Aggregation-Induced Emission of Organogels Based on Self

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Aggregation-induced emission of organogels based on self-assembled 5-(4-nonylphenyl)-7-azaindoles Daniel López‡, Eva M. García-Frutos*† †

Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid (Spain)

‡Instituto de Ciencia y Tecnología de Polímeros, ICTP-CSIC, Juan de la Cierva 3, 28006 Madrid (Spain) *E-mail: [email protected], Phone: +34913349038 KEYWORDS: Organogelator, Luminescence, H-bonding, Self-assembly ABSTRACT A new self-assembled organogel based on 5-(4-nonylphenyl)-7-azaindole (1), possessing an aggregation-induced emission phenomenon (AIE), is described. The incorporation of phenyl alkyl chains improves processability of the platform to form a new class of gelator. The fluorescence spectrum of 1 suffers changes in the gelation process, and an AIE phenomenon is observed during the phase transition from sol to gel state. The fluorescence is decreased slowly by heating the gel, and no emission is detected in concentrated solutions of 1. The AIE effect is due to the formation of the supramolecular organogel, where a self-association of the 7-azaindole moieties by dual hydrogen-bonded dimers is present. Regarding the solid state emission

ACS Paragon Plus Environment

1

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 28

properties, the xerogel 1 exhibits blue emission as well as in its organogel form. Therefore, it could be considered as a promising blue emitter in solid state. INTRODUCTION The self-assembly of low molecular weight organogelators (LMOGs) shows an increased interest in the latest years.1,2,3. Their unique supramolecular organization allows a wide range of applications,2,4,5 e.g. optoelectronics,6,7 fluorescence sensors,8,9 or logic states.10 Generation of such organogels has been achieved by non-covalent H bonds, hydrophobic or van der Waals’ interactions, where the entangled fibrous network can capture and confine the bulk solvent. Organic compounds are expected to be gelators if they possess functional groups to facilitate organogelation in different solvents, such as amide, hydroxyl, urea, acid, large aromatic units, cholesterol, chiral /achiral long aliphatic chains, etc.2,3 Although to date, a large number of organogels are formed with temperature, via a heating-cooling process, other different external stimuli such as sonication, light and shear stress have demonstrated to afford organogels that exhibit reversible gel-solution phase. Organogelation brings significant changes in the optical and electronic properties. In this context, supramolecular self-assembly in organogels has been employed as a method to fabricate aggregation-induced emission (AIE) materials.11-14 Several AIE luminogens with emissions in the whole visible light have been described.15-18 More specifically, those luminescent materials emitting blue light are of high interest, as they are necessary for high quality full color displays and white lightening.19-21 Thus, solid-state blue emitters have been developed, constructed with AIE-generating moieties.22,23


2

Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Scheme 1. Synthesis of compound 1 In this paper, a new organogel based on 5-(4-nonylphenyl)-7-azaindole (1) is reported (Scheme 1), showing an aggregation-induced emission (AIE). The gelation of 1 in cyclohexane displays an emission band in the blue region of the visible spectrum, which is red-shifted respect to the emission band observed in highly diluted solutions. Thus, this organogel with a simple structure and low molecular-weight displays a gelation-enhanced emission. Regarding the solid state emission properties, the xerogel exhibits blue emission similarity the organogel. EXPERIMENT Materials: All starting materials were acquired from commercial suppliers. Oxygen gas sensitive reaction was performed under an atmosphere of nitrogen. Column chromatography was carried out on silica gel (200-300 mesh). Synthesis The commercially available 5-bromo-7-azaindole served as starting material for the preparation of 5-(4-nonylphenyl)-7-azaindole 1 ,through a Suzuki coupling with 4(nonyl)phenylboronic acid, in the presence of Pd(PPh3)4, aqueous K2CO3 (2 M), using tetrahydrofuran as solvent. The synthesis detail is a follow. Preparation of 1: A mixture of 5-bromo-7-azaindole (100 mg, 0.51 mmol), Pd(PPh3)4 (66 mg, 0.057 mmol), 4-nonylbenzeneboronic acid (130 mg, 0.52 mmol) was degassed under nitrogen. Then 0.5 mL of 2 M aqueous K2CO3 and 4 mL of tetrahydrofuran was added. The mixture was heated at 130 ºC for 24 hours under N2. The yellow suspension was partitioned between H2O and CH2Cl2, and dried (MgSO4). The solvent was evaporated and the residue chromatographed on


