Supramolecular Hybrids of MoS2 and Graphene Nanosheets with

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Supramolecular Hybrids of MoS2 and Graphene Nanosheets with Organic Chromophores for Optoelectronic Applications Manjodh Kaur, Navin Kumar Singh, Aritra Sarkar, Subi J. George, and Chrintamani Nages Ramachandra Rao ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01189 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

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ACS Applied Nano Materials

Supramolecular Hybrids of MoS2 and Graphene Nanosheets with Organic Chromophores for Optoelectronic Applications Manjodh Kaur†, Navin Kumar Singh‡, Aritra Sarkar‡, Subi J. George*‡ and C. N. R. Rao*†‡

†

Sheikh Saqr Laboratory, International Centre for Materials Science, School of Advanced

Materials, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, P. O., Bangalore 560064, India ‡

School of Advanced Materials and New Chemistry Unit, Jawaharlal Nehru Centre for

Advanced Scientific Research, Jakkur, P. O., Bangalore 560064, India *E-mail: [email protected]; [email protected] Abstract A novel supramolecular strategy based on host-guest chemistry, has been proposed for the noncovalent functionalization of MoS2 nanosheets with organic chromophores. Stable ternary complexes are formed between benzyl imidazole functionalized MoS2 and chromophores in presence of cucurbit[8]uril host, resulting in non-covalent anchoring of donor and acceptor chromophores on the surface. On the other hand, in the case of graphene, we observe, π-π stacking resulting in the face-on organization of chromophore on the surface. The MoS2 and graphene hybrids have been characterized by various spectroscopic techniques. The present design opens up new possibilities for the non-covalent functionalization of transition metal dichalcogenides even in the absence of delocalised π-electrons which is essential to alter their optoelectronic properties. In addition, these novel supramolecular hybrid materials with strong electronic communication between non-covalently anchored chromophores and the 2-D nanosheets may find applications in optoelectronics and related areas. Keywords: 2D-Materials, MoS2, Non-covalent Functionalization, Host-Guest Chemistry, Chromophores Introduction 2D materials have emerged as an exciting area of research over the last few years, graphene and MoS2 being the two most important members of this family. Molecular 1 ACS Paragon Plus Environment

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engineering by non-covalent functionalization of graphene and MoS2 is useful in tailoring their optical, electronic and optoelectronic properties,1-4 still maintaining the intrinsic features.5-10 There has been considerable success in functionalization and solubilization of graphene with donor and acceptor organic chromophores by virtue of its planar π surface.11-25 However, graphene is a zero-band gap semiconductor, imposing limitations in using it in photovoltaics and other applications.26 In this scenario transition metal dichalcogenides (TMD) have gained immense attention since they have band gaps.27 However, in order to tailor their properties molecular engineering is required with organic chromophores. Functionalization of the MoS2 surface offers the opportunity to tailor its electronic, optical, and catalytic properties. For example, doping MoS2 with electronically active chromophores has been shown to affect the electron transfer rate and excitonic recombination in MoS2. Unlike graphene, functionalization of TMD has not been explored sufficiently. Unlike graphene, where the conjugated, planar surfaces can be easily functionalized via π-π stacking or charge-transfer (CT) interactions,28,29 the MoS2 surface demands alternative means for functionalization. Functionalization of MoS2 is not a straight forward reaction and it is only recently that easy method of functionalization has been found. This method involves generation of the metallic 1T-phase of MoS2 by Li intercalation followed by reaction with organic halides.30 In this process, C-S bonds are formed between MoS2 and the organic moiety. Although alternate methods such as covalent modification,31-33 physical or chemical adsorption34-38 and binding of sulfur containing ligand in the unsaturated molybdenum sites have been reported recently to functionalize MoS2,39 non-covalent functionalization of the MoS2 surface using supramolecular strategies has not been explored. Unlike graphene, where the planar conjugated surface can be easily functionalized via π-π stacking or CT interactions,40,41 the charged π-surface of MoS2 demands alternative non-covalent designs for functionalization. To alleviate this problem, we have employed a novel supramolecular strategy based on host-guest ternary complexation, for the non-covalent functionalization of MoS2 with organic chromophores which can be further generalized for other TMDs. We further show that the host-guest supramolecular design can also be used for the noncovalent functionalization of graphene, although competing π-stacking interaction between graphene and chromophores can lead to competing mode of functionalization. We envisage that the supramolecular host-guest strategy, opens new pathways for the non-covalent functionalization of 2D nanomaterials like MoS2, which does not have the delocalised electron density on the basal plane of the nanosheets for π-stacking mediated modification. Furthermore, since supramolecular host-guest complexation can be reversibly modulated, it brings additional 2 ACS Paragon Plus Environment

