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Monopyrrolotetrathiafulvalene–succinamide conjugates and their TCNQ charge transfer complex based supramolecular gels with multiple stimulus responsiveness† Yucun Liu, Ningjuan Zheng, Tie Chen,* Longyi Jin and Bingzhu Yin* A series of monopyrrolotetrathiafulvalene–succinamide conjugates and their 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) charge transfer (CT) complexes have been synthesized and investigated as new lowmolecular mass organogelators. The gelation capability of these conjugates is highly dependent on the length of the alkyl chain of the terminal amide. Thus, only the short alkyl chain derivatives 1a and 1b could efficiently gelate cyclohexane and methylcyclohexane (MCH). Surprisingly, these gelators react with TCNQ to form stable CT complex gels in both cyclohexane and MCH. The FE-SEM images of the native gels reveal the characteristic gelation morphologies of microporous or fibrous structures, whereas the morphologies of CT complex gels show the fibrillar and globular aggregates in cyclohexane and MCH, respectively. SAXS study of the native gel and the CT complex gel of 1a in cyclohexane suggests that the molecules maintain rectangular and hexagonal columnar molecular packing models in the gel phase, respectively. The native gels undergo a reversible gel–sol phase transition upon exposure to external stimuli, such as temperature and chemical oxidation/reduction. Alternatively, the corresponding CT
Received 4th July 2014, Accepted 15th July 2014 DOI: 10.1039/c4ob01397b www.rsc.org/obc
complex gels exhibit a complicated response to external stimuli. Chemical oxidation by I2 results in the destruction of the gel state. However, neither Fe3+ nor Cu2+ can induce the collapse of the gel phase. Interestingly, all the gels show an irreversible gel–sol transition on successively triggering with trifluoroacetic acid and triethylamine. The reformation of the gel from the sol state is achieved just by the addition of water, showing the phase-selective gelation of the solvents from their mixtures with water.
Introduction Organic molecular self-assembly through various noncovalent interactions, such as hydrogen bonding, π–π stacking, van der Waals interactions, hydrophobic interactions, donor–acceptor interactions, and host–guest interactions, is a powerful approach that is used to make new supramolecular soft materials.1 Low-molecular mass organogelators (LMOGs)2 are capable of entrapping a large amount of organic solvents to form 3D self-assembled fibrillar networks and have various potential applications in molecular electronic devices,3 lightharvesting materials,4 template synthesis,5 controlled drug release,6 biosensing/chemosensing,7 crystal growth,8 and others.9 Furthermore, molecular gels are dynamic supramole-
Key Laboratory of Natural Resources of Changbai Mountain & Functional Molecules, Yanbian University, Ministry of Education, Yanji, Jilin 133002, PR China. E-mail: [email protected]; Fax: +86 433 2732456; Tel: +86 433 2732298 † Electronic supplementary information (ESI) available: General experimental details, procedures, NMR and MALDI-TOF-MS data for new compounds. See DOI: 10.1039/c4ob01397b
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cular systems in which free molecular entities and aggregates are in equilibrium, governed by the solubility of the gel phase.2c Therefore, this equilibrium can be manipulated by some external stimuli, leading to a gel-to-sol state transition with disassembly of the gelators, such as photosensitiveness,10 acid–base responsiveness,11 ion sensitivity,8,12 redox sensitivity,13 enzyme responsiveness,14 complex responsiveness,13a,15 and pH sensitivity.16 Functional organogels containing electroactive units have drawn significant attention as these supramolecular structures have been the focus of immense interest in optoelectronic applications.17 Based on this view, the ability of the tetrathiafulvalene (TTF) core to act as a redox-active subunit is now well established because of its ability to self-assemble in solution and in the solid state;18 it has been extensively used in organic field-effect transistors,19 liquid crystalline materials,20 sensors,21 molecular switches,22 conductive material,23 and other applications.24 The self-assembled molecular wires of a TTF gel were introduced in 1994 by JØrgensen and coworkers.25 Recently, preparation of electroactive fibrillar nanowires from the functionalization of the TTF organogel would
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have a potential for conductivity at room temperature after doping with iodine.26 Amabilino et al. synthesized and described the formation of helical self-assembled fibres by a C3 symmetric molecule incorporating tetrathiafulvalene units which was shown to be influenced dramatically by the processing conditions and different chiral structures.27 Results indicate that the use of intermolecular noncovalent interactions via a selfassembly process to build TTF-based supramolecular organizations is beneficial. More interestingly, the combination of TTF and electron acceptor charge transfer (CT) salt is recognized as an advantageous strategy of molecular design, and its conductivity has been studied extensively.28 For example, Shirai et al. have demonstrated that the amphiphilic TTFs can form conductive helical nanofibers by the formation of CT complexes with F4TCNQ and intermolecular hydrogen bonding among the chiral amide end groups.29 Park et al. measured the electrical conductivity of ClO4−-doped mixed valence nanofibers, which have self-assembled doubly coiled and triplex structures. The conductance decreased with a decrease in T, and the I–V characteristics were nonlinear in the temperature range of 70–300 K.30 As such, progress in the synthesis of TTF derivatives has been closely related to the discovery of new conducting, semiconducting or superconducting materials. In earlier work, we have reported that the synthesis, gelation behaviours, CT complex gels and ion-sensing of a series of organic gelators consisting of an electroactive monopyrrolotetrathiafulvalene (MPTTF) and an amide group with a longer alkyl chain.31 To explore the effect of structure on the gelation ability of new MPTTF systems, we selected succinamide as the hydrogen-bonding site and introduced it to the nitrogen atom of the pyrrole ring to synthesize a new series of MPTTF–succinamide conjugates (Scheme 1). We expected that the introduction of two hydrogen-bonding sites might influence the supramolecular assemblies. In this paper, we report the gelation of the new conjugates and their 7,7,8,8-tetracyano-p-qui-
Scheme 1
Synthesis routes of conjugates 1a–e.
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nodimethane (TCNQ) CT complexes, as well as the multiple stimulus responsiveness of the resulting gels upon exposure to environmental stimuli, such as temperature, chemical redox, and acid–base.
