Ultrasound- and protonation-induced gelation of a carbazole-substituted divinylquinoxaline derivative with short alkyl chain

Accepted Manuscript Ultrasound- and protonation-induced gelation of a carbazole-substituted divinylquinoxaline derivative with short alkyl chain Kechang Li, Pengchong Xue, Yanbin Shen, Jiaxi Liu PII:

S0143-7208(17)32269-6

DOI:

10.1016/j.dyepig.2018.01.010

Reference:

DYPI 6484

To appear in:

Dyes and Pigments

Received Date: 1 November 2017 Revised Date:

8 December 2017

Accepted Date: 6 January 2018

Please cite this article as: Li K, Xue P, Shen Y, Liu J, Ultrasound- and protonation-induced gelation of a carbazole-substituted divinylquinoxaline derivative with short alkyl chain, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.01.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphic abstract

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A divinylquinoxaline derivative with short alkyl chain was found to form gel under ultrasound and partial protonation.

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Ultrasound-

and

protonation-induced

gelation

of

a

carbazole-substituted divinylquinoxaline derivative with short alkyl chain

a

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Kechang Li,a∗ Pengchong Xue,b* Yanbin Shen,a Jiaxi Liua College of Chemistry, Jilin University, No. 2699, Qianjin Street, Changchun, 130012, P. R.

China b

Tianjin Key Laboratory of Structure and Performance for Functional Molecules, Key Laboratory

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of Inorganic-Organic Hybrid Functional Material Chemistry, Ministry of Education, College of

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Chemistry, Tianjin Normal University, No. 393, Bin Shui West Road, Tianjin, 300387, P.R. China

Abstract: A carbazole-substituted divinylquinoxaline derivative with a short alkyl chain was found not to form a gel in common solvents by the heating–cooling method. However, stable gel phases were obtained by ultrasonic treatment. Scanning electron

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microscopy analysis showed that large nodes linking thin fibers induced the formation of a suspension liquid when hot solution was cooled to room temperature. The ultrasonic treatment of the hot solution induced the formation of an orange gel by

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generating numerous small nodes. Spectral analysis suggested that the molecules self-assembled into nanofibers with the aid of intermolecular hydrogen bonds and π–π interactions in the gel. Gelation also caused an emission color change from yellow to

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orange. The gelator bound a proton to form a blue cation in solution, and a phase transition occurred from orange gel to blue sol while excess acid was added. Moreover, the partial protonation of the gelator resulted in the formation of a green gel without ultrasound stimulus. The protonated and unprotonated gelators formed co-assemblies in the partially protonated green gel along with fluorescent complete quenching. Keywords: organogel; stimulus-responsive; ultrasound, self-assembly, protonation

∗

Corresponding Author: [email protected]; [email protected]

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1. Introduction Supramolecular organogels formed by low-molecular-mass gelators (LMMGs) have been paid considerable attention by material researchers because of the materials’ unique functional properties.1-7 These supramolecular soft materials comprised a large

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amount of solvent and low concentration gelator (typically <2.0 wt%). Gelator molecules tend to self-assemble into 1D nanofibers, which further form 3D networks that immobilize the liquid component to a variable extent (principally) by surface tension.2 The driving force of a gelator’s self-assembly involves various noncovalent

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intermolecular interactions, such as hydrogen bond, Van der Waals, π–π stacking interaction, and coordinate bonds.3 These intermolecular weak interactions may be

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easily destroyed reversibly or irreversibly by external stimuli, including heating,4 light irradiation,5 cation,6 anion,7 water,8 and redox reagents.9 Thus, the phase switches from gel to sol is accompanied by sharp changes in physicochemical properties. To date, these smart soft materials have been developed for significant applications, such as in solar cells,10 field-effect transistors,11 and gas sensors.12

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In general, typical gelators contain amide, amino acid, or urea moieties as well as long alkyl chains, sugars, and cholesterol units as auxiliary groups for 1D noncovalent weak interaction. Recently, some nontraditional LMMGs have been developed, and

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these molecules possess nonplanar13 and banana14 configurations or some specific auxiliary groups, such as tetrabutyl,15 triphenylamine,16 CF3,17 methyl ester.18 The

