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Fabrication of High-Resolution 4,82-Type Archimedean Nanolattices Composed of Solution Processable Spin Cross-Over Fe(II) Metallosupramolecular Polymers Uppari Venkataramudu, Naisa Chandrasekhar, Supratim Basak, Muvva D. Prasad, Rajadurai Chandrasekar*
This paper presents the synthesis of two highly soluble Fe(II) metallosupramolecular polymers with two counter anions from a novel back-to-back coupled hybrid ligand. The spin cross-over (SCO) temperature of polymers with BF4 and ClO4 counter anions is T1/2 = 313 K and T1/2 = 326 K, respectively. By following the top-down approach, one of the polymers (with ClO4 counter anion) is successfully solution processed using a lithographically controlled wetting technique to create laser readable high-resolution Archimedean (4,82) nanolattices (consist of diamagnetic octagons and SCO squares). The thickness and top area of each SCO square are ≈75 nm and ≈2 × 2 μm2, respectively.
1. Introduction Spin cross-over (SCO) Fe(II) compounds act as inorganic spin switches by reversibly changing the electron configurations to high-spin (paramagnetic; S = 2; t2g4eg*2) or low-spin (diamagnetic; S = 0; t2g6 eg*0) states with respect to external stimuli such as temperature,[1–6] light irradiation,[7,8] pressure,[9,10] magnetic field,[11,12] and electric field.[13] The SCO phenomenon is very sensitive to the nature of ligands, counter anions, lattice solvents, polymorphism, and physical states (solid or solution).[1–7] A plethora of mono nuclear pseudo-octahedral Fe(II) complexes displaying various SCO curves and temperatures
U. Venkataramudu, Dr. N. Chandrasekhar, Dr. S. Basak, M. D. Prasad, Prof. R. Chandrasekar Functional Molecular Nano/Micro Solids Laboratory, School of Chemistry, University of Hyderabad, Prof. C. R Rao Road, GachiBowli 500046, India E-mail: [email protected]
are reported in the literature.[1–21] Recently, there is a substantial interest to exploit SCO complexes for applications such as displays, sensors, and digital memory using the bottom-up and top-down nanotechnology approaches.[1–3] To fabricate SCO devices, various lithographic techniques have been employed to pattern molecular compounds down to nano- or micro-scale on various surfaces.[19,22,23] In this context, Cavallini et al.[23b] have demonstrated the micro- and nanolithographic patterning of a classical cis-bis(thiocyanato)bis(1,10phenanthroline)iron(II) SCO compound (T1/2 = 176 K) into crystalline logical structures on a silicon surface. In such discrete systems, the solid- state intermolecular interactions (π-stacking, hydrogen bonding, and van der Waals interactions), play a crucial role in the transmission of magneto-elastic cooperative effects and subsequent magnetic bistability.[4] Processing point of view, compared to mononuclear SCO compounds, soluble coordination polymers are suitable for spintronic device fabrication due to their thermal stability, film-forming characteristics, and ability to form
Macromol. Rapid Commun. 2015, DOI: 10.1002/marc.201400628
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high-resolution patterns.[22–39] Recently, we have reported the fabrication of 1D micro arrays composed of a highly soluble Zn(II) coordination polymer (Mn ≈ 9600 g mol−1) using a solution lithography technique.[27] Conceptually, the introduction of metallosupramolecular interactions in SCO polymers is expected to increase SCO temperature and widen the hysteresis loop (ΔT1/2) in comparison to monomeric entities.[15,16] Therefore, the synthesis of novel solution processable metallosupramolecular SCO polymers and fabrication of technologically demanding high-resolution patterns are two essential steps to achieve SCO materialsbased miniaturized devices. For example, patterning of a laser readable SCO square nanolattice is quite promising strategy to implement ON (S = 2) or OFF (S = 0) spin states by using focused lasers. To achieve well separated SCO nanosquares in the range of laser spot size, creation of a highresolution 4,82-type Archimedean lattice (each vertices surrounded by one square and two octagons) on a laser transparent glass surface is an important strategy (see the box in Scheme 1).
Earlier, we have reported a zero-dimensional SCO complex, FeII[ethyl-2-(1-butyl-1H-1,2,3-triazol-4-yl)-6-(1Hpyrazol-1yl)isonicotinate]2(ClO4)2 exhibiting T1/2 at 287 K with a 10 K wide hysteresis loop.[40] To achieve soluble metallosupramolecular polymers, we have rationally designed a back-to-back coupled 2-(1-butyl-1H-1,2,3triazol-4-yl)-6-(1H-pyrazol-1-yl)pyridine (tpp) ligands bridged by a phenyl unit (L1) with soluble butyl chains and synthesized two extended Fe(II) SCO coordination polymers (Scheme 1). In this article, we report two novel aspects: i) first synthesis of a linear ditopic building block ligand (L1) from tpp units carrying butyl chains, and subsequent conversion L1 into two highly soluble linear metallosupramolecular SCO polymers {[FeII(L1)]n(X)2n}, where X = ClO4 (I) and BF4 (II); ii) demonstration of the solution processability of I by creating a ≈75 nm thick 4,82–type Archimedean lattice consisting of a well-separated SCO square of dimension ≈2 × 2 μm2 on a glass substrate using a cost-effective lithographically controlled wetting (LCW) technique.[22]
Scheme 1. Graphical representation of the high-resolution 4,82- type Archimedean nanolattices fabricated from solution processable spin cross-over Fe(II) coordination polymer.