Results and discussion Gelation behaviour The gelation ability of gelators 1a to 1e was examined in 16 different organic solvents through a standard heating and cooling method.32 As summarized in Table S1,† compounds 1a to 1e cannot be dissolved in saturated straight-chain alkanes (such as n-hexane) even under heating. By contrast, compounds 1a to 1e are soluble in benzene, haloalkanes, tetrahydrofuran, and dimethylformamide at room temperature. Compared with our previously reported MPTTF-based gelators,31 we expected that an additional amide moiety contributed to the poor solubility of compounds 1a to 1e in straightchain alkane solvents and promoted the solvation of gelator molecules in polar solvents. Notably, the gelation capability of these conjugates was dependent on the length of the alkyl chain of the terminal amide. In the present series, only shorter alkyl chain derivatives 1a and 1b could efficiently gelate cycloalkanes. Compound 1a with an n-butyl could gelate cyclohexane and methylcyclohexane (MCH) to form yellow transparent gels at lower critical gelation concentrations (CGC) of 1.0 and 1.1 mg mL−1, respectively. By contrast, compound 1b with a longer octyl only gelates cyclohexane to form a yellow opaque gel at a higher CGC (2.5 mg mL−1), which would change into part organogel in a few hours at room temperature (Fig. 1 and S1†). By comparison, the conjugate with long alkyl chains (1c to 1e) did not gelate the 16 organic solvents tested under
Fig. 1 The gel–sol transitions of the gels (gelator 1a/cyclohexane) triggered by a variety of stimuli.
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the same conditions. The inability of the 1c–1e molecules to properly self-assemble and induce gel formation could contribute to enhance the molecular aggregation through the stronger van der Waals interactions between the side-chain in cyclohexane and MCH, thus destroying the dynamic supramolecular systems in which free molecular entities and aggregates are in equilibrium. To investigate the thermal stability of the resulting gel phase material, the gel–sol phase transition temperature (Tgel ) was determined through a conventional ball-drop method. As shown in Fig. 2, Tgel increased as the concentration of the gelator molecule increased until, finally, a plateau appeared. Tgel of the gels of 1a in cyclohexane and MCH were much higher than the gel of 1b in cyclohexane, indicating that the gels of 1a had more stable supramolecular structures than that of 1b in the initial formation of the organogel systems. Moreover, based on Tgel data, gelator 1a formed a more stable gel than 1b at the same concentration. To determine further whether the formation of gels was a spontaneous process, a linear dependence of the melting temperature with the natural logarithm of the mole fraction of the gelator was observed (Fig. 3). The thermodynamic parameters of gelation were calculated using eqn (1)–(3) as follows:33 ΔG o ¼ RT lnðϕÞ
ð1Þ
ΔH o ¼ RT 2 ½δ lnðϕÞ=δT
ð2Þ
ΔS o ¼ R lnðϕÞ RT ½δ lnðϕÞ=δT
ð3Þ
Here, ϕ (mole fraction of gelator) = a/(a + b), where a is the number of moles of the gelator, b is the number of moles of the solvent, and T is the corresponding gel melting temperature. The value of the slope [δ ln(ϕ)/δT] in these equations was calculated and is shown in Fig. 3. The calculated enthalpy (ΔH°) and entropy (ΔS°) for the sol–gel transformation are shown in Table 1. The negative values of the Gibbs free energy showed that gel formation was a spontaneous process. From
Fig. 3 lnϕ vs. T plots of organogels 1a in cyclohexane (■), MCH (●) and 1b in cyclohexane (▲).
Table 1
Parameters
1a/cyclohexane
1a/MCH
1b/cyclohexane
δ ln(ϕ)/δT ΔGo (kJ mol−1 K−1) ΔHo (kJ mol−1) ΔSo (J mol−1 K−1)
0.0330 −22.849 −34.411 −32.649
0.0361 −21.649 −39.254 −48.615
0.0168 −20.399 −13.794 −21.027
these results, we observed that the gel system of 1a in cyclohexane had the lowest ΔG° with respect to its MCH gel and the cyclohexane gel of 1b, indicating that the gelator 1a easily formed a stable organogel in cyclohexane and had a more stable supramolecular structure. An advantageous property of the donor–acceptor system is its ability to assemble and form a stable CT complex gel without forming a precipitate, which enhances the dimensionality of the electronic structure by noncovalent interactions.34 With the electron-active gelators 1a and 1b in hand, we were able to explore in detail the gelation behaviour of their CT complexes with TCNQ. Interestingly, both CT complexes of 1a or 1b with TCNQ could gel cyclohexane or MCH quickly, although both CT complexes had high CGC (Table 2 and Fig. S1†). In fact, when 1 equivalent of TCNQ was directly added on top of a cyclohexane gel of 1a, the gel was gradually destroyed after several minutes, resulting in a brown solution under heating. Upon cooling, the solution changed to a dark green CT complex gel (Fig. 1). Our earlier work had also pointed out that the most effective gelation could be achieved using a 1 : 1 stoichiometry of the two components.31 Notably, from the Tgel data, we observed that the CT gels of 1a with TCNQ could not be transformed into the sols until the boiling
Table 2
Fig. 2 Plots of Tgel versus the concentration of 1a in cyclohexane (■), MCH (●) and 1b in cyclohexane (▲).
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The calculated thermodynamic parameters of gelation
Gelation properties of CT complexes
Solvents
1a
CGC (mg mL−1)
1b
CGC (mg mL−1)
Cyclohexane MCH
Gel Gel
2.5 4.5
Gel Suspension
3.0 —
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point of solvents was reached under different CT salt concentrations (Fig. S2†). This finding implied that the stabilities of CT gels were higher than those of the native gels. Next, the thermal stability of the conjugates was measured through thermogravimetric analysis (TGA) to evaluate the potential of the conjugates to act as soft materials. The results are shown in Table S2.† The temperatures were higher than 245 °C when 5% weight loss occurred, which revealed that the target conjugates had high thermal stability (Fig. S3†).
the native gels, as shown in Fig. 4b, 4d and 4f. For example, the microporous structures of the 1a gel in cyclohexane were transformed to a fibrous structure with diameters of approximately 10 nm to 40 nm and lengths of up to tens of microns in the CT complex gel. For the CT complex gels of 1a in MCH and 1b in cyclohexane, the morphologies also changed from fibrous to globular. These changes in morphology showed that CT interaction played a key role on the formation of the CT complex gels.
