Large Red-Shifted Fluorescent Emission via Intermolecular π−πStacking in 4‑Ethynyl-1,8-naphthalimide-Based SupramolecularAssembliesXinhua Cao,†,‡ Luyan Meng,† Zhenhua Li,† Yueyuan Mao,† Haichuang Lan,† Liming Chen,† Yang Fan,‡

and Tao Yi*,†

†Department of Chemistry, Fudan University, Shanghai 200433, China‡College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, China

*S Supporting Information

ABSTRACT: Two low molecular weight gelators containing 4-ethynyl-1,8-naphthalimide groups with large conjugated structure via different length of alkylchains were synthesized and fully characterized. The gelation properties,structural character, and fluorescence of the gels were investigated via methodsof scanning electron microscopy, X-ray diffraction, and spectral studies. Thegelators have high fluorescence quantum yields in both solution and solid state.Interestingly, the wavelength of the fluorescent emission in the reversible sol−geltransition process of the gels has a large red-shift of 80 nm in DMF, which isextremely sparse for 1,8-naphthalimide derivatives in the literature. Theintermolecular π−π stacking between naphthalimide is suggested to be themain driving force for the gel formation and fluorescent variation by means oftemperature-dependent 1H NMR study and theoretical calculation.

■ INTRODUCTION

Low molecular weight gels (LMWGs) are a class of soft matterscharacterized by the existence of three-dimensional microscopicnetworks through specific noncovalent interactions includinghydrogen-bonds, π−π interactions, hydrophobic interactions,and so forth.1−7 Among the various organogels, those with π-conjugated system were appeared particularly noticeable due totheir optical and electronic functionality, which would havepotential applications in organic light-emitting diodes, organicfield-effect transistors, organic photovoltaic cells, gas sensors,high-resolution laser printers, and photocopying machines.8−12

It is well-known that the “aggregation-caused quenching”(ACQ) effect exists in the gel formation process in most cases,which is undesirable for their light-emitting properties.13−15 Onthe contrary, “aggregation-induced emission” (AIE) wasreported by Tang and Wu in the molecules with a structuralfeature of one conjugated stator carrying multiple aromaticperipheral rotors due to the restriction of the intramolecularrotation.16,17 Other types of aggregation induced or enhancedemission were also reported recently. For example, aggregation-induced emission enhancement (AIEE) was found inderivatives of polyacetylene with the formation of intra-molecular excimer;18 J-aggregation triggered emission enhance-ment was reported in the gelation process of urea-function-alized naphthalene and anthracene derivatives.19 The creationof AIE or AIEE is particularly important for techniqueapplication of the gels in the field of organic electronics andsensors.

1,8-Naphthalimide derivatives are important fluorescent dyeswith excellent spectroscopic properties, such as good photo-stability, high fluorescent quantum yield, and tunablefluorescence emission (from blue to yellow, green, and red).1,8-Naphthalimides are widely applied in design of molecularfluorescent device such as chemosensing,20−22 switching,23,24

and logic operation.25,26 More often 1,8-naphthalimides areacted as fluorescence group for participation in the supra-molecular gel system. In our previous work, tunable red−green−blue fluorescent and white-light-emission supramolecu-lar gels were successfully prepared through intermolecularenergy transfer of the luminescent groups based on 1,8-naphthalimide gelators.27−29 However, the fluorescence of 1,8-naphthalimides generally shows typical ACQ in aggregationprocess due to the strong π−π interaction, and the fluorescentemission wavelength changed very slightly between solutionand the aggregation state. To our knowledge, the red-shift offluorescent emission wavelength for 1,8-naphthalimide gelatorsfrom the solution to gel state was less than 30 nm up to now.30

The great variation of the fluorescent emissions between geland sol may act as a switch for some specific purpose such asvisual recognition sensors and switchable molecular devices.In this work, a new kind of low molecular weight gelators

based on 4-ethynyl-1,8-naphthalimide with large conjugatedstructure were synthesized (compounds 1a and 1b in Scheme

Received: January 15, 2014Revised: September 4, 2014Published: September 11, 2014

Article

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1). The large conjugated structure allowed strong π−πinteractions which played the main role in the formation oforganogel by 1a and 1b. The fluorescent emission wavelengthin the reversible sol−gel transition process had a large red-shiftof 80 nm, which was extremely sparse in the literature.31,32

