1、Monopyrrolo-tetrathiafulvalene based low-molecular-massorganogel with multistimuli-responsive properties#51015202530Liu Yucun, Zheng Ningjuan, Yin Bingzhu*(Department of Chemistry, College of Science, Key Laboratory of Natural Resources of ChangbaiMountain & Functional Molecules, Yanbian University,

2、 Ministry of Education, Yanji, Jilin133002,)Abstract: A new monopyrrolo-tetrathiafulvalene (monopyrrolo-TTF) based low molecular-massorganic gelators (LMOGs) was synthesized and characterized. The gelator could form a transparentorganogel in cyclohexane, while it was soluble in aromatic, haloalkanes

3、, THF, DMF, ethyl acetate, etc.when cooled after heating. And the gel has been thoroughly characterized by using various microscopictechniques including field-emission scanning electron microscopy (FE-SEM), Small-angle X-rayscattering (SAXS), UV-visible and Fourier Transform Infrared Spectroscopy (F

4、T-IR). Also, the gelatorreacted with tetracyano-p-quinodimethane (TCNQ) to form the charge-transfer (CT) complex binaryorganogel in cyclohexane. Experiments results indicated that the molecules arrangement in one- andtwo-components were rectangular and hexagonal columnar structure, which further sta

5、cked intomicroporous and fiber networks, respectively. Interestingly, the gel-sol transition could be reversiblytuned by chemical redox reaction, and trifluoro-acetic acid stimulation could also induce gel-soltransition, but it was irreversible by addition of triethylamine even heated or sonication.

6、 Therefore, themulti-responsive properties endowed the gel to be a favourable candidate for potential functionalmaterials.Key words: organogelator; charge-transfer; chemical redox; gel-sol transition0 IntroductionOrganic molecular self-assembly through various noncovalent interactions, such asH-bond

7、ing, - stacking, van der Waals, hydrophobic interactions, donor-acceptorinteractions, and so forth, is a powerful approach that is used to make new supramolecular softmaterials. Low-molecular-mass organogelators (LMOGs) 1 are capable of entrapping a largeamount of organic solvent to form three-dimen

8、sional self-assembled fibrillar networks(SAFINs), and have various potential applications in molecular electronic devices,2light-harvestingmaterials,3templatesynthesis,4controlleddrug release,5bio-/chemosensing, 6 crystal growth, 7 and others.8 Furthermore, molecular gels aredynamic supramolecular s

9、ystems in which free molecular entities and aggregates are in354045equilibrium governed by the solubility of the gel phase. 1c Therefore, this equilibrium can bemanipulated by some external stimuli, leading to a gel-to-sol state transition with disassemblyof the gelators, such as photosensitive,9 ac

10、id-base sensitive,10 ion sensitive,7,11 redoxsensitive,12 enzyme- responsive, 13 complex responsive, 12a, 14 pH-responsive,15 and so on.Recent studies have shown that functional organogels containing electroactive unit havedrawn significant attention as these supra-molecular structure have found imm

11、ense interest inoptoelectronic applications.16 With this objective in mind, the ability of the tetrathiafulvalene(TTF) core to act as the redox-active subunit is now well established due to their ability toself-assemble both in solution and at solid state,17 which have been widely used in OrganicFie

12、ld-Effect Transistors (OFETs),18 liquid crystalline materials,19 sensors,20 molecularswitches,21 conductive material22 and other applications. 23 All of the results indicate thatFoundations: the National Natural Science Foundation of China (grant No.21062022), the Specialized ResearchFund for the Do

13、ctoral Program of Higher Education (Grant No. 20102201110001)Brief author introduction:Liu Yucun (1987-),Male,Doctor,Supramolecule chemistryCorrespondance author: Yin Bingzhu(1952-),Male,Professor and PhD supervisor,Supramolecule chemistry. zqcong-1-the use of intermolecular noncovalent interactions

