Cu(II), and Zn(II) Binuclear Complexes as Pol - MDPI

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May 7, 2018 - IR(KBr): 1620 (s) (C=N), 1530 (s), 1548, 1385, 1334, 1248, 1204, 1045, ..... CB2 1EZ Cambridge, England, UK; Fax: (+44) 01223 336033;.
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Polarized Light-Induced Molecular Orientation Control of Rigid Schiff Base Ni(II), Cu(II), and Zn(II) Binuclear Complexes as Polymer Composites Hiroyuki Nakatori, Tomoyuki Haraguchi and Takashiro Akitsu * Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Tokyo 162-8601, Japan; [email protected] (H.N.); [email protected] (T.H.) * Correspondence: [email protected]; Tel.: +81-3-5228-8271 Received: 15 March 2018; Accepted: 23 April 2018; Published: 7 May 2018

 

Abstract: We have investigated linearly polarized UV light-induced molecular orientation due to Weigert effect of composite materials of new six binuclear nickel(II), copper(II), and zinc(II) complexes of two rigid Schiff base ring ligands (L1 and L2 ) composite materials with methyl orange (MO), an azo-dye, in polyvinylalchol (PVA) cast films. To compare the degree of molecular orientation, two ligands, namely flexible aliphatic cyclohexane (ML1 : NiL1 , CuL1 , ZnL1 ) and rigid aromatic (ML2 : NiL2 , CuL2 , ZnL2 ), were synthesized using amine moiety. We have also characterized these complexes by means of elemental analysis, IR, and UV-vis spectra, single crystal or powder X-ray diffraction (XRD) analysis, and so on. Composite materials of ML1 or ML2 +MO+PVA were also prepared to separately disperse the solutes in a polymer matrix. For any metal complexes, optical anisotropy (represented as the R parameters) of ML2 +MO+PVA was larger than ML1 +MO+PVA because of the rigidness of the ligands. Keywords: Schiff base; binuclear complex; nickel; copper; zinc; polarized light; PVA; methyl orange; Weigert effect

1. Introduction Nowadays, specific trans-cis photoisomerization of photochromic azobenzene (AZ) and its related compounds are academically and commercially interesting and have been widely studied to date. Many industrial applications of photoisomerization have been reported in the fields such as molecular devices, molecular switches, polymers, and biochemistry, etc. [1–3]. In particular, trans-cis photoisomerization of AZ may also lead to the appearance of nonlinear optics, anisotropy (dichroism), and birefringence [4,5], which must be important features in the field of photofunctional materials. Additionally, it is also well-known that azo-compounds show molecular reorientation in polymers due to Weigert effect [6,7] (namely polarized light induces optical anisotropy of azo-compounds directly). When linearly polarized (UV, and vis under a certain condition), light continues to be irradiated to azo-compounds under a restricted environment to some extent; azo-compounds repeat photoisomerization and finally arrange in one direction that does not absorb linearly polarized light. By such mechanism and reason, molecular angular mobility, as well as birefringence and dichroism, also emerged due to azo-compound exhibiting trans-cis photoisomerization in a polymer matrix [8]. Many studies on orientation control based on the reorientation of azo-compounds have been reported so far. The motivation of this work is to introduce such metal complexes into photosensitive polymer films, as follows. External (mechanical, electric, or magnetic) fields-induced molecular alignment of solutes in a matrix is a promising method for preparing switching or other functional materials