■ EXPERIMENTMaterials. All starting materials were obtained from commercial

suppliers and used as received. Oxygen gas sensitive reactions wereperformed under an atmosphere of dry argon. Trimethysilylacetyleneand bis(triphenylphosphine)palladium(II) chloride were providedfrom Alfa-Aesar. 4-Bromo-1,8-naphthalic anhydride (95%), dodecyl-amine (CP), and other chemicals were supplied from SinopharmChemical Reagent Co., Ltd. (Shanghai). Column chromatography wascarried out on silica gel (200−300 mesh).Synthesis. Compounds 1a and 1b were prepared according to the

previously reported method (Scheme 1).33,34 First, 2a or 2b wasprepared through the condensation of 4-bromo-1,8-naphthalicanhydride with dodecan-1-amine or n-hexylamine in toluene in agood yield. 3a or 3b was obtained after 2a or 2b reacted withtrimethylsilylacetylene via a Sonogashira coupling reaction, followedby removing of the trimethysilyl protective group. The target products1a and 1b were synthesized by the coupling reaction of 3a and 3b,respectively. The synthesis details are as follows.Preparation of 3a. 2a (2.00 g, 4.51 mmol), ethynyltrimethylsilane

(0.66 g, 6.76 mmol), Pd(PPh3)2Cl2 (63.1 mg, 0.09 mmol), and CuI(34.5 mg, 0.18 mmol) were added to a solution of THF (45.0 mL)and triethylamine (15.0 mL) under an Ar atmosphere. The reactionmixture was heated to reflux for 10 h and cooled to room temperature.The solvent was removed under reduced pressure. The crude productwas purified with the column chromatography (silica gel, dichloro-methane as eluent). The silyl group was removed by treatment withanhydrous potassium carbonate in methanol/dichloromethane (3/50,v/v) at room temperature. The reaction process was checked by TLC.After the reaction was over, the solid was filtered out and the solventwas removed under reduced pressure. The product was purifiedthrough column chromatography (silica gel, dichloromethane aseluent) to give a pale yellow powder (1.40 g, yield 80%); mp 78−80 °C. 1H NMR (400 MHz, CDCl3): δ 8.63 (t, J = 8.80 Hz, 2H), 8.51(d, J = 7.6 Hz, 1H), 7.91 (d, J = 7.6 Hz, 1H), 7.81 (t, J = 7.8 Hz, 1H),4.15 (t, J = 7.6 Hz, 2H), 3.72 (s, 1H), 1.71 (m, 2H), 1.27 (m, 18 H),0.88 (t, J = 6.2 Hz, 3H). 13C NMR (100 MHz, CDCl3): 163.9, 163.6,132.1, 131.9, 131.6, 130.1, 127.9, 127.7, 126.2, 123.0, 122.9, 86.5, 80.4,40.7, 32.0, 29.8, 29.7, 29.7, 29.7, 29.6, 29.5, 29.4, 28.2, 27.2, 22.8, 14.2.HRMS (ESI+) calcd for C26H32NO2 [M + H+]: 390.2433; found:390.2453.Preparation of 3b. 3b was prepared by the same method with that

of 3a. Yield 78%; mp 87−89 °C. 1H NMR (400 MHz, CDCl3): δ 8.63(t, J = 7.2 Hz, 2H), 8.52 (d, J = 7.6 Hz, 1H), 7.92 (d, J = 7.6 Hz, 1H),7.81 (t, J = 8.0 Hz, 1H), 4.17 (t, J = 7.6 Hz, 2H), 3.75 (s, 1H), 1.74

(m, 2H), 1.45 (m, 2H), 1.36 (m, 4H), 0.91 (t, J = 7.0 Hz, 3H). 13CNMR (100 MHz, CDCl3): 163.9, 163.6, 132.1, 131.9, 131.6, 130.1,127.6, 126.1, 123.0, 122.8, 86.4, 80.3, 40.6, 31.5, 28.1, 26.8, 22.6, 14.1.HRMS (ESI+) calcd for C20H20NO2 [M + H+]: 306.1494; found:306.1456.

