J. Phys. Chem. B 2008, 112, 10107–10115
10107
Fluorescent Gel from a Self-Assembling New Chromophoric Moiety Containing Azobenzene Based Tetraamide Goutam Palui‡ and Arindam Banerjee*,†,‡ Chemistry DiVision, Indian Institute of Chemical Biology, JadaVpur, Kolkata- 700 032, India, and Department of Biological Chemistry, Indian Association for the CultiVation of Science, JadaVpur, Kolkata- 700 032, India ReceiVed: February 26, 2008; ReVised Manuscript ReceiVed: June 11, 2008
A new chromophoric low molecular weight (LMW) organic molecule, 1, was synthesized, and it forms gels in various organic solvents including toluene, o-xylene, m-xylene and p-xylene. The resultant gel phase materials exhibit enhanced and red-shifted fluorescence emission in the respective gelling solvents. This gelator molecule is self-assembled using various noncovalent interactions including hydrogen bonding, π-π staking and van der Waals interactions to get the gel phase materials. The molecule 1 is very weakly fluorescent in solution, but its intensity is increased by almost 40 times in their respective gelled state depending on the nature of the gelling solvents. Self-assembly of this molecule in the above-mentioned organic solvents gives an elongated nanofibrillar network that can be visualized through Field Emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). Introduction Discovery and developments of low molecular-weight organogelators (LMOG) are important due to their unique features1 and their potential applications in nanotechnology,2 pollution control,3 oil-spill recovery,4 and light-harvesting materials,5 in preparing dye sensitized solar cells,6 and in other uses.7 In order to generate a gel-phase material it is necessary to select or construct a molecular building block that will provide a supramolecular three-dimensional network using various noncovalent interactions including hydrogen bonds,8 π-π staking,9 metal coordination,10 and van der Waals interactions.11 A wide range of different types of organic molecules have been investigated as low molecular weight organogelators including saccharides,12 peptides,13 amides,14 ureas,15 nucleobase,16 steroid derivatives17 and others.18 Many of these above-mentioned organogelators self-assemble utilizing hydrogen bonds and/or π-π interactions to form corresponding gels. Smith and his co-workers are the pioneer of amino acid based dendritic organogelator.19 Organogelators with π-conjugated molecular structure have drawn a special attention due to their advantages for displaying various applications in increased charge transport, fluorescence and sensing capabilities.20 However, chromophorebased organogelators, particularly gels based on extended π-conjugated systems21,24 are relatively less in number. Weiss et al.22 and Desvergne et al.23 synthesized anthracene-containing gelators and examined their unique fluorescence properties. Ajoyghosh et al.21g–k synthesized π-conjugated-oligo (p-phenylenevinylene) based gelators, which show unique fluorescent properties. Shinkai et al.24 also designed a triphenylene-based gels showing excimer emission arising from the unusual π-electron overlapping mode of a triphenylene moiety. For all these gelator molecules, long alkyl chains, steroidal groups or trifluoromethyl substituents have been introduced into the gelator structure for attaining the effective gelation process. However, * Corresponding author. E-mail:
[email protected], arindam.bolpur@ yahoo.co.in. Fax: 0091-33-2473-5197. Telephone: +91-033-2473-3491. † Chemistry Division, Indian Institute of Chemical Biology. ‡ Department of Biological Chemistry, Indian Association for the Cultivation of Science.