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beyond single crystalline materials in solid states [9,10]. Classically, planar metal complexes (lowered symmetry from D)D 4h ) may important example with which elucidate their electronic states symmetry from OhO tohhto Dto 4h may bebe anan important example with which to to elucidate their electronic states symmetry from O 4h ) may be an important example with which to elucidate their electronic (assignment d-d π-π* transitions and other established associated electronic structures) (assignment of of d-d and π-π* transitions and other established associated electronic structures) byby states (assignment ofand d-d and π-π* transitions and other established associated electronic structures) means of optical experiments such as polarized crystal (electronic) spectra [11]. However, spectral means of optical experiments such assuch polarized crystal (electronic) spectra [11]. However, by means of optical experiments as polarized crystal (electronic) spectra [11]. spectral However, measurements single crystals from a certain face (or along one two axis) are not agreement measurements forfor single crystals from a certain face along one or or two axis) areor not in in agreement spectral measurements for single crystals from a(or certain face (or along one two axis) are not with plane-of-coordination-sphere molecular coordinates about d-orbitals symmetry dueto to with plane-of-coordination-sphere molecular coordinates about d-orbitals symmetry due in agreement with plane-of-coordination-sphere molecular coordinates about d-orbitals symmetry orientation in the crystal packing of molecules. Thus, to overcome this experimental difficulty, orientation in the crystal packingpacking of molecules. Thus,Thus, to overcome this this experimental difficulty, due to orientation in the crystal of molecules. to overcome experimental difficulty, alignment-tunable (or at least alignment-oriented) molecules of planer metal complexes may be alignment-tunable (or at least alignment-oriented) molecules of planer metal complexes may bebe alignment-tunable (or at least alignment-oriented) molecules of planer metal complexes may preferable samples with which to study polarized spectral measurements. Moreover, such oriented preferable samples with which to to study polarized spectral measurements. Moreover, such oriented preferable samples with which study polarized spectral measurements. Moreover, such oriented −))in metal complexes (e.g., [CuCl ]22−in single crystals may also exhibit important other physical behavior metal complexes (e.g., [CuCl 4]2−4] ) single crystals may also exhibit important other physical behavior metal complexes (e.g., [CuCl in single crystals may also exhibit important other physical behavior 4 such as thermochromism, as well as optical properties [12]. In this context, organic/inorganic hybrid such as as thermochromism, asas well asas optical properties [12]. InIn this context, organic/inorganic hybrid such thermochromism, well optical properties [12]. this context, organic/inorganic hybrid materials of AZ and (planar) metal complexes deserve to be investigated by means of optical or other materials of of AZAZ and (planar) metal complexes deserve to to bebe investigated byby means of of optical or or other materials and (planar) metal complexes deserve investigated means optical other physicalexperiments. experiments.Organic/inorganic Organic/inorganic hybrid materials have advantage exhibiting physical Organic/inorganic hybrid materials have thetheadvantage of ofexhibiting physical experiments. hybrid materials have the advantage of exhibiting additional additional electronic properties due to inorganic metal complexes, which is difficult for organic additional electronic properties due to inorganic metal complexes, which is difficult for organic electronic properties due to inorganic metal complexes, which is difficult for organic photofunctional photofunctional polymers solely. However, light or laser (at the same time electric field) controlling photofunctional polymers solely. However, or same laser (at theelectric same time electric field) controlling polymers solely. However, light or laser light (at the time field) controlling of molecular of molecular alignment and its evaluation are not simple generally [13,14]. Based on aboveof alignment molecular alignment and its evaluation are not simple generally [13,14]. Based on thethe aboveand its evaluation are not simple generally [13,14]. Based on the above-mentioned factors, mentioned factors, we aimed to develop new functions and physical properties through the anisotropic mentioned factors, we aimed develop new and physical properties the anisotropic we aimed to develop newtofunctions and functions physical properties through the through anisotropic arrangement arrangement metal complexes thathave haveazo-components azo-components polymers. haveprepared prepared arrangement of ofmetal complexes that in in polymers. WeWe have of metal complexes that have azo-components in polymers. We have prepared organic/inorganic organic/inorganic composite materials of methyl methacrylate (PMMA) polymer films separately organic/inorganic composite materials of methyl methacrylate (PMMA) polymer films separately composite materials of methyl methacrylate (PMMA) polymer films separately that contain Schiff bases that contain Schiff bases Ni(II), Cu(II), Zn(II) complexes, and with various substituents [15–19]. that contain Schiff bases Ni(II), Cu(II), Zn(II) and AZAZ with various substituents [15–19]. Ni(II), Cu(II), Zn(II) complexes, and AZ withcomplexes, various substituents [15–19]. In recent years, much effort In recent years, much effort has been devoted to controlling optical anisotropy using not AZ and In has recent years, much effort has been devoted to controlling optical anisotropy using not AZ and been devoted to controlling optical anisotropy using not AZ and PMMA but water-soluble methyl PMMA but water-soluble methyl orange (MO) and polyvinyl alcohol (PVA) [20], by which PMMA but water-soluble methyl orange (MO) and polyvinyl alcohol (PVA) [20], by which orange (MO) and polyvinyl alcohol (PVA) [20], by which application in protein systems may be also application protein systems may also expected potentially. this research, herein, as molecular application in in protein systems bebe also expected potentially. In In this research, herein, asviewpoint molecular expected potentially. In this may research, herein, as molecular design comparison from the of design comparison from the viewpoint of receiving intermolecular interaction, we discuss the degree design comparison from the viewpoint of receiving intermolecular interaction, we discuss the degree receiving intermolecular interaction, we discuss the degree of induced optical anisotropy for binuclear of induced optical anisotropy for binuclear complexes that have flexible cyclohexane moieties (ML of complexes induced optical anisotropy binuclear complexes that flexible cyclohexane (ML 1) 1) that have flexiblefor cyclohexane moieties (ML (Scheme 1) and more rigidmoieties aromatic phenyl 1 ) have (Scheme 1) and more rigid aromatic phenyl moieties (ML 2 ) (Scheme 2) at the amine site. (Scheme 1) (ML and more rigid 2) aromatic phenyl moieties (ML2) (Scheme 2) at the amine site. moieties ) (Scheme at the amine site. 2

Scheme 1. Preparation Preparation of ML ML Scheme 1. Preparation of of ML 1. 1.. Scheme 1. 1

Scheme 2. Preparation Preparation of ML ML Scheme 2. Scheme 2. Preparation of of ML 2. 22.

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2. Materials and Methods 2.1. Preparations of Complexes All the reagents and solvents were of the highest commercial grade available (Aldrich, Wako, or TCI) were used as received without further purification. Those dissolved in methanol (20 mL), metal sources of chlorides (0.67 mmol), amines, and (1R,2R)-(−)-1,2-cyclohexanediamine for L1 or o-phenylenediamine for L2 (0.067 mmol) were refluxed for 1 day at room temperature to give rise to crude products. The solution was left overnight at room temperature. The resulting crude precipitates were filtered, thoroughly washed with methanol, and dried in vacuo. Only for CuL1 , the resulting precipitate was recrystallized in methanol and ethanol (1:1, v/v), taken in sample tube, and left alone for a few days to yield deep green single crystals suitable for single-crystal XRD analysis. NiL1 : Yield: 21.8%. Anal. Found: C, 51.31%; H, 4.70%; N, 8.72%. Calcd. for C28 H30 Cl2 N4 Ni2 O2 ·H2 O: C, 50.89%; H, 4.88%; N, 8.48%. IR(KBr); 1637 (s) (C=N), 1561 (s), 1481, 1446, 1334, 1236, 1055, 568(w), 3420(w) cm−1 . CuL1 : Yield: 13.9%. Anal. Found: C, 50.54%; H, 4.63%; N, 8.59%. Calcd. for C28 H30 Cl2 Cu2 N4 O2 ·H2 O: C, 50.15%; H, 4.81%; N, 8.36%. IR(KBr): 1626 (s) (C=N), 2364 (s), 2345(s), 1553, 1438, 1380, 1344, 1237, 1090, 1032, 889, 2930, 3434(w) cm−1 . ZnL1 : Yield: 20.5%. Anal. Found: C, 51.25%; H, 4.61%; N, 8.54%. Calcd. for C28 H30 Cl2 N4 O2 Zn2 : C, 51.25%; H, 4.61%; N, 8.54%. IR(KBr): 1623 (s) (C=N), 1544, 1430, 1378, 1347, 1230, 1086, 1028, 884, 761, 2922, 3424(w) cm−1 . NiL2 : Yield: 36.5%. Anal. Found: C, 61.04%; H, 2.88%; N, 9.87%. Calc. for C28 H18 Cl2 N4 Ni2 O2 : C, 53.32%; H, 2.88%; N, 8.88%. IR(KBr): 1625 (s) (C=N), 1536 (s), 1490, 1457, 1431, 1385, 1339, 1273, 1235, 1208, 1060, 909, 748, 3424 (w) cm−1 . CuL2 : Yield: 32.7%. Anal. Found; C, 52.51%; H, 2.83%; N, 8.75%. Calcd. for C28 H18 Cl2 Cu2 N4 O2 : C, 52.51%; H, 2.83%; N, 8.75%. IR(KBr): 1620 (s) (C=N), 1530 (s), 1548, 1385, 1334, 1248, 1204, 1045, 755, 3459 (w) cm−1 . ZnL2 : Yield: 32.7%. Anal. Found: C, 52.73%; H, 2.73%; N, 8.50%. Calcd. for C28 H18 Cl2 N4 O2 Zn2 : C, 52.21%; H, 2.82%; N, 8.70%. IR(KBr): 1623 (s) (C=N), 1552 (s), 1489, 1395, 1371, 1334, 1267, 1204, 1038, 760, 3451 (w) cm−1 . 2.2. Preparations of Composite Materials Aqueous solutions of each complex (0.1 mM), MO (0.1 mM), and PVA (10 wt%) were mixed and cast onto a slide glass on the hot plate (333 K) for about 60 min to obtains cast films of the composite materials (MLn +MO+PVA). 2.3. Physical Measurements Elemental analyses were carried out with a Perkin-Elmer 2400II CHNS/O analyzer Perkin-Elmer, Waltham, MA, USA) at Tokyo University of Science. Infrared (IR) spectra were recorded on a JASCO FT-IR 4200 spectrophotometer (JASCO, Tokyo, Japan) in the range of 4000–400 cm−1 at 298 K. (Polarized) electronic (UV-vis) spectra were measured on a JASCO V-650 spectrophotometer (JASCO, Tokyo, Japan) equipped with polarizer in the range of 800–220 nm at 298 K. Circular dichroism (CD) spectra were measured on a JASCO J-725 spectropolarimeter (JASCO, Tokyo, Japan) in the range of 800–200 nm at 298 K. Photo-illumination was carried out using a lamp (1.0 mW/cm2 ) with optical filters (UV λ = 200–400 nm), producing a sample using optical fibers and polarizer through optical filters. 2.4. X-ray Crystallography Deep greenish prismatic single crystals of CuL1 were glued on top of a glass fiber and coated with a thin layer of epoxy resin to measure the diffraction data. Intensity data were collected on a Bruker APEX2 CCD diffractometer (Bruker, Billerica, MA, USA) with graphite-monochromated Mo Kα

