Home > Resources > Chiral alkylated poly(m-phenylene)s: Optical activity and thermal stability of helical structures

Chiral alkylated poly(m-phenylene)s: Optical activity and thermal stability of helical structures

Risa Sone, Ichiro Takemura, Kenichi Oyaizu, Hiroyuki Nishide∗ Department of Applied Chemistry, Waseda University, Tokyo 169-8555, Japan

a b s t r a c t

Chiral poly[4,6-bis(alkylthio)-1,3-phenylene-alt-2-methyl-1,3-phenylene] was synthesized from 1,3-dibromo-2,6-bis(3-dodecyl-2-methylthio)benzene and 2-methyl-1,3-phenylenebis(pinacol borate) as aprecursor of chiral poly(thiaheterohelicene). Circular dichroism (CD) spectra that arise from the poly(1,3-phenylene) backbone inverted according to the chirality of the side chains, which indicated that a helical conformation of the polymer was induced by the interaction between the side chains. The CD intensity of the polymer increased in non-polar solvents such as hexane. The decrease in the molar CD intensity and the broadening of a fluorescence band at higher concentrations suggested that the aggregation of the polymer suppressed the formation of the helical structure. The conformational changes were monitored by the CD and the 1H NMR spectra at different temperatures. In a good solvent such as dichloromethane, the CD intensity increased, and the 1H NMR signal of benzene protons shifted to lower fields at low temperatures. In hexane, the CD spectra and the 1H NMR signals were less dependent on temperatures, as a result of the strong interaction between the chiral alkyl chains in the polymer to freeze the helical conformation.

1. Introduction

Helical _-conjugated polymers have been paid much attention for a variety of electronic functionalities such as chiroptical properties that originate from their unique three-dimensional structures. In particular, helically fused aromatic rings are the target of intense research due to their rigid helical structures, which could be applied to many kinds of electro-optic devices by taking advantages of their stability [1]. There have been many reports on the sophisticated synthesis of helicene derivatives, such as the annulation of aromatic rings with heteroatoms in the presence of metal catalysts [2], the aromatizing reactions of bis(enol ether)s of arylmethyl ketones with p-benzoquinone [3], and the metal-catalyzed couplings of heteroatoms and aromatic rings [4]. They were synthesized via stepwise reactions to control their helicity. However, there have been much difficulties in synthesizing helicenes which are characterized by extended _-conjugation lengths enough to show _-conjugated properties such as an electric conductivity. Furthermore, oligomeric helicenes having several pitches are usually very poor in solubility [5]. m-Phenylene polymers bearing _4-alkylsulfanyliumdiyl linkages and poly(benzothiophene)s fused with the alkylsulfonium cations have been studied as the precursor of poly(helicene)s with improved solubility, resulting in a developmentof their synthetic processes and some applications based on their handling use [6]. Our group has reported the first synthesis of a macromolecular helicene, poly(thiaheterohelicene), by the annulation of the precursor, alkylsulfinyl-substituted poly(mphenylene) s, by way of the corresponding sulfonium cations (Scheme 1) [7]Excess helix sense in polymers has been achieved by the introduction of chiral side groups into the polymer backbones. To controlthe helicity of the poly(thiaheterohelicene), the polysulfoxide precursorsneed to be annealed into the fused rings under appropriateconditions where the helical conformation is induced in a specificdirection by effective chiral side chains. _-Conjugated polymerswith incurvated backbones such as poly(m-phenylene) [8], poly(mphenylenethynylene)[9] and poly(m-pyridine) [10] are known toshow helical conformations in solutions caused by the curvedmolecular shape of the m-phenylene linkages, although they aredynamic and reversible in helical inversion. A strong interactionbetween chiral side chains is required to induce the particular conformationof poly(m-phenylene)s.In this report,we describe a novel poly(m-phenylene) derivativethat is characterized by a remarkably stable helical conformation,as a highly promising precursor for the helix sense-defined poly(thiaheterohelicene).

2. Experimental

2.1. Materials

All reagents were purchased from Kanto Chemical Co., Tokyo Kasei Co. and Aldrich Co., and used without further purification. Synthesis of 2-methyl-1,3-phenylenbis(pinacol borate) was prepared

according to the previous literature [7].

