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Salt Tunable Rheology of Thixotropic Supramolecular Organogels and Their Applications for Crystallization of Organic Semiconductors Sheng Gao, Suansuan Wang, Jing Ma, Ying Wu, Xuwei Fu, Ravi Kumar Marella, Kaiqiang Liu, and Yu Fang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03375 • Publication Date (Web): 30 Oct 2016 Downloaded from http://pubs.acs.org on October 31, 2016

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Salt Tunable Rheology of Thixotropic Supramolecular Organogels and Their Applications for Crystallization of Organic Semiconductors Sheng Gao, Suansuan Wang, Jing Ma, Ying Wu, Xuwei Fu, Ravi Kumar Marella, Kaiqiang Liu* and Yu Fang Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), School of Chemistry and Chemical Engineering, Shaanxi Normal University, No. 620, West Chang'an Avenue, Chang'an District, Xi'an 710119 [email protected]

ABSTRACT: Physical gelation behaviors of a series of novel bisurea-based derivatives bearing fatty alkyl tertiary amine moieties have been explored in water and common organic solvents. One of these amines exhibits very good thixotropic gels in apolar aromatic solvents (e.g. xylenes). The corresponding sol-gel transition is instantaneous and could be repeated for at least 50 cycles. Interestingly, the elasticity and strength of the resulting gels can be remarkably enhanced initially by the addition of a trace amount of tetrabutylammonium acetate (TBA) followed by a subsequent drop with further salt addition. Temperature-dependent 1H NMR confirmed that hydrogen bonding is the main driving force for the physical gelation. TEM, rheology, 1H NMR titration and examination of critical gelation concentration (CGC) reveal that the phenomenon is due to the dominated effects, the salting out effect at lower gelator concentration or the anion-urea hydrogen bonding at higher gelator concentration. Furthermore, the obtained transparent gels in this work can be used as good media for growing crystals of several organic semiconductors.

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1. INTRODUCTION Gels derived from low molecular mass gelators (LMMGs) are the typical class of physical gels [117]

. Unlike chemical gels, they can be obtained from the self-assembly of LMMGs via

supramolecular interactions, e.g. hydrogen bonding, van der Waals interactions, and π-π stacking, etc. In general, the formation of a gel occurs firstly by dissolving a LMMG in a suitable solvent at elevated temperature and then cooling down the solution to the ambient temperature. Over the last few decades, there has been rapid progress in the synthesis of supramolecular gels derived from LMMGs owing to the dynamic interaction nature of the gel networks. The LMMGs-based organogels have potential applications in the recovery of spilled oil [18-20], biomineralization [21, 22], pharmaceutical crystallization [23, 24], and for the template preparation of nanomaterials

[25]

, etc.

However, all these applications are critically based on their unusual thixotropic properties, and the tunability of the rheology of the gel systems. Therefore, to understand the relationship between gelator molecular structure and gelation ability, and to enhance the application of the resulting gels, development of molecular gels with tunable rheological properties is of pivotal importance. In general, rheological behaviours of molecular gels can be modulated by introducing new component into the original gel systems. These gel systems cover various LMMGs and have gained more attention in the recent years

[26-32]

. It is well-known that bis(ureas) can be employed

as efficient gelators in solvents due to the formation of urea α-tape motifs affording efficiently physical gelation. Among the various LMMGs, urea derivatives containing tertiary amine moieties (e.g. pyridyl ureas, and imidazole ureas, etc.) have drawn more attention partially due to their importance in the synthesis of novel functional materials via supramolecular complex [33, 34]. It has been reported that pyridyl ureas exhibit poor gelation abilities in water or organic solvents, and they are considered as “inhibited gelators” because the pyridyl nitrogen may compete with αtape urea-urea hydrogen bonding

[33, 34]

. Therefore, co-gelators, e.g. dicarboxylic acids

2


[35]

