3.3 Synthesis of the nonlinear optical thiophene compounds

Small-molecule semiconductors with an A-D-A core structure (D is an electronrich unit and A is an electron-deficient unit) function as an electron donor or electron acceptor in organic photovoltaic cell devices [97].

a solid film. Compound 19 forms an ordered lamellar packed structure with compact π-π stacking, whereas 20 shows only a weak π-π stacking effect in solid film. Also, the authors correlate chemical structure of the A-D-A semiconductors with their aggregation structure in solid state and their electron-donating and electronaccepting properties in OPV devices (see Figure 5). Zhu et al. synthesized the

Synthesis and Nonlinear Optical Studies on Organic Compounds in Laser-Deposited Films

core and alkoxy side chains using a Stille reaction between an organotin compound and ethyl 2-bromothiophene-3-carboxylate [105]. It was found that change in the side chains affects electronic, optical, charge transport and morphological properties of these analogues. Compound 22 exhibits a narrower bandgap, redshifted absorption spectra, and higher electron mobility compared with 21. The introduction of alkoxy side chains in fused-ring electron acceptors appears to be a promising approach to enhance absorption and electron mobility and therefore efficiencies in organic solar cells. Dai et al. synthesize fused-ring electron acceptors 23–24 using a Suzuki reaction and found that extending the core size leads to upshift of the energy

]dithiophene


fused-ring electron acceptors 21 and 22 with a naphtho[1,2-b,5,6-b<sup>0</sup>

levels, enhancement of molecular packing, higher electron mobility, more balanced-charged transport, and reduced bimolecular charge recombination

using the Friedel-Crafts and the Lawesson reactions. The compounds 25 have proved promising NLO chromophores (see Figure 7), exhibiting β<sup>o</sup> values of 2.5–4 times higher than that of 4-nitroaniline [106]. Also, azo-dyes 26 possessed great values of molecular hyperpolarizability β<sup>0</sup> [107]. Marinescu et al. synthesized by azo-coupling reactions a series of pyrazole-5-ones 27–30, which possessed only hydrazo tautomers and torsional structures, correlated with their NLO properties and the efficiency of compounds was assessed by the relationship between high βtot and low HOMO-LUMO energy gap [89]. Liu et al. synthesized porphyrin dyes 31 by [2+2] click reaction, with very good yields and small energy gaps HOMO-LUMO for each compound, and demonstrated that click moieties affected the third-order

Pascal et al. obtained the push-pull dyes 33–34 with tricyanofuran acceptor linked to donors of various strengths, by two consecutive condensations, and investigated their second-order nonlinear optical properties [109]. It was found that a high intrinsic hyperpolarizability coupled with a high static dipolar moment results in excellent μβ values for polymethine dyes containing pyran, thiopyran,

3.4 Synthesis of the nonlinear optical dyes

DOI: http://dx.doi.org/10.5772/intechopen.83234

Raposo et al. reported the synthesis of the 2,2<sup>0</sup>

nonlinear optical responses [108] (see Figure 8).

Small-molecule semiconductors based on an A-D-A structure [96].

(Figure 6).

Figure 5.

