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Diamagnetic Molecules Exhibiting Room-Temperature Ferromagnetism in Supramolecular Aggregates Barun Dhara, Plawan Kumar Jha, Kriti Gupta, Vimlesh Kumar Bind, and Nirmalya Ballav J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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The Journal of Physical Chemistry

Diamagnetic Molecules Exhibiting RoomTemperature Ferromagnetism in Supramolecular Aggregates Barun Dhara,§ Plawan K. Jha,§ Kriti Gupta, Vimlesh Kumar Bind, and Nirmalya Ballav* Department of Chemistry, Indian Institute of Science Education and Research (IISER), Pune – 411008, India

Author Information *[email protected]

ACS Paragon Plus Environment

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ABSTRACT: Molecule-based materials exhibiting room-temperature ferromagnetism and semiconducting property are promising for molecular spintronic applications. Chemically tunable electronic and magnetic properties of metallo-phthalocyanine (MPc) molecules make them potential candidates in the frame. Here, we show room-temperature ferromagnetism in supramolecular aggregates of two diamagnetic MPcs, nickel(II) phthalocyanine (NiPc; S=0) and zinc(II) hexadecafluorophthalocyanine (ZnFPc; S=0). In the magnetization versus applied field (M–H) plot, recorded at room-temperature, the supramolecular NiPc---ZnFPc aggregate revealed a clear hysteresis-loop with coercive field (Hc) of ∼180 Oe. The Hc values were further increased with decreasing the temperature down to 95 K. The direct current (DC) electrical conductivity value of the supramolecular NiPc---ZnFPc system was observed to be significantly higher than that of a mechanical mixture of NiPc+ZnFPc. An optical band-gap of ∼1.25 eV for the supramolecular solid was estimated from the Tauc plot and no appreciable charge-transfer interaction between NiPc and ZnFPc was detected. The origin of such unusual ferromagnetism is understood with the help of Goodenough-Kanamori-Anderson (GKA) empirical rules and the Zener model of sp-d exchange interaction.

1. INTRODUCTION In the domain of molecule/organic-based materials, metallo-phthalocyanines (MPcs) appear suitable candidates to deal with long-range magnetic order as well as semiconductivity in a single platform.1-3 The capability of MPcs to hold spin-bearing metal center in the core and to involve delocalized π-electrons of the macrocycle for intermolecular magnetic exchange interaction gives rise to a variety of magnetic properties in bulk and/or thin-films.4-6 Changing the metal ion, from chromium(II) phthalocyanine (CrPc) to manganese(II) phthalocyanine


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(MnPc), antiferromagnetic (AFM) exchange interaction switches to ferromagnetic (FM) exchange interaction at low-temperatures.6 Also, iron(II) phthalocyanine (FePc) is known to exhibit ferromagnetism at low-temperatures, whereas, under similar conditions, cobalt(II) phthalocyanine (CoPc), nickel(II) phthalocyanine (NiPc) and copper(II) phthalocyanine (CuPc) are antiferromagnetic, diamagnetic and paramagnetic, respectively.6 Apart from these attractive magnetic properties, MPcs show interesting optical properties by revealing characteristics Q- and Soret-bands (π-π* transitions) in the UV-vis absorption spectra.7-8 While the former-band is mainly sensitive to the nature of metal ion in the core, the latter-band can be highly-influenced by substituents on the periphery of Pc ring.7-8 In the solid-state, MPc can adopt various π-stacked structures depending on inter-planar distance and sliding angle which are commonly referred as α-, β-, γ-, δ-, ε-, ζ- and π- polymorphs.9 Also, peripheral substitution plays an important role in defining the polymorphic phase of MPc. These polymorphic phases of MPcs have distinctive electronic, magnetic and optical properties originating from various non-covalent interactions. The physical properties of MPcs can be further tuned by bringing two different MPc units in a single material (so called a binary combination), as elegantly shown by Torres and co-workers in a donor-acceptor aggregate of zinc(II) phthalocyanine (ZnPc) and NiPc derivates separated via vinylene spacer.10 Several binary combinations of commercially available MPcs, in particular, fluorinated and non-fluorinated, were explored to bring new properties in the resultant materials. For examples, (i) single-crystalline nanoribbons with p-n junction characteristic were fabricated upon deposition of copper(II) hexadecafluorophthalocyanine (F16CuPc) onto CuPc11; and (ii) formation of a hybrid-state at the heterojunction of cobalt(II) hexadecafluorophthalocyanine (F16CoPc) and MnPc was realized due to strong charge-transfer interaction at the interface (MnPcδ+/F16CoPcδ-).12 Apart from these hetero-structures, fluorinated MPcs were successfully