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typical process of preparing gels is heating–cooling. Recently, other methods, such as chemical reaction,19 metal coordination by adding metal cations,20 and ultrasonic treatment,21 have been developed to prepare supramolecular gels. Since the two works on

ultrasound-induced

gelation

reported

in

2005,21,22

some

works

on

ultrasound-induced gelation have emerged.23 In these reports, ultrasound aided a nongel compound, especially nontraditional LMMGs, to acquire a gelling state, or a weak gel to transform into a strong gel. Ultrasound potentially promotes a low supersaturation point by quickly producing a numerous nodes, which gives rise to side branching and low-density branching during crystal network formation.24 Moreover,

ACCEPTED MANUSCRIPT additional additives or seeds may also promote gel formation and adjust gel macroscopic properties by mismatch nucleation and enhanced tip branching. However, whether one compound may form gels under ultrasound and partial protonation stimuli has not been reported.

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In this work, a carbazole-substituted divinylquinoxaline derivative with a short alkyl chain (A4CQ, Scheme 1) was found to form a suspension when its hot solution was cooled to room temperature (RT). However, this derivative may form an orange gel under ultrasound stimulus. Scanning electron microscopy (SEM) images showed that

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the node-linked gel fibers shrank and increased in number after ultrasound treatment. As a result, a stable 3D network was achieved and prevented the solvent from flowing.

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Interestingly, when A4CQ was partially protonated by trifluoroacetic acid (TFA), a dark-green gel was obtained by the normal heating-to-cooling process, in which protonated A4CQ likely serve as additive to induce gelation.

2. Experimental section

2.1 Instruments and method: Infrared spectra were measured using a

The

UV-vis

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Nicolet-360 FT-IR spectrometer by incorporating the samples in KBr disks. spectra

were

determined

on

a

Mapada

UV-1800pc

spectrophotometer. Different temperature absorption spectra were obtained

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through heating mixture and then natural cooling. C, H, and N elemental analyses were performed on a PerkineElmer 240C elemental analyzer.

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Photoluminescence measurements were taken on a Cary Eclipse Luminescence Spectrophotometer. NMR spectra were carried out on a Mercury plus 400 MHz instrument. Mass spectra were obtained with Agilent 1100 MS series and AXIMA CFR MALDI-TOF (Compact) mass spectrometers. The fluorescence quantum yields of A4CQ in THF were measured by comparing to standards (Rhodamine 6G in water, ΦF = 0.75, λex = 488 nm). Rheological measurements were carried out on gels using a TA Instruments AR 2000 and using parallel plates (25 mm diameter). The molecular configuration was used to obtain the frontier orbitals of A4CQ and A4CQH+ by density functional theory (DFT)

ACCEPTED MANUSCRIPT calculation at B3LYP/6-31G level with the Gaussian 09W program package, and stimulated electron transition were performed by time-dependent density functional theory (TD-DFT) at CAM-B3LYP/631G level. 2.2 Gelation measurement

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The mixture of weighed compound and organic solvent was heated in a sealed bottle with 0.5 cm diameter by a heating panel until the solid was dissolved. After the solution was put at room temperature for 6 h or the hot solution was treated by an ultrasonic bath, the state of the mixture was evaluated by the

Synthesis

and

characteristics.

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“stable to inversion of a test tube” method. Compound

1,

2

and

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5,8-dibromo-2,3-dimethylquinoxaline were synthesized according to the lecture

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procedures.25 The synthesis route of A4CQ is shown in Scheme 1.

Scheme 1. Synthesis route of A4CQ. (3).

Methyltriphenylphosphonium

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3-nitro-9-butyl-6-vinyl-9H-carbazole

iodine (4.0 g, 9.9 mmol), compound 2 (2.93g, 9.9 mmol), and t-BuOK (2.30 g, 20 mmol) were dissolved in 30 mL dry THF at 0 ºC, and the solution was stirred for 2 h at room temperature. After filtrated to remove the solid, the solvent was removed under reduced pressure. The residue was purified using column chromatography (petroleum ether/CH2Cl2 = 1:1) to give light yellow solid (2.65 g, 91% in yield). Element analysis (%): calculated for C18H18N2O2: C, 73.45; H, 6.16; N, 9.52; Found: C, 73.51; H, 6.12; N, 9.55. 1H NMR (400 MHz, CDCl3) δ 9.02 (d, J = 2.1 Hz, 1H), 8.39 (dd, J = 9.0, 2.1 Hz, 1H), 8.17 (s,

ACCEPTED MANUSCRIPT 1H), 7.67 (dt, J = 10.0, 5.0 Hz, 1H), 7.42 (dd, J = 10.8, 9.0 Hz, 2H), 6.93 (dd, J = 17.4, 10.8 Hz, 1H), 5.85 (d, J = 17.5 Hz, 1H), 5.30 (t, J = 9.1 Hz, 1H), 4.44−4.29 (m, 2H), 1.96–1.80 (m, 2H), 1.48–1.36 (m, 2H), 0.99 (t, J = 7.4 Hz, 6H).