7

Thiophenes are one of the most studied heterocyclic compounds for D-π-A systems due to their relatively low resonance energies, the facile and cheap preparation of chromophores with high stabilities, and good nonlinearities [98]. The hyperpolarizabilities β of derivatives 7–9 were measured using hyper-Rayleigh scattering, and also the solvatochromic behavior of these thiophenes was investigated [99] (see Figure 4). A series of formyl-5-aryl-2,2<sup>0</sup> -bithiophenes 10 were synthesized by Herbivo et al. through two methods: Vilsmeier-Haack-Arnold reaction or through Suzuki coupling as precursors for NLO materials [100]. Raposo et al. synthesized two series of donor-acceptor conjugated heterocyclic azothienylpyrroles 11–12. Thienylpyrroles 11 have largest first-order hyperpolarizabilities (<sup>β</sup> = 460–<sup>660</sup> � <sup>10</sup>�<sup>30</sup> esu, T convention) [101]. Two series of novel pushpull 1-(4-(thiophen-2-yl)phenyl)-1H-pyrroles 13–14, reported by Castro et al., were studied considering the electron-accepting moieties linked at the arylthiophene bridge or to the pyrrole heterocycle, which plays the role of donor group. Thiophene ring with functionalized thiobarbituric acid derivative 14 shows the largest first hyperpolarizability (<sup>β</sup> =2480 � <sup>10</sup>�<sup>30</sup> esu) [102]. Batista et al. developed a series of thienylpyrrolyl with π-conjugated system attached to functionalized benzimidazole heterocycles 15–16 [103]. The new chromophores possess excellent solvatochromic properties and good molecular optical nonlinearities. By condensation of 5,6-phenanthroline-dione with formyl-thiophene derivatives in the presence of ammonium acetate in glacial acetic acid, oligothienyl-imidazophenanthroline compounds 17–18 were synthesized by Batista et al. These p-conjugated compounds possessed good values of β hyperpolarizabilities and solvatochromic properties [104]. Liu et al. synthesized isomers 19 and 20 [97], very similar in chemical structure, differing only in the substitution position of the alkyl groups attached to their conjugated cores, two novel small molecule photovoltaic materials, by implying a Knoevenagel reaction and investigated their photovoltaic properties by blending each of them with a polymer donor and a fullerene acceptor. Compounds present similar optical absorption spectra in solution and molecular energy levels in

Figure 4. Thienyl compounds with nonlinear optical properties [97–104]. Synthesis and Nonlinear Optical Studies on Organic Compounds in Laser-Deposited Films DOI: http://dx.doi.org/10.5772/intechopen.83234

a solid film. Compound 19 forms an ordered lamellar packed structure with compact π-π stacking, whereas 20 shows only a weak π-π stacking effect in solid film. Also, the authors correlate chemical structure of the A-D-A semiconductors with their aggregation structure in solid state and their electron-donating and electronaccepting properties in OPV devices (see Figure 5). Zhu et al. synthesized the fused-ring electron acceptors 21 and 22 with a naphtho[1,2-b,5,6-b<sup>0</sup> ]dithiophene core and alkoxy side chains using a Stille reaction between an organotin compound and ethyl 2-bromothiophene-3-carboxylate [105]. It was found that change in the side chains affects electronic, optical, charge transport and morphological properties of these analogues. Compound 22 exhibits a narrower bandgap, redshifted absorption spectra, and higher electron mobility compared with 21. The introduction of alkoxy side chains in fused-ring electron acceptors appears to be a promising approach to enhance absorption and electron mobility and therefore efficiencies in organic solar cells. Dai et al. synthesize fused-ring electron acceptors 23–24 using a Suzuki reaction and found that extending the core size leads to upshift of the energy levels, enhancement of molecular packing, higher electron mobility, more balanced-charged transport, and reduced bimolecular charge recombination (Figure 6).

#### 3.4 Synthesis of the nonlinear optical dyes

Raposo et al. reported the synthesis of the 2,2<sup>0</sup> -bithiophene-conjugated dyes 25, using the Friedel-Crafts and the Lawesson reactions. The compounds 25 have proved promising NLO chromophores (see Figure 7), exhibiting β<sup>o</sup> values of 2.5–4 times higher than that of 4-nitroaniline [106]. Also, azo-dyes 26 possessed great values of molecular hyperpolarizability β<sup>0</sup> [107]. Marinescu et al. synthesized by azo-coupling reactions a series of pyrazole-5-ones 27–30, which possessed only hydrazo tautomers and torsional structures, correlated with their NLO properties and the efficiency of compounds was assessed by the relationship between high βtot and low HOMO-LUMO energy gap [89]. Liu et al. synthesized porphyrin dyes 31 by [2+2] click reaction, with very good yields and small energy gaps HOMO-LUMO for each compound, and demonstrated that click moieties affected the third-order nonlinear optical responses [108] (see Figure 8).