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2. Experimental Section Compound 1 was procedures.[15,16,40]
achieved
as
per
our
reported
2.1. Synthesis of 1,4-Bis(2-(1-butyl-1H-1,2,3-triazol-4-yl)6-(1H-pyrazol-1-yl)pyridin-4-yl)benzene (L1) Compound 1 (0.2 g, 0.507 mmol), 1,4-phenylenediboronic acid (0.04 g, 0.246 mmol), and Pd(PPh3)4 (0.027 g, 0.023 mmol, 0.05 equiv) were suspended in degassed 1,4-dioxane (30 mL). Na2CO3 (2 M, 5 mL) was added to the mixture and heated at 70 °C for 3 d under nitrogen atmosphere. The course of the reaction was monitored by thin layer chromatography (1:19 methanol: CH2Cl2 (DCM)). After disappearance of the starting material, the reaction mixture was cooled to rt and then 1,4-dioxane was removed under vacuo. The resulting residue was treated with water and extracted with CH2Cl2 solvent. The separated organic layer was dried over MgSO4 and evaporated under vacuo. The orange residue was washed with a small amount of petroleum ether (b.p. 60 °C) to remove the colored impurities and isolate white powder of compound L1. Yield 48% (0.105 g). 1H NMR (400 MHz, CDCl -d , δ): 8.68 (s, 2H), 8.41 (s, 2H), 8.25 (s, 3 3 2H), 8.21 (s, 2H), 8.01 (s, 4H), 7.81 (s, 2H), 6.52 (s, 2H), 4.47–4.5 (t, 4H), 1.98–2.02 (q, 4H), 1.42–1.05 (q, 4H), 0.99–1.03 (t, 6H); 13C NMR (100 MHz, CDCl3-d3, δ): 151.9, 151.2, 149.4, 142.2, 138.6, 127.8, 127.1, 122.3, 115.5, 108.9, 107.7, 50.3, 32.3, 19.7, 13.5; IR (KBr): ν = 3112, 3084, 2953, 2920, 2871, 1610, 1550, 1512, 1446, 1391, 1210, 1068, 1046, 936, 893, 838, 783cm−1; ESI-MS m/z: [M+H] calcd for C34H34N12, 610.3 found 631.27. Anal. calcd for C34H34N12: C 66.87, H 5.61, N 27.52. found: C 68.72, H 5.68, N 27.61.
and Fe(BF4)2·6H2O (26.5 mg, 0.079 mmol) to get complex II as yellow powder. Yield 75%–80%. IR (KBr): ν = 1460, 1385, 1258 (aromatic C C), 1075 cm−1 (broad, B-F); Raman (488 laser, rt): 240, 413, 524, 968, 1028, 1264, 1303, 1374, 1445, 1619 cm−1; Magnetic Data (350 K): χmT = 3.37 emu K mol−1. Poly(dimethylsiloxane) (PDMS) stamps for lithography: Elastomeric PDMS (Sylgard 184 Down Corning) stamps were prepared by replica molding of a structured master (NTMDT AFM test gratings-TGX1). The curing process was carried out at 60 °C for 6 h. Once cured, the replica was carefully peeled-off from the master and used as such for micropatterning techniques.
3. Results and Discussion 3.1. Synthesis and Characterization To synthesize ditopic ligand L1, the key compound 1 was prepared from low-cost citrazinic acid as per our earlier reported procedure.[20] Ensuing Suzuki cross-coupling reaction of compound 1 with 1,4-phenyldiboronic acid provided the target ligand L1 in ≈50% yield (Scheme 2). The chemical structures of ligand L1 were confirmed unambiguously by 1H, 13C NMR spectroscopy, mass and elemental analyses (see the Supporting Information). The metallosupramolecular SCO Fe(II) polymer {[FeII(L1)]n(ClO4)2n} (I) and {[FeII(L1)]n(BF4)2n} (II) were synthesized from L1 by using the respective FeII salts in 2:1 DCM/MeOH solvent mixture to get brown- orange (I) and yellow (II) powders, respectively.
