*Polymer Architectures for Optical and Photonic Applications DOI: http://dx.doi.org/10.5772/intechopen.99695*

dependence of the SHG signal magnitude on the material structure. Although considerable efforts have been made to create new "push-pull" conjugate sequences, that generate a large charge in the excited state, the predicted response values have not always been obtained.

The correlation of the material response with the extension of double/single bonds alternation in the polyene structures served as a model in development of an elegant model for understanding the relationship of the chemical structure-nonlinear optical response. The model is still a useful guide to design the materials with predicted SHG properties, highlighting, as a key element in generating the NLO response, the size of the electronic conjugation at the level of the donor-acceptor substituted chains [7–15].

Centrosymmetric media are materials with a high density of polarizable electrons, which stimulates the apparition of strong third order NLO phenomena.

Increasing the magnitude of NLO phenomena in polarizable polymers involves either maximizing the intensity of the applied polarization field or designing molecules with intrinsic steric hindrance and an adequate maximization of electronic conjugation at level of the substituted donor-acceptor sequence [16–18].

As extended conjugation is a key element for third-order NLO materials, several constructive restrictions need to be observed [19]:


Theoretical and experimental paper of the field highlights that the best candidates for these applications must be characterized by a high level of extended conjugation to the chain level as well as by strong monophotonic absorptions. Thus, structures similar to polyene compounds are ideal, such as: poly (di)acetylenes, polyarylenes or poly (vinylarylenes) as good as the aromatic structures of chinoids (push-pull). [10, 11, 20–23].

The length of the conjugate sequences as well as the molecular geometry and the electrical charge of the ground state are the key factors for the type of application of the material.
