Wholly π-Conjugated Low-Molecular-Weight Organogelator That

Feb 24, 2014 - A novel smart supramolecular organic gelator exhibiting dual-channel responsive sensing behaviours towards fluoride ion via gel–gel s...
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Wholly π‑Conjugated Low-Molecular-Weight Organogelator That Displays Triple-Channel Responses to Fluoride Ions Jongha Lee,† Ji Eon Kwon,† Youngmin You,‡ and Soo Young Park*,† †

Center for Supramolecular Optoelectronic Materials and Department of Materials Science & Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Korea ‡ Department of Advanced Materials Engineering for Information & Electronics, Kyung Hee University, 1 Seocheon-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Korea S Supporting Information *

ABSTRACT: A novel salicylidene aniline-based wholly π-conjugated molecule that could be self-assembled into an organogel was synthesized. The rigid organogel collapsed into fluid solutions with significant changes in UV−vis absorption and fluorescence colors in response to fluoride ions. It was found that the hydroxyl group in the salicylidene aniline moiety played a key role not only in the gelation but also in fluoride ion responses.

report the interaction with fluoride ions can be integrated into LMOGs to improve the sensitivity further. Recently, An et al. reported a series of simple and wholly π-conjugated LMOGs that consisted of π-cyanostilbenes and quadruple trifluoromethyl (CF3)-substituted aromatic rings.31−33 Despite the absence of typical structural motifs for gelation such as long alkyl chains or hydrogen-bonding groups, the compounds readily formed gels in various organic solvents and also exhibited aggregation-induced enhanced emission (AIEE) behaviors. To give these peculiar aromatic gelators a fluoride-responsive property, we constructed LMOG 1 (Scheme 1) carrying a salicylidene aniline moiety instead of the π-cyanostilbene linker in this work. The salicylidene aniline-based ortho-hydroxy Schiff base derivatives produce unique excited-state intramolecular proton-transfer (ESIPT) fluorescence,34−40 which is characterized by a phototautomerization process between the cyclic Brønsted acid and base moieties. Basic anions such as fluoride ions can abolish the ESIPT process by eliminating the proton from the salicylidene aniline moieties, resulting in large spectral shifts in both absorption and fluorescence.34−38,41−43 It is therefore envisioned that organogels of 1 can respond to fluoride ions in a multimodal manner. Herein, we report the design, syntheses, and fluoride ion detection properties of a novel wholly π-conjugated LMOG, 1. 1 displays fluoride-ion-responsive gel-to-sol transition that accompanies unique absorption and fluorescence colorimetric changes.

1. INTRODUCTION Low-molecular-weight organogelators (LMOGs) are molecules that are able to self-assemble into entangled 3D networks through weak noncovalent interactions such as hydrogen bonding, π−π interaction, and van der Waals forces.1−4 The LMOGs are extremely sensitive to small variations in surrounding environments including temperature, pH, light, ions, and solvents, leading to significant changes in the structural and physical properties.4−8 Such stimuli-responsive LMOGs can serve as smart functional materials for potential use in tunable templates,9 reaction vessels,10 sensors,11−14 and drugdelivery systems.3 In particular, fluoride-ion-responsive LMOGs have drawn special attention because of the significance of the fluoride ion in phathophysiological and environmental processes.15,16 To create fluoride-responsive LMOGs, multiple urea and amide groups are typically employed as molecular building blocks so that their intermolecular hydrogen bonds in the supramolecular structure of the gels could be collapsed by basic fluoride ions.15,17−21 Additional auxiliary gelating moieties such as cholesterol, long alkyl groups, and dendrons are frequently combined in addition to the hydrogen bonding groups to achieve sufficient intermolecular interaction for gelation.15,22−30 Although a few fluoride-responsive LMOGs showed excellent sensitivity,20,22,30 the fluoride detection ability of the LMOGs relying on the auxiliary gelating moieties is usually limited because their strong van der Waals interaction may reduce their sensitivity toward interactions with highly polar fluoride ion. Thus, it is envisioned that wholly π-conjugated LMOGs with the minimized structural modifications can offer highly sensitive fluoride responses. In addition, fluorophores that directly © 2014 American Chemical Society