3

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

silica gel (hexane:acetone, 4:1) to give a white solid 1 (75 mg, 46%): Rf (4:1 hexane: acetone): 0.17, mp 114-116º, 1H NMR (300 MHz, acetone-d6) δ 10.64 (s, 1H), 8.45 (s, 1H), 8.10 (s, 1H), 7.57-7.54 (m, 2H), 7.46-7.44 (dd, J = 3.5, J = 2.4, 1H), 7.28-7.26 (m, 2H), 6.49-6.47 (dd, J = 3.5, J = 2.4, 1H), 2.62 (t, J = 7.5, 2H), 1.65-1.60 (m, 2H), 1.31-1.24 (m, 12H), 0.83 (t, J = 6.6, 3H);

13

C NMR (50 MHz, acetone-d6) δ 148.9, 142.5, 142.5, 141.9, 137.6, 129.7, 127.4, 126.7,

126.5, 120.5, 100.9, 35.7, 32.2, 31.9, 29.6, 29.3, 22.9, 13.9; UV-vis (CH2Cl2, 25 ºC) λmax (ε) 250 (125370); FAB MS m/z 321.2 [M+1]+; HRMS (FAB) calcd for C22H29N2: 321.2331, found: 321.2338. Techniques:1H and NMR, 200 MHz and

13

C NMR spectra were recorded on a Bruker AC 200 spectrometer (1H

13

C NMR (50 MHz), Brucker Avance 300 MHz and Brucker DRX-500 .

Samples were recorded as solutions in deuterated NMR solvents as stated. Chemical shifts (δ) are quoted in parts per million (ppm), referenced to residual solvent. UV–visible absorption spectra studies were carried out on a Perkin–Elmer Lambda XLS+ spectrometer. Fluorescence spectroscopy was recorded on a Varian Cary Eclipse and JASCO-8600 spectrophotometers. Solution experiments were recorded in cyclohexane and chloroform solutions in a 1 cm quartz cell. Fluorescence of the xerogel was recorded in thin film in a quartz plate. Temperature dependent Fluorescence spectroscopy was carried out using JASCO-8600 spectrophotometer and temperature was controlled using Peltier. Solution experiments were recorded in cyclohexane solutions in a 1 cm quartz cell. Field Emission Scanning Electron Microscopy (FE-SEM) image of the xerogels were recorded on an FE-SEM FEI Nova NanoSEM 230 instrument with vCD detector and Simatzu S-8000 with field-emission filament. Rheological measurements: Oscillatory viscoelastic measurements were performed in a TA Instruments ARG2 controlled stress rheometer, using the 40 mm aluminum parallel-plate shear mode to measure the storage


4

Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

modulus, G’ and the loss modulus, G’’. Since the formation of the gel takes place rapidly after the solution is left at room temperature, gel samples were formed directly on the lower plate of the rheometer by placing solutions of compound 1 in cyclohexane at a concentration of 0.09 M. Samples were put on the rheometer at 30ºC to avoid gelation. Then, temperature was lowered to 20 ºC and samples were kept at this temperature for 5 min before each measurement to ensure the formation of the gel. The linear viscoelastic zone (LVZ) defined as the region in which the storage and the loss modulus are independent of the strain amplitude, was located with the aid of a strain sweep (from 0.01 to 1000 %) at the frequency of 1 Hz. Recovery tests were performed by a consecutive strain sweep from 1000 to 0.01%. Isothermal frequency sweeps at 20 ºC and at a constant strain amplitude of 0.1 % were carried out between 100 and 0,1 Hz. Finally, temperature sweeps from 10 to 60 ºC at constant frequency of 1Hz and strain amplitude of 0.1% were performed. Gelation Test: The gelation process involves three steps. The first is mixing the organogelator with solvent at ambient temperature. The second is warming the mixture to produce a clear solution (40ºC). Finally, the sample vial is cooled slowly to the ambient temperature. Gelation process is during the cooling step. The gelation capability of 1 was determined in frequently used solvents by a “stable to inversion of the test tube” method. The xerogel of 1 was prepared from the gel state removing the cyclohexane by capillarity using a piece of absorbent paper to be ensure the xerogel maintains the fiber structure present in the gel. RESULTS AND DISCUSSION Organogelation behavior. The gelation behaviour of the compound 1 was evaluated in a number of solvents (Table S1) As summarized in Table S1, 1 could gelate some solvents such as