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used to generate various supramolecular 2D structures;13-14 of particular mention is chemically programmable chessboard-like spin-arrays of iron(II) hexadecafluorophthalocyanine (F16FePc; S=1) and MnPc (S=3/2).14 Recently, we have demonstrated room-temperature (RT) ferromagnetism in self-assembled 3D aggregates of paramagnetic FePc (S=1) and diamagnetic zinc(II) hexadecafluorophthalocyanine (F16ZnPc; S=0).15 Herein, we present supramolecular 3D aggregates of two diamagnetic molecules viz. NiPc (S=0) and ZnFPc (S=0) (see Figure 1 for molecular schemes and self-assembly) exhibiting ferromagnetism at RT. Our results showcase the first example on RT ferromagnetism in a material originating from the supramolecular arrangement of diamagnetic molecules and without the involvement of any appreciable charge-transfer interaction between the constituents.

2. EXPERIMENTAL Zinc(II) phthalocyanine (ZnPc), hexadecafluorinated zinc(II) phthalocyanine (ZnFPc), and nickel(II) phthalocyanine (NiPc) were purchased from Sigma-Aldrich (USA). Millimolar solutions of ZnFPc and NiPc were prepared in CHCl3, mixed together in a conical flask, kept the solution overnight at room-temperature, and precipitation took place. Under similar experimental conditions, no precipitation occurred from the mixed solution NiPc and ZnPc. The precipitate was filtered and thoroughly washed with CHCl3 to remove undesirable NiPc and ZnFPc, and then dried under vacuum. The obtained powder of supramolecular NiPc---ZnFPc aggregate as well as other samples were subsequently characterized15 viz. fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, matrix-assisted laser desorption-ionization time-of-flight (MALDI-TOF) mass spectrometry, differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD), field-emission scanning electron microscopy (FESEM), transmission


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electron microscopy (TEM), magnetization versus field (M-H) measurements, X-ray photoelectron spectroscopy (XPS), and electron spin resonance (ESR) spectroscopy. The direct current (DC) electrical conductivity values were measured on pressed-pellets by conventional four-probe method. Solid-state and liquid-state UV-vis spectra were also collected. From an extensive analysis of energy dispersive X-ray spectroscopy (EDXS) data, the elemental composition of Ni and Zn in supramolecular NiPc---ZnFPc sample was estimated to be ~1:1.

Figure 1. (a) Molecular schemes of NiPc (left) and ZnFPc (right). Pink and yellow balls are used to discriminate peripheral H and F atoms respectively. (b) Self-assembly of NiPc and ZnFPc (1:1; mol/mol) at room-temperature; solvent is CHCl3.

3. RESULTS AND DISCUSSION Equimolar solutions of NiPc and ZnFPc in CHCl3 were mixed in ∼1:1 (v/v) ratio and kept overnight at ambient conditions. Precipitation took place as a result of the spontaneous selfassembly of NiPc and ZnFPc. The isolated solid was named as supramolecular NiPc---ZnFPc aggregate. Such precipitation was neither observed in the individual solutions of NiPc, ZnPc, and ZnFPc, nor from the mixture of NiPc and ZnPc which clearly necessitates the role of peripheral F and H atoms in various non-covalent interactions (Figure S1). The presence of NiPc and


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ZnFPc in the self-assembled product was investigated by various complementary spectroscopic techniques and the composition was found to be ∼1:1. The morphological features of ZnFPc, NiPc, and NiPc---ZnFPc aggregate samples were evaluated by FESEM: (i) in case of ZnFPc, small crystallites were observed and can be assigned to short-range ordered α/γ -phase phthalocyanine (Figure 2a)9; (ii) twisted columnar crystallites of NiPc represent an α-phase material (Figure 2b)16; and (iii) micron-sized columnar crystallites of NiPc---ZnFPc aggregate is characteristic of a γ-phase material (Figure 2c).17 TEM image of NiPc---ZnFPc aggregate revealed the layered-structure originating mainly from the π-π stacking of the phthalocyanine rings (Figure 2d). A high-resolution TEM image showed molecular chains with inter-chain spacing of ~12.9 Å (Ni---Zn distance) which is close to the intermolecular separation of NiPc and ZnFPc in a γ-phase with a tilt angle of ~27° (Figure 2e).17 A homogeneous distribution of NiPc and ZnFPc in the self-assembled NiPc---ZnFPc aggregate was meticulously assessed by EDXS analysis and an elemental composition of ~1:1 was observed (Figure 2f and Figure S2; and Table S1). Existence of supramolecular NiPc---ZnFPc aggregate as a single-phase material was supported by DSC analysis (Figure S3): an equimolar mechanical mixture of NiPc+ZnFPc showed characteristic α→β/γ phase-transition of ZnFPc whereas no phase-transition was observed in the supramolecular NiPc---ZnFPc aggregate. Thus, our supramoleular approach indeed resulted in the formation of a single-phase material from a binary combination of MPcs. Room-temperature PXRD patterns of NiPc, ZnFPc, and NiPc---ZnFPc aggregate suggest the existence of isomorphic α-phase in all the three solid samples (Figure 2a-c and Figure S4). Due to the lack of X-ray quality single crystals, an indirect approach was adopted to look into the structural aspects of NiPc---ZnFPc aggregate. Experimental PXRD patterns of our MPc samples


