Polarized Light-Induced Molecular Orientation Control of Rigid Schiff Base Ni(II), Cu(II), and Zn(II) Binuclear Complexes as Polymer Composites

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.

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. [1,2,3]. 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 beyond single crystalline materials in solid states [9,10]. Classically, planar metal complexes (lowered symmetry from O_h to D_4h) may be an important example with which to elucidate their electronic states (assignment of d-d and π-π* transitions and other established associated electronic structures) by means of optical experiments such as polarized crystal (electronic) spectra [11]. However, spectral measurements for single crystals from a certain face (or along one or two axis) are not in agreement with plane-of-coordination-sphere molecular coordinates about d-orbitals symmetry due to orientation in the crystal packing of molecules. Thus, to overcome this experimental difficulty, alignment-tunable (or at least alignment-oriented) molecules of planer metal complexes may be preferable samples with which to study polarized spectral measurements. Moreover, such oriented metal complexes (e.g., [CuCl₄]²⁻) in single crystals may also exhibit important other physical behavior such as thermochromism, as well as optical properties [12]. In this context, organic/inorganic hybrid materials of AZ and (planar) metal complexes deserve to be investigated by means of optical or other physical experiments. Organic/inorganic hybrid materials have the advantage of exhibiting additional electronic properties due to inorganic metal complexes, which is difficult for organic photofunctional polymers solely. However, light or laser (at the same time electric field) controlling of molecular alignment and its evaluation are not simple generally [13,14]. Based on the above-mentioned factors, we aimed to develop new functions and physical properties through the anisotropic arrangement of metal complexes that have azo-components in polymers. We have prepared organic/inorganic composite materials of methyl methacrylate (PMMA) polymer films separately that contain Schiff bases Ni(II), Cu(II), Zn(II) complexes, and AZ with various substituents [15,16,17,18,19]. In recent years, much effort has been devoted to controlling optical anisotropy using not AZ and PMMA but water-soluble methyl orange (MO) and polyvinyl alcohol (PVA) [20], by which application in protein systems may be also expected potentially. In this research, herein, as molecular design comparison from the viewpoint of receiving intermolecular interaction, we discuss the degree of induced optical anisotropy for binuclear complexes that have flexible cyclohexane moieties (ML₁) (Scheme 1) and more rigid aromatic phenyl moieties (ML₂) (Scheme 2) at the amine site.

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 C₂₈H₃₀Cl₂N₄Ni₂O₂·H₂O: 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 C₂₈H₃₀Cl₂Cu₂N₄O₂·H₂O: 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 C₂₈H₃₀Cl₂N₄O₂Zn₂: 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 C₂₈H₁₈Cl₂N₄Ni₂O₂: 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 C₂₈H₁₈Cl₂Cu₂N₄O₂: 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 C₂₈H₁₈Cl₂N₄O₂Zn₂: 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 (ML_n+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 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 and refined using riding models. Unfortunately, single crystals of the other complexes could not be obtained.

The powder X-ray diffraction patterns for CuL₂ and ZnL₂ were collected at 298 K on Rigaku Smart lab with CuKα radiation (λ = 1.5418 Å) at the University of Tokyo. Powder crystal structures were determined by means of Rietveld refinement using a Rigaku PDXL2 program package (Rigaku, Tokyo, Japan).

Crystallographic data for CuL₁ (CCDC 1826446): C₂₈H₃₀Cl₂Cu₂N₄O₂, Orthorhombic, P2₁2₁2 (#18), Z = 4, a = 13.841(15) Å, b = 24.705(3) Å, c = 8.198(9) Å, V = 2803.2(6) ų, ρ_calc = 1.660 g/cm³, μ = 1.775 mm⁻¹, F(000) = 1432.0, S = 0.765, R₁[I>2σ(I)] = 0.0335, wR2 = 0.1096. Flack parameter = 0.014(10).

Crystallographic data for CuL₂ (CCDC 1826434): C₂₈H₁₈Cl₂Cu₂N₄O₂, Triclinic, P-1 (#2), Z = 2, a = 7.750(4) Å, b = 8.460(4) Å, c = 15.256(7) Å, α = 85.59(12)°, β = 110.77(9)°, γ = 112.89(7)°, V = 2510(6) ų, R_wp = 5.28%.

