Tunable Morphology and Mesophase Formation by Naphthalene

Publication Date (Web): January 8, 2013 ... and the self-assembly of the system has been examined in a variety of solvents and solvent ...... that all...
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Tunable Morphology and Mesophase Formation by NaphthaleneContaining Poly(aryl ether) Dendron-Based Low-Molecular-Weight Fluorescent Gels P. Rajamalli and Edamana Prasad* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India S Supporting Information *

ABSTRACT: Novel poly(aryl ether) dendron-based low-molecular-weight organogelaters (LMWG) containing naphthalene units at the core have been synthesized, and the self-assembly of the system has been examined in a variety of solvents and solvent mixtures. The compounds readily form gels with attractive critical gel concentration values associated with gelation-induced enhanced emission (GIEE). In addition to the remarkable properties of the previously reported anthracene and pyrene analogues (Rajamalli, P.; Prasad, E. Org. Lett. 2011, 13, 3714 and Rajamalli, P.; Prasad, E. Sof t Matter 2012, 8, 8896), the selfassembled systems exhibit distinctly different structure−property relationships. Unlike the reported ones, the present system forms sheetlike morphology in nonpolar solvent mixtures, giant vesicles in polar solvent mixtures, and lamellar or hexagonal columnar phases in single solvents. The unique properties of the selfassembled systems, which were analyzed through electron microscopic (SEM, TEM, AFM) and spectroscopic techniques (POM, fluorescence), are attributed to the replacement of anthracene/pyrene units by naphthalene units. The present work unravels the subtle role of minute structural change in altering the properties of LMWGs based on poly(aryl ether) dendrons.

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INTRODUCTION The self-assembly of small molecules into various supramolecular architectures is a powerful approach to developing new nanoscale materials and devices.1−5 The motivation to study the supramolecular organization of small molecules is to gain precise control over their interchromophoric interaction, which is highly relevant in the context of their utility in organic−electronic device applications.6−12 Considerable attention has been generated in understanding the self-assembly of low-molecular-weight gels in which small molecules are organized in regular nanoarchitectures through specific noncovalent interactions including hydrogen bonds, hydrophobic interactions, π−π interactions, and van der Waals forces.13−19 In particular, there is strong interest in smart organogelators for their potential applications in organic light-emitting diodes (OLEDs), photovoltaic cells, and light-harvesting systems.20−26 Appropriate molecular design to achieving long-range order of the functional chromophore in the self-assembled structure is extremely important for generating a continuous pathway for better transport of the charge carriers.27−30 In this context, the formation of fibrous structures with long-range order, as often found in organogels formed from small molecular building blocks, has been recognized as one of the attractive selfassembly models.31,32 Results from our laboratory have recently shown that robust gel systems can be generated from poly(aryl ether) dendrons with anthracene and pyrene units attached through an acylhydrazone linkage.33−35 Although the dendrons form useful © 2013 American Chemical Society

gel systems, the rigid nature of the aromatic units (anthracene and pyrene) present in the system limit conformational flexibility. It has been known in the literature that relatively small structural changes in the monomer units can result in large conformational flexibility, leading to a wide array of applications for gel systems. For example, a recent study on the structure−property relationship in the self-assembled systems from bis(trialkoxybenzamide)-functionalized naphthalene tetracarboxylic acid diamide suggests the striking role of minute structural changes in altering properties such as the propensity to self-assemble, mesophase properties, and the stability and morphology of the gel system.36−38 Intrigued by this, we have synthesized poly(aryl ether) dendrons with naphthalene units attached as the core unit. There are only limited numbers of low-molecular-weight hydrogels and organogel-based naphthalene derivatives. An elegant study by Yang et al. has described the synthesis and characterization of mesomorphic superstructures of β-amino acid-based hydrogelators containing naphthalene units that form switchable fluorescent systems.39 In another attempt, Das and co-workers reported the charge-transfer (CT) interaction, which leads to the gelation of bis(amide)-functionalized dialkoxynaphthalene (donor) and naphthalene diimide (acceptor) donor−acceptor (D−A) pairs in a moderately nonpolar Received: October 25, 2012 Revised: January 2, 2013 Published: January 8, 2013 1609

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solvent.40 However, there is a dearth of examples for naphthalene-based gel systems containing dendron units. The presence of dendritic structures provides a better molecular framework for enhanced intermolecular interaction, which assists in supramolecular self-assembly as well as gelation. More importantly, unique morphology and mesophase properties can be expected from naphthalene-containing dendron-based gels compared to those of anthracene or pyrene analogues. A detailed investigation regarding the structure−property relationships in the present gel systems has been carried out using spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), laser scanning confocal microscopy (LSCM), polarizing optical microscopy (POM), and rheology experiments.

