2. Synthesis of compounds with NLO properties

Recent literature shows some classes of organic compounds suitable for applications for molecular switches that possess certain characteristics, such as high molecular hyperpolarizability coefficients (β), special geometry, and in most cases small HOMO-LUMO energy gaps [25–27]. In the following sections, we will refer to the recent synthesis of several classes of organic compounds possessing the abovementioned characteristics.

There are several classes of organic compounds in this category, from which some representa-

Figure 2. Organic structures that contain a couple: donor-acceptor or a "push-pull" system [32].

the Friedel-Crafts and the Lawesson reactions by Raposo et al., have been proven as promising NLO chromophores (see Figure 2). Compounds 11 exhibit high molecular nonlinearities, as their values are 13–16 times higher than that of the well-known 4-nitroaniline (PNA) and the

Kleinpeter et al. demonstrated that compounds 12 possessed excellent values of molecular hyperpolarizability β0; therefore, they can be considered as candidates for nonlinear optical (NLO) applications [34]. A series of pyrazole-5-ones 13, synthesized by azo-coupling reactions, which present only hydrazo tautomers with a planar structure, responsible for their nonlinear optical properties were discussed in terms of NLO parameters by Marinescu et al. [32]. Herein, the NLO efficiency of compounds was assessed by the relationship between high βtot and low HOMO-LUMO energy gap. Also, these compounds can function as ligands for erbium complexes, which possessed solvatochromic properties and have NLO material characteristics [35]. Zajac et al. reported that the synthesis of benzothiazole dyes 17 and their NLO properties was investigated [36]. Formulation of 2-heteroaryl-benzothiazoles by Vilsmeier-Haack reaction gives the corresponding carbaldehydes, which subsequently underwent a Wittig reaction

corresponding β<sup>0</sup> values are 2.5 to 4 times higher than that of PNA (see Figure 3) [33].


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tives will be presented.

Figure 1. Alkyne compounds with NLO properties [28–30].

providing target compounds in good yields [36].

2.2.1. Push-pull dyes Arylazo-5-alkoxy-2,2<sup>0</sup>

### 2.1. Alkyne compounds

Ryhding et al. synthesized by Sonogashira Pd-catalyzed cross-coupling reactions a series of new oligo(p-phenyleneethynylene) 1–4 (Scheme 1) [28]. Miao et al. successfully synthesized a series of novel donor-acceptor chromophores 5–8 in an excellent yield by metal-free [2 + 2] click chemistry. These compounds possessed energy-level modulation of the organic π-conjugated aniline derivatives (see Figure 1) [29].

The new derivatives 9–10 were obtained by Michinobu et al. in quantitative yield (96–100%) by [2 + 2] cycloaddition between tetracyanoethylene- and dimethylanilino-substituted alkynes, followed by electrocyclic ring opening of the initially formed cyclobutenes [30].

#### 2.2. Push-pull derivatives as NLO chromophores

The organic molecules which contain a couple, donor (D)-acceptor (A) (or "push-pull" system), connected to a system which contributes to the delocalization of the π-electrons make the classic structures with optical response due to large hyperpolarizabilities that arise from a combination of strong electron-donating groups (e.g., ▬NR2, ▬OR) and strong electronwithdrawing groups (e.g., ▬NO2, ▬CN), positioned at the opposite ends of a conjugated system (see Figure 2) [16, 23, 31, 32]. Therefore, the synthesis of the classic structures occupies a central place even in the current research.

Synthesis and Nonlinear Studies on Selected Organic Compounds in Nanostructured Thin Films http://dx.doi.org/10.5772/intechopen.79522 3

Figure 1. Alkyne compounds with NLO properties [28–30].

Figure 2. Organic structures that contain a couple: donor-acceptor or a "push-pull" system [32].

There are several classes of organic compounds in this category, from which some representatives will be presented.