Received: September 23, 2013 Revised: February 17, 2014 Published: February 24, 2014 2842

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Scheme 1. Chemical Structures and Synthesis Routes of Organogelator 1 and Control Compound 2

334.0503. Anal. Calcd for C 53.91, H 2.41, O 9.57. Found: C 53.97, H 2.36, O 9.96. 2.1.2. Synthesis of 3′,5′-Bis(trifluoromethyl)biphenyl-4-amine (1b). 3,5-Bis(trifluoromethyl)phenylboronic acid (0.90 g, 3.49 mmol) and Pd(PPh3)4 (0.17 g, 0.14 mmol) were added three times to solution of 4-bromoaniline (1.50 g, 2.64 mmol) in 30 mL of dimethyl ether solution with 15 mL of an aqueous 2.0 M sodium carbonate solution. Reaction mixtures were vigorously stirred and refluxed. After 18 h, the solution became black. The crude mixture was poured into distilled water and titrated to pH 7.0 with 1.0 M hydrochloric acid. The organic layer was extracted with 150 mL of dichloromethane three times and dried with MgSO4. Silica column chromatography (ethyl acetate/ n-hexane = 1:5) gave 0.70 g of white powder (yield = 46%). 1H NMR (300 MHz, CDCl3, 298 K) δ = 7.94 (s, 2H), 7.75 (s, 1H), 7.44 (d, 2H, J = 8.5 Hz), 6.79 (d, 2H, J = 8.5 Hz), 3.95 (s, 1H). 13C NMR (125 MHz, CDCl3, 298 K) δ = 147.3, 143.2, 132.1, 131.7, 128.2, 128.2, 126.2, 125.3, 121.7, 119.7, 116.7, 115.5. HRMS (EI, positive): [M]+ calcd for C14H9F6N 305.0639; found 305.0642. Anal. Calcd for C 55.09, H 2.97, N 4.59. Found C 55.45, H 2.90, N 4.65. 2.1.3. Synthesis of (E)-4-((3′,5′-Bis(trifluoromethyl)biphenyl-4ylimino)methyl)-3′,5′-bis(trifluoromethyl)biphenyl-3-ol (1). A solution of 3-hydroxy-3′,5′-bis(trifluoromethyl)biphenyl-4-carbaldehyde (0.70 g, 1a, 2.09 mmol) and 3′,5′-bis(trifluoromethyl)biphenyl-4amine (0.64 g, 1b, 2.09 mmol) in 50 mL of ethanol was refluxed. After 24 h, the crude solutions were cooled for 0 °C and filtered. The filtered powder was recrystallized in 50 mL of hot ethanol. After the solution was filtered, 1.08 g of yellowish needles of 1 was obatined (yield = 82%). 1H NMR (400 MHz, CDCl3, 298 K) δ = 13.31 (s, 1H), 8.76 (s, 1H), 8.09 (d, 4H, J = 12.5 Hz), 7.91 (d, 2H), 7.72 (d, 2H, J = 8.5 Hz), 7.58 (d, 1H, J = 8.2 Hz), 7.48 (d, 2H, J = 8.2 Hz), 7.32 (d, 1H, J = 1.5 Hz), 7.25 (dd, 1H, J = 7.9 Hz, 1.5 Hz). 13C NMR

