Synergistic Tricolor Emission-Based White Light from Supramolecular

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Interface-Rich Materials and Assemblies

Synergistic Tricolor Emission-Based White light from Supramolecular Organic-Inorganic Hybrid Gel Debasish Podder, Sujay Kumar Nandi, Supriya Sasmal, and Debasish Haldar Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00456 • Publication Date (Web): 18 Apr 2019 Downloaded from http://pubs.acs.org on April 19, 2019

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Synergistic Tricolor Emission-Based White light from Supramolecular Organic-Inorganic Hybrid Gel Debasish Podder, Sujay Kumar Nandi, Supriya Sasmal and Debasish Haldar* Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, West Bengal, India.

ABSTRACT

The development of engineered hybrid systems by encapsulating nanoparticles in gel scaffolds and their synergistic effects are highly crucial for the fabrication of advanced functional materials. Herein, a series of dipeptides containing an aromatic amino acid at the N-terminal and an aliphatic amino acid at the C- terminal were synthesized and studied. Among them, only the dipeptide LPhe-L-Val can form both hydro- and organogelator, depending on the N and C terminal protecting groups. The organogel shows bright blue emission under 366 nm UV irradiation; however, the hydrogel does not show such blue emission. Such kind of emission may be due to the self-assembly and high degree of aggregation in the gel state of the phenyl ring. The blue emitting organogel efficiently encapsulate green emission source CdSe quantum dots and red emission source LD 700 perchlorate dye. The resulting organic-inorganic hybrid gel exhibits white light emission due to the synergistic effect under 366 nm UV irradiation.

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INTRODUCTION Supramolecular gels are a novel class of self-assembled soft materials. In modern days, supramolecular gels are highly important due to their applications in various fields such as drug delivery,1-2 tissue engineering,3-5 biomedical applications,6 optoelectronics,7-8 and stimuli-responsive materials.9-11 Although it is well known that supramolecular gels are formed by different kinds of non-covalent interactions such as hydrogen bonding, π-π stacking, dipole-dipole interactions, and Van der Waals interaction,12-16 designing of a low molecular weight gelator is still challenging and difficult.17-18 In literature, there are various reports19-21 on low molecular weight gelators (organogelators and hydrogelators) from small peptides22-26. There are also few reports on ambidextrous gelators27-28 that efficiently solidified both organic and aqueous solvent simultaneously. Ambidextrous gels can form organogel as well as hydrogel from the same scaffold at a time. But designing of a simple peptide precursor that can form both hydrogelator and organogelator by a simple reaction is relatively rare. Das and co-workers have developed the organogelator and hydrogelator from a common scaffold.29 They have also reported a simple reaction for the transformation of hydrogelator to organogelator and vice versa.30 Recently, balanced white-light-emitting organic materials are center of interest due to their potential application in electronic and light emitting devices.31-34 It is also established that by the combination of three primary colours (blue, green and red) or at least two complementary colours white light can be obtained. So, white light emitting materials can be developed by mixing different luminophores through chemical bonding or selfassembly.35-37 There are many reports of white light emitting materials from organic-


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inorganic hybrid materials,38-40 single molecules,41-43supramolecular polymers,44-45 organic π-conjugated systems,44 gels,46-48 and others.49-50 Banerjee and co-workers have reported two component synergistic white light emitting system of a PDI containing peptide system as an acceptor and a stilbene containing peptide system as a donor in organic solvents.51

Intriguing the previous knowledge, herein we have developed both organogelator and hydrogelator from a simple dipeptide precursor by a simple reaction. Depending on the Nterminal or C-terminal protecting groups, the dipeptide Phe-Val forms both hydrogelator as well as organogelator. The blue emitting organogel efficiently encapsulates CdSe quantum dots and LD 700 perchlorate dye and resulting organic-inorganic hybrid gel exhibits pure white light emission under 366 nm UV irradiation.

Figure 1. Schematic structure of all the peptides. We have synthesized a series of terminally protected dipeptides with the combination of phenylalanine, tyrosine, valine, leucine, phenyl glycine and alanine (Figure 1).