#### 2.2.1. Push-pull dyes

electrochemical sensing, microfabrication and imaging, laser technology, data storage, and telecommunication [3]. Recent studies were focused on the synthesis of the organic molecules with special geometry and certain electronic molecular parameters to possess nonlinear optical (NLO) properties [3–24]. The demand of substances with NLO properties for numerous industrial applications has resulted in many articles reporting the manufacture of various novel molecules, with highly active chromophores and superior optical activity. However, it remains the issue of processing these materials, as long as the specific optical properties have to be preserved, together with new requirements added, such as chemical stability, mechanical strength, etc. The main materials used as host for the embedding of chromophores and fabrication methods are discussed. The influence of parameters (such as chemical structure of the host material, synthesis conditions, and external stimuli)

Recent literature shows some classes of organic compounds suitable for applications for molecular switches that possess certain characteristics, such as high molecular hyperpolarizability coefficients (β), special geometry, and in most cases small HOMO-LUMO energy gaps [25–27]. In the following sections, we will refer to the recent synthesis of several classes of

Ryhding et al. synthesized by Sonogashira Pd-catalyzed cross-coupling reactions a series of new oligo(p-phenyleneethynylene) 1–4 (Scheme 1) [28]. Miao et al. successfully synthesized a series of novel donor-acceptor chromophores 5–8 in an excellent yield by metal-free [2 + 2] click chemistry. These compounds possessed energy-level modulation of the organic π-conjugated

The new derivatives 9–10 were obtained by Michinobu et al. in quantitative yield (96–100%) by [2 + 2] cycloaddition between tetracyanoethylene- and dimethylanilino-substituted alkynes,

The organic molecules which contain a couple, donor (D)-acceptor (A) (or "push-pull" system), connected to a system which contributes to the delocalization of the π-electrons make the classic structures with optical response due to large hyperpolarizabilities that arise from a combination of strong electron-donating groups (e.g., ▬NR2, ▬OR) and strong electronwithdrawing groups (e.g., ▬NO2, ▬CN), positioned at the opposite ends of a conjugated system (see Figure 2) [16, 23, 31, 32]. Therefore, the synthesis of the classic structures occupies

followed by electrocyclic ring opening of the initially formed cyclobutenes [30].

on the optical and electrical properties of the final product is also evidenced.

2. Synthesis of compounds with NLO properties

organic compounds possessing the abovementioned characteristics.

2.1. Alkyne compounds

2 Advanced Surface Engineering Research

aniline derivatives (see Figure 1) [29].

2.2. Push-pull derivatives as NLO chromophores

a central place even in the current research.

Arylazo-5-alkoxy-2,2<sup>0</sup> -bithiophene-conjugated dyes 11, synthesized through a combination of the Friedel-Crafts and the Lawesson reactions by Raposo et al., have been proven as promising NLO chromophores (see Figure 2). Compounds 11 exhibit high molecular nonlinearities, as their values are 13–16 times higher than that of the well-known 4-nitroaniline (PNA) and the corresponding β<sup>0</sup> values are 2.5 to 4 times higher than that of PNA (see Figure 3) [33].

Kleinpeter et al. demonstrated that compounds 12 possessed excellent values of molecular hyperpolarizability β0; therefore, they can be considered as candidates for nonlinear optical (NLO) applications [34]. A series of pyrazole-5-ones 13, synthesized by azo-coupling reactions, which present only hydrazo tautomers with a planar structure, responsible for their nonlinear optical properties were discussed in terms of NLO parameters by Marinescu et al. [32]. Herein, the NLO efficiency of compounds was assessed by the relationship between high βtot and low HOMO-LUMO energy gap. Also, these compounds can function as ligands for erbium complexes, which possessed solvatochromic properties and have NLO material characteristics [35]. Zajac et al. reported that the synthesis of benzothiazole dyes 17 and their NLO properties was investigated [36]. Formulation of 2-heteroaryl-benzothiazoles by Vilsmeier-Haack reaction gives the corresponding carbaldehydes, which subsequently underwent a Wittig reaction providing target compounds in good yields [36].

Figure 3. Push-pull dyes as NLO chromophores [33–36].