2.2. General Procedure for the Syntheses of Complex I and II and Soft Lithography In a 100 mL flask, L1 (50 mg, 0.082 mmol) was dissolved in DCM (10 mL). A clear methanolic (10 mL) solution of Fe(ClO4)2·xH2O (21 mg, 0.082 mmol) was added to the above solution. The mixture was heated to reflux for 6 h under nitrogen atmosphere. After cooling, the solvent was evaporated in vacuo. The solid residue was washed with diisopropyl ether (3 × 20 mL) and dried in vacuum to get complex I [FeII(L1)]n(ClO4)2n as orange brown powder. Yield 73%–80%. UV–vis (solid): λmax 453 nm (LMCT); IR (KBr): ν = 1460, 1385, 1258 (aromatic C C), 1080 cm−1 (broad, Cl O); Raman (488 laser, rt): 413, 524, 968, 1028, 1264, 1303, 1374, 1445, 1619 cm−1; Magnetic Data (350 K): χmT = 3.51 emu K mol−1; Anal. calcd for C34H34N12Cl2O8Fe•5CH3CN: C 48.99, H 4.50, N 21.76. found: C 50.28, H 4.29, N 20.61. Similarly, complex II {[FeII(L1)]n(BF4)2n} was prepared as per the above-mentioned procedure by using L1 (48 mg, 0.079 mmol)
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Scheme 2. Reaction scheme and conditions: a) [Pd(PPh3)4], K2CO3, 1,4-bisphenyleneboronicacid in 1,4-dioxane at 80 °C. b) for I: L1, Fe(ClO4)2, DCM/MeOH, 73%–80%. For II: L1, Fe(BF4)2 DCM/MeOH, 75%–80%.
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The solid-state UV–vis titration studies of the complexes showed emergence of a metal-to-ligand charge transfer (MLCT) band at 470 nm confirming the coordination of Fe2+ to L1 (Figure S2, Supporting Information). Laser Raman spectroscopy investigations of the solid complexes (I and II) showed identical spectra, but the spectra were significantly different from L1. For example, the 948 and 1002 cm−1 Raman peaks of L1 were shifted to high Figure 1. χT f(T) plot for the SCO polymers measured in the temperature range of frequencies at 968 and 1028 cm−1 after 2↔350 K in the cooling, and heating mode cycles with an applied DC magnetic field of the metallosupramolecular polymer 0.5 T. a) Complex I: {[FeII(L1)]n(ClO4)2n}, b) Complex II: {[FeII(L1)]n(BF4)2n}. formation. The low-intensity peak at 1386 cm−1 of L1 disappeared completely for II, the χmT value decreased sharply and reached the from the complexes with the appearance of high-intensity Raman shifts at 524, 1264, 1303, and 1374 cm−1 (Figure S5, value of about 0.1 emu K mol−1 at 2 K. Supporting Information). 3.3. Variable Temperature Raman Spectroscopy Studies 3.2. Bulk Magnetic Studies
SCO effect is normally accompanied by a change in the The temperature-dependent magnetic susceptibility data of average Fe N bond distance of ca. 0.2 Å because during polymers (I and II) recorded in both heating (↑) and cooling the LS- to HS-state transition, the average Fe N bond (↓) cycles in the range of 2↔350 K are shown in Figure 1. distance also correspondingly increases from 1.9 to The plot of χmT as a function of temperature (T) revealed 2.1 Å, due to the population of HS-state eg* orbitals. This reversible SCO behaviors of I and II with hysteresis loops average Fe N bond distance variation also useful to inves(Figure 1). The observation of two different SCO curves for tigate the slight structural alteration and the resultant I and II while changing the counter ions indicated the change in the bond polarizabilty between the HS- and LSsensitivity of the SCO phenomenon. Polymer I showed a state molecules. To investigate this vibration frequency χmT value of 3.51 emu K mol−1 at 350 K, which is almost changes associated with SCO effect, variable temperature solid-state Raman spectroscopy measurements of close to the expected value for a HS iron(II) ion in the S = complex I (thin pellet state) were performed using a con2 state (Figure 1a). The SCO temperatures for the heating focal Raman microscope set-up with an Ar+ 488 nm laser (↑T1/2 = 318 K) and cooling (↓T1/2 = 308 K) modes revealed the occurrence of a ca. 10 K wide thermal hysteresis (ΔT1/2) (power: 40 mW). The obtained Raman spectra at 77 and 400 K clearly showed peak shifts and intensity variations loop. For polymer II, the value of χmT was 3.37 emu K mol−1 between the HS and LS states (Figure 2). For example, an LSat 350 K, which is lower than the expected value for a HS state asymmetric band (high intensity at 520 cm−1) nearly iron(II) ion in the S = 2 state (Figure 1b). Additionally, the SCO temperatures for the heating (↑T1/2 = 336 K) and cooling splits into two equal intensity peaks in the HS state with a shift of one of the peaks to 531 cm−1. Two high-frequency (↓T1/2 = 315 K) modes revealed an occurrence of a ca. 21 K wide thermal hysteresis loop. The observed wide hysteresis loop in both polymers is probably due to the existence of weak intermolecular interactions such as H-bonds and π–π stacking. At very low temperatures, for I the residual fraction of HS Fe(II) ions showed the expected zerofield splitting (ZFS), leading to a slight decrease of the χmT value, reaching a minimum value of ≈0.3 emu K mol−1 at 2 K. The lowest χmT value (≈0.5 emu K mol−1) of the sluggish plateau indicated the presence of paramagnetic impurities or smaller amount of unswitched HS-state Figure 2. Variable temperature Raman spectra of a pellet-state complex {[FeII(L2)]n(ClO4)2n} I at 77 K and 400 K displaying selected regions. polymer fraction in the sample, whereas
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Fabrication of High-Resolution 4,82-Type Archimedean Nanolattices Composed of Solution. . .