13

C NMR (101 MHz, CDCl3) δ 143.83, 141.33, 140.62, 136.81, 130.79,

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125.66, 123.03, 122.55, 121.65, 118.76, 117.31, 112.57, 109.70, 108.37, 43.45, 31.04, 20.48, 13.81.

6,6'-(1E,1'E)-2,2'-(2,3-dimethylquinoxaline-5,8-diyl)bis(ethene-2,1-diyl)bis( 9-butyl-9H-carbazol-3-amine) (5). To a 5,8-dibromo-2,3-dimethylquinoxaline

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(0.26 g, 0.82 mmol), compound 3 (0.54 g, 1.84 mmol), anhydrous K2CO3 (0.50 g, 2.9 mmol), and Pd(OAc)2 (2.5 mg, 0.01 mmol) and Bu4NBr (3.1 g, 7.4

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mmol) were added in dry DMF (15 mL). The mixture was stirred under a N2 atmosphere at 120 °C for 24 h. After cooling to room temperature, the mixture was poured into water (200 mL) and crude 4 was obtained by filtration and drying. Crude 4 was dispersed in the mixture of ethanol and THF (1:1). 10% Pd/C (10 mg) and hydrazine hydrate (3.0 mL, 80% in water) were added and

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the mixture was refluxed for 4 h. Pd/C was removed by filtration and then the filtrate was poured into water (100 mL). Crude product was collected by filtration

and

drying

and

purified

using

column

chromatography

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(methanol/CH2Cl2 = 1:30) to give light yellow solid (0.41 g, 65 % in yield). Element analysis (%): calculated for C46H46N6: C, 80.90; H, 6.79; N; Found: C, 80.94; H, 6.71; N. 1H NMR (400 MHz, CDCl3) δ 8.46 (d, J = 16.4 Hz, 2H),

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8.29 (s, 2H), 8.10 (s, 2H), 7.84 (d, J = 9.6 Hz, 2H), 7.63 (d, J = 16.4 Hz, 2H), 7.53 (d, J = 2.0 Hz, 2H), 7.39 (d, J = 9.1 Hz, 2H), 7.26 (d, J = 8.4 Hz, 2H), 6.95 (dd, J = 8.2, 2.0 Hz, 2H), 4.29 (t, J = 6.8 Hz, 4H), 3.70 (b, 4H), 2.94–2.76 (m, 6H), 1.87 (m, 4H), 1.43 (m, 4H), 0.98 (t, J = 7.3 Hz, 6H). 13C NMR (101 MHz, d6-DMSO) δ 153.03, 142.36, 140.68, 138.36, 134.15, 133.80, 131.57, 127.99, 124.38, 123.93, 123.32, 122.65, 120.00, 119.26, 115.79, 110.27, 109.83, 104.62, 42.59, 31.32, 23.58, 20.30, 14.23. N,N'-(6,6'-(1E,1'E)-2,2'-(2,3-dimethylquinoxaline-5,8-diyl)bis(ethene-2,1-diyl)bis( 9-butyl-9H-carbazole-6,3-diyl))diacetamide (A4CQ). 5 (0.40 g, 0.58 mmol) and

ACCEPTED MANUSCRIPT acetic anhydride (0.50 g, 4.9 mmol) was dissolved in 10 mL dry THF and stirred for 10 h at room temperature. After removing solvent, the residue was dispersed in ethanol and treated for 5 min by ultrasonic. The orange product was gained by filtration. Yield = 95%. Element analysis (%): calculated for C50H50N6O2: C, 78.30; H,

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6.57; N, 10.96; Found: C, 78.34; H, 6.51; N, 10.99. 1H NMR (400 MHz, DMSO) δ 9.98 (s, 2H), 8.50 (s, 2H), 8.37 (d, J = 16.7 Hz, 2H), 8.33 (s, 2H), 8.21 (s, 2H), 7.84 (d, J = 8.5 Hz, 2H), 7.79 (d, J = 16.6 Hz, 2H), 7.64 (d, J = 8.6 Hz, 2H), 7.56 (s, 4H), 4.39 (t, J = 6.7 Hz, 4H), 2.80 (s, 6H), 2.09 (s, 6H), 1.77 (m, 4H), 1.31 (m, 4H), 0.90 (t, J = 13

C NMR (101 MHz, d6-DMSO) δ 168.34, 153.02, 140.81,

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7.3 Hz, 6H) (Fig. S1).

