Selected Organometallic Compounds for Third Order Nonlinear Optical Application

In this paper, we present the third harmonic generation response of Znq2 (Bis-(8-hydroxyquinolinato)zinc), Cuq2 (8-Hydroxyquinoline copper(II)), and Alq3 (Tris-(8-hydroxyquinoline)aluminum) organometallic compounds. An experiment was conducted for s and p polarizations of incident beam, using the Maker fringes technique. The third order nonlinear susceptibility χ(3) was estimated using the Kubodera and Kobayashi comparative model, on the grounds that presented compounds exhibit high linear absorption of the generated third harmonic wavelength (355 nm). These complexes were deposited as thin films using the physical vapor deposition (PVD) method. Investigated complexes vary in terms of the coordination center and number of quinoline ligands, which visibly influence their nonlinear response. The global hybrid B3LYP functional with the basis set 6-31G(d) was used in computing the linear and non-linear optical properties. The computed γtot value (8765.36 × 10−36 esu for Cuq2) is superior to that of methylene blue (γ = 32.00 × 10−36 esu). The calculated theoretical values were found to be in good agreement with the experimental results.


Introduction
Nowadays, a great number of researchers pay attention to the metal complexes that represent promising candidates for nonlinear optics (NLO) [1]. These kind of compounds attracted enormous attention due to their applications in different fields, such as medicine [2], organic light-emitting diodes (OLED) [3,4], photovoltaic [5], and photonics and optoelectronics [6,7]. Organometallic compounds exhibit potential as a third order nonlinear material, due to the important charge transfer between the ligands and the metal, as well as the switchable nonlinearity that is related to multiple electronic states of the central metal atom [8,9]. It has been already proven that the abovementioned properties allow for significant values of first molecular hyperpolarizability β [10,11]. Bis(8-Hydroxyquinoline) zinc-Znq 2 , bis(8-Hydroxyquinoline) copper (II)-Cuq 2 , and tris(8-Hydroxyquinoline) aluminum-Alq 3 have The experimental studies used have allowed us to verify the correctness of the construction of the applied computational theoretical model, which together allowed for analysis of the mechanisms of the physical processes responsible for nonlinear effects.

Preparation of Thin Films
Presented compounds were obtained commercially from Sigma Aldrich Chemical Company (99% for Znq2, 98% for Cuq2, and 99.995% for Alq3, St. Louis, MO, USA). The Znq2, Cuq2, and Alq3 thin films were fabricated with the Physical Vapor Deposition (PVD) technique using Thin Film Deposition System-NANO 36™ (Kurt J. Lesker Company, Jefferson Hills, PA, USA) in a high vacuum chamber under 2 × 10 −6 Torr. Compounds were evaporated from the ceramic effusion cell with source material and deposited successfully on BK7 glass substrates kept at room temperature during the deposition process. Thickness of the thermally evaporated films depends on properties of the source, time of evaporation, and distance between source and substrate. The powdered source material was evaporated from a ceramic effusion cell with a nozzle 10 mm in diameter on the top. The distance between source and substrate was equal to 20 cm and substrates were rotated at a speed equal to 20 rot/min during the whole deposition process. The deposition rate was considered the most important parameter for thin films forming and for all samples it was equal to 0.1 Å/s. Initial power necessary to achieve the desired rate of the source material's evaporation depends on properties of the source material, i.e., molecular mass, density, and temperature of the sublimation. The initial power required to fix the rate at 0.1 Å/s was different for all studied complexes and was equal to 280 W (Znq2), 220 W (Cuq2), and 150 W (Alq3), respectively. The deposition rate and the final film's thickness were controlled by the piezoelectric controller.

UV-Vis Absorption Spectra
A Lambda 950 UV/Vis/NIR spectrophotometer (Perkin Elmer, Waltham, MA, USA) was used to measure the absorption spectra of Znq2, Cuq2, and Alq3 thin films in the range 300-1100 nm at room temperature.

Atomic Force Microscopy (AFM)
An Agilent 5500 Atomic Force Microscope equipped with a MSNL-D Bruker cantilever was used to measure AFM images. AFM imaging was carried out in the tapping mode. Measurements were performed in an isolated sound and vibration proof chamber at a temperature equal to 16 °C. The experimental studies used have allowed us to verify the correctness of the construction of the applied computational theoretical model, which together allowed for analysis of the mechanisms of the physical processes responsible for nonlinear effects.

