Supramolecular conducting micro fibers from amphiphilic tetrathiafulvalene-based organogelator

Xuemei Ling,Lei Wng,Kw ngun Jeong*,Myonghoon Lee*

a College of Information Technology,Jilin Agricultural University,Changchun 130118,China

b Department of Polymer/Nano Science and Technology,Chonbuk National University,Jeonju 561756,Republic of Korea

c National Center for Nanoscience and Technology(NCNST),Beijing 100190,China

Key words:Tetrathiafulvalene Fiber Gelator Supramolecular Conducting

ABSTRACT An amphiphilic tetrathiafulvalene molecule was designed and readily synthesized.The am-TTFcan gelate a variety of organic solvents in view of multiple intermolecular interactions,especially in polar solvent.SEM observation provided clear evidence for the self-assembled micro/nano fibers morphologies in gelation state.Moreover,XRD measurements indicated the formation of highly-ordered columnar structures.The FT-IR spectra revealed that the formation of mixed-valence states with the absorption over[38_TD DIFF]1700 cm?1,show ing the semiconductive behaviors with the conductivity of 10?4 S/cm.The am-TTF based conducting fibers could be promising candidates for organic electronics.

The exploitation of one-dimensional(1D)conductive micro/nanostructures is expected to lead to a novel generation of miniaturized processors and sensors which are different from those of current silicon-based electronics[1–3].Therefore,fabrication of supramolecular conducting nano fibers,which might enable the electrical connection of micro/nanometer scale electronic structures,has been received extensive attention[4,5].Self-assembly of organic p-conjugated molecules into low molecular mass organic gelators(LMOGs)offers a simple and powerful approach to the spontaneousformation of micro/nano fibers[6–8].On theonehand,the formation of supramolecular micro/nano fibers occurs in a selfassembled bottom-up approach through non-covalent forces,such as p–p stacking,hydrogen bonding or hydrophobic interactions[9,10].On the other hand,gelatorsassoft m aterialscan be deposited on any surface,and thesolvent immobilized upon gel formation can be removed by evaporation to leave the active micro/nanostructures of xerogels,which provide the necessary processability and fl exibility for practical applications.Intense research has been devoted to build the family of conductive architectures from organogelators[11].

Tetrathiafulvalene(TTF)is know n as an organic conductive material,high electron conductivity of which is originated from their p-stacked columnar structures in its single crystals[12–16].Recently,TTFderivatives have been intensely studied with a focus on fi brousgelatorsfor electrically conducting materials.The micro/nano fibers from 1D columnar structure w ould furnish key materials for advanced nanoscience and electronics[17,18].Previous reports indicate that the use of intermolecular noncovalent interactions via a gelation process to construct TTF-based conducting fibersisvery appealing to construct supramolecular 1D micro/nano fibers.In the gel state,TTF molecules aggregate and immobilize the surrounding solvent,offering a good w ay to form conducting TTF-based micro/nano fiberswith mixed valence states.Electrical conduction in these TTF-based fibers occurs because of the stacking between the p-surfaces of the functional unit as well as the network of short S???Scontacts between the ‘sides’of the molecules,and the existence of an un fi lled state upon oxidation of TTF.These studies validate that a low molecular-weight gel strategy show s potential for the construction 1D columnar structure of TTF[19].The mixed-valence state of the TTF fi brous structures upon doping further generates the conductivity,potentially providing soft materials for organic electronics.

Fig.1.Schematic synthetic route for am-TTF.i:DCC,DMAP,room temperature,CH2Cl2;ii:Hg(OAc)2,CH2Cl2,iii:P(OEt)3,120?C.

We are interested in supramolecular interactionsand their selfassembled structures of TTFderivatives[20–24].Through extended TTF based p-p interactions and hydrophobic interactions.TTF derivatives show liquid crystalline properties,which are promising active materials for organic electronics.Moreover,the particular interest in building 1D conductive micro/nanostructures and understanding cooperative intermolecular interactionsinvolved in self-assembly process promote us to prepare the amphiphilic TTF derivative(am-TTF)(Fig.1)[24].The TTF core is fused with a naphenyl group to increase the p-p stacking,and the long hydrophobic long alkyl chains and tri(ethylene glycol)chains are employed to impart the van der Waals forces.The am-TTFmolecule could self-assemble into columnar structure,which further forms fi brous structures with various diameters in the micrometer length scale through the cooperative multiple intermolecular interactions.Herein,we report the self-assembly behavior of am-TTF in solution.In some polar solvent,it can form organogel with micro/nano fi brous structures,which provide highly-ordered columnar structures.Moreover,the oxidized am-TTForganogel forms charge transfer(CT)state and show s a semiconducting feature.The organic fi brous structures of am-TTF show great potential for organic electronics.