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silicon wafers, and images were recorded using a Park System XE-100 in the noncontact mode regime. Confocal laser scanning microscopy images were obtained from an LSM710 laser scanning microscope. Powder XRD patterns were recorded on a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (λ = 1.54178 Å). Rheological studies were conducted in an MCR-301 Anton Paar rheometer. Dynamic light scattering (DLS) experiments were carried out with a Malvern Zetasizer nano series 25 °C with a path length of 1 cm. The wavelength of the laser used was 632.8 nm, and the scattering angle was kept at 90°. A confocal laser scanning microscopy image was obtained from an LSM710 laser scanning microscope.

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RESULTS AND DISCUSSION Gelation Properties. The structures of the naphthalenecored dendrons (I−IV) used in the present study are shown in Chart 1. The compounds are synthesized according to a reported procedure, after a slight modification.34 The purified compounds (I−IV) have been dissolved in the selected solvent, followed by gentle heating until a homogeneous, clear solution is obtained. Upon cooling to room temperature, the solution turned to gel in less than 1 h. Conversely, the solution becomes a gel within a few seconds if a sonication method is adopted. Although compound II forms a gel with a critical gel concentration (CGC) of 0.40 wt % in toluene, the remaining compounds exhibited relatively higher CGC values. The solvent and solvent mixtures that can be gelled by compounds I−IV are summarized in Table 1, along with their CGC values. The data shown in Table 1 reflect the influence of the gelation behavior of the compounds. For example, between compounds II and IV, compound II exhibits gelation at lower CGC values because the presence of an increased number of benzyl units enhances the propensity of self-assembly of the individual units. The gelation dependency on dendron generation has been similar to that of anthracene and pyrene analogues. Similarly, the addition of hexane to a solution of the gelator in chloroform leads to greater π stacking and hydrogen bonding, leading to the formation of an opaque gel. Interestingly, the use of aromatic solvents such as toluene and benzene results in the formation of translucent gels. To determine the intermolecular interactions prevailing in the gel phase, we have analyzed the system using FTIR and UV−vis spectroscopy. The FTIR spectrum of xerogel I (from toluene) shows a single amide (CO) stretching vibrational band at 1649 cm−1 and an NH stretching vibration at 3196 cm−1 (Figure S1). This is an unambiguous signature of the presence of a network of intermolecular H-bonded amides in the system. The UV−vis spectrum (Figure S2) indicates a red-shifted band upon gelation, presumably resulting from the enhanced π−π interactions in the gel phase. These gels have been stable for more than 6 months. Although the poly(aryl ether) dendron derivatives shown in Chart 1 are almost nonfluorescent in solution, a significant enhancement in fluorescence emission intensity is observed as a result of the gelation process. Figure S3 shows the photographs of the compound II gel under UV-light illumination. It is evident from Figure S3 that the gel system emits bright-blue luminescence under UV irradiation. The intensity of photoluminescence in the gel phase is enhanced by orders of magnitude compared to that in the solution phase (Figures S3 and S4). This is generally referred to as gelation-induced enhanced emission (GIEE). In solution, nonradiative decay process such as internal conversion (IC) and photoinduced electron transfer (PET) take place to a large extent. In contrast,

EXPERIMENTAL SECTION

Materials. The poly(aryl ether) dendron derivatives are synthesized according to the reported procedure with slight modifications.34 All of the starting materials are obtained from Sigma-Aldrich (USA) and Sd Fine Chemicals Pvt. Ltd. (India). The chemicals are used as received unless otherwise mentioned. The organic solvents are purified according to the standard procedures. The synthesis procedures for compounds I−IV (Chart 1) and the characterization data are given in the Supporting Information.