5

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

cyclohexane, hexane and dodecane with the critical gelation concentration of 90, 100, 120 mM, respectively, where the cyclohexane proved to be the best solvent for the gelation. Compound 1 was dissolved (3.5 wt %) in cyclohexane at 70 ºC, forming a non-flowing gel-like material when cooled down. Moreover, the organogel of 1 is white-opaque and the gelation process completely thermoreversible. The sol-to-gel transition temperature of gel in cyclohexane is between 34-43 ºC. The process of gel formation was observed after resting 16 min at room temperature and the formation of the organogel was tested by the "tube inversion" method (Figure 1a). On the other hand, the sonication irradation can also promote rapidly the gel formation in 15 seconds. Field Emission Scanning Electron Microscopy (FE-SEM). The morphologies of the xerogel (dried gel) of 1 were examined by scanning electron microscopy (FE-SEM) (Figure 1b). Morphology of the xerogel sample showed fibrillar aggregates having various diameters (Figure 1b, S1). Similar fibrillar networks were also found when the organogel is formed by sonication irradation possessing a 3D network formed by the entanglement of 1D fibrous nanoaggregates (Figure S2).


6

Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 1. a) Process gel formation 1 (3.5 wt %) organogel in cyclohexane at room temperature and the "tube inversion" method; b) SEM image of a dried gel of 1 in cyclohexane. Rheological Properties. In order to ascertain whether compound 1 forms gels in cyclohexane, samples of 1 in CH at a concentration of 1 were characterized for their rheological properties. A system must meet some requirements according to its viscoelastic properties to be considered as a gel. Basically, it has to present a solid-like behavior, which is the mechanical definition of gel.24 Its dynamic storage modulus, G’, must be greater than the loss modulus, G’’ and must also be independent of the oscillatory frequency.25 The results of the rheological experiments performed at varying oscillatory strains and constant frequency of 1 Hz and temperature of 20 ºC are depicted in Figure 2a


7

Langmuir

Page 8 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

8

Page 9 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 2. a) Variation of G’(circles) and G’’(squares) with oscillatory strain for a 3.5 wt % gel of 1 in CH at 20 ºC and frequency of 1 Hz. Full symbols corresponds to curves obtained at increasing strains, empty symbols corresponds to curves obtained at decreasing strains; b) Variation of G’(cicles) and G’’(squares) with oscillatory frequency for a gel of 1 at a concentration of 0.09 M in CH. Experiment performed at 20 ºC and constant oscillatory strain amplitude of 0.1%; c) Variation of G’(cicles) and G’’(squares) with temperature for a gel of 1 at a concentration of 0.09 M in CH. Experiment performed at constant frequency of 1Hz and constant oscillatory strain amplitude of 0.1%.

The elastic modulus, G’, and the loss modulus, G’’ remain approximately constant below an oscillatory strain value of around 0.3 %, which is quite low and can be considered as the upper limit of the linear viscoelastic zone. Above a strain of 0.3 %, a decrease in G’ and G’’ is observed, which can be attributed to a partial breakup of the gel structure that provokes the sample to flow. This breakup of the gel structure is completed at a strain amplitude value of 400% at which the crossover of G’ and G’’ takes place and the solid-like behavior is lost. The thixotropic character of the organogel was analyzed by performing a recovery test once the structure is destroyed after submitted to strains above the yield strain at 400% . As can be observed in Figure 2a (down curves), the application of decreasing oscillation strains provokes the recovery of the elastic properties of the organogel. However, both G’ and G’’ does not attain the original values of the starting organogel. This maybe due to the fact that longer times are required for the recovery of the elastic properties to be complete, as it corresponds to proper thixotropic materials.


