3.6 Synthesis of other classes of organic compounds with NLO properties

Other classes of heterocyclic compounds were reported to have nonlinear optical properties, and some of them are presented below. 1,2,3,4,5,6,7,8- Octahydroacridine (OHA) 47 (see Figure 12) thin films grown by matrix-assisted pulsed laser evaporation (MAPLE) showed the SHG (second harmonic generation) signal of the conformational asymmetry of OHA and therefore the nonlinear optical applications of it [114]. Matei et al. studied the morphology and optical properties of ferrocene 48 thin films grown by MAPLE technique, films that were further used in Synthesis and Nonlinear Optical Studies on Organic Compounds in Laser-Deposited Films DOI: http://dx.doi.org/10.5772/intechopen.83234

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

#### 4. Matrix-assisted pulsed laser evaporation

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 good surface coverage [110].

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 [13, 110, 114, 115].

Three steps are necessary in the MAPLE technique:


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

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 dissolved material only.

polymer repeat unit is a key strategy that could be used to obtain donor-acceptor

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

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

3.6 Synthesis of other classes of organic compounds with NLO properties

Other classes of heterocyclic compounds were reported to have nonlinear optical properties, and some of them are presented below. 1,2,3,4,5,6,7,8-

Octahydroacridine (OHA) 47 (see Figure 12) thin films grown by matrix-assisted pulsed laser evaporation (MAPLE) showed the SHG (second harmonic generation) signal of the conformational asymmetry of OHA and therefore the nonlinear optical applications of it [114]. Matei et al. studied the morphology and optical properties of ferrocene 48 thin films grown by MAPLE technique, films that were further used in

systems with fine properties and better materials.

Structures of compounds 47–50 with NLO properties.

Figure 10.

Applied Surface Science

Figure 11.

Figure 12.

10

Most of the laser energy is absorbed by the volatile matrix, not the dissolved molecules, which minimize the photochemical decomposition of the precursor solution. In addition, the use of low fluences prevents or reduces thermal damage and decomposition of molecules, so deposition can take place at low fluctuations (0.05–0.5 J/cm<sup>2</sup> ) compared to conventional pulsed laser deposition (PLD) (typically few J/cm<sup>2</sup> ).

moment μtot, the quadrupole moment Q, and the mean first polarizability βtot may be calculated by using DFT theory. The x, y, z components are defined as follows:

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

<sup>α</sup>tot <sup>¼</sup> <sup>α</sup>xx <sup>þ</sup> <sup>α</sup>yy <sup>þ</sup> <sup>α</sup>zz

<sup>x</sup> <sup>þ</sup> <sup>μ</sup><sup>2</sup>

<sup>x</sup> <sup>þ</sup> <sup>q</sup><sup>3</sup>

β2 <sup>x</sup> <sup>þ</sup> <sup>β</sup><sup>2</sup>

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

<sup>þ</sup> <sup>β</sup>yyy <sup>þ</sup> <sup>β</sup>yzz <sup>þ</sup> <sup>β</sup>yxx � �<sup>2</sup>

The larger hyperpolarizability value of one component over the other components means that the electronic charge delocalization is larger in that direction [11]. There are a lot of factors that contribute to enhance the NLO properties of the compounds, which are cumulated with the prospective βtot calculated values [3].

4.1.2 Experimental determination of second harmonic generation (SHG) in thin films

Experimental setup used to investigate the SHG behavior [117] in thin-film samples is represented in Figure 14. The component parts of the system used for the determination of second harmonic generation (SHG) are a sapphire laser ("Tsunami," from Spectra-Physics; 780 nm, 60–100 fs pulse duration, 80 MHz repetition rate); an optical system made of a half-wave plate and a Glan-Taylor polarizing prism that allows the variation of beam intensity; a microscope's objective is to focus the laser beam onto the thin-film samples and collect the emitted SHG radiation. A dichroic mirror (DM) separated the excitation radiation, and the

1 3 ∑ i6¼j <sup>y</sup> <sup>þ</sup> <sup>μ</sup><sup>2</sup> z

<sup>y</sup> <sup>þ</sup> <sup>q</sup><sup>3</sup> z

> <sup>y</sup> <sup>þ</sup> <sup>β</sup><sup>2</sup> z

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

� �1=<sup>2</sup>

� �1=<sup>3</sup>

<sup>μ</sup>tot <sup>¼</sup> <sup>μ</sup><sup>2</sup>

Qtot <sup>¼</sup> <sup>q</sup><sup>3</sup>

βtot ¼

β<sup>j</sup> ¼ βiii þ

SHG intensity is measured by a camera spectrograph [117].

