FULL PAPER DOI: 10.1002/asia.201402358

Pyrene-Based Fluorescent Supramolecular Hydrogel: Scaffold for Energy Transfer Subrata Mukherjee, Tanmoy Kar, and Prasanta Kumar Das*[a] Abstract: The self-assembled gelation of an amino-acid-based low molecular weight gelator having a pyrene moiety at the N terminus and a bis-ethyleneoxy unit linked with succinic acid at the C terminus is reported. This amphiphile is capable of gelating binary mixtures (1/3 v/v) of CH3CN/water, DMSO/water, and DMF/water, and the minimum gelation concentration (MGC) varied from 0.2 to 0.3 % w/v.

The sodium salt of the amphiphile efficiently gelates water with an MGC of 1.5 % w/v. The participation of different noncovalent interactions in supramolecular gelation by formation of fibrillar networks was investigated by Keywords: amphiphiles · fluorescence · FRET · gels · supramolecular chemistry

Introduction

In this regard, fluorescence resonance energy transfer (FRET) between two properly arranged fluorophores has gained enormous interest in recent years because of its applications in molecular electronics, light-harvesting systems, signal amplifiers, and so forth.[42–56] This work is primarily aimed at mimicking the unidirectional energy transfer between chromophores in photosynthetic systems. The chromophores (i.e., chlorophyll molecules) are arranged by noncovalent interactions at suitable distances and in appropriate orientations within the peptide matrix.[57] LMW gels have also greatly motivated supramolecular chemists, because gel formation is considered to occur by 1D alignment of gelator molecules through noncovalent interactions. Energy transfer from an organogelator to a hosted dye in the gel state was reported earlier.[57–61] There are also a few reports on FRET involving naphthalene-based hydrogelators. For instance, Friggeri and co-workers as well as Adams and co-workers have illustrated effective energy transfer between a naphthalene-based hydrogelator and an externally added dansyl derivative in the gel state.[62–64] Pyrene is another well-known fluorophore that has been widely used across this discipline because of its photoluminescence properties.[65–67] However, reports on pyrene-based hydrogelators and in particular their utilization in energy transfer are scarce. Herein, we report the synthesis and development of a pyrene-based fluorescent supramolecular hydrogel and its utilization in energy transfer to a hosted fluorophore (acridine orange). The gelator was found to gelate CH3CN/ water, DMSO/water, and DMF/water binary mixtures, and the minimum gelation concentration (MGC) varied from 0.2 to 0.3 % w/v in mixed solvents. The self-aggregation behavior of these thermoreversible gels was investigated by spectroscopic and microscopic techniques such as circular dichroism (CD), FTIR, temperature-dependent NMR, and lu-

Supramolecular gels[1–11] have attracted widespread attention due to their potential applications in diverse fields such as drug delivery,[12, 13] tissue engineering,[14, 15] templates for nanostructured materials,[16, 17] removal of pollutants,[18, 19] enzyme-immobilization matrices,[20–22] and so on. This has augmented the need for the rational design and synthesis of gelators with appropriate functionality for task-specific applications. Low molecular weight (LMW) gelators can immobilize a free-flowing solvent into a semisolid phase on addition of very small amount of gelator. LMW gelators can self-assemble into fibers, rods, ribbons, or other morphologies in suitable liquids through numerous weak interactions such as hydrogen bonding, p–p stacking, and van der Waals interactions, which lead to the formation of 3D self-assembled fibrillar networks (SAFINs).[23–37] The gelation process is controlled in part by the optimum balance between hydrophilicity and lipophilicity within the structure of the gelator, which dictates the tendency of the molecules to remain dissolved or to aggregate in a liquid. The amphiphilic character of the SAFINs of LMW gels make them capable of hosting externally doped hydrophilic or lipophilic molecules inside the interstitial space of the networks and arranging them along the alignment of the gelators.[38–41]

[a] S. Mukherjee, T. Kar, Prof. P. Kumar Das Department of Biological Chemistry Indian Association for the Cultivation of Science Jadavpur, Kolkata 700032 (India) Fax: (+ 91) 33-24732805 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402358.

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spectroscopic and microscopic methods. High mechanical strength of the supramolecular gels is indicated by storage moduli on the order of 103 Pa. The hydrogel was utilized for energy transfer, whereby inclusion of only 0.00075 % w/v of acridine orange resulted in about 50 % quenching of the fluorescence intensity of the gel through fluorescence resonance energy transfer.

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minescence spectroscopy, as well as high-resolution transmission electron microscopy (HRTEM) and fluorescence microscopy. A balanced participation of hydrogen bonding, p–p stacking, and van der Waals interactions was found to be the driving force for gelation. Rheological studies confirmed the viscoelastic nature of the prepared gels. Furthermore, inclusion of only 0.00075 % w/v of acceptor fluorophore (acridine orange) in the hydrogel resulted in about 50 % quenching of the fluorescence intensity of the hydrogel through FRET.