Chart 1. Structure of Naphthalene-Based Dendrons I−IV

Instruments. 1H and 13C NMR data were collected on a Bruker 400 MHz spectrometer (1H, 400 MHz; 13C, 100 MHz). Mass spectra were recorded using a Micromass Q-TOF mass spectrometer and a Voyager-DE PRO MALDI/TOF mass spectrometer with α-cyano-4hydroxylcinnamic acid (CCA) as the matrix. The UV−vis spectroscopic studies were carried using a Jasco V-660 spectrophotometer. Luminescence experiments were carried out on a Horiba Jobin Yvon Fluoromax-4 fluorescence spectrophotometer. The fluorescence decay measurements were carried out by the time-correlated single-photoncounting technique (TCSPC) with a microchannel plate photomultiplier tube (MCP-PMT) as the detector and a picosecond laser as the excitation source (model 5000 U, IBH, U.K.). The scanning electron microscopy studies were carried out using an FEI-Quanta microscope. AFM samples were prepared by a spin-coating method on 1610

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Table 1. Gelation Properties, Critical Gel Concentrations (CGCs, mg/mL), and Tgel Values (°C) of the Dendrons in Various Organic Solvents and Solvent Mixturesa

a

solvent

I

II

III

toluene benzene CCl4 CH3CN MeOH xylene CHCl3 DCM CHCl3:hexane chlorobenzene DCM: hexane THF

G (9 mg/mL) 80 °C G (10 mg/mL) 76 °C G (7 mg/mL) 62 °C G (9 mg/mL) 73 °C I G (7.5 mg/mL) 82 °C G (14 mg/mL) 58 °C G(15 mg/mL) 52 °C G (6 mg/mL) 82 °C G (10 mg/mL) 74 °C G (7 mg/mL) 70 °C G (20 mg/mL) 60 °C

TG (4 mg/mL) 50 °C TG (5 mg/mL) 54 °C G (3 mg/mL) 56 °C G (3 mg/mL) 62 °C G (2.5 mg/mL) 54 °C G (3.5 mg/mL) 58 °C S S G (2.5 mg/mL) 74 °C G (6 mg/mL) 59 °C G (3 mg/mL) 64 °C S

PG PG PG G (6 mg/mL) 70 °C I PG G (9 mg/mL) 55 °C G (10 mg/mL) 49 °C G (7 mg/mL) 78 °C PG G (8 mg/mL) 70 °C G (25 mg/mL) 56 °C

IV G G G G G G S S G G G S

(4 mg/mL) 58 °C (8 mg/mL) 60 °C (5 mg/mL) 62 °C (4.5 mg/mL) 65 °C (4 mg/mL) 57 °C (4 mg/mL) 63 °C

(4.5 mg/mL) 75 °C (7.5 mg/mL) 62 °C (5 mg/mL) 65 °C

G, gel; TG, translucent gel; S, solution; PG, partial gel; I, insoluble.

Figure 1. (a) SEM, (b) AFM, and (c) TEM images of the xerogel formed from compound II in toluene.

these detrimental nonradiative decay processes are restricted in the gel phase because of reduced motional collision and the nonavailability of lone pair electrons for PET.41 As a representative case, the excited-state lifetime of compound I has been determined using a time-correlated single-photoncounting (TCSPC) instrument (Figure S5). The decay was double exponential in nature, and the lifetime values were τ1 = 0.70 ns and τ2 = 4.30 ns. The lifetime decay values indicate that the emission originated from naphthalene excimers.42 Morphological Investigation. The morphology of the xerogels of the dendron derivatives formed from solvent and a solvent mixture has been examined by SEM, TEM, and AFM. Figure 1a shows the SEM image of compound II, and Figure S6 shows the SEM images of the remaining compounds. It is clear from SEM images that all compounds exhibit well-defined fibrillar-type self-assembly similar to that of pyrene and anthracene analogues. A careful examination of the SEM images illustrates that the fiber thickness of the self-assembled poly(aryl ether) dendron derivatives can be correlated with the dendron generation. For example, the first-generation dendrons form fibers with approximately 1 μm width, whereas the fiber width of compounds from the second generation is only a few nanometers (200 nm). This is presumably due to the closely packed molecular arrangement in second-generation dendrons, which results in the enhanced π−π interactions between the relatively large numbers of aryl units present compared to those in first-generation dendrons. These observations are consistent with the relatively higher CGC value of first-generation dendrons compared to that of second-generation dendrons (Table 1). Furthermore, AFM analysis reveals the formation of well-defined fibrils by the poly(aryl ether) dendron derivatives

