Molecular structure-optical property relationship of salicylidene derivatives: a study on the first-order hyperpolarizability

The first-order hyperpolarizability of π-conjugated organic molecules is of particular interest for the fabrication of electro-optical modulators. Thus, we investigated the relationship between the molecular structure and the incoherent second-order nonlinear optical response (βHRS) of four salicylidene derivatives (salophen, [Zn(salophen)(OH2)], 3,4-benzophen, [Zn(3,4-benzophen)(OH2)]) dissolved in DMSO. For that, we employed the Hyper-Rayleigh Scattering technique with picosecond pulse trains. Our experimental results pointed out dynamic βHRS values between 32.0 ± 4.8 × 10-30 cm5/esu and 58.5 ± 8.0 × 10-30 cm5/esu at 1064 nm, depending on the molecular geometry of the salicylidene molecules. More specifically, the outcomes indicate a considerable increase of βHRS magnitude (∼30%) when in the ligands are incorporated the Zn(II) ion. We ascribed such results to the rise of the planarity of the π-conjugated backbone of the chromophores caused by the Zn(II). Furthermore, we observed an increase of ∼50% in dynamic βHRS when there is a replacement of one hydrogen atom (salophen molecule) by an acetophenone group (3,4-benzophen). This result is related to the increase of the effective π-electron number and the higher charge transfer induced at the excited state. All these findings were interpreted and supported in the light of time-dependent density functional theory (DFT) calculations. Solvent effects were considered in the quantum chemical calculations using the integral equation formalism variant of the polarizable continuum model.