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

the establishment of techniques for incorporating these NLO sequences into the global polymeric architecture.

A particular aspect in the synthesis of NLO polymers, beyond optimization of the individual response of the constituent chromophore, is how the chromophore density can be maximized while preserving micro- and supra-structuring properties, respectively. The experimental studies, developed in this context, outlined two alternative strategies (**Figure 3**):


Based on theoretical prediction and quantum the most promising macromolecular materials were, initially, the electron -conductive polymers such polyacetylenes, polythiophene or poly(p-vinyl phenylene)'s. Few research groups underlined that the lengths of conjugate sequences is not a necessary preconditions for increasing the intern polarity value (implicit χ(2), χ(3)). Therefore the oligomers segments can be understood as inactive spacer with broaden the NLO activity,

**Figure 3.** *Design Motifs of NLO polymers materials.* simultaneous facilitate the processability of materials [43–45]. Moreover, the susceptibility tensor calculus, with theory the disturbing torque- Hertree-Fock, for polyene series C22H24, underline the dependence γxxxx ≈ L4 ; (L chain length) [45]. Afterwards were demonstrate that γxxxx ≈ L4,6 0,22 [46].

For poly (p-vinyl phenylene), the predictions of the value rapport γ/N, pointing out a decreasing with increasing N(dipols number). In this context, it is outlined the idea that the best polymeric structures are those consisting of conjugated oligomeric segments, connected with spacers with or without NLO activity (**Figure 4**) [22, 47].

Through processing limitations (low solubility in the usual solvents) the researcher works were directed towards finding a new modality for including these sequences in polymer structure. So, the ladder hyperconjugate moieties thiophene oligomers and/or polyamide/imide were attached to the polyenic skeletons. Such materials were characterized by high values of hyperpolarizabillity, 1011÷10<sup>12</sup> esu/cm.

The copolyamides structures with polyvinylthiophene, obtained by interfacial polymerization (Mn = 10<sup>5</sup> ), were characterized by thermal resistance until 400°C. The electro-optical analysis of these films denotes the existence of conjugate phase which generated <sup>χ</sup>(3)/<sup>α</sup> <sup>10</sup><sup>3</sup> esu/cm. This technique offers the possibility to tune the NLO activity by the number of the electrooptic segments of different species [48, 49].

The similarity of poly(urea)s properties with NLO "piezoelectrical polymers" (poly(vinyl fluorine), poly(vinyl triflourine), copolymers of the triflourine ethylene and tetra- fluorine ethylene, nylon, vinyl cyanide entitle their NLO activity. The strong interchains bond (hydrogen bonds) rule the dipoles alignment in the electric field, [50].

The d33 coefficients evolution highlight a very slow relaxation, emphasize the lead of hydrogen bond for maintaining the NLO properties of the material. Starting with these conclusions great efforts were dedicated to investigation of the polymer structures showed in the **Figure 5** [51]. The NLO properties are due to the junction of the donating electrons (2) with the π electrons system. The flexible chain (σ bonds (3) (**Figure 5**) play an important role regarding the solubility, transparence and fusibility of the polymers.

The constitutive- compositional similitude with polyureeas and the peculiar features: (i) the easy chromophores alignment for each synthesis step, the relative low viscosity; (ii) the high rate of polymerization (few seconds to several minutes) –reaction izocianate/alcohol; stabilization of NLO response by the hydrogen bonds, characteristics for polyurethanes; justifying the investigations of NLO polyurethane comportment [52–54]. The study of stability discloses in this case the importance of simultaneous development of polymerization and poling. So, by this procedure were diminished the phenomena of loss of property to reorientation (keeping 40– 60% of the initial value) [54].

#### **Figure 4.**

*The structural scheme proposed for NLO copolymers: A = segments with low NLO activity; B = oligomers segments with high NLO activity; G = functional groups [22].*

**Figure 5.** *Macromolecular materials structure used for waveguide fabrication.*

#### **Figure 6.**

*Representation of the two excited states which contribute to the γ value: E: energy of band gapes; P: transition dipole moment; for conversion the three electronic states are denoted as: 0 = G(1Ag); 1 = 1Bu; 2 = 2Ag [51].*

The contribution of π-electron sequence, their length, is not crucial for increasing the hyperporarizability value (specially **χ(3**) ). In this case significant is the sequence planarity. On the other hand, the symmetrical substitution, broadening results the enlargement of the transition bands fromHOMO-1 to HOMO and similarly from LUMO to LUMO-1. These transitions promote the third order susceptibility, theoretical prevue by relation showed in the **Figure 6**.