The

dipeptides have an aromatic amino acid in the N-terminus and aliphatic amino acid in the C-terminus. Hydrolysis of these peptides with NaOH develops dipeptide acids 1a-5a


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(Figure 1). The N-terminal deprotection of these dipeptides by TFA provides dipeptide amines 1b-5b (Figure 1). The peptides were purified and characterized by 1H-NMR,

13C-

NMR, Mass spectrometry analysis. EXPERIMENTAL Materials L-amino acids (L-phenylalanine, L-tyrosine, L-phenyl glycine, L-valine, L-leucine, and L-alanine) were supplied by Sigma chemicals. We have purchased HOBt (1- hydroxybenzotriazole) and DCC (dicyclohexylcarbodiimide), from Sisco Research Laboratories (SRL). . Peptide Synthesis We have synthesized all the peptides following standard solution phase methodology. Here we have used the Boc group for the N-terminal protection, and C-terminus was protected as methyl ester. Coupling reactions were done using DCC as an activator and HOBt to stop the racemization. Methylester deprotecton has done by NaOH in methanol. Boc group deprotection hass performed by TFA (Trifluoroacetic acid). We have used 1H NMR and 13C

NMR to characterize all the intermediates. The final compounds were also fully

characterized by 1H NMR spectroscopy, 13C NMR spectroscopy and mass spectrometry. NMR Experiments NMR of all the compounds were recorded on a JEOL 400 MHz or Bruker 500 MHz spectrometer. We have used CDCl3 or DMSO-d6 as solvent. Chemical Shifts of the NMR spectra were measured by taking tetramethylsilane (d = 0.0 ppm) as an internal standard. Mass Spectrometry


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Mass spectra of the final compounds were recorded on a Waters Corporation Q-Tof Micro YA263 high-resolution mass spectrometer by electrospray ionization (positive-mode). Field Emission Scanning Electron Microscopy JEOL Scanning Microscope-JSM-6700F was utilized to obtain the SEM images of the xerogel materials. SEM samples were prepared by drop-casting the peptide solution on a clean microscopic cover slip and dried it under vacuum at room temperature for 24 h. Finally the samples have coated with gold and examined by the FE-SEM apparatus. Fluorescence spectroscopy Perkin Elmer fluorescent spectrometer (LS 55) was used to record the fluorescence spectra at room temperature. For measurement, a quartz cell of 1 cm path length and slit widths 2.5/2.5 have been used. Rheology Experiments MCR 102 rheometer (Anton Paar, Modular Compact Rheometer) having a steel parallel plate geometry with 40 mm diameter was used for rheology experiment at room temperature. To know the mechanical strength of the gel, we have measured storage modulus and loss modulus as a function of frequency and oscillatory strain and stress for both the organogel and hydrogel. X-ray crystallography XRD data of peptide 2 and 4b were measured by Bruker high-resolution X-ray diffractometer instruments with MoK radiation. Bruker SAINT package was used for the data processing. For the structure solution and refinement procedures, SHELX97 was used. CCDC 1855050 and 1855051contain the supplementary crystallographic data for peptides 2 and 4b respectively.


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RESULTS AND DISCUSSION Initially, we have designed and synthesized three dipeptides. As phenylalanine is the most used amino acid in literature to develop peptide-based gelators, we have kept phenylalanine as N-terminal amino acid and varying the C-terminal with different aliphatic amino acids to investigate the effect of hydrophobicity. We have used alanine, valine, and leucine as a C-terminal amino acid for peptide 1, 2, and 3 respectively. Peptides 1-3 are soluble in organic aromatic solvents but did not form organogel. Peptides 1-3 are insoluble in water even at high temperature and thus failed to form hydrogel. Then we have synthesized peptide 1a-3a (C-terminal deprotection) by the hydrolysis of peptide 1-3 with NaOH in methanol. The N-terminal deprotection of these peptides 1-3 by TFA provides dipeptide amine 1b-3b. Then we have checked the gelation abilities of the peptide 1a-3a and 1b-3b in different organic solvent as well as in water. The alanine residue (peptide 1a and 1b) failed to form any gel (hydrogel as well as organogel) (Figure S1). Peptide 3a failed to form a gel, but peptide 3b forms organogel in the different organic solvent (Figure S1). But surprisingly, peptide 2a can form hydrogel and peptide 2b forms organogel. Thus, from this dipeptides series, only peptide 2, Boc-Phe-Val-OMe works as a common precursor for organogelator and hydrogelator by one side deprotection reaction. Further, we have designed and synthesized more two dipeptides and their respective derivatives by changing the N-terminal amino acid phenylalanine and keeping valine as a C-terminal amino acid. For peptide 4, 4a and 4b, we have used tyrosine as a N-terminal amino acid to incorporate one additional hydrogen bonding site in the peptide backbone. We have also used phenyl glycine (one CH2 less compare to phenylalanine) as a N-terminal