Pascal et al. obtained the push-pull dyes 33–34 with tricyanofuran acceptor linked to donors of various strengths, by two consecutive condensations, and investigated their second-order nonlinear optical properties [109]. It was found that a high intrinsic hyperpolarizability coupled with a high static dipolar moment results in excellent μβ values for polymethine dyes containing pyran, thiopyran,

Figure 5. Small-molecule semiconductors based on an A-D-A structure [96].

3.3 Synthesis of the nonlinear optical thiophene compounds

electron acceptor in organic photovoltaic cell devices [97].

Applied Surface Science

gated [99] (see Figure 4). A series of formyl-5-aryl-2,2<sup>0</sup>

Figure 4.

6

Thienyl compounds with nonlinear optical properties [97–104].

Small-molecule semiconductors with an A-D-A core structure (D is an electron-


rich unit and A is an electron-deficient unit) function as an electron donor or

Thiophenes are one of the most studied heterocyclic compounds for D-π-A systems due to their relatively low resonance energies, the facile and cheap preparation of chromophores with high stabilities, and good nonlinearities [98]. The hyperpolarizabilities β of derivatives 7–9 were measured using hyper-Rayleigh scattering, and also the solvatochromic behavior of these thiophenes was investi-

synthesized by Herbivo et al. through two methods: Vilsmeier-Haack-Arnold reaction or through Suzuki coupling as precursors for NLO materials [100]. Raposo et al. synthesized two series of donor-acceptor conjugated heterocyclic azothienylpyrroles 11–12. Thienylpyrroles 11 have largest first-order hyperpolarizabilities (<sup>β</sup> = 460–<sup>660</sup> � <sup>10</sup>�<sup>30</sup> esu, T convention) [101]. Two series of novel pushpull 1-(4-(thiophen-2-yl)phenyl)-1H-pyrroles 13–14, reported by Castro et al., were studied considering the electron-accepting moieties linked at the arylthiophene bridge or to the pyrrole heterocycle, which plays the role of donor group. Thiophene ring with functionalized thiobarbituric acid derivative 14 shows the largest first hyperpolarizability (<sup>β</sup> =2480 � <sup>10</sup>�<sup>30</sup> esu) [102]. Batista et al. developed a series of thienylpyrrolyl with π-conjugated system attached to functionalized benzimidazole heterocycles 15–16 [103]. The new chromophores possess excellent solvatochromic

properties and good molecular optical nonlinearities. By condensation of 5,6-phenanthroline-dione with formyl-thiophene derivatives in the presence of ammonium acetate in glacial acetic acid, oligothienyl-imidazophenanthroline compounds 17–18 were synthesized by Batista et al. These p-conjugated compounds possessed good values of β hyperpolarizabilities and solvatochromic properties [104]. Liu et al. synthesized isomers 19 and 20 [97], very similar in chemical structure, differing only in the substitution position of the alkyl groups attached to their conjugated cores, two novel small molecule photovoltaic materials, by implying a Knoevenagel reaction and investigated their photovoltaic properties by blending each of them with a polymer donor and a fullerene acceptor. Compounds present similar optical absorption spectra in solution and molecular energy levels in

selenopyran, and benzoindoline moieties, and therefore, these dyes are promising

Synthesis and Nonlinear Optical Studies on Organic Compounds in Laser-Deposited Films

A review of all the compounds deposited by MAPLE, including organics, with their applications was carried out by Caricato et al. [33]. Consequently, here we point out the newest structures reported in the literature with the most notable

Mariano et al. combine spin coating with the MAPLE technique, to realize polymeric multilayered thin films using three polymers 35–37 [110] (see Figure 9). This is the first white light-emitting diode produced by multilayer polymeric thin films reported in literature [110]. Constantinescu et al. reported fabrication of thin solid layers of poly(3,3‴-didodecyl-quaterthiophene) <sup>38</sup> by MAPLE, on silicon and quartz substrates and studied films with different thicknesses, morphologies, and optical properties [111]. Thin films of polyvinyl chloride 39, polyacrylic acid 40, and polyaniline 41 with potential use in optoelectronic were

poly(9,9-dioctylfluorene) 42 deposited by MAPLE have been investigated at different laser fluence values and at different laser repetition rate. Authors reported