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advantage of reversible non-covalent functionalization. Such supramolecular hybrids are likely to be useful in optoelectronics and other areas. Scheme 1. Schematic illustration of the host-guest supramolecular strategy proposed to noncovalently functionalize MoS2 and graphene nanosheets with organic chromophores. Highly efficient host-guest ternary complex formation between benzyl imidazole functionalized organic chromophores and MoS2 in presence of CB[8] ensures non-covalent anchoring of the chromophores on the surface. In the case of graphene, non-covalent attachment of chromophore competes with the charge-transfer/ π-π interactions between the aromatic surfaces of chromophore and graphene. C, Mo, and S in the sheet structures are represented by pink, navy blue, and pale-yellow colour spheres, respectively.

N N

N

N

N

N

N O

N

O

O O

O O

N

N

N

N

Non-covalent hybrid of graphene with chromophores

Non-covalent hybrid of MoS2 with chromophores

NMI-im (Acceptor)

Cucurbit[8]uril (CB[8])

Nap-im (Donor)

In order to non-covalently functionalize 2D nanosheets with organic chromophores, we have functionalized MoS2 and graphene with benzyl imidazole groups (Scheme 1) followed by complexation with organic chromophores also containing benzyl imidazole groups in the presence of the host, cucurbit[8]uril CB[8]. We have employed two fluorescent chromophores, one donor and once acceptor, appended with benzyl imidazole groups to examine the non3 ACS Paragon Plus Environment


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covalent functionalization. We have selected naphthol derivative (Nap-im) and naphthalene monoimide derivative (NMI-im) as the donor and acceptor model chromophores, respectively as they have been used extensively in the design of charge-transfer systems.42,43 The benzyl imidazole group of the organic chromophores and that bound to the MoS2 layer yield a strong supramolecular ternary adduct with CB[8] with a very high association constant,44 naphthol and naphthalene monoimide, as donor and acceptor model systems, Unlike MoS2 which forms a ternary supramolecular adduct, the situation with graphene is different where the chromophore forms a π-π adduct. Scheme 2. a) Imidazole functionalization of MoS2 by nucleophilic substitution with L1; b) Imidazole functionalization of carboxylate functionalized exfoliated graphene (FEG) by esterification reaction with L2.

N

a)

N

Br

N

N

Br

L1

Nucleophilic substitution Exfoliated MoS2

MoS2-L1

b)

N

N

HO

O

N

N

N

N

N

O

HO

O

HO

O

Br

N

OH

O

O O

O O

L2

Esterification Graphene-L2

FEG

Experimental section Reagents and precursors: All the starting materials used in synthesis were of high purity and obtained commercially. Pre-dried DMF was used for all coupling reactions. Carboxylate functionalized graphene was prepared by using the synthesis method reported elsewhere.45 Synthesis of NMI-im and Nap-im: The imidazole functionalized naphthalene monoimide acceptor (NMI-im) as well as the naphthol donor (Nap-im) were synthesized by the steps shown in Scheme S1 and characterized by 1H NMR and mass spectrometry. Detailed synthesis along with characterization has been described in SI. Synthesis of L1 and L2: Detailed synthesis along with characterization has been described in SI (Scheme S2). 4 ACS Paragon Plus Environment