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transparent and colorless (Figure 1 b). Interestingly, they show bright blue emission (Figure 1 c) on exposure to UV light due to the strong fluorescence of the pyrenyl moiety. The gels were thermoreversible in nature and stable over a period of months at room temperature. They melted on heating and turned into gels again on cooling. The temperature at which the gel-to-sol transition takes place is defined as the gel-melting temperature Tgel. The Tgel values of 1 (in binary solvent mixtures) and 1 a (in water) varied from 42 to 51 8C at their MGC (Table 1). This gel-to-sol transition takes place due to the breaking of noncovalent interactions that were involved in self-assembled gelation. The Tgel value increased with increasing gelator concentration (Figure 2) due to the increasing participation of intermolecular noncovalent interactions.[36]

Results and Discussion Synthesis and Gelation Small-molecule amphiphilic gelator 1 comprising a pyrene moiety (hydrophobic group), an amino acid residue (linker), and a bis-ethyleneoxy unit linked with succinic acid (hydrophilic segment) was synthesized (Figure 1 a, see Figure S1 in

Figure 1. a) Structures of gelators. b) Photograph of gel at 0.2 % w/v in CH3CN/water (1:3, v/v).c) Photograph of the same gel under UV light.

Figure 2. Variation of Tgel with gelator concentration. 1 in DMSO/H2O 1:3 (*), 1 in CH3CN/H2O 1:3 (&), 1 in CH3CN/H2O 1:3 (~), 1 a (open stars).

the Supporting Information). Compound 1 is soluble in various polar organic solvents (e.g., CH3CN, DMSO, DMF) but insoluble in water. Interestingly, amphiphile 1 was found to gelate CH3CN/water, DMSO/water, and DMF/water (1/3 v/ v). Gel formation was verified by inversion of glass vials. The minimum gelation concentration (MGC) of 1 varied from 0.2 to 0.3 % w/v in mixed solvents (Table 1). This gel

Microscopy To gain insight into the self-assembled structure, the supramolecular morphology of the gels was examined by high-resolution transmission electron microscopy (HRTEM).[36, 68] The xerogel (dried gel) of 1 [CH3CN/H2O (1/3, v/v)] showed an entangled fibrillar network with fiber diameters of 25– 40 nm and lengths of several micrometers (Figure 3 a) in which two or more fibers were linked with each other to form thicker fibers of 120–150 nm in diameter. Similarly,

Table 1. MGC of 1 and 1 a at 25 8C and Tgel in different solvents. Gelator

Solvent system (v/v)

MGC [% w/v]

Tgel [8C]

1

CH3CN/H2O (1/3) DMSO/H2O (1/3) DMF/H2O (1/3) H2O

0.2 0.25 0.3 1.5

45 51 47 42

1a

can hold a large number of solvent molecules in its network (ca. 14 000 solvent molecules per gelator molecule at MGC). The contrasting solubility of 1 in water and polar organic solvents may have promoted the self-assembled gelation, which is considered to be a phenomenon in between solubilization and precipitation. The sodium salt of 1 showed gelation ability in pure water with an MGC of 1.5 % w/v (Table 1), possibly due to the increased hydrophilicity of the corresponding carboxylate 1 a (Figure 1). The gels were

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Figure 3. TEM images of a) xerogel 1 [CH3CN/water (1/3, v/v)] and b) hydrogel 1 a. c) Fluorescence microscopic image of 1 in CH3CN/water (1/3, v/v).

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thin fibers with average thickness of 20–30 nm were observed for the xerogel obtained from DMF/water (1/3, v/v), and the fibers were entangled with each other to form fibrillar networks (see Figure S2 a in the Supporting Information). The xerogel of 1 a (from water) showed a similar interconnected fibrillar network with a thickness of 20–30 nm and thicker fibers of 80–100 nm in diameter (Figure 3 b). The field-emission scanning electron microscopy (FESEM) image of 1 a also revealed the formation of intertwined fibrillar networks (see Figure S2 b in the Supporting Information). The fibrous morphology in the gel state was also evident in the corresponding fluorescence microscopic image of 1 (Figure 3 c) due to the intrinsic fluorescence emission of pyrene.

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of the amide bond, which is extremely sensitive to coupling with neighboring amide groups. The intensity of the CD peak steadily increased with increasing gelator concentration, and this suggested a highly ordered arrangement of monomers at the supramolecular level. The noncovalent supramolecular association of the gelator molecules was also studied by temperature-dependent CD spectroscopy of 1 a in aqueous medium. Initially, CD spectra of 1 a in water were acquired with increasing temperature from 20 to 70 8C while keeping the gelator concentration fixed at 0.05 % w/v (Figure 4 b). The nature of the CD peaks of 1 a was similar at all the temperatures; however, the peak intensity notably decreased at higher temperature. At elevated temperature, loss of the higher ordered aggregates that were formed through the supramolecular association of the amphiphilic gelator occurred. With further lowering of temperature to 20 8C, the self-assembled aggregate of the gelator molecule was restored, and the observed CD spectrum showed increased peak intensity almost matching that of the initial spectra taken at 20 8C (Figure 4 b). The temperature-dependent CD study confirmed the thermoreversibility of the hydrogel by the decrease and increase in peak intensity with variation in temperature (Figure 4 b).[36, 68] Temperature-dependent absorption spectra of 1 a in water also revealed thermoreversible aggregation of 1 a (see Figure S3 in the Supporting Information).