Preparation of Thin Films
Presented compounds were obtained commercially from Sigma Aldrich Chemical Company (99% for Znq 2 , 98% for Cuq 2 , and 99.995% for Alq 3 , St. Louis, MO, USA). The Znq 2 , Cuq 2 , and Alq 3 thin films were fabricated with the Physical Vapor Deposition (PVD) technique using Thin Film Deposition System-NANO 36™ (Kurt J. Lesker Company, Jefferson Hills, PA, USA) in a high vacuum chamber under 2 × 10 −6 Torr. Compounds were evaporated from the ceramic effusion cell with source material and deposited successfully on BK7 glass substrates kept at room temperature during the deposition process. Thickness of the thermally evaporated films depends on properties of the source, time of evaporation, and distance between source and substrate. The powdered source material was evaporated from a ceramic effusion cell with a nozzle 10 mm in diameter on the top. The distance between source and substrate was equal to 20 cm and substrates were rotated at a speed equal to 20 rot/min during the whole deposition process. The deposition rate was considered the most important parameter for thin films forming and for all samples it was equal to 0.1 Å/s. Initial power necessary to achieve the desired rate of the source material's evaporation depends on properties of the source material, i.e., molecular mass, density, and temperature of the sublimation. The initial power required to fix the rate at 0.1 Å/s was different for all studied complexes and was equal to 280 W (Znq 2 ), 220 W (Cuq 2 ), and 150 W (Alq 3 ), respectively. The deposition rate and the final film's thickness were controlled by the piezoelectric controller.

UV-Vis Absorption Spectra
A Lambda 950 UV/Vis/NIR spectrophotometer (Perkin Elmer, Waltham, MA, USA) was used to measure the absorption spectra of Znq 2 , Cuq 2 , and Alq 3 thin films in the range 300-1100 nm at room temperature.

Atomic Force Microscopy (AFM)
An Agilent 5500 Atomic Force Microscope equipped with a MSNL-D Bruker cantilever was used to measure AFM images. AFM imaging was carried out in the tapping mode. Measurements were performed in an isolated sound and vibration proof chamber at a temperature equal to 16 • C.

NLO Measurements
The nonlinear optical measurements were carried out using the rotational Maker fringe technique [36][37][38][39]. The Nd:YAG laser generating at λ = 1064 nm was used as the fundamental beam, with 10 Hz repetition rate and 30 ps pulse duration. The energy of every pulse was 100 µJ and it was controlled by the laser powermeter (LabMax TOP, Coherent, Santa Clara, CA, USA). The polarizer and the half wave plate were used for polarization adjustments of the laser beam. The incident beam was focused on the sample by the lens with the focal point equal to 25 cm. The motorized rotation stage changed angle of incidence from −60 • to +60 • . The interference filter was used to ensure only measurement of generated third harmonic wavelength (355 nm). The signal was recorded by the Hamamatsu R1828-01 photomultiplier which collected intensity of THG every 0.5 • . The boxcar average system integrated the signal which was afterwards processed by the computer. As the reference material, we used a silica glass plate (SiO 2 ) with a well-established value of χ (3) equal to 2.0 × 10 −22 m 2 V −2 . Due to the high linear absorption in the region of the generated third harmonic wavelength, the Kubodera and Kobayashi theoretical model was used to calculate the third order susceptibility of investigated compounds [40][41][42]. This model makes a comparison between the maximum light intensity of the generated signal of THG and the maximum light intensity of the reference signal (silica).
where, L coh represents the coherence length of the silica equal to 6.7 µm. The thickness of the studied thin films is represented as d, χ silica is the third nonlinear susceptibility of fused silica, I 3ω comp and I 3ω silica represent the average peak intensity of the recorded Maker fringes signal. The thicknesses of the prepared thin films were measured using the profilometer Veeco Dektak 3.

Computational Details
The ground-state geometries were optimized in gas phase by a density functional theory DFT/B3LYP called the hybrid method based on three Becke parameters using the base 6-31G(d) level of theories implemented in the GAUSSIAN 09 program package [43,44]. With this method the dynamic second order hyperpolarizability γ (−2ω; ω, ω, 0) and the HOMO and LUMO energy levels of the investigated molecules were calculated. The addition of d polarization functions on the carbon and nitrogen atoms are critical in order to have a precise estimation of the hyperpolarizabilities.