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radiation (λ = 0.7107 Å). Data analysis was carried out with a SAINT program package. The structures were solved by direct methods with a SHELXS-97, expanded by Fourier techniques, and refined by full-matrix least-squares methods based on F2 using the program SHELXL-97 [21]. An empirical absorption correction was applied by a program SADABS. All non-hydrogen atoms were readily located and refined anisotropic thermal parameters. All hydrogen atoms were located at geometrically calculated positions refined using riding models. Unfortunately, single crystals of4 ofthe Symmetry 2018, 10, x and FOR PEER REVIEW 10 other complexes could not be obtained. refined by full-matrix least-squares methods based on F2 using the program SHELXL-97 [21]. An The powder X-ray diffraction patterns for CuL2 and ZnL2 were collected at 298 K on Rigaku empirical absorption correction was applied by a program SADABS. All non-hydrogen atoms were Smart lab with CuKα radiation (λ = 1.5418 Å) at the University of Tokyo. Powder crystal structures readily located and refined anisotropic thermal parameters. All hydrogen atoms were located at were determined bycalculated means of Rietveld refinement using a Rigaku PDXL2 program package geometrically positions and refined using riding models. Unfortunately, single crystals (Rigaku, of Tokyo, the Japan). other complexes could not be obtained. The powder X-ray for CuL 2 and ZnL2 C were collected Rigaku Crystallographic datadiffraction for CuLpatterns 1826446): Cl2 Cu2atN298 , on Orthorhombic, 1 (CCDC 28 H30 4 O2K CuKα (λ = 1.5418 the University Powder P21 21 2 Smart (#18),labZwith = 4, a =radiation 13.841(15) Å, bÅ)=at24.705(3) Å, of c Tokyo. = 8.198(9) Å,crystal V =structures 2803.2(6) Å3 , were determined meansmm of Rietveld refinement using program package wR2 (Rigaku, −1 , F(000) ρcalc = 1.660 g/cm3 , µ by = 1.775 = 1432.0, S a=Rigaku 0.765,PDXL2 R1 [I>2σ(I)] = 0.0335, = 0.1096. Tokyo, Japan). Flack parameter = 0.014(10). Crystallographic data for CuL1 (CCDC 1826446): C28H30Cl2Cu2N4O2, Orthorhombic, P21212 (#18), Crystallographic data CuL2 (CCDC 1826434): H18 Cl2 Cu , Triclinic, (#2), Z = 2, 4O Z = 4, a = 13.841(15) Å , bfor = 24.705(3) Å , c = 8.198(9) Å , VC=282803.2(6) Å 3,2 N ρcalc =21.660 g/cm3, P-1 μ = 1.775 ◦ , β = 110.77(9)◦ , γ = 112.89(7)◦ , a = 7.750(4) Å, b = 8.460(4) Å, c = 15.256(7) Å, α = 85.59(12) −1 mm , F(000) = 1432.0, S = 0.765, R1[I>2σ(I)] = 0.0335, wR2 = 0.1096. Flack parameter = 0.014(10). V = 2510(6)Crystallographic Å3 , Rwp = 5.28%. data for CuL2 (CCDC 1826434): C28H18Cl2Cu2N4O2, Triclinic, P-1 (#2), Z = 2, a = 7.750(4) Å , b = 8.460(4) , c =ZnL 15.256(7) Å , α =1826435): 85.59(12)°, C β 28 = 110.77(9)°, V = 2510(6) Å 3, Z = 2, Crystallographic dataÅfor H18 Cl2 N4γO=2 112.89(7)°, Zn2 Triclinic, P-1 (#2), 2 (CCDC ◦ ◦ R wp = 5.28%. a = 13.87(4) Å, b = 14.56(4) Å, c = 13.18(4) Å, α = 85.19(15) , β = 110.60(17) , γ = 112.84(14)◦ , Crystallographic data for ZnL2 (CCDC 1826435): C28H18Cl2N4O2Zn2 Triclinic, P-1 (#2), Z = 2, a = V = 2293(11) Å3 , Rwp = 4.25%. (See Supplementary Materials) 3 13.87(4) Å , b = 14.56(4) Å , c = 13.18(4) Å , α = 85.19(15)°, β = 110.60(17)°, γ = 112.84(14)°, V = 2293(11) Å , Rwp = 4.25%. (See Supplementary Materials)