Preparation of 1a. Under an Ar atmosphere, 3a (1.40 g, 3.59mmol), Pd(PPh3)2Cl2 (49.1 mg, 0.07 mmol), and CuI (26.8 mg, 0.14mmol) were added to a solution of THF (45.0 mL) and triethylamine(15.0 mL). The mixture was heated to reflux for 10 h. The reactionprocess was checked by TLC. After the reaction was over, the mixturewas cooled to room temperature. The solvent was removed underreduced pressure. The crude product was purified with the columnchromatography (silica gel, dichloromethane as eluent) to give ayellow powder with a yield of 90% (1.25 g); mp 194−196 °C. 1HNMR (400 MHz, CDCl3): δ 8.67 (t, J = 9.4 Hz, 4H), 8.56 (d, J = 7.6Hz, 2H), 8.03 (d, J = 7.2 Hz, 2H), 7.88 (t, J = 7.6 Hz, 2H), 4.16 (t, J =7.2 Hz, 4H), 1.71 (m, 4H), 1.24 (m, 36 H), 0.86 (t, J = 6.8 Hz, 6H).13C NMR (100 MHz, CDCl3): 163.8, 163.5, 132.7, 132.4, 132.0, 130.1,128.2, 128.1, 125.3, 123.5, 123.3, 82.0, 81.6, 40.8, 32.0, 29.7, 29.6, 29.4,28.2, 27.2, 22.8, 14.2. HRMS (ESI+) calcd for C52H61N2O4 [M + H+]:777.4631; found: 777.4625.

Preparation of 1b. 1b was prepared by the same method with thatof 1a. Yield 74%; mp 205−207 °C. 1H NMR (400 MHz, CDCl3): δ8.66 (t, J = 8.0 Hz, 4H), 8.56 (d, J = 7.6 Hz, 2H), 8.04 (d, J = 7.6 Hz,2H), 7.87 (t, J = 7.8 Hz, 2H), 4.17 (t, J = 7.6 Hz, 4H), 1.73 (m, 4H),1.43 (m, 4H), 1.34 (m, 8 H), 0.89 (t, J = 6.8 Hz, 6H). 13C NMR (100MHz, CDCl3): 163.7, 163.4, 132.6, 132.3, 132.0, 131.8, 128.4, 128.1,125.2, 123.5, 123.3, 81.9, 81.5, 40.7, 31.6, 29.7, 29.6, 29.4, 28.1, 27.2,22.6, 14.1. HRMS (ESI+) calcd for C40H37N2O4 [M + H+]: 609.2753;found: 609.2730.

Techniques. The 1H NMR (400 MHz) and 13C NMR (100 MHz)spectra were recorded on a Mercury plus-Varian instrument. Protonchemical shifts are reported in parts per million downfield fromtetramethylsilane (TMS). HRMS was recorded on an LTQ-Orbitrapmass spectrometer (ThermoFIsher, San Jose, CA). SEM images wereobtained using a TS 5136MM instrument. Samples were prepared byspinning the samples on glass slices and coating with Au. Powder X-raydiffractions were generated by using a Philips PW3830 sealed-tube. X-ray generator (Cu target, λ = 0.1542 nm) with a power of 40 kV and40 mA. UV−vis absorption spectra were recorded on a UV−vis 2550spectroscope (Shimadzu). Fluorescent spectra and fluorescencelifetime were recorded on an Edinburgh Instruments FLS 900. Theabsolute fluorescent quantum yields for all the samples weredetermined on a Hamamatsu absolute PL quantum yield spectrometerC11347 excited at 365 or 390 nm. Rheology experiments wereperformed on a MCR 301 Anton Paar (Austria) rheometer, with aCouette cell and a temperature control unit. The measurements werecarried out on freshly prepared gels by using a controlled-stressrheometer. Parallel-plate geometry of 25 mm diameter and 1 mm gapwas employed throughout the dynamic oscillatory tests. The followingtests were performed: increasing the amplitude of oscillation up to

Scheme 1. Synthetic Route for 1a, 1b and 3a, 3b

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100% apparent strain shear (kept a frequency of 1 rad s−1) andfrequency sweeps at 20 °C (from 100 to 0.1 rad s−1, 0.1% strain). DFTcalculation of 3a and 1a were done using B3LYP/6-31G*.Gelation Test for Organic Fluids. The gelator and solvent were

put in a septum-capped test tube and heated (>75 °C) until the solidwas dissolved. The sample vial was then cooled to 25 °C (roomtemperature). Qualitatively, gelation was considered successful if nosample flow was observed upon inversion of the container at roomtemperature (the inverse flow method).

