**3. Structural features of NLO polymer materials**

In the previously outlined context, the researchers' attention was focused on studies to generalize the NLO response-chemical structure correlation, while spending efforts on applied research aimed at creating new photonics and photoelectric devices. The variety of physicomechanical properties as well as the compositional versatility of conjugate organic materials placed them at the center of these studies. The π conjugate organic materials with large electronic polarizabilities and, since the nonlinear response is a weighted summation of higher molecular polarizabilities, it becomes reasonable for conjugate molecules to possess substantial hyperpolarizabilities.

#### **3.1 NLO chromophors**

The researches activities in the materials fields as well as application development mainly based on multiphoton absorption (MPA) process continue with

consistency to demonstrate the successful proof-of-concept in diverse areas, such as microfabrication, bioimaging, photodynamic therapy, frequency upconverted lasing, etc. The mid-1990 is marked by tempestuously development in the design and synthesis of the new dyes, which are more efficient than those commercially available. Consequently, in the coming decades it became evident that these together with theirs processability, photostability, and durability, that depends on the application, are the basic elements for the specificity of physical motif. The great continuous challenge for technological exploration remain how to establish and fine-tune structure-properties relationships for these large numbers of organic, organic–inorganic materials assemblies, taking into account to the varied molecular structural factors preserving the reproducible NLO properties. It was known that molecular hyperpolarizability increases with the π-electron conjugation length of the chromophore. Although it would be expected that the increase in chemical reactivity and optical absorption would be directly proportional to the length of the conjugation at the chromophore level, it has experimentally proved that it is not the best approach to optimize nonlinearity. Theoretical models developed for molecules prone to charge transfer show that the strength and energy balance of donoracceptor sequences, correlated with the length of the electronic bridge, directly affect the value of molecular hyperpolarization [24–26]. Thus, by adjusting the strength of the donor and acceptor sequences, the moment of the fundamental state, respectively excited, is directly determined. These peculiarities claim that the increase of the value β is the consequence of the mesomeric interactions of the strong donor-acceptor groups. In conclusion, in the case of organic compounds, the structure-properties relationship is closely related to the strength of the D-A bond, as well as to the widening of the conjugation in the structural unit:

$$
\beta \approx \left(\mu\_{\rm ee} - \mu\_{\rm gg}\right) \mu\_{\rm ge}^2 / \mathbf{E}\_{\rm ge}^2,
$$

where: g = ground sate index; e = excited state index; μgg = ground state molecular dipole; μee = excite state molecular dipole; μge = transition state molecular dipole; Ege = transition energy.

Pursuant to this equation, hyperpolarizability is directly determined by the change of the dipole moment and the oscillator power (expressed by the square of the matrix of the dipolar elements) and is inversely proportional to the square of the transition energy. Each of these parameters has a maximum value closely related to the chemical structure of the molecule.The specific function (electronic exchange) which determine the NLO character, as well as the strength D-A, shall decided by the variation of all mentioned factors.This dependence can be found in the dominant β value, approximate by Hückel series for molecular orbitals calculation.

Thus the coulombic energetical difference between donor (αD) and acceptor (αA) can be explained as energetical HOMO-LUMO dissimilarity. The parameters μ2 ge and E<sup>2</sup> ge show maxima value for the null molecular asymmetry, so: μee μgg ≈α<sup>D</sup> α<sup>A</sup> (**Figure 1**) [8]. Consequently, at increasing of molecular asymmetry α<sup>D</sup> -α<sup>A</sup> > 0, μee-μgg increase with the difference αD-αA. Increasing the αD-αA, the mixing of bridge orbital's D-A decrease in the same time with transition HOMO-LUMO. At the same time, there is a decrease in the charge transfer character, specific to stilbenic molecules substituted with D -A groups.

The D-A molecular structures are limited on the one hand by the coulombic factor (which may have negative, positive or zero values depending on the level of charge separation in the molecule) and on the other hand by the resonance energy, the consequence of the aromatic electrons involved in the conjugate DA.

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

**Figure 1.** *The dependence of μee-μgg (1), β (2), μge <sup>2</sup> (3), and 1/E<sup>2</sup> ge (4) as function to* α<sup>D</sup> αA*.*

In addition, the dipolar transition moment show a sinusoidal variation for β with D-A strength while maintaining a consistent bridge of the conjugate bridge (**Figure 1**). Chromophores that were initially used in the NLO study were of limited success as they failed to reach the required absorption maxima in the near IR wavelength range (**Table 1**). There is a strong correlation between intramolecular charge transfer processes and NLO response, if discussed in terms of electronic structures and physical processes. Therefore, the design of an ideal chromophore structure with high NLO activity is based on some fundamental aspects at the molecular level. In this idea the intermolecular charge transfer, as a driving force, is necessary but not enough, so it is clear that there must be a high electronic density in the material. On the other hand, another fundamental problem in the operation of NLO materials is the length of the conjugate (i.e., the path length), whit determine the hight and permanent charge separation at molecular level [18, 29–35].

At the outset of reviling the particularities of molecular structures with NLO properties, two general types of organic molecules predisposed for these phenomena are defined (**Figure 2**) [36]:


Improved solubility required the attachment of pendant alkyl chains to the aromatic sequence. At the same time, the attachment of electron-donor/acceptor groups models the intrinsic electronic density of high molecular weight polymers. The criteria for achieving symmetrical structures (type I (a) and I (b)) are based on the correlation between the large loads of the quadrupole moment during photoexcitation and the cross section. Consequently, molecules characterized by the alternation of vinyl and 1,4-arylene groups as well as the connectors between two identical end groups (i.e., electron- acceptor or donor) are preferred to realise of many NLO chromophores. Such structures are described generically as "push-push"
