Synthesis of some Heterocyclic Compounds with Nonlinear Optical Properties ^†

Organic compounds have been mentioned recent decades as an alternative to their inorganic counterparts, due to their strong nonlinear optical (NLO) properties, with many applications [1]. Field effect transistors, organic light-emitting diodes (OLEDs), photovoltaic devices and white light sources for indoor and outdoor lighting are some of the applications of organic materials [2,3]. In addition to the classic organic push–pull molecules with high values of dipole moment and polarizability, there are a number of other compounds, such as fullerenes, perylenes, polymers or dyes that possess very notable nonlinear optical properties [4,5]. Organic commercial and synthetic materials were used for the synthesis of the heterocyclic compounds. All compounds were characterized with physicochemical techniques (elemental analysis, ¹H, ¹³C, FTIR and UV-Vis spectroscopy). The SHG capability of samples was measured by using an experimental set-up. Three series of heterocyclic compounds (octahydroacridines [6,7,8,9], thioamides [2] and azo-pyrazolone [5] with classical push-pull structures) were synthesized and characterized. The SHG (second harmonic generation) value was determined for each compound. The molecular polarizability (α), first order hyperpolarizabilities (βtot), dipole (μtot) and quadrupole (Q) moments, were calculated using DFT (density functional theory) method. It was found that the NLO efficiency was best in the case of one compound with low HOMO-LUMO energy gap (HOMO = high occupied molecular orbital; LUMO = low unoccupied molecular orbital) and high total hyperpolarizability.