#### 2.2.2. Push-pull structures from different classes of heterocyclic compounds

Push-pull molecular structures based on angular benzobisthiazolium acceptor were synthesized by Cibova et al. A comparison of selected spectral and nonlinear optical properties of the prepared compounds shows that compound 18 possess the best NLO properties (see Figure 4) [37]. Hebbar et al. describe the synthesis of various push-pull molecules 19–20 with a central pyrazine unit connected to a hexatriene chain terminated by various p-substituted phenyl groups. The compounds were synthesized by a Negishi coupling reaction followed by Wittig and Wadsworth-Emmons reactions [38]. Cho et al. synthesized new efficient pushpull organic semiconductors comprising of the bis(9,9-dimethyl-9H-fluoren-2-yl)aniline donor and the various acceptors such as NO2, 4-dicyanomethylene-6-tert-butyl-4H-pyran, and 2-dicyanomethylene-3-cyano-5-dimethyl-2,5-dihydrofuran, which were linked with bithiophene or vinyl bithiophene p-conjugation bridges, were synthesized, and their photovoltaic characteristics were investigated in solution-processed small molecule organic solar cells (SMOSCs) [39].

for tuning the electrical and optical properties of surfaces with controlled self-assembled monolayer (SAM) formation [42]. Ion et al. reported for smooth/compact 1,2,3,4,5,6,7,8 octahydroacridine 29 thin films grown by matrix-assisted pulsed laser evaporation (MAPLE) the presence of the SHG (second-harmonic generation) signal related to the conformational asymmetry of the OHA molecule and therefore the nonlinear optical applications of it [43].

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Thiophenes are among the most studied heterocyclic spacers for D-π-A systems due to their relatively low resonance energies and have allowed the preparation of chromophores with high stabilities and nonlinearities [44]. Raposo et al. reported the synthesis of formyl-

rials, by functionalization of the pyrrole or thiophene ring of thienylpyrroles using different methods: Vilsmeier formylation or metalation followed by reaction with DMF (see Figure 5) [45]. Batista et al. reported the synthesis of new chromophores with second-order nonlinearities containing thienylpyrrolyl and benzothiazolyl moieties. The solvatochromic behavior of the compounds was investigated, and the hyperpolarizabilities β of derivatives 33–35 were measured using hyper-Rayleigh scattering [46]. A series of thienylpyrrolyl π-conjugated systems attached to functionalized benzimidazole heterocycles 36–37 were developed by Batista et al. [47]. These chromophores possess excellent solvatochromic properties and good molecular optical nonlinearities. Herbivo et al. reported the synthesis of a series of formyl-substituted


2.3. Thienyl compounds as NLO materials

Figure 4. Push-pull molecules as NLO chromophores [37–43].

substituted 1-alkyl(aryl)-2-(2<sup>0</sup>

The compounds 21–23 were readily prepared through Knoevenagel condensation reaction with 5-bromo-5'-nitro-2,2<sup>0</sup> -bithiophene, 2-(2-tert-butyl-6-methyl-4H-pyran-4-ylidene) malononitrile, and 2-(3-cyano-4,5,5-trimethyl-5H-furan-2-ylidene)malononitrile, respectively, and a palladiumcatalyzed Suzuki coupling reaction with 2-bromo-5-nitrothiophene using phase-transfer catalyst. Shi et al. synthesized boron dipyrromethene 24 derivative bearing an electron-donating 4- (dimethylamino)phenylethynyl group and an electron-withdrawing 4-nitrophenylethynyl group in the opposite two and six positions by Knoevenagel condensation followed by sequential Sonogashira coupling reactions and the second-order nonlinear optical properties of this compound has been studied [40]. Farat et al. synthesized novel xanthene push-pull chromophores and luminophores 25–26 and show these structures as the more pronounced vibrational bands in the luminescence spectra, which indicate the more rigid structure of these molecules in the excited state [41]. Malytskyi et al. synthesized thiophene-based push-pull chromophores 27–28 Synthesis and Nonlinear Studies on Selected Organic Compounds in Nanostructured Thin Films http://dx.doi.org/10.5772/intechopen.79522 5

Figure 4. Push-pull molecules as NLO chromophores [37–43].