2.2. Synthesis

2.2.1. 1,3-Bis(3-hydroxy-2-methylpropylthio)benzene (1To a DMF solution (30 ml) of 1,3-benzenedithiol (2.0 g,14.1 mmol) was added sodium bicarbonate (0.60 g, 25 mmol). Theresulting mixture was stirred at room temperature for 30 minunder N2 atmosphere. A DMF solution (10ml) of (S)-(+)- or (R)-()-3-bromo-2-methyl-1-propanol (4.0 g, 26.3 mmol) was addedto the mixture and allowed to react at room temperature for 5 h.After distillation of DMF under vacuum, the organic residue wasneutralized with diluted HCl and extracted with chloroform. Theextractwas dehydrated with sodium sulfate, concentrated by rotaryevaporation, and purified by column chromatography using chloroform/hexane (20/1) as an eluent to give a pale-yellowviscous liquid(3.07 g, 10.7 mmol, 76%). 1H NMR (CDCl3, 500 MHz, ppm): ı 7.29 (s,1H, Ph), 7.09 (m, 3H, Ph), 3.51 (m, 4H, –OCH2–), 3.35 (s, 2H, –OH),3.07 (q, 2H, –SCH2–), 2.73 (q, 2H, –SCH2–), 1.89 (m, 2H, –CH–), 1.00(d, 6H, –CH3). 13C NMR(CDCl3, 125 MHz, ppm): ı 137.8, 129.1, 127.8,125.9, 66.4, 36.9, 35.6, 16.3. IR(KBr, cm1): 3347 (_O–H), 2924 (_C–H),778 (ıC–H). MS (EI, m/z) 286 (found), 286.11 (calcd. for M+).2.2.2. 1,3-Bis(3-dodecyloxy-2-methylpropylthio)benzene (2)1 (4.47 g, 15.6 mmol) was dissolved in THF (50 ml), to whichwas added KOH (3.5 g, 62.5 mmol) and iodododecane (18.5 g,62.5 mmol). The solution was heated at 70 Cwith constant stirringfor 24 h. Then, the solvent was removed by distillation and theresulting organic residuewaswashed with H2O and extracted withCHCl3. The extract was dehydrated over sodium sulfate, dried byrotary evaporation, and purified by column chromatography usingCHCl3/hexane (1/1) as an eluent to yield a pale-yellowliquid (4.37 g,7.0 mmol, 45%). 1H NMR (CDCl3, 500 MHz, ppm): ı 7.27 (s, 1H, Ph),7.08 (m, 3H, Ph), 3.33 (m, 8H, –OCH2–), 3.09 (q, 2H, –SCH2–), 2.73(q, 2H, –SCH2–), 2.00 (m, 2H, –CH–), 1.54 (m, 4H, –CH2–), 1.25 (m,36H, –CH2–), 1.01 (d, 6H, –CH3), 0.84 (t, 6H, –CH3). 13C NMR (CDCl3,125MHz, ppm): ı 138.2, 128.9, 127.9, 125.4, 74.4, 71.2, 37.2, 33.7,31.9, 29.7, 29.6, 29.5, 29.3, 26.1, 22.6, 16.6, 14.0. MS (EI, m/z): 622

found), 622.48 (calcd. for M+).2.2.3. 1,5-Dibromo-2,4-bis(3-dodecyloxy-2-methylpropylthio)benzene (3)2 (1.24 g, 2.0 mmol) was dissolved in CHCl3 (60 ml) whichwas maintained at 0 C with constantstirring. Bromine (0.96 g,6.0 mmol) was added to the solution and stirred at 0 C for further5 h. The resulting reaction mixturewas neutralizedwith a saturatedsolution of aqueous sodium sulfite. The organic layerwas extracted,whichwaswashedwith an aqueous sodium carbonate solution andH2O. The organic solvent was removed by rotary evaporation, andthe residue was purified byliquid chromatography to yield a paleyellowliquid (1.15 g, 1.48 mmol, 74%). 1H NMR (CDCl3, 500 MHz,ppm): ı 7.66 (s, 1H, Ph), 7.19 (s, 1H, Ph), 3.37 (m, 8H, –OCH2–), 3.12(q, 2H, –SCH2–), 2.75 (q, 2H, –SCH2–), 2.04 (m, 2H, –CH–), 1.55 (m,4H, –CH2–), 1.25 (m, 36H, –CH2–), 1.06 (d, 6H, –CH3), 0.88 (t, 6H,–CH3). 13C NMR (CDCl3, 125 MHz, ppm): ı 138.4, 135.7, 127.1, 120.0,74.3, 71.3, 37.3, 33.4, 31.9, 29.9, 29.6, 29.5, 29.3, 26.1, 22.6, 16.7, 14.1.MS (EI, m/z): 778 (found), 778.30 (calcd. for M+).2.2.4. Poly[4,6-bis(3-dodecyloxy-2-methylpropylthio)-1,3-phenylene-alt-2-methyl-1,3-phenylene](Poly-S*)3 (194.6 mg, 0.25 mmol), 2-methyl-1,3-phenylenbis(pinacolborate) (86.1mg, 0.25 mmol), tetrakis(triphenylphosphine)-palladium(0) (5.8 mg, 0.005 mmol) and THF (2.5 ml) were carefullyplaced in a 10ml ampoule in a glove box which was maintainedstrictly under pure argon. After the addition of a 2M solution ofaqueous sodium carbonate (0.5 ml), the ampoule was sealed andheated at 80 C for 72 h. The resulting mixture was poured intomethanol (50 ml) to precipitate the crude product, which wascollected by filtration. The crude product was washed with H2Oand extracted with CHCl3. Afterdehydrating the organic layer oversodium sulfate, the solvent was removed by rotary evaporationto give a yellow viscous liquid (143 mg, 0.2 unit mmol, 80%). 1H