Circular Dichroism (CD) The supramolecular chirality originating from the noncovalent self-assembly of chiral monomers was studied by recording CD spectra of 1 a in water.[36, 68] The CD spectra showed a negative peak around 215, 250 nm and a positive peak at 285 nm (Figure 4 a). The observed Cotton effect at about 210–220 nm could be attributed to the p–p* transition

FTIR Spectroscopy The participation of amide moieties in hydrogen-bonding interactions during gelation was examined by recording FTIR spectra of 1 a in D2O in the gel state and the non-self-assembled state of 1 a in CHCl3. The FTIR spectrum of 1 a in CHCl3 showed transmission bands at 3448, 1652, and 1520 cm1, which correspond to the non-hydrogen-bonded nACHTUNGRE(N-H) (amide A), nACHTUNGRE(C=O) (amide I), and dACHTUNGRE(N-H) (amide II) modes, respectively (Figure 5). On the contrary, the trans-

Figure 4. CD spectra of a) 1 a with varying concentration in water at room temperature and b) CD spectra of 1 a (0.05 % w/v) with varying temperature in water.

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Figure 5. FTIR spectra of 1 a in D2O (gel state) and in CHCl3 (non-selfassembled state). 1 a in D2O (a), 1 a in CHCl3 (b).

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mission bands for hydrogel 1 a in D2O, which appeared at 3290, 1633, and 1565 cm1, are characteristic of hydrogenbonded amide N-H stretching, C=O stretching, and NH bending, respectively (Figure 5). Carboxylate stretching bands were observed at 1420 and 1422 cm1 in the FTIR spectra of 1 a in D2O gel and in CHCl3, respectively. The shifts of transmission bands indicate the involvement of hydrogen bonding between carbonyl group and amide N-H group in the gel state, possibly via intermolecular interactions.[36, 68]

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oped at higher wavelength, centered at 470 nm. This is a characteristic excimer peak of pyrene that is known to appear from at least the dimeric form of pyrene.[70, 71] Similar observations were made for 1 in the DMSO/water (1/3, v/v) system (Figure 6 B). The eximer peak clearly indicates the presence of strong p–p interactions between the pyrenyl groups of gelator molecules in the self-assembled state. NMR Spectroscopy The driving forces behind the supramolecular arrangement of the pyrene-based gelator were also investigated by 1 H NMR spectroscopy. NMR spectra of 1 a (2.0 % w/v) were recorded in [D6]DMSO with varying amounts of water (Figure 7). First, the NMR spectrum of 1 a (2.0 % w/v) was

Photoluminescence Spectroscopy Hydrophobic interaction is known to be a crucial parameter in particular for self-assembled hydrogelation. The intrinsic fluorescence property of the pyrene moiety in the structure of the gelator was exploited to decipher the role of hydrophobic interaction in gelation of 1.[69] The fluorescence spectra of 1 at 0.0001 % w/v (solution state) in CH3CN/water (1/ 3, v/v) showed characteristic peaks for molecular pyrene at 375, 396, and 417 nm on excitation at 335 nm (Figure 6 A). However, at the MGC (0.2 % w/v), the intensity of these emission peaks decreased and a new emission peak devel-

Figure 7. 1H NMR spectra of 1 a (1.5 % w/v) in [D6]DMSO with varying H2O content.

recorded in [D6]DMSO, in which the gelator is in the nonself-assembled state. The amide protons showed characteristic sharp signals at d = 9.44, 9.02, and 8.63 ppm (Figure 7). On addition of water, the amide protons of 1 a showed significant signal broadening with changes in their 1H NMR peak positions. The signal for the amide proton of 1 a shifted upfield (from d = 9.44, 9.02, 8.63, to 8.31–8.01 ppm) as water content increased to 20 %, and the shifting continued up to

Figure 6. Fluorescence spectra (lex = 335 nm) of gelator 1 A) in CH3CN/ water (1:3, v/v) and B) DMSO/water (1:3, v/v). 0.0001 % w/v gelator 1 (a), 0.2 % w/v gelator 1 (b).

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d = 7.89 at 40 % water content. These shifts suggest the involvement of amide protons in H-bonding during self-assembled hydrogelation. In a temperature-dependent NMR study, 1H NMR spectra of 1 a (2.0 % w/v) in D2O exhibited broad peaks in the aromatic region at 25 8C in the self-assembled state. With increasing temperature, the broad peak of aromatic protons gradually transformed into a sharper peak at a higher d value at 80 8C, which is much higher than Tgel of the gelator (Figure 8). This clearly indicates the involvement of the aro-

Figure 9. Plots of G’ and G’’ of 1 [1.0 % w/v in DMF/water, CH3CN/ water, DMSO/water (1:3, v/v)] as a function of angular frequency. G’ of 1 in DMF/H2O (~), G’’ of 1 in DMF/H2O (~), G’ of 1 in CH3CN/H2O (*), G’’ of 1 in CH3CN/H2O (*), G’ of 1 in DMSO/H2O (&), G’’ of 1 in DMSO/H2O (&).

region under the experimental conditions, which is a typical feature of viscoelastic materials. The G’ values of all gels were on the order of 103 Pa and thus indicate high mechanical strength of the supramolecular gels. Fluorescence Resonance Energy Transfer (FRET)

Figure 8. Temperature-dependent 1H NMR spectra of 1 a (2.0 % w/v) in D2O.