that are entangled with each other upon gel formation. Figure 1b contains the AFM images of compound II, and Figure S7 contains the AFM images of the remaining compounds utilized in the present study. The images show fibers with two distinct heights for the first and second generations (200 and 50 nm, respectively). Figure 1c shows the TEM image of the xerogel formed from compound II in toluene, which also supports the entangled fiber formation in naphthalene-based poly(aryl ether) dendron-based gels. The electron microscopy analysis of the gel morphology indicates that the compounds form a fiber-type assembly in single solvents. Solvent-Controlled Tunable Morphology. Compounds I−IV were dissolved in a series of solvent mixtures to analyze the effect of varied polarity on the gelation propensity. The dendrons utilized in the present study are partially soluble in a 1:1 (v/v) mixture of THF/H2O at room temperature, and a clear solution has been formed upon heating. Interestingly, instead of fiber formation, spherical aggregates were formed in the system. The hydrodynamic diameter of spherical aggregates formed from compounds I−IV is relatively larger than that of micelles (1−40 nm), which suggests that these aggregates might be forming vesicles (vide infra).43 Figures 2 and S8a show the SEM images of compounds I−IV prepared in 1:1 (v/ v) THF/water. The results also indicate that the dendron derivatives show two different types of aggregates with distinct diameter ranges: one with an average diameter of approximately 400 nm (compounds I and III) and the other with an average diameter of approximately 1.2 μm (compounds II and IV). Dynamic light scattering (DLS) experiments have been carried out to confirm the size of the aggregates formed in the self-assembly. The DLS histograms corroborate the formation 1611

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Figure 3. CLSM image of compound IV in THF/water (a) in the absence and (b) in the presence of rhodamine B.

between the outer ring and the inner areas. This indicates that vesicles are centrally hollow and filled with fluorescently silent solvent molecules. To substantiate this hypothesis, rhodamine B dye in water is added dropwise to the dendron derivatives dissolved in THF. Figure 3b clearly shows that the dye molecules are encapsulated inside the hollow microvesicles. The vesicle nature of the self-assembly has been further confirmed by TEM studies. A small drop of compound II from the 1:1 (v/v) THF/water mixture is deposited on a carboncoated copper grid (200 mesh) and dried. The TEM image is recorded without staining the sample. Figures 4 and S10 show the 2D projection of the polydisperse spherical vesicles. These vesicles are 1 μm in diameter, which suggests that they are medium-sized vesicles.

Figure 2. (a) Dynamic light scattering histograms and (b) SEM image of compound II in 1:1 (v/v) THF/water.

of two distinctly differently sized aggregates depending upon the dendron generation (Figures 2 and S8b). The correlation between the SEM and DLS data indicates that the aggregates are stable even in the dry state. The interesting morphology of the self-assembled structures has been further analyzed by AFM. The results suggest that the vesicle-type aggregates formed by the dendrons in the THF/ water mixture are flattened in shape with rounded edges. Figure S9 shows the AFM images for compounds I−IV, where the sample has been initially prepared from a THF/water mixture. The vesicles formed from compounds II and IV are ellipsoidal in shape instead of spherical because solvents are not dried instantly from the deposited droplet on a silicon wafer. It is likely that there is a moving drying front that allows these vesicles to orient along its direction, which accounts for the ellipsoid shape. AFM cross-sectional analysis indicates that the length, width, aspect ratio, and length to height ratios of the ellipsoidal assembly fall in the ranges of 350−1400, 200−700, 2.3−3.6, and 2.8−7.1 nm, respectively, for the compound II. Similar results have also been obtained for compound IV, where the length, width, aspect ratio, and length to height ratios are in the order of 350−1000, 300−700, 2.2−3.0, and 3.3−3.6 nm, respectively. It has been observed that the first-generation dendron derivatives exhibit a spherical shape instead of an ellipsoidal shape. This is presumably due to the formation of relatively smaller vesicles during self-assembly, where the driving force of solvent evaporation is not sufficient to change the morphology to an ellipsoidal shape. It is worth noting that the ellipsoidal morphology was completely absent in pyrene/ anthracene analogues. The hollow features of the vesicles are evidenced by a laser scanning confocal microscopy (LSCM) image (Figure 3a), where a clear contrast in the luminescence has been observed

Figure 4. TEM images of compound II in THF/water and its magnified image.