9

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 28

In Figure 2b G’ and G’’ are depicted as a function of oscillatory frequency for a gel of 1 at a concentration of 0.09 M at 20 ºC and a constant oscillatory strain amplitude of 0.1%. As can be observed, the two requirements for a sample to be considered as a gel are approximately satisfied: i) G’ is almost independent of the frequency of oscillation and ii) G’ is greater than G’’ at all frequencies. It is important to note that G’ slightly deceases at low frequencies, which might indicate a possible crossover of G’ and G’’ at very low frequency values, so that the characteristic relaxation time of the system is rather long. These results confirm that we are dealing with rather consistent physical gels of relatively high elastic modulus. To deepen on the characterization of the organogel, the thermal behavior of the system was analyzed through temperature sweeps. In Figure 2c the evolution of the storage modulus and the loss modulus as a function of temperature is depicted for a sample of 1 at a concentration of 0.09 M. The storage modulus is relatively independent of temperature up to 20 ºC which points to a predominantly enthalpic contribution to the elasticity of this organogel. This type of elasticity is characteristic of gels presenting rigid elements as constituents of the network structure.26 At temperatures above 25 ºC the elastic modulus drops rapidly and becomes lower than the loss modulus at around 29 ºC, indicating the melting of the organogel. It is important to note that due to experimental conditions the curves of G’ and G’’ shows a behavior above 30 ºC that is not related to the sample itself but to the evaporation of the cyclohexane from the melted sample. The evaporation of CH causes the rigidification of the surface of the sample through film formation and therefore the increase of both G’ and G’’.

1

H NMR experiments. 1H NMR experiments using cyclohexane-d12 as deuterated solvent

(Figure S3), show that the self-association of these organogelators do not occur through π-π


10

Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

interactions, as only a small shift in the aromatic signals was observed, demonstrating very weak π-π interactions between 7-azaindole units. However, it was found that protons corresponding to the pyrrolic N−H show a huge downfield shift (12.6−13.3 ppm) upon increasing the concentration from 9 to 58 mM, indicating that the hydrogen bond has a remarkable influence, in the formation of the organogelator 1. To confirm the nature of the intermolecular interaction constituting the self-assembly process and to corroborate the presence of H−bonds, temperature-dependent NMR experiments were carried out (Figure 3). 1H-NMR spectra of compound 1 in cyclohexane-d12 at 10-2 M shows a upfield-shift of the pyrrolic hydrogen when increasing the temperature, due to the fact that such hydrogens losses the ability to form intermolecular H-bonds.

Figure 3: Temperature-dependent 1H NMR spectrum of 1 in cyclohexane-d12 at 10-2M.


11

Langmuir

On the other hand, NOESY experiments were also performed on 1 in two different solvents, aggregating and non-aggregating, such as cyclohexane-d12 and CDCl3 solutions at 10-2 M. NOE effect corroborates the formation of hydrogen bond in the case of using cyclohexane-d12 between two 7-azaindole molecules, where hydrogen of the N atom of five membered ring has NOE effect with the proton of the pyridyl ring (Figure S4). The intermolecular hydrogen bond Npyrrolic−H····Npyridinic was not detect in CDCl3 solutions (Figure S5). The resulting hydrogen-bonded complexes form a supramolecular self-assembly and immobilize cyclohexane efficiently to give the organogel aggregates.

a) 3497

Transmitance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 28

b) 3440

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

Figure 4. FT-IR spectra a) solid state and b) xerogel of 1. The formation of hydrogen bond in gel state was corroborated by a comparison of FT-IR spectra of the xerogel and solid compound of 1. The red-shift of the N-H stretching peak to 3440 cm-1, which suggested a weaker hydrogen bonding interaction (Figure 4). Absorption and fluorescence studies. The absorption spectra of 1 in both solution and xerogel state were studied (Figure S6). The absorption spectrum of 1 (10-5 M) in cyclohexane


12

Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

showed a band centered at 248 nm and a shoulder at 303 nm. The absorption band of a film of xerogel 1 onto quartz slide showed two bands at 264 and 309 nm, with a markedly broader spectrum in comparison with the solution state. The absorption spectrum of xerogel in the film state is more bathochromic-shifted respect to the spectrum in solution state, presumably due to interactions of adjacent π-clouds of the molecules. The fluorescence spectrum of 1 in cyclohexane solution (10-5 M) showed a single band with a maximum at 349 nm (λexc = 306 nm) which corresponds to monomeric species in solution (Figure S7) (Φ =0.56, using quinine sulfate in 0.1 N H2SO4 as reference) ( See SI). When the concentration of 1 was increased, up to 10-2 M, fluorescence was quenched (Figure S8). The gel state (10-2 M ) at room temperature presented fluorescence at higher wavelengths (blue region) than those observed in 10-5 M solution in cyclohexane. By contrast, the 10-2 M solution did not show fluorescence in any region, as observed with the solution obtained when heating the gel of 1 at this concentration. The fact that gel emits at longer wavelengths than the 10-5 M solution can be ascribed to the formation of complexes shaping a more planar conformation, producing the AIE effect (Figure 5).