NMR (CD2Cl2, 500 MHz, ppm): ı 7.32 (br, 5H, Ph), 3.45 (br, 8H,–OCH2–), 3.18 (br, 2H, –SCH2–), 2.79 (br, 4H, –SCH2–), 2.11 (br, 5H,–CH–, Ph-CH3), 1.67 (br, 4H, –CH2–), 1.40 (br, 36H, –CH2–), 1.13 (br,

6H, –CH3), 1.01 (s, 6H, –CH3). GPC (CHCl3 with polystyrene as anexternal standard): Mw =1.5×104, Mw/Mn = Poly[4,6-bis(3-dodecyloxy-2-methylpropylsulfinyl)-1,3-phenylene-alt-2-methyl-1,3-phenylene](Poly-S*O)A CH2Cl2 (5 ml) solution of Poly-S* (71.2mg, 0.1 mmol), a 30%aqueous hydrogen peroxide (1 ml) and acetic acid (0.5 ml) wereplaced in a flask and stirred at 30 C for 24 h. The organic layerwas extracted with CHCl3 and washed with H2O. After dehydrationand evaporation, the residue was dissolved in CHCl3 andpoured into diethylether to precipitate the product. The productwas dried under vacuum to give a yellow viscous liquid (54.3 mg,0.073 unit mmol, 73%). 1H NMR (CD2Cl2, 500 MHz, ppm): ı 8.85(s, 1H, Ph), 7.33 (br, 4H, Ph), 3.43 (br, 8H, –OCH2–), 2.93 (br, 2H,–SCH2–), 2.63 (br, 4H, –SCH2–), 2.10 (br, 5H, –CH–, Ph-CH3), 1.62(br, 4H, –CH2–), 1.37 (br, 36H, –CH2–), 1.00 (s, 12H, –CH3). IR (KBr,cm1): 1042 (_S O). GPC (CHCl3 with polystyrene as an externalstandard): Mw =7.5×103, Mw/Mn = 1.4.

2.3. Measurements

1H and 13C NMR, mass, and infrared spectra were recorded on aJEOL Lambda 500, a Shimadzu GCMS-QP5050, and a JASCO FT/IR-410 spectrometers, respectively.Molecularweights of thepolymerswere determined by GPC using a Tosoh highly sensitive GPC systemHLC-8220GPC equipped with a UV-8220 at a detection wavelengthof 254 nm. The UV–vis and circular dichroism (CD)spectra wereobtained using a JASCO V-550 and a JASCO J-820 spectrometers,respectively.

3. Results and discussions

Chiral 3-dodecyloxy-2-methylthio group was selected as theside chain of poly(m-phenylene) for helical induction, based onthe capability of long alkoxy chains to allow strong interactionand to promote gel formation [11]. The synthesis of a chiral alkylthio-substituted poly(m-phenylene) and its helical induc