14、 via a self-assemble process to buildTTF-based supramolecular organizations are very appealing. Besides, the combination oftetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) charge-transfer (CT) salt isrecognized as an attractive strategy of molecular design, and its conductivity have been relat

15、ed505560studied extensively. 24 So, the progress in the synthesis of TTF derivatives has been closely tothe discovery of new conducting or superconducting materials.In earlier work, we have reported a series of organogels which contain an electroactivemonopyrrolo-TTF and an amide group with a longer

16、 alkyl chain, and discussed theirself-assemble behaviors, CT complex binary gels and ion sensing. 25 Over the course of ourexaminations of the self-assembly and gelation of monopyrrolo-TTF -based molecules, and inorder to reduce the critical gelation concentration (CGC) compared with the previous wo

17、rk,we reported a new organic gelator which consisted of two amide groups and a shorter alkylchain (Scheme 1). The hydrogen-bonding, p-p stacking, SS interactions and solvophobiceffect were expected to be the driving forces for gelation. Importantly, the monopyrrolo-TTF-based organogel showed better

18、gelation ability in cyclohexane and exhibited gel-solphase transition in response to the external factors stimulation such as temperature,charge-transfer complex, chemical redox reaction and trifluoroacetic acid-triethylamine. Itwas believed that the multiple stimuli responsive properties would crea

19、te more potentialapplications in soft materials.65Scheme 1 Synthesis routes of molecules 1a-e.1 Results and Discussion1.1Gelation behaviorThe gelation ability of gelator 1a-e was examined in sixteen different organic solvents by the7075standard heating-and-cooling method. 26 Compound 1 could not be

20、dissolved in saturatedstraight-chain alkanes (such as n-hexane) even under heating, while it was soluble in Benzene,haloalkanes, THF, DMF, etc. when cooled after heating. Compared with our previous report aboutmonopyrrolo-TTF-based gelators, we deduced that the amide moieties contributed to its poor

21、ersolubility in straight-chain alkane solvents, and might be broken the noncovalent interactionbetween molecules in polar solvents. In addition, compound 1a could soluble in cyclohexane andmethylcyclohexane (MCH) under heating. After cooling to the room temperature, these solutions-2-turned into a t

22、hermoreversible organogel. The difference was the formation of a transparent gel incyclohexane (Figure 1), and the critical gelation concentration (CGC) was as low as 1.0 mg/mL.However, 1b could only form the opaque gel in the cyclohexane, and the CGC was 2.5 mg/mL.808590But，it had a weaker stabilit

23、y and would change into a part organogel in few hours after gelation.On the other hand, we compared the gelation ability end with longer alkyl chains. The experimentconsequence indicated that the number of carbons on the terminal alkyl chain more than twelvedid not have gelation ability in any organ

24、ic solvents. This result showed that alkyl chain played animportant role in gelation, and longer chains were distorted in cyclohexane or MCH so thathampered the intermolecular noncovalent interaction. Meanwhile, the thermal stability of thecompounds was measured by thermogravimetric analysis (TGA).

25、The results revealed that allcompounds had high decomposition temperature up to 246 oC, indicating good thermal stability.And the melting temperature (Tm) and decomposition temperature (Td) were depicted in Table 1.The Td was highest for 1a and decreased upon an increase in alkyl chain length (1b-e)

26、 due to thebond strength of the molecules. Similarly, 1a had the highest Tm compared with others.Fig. 1 Tuning the gel formation by adding TCNQ： (left) A cyclohexane organogel of 1a; (middle) a brownsolution of the CT complex; (right) a dark-green binary gel of the CT complex.Table 1 Thermal propert

27、ies of molecules.95Fig. 2 Plots of Tgel versus the concentration of 1a in cyclohexane (), MCH () and 1b in cyclohexane ().To investigate the thermal stability of the resulting gel-phase material, the gelsol phasetransition temperature (Tgel) was determined by a convenient ball-drop method. As shown