Recent works reported new conjugated copolymers with different donor (D) acceptor (A) motifs (see Figure 10) for optoelectronic devices [113]. Authors synthesized a series of four DAA copolymers 43–46 (see Figure 11) with thiophene donors and isoindigo-based acceptors and compared the optoelectronic properties of these four copolymers with those of the analogous. It was found that "increasing the number of acceptors in the polymer repeat unit decreased the LUMO energy of the polymer and increased the electron mobility" [113]. The authors studied the influence of acceptor length and planarity on the optoelectronic properties of donor-acceptor systems for the copolymers 43–46 with thiophene donors and isoindigo acceptors and formulated that changing the number of acceptors in the

bubbles presenting the intrinsic blue emission. At smooth surfaces, it can be

), the dioctylfluorene surface showed large

reported by Constantinescu et al. [112]. Thin films of the blue-emitting

observed that green emission becomes predominant [110].

3.5 Synthesis of the polymers with nonlinear optical properties

for electro-optical modulation.

nonlinear optical properties mentioned.

DOI: http://dx.doi.org/10.5772/intechopen.83234

that at high laser fluence (450 mJ/cm<sup>2</sup>

Figure 9.

9

Polymers deposited by MAPLE technique [110–112].

Figure 6. Chemical structure of fused-ring electron acceptor 21–24 [104, 105].

Figure 7. Azo-dyes as NLO chromophores 25–30 [89, 106, 107].

Figure 8. Porphyrins 31–32 and push-pull polymethine dyes 33–34 [108, 109].

selenopyran, and benzoindoline moieties, and therefore, these dyes are promising for electro-optical modulation.

## 3.5 Synthesis of the polymers with nonlinear optical properties

A review of all the compounds deposited by MAPLE, including organics, with their applications was carried out by Caricato et al. [33]. Consequently, here we point out the newest structures reported in the literature with the most notable nonlinear optical properties mentioned.

Mariano et al. combine spin coating with the MAPLE technique, to realize polymeric multilayered thin films using three polymers 35–37 [110] (see Figure 9). This is the first white light-emitting diode produced by multilayer polymeric thin films reported in literature [110]. Constantinescu et al. reported fabrication of thin solid layers of poly(3,3‴-didodecyl-quaterthiophene) <sup>38</sup> by MAPLE, on silicon and quartz substrates and studied films with different thicknesses, morphologies, and optical properties [111]. Thin films of polyvinyl chloride 39, polyacrylic acid 40, and polyaniline 41 with potential use in optoelectronic were reported by Constantinescu et al. [112]. Thin films of the blue-emitting poly(9,9-dioctylfluorene) 42 deposited by MAPLE have been investigated at different laser fluence values and at different laser repetition rate. Authors reported that at high laser fluence (450 mJ/cm<sup>2</sup> ), the dioctylfluorene surface showed large bubbles presenting the intrinsic blue emission. At smooth surfaces, it can be observed that green emission becomes predominant [110].

Recent works reported new conjugated copolymers with different donor (D) acceptor (A) motifs (see Figure 10) for optoelectronic devices [113]. Authors synthesized a series of four DAA copolymers 43–46 (see Figure 11) with thiophene donors and isoindigo-based acceptors and compared the optoelectronic properties of these four copolymers with those of the analogous. It was found that "increasing the number of acceptors in the polymer repeat unit decreased the LUMO energy of the polymer and increased the electron mobility" [113]. The authors studied the influence of acceptor length and planarity on the optoelectronic properties of donor-acceptor systems for the copolymers 43–46 with thiophene donors and isoindigo acceptors and formulated that changing the number of acceptors in the

Figure 9. Polymers deposited by MAPLE technique [110–112].