tion was attempted with a view to develop novel _-conjugatedprepolymers of heterohelicenes. Chiral 3-dodecyloxy-2-methylgroups were introduced into 1,3-benzenedithiol, and the subsequentbromination gave the monomer of the correspondingfunctionalized poly(m-phenylene). Thus, 1,3-benzenedithiol wasreacted with (S)-(+)- or (R)-()-3-bromo-2-methyl-1-propanol.The following Williamson reaction gave 1,3-bis(3-dodecyloxy-2-methylpropylthio)benzene. The chiral alkylthio-functionalizedmonomer was brominated to yield 1,5-dibromo-2,4-bis(3-dodecyloxy-2-methylpropylthio)benzene. For the cross-couplingpolymerization, 2-methyl-1,3-phenylenebis(pinacol borate), wassynthesized from the Grignard reagent of 2,6-dibromotoluene andtri(i-propyl)borate (Scheme 2).The monomers 3 and 4 were polymerized by the Pd-catalyzedSuzuki-Miyaura condensation to yield Poly-S*. The molecularweight of Poly-S* was sufficiently large (Mw =1.5×104) which correspondedto a polymer containing 21 phenylene units. Oxidationof the sulfide bond withH2O2 proceeded tocompletion to give Poly-S*O (Scheme 3), which was confirmed by the integration of the 1H NMR signal at 8.85ppm ascribed to the phenyl proton between thetwo alkylsulfinyl groups (see Section 2).The helical conformation of Poly-S*O was analyzed by CD and1H NMR spectroscopy, whichwere also used for optimizing thehelicity-inducing conditions. CD spectra of Poly-S* and Poly-S*Owere obtained in hexane solutions, as shown in Figs. 1 and 2,respectively. Cotton effects were observed at 265 and 220nm forPoly-S*, and 285 and 230nm for Poly-S*O. The band at the longerwavelength was assigned to the _–_* electronic transition of them-phenylene main chain, while those at the shorter wavelengthwas attributed to the transition in the phenyl rings. The invertedCD spectra obtained for (S)- and (R)-derivatives clearly indicatedthat the helical conformation of the polymer was induced by thechiral side chains. The molar CD intensity of Poly-S*O was higherthan that of Poly-S*, which suggested the stabilization of the helicalstructure by the enhanced interaction between the more polarsulfoxide groups.The CD and the fluorescence spectra at different concentrationswere recorded to obtain information about the possible

concentration effects on the helical conformation of Poly-S*O.

The CD intensity significantly increased at lower concentrations(Fig. 3(a)). The conformational transition was suggested by Kuhn’sdissymmetry factor gabs as a measure of helicity, which was determinedas a function of concentration from the ratio of circulardichroism to absorbency (/ε) (Fig. 3(a), inset). The noticeabledecrease in /ε at higher concentrations indicated that the helicalconformationwas suppressed, due probably to the undesired intermolecularinteraction. The fluorescence spectra of Poly-S*O showeda broadening at higher concentrations (Fig. 3(b)), which supportedthe presence of the significant intermolecular interaction.The helical conformation of Poly-S*O was influenced by thenature of the solvent. An intense CD spectrumwas obtained in hexane,rather than in CH2Cl2 and CH2Cl2/CH3CN mixtures, which canbe ascribed to the enhanced interaction between the chiral sidechains in poor solvents. The thermal stability of the helix in solutionswas estimated by the CD and the 1H NMR spectra recorded atdifferent temperatures. The shapes and intensities of the CD bandsin CH2Cl2 clearly depended on temperatures (Fig. 4) but showed nochanges in hexane (Fig. 5), suggesting that the helical conformationwas frozen in hexane due to the enhanced interaction between thepolar groups.The conformational changes of Poly-S*O in hexane and CH2Cl2were monitored by 1H NMR spectroscopy (Fig. 6). While the protonsignals ascribed to phenyl rings and alkylsulfinyl substituentsshowedanupfield shift at higher temperatures inCH2Cl2, the chemicalshiftwas independent of the temperature in hexane although aslightly broadened aromatic proton signal at ı = 7.2 was sharpenedat higher temperatures. The broadening of the signal upon coolingsuggested that the aromatic rings and the chiral side chains werefrozen due to the enhanced interaction at reduced temperatures.The proposed freezing of the helical structure is regarded as agreat advantage for the assumed ring-closing reaction (Scheme 1),which is the topics of our continuous research.

4. Conclusion

The precursor polymer of chiral poly(thiaheterohelicene), Poly-S*O, was synthesized. The helical conformation of the sulfoxidepolymerwas successfully induced in non-polar solvents. Thehelicalstructure was static and frozen in hexane. The long alkyl substituent,3-doceyloxy-2-methylpropyl group, was found to be apotential side group to induce the helicity of poly(m-phenylene).


This work was partially supported by a Grant-in-Aid forScientific Research on the Priority Area “Super-Hierarchical Structures”(No. 17067017), Grants-in-Aid for Scientific Research (Nos.19105003 and 19655043), and the Waseda University Global COEProgram from MEXT, Japan.


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