■ RESULTS AND DISCUSSIONThe gelation capability of 1a and 1b was determined in 12frequently used solvents by a “stable to inversion of the testtube” method.36 As summarized in Table 1, 1a could gelate

some polar solvents such as DMF, DMSO, EtOH, ethyl acetate,and acetone with the critical gelation concentration of 16, 32,32, 32, and 16 mM, respectively. When the alkyl chain wasshortened from C12 to C6, 1b could only gelate DMF, DMSO,and EtOH with the critical gelation concentration of 20.5, 20.5,and 41 mM, respectively. In contrast, the reference compounds3a and 3b could not gelate any of the above solvents. Thephotographs of sol−gel transitions of 1a in DMF are shown inFigure 1. The color of hot solution of 1a in DMF at theconcentration of 16 mM (0.6 wt %) was yellow with a blue lightemission under the irradiation of 365 nm UV light. The gel 1ain DMF was formed after the hot solution (120 °C) was cooledto 25 °C. The sol-to-gel transition temperature of gel 1a 16mM in DMF was 90 °C. The color of the gel was still yellow,

but its emission color was changed from blue to yellow. TheDMF gel of 1b had the same behavior in the process of gelationas that of 1a. These gels were stable at room temperature formore than 1 week. Here taking the DMF gel as an example, thegel behavior and photophysical properties of 1a and 1b wereinvestigated in detail.The UV−vis absorption and emission spectra of 1a, 1b, 3a,

and 3b in DMF solutions, gels, and solid state were carried outto understand the intermolecular interaction and the self-assembly process. The details of the absorption and emissioncharacteristics of those compounds together with the quantumyield (φf) and lifetime τ (ns) are shown in Table 2. 3a in the

solution of DMF (1.0 × 10−4 M, 25 °C) showed twoabsorption bands centered at 350 nm (ε = 6.18 × 103 dm3

mol−1 cm−1) and 366 nm (ε = 5.69 × 103 dm3 mol−1 cm−1)which were ascribed to the π−π* transition of the conjugatedmolecular backbone (Figure 2a).37 3b has very similarabsorption spectral character with that of 3a in solution. Thetwo maximum absorption bands of 1a were at 390 nm (ε = 3.04× 104 dm3 mol−1 cm−1) and 423 nm (ε = 2.48 × 104 dm3 mol−1

cm−1), respectively, 40−50 nm red-shifted comparing with thatof 3a (Figure 2b). The absorption bands in 1b are at 391 and422 nm. Those two absorption bands of 1a and 1b have almostno change in the gel state, but a new broad shoulder band atabout 360 nm appeared in the gel state. This may indicate anH-type aggregation existing in the gel state of 1a and 1b. Inorder to further understand the self-assembly process, theabsorption spectral change of the sol−gel transition process of1a was studied (Figure S1). The two absorption bands of thehot sol were gradually red-shifted with the temperaturedecreasing from 88 to 20 °C. For example, the two absorptionband at 387 and 418 nm at 88 °C were slightly red-shifted to392 and 424 nm at 20 °C, respectively. At the same time, abroad shoulder absorption band at about 360 nm appeared.This result clearly showed that the π−π stacking occurred inthe sol−gel transition process.

Table 1. Gelation Properties of 1a and 1b in a Variety ofSolventsa

solvent 1a 1b solvent 1a 1b

cyclohexane P P DMF G (16/0.6) G (20.5/1.3)toluene p p DMSO G (32/1.1) G (20.5/1.1)acetonitrile p p MeOH NS NShexane P P EtOH G (32/1.5) G (41/3.0)ethyl acetate G (32/1.3) p acetone G (16/0.8) NSmethylene P P dioxane P Pchloroform PG P THF P PaG: gel; P: precipitation; NS: not soluble. The critical gelationconcentrations of the gelator are given in parentheses (mM/wt %).

Figure 1. Photographs of sol−gel transition of 1a in DMF at theconcentration of 0.6% (16 mM): (a, c) the solution and gel statesunder a bright lamp, respectively; (b, d) the solution and gel states inthe dark under illumination of 365 nm light, respectively.

