Mutual influence on aggregation and magnetic properties of graphene oxide and copper phthalocyanine through non-covalent, charge transfer interaction

Journal Pre-proofs Full Length Article Mutual influence on aggregation and magnetic properties of graphene oxide and copper phthalocyanine through non-covalent, charge transfer interaction G.B. Markad, N. Padma, R. Chadha, K.C. Gupta, A.K. Rajarajan, P. Deb, S. Kapoor PII: DOI: Reference:

S0169-4332(19)33440-3 https://doi.org/10.1016/j.apsusc.2019.144624 APSUSC 144624

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

6 August 2019 1 November 2019 6 November 2019

Please cite this article as: G.B. Markad, N. Padma, R. Chadha, K.C. Gupta, A.K. Rajarajan, P. Deb, S. Kapoor, Mutual influence on aggregation and magnetic properties of graphene oxide and copper phthalocyanine through non-covalent, charge transfer interaction, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc. 2019.144624

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Mutual influence on aggregation and magnetic properties of graphene oxide and copper phthalocyanine through non-covalent, charge transfer interaction G. B. Markad1, N. Padma2,3,*, R. Chadha4, K. C. Gupta5, A. K. Rajarajan3,6, P. Deb5 and S. Kapoor3,4,* 1Department 2Technical 3Homi

Physics Division, Bhabha Atomic Research Centre, Mumbai-400085, India

Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai-400094, India

4Radiation 5High

of Chemistry, Savitribai Phule Pune University, Pune-411007, India

and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai-400085, India

Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai-400085,

India 6Solid

State Physics Division, Bhabha Atomic Research Centre, Mumbai-400085, India

Abstract Investigations on the non-covalent, charge transfer interaction between graphene oxide (GO) sheets and unsubstituted copper phthalocyanine (CuPc) revealed their mutual influence on their structural, optical and magnetic properties. While charge transfer interaction is found to change the aggregation of CuPc from J to H state, and this change increasing with GO concentration, the aggregation/stacking of GO is also found to be getting modified (increasing in this study) on interaction with CuPc. The charge transfer interaction is suggested to be mostly occurring through carboxyl groups at the edges of GO sheets, which could be causing this stacking/aggregation change of GO. While GO, CuPc and the nanocomposites exhibited paramagnetic behaviour, the saturation magnetization of the nanocomposites is observed to be lower than that expected. This is correlated to the reduction in the total spin (J) of GO and its subsequent altered magnetic response, which is attributed to the charge transfer interaction with CuPc. This study shows the possibility of tuning the magnetic properties of GO through such non-covalent, charge transfer routes. The improved optical limiting behaviour of the nanocomposites is further attributed to the charge transfer interaction and the increase in stacking of GO. Keywords: non-covalent interaction, charge transfer, aggregation, spin, magnetic moment Corresponding authors: Dr. N. Padma,

Dr. S. Kapoor,

Email: [email protected],

[email protected],

Tel. No.: 91 22 25592317

91 22 25590298

1.Introduction In the past decade, graphene and its derivative graphene oxide (GO) have been shown to be very useful candidates in various applications like field effect transistors (FETs), supercapacitors, solar cells, batteries, gas sensors etc.[1,2]. GO is basically decorated with oxygen containing pendant groups like hydroxyl, epoxy, carboxylic, and carbonyl moieties, with former two attached in the basal plane and the latter two along the edges of the graphene sheets [2-4]. These oxygen and hydrogen containing moieties can tune the hydrophilic nature of graphene sheets and also their electronic properties. These functional groups also improve the efficiency of interaction of GO and RGO sheets with other organic/inorganic materials through covalent or non-covalent routes [5,6]. Such composites can combine the unique properties of each of these materials and offer improved electronic and optical properties where the interaction with functional molecules can further tune the electronic properties like band gap etc. of GO. Many studies have reported covalent attachment of functional materials like CuS, polymers, small molecules like metallophtalocyanines (MPcs), porphyrins and fullerenes, redox and/or photoactive entities etc., similar to other nanocarbons like carbon nanotubes, fullerenes etc. [4,6-17]. It has been reported that covalent interaction between porphyrins/phthalocyanines etc. with carbon allotropes like graphene, GO, RGO, carbon nanotubes (CNTs), fullerenes, etc. lead to stable dispersions/solutions in aqueous and/or common organic solvents, with the altered structure and electronic properties of the carbon nanostructures. It has also been reported that interaction of multi walled carbon nanotubes (MWCNTs) with water soluble cobalt octacarboxyphthalocyanine derivative, under ultrasonication, leads to unfolding of MWCNTs and transforming them to graphene sheets [18]. The non-covalent interaction between the phthalocyanines/porphyrins generally occurs through π-π interactions, electrostatic/van der Waals interactions, hydrogen bonding etc. This results in lesser stability of the hybrid systems as compared to that formed with covalent interactions [6-10]. Such covalent interaction of the above mentioned macromolecules has been exploited to further tune the electronic properties of graphene, RGO and GO, enabling them to be potentially employed in applications like LEDs, solar cells, phototransistors, photocatalysis, photoreduction of CO2, medical applications etc. [4, 12-17]. These functional materials also act as intercalants, thereby increasing the spacing between the GO/RGO sheets [4,5,19]. The extentof interaction between the individual constituent materials in a composite is gauged by identifying and estimating the extent of charge transfer interaction between the two. Hence, some other studies have focussed mainly on the charge transfer interactions