3. Results

3. Results

3.1. Spectral Characterization 3.1. Spectral Characterization Figure 1 depicts CD spectra of chiral complexes (ML1 ). As prepared using chiral amine, it could Figure 1 depicts spectra of chiralproducts. complexesSince (ML1).these As prepared usingafford chiral amine, it could be confirmed that they areCD optically active complexes identical structures be confirmed that they are optically active products. Since these complexes afford identical structures except for metal ions, the spectral difference is attributed to transition associated with metal ions for metal ions, thecoordination spectral difference is attributed to transition associated with metal ions (d-d) orexcept between metal and atoms (CT) transitions mainly. The reason why the(d-spectra d) or between metal and coordination atoms (CT) transitions mainly. The reason why the spectra in in solutions are slightly different from that in solid state (KBr) may be artifact peaks due to LD solutions are slightly different from that in solid state (KBr) may be artifact peaks due to LD components besides CD components [22]. components besides CD components [22].

Figure 1. Cont.

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Figure 1. CD spectra in aqueous solutions (blue) and KBr pellets (orange) for chiral complexes (NiL1, Figure 1.1.CD spectra FigureCuL CD spectrain inaqueous aqueoussolutions solutions(blue) (blue)and andKBr KBrpellets pellets(orange) (orange)for forchiral chiralcomplexes complexes(NiL (NiL11,, 1, and ZnL1). CuL , and ZnL ). 1 1 CuL1, and ZnL1).

3.2. Structural Characterization

3.2. Characterization 3.2.Structural Structural Characterization Figures 2–4 exhibits crystal structures of CuL1, CuL2, and ZnL2, respectively. Each metal ion of binuclear chromophores affords a five-coordinated square pyramidal coordination geometry, in ion Figures 2–4 exhibits structures ofofCuL ZnL Each 22,,and Figures 2–4 exhibitscrystal crystal structures CuL1 ,1,CuL CuL and ZnL22,, respectively. respectively. Eachmetal metal ionof of which metal ions are slightly lifted up towards the apical Cl atom from the [MN 2O2] basal planes. binuclear chromophores affords a five-coordinated square pyramidal coordination geometry, in which binuclear chromophores affords a five-coordinated square pyramidal coordination geometry, in Both apical Cl atoms are located on the opposite sides of a ligand plane in a symmetric manner. metal are ions slightly up towards apical Cl from the [MN2 O2the ] basal planes. Bothplanes. apical whichions metal arelifted slightly upthe towards theatom apical Cl1 are atom [MN 2O2] basal Rigidity of six-membered C lifted ring derived from amines for CuL CuLfrom 2 resulted in conformational Cl atoms areCl located on the opposite sides of a ligand plane in a symmetric Rigidity of Both apical atoms located on the opposite a ligand plane are in amanner. symmetric manner. difference of wholeare ligands. Besides them, most ofsides bond of lengths and angles within common six-membered C ring derived from amines for CuL are CuL resulted in conformational difference 1 2 1 are CuL2 resulted in conformational Rigidity of six-membered ring derived for CuL ranges of the analogousCcompounds [20].from amines of whole ligands. Besides them, most of bondmost lengths and angles within common ranges of the difference of whole ligands. Besides them, of bond lengthsare and angles are within common analogous compounds [20]. ranges of the analogous compounds [20].

Figure 2. Crystal structures of CuL1 showing atom labeling schemes. Hydrogen atoms and crystalline water were omitted for clarity. Selected bond lengths [Å ] and angles [°]: Cu1-N2 = 1.900(3), Cu1-O2 = 1.917(3), Cu1-N4 = 1.919(4), Cu1-O1 = 1.928(3), Cu1-Cl2 = 2.5297(13), Cu1-Cu2 = 2.9823(7), Cu2-N3 = 1.912(4), Cu2-N1 = 1.917(4), Cu2-O2 = 1.917(3), Cu2-O1 = 1.926(3), Cu2-Cl = 2.5290(13), N2-Cu1-O2 = 160.33(15), N2-Cu1-N4 = 89.07(15), O2-Cu1-N4 = 92.47(14), N2-Cu1-O1 = 92.97(14), O2-Cu1-O1 = Figure2.2.Crystal Crystalstructures structuresofofCuL CuL11 showing showingatom atom labelingschemes. schemes.Hydrogen Hydrogenatoms atomsand andcrystalline crystalline Figure 78.19(12), N4-Cu1-O1 = 157.03(15), N3-Cu2-N1 = labeling 88.85(15), N3-Cu2-O2 = 92.52(14), N1-Cu2-O2 = ◦ ]: Cu1-N2 water162.59(16), were omitted for clarity. Selected bond lengths [Å ] and angles [°]: Cu1-N2 = 1.900(3), = water were omitted for clarity. Selected bond lengths [Å] and angles [ = 1.900(3), N3-Cu2-O1 = 155.27(15), N1-Cu2-O1 = 93.46(14), O2-Cu2-O1 = 78.23(12), Cu1-O2-Cu2Cu1-O2 = 1.917(3), Cu1-N4 = 1.919(4), Cu1-O1 = 1.928(3), Cu1-Cl2 = 2.5297(13), Cu1-Cu2 = 2.9823(7), Cu2-N3 Cu1-O2102.12(14), = 1.917(3), Cu1-N4 == 101.42(13). 1.919(4), Cu1-O1 = 1.928(3), Cu1-Cl2 = 2.5297(13), Cu1-Cu2 = 2.9823(7),= Cu2-O1-Cu1

