Many chiral compounds exist in nature,such as natural amino acids,sugars,DNA,and proteins. The preparation and application of chiral materials is a major topic in the fields of optical devices, pharmacy,and enantioseparation. In recent decades,many efforts have been devoted to studying chiral and helical silica nanomaterials,which are usually synthesized using a supramolecular templating approach [1, 2]. Varieties of silicas such as helical nanowires,helical nanotubes,inner helical nanotubes,and hybrid helical bundles have been obtained using the self-assemblies of surfactants,lipids,gelators,and thickeners as the templates [3, 4, 5, 6]. Recently,many helical polymers,such as polypyrrole [7, 8], polythiophene ,polyaniline ,and polysilsesquioxanes [11, 12, 13],prepared using this method have been reported.
As important organopolymers,phenolic resins have been widely used in industrial production owing to their thermostability,mechanical stability,and acid resistivity. Mesoporous phenolic resins have been synthesized by an organic-organic self-assembly approach using the amphiphilic triblock copolymers as the structure-directing agent and phenol-formaldehyde based resins as the precursors [14, 15, 16, 17, 18]. After carbonization of phenolic resins, mesoporous carbon with the characteristics of high specific surface area and porosity were obtained . Hence,a curious question arises: can chiral phenolic resins be created by the supramolecular templating approach,which may find promising applications in chiral catalysis and enantiomeric drug separation?
The answer is ‘‘yes’’. Due to the fact that the phenolicformaldehyde precursors exhibit structural similarities to silanes ,we have synthesized chiral helical phenolic resin nanofibers, and our strategy is similar to that of preparing chiral silica nanostructures. In this work,a pair of chiral low molecular weight gelator (LMWG) enantiomers derived from valine was synthesized. Gels are formed through an organic-organic self-assembly process using the LMWGs and 2,4-dihydroxybenzoic acid. After the polycondensation of 2,4-dihydroxybenzoic acid and formaldehyde followed by a thermosetting treatment,single-handed helical phenolic resins were successfully obtained. These phenolic resins exhibited chirality at an Ångstrom scale and helicity at nanoscale. 2. Experimental
Materials: 2,4-Dihydroxybenzoic acid and formaldehyde (37-40 wt%) were purchased from Aldrich. Concentrated aqueous ammonium hydroxide (25-28 wt%) and ethanol were purchased from Sinopharm Chemical Reagent Co.,Ltd. The synthesis and characterization of the LMWGs,L-,and D-18Val8PyBr have been reported previously .
General methods: Transmission electron microscopy (TEM) images were obtained using a Tecnai G220 instrument operating at 200 kV. Field-emission scanning electron microscopy (FE-SEM) was performed using a Hitachi 4800 instrument at 3.0 kV. Diffused reflection circular dichroism (DRCD) and diffused reflection ultraviolet visible absorption (DRUV-vis) spectra were obtained using a JASCO 815 spectrophotometer.
Synthetic procedure for the helical phenolic resin nanofibers: L-18Val8PyBr (20 mg,0.031 mmol) and 2,4-dihydroxybenzoic acid (20 mg,0.13 mmol) were dissolved in deionized water (2.0 mL) at 80 ℃. Concentrated aqueous ammonium hydroxide (20 mL, 25-28 wt%) and formaldehyde (12 mL) were added to the solution under stirring. The reaction mixture was kept at 80 ℃ for 24 h. After that,the as-prepared product was collected by filtration and extracted with ethanol for 48 h,then dried in an oven at 50 ℃ for 24 h. The final obtained phenolic resins were yellow brown powder. 3. Results and discussion
Because the LMWGs can self-assemble into chiral aggregates ,they can be used as templates for the preparation of chiral nanomaterials. Herein,the gelators self-assembled into nanofibers, and left- and right-handed helical phenolic resin nanofibers were obtained using L- and D-18Val8PyBr,respectively. The FE-SEM and TEM images of the products are shown in Fig. 1. These nanofibers are about 75-150 nm in width and their helical pitches are about 500-1000 nm. The FE-SEM images indicated that these nanofibers were composed of many ultrafine nanofibers. However,no mesopores were identified in the TEM images. It was proposed that the phenolic resin framework was not highly cross-linked and was not stable. This might be due to a lack of 3D interconnecting sites in the precursors (2,4-dihydroxybenzoic acid),and we obtained linear novolac resins . It shrank after removing the templates.
