SCIENCE CHINA Chemistry • ARTICLES •

February 2010 Vol.53 No.2: 432–437 doi: 10.1007/s11426-010-0067-2

Preparation of optical active polydiacetylene through gelating and the control of supramolecular chirality DUAN PengFei1, LI YuanGang2 & LIU MingHua1* 1

Beijing National Laboratory for Molecular Science; CAS Key Laboratory of Colloid, Interface, and Chemical Thermodynamics; Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China; 2 College of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an 710054, China Received October 9, 2009; accepted December 7, 2009

Achiral diacetylene 10,12-pentacosadinoic acid (PCDA) and a chiral low-molecular-weight organogelator could form co-gel in organic solvent and it could be polymerized in the presence of Zn(II) ion or in the corresponding xerogel under UV-irradiation. Optically active polydiacetylene (PDA) were subsequently obtained. Supramolecular chirality of PDA could be controlled by the chirality of gelators. Left-handed and right-handed helical fibers were obtained by using L- and D-gelators in xerogels respectively, and CD spectra exhibited mirror-image circular dichroism. The PDA in xerogel exhibited typical blue-to-red transition responsive to the temperature and pH, while the supramolecular chirality of PDA showed a corresponding change. diacetylene, low-molecular-weight organogel, induced chirality, supramolecular chirality

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Introduction

The fabrication of optically active polymer is one of the important topics in polymer and materials sciences [1–3]. All the natural polymers such as proteins and DNA have precise control on their chirality. In order to fabricate the chiral polymers, the general way is by using the chiral monomer unit or polymerized in certain chiral catalyst. Recently, the supramolecular control on the conformation of polymers by non-covalent bond proved to be an alternative way to obtain the optically active polymers [4–7]. Polydiacetylene (PDA), as one of the most investigated polymers, exhibited many potential applications as biosensors, pathogenic agents, and functional materials [8–11]. It is well known that diacetylene undergoes topochemical reactions upon UV-irradiation by 1,4-addition reactions when the monomer units are aligned appropriately [12, 13]. Strict steric conditions in the arrangement of diacetylene monomers are usually required in such topochemical reactions, *Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2010

which are largely depended on the substituent group in the monomeric diacetylene [14–20]. In order to obtain the chiral PDA, a chiral unit is generally attached to the end group. Since the synthesis of the diacetylene derivatives is difficult, so far, there are a few studies of chiral PDA reported, especially fabricating chiral PDAs from achiral diacetylenes [21–23]. We have been interested in creating optically active material through supramolecular self-assembly and showed that both achiral and chiral building block can be organized into chiral assemblies through certain assembly way [24–26]. In this paper we report a new approach to fabricate optically active helical PDA fibers through gelation and the control on the chirality. We show that through gel formation of an achiral diacetylene PCDA with a chiral organogeltor, the PCDA can be polymerized into the optically active polymers by UV irradiating, and helical structures were obtained. We have found that although PCDA cannot form organogel itself, when it was mixed with chiral LMWGs: N,N-9-bis (octadecyl)-L-Boc-glutamic diamide (LBG) or N,N-9-bis (octadecyl)-D-Boc-glutamic diamide (DBG), it could form the organogels. After polymerization chem.scichina.com

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in organogels or xerogels, induced circular dichroism of the PDA was obtained. It revealed that the chirality of the gelator was transferred to the achiral PCDA aggregates through the interchain interaction between the alkyl chains of both diacetylene and gelator during the gelation, and the supramolecular chirality of PDA was expressed after polymerization upon UV-irradiation. Furthermore, the supramolecular chirality of PDA could be controlled by the chirality of gelator.

The FESEM was performed using a Hitachi S-4300 system and TEM images were obtained on a JEM-100CXII electron microscope operating at accelerating voltages of 15 kV and 100 kV, respectively. In order to obtain SEM figures of the xerogel, the same procedure of preparation of co-xerogel was performed on a hydrophobic silicon slide. Then, the solvent was removed in vacuum.