Morphologies and architectures
SAXS study
To gain direct insight into the nature of the self-assembled microstructures, field emission scanning electron microscopy (FE-SEM) was used to observe two series of gels, namely, the native gels of 1a and 1b and their CT complex gels with TCNQ.35 All the gels exhibited well-defined structures with different morphologies. The native gel of 1a in cyclohexane showed a microporous architecture upon self-association, with average inner diameters of approximately 1 µm to 2 µm for the cavities, which were constructed from well-grown entangled nanometer fibers and then formed a larger 3D microporous structure (Fig. 4a). By contrast, the morphology of the native gels of 1a in MCH and 1b in cyclohexane were quite different from that of 1a in cyclohexane. The aggregates consisted of an entanglement of fibers with lengths of several micrometers, generating an extended and 3D fibrillar network that stabilized the gel state (Fig. 4c and 4e). More interestingly, an evident change took place in the morphologies of the CT complex gels compared with those of
To evaluate further the structures of the self-assembly, a smallangle X-ray scattering (SAXS) experiment was conducted. Taking the 1a gel formed from cyclohexane as an example, the scattering pattern of the xerogel was characterized by reflection peaks at 3.09 and 2.14 nm in the low-angle region with a scattering vector ratio of 1 : √2, corresponding to a 2D rectangular columnar structure with a = 3.09 nm and b = 2.93 nm, as shown in Fig. 5a. The number of molecules within the cell unit was approximately four based on calculations using the equation.20b Considering that the molecular length was approximately 3.02 nm (based on the CPK model), the molecules have a monomolecular arrangement in the matrix. By contrast, the packing mode of the molecules in the CT complex gel of 1a with TCNQ showed a significant change. As shown in Fig. 5b, the CT complex gel showed a series of strong scattering peaks with a d-spacing of 5.50, 3.19, and 1.85 nm in the low-angle region, with a scattering vector ratio of 1 : √3 : 3, indicating a 2D hexagonal columnar structure with a column diameter of 6.35 nm. The distance corresponding to the first scattering peak (100) was shorter than the length of two molecules, showing that the flexible chains of molecules interdigitated in the columns. The number of molecules in a disk was calculated to be approximately 12. Given the packing structural differences between the gels, we could presume that the CT interaction had a significant effect on their packing modes. The packing modes of the aggregates of 1a and its CT complex
Fig. 4 FE-SEM images of xerogels of 1a and 1b obtained from cyclohexane (a, e) and 1a obtained from MCH (c), and CT complex xerogels of 1a and 1b obtained from cyclohexane (b, f ) and 1a from MCH (d).
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Fig. 5 SAXS patterns of xerogels of 1a (a) and 1a/TCNQ complex (b) from cyclohexane.
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Fig. 6 Cartoon representation of the self-assembly of 1a and 1a/TCNQ (mole ratio = 1 : 1) in cyclohexane.
in cyclohexane are shown in Fig. 6. The X-ray scattering patterns of the native gel of 1a and its CT complex gel in MCH showed two scattering peaks, with a scattering vector ratio of 1 : √2, indicating that the molecules adopted a rectangular column-packing mode in self-assembled aggregates (Fig. S4a and b†). As for the gelator 1b, the SAXS experiments of the native gel and its CT complex gel from cyclohexane showed similar patterns to those of 1a (Fig. S4c and d†). These results revealed that the solvent molecules had an important role in the assembly process of gelators and CT complexes. Driving-force analysis To ascertain how the gelator molecules aggregated into the gels, Fourier transform infrared spectroscopy (FT-IR), ultraviolet–visible spectroscopy (UV–Vis), and proton nuclear magnetic resonance (1H NMR) investigations were performed. From the FT-IR absorption data, we observed that the nongelled CHCl3 solution (1.6 mM) of 1a showed relatively broad absorption bands at 3377, 1665, and 1525 cm−1, characteristic of stretching vibrations of N–H, CvO (amide II), and N–H (amide II) bonds, respectively, as shown in Fig. 7a.36 However, the FT-IR spectra of the corresponding gel in cyclohexane (1.6 mM) showed different characteristics. The corresponding absorption bands at 3303 cm−1 (N–H) and 1633 cm−1 (amide I) red-shifted by 74 and 32 cm−1, respectively, and the band at
Fig. 7 FT-IR spectra of 1a in CHCl3 solution (a), 1a gel from cyclohexane (b), the CT complex gel of 1a with TCNQ (mole ratio = 1 : 1) from cyclohexane (c) and TCNQ in chloroform (d).
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1542 cm−1 (amide II) blue-shifted by 17 cm−1 (Fig. 7b). Similar infrared spectrum changes were also observed in both gels of 1b in cyclohexane and 1a in MCH (Fig. S5†). Notably, the absorption band of N–H stretching vibration of the gel of 1a blue-shifted by 11 cm−1 in MCH with respect to that in cyclohexane, indicating that the gelator showed a stronger hydrogen-bonding ability in cyclohexane. This phenomenon was consistent with a lower CGC of 1a in cyclohexane relative to that in MCH. In addition, the 1H NMR spectrum of 1a was recorded at different concentrations (Fig. S6†). By increasing the concentration from 8 mg mL−1 to 32 mg mL−1, the signals at 5.85 and 6.33 ppm ascribed to the amide group gradually shifted downfield to 6.17 and 6.70 ppm, respectively, consistent with the formation of intermolecular hydrogen-bonding interactions. These results suggested that all amide groups were involved in hydrogen-bonding interactions in the gel state. By contrast, we focused our attention on the CN stretching band of TCNQ in the CT complex gels (4 mM) in the FT-IR spectra (Fig. 7c and d). The CN stretching band of the 1a CT complex gel in cyclohexane appeared at 2189 cm−1, exhibited a low frequency, and shifted by 32 cm−1 in comparison with that of TCNQ in chloroform solution (1.6 mM), indicating that CT interaction had a key role on the formation of the CT complex gels. However, we did not observe any shifts of the amide I and II bands in the CT complex gel with respect to the 1a gel, implying that intermolecular hydrogen-bonding interaction also had a key role on the formation of the CT complex gel. Next, to monitor the π–π interaction between aromatic moieties during gelation, the UV–Vis absorption spectra for the gels were obtained and compared with the corresponding dilute CHCl3 solution in which molecules were considered to be in the monomer state. As shown in Fig. 8, for compound 1a, the peak assignable to MPTTF absorption showed a broader and red-shifted band in the gel phase, which was quite different from that of the monomer state in CHCl3
Fig. 8 Absorption spectra of 1a in CHCl3 solution (0.1 mM, a), native gel of 1a in cyclohexane (1 mM, b) and the CT complex gel (molar ratio = 1 : 1) in cyclohexane (2 mM, c).
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solution, indicative of the formation of J-aggregation within the gel state. The π–π interaction also had a key role on the formation of the gels.37 By comparison, the UV–Vis spectrum of the CT complex gel of 1a showed two sharper absorption bands at λmax 754 and 856 nm and a shoulder-like band at 686 nm. The three bands were unambiguously assigned to the intramolecular transition of the TCNQ anion radical, proving that the CT complex gel was really formed and the CT interaction of TTF•+ with TCNQ•− contributed to the self-assembly process of CT complexes. Electrochemical properties The electrochemical properties of the conjugates were evaluated by cyclic voltammetry in CH2Cl2–CH3CN (v/v = 1 : 1, 1 mM, vs. Ag/AgCl). The numeric data are summarized in Table S3.† Gelator 1a showed two reversible single-electron oxiox dation waves at approximately Eox 1 = 0.564 V and E2 = 0.931 V, corresponding to the formation of radical cations and dications of TTF, respectively, indicating two sequential reversible one-electron transfer steps (Fig. 9a).38 Substantial positive shifts of the two oxidation peaks were observed, with ΔE1 = 96 mV for the first oxidation potential and with ΔE2 = 120 mV for the second oxidation potential compared with the previously reported gelators with one amide moiety.31 The number of the amide group exerted a significant influence on the oxidation potential of the gelator molecule. Compounds 1b to 1f exhibited similar electrochemical behaviour to that of 1a. All compounds showed two reversible single-electron oxidation waves in similar potential windows, illustrating that the length of the alkyl chain barely affected their redox peak profiles (seen in Fig. S7†). Next, we examined the electrochemical behaviour of the gels of 1a and its CT complex, both obtained from cyclohexane. The native gel of 1a exhibited two reversible oxidation
Fig. 9 Cyclic voltammograms of 1a (a) and TCNQ (d) in CH2Cl2– CH3CN (1 mM, 1 : 1, v/v), dried gel of 1a (b) and the dried CT complex gel (c) in CH2Cl2–CH3CN (1 : 1, v/v). Containing 0.1 M Bu4NPF6 and the scan rate was 100 mV s−1.