28、inFigure 2, The Tgel increased as the concentration of the gelator molecule increases and finally a100plateau appears. Notably, the Tgel reached the maximum value even at low concentration of the-3-Temperature1a 1b 1c 1d 1eoTm ( C)oTd ( C)142.5 131.4 130.7 130.1 129.9252.6 251.7 249.4 248.3 247.9org

29、anogel, indicating that the gelators 1a and 1b in cyclohexane or MCH owned stablesupramolecular structures at low concentration in the initial formation of the organogel systems.Besides, by the Tgel, the gelator 1a could form more stable micro/nanometer supramoleculararchitecture related to 1b in cy

30、clohexane at the same concentration. This result also stated that the105gelator 1a had a lower CGC in cyclohexane.In order to further determine the forming gels whether were spontaneous process, a lineardependence of the melting temperature with the natural logarithm of the mole fraction of gelatorw

31、as observed. The thermodynamic parameters of the gelation have been calculated fromEquations (1)-(3): 27110Go = RT ln (f)Ho= -RT2 ln (f)/TSo = -R ln (f) RT ln (f)/T(1)(2)(3)Here, f (mole fraction of gelator) = a/ (a+b), where a is the number of moles of gelator, b isthe number of moles of solvent, a

32、nd T is the corresponding gel-melting temperature. The value of115120125130135the slope ln(f)/T in these equations was calculated in Figure 4. And the calculated enthalpy(H) and entropy (S) for the solgel transformation were depicted in Table 2. And furthermore,the negative values of the Gibbs free

33、energy showed that the gel formation were a spontaneousprocess. On the other hand, the gel system of 1a in cyclohexane had the lowest G o with respectto its MCH organogel and the cyclohexane gel of 1b, indicating that the gelator 1a was easier toform a stable organogel in cyclohexane and had a more

34、supramolecular micro/nanometer sructure.Table 2 The calculated thermodynamic parameters of gelation.As an electron rich compound 1, we were curious to examine its gel-phase materials with anelectron deficient system in gel state. The attractive property of donor-acceptor system was theirability to a

35、ssemble and form a stable binary organogel without forming a precipitate. 28 Thegelator 1a and 1b formed dark-green binary gels in cyclohexane or MCH in the presence ofTCNQ (Figure 1), and the binary gel of 1a with TCNQ owned the lowest CGC in cyclohexanecompared with others. These were ascribed to

36、the formation of stacked superstructure withinteractions between alternating TTF and TCNQ enhancing the assembly process. According toour earlier reported, the most effective gelation was achieved using a 1:1 stoichiometry of the twocomponents.25 We also considered the Tgel under the different conce

37、ntrations of binary gels,and found that the gel could change to the sol only at boiling point of solvents even in the CGC(the data were not listed). Representation of 1a, UV-vis spectra of the CT complex binary gels (2mM) showed three new absorption bands in the 650 900 nm regions (Figure 3), center

38、ed at lmax682 nm, lmax 753 nm and lmax 855 nm, which represented the absorption of TTF.+ and TCNQ-,proving that the CT complex gels were really formed. Furthermore, the formation of the CTcomplex gel indicated that some driving forces for gelation were changed from SS andp-p interactions between gel

39、ators to the CT interaction of TTF with TCNQ.-4-parameters1a1bCyclohexaneMCHCyclohexane ln(f)/To -1 -1G (KJ mol K )o -1H (KJ mol )o -1 -1S (J mol K )0.0330 0.0361 0.0168-22.849 -21.649 -20.399-34.411 -39.254 -13.794-32.649 -48.615 -21.027Fig. 3 Absorption spectra of 1a in CHCl3 solution (0.1 mM, bla

40、ck line), the dilution of the gel in cyclohexane (1140145150155160165mM, red line) and the CT complex gel (molar ratio = 1 : 1) in cyclohexane (2 mM, blue line).1.2 Morphological characterizationTo investigate the morphological changes of the one and two-component gels, field emissionscanning electr