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were compared with the patterns of MPcs available in the Cambridge Crystallographic Data Centre (CCDC) (Figure S5).17-23 Among various polymorphs of MPcs, β-phase is most stable due to an additional metal-ligand coordination interaction providing crystal-field stabilization energy as well as herring-bone arrangement of the π-stacked molecular columns bringing mechanical rigidity.9 In accordance with previous observations on Cl and F substituted MPcs, the feasibility of a β-phase for ZnFPc is enormously suppressed due to larger size of the peripheral atoms,9 also the PXRD pattern of ZnFPc is in good agreement with that of γ-CuFPc (Figure S5).17 Considering the isomorphic nature of NiPc and ZnFPc, supramolecular NiPc---ZnFPc aggregate is expected to be in the α-phase at room-temperature; however, γ-phase could favor more stability by allowing additional non-covalent interactions similar to those in the β-phase, as nicely described in the crystal packing of γ-CuFPc (Figure S5).17 Since formation of β-phase will be strongly repelled by the ZnFPc, the PXRD pattern of NiPc---ZnFPc aggregate showed much resemblance to the γ-phase than to the β-phase (Figure S5). The phase-change of NiPc in the course of self-assembly with ZnFPc was also detected in the FTIR spectra (Figure S6). In supramolecular NiPc---ZnFPc aggregate, some characteristic vibration modes were either vanished or blue-shifted with respect to α-phase NiPc. On particular mention are the peaks at ∼721 cm-1 (C-H out-of-plane deformation),24 ~1090 cm-1 (C-H in-plane deformation), ~1120 cm1

/ ~1469 cm-1 (C-H in-plane bending) and ~1288 cm-1 (C-N stretching in isoindole). Macrocycle

ring deformation involving C=C of NiPc (∼1610 cm-1) was also noticeable.24 Such a remarkable change in the FTIR spectra of NiPc in NiPc---ZnFPc aggregate could be because of the presence of additional non-covalent interactions viz. C-H---π and metal-ligand coordination (M-Nim; Nim is imine nitrogen atom). Upon coalescing the Raman modes of NiPc and ZnFPc molecules, we could not find any notable additions of auxiliary modes which suggest the absence of appreciable


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charge-transfer between NiPc and ZnFPc in the supramolecular NiPc---ZnFPc aggregate material – likewise our recently reported FePc---ZnFPc system (Figure S7 and Table S2).15

Figure 2. (a-c) Field-emission scanning electron microscopy (FESEM) images of ZnFPc (a), NiPc (b) and NiPc---ZnFPc aggregate (c). The white scale-bars are 500 nm in length. (d) Transmission electron microscopy (TEM) image of the layered-structure (from color contrast) of NiPc---ZnFPc crystallite. Insets: layered-structure in black-white contrast. The black scale-bar represents a length of 100 nm. (e) A high-resolution TEM image from the selected zone (white box in d) showing the uniformity in the sample and a spacing of ~12.9 Å (yellow dotted lines) in the (001) crystallographic direction indicative of an inter-chain distance between NiPc and ZnFPc chains. The scale bar is 10 nm. (f) Elemental analysis across a molecular crystal of NiPc--ZnFPc and an average value of Ni:Zn from twelve measurement spots (yellow dots) was estimated to be ~1:1. The scale bar is 250 nm.