2.2.2. Push-pull structures from different classes of heterocyclic compounds

Figure 3. Push-pull dyes as NLO chromophores [33–36].

4 Advanced Surface Engineering Research

cells (SMOSCs) [39].

5-bromo-5'-nitro-2,2<sup>0</sup>

Push-pull molecular structures based on angular benzobisthiazolium acceptor were synthesized by Cibova et al. A comparison of selected spectral and nonlinear optical properties of the prepared compounds shows that compound 18 possess the best NLO properties (see Figure 4) [37]. Hebbar et al. describe the synthesis of various push-pull molecules 19–20 with a central pyrazine unit connected to a hexatriene chain terminated by various p-substituted phenyl groups. The compounds were synthesized by a Negishi coupling reaction followed by Wittig and Wadsworth-Emmons reactions [38]. Cho et al. synthesized new efficient pushpull organic semiconductors comprising of the bis(9,9-dimethyl-9H-fluoren-2-yl)aniline donor and the various acceptors such as NO2, 4-dicyanomethylene-6-tert-butyl-4H-pyran, and 2-dicyanomethylene-3-cyano-5-dimethyl-2,5-dihydrofuran, which were linked with bithiophene or vinyl bithiophene p-conjugation bridges, were synthesized, and their photovoltaic characteristics were investigated in solution-processed small molecule organic solar

The compounds 21–23 were readily prepared through Knoevenagel condensation reaction with

and 2-(3-cyano-4,5,5-trimethyl-5H-furan-2-ylidene)malononitrile, respectively, and a palladiumcatalyzed Suzuki coupling reaction with 2-bromo-5-nitrothiophene using phase-transfer catalyst. Shi et al. synthesized boron dipyrromethene 24 derivative bearing an electron-donating 4- (dimethylamino)phenylethynyl group and an electron-withdrawing 4-nitrophenylethynyl group in the opposite two and six positions by Knoevenagel condensation followed by sequential Sonogashira coupling reactions and the second-order nonlinear optical properties of this compound has been studied [40]. Farat et al. synthesized novel xanthene push-pull chromophores and luminophores 25–26 and show these structures as the more pronounced vibrational bands in the luminescence spectra, which indicate the more rigid structure of these molecules in the excited state [41]. Malytskyi et al. synthesized thiophene-based push-pull chromophores 27–28


for tuning the electrical and optical properties of surfaces with controlled self-assembled monolayer (SAM) formation [42]. Ion et al. reported for smooth/compact 1,2,3,4,5,6,7,8 octahydroacridine 29 thin films grown by matrix-assisted pulsed laser evaporation (MAPLE) the presence of the SHG (second-harmonic generation) signal related to the conformational asymmetry of the OHA molecule and therefore the nonlinear optical applications of it [43].

#### 2.3. Thienyl compounds as NLO materials

Thiophenes are among the most studied heterocyclic spacers for D-π-A systems due to their relatively low resonance energies and have allowed the preparation of chromophores with high stabilities and nonlinearities [44]. Raposo et al. reported the synthesis of formylsubstituted 1-alkyl(aryl)-2-(2<sup>0</sup> -thienyl)pyrroles 30–32 as versatile building blocks for NLO materials, by functionalization of the pyrrole or thiophene ring of thienylpyrroles using different methods: Vilsmeier formylation or metalation followed by reaction with DMF (see Figure 5) [45]. Batista et al. reported the synthesis of new chromophores with second-order nonlinearities containing thienylpyrrolyl and benzothiazolyl moieties. The solvatochromic behavior of the compounds was investigated, and the hyperpolarizabilities β of derivatives 33–35 were measured using hyper-Rayleigh scattering [46]. A series of thienylpyrrolyl π-conjugated systems attached to functionalized benzimidazole heterocycles 36–37 were developed by Batista et al. [47]. These chromophores possess excellent solvatochromic properties and good molecular optical nonlinearities. Herbivo et al. reported the synthesis of a series of formyl-substituted

2.4. Ferrocene derivatives

toward applications in optoelectronics.