138.37, 137.45, 133.85, 132.17, 131.36, 128.94, 124.90, 124.04, 122.86, 122.32,

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120.67, 119.56, 119.41, 111.64, 110.25, 109.90, 42.70, 31.27, 24.42, 23.57, 20.27, 14.20 (Fig. S2). MS, m/z: cal. 766.4, found 767.9, [M+H]+ (Fig. S3).

3. Results and discussion

3.1 Photophysical properties in solution

Initially, the UV–visible (UV–vis) absorption and emission in different solvents were

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determined. The maximal absorption peaks in THF were located at 450 nm, which gives the solution a yellow color. The absorption peaks in CH2Cl2 and DMF were similar to that in THF (Fig. 1a). Under 365 nm light, the THF solution emitted a

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yellow-green fluorescence. The emission peak was at 545 nm in THF (Fig. 1b), and the fluorescence quantum yield (ΦF) reached 0.49. The emission peak shifted to 554

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nm in CH2Cl2 and to 568 nm in polar DMF. The ΦFs in CH2Cl2 and DMF were determined to be 0.47 and 0.41, respectively. The slight shift in absorption spectra and large shift in emission spectra in different solvents suggest that A4CQ possesses a higher polarity in the excited state than in the ground state.26 To understand this phenomenon, we performed quantum chemical calculation on A4CQ by density functional theory calculations at the B3LYP/6-31G(d) level. After geometric optimization, frontier orbitals were obtained. The highest occupied molecular orbital (HOMO) mainly distributed at the phenyl rings of carbazole and quinoxaline units as well as vinyl groups (Fig. 1c). By contrast, the lowest unoccupied molecular orbital

ACCEPTED MANUSCRIPT (LUMO) was mainly located at the electron-withdrawing quinoxaline moiety and vinyl groups, and the orbital density was very low in the electron-donating carbazole. Moreover, the stimulated electron transition of A4CQ at the CAM-B3LYP/6-31G(d) level appeared at 425.7 nm. This transition was mainly ascribed to an intramolecular

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charge transition (ICT) from HOMO to LUMO (86%, Table S1). Therefore, light excitation led to an electron transfer from the donor unit to the acceptor and elicited a

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strong polar excited state.27

Fig. 1 (a) Normalized UV-vis absorption and (b) emission spectra of A4CQ in different solvents (10-5 M). (c) Frontier orbitals of A4CQ.

A4CQ achieved an ICT transition at 450 nm owing to the electron-withdrawing feature of the quinoxaline moiety. Thus, A4CQ would show a red-shifted absorption band when the quinoxaline moiety bound a proton to form a stronger

ACCEPTED MANUSCRIPT electron-withdrawing group.28 As a result, the response of A4CQ to the proton was investigated. When excess TFA was added, the yellow A4CQ solution was transformed into a blue solution (Fig. 2c). The yellow fluorescence completely disappeared (Fig. 2d), which implies the binding of the A4CQ to a proton.

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Meanwhile, Fig. 2a shows the absorption spectral change of A4CQ after TFA was added. The absorption peak at 454 nm gradually decreased with increasing TFA concentration. At the same time, a new absorption peak emerged at 611 nm and was ascribed to the protonated A4CQ. This peak was also gradually

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enhanced. Consequently, the solution became blue. An isosbestic point at 511 nm was observed and implied a reactive equilibrium between the two

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components. The number of binding protons of one A4CQ cannot be obtained intuitively from the curve of absorbance (454 nm) versus [TFA]/[A4CQ] because of the need for excess TFA to reach a saturation point. Therefore, trifluoromethanesulfonic acid (TfOH), an acid stronger than TFA, was selected to confirm the proton binding number. The absorption spectral change of A4CQ