organogelator containing azobenzene based amide moiety with an aminoacid substituent is very rare in the literature.25 Here, we report a new class of LMOG, 1 (Scheme 1), an azobenzene based tertaamide with a proteinogenic amino acid side chain (isoleucine) as substituents endowing gel phase material with a potential for exhibiting an enhanced fluorescence property that is important for functional gel-phase material. Other azobenzene based tetraamides (2 and 3), each containing amino acid side chains have also been synthesized, purified, characterized and tested for gelation to probe the role of amino acid side chain in gelation. Experimental Section General Methods and Materials. 5-Nitroisophthlic acid, HOBt (1-hydroxy benzotriazole), DCC (dicyclohexyl carbodiimide), and amino acids were purchased from Sigma chemicals. Synthesis. The azobenzene based tetraacid was synthesized by using few synthetic steps (Scheme 1) and then tertaamide with a proteinogenic amino acid side chain were synthesized by conventional solution phase method by using racemizationfree fragment condensation strategy.26 Couplings were mediated by dicyclohexylcarbodiimide-1-hydroxybenzotriazole (DCC/ HOBt). The final products (dendrons) were purified by column chromatography using silica (100-200-mesh size) gel as stationary phase and chloroform-methanol (95:5) mixture as eluent. The purified dendrons have been fully characterized by 300 MHz 1H NMR spectroscopy. (i) Synthesis of B [trans-3,5-Dicarboxyl-(3′,5′-dicarboxylazophenyl)benzene]. A mixture of 5-nitroisophthlic acid (4.2 g, 20 mmol), Zn (2.6 g, 40 mmol) and NaOH (1.6 g, 40 mmol) in a mixture of ethanol (100 mL) and water (40 mL) was heated under reflux. After the mixture was refluxed for 14 h, a yellow solid was obtained and collected by filtration. The resultant solid was dissolved in 70 mL of NaOH (aq, 1 M) and filtered to remove the insoluble solids. The filtrate was then acidified with 3 M HCl (aq). The pH of solution should be nearly 3.0 to afford the orange precipitate of B. Yield: 4.67 (65%). 1H NMR (CDCl3, 300 MHz, δ ppm): 8.94 (s, 4H), 8.79 (s, 2H). Anal. Calcd. for C16H10N2O8 (358.2): C, 53.64; N, 7.82; H, 2.81. Found: C,
10.1021/jp801657h CCC: $40.75 2008 American Chemical Society Published on Web 07/30/2008
10108 J. Phys. Chem. B, Vol. 112, No. 33, 2008
Palui and Banerjee
SCHEME 1: Reagents and Conditionsa
a
Key: (i) Zn/NaOH/EtOH/H2O, refluxed for 24 h; (ii) HOBt and DCC; HOBt ) N-hydroxybenzotriazole, DCC ) N,N-dicyclohexylcarbodiimide.
53.51; N, 7.89; H, 2.80. MS (HRMS): m/z 358.2143 (M)+, m/z 381.3451 (M + Na)+. (ii) Synthesis of Compound 1. A sample of B (896 mg, 2.5 mmol) dissolved in dimethylformamide (DMF) (15 mL) was cooled in an ice-water bath and H-Ile-OMe was isolated from 2.73 g (15.0 mmol) of the corresponding methyl ester hydrochloride by neutralization and subsequent extraction with ethyl acetate and the ethyl acetate extract was concentrated to 8 mL. It was then added to the reaction mixture, followed immediately by 2.06 g (10.0 mmol) DCC and 1.35 g (10.0 mmol) of HOBt. The reaction mixture was stirred for four days. The residue was taken up in ethyl acetate (40 mL) and the DCU was filtered off. The organic layer was washed with 2N HCl (3 × 40 mL), brine (2 × 50 mL), 1 M sodium carbonate (3 × 40 mL), brine (2 × 40 mL), dried over anhydrous sodium sulfate and evaporated in a vacuum to yield 1 of yellowish solid. Purification was done by silica gel column (100-200 mesh) using chloroform-methanol (95: 5) as eluent. Yield: 1.73 g (80%). [R]D20 +7.8 (c ) 0.5 M, MeOH). 1H NMR (CDCl , 300 MHz, δ ppm): 8.81 (s, 4H), 8.43 (s, 3 2H), 7.22 (d, J ) 8.5, 4H), 4.89-4.82 (m, 4H), 3.77 (s, 12H), 2.17 (b, 4H), 1.61-1.54 (m, 8H), 1.03-0.95 (m, 24H). 13C NMR (75 MHz, CDCl3, δ ppm): 11.61, 15.76, 25.59, 25.62, 25.66, 25.75, 37.81, 37.84, 37.91, 37.96, 52.44, 57.53, 124.67, 128.22, 135.51, 135.53, 135.55, 135.59, 152.28, 165.85, 165.89, 165.98, 166.10, 172.94, 172.99, 173.04, 173.13. Anal. Calcd. for C44H62N6O12 (866.996): C, 60.95; N, 9.69; H, 7.21. Found: C, 60.80; N, 9.75; H, 7.20. MS (HRMS): m/z 867.2701 (M)+, m/z 890.3244 (M + Na)+. (iii) Synthesis of Compound 2. A sample of B (896 mg, 2.5 mmol) dissolved in dimethylformamide (DMF) (15 mL) was cooled in an ice-water bath and H-Val-OMe was isolated from 2.51 g (15.0 mmol) of the corresponding methyl ester hydrochloride by neutralization and subsequent extraction with ethyl acetate and the ethyl acetate extract was concentrated to 8 mL.