In a polar medium, the molecules may form folded structures because of the presence of both polar (acylhydrazone) and nonpolar {naphthalene and poly(aryl ether)} moieties in the molecule. The folded structures can further self-assemble to form relatively smaller vesicles. The larger vesicles are formed by the fusion of smaller vesicles as observed in the TEM images. Although the energy-minimized geometry of individual molecules of II shows ∼1.9 nm as the folded length (Figure S11), the TEM images show a wall thickness of more than ∼7 nm. This suggests that the vesicles that are formed are multilayered in nature. Next, the gelation and morphology of the compounds are investigated in nonpolar solvent mixtures. As stated earlier, these dendrons form vesicles in polar solvent mixtures because of the self-assembly of folded structures. Such folded molecules are expected to form inverted vesicles in a nonpolar medium such as CHCl3/hexane. Surprisingly, it is found that the dendron derivatives form extended sheet-type morphology in CHCl3/hexane mixtures. The representative SEM images of the dendron derivatives in CHCl3/hexane mixtures are given in Figures 5 and S12. The SEM images clearly indicate that two 1612

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To understand the molecular packing pattern of the lyotropic liquid-crystalline phase, the X-ray diffraction pattern of the xerogels of compounds I, II, and IV was analyzed. The XRD pattern of I (generation I) follows the scattering vector ratio 1:2, which clearly reveals that molecules are arranged in a lamellar pattern. The end-to-end distance in the dendron wedges of I, calculated by the CPK molecular modeling method, is approximately 1.4 nm. XRD results also suggest that d = 1.33 nm, which is in good agreement with molecular arrangements indicated by theory. Interestingly, the XRD pattern of II and IV (generation II) follows the scattering vector ratio 1:31/2:41/2 in the low-angle region, which clearly suggest that molecules self-organize in a hexagonal columnar fashion with lattice parameters of a = b = 3.01 and a = b = 2.29 nm for II and IV, respectively (Figure 7 and Table S1). The XRD pattern in the wide-angle region is given in Figure S13. It can be envisaged that the molecules are self-organized in a columnar hexagonal (Colh) fashion through the stacking of naphthalene units, along with the hydrogen bonding between the acylhydrazone moieties. The stacked naphthalene units are located in the middle of the column, and the dendron parts can be oriented outward. The end-to-end distances in the dendrons of compounds II and IV, calculated by the CPK molecular modeling method, are approximately 2.6 and 2.1 nm, respectively. However, the expected column diameter should be around 4.0 nm on the basis of the length of the molecule. This could presumably be due to the interdigitation of the naphthalene moiety through π−π stacking. The XRD results indicating the formation of the hexagonal columnar phase with column diameters of 3.01 and 2.29 nm were consistent with the results obtained from geometry optimization for compounds II and IV. The comparison between the naphthalene-based gels and anthracene- or pyrene-based analogue gel systems suggests the following: although the anthracene- and pyrene-based gel systems exhibit only fiber-type morphology above the CGC values, the present systems exhibit various types of morphologies depending upon the solvent polarity. This suggests that the role of various noncovalent intermolecular forces in the two systems is not identical. Although the deciding parameter in the self-assembly of anthracene- and pyrene-based gel systems was π−π stacking interactions between the aromatic units as a result of the availability of a large π-cloud in those systems, hydrogen bonding plays the leading role in naphthalene-based systems. In single solvents, entangled fibers are preferred because of the linear overlap of the dendron wedges of the monomer units, as previously found in similar systems.35 Powder XRD patterns suggest that the molecules are

Figure 5. SEM images of the gel formed by (a) compound I in CHCl3/hexane and (b) compound IV in CHCl3/hexane.

differently sized sheets are formed by the dendron derivatives with average diameters of 1 to 2 and 5 to 8 μm for first and second generations, respectively. The sheet structure is preferred, presumably because of the formation of laterally self-assembled structures during the self-assembly of the dendrons. Lyotropic Liquid-Crystalline Behavior. Lyotropic liquid crystals have emerging applications in a large number of areas such as switchable display materials, optical gratings, polarizers, biosensors, light-emitting diodes, organic transistors, and drugdelivery materials.44−49 Aside from the potential applications, dendrimer-based lyotropic liquid crystals have been rarely reported in the literature.50−52 It has also been reported that relatively minor structural changes might lead to large differences in the self-assembling nature of lyotropic liquid crystals.50 To achieve lyotropic liquid-crystalline behavior, it is important to have rigid and flexible parts in the molecular system. Because the introduction of a naphthalene unit into the system might increase the flexibility of the monomer unit in a significant way, the naphthalene-cored poly(aryl ether) dendron derivatives were examined under polarizing optical microscopy (POM) to determine whether the molecular assembly forms a mesophase. POM studies indicate that the first- and second-generation (compounds I, II, and IV) dendron derivatives self-organize into a lyotropic liquidcrystalline phase in toluene upon increasing their concentration above the critical gel concentration values. For example, compounds II and IV self-assemble into a lyotropic liquidcrystalline phase when the concentration in toluene exceeds 4 wt % (i.e., >CGC of II and IV in toluene). The POM images of the lyotropic phases for compounds I, II, and IV are given in Figure 6. Although the textures can be deformed with pressure, they possess the property of self-healing. No evidence of microcrystallite formation has been observed.