13

Langmuir

250 250

200

GEL

200

SOL

150

GEL Flu.

Fluorescence Intensity (u.a.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 28

150

100 50 0

SOL

1

100

2

3 4 Cycles

5

6

50

0 400

500

600

700

800

Wavelength (nm)

Figure 5. Fluorescence spectra of 1 in cyclohexane (32.9 mg mL-1) before and after gel formation. The inset shows the reversible emission switching at 306 nm by repeated sol-gel phase transition.

The emission spectrum of the xerogel of 1, obtained from the gel state in cyclohexane, also exhibited a considerable bathochromic shift in comparison with the diluted solution. The xerogel of 1 displays two broad emission bands at 385 and 456 nm (Figure S9), probably due to different aggregated species. It also emits blue fluorescence in its solid-state films. It is worthwhile mentioning that most luminophoric materials are used as solid films for their practical applications, so the xerogel of 1 could be used for the construction of solid-state blue emitters. Optical images of the organogel of 1 demonstrate it is almost non-fluorescent in solution (Figure S10), but a significant enhancement in the emission intensity was observed as a result of the gelation process, emitting blue fluorescence upon exposure to UV light (illuminated at 366 nm) (Figure 6).


14

Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 6. Optical images of 1 at 10-2 M in cyclohexane solution (a) and in organogel state (b) under daylight and under UV light (366 nm).

Finally, the single crystals of 1 were obtained upon slow evaporation from cyclohexane solution (Figure 7, Figure S10, S11, S12, Table S2, S3). X-ray analysis revealed the planarity of the 7azaindole core. The intermolecular hydrogen bond Npyrrolic−H···Npyridinic, with a distance of 2.092 Å, plays a significant role in the crystal packing, forming efficiently a dimer, by two 1D H−bond (Figure 7, Figure S10). Finally, the crystallographic packing evidences the little contribution of cooperative C−H···π interactions between some of methylene groups and the aromatic moieties. Distances between alkyl chains respect to the centroid of the phenyl ring of adjacent molecule are about 3.6 Å (Figure S10). The intramolecular rotation would be restricted by the interaction of long phenyl alkyl chains from another azaindole. As hypothesized, the presence of long alkyl chains would stabilize 1 assemblies, by van der Waals’ interactions between alkyl chains (Figure 11), with distances about 4.89 Å. Presumably, inhibition of the intramolecular rotation helps to the stability of the molecule and the formation of the gel state.


15

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

Figure 7. Crystallographic packing structure of the compound 1. CONCLUSIONS AND OUTLOOK In summary, a new low molecular weight-based organic gelators with AIE capacity have been synthesized. The self-assembly of the molecule of 5-(4-nonylphenyl)-7-azaindole in cyclohexane affords a supramolecular soft material. The 10-2 M solution in cyclohexane did not show fluorescence in any region, however when the organogel is formed it emits blue fluorescence emission with a bathochromic shift with respect to the diluted solution, showing a typical AIE characteristic. The AIE effect could be due to the formation of the supramolecular organogel in which exist a self-association of the 5-(4-nonylphenyl)-7-azaindole by hydrogen-bonded and planarity of the phenyl ring. Regarding the solid state emission properties, the xerogel 1 exhibits blue emission as well as in its organogel form. Therefore, it could be considered as a promising blue emitter in solid state. This paper shows a novel strategy for creating new AIE organogels. The structural simplicity and easy synthesis make it a potential emitting material for the future applications.

ASSOCIATED CONTENT


16

Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Experimental details, absorption and fluorescence studies, FE-SEM, 1H NMR and 13C NMR spectra, X-Ray data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Instituto de Ciencia de Materiales de Madrid, CSIC, C/ Sor Juana Inés de la Cruz 3, 28049 Madrid, Spain. *E-mail: [email protected], Phone: +34913349038 ACKNOWLEDGMENT This work was financially supported by the MICINN of Spain (CTQ2010-18813) and CAM (Project S2009/MAT-1756/CAM). Thanks to Dr. Perles (SIdI, UAM) for resolution of singlecrystal X-ray diffraction analysis, and M. Dolores Penin and Antonio Almorin (SIdI, UAM) for 2D-NMR studies. Thanks to Dr. González-Rodríguez for technical support. REFERENCES (1) Steed, J. W. Supramolecular gel chemistry: developments over the last decade. Chem. Commun. 2011, 47, 1379−1383. (2) Babu, S. S.; Praveen, V. K.; Ajayaghosh, A. Functional π‑Gelators and Their Applications. Chem. Rev. 2014, 114, 1973−2129. (3) Dastidar, P. Supramolecular gelling agents: can they be designed?. Chem. Soc. Rev. 2008, 37, 2699−2715.