between GO and the functional molecules [2,3,19-23]. Many studies have paid attention to metallophthalocyanines (MPcs) for this purpose due to their interesting and useful properties such as semiconductivity, photoconductivity and excellent thermal and chemical stability [2,12]. MPcs, mostly acting as electron donor, are basically planar complexes of macrocyclic nature with extended π electron density that is well suited for charge transfer interactions with GO sheets [2, 16]. They are also reported to exhibit additional electrostatic interaction with GO sheets [21]. While most of the studies discuss the structural and optical properties, there are only a few studies discussing the magnetic properties of GO [24-27]. Studies on magnetic properties of graphene, GO and RGO have been recognized as the useful field of research in the last few years as they can find potential applications in flexible light weight magnetic memory and spintronic devices. The long spin diffusion times and weak spin-orbit and hyperfine interactions, along withhigh charge carrier mobility, identify these materials as potential candidates for spintronic applications, provided they can be made magnetic [23,28]. While pristine graphene is reported to be non-magnetic, zig-zag edges, defects, vacancies, adatoms etc. have been identified as sources for inducing localised magnetic moment and hence magnetization in the same. Presence of sp3 carbons in GO, brought about by various oxygen based functional groups, leave the unpaired electrons to form the localised magnetic moments that contribute to paramagnetism. These localised moments can also interact with each other to magnetically order at low temperature, through an exchange interaction facilitated by the conducting delocalised π electrons [29]. While doping of foreign atoms like F, N, S, H etc. in graphene/GO/RGO has been reported to be a useful strategy to induce and/or enhance magnetization, studies on the effect of charge transfer interaction of functional molecules on magnetization of graphene/GO are scarce [23,24,28,30,31]. Since covalent/non-covalent attachment of functional molecules can modify the electron density, it can also be expected to alter the magnetic properties. While the above mentioned studies restrict themselves to identifying the interaction between GO and Pcs through shift in the absorption band positions, to our knowledge, there are no reports on the effect of graphene/GO/RGO on the aggregation state of Pc molecules. Though Song et al observed change in relative intensity in absorption bands of ZnPc and attributed it to the change in their electronic state on interaction with graphene, they do not discuss about their aggregation states [32]. Our earlier study on the nanocomposites of ZnO and CuPc had clearly shown the change in the aggregation of the latter from J state to H state [33]. Such change in the aggregation state, manifested mainly as the change in the absorption

band positions as well as in their relative intensity, can be expected to lead to improvement in the solar light harvesting and hence could help in the photovoltaic applications. In the present study we have shown that the non-covalent, charge transfer interaction between GO and un-substituted copper phthalocyanine (CuPc) influences, not only the structural and optical properties, but also the magnetic properties of GO. The study also shows that interaction with GO modifies the aggregation state of CuPc molecules and the extent of modification depends on the concentration of GO. Interestingly, our results also show increase in stacking of GO sheets on interaction with CuPc molecules, which is in contradiction to the common belief that the organic molecules intercalate between GO sheets causing enhanced interlayer separation and exfoliation of the latter. The research on high performing materials like graphene and GO show interesting non-linear optical properties, that have application in fibre-optic communications, saturable absorbers, optical limiters etc. Many studies have been investigating the development of optical limiters using these materials which are highly essential for protecting detectors and human eyes from strong laser radiations [29, 34-37]. Improving the optical limiting behaviour of GO through charge transfer interaction between other inorganic and organic materials has been the commonly employed method in all these studies [32,38-45]. Among the functional materials, metallophthalocyanines (MPcs) are most preferred due to their excellent non-linear optical properties that have identified them as the most suitable candidate to be in composite with GO sheets [31,43,45,46]. Most of the MPcs studied in combination with GO for nonlinear opticalapplications are in derivatised/substituted form such that they can interact through covalent bonding with the functional groups of GO [32,43,45,46]. Studies on charge transfer interaction between GO and unsubstituted Pcs are negligible, especially for that on the effect of non-covalent interaction between the two, on non-linear optical properties is negligible [21,23]. The present study shows that the improvement in optical limiting behaviour of GO can be obtained even with non-covalent interaction with CuPc. 2. Experimental methods 2.1 Preparation of graphene oxide (GO) Graphene oxide (GO) was prepared by Hummers method with slight modification and reported elsewhere by us [20,47]. In detail, graphite powder (0.5 g) and sodium nitrate (0.5 g) were added in sulfuric acid (23 ml) with continuous stirring followed by cooling in an ice