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upon TfOH addition was similar to that in the TFA system. The original peak at 454 nm gradually disappeared, and the new peak at 614 nm was enhanced (Fig. S4). Moreover, the plot of absorbance at 614 nm versus concentration ratio

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clearly illustrated that one A4CQ only bound one proton to form a cation, namely, A4CQH+. Quantum chemical calculation was performed to further achieve insight into the increased absorption wavelength of A4CQH+. The

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stimulated absorption peak located at 786.77 nm was mainly derived from the transition from HOMO to LUMO (Table S2). Its HOMO largely distributed among carbazole moieties, and the LUMO was completely located at the quinoxaline unit (Fig. S5). This result suggests a considerable charge-separated state, and the quinoxaline unit that bound a proton became a stronger electron-withdrawing group than the original moiety.

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Fig. 2 (a) Absorption and (b) emission spectra of A4CQ in o-dichlorobenzene (10-5 M) in the presence of TFA; (c) and (d) photos of solution before and after

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adding excess TFA under natural and 365 nm light, respectively. Insets are the plots of absorbance and emission intensity vs [TFA]/[A4CQ]. λex = 425 nm.

Fluorescence spectra were obtained to monitor the fluorescent change of A4CQ

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solution upon TFA addition. The maximal emission peak was at 552 nm (Fig. 2b) and gradually decreased with increasing TFA concentration. Moreover, no new peak in the

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long-wavelength region was noted. This result suggests that A4CQH+ is nonluminescent, given the strong electron-withdrawing property of the protonated quinoxaline.

3.2 Photophysical properties and self-assemblies in gel phase The gelation ability of A4CQ was initially examined by a standard heating– cooling process (Table 1). A4CQ did not dissolve in nonpolar alkane solvents, such as cyclohexane, n-hexane, and n-octane, even upon heating. In polar alcohol solvents, including ethanol and n-butanol, only a yellow precipitate formed after hot solution was cooled to RT. A4CQ may dissolve in some solvents, such as THF, CH2Cl2,

ACCEPTED MANUSCRIPT o-dichlorobenzene (ODCB), and dimethylsulfoxide, under heating. Then, yellow solutions were obtained in these solvent systems after cooling to RT for over 2 h. Only a yellow suspension was generated when hot chlorobenzene (CB) solution was naturally cooled to RT. However, a translucent gel was attained when the hot CB

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solution was treated by an ultrasonic cleaner washing for 10 s (Fig. 3). The rheological behavior of the sample after ultrasound treatment also confirms the gel phase (Fig. S6). The strength (storage modulus, G') of the gel is larger than the loss modulus (G"), whereas the strain lgγ was less than ‒0.1, suggesting a gel phase. The

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critical gelation concentration (CGC) in CB was 1.0 mg/mL, which corresponded to 1.3 mM. Moreover, A4CQ also formed a gel in the mixed solvent of ODCB and

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n-octane (V/V = 1:1) under ultrasound stimulus, and CGC was as low as 0.5 mg/mL (0.65 mM).

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Table 1. Gelation ability of A4CQ in different solvents.a Solvent Status Solvent Status Cyclohexane I Toluene P n-Hexane I Chlorobenzene SP,b G (1.0)c n-Octane I ODCB S b Ethanol P ODCB/n-octane (v/v =1/1) SP, G (0.5)c n-Butanol P CH2Cl2 S THF S DMSO S G = gel; P = precipitate; S = soluble; I = insoluble; SP = suspension.

the sample was treated by

Gel was formed by ultrasound treatment for 10 s; the critical gelation

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heating-cooling process.

C

b

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concentration (CGC) [mg/mL].

Fig. 3 Photos of A4CQ in different conditions.

ACCEPTED MANUSCRIPT Generally, ultrasound exerts two distinct effects on gelator self-assemblies. Many molecular gels may be damaged by prolonged ultrasound treatment because 3D fibrillar networks are potentially broken down in this case.23b,29 By contrast, some examples indicate that ultrasound positively influences gel

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formation through ultrasound-induced rapid nucleation.30 Hereafter, the morphologies of the A4CQ aggregates in different conditions are compared to clarify the influence of ultrasonic stimulus. After the hot solution was naturally

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cooled, the A4CQ suspension comprised many aggregates, in which many thin

ACCEPTED MANUSCRIPT Fig. 4 SEM images of A4CQ aggregates in CB by (a) natural cooling, (b) ultrasonic stimulus, and natural cooling in the presence of 1.4 equiv. TFA.