17

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

(4) Skilling, K. J.; Citossi, F.; Bradshaw, T. D.; Ashford, M.; Kellam, B.; Marlow, M. Insights into low molecular mass organic gelators:a focus on drug delivery and tissue engineering applications. Soft Matter 2014, 10, 237−256. (5) Yu, H.; Lü, Y.; Chen, X.; Liu K.; Fang Y. Functionality-oriented molecular gels: synthesis and properties of nitrobenzoxadiazole (NBD)-containing low-molecular mass gelators. Soft Matter 2014, 10, 9159−9166. (6) Babu, S. S.; Prasanthkumar, S.; Ajayaghosh, A. Self-Assembled Gelators for Organic Electronics. Angew. Chem., Int. Ed. Engl. 2012, 51, 1766−1776. (7) Diring, S.; Camerel, F.; Donnio, B.; Dintzer, T.; Toffanin, S.; Capelli, R.; Muccini, M.; Ziessel, R. Luminescent Ethynyl-Pyrene Liquid Crystals and Gels for Optoelectronic Devices. J. Am. Chem. Soc. 2009, 131, 18177−18185. (8) Liu, K.; Liu, T.; Chen, X.; Sun, X.; Fang, Y. Fluorescent Films Based on Molecular-Gel Networks and Their Sensing Performances. ACS Appl. Mater. Interfaces 2013, 5, 9830−9836. (9) Wang, C.; Chen, Q.; Sun, F.; Zhang, D.; Zhang, G.; Huang, Y.; Zhao, R.; Zhu, D. Multistimuli Responsive Organogels Based on a New Gelator Featuring Tetrathiafulvalene and Azobenzene Groups: Reversible Tuning of the Gel-Sol Transition by Redox Reactions and Light Irradiation. J. Am. Chem. Soc. 2010, 132, 3092−3096. (10) Xue, P.; Lu, R.; Jia, J.; Takafuji, M.; Ihara, H. A Smart Gelator as a Chemosensor: Application to Integrated Logic Gates in Solution, Gel, and Film. Chem. Eur. J. 2012, 18, 3549−3558. (11) Zhao, Z.; Lam, J. W. Y.; Tang, B. Z. Self-assembly of organic luminophores with gelation-enhanced emission characteristics Soft Matter 2013, 9, 4564−4579.


18

Page 19 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(12) Xue, P.; Yao, B.; Sun, J.; Zhang, Z.; Lu; R. Emission enhancement of a coplanar πconjugated gelator without any auxiliary substituents Chem. Commun. 2014, 50, 10284–10286. (13) Xue, P.; Lu, R.; Chen, G.; Zhang, Y.; Nomoto, H.; Takafuji, M; Ihara, H. Functional Organogel Based on a Salicylideneaniline Derivative with Enhanced Fluorescence Emission and Photochromism Chem. Eur. J. 2007, 13, 8231–8239. (14) Zhang, Y.; Liang, C.; Shang, H.; Ma,Y.; Jiang, S. Supramolecular organogels and nanowires based on a V-shaped cyanostilbene amide derivative with aggregation-induced emission (AIE) properties J. Mater. Chem. C. 2013, 1, 4472–4480. (15) Hu, R.; Leung, N. L. C.; Tang, B. Z. AIE macromolecules: syntheses, structures and functionalities. Chem. Soc. Rev. 2014, 43, 4494−4562. (16) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Aggregation-induced emission. Chem. Soc. Rev. 2011, 40, 5361−5388. (17) Tang, B. Z.; Qin, A. Aggregation-Induced Emission: Fundamentals; Wiley-VCH Verlag GMBH Weinheim, 2013. (18) Hong, Y.; Lama, J. W. Y.; Zhong Tang; B. Aggregation-induced emission: phenomenon, mechanism and applications Chem. Commun., 2009, 4332–4353. (19) Shih, P. I.; Chuang, C. Y.; Chien, C. H.; Diau, E. W. G.; Shu, C. F. Highly Efficient NonDoped Blue-Light-Emitting Diodes Based on an Anthrancene Derivative End-Capped with Tetraphenylethylene Groups. Adv. Funct. Mater. 2007, 17, 3141−3146. (20) Li, H. C.; Lin, Y. P.; Chou, P. T.; Cheng, Y. M.; Liu, R. S. Color Tuning and Highly Efficient Blue Emitters of Finite Diphenylamino-Containing Oligo(arylenevinylene) Derivatives Using Fluoro Substituents. Adv. Funct. Mater. 2007, 17, 520−530.