bath. Potassium permanganate (3.0 g) was added to the suspension slowly so that the temperature does not exceed 20 °C. Following this, the reaction system was transferred to a 35 ± 5 °C water bath for 1 hour, forming a thick paste. At the end of 1 hour, 100 ml of water was gradually added, and the solution was stirred for 15 minutes at 90 ± 5 °C. Additional 500 ml of water was added and treated with 3 ml of H2O2 (30%), turning the colour of the suspension to yellow–brown. On completion of the reaction, the mixture was cooled to room temperature and then washed with water several times until the pH of the filtrate was neutral. The remaining solid was dried under vacuum. 2.2 GO-CuPc nanocomposites 6 mg of CuPc was added in 25 ml methanol and sonicated for several hours to get CuPc dispersion in methanol. Similarly, 12.5 mg of GO was added in 25 ml methanol and sonicated for several hours to get GO dispersion. For preparing nanocomposites (NC-A and NC-B) of GO and CuPc, 6 mg of CuPc was added in methanol along with 12 mg of GO for NC-A and 24 mg GO for NC-B, and sonicated for 3 hours to get uniform dispersion. Methanol was evaporated by drying them under IR lamp to get the solid powder. The dispersion of these nanocomposites were used for UV-visible, photoluminescence (PL) and Fourier transform infra-red (FTIR) measurements. While UV-visible and steady state PL measurements were carried out in solution using methanol as blank, FTIR measurements were carried out on films formed by drop casting each of them on glass plates and drying under IR lamp. To obtain thick films, the drop casting procedure was repeated several times. For optical limiting measurements, about 1.5 ml of the above dispersions were drop cast on the quartz plates of size around 5mm x 5mm to form the corresponding films. For Raman, X ray photoelectron spectroscopy (XPS) and X ray diffraction (XRD) measurements, samples were used in powder form. To prepare samples 1 to 7 (see supplementary material), constant volume of CuPc, i.e. 3 ml of the above mentioned CuPc dispersion, was mixed with different volumes of GO dispersion which was subsequently diluted to 10 ml by methanol followed by sonication for 30 minutes. The corresponding volume and concentration details are provided in Table S1 in supporting information along with the details for NC-A and NC-B. FTIR spectra were measured using a Bruker tensor-37 FTIR-spectrophotometer, having the attenuated total reflection (ATR) attachment and with resolution better than 1 cm1.

UV-visible absorption measurements were carried out using an Agilent 8453 diode array

single beam spectrophotometer. Steady state PL spectra were measured using a Shimadzu RF-5301PC spectrofluorimeter. The PL spectra were obtained at 260 nm excitation wavelength. Powder XRD patterns were obtained using a Bruker D8 Advance x-ray diffractometer using CuKα radiation (1.54 Å). XPS measurements were carried out using MgK (1253.6 eV) source and DESA-150 electron analyzer (Staib Instruments, Germany). The binding-energy scale was calibrated to Au-4f7/2 line of 84.0 eV. The analyzer was operated at 40 eV pass energy. For the optical limiting measurements, the drop cast films of GO, CuPc, NC-A and NC-B dispersed in methanol, on quartz substrate, were exposed to 1064 nm laser pulses (single shot) of 600 ps pulse duration. The laser spot size on the sample was about 2 mm in diameter. The incident pulse energy and the transmitted pulse energy were measured to obtain the absorption/ transmission characteristics of GO, CuPc, NC-A and NC-B. Magnetization measurements were carried out using a SQUID magnetometer (model MPMS, M/s Quantum design). Raman spectra were recorded at room temperature using 633 nm (HeNe) laser line. The sample was kept on glass slide and scattered light was collected at 180° scattering geometry with a 50X LWD (long working distance) objective by keeping the glass slide under the Raman microscope. The scattered light was detected using a CCD (Synapse, Horiba Jobin Yvon) based monochromator (LabRAM HR800, Horiba Jobin Yvon, France) together with an edge filter, covering a spectral range 200−1800 cm−1. The spot size on the sample was ~0.5 mm in diameter, and the laser power and the power density at the sampling position was 12.7 mW and 12.95 W/cm2, respectively for the excitation wavelength of 633 nm. The Raman band of a silicon wafer at 520 cm−1 was used to calibrate the spectrometer, and the accuracy of the spectral measurement was estimated to be better than 1 cm−1. 3. Results and Discussion 3.1 Structural studies Fig. 1 shows the XRD patterns of GO, CuPc, NC-A and NC-B powders. A diffraction peak at 2θ = 10.72 º corresponding to (002) reflection of the stacks of GO sheets with an interlayer spacing of 0.804 nm is observed. Using the Scherrer formula and the FWHM of around 1.11 º observed from this major diffraction peak, the average crystallite size is found to be around 7.01 nm. The XRD patterns of CuPc reveal polycrystalline nature of the same

with reflections corresponding to its β-phase form. In the XRD pattern of NC-A and NC-B, peaks corresponding to both GO and CuPc are present with their peak positions almost same as that of pristine GO and CuPc. But the FWHM of GO peak was found to be significantly reduced to about 0.7º in NC-A and 0.8º in NC-B, indicating increase in crystallite size and hence the stacking of GO sheets. The relative intensity of the diffraction peaks of CuPc in both the nanocomposites was found to be significantly altered on mixing with GO and this change in relative intensity was also found to be varying with GO concentration. All the above changes in the diffraction peaks indicate the clear interaction between GO and CuPc molecules. Negligible peak shift for GO indicates that there is no further expansion of the interlayer spacing of GO sheets, contrary to that reported by some studies showing expansion of interlayer spacing due to intercalation of the organic molecules between the sheets [16,21,32]. Therefore, the XRD patterns observed in the present study indicate absence of intercalation of GO sheets by CuPc molecules. This was further suggested by the fact that the GO peak was distinctly observed in the nanocomposites. If CuPc had intercalated between the GO sheets, the GO peak would have disappeared due to the strong scattering from Cu atoms, as also reported by Yu et al [4]. In that study, disappearance of the [001] reflection of GO due to intercalation of CuS nanoparticles and the destroying of the stacking of GO sheets is reported.