, metal

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ions

[36]

or halogen bond donors

[37]

were introduced into the pyridyl urea systems to release the

urea functionality and switch off urea-pyridyl hydrogen bonding motif for triggering efficient gelation of bis(pyridyl urea)s in water or organic solvents. In case of imidazole urea, the imidazole nitrogen atom in the molecular structure can form hydrogen bond with imidazole NH group, also leaving the urea moiety free and hence act as an efficient hydrogelator [38]. To the best of our knowledge, ureas derived from fatty alkyl tertiary amines are still unexplored as LMMGs. In the view of molecular structure, urea derivatives bearing fatty alkyl tertiary amine moieties, N atoms in tertiary amines are more sterically hindered and far from the urea groups during molecular self-assembly in solvents unlike pyridyl or imidazole ureas. Therefore, existence of fatty alkyl tertiary amine in these urea derivatives wouldn′t prevent the formation of urea tapes, and logically these amines are expected to be efficient LMMGs. Furthermore, like other urea derivatives, these amines are also receptive to the competition between urea-urea and anion-urea interactions. This approach can be used efficiently to tune rheology of molecular gels by changing types and amounts of the selected anions, which parallels with the literatures reported functions of metal-complexes, neutral molecules, and inorganic incorporates. In general, anions can reduce strength and elasticity of molecular gels up to dissolution by the addition of carboxylates above 0.1 equivalents to the selected gelator amount. It has been proved to be the result of the competition between urea-anion and urea-urea hydrogen bonding, and was used successfully to recover grown pharmaceutical crystals in the gels

[24]

. To the best of our knowledge, no reports

have been disclosed with tuneable rheology of supramolecular gels by addition of different amount of tetrabutylammonium acetate (TBA) starting from trace amount (e.g. 0.001 equiv.). In particular, no reports do also consider competition between the salt effect (e.g. salting out or salting in effect) and the anion-urea hydrogen bonding during molecular assembly of the selected gelator in solvents. In this paper, we have designed a series of six organic compounds derived from ureas bearing

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alkyl tertiary amine moieties, and gotten a novel and highly efficient LMMG (c.f. Figure 1). Furthermore, we explore which is dominant during salt tuneable rheology of supramolecular gels for two effects, either the salting out effect or the anion-urea hydrogen bonding. Finally, we investigate further applications of the resulting gels for growing crystals of organic semiconductor in one of the transparent gel.

Figure 1. Molecular structures of alkyl tertiary amine ureas (a-f).

2. EXPERIMENTAL SECTION 2.1 Materials. All the fatty alkyl tertiary primary amines (N,N-dimethylethylenediamine, N,N,2,2-tetramethyl-1,3-propanediamine, diaminopropane,

N,N-diethylethylenediamine,

4-amino-1-diethylaminopentane,

N,N-diethyl-1,3-

N,N-diisopropylethylenediamine),

4,4'-

diisocyanato-3,3'-dimethylbiphenyl and xylenes were purchased from Shanghai District of Japan TCI Co. Ltd. (~98%) and tetrabutylammonium acetate (TBA) were purchased from SigmaAldrich Co. Ltd. (~99.5%). The other solvents were provided by China Shanghai Pharmaceutical Group Co. Ltd. And all the chemicals available were used directly without further purification. 2.2 Synthesis of Compounds (a-f). 10 mmol 4,4'-diisocyanato-3,3'-dimethylbiphenyl was dissolved in 150 mL CH2Cl2 in 250 mL three-neck flask under vigorous stirring. Then 20 mmol fatty alkyl tertiary primary amines were added into the diisocyanate solution, and the mixture was