A special attention was granted to the epoxy macromolecular structures. These investigations are supported by the potential stabilization of the chromophore, by its covalent binding to the carrier matrix [55–57]. Another very important aspect, especially if the polymer precursor has a high reactivity, was giving rise to the crosslinked structures. The major advantage, in these cases, is using an amine chromophore monomer. So, the dipoles chromophores alignment is accomplished by crosslinking control simultaneous with polymerization process. A wide range are getting the epoxydic compounds, which is a gain for the electrooptics coefficients (10–50 pm/V) [58–63].

Also of interest for the NLO has proved to be polysiloxane structures (**Figure 7**). This orientation is justified by their versatility, ability to form the films, very useful for monolayers deposition (Lagmuir-Blodgett) and the possibility of obtaining the self- stabilized dipolar structures [64–66]. The assessment of the SHG value, for all

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

**Figure 7.**

*General structure of polyorganosiloxanes for NLO applications: R",m=6 2; n = 8 2 (R and R' are hydrogen's atoms) [65].*

monolayers, highlights a significant increase while increasing the substituant polarity. This suggests that the package of each polymer chain in the monolayer is orderly, and the interchain bonds are essential in achieving the noncentrosymmetric structure.

Perhaps the most investigated NLO material class is azobenzene derivatives, known and studied for a very long time, their initial use being dye for a variety of substrates. Azobenzenes Photoisomerization is a well-known phenomenon, dedicated to reference materials [67, 68].

The key to exploiting the unique behavior of the azobenzene sequence is defined by how it can be included in the functional materials. For the generation of new properties, the most effective method of incorporation of azobenzene is the covalent attachment to polymers. The materials thus obtained will show both properties specific to polymers: inherent stability, rigidity and processability as well as conformational change under the action of electromagnetic radiation, specific to azo segments - photoisomerization processes. The covalent incorporation of the azo sequence in the polymer, through its photoisomerization process can generate a wide range of phenomena, even unexpected ones [69].

About 25 years ago, at dispersing of azobenzene groups in the polymer matrix, the unique phenomenon recorded was associated with their photoisomerization [70].

Under the action of polarized laser radiation, in polymer films with azobenzene sequence, their orientation is perpendicular to the polarization direction. Dichroic phenomena are thus generated.

This type of phenomenon is reported in the years '80 in liquid crystal polymers with photoactive azobenzene mesogenic sequences [71, 72]. Simultaneously, in Japan, the "command surfaces"concept [73] is opened, in which the azobenzene group acts as 'commander' and the Langmuir — Blodgett liquid films play as 'soldiers', which are aligned in trans/cis conformations by azoderivatives —the switching initiated by illumination [69]. After 1991, the applicative exploitation of these phenomena expanded worldwide, highlighting interesting and unexpected phenomena, which diversify use in reversible optical memory applications as well as photonic applications. Over the last 40 years, an exponential development of the domain has been observed and there is still no analysis covering all these new implications of azobenzene photoisomerization in polymeric structures. Remarkable and still of real interest remain the reviews which treat, generally or in a specific way, phenomena generated by the presence of azoderived sequences

[74–78]. However, the best available analysis covering both photoinduced birefringence and surface relief grating dates back to 2000 [79].

The special interest in azobenzene polymers is the result of their particular properties, which are useful in the processing and storage of optical information [69, 80–82]. The sin-anti conformational conversion process at the bond N = N, presumes some distinct transformations, highlighted in **Figure 8a**): (i) decoupling of the π electrons from the double bond, (ii) rotation around the single bond N-N followed by (iii) restoring the double bond, N=N, in a new configuration. Assisted by the adequate wavelength radiation, this phenomenon is governed by electronic transitions π-π\*, respectively n-π [23].

The molecular photoorientation of chromophe sequences is governed by anisotropic absorption, so that under the action of incident radiation, the dipole moment of chromophe sequence aligns along the molecular axis (**Figure 8b**). The linear polarized light is capable to induce a centro-symmetrical orientation in which the azoic sequence is directed perpendicular to the light plane. These oriental oscillations of the dipole moment's are transferred to the macro-scopical level, stimulating the emergence of optical dicroism and birefringence.

Therefore, the energy metastability of the sin-isomer plays a decisive role in both the theoretical interpretation and the practical application of the photoinduced phenomena in azoic polymers. For example, applications that rely on the modulation of the molecular alignment of CL will use materials with high stability of the sin-isomer (holography, optical data storage) while operational devices based on optical birefringence and dicroism phenomena will only use structures with rapid reversion of the sin-isomer.