Before we start discussing our results on the magnetism of bulk NiPc, let us present some frequently referred information on magnetic property of NiPc: (i) magnetic susceptibility measurements in 1939 (perhaps the first) claiming NiPc to be diamagnetic,25 (ii) report of diamagnetic anisotropy at room-temperature, however, clearly mentioning a major contribution of the temperature-independent paramagnetism,26 (iii) magnetic susceptibility measurements up to 77 K revealing diamagnetism of NiPc,21 (iv) theoretical calculations on discrete NiPc


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molecule27 showing spin-state of S=0, and (v) finally, two book-chapters on phthalocyaninebased magnets6 and magnetism of metal phthalocyanines28 mentioning diamagnetism of NiPc on account of their completely filled orbitals by citing earlier work. Our experience with magnetic characteristics of bulk NiPc in the temperature range of 2-300 K is unusually noteworthy and was found to be reproducible from different batches of NiPc sample bought from Sigma. In short, diamagnetism, paramagnetism and ferromagnetism – all three phases were observed in NiPc in the M-H plots measured at different temperatures (Figure S8). Such temperature dependent magnetic phase transitions, on the one hand, could well be related to various structural transitions as was nicely shown in the case of molecular thin-films of spin-bearing CuPc and MnPc revealing temperature-dependent ferromagnetism and antiferromagnetism3; and be understood in the framework of the spin-transition molecular materials.29 On the other hand, a recent report on an extensive evaluation of magnetic properties of bulk β-NiPc (by physical property measurements system PPMS, from Quantum Design) suggested that the unwanted ferromagnetism at 2 K could be due to the presence of Fe impurity.30 Notably however, such Fe impurity could not bring long-range magnetic order in β-NiPc, specifically ferromagnetism at room-temperature. In the present study, room-temperature M-H plots clearly show ferromagnetic ordering in supramolecular NiPc---ZnFPc aggregate whereas both NiPc and ZnFPc are found to be diamagnetic (Figure 3a). In line with the previous report on room-temperature ferromagnetism in inorganic-based systems like Mn-doped ZnO,31 we have employed appropriate diamagnetic correction in the M-H plot of NiPc---ZnFPc aggregate. As such the coercive magnetic field (Hc~180 Oe) and the saturation magnetization (Ms~0.003 emu/gm) values of molecule-based magnetic materials like NiPc---ZnFPc aggregate are comparatively better than those reported


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inorganic-based dilute magnetic semiconductors (DMSs). We have also recorded M-H plots at different temperatures and consistent appearance of hysteresis loops is a strong signature of intrinsic ferromagnetism in the supramolecular NiPc---ZnFPc aggregate (Figure 3b and Figure S9). Furthermore, coercive field vs. temperature plot (Hc-T) showed gradual increase of Hc from 180 Oe to 220 Oe upon decreasing the temperature from 300 K down to 95 K, which is typical of ferromagnetic materials (Figure 3b).

Figure 3. (a) M-H plots of ZnFPc (orange; filled-triangles down), NiPc (green; filled-triangles up), supramolecular NiPc---ZnFPc aggregate (Magenta; sphere) recorded in superconducting quantum interference device (SQUID) from Quantum Design at 300 K. (b) Coercive field (Hc) versus temperature plot obtained from M-H plots recorded at different temperatures from 300 K to 95 K.

Quenching of magnetic moments due to direct overlap of d-orbital(s) of adjacent metal centers (antiferromagnetic coupling) in the α-phase could be easily avoided in the γ-phase (or β-phase)


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(ferromagnetic coupling) and an elegant example in this respect is the α-MnPc and β-MnPc materials in bulk or thin-films.4, 32 Interestingly, both α-CoPc and β-CoPc exhibited the presence of antiferromagnetic molecular chains in bulk as well as in thin-films.33 Thus, if one assumes the paramagnetic metal centers in phthalocyanines to be the key sources of magnetic ordering, then extent of direct and indirect exchange paths need to be carefully taken into consideration along with the spin-multiplicity of the magnetic center. Let us present important bond lengths in the view of polymorphism and bulk magnetism of some metal-phthalocyanines taken from CCDC:1723

β-NiPc (Ni---Ni ~4.7 Å and Ni---Nim ~3.3 Å), γ-CuFPc (Cu---Cu ~ 4.7 Å and Cu---Nim ~ 3.3