|Fig. 1. FESEM (a and c) and TEM (b and d) images of the single-handed helical phenolic resin nanofibers prepared using L-18Val8PyBr (a and b) and D-18Val8PyBr (c and d)|
The template L-18Val8PyBr and the left-handed helical phenolic resin nanofibers were characterized using FT-IR spectroscopy. As shown in Fig. 2,the spectrum of L-18Val8PyBr shows a broad band at 3440 cm-1 arising from the stretching vibration of N-H of the non-hydrogen-bonded amide groups. A band at 3290 cm-1 and a strong band at 1550 cm-1 arise from the stretching vibration and bending vibration of N-H of the hydrogen-bonded amide groups, respectively. These two bands are not found in the spectrum of phenolic resin nanofibers,indicating that the templates were removed absolutely from the final nanofibers. The spectrum of phenolic resin nanofibers shows a broad band at 3416 cm-1 arising from the vibrational stretching of the OH groups,which is coincident with the stretching vibration of N-H in the spectrum of the template. The bands at 2856-2960 cm-1 are related to the alkyl C-H stretching in the CH2unit,while the bands at 1620 cm-1 and 1462 cm-1 are the characteristic absorptions of an aromatic ring. The IR results indicate that the phenolic resin nanofibers were synthesized successfully.
|Fig. 2. FT-IR spectra of L-18Val8PyBr and the as-made phenolic resin nanofibers.|
The DRUV-vis and DRCD spectra of the single-handed helical phenolic resin nanofibers are shown in Fig. 3. The DRUV-vis spectrum shows broad absorption bands in the range of 200-700 nm. For the left-handed helical nanofibers,the DRCD spectrum shows two negative signals at 330 and 268 nm. On the contrary,the right-handed helical fibers exhibit two positive signals at 335 and 272 nm. The first DRCD signal is negative for the left-handed helical nanofibers,indicating that the aromatic rings in the phenolic resin stack in a left-handed manner. Meanwhile,the aromatic rings in the right-handed helical phenolic resins stack in a right-handed manner. Therefore,both the A˚ ngstrom scale chirality and nanoscale helicity of the organic self-assemblies have successfully transferred to the phenolic resin nanofibers. The chiral transfer should be due to the hydrogen bonds between the amide and phenolic resin,and the helical transfer should be due to the morphological transcription.
|Fig. 3. DRUV-vis and DRCD spectra of the left-handed and right-handed helical phenolic resins.|
For a better understanding of the formation process,the FE-SEM (Fig. 4a-e) and TEM (Fig. 4f) images of the reaction mixture were taken at different reaction time. First,the gelator L-18Val8PyBr self-assembled into fibrous aggregates through non-covalent interactions; nanofibers with lengths of tens of microns were shown in Fig. 4a. After adding NH3·H2O,the fibers twisted and aggregated closely (Fig. 4b and c). Some nanofibers exhibited lefthandedness. This might be because the carboxylate radical with negative charge of the 2,4-dihydroxybenzoic acid adsorbed onto the surfaces of the assemblies through electrostatic force and coassembled with the templates. Meanwhile,these nanofibers were shorter than those gel fibers shown in Fig. 4a,which might be due to the stirring. After the addition of formaldehyde,left-handed helical nanofibers were shown in Fig. 4d-f. In this process,the formaldehyde molecules adsorbed onto the surfaces of nanofibers, and polymerized with 2,4-dihydroxybenzoic acid. Along with thermosetting for a certain time,the crosslinking degree of phenolic resins increased. Finally,after removing the templates, single-handed helical phenolic resins were obtained. The helical nanostructure of the organic self-assemblies was transferred to the phenolic resins.
|Fig. 4. FE-SEM (a-e) and TEM (f) images of the reaction mixture after different reaction steps. (a) Before adding NH3·H2O,(b) 30 s after adding NH3·H2O,(c) 2.0 min after adding NH3·H2O,(d) 5.0 min after adding formaldehyde,(e) 1.0 h after adding formaldehyde and (f) TEM image of the obtained helical phenolic resin fibers without removing templates.|
In summary,single-handed helical phenolic resin nanofibers were prepared using a pair of chiral low-molecular-weight amphiphiles as templates. These nanofibers exhibited chirality at both the nano and the Ångstrom level. As far as we know,this is the first report on the preparation of chiral phenolic resins. This chiral helical polymer can potentially be applied in the fields of adsorption,separation,and catalysis. Further,the chiral phenolic resins might be used as precursors to prepared chiral helical carbonaceous nanostructures.
Acknowledgments This work was supported by the Natural Science Foundation of Jiangsu Province (No. BK2011354),the Priority Academic Program Development of Jiangsu High Education Institutions (PAPD,No. YX10900114),and the National Natural Science Foundation of China (No. 21104053).(PAPD,No. YX10900114),
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