19

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

(21) Tonzola, C. J.; Kulkarni, A. P.; Gifford, A. P.; Kaminsky, W.; Jenekhe, S. A. Blue-LightEmitting Oligoquinolines: Synthesis, Properties, and High-Efficiency Blue-Light-Emitting Diodes. Adv. Funct. Mater. 2007, 17, 863−874. (22) Zhao, Z.; Chen, S.; Lam, J. W. Y.; Lu, P.; Zhong, Y.; Wong, K. S.; Kwok, H. S.; Tang, B. Z. Creation of highly efficient solid emitter by decorating pyrene core with AIE-active tetraphenylethene peripheries. Chem. Commun. 2010, 46, 2221−2223. (23) Zhao, Z.; Lam, J. W. Y.; Tang, B. Z. Tetraphenylethene: a versatile AIE building block for the construction of efficient luminescent materials for organic light-emitting diodes. J. Mater. Chem. 2012, 22, 23726−23740. (24) Guenet, J. M.; Thermoreversible gelation of polymers and biopolymers; Academic Press: London, 1992. (25) Kavanagh, G. M.; Ross-Murphy, S. B.; Rheological characterisation of polymer gels, Prog. Polym. Sci 1998, 23, 533−562. (26) Jones, J. L.; Marques, C. M. Rigid polymer network models J. Phys. France 1990, 51, 1113−1127.


20

Page 21 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

TOC Graphic This letter presents a new simple fluorescent self-assembled organogel based on 5-(4nonylphenyl)-7-azaindole. The organogel affords a strong red-shifted emission respect to diluted solutions with a blue light emitting as well as an aggregation-induced emission (AIE) behavior.


21

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. a) Process gel formation 1 (3.5 wt %) organogel in cyclohexane at room temperature and the "tube inversion" method; b) SEM image of a dried gel of 1 in cyclohexane. 31x34mm (300 x 300 DPI)


Page 22 of 28

Page 23 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 2. a) Variation of G’(circles) and G’’(squares) with oscillatory strain for a 3.5 wt % gel of 1 in CH at 20 ºC and frequency of 1 Hz. Full symbols corresponds to curves obtained at increasing strains, empty symbols corresponds to curves obtained at decreasing strains; b) Variation of G’(cicles) and G’’(squares) with oscillatory frequency for a gel of 1 at a concentration of 0.09 M in CH. Experiment performed at 20 ºC and constant oscillatory strain amplitude of 0.1%; c) Variation of G’(cicles) and G’’(squares) with temperature for a gel of 1 at a concentration of 0.09 M in CH. Experiment performed at constant frequency of 1Hz and constant oscillatory strain amplitude of 0.1%. 184x410mm (288 x 288 DPI)


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3: Temperature-dependent 1H NMR spectrum of 1 in cyclohexane-d12 at 10-2M. 81x57mm (300 x 300 DPI)


Page 24 of 28

Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 4. FT-IR spectra a) solid state and b) xerogel of 1 79x62mm (300 x 300 DPI)


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. Fluorescence spectra of 1 in cyclohexane (32.9 mg mL-1) before and after gel formation. The inset shows the reversible emission switching at 306 nm by repeated sol-gel phase transition. 75x55mm (300 x 300 DPI)


Page 26 of 28

Page 27 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 6. Optical images of 1 at 10-2 M in cyclohexane solution (a) and in organogel state (b) under daylight and under UV light (366 nm). 55x26mm (300 x 300 DPI)


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. Crystallographic packing structure of the compound 1. 92x54mm (150 x 150 DPI)


Page 28 of 28

Aggregation-Induced Emission of Organogels Based on Self

Recommend Documents