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waves at 0.513 and 0.885 V (Fig. 9b), respectively, which were negatively shifted compared with those in solution under the same conditions. This finding was probably due to the stabilization effect arising from the strong π–π interaction. These results were consistent with the previous reports in the literature.13a,b As expected, the CT complex gel of 1a showed five oxidation potentials at approximately E = −0.713 V (I), −0.206 V (II), +0.355 V (III), +0.524 V (IV), and +0.837 V (V) vs. Ag/AgCl (Fig. 9c). Processes (I), (II), and (III) were assigned to the TCNQ3−/TCNQ2−, TCNQ2−/TCNQ•−, and TCNQ•−/TCNQ couples, respectively, which were higher than those of TCNQ in the solution (Fig. 9d). In addition, processes (IV) and (V) were assigned to the MPTTF•+/MPTTF and MPTTF2+/MPTTF•+ couples, respectively. Interestingly, the first oxidation potential of the MPTTF moiety anodically shifted by 11 mV compared with that of the native gel because of the CT interaction from MPTTF to TCNQ. By contrast, the second oxidation potential of the MPTTF moiety cathodically shifted by 48 mV, which was different from the responses of other TTF derivatives to metal ions in which the Eox 1 value usually shifted to higher potential and the Eox 2 value showed no evident change because of the leaving of the guest cation due to the increased electrostatic repulsion from the TTF2+ cation.39 The large negative shifts of the second oxidation potential corresponded to an increase in the π-donating ability of the MPTTF unit and indicated that the redox-active MPTTF moiety seemed to be more easily oxidized upon addition of TCNQ. The shift of the second oxidation wave to more cathodic potentials was interpreted to be the result of the CT interaction between MPTTF•+ (electron acceptor) and TCNQ•− (electron donor) that increased the electron density of the MPTTF moiety. Multiple-stimulus-responsiveness Reversible gel–sol transition by redox reactions. With these gels in hand, we subsequently examined their stimuli-responsive properties. As we know, the TTF moiety could be oxidized in an oxidative environment, leading to the gel’s response to certain external chemical stimuli. When 1 equivalent of I2 was carefully placed above the native gel of 1a obtained from cyclohexane, the gel was gradually destroyed within a few minutes, leading to a brownish green suspension.40 Notably, when excess ascorbic acid (Vc) was added to the suspension, which was able to reduce TTF•+ to the neutral TTF, the brownish green suspension became a yellow solution instantly. After further heating and cooling, the gel phase was regenerated (Fig. 1). Such a gel–sol transition could be repeated for several cycles. Chemical oxidations in cyclohexane were also conducted by the successive addition of FeCl3 or Cu(ClO4)2 as oxidizing reagents. As shown in Fig. 1, upon addition of 1 equivalent of FeCl3 or Cu(ClO4)2, the gel collapsed and turned to a green suspension. Likewise, upon addition of the Vc solution, the green suspension changed to yellow in a few seconds. By further heating and cooling processes, the yellow gel was regenerated. Next, the gelation ability of the radical cation salt of 1a in which the TTF unit was oxidized to TTF•+ by chemical oxidants was examined in cyclohexane and ethanol. However,
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no gels were formed by cooling the hot solution of the radical cation salt of 1a with concentrations as high as 30 mg mL−1, which was different from the case of TCNQ. We note that the corresponding CT complex gel was not destroyed within 1 day or even after further heating and cooling in the presence of 2 equivalents of FeCl3 or Cu(ClO4)2, suggesting that the CT complex gels had higher stability than the native gels. However, upon addition of 1 equivalent of I2, the CT complex gels collapsed and changed to precipitates (Fig. S10†). To monitor the redox reaction processes of 1a with the oxidants, the UV–Vis absorption spectra were recorded. The UV– Vis spectrum of 1a in ethanol did not show any absorption band in the range of 400 nm to 900 nm. At first, the addition of 1 equivalent of I2 led to the development of two broad bands at approximately 433 and 780 nm, which then increased gradually upon the addition of increasing amounts of I2. However, even upon the addition of 5 equivalents of I2, no new band appeared (Fig. 10). The result meant that MPTTF could only be oxidized to the radical cation TTF•+ by I2 and could not be further oxidized to dication TTF2+.41 In turn, two characteristic bands of the MPTTF•+ cation gradually decreased with the increase in Vc concentration. In addition, the original spectrum of 1a was totally regenerated upon the addition of 5 equivalents of Vc. In the cases of both FeCl3 and Cu(ClO4)2 as oxidizing reagents, gelator 1a showed similar changes in UV–Vis spectra under the same experimental conditions (Fig. S11-b†). These results were perfectly consistent with those of the gel–sol transition experiments. Trifluoroacetic acid–triethylamine responsive properties. The amide group was an especially good hydrogen bond donor, so the introduction of a hydrogen bond acceptor guest could change the gel into a solution phase.42 Native gel of 1a in cyclo-hexane gradually collapsed and changed into a yellow suspension upon addition of 4 equivalents of trifluoroacetic acid (TFA). Interestingly, the color of the solution changed significantly on the addition of increasing amounts of TFA (Fig. 1
Fig. 10 Changes of UV-vis absorption of 1a (5 × 10−5 M) in ethanol upon chemical redox by successive addition of I2 (0–5 eq.) and ascorbic acid (1–5 eq.), respectively.