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stirred for about 16 h at 25 oC under the protection of nitrogen atmosphere. The resulting white precipitates was filtered and washed thoroughly with copious amounts of benzene and THF and then dried under vacuum as white powders in 80-95 % yield. 2.3 Gel Preparation. A known amount of the resulting bisurea compound was added into the tested solvent (40 mg/2 mL) and the mixture was heated until it forms a clear solution and then allowed to cool down to ambient temperature (named as W-C cycle), or was sonicated for about 20 mins at ambient temperature. The gel formed was recognized by the vial inversion tests. If the solvent was fully immobilized it was considered to be gelled (named as G and G* under W-C treatment and sonication, respectively). When the gelator formed transparent gels by immobilizing the solvent at this stage, it was denoted as “TG”. The term precipitate (P) is referred to the systems which give a clear warm solution by dissolving the gelator under heating but if form a precipitate at ambient condition. The systems in which only clear solution, viscous solution, turbid solution or an insoluble system remained until the end of the tests were referred as S, VS, TUS and I, respectively. 2.4 Temperature-Dependent 1H NMR and 1H NMR Titration. 1H NMR and temperature-dependent NMR spectroscopic measurements were carried out on a Bruker Avance-400. In the temperature experiments, 10 mg of gelator e was dissolved in 0.5 mL of d6-DMSO. The temperature was increased from 25 oC to 65 oC, and then cooled down to 25 oC at a regular time intervals of 10 min. In the titration experiments, stock solutions of the urea gelator were prepared by dissolving a known amount of gelator e in d6-DMSO (i.e. Se, 3.15 × 10-5 mol per 1.0 mL). Then TBA was dissolved in an appropriate volume of the Se solution under sonication for 10 min to obtain the required concentration of the titrant (namely STBA1, 39.8 × 10-5 mol per 1.0 mL). To explore the effect of TBA at very low concentration on the chemical shift, a small amount of TBA was dissolved in another appropriate volume of the Se solution to get the very low concentration of the titrant under sonication for 10 min (i.e. STBA2, 3.98 × 10-6 mol per 1.0 mL). Aliquots of the latter solution (STBA1 or STBA2) were added to the solution (Se) which contains e without negligible dilution

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effects on the titrated species. 2.5 Preparation of ‘Co-Gels’ for Salt-Effects on Gel Rheology. 40 mg of gelator e and 100 µL of TBA acetonitrile solutions were added into 2.0 mL of selected solvent, e.g. m-xylene, and the resulting mixture was heated until it forms a clear solution and allowed to cool down to ambient temperature to form cogels. For comparison, a reference gel was prepared only by introducing 100 µL of acetonitrile without TBA into the mixture of the gelator and the solvent. 2.6 Crystallization of Organic Semiconductors in Gel Media. The selected semiconductor was dissolved in the aromatic solvents till the saturation (in case of toluene). 10 mg of gelator e was dissolved in 2 mL of toluene solution of the semiconductor under heating treatment and cooling treatment. The resulting co-gels are placed in a refrigerator and kept at 2 oC for two days for the formation of semiconductor crystals. The molecular gels with grown crystals were dissolved by the addition of a liquid carboxylic acid. The resulting crystals were washed several times using cold methanol. 2.7 Rheological Measurements. Rheology experiments were performed using a TA Instruments Advanced Rheometer G2 (shear-controlled mode). All the measurements were made on a 20 mm roughsurface parallel plate with a gap of 500 micro meter at a fixed temperature of 20 oC and 2 mL of the gel sample. The stress sweep was chosen to determine the liner viscoelasticity (LVR) region and dynamic stress yield values of the tested gels (frequency=1 Hz, oscillatory stress=0.1–2000.0 Pa). A constant oscillatory shear stress within the LVR region (10.0 Pa) was applied to monitor the dependences of rheology of the formed gels on angle frequency from 0.0628 to 628.0 rad s-1. Complex viscosity was calculated from frequency sweep by using the formula (|η*(ω)| =ext ((G′(ω)2+ G′′(ω)2)/ ω2) to show whether the tested gels are shear thinning or not. Thixotropic measurements were conducted at 50 cycles, with two steps per cycle as follows: (a) time sweep under the dynamic stress beyond LVR region as a deformation process: oscillatory stress = 1000.0 Pa, angle frequency= 6.28 rad s-1 and time=120 s; (b) time sweep under the dynamic stress within LVR as a recovery process: oscillatory stress=5.0 Pa, angle frequency= 6.28 rad s-1 and time=120 s. In the comparison of gel thixotropy, the term, the loss tangent or