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In our previous work, we have discovered that the gelator was capable of gelling a broad array of organic solvents ranging from highly hydrophilic DMSO (0.2%wt) to highly hydrophobic toluene (0.5%wt) at low concentrations. The low critical gelation concentration and the large range of the solvents being gelled indicated that the compounds could serve as super organogelators. Furthermore, they could form co-gels with some other functional compounds such as porphyrins, although the selected porphyrins themselves could not form organogels. In the current work, we selected PCDA as the dopant, and examine the photoreaction of PCDA in gels and xerogels. The co-gels of PCDA and gelators showed slightly dissimilar properties in different solvents. In some solvents such as DMSO, toluene, benzene and 2-pyrrolidone, the transparent co-gels were obtained, while in other solvents opaque or translucent co-gels were given, as shown in Figure 1. But the photo-polymerization reaction could not happen, neither transparent co-gel nor opaque one. This indicated that in the organogels, PCDA monomers did not packing well enough to facilitate the steric requirement of PDA. However, by adding Zn(Ac)2 into the system, the formed organogels can be photo-polymerized.

Materials and method

The LBG (DBG) was synthesized by the amidation of Boc-L(D)-glutamic acid with octadecylamine. 10,12-Pentacosadinoic acid was purchased from Lancaster Co. (U.K.) and used without further purification. Zinc acetate was analytical reagent and used as received. All of the other chemical reagents and solvents were used without further purification. PCDA itself could not form organogel. In order to form the organogel, PCDA was firstly dissolved in chloroform solution. 1 mL of a PCDA chloroform solution (1.8 mg/mL) was mixed with 5.0 mg gelator in a test tube and then heated until the dissolution of the solid. The solution was cooled to room temperature and a homogeneous gel was formed. In order to form the co-gel of the zinc salt of PCDA with the gelator, a 1:1 mixture of PCDA in chloroform and zinc acetate in methanol was mixed first for one hour and the solvent was then evaporated under vacuum, and white powder was obtained. Gelator (5 mg) was added into the test tube and the mixture was dissolved with DMSO under heating until the dissolution of both Zn-PCDA and the gelator. Upon cooling to room temperature, transparent gel was formed. The photo-polymerization of the gel was performed by irradiation with a UV lamp (= 254 nm 25 W) at a distance of 25 cm from the cell and the progress of the reaction was monitored by the UV-Vis spectra at different time intervals. In order to investigate the photo-polymerization in the xerogels, the organogel was cast on the quartz slide and their photo-polymerization was also performed by the UV-irradiation under the same condition as in the organogel in the UV-cell. 2.2

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Results and discussion Gel formation with LBG or DBG

Photo-polymerization in the gels

We added 1:1 Zn(Ac)2 in the PCDA and DMSO. The solution formed a translucent organogel. Upon irradiation with a

Apparatus and measurements

CD and UV-Vis spectra were obtained using a JASCO J-810 CD and a JASCO UV-550 spectrophotometer, respectively. In the process of CD spectral measurement of the xerogel, the quartz slide was placed perpendicular to the light path and rotated within the plane of the slide wall to avoid polarization-dependent reflections and eliminate the possible angle dependence of the CD signals.

Figure 1 The co-gels formed by LBG and PCDA in polar solvent (methanol and DMSO) and apolar solvents (toluene and hexane).

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UV254 nm light, the transparent gels became blue. Figure 2 shows the UV-Vis spectral changes of the organogel of LBG and DMSO in the presence of Zn(Ac)2. Maximum absorptions appeared at 647 nm, with two shoulder peaks at 592 nm and 681 nm, which indicated that PCDA was polymerized into blue phase. This suggested that the addition of Zn(II) ion in the system caused the packing of PCDA to favor the photopolymerization. FTIR spectra of PCDA before and after adding Zn(Ac)2 clearly show the coordination between Zn(II) ion and PCDA, as shown in Figure 2(c). The coordination may pull the diacetylene monomer closely which would favor the topochemical reaction. 3.2.1 Photo-polymerization in the xerogel Although in the gel state, the packing of PCDA did not favor the photo-polymerization, however, the xerogel from the gel showed the polymerization ability. Uniform xerogel was obtained by the method of experimental section, and was irradiated at 254 nm with a 25 W UV lamp for 1 min at room temperature, giving the material a dark blue appearance. The rapid polymerization indicated a highly ordered assembly and the good alignment of PCDA units. The dark blue was due to the formation of polymers that had strong absorption bands between 600 and 700 nm. Figure 3 shows the UV-Vis spectra of poly-PCDAs in LBG xerogel at various UV254 nm irradiation time. At early irradiation time, maximum absorption appeared at 682 nm with a shoulder peak at 621 nm. Along with the increase of irradiation time, the intensity of poly-PCDA was increased, and the maximum absorption was blue-shifted to 657 nm with a shoulder peak at 599 nm. The blue-shift

Figure 2 (a) UV-Vis spectra of poly-PCDA-Zn(OAc)2 and LBG in DMSO co-gel irradiated by UV254 nm at various time; (b) photography of the co-gel after UV254 nm irradiating for 28 min; (c) FTIR spectra of PCDA before (dash line) and after (solid line) adding Zn(Ac)2.