2. EXPERIMENTAL SECTION 2.1. Materials and Characterization. Commercially available 4-bromo-2-hydroxybenzaldehyde, 4-bromoaniline, 4-bromobenzaldehyde, 3,5-bis(trifluoromethyl)phenylboronic acid, tetrakis(triphenylphosphine)palladium(0), and all kinds of solvents were purchased from SigmaAldrich Chemical Co. and used without further purification. 1H NMR were obtained with a Bruker-300 spectrometer, and 13C NMR were obtained with a Bruker-500 using chloroform-d as a solvent with tetramethylsilane as the internal standard. High-resolution mass spectroscopy (HRMS) analysis was carried out with a JEOL JMS-700. Elemental analysis was carried out with an EA1112 (CE Instrument, Italy). XRD measurements were performed with a D8-Advance (Bruker, CuKα, λ = 1.5418 Å). 2.1.1. Synthesis of 3-Hydroxy-3′,5′-bis(trifluoromethyl)biphenyl4-carbaldehyde (1a). 4-Bromo-2-hydroxybenzaldehyde (0.53 g, 2.64 mmol), 3,5-bis(trifluoromethyl)phenylboronic acid (0.82 g, 3.16 mmol), and Pd(PPh3)4 (0.15 g, 0.13 mmol) were added to 30 mL of a tetrahydrofuran solution in the dark. Then, 15 mL of a 2.0 M aqueous sodium carbonate solution was added. Reaction mixtures were vigorously stirred and reflux. After 18 h, the reaction mixtures became black. The crude mixture was poured into distilled water and titrated to pH 7.0 with 1.0 M hydrochloric acid. The organic layer was extracted with 150 mL of dichloromethane three times and dried with MgSO4. Silica column chromatography (ethyl acetate/ n-hexane = 1:30) gave 1.12 g of white powder (yield = 79%). 1H NMR (300 MHz, CDCl3, 298 K) δ = 11.14 (s, 1H), 9.98 (s, 1H), 8.04 (s, 2H), 7.94 (s, 1H), 7.71 (d, 1H, J = 7.9 Hz), 7.28−7.24 (m, 2H). 13 C NMR (125 MHz, CDCl3, 298 K) δ = 196.3, 162.2, 146.5, 141.8, 134.8, 127.7, 125.2, 122.6, 122.6, 122.5, 121.6, 120.7, 116.7. HRMS (EI, positive): [M + H]+ calcd for C15H9F6O2, 334.0462; found, 2843

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(125 MHz, CDCl3, 298 K) δ = 160.1, 152.6, 142.9, 142.6, 141.3, 136.7, 136.2, 130.0, 128.4, 127.9, 127.5, 127.4, 125.4, 125.3, 122.1, 121.8, 121.7, 121.1. HRMS (FAB, positive): [M + H] + calcd for C29H16F12NO 621.1040; found 622.1042. Anal. Calcd for C 56.05, H 2.43, O 2.57, N 2.25. Found C 56.06, H 2.33, O 2.89, N 2.27. 2.1.4. Synthesis of 3′,5′-Bis(trifluoromethyl)biphenyl-4-carbaldehyde (2a). 4-Bromobenzaldehyde (0.90 g, 4.86 mmol), 3,5-bis(trifluoromethyl)phenylboronic acid (1.51 g, 5.84 mmol), and Pd(PPh3)4 (0.28 g, 0.24 mmol) were added to 30 mL of tetrahydrofuran solution in the dark. Then, 15 mL of a 2.0 M aqueous sodium carbonate solution was added. The reaction mixture was vigorously stirred and reflux. After 18 h, the reaction changed to a black solution. The crude mixture was poured into distilled water and titrated to pH 7.0 with 1.0 M hydrochloric acid. The organic layer was extracted with 150 mL of dichloromethane three times and dried with MgSO4. Silica column chromatography (ethyl acetate/n-hexane = 1:30) gave 1.49 g of white powder (yield = 96%). 1H NMR (300 MHz, CDCl3, 298 K) δ = 10.40 (s, 1H), 8.05 (s, 2H), 8.03 (d, 2H), 7.93 (s, 1H), 7.79 (d, 2H). 13C NMR (125 MHz, CDCl3, 298 K) δ = 191.7, 144.1, 142.2, 136.6, 132.9, 130.8, 128.2, 127.7, 124.5, 122.3. HRMS (CI, positive): [M]+ calcd for C15H9F6O 319.0558; found 319.0557. Anal. Calcd for C 56.62, H 2.53, O 5.03. Found C 56.66, H 2.47, O 5.13.