It was then added to the reaction mixture, followed immediately by 2.06 g (10.0 mmol) DCC and 1.35 g (10.0 mmol) of HOBt. The reaction mixture was stirred for four days. The residue was taken up in ethyl acetate (40 mL) and the DCU was filtered off. The organic layer was washed with 2N HCl (3 × 40 mL), brine (2 × 50 mL), 1 M sodium carbonate (3 × 40 mL), brine (2 × 40 mL), dried over anhydrous sodium sulfate and evaporated in a vacuum to yield 2 of yellowish solid. Purification was done by silica gel column (100-200mesh) using chloroform-methanol (98:2) as eluent. Yield: 1.46 g (72%). [R]D20 +27.10 (c ) 0.5 M, CHCl3). 1H NMR (CDCl , 300 MHz, δ ppm): 8.83 (s, 4H), 8.43 (s, 3 2H), 7.47 (d, J ) 8.55, 4H), 4.86-4.80 (m, 4H), 3.81 (s, 12H), 2.17 (m, 4H), 1.07-1.01 (m, 8H). 13C NMR (75 MHz, room temperature, CDCl3, δ ppm): 19.30, 31.25, 31.33, 31.44, 31.49, 52.44, 58.27, 124.68, 126.96, 135.14, 135.26, 135.52, 135.57, 152.15, 165.84, 165.89, 166.14, 166.22, 172.92, 172.99, 173.06, 173.15. Anal. Calcd. for C40H54N6O12 (810.890): C, 59.25; N, 10.36; H, 6.71. Found: C, 59.28; N, 10.48; H, 6.69. MS (HRMS): m/z 833.1996 (M + Na)+, m/z 849.2055 (M + K)+, m/z 850.1602 (M + K + H)+. (iv) Synthesis of Compound 3. A sample of B (896 mg, 2.5 mmol) dissolved in dimethylformamide (DMF) (15 mL) was cooled in an ice-water bath and H-Ala-OMe was isolated from 2.09 g (15.0 mmol) of the corresponding methyl ester hydrochloride by neutralization and subsequent extraction with ethyl acetate and the ethyl acetate extract was concentrated to 8 mL. It was then added to the reaction mixture, followed immediately by 2.06 g (10.0 mmol) of DCC and 1.35 g (10.0 mmol) of HOBt. The reaction mixture was stirred for four days. The residue was taken up in ethyl acetate (40 mL) and the DCU was filtered off. The organic layer was washed with 2N HCl (3 × 40 mL), brine (2 × 50 mL), 1 M sodium carbonate (3 × 40 mL), brine (2 × 40 mL), dried over anhydrous sodium sulfate and evaporated in a vacuum to yield 3 of yellowish solid.