1.912(4),=Cu2-N1 1.917(4), = Cu2-O2 = 1.917(3), = 1.926(3), = 2.5290(13), N2-Cu1-O2 Cu2-N3 1.912(4),= Cu2-N1 1.917(4), Cu2-O2 Cu2-O1 = 1.917(3), Cu2-O1Cu2-Cl = 1.926(3), Cu2-Cl = 2.5290(13),= 160.33(15), N2-Cu1-N4 89.07(15), O2-Cu1-N4 92.47(14), N2-Cu1-O1 92.97(14), O2-Cu1-O1 N2-Cu1-O2 = 160.33(15),= N2-Cu1-N4 = 89.07(15),= O2-Cu1-N4 = 92.47(14),= N2-Cu1-O1 = 92.97(14),= 78.19(12), N4-Cu1-O1 157.03(15), N3-Cu2-N1 88.85(15), N3-Cu2-O2 92.52(14), N1-Cu2-O2 O2-Cu1-O1 = 78.19(12),= N4-Cu1-O1 = 157.03(15),= N3-Cu2-N1 = 88.85(15),= N3-Cu2-O2 = 92.52(14),= 162.59(16), N3-Cu2-O1 155.27(15), N1-Cu2-O1 93.46(14), O2-Cu2-O1 = 78.23(12), Cu1-O2-Cu2 N1-Cu2-O2 = 162.59(16),= N3-Cu2-O1 = 155.27(15),= N1-Cu2-O1 = 93.46(14), O2-Cu2-O1 = 78.23(12),= 102.12(14), Cu2-O1-Cu1 101.42(13). = 101.42(13). Cu1-O2-Cu2 = 102.12(14),=Cu2-O1-Cu1

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Figure 3. Crystal structures of CuL2 showing atom labeling schemes. Hydrogen atoms were omitted Figure 3. Crystal structures of CuL2 showing atom labeling schemes. Hydrogen atoms were omitted Figure 3. Crystal structures of CuL[Å 2 showing atom[°]: labeling schemes. Hydrogen atoms were omitted for clarity. Selected bond lengths ] and angles Cu1-Cu2 = 3.142(4), Cu1-Cl2 = 2.315(4), Cu2-Cl1 for clarity. Selected bond lengths [Å] and angles [◦ ]: Cu1-Cu2 = 3.142(4), Cu1-Cl2 = 2.315(4), for clarity. Selected lengthsCu1-O2 [Å ] and= angles [°]:Cu2-O1 Cu1-Cu2= =2.041(3), 3.142(4),Cu2-O2 Cu1-Cl2= =2.018(3), 2.315(4),Cu1-N3 Cu2-Cl1= = 2.326(3), Cu1-O1bond = 1.989(2), 2.306(4), Cu2-Cl1 = 2.326(3), Cu1-O1 = 1.989(2), Cu1-O2 = 2.306(4), Cu2-O1 = 2.041(3), Cu2-O2 = 2.018(3), =2.061(3), 2.326(3),Cu1-N4 Cu1-O1= =2.110(3), 1.989(2), Cu1-O2 = 2.306(4), Cu2-O1 = 2.041(3), Cu2-O2 = 78.53(9), 2.018(3), O1-Cu2-O2 Cu1-N3 = Cu2-N1 = 2.077(3), Cu2-N2 = 2.254(3), O1-Cu1-O2 Cu1-N3 = 2.061(3), Cu1-N4 = 2.110(3), Cu2-N1 = 2.077(3), Cu2-N2 = 2.254(3), O1-Cu1-O2 = 78.53(9), 2.061(3), Cu1-N4 = 2.110(3), Cu2-N1 = O1-Cu1-N4 2.077(3), Cu2-N2 = 2.254(3), O1-Cu1-O2 = 78.53(9),O2-Cu1-N4 O1-Cu2-O2= = 84.47(10), O1-Cu1-N3 = 140.88(9), = 84.37(11), O2-Cu1-N3 = 77.09(12), O1-Cu2-O2 = 84.47(10), O1-Cu1-N3 = 140.88(9), O1-Cu1-N4 = 84.37(11), O2-Cu1-N3 = 77.09(12), =124.09(9), 84.47(10),O2-Cu2-N2 O1-Cu1-N3 == 140.00(9), 140.88(9), O1-Cu2-N1 O1-Cu1-N4 = 136.46(8), 84.37(11), O2-Cu1-N3 == 77.09(12), O2-Cu1-N4 == O1-Cu2-N2 78.62(14), O2-Cu2-N1 O2-Cu1-N4 = 124.09(9), O2-Cu2-N2 = 140.00(9), O1-Cu2-N1 = 136.46(8), O1-Cu2-N2 = 78.62(14), 124.09(9), O2-Cu2-N2 = 140.00(9), O1-Cu2-N1 = 136.46(8), O1-Cu2-N2 = 78.62(14), O2-Cu2-N1 = 87.36(11), Cu1-O1-Cu2 = 102.45(9), Cu1-O2-Cu2 = 92.97(10). O2-Cu2-N1 = 87.36(11), Cu1-O1-Cu2 = 102.45(9), Cu1-O2-Cu2 = 92.97(10). 87.36(11), Cu1-O1-Cu2 = 102.45(9), Cu1-O2-Cu2 = 92.97(10).