The am-TTFwas synthesized according to the route as show n in Fig.1,and the molecular structure was characterized by NMR,MS,and elementary analysis(details see Supporting information)[24].The am-TTFwas expected to be gelation in some organic solvents with the aid of the cooperative multiple intermolecular interactions,including p-p stacking,hydrophobic interactions,and S???S interactions.In general,the am-TTF was dissolved in most organic solvent,such as toluene,diethyl ether and methylene chloride,forming solution with the concentration of 10 mg/m L(Table 1).In none-polar solvent,hexane and n-heptane,compound was not soluble,even under heating.In strong polar solvent,such as dimethyl sulfoxide(DMSO),methanol(MeOH),ethanol and nbutanol,the am-TTF was not dissolve at room temperature,but heating allowed the dissolution of am-TTFin above polar solvent,such as in DMSO(Fig.2a).Subsequently,yellow gel formed with the concentration of 10 mg/m Lupon cooling(Fig.2b).Interestingly,in DMF,am-TTFformed solution at 25?Cand precipitation at 0?C.Transparent yellow organogels were stable for several days w hen am-TTF was dissolved in DMSO and MeOH by heating to around 50?C,then air cooled and left to stand.The rheological property of the am-TTF gel was further investigated by dynamic time sweep rheological experiments.Asshow n in Fig.2c,the magnitude of G’is much higher than that of G’,revealing that the mechanical properties of the sample were dominated by the solid phase rather than the liquid phase,i.e.,low-mass am-TTF based organogels.

Table 1 Gelation properties of am-TTF in selected organic solvents at 25?C and 0?C,respectively.

In order to evaluate electrochemical properties,the cyclic voltammetry(CV)measurements were carried out in a dry dichloromethane solution of Bu4NBF4(0.1 mol/L)with a scan rate of 100 m V/s at room temperature.As show n in Fig.2d,tw o irreversible single-electron oxidation peaks at?0.89 V and 1.20 V were observed,indicating the formation of radical cations and dications of am-TTF,respectively.Due to the introduction of electron-withdraw ing ester into TTFcore structure,both oxidation potentials of am-TTF are higher than those of TTF[24].As expectation,the am-TTF could be oxidized by FeCl3in dichloromethane solution(ca.2?10?5mol/L).This process was achieved by a stepw ise addition of FeCl3,and monitored by UV–vis absorption spectral change(Fig.2e).The results indicated that am-TTFwas chemically oxidized to its radical(am-TTF?+)by FeCl3,which was con fi rmed by the new absorption bands at 469 and 807 nm.The absorptions at 469 was assigned to an intramolecular charge transfer of radical cation,am-TTF?+,while the absorption band at 807 nm was due to an intermolecular electron transfer of the p-dimer of am-TTFdications[25].Therefore,FeCl3gel(DMSO)was utilized for the oxidation of the am-TTF gel.First,the FeCl3solution was carefully put on top of am-TTF gelator(Fig.2f).The am-TTF gel was oxidized in 2 h with the gel color changing from yellow to brow n,indicating formation of the charge transfer state(Fig.2g).

In order to further study the gel,the scanning electron microscope(SEM)was used to measure the morphologies of the am-TTF based gel in DMSO.As show n in Fig.3a,the am-TTF gel showed highly ordered micro/nano fibers to form fi brous network.

Fig.2.Tuning the formation of solution(a)from heating to the formation of gel(b)upon cooling to room temperature of am-TTF in DMSO with the concentration of 10 mg/m L.(c)Rheological characterization of am-TTF gel.(d)The cyclic voltammetry measurement of am-TTF in a solution of Bu4NBF4(0.1 mol/L)in w ater-free dichloromethane with a scan rate of 100 m V/s at room temperature.(e)The UV spectra for monitoring am-TTF oxidation in DMSO.(f)The am-TTF gel in DMSOwith 10 mg/m Lat 25?Coxidized by FeCl3 solution,and the resultant oxidized am-TTF gel(g).

