Abstract

Organic semiconductors as active materials in thin-film electronic devices such as alkynes, heterocycles, dyes, ferrocenes, spiranes, or porphyrins, with special geometries and certain electronic molecular parameters, which possess nonlinear optical (NLO) properties and offer several major advantages over their inorganic counterparts, are presented in this chapter. There are a number of simple and versatile techniques that can be employed for the deposition of these important classes of materials. The matrix-assisted pulsed laser evaporation (MAPLE) technique provides advantages with regard to making organic films of different morphologies on different types of substrates. New insights into the crystallization growth mechanisms in MAPLE-deposited conjugated polymer films, which realize the connection between the structure and the carrier transport properties, are discussed herein. Second harmonic generation (SHG) capabilities of the thin films were also investigated.

Keywords: organic synthesis, laser deposition, nonlinear optical properties, thin films

## 1. Introduction

During the last decades, the nonlinear optical (NLO) materials have gained significant role because of their various applications in medicine, molecular switches, luminescent materials, laser technology, spectroscopic and electrochemical sensors, data storage, microfabrication and imaging, modulation of optical signals, and telecommunication [1–3]. Organic materials are distinguished by the fact that they exhibit strong nonlinear optical (NLO) properties [4–8]. In the last years, researchers have based on the synthesis of the target organic molecules with particular geometries and certain electronic molecular parameters, in order to have the desired nonlinear optic properties [9–31].

The changes of optical properties (absorption coefficient, index of refraction), through the increasing intensity of the input light, led to the discovery of the nonlinear optical phenomenon, second harmonic generation (SHG), detectable only after the improvement of the laser in 1962 [32]. Thus, nonlinear optics developed as a tremendous field of research, especially after the profound understanding of nonlinear optic phenomena (NLO) and the structure-property relations of

chromophores, after the development of different tools to accurately measure and calculate hyperpolarizabilities [33].

reveals the internal charge transfer responsible for the non linear optical properties. Nonlinear materials are defined as optical media in which the refractive index depends on light intensity [87]. So, the HOMO-LUMO gap energy is involved in

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

The designing and obtaining (synthesis) of the new molecules with high first hyperpolarizability β (theoretical and experimental) is central in discovery of the second-order and higher-order nonlinear optical materials and is quantified by the

with μ<sup>i</sup> the ith component of the induced dipole moment, Ej the corresponding component of the applied electromagnetic field, and αij, βijk, γijkl, the components of the linear polarizability, the first hyperpolarizability, and the second hyperpolarizability. In case of an ensemble of molecules, the macroscopic polarization is

μ<sup>i</sup> ¼ αijEj þ βijkEjEk þ γijklEjEkEl þ … (1)

ð Þ<sup>1</sup> Ej <sup>þ</sup> <sup>χ</sup>ijkð Þ<sup>2</sup> EjEk <sup>þ</sup> <sup>χ</sup>ijklð Þ<sup>3</sup> EjEkEl <sup>þ</sup> …: (2)

(1), (2), (3) the macroscopic susceptibilities of the first (1), second (2), and

third (3) order, which can be directly related to the density of the organic chromophore [88]. Recent advances in chromophore design report some features for classic dipolar organic structures with good nonlinear optic properties [89]: (1) presence of a π-conjugated systems with π electron delocalization, (2) a "push-pull" system, which is a couple donor-acceptor or connected to a system that contributes to the delocalization of the π electrons; (3) presence of a strong electron donor groups (e.g., ─NR2, ─NHR, ─OR, ─OH), and strong electron withdrawing groups

(e.g., ─CF3, SO2CF3, ─SO3H, ─NO2, ─CN), positioned at opposite ends of a conjugated molecule in case of dipolar molecules; (4) great values of dipole moment and polarizability; (5) small HOMO-LUMO energy gap; (6) planarity of the molecule for neutral, polar, and zwitterionic resonance structures. Dipole organic molecules have an intrinsic matter: the dipoles prefer to align antiparallel with each other in the solid-state film to nullify the bulk effect. Octupolar molecules, alternative NLO materials, present more advantages compared with dipole molecules [90]: (1) the second harmonic response (SHG) does not depend on the polarization of the incident light because they are more isotropic than the dipolar molecules; (2) β values of the octupoles can be increased by increasing of intramolecular charge transfer; (3) octupoles form noncentrosymmetric crystals; and (4) they are less likely to

In the last decades, literature reveals some classes of organic compounds suitable for organic electronic devices, such as organic photovoltaics (OPVs) and organic thin-film transistors (OTFTs), which possess certain characteristics, such as high molecular hyperpolarizability coefficients (β), special geometry, and in most cases,

small HOMO-LUMO energy gaps [25–27]. Among these classes of organic compounds, there are highlighting fullerenes, perylenes, thiophene compounds,

Furthermore, the polymers represent one of the most used classes of substances in pulsed laser deposition (PLD), but also in the other methods for preparing thin films. Organic compounds with nonlinear optical properties and

undergo relaxation due to the lack of ground-state dipole moment.