Figure 3 UV-Vis spectra of poly-PCDAs in LBG xerogel under UV254 nm irradiation at various times.

might be due to the further adjusting of the poly-PCDAs side chains and the arrangement of the chains might be more orderly. According to ref. [27], the shoulder peak was ascribed to the chain segments in the amorphous phase and the maximum peak to the ones in the crystalline phase. The relative intensity of the two peaks depends on the preparation conditions, i.e., on the degree of crystallinity. 3.2.2 Micro- and nanoscopic characterizations: morphologies of xerogels before and after photoreaction All the transparent toluene gels were quite stable at room temperature for several months. The morphology of the gel network was inspected through SEM, where all xerogels displayed a typical entangled nano- or sub-micrometer fibrous network of varying thickness. Figures 4(a) and 4(b) show the xerogels morphologies of PCDA and gelator from toluene. In the system of co-gel, the ratio of PCDA/gelator was low (1.8 mg/5 mg), that is to say, PCDA was on a kind of doping state, and it might tangle with the long alkyl chains of gelator to form gel network.

Figure 4 SEM images of the xerogels of (a) PCDA-LBG, (b) PCDADBG before UV irradiation and TEM images of the xerogels of (c) PCDALBG, (d) PCDA-DBG before UV irradiation.

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Figure 5 SEM images of the xerogels of (a) PCDA-LBG, (b) PCDADBG after UV-irradiation and TEM images of the xerogels of (c) PCDALBG, (d) PCDA-DBG after UV-irradiation.

The TEM pictures confirmed the above results, and even clearer fibers structures could be seen from the TEM image (Figures 4(c) and 4(d)). Both the SEM and TEM observations confirmed that the PCDA and gelator molecules self-assembled to form nano- or sub-micrometer fibers. The morphologies of the xerogels changed a lot after UV irradiation. Those nano- or sub-micrometer fibers further self-assembled into helical bundle. As seen in Figures 5, left-handed helical bundles were obtained in PPL (polyPCDAs in LBG ) (SEM Figure 5(a), TEM Figure 5(c)), while right-handed helical bundles were produced in PPD (poly-PCDAs in DBG) (SEM Figure 5(b), TEM Figure 5(d)). Furthermore, large number of slim fibers and ribbons could be seen under these helical bundles, which might due to the gelators and unpolymerized PCDA aggregates. TEM pictures gave a more clear vision. Those PCDA molecules that met the requirement of topochemical reactions would polymerize upon UV irradiation and twist to form helical structure while others not. That is why there existed large number of slim fibers and ribbons under helical bundles. 3.3

Supramolecular chirality of PDA

When the co-gel of PCDA-Zn(OAc)2 and LBG was subjected to CD measurements, it was interesting to find that the co-gel displayed a negative exciton-type Cotton effect (CE) at 644 and 690 nm with a crossover at 664 nm, as shown in Figure 6(a). Another positive CD signal appeared at 567 nm and was blue-shifted to 511 nm with the increase of UV irradiation time. The diverse CD signals were assignable to -* transition of the polymer main chains which consisted of alternating double and triple bonds (=CR1–C≡C–CR2=). Obviously, multiform electronic transition exists in the polymer main chains. Due to the achiral PCDA molecules, the poly-PCDAs should be achiral too. Therefore, the supramolecular chirality of poly-PCDAs was

Figure 6 (a) CD spectra of poly-PCDA-Zn(OAc)2 coordination compound and LBG in DMSO co-gel irradiated by UV254 nm at various time; (b) PCDA-LBG xerogel (PPL) after UV254 nm irradiation for 30 min (solid line), PCDA-DBG (PPD)xerogel after UV254 nm irradiation for 30 min (dash line).