The intrinsic fluorescence property of pyrene moiety encouraged us to employ our gelator as an efficient donor in a FRET process. Molecular pyrene in solution has characteristics emission peaks at 375, 396, and 417 nm on excitation at 335 nm (see Figures S4 and S5 in the Supporting Information).[65–67, 69–71] Moreover, the hydrogel of 1 a (1.5 % w/v) exhibits a strong excimer emission peak centered at 477 nm (Figure 10) in the self-assembled state.[69–71] To evaluate whether efficient energy transfer can take place from 1 a to other fluorophores, we doped acridine orange (AO) into hy-

matic rings in supramolecular gelation. In the self-assembled state (in D2O, 2.0 % w/v of 1 a), the aromatic rings of 1 a are involved in strong p–p interactions. As a result, the protons are not in their characteristic spinning motion and could not exhibit sharp individual peaks in the NMR spectra. With increasing temperature, the intermolecular noncovalent interactions are destroyed, and loss of the gelation efficiency of the amphiphile leads to its transformation from gel to sol, for which the protons showed their characteristic signals.[36, 68] Rheology The viscoelastic properties of the supramolecular gels were investigated in rheological experiments. Rheological studies provide information about the flow behavior and rigidity of the gel. Two major rheological parameters are the storage modulus G’, which signifies the ability of the deformed material to store energy, and the loss modulus G’’, which corresponds to the flow behavior of the material under stress.[69, 72] Gels were prepared in CH3CN/water, DMF/water, and DMSO/water at 1.0 % w/v and subjected to a oscillatory frequency-sweep experiment. In a typical frequency-sweep experiment, the variation of G’ and G’’ is plotted as a function of angular frequency w under a constant strain of 0.01 %. For all the gels, G’ was higher than G’, and their curves did not cross each other over the entire w range (0.1–300 rad s1, Figure 9). Both G’ and G’ (G’ > G’’) showed a plateau

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Figure 10. Fluorescence spectra of hydrogel 1 a (1.5 % w/v) and fluorescence spectra of 1 a in the presence of varying concentration of AO. 1 a (1.5 % w/v) in water (a), 1 a and acridine orange (0.00025 % w/v) (b), 1 a and acridine orange (0.0005 % w/v) (c), 1 a and acridine orange (0.00075 % w/v) (d).

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drogel of 1 a. AO was the fluorophore of choice as its excitation wavelength (lex = 470 nm, see Figure S6 in the Supporting Information) overlaps with the emission wavelength of the hydrogel and it emits at about 525 nm (see Figure S7 in the Supporting Information). Addition of AO to the hydrogel did not alter the gelation efficacy and viscoelastic properties of the hydrogel. Interestingly, on excitation the gel sample containing AO (0.00025 % w/v, Figure 10) at 335 nm, a strong fluorescence band centered at 525 nm appeared while the fluorescence intensity of 1 a (1.5 % w/v) decreased slightly. With further increase in dye concentration (up to 0.00075 % w/v) the peak intensity at 477 nm decreased to about 50 % and subsequently the peak intensity at 525 nm gradually increased (Figure 10). However, similar energy transfer from 1 a to the host fluorophore was not observed in its non-self-assembled state (0.0025 % w/v; see Figure S8 in the Supporting Information). This observation indicates that energy was effectively transferred from the donor pyrene moiety to the added chromophore in the self-assembled gel state. Hence, the acceptor AO molecules were included within the SAFINs of hydrogelator 1 a and energy was transferred efficiently from the gel system to the fluorophore. Furthermore, fluorescence lifetime decay profiles of gelator 1 a (lex = 375 nm) in the absence and presence of AO were monitored at the emission maximum 470 nm (see Figure S9 in the Supporting Information). In both cases, the decay curves were fitted by a triexponential function. The components were 0.59 ns (72.0 %), 4.06 ns (25.0 %), and 18.45 ns (3.0 %) for 1 a (see Table S1 in the Supporting Information) and the average lifetime of 1 a (1.5 % w/v) was 1.99 ns. In the presence of AO, the components were 0.23 ns (83.0 %), 2.55 ns (15.0 %), and 12.68 ns (2.0 %) and the average lifetime of 1 a in the presence of 0.005 % w/v AO was 0.83 ns. Therefore, shortening of the decay time was observed in the presence of AO, which confirmed energy transfer from 1 a to acridine orange. The energy-transfer efficiency from 1 a to AO was calculated by using the equation FET = 1ACHTUNGRE(tDA/tD), where tDA and tD are the decay times of gelator 1 a in the presence and absence of AO.[73] The calculated energy transfer was found to be about 58 % (see Table S1 in the Supporting Information).

find use in fluorescence sensors, energy filters, light-harvesting systems, and other photonic devices.