fibers grew and extended from both ends of large nodes (Fig. 4a). Although these

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large aggregates came in contact with each other to a certain extent, such cross-linking was insufficient to support a stable gel phase. We were convinced that naturally cooling hot solutions in this system cannot provide numerous small crystal nuclei because of the difficulty of primary nucleation in this situation. Moreover,

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fibers would grow on the surface of these small nuclei as seeds that would form into isolated fibrous aggregates. However, these isolated aggregates only weakly mutually

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cross-link, and only a suspension emerges. On the contrary, large nodes disappeared after ultrasonic treatment and thin fibers were cross-linked by many small nodes to construct 3D fibrous networks (Fig. 4b). Such networks behaved similarly to a single fibrous network, in which fibers interpenetrated and interlocked with each other to

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support a gel. Therefore, ultrasound promoted gel formation in this case.

ACCEPTED MANUSCRIPT Fig. 5 (a) UV-vis absorption and (b) fluorescence spectral changes of A4CQ during aggregation from 120 ºC to room temperature in CB. Time interval is 0.5 min. λex = 440 nm. Inset in Fig. 5a is the absorption spectra of hot solution at 120 ºC and suspension at room temperature, and inset in Fig. 5b is the plot of fluorescence

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intensity at 541 nm vs. aging time.

UV–vis absorption, emission and infrared (IR) spectra were obtained to further investigate self-assemblies in gel phase. Initially, the IR spectrum of the gel was

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acquired. The vibration peaks of N–H and C=O were located at 3277 and 1650 cm–1, respectively (Fig. S7), and indicated that intermolecular hydrogen bonds between

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amide groups exist.31 Subsequently, the IR spectra of ultrasound-induced gel and suspension were measured and compared. We found that the IR spectra of two samples were almost the same and suggest that intermolecular hydrogen bonds in the two kinds of samples were consistent. The ultrasound stimulus did not change the intermolecular hydrogen bonds. The UV–vis absorption spectra of the gel and

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suspension were also similar; thus, only the absorption spectral change in the absence of ultrasound treatment during cooling was observed. The maximal absorption peak was located at 447 nm, increase and red-shifted to 453 nm after 0.5 min (Fig. 5a, Fig.

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S8). After 1 min, the absorption band gradually decreased and showed molecular aggregation. After 15 min, the absorption peak was at 445 nm, and the absorbance from 500 nm to 600 nm was enhanced. Thus, a color change from yellow to orange

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was occurred during aggregation (Fig. 3). Fig. 5b reveals the fluorescence spectral alteration. The hot solution emitted a yellow fluorescence with a maximal emission peak at 541 nm, and the suspension displayed a maximal emission peak at 560 nm. However, while the suspension was centrifuged to obtain two phases, the supernatant exhibited a yellow fluorescence (λem = 549 nm), and the aggregates emitted orange red fluorescence (λem = 617 nm) (Fig. S9). These results show that the driving forces for molecular aggregation were intermolecular hydrogen bonds between amide moieties and π–π packing between aromatic units.

ACCEPTED MANUSCRIPT As shown, A4CQ manifested an obvious response to TFA in dilute solution. Thus, the gel in CB appeared also sensitive to TFA. Adding TFA did not only alter the absorption and emission spectra, but also regulated the gel formation condition. The peak at 446 nm gradually decreased, and a new peak at 610 nm

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emerged and enhanced while TFA was continuously added (Fig. 6a). As 6.4 equiv. TFA was added, an orange gel transformed into a blue solution (Fig. 6c). To understand the high solubility of A4CQH+ in CB, we obtained the substance’s optimal configuration (Fig. S10). A4CQH+ exhibits a nonplanar

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configuration derived from the steric hindrance between the bound proton and the hydrogen atom of the vinyl group, which may be responsible for the high

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solubility of A4CQH+. At the same time, fluorescence color changed from orange red to green (λem = 539 nm); this change was ascribed to the free A4CQ in solution. When excess triethylamine was added to the blue solution, an orange suspension re-emerged. Then, an orange gel reformed when this orange

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suspension was heated to dissolution and treated by ultrasound.

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Fig. 6 (a) Absorption and (b) emission spectral changes of A4CQ in CB in the

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presence of TFA. (c) Photos of A4CQ in different conditions.