Fig. 1 XRD patterns of GO, CuPc, NC-A and NC-B. Fig.2 shows the comparison of FTIR spectra of GO, CuPc, and the nanocomposites NC-A and NC-B. The spectrum of GO shows a broad band within the range 3100- 3600 cm-1 corresponding to the stretching vibrations of OH groups attached to basal plane of GO, from

the adsorbed water molecules and from those in the carboxyl groups [37,40,48]. The absorption peaks at around 1095, 1409, 1618 and 1718 cm-1 corresponding to the C-O-C epoxy symmetric stretching vibrations, C-OH stretching of carboxyl groups, C=C stretching vibration of unoxidized graphitic domains merging with deformation vibration of OH groups in adsorbed water molecules, -C=O stretching from carboxyl and carbonyl groups respectively are also observed [21,36,37,42,49]. For pristine CuPc, strong peaks corresponding to ν(C-H) out of plane bending and ν(C-N) stretching modes at 725 and 777 cm-1 respectively, in-plane bending and stretching modes of tetrapyrrolic macrocycle and porphyrin ring systems at 1065, 1086, 1117, 1162, 1284, 1330, 1416, 1504, 1587, 1606 cm-1 are seen [33].

Fig. 2 FTIR spectrum of a) GO, b) CuPc, c) NC-A and d) NC-B. Both GO and CuPc peaks are present in NC-A and NC-B indicating a mixture of both the constituents. CuPc peak at 725 is shifted to 729 cm-1for NC-A and to 728 cm-1 for NC-B while the peak at 1330 cm-1 is shifted by about 3 cm-1 to higher wavenumber for both the nanocomposites. Other peaks at 1086, 1162, 1284 and 1606 cm-1 etc. are shifted to higher wavenumber by 2-2.5 cm-1. The shift to higher wavenumbers indicates hole doping of CuPc in the nanocomposites. Due to the overlap of CuPc peaks with many of the GO peaks mentioned above, the shift in that of the latter could not be estimated. But the shift in the peak at 1718 cm-1 of GO to around 1711 cm-1 could be clearly identified in both the nanocomposites, indicating electron doping of GO in the same. The change in relative intensities of the peaks of both GO and CuPc in the nanocomposites, could be clearly observed, revealing the clear interaction between the two. Change in relative intensity of GO

within the region of 1500 to 1800 cm-1 suggests that CuPc could be interacting with GO through carboxyl groups and OH groups. FTIR of samples 1 to 7 have also revealed similar changes (Fig. S1 in supplementary material). Raman spectroscopy is a versatile, non-destructive and powerful tool widely employed for characterization of graphene, which can reveal information regarding number of layers, defects, doping etc. It has also been shown to provide useful information on charge transfer interaction and the subsequent hole/electron doping of the materials. The Raman spectrum of GO, displayed in Fig. 3, shows two major bands G and D, at 1595 and 1330 cm1,

where the former is attributed to the sp2 carbon clusters of GO and the latter to the defects

produced due to hydroxyl, epoxy, carboxyl functional groups attached to graphene sheets. Raman spectrum of CuPc reveals strong peaks at 676 and 743 cm-1 corresponding to vibrations involving both bridging and pyrrolic nitrogen atoms bonding with carbon in the macrocycle ring. The strong peak at 1517 cm-1 and the weaker peak at 1444 cm-1 correspond to C-C vibrations and C-N-C vibrations of the bridging nitrogen of the ring. Another weak peak observed at 1333 cm-1 corresponds to mainly C-C vibrations in the macrocycle [50]. In the case of nanocomposites, G peak has shifted to lower wavenumber by about 4 cm-1 in NCA and by 2 cm-1 in NC-B. Some studies discussing interaction of different materials with GO/RGO have shown shift in G peak to lower wavenumber due to electron doping of the latter while some others have shown the shift to higher wavenumber [6,32,42]. The shift to lower wavenumber of G peak in the present study indicates electron doping of GO. The peaks of CuPc show significant changes in relative intensity in the nanocomposites indicating strong charge transfer interaction. The peaks at 676 and 743 cm-1 for CuPc shift to higher wavenumber by about 8 cm-1 and 7 cm-1 for NC-A respectively, and by about 9 cm-1 each for NC-B. Such strong shifts to higher wavenumbers indicate clear hole doping of CuPc [51]. The peaks at 1333, 1444 and 1517 cm-1 do not show significant shift. This indicates that involvement of pyrrolic nitrogen atoms in the interaction are stronger than that from the bridging nitrogen and carbon atoms in the macrocycle ring.

Fig.3 Raman spectrum of a) GO, b) CuPc, c) NC-A and d) NC-B. Charge transfer interactions revealed by FTIR and Raman studies are further supported by XPS measurements. Fig. 4a shows the C1s spectrum of pristine GO that is deconvoluted into 5 peaks at 284.6, 285.4, 286.1, 287 and 289 eV assigned to sp2 hybridization of carbon atoms, hydroxyl (C-OH), epoxy (C-O-C), carbonyl (>C=O) and carboxyl (HO-C=O) respectively [29,48,52].