Experimental Section Materials Amino acids, tert-butyloxycarbonyl (Boc) anhydride, dicyclohexyl carbodiimide (DCC), 4-(N,N-dimethyl)aminopyridine (DMAP), 1-hydroxybenzotriazole (HOBT), succinic anhydride, and solvents were procured from SRL, India. Trifluoroacetic acid (TFA) and sodium hydroxide were procured from Spectrochem, India. Pyrenebutyric acid, 2,2’-(ethylenedioxy)bis(ethylamine) and acridine orange were bought from Sigma. All deuterated solvents for NMR and FTIR experiments were obtained from Aldrich Chemical Co. TLC was performed on Merck precoated silica gel 60-F254 plates. 1H NMR spectra were recorded in AVANCE 500 MHz (Bruker) spectrometer. Mass spectrometric data were acquired by electron spray ionization (ESI) technique on a Q-tof-micro quadruple mass spectrometer (Micromass). Elemental analyses were performed on a PerkinElmer 2400 CHN analyzer. Synthetic Procedure Amphiphiles 1 and 1 a were synthesized by following well-established procedures of peptide chemistry.[35, 36] Briefly, pyrene butyric acid was coupled with the methyl ester of protected l-amino acid in dichloromethane by using DCC (1.1 equivalent), DMAP (1.1 equivalent), and HOBT (1.1 equiv) (see Figure S1 in the Supporting Information). The coupled product was subjected to hydrolysis with 1 N NaOH solution followed by workup with 1 N HCl. The free acid terminus of the l-amino acid was further coupled with mono-Boc-protected 2,2’-(ethylenedioxy)bis(ethylamine). Boc protection was removed from the resulting product by stirring with TFA. The purified product was obtained by column chromatography on 60–120 mesh silica gel with ethyl acetate/toluene as eluent. The purified product was then heated to reflux with succinic anhydride in dry dichloromethane to obtain the final product 1, which was then further purified by column chromatography. The carboxylic acids were converted to corresponding sodium salts (1 a) by adding one equivalent of 0.1 N NaOH (standardized) to a methanolic solution of the acid. After brief stirring, the solvent was removed and the residue dried under vacuum to get the sodium salt. Characterization of Amphiphile 1 1

H NMR (500 MHz, RT, CDCl3): d = 8.22–8.20 (d, 1 H), 8.17–8.15 (m, 2 H), 8.10–8.07 (m, 2 H), 8.05–7.97 (m, 2 H), 7.78–7.76 (d, 1 H), 7.25–7.22 (m, 5 H), 7.17–7.14 (m, 3 H), 6.74–6.64 (d, 2 H), 4.88 (s, 1 H), 3.47–3.38 (m, 12 H), 3.28–3.25 (m, 2 H), 3.08–3.04 (m, 1 H), 2.99–2.98 (m, 1 H), 2.70–2.65 (m, 2 H), 2.46 (br, 2 H), 2.30 (br, 2 H), 2.11 ppm (br, 2 H); elemental analysis (%) calcd for C39H43N3O7: C 70.36, H 6.51, N 6.31; found: C 70.31, H 6.55, N 6.33; ESI-MS calcd for C39H42N3O7Na + : m/z = 687.2920 [M] + ; found: 687.1577.

Preparation of Gel

Conclusions

The requisite amount of compound was added to a screw-capped vial with internal diameter (i.d.) of 10 mm and slowly heated to dissolve it in water, CH3CN, DMSO, DMF, CH3CN/water, DMSO/water, or DMF/ water. The solution was then allowed to cool slowly (undisturbed) to room temperature. The gelation was checked by stability to inversion of the aggregated material in the glass vial.

We have developed a pyrene-based, amino-acid-containing amphiphilic gelator that exhibited efficient gelation properties in binary solvent/water mixtures and also in water with formation of gels of notable mechanical strength. The gelator molecules self-assemble into higher-order aggregates through several noncovalent interactions. A light-harvesting system could be designed by utilizing the supramolecular aggregates formed by the pyrene-based gelator in aqueous medium, since about 50 % quenching of the fluorescence intensity of the gel was observed in presence of only 0.00075 % w/v of AO through efficient energy transfer. These soft materials based on molecular assemblies may

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Determination of Tgel The gel-to-sol transition temperature Tgel was recorded by gradually increasing the temperature (at a rate of 2 8C min1) of a thermostatted oil bath in which a hydrogel-containing glass vial (i.d. 10 mm) was placed. The temperature ( 0.5 8C) at which the gel melted and started to flow was taken as Tgel.

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FESEM Studies

NMR Experiments

A drop of native hydrogel was placed on a piece of cover slip and dried in vacuum for few hours before imaging. Then it was coated with platinum vapor to make it conducting and analyzed on a JEOL-6700F microscope.

Compound 1 a was dissolved in D2O (2 % w/v), and temperature-dependent NMR spectra were acquired from 25 to 75 8C. For NMR studies using varying solvent systems, initially 1 a was dissolved in [D6]DMSO (1.5 % w/v), and an NMR spectrum was taken. Subsequently, samples containing 20 and 40 % water were prepared (keeping the total solvent volume and amount of gelator constant), and the corresponding NMR spectra were acquired.

HRTEM Studies A drop of dilute solution of hydrogel was placed on a 300 mesh carbon coated Cu grid and dried for about 4 h under vacuum before imaging. The TEM images were taken on a JEOL JEM 2010 high-resolution microscope operating at 200 kV.

Rheology The rheological experiments were performed in cone-and-plate geometry (diameter: 40 mm) on a rheometer plate by using an Anton Paar MCR 302 instrument. The native gel was scooped on the rheometer plate so that there was no air gap to the cone. Frequency-sweep experiments were done as a function of angular frequency (0.1–300 rad s1) at a fixed strain of 0.01 % at 25 8C and the storage modulus G’ and the loss modulus G’ were plotted against angular frequency w.

Fluorescence Microscopy Fluorescence microscopic images of the gel samples were recorded by using a light microscope (BX61, Olympus) equipped with a BP330– 385 nm filter for the exciter and a band absorbance filter covering wavelengths below 420 nm. A small quantity of gel sample (20-fold diluted from MGC) was placed on a glass slide, which was sealed with a glass cover slip, and images were recorded.