Å) and α-CuPc (Cu---Cu ~ 3.7 Å and Cu---Nim ~4.23 Å). Note that the M---Nim bond length can serve as a good marker for the π-π stacking distance in the molecular stacks of metalphthalocyanines. In our PXRD data of NiPc---ZnFPc aggregate, a sharp peak at 2θ ≈ 29o, reflecting a π-π stacking distance of ~3.00 Å which was also present in the ZnFPc, and perhaps enabled NiPc to commensurate in course of their self-assembly (Figure S4). So as to say, a significant shortening of π-π stacking distance not only stabilized the molecular chains in the NiPc---ZnFPc aggregate, but also lead to the emergence of a tetragonally-distorted octahedral crystal field around Ni2+ ions which have been usually observed to be paramagnetic (discussed later). In the scenario of single-phase NiPc---ZnFPc aggregate, two distinctive molecular arrangements are apparently possible: (i) uniformly distributed and alternate π-stacks of NiPc and ZnFPc or (ii) molecular blending upon π-stacking of NiPc and ZnFPc on top of each other. We would like to preferably assign the former structure to the self-assembled NiPc---ZnFPc aggregate and a tentative scheme of molecular packing adopted from the γ-CuFPc structure17 is provided (Figure S10a). The sign of superlattice in the structure of NiPc---ZnFPc aggregate can


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be detected in the SAED pattern (Figure S10b) resembling similarity with the CuPcδ+/CuFPcδinterface11 and the overall pattern suggests a triclinic lattice.10, 34-35 Although it is difficult for us to unambiguously speculate the actual structures of NiPc, ZnFPc and NiPc---ZnFPc aggregate, however it is worthwhile to mention that powder NiPc and ZnFPc samples are commercially available robust materials and experimental data look alike from different batches of production from an internationally reputed chemical company like Sigma (Figure S11). Also, experimental results from various batches of self-assembled NiPc---ZnFPc aggregate remained almost identical (Figure S11). The origin of ferromagnetic ordering in NiPc---ZnFPc aggregate could be explained by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). The Zn2+ ion (3d10) is intrinsically diamagnetic in both square-planar and octahedral ligand-fields. Thus, paramagnetic Ni2+ ions (3d8) could be the driving force for the observed ferromagnetic ordering in the self-assembled aggregate of NiPc and ZnFPc. Ni2+ ion in a square-planar ligand field (D4h symmetry) like NiPc is usually diamagnetic (Figure 4a), whereas Ni2+ ion in an octahedral ligand-field (Oh symmetry, tetragonally distorted (TD)) is paramagnetic (Figure 4a). Such a diamagnetic to paramagnetic transition (so called spin-cross over phenomenon) should be accompanied by a significant change in the electronic configuration of Ni2+ ion i.e. from closedshell to open-shell. Earlier it has been shown that diamagnetic Ni2+ ion in porphyrin was converted to paramagnetic Ni2+ upon coordination with suitable neutral ligand, so called coordination-induced spin-crossover.36-38


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Figure 4. (a) Ni2+ ion (NiPc) with paired-electronic configuration (dxz)2, (dyz)2, (dz2)2, (dxy)2 , and (dx2-y2) (dx2-y2 orbital is empty) (left panel). Ni2+ ion (NiPc---ZnFPc) with unpaired-electronic configuration (dxy)2, (dxz)2, (dyz)2, (dz2)1, and (dx2-y2)1(dx2-y2 and dz2 orbitals are singly-occupied) (right panel). (b) X-ray photoelectron spectroscopy (XPS) signals revealing the presence of diamagnetic (closed-shell) and paramagnetic (open-shell) Ni2+ ions in NiPc (blue fitted-line) and NiPc---ZnFPc aggregate (red fitted-line), respectively (top panel). In XPS, 2p signal is doublet due to spin-orbit coupling (2p3/2 and 2p1/2) and for clarity Ni2p3/2 region is presented. Also, N1s XPS signals clearly indicate the coordination of Nim ligands (pink colored N atoms in molecular schemes above) to Ni2+ and Zn2+ ions, specifically the peak at ~401 eV (pink shed) is the signature of additional metal-N coordination interaction. (c) MALDI-TOF patterns of NiPc and NiPc---ZnFPc aggregate whereby additional Ni-N bonds are clearly detected in the latter; specifically, the m/z value characteristic of [N-NiPc-N] moiety is almost absent in pure NiPc. Note also that such characteristic m/z values were also observed around the dimeric NiPc peak in NiPc---ZnFPc aggregate.