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the dissipation factor (tanδ) was applied, which is generally equal to G′′(ω)/G′(ω). Temperature sweep was carried out to monitor the sol-gel transition beahvior from 20 oC to 100 oC at 2 oC per min under 5 Pa with 6.28 rad s-1. It was observed that all the rheological measurements were carried out after stable gels formed from sealed hot solutions of the gelator or the ′co-gels′ for about 45 min in a by-pass specimen tube (Ø=25 mm) above the lower plate of the rheometer at 20 oC. 2.8 TEM Measurements. Molecular aggregates of the examined gelator (e) in m-xylene were examined by FEI Tecnai G2 F20 field transmission electron microscopy (TEM) at 200 KV. The gelator e was dissolved in m-xylene at a concentration of 0.033% (w/v) via heating and cooling treatment and left for about 12 h at ambient temperature. Pure carbon-supported copper grids were immerged into e/m-xylene solution to drag gelator molecular aggregates, and the copper grids were dried in a mini vacuum system (DAP-6D, ULVAC KIKO. Inc. Japan).

3. RESULTS AND DISCUSSION 3.1 Gelation Test. A new types of bisureas containing fatty alkyl tertiary amine moieties (R,R'N-) have been designed and synthesized as depicted in the Figure 1. Gelation experiments were carried out in two ways for each sample either by heating and cooling (c.f. Table 1) or by sonicating at room temperature (c.f. Table S1) (a~f%=1% weight/volume, w/v). From the tables (c.f. Table 1 and Table S1), it can be revealed that two of these bisurea derivatives are efficient gelators of apolar organic solvents. In the process of heating and cooling treatment, compound d forms turbid gel in four solvents (viz. toluene, o-xylene, p-xylene and m-xylene), and e forms transparent gel in eight solvents (viz. CH2Cl2, benzene, toluene, o-xylene, p-xylene, m-xylene, styrene and butyl acrylate) and five turbid gels (cyclohexane, acetone, THF, t-buyl methacrylate and methyltrimethoxysilane). Interestingly, compound d forms turbid gel in CH2Cl2, and compound e forms turbid gel in benzene, acetone, butyl acrylate and methyltrimethoxysilane after sonication for 30 min at room temperature. In particular, compound e can form transparent gel in o-xylene, and m-xylene after sonication for 10 min at room temperature. In comparison,

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Table 1. Gelation behaviors of compound a-f at the concentration of 1.0% (w/v) via a heating and cooling treatment Compounds Solvents a

b

c

d

e

f

Water

TuS

I

I

I

I

I

CH2Cl2

TuS

TuS

S

S

TG

I

Benzene

P

I

I

S

TG

I

p-Xylene

I

I

I

G

TG

I

o-Xylene

I

I

P

G

TG

I

m-Xylene

I

I

I

G

TG

I

Toluene

P

I

I

G

TG

I

Pyridine

S

S

S

S

S

I

DMSO

S

S

S

S

S

I

Cyclohexane

P

TuS

I

P

G

I

n-Hexane

TuS

I

I

P

TuS

I

Acetone

P

I

I

VS

G

I

Methanol

S

I

S

S

S

I

Ethanol

S

I

S

S

S

I

THF

TuS

I

S

S

G

I

Styrene

P

I

I

S

TG

I

Butyl acrylate

P

I

I

TuS

TG

I

T-butyl methacrylate

P

I

I

TuS

G

I

Chloroform

TuS

S

S

S

S

I

MTOS

TuS

I

I

TuS

G

I

Notes: *MTOS=Methyltrimethoxysilane. G=gel formed after warming and cooling treatment (W-C), P=Precipitate from clear warm solution, S=Solution, I=Insoluble under W-C cycle, TuS=Turbid solution, VS=Viscous solution, TG=Transparent gel.

compound e is the most efficient gelator among five compounds. Furthermore, critical gelation concentrations (CGC) of e in gelated solvents were tested and the results have shown that the gelator e forms gels in most of gelled solvents below ~0.3% (w/v) (c.f. Table S2 in the ESI). More interestingly, the CGC values of e in m-xylene, butyl acrylate and t-butyl methacrylate are lower to 0.06 %, 0.07 % and 0.08 % (w/v), respectively, which is significantly lower than 0.1% (w/v), a