a kind of induced chirality, which was transferred from the chirality of gelators through the interchain interaction between the alkyl chains. This induced chirality could be controlled by the chirality of gelators. Here, the organogel constructed a chiral micro-environment which offered the possibility to induce supramolecular chirality of poly-PCDAs. That’s to say, the organogel afford the chiral template to create a kind of supramolecular aggregates which expressed the induced supramoleclar chirality after photo-polymerization. The CD spectra of xerogels were measured following the procedure of experimental section, as shown in Figure 6(b). The PPL xerogel showed a positive exciton-coupling-type CD spectrum with a crossover at 618 nm and a negative Cotton effect at a maximum of 530 nm which were due to multiform electronic transition in the polymer main chains. The CD spectrum of PPD xerogel was completely opposite to the PPL system. These data indicated that packing and conformation of the polymer main chains were twisted in the case of chiral gelators and the induced chirality of poly-PCDAs was really formed in the xerogel. Interestingly,

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the CD spectrum of PPL in xerogel was completely opposite to the poly-PCDA-Zn(OAc)2, which might be due to the change of the environment. According to references, the formation of optical activity of conjugated polymers and oligomers follow two different mechanisms [28, 29]. One is that conjugated polymer chains preferentially adopt either a left-handed or right-handed screw with chiral induction, due to some non-covalent interactions. The other is that chiral species induce either a predominantly left-handed or a predominantly right-handed helical packing of molecules into a chiral superstructures. The appropriate mechanism for the current work is that the chiral gelators induced the PCDA molecules to form potential helical aggregates before UV irradiation, and the potential helical PCDA aggregates would polymerized upon UV irradiation to form helical fibers. These helical conjugated chains further self-assembled to form micrometers helical bundles twisted with gelator, as illustrated in Figure 7. 3.4

Thermo- and pH responsive property

As is commonly known that PDA conjugated backbone is very sensitive to environment and two kinds of color (blue and red) can appear depending subtly on the external environment such as pH, temperature, mechanical stress, and solvent [30, 31]. Our chiral polymer showed a positive response to the pH and temperatures not only in color but also in the supramolecular chirality. When the blue xerogels were heated to ca. 90 °C, a sharp blue-to-red color change was observed. The UV-Vis absorbance spectra of the red forms of the xerogels are shown in Figure 8B. A maximum absorption appears at 544 nm with a shoulder peak at 500 nm. As seen in Figure 8(A), the Cotton effects also shifted to the red band, appearing at 544 and 462 nm, with a cross over at 493 nm and fits well with the absorptions in the UV-Vis spectra. Furthermore, the CD spectra of PPL and PPD are still mirror images in the

Figure 8 (A) CD spectra and (B) UV-Vis spectra of (a) PCDA-LBG xerogel, (b) PCDA-DBG xerogel after heating to 90 °C. (C) CD spectra and (D) UV-Vis spectra of (a) PCDA-LBG xerogel, (b) PCDA-DBG xerogel after dipping into NaOH ethanol solution (0.1 mmol/L).

red forms, which indicate that although the film is transferred from blue to red with some structural changes, the supramolecular chirality of PDAs is preserved by appearing in another band. Because of the existence of a carboxylate group in the PCDA molecule, the pH elevation might lead to the typical blue-to-red transition of the conjugated PDAs polymer. Put the blue xerogels into a NaOH ethanol solution (0.1 mmol/L), the color turned to red immediately, and the absorptions blue-shifted to 544 nm with a shoulder peak at 500 nm (Figure 8(D)), which was similar with the condition of thermochromism. Thus, the supramolecular chirality of pDAs was also preserved. The CD spectras was still mirror images and have the similar shape with the condition of thermochromism (Figure 8(C)).

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Figure 7 Possible mechanism of the formation of induced circular dichromism (ICD) of helical PDA by the gelation with LBG. The chirality was transferred from gelator molecules to PCDA aggregates through interchain interaction between the alkyl chains and expressed after photopolymerization.

Conclusion

In summary, we have prepared optically active PDAs from achiral diacetylene through gelation method. Upon mixing with a chiral gelator, PCDA formed organogels. In the presence of Zn(II) ion, which is a coordination agent to PCDA, PCDA can be polymerized into PDA in organogel.

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In the absence of the Zn(II), although the organogel could not be polymerized, they can be polymerized in the xerogels to form PDA. Although achiral PCDA was used, the formed composite PDA showed supramolecular chirality and helical structures. Chirality was transcribed from the gelators to PDA, leading to the left- and right-handed helical fibers. Furthermore, during the structural changes responding to the pH and temperature, the supramolecular chirality of PDAs shows corresponding changes without disappearance.

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17 This work was supported by the National Natural Science Foundation of China (Grant Nos. 50673095 and 20533050) and 973 Project (Grant No. 2007CB808005). 1

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