The Red Dispers-based materials are the most representative in the class of polymers with the azobenzene side-chain NLO sequence [83–87]. Although noted by good temporal stability of the orientation (preserving about 80–90% of the maximum property value at 1000 hours after orientation) and, in addition, they admit a wide range of refractive index values from UV exposure, a still ongoing problem remains the stabilization of their orientation through photo crosslinking [88].

As the free radical mechanism is easily accessible, an informational richness covers the synthesis, characterization and use of (meta)acrylic and styrenic (co) polymers, with side-chain azo sequence (**Figure 9**) [74, 88–98].

#### **Figure 8.**

*Conformational changes in azobenzene: (a) A: Rotation of substitutes around–N = N- bond; B: Inversion of substitutes; (b) the azobenzene - light interaction.*

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

#### **Figure 9.**

*Structures for methacrylate and styrene materials for NLO applications: a) R = CN; NO2; N(CH3)2; OCH3, CH3; X = alkyl; Chemical modifications of polystyrene and/or chloromethylestyrene; c) synthesis of NLO styrene monomer structure.*

The versatility and easily processing of methacrylic polymers recommend these for NLO applications. The new monomers structures are investigated for chemical reactivity in the (co)polymerization [99–101] as well their electrooptic characterization [102–105]. These research points the inclusion of these cromogene sequences in copolymer structures without major difficulties. A peculiarity of these monomers is their predisposition to an hight chain transfer, which involves the design of syntheses in compositional-operational conditions so as not to drastically reduce the degree of polymerization. In addition, the presence of coloured monomers leads an increase in the rate of polymerization, the phenomenon of self-acceleration becoming obvious. The sequential distribution confirms the predictions obtained from the capitalization of the reactivity ratios and at the same time, the characterization of the material proves the preservation of the optical characteristics of the chromophore.

The styrene materials are preferred relative to methacrylic materials. They are so much easier obtained by chemical modifications (**Figure 9b**): usually is used one of the vinyl benzyle chloride isomers. The drawback of these polymers is the relative rigidity of the chromogen sequence due to the aromatic spacer. The major advantage for these SC structures is the decupling of the chromogene, which facilitates the molecular dipole orientation; but the poling-induced order in organic/polymeric materials is thermodynamic unstable. Thus, an important objective for the NLO polymer synthesis is the structural induction of slow relaxation in the acquired polarization orientation.

A promising alternative is azobenzene SC liquid crystalline polyester architecture [106, 107]. These, by their own nature, extend flexibility at the base-catene level by interactions with both acid and glycolic sequences that can serve as the link sequence of the mesogene. Thus, by modular construction, can be adjusted the length of the flexible methylene spacer in the lateral chain, the substitute on the azobenzoic fragment, the length of the methylene sequence in the main chain (all aliphatic) and the molecular weight of the polyester. Moreover, each parameter obtained by the versatility of the esthetically bond significantly influences the optical storage behaviour of the materials [108]. All materials (with a wide range of substitutes: cyano, nitro, methoxy, hydrogen, methyl, n-butyl, phenyl, fluorine, trifluoromethyl, chlorine, brom) have a diffraction efficiency of over 50%, giving these materials great diffraction properties. The stored information on azo copolymer media (75–100% azo- dye content) can be partially erased up to 80°C [109].

The copolymers of maleamic monomers with styrene and vinyl benzyl chloride and theirs preliminary characterizations of these materials proves that these monomers can be used for build NLO materials [110–112].

A critical issue of materials used in applications based on NLO properties is their thermal stability. This shortcoming can be addressed by promoting (after polarization) cross-linked structures, in particular by photochemical processes. Preferred for this direction are aromatic polyimides (PIm), attached to the polymer frame. The chromophore can be 'grafted' onto PIm sequence through transformation reactions. The method is advantageous because it allows the attachment of a different range of chromophores to polymer supports with a wide compositional variety, promoting the production of materials with high Tg (around 220°C), excellent solubility and processability. In addition, the topological stiffness, induced by the aromatic sequence, results in the preservation of high values of electro-optical coefficients during long periods of use [113]. In this way, by coupling reaction, was synthesized the poly(phenylene- imide thiophene) materials (PPIT) (**Figure 10**) [114]. They showed high Tg and consequently high temporal stability of the SHG, as demonstrated by dynamic signal decay behaviour studies that attest their stability at temperatures up to 150°C, preserved more than 82% of the SHG signal after 1500 h of operation**.**

An elegant and efficient method of synthesis of PImss functionalized with second-order NLO, is the one-step synthesis (**Figure 11**) [115], which allows the use of the most popular electron-acceptor moieties. This give the materials with Tg values in the range 205–224°C [113, 116–119], whose thin films, corona-poled and SHG analyzed, are characterized by enhanced d33 value (30 pm / V at λ = 1064 nm). The study of the preservation of SHG properties under thermal stress – as a function of annealing time at 125°C - reveals that the NLO response, after 9–20% initial decrease, does not change significantly over a period of 210 h for polymers: (PIc-e). (PIc), (PId) and (PIe), with better stability than PIb at 125° C, retaining about 70% of the NLO properties (**Figure 11**).