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[39]

well-recognized standard for supergelation

. However, irrespective of the method we adopted

to prepare gels in the tested solvents, a, b, c and f cannot undergo gelation in any provided solvents, and they give turbid solution or precipitates in these solvents. It clearly suggests that the subtle differences in gelator structures results in generally significant differences in the gelation ability. 3.2 Rheological Behaviours. The rheological behaviours of the supramolecular gels are important for their real-life applications, in particular thixotropy. In this section, to explore the effect of the solvents gelled by the compound e studied upon the mechanical and thixotropic behaviors of their gels, the rheological properties of the gels of in three allotropic solvents (m-xylene, oxylene, and p-xylene, 2% w/v) were examined. As stress sweep was shown in the Figure 2, gels derived from gelator e in

o-xylene,

p-xylene

and m-xylene

exhibit different values of yield stress and similar values of G' and G'', of which the

Figure 2. Determination of the liner region. Measurement of the evolution of G′ and G′′ as a function of the applied shear stress. The samples are gels of e in o-xylene, pxylene and m-xylene concentration of 2.0% (w/v).

yield stresses of the three xylene gels are 500.9 Pa (o-xylene), 630.8 Pa (p-xylene) and 891.2 Pa (m-xylene), respectively. Surprisingly, the three xylene gels show very good thixotropic properties. The corresponding sol-gel transitions are instantaneous and could be repeated for at least 50 cycles (c.f. Figures 3a, S1, S2 and S3). During the sol-gel phase transition process, loss tangents (tan δ= G''/G') showing dominant rheological properties (G' or G'') of the three gels exhibit different amplitude of variation, p-xylene gel < o-xylene gel < m-xylene gel in sequence (c.f. Figure 3b). In view of gel thixotropy, a higher amplitude of tan δ shows better thixotropic

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Figure 3. Measurement of the evolution of G′ and G′′ as a function of time sweep at 50 cycles with the deformation and the recovery process (2.0 %, w/v, m-xylene, a) and change of tan δ during sol-gel process of molecular gels from three xylenes under oscillation stress stimuli (b, 2%, w/v).

properties when a stress beyond their yield values of the gels is imposed upon and released from the examined gel. It can be attributed to impose a certain stress turns the gel into a more liquidlike material, and to release the stress results in rapid transition from liquid-like state to solid-like

materials.

Such

well-tuned

thixotropic properties are less reported for supramolecular gels derived from LMMGs. It is very obvious that solvent nature, even allotropic solvents, can also result in clear differences in rheology of molecular gels. At

Figure

4.

Determination

of

the

liner

regime.

Measurement of the evolution of G′ and G′′ as a

the same time, as expected, the elasticity and the yield stress of the e/m-xylene gel are

function of the applied shear stress at different concentration of e from 0.5 to 3.0% (w/v) in m-xylene.

gelator-concentration-dependent. Specifically, with increasing the concentration of e from 0.5% to 1.0%, then to 2.0%, and finally to 3.0% (w/v), the values of the yield stress of the corresponding molecular gels changes from 50.1 to 223.9, then to 891.2, and finally to 1585.0 Pa (c.f. Figure 4).

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3.3 Salt-Effects on Gel Rheology. Moreover, rheological behaviour of molecular gels can be tuned by adding new component, one organic carboxylate, TBA, as a route of supramolecular modulation. As reported, addition of carboxylate into the molecular gels derived from urea-based LMMGs could induce molecular gels into solution or sol [24]. However, for the gel systems used in this study, it was found with surprise that introduction of a trace amount of TBA into the systems results in a sharp enhancement of the elasticity and strength of molecular gels. In the whole, G', G'' (c.f. Figure 5a), yield values (c.f. Figure 5b), and complex viscosity (c.f. Figure S4) of the e/m-xylene gels enhanced initially followed by a subsequent drop with further addition of TBA. When 100 µL of acetonitrile was added into the pristine gel (e=0.04 g, m-xylene=2 mL), the G' and G'' values of the resulting gel are increased up to about 1500 and 230 Pa from 470 and 49 Pa. However, the introduction of 100 µL TBA/acetonitrile solution (TBA=0.01 equiv.) into 2 mL of the pristine gel results in great enhancement of the G' and G'' values to ~58000 Pa and ~8900 Pa, respectively. Meanwhile, yield values of the gels were also improved from 800 Pa to 1600 Pa. Moreover, the sol-gel transition temperatures (Tgel) estimating the thermal stability of the examined gels are changing in the similar trend (c.f. Figure S5). By the addition of small amounts of TBA, the obtained gels are clearly embrittled into many separated gel pieces under a short time

Figure 5. Dependence of G', G'' (a) and yield values (b) on equivalent fractions of salt TBA in e/m-xylene molecular gels.