2.1.5. Synthesis of (E)-N-((3′,5′-Bis(trifluoromethyl)biphenyl-4yl)methylene)-3′,5′-bis(trifluoromethyl)biphenyl-4-amine (2). A solution of 3′,5′-bis(trifluoromethyl)biphenyl-4-carbaldehyde (0.63 g, 2a, 1.96 mmol) and 3′,5′-bis(trifluoromethyl) biphenyl-4-amine (0.60 g, 1b, 1.96 mmol) in 50 mL of ethanol was refluxed. After 24 h, the crude solutions were cooled to 0 °C and filtered. The filtered powder was recrystallized in 50 mL of hot ethanol. After the solution was filtered, 0.77 g of a white powder (2) was obtained (yield = 65%). 1H NMR (300 MHz, CDCl3, 298 K) δ = 8.58 (s, 1H), 8.07 (m, 6H), 7.89 (d, 1H, J = 15.2 Hz), 7.75 (d, 2H, J = 8.3 Hz), 7.69 (d, 2H, J = 8.4 Hz), 7.39 (d, 2H, J = 8.4 Hz). 13C NMR (125 MHz, CDCl3, 298 K) δ = 162.6, 161.9, 149.0, 143.1, 142.6, 142.4, 137.2, 133.5, 128.6, 122.4, 119.5, 118.3, 116.3. HRMS (FAB, positive): [M]+ calcd for C29H16F12N

Table 1. Gelation Properties of 1 in Organic Solventsa solvent CHCl3 CH2Cl2 THF ethanol 1-butanol 1-octanol ethyl acetate butyl acetate

1b CGC (wt %)c S S S I G G I S

1.6 0.9

solvent benzene toluene cyclohexane n-hexane n-decane n-dodecane 1,2-dichloroethane 1,1,2,2tetrachloethane

1b CGC (wt %)c S S G I G G G G

2.3 3.0 2.5 2.0 2.7

Figure 2. Comparison of normalized UV−vis absorption and fluorescence spectra of a solution of 1 (dotted lines; 10 μM in 1,2,dichloroethane) and xerogel (solid lines; 0.1 wt % in sodium chloride solid solution), respectively. A diffuse reflectance technique was employed for the measurement of the UV−vis absorption spectrm (corrected by the Kubelka−Munk function46).

a

G, stable gel formed at room temperature; S, soluble; I, insoluble. The concentration of 1 was 3.0 wt %. cCritical gelation concentration (CGC) of 1 in each solvent. b

Figure 1. (a) Optical and (b) fluorescence (λex = 365 nm) microscope images of xerogel 1 cast on a glass substrate. Scale bar = 50 μm. (c, d) FESEM images of the xerogel of 1 taken at 15.0 kV. The scale bars correspond to (c) 50 μm and (d) 5 μm, respectively. 2844

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Figure 3. (a) XRD profile of xerogel 1. (b) Lowest-energy geometry of 1 in the gas phase calculated using density functional theory with the B3LYP functional and 6-31g+(d,p) basis set. (c) Schematic illustration of the suggested packing structure of 1 in the gel state.

Figure 4. (a) Electrostatic potential isosurfaces of 1 and schematic illustration of the intermolecular electrostatic attraction due to the anisotropic electronic distribution of 1 and steric hindrance of bulky CF3 groups. Electrostatic potential isosurfaces of (b) 2 and (c) deprotonated 1. The Gaussian 09 software package was employed for the quantum chemical calculations. Potential ranges were set to a constant value from −1.600 × 10−2 to 1.600 × 10−2. functional theory using the hybrid B3LYP functional with the splitvalence polarized 6-31g+(d,p) basis set for all species. To obtain estimates of the vertical electronic excitation energies that include some account of electron correlation, time-dependent density functional theory (TD-DFT) calculations with the B3LYP functional and 6-31g+(d,p) basis set were utilized. B3LYP corresponds to the combination of Becke’s three-parameter exchange functional (B3) with a Lee−Yang−Parr fit for the correlation functional (LYP).