Figure 4. Crystal structures of ZnL2 showing atom labeling schemes. Hydrogen atoms were omitted Figure 4. Crystal structures of ZnL[Å 2 showing atom[°]: labeling schemes. Hydrogen atoms were omitted for clarity. Selected bond lengths ] and angles Zn1-Zn2 = 3.244(10), Zn1-Cl2 = 2.250(5), Zn2-Cl1 Figure 4. Crystal structures of ZnL2 showing atom labeling schemes. Hydrogen atoms were omitted for clarity. Selected lengthsZn1-O2 [Å ] and=angles [°]: Zn2-O1 Zn1-Zn2 = 3.244(10), Zn1-Cl2 = 2.250(5), Zn2-Cl1 = 2.250(5), Zn1-O1bond = 1.998(7), 2.120(5), = 2.120(5), Zn2-O2 = 1.998(7), Zn1-N1 = for clarity. Selected bond lengths [Å] and angles [◦ ]: Zn1-Zn2 = 3.244(10), Zn1-Cl2 = 2.250(5), =2.151(6), 2.250(5),Zn1-N4 Zn1-O1= =2.231(6), 1.998(7), Zn1-O2 = 2.120(5), Zn2-O1 = 2.120(5), Zn2-O2 ==76.1(3), 1.998(7), Zn1-N1 == Zn2-N2 = 2.231(6), Zn2-N3 = 2.151(6), O1-Zn1-O2 O1-Zn2-O2 Zn2-Cl1 = 2.250(5), Zn1-O1 = 1.998(7), Zn1-O2 = 2.120(5), Zn2-O1 = 2.120(5), Zn2-O2 = 1.998(7), 2.151(6), Zn1-N4 = 2.231(6), Zn2-N2 = 2.231(6), Zn2-N3O2-Zn1-N1 = 2.151(6), O1-Zn1-O2 = 76.1(3), O1-Zn2-O2 = 76.1(3), O1-Zn1-N1 = 127.87(15), O1-Zn1-N4 = 82.8(2), = 81.61(18), O2-Zn1-N4 = 130.50(15), Zn1-N1 = 2.151(6), Zn1-N4 = 2.231(6), Zn2-N2 = 2.231(6), Zn2-N3 = 2.151(6), O1-Zn1-O2 = 76.1(3), 76.1(3), O1-Zn1-N1 = 127.87(15), O1-Zn1-N4 = 82.8(2), O2-Zn1-N1 = 81.61(18), O2-Zn1-N4 = 130.50(15), O2-Zn2-N2 = 82.8(2), O1-Zn2-N2 = 130.51(15), O1-Zn2-N3 = 81.61(18), O2-Zn2-N3 = 127.88(15), Zn1O1-Zn2-O2 = 76.1(3), O1-Zn1-N1 = 127.87(15), O1-Zn1-N4 = 82.8(2), O2-Zn1-N1 = 81.61(18), O2-Zn2-N2 = 82.8(2), O1-Zn2-N2==103.9(3). 130.51(15), O1-Zn2-N3 = 81.61(18), O2-Zn2-N3 = 127.88(15), Zn1O1-Zn2 = 103.9(3), Zn1-O2-Zn2 O2-Zn1-N4 = 130.50(15), O2-Zn2-N2 = 82.8(2), O1-Zn2-N2 = 130.51(15), O1-Zn2-N3 = 81.61(18), O1-Zn2 = 103.9(3), Zn1-O2-Zn2 = 103.9(3). O2-Zn2-N3 = 127.88(15), Zn1-O1-Zn2 = 103.9(3), Zn1-O2-Zn2 = 103.9(3).

3.3. Linearly Polarized UV Light Induced Optical Anisotropy 3.3. Linearly Polarized UV Light Induced Optical Anisotropy At first, Polarized induced optical anisotropy was examined with polarized UV-vis spectra of MO+PVA and 3.3. Linearly UV Light Induced Optical Anisotropy At first, induced optical anisotropy the wasrelative examined with (to polarized of MO+PVA andR MO+ML 1+PVA (Table 1). Regarding values 0 min)UV-vis of an spectra anisotropy parameter At first, induced optical anisotropy was examined with polarized UV-vis spectra of MO+PVA MO+ML +PVA 1).parameters RegardingRthe relativewith values (to polarized 0 min) ofUV anlight anisotropy parameter R (=Abs0deg1/Abs 90deg)(Table [23], the increased linearly irradiation time based and MO+ML1 +PVA (Table 1). Regarding the relative values (to 0 min) of an anisotropy parameter (=Abs 0deg/Abs90deg ) [23], the parameters with linearly polarizedbefore UV light irradiation timetobased on polarized UV-vis spectra (FigureR increased 5). The anisotropy parameter irradiation is set 1 for R (=Abs0deg /Abs90deg ) [23], the parameters R increased with linearly polarized UV light irradiation on polarized UV-vis spectrathe (Figure 5). TheR anisotropy parameter before irradiation set toimplies 1 for simplicity. From the results, parameters increased with increasing irradiation time,iswhich time based on polarized UV-vis spectra (Figure 5). The anisotropy parameter before irradiation is simplicity. Fromwere the results, R avoid increased with increasing implies that molecules alignedthe in parameters a direction to absorbing polarizedirradiation light. Thus,time, it is which suggested that set to 1 for simplicity. From the results, the parameters R increased with increasing irradiation time, that molecules aligned in direction, a directionleading to avoid it is suggested that molecules are were aligned in one toabsorbing a situationpolarized in whichlight. theyThus, cannot absorb irradiated molecules in one[6]. direction, to a situation inMO+ML which they cannot absorb irradiated polarized are lightaligned furthermore Next, theleading corresponding data for 2+PVA (Table 2) indicated that polarized light furthermore [6]. Next, the corresponding data for MO+ML 2 +PVA (Table 2) indicated that the maximum values of the anisotropy parameter for MO+ML2+PVA (1.0279, 1.0222, and 1.0452 for NiL2, the maximum values of the anisotropy parameter for MO+ML2+PVA (1.0279, 1.0222, and 1.0452 for NiL2,

CuL2, and ZnL2, respectively) are larger than those of MO+ML1+PVA (0.9850, 1.0141, and 1.0230 for NiL1, CuL1, and ZnL1, respectively), which expressed that MO+ML2+PVA (incorporating more rigid aromatic ligands) are easier to align due to intermolecular interaction between MO and ML2 in PVA in which only MO is affected on Weigert effect directly [18,19]. Symmetry 2018, 10, 147

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Table 1. The relative R parameters for MO+PVA and MO+ML1+PVA after linearly polarized UV light irradiation with detected wavelengths.

which implies that molecules were aligned in a direction to avoid absorbing polarized light. Thus, it is MO+PVA MO+NiL1+PVA MO+CuL1+PVA MO+ZnL1+PVA Time/min suggested that molecules are aligned in one direction, leading to a situation in which they cannot absorb n-π* 436 nm π-π* 245 nm π-π* 253 nm π-π* 242 nm irradiated polarized light furthermore [6]. Next, the corresponding data for MO+ML2 +PVA (Table 2) 0 1 1 1 1 indicated that the maximum values of the anisotropy parameter for MO+ML2 +PVA (1.0279, 1.0222, 1 1.0208 0.9850 1.0141 1.0119 and 1.0452 for NiL2 , CuL2 , and ZnL2 , respectively) are larger than those of MO+ML1 +PVA (0.9850, 3 1.0311 0.9939 1.0033 1.0140 1.0141, and 1.0230 for NiL1 , CuL1 , and ZnL1 , respectively), which expressed that MO+ML2 +PVA 5 1.0382 0.9927 1.0055 1.0230 (incorporating more rigid aromatic ligands) are easier to align due to intermolecular interaction 10 1.0459 0.9983 1.0049 1.0005 between MO and ML2 in PVA in which only MO is affected on Weigert effect directly [18,19]. Table 2. The Therelative relativeRRparameters parametersforfor MO+MLand 2+PVA after linearly polarized UV light irradiation Table 1. MO+PVA MO+ML1 +PVA after linearly polarized UV light with detected wavelengths. irradiation with detected wavelengths.