Preparation of Fluorophore-Doped Hydrogel The required amount of AO solution (1–3 mL of a 1 mg mL1 stock solution in methanol) was added to 400 mL of a solution of gelator 1 a in water (1.5 % w/v), and the mixture kept undisturbed to allow gel formation.

Circular Dichroism CD spectra of aqueous solutions of gelators 1 and 1 a were recorded with varying concentrations and changing temperature in a quartz cuvette (1 mm path length) on a JASCO J-815 spectrometer. UV/Vis spectroscopy The UV/Vis spectra of the gelator and acridine orange and temperaturedependent absorption spectrum of gelator were recorded in a PerkinElmer Lambda 25 spectrophotometer.

Acknowledgements

FTIR Spectroscopy

P.K.D. is thankful to Department of Science and Technology, India (SR/ S1/OC-25/2011) for financial assistance. S.M. and T.K. acknowledge Council of Scientific and Industrial Research, India for their Research Fellowships. We are thankful to Mr. Sadananda Mandal and Prof. Amitava Patra for valuable help with fluorescence lifetime decay experiments.

FTIR measurements on gelators in CHCl3 solution and D2O for hydrogel were carried out on a PerkinElmer Spectrum 100 FTIR Spectrometer by using KBr pellets and CaF2 cell, respectively. Fluorescence Spectroscopy The emission spectra of the amphiphiles at varying concentration were recorded on a Varian Cary Eclipse luminescence spectrometer. Gel samples (at MGC) in different solvent systems were prepared and subsequently diluted in the respective mixed solvents to obtain the spectra. Solutions were excited at lex = 335 nm. The excitation and emission slit width were both 5 nm.

[1] P. Terech, R. G. Weiss, Chem. Rev. 1997, 97, 3133 – 3160. [2] Molecular Gels, Materials with Self-Assembled Fibrillar Networks (Eds.: R. G. Weiss, P. Terech), Springer, 2006. [3] S. S. Babu, V. K. Praveen, A. Ajayaghosh, Chem. Rev. 2014, 114, 1973 – 2129. [4] D. Das, T. Kar, P. K. Das, Soft Matter 2012, 8, 2348 – 2365. [5] A. Ajayaghosh, V. Praveen, Acc. Chem. Res. 2007, 40, 644 – 656. [6] M. Zelzer, R. V. Ulijn, Chem. Soc. Rev. 2010, 39, 3351 – 3357. [7] S. Banerjee, R. K. Das, U. Maitra, J. Mater. Chem. 2009, 19, 6649 – 6687. [8] A. Dawn, T. Shiraki, S. Haraguchi, S. Tamaru, S. Shinkai, Chem. Asian J. 2011, 6, 266 – 282. [9] V. K. Praveen, C. Ranjith, N. Armaroli, Angew. Chem. Int. Ed. 2014, 53, 365 – 368; Angew. Chem. 2014, 126, 373 – 376. [10] M. De Loos, B. L. Feringa, J. H. van Esch, Eur. J. Org. Chem. 2005, 3615 – 3631. [11] D. K. Smith, Chem. Soc. Rev. 2009, 38, 684 – 694. [12] J. Y. Li, Y. Kuang, Y. Gao, X. W. Du, J. F. Shi, B. Xu, J. Am. Chem. Soc. 2013, 135, 542 – 545. [13] “Enzyme-responsive Drug-delivery Systems”, P. F. Caponi, R. V. Ulijn in Smart Materials for Drug Delivery, Vol. 1 (Eds.: C. AlvarezLorenzo, A. Concheiro), The Royal Society of Chemistry, 2013, DOI: 10.1039/9781849736800-00232. [14] V. Jayawarna, M. Ali, T. A. Jowitt, A. F. Miller, A. Saiani, J. E. Gough, R. V. Ulijn, Adv. Mater. 2006, 18, 611 – 614. [15] F. Zhao, M. L. Ma, B. Xu, Chem. Soc. Rev. 2009, 38, 883 – 891. [16] N. M. Sangeetha, S. Bhat, G. Raffy, C. Belin, A. Loppinet-Serani, C. Aymonier, P. Terech, U. Maitra, J. P. Desvergne, A. D. Guerzo, Chem. Mater. 2009, 21, 3424 – 3432. [17] T. Kar, S. Dutta, P. K. Das, Soft Matter 2010, 6, 4777 – 4787. [18] T. Kar, S. Mukherjee, P. K. Das, New J. Chem. 2014, 38, 1158 – 1167.

For the time-correlated single-photon counting measurements, the samples were excited at 375 nm by using a nanosecond diode laser (IBH Nanoled) in an IBH Fluorocube apparatus. The typical full width at halfmaximum of the system response with a liquid scatterer was about 90 ps. The repetition rate was 1 MHz.The fluorescence decays were analyzed with IBH DAS6 software. Equation (1) was used to analyze the experimental time resolved fluorescence decay PðtÞ:[74]

PðtÞ ¼ b þ

n X i

t ai expð Þ ti

ð1Þ

where n is the number of discrete emissive species, b is a baseline correction (“dc” offset), and ai and ti are pre-exponential factors and excitedstate fluorescence lifetimes associated with the ith component, respectively. For multiexponential decays the average lifetime hti was calculated from Equation (2)[75]

h ti ¼

n X

ai ti

ð2Þ

i¼1

where ai ¼ ai =

P

ai and ai is the contribution of a decay component.