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Synthesis of 1T-MoS2: Bulk MoS2 (2g) was mixed with n-BuLi (15 mL) and was allowed to stir at RT for 7 days under N2 atmosphere. The black precipitate was washed with dry hexane thoroughly to remove unreacted n-BuLi. Thereafter it was immediately mixed with required amount of deionised, deaerated water under sonication for 1 hour. The resulting dispersion was centrifuged at 8000 rpm to remove the unexfoliated part. The supernatant was collected and utilized further for functionalization (Scheme S3a). Synthesis of MoS2-L1: 100 mL of the exfoliated 1T-MoS2 solution and 1 g of L1 in 25 mL of dry DMF were mixed in a round bottomed flask. The resulting mixture was stirred for 2 days under ambient conditions. The black precipitate was separated by centrifugation and washed with absolute ethanol several times and dried in vacuum oven for 6 hours (Scheme 2a, S3a). Synthesis of carboxylate functionalized exfoliated graphene (FEG): Graphene oxide (GO) was synthesised by modified Hummer’s method. Thereafter GO was subjected to exfoliation at 1050 o

C to obtain exfoliated graphene (EG). EG was acid functionalised in the presence of mixture of

H2SO4/HNO3 in a hydrothermal bomb. The resulting black precipitate of carboxylate functionalised exfoliated graphene (FEG) was washed with ample amount of water and dried at 70 oC for further functionalization (Scheme S3b). Synthesis of Graphene-L2: 100 mg of FEG was mixed with 15 mL dry DMF. The resulting solution was heated at 80 oC for few hours. 86.24 mg of N,N'-Dicyclohexylcarbodiimide (DCC) and 6.24 mg of 4-Dimethylaminopyridine (DMAP) coupling agents were added to the above dispersion. To the above dispersion of 100 mg of L2 was added. The mixture was allowed to stir at RT for 3 days under N2. The product was separated by filtration and washed with copious amount of ethanol and DMF (Scheme 2b, S3b). Synthesis of BN: Boric acid and urea were mixed in the ratio of 1 : 48 molar ratio at 900 oC for 5 hours under ammonia atmosphere to obtain amine functionalized BN nanosheets. Synthesis of BN-L3: 100 mg of BN was mixed with 15 mL dry DMF. 86.24 mg of N,N'dicyclohexylcarbodiimide (DCC) and 6.24 mg of 4-Dimethylaminopyridine (DMAP) coupling agents were added to the above dispersion. To the above dispersion of 100 mg of L3 was added. The mixture was allowed to stir at RT for 3 days under N2 and the product was separated by filtration and washed with copious amount of ethanol and DMF (Scheme S4b). Synthesis of non-covalent hybrids: The functionalized MoS2 and graphene nanosheets were dispersed in water, enabling further non-covalent functionalization with water soluble organic 5 ACS Paragon Plus Environment


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chromophores in the presence of CB[8]. The required amount of MoS2-L1 and Graphene-L2 were dispersed in aqueous solution of CB[8] (10-4 M) and sonicated for few hours for obtaining uniform dispersions. Thereafter, equimolar concentration of NMI-im and Nap-im (10-4 M) was added to the above dispersion and sonicated for few minutes and spectroscopic studies were performed to confirm the non-covalent functionalization. Characterizations: UV-Vis absorption spectra was recorded on a Perkin Elmer UV-Vis-NIR spectrometer lambda 750 and emission spectra was recorded on Perkin Elmer LS 55 fluorescence spectrometer. The lifetime decay measurements were performed on Deltaflex TM HORIBA scientific. The instrument response function (IRF) was recorded by using a scatterer (Ludox AS40 colloidal silica, Sigma-Aldrich). The FTIR spectra were recorded by making KBr pellets using the bruker IFS 66v/s. TEM and HRTEM images were recorded using FEI Tecnai T20 operating at 200 keV. The thermogravimetric analysis were performed on Mettlar Toledo TGA/DSC STARe system instrument. Alumina crucible was used for the analysis and were performed in the temperature range of 28 oC to 900 oC with continuous flow of N2 (40 mL min-1) with a heating rate of 3 oC min-1. The diffraction measurements were performed on BrukerAXS D8Discover using Cu Kα radiation.

