3. Fabrication of thin films with NLO properties

For most practical uses, substances with NLO properties are incorporated into a host material to ensure good dispersibility and stability. Thus, in most of the cases, polymers are used, usually with electrical and optical properties to improve the performance of the hybrid system. The polymeric derivatives applicable as matrices are the ones with high thermal stability and optical transparency. A large variety of polymers were tested as host matrix, such as polymethyl methacrylates, polyether ketones, polyimides, and polycarbonates. The most used material is polymethyl methacrylate, due to its intrinsic optical properties, stability, functionalization possibilities, and processability. Another material used as the host for NLO substances is silica matrix, which has important advantages: the facility and flexibility of syntheses as well as the possibility of processing them as thin films. Ji et al. [64, 65] report the successful embedding of Disperse Red 1 as chromophore in a silica glass prepared by using sol-gel synthesis. The silicate precursor was a new derivative 1,4 phenylene bis(4-trimethoxysilyl ethyl benzoate), with a structure that allows uniform dispersion and accommodates the chromophore molecules in the cross-linked polymeric matrix (see Figure 8).

The hybrid film is fabricated by in situ poling and sol-gel synthesis. The final material exhibits an efficient NLO behavior, with a very good thermal stability (the SHG signal is stable until 120C, and the half decay temperature is 170C.

recording [61]. Lijanova et al. synthesized dendrimers 52 with ferrocenyl-ended groups joined

The synthesis of the dendrimers with vinyl ferrocene was carried out applying the convergent Frechet approach that consists of three steps: (1) the synthesis of the two conjugated dendrons, (2) the selective formation of resorcinarenes bearing three different solubilizing groups, and (3) the alkylation of the dendrons to the resorcinarenes. Under this order, dendrons containing ferrocenylvinyl groups were prepared starting from a Heck reaction coupling of 3,5-dibromobenzaldehyde with vinyl ferrocene in dimethylformamide/triethylamine (1:0.25) using palladium acetate as catalyst. The χ(3) values estimated from the THG Maker fringe technique for ferrocenyl-ended resorcinarene dendrimers dispersed in thin solid films are of the order of

Villalonga-Barber et al. synthesized new dodecaferrocenyl dendrimer 53 using sixfold Huisgen cycloaddition. Each dendron containing four ferrocenyl groups has been synthesized by conventional synthetic methods in good yields [63]. The cyclic voltammetry studies revealed that they all undergo a reversible single-electron transfer process. The absorptions at around

For most practical uses, substances with NLO properties are incorporated into a host material to ensure good dispersibility and stability. Thus, in most of the cases, polymers are used,

450 nm for the ferrocene-containing carbinols 52 are due to ferrocene d-d bands.

3. Fabrication of thin films with NLO properties

by vinyl moieties (see Figure 7) [62].

8 Advanced Surface Engineering Research

Figure 7. Ferrocene dendrimers with nonlinear optical properties [62, 63].

10<sup>12</sup> esu.

Figure 8. Schematic representation of the silica-based cross-linked film with chromophore Disperse Red 1 (adapted with the permission from reference [64]).

Another advantage of silica as host material is the possibility to prepare polymer-silica hybrid matrix that combines the best properties from both substances. An interpenetrating polymersilica matrix was prepared [66] from polymerization of γ-methacrylpropyl)-silsesquioxane and an allyl glycidyl ether modified chromophore (Disperse Orange 3 (DO3)). The high degree of cross-link density and order leads to enhancement of the nonresonant second-order nonlinearity in the film obtained. Unfortunately, the process is very difficult and laborious, since the incorporation of the NLO chromophore in the polymer-silica network is a three-step synthesis: (i) functionalization of DO3 with glycidyl ether, (ii) synthesis of monomer (γ-methacrylpropyl) silsesquioxane using hydrolytic condensation of commercially available silane derivative, and (iii) the preparation of NLO embedded matrix through the free radical polymerization of modified chromophore and methacrylpropyl compound and the simultaneous cross-linking of the network [66].

The AFM images of the DO3-doped methacrylate-silica film before and after poling demonstrate that the poling process allows a better orientational order of the chromophore molecules (see Figure 9).