3. Synthesis of the compounds with NLO properties

induced dipole moment under an intense light field E in Eq. (1):

molecular electrical transport properties.

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

Pi ¼ χij

defined by Eq. (2):

polymers, and dyes.

3

with χ<sup>s</sup>

Recent literature highlights the increased interest in organic materials in recent decades, as an alternative to their inorganic counterparts, and having several advantages, such as their low cost, low toxicity, ease of solution processability, flexibility for device fabrications [34], and modulation of their optical, electronic, and chemical properties by adapting their molecular structure. Field effect transistors, photovoltaic devices, organic light-emitting diodes (OLEDs), and white light sources for indoor and outdoor lighting are some of the applications of organic materials [33].

The deposition of organic materials in thin films, required for the design of new, successful devices, implied the precise monitoring of their chemical, structural, and morphological properties [35]. The deposition of organic substances in thin films has to meet the requirements of the market: (1) good uniformity of simple or multilayer structures of organic, polymeric, or composite materials—in the electronics industry; (2) thickness control, film uniformity of coating, and good interfacing properties—in OLED polymer applications; (3) conformal coatings required to modify the interior surfaces of porous materials (membranes, foams, textiles) or irregular geometries of surfaces—for optoelectronic and medical devices [36].

Several classes of organic compounds, including conjugated molecules, fullerenes, polymers, perylenes, dyes, and thiophenes, have been studied as materials and investigated for their NLO responses [5]. Conjugated organic polymers with large nonlinear responses correlated with rapid response time have been observed as NLO materials with great expectations [37]. Although organic compounds have been considered as frail, the experiments showed, with the optical damage, threshold for polymeric materials can be greater than 10 GW/cm<sup>2</sup> [37].

Two deposition techniques, physical and chemical, are used in order to obtain organic thin films with good quality. For each type, there are several techniques applied. Physical deposition techniques for thin organic films include physical vapor deposition (PVD) [38–42], organic vapor phase deposition (OVPD) [43–45], organic molecular beam deposition (OMBD) [46–51], solvent vapor annealing (SVA) [52–56], self-assembled monolayers (SAMs) [57, 58], inkjet printing [59, 60], pulsed laser deposition (PDL) [61–64], and laser evaporation [65–67] techniques. The chemical methods include solution techniques and gas-phase deposition methods. Techniques that use solutions include Langmuir-Blodgett (LB) [68, 69], spin coating [70, 71], dip coating [72, 73], sol-gel [74, 75], and spray pyrolysis [76]. Chemical vapor deposition (CVD) [77–82] uses the gas phase of organic compounds.

Many articles report the synthesis of the novel organic molecules or polymers with highly active chromophores and superior optical activity, as response to the demand of substances with NLO properties for various applications [83–86].

This chapter refers to synthesis of organic compounds with nonlinear optical properties in one of the techniques mentioned above, laser-deposited films.

#### 2. Nonlinear optical (NLO) response in organic molecules

The optical response is due to a transition of the dipole moment from the ground state to the excited state due to the transition of an electron between frontier orbitals, from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The chemical activity of the molecule and the availability of the internal charge transfer are due to the balance between the redox ability of HOMO (as reducing agent) and LUMO (as oxidizing agent), which

Synthesis and Nonlinear Optical Studies on Organic Compounds in Laser-Deposited Films DOI: http://dx.doi.org/10.5772/intechopen.83234

reveals the internal charge transfer responsible for the non linear optical properties. Nonlinear materials are defined as optical media in which the refractive index depends on light intensity [87]. So, the HOMO-LUMO gap energy is involved in molecular electrical transport properties.