13

C-NMR solid state were performed on Bruker AscendTM

400 MHz spectrometer. 1H-NMR measurements were performed on JEOL RESONANCE ECZ600R operating at 600.12 MHz. and with a Bruker AVANCE 400 (400 MHz). Fourier transform NMR spectrometer with chemical shifts reported in parts per million (ppm) with respect to TMS. Splitting patterns are designated as s, singlet; d, doublet; bs, broad singlet; m, multiplet; t, triplet; q, quartet; quin, quintet and br, broad. High-Resolution Mass Spectra (HRMS) were recorded on an Agilent 6538 Ultra High Definition (UHD) AccurateMass Q-TOFLC/MS system using electrospray ionization (ESI) and Atmospheric Pressure Chemical Ionization (APCI) modes. MALDI was performed on a Bruker daltonics Autoflex Speed MALDI TOF System (GT0263G201) spectrometer using α-Cyano-4-hydroxy-cinnamic acid (CCA) as the matrix. XPS data were collected with an Omicron Nanotechnology spectrometer employing a 3 monochromatic Al Kα X-ray source (1486.72 eV).

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Results and Discussion Powder X-Ray diffraction (PXRD) patterns of the functionalized nanosheets of MoS2 and graphene indicate that they remain in the exfoliated state after functionalization with the ligands (Figure S1). After functionalization, the 1T-MoS2 sheets transforms to the stable 2H form (Figure S1a). Functionalization with L1 is also confirmed by the appearance of the C-S stretching band at 666 cm-1 (Figure 1a) while, L2 functionalized graphene show C=O and C=C stretching bands at 1734 and 1580 cm-1, and the C-O stretching bands at 1278 and 1153 cm-1 (Figure 1b).

.

a)

b)

C-S -1 666 cm

Graphene-L2 C=O -1 1734 cm

MoS2-L1

1200

1000 800 -1 Wavenumber(cm )

600

3500

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c)

C=C GO -1 1627 cm

C=O -1 1725 cm

O-H -1 3414 cm

Bulk-MoS2 1400

FEG

C=O -1 1725 cm

Exfoliated MoS2

3000 2500 2000 1500 -1 Wavenumber (cm )

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d) C=C

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C-C -CH 3

C=O Graphene-L2

MoS2-L1

C=C

Bulk MoS2

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ppm

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FEG 160

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Figure 1. a) Comparison of FT-IR spectra of bulk MoS2, exfoliated MoS2 and MoS2-L1 and the peak at 666 cm-1 confirms the presence of C-S bond in MoS2-L1 (marked with black dot); b) Comparison of FT-IR spectra of graphene oxide (GO), carboxylate functionalized exfoliated graphene (FEG) and Graphene-L2, showing two weak bands at 1278 and 1153 cm-1 (marked with black dots) corresponding to ester bond formation in Graphene-L2 and a shift in the C=O stretching frequency upon esterification with L2; c) Solid state 13C-NMR for MoS2-L1 showing the presence of aromatic carbon atoms from the L1 which is absent in unfunctionalized MoS2; d) Solid state

13

C-NMR for Graphene-L2 showing the presence of aromatic and aliphatic carbon

atoms from the L2 compared to the carboxylate functionalized exfoliated graphene (FEG). 7 ACS Paragon Plus Environment


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Further, evidence of functionalization of MoS2 was also obtained from the solid state 13

C-NMR spectrum, showing signals corresponding to the aromatic carbons of the L1, around