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sweep beyond yield values. Clearly, it is difficult to explore salt effects on gel thixotropy in the procedure shown in the Figure 3a. 3.4 Temperature-Dependent 1H NMR and 1H NMR Titration. It is important understand the driving forces that can tune the rheology of molecular gels during addition of TBA, in particular how a trace amount of TBA can trigger a sharp enhancement of strength and elasticity of the examined gels. By taking the consideration of possible interactions between salts and compound e in m-xylene suggests that there exist a competition between α-tape urea-urea and anion-urea hydrogen bonding, and a nonnegligible salting effect. In fact, urea α-tape hydrogen bonding was recognized as one of the main driving force for the physical gelation of gelator e in the solvent as further evidenced by temperature-dependent 1H NMR measurements (c.f. Figure 6). 1

H NMR titration rationalizes the salt effects on elasticity and strengths are in decreasing

trend (c.f. Figure 6 and Figures S6 and S7 in the ESI). The 1H NMR spectra from the NMR titration show that chemical shifts of hydrogen atoms of urea groups in the gelator shift to downfield as a whole, indicating that addition of RCOO- weakens the urea-urea hydrogen

Figure 6. Temperature-dependent 1H NMR spectra of e in d6-DMSO (left) and 1H NMR titration spectra of e in d6-DMSO via addition of TBA (right).

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bonding via the new formation of hydrogen bonding between RCOO- and urea groups in the gelator molecules (c.f. Figure 7). Within the lower TBA concentration (below 0.01 equi.), chemical shift of Ha and Hb exhibited the similar trend, but it cannot explain the salt-induced enhancements of gel strength (e.g. Figure S6).

Figure 7. The urea α-tape hydrogen bonding motif for fibrous aggregation in bis(urea) gelator, and competitive anion binding with urea groups upon addition of TBA into gel system

[37]

. This estimates that

addition of TBA into gel results in the decrease of the strength and the elasticity of the gels.

3.5 TEM Characterization. TEM images were recorded to examine the molecular aggregation of e in m-xylene with different amounts of TBA. Seen in Figures 8a and 8b, gelator e forms very thinner fibrous networks in pure m-xylene, and gelator fibers begin to aggregate together into thicker ones when 100 µL of acetonitrile was added into the above system (m-xylene=2 mL). Further increase in the amount of TBA, specifically from 0.001, 0.01, 0.1 to 1.0 equivalent of the gelator, results in various aggregates of the gelator fibers (c.f. Figures 8c, 8d, 8e and 8f), but the presence of TBA in gel systems doesn′t change fibrous structures of gelator aggregates. However, TEM measurements cannot give any information about the sharp enhancement in the strength and elasticity of the gels by addition of trace TBA although it reveals salt-tuneable aggregates of gelator fibres.

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(a)

(b)

(c)

(d)

(e)

(f)

Figure 8. TEM images of e aggregates in 2 mL m-xylene (a), m-xylene with 100 µL acetonitrile (b), and with 100 µL different equivalent fractions of TBA acetonitrile solution 0.001 (c), 0.01 (d), 0.1 (e) and 1.0 (f).