606.1091; found 606.1089. Anal. Calcd for C 57.53, H 2.50, N 2.31. Found C 57.57, H 2.50, N 2.31. 2.2. Measurements. All of the solvents used for spectroscopy were spectroscopic grade purchased from Sigma-Aldrich Chemical Co. and SDS. Steady state absorbance, fluorescence excitation, and emission spectra of the solution were recorded on a Uvikon 933 (Kontron) and a Fluoromax 3 (Jobin Yvon Horiba). Each fluorescence spectrum was corrected by detector sensitivity and subtracted from the baseline. Solid-state UV-reflection absorbance and emission spectra of xerogel 1 were recorded in a Cary 5000 (Varian). Sodium chloride was purchased from SDS and dried over 3 days at 100 °C. The relative quantum yield was calculated by using quinine sulfate in 0.5 M sulfuric acid as the fluorescence reference standard (Φ = 0.546). FT-IR spectra over the range from 4000 to 650 cm−1 were collected at room temperature using a Nicolet 6700 (Thermo Scientific) spectrometer, equipped with attenuated total reflection (ATR) accessories (ZnSe/ diamond). Diffuse reflectance UV−vis spectra of organogels were recorded using a Lambda 1050 (PerkinElmer) spectrophotometer equipped with a 150 mm integrating sphere. 2.3. Calculations. Gaussian 09 and GaussView 5.0 have been employed for quantum chemical drawing and calculation. 44 The geometry of the ground-state structures was optimized by density

3. RESULTS AND DISCUSSION The chemical structures and synthesis routes of newly designed salicylidene aniline LMOG 1 and control compound 2 are shown in Scheme 1. These compounds were synthesized in three simple steps through Suzuki coupling and Schiff base reactions in good yields. Details of synthesis of compounds 1 and 2 are described in the Experimental Section. Spectroscopic and analytical data fully agreed with the anticipated structures (Experimental Section). When various organic solutions (1.00 g) containing 1 were heated to complete dissolution and subsequently cooled to room temperature, yellow organogels were formed (Table 1). Compound 1 could form organogels in higher 2845

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Figure 5. FT-IR spectra of compound 1 (a) calculated in the gas phase using DFT and measured in (b) 0.1 M chloroform solution and (c) 2.0 wt % gel in 1,2-dichloroethane. Dotted lines indicate the main peaks coupled with C−F stretching vibrations. The strong peak at 1215 cm−1 in b is assigned to the C−H bending vibration of the chloroform solvent.

alcohols, aliphatic hydrocarbons, and chlorinated ethanes such as 1-butanol, 1-octanol, cyclohexane, n-decane, n-dodecane, 1,2dichloroethane, and 1,1,2,2-tetrachloroethane. The critical gelation concentration (CGC) in higher alcohols was relatively low (1.6 wt % for 1-butanol and 0.9 wt % for 1-octanol, respectively) compared to that in other gelatable solvents (CGC > 2.0 wt %). The “dropping-ball” method was employed to determine gel-to-sol transition temperatures,45 which were 44 °C in 1,2-dichloroethane, 17 °C in 1,1,2,2-tetrachloroethane, and 55 °C in 1-octanol, respectively. The organogels in the chlorinated solvents and 1-octanol were very stable over several months. However, 1 readily dissolved in chloroform, dichloromethane, THF, and nonpolar aromatic solvents including benzene and toluene at room temperature. In contrast, 1 was

Figure 7. (a) Normalized absorbance and (b) fluorescence spectra of the original as-prepared organogel 1 (□, 2.0 wt % in 1,2dichloroethane), the collapsed gel (△), the restored gel (○), and the xerogel (◊) made from the restored gel by the drying process.

insoluble in ethanol, n-hexane, and ethyl acetate. Optical microscope and field emission scanning electron microscope (FE-SEM) images reveal that organogel 1 consists of entangled

Figure 6. Fluoride ion induces the collapse of the organogel of 1 (2.0 wt % in 1,2-dichloroethane): Photographs showing (a) the fluorescent image of an as-prepared organogel under a 365 nm hand-held UV lamp; (b) the image of an as-prepared organogel under room light; (c) the fluorescence image under a UV lamp; (d) the image of the xerogel under room light; (e−i) the time course of the gel collapse after the addition of TBAF (4 equiv): (e) 0, (f) 10, (g) 20, and (h) 30 min and (i) the fully collapsed organogel; (j) fluorescent image of the collapsed organogel under a 365 nm hand-held UV lamp; (k) the room-light image and (l) the fluorescent image of a restored organogel after the addition of 0.1 mL of methanol followed by the subsequent heat-and-cool process; (m) the room-light image; and (n) the fluorescent image of the xerogel made from the restored organogel by the drying process. 2846