Fig.3.SEM images of am-TTF gel(a)and oxidized am-TTF gel(b)in DMSO with 10 mg/mL at 25?C.(c)The XRD pro files of am-TTFgel and oxidized am-TTFgel in DMSO with 10 mg/m L at 25?C.(d)The schematic illustration of am-TTFmolecular packing in gel state.

The w idth was about 1.9?0.8 m m,and the length was more than 50 m m.In order to get the insight,we measured the X-ray diffraction(XRD)patterns of the supramolecular structure of am-TTF gel.As show n in Fig.3c,there were many sharp re fl ection peaks appeared in both low(3.62?,4.05?,5.70?)and w ide angle regions(21.71?).These results indicated that the asymmetric TTF molecules formed highly ordered columnar structures.The dspacing calculated from diffraction at 3.62?was 2.43 nm,which may suggest the side by side molecular packing mode.Moreover,the diffraction at 21.71?indicated the p-p stacking(0.41 nm)of am-TTF self-assembled structures[26].Together with the nanophase separation between TTF and hydrophobic alkyl and hydrophilic tri(ethylene oxide)tails,the am-TTFmay self-assemble into one-dimensional columnar structures,which was show n in Fig.3d.

Fig.4.FT-IRof am-TTFoxidized by FeCl3 in 23 days and the conductivity of oxidized am-TTF gel after 23 days.

The formation of CT complex and conductivity of am-TTF gel was expected to be achieved by oxidation.The oxidized am-TTFgel was fi rstly characterized by time-dependent IRspectra.As show n in Fig.4,the changes in the IRspectra indicated the formation of the CT complex.A broad CT band of the mixed valence state appeared in the IRregion over 1700 cm?1.Furthermore,there was a new sharp band at 1396 cm?1upon oxidation due to the coupling of a conduction electron with the vibrational mode of the TTF moiety[27,28].This band became broader and broader with the formation of a mixed-valence state.No signi fi cant change was observed after 23 days[28].These results were in good agreement with the previous reports on iodine doped fi lms having TTF moieties.The formation of am-TTFbased CTcomplex was further characterized by SEM and XRD.The oxidized am-TTF gel showed similar morphologies with am-TTF gel as show n in Fig.3b.In addition,the XRD measurements still exhibited the highly ordered columnar structure with similar with strong diffractions at 3.62?,5.34?,5.55?,7.12?,10.77?,21.42?,similar to XRDpatterns of am-TTF gel(Fig.3c)[29].These results indicated that molecular assembled structures were not disturbed by oxidation.

Finally,the xerogels from as-prepared gel of am-TTFmolecules were prepared for the measurement of conductivity by four-probe method at room temperature[30,31].The previousreport revealed that the conductivity increased with the oxidizing time until it reached a plateau value,which corresponded to the formation of the full CTstate and the subsequent the formation of mixed valence state and could be announced by the IRspectra[28].Therefore,we measured the conductivity at day 23 after oxidation.As show n in Fig.4,the conductivity of oxidized am-TTF based gel was up to 6.27?10?4S/cm.However,there were no signals detected by this four-probe method for the am-TTF based xerogels without oxidation,indicating the electrical conductivity was smaller than 10-7S/cm.Therefore,it could be concluded that the formation of the mixed-valence state was essential to acquire the electric conducting TTFassemblies.

In summary,we described the self-assembly and conductive properties of supramolecular organogels based on am-TTF molecules.The XRD measurements indicated the formation of highly-ordered columnar structures,and SEM observation provided clear evidence for the self-assembled micro/nano fibers structures based on am-TTF molecules.The organogels can be oxidized and form charge transfer state,remaining the micronano fi brous structures with the semiconducting values of about 10?4S/cm.This may lead to the fabrication of self-assembled 1D solid conductive fibers.

Acknow ledgm ents

This work was supported by the National Natural Science Foundation of China(No.61106068),the Scienti fi c Research Foundation for the Returned Overseas Chinese Scholars,State Education Ministry,Jilin Province Key Laboratory of Organic Functional Molecular Design&Synthesis(No.130028831).

Appendix A.Supplem entary data

Supplementary materialrelated to thisarticlecan befound,in the online version,at doi:https://doi.org/10.1016/j.cclet.2018.07.001.