Figure 6. Ferrocene derivatives with nonlinear optical properties [52–60].

Ferrocene (47), or bis-cyclopentadienyl iron, is a sandwich-like compound known since 1951 when Pauson and Kealy reacted cyclopentadienyl magnesium bromide with anhydrous ferric chloride (see Figure 6) [52, 53]. Ferrocene, in bulk or as thin films, is one of the most studied metallocenes. Ferrocene found itself applications not only in a multitude of areas, from chemical synthesis and engineering, e.g., materials chemistry (catalyst in carbon nanotechnology, precursor material, ligand and scaffolding, fuel additives, etc.), and pharmaceutics (anticancer and other drugs), but also in theoretical and basic research [54–57]. Matei et al. present results on the morphology and optical properties of ferrocene thin films grown by a nonconventional laser-assisted technique, films that were further used in two-photon absorption investigations, evidencing that the compound has low-to-medium SHG capabilities [58]. Constantinescu et al. studied nonlinear optical applications of ferrocene carboxaldehyde 48 thin films grown by matrix-assisted pulsed laser evaporation (MAPLE) [59]. Two-photon absorption investigations reveal that the ferrocene carboxaldehyde thin films deposited by MAPLE have secondharmonic generation capabilities improved compared to simple ferrocene, opening the path

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Yang et al. synthesized two electron-donating π-acceptor (D-π-A) chromospheres 49–50, with ferrocene as the electron donor and pyridinium as the electron acceptor, and investigated the nonlinear optical absorption (NOA) properties in the solution state by the Z-scan technique [60]. Both compounds exhibited reverse saturable absorption (RSA) and optical limiting effect under nanosecond pulse irradiation. Matei et al. synthesized 4-(ferrocenylmethylimino)- 2-hydroxy-benzoic 51 acid using a Schiff reaction and deposited thin films of it with controlled thickness by matrix-assisted pulsed laser evaporation (MAPLE), on quartz and silicon substrates, with the aim of evaluating the nonlinear optical properties for potential optoelectronic applications. Second-harmonic generation signals were weak due to the relatively short time of

Figure 5. Structures of thienyl compounds with nonlinear optical properties [44–51].

5-aryl-2,2<sup>0</sup> -bithiophenes 38 using two different methods: Vilsmeier-Haack-Arnold reaction and through Suzuki coupling as precursors for NLO materials [48]. Batista et al. synthesized that chromophores possess an (oligo)thienyl p-conjugated system attached to an imidazo-phenanthroline moiety by condensation of phenanthroline-5,6-dione with formyl-thiophene derivatives in the presence of ammonium acetate in glacial acetic acid. Compounds 39–40 possess good values of β hyperpolarizabilities and solvatochromic properties [49]. Raposo et al. reported the synthesis of the two series of donor-acceptor conjugated heterocyclic azo-thienylpyrrole system 41–42. The compounds 41 have the largest first-order hyperpolarizabilities (β = 460–660 10�<sup>30</sup> esu, T convention) [50]. Castro et al. designed two series of novel push-pull 1-(4-(thiophen-2-yl)phenyl)-1H-pyrroles 43–44 to explore the consequence of using different electron-accepting moieties linked to the thiophene at the arylthiophene bridge or to the pyrrole heterocycle, which plays the role of donor group. Thiobarbituric acid derivative 5b functionalized in the thiophene ring exhibits the largest first-order hyperpolarizability (<sup>β</sup> =2480 � <sup>10</sup>�<sup>30</sup> esu) [51]. Marco et al. synthesized two series of compounds bearing dithienothiophene or thienothiophene as π-conjugated spacer 45–46 with high second-order molecular nonlinearity [44].