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ribbons and the width of the ribbons ranges over several tens of micrometers (Figure 1). J-type aggregation of the organogel is inferred from the UV−vis absorption and photoluminescence spectra; an 18 nm red shift in the absorption maxima and a new shoulder band at 420 nm are observed in the UV−vis absorption spectra upon gelation, and the fluorescence intensity increases by 24 fold (λmax = 554 nm, Figure 2 and SI Figure S1). These spectroscopic behaviors are well-known indications of J-aggregation formation,31−33,47 also in accordance with the results from previously reported salicylidene aniline-based molecules in aggregated states.48,49 To obtain better insight into the stacking structure of organogel 1, we acquired and analyzed the out-of-plane X-ray diffraction (XRD) pattern of the xerogel of 1. As shown in Figure 3a, the XRD pattern exhibits strong diffractions at 11.43° (d100 = 7.75 Å) and higher-order diffractions. By comparing with the molecular length of 1 (21.29 Å, Figure 3b), it is inferred that molecules of 1 are stacked over each other in a lamellar fashion with a tilt angle of 21.3° (Figure 3c). Several strong peaks originating from d spacings of 7.84, 4.95, 4.38, and 3.64 Å were also observed in the wide-angle X-ray scattering (WAXS) (SI Figure S2). Importantly, the d spacing of 3.64 Å is the typical π−π stacking distance of aromatic molecules, suggesting that the π−π interaction plays an important role in the gelation of 1. It is noted that these structural assignments are consistent with the J-aggregation structure deduced from the UV−vis absorption spectra (vide supra). To determine the driving force for the gelation process of 1, we synthesized compound 2 (Scheme 1) as a control that lacked the hydroxyl group. In contrast to 1, attempts to prepare an organogel of 2 were unsuccessful (SI Figure S3), indicating that the hydroxyl group is crucial in the gelation process. It is speculated that the gelation is driven by the combined effects of the hydroxyl group, involving the planarization effect of the molecular backbone through an intramolecular hydrogen bond between the hydroxyl group and nitrogen atom and the increased dipolar interactions. The latter effect is particularly supported by the increase in polarity of 1 (2.28 D) relative to that of 2 (1.55 D) (Figure 4a,b). As An et al. suggested,31−33 the anisotropic charge distribution in 1 caused by the polar hydroxyl group may strengthen the intermolecular electrostatic attraction between neighboring molecules (Figure 4a). Moreover, quadruple CF3 groups in 1 corroborate the strong intermolecular interaction by donating C−F···H−C hydrogen bonds and induce a J-stacking structure resulting from steric hindrance from their bulkiness. To investigate the noncovalent interactions between C−F and H−C, FT-IR spectra of 1 were recorded in solution (0.1 M in chloroform) and gel (2.0 wt % in 1,2-dichloroethane). In the solution state, four main characteristic peaks appeared at 1385, 1281, 1184, and 1142 cm−1, well correlated with the calculated FT-IR spectrum of 1 in the gas phase using the DFT method (Figure 5a,b). The bands at 1184 and 1142 cm−1 are mainly attributed to C−F stretches of the CF3 groups.50,51 The strong band at 1281 cm−1 is assigned to the phenolic C−O stretching vibrational mode.52 According to the calculation results and the literature, it is found that the two bands at 1385 and 1281 cm−1 should also be coupled to the C−F stretching modes.53 In sharp contrast, these bands are shifted to 1377, 1277, 1161, and 1111 cm−1, respectively, in the organogel (Figure 5c). The red shifts of the C−F stretching bands indicate that intermolecular hydrogen bond formation between C−F and H−C occurs in the gel state.50,54 Taken together, the results suggest that the cooperative combination

Figure 8. (a) UV−vis absorption and (b) fluorescence (λex = 405 nm) spectra of 1 (40 μM in THF) with increasing concentration of TBAF (0−5 equiv). (Inset) Absorbance change at 481 nm and fluorescence change at 602 nm with increasing concentration of TBAF. (c) Schematic representation of the photophysical mechanism for the fluorescence responses of 1 to fluoride ion.