3.6 Salting Out Effects. Further exploration of TBA effects (≤0.01 equi.) on CGC of the gelator e in m-xylene before and after addition of TBA for the examined gel systems are carried out, and it is expected to confirm whether the sharp enhancement of gel strength and elasticity can be ascribed to a salting out effect or not. In 1870s, Frank Hofmeister described the effect of different salts to decrease (salting-out species) or increase (salting-in species) the solubility of proteins, some other colloids [40-44], amino acids [45] and water-soluble polymers [46]. To date, there are a few nice works to explore such salt-effects on gelation mechanism or gelation behaviors of supramolecular hydrogels derived from LMMGs

[47-51]

. In this work, three samples of the e/m-

xylene gel (the gel 1) and the e/m-xylene/acetonitrile gel without TBA (the gel 2) and the e/mxylene/acetonitrile gel with 0.01 equivalent of TBA (the gel 3) were diluted gradually to form a clear solution or sol by adding their corresponding solvents. Interestingly, when two systems of the gel 1 and the gel 2 turned into clear solutions, the gel 3 does still keep gel state (c.f. Figure 9). It shows that the addition of TBA in the system results in the decrease of the CGC of gelator e in m-xylene with trace acetonitrile. Quantitatively, all these systems are studied further in detail to

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get their CGC values by taking average of three parallel experiments. As a result, the CGC values are in the order of the gel (0.080 %) > the gel 2 (0.070 %) > the gel 3 (0.055 %, w/v), respectively. Furthermore, the salt out effect was further

confirmed

by

stress

sweep

measurements of the four gel systems consisting different gelator concentration with the certain amount of TBA (e% = 0.5 %, 1.0%, 2.0%, and 3.0%, w/v, respectively). The experimental results

(a)

(b)

(c)

showed that the salt effect on the elasticity and the strength of molecular

Figure 9. Salt effect on physical gelation during the dilution with corresponding solvents: (a) e/m-xylene gel,

gels

were

clearer

at

low

gelator

(b) and (c) e/m-xylene/acetonitrile gel without and with

high

gelator

TBA. The results provide possible reasons for the

concentrations

than

at

concentration

(Salt

effect=

enhancements of the elasticity and the strength of the gels

(G′(salt)-

G′0)/G′0, G′0 = storage modulus of the gel

with addition of trace TBA. The photo was recorded in the concentration of gelator with 0.065% (w/v).

without TBA, G′(salt) = storage modulus of the gel with TBA, c.f. Figure S8 in the ESI). This again indicates that this phenomenon is particularly due to a salting out effect. Therefore, addition of TBA results in the decrease of the solubility of the gelator e in the studied solvent, and the gelator dissolving in the solvent re-assemble into three dimensional networks. It can be predicated that more gelator molecules aggregate into fibrous networks of the resulting gels with TBA below 0.01 equiv. Therefore, it can enhance elasticity and strengths of the molecular gels. However, when the amount of TBA increased above 0.01 equivalents to the gelator e, the anion-urea interactions are dominated although the salting out effect still exist which is monitored by NMR titration as shown in Figure 6. Consequently, the elasticity and strengths of the molecular gels begin to decrease. In summary, salt-tuneable elasticity and strengths of the examined gels results

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from the combined influence of the salting out and the urea-anion interactions. The salting out effect is dominated at lower TBA concentrations (below 0.01 equiv.) which can enhance of the gel strength, while at higher TBA concentrations (above 0.01 equiv.) carboxylic anions from TBA can destroy the α-tape urea hydrogen bonding in the gels via the formation of more anion-urea hydrogen bonds. 3.7 Crystallization of Organic Semiconductors in Gel Media. As well-known, crystallization is very important in supramolecular electrics

[52, 53]

, pharmaceuticals [54, 55], catalysts

[56]

and solid

explosives [57], etc. Among them, organic semiconductor compounds exhibit potential applications in generating highly efficient devices from single crystals through supramolecular assembly

[58-60]

Figure 10. Growing crystals of organic semiconductors in molecular gel derived from e in toluene. The selected semiconductors are 6,13-pentacenequinone (a), 9,10-bis-phenylethynyl-anthracene (b), N,N,N',N'tetraphenylbenzidine (c), and triphenylamine (d), respectively.