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amino acid for peptide 5, 5a, 5b. Both tyrosine and phenyl glycine based peptide and their respective deprotected derivatives failed to form a gel (Figure S1). Colourless crystals for peptide 4b were obtained from the methanol-water solution by slow evaporation. Peptide 4b crystalizes with one peptide molecule in the asymmetric unit and the molecule is stabilized by an O…. interaction (Figure S2). Thus among these five dipeptides, only the peptide 2, L-Phe-L-Val can form both hydrogelator and organogelator, depending on the N and C terminal protecting groups. Structure analysis The molecular packing and solid state self-assembly of the reported peptide 2 were explained by X-ray crystallography. Colorless crystals were obtained from the methanolwater solution. Peptide 2 crystalizes with three peptide molecules along with a methanol molecule in the asymmetric unit (Figure S3). There is no intramolecular hydrogen bond. The peptide 2 adopts extended conformation. The molecules A and B are connected through two intermolecular N-H...O=C hydrogen bonds between Boc C=O and Phe NH, and Val NH and Phe C=O. But the molecule B and C are interlinked by N-H...O=C intermolecular hydrogen bond between Val NH and Phe C=O and two methanol mediated hydrogen bonds between Boc C=O and methanol OH as well as Phe NH and methanol O (Fig 2a). There also an edge to face π-π interaction between molecule A and B (centroid to C distance 4.95Å). In higher order packing, the individual subunits of peptide 2 are regularly interlinked through multiple intermolecular hydrogen bonding interactions and thereby form a supramolecular twisted sheet-like structure along the crystallographic c direction (Fig 2b). The Phenyl rings are arranged in a helical pattern around the twisted sheet (Fig 2b). Peptides 2a and 2b failed to generate X-ray quality crystals. The peptide 2


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exhibits microtube like morphology (Fig 2c). The microtubes are triangular in shape and several micrometers in length (Fig 2c).

Figure 2. (a) The asymmetric unit containing three peptide 2 molecules. Intermolecular hydrogen bonds and π-π interactions are shown as dotted lines. t-butyl groups here appear as violet spheres and i-butyl groups as orange spheres. (b) The supramolecular twisted sheet like structure of peptide 2 along the crystallographic c direction. For clarity, phenyl rings are presented in spacefill model. (c) The triangular microtube like morphology of peptide 2. Inset shows the hollow triangle of a microtube.

Gelation Study Peptide 2a selectively forms hydrogel with NaOH only. When the dipeptide 2a (5mg in 1 ml) was mixed with 5 equivalent of NaOH and shaken for 1 min, instant gelation was observed (Figure3a). We have also tried the gelation of peptide 2a using different salt such


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as K2CO3, KOH, LiOH, Na2HPO4, NaHCO3 under the same condition but they failed to form a hydrogel with Peptide 2a (Figure 3b-f). We have also tried the gelation of peptide 2a with a mixture of salts. The dipeptide BocPhe-Val-OH, forms gel in the mixture of NH4OH and NaCl; Na2CO3 and LiOH; and NaCl and KOH. This indicates that not only sodium but also hydroxide ions have a significant role in the hydrogel formation. The minimum gelation concentration for peptide 2a is 5 mg/mL. The minimum concentration of NaOH required is 2.5 mg/mL. The mechanical strength of the NaOH responsive hydrogel was examined by rheology experiments (Figure S4). In rheology, there are two main parameters, storage modulus (G’) and loss modulus (G”). In case of NaOH responsive hydrogel of peptide 2a, for frequency sweep experiment as a function of stress, storage modulus (G’) is greater than loss modulus (G”) entire the range which indicates the formation of a gel. Then we have carried out oscillatory stress sweep experiments where G’ and G” were measured as a function of stress at a constant frequency. Initially, G’ value is greater than G” but after certain stress, the value of G” becomes greater than G’. This crossover point is defined as yield stress. Here the crossover point is at 340 Pa that indicates the formation of a strong gel. Field emission scanning electron microscopy was done to understand the supramolecular arrangement in the gel state. FE-SEM images show polydisperse fibers which are highly entangled to form 3D network structure (Figure 3g). Energy dispersive X-ray spectroscopy (EDS) measurement have proved the presence of alkali metal sodium in the hydrogel (Fig 3h).


