Abstract

We have here synthesized new chiral Schiff base Ni(II), Cu(II), Zn(II) complexes (Ni, Cu, Zn) and hybrid materials with azobenzene (AZ) in polymethyl methacrylate (PMMA). Linearly polarized UV light irradiation of these hybrid materials slightly increased their optical anisotropy of AZ as well as the complexes, which were measured with polarized IR and UV-Vis spectra and discussed based on TD-DFT calculations. Non-linear concentration (viscosity) dependence of PMMA solutions about artifact peaks suggested weak intermolecular interactions due to the flexibility of complexes by inserted methylene chains. Molecular modeling indicated that large spaces around complexes in PMMA resulted in easy molecular orientation (Ni > Cu > Zn) as short-term saturation of the UV light irradiation.

1. Introduction

The study of organic–inorganic materials of Schiff base complexes and polymers is progressing to impart new features such as fluorescence properties [1]. Indeed, we have also systematically prepared organic/inorganic hybrid materials composed of chiral Schiff base mononuclear and dinuclear complexes including or mixing azo-moiety in synthetic polymer (e.g., polymethyl methacrylate (PMMA) and polyvinyl alcohol [2]) or biopolymer (albumin [3] or laccase [4]) matrices. In addition, optical alignment by the Weigert effect of azobenzene (AZ) has been widely used [5]. By the UV irradiation of polarized light to PMMA cast film containing AZ (azo-moieties) and chiral Schiff base complexes, an increase in optical anisotropy was observed in our previous studies [6,7,8,9]. After linearly or circularly polarized UV light irradiation, we have also measured polarized electronic (UV-Vis), infrared (IR), and circular dichroism (CD) spectra in order to elucidate light-induced molecular orientation of each component by the direct Weigert effect of AZ (azo-moiety) or supramolecular transmission to metal complexes. However, a detailed mechanism and intermolecular interaction is not reasonably understood. In other words, the mechanism has not been clarified; specifically, the interaction between AZ and metal complex in PMMA film is still unclear.

Previously, to elucidate intermolecular interactions, we designed metal complexes involving an electron-withdrawing (Br– or halogen) group, an UV absorbing and hydrophobic (Ph–) group, and a strong hydrogen-bonding (HO–) group to PMMA [10]. Here, we prepared new similar Ni(II), Cu(II), and Zn(II) complexes (abbreviated as Ni, Cu, and Zn, respectively) that enhance flexibility by inserting one more methylene chains between the Ph– group and the asymmetric carbon atom (Figure 1). We focused on the polarized light-induced molecular orientation control of the present complexes. In order to examine intermolecular interactions further, we employed concentration (viscosity) dependence of PMMA solutions and molecular modeling.

2. Results

2.1. Polarized IR Spectra

The angular dependence of polarized IR spectra after irradiation of linearly polarized UV light is exhibited in Figure 2. Linearly polarized UV light irradiation resulted in the induction of anisotropic molecular orientation, not only AZ directly but also the complexes though supramolecular transmission in the PMMA matrix. Polarized IR spectra of C=N bands can provide selective information about the molecular orientation of complexes only [11]. In order for a discussion of the Weigert effect, namely, the orientation of dyes in general and among many methods [12], we employed conventional polarized absorption spectra [13] (Tables S1–S3), and these two parameters (R and S) for the degree of photoinduced optical anisotropy (spectral dichroism):

$$R=\frac{A_0}{A_{90}};$$

$$S=\frac{A_0- A_{90}}{A_0+ A_{90}},$$

where A₉₀ and A₀ denote the absorbance measured with the measuring polarizer perpendicular and parallel, respectively, to the electric vector of irradiation polarized light. Ideal isotropic systems of S = 0 and R = 1 and both S and R parameters are changed as dichroism by molecular alignment increases.

As for Ni+AZ+PMMA, saturation of induced molecular orientation was observed at 0.5 min with R = 1.29 and S = 0.0888. Though induced anisotropy of Ni was also confirmed, the degree of orientation was considered to be the weakest among them.

As for Cu+AZ+PMMA, saturation of induced molecular orientation was observed at 10 min with R = 1.32 and S = 0.0970. The longest time for saturation of molecular orientation may be ascribed to the flexibility of the coordination environment of copper(II) complexes.

As for Zn+AZ+PMMA, saturation of induced molecular orientation was observed at 0.5 min with R = 1.31 and S = 0.0927. Transmission of molecular anisotropy was quickly observed for a zinc(II) complex because of their stiffness of compressed tetrahedral coordination environment. The order of saturation time is Ni < Cu < Zn, which may be attributed to the difference in transmission of molecular orientation due to the molecular geometry and molecular flexibility of the complexes.

2.2. Polarized UV-Vis and CD Spectra with Thoretical Calculations

Figure 3, Figure 4 and Figure 5 exhibit experimental (in acetone solution) and simulated (UB3LYP/6-31G(d)) CD and UV-Vis spectra of Ni, Cu, and Zn, respectively, based on optimized structures (Figures S1–S3). The π–π*, n–π*, and d–d bands could be reasonably assigned based on a theoretical simulation using Gaussian09 [14]. As the model structures in PMMA, the optimized structures of Ni and Cu afford a tetrahedrally distorted square planar geometry indicating dipole moment 3.2694 Debye with direction vector (x, y, x) = (1.7223, 0.7499, −2.6759)) and 5.2898 Debye with (−1.5744, 4.2113, 2.7872), while that of Zn affords a compressed tetrahedral geometry indicating 8.2269 Debye with (4.6534, −5.7037, 3.6736).