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Figure 3. AFM investigations of growth of Archimedean (4, 82) nanolattice at different concentrations of SCO polymer ink. a) 2 mg/2 mL, b) 4 mg/2 mL, c) 6 mg/2 mL, d) 8 mg/2 mL.
LS state bands appeared at 656 and 1027 cm−1 were also shifted to 652 and 1022 cm−1 in the HS states. This result clearly supported the SCO effect observed in the bulk magnetic susceptibility measurements.
that different ink concentrations (2 mg/2 mL, 4 mg/2 mL, 6 mg /2 mL, and 8 mg/2 mL) were used for the optimization of the quality of the nanopatterns (see Figure 3). Only at a higher concentration of 8 mg/2 mL, the ink formed
3.4. Nanolattice Formation from Solution Processable Polymer 1 In the processing step, we thought of generating 4,82-type Archimedean-type nanosquare lattices from the SCO polymers by using a solution-based LCW technique[22c] (Scheme 1). At first, a novel PDMS stamp was prepared from a commercially available master having 4,82-type Archimedean lattice features, i.e., projected octagons and shallow squares. For the nanofabrication, a 20 μL dimethyl formamide (DMF) solution of complex I (ink) was drop casted on a clean glass slide. Afterward the PDMS stamp was gently placed on the solution meniscus, and the stamp was uniformly pressed with a weight of ≈10 g. This pressure allowed the solution to stay within the microsquare channels. As the solvent evaporated, the solute nicely precipitated onto the substrate only below the microchannels, forming a pattern that replicated the negative feature of the stamp. It is important to mention here that finding the right concentration of DMF solution of I is crucial to achieve high-resolution 4,82-type Archimedean nanolattice. For
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Figure 4. a) Confocal micrograph of nanolattice pattern. b,c) FESEM image displaying the Archimedean (4, 82) nanolattice. c–f) False color elemental mapping of a square (blue square) indicating the distribution of C, N and Fe elements.
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Figure 5. a,b) Laser confocal Raman microscopy 3D images of nanolattice. c) Raman spectrum from nanolattice composed of polymer I (bottom) and no Raman signal from the empty areas (top). d,e) Raman intensity profiles of nanolattices corresponding to the lines shown in a and b.
high-resolution patterns with uniform thickness covering large surface area as evidenced by the AFM images. The confocal optical microscope image of a 4,82-type Archimedean nanolattice showed projected SCO squares and empty octagons covering large surface area with identical periodicity (Figure 4a). Atomic force microscopy (AFM) investigations of the pattern revealed the formation of a nearly perfect SCO squares. The lateral dimension (length = a) of each square was 2 μm2 with a thickness of ≈75 nm (Figure 3d). The top surface area (a2) of each square was of about 4 μm. Each SCO square pattern (HS↔LS) was well separated from each other via octagonal empty spaces (diamagnetic). To verify the chemical composition of the square pattern, Field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), and Raman imaging and spectroscopy (back-scattering geometry) studies were performed. Compared with optical micrographs, the FESEM image clearly displayed the 4, 82-type Archimedean nanolattice patterns. Further elemental mapping of a single SCO square using EDS indicated the presence of C, N, and Fe elements (Figure 4). The confocal Raman spectroscopy and microscopy studies (488 Ar+ laser; 150× objective) of nanolattices showed a clear Raman spectrum matching with the bulk complex (Figure 5 c). From the Raman spectrum, a high intensity 1619 cm−1 peak (C C) was picked up and used as marker peak to generate 3D Raman images (Figure 5a, b). The resultant Raman images clearly showed square pattern composed of polymer I and the nearly octagonal dark areas without any Raman spectrum, indicating the accuracy and high resolution of the printing (Figure 5c). The intensity profile mapping (Figure 5d,e) clearly demonstrated the nearly equal Raman intensity from the squares
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made from polymer I indicating distribution of almost same amount of compound in each square.
4. Conclusion Two novel solution processable metallosupramolecular Fe(II) polymers were successfully synthesized from a back-to-back coupled (2-(1-butyl-1H-1,2,3-triazol-4-yl)-6-(1H-pyrazol-1-yl) pyridine ligand. The iron(II) polymers showed SCO effects with respect to temperature and displayed above room temperatures transitions. Fabrication of a novel SCO coordination polymer based 4,82-type Archimedean nanolattice was efficiently achieved by optimization of the amount of sample used in the top-down LCW technique. Laser Raman spectroscopy and imaging investigation of the thin (≈75 nm) nanolattice was succesfully carried out. This methodology can be applied to prepare soluble functional coordination polymers with diverse metal centers and their corresponding nanostructures. Additionally, the reversible HS and LS inter conversion of the nanolattice pattern can also be followed by Raman spectroscopy by laser scanning. Similarly one can realize a SCO nanodevice by point focusing laser beam of diameter in the range of 2 μm (corresponding to the each SCO square size) to generate heat and thereby selectively switch the spin states of each square from LS (at room temperature) to HS (laser heat) and vice versa.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Macromol. Rapid Commun. 2015, DOI: 10.1002/marc.201400628 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Acknowledgements: The authors thank the CSIR New Delhi (01/2409/10/EMR-II) for funding. Received: November 3, 2014; Revised: November 25, 2014; Published online: ; DOI: 10.1002/marc.201400628 Keywords: coordination polymers; Fe(II) nanopatterns; spin cross-over effects
ions;
lithography;
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Communication
Fabrication of High-Resolution 4,82-Type Archimedean Nanolattices Composed of Solution Processable Spin Cross-Over Fe(II) Metallosupramolecular Polymers Uppari Venkataramudu, Naisa Chandrasekhar, Supratim Basak, Muvva D. Prasad, Rajadurai Chandrasekar*
This paper presents the synthesis of two highly soluble Fe(II) metallosupramolecular polymers with two counter anions from a novel back-to-back coupled hybrid ligand. The spin cross-over (SCO) temperature of polymers with BF4 and ClO4 counter anions is T1/2 = 313 K and T1/2 = 326 K, respectively. By following the top-down approach, one of the polymers (with ClO4 counter anion) is successfully solution processed using a lithographically controlled wetting technique to create laser readable high-resolution Archimedean (4,82) nanolattices (consist of diamagnetic octagons and SCO squares). The thickness and top area of each SCO square are ≈75 nm and ≈2 × 2 μm2, respectively.