Table 2. Photophysical Parameters of 1a, 1b, 3a, and 3b inDifferent Conditions

compounds λabs (nm) log ελem(nm)

Φfa

(%) t (ns)

1a (solution) 390 4.48 472 47.6 1.27 (100%)423 4.39

1a (solid) 552 9.3 8.50 (100%)1a (gel) 376, 402,

435491,552

9.5 10.84 (100%)

1b (solution) 391 4.60 466 49.5 1.26 (100%)422 4.54

1b (solid) 542 9.9 11.07 (100%)1b (gel) 391, 422 506,

54811.8 13.31 (100%)

3a (solution) 350 3.79 420 1.2 0.48 (37.9%), 2.57(62.1%)

366 3.753a (solid) 490 2.5 32.64 (81.8%), 8.90

(18.2%)3b (solution) 350 4.00 422 0.9 1.10 (57.8%), 1.86

(42.2%)366 3.97

3b (solid) 504 2.2 1.90 (85.2%), 4.66(14.8%)

aAbsolute fluorescence quantum yields (Φf) of all the samples weremeasured by using integraph excited at 365 or 390 nm.

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All the compounds show a large red-shift in fluorescencespectra between solution and aggregated states. The DMFsolution of 3a (1.0 × 10−4 M, 25 °C) emitted weak bluefluorescence at the maximal wavelength of 415 nm with thefluorescence decay times of 0.48 ns (37.9%) and 2.57 ns(62.1%), and an absolute fluorescence quantum yield (Φf) of1.2%, under the excitation of 365 nm light (Figure 2c,e). Onthe contrary, the solid powder of 3a emitted much strongerblue light (Φf = 2.2%), and the maximal wavelength was red-shifted to 489 nm with the fluorescence decay times of 8.90 ns(18.2%) and 32.64 ns (81.8%) under the excitation of 365 nmlight. The longer lifetime in the solid state suggested a strongπ···π interaction in the aggregated state. It was interesting thatthe fluorescence emission of 1a was obviously different fromthat of the precursor 3a. The fluorescent emission spectra of 1ain three states are shown in Figure 2d. The maximal emissionwavelength of 1a in DMF solution (1.0 × 10−4 M) was at 472nm with a high quantum yield of 47.6% and a lifetime of 1.27ns (Figure 2f). This quantum yield of 1a was almost 40-foldthat of 3a in the solution. The maximum emission wavelengthof 1a in solid powder was red-shifted from 472 nm in DMFsolution to 552 nm with the lifetime of 5.58 ns and the Φf of9.3%. Two emission wavelengths of 491 and 552 nm wereobserved in the gel state (16 mM), indicating that themolecules of 1a exhibited both solution and solid state

characters in the gel state. The gel 1a had an absolutefluorescence quantum yield of 9.5% with the lifetime of 10.84ns. The drastic red-shift of the emission of 1a in both gel andsolid state compared to the solution demonstrated that theadjacent π clouds of 1a were interacted much more stronglythan that in molecule 3a.38 The same change trend existed inthe compounds 3b and 1b (Figure S1 and Table 2).To further understand the role of π−π stacking in the

emission properties of aggregated state, the fluorescenceemission changing of 1a in the sol−gel transition was trackedby naturally cooling the hot sol to the gel state (Figure 3a). Thefluorescence emission of 1a in the hot solution of DMF (16mM) was blue light with the maximum peak at 488 nm. Withthe aging time at room temperature, the fluorescent emissionintensity of 488 nm was gradually decreased; simultaneously, anew emission at 552 nm was gradually strengthened. When thehot solution of 1 in DMF was naturally cooled for 9 min, theprocess of sol−gel transition was finished and the emission ofthe gel was changed from blue to yellow light. The fluorescenceemission intensity at 552 nm was 2.5-fold that of 488 nm. Theaggregation character of 1a was further confirmed by additionof water (poor solvent) to the dilute solution of DMF (1.0 ×10−5 M). The change of the fluorescent emission on the watertitration process is shown in Figure 3b,c, from which we couldsee an obviously red-shift of the emission from 471 to 552 nm

Figure 2. Absorption spectra of (a) DMF solution of 3a (1.0 × 10−4 M), (b) DMF solution (1.0 × 10−5 M) and gel (16 mM) of 1a, and (c) DMFsolution (1.0 × 10−5 M) and gel (20.5 mM) of 1b; normalized fluorescent emission spectra of (d) 3 in DMF solution (1.0 × 10−4 M) and the solidpowder (λex = 365 nm), (e) 1a in solution (1.0 × 10−4 M in DMF), solid powder and gel states, and (f) 1b in solution (1.0 × 10−4 M in DMF), solidpowder and gel states (λex = 390 nm); typical fluorescence decay curves of (g) 3a in DMF solution and solid powder (λex = 365 nm, monitored at420 and 480 nm, respectively), (h) 1a in DMF solution, solid powder and gel state (16 mM), and (i) 1b in DMF solution, solid powder and gel state(20.5 mM) (λex = 390 nm, monitored at 470 and 552 nm, respectively, cell length for solution, 1 cm).