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[19] S. R. Jadhav, P. K. Vemula, R. Kumar, S. R. Raghavan, G. John, Angew. Chem. Int. Ed. 2010, 49, 7695 – 7698; Angew. Chem. 2010, 122, 7861 – 7864. [20] D. Das, S. Roy, S. Debnath, P. K. Das, Chem. Eur. J. 2010, 16, 4911 – 4922. [21] N. Bruns, J. C. Tiller, Nano Lett. 2005, 5, 45 – 48. [22] Q. Wang, Z. Yang, X. Zhang, X. Xiao, C. K. Chang, B. Xu, Angew. Chem. Int. Ed. 2007, 46, 4285 – 4289; Angew. Chem. 2007, 119, 4363 – 4367. [23] X. Li, Y. Kuang, B. Xu, Soft Matter 2012, 8, 2801 – 2806. [24] S. K. Samanta, S. Bhattacharya, Chem. Commun. 2013, 49, 1425 – 1427. [25] L. Chen, S. Revel, K. Morris, L. C. Serpell, D. J. Adams, Langmuir 2010, 26, 13466 – 13471. [26] M. Hughes, P. W. J. Frederix, J. Raeburn, L. S. Birchall, J. Sadownik, F. C. Coomer, I.-H. Lin, E. J. Cussen, N. T. Hunt, T. Tuttle, S. J. Webb, D. J. Adams, R. V. Ulijn, Soft Matter 2012, 8, 5595 – 5602. [27] A. Chakrabarty, U. Maitra, A. Devi Das, J. Mater. Chem. 2012, 22, 18268 – 18274. [28] A. R. Hirst, S. Roy, M. Arora, A. K. Das, N. Hodson, P. Murray, N. Javid, J. Sefcik, J. Boekhoven, J. H. van Esch, S. Santabarbara, N. T. Hunt, R. V. Ulijn, Nat. Chem. 2010, 2, 1089 – 1094. [29] S. Fleming, S. Debnath, P. W. J. M. Frederix, T. Tuttle, R. V. Ulijn, Chem. Commun. 2013, 49, 10587 – 10589. [30] D. J. Adams, P. D. Topham, Soft Matter 2010, 6, 3707 – 3721. [31] J. Nanda, A. Biswas, B. Adhikari, A. Banerjee, Angew. Chem. Int. Ed. 2013, 52, 5041 – 5045; Angew. Chem. 2013, 125, 5145 – 5149. [32] B. Escuder, F. R. Llansola, J. F. Miravet, New J. Chem. 2010, 34, 1044 – 1054. [33] A. M. Bieser, J. C. Tiller, J. Phys. Chem. B 2007, 111, 13180 – 13185. [34] E. K. Johnson, D. J. Adams, P. J. Cameron, J. Mater. Chem. 2011, 21, 2024 – 2027. [35] S. Dutta, D. Das, A. Dasgupta, P. K. Das, Chem. Eur. J. 2010, 16, 1493 – 1505. [36] T. Kar, S. Debnath, D. Das, A. Shome, P. K. Das, Langmuir 2009, 25, 8639 – 8648. [37] S. Banerjee, R. K. Das, P. Terech, A. de Geyer, C. Aymonier, A. L. Serani, G. Raffy, U. Maitra, A. D. Guerzon, J.-P. Desvergne, J. Mater. Chem. C 2013, 1, 3305 – 3316. [38] S. Bhowmik, S. Banerjee, U. Maitra, Chem. Commun. 2010, 46, 8642 – 8644. [39] S. Bhattacharya, A. Srivastava, A. Pal, Angew. Chem. Int. Ed. 2006, 45, 2934 – 2937; Angew. Chem. 2006, 118, 3000 – 3003. [40] A. Shome, S. Debnath, P. K. Das, Langmuir 2008, 24, 4280 – 4288. [41] T. Kar, S. K. Mandal, P. K. Das, Chem. Commun. 2012, 48, 8389 – 8391. [42] J. F. Miravet, B. Escuder, Chem. Commun. 2005, 5796 – 5798. [43] D. M. Wood, B. W. Greenland, A. L. Acton, F. Rodriguez-Llansola, C. A. Murray, C. J. Cardin, J. F. Miravet, B. Escuder, I. W. Hamley, W. Hayes, Chem. Eur. J. 2012, 18, 2692 – 2699. [44] V. K. Praveen, S. J. George, R. Varghese, C. Vijayakumar, A. Ajayaghosh, J. Am. Chem. Soc. 2006, 128, 7542 – 7550. [45] T. Nakashima, N. Kimizuka, Adv. Mater. 2002, 14, 1113 – 1116. [46] A. Ajayaghosh, V. K. Praveen, S. Srinivasan, R. Varghese, Adv. Mater. 2007, 19, 411 – 415. [47] K. V. Rao, K. K. R. Datta, M. Eswaramoorthy, S. J. George, Adv. Mater. 2013, 25, 1713 – 1718.

