4. The arrangement of chromophores in polymeric host matrix

The macromolecular host matrix encapsulates the chromophores either dispersed or attached on the polymeric chain. In both cases, the most important issue that appears is the proper arrangement of NLO active molecules. The common procedure to obtain the required noncentrosymmetric arrangement for the dipolar push-pull chromophores embedded in a polymeric matrix is based on electric field poling process. It consists in the application of the electric field across the material, when heating the system to the glass transition temperature (Tg) of the polymer.

When photosensitive dyes are encapsulated, for example, azo-derivatives, light can be used in order to replace the heating procedure (photoassisted poling). This method is a choice when temperature-sensitive NLO molecules are involved, enabling poling process preformed at temperatures well below Tg. A large variety of azo-derivatives have been poled using this method (1).

Other guest molecules may be covalently bonded to the polymer chain, but the chemistry involved in the attachment of the chromophore to the macromolecule still remains a major challenge. Severe conditions in synthesis and processing employed to prepare most of sidechain NLO polymers (polymers with grafted chromophore as side chain) limit their practical application. The use of these polymer-based materials generates several film properties required for photonic applications, such as thermal and chemical stability, high loading of the active molecule, and stability of the chromophore orientation [68]. Highly cross-linking of sidechain polymers enhances the stability of the thin film together with the chromophore moiety arrangement.

Another covalent bonded variant is main-chain NLO polymer, with chromophores linked in the backbone at either end of the chromophore or linked at both ends of the chromophores, to form the polymer backbone. The dye fragment can be processed in various configurations: a head-to-tail (isoregic), head-to-head (syndioregic), or in random head-to-tail and head-to-head configurations (aregic) (2) [69].

> Usually, Langmuir-Blodgett films are obtained from Langmuir film deposited at the air-water interfaces with the well-known orientation of the molecule with the hydrophobic groups in air

> Figure 11. The representation of monolayer structure with various packing densities for azobenzene NLO compounds

Synthesis and Nonlinear Studies on Selected Organic Compounds in Nanostructured Thin Films

The driving forces of coherent monolayer formation are the hydrophobic interactions between the nonpolar parts and the interaction between the polar groups and the water subphase. Wang et al. [71] propose an unconventional method to prepare NLO LB films based on the molecular electrostatic interaction of hydrogen bonding in a chromophore modified with urea. The new synthesized optic active compound is 1-(10'-[(10-nitro)-6,7-azobenzenl]-ether-decyl)- 3-(tetracosa-12,14-diynyl)urea (NAEDTDU), which exhibits an unusual packing behavior at

/molecule, smaller than the molec-

http://dx.doi.org/10.5772/intechopen.79522

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/molecule).

air-water interface as it is proven by the П-A isotherm (see Figure 12).

bearing acetyl, nitro, and cyano groups (adapted with the permission from reference [70]).

ular area of the usual urea-containing derivatives in Langmuir film (50 Å<sup>2</sup>

Figure 12. П-A isotherm of Langmuir film of NAEDTDU (adapted with the permission from reference [71]).

The recorded specific molecular area of NAEDTDU is 35 Å<sup>2</sup>

and hydrophilic ones immersed in water.