To probe electronic structure of Ni2+ ions in NiPc and NiPc---ZnFPc aggregate, we have employed XPS to record the Ni2p spectra (Figure 4b). In case of NiPc, Ni2p3/2 signal can be almost fitted by a single-peak at ~854.7 eV and no multiplet feature was observed. On the contrary, a multiplet feature of the Ni2p XPS spectrum with main Ni2p3/2 signal at ~855.7 eV was observed for the NiPc---ZnFPc aggregate. These photoemission signatures are strong


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evidence towards the presence of diamagnetic and paramagnetic Ni2+ ion in NiPc and NiPc--ZnFPc aggregate materials, respectively.39-40 Such a significant change in the Ni2p XPS signal from single-peak to multiple spectral feature is reminiscent of diamagnetic to paramagnetic conversion of the Ni2+ ion in Ni-porphyrin upon coordination with neutral ligand ammonia (NH3) corroborating the Ni L-edge X-ray absorption as well as 2p XPS signals.36 In the case of thin-films of MnPcδ+/CoFPcδ- dimer, two-paramagnetic molecules (MnPc, S=3/2 and CoFPc, S=1/2) were leading to a blended material of S=2. Besides relative shift in binding energy positions, the open-shell electronic configurations (multiplet feature) of Mn2+ and Co2+ ions were not significantly affected, specifically in thin-films regime.12, 41 A strong charge-transfer between redox-active Mn and Co ions having a range of stable oxidation states42 was suggested in the MnPcδ+/CoFPcδ- dimer through direct overlap of out-of-plane d-orbital(s). However, in the present study, two diamagnetic molecules (NiPc, S=0 and ZnFPc, S=0) were resulting into a self-assembled ferromagnetic binary aggregate and in addition, closeshellopen-shell electronic configuration of Ni2+ ion was observed. Even if we assume that ZnFPc is acting as a mild oxidizing agent and leading to a configuration of NiPcδ+/ZnFPcδ- dimer via strong charge-transfer between Ni and Zn ions, it has two important implications. First, the delta positive charge would prefer to lie on the delocalized π-ring rather than on Ni ion, as was confirmed in the case of NiPc-I2 adduct.21. Second, Zn ion has been found to be much more stable in the 2+ instead of 1+ oxidation state with unusual Zn-Zn bond.42-43 In fact, upon mixing NiPc and ZnFPc solutions, no appreciable charge-transfer band could be detected in the UV-vis absorption spectra (Figure S12). Earlier, Miller and coworkers elegantly demonstrated ferromagnetism at room-temperature in a complex generated via strong charge-transfer interaction involving two diamagnetic species.44 The here observed self-assembly-induced spin-


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crossover at room-temperature is perhaps new to the existing literature and is attributed to the influence of additional non-covalent interaction involving F atoms that changed the sliding angle of NiPc close to that in the typical γ-phase stacking. Coordination of two imine-N atoms (Nim) to Ni2+ and Zn2+ ions is also evidenced from the N1s XPS signal (Figure 4b); specifically the peak at ~401.0 eV weighing ~25% of the total spectral intensity. Observation on the coordination of Nim ligands to the Ni2+ ion by XPS is truly complemented by the MALDI-TOF data (Figure 4c and Figure S13). Ionization pattern of the supramolecular NiPc---ZnFPc aggregate is appreciably different from that of pure NiPc, apart from the fact that in both patterns, characteristic monomeric and dimeric peaks at m/z~571 and m/z~1142 were observed. However, in case of the NiPc---ZnFPc aggregate sample additional peaks of equal intensity at m/z~571+14 and m/z~571+28 (also at m/z~1142+14 and m/z~1142+28) clearly indicated coordination of Nim ligands to the Ni2+ ion in the π-π stack of NiPc. On the contrary, such coordination of Nim ligands to Ni2+ ion was noted to be almost negligible for pure NiPc. Thus, NiPc moieties in the NiPc---ZnFPc aggregate are clearly in the arrangement likewise in the β- or γ-phase. In order to unambiguously assign the origin of ferromagnetic signature in the supramolecular NiPc---ZnFPc aggregate of two diamagnetic species, we have employed room-temperature ESR spectroscopy (Figure 5a). Intense paramagnetic signals with g=2.0 (free electron) for the so called diamagnetic NiPc and ZnFPc was realized. Such free radical ESR signals of NiPc (∆H ~4 mT) and ZnFPc (∆H ~10 mT) are not surprising and observed earlier for several metallophthalocyanines with spin-state of S=0, for examples, NiPc, ZnPc, PdPc, PtPc and H2Pc (metalfree phthalocyanine).45 The source of these electrons is not fully understood yet and attributed to modulation of ionization potential as well as electron affinity and/or oxygen impurity involved in


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charge-transfer reaction45; though ESR signals clearly show that that there are ‘free electrons’ in diamagnetic phthalocyanines.27,