100-150 ppm (Figure 1c). The L2 functionalized graphene also showed signals due to the aromatic carbons of the ligand L2 (Figure 1d). We also observe an increase in the intensity of the signals of Graphene-L2 in the aromatic regions (Figure 1d). Signals due to aliphatic carbons and C=O are observed at 32, 56 and 164 ppm, respectively. Thermogravimetric analysis (TGA) of the functionalized nanosheets show 15 wt% loss due to L1 and 6 wt% loss due to L2 (Figure S1c, d). Transmission electron microscope (TEM) images (Figure S2a) shows the presence of nanosheets of MoS2-L1, and an increase in the interlayer spacing up to 1 nm (Figure S2b) signifying the presence of exfoliated functionalized nanosheets. Graphene-L2 (Figure S3a) also shows the presence of nanosheets. Raman spectra also confirms the identity of the ligand functionalized nanosheets (Figure S4a and b). X-ray photoelectron spectra of MoS2-L1 (Figure S5a) confirms the functionalization of MoS2 with L1, as revealed by the shift in the binding energies of Mo 3d and S 2p in comparison to bulk MoS2 (Table S1). The binding energies of Mo4+ 3d5/2 and Mo4+ 3d3/2 (Figure S5b) in bulk MoS246 and MoS2-L1 are found at 229.4, 232.6 eV and 230.1, 233.72 eV respectively showing considerable shift, indicating the change in electronic environment of MoS2 due to the presence of L1, thus confirming the functionalization. Similarly, deconvoluted signals for S 2- 2p3/2 and S22p1/2 (Figure S5b) in bulk MoS2 and MoS2-L1 (Figure S5d) occur at 163.2, 163.5 eV and 162.8, 164.1 eV respectively. High resolution XPS (Figure S5c) shows the absence of the Br signal indicating the absence of any unreacted L1 on the sheets of MoS2. Atomic force microscopy of MoS2-L1 (Figure S6a) reveals the bilayer nature (inset Figure S6a) of the MoS2-L1 nanosheets with a height profile of 2 nm. Graphene-L2 (Figure S6b) gives a height profile of 1.32 nm signifying the presence of 2 layers of the Graphene-L2 nanosheets (inset Figure S6a). We first examined the formation of the ternary complex of imidazole functionalized MoS2 with NMI in the presence of CB[8]. This was done by sonicating a mixture of functionalized MoS2/graphene (0.03/0.015 wt%) with an equimolar mixture of CB[8] (10-4 M) and NMI (NMI-im) or naphthol (Nap-im) (10-4 M). The formation of the ternary complex was confirmed by 1H-NMR in D2O (Figure 2). In the case of MoS2, the phenyl (6.90-7.03 ppm) and imidazole (7.2-7.5 ppm) ring protons of the NMI-im (acceptor) exhibit upfield shifts to 5.30 and 5.60 ppm, respectively in presence of the CB[8]. The aromatic protons of the NMI (8.2-8.3 ppm) 8 ACS Paragon Plus Environment

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remains unaffected, thus ruling out any physisorption or charge-transfer based interaction of the aromatic core with the MoS2-L1 surface. In the absence of CB[8] 1H-NMR spectra (Figure S7a) of a mixture of MoS2-L1 (0.03 wt%) and NMI-im (10-4 M), we did not observe shifts in the aromatic and phenyl protons of the acceptor. Similar were the findings on mixing Nap-im (donor) and MoS2-L1 in the presence of CB[8], where the phenyl (6.90 ppm) and imidazole (7.6-7.7 ppm) ring protons of the Nap-im exhibits an upfield shift to 5.6 ppm in presence of the CB[8]. Control experiments with CB[8] and Nap-im in the absence of MoS2 showed broadening and disappearance of the protons suggesting host-guest complex formation between naphthol and CB[8] (Figure S7b), but the reappearance of the naphthol protons upon addition of MoS2-L1 confirms the formation of the stable ternary complex between benzyl imidazole group and CB[8] rather than naphthol-CB[8] host-guest complex.

