Molecular Structure-Optical Property Relationship of Salicylidene Derivatives: A Study on the First-Order Hyperpolarizability

The first-order hyperpolarizability of pi-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 (beta(HRS)) 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 beta(HRS) values between 32.0 +/- 4.8 x 10(-30) cm(5)/esu and 58.5 +/- 8.0 x 10(-30) cm(5)/esu at 1064 nm, depending on the molecular geometry of the salicylidene molecules. More specifically, the outcomes indicate a considerable increase of beta(HRS) magnitude (similar to 30%) when in the ligands are incorporated the Zn(II) ion. We ascribed such results to the rise of the planarity of the pi-conjugated backbone of the chromophores caused by the Zn(II). Furthermore, we observed an increase of similar to 50% in dynamic films 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 pi-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. (AU)