The designing and obtaining (synthesis) of the new molecules with high first hyperpolarizability β (theoretical and experimental) is central in discovery of the second-order and higher-order nonlinear optical materials and is quantified by the induced dipole moment under an intense light field E in Eq. (1):

$$
\mu\_{\rm i} = \alpha\_{\rm i} \mathbf{E}\_{\rm j} + \beta\_{\rm ijk} \mathbf{E}\_{\rm j} \mathbf{E}\_{\rm k} + \gamma\_{\rm ijkl} \mathbf{E}\_{\rm j} \mathbf{E}\_{\rm k} \mathbf{E}\_{\rm l} + \dots \tag{1}
$$

with μ<sup>i</sup> the ith component of the induced dipole moment, Ej the corresponding component of the applied electromagnetic field, and αij, βijk, γijkl, the components of the linear polarizability, the first hyperpolarizability, and the second hyperpolarizability. In case of an ensemble of molecules, the macroscopic polarization is defined by Eq. (2):

$$P\_i = \chi\_{\rm ij}^{(1)} \to\_{\rm j} + \chi\_{\rm ijk}^{(2)} \to\_{\rm j} \mathcal{E}\_{\rm k} + \chi\_{\rm ijkl}^{(3)} \to\_{\rm j} \mathcal{E}\_{\rm k} \mathcal{E}\_{\rm l} + \dots \tag{2}$$

with χ<sup>s</sup> (1), (2), (3) the macroscopic susceptibilities of the first (1), second (2), and third (3) order, which can be directly related to the density of the organic chromophore [88]. Recent advances in chromophore design report some features for classic dipolar organic structures with good nonlinear optic properties [89]: (1) presence of a π-conjugated systems with π electron delocalization, (2) a "push-pull" system, which is a couple donor-acceptor or connected to a system that contributes to the delocalization of the π electrons; (3) presence of a strong electron donor groups (e.g., ─NR2, ─NHR, ─OR, ─OH), and strong electron withdrawing groups (e.g., ─CF3, SO2CF3, ─SO3H, ─NO2, ─CN), positioned at opposite ends of a conjugated molecule in case of dipolar molecules; (4) great values of dipole moment and polarizability; (5) small HOMO-LUMO energy gap; (6) planarity of the molecule for neutral, polar, and zwitterionic resonance structures. Dipole organic molecules have an intrinsic matter: the dipoles prefer to align antiparallel with each other in the solid-state film to nullify the bulk effect. Octupolar molecules, alternative NLO materials, present more advantages compared with dipole molecules [90]: (1) the second harmonic response (SHG) does not depend on the polarization of the incident light because they are more isotropic than the dipolar molecules; (2) β values of the octupoles can be increased by increasing of intramolecular charge transfer; (3) octupoles form noncentrosymmetric crystals; and (4) they are less likely to undergo relaxation due to the lack of ground-state dipole moment.

## 3. Synthesis of the compounds with NLO properties

In the last decades, literature reveals some classes of organic compounds suitable for organic electronic devices, such as organic photovoltaics (OPVs) and organic thin-film transistors (OTFTs), which possess certain characteristics, such as high molecular hyperpolarizability coefficients (β), special geometry, and in most cases, small HOMO-LUMO energy gaps [25–27]. Among these classes of organic compounds, there are highlighting fullerenes, perylenes, thiophene compounds, polymers, and dyes.

Furthermore, the polymers represent one of the most used classes of substances in pulsed laser deposition (PLD), but also in the other methods for preparing thin films. Organic compounds with nonlinear optical properties and

chromophores, after the development of different tools to accurately measure and

decades, as an alternative to their inorganic counterparts, and having several advantages, such as their low cost, low toxicity, ease of solution processability, flexibility for device fabrications [34], and modulation of their optical, electronic, and chemical properties by adapting their molecular structure. Field effect transistors, photovoltaic devices, organic light-emitting diodes (OLEDs), and white light sources for indoor and outdoor lighting are some of the applications of organic

old for polymeric materials can be greater than 10 GW/cm<sup>2</sup> [37].

Recent literature highlights the increased interest in organic materials in recent

The deposition of organic materials in thin films, required for the design of new, successful devices, implied the precise monitoring of their chemical, structural, and morphological properties [35]. The deposition of organic substances in thin films has to meet the requirements of the market: (1) good uniformity of simple or multilayer structures of organic, polymeric, or composite materials—in the electronics industry; (2) thickness control, film uniformity of coating, and good interfacing properties—in OLED polymer applications; (3) conformal coatings required to modify the interior surfaces of porous materials (membranes, foams, textiles) or irregular geometries of surfaces—for optoelectronic and medical devices [36]. Several classes of organic compounds, including conjugated molecules, fullerenes, polymers, perylenes, dyes, and thiophenes, have been studied as materials and investigated for their NLO responses [5]. Conjugated organic polymers with large nonlinear responses correlated with rapid response time have been observed as NLO materials with great expectations [37]. Although organic compounds have been considered as frail, the experiments showed, with the optical damage, thresh-