Crystallographic data for ZnL₂ (CCDC 1826435): C₂₈H₁₈Cl₂N₄O₂Zn₂ Triclinic, P-1 (#2), Z = 2, 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)°, V = 2293(11) ų, R_wp = 4.25%. (See Supplementary Materials)

3. Results

3.1. Spectral Characterization

Figure 1 depicts CD spectra of chiral complexes (ML₁). As prepared using chiral amine, it could 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 (d-d) or between metal and coordination atoms (CT) transitions mainly. The reason why the spectra in solutions are slightly different from that in solid state (KBr) may be artifact peaks due to LD components besides CD components [22].

3.2. Structural Characterization

Figure 2, Figure 3 and Figure 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 which metal ions are slightly lifted up towards the apical Cl atom from the [MN₂O₂] basal planes. Both apical Cl atoms are located on the opposite sides of a ligand plane in a symmetric manner. Rigidity of six-membered C ring derived from amines for CuL₁ are CuL₂ resulted in conformational difference of whole ligands. Besides them, most of bond lengths and angles are within common ranges of the analogous compounds [20].

3.3. Linearly Polarized UV Light Induced Optical Anisotropy

At first, induced optical anisotropy was examined with polarized UV-vis spectra of MO+PVA and MO+ML₁+PVA (

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

Time/min	MO+PVA n-π* 436 nm	MO+NiL1+PVA π-π* 245 nm	MO+CuL1+PVA π-π* 253 nm	MO+ZnL1+PVA π-π* 242 nm
0	1	1	1	1
1	1.0208	0.9850	1.0141	1.0119
3	1.0311	0.9939	1.0033	1.0140
5	1.0382	0.9927	1.0055	1.0230
10	1.0459	0.9983	1.0049	1.0005

). Regarding the relative values (to 0 min) of an anisotropy parameter R (=Abs_0deg/Abs_90deg) [23], the parameters R increased with linearly polarized UV light irradiation time based on polarized UV-vis spectra (Figure 5). The anisotropy parameter before irradiation is set to 1 for simplicity. From the results, the parameters R increased with increasing irradiation time, which implies that molecules were aligned in a direction to avoid absorbing polarized light. Thus, it is suggested that molecules are aligned in one direction, leading to a situation in which they cannot absorb irradiated polarized light furthermore [6]. Next, the corresponding data for MO+ML₂+PVA (

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

Time/min	MO+NiL2+PVA π-π* 294 nm	MO+CuL2+PVA π-π* 306 nm	MO+ZnL2+PVA π-π* 306 nm
0	1	1	1
1	1.0082	1.0126	1.0223
3	1.0215	1.0109	1.0258
5	1.0130	1.0222	1.0382
10	1.0279	1.0192	1.0452

) indicated that 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].

4. Discussion

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 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 flexible mononuclear Cu(II) complexes was not as suitable for transferring intermolecular interaction from azo-dyes as that of rigid mononuclear Zn(II) complexes. Since cyclohexane possessed by ML₁ is composed of a single bond, it is hardly affected by intermolecular effect [26,27] due to molecular reorientation of MO. Because cyclohexane moiety of ML₁ is composed of a single bond, it would be hardly affected by molecular reorientation of MO. On the other hand, phenylenediamine of ML₂ is rigid and flat aromatic ring, resulting in its susceptible nature to molecular reorientation of MO. Thus, the change of anisotropic parameter R would be affected by the difference in the susceptibility of ML₁ and ML₂ to the molecular reorientation of MO. In other words, since phenylenediamine possessed by ML₂ is an aromatic ring and a planar shape, it is susceptible to intermolecular effect due to molecular reorientation of MO. As a result, ML₂ is oriented without inhibiting the molecular reorientation of MO. However, ML₁ inhibits the molecular reorientation of MO [28]. This is related to the magnitude of the change in the anisotropic parameter R depicted in Scheme 4. The quality of data (statistical dispersion) and magnitude of optical anisotropy induced are common ranges for that of the recent examples [29]. By using these mechanisms, potential applications can be expected such as chiroptical probes of spectroscopy [30,31] or sensors [32], and chiral materials of nanostructure in oriented ones [32] or surface [33,34].

5. Conclusions

In summary, we have synthesized two series of new binuclear Schiff base Ni(II), Cu(II), and Zn(II) complexes. One series is the complexes that have flexible cyclohexane moiety on the amine site (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 ML_2, 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).