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or supramolecular complex

[61, 62]

. Therefore, the purity, morphologies, or habits of their crystals

cannot be ignored. In a few reported cases

[21-25]

, supramolecular gels derived from LMMGs are

proved to be another novel media for controlling crystallization except conventional gels (polymeric or clay-like materials). In this work, the gels we adopted were transparent and chemical stimulus responsive (c.f. Figure S9) for the convenient viewing of organic crystallization and the easy recovery of crystal from them. Several organic semiconductor compounds were chosen to grow their crystals in supramolecular gels from gelator e in aromatic solvents at low temperature. In one typical case, the selected organic semiconductor compounds, e.g. 6,13-pentacenequinone, 9,10-bis-phenyl-ethynyl-anthracene, N,N,N',N'-tetraphenylbenzidine, and triphenylamine, etc., can easily form their crystals in the e/toluene gels (e%=1%, w/v, ~2 oC, c.f. Figure 10). This has lot of scopes for the use of supramolecular gels derived from LMMGs to grow crystals of organic semiconductors and further to generate highly efficient electronic devices.

4. CONCLUSIONS A series of novel urea-based derivatives were prepared, and their gelation behaviors were tested in various solvents. Interestingly, we have found two efficient LMMGs. The produced xylene gels from the gelator e exhibit very good thixotropic properties and the sol-gel transition is instantaneous and fully reversible. Thixotropic test could be repeated for more than 50 cycles. In particular, addition of only 0.01 equivalent of TBA into the pristine gel results in a sharp enhancement of the strengths and the elasticity of the gel. It has been proved to be the result that the salting out effect reduces the CGC values of gelator e in m-xylene, and reasonably the salting out effect is dominant at low concentration of TBA (below 0.01 equiv.) in the examined systems. When the TBA amount is increasing above 0.01 equiv., the competition between anion-urea interactions and urea-urea interactions is dominated. As a result, the strength and the elasticity of the gel are beginning to decrease when TBA amounts are increased from this concentration. The effects are further confirmed by CGC tests, rheometry, TEM and NMR measurements. To date,

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there has been no report in LMMG-based organogels, which provides a new strategy for tuning rheology of organogels derived from LMMGs. This parallels the literature reported functions of metal-complexes and other inorganic incorporates which can also enhance the molecular gel′s strength or triggers gel formation. Moreover, our molecular gels can be also proved to be good media for growing crystals of organic semiconductor compounds. ASSOCIATED CONTENTS Supporting Information. Characterization of Compounds (a-f), preparation of TBA acetonitrile solution, gelation behaviours of all the resulting compounds (a-f) in the solvents under sonication, critical gelation concentration of e in selected solvents, thixotropic behaviours, salt effects on gel rheology, 1H NMR titration data, and chemical-stimulus responsiveness of the gel. AUTHOR INFORMATION *Fax +86-(0)29-81530811; e-mail: [email protected] ACKNOWLEDGEMENTS The authors thank for the financial supports of this work provided by the National Natural Science Foundation of China (Grant 21473110 and 21527802), the Natural Science Foundation of Shaanxi Province (Grant 2015JM2068), the 111 project (B14041) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT-14R33). We are grateful for his helpful discussion with Prof. Richard G. Weiss in Georgetown University, USA, in exploring salt effects on the rheology of molecular gels. We also express our gratitude for the technical supports by Mr Mingzhen Wang, and Mrs Juan Fan in Shaanxi Normal University, Xi’an China. REFERENCES (1) Terech, P.; Weiss, R. G. Low molecular mass gelators of organic liquids and the properties of their gels. Chem. Rev. 1997, 97, 3133-3160. (2) Estroff, L. A.; Hamilton, A. D. Water gelation by small organic molecules. Chem. Rev. 2004, 104, 1201-1218. (3) Liu, K. Q.; He, P. L.; Fang, Y. Progress in the studies of low-molecular mass gelators with unusual properties. Sci. China. 2011, 54, 575-586.

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GRAPHIC ABSTRACT The elasticity and strength of the resulting gels can be remarkably enhanced initially by the addition of a trace amount of tetrabutylammonium acetate (TBA) followed by a subsequent drop with further salt addition. This phenomenon is due to the dominated effects, the salting out effect at lower gelator concentration or the anion-urea hydrogen bonding at higher gelator concentration.

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Salt Tunable Rheology of Thixotropic ... - ACS Publications

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