of the π−π interaction of the backbone, electrostatic attraction, and four C−F···H−C interactions produces the J-type lamellar stacking of 1, promoting 1D growth and gelation. Figure 6 displays the fluoride ion response of organogel 1. The addition of tetrabutylammonium fluoride (TBAF) to the organogel immediately induced a gel-to-sol transition accompanied by significant changes in absorption and fluorescence colors at the sol−gel interface. The original yellow organogel was totally collapsed after 30 min and became a reddish solution showing orange-red fluorescence at 590 nm (Figures 6a−j and 7). On the contrary, the addition of TBA salts of other anions such as Cl−, Br−, I−, NO3−, and H2PO4− produced no such response, whereas much weaker responses to AcO− were observed (SI Figure S4). It should be noted that the absorption and fluorescence colors of the collapsed organogel of 1 differed markedly from those of solutions of original 1. This disparity suggests that the fluoride-ion-induced gel-to-sol transition may involve a direct alteration of the chromophoric salicylidene aniline moiety. To clarify this, steady-state photophysical responses of THF solutions of 1 (40 μM) to fluoride ions were investigated. In the absence of TBAF, the THF solutions of 1 displayed a 2847

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Figure 9. 1H NMR titration spectra of 1 (a) in the absence and in the presence of (b) 0.5, (c) 1.0, and (d) 2.0 equiv of TBAF. The dotted lines indicate the directions of peak shifts, and the corresponding protons are shown in the chemical structure at the bottom. The inset shows the disappearance of the hydroxyl peak of 1 after the addition of TBAF.

strong π−π* absorption band at 363 nm (Figure 8a) as well as 180 nm of largely Stokes-shifted fluorescence emission at 546 nm (Φem = 0.0042, Figure 8b). These spectral properties are mostly reserved in the organogel of 1, however, with a very large emission enhancement (vide supra, Figure 2 and SI Figure S2). In particular, the large Stokes-shifted fluorescence of 1 originates from the ESIPT process because control compound 2 shows a very weak blue fluorescence emission with a normal Stokes shift (λmax = 450 nm, Φem < 0.0001, SI Figure S5). Upon addition of TBAF, the THF solution containing 1 produces significant bathochromic shifts in both the UV−vis absorption (λabs = 481 nm) and the fluorescence spectra (λem = 602 nm, Φem = 0.025) (Figure 8a,b). The behaviors are qualitatively identical to those of the organogel of 1 (Figure 7). The stoichiometry of the reaction between 1 and fluoride turns out to be 1/fluoride = 1:2 from the Job plot (SI Figure S6). 1 H NMR titration experiments in CDCl3 were conducted to determine the origin of the arising absorption and fluorescence bands. Upon fluoride addition, 1H NMR peaks of the salicylidene aniline ring of 1 gradually shifted (protons labeled as Ha, Hb, and Hc in Figure 9). In addition, we observed the disappearance of a hydroxyl peak at 13.3 ppm in the 1H NMR spectra (inset in Figure 9) as well as the appearance of an [HF2]− peak at −155.33 ppm in the 19F NMR spectra (SI Figure S7). These shifts in the 1H NMR spectra are indicative of the increased electron density of the salicylidene aniline ring by deprotonation,34−38,41−43 and thus the spectral shifts in UV−vis and fluorescence by fluoride ion addition can

reasonably be attributed to the following deprotonation reaction.41−43 K

1 + 2F− ⇄ [1−H+]− + [HF2]−

(1)

Accordingly, it is most likely that the fluoride-induced deprotonation facilitates the intramolecular charge transfer (ICT) transition, producing the observed bathochromic shifts in the UV−vis absorption and the fluorescence spectra (Figure 8c). Actually, the deprotonation process of 1 by fluoride ions can be thought to be a two-step process as follows.41,42,55 1 + F− ⇄ [1−H+]− + HF

(2)

HF + F− ⇄ [HF2]−

(3)

However, the UV−vis and fluorescence titration spectra of 1 exhibited no such stepwise changes (insets in Figure 8). By considering the high stability of the HF2− anion,55 it can be concluded that subsequent reaction 3 instantaneously proceeds to form HF2−. Thus, the equilibrium constant K of fluorideinduced deprotonation reaction 1 could be calculated from the titration spectra using nonlinear curve fitting for the 1:2 reaction model (K = 4.42 × 108 M−2; SI Figure S6).42,55 The selectivity of the fluoride ion over AcO− is attributable to the greater stability of the corresponding conjugate base (i.e., [HF2]− > [H(CH3COO)2]−).55 As expected, anions including NO3−, H2PO4−, and other halides ions produce no significant changes in the UV−vis absorption spectra (SI Figure S9). Taken together, it is deduced that the gel-to-sol transition of the organogel of 1 is triggered by the fluoride-ion-induced 2848