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Figure 3 Hydrogelation studies of peptide 2a in presence of (a) NaOH, (b) K2CO3 (c) LiOH (d) Na2HPO4 (e) NaHCO3 (f) KOH (g) FE-SEM image shows entangled fibers in xerogel obtained from NaOH responsive peptide 2a hydrogel. (h) EDS proves the presence of Na in the hydrogel network. Initially, the gelation ability of Peptide 2b was tested by dissolving 10mg of the compound in various organic solvents and water. It was found that peptide 2b can form transparent gel only in organic solvent by conventional heating-cooling technique. The peptide 2b was insoluble in water even after heating and thus failed to form the gel. We have also tried for pH-responsive hydrogelation of peptide 2b, but it failed to form any hydrogel. Gelation of peptide 2b in organic solvents was confirmed by the inverted vial method (Figure4). The minimum gel concentration (MGC) of Peptide 2b in toluene is 2 mg/ml. The MGC of the organogel in various organic solvents is listed in table S1. We have also measured the Tgel


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(gel to sol transition temperature) for the organogels in different organic solvent at their minimum gel concentration (Table S2).

Figure 4. Organogel of peptide 2b from different solvent.

The mechanical strength of the organogel was measured by rheology experiments. Fig 5 depicts the rheology data as a function of angular frequency and oscillatory strain. In case of frequency sweep rheology experiments, storage modulus (G’) is greater than loss modulus (G”) entire the range. Strain sweep rheology experiment also supports the formation of strong gel. We have also done concentration-dependent frequency sweep rheology experiment in toluene (Figure S5). In this case, both storage modulus G’ and loss modulus G” increases with increasing concentration as expected. Thus, both the stability of the gel and elastic nature increases with increasing the concentration. Variable temperature 1H NMR experiment is an interesting tool to understand the selfassembly behavior of the peptide in the gel state. We have performed the experiment with peptide 2b gel in CDCl3. From the stack plot (Figure S6), it was observed that with increasing temperature the NH proton has shifted towards upfield. This is due to the breaking of the intermolecular hydrogen bonding with increasing temperature and the gel


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to sol transition. However, the other 1H NMR peaks have not been influenced by increasing the temperature.

Figure 5 (a) Frequency sweep and (b) Amplitude sweep experiment for peptide 2b organogel containing 2.0% (w/v) gelator.


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FE-SEM was performed to study the morphology of the peptide 2b organogel. The xerogels of peptide 2b obtained from different organic solvents show the formation of 3D fibrillar network (Figure 7d). However, the size, shape, and nature of the fibers vary from solvent to solvent (Figure 6). The xerogels obtained from toluene, m-xylene, p-xylene show fibers with diameters in the range 30-40 nm and length about few micrometers, whereas in benzene, chlorobenzene, and chloroform the diameters of the fibers are in the order of 100 nm (Figure 6).

Figure 6 FE-SEM images of the xerogel of peptide 2b from different solvent (a) Toluene (b) Benzene (c) m-Xylene (d) Chlorobenzene Xylene (e) Chloroform (f) p-Xylene. The dipeptide 2 failed to form a gel, but it can form hydrogelator and organogelator by two different simple organic reactions. Thus peptide 2 is not an ambidextrous gelator but it can be used as a common precursor for the easy and simple synthesis of organogelator and hydrogelator (Figure S7). Though the dipeptide scaffold is same for hydrogel and organogel, they have different assembly pattern and conditions. So, they have different emission property.


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White-light emitting study Previously we have reported NaOH responsive hydrogel that exhibits blue emission under excitation at 366 nm.52 Here also we have examined the emission property of NaOH responsive hydrogel, but that type of blue emission is absent, and the hydrogel is also nontransparent. Surprisingly, peptide 2b gel in toluene shows blue emission under excitation at 366 nm although in solution such kind of blue emission was not observed (Figure 7a). This is may be due to self-assembly and the high degree of aggregation in the gel state. Previously it has been reported that supramolecular gels can be used as a white light emitting material.51

Figure 7 (a) Normalized fluorescence spectra of peptide 2b organogel, CdSe quantum dots and LD 700 perchlorate dye. (b) Fluorescence spectra of white light emitting organic-inorganic hybrid gel


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under excitation at 360 nm. (c) CIE color coordinate of the white light emitting hybrid gel, only peptide 2b organogel, CdSe quantum dots and LD 700 perchlorate dye. (d) The FE-SEM image showing the entangled fiber network in xerogel of peptide 2b. Inset: transparent organogel of peptide 2b. (e) Photograph of white light emitting hybrid gel under UV light at 365 nm.