This material exhibits a very good temporal stability of the chromophore orientation in the interpenetrated hybrid polymer-silica matrix, together with superior NLO properties.

hexagonal pores, regularly 2D distributed and aligned perpendicularly to the substrate, provide large specific area for easy access to the pore interior and lead to a high loading of the active chromophore. Moreover, the optical active moieties are very uniformly distributed.

Figure 10. Schematic representation of the structure of mesoporous silica thin layer with Cu2+ propyl phosphonate

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

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

11

Since the SHG signal obtained depends on the concentration of the NLO fragments, the material with tunable NLO response could be obtained by adjusting the functionalization

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

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

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

4. The arrangement of chromophores in polymeric host matrix

chromophore bonded into pores (adapted with the permission from reference [67]).

degree from the synthesis parameters.

(Tg) of the polymer.

method (1).

Silica material mentioned above could also provide the sites to adjust covalent bonding of the chromophores. Recently, Laskowska et al. [67] reported the fabrication of NLO thin films from a novel functionalized mesoporous silica material as host matrix. To show the efficiency of the concept in tuning the NLO behavior from the morphology of the silica matrix, a very simple optically active compound is proposed, a dipolar copper propyl phosphonate fragment. The copper-containing functional groups are covalently linked to the silica wall inside the pore of the matrix. The structure of the proposed material is presented schematically in Figure 10.

The host matrix consists in mesoporous silica thin films with 2D hexagonally distributed nanometric pores. The suitable morphological characteristics of the silica material, with

Figure 9. The AFM images of the film before and after poling 2 h at 150C (reproduced with permission from reference [66]).

Synthesis and Nonlinear Studies on Selected Organic Compounds in Nanostructured Thin Films http://dx.doi.org/10.5772/intechopen.79522 11

Another advantage of silica as host material is the possibility to prepare polymer-silica hybrid matrix that combines the best properties from both substances. An interpenetrating polymersilica matrix was prepared [66] from polymerization of γ-methacrylpropyl)-silsesquioxane and an allyl glycidyl ether modified chromophore (Disperse Orange 3 (DO3)). The high degree of cross-link density and order leads to enhancement of the nonresonant second-order nonlinearity in the film obtained. Unfortunately, the process is very difficult and laborious, since the incorporation of the NLO chromophore in the polymer-silica network is a three-step synthesis: (i) functionalization of DO3 with glycidyl ether, (ii) synthesis of monomer (γ-methacrylpropyl) silsesquioxane using hydrolytic condensation of commercially available silane derivative, and (iii) the preparation of NLO embedded matrix through the free radical polymerization of modified chromophore and methacrylpropyl compound and the simultaneous cross-linking of the

The AFM images of the DO3-doped methacrylate-silica film before and after poling demonstrate that the poling process allows a better orientational order of the chromophore molecules

This material exhibits a very good temporal stability of the chromophore orientation in the

Silica material mentioned above could also provide the sites to adjust covalent bonding of the chromophores. Recently, Laskowska et al. [67] reported the fabrication of NLO thin films from a novel functionalized mesoporous silica material as host matrix. To show the efficiency of the concept in tuning the NLO behavior from the morphology of the silica matrix, a very simple optically active compound is proposed, a dipolar copper propyl phosphonate fragment. The copper-containing functional groups are covalently linked to the silica wall inside the pore of the matrix. The structure of the proposed material is presented schematically in Figure 10.

The host matrix consists in mesoporous silica thin films with 2D hexagonally distributed nanometric pores. The suitable morphological characteristics of the silica material, with

Figure 9. The AFM images of the film before and after poling 2 h at 150C (reproduced with permission from reference

interpenetrated hybrid polymer-silica matrix, together with superior NLO properties.

network [66].

10 Advanced Surface Engineering Research

(see Figure 9).

[66]).

Figure 10. Schematic representation of the structure of mesoporous silica thin layer with Cu2+ propyl phosphonate chromophore bonded into pores (adapted with the permission from reference [67]).

hexagonal pores, regularly 2D distributed and aligned perpendicularly to the substrate, provide large specific area for easy access to the pore interior and lead to a high loading of the active chromophore. Moreover, the optical active moieties are very uniformly distributed.

Since the SHG signal obtained depends on the concentration of the NLO fragments, the material with tunable NLO response could be obtained by adjusting the functionalization degree from the synthesis parameters.