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and a drastic emission color change from blue to green whenthe amount of the additional water larger than 33% (v/v)(Figure 3c).It is obvious that both 1a and 1b have a much larger

fluorescence quantum yield than that of 3a and 3b in bothsolution and aggregated states, whereas the length of the alkylchain has no large effect on the photophysical properties of thecompounds. Insight into the large variation in optical propertiesbetween compound 3a (or 3b) and 1a (or 1b) in solution andsolid state, the optimized conformation of the truncated models(dodecyl chains are replaced by Me groups, named as 1 and 3)of those compounds in monomer and dimer was gained by

DFT calculations. Optimized structures (M06-2x/6-31G+(d))and MOs of 1 and 3 are shown in Figure 4.39 The DFT-calculated values for the HOMO−LUMO energy gaps of 3 and1 reflect the trend shown in the optical data. The HOMO−LUMO energy gap of 3 in monomer model is 5.75 eV. Becauseof the π−π stacking between molecules of 3, the HOMO−LUMO energy gap of the dimer of 3 decreases to 5.27 eV.Comparing with 3, the HOMO−LUMO energy gaps of 1 inmonomer and dimer models were obviously low and were 4.93and 4.82 eV, respectively. Optimized models of 3 and 1 showvery high planarity of the naphthalimide chromophore in thestructures, which is in favor of intermolecular π−π stacking

Figure 3. (a) Fluorescent spectra of the sol−gel transition of 1a in the naturally cooling process from 120 to 59 °C (csol = 16 mM; λex = 390 nm). (b)Fluorescence spectra of 1 in DMF (1.0 × 10−5 M) with addition of different amount of H2O at room temperature (λex = 365 nm) and (c) thecorresponding images of (b) (the top row was under a bright lamp, and the bottom row was in the dark under 365 nm light).

Figure 4. DFT-optimized structures and FMO plots of truncated models of 3, 1 and the dimer of 3, 1 at the M06-2x/6-31G+(d) level usingGaussian 09.

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formation. The shortest distance between the two planes in thedimer is about 3.29 and 2.86 Å for 3 and 1, respectively, whenthe two molecules are parallel to each other. Clearly, there issignificant orbital overlap between the orbitals of the twomonomers, and the π−π stacking interaction betweenmolecules of 1 is obviously stronger than that of 3. So, themaximum absorption and emission spectra of 1 in solution andsolid states have a large red-shift compared with that of 3. Thelarger fluorescence quantum yield in both the solution and solidstate of 1 than that of 3 may be due to the planar character ofthe large conjugated system and the restriction of the singlebond rotation between ethynyl and naphthalimide.To verify the nature of the intermolecular interactions

constituting the self-assembly process, the temperature-depend-ent NMR spectra were studied for 1a as an example (Figure5a). All the signals of the aromatic protons of H1, H2, H3, H4,and H5 in the naphthalimide units of the organogel 1a in DMF(32 mM) shifted upfield with the increase of temperature. Thecomparison of the chemical shift of the aromatic protons in 1awith temperature change is summarized in Figure 5b, fromwhich we could see that the chemical shifts of the protons H1,H2, H3, and H5 were drastically upfield shifted from 8.812 to8.727, 8.590 to 8.531, 8.337 to 8.231, and 8.670 to 8.613 ppm,respectively, from 30 to 90 °C and kept constant until 140 °C.Considering that the temperature of 90 °C is just the gel to soltransition point of 1a, the 1H NMR spectra strongly suggest thepresence of π−π stacking in the gel state.40 The π−π stackingshould be the main driving force for the organogel formation of1a.Low molecular weight gelators were generally self-assembled

into one-dimensional nanostructures, and then 3D networkswere formed in solvent.41,42 The morphologies of the xerogelsof 1a obtained from ethanol, acetone, ethyl acetate, and DMFwere investigated by scanning electron microscopy (SEM) andshowed similar architectures of microbelts (Figure 6). Thewidth of the microbelts obtained from the four solvents was inthe range of 0.5−2.0 μm. The microbelts in the xerogels fromethanol (less than 10 μm) were shorter than that obtained fromethyl acetate, acetone, and DMF (greater than 10 μm).In order to investigate the molecular packing models of