Here we have tried to fabricate a white light emitting gel from this blue emitting organogel. It is well known that white light can be generated by mixing three primary colours (blue, green and red). In this case, we have incorporated two more primary colours into the organogel matrix to produce white light. For green light, we have used CdSe quantum dots as a source, and for the red light source, LD 700 perchlorate dye was used (Figure 7a). Initially, we have mixed the components and observed emission changes. Then we have tuned the ratio of the three components to achieve the white light emitting materials (Figure 7b). We have optimized the ratio of the peptide 2b, CdSe quantum dot,53-54 and LD 700 perchlorate dye. We have first dissolved 3.4 mg of peptide 2b in 1 ml of toluene and 10μl of the quantum dot of concentration 10-6(M) and 10μl of the dye of concentration 10-5(M) was added to it. Then the mixture was gently heated until a homogeneous solution was obtained. Finally, the organic-inorganic hybrid gel was obtained after cooling the mixture at room temperature. FE-SEM and TEM experiments show that the nanoparticles are attached to the organogel fibers (Figure S9). Fig 7c shows the CIE colour coordinate of the white light

emitting hybrid gel, only peptide 2b organogel, CdSe quantum dots and LD 700 perchlorate dye. Upon excitation at 366 nm, emission of white light from the hybrid gelator was observed (Figure 7e). The generation of white light is due to the synergistic effect. From the chromaticity diagram (CIE), the coordinates for the white light emission are 0.31, 0.32 (Figure 2c) which is very close to those of pure white light emission (0.33, 0.33) and comparable with the stateof-art other white light emitting gel-based device.39, 51


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Conclusions In Conclusion, we have demonstrated that depending upon the N- and C- terminal protecting groups, a dipeptide scaffold Phe-Val can form organogel and NaOH responsive hydrogel. The organogel shows bright blue emission under UV irradiation. The organogel can efficiently accommodate green emission source CdSe quantum dots and red emission source LD 700 perchlorate dye. The resultant organic-inorganic hybrid gel shows pure white light emission. The generation of white light is due to the synergistic effect. The results are important for colloid and interface science. This white light emitting gel holds future promises for the potential application as advanced material that may be used in organic electronic devices.

ASSOCIATED CONTENT Supporting Information The Supporting Information (PDF) is available free of charge on the ACS Publications website. Synthesis and characterization of compounds, 1H NMR, 13C NMR, compositions of gel prepared in the present study, Figures S1-S43, CCDC 1855050, 1855051 (PDF). Corresponding Author E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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We acknowledge the IISER Kolkata, India, for financial assistance. D. Podder and S. K. Nandi thanks CSIR, India for research fellowship. S. Sasmal acknowledges the IISER-K, India for fellowship. Authors thank Dr. Raju Mondal and Mr. Krishna Sundar Das of IACS, Kolkata for the rheology experiment. We thank Dr. Debjit Roy, Mr. Chayan Kumar De, and Mr. Mrinal Mandal of IISER Kolkata for their help during Spectroscopic studies of the white light emitting gel. REFERENCES 1 Hoare, T. R.; Kohane, D. S. Hydrogels in drug delivery: Progress and challenges. Polymer. 2008, 49, 1993-2007. 2 Vintiloiu, A.; Leroux, J. C. Organogels and their use in drug delivery — A review. J. Controlled Release. 2008,125, 179-192. 3 Hirst, A. R.; Escuder, B.; Miravet, J. F.; Smith, D K. High-Tech Applications of SelfAssembling Supramolecular Nanostructured Gel-Phase Materials: From Regenerative Medicine to Electronic Devices. Angew. Chem. Int. Ed. 2008, 47, 8002-8018. 4 Kurisawa, M.; Chung, J. E.; Yang, Y. Y.; Gaoa, S. J.; Uyama, H. Injectable biodegradable hydrogels composed of hyaluronic acid–tyramine conjugates for drug delivery and tissue engineering. Chem. Commun., 2005, 4312-4314. 5 Lee. K. Y; Mooney, D. J. Hydrogels for Tissue Engineering. Chem. Rev. 2001, 101, 18691879. 6 Jayawarna, V.; Ali, M.; Jowitt, T. A.; Miller, A. F.; Saiani, A.; Gough, J. E.; Ulijn, R. V. Nanostructured Hydrogels for Three-Dimensional Cell Culture Through Self-Assembly of Fluorenylmethoxycarbonyl– Dipeptides. Adv. Mater. 2006, 18, 611-614. 7 Xue, P.; Wang, P.; Yao, B.; Sun, J.; Gong, P.; Zhang, Z.; Qian, C.; Lu, R. Nanofibers of Hydrogen-Bonded Two-Component Gel with Closely Connected p- and n‑Channels and Photoinduced Electron Transfer. ACS Appl. Mater. Interfaces, 2014, 6, 21426-21434.


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