1. Introduction Spin cross-over (SCO) Fe(II) compounds act as inorganic spin switches by reversibly changing the electron configurations to high-spin (paramagnetic; S = 2; t2g4eg*2) or low-spin (diamagnetic; S = 0; t2g6 eg*0) states with respect to external stimuli such as temperature,[1–6] light irradiation,[7,8] pressure,[9,10] magnetic field,[11,12] and electric field.[13] The SCO phenomenon is very sensitive to the nature of ligands, counter anions, lattice solvents, polymorphism, and physical states (solid or solution).[1–7] A plethora of mono nuclear pseudo-octahedral Fe(II) complexes displaying various SCO curves and temperatures
U. Venkataramudu, Dr. N. Chandrasekhar, Dr. S. Basak, M. D. Prasad, Prof. R. Chandrasekar Functional Molecular Nano/Micro Solids Laboratory, School of Chemistry, University of Hyderabad, Prof. C. R Rao Road, GachiBowli 500046, India E-mail: [email protected]
are reported in the literature.[1–21] Recently, there is a substantial interest to exploit SCO complexes for applications such as displays, sensors, and digital memory using the bottom-up and top-down nanotechnology approaches.[1–3] To fabricate SCO devices, various lithographic techniques have been employed to pattern molecular compounds down to nano- or micro-scale on various surfaces.[19,22,23] In this context, Cavallini et al.[23b] have demonstrated the micro- and nanolithographic patterning of a classical cis-bis(thiocyanato)bis(1,10phenanthroline)iron(II) SCO compound (T1/2 = 176 K) into crystalline logical structures on a silicon surface. In such discrete systems, the solid- state intermolecular interactions (π-stacking, hydrogen bonding, and van der Waals interactions), play a crucial role in the transmission of magneto-elastic cooperative effects and subsequent magnetic bistability.[4] Processing point of view, compared to mononuclear SCO compounds, soluble coordination polymers are suitable for spintronic device fabrication due to their thermal stability, film-forming characteristics, and ability to form
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high-resolution patterns.[22–39] Recently, we have reported the fabrication of 1D micro arrays composed of a highly soluble Zn(II) coordination polymer (Mn ≈ 9600 g mol−1) using a solution lithography technique.[27] Conceptually, the introduction of metallosupramolecular interactions in SCO polymers is expected to increase SCO temperature and widen the hysteresis loop (ΔT1/2) in comparison to monomeric entities.[15,16] Therefore, the synthesis of novel solution processable metallosupramolecular SCO polymers and fabrication of technologically demanding high-resolution patterns are two essential steps to achieve SCO materialsbased miniaturized devices. For example, patterning of a laser readable SCO square nanolattice is quite promising strategy to implement ON (S = 2) or OFF (S = 0) spin states by using focused lasers. To achieve well separated SCO nanosquares in the range of laser spot size, creation of a highresolution 4,82-type Archimedean lattice (each vertices surrounded by one square and two octagons) on a laser transparent glass surface is an important strategy (see the box in Scheme 1).
Earlier, we have reported a zero-dimensional SCO complex, FeII[ethyl-2-(1-butyl-1H-1,2,3-triazol-4-yl)-6-(1Hpyrazol-1yl)isonicotinate]2(ClO4)2 exhibiting T1/2 at 287 K with a 10 K wide hysteresis loop.[40] To achieve soluble metallosupramolecular polymers, we have rationally designed a back-to-back coupled 2-(1-butyl-1H-1,2,3triazol-4-yl)-6-(1H-pyrazol-1-yl)pyridine (tpp) ligands bridged by a phenyl unit (L1) with soluble butyl chains and synthesized two extended Fe(II) SCO coordination polymers (Scheme 1). In this article, we report two novel aspects: i) first synthesis of a linear ditopic building block ligand (L1) from tpp units carrying butyl chains, and subsequent conversion L1 into two highly soluble linear metallosupramolecular SCO polymers {[FeII(L1)]n(X)2n}, where X = ClO4 (I) and BF4 (II); ii) demonstration of the solution processability of I by creating a ≈75 nm thick 4,82–type Archimedean lattice consisting of a well-separated SCO square of dimension ≈2 × 2 μm2 on a glass substrate using a cost-effective lithographically controlled wetting (LCW) technique.[22]
Scheme 1. Graphical representation of the high-resolution 4,82- type Archimedean nanolattices fabricated from solution processable spin cross-over Fe(II) coordination polymer.