Contrary to IR spectra, UV-Vis spectra (Tables S4–S6) contain overlap of AZ and complexes with initially drastic spectral changes by trans to cis photoisomerization of AZ. Thus, polarized UV-Vis spectra contain information about molecular orientation of each component and their conformational (both ligands and coordination environment) changes—as expected, deviated from crystal structures [15].

As for Ni+AZ+PMMA, after 10 min, π–π* (318 nm), n–π* (440 nm), and d–d (610 nm) bands exhibited R = 0.958; S = −0.0142, R = 1.02; S = 0.0078, and R = 1.08; S = 0.0269, respectively.

As for Cu+AZ+PMMA, after 10 min, π–π* (318 nm), n–π* (440 nm), and d–d (614 nm) bands exhibited R = 1.00; S = −0.0011, R = 0.954; S = −0.0055, and R = 0.998; S = −0.0007, respectively.

As for Zn+AZ+PMMA, after 10 min, π–π* (318 nm) and n–π* (440 nm) bands exhibited R = 0.969; S = −0.0105 and R = 1.05; S = −0.00152, respectively.

In contrast to previous studies [15,16], however, supramolecular chirality resulting from helical orientation could not be observed as detectable changes of CD spectra (220–900 nm), even following circularly polarized UV light irradiation for 10 min (not shown). Long and flexible ligand conformation [17,18] exhibited a disadvantage in the supramolecular transmission of molecular orientation.

3. Discussion

Viscosity and CD Spectra with Molecular Modeling

In order to discuss intermolecular interaction in PMMA, Figure 6 shows a correlation between concentration of PMMA acetone solutions and the intensity of so-called artifact peak of solid-state CD spectra. Our previous studies [19,20] have successfully elucidated that there is a good correlation between viscosity (namely, concentration of PMMA acetone solutions of stiff chiral metal complexes [21]) and intensity of artifact CD peaks, accompanying a gradual restriction of free rotation and losing isotropy of a chiral complex (Figure S4). In principle, isotropy of a chiral material reduces artifact peaks of CD spectra.

In this study, we tested 1:2 mixtures of PMMA acetone solutions (2.5, 5, 10, 15, 20, 25 wt %) and 0.05 mM acetone solutions of complexes and AZ. The strongest artifact CD peaks could be observed at 10%, 15%, and 5% for Ni, Cu, and Zn, respectively. Contrary to other examples of relatively stiff complexes, a poor correlation between viscosity and intensity was found. Both the flexible conformation of ligands and the whole structure of the complexes resulted in a different fashion of intermolecular interaction between the complexes, AZ, and PMMA.

Molecular modeling (Figure 7) helps the visualization of intermolecular interaction fashion between complexes, AZ, and PMMA from the viewpoint of stereochemistry. In the PMMA matrix, free volume around AZ enables AZ to photoisomerize smoothly. Similar to the discussion of AZ, the order of free volume around complexes is Ni > Cu > Zn, which is in agreement with the easy-to-move complexes in PMMA.

4. Materials and Methods

Complexes (Ni, Cu, and Zn) were prepared according to literature methods [10,16,18] using the corresponding chiral amines and aldehydes having halogen. Ni: Yield: 52.5%. IR 1631 cm⁻¹ (C=N). Cu: Yield: 46.2%. IR 1628 cm⁻¹ (C=N). Zn: Yield: 71.8%. IR 1615 cm⁻¹ (C=N). Hybrid materials of complexes (Ni, Cu, and Zn), AZ, and PMMA were prepared according to literature procedures [16,18].

Infrared (IR) spectra were recorded with Nujol mull on a JASCO FT-IR 4200 plus spectrophotometer (JASCO, Tokyo, Japan) equipped with a polarizer in the range of 4000–400 cm⁻¹ at 298 K. Absorption electronic spectra were measured on a JASCO V-570 spectrophotometer equipped with a polarizer in the range of 900–200 nm at 298 K. Circular dichroism (CD) spectra were measured on a JASCO J-725 spectropolarimeter in the range of 800–200 nm at 298 K. Viscosity was measured on an A&D SV-10A type SV (A&D, Tokyo, Japan, 30 Hz frequency) at 298 K. Photo-illumination were carried out using a lamp (1.0 mW/cm²) with optical filters (UV λ = 200–400 nm) and a polarizer.

All calculations were performed using the Gaussian 09W software Revision D.01 (Gaussian, Inc., Wallingford, CT, USA) [14]. The vertical excitation energy was calculated using the TD-DFT method based on the singlet ground state geometry. The exchange functional, the correlation functional, and the basis set were UB3LYP/6-31G8d.

5. Conclusions

In summary, we prepared organic/inorganic hybrid materials containing three new chiral complexes having Br–, Ph–, and –OH groups. Stereochemistry of ligands resulted in weakening propagation of optical anisotropy from azobenzene to chiral Schiff base metal complexes in PMMA polymer matrix. In contrast to previous analogous chiral Schiff base metal complexes without the methylene group (namely, connecting (R)-asymmetric carbon and the Ph– group directly), flexibility for Ni, Cu, and Zn was attributed to a methylene carbon between the (R)-asymmetric carbon and the Ph– group. Linearly polarized UV light irradiation induced anisotropic molecular orientation of AZ as well as complexes, and saturated irradiation time was Ni < Cu < Zn. Viscosity and CD intensity test suggested weak intermolecular interaction between flexible complexes and PMMA. This may be in agreement with the free volume of PMMA around complexes Ni > Cu > Zn.