Two deposition techniques, physical and chemical, are used in order to obtain organic thin films with good quality. For each type, there are several techniques applied. Physical deposition techniques for thin organic films include physical vapor deposition (PVD) [38–42], organic vapor phase deposition (OVPD) [43–45], organic molecular beam deposition (OMBD) [46–51], solvent vapor annealing (SVA) [52–56], self-assembled monolayers (SAMs) [57, 58], inkjet printing [59, 60], pulsed laser deposition (PDL) [61–64], and laser evaporation [65–67] techniques. The chemical methods include solution techniques and gas-phase deposition methods. Techniques that use solutions include Langmuir-Blodgett (LB) [68, 69], spin coating [70, 71], dip coating [72, 73], sol-gel [74, 75], and spray pyrolysis [76]. Chemical vapor deposition (CVD) [77–82] uses the gas phase of

Many articles report the synthesis of the novel organic molecules or polymers with highly active chromophores and superior optical activity, as response to the demand of substances with NLO properties for various applications [83–86]. This chapter refers to synthesis of organic compounds with nonlinear optical

The optical response is due to a transition of the dipole moment from the ground

properties in one of the techniques mentioned above, laser-deposited films.

state to the excited state due to the transition of an electron between frontier orbitals, from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The chemical activity of the molecule and the availability of the internal charge transfer are due to the balance between the redox ability of HOMO (as reducing agent) and LUMO (as oxidizing agent), which

2. Nonlinear optical (NLO) response in organic molecules

calculate hyperpolarizabilities [33].

materials [33].

Applied Surface Science

organic compounds.

2

organic compounds reported in laser deposition (PLD) will be presented in the following two sections.

3.2 Synthesis of the nonlinear optical perylenes

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

able for organic solar cells (OSCs) (see Figure 2).

excitation.

Figure 2.

Figure 3.

5

Perylene isomers 5 and 6 [96].

Perylene 4 with NLO properties [83].

Perylene compound 4 was synthesized by a catalyzed heteroarylation reaction by McAfee et al. [83]. The geometry of perylene 4 exhibits planes of diketopyrrolopyrrole and perylene diimide at one dihedral angle too high for a good π-orbital overlapping, which determines HOMO-LUMO orbitals on specific atoms, supposition confirmed by TDDFT calculations at B3LYP/6-31G(d,p) level of theory. Optical, electronic, and self-assembly properties of the thin films of perylene 4 fabricated by solvent vapor annealing (SVA) recommended this compound as suit-

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

Perylene diimides (PDIs) are a new class of nonfullerene electron acceptors for organic solar cells with many attracting features, like low cost; significant thermal, chemical, and light stability; good electron-accepting ability; and excellent electron mobility [96]. Carlotti et al. investigated PDI dimers as nonfullerene electron acceptors [96] for organic solar cells. Two isomers 5 and 6 have planar and twisted geometries, which determined very diverse spectral and photophysical properties (see Figure 3). Theoretical calculations and also the experimental time-resolved investigation confirm isomers 5 and 6 show charge transfer following light

## 3.1 Synthesis of the nonlinear optical fullerenes

Canulescu group [91] studied thin films of fullerenes (C60) 1 deposited onto silicon using matrix-assisted pulsed laser evaporation (MAPLE). MALDI analysis showed that a dominant transfer of intact C60 molecules onto a silicon wafer is realized when the laser fluence is carefully selected, for example, below a threshold of 1.5≈/J cm<sup>2</sup> . Labrunie et al. synthesized triphenylamine-based push-pull σ-C60 dyad, as photoactive molecular material for single-component organic solar cells, using a copper(I)-catalyzed 1,3-dipolar Huisgen cycloaddition under strict anaerobic conditions, leading to the selective formation of a 1,2,3-triazole ring and affording fullerene 2 in 80% yield (see Figure 1).

Thin films of 2, prepared by spin coating of a CHCl3 solution, highlight the ambipolar semiconducting behavior and also very good electron-transporting properties for fullerene 2 [92]. Kim et al. synthesized new fullerene 3 soluble in ethanol/water solvent mixtures and implemented these materials to fabricate polymer solar cells (PSCs) using environmentally friendly solvents [93]. The results of this paper provide important guidelines for the design of aqueouselectroactive materials having high carrier mobilities suitable to achieve very efficient eco-PSCs.

