Copper(II) Complexes of salen Analogues with Two Differently Substituted (Push−Pull) Salicylaldehyde Moieties. A Study on the Modulation of Electronic Asymmetry and Nonlinear Optical Properties

This paper presents some copper(II) complexes of salen analogues in which the two salicylaldehyde moieties carry
different (electron donor, D, and acceptor, A) substituents in position 5, producing a push−pull charge asymmetry.
The X-ray structures of some compounds show the presence of pairs of stacked molecules with head-to-tail
intermolecular associations. The geometries of all complexes have been optimized through density functional theory
(DFT) studies, which have shown that a major influence on the coordination bond lengths is given by the presence
of the electron acceptor NO2 group. Such an influence operates mainly on the Cu−phenolato bonds:  elongation
of the Cu−O distance of the 5-nitrosalicylaldehyde moiety, with a concomitant decrease of the other Cu−O distance;
the Cu−N bonds are less affected. The D groups have only a minor influence. The nonlinear optical responses,
μgβvec, of some molecules have been determined by EFISH measurements, and the βvec values have been obtained
using the DFT-calculated μg values because solubility problems hampered the experimental measurements of μg
of some derivatives; the former, however, have been found to be in agreement with the experimental values that
could be obtained. Deconvolution of the absorption bands in the near-UV region has allowed recognition of the
charge-transfer (CT) transition, assigned to a ligand-to-metal CT (LMCT) by time-dependent DFT computations;
we have then used the solvatochromism of this transition to obtain β0 and βCT values using the two-state model.
These values were compared with those obtained by computational studies, which have also allowed evaluation
of the influence of the substituents on the directions of μg and βtot.