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):
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.