Graphene-L2 : CB[8] : NMI-im

Graphene-L2 : CB[8] : Nap-im

MoS2-L1 : CB[8] : NMI-im

MoS2-L1 : CB[8] : Nap-im

NMI-im

Nap-im

9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 8.5  (ppm)

8.0

7.5

7.0

6.5

6.0

5.5

5.0

 (ppm)

Figure 2. 1H-NMR spectra showing the non-covalent functionalization of MoS2-L1 with NMIim and Nap-im using efficient ternary complexation in presence of CB[8] and corresponding non-directional non-covalent functionalization in case of Graphene-L2. Shift in only benzene and imidazole protons of MoS2-L1 : CB[8] : NMI-im and MoS2-L1 : CB[8] : Nap-im noncovalent hybrid confirms the ternary complex formation (Red curves). Corresponding 1H-NMR spectra of Graphene-L2 : CB[8] : NMI-im and Graphene-L2 : CB[8] : Nap-im (blue curves) demonstrating broadening of all aromatic protons of NMI-im and Nap-im and non-directional π-π interaction between acceptor and graphene. Corresponding schematics are also shown. Non-covalently functionalized MoS2 was examined by electronic absorption and emission spectroscopy techniques. NMI-im exhibits an absorption and emission maxima at 345 nm and 400 nm, respectively and Nap-im exhibits absorption and emission maxima at 271 nm 9 ACS Paragon Plus Environment


and 347 nm, respectively in aqueous media (Figure S8a, b). The spectral properties of the acceptor remains unchanged on interaction with CB[8] (Figure S9), but the spectral properties of the donor changes in the presence of CB[8] due to host-guest complex formation corroborating with 1H-NMR experiments (Figure S10). b)

400 200

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Prompt CB[8] : NMI-im 0.030 wt% of MoS2-L1 N N

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e) 80

Nap-im in CB[8] 0.0012 wt% 0.005 wt% 0.0075 wt% 0.01 wt% 0.015 wt%

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35 Time (ns)

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Intensity (counts)

Intensity

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CB[8] : NMI-im 0.005 wt % 0.0075 wt% 0.01 wt% 0.015 wt% 0.020 wt% 0.025 wt% 0.030 wt%

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Intensity400

a)

Intensity

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0.005 0.010 0.015 wt% of MoS2-L1

0.020

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Prompt CB[8] : Nap-im 0.015 wt% of MoS2-L1

40

60 80 Time (ns)

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Figure 3. a), d) Fluorescence spectra of NMI-im and Nap-im in presence of CB[8] and with

increasing wt% of MoS2-L1 showing gradual quenching of fluorescence; b), e) corresponding titration plot showing saturation of fluorescence quenching with 0.03 wt% of MoS2-L1 in case of NMI-im and with 0.015 wt% in case of Nap-im, respectively; c), f) Lifetime decay profiles of NMI-im and Nap-im in presence of MoS2-L1 and CB[8] showing decrease in acceptor and donor lifetime and electronic communication between acceptor/donor and MoS2. A more efficient quenching in case of donor suggests effective electronic interaction of MoS 2 with donor than acceptor; corresponding schematic showing the ternary complex formation between MoS2L1, CB[8] and NMI-im and Nap-im respectively. (NMI-im = Nap-im = 10-4 M, CB[8] = 10-4 M, H2O). Addition of MoS2-L1 to this mixture, facilitates the formation of stronger ternary complexation of the benzyl imidazole groups with CB[8] resulting in non-covalent anchoring and a gradual decrease in the fluorescence intensity (Figure 3a, d), which get completely quenched at 0.03 wt% of the MoS2-L1 for the acceptor and at 0.015 wt% for the donor (Figure 3b and 3e). This observation confirms electronic interaction between the acceptor and the donor molecules with MoS2-L1 through non-covalent anchoring. Requirement of lower wt% of MoS2 to completely quench the fluorescence in the case of naphthol compared to NMI suggests a more efficient electronic interaction between the MoS2 and the donor chromophore compared to the 10 ACS Paragon Plus Environment