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2. Experimental Section Compound 1 was procedures.[15,16,40]
achieved
as
per
our
reported
2.1. Synthesis of 1,4-Bis(2-(1-butyl-1H-1,2,3-triazol-4-yl)6-(1H-pyrazol-1-yl)pyridin-4-yl)benzene (L1) Compound 1 (0.2 g, 0.507 mmol), 1,4-phenylenediboronic acid (0.04 g, 0.246 mmol), and Pd(PPh3)4 (0.027 g, 0.023 mmol, 0.05 equiv) were suspended in degassed 1,4-dioxane (30 mL). Na2CO3 (2 M, 5 mL) was added to the mixture and heated at 70 °C for 3 d under nitrogen atmosphere. The course of the reaction was monitored by thin layer chromatography (1:19 methanol: CH2Cl2 (DCM)). After disappearance of the starting material, the reaction mixture was cooled to rt and then 1,4-dioxane was removed under vacuo. The resulting residue was treated with water and extracted with CH2Cl2 solvent. The separated organic layer was dried over MgSO4 and evaporated under vacuo. The orange residue was washed with a small amount of petroleum ether (b.p. 60 °C) to remove the colored impurities and isolate white powder of compound L1. Yield 48% (0.105 g). 1H NMR (400 MHz, CDCl -d , δ): 8.68 (s, 2H), 8.41 (s, 2H), 8.25 (s, 3 3 2H), 8.21 (s, 2H), 8.01 (s, 4H), 7.81 (s, 2H), 6.52 (s, 2H), 4.47–4.5 (t, 4H), 1.98–2.02 (q, 4H), 1.42–1.05 (q, 4H), 0.99–1.03 (t, 6H); 13C NMR (100 MHz, CDCl3-d3, δ): 151.9, 151.2, 149.4, 142.2, 138.6, 127.8, 127.1, 122.3, 115.5, 108.9, 107.7, 50.3, 32.3, 19.7, 13.5; IR (KBr): ν = 3112, 3084, 2953, 2920, 2871, 1610, 1550, 1512, 1446, 1391, 1210, 1068, 1046, 936, 893, 838, 783cm−1; ESI-MS m/z: [M+H] calcd for C34H34N12, 610.3 found 631.27. Anal. calcd for C34H34N12: C 66.87, H 5.61, N 27.52. found: C 68.72, H 5.68, N 27.61.
and Fe(BF4)2·6H2O (26.5 mg, 0.079 mmol) to get complex II as yellow powder. Yield 75%–80%. IR (KBr): ν = 1460, 1385, 1258 (aromatic C C), 1075 cm−1 (broad, B-F); Raman (488 laser, rt): 240, 413, 524, 968, 1028, 1264, 1303, 1374, 1445, 1619 cm−1; Magnetic Data (350 K): χmT = 3.37 emu K mol−1. Poly(dimethylsiloxane) (PDMS) stamps for lithography: Elastomeric PDMS (Sylgard 184 Down Corning) stamps were prepared by replica molding of a structured master (NTMDT AFM test gratings-TGX1). The curing process was carried out at 60 °C for 6 h. Once cured, the replica was carefully peeled-off from the master and used as such for micropatterning techniques.
3. Results and Discussion 3.1. Synthesis and Characterization To synthesize ditopic ligand L1, the key compound 1 was prepared from low-cost citrazinic acid as per our earlier reported procedure.[20] Ensuing Suzuki cross-coupling reaction of compound 1 with 1,4-phenyldiboronic acid provided the target ligand L1 in ≈50% yield (Scheme 2). The chemical structures of ligand L1 were confirmed unambiguously by 1H, 13C NMR spectroscopy, mass and elemental analyses (see the Supporting Information). The metallosupramolecular SCO Fe(II) polymer {[FeII(L1)]n(ClO4)2n} (I) and {[FeII(L1)]n(BF4)2n} (II) were synthesized from L1 by using the respective FeII salts in 2:1 DCM/MeOH solvent mixture to get brown- orange (I) and yellow (II) powders, respectively.