Kamanina highlights that there are two reasons for the importance of fullerenes: their unique energy levels and high value of electron affinity energy (0.65–0.7 eV). This value is larger than the one for most dyes and organic molecules with intramolecular acceptor fragment and can stimulate the efficient intermolecular charge transfer complex formation in the fullerene-doped organic conjugated materials [94].

Although fullerene acceptors were the predominant choice in the acceptor materials for two decades, the limited tunability of electronic properties and weak absorption of fullerene derivatives in visible range prevent further development of organic solar [95]. Therefore, other classes of organic molecules have been researched to obtain the desired properties of the electronic materials.

Figure 1. Fullerenes with NLO properties [91–93].

Synthesis and Nonlinear Optical Studies on Organic Compounds in Laser-Deposited Films DOI: http://dx.doi.org/10.5772/intechopen.83234

## 3.2 Synthesis of the nonlinear optical perylenes

organic compounds reported in laser deposition (PLD) will be presented in the

Canulescu group [91] studied thin films of fullerenes (C60) 1 deposited onto silicon using matrix-assisted pulsed laser evaporation (MAPLE). MALDI analysis showed that a dominant transfer of intact C60 molecules onto a silicon wafer is realized when the laser fluence is carefully selected, for example, below a threshold

dyad, as photoactive molecular material for single-component organic solar cells, using a copper(I)-catalyzed 1,3-dipolar Huisgen cycloaddition under strict anaerobic conditions, leading to the selective formation of a 1,2,3-triazole ring and

Thin films of 2, prepared by spin coating of a CHCl3 solution, highlight the ambipolar semiconducting behavior and also very good electron-transporting properties for fullerene 2 [92]. Kim et al. synthesized new fullerene 3 soluble in ethanol/water solvent mixtures and implemented these materials to fabricate polymer solar cells (PSCs) using environmentally friendly solvents [93]. The results of this paper provide important guidelines for the design of aqueouselectroactive materials having high carrier mobilities suitable to achieve very

Kamanina highlights that there are two reasons for the importance of fullerenes: their unique energy levels and high value of electron affinity energy (0.65–0.7 eV). This value is larger than the one for most dyes and organic molecules with intramolecular acceptor fragment and can stimulate the efficient intermolecular charge transfer complex formation in the fullerene-doped organic conjugated

Although fullerene acceptors were the predominant choice in the acceptor materials for two decades, the limited tunability of electronic properties and weak absorption of fullerene derivatives in visible range prevent further development of

organic solar [95]. Therefore, other classes of organic molecules have been researched to obtain the desired properties of the electronic materials.

. Labrunie et al. synthesized triphenylamine-based push-pull σ-C60

following two sections.

Applied Surface Science

of 1.5≈/J cm<sup>2</sup>

efficient eco-PSCs.

materials [94].

Figure 1.

4

Fullerenes with NLO properties [91–93].

3.1 Synthesis of the nonlinear optical fullerenes

affording fullerene 2 in 80% yield (see Figure 1).

Perylene compound 4 was synthesized by a catalyzed heteroarylation reaction by McAfee et al. [83]. The geometry of perylene 4 exhibits planes of diketopyrrolopyrrole and perylene diimide at one dihedral angle too high for a good π-orbital overlapping, which determines HOMO-LUMO orbitals on specific atoms, supposition confirmed by TDDFT calculations at B3LYP/6-31G(d,p) level of theory. Optical, electronic, and self-assembly properties of the thin films of perylene 4 fabricated by solvent vapor annealing (SVA) recommended this compound as suitable for organic solar cells (OSCs) (see Figure 2).

Perylene diimides (PDIs) are a new class of nonfullerene electron acceptors for organic solar cells with many attracting features, like low cost; significant thermal, chemical, and light stability; good electron-accepting ability; and excellent electron mobility [96]. Carlotti et al. investigated PDI dimers as nonfullerene electron acceptors [96] for organic solar cells. Two isomers 5 and 6 have planar and twisted geometries, which determined very diverse spectral and photophysical properties (see Figure 3). Theoretical calculations and also the experimental time-resolved investigation confirm isomers 5 and 6 show charge transfer following light excitation.

Figure 2. Perylene 4 with NLO properties [83].

Figure 3. Perylene isomers 5 and 6 [96].