Fig. 4 XPS C1s spectrum of a) GO, b) NC-Aand c) NC-B, N1s spectrum of d) CuPc, e) NCA and f) NC-B. The deconvoluted C1s peaks in the nanocomposites NC-A and NC-B are shown in Fig. 4b and 4c. In NC-A, while there is no significant shift in hydroxyl peak, epoxy, carbonyl and carboxyl peaks shift to lower energy by 0.2, 0.5 and 0.8 eV respectively. In the case of NC-B, hydroxyl group exhibits a down shift by 0.2 eV and other peaks mentioned above shift by 0.4, 0.5 and 0.4 eV respectively. This shows that charge transfer interaction of GO with CuPc is found to vary with concentration of the former and is found to occur majorly through carbonyl and carboxyl groups of GO. The N1s and Cu 2p spectrum of pristine CuPc, NC-A and NC-B are shown in Fig. 4d-f and Fig. 5 respectively. The N1s peak of CuPc deconvoluted into two components ascribed to Cu-N and C-N bonds are observed at 398.6 and 399.5 eV (Fig. 4d). While the Cu-N component is found to shift by about 0.2 eV to higher binding energy in nanocomposite A, both Cu-N and C-N component of nanocomposite B are shifted by 0.6 and 0.5 eV respectively (Fig. 4e and f). The Cu 2p1/2 and 2p3/2 peaks of CuPc (Fig. 5a) are shifted by 0.2 and 0.4 eV in NC-A (Fig. 5b) and by 0.5 and

0.6 eV in NC-B (Fig. 5c), respectively, to higher binding energy. These shifts clearly imply charge transfer from CuPc to GO through Cu and N atoms of CuPc. The present study shows clear interaction, though non-covalent, between unsubstituted CuPc to GO, as also reported by Yang et al [21]. But unlike the bi-directional charge transfer between CoPc and GO suggested in that study, the present one reveals only one way charge transfer from CuPc to GO.

Fig. 5 XPS- Cu 2p spectrum of a) CuPc, b) NC-A and c) NC-B. All the above results from XRD, FTIR, Raman and XPS measurements suggest that the CuPc molecules may not be intercalating between the GO sheets, but could be interacting with the latter through mainly carboxylic groups at the edges of GO sheets.

3.2 Optical studies The UV-visible absorption spectrum of GO, CuPc, NC-A and NC-B are shown in Fig. 6a. The absorption spectrum of GO reveals two bands at 230 and 308 nm with the former being stronger and the latter appearing as shoulder. The former band corresponds to π-π* transitions of the aromatic planar network of C-C and C=C bonds of the sp2 hybrid regions and the latter due to n-π* transition of the sp3 regions [35,53]. The absorption spectrum of pristine CuPc shows two maxima of the Q bands at 648 (Q1) and 742 nm (Q2), occurring due to π-π* transitions. Another maximum corresponding to B band is observed at 379 nm [21,33,43]. On mixing with GO, the Q1 and Q2 bands of CuPc are respectively blue shifted to 638 and 718 nm for NC-A and to 631 and 715 nm for NC-B. Absorption spectrum of samples 1 to 7, revealing similar behaviour, are shown in Fig. S2 in supplementary material. Fig. 6b shows the peak positions of Q1 and Q2 with increasing GO:CuPc weight ratio including that of samples 1 to 7, and Fig. 6c shows the absorbance ratio of Q1 and Q2 with respect to the same. It can be seen that the band positions systematically shift to lower wavelength and the intensity ratio (Q1/Q2) increases with increase in GO weight ratio, tending towards saturation at higher ratios. Many studies report red shift of the Q bands of MPcs on interaction with GO, contrary to that observed here [21,32,46]. Das et al have mentioned blue shift of the absorption band of Pcs on interaction with RGO, but they have restricted their discussion to B band of the same and not mentioned anything about the Q bands [16]. Phthalocyanines are reported to undergo strong aggregation depicted as coplanar interaction between the molecules, progressing from monomer to dimer and higher order arrangement through hydrogen bonds, electrostatic interaction, π-π interaction etc. Depending on their aggregation state, the electronic structure of the Pc molecules is reported to be modified, as predicted by the molecular exciton theory based on excited state resonance interaction between the molecules [54,55]. The transition dipole moments are said to be strongly coupled. Depending on the type of stacking between CuPc molecules or the angle between the coupled transition dipole moments and the line joining the centre of the molecules, the absorption band shifts towards lower or higher wavelength side compared to that of the monomers. On non-covalent interaction between a pair of molecules, the ground state of the molecules remains localised, but the excited states undergo exciton splitting, thereby lowering the degeneracy of the excited state as compared to that of the monomer. When the molecules are stacked head-to-head (H aggregates), blue shift in the absorption

band occurs whereas head-to-tail stacking (J aggregates) of the molecules gives rise to red shift in the same, as per the allowed transitions from the ground state to the corresponding excited state [54].