2.2. General Procedure for the Syntheses of Complex I and II and Soft Lithography In a 100 mL flask, L1 (50 mg, 0.082 mmol) was dissolved in DCM (10 mL). A clear methanolic (10 mL) solution of Fe(ClO4)2·xH2O (21 mg, 0.082 mmol) was added to the above solution. The mixture was heated to reflux for 6 h under nitrogen atmosphere. After cooling, the solvent was evaporated in vacuo. The solid residue was washed with diisopropyl ether (3 × 20 mL) and dried in vacuum to get complex I [FeII(L1)]n(ClO4)2n as orange brown powder. Yield 73%–80%. UV–vis (solid): λmax 453 nm (LMCT); IR (KBr): ν = 1460, 1385, 1258 (aromatic C C), 1080 cm−1 (broad, Cl O); Raman (488 laser, rt): 413, 524, 968, 1028, 1264, 1303, 1374, 1445, 1619 cm−1; Magnetic Data (350 K): χmT = 3.51 emu K mol−1; Anal. calcd for C34H34N12Cl2O8Fe•5CH3CN: C 48.99, H 4.50, N 21.76. found: C 50.28, H 4.29, N 20.61. Similarly, complex II {[FeII(L1)]n(BF4)2n} was prepared as per the above-mentioned procedure by using L1 (48 mg, 0.079 mmol)
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Scheme 2. Reaction scheme and conditions: a) [Pd(PPh3)4], K2CO3, 1,4-bisphenyleneboronicacid in 1,4-dioxane at 80 °C. b) for I: L1, Fe(ClO4)2, DCM/MeOH, 73%–80%. For II: L1, Fe(BF4)2 DCM/MeOH, 75%–80%.
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The solid-state UV–vis titration studies of the complexes showed emergence of a metal-to-ligand charge transfer (MLCT) band at 470 nm confirming the coordination of Fe2+ to L1 (Figure S2, Supporting Information). Laser Raman spectroscopy investigations of the solid complexes (I and II) showed identical spectra, but the spectra were significantly different from L1. For example, the 948 and 1002 cm−1 Raman peaks of L1 were shifted to high Figure 1. χT f(T) plot for the SCO polymers measured in the temperature range of frequencies at 968 and 1028 cm−1 after 2↔350 K in the cooling, and heating mode cycles with an applied DC magnetic field of the metallosupramolecular polymer 0.5 T. a) Complex I: {[FeII(L1)]n(ClO4)2n}, b) Complex II: {[FeII(L1)]n(BF4)2n}. formation. The low-intensity peak at 1386 cm−1 of L1 disappeared completely for II, the χmT value decreased sharply and reached the from the complexes with the appearance of high-intensity Raman shifts at 524, 1264, 1303, and 1374 cm−1 (Figure S5, value of about 0.1 emu K mol−1 at 2 K. Supporting Information). 3.3. Variable Temperature Raman Spectroscopy Studies 3.2. Bulk Magnetic Studies
SCO effect is normally accompanied by a change in the The temperature-dependent magnetic susceptibility data of average Fe N bond distance of ca. 0.2 Å because during polymers (I and II) recorded in both heating (↑) and cooling the LS- to HS-state transition, the average Fe N bond (↓) cycles in the range of 2↔350 K are shown in Figure 1. distance also correspondingly increases from 1.9 to The plot of χmT as a function of temperature (T) revealed 2.1 Å, due to the population of HS-state eg* orbitals. This reversible SCO behaviors of I and II with hysteresis loops average Fe N bond distance variation also useful to inves(Figure 1). The observation of two different SCO curves for tigate the slight structural alteration and the resultant I and II while changing the counter ions indicated the change in the bond polarizabilty between the HS- and LSsensitivity of the SCO phenomenon. Polymer I showed a state molecules. To investigate this vibration frequency χmT value of 3.51 emu K mol−1 at 350 K, which is almost changes associated with SCO effect, variable temperature solid-state Raman spectroscopy measurements of close to the expected value for a HS iron(II) ion in the S = complex I (thin pellet state) were performed using a con2 state (Figure 1a). The SCO temperatures for the heating focal Raman microscope set-up with an Ar+ 488 nm laser (↑T1/2 = 318 K) and cooling (↓T1/2 = 308 K) modes revealed the occurrence of a ca. 10 K wide thermal hysteresis (ΔT1/2) (power: 40 mW). The obtained Raman spectra at 77 and 400 K clearly showed peak shifts and intensity variations loop. For polymer II, the value of χmT was 3.37 emu K mol−1 between the HS and LS states (Figure 2). For example, an LSat 350 K, which is lower than the expected value for a HS state asymmetric band (high intensity at 520 cm−1) nearly iron(II) ion in the S = 2 state (Figure 1b). Additionally, the SCO temperatures for the heating (↑T1/2 = 336 K) and cooling splits into two equal intensity peaks in the HS state with a shift of one of the peaks to 531 cm−1. Two high-frequency (↓T1/2 = 315 K) modes revealed an occurrence of a ca. 21 K wide thermal hysteresis loop. The observed wide hysteresis loop in both polymers is probably due to the existence of weak intermolecular interactions such as H-bonds and π–π stacking. At very low temperatures, for I the residual fraction of HS Fe(II) ions showed the expected zerofield splitting (ZFS), leading to a slight decrease of the χmT value, reaching a minimum value of ≈0.3 emu K mol−1 at 2 K. The lowest χmT value (≈0.5 emu K mol−1) of the sluggish plateau indicated the presence of paramagnetic impurities or smaller amount of unswitched HS-state Figure 2. Variable temperature Raman spectra of a pellet-state complex {[FeII(L2)]n(ClO4)2n} I at 77 K and 400 K displaying selected regions. polymer fraction in the sample, whereas
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Figure 3. AFM investigations of growth of Archimedean (4, 82) nanolattice at different concentrations of SCO polymer ink. a) 2 mg/2 mL, b) 4 mg/2 mL, c) 6 mg/2 mL, d) 8 mg/2 mL.