Interestingly, a green gel was generated in the absence of ultrasound stimulus

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after 1.4 equiv. TFA was added. This green gel emitted green fluorescence (Fig. S11a). When this green gel was destroyed, the supernatant was yellow with green fluorescence, and the solid was dark green without fluorescence. This observation indicates that the green fluorescence of green gel was derived from the free A4CQ in the supernatant. After gel formation, the absorption band at 445 (yellow color) and 610 (blue color) nm co-existed (Fig. S11b), which led to green color of gel. Moreover, the green color of aggregates indicated that A4CQ and A4CQH+ co-assembled together in the green gel. The FT-IR spectrum of green gel also proves the co-assembly (Fig. S12). The vibrational

ACCEPTED MANUSCRIPT peak of N‒H became wide and slightly shifted to 3283 from 3277 cm‒1, and the band of C=O divided into two peaks at 1653 and 1743 cm‒1. This result indicates that A4CQH+ disturbed the intermolecular hydrogen bonding to a certain extent. Based on the absorbance at 610 nm, it is determined that 22%

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A4CQ was protonated while 1.4 equiv. TFA was added. Adding excess TFA, such as 6.4 equiv., produced a blue solution; hence, appropriate TFA amounts are needed to construct a green gel without ultrasound treatment.

Fig. 4c displays the SEM image of the green gel, which reveals the lack of

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large aggregates and the presence of a fibrous network. Moreover, the absorption band at around 610 nm was gradually enhanced during the

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formation of green gel (Fig. S11b), indicating that A4CQ gradually bound a proton to form A4CQH+ during gelation. Then, the A4CQ concentration may have rapidly decreased relative to the system without TFA. Such rapid decrease in A4CQ gelator concentration induced a low supersaturation, and then promoted the loose branching of fiber networks. Moreover, considering the

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co-assembly of A4CQ and A4CQH+ under low TFA concentrations, A4CQH+ may additionally provide mismatch nucleation, which promotes branching

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because A4CQH+ strongly adsorbs onto the tip surface of A4CQ fibers.32

ACCEPTED MANUSCRIPT Fig. 7 (a) Schematic of A4CQ aggregate formation in different conditions; and stacking models in (b) neat orange gel and (c) green gel. Based on the above results, the stacking models and aggregation states of A4CQ in different conditions are summarized in Fig. 7. A high supersaturation

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during natural cooling promotes the formation of few nuclei; then, numerous fibers grow on the tip surface of the nuclei to produce larger radiating fiber clusters. However, these fiber clusters only weakly cross-link and merely produce a suspension. By contrast, numerous small nuclei are generated when

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the hot solution is treated by an ultrasound bath, which causes low supersaturation. Then, the presence of branching growth and numerous nuclei

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resulted in a whole 3D network and the emergence of gel. In both conditions, molecular stacking models were the same (Fig. 7b). Intermolecular hydrogen bonding and π-π interactions induced 1D stacking. As a small TFA amount was added, A4CQH+ formed. Then, a low supersaturation of A4CQ was achieved. During the aggregation of A4CQ in the help of hydrogen bonding and π-π

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interaction, A4CQH+ assembled with A4CQ because of their same amide group (Fig. 7c). Moreover, insertion of A4CQH+ might result in mismatch nucleation, which also allowed branching growth and resulted in the formation

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of a green gel.

In summary, a carbazole-based divinylquinoxaline derivative with two butyl groups was found to form a suspension when the hot solution was naturally cooled. However,

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an orange gel was generated when the hot solution was treated by ultrasound. UV–vis absorption and IR spectra showed that intermolecular hydrogen bonds and π–π interactions were the driving forces of molecular aggregation, and ultrasound treatment did not change the intermolecular interaction. SEM observations suggested that ultrasound can accelerate nucleation and promote branching growth to form a cross-linking fibrous network. More interestingly, partial protonation supported the production of a green gel in the absence of ultrasound stimulus because of the low supersaturation and mismatch nucleation. Thus, this work implied that, in addition to ultrasound, partial protonation can also facilitate molecules to acquire the ability to

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ACCEPTED MANUSCRIPT 1. A carbazole-substituted divinylquinoxaline derivative with a short alkyl chain was

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found to form a gel under ultrasound stimulus. 2. The gel could change into a blue sol upon addition of acid, indicating acid responsive gel-sol phase transition. 3. Partial pronation could result in a green gel without ultrasound treatment.