Figure 6. POM images of (a) compound I, (b) compound II, and (c) compound IV in toluene. 1613

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Figure 7. Powder XRD patterns of gels formed from (a) compound I, (b) compound II, and (c) compound IV.

value compared to those of other compounds. The difference in the two moduli (G′ − G″ = ΔG) is considered to be a measure of the dominance of the elastic behavior of the material over its viscous properties. ΔG values for compounds I, II, and IV are 7833, 1354, and 748 Pa, respectively. As shown in Figure S15b, the G′ values depend on the concentration of the organogel. For example, compound II had a larger G′ (10 134 Pa) value at a higher concentration compared to that at lower concentration (G′ = 1526 Pa). The highest value obtained for G′ is 6.6 times greater than that of the value at lower concentration, which suggests that the gel becomes stronger in the higher concentration range of the organogel. The difference in these two moduli (G′ − G″= ΔG) is higher for the higher-concentration gel (HCG) (ΔG = 8912.27 Pa) compared to that of the lower-concentration gel (LCG) (ΔG = 1383.96 Pa). This indicates that the elastic property is more dominant than the viscous nature in the case of HCG relative to that of the lower-concentration gel. This is consistent with the observation that the gel−sol phase-transition temperature (Tg), which is the transition temperature from an immobile to a mobile self-assembly state, increases as the concentration of the gel increases (Figure S16). A comparison of the G′ values between anthracene-based analogues gel systems indicates that anthracene gels are approximately 5 times stronger than the corresponding naphthalene gels (G′ for anthracene-based gel systems = 53417 Pa and G′ for naphthalene-based gel systems = 10 134 Pa). Corresponding rheological experimental data of the anthracene compound is given in the Supporting Information (Figure S17). Considering the fact that the structural changes in the present system are quite minimal compared to those for the previously reported anthracene analogues, the impact on the rigidity of the system is quite intriguing. Fluoride Ion Detection. Because the earlier version of the acylhydrazone-based gel systems exhibited excellent fluoride ion sensitivity,34 the anion binding properties of the current gelators toward a number of selected anions (F−, Cl−, Br−, I−, ClO4−, CH3COO−, H2PO4−, PF6−, ClO4−, and HSO4−) as Bu4N+ salts have been examined. The results suggest that naphthalene derivatives described in the present study are less sensitive toward the fluoride ion in their gel phase compared to that of the anthracene and pyrene analogues. The gel−sol phase transition has been induced only in the presence of excess F−, hence an anion detection study has not been carried out in the gel phase. However, further analysis has shown that the molecules detect fluoride ions in the solution phase within the micromolar concentration range of the analyte. More interestingly, the detection event has been associated with a color change observable by the naked eye. The above-

arranged in the lamellar or hexagonal columnar arrangement in single solvents, depending on the dendron generation (vide supra). The self-assembled units further interact with similar units to form a fiber, and several of such individual fibers selfassemble to form the fibrillar-type assembly that is observed via SEM analysis.33,50 In nonpolar solvent mixtures, the propensity to self-assemble through hydrogen bonding will be maximized because no solvent molecules compete for hydrogen bonding. This results in closely packed fiber structures, leading to sheettype morphology. This is further substantiated by the sharp peaks obtained in the powder XRD analysis, indicative of the crystalline nature of the opaque gel systems (Figure S14). Such gel systems are unstable, and easy phase separation was observed when nonpolar solvents have been utilized. Conversely, molecules are folded into a polar solvent mixture to form multilayer structures, and multilayers are further folded to form vesicle structures. On the basis of the proposed morphology variation in the solvent and solvent mixtures, a schematic representation of self-assembly is given in Figure 8.