Fig. 6 a) UV-visible absorption measurements of GO, CuPc, NC-A and NC-B. Inset shows the absorption spectrum of GO with larger range, b) Q1 and Q2 band positions as a function of GO:CuPc weight ratio, c) Absorbance ratio (Q1/Q2) as a function of GO:CuPc weight ratio. The monomer peak of CuPc is reported to occur at around 690 nm indicating that the red shift of the Q2 band with respect to this monomer peak is due to J aggregation of the CuPc molecules and the blue shift of the Q1 band due to H aggregation of the same [33,56]. The higher intensity of the Q2 band than that of Q1 band suggests that more number of molecules are stacked head-to-tail in pristine CuPc. The blue shift of both the maxima and the change in the relative intensity of the two bands occurs, with Q2 decreasing and Q1 increasing in NC-A indicates that the CuPc molecules undergo change in the aggregation state on interaction with GO sheets, with more number of molecules rearranging themselves in cofacial manner (H aggregation) from their original slipped cofacial state. This behaviour appears to be more enhanced with increase in GO concentration (NC-B) where further blue shift of both the bands and increase in Q1/Q2 intensity ratio can be seen with the same. Similar behaviour was observed in our earlier study on CuPc interaction with ZnO [33].

It is a commonly known fact that the PL emission occurs due to the recombination of electrons and holes either through band gap state or from band to band states, implying that higher recombination rate results in higher PL [40]. Quenching of PL occurs when such recombination is hinderedeither by electron or energy transfer to the neighbouring material [4,46]. Therefore, PL spectroscopy has been identified as a useful tool to investigate charge transfer interaction between two materials in a nanocomposite. While zero band gap of graphene does not allow it to exhibit fluorescence easily, the heterogenous atomic and electronic structures enables GO to fluoresce in a wide wavelength ranges like near-infra red (NIR), visible and ultraviolet regions [34]. In the present study, GO exhibits weak shoulder like emission peaks at around 420, 540 and 603 nm (Fig.7). CuPc shows much stronger emission than GO at around 450 nm and weak shoulder like peaks at about 413 and 425 nm. It also shows broad and smaller bandsat 565 and 615 nm. In NC-A and NC-B, the emission bands of CuPc appear strongly quenched. Generally, in nanocomposites, PL quenching takes place due to electron or energy transfer from one material to another where the former is said to be dominating in most cases, the latter is possible if the absorption wavelength of acceptor and the emission wavelength of donor overlaps [16,32,57]. Since in our study, the absorption band of GO does not overlap with the emission band of CuPc, electron transfer can be considered to be causing this quenching. It can be seen that the band at around 565 nm strongly blue shifts to 555 and 544 nm in NC-A and NC-B respectively. Additionally, the relative intensity of this band with respect to that at 450 nm (having maximum intensity), seems to be increasing. Such effects on this band could be due to the change in the aggregation state of CuPc molecules. Aggregation of organic molecules is reported to be strongly affecting fluorescence in earlier studies [57-59]. While fluorescence is reported to be quenched for molecules arranged cofacial with increased π-π interaction, some other studies also report increase in fluorescence due to the restriction posed by the aggregated molecules to intramolecular rotation, thereby minimising the nonradiative recombination of the excitons [57-59]. This in-turn is suggested to enhance radiative recombination, as also observed in the increase in relative intensity of 565 nm band (Fig. 7). The blue shift of this band could be due to the formation of more H aggregated molecules.

Fig. 7 PL spectrum of a) GO, b) CuPc, c) NC-A and d) NC-B. The shift of both the absorption and fluorescence bands clearly indicate the influence of GO on the packing mode of CuPc molecules. As mentioned before, most of previous studies have reported red shift of the absorption bands of Pcs, which could be due to the fact that they ae covalently linked to GO/RGO. In such cases the movement of the Pc molecules are restricted and hence they are able to overlap sideways with the neighbouring Pc molecules, as suggested by Ashokkumar et al [57]. Such sideways overlap could be increasing J aggregation of Pc molecules causing red shift of the optical bands. In the present study, since the Pc molecules are non covalently interacting with GO, they can move freely and hence are able to arrange in face-to-face configuration, causing blue shift in the absorption bands. Such free movement could also be allowing the bunch of aggregated molecules to be arranged in face-to-edge manner, causing increase in relative intensity, as suggested by Li et al, for hexaphenylsilole (HPS) [59]. It should be pointed out here that having cofacial molecules and hence improved π-π stacking enhances intermolecular interaction, thereby improving charge carried mobility which is a prerequisite for many devices like organic field effect transistors (OFETs), organic light emitting diodes (OLEDs) etc. [60]. Additionally, co-existence of both J and H aggregated molecules can widen the range of absorption window leading to efficient light harvesting which could be very useful in optoelectronic applications like photovoltaic devices [61]. H aggregation, with the higher lowest excited level, is suggested to improve the driving force for exciton dissociation which could prove to be beneficial for solar cell applications.

3.3 Magnetic studies Temperature dependent susceptibility (𝜒 ― 𝑇) measurements carried out on GO, CuPc, NC-A and NC-B are shown in Fig. 8. Within the experimental precision, no difference was observed in the magnetization values between the Field Cooled (FC) and Zero Field Cooled (ZFC) measurements (FC not shown).