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and Fig. S12†). Upon addition of TFA from 5 equivalents to >20 equivalents, the color of the solution changed from yellow to greenish yellow, then to brownish green, and finally to a dark green isotropic solution (>20 equivalents). However, the addition of excess acetic acid or formic acid did not induce color changes in the system, but the gel–sol transition occurred (Fig. S13†). These results indicated that TFA would interact with the amide group first and then destroy the hydrogen bonds between the molecules when TFA was
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Monopyrrolotetrathiafulvalene–succinamide conjugates and their TCNQ charge transfer complex based supramolecular gels with multiple stimulus responsiveness† Yucun Liu, Ningjuan Zheng, Tie Chen,* Longyi Jin and Bingzhu Yin* A series of monopyrrolotetrathiafulvalene–succinamide conjugates and their 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) charge transfer (CT) complexes have been synthesized and investigated as new lowmolecular mass organogelators. The gelation capability of these conjugates is highly dependent on the length of the alkyl chain of the terminal amide. Thus, only the short alkyl chain derivatives 1a and 1b could efficiently gelate cyclohexane and methylcyclohexane (MCH). Surprisingly, these gelators react with TCNQ to form stable CT complex gels in both cyclohexane and MCH. The FE-SEM images of the native gels reveal the characteristic gelation morphologies of microporous or fibrous structures, whereas the morphologies of CT complex gels show the fibrillar and globular aggregates in cyclohexane and MCH, respectively. SAXS study of the native gel and the CT complex gel of 1a in cyclohexane suggests that the molecules maintain rectangular and hexagonal columnar molecular packing models in the gel phase, respectively. The native gels undergo a reversible gel–sol phase transition upon exposure to external stimuli, such as temperature and chemical oxidation/reduction. Alternatively, the corresponding CT
Received 4th July 2014, Accepted 15th July 2014 DOI: 10.1039/c4ob01397b www.rsc.org/obc
complex gels exhibit a complicated response to external stimuli. Chemical oxidation by I2 results in the destruction of the gel state. However, neither Fe3+ nor Cu2+ can induce the collapse of the gel phase. Interestingly, all the gels show an irreversible gel–sol transition on successively triggering with trifluoroacetic acid and triethylamine. The reformation of the gel from the sol state is achieved just by the addition of water, showing the phase-selective gelation of the solvents from their mixtures with water.
Introduction Organic molecular self-assembly through various noncovalent interactions, such as hydrogen bonding, π–π stacking, van der Waals interactions, hydrophobic interactions, donor–acceptor interactions, and host–guest interactions, is a powerful approach that is used to make new supramolecular soft materials.1 Low-molecular mass organogelators (LMOGs)2 are capable of entrapping a large amount of organic solvents to form 3D self-assembled fibrillar networks and have various potential applications in molecular electronic devices,3 lightharvesting materials,4 template synthesis,5 controlled drug release,6 biosensing/chemosensing,7 crystal growth,8 and others.9 Furthermore, molecular gels are dynamic supramole-
Key Laboratory of Natural Resources of Changbai Mountain & Functional Molecules, Yanbian University, Ministry of Education, Yanji, Jilin 133002, PR China. E-mail: [email protected]; Fax: +86 433 2732456; Tel: +86 433 2732298 † Electronic supplementary information (ESI) available: General experimental details, procedures, NMR and MALDI-TOF-MS data for new compounds. See DOI: 10.1039/c4ob01397b
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cular systems in which free molecular entities and aggregates are in equilibrium, governed by the solubility of the gel phase.2c Therefore, this equilibrium can be manipulated by some external stimuli, leading to a gel-to-sol state transition with disassembly of the gelators, such as photosensitiveness,10 acid–base responsiveness,11 ion sensitivity,8,12 redox sensitivity,13 enzyme responsiveness,14 complex responsiveness,13a,15 and pH sensitivity.16 Functional organogels containing electroactive units have drawn significant attention as these supramolecular structures have been the focus of immense interest in optoelectronic applications.17 Based on this view, the ability of the tetrathiafulvalene (TTF) core to act as a redox-active subunit is now well established because of its ability to self-assemble in solution and in the solid state;18 it has been extensively used in organic field-effect transistors,19 liquid crystalline materials,20 sensors,21 molecular switches,22 conductive material,23 and other applications.24 The self-assembled molecular wires of a TTF gel were introduced in 1994 by JØrgensen and coworkers.25 Recently, preparation of electroactive fibrillar nanowires from the functionalization of the TTF organogel would
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have a potential for conductivity at room temperature after doping with iodine.26 Amabilino et al. synthesized and described the formation of helical self-assembled fibres by a C3 symmetric molecule incorporating tetrathiafulvalene units which was shown to be influenced dramatically by the processing conditions and different chiral structures.27 Results indicate that the use of intermolecular noncovalent interactions via a selfassembly process to build TTF-based supramolecular organizations is beneficial. More interestingly, the combination of TTF and electron acceptor charge transfer (CT) salt is recognized as an advantageous strategy of molecular design, and its conductivity has been studied extensively.28 For example, Shirai et al. have demonstrated that the amphiphilic TTFs can form conductive helical nanofibers by the formation of CT complexes with F4TCNQ and intermolecular hydrogen bonding among the chiral amide end groups.29 Park et al. measured the electrical conductivity of ClO4−-doped mixed valence nanofibers, which have self-assembled doubly coiled and triplex structures. The conductance decreased with a decrease in T, and the I–V characteristics were nonlinear in the temperature range of 70–300 K.30 As such, progress in the synthesis of TTF derivatives has been closely related to the discovery of new conducting, semiconducting or superconducting materials. In earlier work, we have reported that the synthesis, gelation behaviours, CT complex gels and ion-sensing of a series of organic gelators consisting of an electroactive monopyrrolotetrathiafulvalene (MPTTF) and an amide group with a longer alkyl chain.31 To explore the effect of structure on the gelation ability of new MPTTF systems, we selected succinamide as the hydrogen-bonding site and introduced it to the nitrogen atom of the pyrrole ring to synthesize a new series of MPTTF–succinamide conjugates (Scheme 1). We expected that the introduction of two hydrogen-bonding sites might influence the supramolecular assemblies. In this paper, we report the gelation of the new conjugates and their 7,7,8,8-tetracyano-p-qui-
Scheme 1
Synthesis routes of conjugates 1a–e.
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nodimethane (TCNQ) CT complexes, as well as the multiple stimulus responsiveness of the resulting gels upon exposure to environmental stimuli, such as temperature, chemical redox, and acid–base.