molecules in the supramolecular gel state, the small- and wide-angle X-ray scattering experiments of the organogel 1a in DMFwere carried out. The scattering patterns of xerogel of 1ashowed a series of d space values at 35.8, 17.9, 11.7, and 8.8 Å,which were in the ratio of d/1:d/2:d/3:d/4, indicating alamellar structure (Figure 7a,b).43,44 A diffraction peak at 2θ =26.6° in Figure 7b was observed corresponding to a d spacing

of 3.4 Å, which could be attributed to the π−π stacking distancebetween two neighboring molecules of 1a.45

The mechanical properties of the gel were the importantparameters for application. The viscoelastic behavior of gel 1ain DMF was studied by dynamic oscillatory measurements. Thelinear viscoelastic regions (LVR) of the gel were determined bystrain amplitudes ranging from 0.01% to 100% at 6.28 rad s−1

(Figure 8a). The strength (storage modulus, G′) of the gel isabout 83 Pa with a small strain of 0.01%. This signifies thestability of the gel toward the shearing process. The storagemodulus (G′) and the loss modulus (G″) remained nearlyconstant up to approximately 0.1% strain (G′ > G″) whichdefined the uppermost boundary of the LVR. Beyond this level,a weak strain overshoot was given (the G′ value decreased,whereas the G″ value increased and then decreased) within therange of 0.1%−1%. When the large strain amplitudes wasimposed, the gel showed a catastrophic disruption accompaniedby a steep decrease in the values of both moduli and thereversal of the viscoelastic signal (G″ > G′).46 Simultaneously,the experiment of a frequency sweep between 0.1 and 100 rads−1 showed G′ > G″, which confirms that the gel has a

Figure 5. (a) Temperature-dependent 1H NMR spectra of gel of 1a from DMF (32 mM). The positions of the labeled protons are marked inScheme 1. (b) Changes of the chemical shift of protons H1, H2, H3, and H5 with temperature.

Figure 6. SEM images of the xerogels of 1a at room temperature (25°C) from (a) ethanol, (b) acetone, (c) ethyl acetate, and (d) DMF.The concentrations for the formation of the original gels are 32, 16, 32,and 16 mM, respectively; scale bar for (a)−(d) is 5 μm.

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predominantly elastic character (Figure 8b). The elasticity ofthe gel is further evident from the fact that the G′ and G″ valuesare minimally sensitive to frequency (ω), which indicated thatthe gel system formed a stable network structure.47 The double-logarithmic plot of the dynamic viscosity (η*) versus theangular frequency (ω) has a gradient close to −1, which ischaracteristic of a typical gel.48

■ CONCLUSIONS

In summary, novel 1,8-naphthalimide derivative gelators withlarge conjugated structure which could effectively gelate somepolarity solvents were designed and synthesized. The π−πstacking of the large conjugated system was the main drivingforce for the gel formation from spectral study and DFTcalculation. Both the fluorescent emission and UV−visabsorption of the gelators had a large red-shift compared withtheir unilateral precursors. The organogels were thermallyreversible with a drastic fluorescent change from blue to yellowlight between sol and gel state. The great variation of thefluorescent emissions between gel and sol may act as a switchfor some specific purpose such as visual recognition sensors andswitchable molecular devices. Further works are in progress.

■ ASSOCIATED CONTENT

*S Supporting InformationFigure S1. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail yitao@fudan.edu.cn; Fax +86 21 5566 4621 (T.Y.).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was supported by the National Basic ResearchProgram of China (2013CB733700), the China National Fundsfor Distinguished Young Scientists (21125104), NationalNatural Science Foundation of China (21002082, 51373039),Program for Innovative Research Team in University(IRT1117), Program of Shanghai Subject Chief Scientist(12XD1405900), the Science and Technology Key Project ofHenan Education Department (13A150760), and the Develop-ment Projects of Henan Province Science and Technology(132300410090).

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Figure 7. (a) Small- and (b) wide-angle X-ray scattering pattern of the xerogel of 1a from DMF.

Figure 8. Dynamic oscillatory data for gel 1a (16 mM) in DMF at 20 °C. (a) Strain sweep of the gel at a frequency of 6.28 rad s−1; (b) frequencysweep of the gel at a strain of 0.1%.

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