41、on microscopy (FE-SEM) measurements were conducted.29 Taking thecyclohexane xerogel of 1a for example, Figure 4a showed that the SEM image of theone-component xerogel was the microporous architecture upon self-association with average innerdiameters of ca. 12 mm for the cavities. The results demonst

42、rated that the molecules in the gelphase were self-assembling into one-dimensional fibers with a few micrometers long and furtherentangled to form a larger three-dimensional microporous structure. The image made clear that themorphological of xerogel had taken place an obvious change after addition

43、of TCNQ. Themicroporous structures obtained from cyclohexane transformed to the fibrous structure withdiameters of ca. 1040 nm and lengths up to tens of microns after formation of a CT complex withTCNQ in cyclohexane (Figure 4b). Changes in morphology illustrated that there existed strong CTinteract

44、ion in binary organogel system. Furthermore, such structures transition of xerogel 1a inMCH and 1b in cyclohexane were occurred after formation the CT complex with TCNQ. As seenin Figure 4c and 4e, the morphology of 1a in MCH and 1b in cyclohexane were quite differentfrom that of 1a in cyclohexane.

45、The images of one-component xerogels showed co-structureconsisting of both fibrillar with 80100 mm diameters and rope-like with length of a few micronsarrangements. The structures of the gels of 1a and 1b were changed after formation the CTcomplex with TCNQ in MCH and cyclohexane, respectively. The

46、results were shown in Figure4d and 4f, some globular aggregates were formed within the binary organogel system, alsoindicated the presence of strong interaction within the CT complex system. The morphologydifference before or after formation of CT complex organogel with TCNQ showed that the gelstruc

47、ture was not only related to the solvent, but also related to the length of the flexible chain.1.3 SAXS studiesIn addition to the microscopic image research, we measured the Small-angle X-ray scattering(SAXS) spectra of both the one- and two-component gels to reveal the molecular packing andorientat

48、ion of the molecules in gel phase. Taking 1a as example, as shown in Figure 5a, thescattering pattern of the cyclohexane xerogel was characterized by reflection peaks at 3.09 and2.14 nm in the low angle region with the scattering vector ratio of 1 :2, corresponding to the-5-170175180185rectangular c

49、olumnar structure with a = 3.09 nm and b = 2.93 nm. The number of moleculeswithin the cell unit was about 4 by Equation.19b Considering that the molecular length was about3.02 nm (by the CPK model), indicating that the molecules may have a mono-moleculararrangement in the matrix.As for the binary ge

50、ls, the packing conformations of the molecules in gel phase had takenplace significant changes. Seeing in Figure 5b, binary gels showing a series of strong scatteringpeak with d-spacing of 5.50, 3.19 and 1.85 nm in the low angle region with the scattering vectorratio of 1:3:3, revealed a hexagonal c

51、olumnar structure with the column diameters of 6.35 nm.The value of the first scattering peak corresponding to the length was shorter than two ofmolecules, showing that the flexible chains of molecules distorted in the columns. The number ofmolecules in a disk was calculated to be nearly 12 by Equat

52、ion. Given the packing structuraldifferences between the gels, one can presume that the CT interaction should have a significanteffect on their packing modes. The packing model of the columnar square phase is shown inFigure 6.Fig. 4 FE-SEM images of (a) one-component xerogel and (b) the CT complex x

53、erogel of 1a with TCNQ (1 : 1)from cyclohexane.Fig. 5 SAXS patterns of xerogel 1a (black line) and 1a/TCNQ (mole ratio = 1:1, red line) from cyclohexane.-6-As for MCH xerogel of 1a, the SAXS data displayed two scattering peaks in single and190binary components xerogels, and the scattering vector rat

54、io are 1:2, indicating that moleculesassembly model all adopted rectangular column in the organogel systems. On the other hand, thegelator 1b had the same assemble patterns with 1a in single or binary components gel systems.Above data showing that the solvent molecules played an important role in the assembly processof gelator and TCNQ molecules in the binary components.195Fig. 6 Cartoon representation of th