Step-by-step polyaddition reactions is another tool for synthesis of thermally stable, second-order NLO chromophore PIms and poly (urea), starting from

#### **Figure 10.**

*Structure of new poly(phenyleneimide) (50.85% chromophores,Tg = 170°C; Td = 245°C, Φ = 23; r33 = 35 pm/V).*

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

**Figure 11.**

*Poly(imide)s containing NLO chromophores based on 2-(5, 5-dimethyl-cyclohex-2-enylidene)-malonitrile and conventional chromophores.*

2,3-bis (4-aminophenyl) -5,6-dicyanopyrazine (BAPDCP), who's the first order molecular hyperpolarizability, β, was evaluated as 123.5 10–30 esu [120].

Using a new polymerization methodology, were produced the high Tg polymers [121] introducing into polymer backbone an imide–siloxane linkage containing NLO-active chromophores. As expected, introduction of dimethylsiloxane linkages in the backbone led to enhanced solubility and thermal stability of the functionalized PIms while retaining their useful physical characteristics, such as Tg: These polymers show SHG efficiencies comparable to those for functionalized PIms or polyurethanes and exhibit high temporal stability of resultant SHG signals.

Poly (maleimide) polymers functionalized with aminoalkyl sequences have a transition temperature in the range of 178–228° C, as well as a stable NLO response at high temperatures: such systems lose 24% of their property after 1000 hours at 125° C. The value d33 of SHG coefficient, 64.0 pm/V measured at 1064 nm, suggests that these polymers could be useful for NLO-applications [122].

The synthesis of polymers with electro-optical characteristics by sequential selfrepetitive reaction (SSRR) [123] offers the possibility of making poly (amide-imide) structures. The promotion, in this case, of the reaction between dysfunctional azo chromophores (DR19, NDPD, DNDA), with an excess of 4,4-methylenediphenylisocyanate (MDI), ends with the formation of a carbamic structure, useful for obtaining poly (carbamate diphenylisocyanate) (poly CDI- **Figure 12**). Subsequent addition of trimellitic anhydride (TMA) to the poly-CDI solution leads to the poly (N-acylurea) intermediate, characterized by good solubility. The in-situ poling and curing process, of this reactive mixture, favors the formation of the amideimide structure of the N-acylurea sequence. The correlation of the thermal behaviour with the electro-optical characteristics, for these polymers, highlights values of r33, in the range of 5.2–25.2 pm / V at 830 nm. There is also a proportionality of these values with the chromophore concentration. Such structures are characterized by good thermal stability- (80°C and optical waveguide losses (3.8–6.6 dB/cm at 830 nm) [124], which argues that when using the SSRR technique, the values of the EO coefficients are the consequence of the orientation of the chromophores on the polarization direction before lattice hardening.

Carbazole and its derivatives remain in the attention of chemists, mainly due to the fact that the carbazole skeleton has proven to be a versatile platform for the development of materials with applications in specialized fields such as optics, thermoelectronics as well as medical and pharmaceutical applications.

#### **Figure 12.**

*Poly(amide-imide)s NLO polymers structures obtained by SSRR.*

Belonging to a very interesting and relatively new class of material-conjugated polymers, carbazole and its derivatives are manly characterized by the existence of π-mobile electrons, which can be used as chromophores, respectively electrophores [125]. The size of the energy difference between the conduction band and valence band, customizes the wavelength of absorption and / or radiative emissions, configuring the conduction capacity. What differentiates conductive polymers, expressly carbazoles, in the vast class of polymeric materials (known as true insulators) is their ability to carry electrical charges. This behaviour brings them closer to the electrical conductors - metallic or semiconductor.

Such materials are used both in the field of powerful light transmitters for sensors or as active components in electronic (opto) devices and batteries [126–129].