Fluorescent Gel Purification was done by silica gel column (100-200 mesh) using chloroform-methanol (98:2) as eluent. Yield: 1.46 g (72%). [R]D20 +22.10 (c ) 0.5 M, CHCl3). 1H NMR (CDCl , 300 MHz, δ ppm): 8.47 (s, 4H), 8.12 (s, 3 2H), 8.06 (d, J ) 8.12, 4H), 4.91-4.86 (m, 4H), 3.88 (s, 12H), 1.65-1.62 (m, 12H). 13C NMR (75 MHz, room temperature, CDCl3, δ ppm): 17.19, 17.20, 17.26, 17.38, 48.73, 48.80, 52.67, 124.18, 125.06, 127.65, 135.08, 135.13, 151.34, 166.29, 166.39, 174.40, and 174.59. Anal. Calcd. for C32H38N6O12 (698.68): C, 55.01; N, 12.03; H, 5.48. Found: C, 53.28; N, 14.48; H, 6.69. MS (HRMS): m/z 833.1996 (M + Na)+, m/z 849.2055 (M + K)+, m/z 850.1602 (M + K + H)+. NMR Experiments. All NMR studies were carried out on a Bru¨ker DPX 300 MHz spectrometer at 300 K. Compounds concentrations were in the range 1-10 mmol in CDCl3 and (CD3)2SO. Mass Spectroscopy. Mass spectra were recorded on a Hewlett Packard Series 1100MSD and Micromass Qtof Micro YA263 mass spectrometer by positive mode electrospray ionization. FT-IR Spectroscopy. All reported FT-IR spectra were taken using Shimadzu (Japan) model FT-IR spectrophotometer. A Nicolet FT-IR instrument [Magna IR-750 spectrometer (series II)] was used to obtain in the solid state FT-IR spectra. For the solid-state measurements, the KBr disk technique was used. Wide Angle X-ray Diffraction Study. The WAXS patterns were made on the gel of 2.5% (w/v) of compound 1 in o-xylene. The experiment was carried out in a Seifert X-ray diffractometer (C 3000) with a parallel beam optics attachment. The instrument was operated at a 35 KV voltage and 30 mA current and was calibrated with a standard silicon sample. The sample was scanned from 2° to 50° (2θ) at the step scan mode (step size 0.03°, preset time 2 s) and the diffraction pattern was recorded using a scintillation scan detector. Scanning Electron Microscopic Study. Morphologies of all reported gel materials were investigated using field emission scanning electron microscopy (FE-SEM). For SEM study, the gel materials were dried and gold coated. Then the micrographs were taken in a SEM apparatus (Jeol Scanning MicroscopeJSM-6700F). Transmission Electron Microscopic Study. The morphologies of the reported gels were investigated using transmission electron microscope (TEM). Transmission electron microscopic studies were done by placing a small amount of gel (at its minimum gelation concentration) of the corresponding compounds on carbon-coated copper grids (300 mesh) and dried by slow evaporation. The grid was then allowed to dry in vacuum at 30 °C for two days. Images were taken by FEI (Tecnai spirit) instrument. Atomic Force Microscopic Study. The morphologies of the reported gels were investigated using atomic force microscopy (AFM). Atomic force microscopic studies were done by placing a small amount of gel (at its minimum gelation concentration) of the corresponding compounds on a microscopic cover glass and then dried by slow evaporation. The material was then allowed to dry in vacuum at 30 °C for two days. Images were taken by AUTOPROBE CP BASE UNIT, di CP-II instrument, model no. AP-0100. Optical Polarizing Microscopic Study. Small amount of gel phase material obtained from compound 1 in o-xylene solvent at its minimum gel concentration (w/v 1.67%) was placed on a glass microscope slide and then placed under the optical polarizing microscope and the material was visualized at 100×
J. Phys. Chem. B, Vol. 112, No. 33, 2008 10109 TABLE 1: Gelation Properties of Compound 1 in Organic Solventsa solvent methanol ethanol chloroform ethyl acetate acetonitrile n-hexane n-heptane cyclohexane benzene toluene o-xylene m-xylene p-xylene chlorobenzene o-dichlorobenzene
compound 1 S S S P S P P S S G G G G S S
(2.5) (1.67) (1.25) (1.25)
G ) gel formed at room temperature, while in parentheses is given the minimum gelation concentration (% w/v); S ) soluble; I ) insoluble; P ) precipitate. a
magnification with white light. A birefringent texture was observed between crossed polarizer. Fluorescence Microscopic Study. A small amount of gel phase material obtained from compound 1 in o-xylene solvent at its minimum gel concentration (w/v 1.67%) was placed on a glass microscope slide and was then placed under the fluorescence microscope with a blue filter and the fibriller morphology was visualized at 40× magnification. Green fibriller network was observed. The image was taken by an Olympus BX 61 instrument. Fluorescence Spectroscopic Study. Fluorescence spectroscopic measurements for the compound 1 in both the solution state (2 mg/mL) and the gel state (2.5% w/v) were obtained using a Perkin-Elmer spectrofluorimeter. Results and Discussion Thermal Behavior of the Gel-Phase Materials. The gel forming nature of these organic chromophoric molecules 1 has been tested in various organic solvents. The compound 1 was readily soluble (
