A series of pyrazole-5-ones 13, synthesized by azo-coupling reactions, which present only hydrazo tautomers with a planar structure, responsible for their nonlinear optical properties were discussed in terms of NLO parameters by Marinescu et al. demonstrated that compounds 12possessed excellent values of molecular hyperpolarizability β 0 therefore, they can be considered as candidates for nonlinear optical (NLO) applications. The influence of parameters (such as chemical structure of the host material, synthesis conditions, and external stimuli) on the optical and electrical properties of the final product is also evidenced. The main materials used as host for the embedding of chromophores and fabrication methods are discussed. However, it remains the issue of processing these materials, as long as the specific optical properties have to be preserved, together with new requirements added, such as chemical stability, mechanical strength, etc. The demand of substances with NLO properties for numerous industrial applications has resulted in many articles reporting the manufacture of various novel molecules, with highly active chromophores and superior optical activity. Recent studies were focused on the synthesis of the organic molecules with special geometry and certain electronic molecular parameters to possess nonlinear optical (NLO) properties. Organic NLO materials, which can be modulated and processed readily, are of much contemporary interest because of their potential applications in modulation of optical signals, medicine, spectroscopic and electrochemical sensing, microfabrication and imaging, laser technology, data storage, and telecommunication. A better understanding was achieved on the origin of nonlinear optical (NLO) phenomena and the structure-property relationships of NLO chromophores in the late 1970s when various tools were developed to accurately measure and calculate hyperpolarizabilities. Nonlinear optics has become a vibrant field of research since the optical second-harmonic generation (SHG) was first observed in the early 1960s. The nonlinear optical properties of some new synthesized compounds were evaluated in thin films with nanometric morphology obtained using various methods: Langmuir-Blodgett (LB) thin films, sol-gel deposition, and layer-by-layer deposition. The NLO efficiency was assessed by the relationship between high (βtot) and low HOMO-LUMO energy gap.
The molecular polarizability (α), first-order hyperpolarizabilities (βtot), dipole (μtot), and quadrupole (Q) moments were computed. In condensed state, ferrocene and other organic molecules were found in a staggered arrangement (D5d) as a nonpolar molecule, but the eclipsed (D5h) and twisted (D5) forms exhibit SHG capabilities. Nonlinear optical (NLO) properties of some new synthesized derivatives are evaluated in relationship with the chemical structure by using DFT molecular modeling. Organic compounds based on ferrocene, pyrazolone, and octahydroacridine exhibit high interest due to their unusual structure, with potential practical applications due to their special optical properties.
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