46-51

Free electron like signals in ESR and clear diamagnetic

characteristics in the M-H plots of NiPc and ZnFPc suggests that in these molecules there are undesirable paramagnetic components. However, these paramagnetic components are suppressed in the M-H plots due to an averaging out by much stronger diamagnetic counter components of NiPc and ZnFPc. These weak paramagnetic signals of NiPc and ZnFPc are not the primary origin of long-range magnetic order observed at room-temperature (300 K) i.e. ferromagnetism in the supramolecular NiPc---ZnFPc aggregates. A mechanical mixture of 50% NiPc + 50% ZnFPc (mol/mol) remained diamagnetic at room-temperature (Figure S14a) and thereby clearly necessitates the concept of supramolecular organization. Also, we realized that a deliberate mixing of intrinsically paramagnetic FePc (S=1) species to diamagnetic ZnFPc (S=0) was unsuccessful in achieving ferromagnetism in a mechanical mixture of 5% FePc + 95% ZnFPc (mol/mol) at room-temperature; the mixture showed diamagnetic characteristic in the M-H plot (Figure S14b). Even, a mechanical mixture of 50% FePc + 50% ZnFPc (mol/mol) behaved as a paramagnetic material at 300 K and did not show any appreciable hysteresis-loop in the M-H plot; only the magnetization value was decreased in comparison to bulk FePc, as expected.15 In our study, two distinctive features in the ESR signals of the supramolecular NiPc---ZnFPc aggregate in comparison to those of NiPc and ZnFPc are noteworthy. Firstly, the line-width is subsequently increased and decreased with respect to NiPc and ZnFPc respectively, meaning that these electrons are strongly interacting in the NiPc---ZnFPc structure. In fact, the ESR signal of a mechanical mixture of NiPc+ZnFPc was found to be just a superimposition of the ESR signals of NiPc and ZnFPc. Secondly, appearance of an additional and relatively broad ESR signal for the case of supramolecular NiPc---ZnFPc aggregate at g=2.6 (absent in both NiPc and ZnFPc)


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reflects a clear signature of ferromagnetic interaction involving Ni2+ ions with S=1 in a tetragonally-distorted octahedral crystal field.52-53 In brief, only one ESR signal for Ni2+ ion with S=1 in perfect Oh crystal field is expected due to degeneracy in the spin-states whereas, in Oh (TD) crystal field two ESR signals are generated because of zero-field splitting (Figure 5a). Since the electrons in the open magnetic shell of Ni2+ in NiPc remained localized in d-orbitals (one electron each in dz2 and dx2-y2) and the so called ‘free electrons’ mostly resides in the delocalized sp-orbitals of Pc ring, spin-polarization of the carrier electron clouds around each localized-spin thus prevails due to sp-d interaction, as was originally proposed in the Zener model.54 To achieve a stable magnetic ground state, carrier electron clouds around diamagnetic ZnPc moieties subsequently favor ferromagnetic spin-polarization. Overall, long-range ferromagnetic order in the NiPc---ZnFPc aggregate is achieved due to sp-d exchange interactions involving localized electrons in the Ni2+ ions and the π-electron clouds around NiPc and ZnFPc enabling the semiconducting-band structure of NiPc---ZnFPc to be spin-polarized (Figure 5b).55 The material can be easily spin-coated on plastic-substrates like PET while keeping the structural integrity almost unaltered (Figure 6a). I-V characteristics of both NiPc and ZnFPc (Figure 6b) suggested Ohmic-type electrical conduction with conductivity in the order of ~10-10 S/cm – similar to high-band gap organic semiconductors like polyaniline.21,

56

It has been

demonstrated earlier that MPcs are excellent candidates for the fabrication of air-stable thin-film devices, for example, transistors, with reasonably good carrier mobility values.11, 57-60 In case of the supramolecular NiPc---ZnFPc aggregate, conductivity increased compared to individual molecules NiPc and ZnFPc; specifically, by ~100 times with respect to a mechanical mixture of NiPc+ZnFPc. To get an idea, we have employed solid-state UV-vis spectroscopy and Tauc plots are presented (Figure 6c) where Q-band (~1.85 eV) and Soret-band (~3.58 eV) are indicated. The


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energy gaps between HOMO and LUMO for NiPc, ZnFPc, and NiPc---ZnFPc samples were estimated to be ~1.18 eV, ~1.40 eV, and ~1.25 eV respectively, which are in close agreement with theoretical values of relevant MPcs.27