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Job’s plot of 1 and TBAOAc. Absorption spectra of 1 in the presence of various tetrabutylammonium anions. Images of gel collapse by the fluoride ion in a 1-butanol gel. This material is available free of charge via the Internet at http://pubs.acs.org.

deprotonation of 1. The deprotonation unlocks the dihedral motion of the salicylidene aniline moiety of 1 by breaking the intramolecular hydrogen bond. At the same time, the negative charge generated by deprotonation produces the electrostatic repulsion between the adjacent molecules of 1. The combination of these effects offsets the driving force for gelation originating from the cooperation of the π−π stacking force, electrostatic attraction, and C−F···H−C interaction. Because the deprotonation involves the equilibrium, the gel-to-sol transition is limited by the mass transport of fluoride ions. It is noteworthy that the gel collapse accompanies dramatic changes in both absorption and fluorescence colors as clearly shown in Figures 6a−j and 7. In addition, the gelation behavior of 1 is reversible as demonstrated through the removal of fluoride ions by partitioning into polar protic solvents such as methanol (Figure 6k). Diffuse reflectance UV−vis spectroscopy revealed that the restored gel (Figure 6k) and the as-prepared original gel (Figure 6b) have virtually identical absorption spectra (λmax = 390 nm with a shoulder band at 450 nm; Figure 7a), but the fluorescence spectrum of the restored gel showed that a new band arises at around 480 nm (λmax = 556 nm; Figures 6l and 7b), which is attributable to the enol emission of 1 caused by protic methanol addition. Interestingly, a red-colored xerogel was obtained by drying the restored gel (Figures 6m,n and 7). The xerogel made from the restored gel exhibited red-shifted absorption (λmax = 414 nm with a shoulder band at 530 nm; Figures 6m and 7a) and fluorescence spectra (λmax = 600 nm; Figures 6n and 7b) compared to those of the restored gel. Considering absorption and fluorescence spectra of the collapsed gel (λmax,abs = 482 nm, λmax,em = 588 nm; Figure 6i, j and Figure 7), this is attributable to the drying process removing methanol from the restored gel and leaving behind the fluoride ions, resulting in changes in the absorption and fluorescence colors of the gel once more. In contrast, the addition of fluoride ions to an organogel of 1 formed in protic solvents such as 1-butanol did not induce such dramatic color changes but brought about just gel collapse (SI Figure S10). It is likely that reaction 1 is at dynamic equilibrium in the 1-butanol gel and thus can affect only the supramolecular structures.

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*E-mail: [email protected]. Author Contributions

J.L and J.E.K. made equal contributions to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (no. 2009-0081571).

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REFERENCES

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4. CONCLUSIONS We have designed and synthesized wholly π-conjugated organic gelator 1. The gelator readily formed fluorescent organogels with entangled nanoribbon structures in a variety of organic solvents because of the cooperative combination of the π−π stacking force, the electrostatic attraction, and the C−F···H−C interaction. The organogel of 1 showed a reversible gel-to-sol transition in response to fluoride ions, with considerable changes in the UV−vis absorption and fluorescence colors. Mechanistic studies strongly indicated that the triple-modal responses were attributed to the fluoride-ion-induced deprotonation of the salicylidene aniline moiety.

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AUTHOR INFORMATION

Corresponding Author

ASSOCIATED CONTENT

S Supporting Information *

Fluorescence spectrum of 1 in a hot 1,2-dichloroethane solution and after gel formation. Small angle X-ray scattering and wide angle X-ray scattering profiles of xerogel 1. Image of recrystallized 2 in 1,2-dichloroethane. Images of gel collapse by the acetate ion. Absorption and fluorescence spectra of 10 μM 1 and 2 in THF. Job’s plot and equilibrium constant of 1 and TBAF. 19F NMR spectra from a 1:1 mixture of 1 and TBAF. 2849

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