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acceptor chromophore. The absorbance spectra (Figure S11a) did not show any significant changes, apart from the appearance of scattering due to the increasing amount of MoS2-L1, signifying the absence of any ground-state interaction. Time-resolved fluorescence spectra showed the appearance of a fast decay component of 0.02 ns with the acceptor and an increased component of 0.03 ns with the donor (Figure 3c, f) (Table S2, S3) in the presence of MoS2-L1, due to excited state interaction between MoS2-L1 and the non-covalently anchored NMI and the naphthol chromophores. This can be either due to energy-transfer or electron transfer. The lifetime of the long-lived component of non-covalent hybrid of NMI and naphthol (Table S2, S3) matches with that of the NMI and naphthol alone suggesting the presence of uncomplexed chromophores in the systems. Unlike MoS2, graphene exhibits strong π-π stacking or charge-transfer interaction between the acceptor and the surface of graphene and hence ternary complex formation does not occur. The 1H-NMR spectrum of the mixture of imidazole functionalized graphene with NMIim and Nap-im even in the presence of CB[8] showed broadening of NMI and Nap protons, suggesting non-covalent functionalization through π-π stacking interactions preferably over ternary complex formation (Figure 2). Non-covalent attachment of the chromophores via π stacking interactions is also evident from spectroscopic properties. Addition of Graphene-L2 also results in an efficient quenching of the NMI and naphthol emission and complete quenching of the fluorescence even with the addition of 0.015 wt% of Graphene-L2 in case of both acceptor and donor chromophores (Figure S12). This is accompanied by broadening of the absorption spectra (Figure S11c, d) in the case of NMI, signifying a strong ground-sate interaction corroborating with the results from NMR spectra. We and others have previously reported experimental and theoretical studies on similar ground state interactions of various donor and acceptor chromophores with graphene via chargetransfer or simple π-stacking interactions.15,16,21 Time-resolved spectra do not show any significant change in the life time of NMI-im (Figure S12c, Table S4), suggesting the absence of any excited state contribution to the observed quenching of the fluorescence. Whereas naphthol increased the contribution of the sharp component of 0.03 ns possibly due to excited state electronic interaction in non-directional non-covalent hybrid formed between graphene and naphthol donor and needs to be investigated further. These results agree with the previous reports, where strong charge-transfer interaction was used for the non-covalent functionalization of graphene resulting in non-directional functionalization. Furthermore, to test the versatility of 11 ACS Paragon Plus Environment


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the host-guest strategy to various 2-D materials, non-covalent functionalization of BN nanosheets with NMI-im was similar to that of graphene (Scheme S4a, Figures S12-S15 and Table S6). Conclusions In conclusion, we have demonstrated a supramolecular host-guest strategy to produce ternary complexes of both MoS2 and graphene although the mode of interaction differs in the two cases. In the case of MoS2, non-covalent functionalization occurs through ternary host-guest complexation of the benzyl imidazole groups in the presence of CB[8]. In the case of graphene, π-π interaction occurs between the organic chromophore and graphene surface. We believe that the supramolecular strategy presented here opens up a new path for functionalization of MoS2 and other transition metal dichalcogenides (TMDs) with a series of donor and acceptor organic chromophores to alter their optoelectronic properties for different applications. This strategy can also be used to generate heterostructures based on 2D materials.47 Another feature of this strategy is that the process is reversible since the supramolecular hybrid dissociates to starting molecules on reaction with competing guest molecules such as adamantylamine. Associated Content Method of synthesis of materials and organic compounds with schemes and explanations for analyses performed with UV-Vis, PL, TEM, 1H and

13

C NMR and TCSPC studies is described

in supporting information. Author Contributions The authors contributed equally to this work and declare no competing financial interest. Author Information Corresponding Authors: *E-mail: [email protected], *E-mail: [email protected] Acknowledgements M. K thanks SSL, ICMS for fellowship. A. S thanks CSIR for fellowship.


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Supramolecular Hybrids of MoS2 and Graphene Nanosheets with

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