#### 2.4. Ferrocene derivatives

5-aryl-2,2<sup>0</sup>

6 Advanced Surface Engineering Research


through Suzuki coupling as precursors for NLO materials [48]. Batista et al. synthesized that chromophores possess an (oligo)thienyl p-conjugated system attached to an imidazo-phenanthroline moiety by condensation of phenanthroline-5,6-dione with formyl-thiophene derivatives in the presence of ammonium acetate in glacial acetic acid. Compounds 39–40 possess good values of β hyperpolarizabilities and solvatochromic properties [49]. Raposo et al. reported the synthesis of the two series of donor-acceptor conjugated heterocyclic azo-thienylpyrrole system 41–42. The compounds 41 have the largest first-order hyperpolarizabilities (β = 460–660 10�<sup>30</sup> esu, T convention) [50]. Castro et al. designed two series of novel push-pull 1-(4-(thiophen-2-yl)phenyl)-1H-pyrroles 43–44 to explore the consequence of using different electron-accepting moieties linked to the thiophene at the arylthiophene bridge or to the pyrrole heterocycle, which plays the role of donor group. Thiobarbituric acid derivative 5b functionalized in the thiophene ring exhibits the largest first-order hyperpolarizability (<sup>β</sup> =2480 � <sup>10</sup>�<sup>30</sup> esu) [51]. Marco et al. synthesized two series of compounds bearing dithienothiophene or thienothiophene as π-conju-

gated spacer 45–46 with high second-order molecular nonlinearity [44].

Figure 5. Structures of thienyl compounds with nonlinear optical properties [44–51].

Ferrocene (47), or bis-cyclopentadienyl iron, is a sandwich-like compound known since 1951 when Pauson and Kealy reacted cyclopentadienyl magnesium bromide with anhydrous ferric chloride (see Figure 6) [52, 53]. Ferrocene, in bulk or as thin films, is one of the most studied metallocenes. Ferrocene found itself applications not only in a multitude of areas, from chemical synthesis and engineering, e.g., materials chemistry (catalyst in carbon nanotechnology, precursor material, ligand and scaffolding, fuel additives, etc.), and pharmaceutics (anticancer and other drugs), but also in theoretical and basic research [54–57]. Matei et al. present results on the morphology and optical properties of ferrocene thin films grown by a nonconventional laser-assisted technique, films that were further used in two-photon absorption investigations, evidencing that the compound has low-to-medium SHG capabilities [58]. Constantinescu et al. studied nonlinear optical applications of ferrocene carboxaldehyde 48 thin films grown by matrix-assisted pulsed laser evaporation (MAPLE) [59]. Two-photon absorption investigations reveal that the ferrocene carboxaldehyde thin films deposited by MAPLE have secondharmonic generation capabilities improved compared to simple ferrocene, opening the path toward applications in optoelectronics.

Yang et al. synthesized two electron-donating π-acceptor (D-π-A) chromospheres 49–50, with ferrocene as the electron donor and pyridinium as the electron acceptor, and investigated the nonlinear optical absorption (NOA) properties in the solution state by the Z-scan technique [60].

Both compounds exhibited reverse saturable absorption (RSA) and optical limiting effect under nanosecond pulse irradiation. Matei et al. synthesized 4-(ferrocenylmethylimino)- 2-hydroxy-benzoic 51 acid using a Schiff reaction and deposited thin films of it with controlled thickness by matrix-assisted pulsed laser evaporation (MAPLE), on quartz and silicon substrates, with the aim of evaluating the nonlinear optical properties for potential optoelectronic applications. Second-harmonic generation signals were weak due to the relatively short time of

Figure 6. Ferrocene derivatives with nonlinear optical properties [52–60].

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 poly-

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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

Figure 8. Schematic representation of the silica-based cross-linked film with chromophore Disperse Red 1 (adapted with

meric matrix (see Figure 8).

the permission from reference [64]).

120C, and the half decay temperature is 170C.

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

recording [61]. Lijanova et al. synthesized dendrimers 52 with ferrocenyl-ended groups joined by vinyl moieties (see Figure 7) [62].

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 10<sup>12</sup> esu.

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