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[48] A. Ajayaghosh, C. Vijayakumar, V. K. Praveen, S. S. Babu, R. Varghese, J. Am. Chem. Soc. 2006, 128, 7174 – 7175. [49] T. Sagawa, T. Yamada, H. Ihara, Langmuir 2002, 18, 7223 – 7228. [50] P. Bairi, B. Roy, P. Chakraborty, A. K. Nandi, ACS Appl. Mater. Interfaces 2013, 5, 5478 – 5485. [51] S. K. M. Nalluri, R. V. Ulijn, Chem. Sci. 2013, 4, 3699 – 3705. [52] T. Cardolaccia, Y. Li, K. S. Schanze, J. Am. Chem. Soc. 2008, 130, 2535 – 2545. [53] C. Giansante, G. Raffy, C. Schafer, H. Rahma, M.-T. Kao, A. G. L. Olive, A. Del Guerzo, J. Am. Chem. Soc. 2011, 133, 316 – 325. [54] T. Shu, J. Wu, M. Lu, L. Chen, T. Yi, F. Li, C. Huang, J. Mater. Chem. 2008, 18, 886 – 893. [55] H. Jintoku, T. Sagawa, M. Takafuji, H. Ihara, Org. Biomol. Chem. 2009, 7, 2430 – 2434. [56] K. Miyamoto, H. Jintoku, T. Sawada, M. Takafuji, T. Sagawa, H. Ihara, Tetrahedron Lett. 2011, 52, 4030 – 4035. [57] K. Sugiyasu, N. Fujita, S. Shinkai, Angew. Chem. Int. Ed. 2004, 43, 1229 – 1233; Angew. Chem. 2004, 116, 1249 – 1253. [58] A. Ajayaghosh, S. J. George, V. K. Praveen, Angew. Chem. Int. Ed. 2003, 42, 332 – 335; Angew. Chem. 2003, 115, 346 – 349. [59] A. Ajayaghosh, V. K. Praveen, C. Vijayakumar, S. J. George, Angew. Chem. Int. Ed. 2007, 46, 6260 – 6265; Angew. Chem. 2007, 119, 6376 – 6381. [60] A. Ajayaghosh, V. K. Praveen, C. Vijayakumar, Chem. Soc. Rev. 2008, 37, 109 – 122. [61] C. Vijayakumar, V. K. Praveen, K. K. Kartha, A. Ajayaghosh, Phys. Chem. Chem. Phys. 2011, 13, 4942 – 4949. [62] M. Montalti, L. S. Dolci, L. Prodi, N. Zaccheroni, M. C. A. Stuart, K. J. C. van Bommel, A. Friggeri, Langmuir 2006, 22, 2299 – 2303. [63] L. Chen, S. Revel, K. Morris, D. J. Adams, Chem. Commun. 2010, 46, 4267 – 4269. [64] K. J. Channon, G. L. Devlin, C. E. MacPhee, J. Am. Chem. Soc. 2009, 131, 12520 – 12521. [65] N. Yan, Z. Xu, K. K. Diehn, S. R. Raghavan, Y. Fang, R. G. Weiss, Langmuir 2013, 29, 793 – 805. [66] N. Yan, Z. Xu, K. K. Diehn, S. R. Raghavan, Y. Fang, R. G. Weiss, J. Am. Chem. Soc. 2013, 135, 8989 – 8999. [67] Th. Fçrster, Angew. Chem. Int. Ed. Engl. 1969, 8, 333 – 343; Angew. Chem. 1969, 81, 364 – 374. [68] S. Dutta, T. Kar, D. Mandal, P. K. Das, Langmuir 2013, 29, 316 – 327. [69] D. Mandal, T. Kar, P. K. Das, Chem. Eur. J. 2014, 20, 1349 – 1358. [70] G. Pistolis, A. Malliaris, Langmuir 2002, 18, 246 – 251. [71] M. Beinhoff, W. Weigel, W. Rettig, I. Bruedgam, H. Hartl, A. D. Schlueter, J. Org. Chem. 2005, 70, 6583 – 6591. [72] T. Kar, S. K. Mandal, P. K. Das, Chem. Eur. J. 2011, 17, 14952 – 14961. [73] S. Bhattacharyya, T. Sen, A. Patra, J. Phys. Chem. C 2010, 114, 11787 – 11795. [74] J. R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd ed., Kluwer Academic/Plenum Publishers, New York, 1999, p. 496. [75] S. Mandal, M. Rahaman, S. Sadhu, S. K. Nayak, A. Patra, J. Phys. Chem. C 2013, 117, 3069 – 3077. Received: April 10, 2014 Revised: May 22, 2014 Published online: && &&, 0000

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FULL PAPER Self-assembled gelation: A pyrenecontaining, low molecular weight gelator (see figure) shows high gelation efficiency for the formation of supramolecular fluorescent gels with notable mechanical strength. The presence of only 0.00075 % w/v of acridine orange resulted in about 50 % quenching of the fluorescence intensity of the gel (yellow spectrum) through efficient fluorescence resonance energy transfer to the hosted fluorophore (red spectrum).

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Supramolecular Gels Subrata Mukherjee, Tanmoy Kar, Prasanta Kumar Das* &&&&—&&&& Pyrene-Based Fluorescent Supramolecular Hydrogel: Scaffold for Energy Transfer

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Pyrene-based fluorescent supramolecular hydrogel: scaffold for energy transfer.

The self-assembled gelation of an amino-acid-based low molecular weight gelator having a pyrene moiety at the N terminus and a bis-ethyleneoxy unit li...
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