MO+NiL2+PVA MO+CuL2+PVA MO+ZnL2+PVA Time/min MO+PVA MO+NiL1 +PVA MO+CuL1 +PVA MO+ZnL1 +PVA π-π* 294 nm π-π* 306 nm π-π* 306 nm Time/min n-π* 436 nm π-π* 245 nm π-π* 253 nm π-π* 242 nm 0 1 1 1 0 1 1 1 1 1 1.0082 0.9850 1.0126 1.0141 1.0223 1.0119 1 1.0208 3 1.0215 0.9939 1.0109 1.0033 1.0258 1.0140 3 1.0311 5 1.0382 5 1.0130 0.9927 1.0222 1.0055 1.0382 1.0230 10 1.0459 0.9983 1.0049 10 1.0279 1.0192 1.0452 1.0005

of of ZnL before andand after 10 min polarized UV light Figure 5. (Left) (Left) Polarized PolarizedUV-vis UV-visspectra spectra ZnL 2+MO+PVA before after 10 min polarized UV 2 +MO+PVA irradiation. (Right) Polarizer angle angle dependence of absorbance of polarized UV-vis UV-vis spectra (at 306 nm, light irradiation. (Right) Polarizer dependence of absorbance of polarized spectra (at π-π* band) of ZnL +MO+PVA before and after 10 min polarized UV light irradiation. 306 nm, π-π* band) 2 of ZnL2+MO+PVA before and after 10 min polarized UV light irradiation.

4. Discussion Table 2. The relative R parameters for MO+ML2 +PVA after linearly polarized UV light irradiation with detected wavelengths.

Similar to the related study with theoretical analysis [24], explanation of the origin of the observed spectral changes in composite films can be also probed using Weigert effect for MO (as MO+NiL2 +PVA MO+CuL2 +PVA MO+ZnL2 +PVA Time/min direct driving force) and intermolecular toπ-π* complexes matrix. π-π* 294interaction nm 306 nm in a polymer π-π* 306 nm According to these results, the maximum change in the anisotropic parameter R was larger for ML2+MO+PVA 0 1 1 1 than ML1+MO+PVA for all metals. From the results, it was found that ZnL 2 in ZnL 2 +MO+PVA is the 1 1.0082 1.0126 1.0223 1.0215 1.0109in Scheme 3 can1.0258 easiest to align. As a3 consideration, assumption summarized be deduced. Flexibility 5 1.0222 1.0382 is more important than chirality of 1.0130 ligand in this case [25]. In previous study, indeed, we exhibited 10 1.0279 1.0192 1.0452 that diastereomers of mononuclear (chiral) complexes indicated similar results (namely slight differences) about anisotropic alignment resulting from Weigert effect. In addition, coordination 4. Discussion of flexible mononuclear Cu(II) complexes was not as suitable for transferring environment Similar to the related study with theoretical analysis [24], explanation of the origin of the observed spectral changes in composite films can be also probed using Weigert effect for MO (as direct driving force) and intermolecular interaction to complexes in a polymer matrix. According to these results, the maximum change in the anisotropic parameter R was larger for ML2 +MO+PVA than ML1 +MO+PVA for all metals. From the results, it was found that ZnL2 in ZnL2 +MO+PVA is the

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easiest to align. As a consideration, assumption summarized in Scheme 3 can be deduced. Flexibility is more important than chirality of ligand in this case [25]. In previous study, indeed, we exhibited that diastereomers of mononuclear (chiral) complexes indicated similar results (namely slight differences) about anisotropic alignment resulting from Weigert effect. In addition, coordination environment of Symmetry 2018, 10, x FOR PEER REVIEW 8 of 10 Symmetrymononuclear 2018, 10, x FOR PEER REVIEW 8 of 10 flexible Cu(II) complexes was not as suitable for transferring intermolecular interaction from azo-dyes asinteraction that of rigid mononuclear Since cyclohexane by ML 1 intermolecular from azo-dyes Zn(II) as thatcomplexes. of rigid mononuclear Zn(II)possessed complexes. Since intermolecular interaction from azo-dyes as thatbyofintermolecular rigid mononuclear Zn(II) due complexes. Since is composed of a single bond, it is hardly affected effect [26,27] to molecular cyclohexane possessed by ML1 is composed of a single bond, it is hardly affected by intermolecular cyclohexane possessed by ML1cyclohexane is composed of a single bond, it is hardlyof affected bybond, intermolecular reorientation MO. Because ML composed a single it would 1 iscyclohexane effect [26,27] of due to molecular reorientationmoiety of MO.of Because moiety of ML 1 is composed effect [26,27] due to molecular reorientation of MO. Because cyclohexane moiety of ML 1 is composed be hardly affected by molecular reorientation of MO. On the other hand, phenylenediamine of hand, ML2 of a single bond, it would be hardly affected by molecular reorientation of MO. On the other ofrigid a single bond, it wouldring, be hardly affected by molecularnature reorientation of MO. On the otherofhand, is and flat aromatic resulting in its susceptible to molecular reorientation MO. phenylenediamine of ML2 is rigid and flat aromatic ring, resulting in its susceptible nature to phenylenediamine of ML 2 is rigid and flat aromatic ring, resulting in its susceptible nature to Thus, the change of anisotropic would of beanisotropic affected by parameter the difference in the be susceptibility molecular reorientation of MO.parameter Thus, theRchange R would affected by molecular reorientation of MO. Thus, the changeofofMO. anisotropic R would be affected by of ML to the molecular reorientation In otherparameter words, since phenylenediamine 1 and ML the difference in2 the susceptibility of ML1 and ML 2 to the molecular reorientation of MO. In other the difference in 2the susceptibility of ML 1 and ML2 shape, to the molecular reorientation of MO. Ineffect other possessed by ML is an aromatic ring and a planar it is susceptible to intermolecular words, since phenylenediamine possessed by ML2 is an aromatic ring and a planar shape, it is words, since phenylenediamine possessed by ML 2 is an aromatic ring and a planar shape, it is due to molecular reorientationeffect of MO. result, ML without the2molecular 2 is oriented of susceptible to intermolecular dueAs toamolecular reorientation MO. Asinhibiting a result, ML is oriented susceptible toofintermolecular effect to molecular reorientation of MO. of As a result, 2 is oriented reorientation MO. the However, ML1due inhibits the molecular [28]. ML This related without inhibiting molecular reorientation of MO. reorientation However, ML1MO inhibits the is molecular without inhibiting the molecular reorientation of MO. However, ML 1 inhibits the molecular toreorientation the magnitude of the change the anisotropic parameter depicted 4. The quality of of MO [28]. This isinrelated to the magnitude of R the change in in Scheme the anisotropic parameter reorientation ofdispersion) MO [28]. This ismagnitude related to of theoptical magnitude of theinduced change in the anisotropic parameter data (statistical and anisotropy are common ranges for that R depicted in Scheme 4. The quality of data (statistical dispersion) and magnitude of optical R the depicted in Scheme[29]. 4. The quality ofmechanisms, data (statistical dispersion) and magnitude of optical of recent induced examples By using these applications expected anisotropy are common ranges for that of potential the recent examples can [29].beBy using such these anisotropy induced are common ranges for that of the recent examples [29]. By using these as chiroptical probes of applications spectroscopycan [30,31] or sensors [32], and chiral materials of nanostructure in mechanisms, potential be expected such as chiroptical probes of spectroscopy [30,31] mechanisms, potential applications can be expected such as chiroptical probes of spectroscopy [30,31] oriented ones [32] or surface [33,34]. or sensors [32], and chiral materials of nanostructure in oriented ones [32] or surface [33,34]. or sensors [32], and chiral materials of nanostructure in oriented ones [32] or surface [33,34].

