*Nonlinear Optics - Nonlinear Nanophotonics and Novel Materials for Nonlinear Optics*


#### **Table 1.**

*Most investigated NLO chromophores and their βμ-values [27, 28].*

(I (a)) or "pull-pull" (Type I (b)), depending on the direction of intramolecular load transfer: from the end to the centre of the molecule, or vice versa. This model has subsequently gained growing popularity, proving very useful in the design of one-dimensional molecules [29]. The molecular coplanarity is another key parameter in enhancing the intramolecular charge transfer efficiency [37]. At the same

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

**Figure 2.**

*Classical molecular structures for NLO chromophors: Ar aromatic block.*

time, for organic molecules, the strength of dipole in the ground- state of nonsymmetric molecules as well as the multipolar transition-dipole strength in the centrosimetric molecules are also energy factors that determine the NLO response magnitude [29, 38, 39]. Both experimental and theoretical, it has been demonstrated that the multiplication of the number of conjugation path or the connection of more than one linear path to generate two- or three-dimensional configurations, leads, in the material, to a major increase of the NLO responses value [31, 32, 38, 40].

Remarkable is the fact that when assessing the properties of NLOs, the energy changes caused by the interaction with the environment should not be neglected [41]. Thus, the polarity of the solvent, expressed by its dielectric constant, is an important structuring parameter determining on the one hand solvation by hydrogen bonds or by promoting colloidal-microaggregate dissolution. Thus, the efficiency of the cross section is more sensitive to the polarity of the local environment than to the intrinsic NLO aspects (properties) of the material [42].

#### **3.2 Functionalized polymer materials systems**

The NLO polymeric films are usually composed of long chains with chromophore sequences. Thus, chromophores, which are seen as net dipoles, are the origin of optical non-linearity in polymers. The interaction between the 'non-polar' molecules from bulk material and the chromophore, the molecular orientation (either random or of a certain symmetry) can introduce an overall orientation of the molecular assembly. This orientation will tend to cancel the individual molecular contributions. It is therefore important to ensure that all molecules are aligned correctly. The first information on this issue can be obtained by analyzing the chromophe sequence.

The starting point in developing technical strategies for the real NLO polymers synthesis is both the design of the assembly at the molecular level and the design and characterization of chromophe molecules with high molecular hyperpolarizabilities (β, γ), able to be, easily and efficiently, incorporated in a polymeric matrix.

The generous literature information, both experimental and theoretical, on the use of organic materials (especially polymers) in NLO applications is based on the composition-properties relationship. A particularly important aspect relates, in addition to practical assembly techniques, to their composition, synthesis and characterization. Therefore, for polymerist researchers, it becomes defining the aspects of the synthesis and reactivity of the monomers bearing NLO sequences as well as
