Mechanism of Nonlinear Optical Enhancement and Supramolecular Isomerism in 1D Polymeric Zn(II) and Cd(II) Sulfates with Pyridine-4-aldoxime Ligands

Interaction
of zinc­(II) and cadmium­(II) sulfates with pyridine-4-aldoxime (4-pyao)
and pyridine-4-amidoxime (4-pyamo) ligands resulted in four 1D metal–organic
materials (MOMs) with identical composition, [M­(SO4)­A2(H2O)2]n, where M = Zn­(II), A = 4-pyao for 1, M = Cd­(II), A
= 4-pyao for 2, M = Zn­(II), A = 4-pyamo for 3, M = Cd­(II), A = 4-pyamo for 4, and mononuclear [Zn­(SO4)­(4-pyamo)2(H2O)3] 5. New coordination polymers represent the mixed-ligand supramolecular
isomers different by the twisting of two pyridine-4-oxime ligands
in the metal coordination environments, and crystallizing in the different
space groups. Conformational preferences and nonlinear optical properties
of the 4-pyao and 4-pyamo complexes were investigated using density
functional theory. Spectral properties of 1–3 have been also evaluated. The solid-state emission of 1D
polymers 1–3 appears to be ligand-based,
as the positions of the emission maxima remain practically unchanged
from free ligand to complexes. The enhancement of luminescence and
two-photon absorption in polymers in comparison with the pure ligands
is attributed to the chelation of the ligand to the metal center.
The detailed mechanism of this enhancement upon complex formation
is analyzed and can be used in future design of metal–organic
nonlinear optical materials.