Conflict of interest

A particular kind of host-guest material exhibiting NLO properties is obtained by using semiconducting nanoparticles (quantum dots) as optical active materials. The interface phe-

An innovative approach is reported by Britton et al. [75], to significantly increase the nonlinear properties when combining in the polymeric matrix a low symmetry phthalocyanine derivative with CdSe/CdS quantum dots. The new synthesized metal-free chromophore


The synthesis of the most important classes of the organic compounds, alkyne, dye, heterocycle, thienyl, and ferrocene compounds, was presented. A lot of important reactions, like Sonogashira, Suzuki, Heck, Huisgen, Wadsworth-Emmons, Vilsmeier-Haack-Arnold, Wittig, etc., were employed for the synthesis of the new compounds with nonlinear optical applications. The synthesis of the new molecules possessing privileged structures, with a couple donor-acceptor or "push-pull" molecules, was the purpose of the last decades. Besides, new unorthodox structures, which do not possess this system, but with very good NLO properties and very good SHG signal, were reported, and the number of these increase every year. The most important parameters in the characterization of the new NLO organic compounds are the HOMO-LUMO energy gap and the first-order hyperpolarizability βtot. Small energy gaps and high hyperpolarizabilities define the best compounds with nonlinear optical applications.

Thin films were fabricated by incorporating substances with NLO properties in a large variety of polymers to improve the performance of the resulted hybrid system. Polymethyl methacrylate is the most used polymer, but silica matrix is preferred. Also, Disperse Red 1 in a silica glass allows uniform dispersion and arrangement of the chromophore molecules in the crosslinked polymeric matrix. The new hybrid films are fabricated by sol-gel synthesis. The morphology of the silica matrix is very important for a good NLO behavior. Each class of organic compounds possesses a specific method for the fabrication of the thin films in order to increase their NLO properties. Thus, azo-photosensitives dyes are encapsulated. Noncentrosymmetric arrangement for the dipolar push-pull chromophores embedded in a polymeric matrix is

nomena play an important role in tuning the NLO properties of these materials.

2,3-bis[2<sup>0</sup>

16 Advanced Surface Engineering Research

6. Conclusion

significant improvement of NLO properties.

The authors have no conflict of interest to declare.