Results and discussion Gelation behaviour The gelation ability of gelators 1a to 1e was examined in 16 different organic solvents through a standard heating and cooling method.32 As summarized in Table S1,† compounds 1a to 1e cannot be dissolved in saturated straight-chain alkanes (such as n-hexane) even under heating. By contrast, compounds 1a to 1e are soluble in benzene, haloalkanes, tetrahydrofuran, and dimethylformamide at room temperature. Compared with our previously reported MPTTF-based gelators,31 we expected that an additional amide moiety contributed to the poor solubility of compounds 1a to 1e in straightchain alkane solvents and promoted the solvation of gelator molecules in polar solvents. Notably, the gelation capability of these conjugates was dependent on the length of the alkyl chain of the terminal amide. In the present series, only shorter alkyl chain derivatives 1a and 1b could efficiently gelate cycloalkanes. Compound 1a with an n-butyl could gelate cyclohexane and methylcyclohexane (MCH) to form yellow transparent gels at lower critical gelation concentrations (CGC) of 1.0 and 1.1 mg mL−1, respectively. By contrast, compound 1b with a longer octyl only gelates cyclohexane to form a yellow opaque gel at a higher CGC (2.5 mg mL−1), which would change into part organogel in a few hours at room temperature (Fig. 1 and S1†). By comparison, the conjugate with long alkyl chains (1c to 1e) did not gelate the 16 organic solvents tested under
Fig. 1 The gel–sol transitions of the gels (gelator 1a/cyclohexane) triggered by a variety of stimuli.
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the same conditions. The inability of the 1c–1e molecules to properly self-assemble and induce gel formation could contribute to enhance the molecular aggregation through the stronger van der Waals interactions between the side-chain in cyclohexane and MCH, thus destroying the dynamic supramolecular systems in which free molecular entities and aggregates are in equilibrium. To investigate the thermal stability of the resulting gel phase material, the gel–sol phase transition temperature (Tgel ) was determined through a conventional ball-drop method. As shown in Fig. 2, Tgel increased as the concentration of the gelator molecule increased until, finally, a plateau appeared. Tgel of the gels of 1a in cyclohexane and MCH were much higher than the gel of 1b in cyclohexane, indicating that the gels of 1a had more stable supramolecular structures than that of 1b in the initial formation of the organogel systems. Moreover, based on Tgel data, gelator 1a formed a more stable gel than 1b at the same concentration. To determine further whether the formation of gels was a spontaneous process, a linear dependence of the melting temperature with the natural logarithm of the mole fraction of the gelator was observed (Fig. 3). The thermodynamic parameters of gelation were calculated using eqn (1)–(3) as follows:33 ΔG o ¼ RT lnðϕÞ
ð1Þ
ΔH o ¼ RT 2 ½δ lnðϕÞ=δT
ð2Þ
ΔS o ¼ R lnðϕÞ RT ½δ lnðϕÞ=δT
ð3Þ
Here, ϕ (mole fraction of gelator) = a/(a + b), where a is the number of moles of the gelator, b is the number of moles of the solvent, and T is the corresponding gel melting temperature. The value of the slope [δ ln(ϕ)/δT] in these equations was calculated and is shown in Fig. 3. The calculated enthalpy (ΔH°) and entropy (ΔS°) for the sol–gel transformation are shown in Table 1. The negative values of the Gibbs free energy showed that gel formation was a spontaneous process. From
Fig. 3 lnϕ vs. T plots of organogels 1a in cyclohexane (■), MCH (●) and 1b in cyclohexane (▲).
Table 1
Parameters
1a/cyclohexane
1a/MCH
1b/cyclohexane
δ ln(ϕ)/δT ΔGo (kJ mol−1 K−1) ΔHo (kJ mol−1) ΔSo (J mol−1 K−1)
0.0330 −22.849 −34.411 −32.649
0.0361 −21.649 −39.254 −48.615
0.0168 −20.399 −13.794 −21.027
these results, we observed that the gel system of 1a in cyclohexane had the lowest ΔG° with respect to its MCH gel and the cyclohexane gel of 1b, indicating that the gelator 1a easily formed a stable organogel in cyclohexane and had a more stable supramolecular structure. An advantageous property of the donor–acceptor system is its ability to assemble and form a stable CT complex gel without forming a precipitate, which enhances the dimensionality of the electronic structure by noncovalent interactions.34 With the electron-active gelators 1a and 1b in hand, we were able to explore in detail the gelation behaviour of their CT complexes with TCNQ. Interestingly, both CT complexes of 1a or 1b with TCNQ could gel cyclohexane or MCH quickly, although both CT complexes had high CGC (Table 2 and Fig. S1†). In fact, when 1 equivalent of TCNQ was directly added on top of a cyclohexane gel of 1a, the gel was gradually destroyed after several minutes, resulting in a brown solution under heating. Upon cooling, the solution changed to a dark green CT complex gel (Fig. 1). Our earlier work had also pointed out that the most effective gelation could be achieved using a 1 : 1 stoichiometry of the two components.31 Notably, from the Tgel data, we observed that the CT gels of 1a with TCNQ could not be transformed into the sols until the boiling
Table 2
Fig. 2 Plots of Tgel versus the concentration of 1a in cyclohexane (■), MCH (●) and 1b in cyclohexane (▲).
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The calculated thermodynamic parameters of gelation
Gelation properties of CT complexes
Solvents
1a
CGC (mg mL−1)
1b
CGC (mg mL−1)
Cyclohexane MCH
Gel Gel
2.5 4.5
Gel Suspension
3.0 —
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point of solvents was reached under different CT salt concentrations (Fig. S2†). This finding implied that the stabilities of CT gels were higher than those of the native gels. Next, the thermal stability of the conjugates was measured through thermogravimetric analysis (TGA) to evaluate the potential of the conjugates to act as soft materials. The results are shown in Table S2.† The temperatures were higher than 245 °C when 5% weight loss occurred, which revealed that the target conjugates had high thermal stability (Fig. S3†).
the native gels, as shown in Fig. 4b, 4d and 4f. For example, the microporous structures of the 1a gel in cyclohexane were transformed to a fibrous structure with diameters of approximately 10 nm to 40 nm and lengths of up to tens of microns in the CT complex gel. For the CT complex gels of 1a in MCH and 1b in cyclohexane, the morphologies also changed from fibrous to globular. These changes in morphology showed that CT interaction played a key role on the formation of the CT complex gels.