Figure 5. (a) X-band electron spin resonance (ESR) spectra of pure NiPc (green), pure ZnFPc (orange), self-assembled NiPc---ZnFPc aggregate (pink), mechanical mixture of NiPc+ZnFPc (grey) and blank (black) reported at room-temperature. Signatures of free electrons with g=2 and ferromagnetic open shell of Ni2+ ion with g=2.6 could only be detected in NiPc---ZnFPc. (b) A schematic representation of the spin-polarization of free electron clouds of NiPc (black arrow with green glow) and ZnFPc (black arrow with orange glow) around localized electron in Ni2+ open magnetic shell (glowing thick pink arrow) (left panel). As a result, the overall band structure of semiconductor NiPc---ZnFPc aggregate is spin-polarized (VB stands for valenceband and CB stands for conduction-band) and ferromagnetism thus prevails (right panel).

Based on all complementary sets of data, we have proposed a γ-phase model of the air-stable supramolecular NiPc---ZnFPc aggregate driven by various non-covalent interactions including π-


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π interaction, tilted edge-to-face Ar-H---π and Ar-F---π interactions,61-62 and metal-ligand bonding interaction.17 Ferromagnetic ordering can be understood upon combining the wellestablished Goodenough-Kanamori-Anderson (GKA) rules63 with the Zener model54 in magnetic exchange interaction. In line with the GKA rules, superexchange of two magnetic ions with partially filled d shells in 180o and 90o (magnetic ion–ligand–magnetic ion angle) configurations are strongly-antiferromagnetic and weakly-ferromagnetic, respectively. Thus in the present case, ca. 90o Ni2+---ligand (imine Nim---Ni2+) superexchange interaction is ferromagnetic. Two eg (dz2) orbitals of two nearest Ni2+ ions are coupled to orthogonal p-orbitals of the imine-N atom thereby making it impossible for the d-electrons to hop from one-site to the other-site (Figure S15) – likewise α-FePc5 with S=1 and β-MnPc4, 32 with S=3/2. As for the Zener model to take into account, the so called sp-d exchange interaction vis-à-vis positive and negative spinpolarizations in respective NiPc and ZnFPc layers is also expected from our recent quantum chemical calculations based on density functional theory (DFT+U).15

4. CONCLUSION In summary, we have explored supramolecular chemistry as a novel platform to bring longrange magnetic order in molecule-based semiconducting material, metallo-phthalocyanine (MPc). Two diamagnetic molecules, namely NiPc and ZnFPc, self-assembled to produce ∼1:1 supramolecular aggregate exhibiting ferromagnetism at room-temperature. The supramolecular NiPc---ZnFPc aggregate solid is air-stable alike its constituents, NiPc and ZnFPc. The origin of ferromagnetism is proposed to be due to tetragonal distortion of the Ni2+ centre of NiPc in the supramolecular configuration with ZnFPc as well as spin-polarization in the π-electrons of NiPc


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and ZnFPc, realised from ESR and XPS data. We foresee our approach to be beneficial in producing molecule-based materials for molecular spintronic applications.

Figure 6. (a) Powder X-ray diffraction pattern (PXRD) of NiPc---ZnFPc aggregate in the solidstate (red line) and in thin-film (blue line) configuration. Inset: spin-coated thin-film on PET sheet. (b) DC electrical characteristics of NiPc, ZnFPc, NiPc+ZnFPc and NiPc---ZnFPc aggregate. (c) Tauc plots of NiPc (green), ZnFPc (orange) and NiPc---ZnFPc aggregate (pink).


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ASSOCIATED CONTENT Supporting Information: This material is available free of charge via the Internet at http://pubs.acs.org. Additional Experimental Data on Self-Assembly, FESEM/EDXS, DSC, PXRD, FTIR, Raman, Magnetic measurements, TEM/SAED, MALDI-TOF, UV-vis and Schemes on Magnetism (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +91(0)20 2590 8215 (N.B.). Author Contributions §

Contributed equally.

Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS Financial support from IISER Pune; DST NanoMission (India; project number SR/NM/NS42); and DAE-BRNS (India; project number 2011/20/37C/17/BRNS) are thankfully acknowledged. B.D. thanks CSIR (India); P.K.J. and K.G. thank IISER Pune for Research Fellowships; and V.K.B. thanks DST (India) for Inspire Fellowship. N.B. thanks Dr. A. Banerjee (IUC-DAE Indore, India) and Dr. S. Nair (IISER Pune, India) for magnetic measurements; Dr. A. Dziwoki (Prevac, Poland) for providing XPS data; and Mrs. V. Vishvanathan (SAIF-IIT Bombay, India) for providing ESR spectra.


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