The main advantages of carbazole compounds are related to the economical, processability and their properties, particularly regarding few aspects:


Due to the mentioned advantages, a wide range of materials for transporting holes [130, 131], molecular glasses [132–135] or light-emitting materials [136–140] *Polymer Architectures for Optical and Photonic Applications DOI: http://dx.doi.org/10.5772/intechopen.99695*

**Figure 13.** *Carbazole structures: reactive positions correlated with material structure and its particular properties.*

have been designed with carbazole. Favourable to the synthesis of different dyes architectures (push-pull dyes; push-push or pull-pull), through substitution reactions to aromatic nuclei, carbazole was and remains an excellent candidate for the design of light harvesting materials for solar cells and numerous dyes with large and wide molar extinction coefficients the absorption spectra were obtained with this plan electron donor [141, 142].

Carbazole derivatives are intensely studied because of their well-known potential as precursors of materials for optical/ optoelectronic applications. The characteristics that recommend them for use are their special photorefractive, electrical, and chemical properties. Carbazoles are well known as a conjugated, good holetransporting, electron-donor, planar compound and ease to introduce solubilizing groups to rigid ring structure. Carbazole can polymerize and couple in positions 3, 6, 9 and 1, 8 respectively. However, due to the very rigid structure of the carbazole, the last pair of positions is sterically hindered [143, 144]. The very high reactivity of positions 3, 6, favors the rapid synthesis of carbazole derivatives starting from 9Hcarbazole, by direct bromination of the carbazole group with N-Bromo succinimide (NBS) [145].

In the context of the progress of new techniques for the manufacture of intelligent devices, the constructive versatility of the carbazole sequence, stimulated the development of design new dyes classes with photoinitiation activity in UV, near-UV or visible light, very useful in the development of 3D printing [146].

Substitution reactions at the carbazole unit can generate poly (3,6-carbazole) and poly (2,7-carbazole), respectively. These structures, due to the effective difference in the length of the conjugate sequence ((2, 7) is the longest, being similar to polyp phenylenes) have different properties and potential applications. In addition, using specialized modification strategies, the properties of both polycarbazole classes can be improved / fine-tuned, thus diversifying high-performance applications in the electronic and electro-optical field, such as polymeric light emitting diodes (PLEDs), organic field-effect transistors (OFETs), and photovoltaic cells (PCs). For both categories of polycarbazoles, an elegant and very complete presentation of the structure-properties-application correlation is summarized in several reviews, among which we recommend [147–149].

From this multitude of papers, the authors summarize studies dealing with novel aspects in poly (3,6-carbazole) synthesis, particular aspects of the photoactive

properties of polyvinyl carbazole copolymers, as well as other derivatives with polymerizable group in position 9 of carbazole, adjacent to the poly (2,7 carbazole) derivatives, strongly predisposed for optoelectronic applications such: organic LED, photovoltaic cells and biosensors.

Thus, the first decades of the 21st century are distinguished by the extension of studies dedicated to the synthesis and characterization of new optically active photochromic polymeric structures, with two distinct functional sequences: azo and a chiral group [150–153]. The simultaneous presence of these functions, sensitive to electromagnetic stimuli, creates the premises to display both the typical properties of asymmetric systems (optical activity, exciton division of dichroic absorption) and the typical characteristics of photochromic materials (photo refractivity, photo reactivity, N properties). In this context, it seemed interesting to investigate the properties of new multifunctional copolymer materials containing both photo responsive azobenzene and the photoconductive carbazole chromophore directly bound to the polymeric side chain through the chiral fragment: the presence of the electron-rich carbazole sequence as partner of the electron-poor azobenzene chromophores can induce charge transfer interactions fundamental in the operation of photoconductive materials [154].

The literature highlights the appearance of charge transfer interactions in achiral copolymers of N-vinyl carbazole (NVK) with methacrylic azoderivatives, the intensity of which is stimulated by the alternation of monomer sequences [155–159].

Due to their complexity of structural behaviour, they are able to generate sophisticated synthetic architectures, with high sensitivity and selectivity, which expands the application range. By consequence, they may be adopted special applicative functions, such: with molecules and ions they can be utilized in different separation and purification techniques; by interactions with the electromagnetic stimulus in solar cells fabrication, organic light emitents and optical elements, by specifical interaction with the biological sequences they developed applications in anti-biofouling or specific binding proteins, by specifical interactions in adhesive assemblies, they can generated bonding-disbonding phenomena's, by peculiarity of theirs response to the stimulus action they can behave as self-healing agent for other different materials, by interactions between different polymer's sequences can be achieved a multi-layered polymer materials with different functionalities in applications: screen printing, stamping ink-jet-as commercial used technology or optical waveguide, elastomeric light-emitting devices and displays or molecularly stretchable electronics- as specialized technologies.