Fig. 8 Susceptibility as a function of temperature. Inset: Magnetization curve with respect to the applied field. The solid lines are Curie-Weiss and Brillouin (inset) fit. All the samples show paramagnetic nature and the temperature dependence is well explained by Curie-Weiss behaviour given by 𝐶

𝜒 = 𝜒0 + 𝑇 ― 𝜃𝐶𝑊

(1)

with a small temperature independent component 𝜒0. 𝜃𝐶𝑊 is the Curie-Weiss temperature indicative of strength of interaction between the magnetic moments. In the present study we observed 𝜃𝐶𝑊 values to be very close to zero indicating an absence of interaction among the magnetic moments.C is a fitting parameterfrom which the effective moment (𝜇𝑒𝑓𝑓) value can be obtained using the formula 𝜇𝑒𝑓𝑓 =

3𝑘𝐵𝐶 𝑁𝜇2𝐵

(2)

where 𝑘𝐵 is Boltzmann constant, 𝜇𝐵is Bohr magneton and N is the number of magnetic moments. In the case of CuPc, since the molecular mass is well known, N can be calculated directly from the mass of the sample. However, due to the uncertain molecular mass of GO, NC-A and NC-B, it is necessary to make an estimate of the number of magnetic moments in

each case by an alternate method. For this purpose, we have fitted the magnetization (M vs H) data measured at 5 K on GO, CuPc, NC-A and NC-B (inset in Fig. 8) with Brillouin function given by

[

𝑀 = 𝑀𝑠

(2𝐽 + 1) 2𝐽

coth

(

(2𝐽 + 1) 2𝐽

)

1

𝑥 ― 2𝐽coth

( 𝑥 )] 1

(3)

2𝐽

Here, 𝑀𝑠 = 𝑁𝑔𝜇𝐵𝐽 where 𝐽 is the total angular momentum, 𝑔 is Lande g-factor which is assumed to be 2 (spin only) and 𝑥 is the ratio of magnetic energy to thermal energy

(

𝑔𝐽𝜇𝐵𝜇0𝐻 𝑘𝐵𝑇

).

The above fit yields the two fitting parameters N and J. The N values obtained from the Brillouin function are used for estimating 𝜇𝑒𝑓𝑓 (eq. 2). The effective paramagnetic moment can also be calculated from J and is given by (4)

𝜇′𝑒𝑓𝑓 = 𝑔 𝐽(𝐽 + 1)

The 𝜇𝑒𝑓𝑓values obtained from the fitting of Curie-Weiss (eq.1) behaviour and the 𝜇′𝑒𝑓𝑓 (eqn. 4) obtained using J values from Brillouin fit (eq. 2) are summarized in Table 1. Table 1: Values obtained by fitting Curie-Weiss ((C), eff) and Brillouin (N, J) functions Sample

C(emu K/gOe)

N (per gram)

J

g

𝒆𝒇𝒇(𝝁𝑩) 𝝁′𝒆𝒇𝒇(𝝁𝑩)

CuPc

7.21E-04

1.01E+21

0.574

2

1.86

1.90

GO

2.00E-04

2.79E+19

2.53

2

5.87

5.97

NC-A

2.89E-04

1.22E+21

1.19

2

1.07

3.22

2.57E-04

7.03E+20

1.48

2

1.33

3.84

NC-B

In the case of GO and CuPc, the magnetization (M vs H) is well explained by the Brillouin function corresponding to the J values of 2.5 and0.5 (𝑔 assumed to be 2) respectively. For CuPc, the value of N obtained from the Brillouin fit matches with that estimated using the formula NA * mass/molar-mass, (where NA is the Avogadro Number). Also the 𝜇𝑒𝑓𝑓values (eq. 2) obtained using Curie-Weiss fit is consistent with 𝜇′𝑒𝑓𝑓 (eq. 4) estimated using J values from Brillouin fit.In the case of GO, the N value obtained from Brillouin fit is incorporated in eq. 2 to calculate 𝜇𝑒𝑓𝑓, whichis found to be very close to 𝜇′𝑒𝑓𝑓 obtained from eq. 4. These results validate our method for estimating the number of moments in the case of GO.While J ~ ½ for CuPc is in accordance with the spin only d9 configuration of Cu2+, the value of J ~ 5/2 for GOis sparsely reported [27].