Figure 8. Schematic representation of morphology variations in solvents and solvent mixtures.

Rheological Studies. Because the results indicate that there are significant differences between the self-assembly between the present gel systems and the earlier reported anthracene analogues, the rigidity and flow behavior of the naphthalene gels have been investigated in toluene for comparison purpose. In Figure S15a, the respective storage moduli, G′, and loss moduli, G″, have been shown for compounds I, II, and IV as functions of the angular elastic response. Because compound III does not forming a gel in toluene, the rheology experiment was not performed for III. As the data shows, the G′ values are always found to be larger than the G″ values over the entire range of frequencies. An organogel formed from compound I provides the greatest G′ 1614

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longer wavelength only in the presence of fluoride ions. The UV−vis spectra of compound I in THF (1 × 10−4 M) in the presence of an increasing number of fluoride ions are given in Figure 10b, which showed an absorbance peak at longer wavelength with an isosbestic point, indicating that the addition of fluoride ions generates a new species in equilibrium.53 The binding constants between the fluoride anion and the ligand are calculated using the Benesi−Hildebrand method, and the value of the binding constant is 5.2 × 104 M−1 in the case of compound I (Figure S18a). Titration experiments suggest that the stoichiometry involved in the equilibrium is 1:1 (Figure S18b). The mechanism is similar to that observed with anthracene- and pyrene-derived gel systems. Before the addition of F−, the 1H NMR chemical shift values of −CH N− and −NH protons in the compounds appear at 9.65 and 12.05 ppm, respectively. After the addition of 1 equiv of F−, the NH signal disappears, indicating a deprotonation reaction (Figure S19).

mentioned anions were dissolved in THF and mixed with 1 equiv of the dendron-based gelators. The THF solution of gelators shows an intense variation in their electronic absorption spectra only upon the addition of tetrabutylammoniumfluoride (TBAF). The presence of F− alters the colorless solution to dark yellow in color. Photographs of compound I in THF in the presence of various ions are shown in Figure 9. The

Figure 9. (a) Photographs of compound I (1 × 10−4 M) in THF after the addition of 1 equiv of various anions: (1) BF4−, (2) Br−, (3) CH3− COO−, (4) Cl−, (5) F−, (6) H2PO4−, (7) HSO4−, (8) I−, (9) PF6−, and (10) ClO4−.

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CONCLUSIONS A new series of naphthalene-cored poly(aryl ether) dendrons (first and second generations) have been prepared, and their gelation ability and structure−property relationships have been investigated. The present systems exhibit various morphologies depending upon the solvent polarity, indicating that significant variations compared to earlier reported pyrene or anthracene analogues exist regarding the intermolecular interactions that drive the gelation. Furthermore, unlike the reported systems, the present gels self-organize into lyotropic and hexagonal columnar-type liquid-crystalline phases. However, the mechanical strength of the naphthalene-based gel is approximately onefourth of that of anthracene-based gel systems. The monomer dendrons act as an efficient “naked eye” detecting system for F− in solution at very low concentrations of the analyte. The simultaneous observation of gelation and mesophase formation along with aggregation-induced enhanced emission properties makes these molecules promising candidates for various optoelectronic applications. The results taken together suggest that minor structural modification leads to significant property changes in poly(aryl ether)-based gel systems.

drastic color change of the gels accompanied by the addition of fluoride ions suggests changes in the electronic structure of the system. The UV−vis absorption spectra of compound I in THF (1 × 10−4 M) in the presence of 1 equiv of various anions are analyzed at room temperature (Figure 10a). As is clear from Figure 10a, the absorption band of compound I is shifted to

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ASSOCIATED CONTENT

S Supporting Information *

Detailed synthesis procedure and characterization data of compounds I−IV, remaining SEM image, AFM image, Job plot, and NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91-44-2257-4202. Tel: +91-442257-4232. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank DST {SR/NM/NS-115/2010(G)}, Government of India, for financial support. We thank the Department of MME and SAIF IIT Madras for SEM and TEM and Professor T. Pradeep, IITM, for MALDI-TOF experiments. We also thank the Bio-Tech Department, IITM, for access to the laser scanning confocal microscopy facility.

Figure 10. (a) UV−vis absorption spectra of I (1 × 10−4 M) in THF in the presence of various anions at room temperature and (b) UV−vis absorption spectra of I (1 × 10−4 M) with increasing concentration of F− (10−6−10−4 M). 1615

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