Scheme 3.Flexibility Flexibility (orrigidity) rigidity) ofligands ligands andintermolecular intermolecular effectfrom from MO. Scheme Scheme 3. 3. Flexibility (or (or rigidity) of of ligands and and intermoleculareffect effect fromMO. MO.

Scheme 4. Molecular orientation and parameter change (proposed mechanism). Scheme4. 4. Molecular Molecular orientation orientation and and parameter parameterchange change(proposed (proposedmechanism). mechanism). Scheme

5. Conclusions 5. Conclusions Conclusions 5. In summary, we have synthesized two series of new binuclear Schiff base Ni(II), Cu(II), and Insummary, summary, wehave have synthesized series of new binuclear base Ni(II), Cu(II), and In synthesized twotwo series ofhave new binuclear SchiffSchiff base moiety Ni(II), Cu(II), Zn(II) Zn(II) complexes.we One series is the complexes that flexible cyclohexane on theand amine site Zn(II) complexes. One series iscomplexes the complexes that have flexible cyclohexane moiety onthe theamine aminesite site complexes. One series is the that have flexible cyclohexane moiety on (ML1: NiL1, CuL1, and ZnL1) and the other series is the complexes that have rigid aromatic six(ML1: NiL1, CuL1, and ZnL1) and the other series is the complexes that have rigid aromatic sixmembered ring moiety on the amine site (ML2: NiL2, CuL2, and ZnL2) instead of the cyclohexane membered ring moiety on the amine site (ML2: NiL2, CuL2, and ZnL2) instead of the cyclohexane moiety. We have also prepared their hybrid materials (ML1+MO+PVA and ML2+MO+PVA) as PVA moiety. We have also prepared their hybrid materials (ML1+MO+PVA and ML2+MO+PVA) as PVA cast films. The polarized UV-vis spectra of the hybrid materials after linearly polarized UV light cast films. The polarized UV-vis spectra of the hybrid materials after linearly polarized UV light irradiation were measured. From the changes of the anisotropy parameter R, it was found that irradiation were measured. From the changes of the anisotropy parameter R, it was found that ZnL2+MO+PVA exhibited the highest anisotropic orientation induced among the six materials. In

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(ML1 : NiL1 , CuL1 , and ZnL1 ) and the other series is the complexes that have rigid aromatic six-membered ring moiety on the amine site (ML2 : NiL2 , CuL2 , and ZnL2 ) instead of the cyclohexane moiety. We have also prepared their hybrid materials (ML1 +MO+PVA and ML2 +MO+PVA) as PVA cast films. The polarized UV-vis spectra of the hybrid materials after linearly polarized UV light irradiation were measured. From the changes of the anisotropy parameter R, it was found that ZnL2 +MO+PVA exhibited the highest anisotropic orientation induced among the six materials. In addition, the magnitude of the anisotropic parameter R was compared among ML1 +MO+PVA and ML2 +MO+PVA. As a result, it was found that the orientation of ML2, of which the ligand had many aromatic groups, was induced the most effectively of all the metals. Therefore, the susceptibility of the molecular reorientation by MO to the complex through intermolecular interactions leads to enhancement in the anisotropic molecular orientation of the complex. In this way, the change of anisotropic parameter R would be affected by the difference in the susceptibility of ML1 , which has flexible cyclohexane moiety, and ML2 , which has a rigid phenyl group with regard to the molecular reorientation of MO. These results also support a mechanism based on intermolecular interaction and emphasize the significance of the rigidity of the ligands even for binuclear complexes for effective receiving such as intermolecular interaction from MO (azo-dyes influenced by Weigert effect directly). Supplementary Materials: The following are available online at http://www.mdpi.com/2073-8994/10/5/147/s1, CCDC 1826446, 1826434, and 1826435 contain the supplementary crystallographic data for CuL1 , CuL2 , and ZnL2 , respectively. These data can be obtained free of charge via www.ccdc.cam.ac.uk, or from the Cambridge Crystallographic Data Centre, 12 Union Road, CB2 1EZ Cambridge, England, UK; Fax: (+44) 01223 336033; or E-Mail: [email protected]. Author Contributions: H.N. performed the experiments, T.H. analyzed the data, and T.A. wrote the paper. Acknowledgments: This XRD work was conducted at Advanced Characterization Nanotechnology Platform of the University of Tokyo (Kazuhiro Fukawa), supported by “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. Conflicts of Interest: The authors declare no conflict of interest.

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