Synthesis, characteristic and theoretical investigation of the structure, electronic properties and second-order nonlinearity of salicylaldehyde Schiff base and their derivatives

Abstract

A series of asymmetric donor–acceptor substituted salen-type Schiff-bases have been synthesized and their structures, electronic properties and second order nonlinearities were investigated by DFT methods. In order to verify the stable of these Schiff-base derivates, the IR spectrum of these Schiff-base derivates were calculated, the result showed that these compounds are stable. The results of TD-DFT calculation indicate that the derivatives with the electron-donating group (CH 3, OCH 3 or N(C 2H 5) 2) have a red shift absorption compared to derivatives with the electron-withdrawing group (NO 2). The analysis of MOS indicates that the CN group has contribution to the LUMO orbital while the groups of OCH 3, N(C 2H 5) 2 and NO 2 have contribution to the HOMO orbital. OCH 3, N(C 2H 5) 2 as electron rich groups, made the derivates have a larger first static hyperpolarizability. However, the compound (II) with a NO 2 substituent, also has a large first static hyperpolarizability. This is probably because of the special transition model, namely the values of two oscillator strength f ( f HOMO−1–LUMO = 0.405, f HOMO–LUMO = 0.321) are almost equal. In order to understand the influence of the energy gap (Δ E) between the HOMO and the LUMO orbitals on the first static hyperpolarizability, we calculated the energy gap (Δ E) of all Schiff-base compounds. The results show that the smaller the HOMO–LUMO energy gap is, the larger the first static hyperpolarizability is.