Experimental setup for determining second harmonic generation.

Therefore:

r

<sup>β</sup>xxx <sup>þ</sup> <sup>β</sup>xyy <sup>þ</sup> <sup>β</sup>xzz � �<sup>2</sup>

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

βtot ¼

Figure 14.

13

<sup>3</sup> (3)

r� � (6)

<sup>β</sup>ijj <sup>þ</sup> <sup>β</sup>jij <sup>þ</sup> <sup>β</sup>jji � � (7)

<sup>þ</sup> <sup>β</sup>zzz <sup>þ</sup> <sup>β</sup>zxx <sup>þ</sup> <sup>β</sup>zyy � �<sup>2</sup>

(4)

(5)

(8)

Particularly important in this technique is the choice of solvent because it has a great impact on the deposition of organic matter, it can interact with the dissolved substance, it can lead to the production of secondary products from it, or it can be present in the deposited films [33]. The role of solvent in MAPLE technique is central; we can say that the solvent (1) must dissolve the solute without interacting with it; (2) has to be volatile; (3) must absorb laser radiation; and (4) must transport the dissolved substance from the target to the substrate.

The experimental setup of MAPLE deposition technique for thin-film fabrication is showed in Figure 13. The solution concentration must be of 0.1–2.0% (mass) because of the hard laser interaction with the frozen solid. The solvent is desirable to have a freezing point as high as possible. Only the solvent (also named matrix) absorbed the radiation when the laser reaches the target, so the matrix evaporates, the "solid" is ablated, and only the material's molecules are deposited on the substrate [119].

#### 4.1 Investigation of NLO properties

#### 4.1.1 Parameters of NLO properties

Interactions of electromagnetic fields in various media produce new fields changed in frequency, phase, amplitude, or other characteristics of the incident fields resulting nonlinear optical (NLO) properties [89]. The parameter used to evaluate the NLO susceptibility is the total hyperpolarizability (βtot), meaning that a compound with large βtot value is predicted to be a potential NLO active one and vice versa [11]. Literature shows that experimental determination of the βtot value and therefore the NLO susceptibility is an expensive and laborious process, which led to using the quantum mechanical calculations including the DFT methods for the designing of NLO materials. The mean polarizability α, the total static dipole

Figure 13. Scheme of the MAPLE setup [119].

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

moment μtot, the quadrupole moment Q, and the mean first polarizability βtot may be calculated by using DFT theory. The x, y, z components are defined as follows:

$$a\_{\text{tot}} = \frac{a\_{\text{xx}} + a\_{\text{yy}} + a\_{\text{xx}}}{3} \tag{3}$$

$$
\mu\_{\rm tot} = \left(\mu\_x^2 + \mu\_y^2 + \mu\_x^2\right)^{1/2} \tag{4}
$$

$$Q\_{\rm tot} = \left(q\_x^3 + q\_y^3 + q\_z^3\right)^{1/3} \tag{5}$$

$$
\beta\_{\text{tot}} = \sqrt{\left(\boldsymbol{\beta}\_{\text{x}}^{2} + \boldsymbol{\beta}\_{\text{y}}^{2} + \boldsymbol{\beta}\_{\text{z}}^{2}\right)}\tag{6}
$$

$$
\beta\_j = \beta\_{\rm iji} + \frac{1}{3} \sum\_{i \neq j} \left( \beta\_{\rm ijj} + \beta\_{j\rm ij} + \beta\_{j\rm ji} \right) \tag{7}
$$

Therefore:

Most of the laser energy is absorbed by the volatile matrix, not the dissolved molecules, which minimize the photochemical decomposition of the precursor solution. In addition, the use of low fluences prevents or reduces thermal damage and decomposition of molecules, so deposition can take place at low fluctuations

Particularly important in this technique is the choice of solvent because it has a great impact on the deposition of organic matter, it can interact with the dissolved substance, it can lead to the production of secondary products from it, or it can be present in the deposited films [33]. The role of solvent in MAPLE technique is central; we can say that the solvent (1) must dissolve the solute without interacting with it; (2) has to be volatile; (3) must absorb laser radiation; and (4) must trans-

The experimental setup of MAPLE deposition technique for thin-film fabrication is showed in Figure 13. The solution concentration must be of 0.1–2.0% (mass) because of the hard laser interaction with the frozen solid. The solvent is desirable to have a freezing point as high as possible. Only the solvent (also named matrix) absorbed the radiation when the laser reaches the target, so the matrix evaporates, the "solid" is ablated, and only the material's molecules are deposited on the

Interactions of electromagnetic fields in various media produce new fields changed in frequency, phase, amplitude, or other characteristics of the incident fields resulting nonlinear optical (NLO) properties [89]. The parameter used to evaluate the NLO susceptibility is the total hyperpolarizability (βtot), meaning that a compound with large βtot value is predicted to be a potential NLO active one and vice versa [11]. Literature shows that experimental determination of the βtot value and therefore the NLO susceptibility is an expensive and laborious process, which led to using the quantum mechanical calculations including the DFT methods for the designing of NLO materials. The mean polarizability α, the total static dipole

port the dissolved substance from the target to the substrate.

) compared to conventional pulsed laser deposition (PLD) (typically

(0.05–0.5 J/cm<sup>2</sup>

substrate [119].

Figure 13.

12

Scheme of the MAPLE setup [119].

4.1 Investigation of NLO properties

4.1.1 Parameters of NLO properties

).

Applied Surface Science

few J/cm<sup>2</sup>

$$\beta\_{\text{tot}} = \sqrt{\left(\beta\_{\text{xxx}} + \beta\_{\text{xy}} + \beta\_{\text{xxx}}\right)^2 + \left(\beta\_{\text{yy}} + \beta\_{\text{yx}} + \beta\_{\text{yx}}\right)^2 + \left(\beta\_{\text{xxx}} + \beta\_{\text{xxx}} + \beta\_{\text{xy}}\right)^2} \tag{8}$$

The larger hyperpolarizability value of one component over the other components means that the electronic charge delocalization is larger in that direction [11]. There are a lot of factors that contribute to enhance the NLO properties of the compounds, which are cumulated with the prospective βtot calculated values [3].

#### 4.1.2 Experimental determination of second harmonic generation (SHG) in thin films

Experimental setup used to investigate the SHG behavior [117] in thin-film samples is represented in Figure 14. The component parts of the system used for the determination of second harmonic generation (SHG) are a sapphire laser ("Tsunami," from Spectra-Physics; 780 nm, 60–100 fs pulse duration, 80 MHz repetition rate); an optical system made of a half-wave plate and a Glan-Taylor polarizing prism that allows the variation of beam intensity; a microscope's objective is to focus the laser beam onto the thin-film samples and collect the emitted SHG radiation. A dichroic mirror (DM) separated the excitation radiation, and the SHG intensity is measured by a camera spectrograph [117].

Figure 14. Experimental setup for determining second harmonic generation.

#### Figure 15.

The SHG experimental spectra of MAPLE-grown OHA thin films (adapted with the permission from reference [114]).

The experimental second harmonic generation of MAPLE-grown 1,2,3,4,5,6,7,8 octahydroacridine (OHA) 47 using laser fluence 0.3 J/cm<sup>2</sup> , laser spot 2 mm<sup>2</sup> , and 40,000 pulses, on Si substrates, under fs-laser irradiation at 788 nm wavelength is represented in Figure 15 [114]. In this case, the presence of the SHG signal is related to the conformational asymmetry of the OHA molecule.
