Dipolar donor-acceptor-substituted Schiff base complexes with large off-diagonal second-order nonlinear optical tensor components

The synthesis, characterization, and two-dimensional second-order nonlinear optical (NLO) response, of a dipolar Ni-II donor-acceptor Schiff base comp lex and the related ligand are reported. Electric-field-induced second-harm onic generation and harmonic light (hyper-Rayleigh) scattering techniques, in combination with INDO/SCI-SOS theoretical calculations, were used to inv estigate the vector part of the hyperpolarizability tensor and the two-dime nsional character of the molecular nonlinearity, respectively. Off-diagonal hyperpolarizability tensors can be related to charge-transfer transitions that are polarized perpendicular to the molecular dipolar axis, while paral lel transitions account for the diagonal hyperpolarizability tensor. The ro le of the metal center in enhancing the two-dimensional NLO response of suc h molecules is twofold since it acts both as the donor and the bridging moi ety of the planar donor-(pi -conjugate-bridge)-acceptor system. These dipol ar two-dimensional molecules are interesting candidates from the perspectiv e of polarization-independent NLO materials.