Table1 shows the 𝜇𝑒𝑓𝑓 values for NC-A and NC-B to be significantly less than that of CuPc and GO. In the absence of any interaction between the two components, the 𝜇𝑒𝑓𝑓values are expected to be intermediate between those of pristine CuPc and GO, as revealed by 𝜇′𝑒𝑓𝑓 values. However, the observed results are in disagreement with this expectation. In order to analyze this situation, the magnetic moments are first calculated assuming the nanocomposites to be merely the mixture of the two components. Using the starting compositions in NC-A and NC-B, and the respective N and J values obtained from the Brillouin fit of the pristine samples, the saturation magnetization 𝑀𝑠 is calculated to be 4.59 emu/g and 3.28 emu/g for NC-A and NC-B respectively. But the observed saturation magnetization values are less than these calculated values (inset in Fig. 8). It can be pointed out that the magnetic moment associated with d9 configuration of Cu2+ is considered rather robust in organometallic complexes [62]. So the variation in the observed magnetization can be explained to be due to the possible reduction in the J values of the GO component of the nanocomposites, on their interaction with CuPc. In addition to this discrepancy in saturation magnetization observed for both the nanocomposite samples, NC-B is found to have larger 𝜇𝑒𝑓𝑓and 𝜇′𝑒𝑓𝑓 than NC-A. However, since the number of moments is less, the resultant saturation magnetization is less than NC-A. 3.4 Non-linear optics studies Charge transfer interaction between GO and phthalocyanines are also reported to strongly affect the optical limiting behaviour of GO by many groups [32,43,45,46]. In all these studies, the phthalocyanines are covalently linked to GO while the present study investigates the influence of non-covalent interaction between GO and CuPc on the optical limiting behaviour of GO. Fig. 9 depicts the laser light transmission characteristics of GO, CuPc and NC-A at different laser pulse energies. All the samples exhibit optical limiting transmittance with NC-A exhibiting improved optical limiting (OL) behaviour with a larger reduction in transmission than that for pristine GO and CuPc. This kind of optical limiting behaviour can arise due to non-linear scattering (NLS), reverse saturable absorption (RSA) brought about by excited state absorption (ESA), two photon absorption (TPA), or due to photon induced electron transfer (PET)/energy transfer (ET) [41,46]. Among these, PET/ET is said to occur when excited by lasers having pulse width in the range of a few nanoseconds. Since the laser used in the current study is in the

picosecond range, this process can be ruled out. Similarly, non-linear scattering is suggested to occur due to micro bubble formation in the solvents in which the material of interest is dispersed [46]. As in the present study drop cast thin films are used (absence of solvent), NLS effect can also be ignored. Hence OL behaviour could be attributed to either ESA or TPA. OL in phthalocyanines are reported to be caused by ESA while in the case of GO, isolated sp2 configurations in the band gap provided by sp3 matrix is suggested to induce ESA or TPA [45,46]. Increase in sp2 domains due to increased stacking of GO sheets (as seen from XRD results) could be enhancing the optical limiting behaviour in NC-A, as also mentioned by other studies [29,46]. The change in optical limiting behaviour in NC-A indicates clearly charge transfer interaction between GO and CuPc and also the change in stacking of GO. The study suggests that Pcs interacting non-covalently with GO can also cause improvement in OL, in a similar fashion as the covalently bonded Pcs [46].

Fig. 9 Transmission measurements of GO, CuPc and NC-A as a function of laser energy density. 4. Conclusions The non-covalent charge transfer interaction between GO and unsubstituted CuPc was studied by probing their structural, optical and magnetic properties. XRD results revealed the absence of intercalation of CuPc between GO sheets and on the contrary implied the increase in stacking of the latter. FTIR, Raman and XPS studies indicated charge transfer through the carboxyl groups at the edges of GO sheets, supporting the suggestion on the absence of intercalation. Optical properties of the nanocomposites mainly determined using UV-vis

absorption spectroscopy, revealed the change in the aggregation state of CuPc molecules from J state to H state, with their absorption Q bands exhibiting systematic blue shift and change in their relative intensity. This change was found to be with the change increasing with increase in concentration of GO. This suggests that interaction between GO and CuPc, even when non-covalent and non-intercalating, influence the aggregation state of each other strongly. The charge transfer interaction between the two materials also was found to influence the magnetisation of the nanocomposites, where the saturation magnetisation was found to be less than that expected from the two constituent materials individually. This was inferred to be due to reduction in spin angular momentum of GO on interaction with CuPc. Charge transfer interaction and the possible increase in stacking of GO was further confirmed by non-linear optical studies which showed improved optical limiting behaviour of the nanocomposites as compared to that of pristine GO and CuPc. In future, the tunability of optical and magnetic properties of these nanocomposites, through such charge transfer interaction of GO with macromolecules, can be employed for optoelectronic/non-linear optic and magnetic devices respectively. Acknowledgements The authors would like to thank Dr. T. V. Chandrasekhar Rao, Technical Physics Division, BARC, for helping in handling SQUID magnetometer. Funding sources No funding sources involved. References: [1] Y. Xue, Y. Liu, F. Lu, J. Qu, H. Chen, L. Dai, Functionalization of Graphene Oxide with polyhedral oligomericsilsesquioxane (POSS) for multifunctional applications, J. Phys. Chem. Lett., 3 (2012) 1607–1612. [2] G. I. C-Jiron, P. L-Plata, D. C-Arriagada, J. M. Seminario, Electron Transport Properties through Graphene oxide-cobalt phthalocyanine complexes, J. Phys. Chem. C, 117 (2013) 23664–23675. [3] S. K. Das, Chandra B. KC, K. Ohkubo, Y. Yamada, S. Fukuzumi, F. D’Souza, Decorating single layer graphene oxide with electron donor and acceptor molecules for the study of photoinduced electron transfer, Chem. Commun. 49 (2013) 2013–2015.

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

Non-covalent charge transfer interaction between GO and CuPc



CuPc aggregation change from J to H state, GO stacking increases



Charge transfer interaction modifies magnetic property of GO



Spin angular momentum of GO is inferred to be reduced on interaction with CuPc



Improvement in optical limiting behaviour in nanocomposites due to increased stacking of GO