å and 147 º for C12−H12…O4) nanofibers (Table 1), G2 should form strongest intermolecular hydrogen bonds to stabilize the ICT states and decrease the participation of non-fluorescent TICT states, which led to the strongest fluorescent emission.55-56 In addition, π-π stacking distance between G3 molecules (the minimum Cg-Cg distances is 3.45 Å) were shortest compared with those in G1 (3.64 Å) and G2 (3.65 Å) nanofibers (Table S2), which should have strongest self-quenching effects in self-assembled G3 nanofibrous aggregate, leading to the lowest fluorescence intensity for G3 nanofibers (Figure 4b).57 The mechanical properties of the nanofibrous hydrogels were further investigated by Rheometer. A continuous increase of the storage modulus was observed from G1, G2, to G3 hydrogels in Figure 5 (G1’G1’’). Since two strong intermolecular hydrogen bonds were formed in G2 nanofibers, it should induce more energy loss during deforming the materials (Table 1).58 Although G1 and G3 had the same intermolecular hydrogen bonds interactions in nanofibrous structures (C12−H12…O4), the relatively strong π-π stacking interaction was observed for G3 assembly due to the shorter minimum Cg-Cg distances between ring centroids (Table S2), which could benefit the stabilization of gel networks and G3 hydrogel needed more energy to improve the tendency to flow under stress.


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Figure 5. Rheological measurement of strain sweep at a frequency 1 rads-1 over a range of strain of 0.01%-100%: (a) G1; (b) G2; (c) G3. Rheological measurement of frequency sweep at a strain of 0.01% over a range of 0.1-100 rads-1: (d) G1; (e) G2; (f) G3.

To investigate the influence of stereo-arrangement regulated CH...O hydrogen bonds on cell proliferation, human hepatoma SMMC-7721 cells were cultured on polystyrene (PS), G1, G2, and G3 hydrogels films for 24h, respectively. By cell counting kit-8 (CCK-8) assays, the hydrogels showed good biocompatibility and could promote SMMC-7721 cells proliferation compared with PS films (Figure 6).59 Typically, the different proliferation were observed though G1, G2, and G3 had completely the same chemical composition. The highest proliferation was obtained on G2 hydrogel films. This phenomenon is obviously induced by different properties of the nanofibers (e.g. loss modulus), which are basically determined by self-assembly mechanisms influenced by the stereo-arrangement of nitrogen atom in isomeric gelator structures. Compare with G1 and G3 (G1 had the same intermolecular hydrogen bonds as G3), G2 could form completely different types of intermolecular CH...O hydrogen bonds and π-π stacking interactions. The similar results were also achieved for MC3T3 osteoblastic and EAhy926 human endothelial cells. The result implied that the tiny variation of hydrogelators could influence the property of the ECM structures and then induced some differences on cell behaviour. The further related research is still going on in our group.


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Figure 6. CCK-8 tests of SMMC-7721 cells cultured on polystyrene, G1, G2, and G3 hydrogels films, respectively. The highest proliferation is achieved on G2 hydrogels. *, **, *** data show significant differences (ANOVA: *p≤0.05, **p≤0.005, ***p≤0.001).

4. Conclusions In conclusion, by varying different location of nitrogen atom in pyridine ring, coumarinderived isomeric supramolecular gelators are obtained and their self-assembly can be controlled by different CH…O hydrogen bonds interaction. The study directly proves the critical role of molecular spatial-structure on CH…O hydrogen bonds and their corresponding influence on overall macroscopic properties of nanofibrous structures. For the first time, it gives an exploration of the influence of molecular spatial-structure on CH…O hydrogen bond interaction modes and such difference may be translated into different macroscopic properties for CH…O bonds regulated assembled materials. The study should pave a novel way to investigate the influence of isomeric molecules assembled aggregates on macroscopic properties and functions. It will be very important for deeply understanding CH...O hydrogen bonds involved cell-subtract


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interactions in tissue engineering and further exploring their role in structural chemistry and biology, e.g. activity of biomacromolecules. ASSOCIATED CONTENT Supporting Information: X-ray crystallographic data, experimental procedures, supplemental figures, synthesis and characterization of all new compounds. This material is available free of charge via the Internet at http://pubs.acs.org. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We thank the National Natural Science Foundation of China (51573092, 51273111, 51173105), the National Basic Research Program of China (973 Program 2012CB933803), SJTU-UM Collaborative Research Program, the Program for Professor of Special Appointment (Eastern Scholar) at the Shanghai Institutions of Higher Learning. REFERENCES (1) Desiraju, G. R. The C−H...O Hydrogen Bond in Crystals: What Is It? Acc. Chem. Res. 1991, 24, 290-296. (2) Desiraju, G. R. The CH···O Hydrogen Bond: Structural Implications and Supramolecular Design. Acc. Chem. Res. 1996, 29, 441-449. (3) Desiraju, G. R. C–H···O and Other Weak Hydrogen Bonds. From Crystal Engineering to Virtual Screening. Chem. Commun. 2005, 24, 2995-3001.


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Table of contents

The coumarin-derived isomeric hydrogelators can form single crystals and self-assemble into nanofibrous networks under the regulation of CH…O hydrogen bonds, which are greatly influenced by gelator spatial-structures, leading to different self-assembly mechanism, absorption and emission, mechanical properties for the assembled aggregates. The study directly proves the critical role of molecular spatial-structure on CH…O hydrogen bonds formation and their corresponding influence on overall macroscopic properties of materials.


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Scheme 1 259x150mm (300 x 300 DPI)


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Scheme 2 18x4mm (300 x 300 DPI)



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Figure 1 216x192mm (150 x 150 DPI)


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Figure 2 276x117mm (300 x 300 DPI)



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Figure 3 102x34mm (300 x 300 DPI)


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Figure 4 284x108mm (300 x 300 DPI)



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Figure 5 405x214mm (300 x 300 DPI)


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Figure 6 230x90mm (300 x 300 DPI)


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