Figure 6.

Applied Surface Science

Figure 7.

Figure 8.

8

Chemical structure of fused-ring electron acceptor 21–24 [104, 105].

Azo-dyes as NLO chromophores 25–30 [89, 106, 107].

Porphyrins 31–32 and push-pull polymethine dyes 33–34 [108, 109].

two-photon absorption investigations, showing that the compound has SHG capabilities [115]. Nonlinear optical properties of ferrocene carboxaldehyde 49 thin films grown by matrix-assisted pulsed laser evaporation (MAPLE) were studied by Constantinescu et al. [116] using two-photon absorption investigations. The experiment revealed that the ferrocene 49 thin films deposited by MAPLE have second harmonic generation capabilities improved compared to simple ferrocene, with better applications in optoelectronics. 4-(Ferrocenylmethylimino)-2-hydroxybenzoic acid 50 was synthesized by a Schiff reaction, and deposited thin films of it with controlled thickness were fabricated by matrix-assisted pulsed laser evapora-

Synthesis and Nonlinear Optical Studies on Organic Compounds in Laser-Deposited Films

The main method used to obtain thin laser films is matrix-assisted pulsed laser evaporation (MAPLE). Most organic compounds deposited by matrix-assisted pulsed laser evaporation reported so far are polymers, so they are very important for this chapter. There are three important advantages of the MAPLE technique compared to solution cast techniques: (1) the control of thickness; (2) possibility to deposit multilayers; and (3) fabrication of thin films on nonplanar substrates with

The fact that method pulsed laser ablation is not convenient for the deposition of soft materials (almost all polymers, proteins, and other materials are chemically and/or thermally modified or destroyed) has led to the invention of a new improved method to remove these limitations. Two researchers McGill and Chrisey gave birth to matrix-assisted pulsed laser evaporation (MAPLE) technique [33] in order to deposit thin and uniform films of polymers and carbohydrates. The new method is suitable for the deposition of the complex organic materials, such as polymers, bioorganic molecules, and coordination compounds [118]. Fabrication of thin films from such materials is very important for new devices with many applications, including light-emitting diodes (LEDs) [110], field-effect transistors, sensors, photovoltaic devices, and white light sources for indoor and outdoor lighting

1. dissolving the substance (solute) of interest in a volatile solvent (matrix) to form a diluted homogeneous solution (concentration of the order of 1 wt%);

3. placing the solution in the vacuum chamber to act as a target for laser-assisted deposition and irradiation of the frozen solution with a pulsed laser beam.

Matrix-assisted pulsed laser evaporation deposition of the desirable molecules is effectuated in a light manner, which implied the passing of the condensed phase to the gas phase. A low kinetic energy is implied in MAPLE process, in advantage to

In the MAPLE method, the laser pulse energy is absorbed by the solvent and converted into thermal and kinetic energy, enabling the solvent to evaporate and carry in the gas phase the solute molecules onto the deposition substrate where they adhere as a thin film. A very volatile solvent is required to be pumped during the flight from target to substrate, and thus, the deposited film is made up of the

2. freezing the solution at the temperature of the liquid nitrogen; and

tion (MAPLE), on quartz and silicon substrates [117].

Three steps are necessary in the MAPLE technique:

laser ablation with a high level of kinetic energy [110].

4. Matrix-assisted pulsed laser evaporation

DOI: http://dx.doi.org/10.5772/intechopen.83234

good surface coverage [110].

[13, 110, 114, 115].

dissolved material only.

11

Figure 10.

Donor (D)-acceptor (A) motifs along with magnetic moment and LUMO energy variations [113].

Figure 11. Structures of DAA copolymers isoindigo-based acceptors and thiophene donors 43–46 [113].

Figure 12. Structures of compounds 47–50 with NLO properties.

polymer repeat unit is a key strategy that could be used to obtain donor-acceptor systems with fine properties and better materials.