LS state bands appeared at 656 and 1027 cm−1 were also shifted to 652 and 1022 cm−1 in the HS states. This result clearly supported the SCO effect observed in the bulk magnetic susceptibility measurements.
that different ink concentrations (2 mg/2 mL, 4 mg/2 mL, 6 mg /2 mL, and 8 mg/2 mL) were used for the optimization of the quality of the nanopatterns (see Figure 3). Only at a higher concentration of 8 mg/2 mL, the ink formed
3.4. Nanolattice Formation from Solution Processable Polymer 1 In the processing step, we thought of generating 4,82-type Archimedean-type nanosquare lattices from the SCO polymers by using a solution-based LCW technique[22c] (Scheme 1). At first, a novel PDMS stamp was prepared from a commercially available master having 4,82-type Archimedean lattice features, i.e., projected octagons and shallow squares. For the nanofabrication, a 20 μL dimethyl formamide (DMF) solution of complex I (ink) was drop casted on a clean glass slide. Afterward the PDMS stamp was gently placed on the solution meniscus, and the stamp was uniformly pressed with a weight of ≈10 g. This pressure allowed the solution to stay within the microsquare channels. As the solvent evaporated, the solute nicely precipitated onto the substrate only below the microchannels, forming a pattern that replicated the negative feature of the stamp. It is important to mention here that finding the right concentration of DMF solution of I is crucial to achieve high-resolution 4,82-type Archimedean nanolattice. For
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Figure 4. a) Confocal micrograph of nanolattice pattern. b,c) FESEM image displaying the Archimedean (4, 82) nanolattice. c–f) False color elemental mapping of a square (blue square) indicating the distribution of C, N and Fe elements.
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Figure 5. a,b) Laser confocal Raman microscopy 3D images of nanolattice. c) Raman spectrum from nanolattice composed of polymer I (bottom) and no Raman signal from the empty areas (top). d,e) Raman intensity profiles of nanolattices corresponding to the lines shown in a and b.
high-resolution patterns with uniform thickness covering large surface area as evidenced by the AFM images. The confocal optical microscope image of a 4,82-type Archimedean nanolattice showed projected SCO squares and empty octagons covering large surface area with identical periodicity (Figure 4a). Atomic force microscopy (AFM) investigations of the pattern revealed the formation of a nearly perfect SCO squares. The lateral dimension (length = a) of each square was 2 μm2 with a thickness of ≈75 nm (Figure 3d). The top surface area (a2) of each square was of about 4 μm. Each SCO square pattern (HS↔LS) was well separated from each other via octagonal empty spaces (diamagnetic). To verify the chemical composition of the square pattern, Field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), and Raman imaging and spectroscopy (back-scattering geometry) studies were performed. Compared with optical micrographs, the FESEM image clearly displayed the 4, 82-type Archimedean nanolattice patterns. Further elemental mapping of a single SCO square using EDS indicated the presence of C, N, and Fe elements (Figure 4). The confocal Raman spectroscopy and microscopy studies (488 Ar+ laser; 150× objective) of nanolattices showed a clear Raman spectrum matching with the bulk complex (Figure 5 c). From the Raman spectrum, a high intensity 1619 cm−1 peak (C C) was picked up and used as marker peak to generate 3D Raman images (Figure 5a, b). The resultant Raman images clearly showed square pattern composed of polymer I and the nearly octagonal dark areas without any Raman spectrum, indicating the accuracy and high resolution of the printing (Figure 5c). The intensity profile mapping (Figure 5d,e) clearly demonstrated the nearly equal Raman intensity from the squares
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made from polymer I indicating distribution of almost same amount of compound in each square.
4. Conclusion Two novel solution processable metallosupramolecular Fe(II) polymers were successfully synthesized from a back-to-back coupled (2-(1-butyl-1H-1,2,3-triazol-4-yl)-6-(1H-pyrazol-1-yl) pyridine ligand. The iron(II) polymers showed SCO effects with respect to temperature and displayed above room temperatures transitions. Fabrication of a novel SCO coordination polymer based 4,82-type Archimedean nanolattice was efficiently achieved by optimization of the amount of sample used in the top-down LCW technique. Laser Raman spectroscopy and imaging investigation of the thin (≈75 nm) nanolattice was succesfully carried out. This methodology can be applied to prepare soluble functional coordination polymers with diverse metal centers and their corresponding nanostructures. Additionally, the reversible HS and LS inter conversion of the nanolattice pattern can also be followed by Raman spectroscopy by laser scanning. Similarly one can realize a SCO nanodevice by point focusing laser beam of diameter in the range of 2 μm (corresponding to the each SCO square size) to generate heat and thereby selectively switch the spin states of each square from LS (at room temperature) to HS (laser heat) and vice versa.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
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Acknowledgements: The authors thank the CSIR New Delhi (01/2409/10/EMR-II) for funding. Received: November 3, 2014; Revised: November 25, 2014; Published online: ; DOI: 10.1002/marc.201400628 Keywords: coordination polymers; Fe(II) nanopatterns; spin cross-over effects
ions;
lithography;
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