Morphologies and architectures
SAXS study
To gain direct insight into the nature of the self-assembled microstructures, field emission scanning electron microscopy (FE-SEM) was used to observe two series of gels, namely, the native gels of 1a and 1b and their CT complex gels with TCNQ.35 All the gels exhibited well-defined structures with different morphologies. The native gel of 1a in cyclohexane showed a microporous architecture upon self-association, with average inner diameters of approximately 1 µm to 2 µm for the cavities, which were constructed from well-grown entangled nanometer fibers and then formed a larger 3D microporous structure (Fig. 4a). By contrast, the morphology of the native gels of 1a in MCH and 1b in cyclohexane were quite different from that of 1a in cyclohexane. The aggregates consisted of an entanglement of fibers with lengths of several micrometers, generating an extended and 3D fibrillar network that stabilized the gel state (Fig. 4c and 4e). More interestingly, an evident change took place in the morphologies of the CT complex gels compared with those of
To evaluate further the structures of the self-assembly, a smallangle X-ray scattering (SAXS) experiment was conducted. Taking the 1a gel formed from cyclohexane as an example, the scattering pattern of the xerogel was characterized by reflection peaks at 3.09 and 2.14 nm in the low-angle region with a scattering vector ratio of 1 : √2, corresponding to a 2D rectangular columnar structure with a = 3.09 nm and b = 2.93 nm, as shown in Fig. 5a. The number of molecules within the cell unit was approximately four based on calculations using the equation.20b Considering that the molecular length was approximately 3.02 nm (based on the CPK model), the molecules have a monomolecular arrangement in the matrix. By contrast, the packing mode of the molecules in the CT complex gel of 1a with TCNQ showed a significant change. As shown in Fig. 5b, the CT complex gel showed a series of strong scattering peaks with a d-spacing of 5.50, 3.19, and 1.85 nm in the low-angle region, with a scattering vector ratio of 1 : √3 : 3, indicating a 2D hexagonal columnar structure with a column diameter of 6.35 nm. The distance corresponding to the first scattering peak (100) was shorter than the length of two molecules, showing that the flexible chains of molecules interdigitated in the columns. The number of molecules in a disk was calculated to be approximately 12. Given the packing structural differences between the gels, we could presume that the CT interaction had a significant effect on their packing modes. The packing modes of the aggregates of 1a and its CT complex
Fig. 4 FE-SEM images of xerogels of 1a and 1b obtained from cyclohexane (a, e) and 1a obtained from MCH (c), and CT complex xerogels of 1a and 1b obtained from cyclohexane (b, f ) and 1a from MCH (d).
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Fig. 5 SAXS patterns of xerogels of 1a (a) and 1a/TCNQ complex (b) from cyclohexane.
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Fig. 6 Cartoon representation of the self-assembly of 1a and 1a/TCNQ (mole ratio = 1 : 1) in cyclohexane.
in cyclohexane are shown in Fig. 6. The X-ray scattering patterns of the native gel of 1a and its CT complex gel in MCH showed two scattering peaks, with a scattering vector ratio of 1 : √2, indicating that the molecules adopted a rectangular column-packing mode in self-assembled aggregates (Fig. S4a and b†). As for the gelator 1b, the SAXS experiments of the native gel and its CT complex gel from cyclohexane showed similar patterns to those of 1a (Fig. S4c and d†). These results revealed that the solvent molecules had an important role in the assembly process of gelators and CT complexes. Driving-force analysis To ascertain how the gelator molecules aggregated into the gels, Fourier transform infrared spectroscopy (FT-IR), ultraviolet–visible spectroscopy (UV–Vis), and proton nuclear magnetic resonance (1H NMR) investigations were performed. From the FT-IR absorption data, we observed that the nongelled CHCl3 solution (1.6 mM) of 1a showed relatively broad absorption bands at 3377, 1665, and 1525 cm−1, characteristic of stretching vibrations of N–H, CvO (amide II), and N–H (amide II) bonds, respectively, as shown in Fig. 7a.36 However, the FT-IR spectra of the corresponding gel in cyclohexane (1.6 mM) showed different characteristics. The corresponding absorption bands at 3303 cm−1 (N–H) and 1633 cm−1 (amide I) red-shifted by 74 and 32 cm−1, respectively, and the band at
Fig. 7 FT-IR spectra of 1a in CHCl3 solution (a), 1a gel from cyclohexane (b), the CT complex gel of 1a with TCNQ (mole ratio = 1 : 1) from cyclohexane (c) and TCNQ in chloroform (d).
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1542 cm−1 (amide II) blue-shifted by 17 cm−1 (Fig. 7b). Similar infrared spectrum changes were also observed in both gels of 1b in cyclohexane and 1a in MCH (Fig. S5†). Notably, the absorption band of N–H stretching vibration of the gel of 1a blue-shifted by 11 cm−1 in MCH with respect to that in cyclohexane, indicating that the gelator showed a stronger hydrogen-bonding ability in cyclohexane. This phenomenon was consistent with a lower CGC of 1a in cyclohexane relative to that in MCH. In addition, the 1H NMR spectrum of 1a was recorded at different concentrations (Fig. S6†). By increasing the concentration from 8 mg mL−1 to 32 mg mL−1, the signals at 5.85 and 6.33 ppm ascribed to the amide group gradually shifted downfield to 6.17 and 6.70 ppm, respectively, consistent with the formation of intermolecular hydrogen-bonding interactions. These results suggested that all amide groups were involved in hydrogen-bonding interactions in the gel state. By contrast, we focused our attention on the CN stretching band of TCNQ in the CT complex gels (4 mM) in the FT-IR spectra (Fig. 7c and d). The CN stretching band of the 1a CT complex gel in cyclohexane appeared at 2189 cm−1, exhibited a low frequency, and shifted by 32 cm−1 in comparison with that of TCNQ in chloroform solution (1.6 mM), indicating that CT interaction had a key role on the formation of the CT complex gels. However, we did not observe any shifts of the amide I and II bands in the CT complex gel with respect to the 1a gel, implying that intermolecular hydrogen-bonding interaction also had a key role on the formation of the CT complex gel. Next, to monitor the π–π interaction between aromatic moieties during gelation, the UV–Vis absorption spectra for the gels were obtained and compared with the corresponding dilute CHCl3 solution in which molecules were considered to be in the monomer state. As shown in Fig. 8, for compound 1a, the peak assignable to MPTTF absorption showed a broader and red-shifted band in the gel phase, which was quite different from that of the monomer state in CHCl3
Fig. 8 Absorption spectra of 1a in CHCl3 solution (0.1 mM, a), native gel of 1a in cyclohexane (1 mM, b) and the CT complex gel (molar ratio = 1 : 1) in cyclohexane (2 mM, c).
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solution, indicative of the formation of J-aggregation within the gel state. The π–π interaction also had a key role on the formation of the gels.37 By comparison, the UV–Vis spectrum of the CT complex gel of 1a showed two sharper absorption bands at λmax 754 and 856 nm and a shoulder-like band at 686 nm. The three bands were unambiguously assigned to the intramolecular transition of the TCNQ anion radical, proving that the CT complex gel was really formed and the CT interaction of TTF•+ with TCNQ•− contributed to the self-assembly process of CT complexes. Electrochemical properties The electrochemical properties of the conjugates were evaluated by cyclic voltammetry in CH2Cl2–CH3CN (v/v = 1 : 1, 1 mM, vs. Ag/AgCl). The numeric data are summarized in Table S3.† Gelator 1a showed two reversible single-electron oxiox dation waves at approximately Eox 1 = 0.564 V and E2 = 0.931 V, corresponding to the formation of radical cations and dications of TTF, respectively, indicating two sequential reversible one-electron transfer steps (Fig. 9a).38 Substantial positive shifts of the two oxidation peaks were observed, with ΔE1 = 96 mV for the first oxidation potential and with ΔE2 = 120 mV for the second oxidation potential compared with the previously reported gelators with one amide moiety.31 The number of the amide group exerted a significant influence on the oxidation potential of the gelator molecule. Compounds 1b to 1f exhibited similar electrochemical behaviour to that of 1a. All compounds showed two reversible single-electron oxidation waves in similar potential windows, illustrating that the length of the alkyl chain barely affected their redox peak profiles (seen in Fig. S7†). Next, we examined the electrochemical behaviour of the gels of 1a and its CT complex, both obtained from cyclohexane. The native gel of 1a exhibited two reversible oxidation
Fig. 9 Cyclic voltammograms of 1a (a) and TCNQ (d) in CH2Cl2– CH3CN (1 mM, 1 : 1, v/v), dried gel of 1a (b) and the dried CT complex gel (c) in CH2Cl2–CH3CN (1 : 1, v/v). Containing 0.1 M Bu4NPF6 and the scan rate was 100 mV s−1.
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waves at 0.513 and 0.885 V (Fig. 9b), respectively, which were negatively shifted compared with those in solution under the same conditions. This finding was probably due to the stabilization effect arising from the strong π–π interaction. These results were consistent with the previous reports in the literature.13a,b As expected, the CT complex gel of 1a showed five oxidation potentials at approximately E = −0.713 V (I), −0.206 V (II), +0.355 V (III), +0.524 V (IV), and +0.837 V (V) vs. Ag/AgCl (Fig. 9c). Processes (I), (II), and (III) were assigned to the TCNQ3−/TCNQ2−, TCNQ2−/TCNQ•−, and TCNQ•−/TCNQ couples, respectively, which were higher than those of TCNQ in the solution (Fig. 9d). In addition, processes (IV) and (V) were assigned to the MPTTF•+/MPTTF and MPTTF2+/MPTTF•+ couples, respectively. Interestingly, the first oxidation potential of the MPTTF moiety anodically shifted by 11 mV compared with that of the native gel because of the CT interaction from MPTTF to TCNQ. By contrast, the second oxidation potential of the MPTTF moiety cathodically shifted by 48 mV, which was different from the responses of other TTF derivatives to metal ions in which the Eox 1 value usually shifted to higher potential and the Eox 2 value showed no evident change because of the leaving of the guest cation due to the increased electrostatic repulsion from the TTF2+ cation.39 The large negative shifts of the second oxidation potential corresponded to an increase in the π-donating ability of the MPTTF unit and indicated that the redox-active MPTTF moiety seemed to be more easily oxidized upon addition of TCNQ. The shift of the second oxidation wave to more cathodic potentials was interpreted to be the result of the CT interaction between MPTTF•+ (electron acceptor) and TCNQ•− (electron donor) that increased the electron density of the MPTTF moiety. Multiple-stimulus-responsiveness Reversible gel–sol transition by redox reactions. With these gels in hand, we subsequently examined their stimuli-responsive properties. As we know, the TTF moiety could be oxidized in an oxidative environment, leading to the gel’s response to certain external chemical stimuli. When 1 equivalent of I2 was carefully placed above the native gel of 1a obtained from cyclohexane, the gel was gradually destroyed within a few minutes, leading to a brownish green suspension.40 Notably, when excess ascorbic acid (Vc) was added to the suspension, which was able to reduce TTF•+ to the neutral TTF, the brownish green suspension became a yellow solution instantly. After further heating and cooling, the gel phase was regenerated (Fig. 1). Such a gel–sol transition could be repeated for several cycles. Chemical oxidations in cyclohexane were also conducted by the successive addition of FeCl3 or Cu(ClO4)2 as oxidizing reagents. As shown in Fig. 1, upon addition of 1 equivalent of FeCl3 or Cu(ClO4)2, the gel collapsed and turned to a green suspension. Likewise, upon addition of the Vc solution, the green suspension changed to yellow in a few seconds. By further heating and cooling processes, the yellow gel was regenerated. Next, the gelation ability of the radical cation salt of 1a in which the TTF unit was oxidized to TTF•+ by chemical oxidants was examined in cyclohexane and ethanol. However,
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no gels were formed by cooling the hot solution of the radical cation salt of 1a with concentrations as high as 30 mg mL−1, which was different from the case of TCNQ. We note that the corresponding CT complex gel was not destroyed within 1 day or even after further heating and cooling in the presence of 2 equivalents of FeCl3 or Cu(ClO4)2, suggesting that the CT complex gels had higher stability than the native gels. However, upon addition of 1 equivalent of I2, the CT complex gels collapsed and changed to precipitates (Fig. S10†). To monitor the redox reaction processes of 1a with the oxidants, the UV–Vis absorption spectra were recorded. The UV– Vis spectrum of 1a in ethanol did not show any absorption band in the range of 400 nm to 900 nm. At first, the addition of 1 equivalent of I2 led to the development of two broad bands at approximately 433 and 780 nm, which then increased gradually upon the addition of increasing amounts of I2. However, even upon the addition of 5 equivalents of I2, no new band appeared (Fig. 10). The result meant that MPTTF could only be oxidized to the radical cation TTF•+ by I2 and could not be further oxidized to dication TTF2+.41 In turn, two characteristic bands of the MPTTF•+ cation gradually decreased with the increase in Vc concentration. In addition, the original spectrum of 1a was totally regenerated upon the addition of 5 equivalents of Vc. In the cases of both FeCl3 and Cu(ClO4)2 as oxidizing reagents, gelator 1a showed similar changes in UV–Vis spectra under the same experimental conditions (Fig. S11-b†). These results were perfectly consistent with those of the gel–sol transition experiments. Trifluoroacetic acid–triethylamine responsive properties. The amide group was an especially good hydrogen bond donor, so the introduction of a hydrogen bond acceptor guest could change the gel into a solution phase.42 Native gel of 1a in cyclo-hexane gradually collapsed and changed into a yellow suspension upon addition of 4 equivalents of trifluoroacetic acid (TFA). Interestingly, the color of the solution changed significantly on the addition of increasing amounts of TFA (Fig. 1
Fig. 10 Changes of UV-vis absorption of 1a (5 × 10−5 M) in ethanol upon chemical redox by successive addition of I2 (0–5 eq.) and ascorbic acid (1–5 eq.), respectively.
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and Fig. S12†). Upon addition of TFA from 5 equivalents to >20 equivalents, the color of the solution changed from yellow to greenish yellow, then to brownish green, and finally to a dark green isotropic solution (>20 equivalents). However, the addition of excess acetic acid or formic acid did not induce color changes in the system, but the gel–sol transition occurred (Fig. S13†). These results indicated that TFA would interact with the amide group first and then destroy the hydrogen bonds between the molecules when TFA was