Topography and Structural Properties
The surface morphology of the investigated metalorganic thin films deposited by the PVD technique was analyzed by Atomic Force Microscopy (AFM). Figure 2a-f shows AFM images of the Znq 2 , Cuq 2 , and Alq 3 thin films for samples deposited on the substrate at room temperature, respectively. The scanning area of the films was equal to 1 × 1 µm 2 . Presented figures were drawn using Gwyddion software and they visualize 2D and 3D images for all of the investigated metalorganic complexes. The Znq2 and Cuq2 thin films deposited at room temperature showed nanocrystalline characters. The structure of the films contained clearly visible hills of nanocrystallites and valleys between them. The average differences between them were equal to ca. 40 nm for Cuq2 and 30 nm for Znq2. The structure of the Alq3 thin film was completely different. The structure also showed a nanocrystalline character, but the size of the nanocrystallites were much smaller and the surface was more homogeneous. The average difference was equal to ca. 4nm. Thin film profile examples of tested compounds are presented in Figure 3. The Znq 2 and Cuq 2 thin films deposited at room temperature showed nanocrystalline characters. The structure of the films contained clearly visible hills of nanocrystallites and valleys between them. The average differences between them were equal to ca. 40 nm for Cuq 2 and 30 nm for Znq 2 . The structure of the Alq 3 thin film was completely different. The structure also showed a nanocrystalline character, but the size of the nanocrystallites were much smaller and the surface was more homogeneous. The average difference was equal to ca. 4nm. Thin film profile examples of tested compounds are presented in Figure 3.  Film growth of Mq2 complexes showed an island formation nature. In the case of the Alq3 complex, film growth showed definitely more layer by layer growth. This phenomenon indicated a much stronger interaction between the molecules Mq2 compared to Mq3 on the substrate's surface, not only during the formation process of the thin films, but also after the deposition inside thin films. Such intermolecular interactions usually affect nonlinear optical properties of the thin films. There are many interesting structural effects that can affect second and third order nonlinear effects. The enhancement of THG responses by surface plasmons [45,46], metal's nanoparticles [47,48], or welloriented nanostructures has been reported among many researchers [49,50]. There is still a need for systems (compounds) that can exhibit large NLO responses for potential photonics applications as nonlinear optical (NLO) systems play a major role in the field of nanophotonics, photonics including optical information processing, sensor protector applications, and optical data storage. As the occurrence of NLO response is not well studied in the case of investigated compounds, we studied these interesting NLO systems. In such systems, intermolecular interactions are not strong enough to create a well-oriented nanostructural film, which was proven by visualization of surface morphology. Moreover, the internal molecular bonds are not broken during the sublimation process. Therefore, the nanostructured nature of the films is entirely random, nanoparticles are not formed on the surface, and there are no plasmonic effects. Figure 4 illustrates the UV-Vis absorption spectra of Znq2, Cuq2, and Alq3 thin films. All of the investigated compounds exhibit absorption in range of the blue light, with maxima positioned at λabs = 380 nm for Znq2, λabs = 390 nm for Alq3, and λabs = 428 nm for Cuq2. These bands might be attributed to the combination of intraligand π→π * transition and the ligand to metal charge transfer (LMCT). The most important observation from this measurement is the fact that none of the compounds absorb wavelengths of 1064 nm, which was used to investigate the third nonlinear optical properties In the case of the Alq 3 complex, film growth showed definitely more layer by layer growth. This phenomenon indicated a much stronger interaction between the molecules Mq 2 compared to Mq 3 on the substrate's surface, not only during the formation process of the thin films, but also after the deposition inside thin films. Such intermolecular interactions usually affect nonlinear optical properties of the thin films. There are many interesting structural effects that can affect second and third order nonlinear effects. The enhancement of THG responses by surface plasmons [45,46], metal's nanoparticles [47,48], or well-oriented nanostructures has been reported among many researchers [49,50]. There is still a need for systems (compounds) that can exhibit large NLO responses for potential photonics applications as nonlinear optical (NLO) systems play a major role in the field of nanophotonics, photonics including optical information processing, sensor protector applications, and optical data storage. As the occurrence of NLO response is not well studied in the case of investigated compounds, we studied these interesting NLO systems. In such systems, intermolecular interactions are not strong enough to create a well-oriented nanostructural film, which was proven by visualization of surface morphology. Moreover, the internal molecular bonds are not broken during the sublimation process. Therefore, the nanostructured nature of the films is entirely random, nanoparticles are not formed on the surface, and there are no plasmonic effects.

Optical Properties
(THG response). These results suggest that in case of third order nonlinear susceptibility calculations, the use of the Kubodera-Kobayashi comparative model is justified, due to the strong absorption of generated third harmonic wavelength (355 nm).

THG Measurements
In Figures 5-7 we present curves with an oscillatory signal of measured sample and reference for two different polarizations of pump beam. There is a good symmetry of signals at incident angle 0° visible for all of investigated samples. In every case, there is no significant difference between applied polarizations of initial laser beam. The angle range of −60° to +60° allowed us to measure a wide spectrum with observable growth and decay of Maker fringes intensity and phase matching. All of the presented compounds exhibit a THG efficiency two magnitudes higher than reference material. Table 1 represents the thickness of studied samples, their absorption coefficients, and their third order susceptibilities, which were calculated using Equation 1.

THG Measurements
In Figures 5-7 we present curves with an oscillatory signal of measured sample and reference for two different polarizations of pump beam. There is a good symmetry of signals at incident angle 0° visible for all of investigated samples. In every case, there is no significant difference between applied polarizations of initial laser beam. The angle range of −60° to +60° allowed us to measure a wide spectrum with observable growth and decay of Maker fringes intensity and phase matching. All of the presented compounds exhibit a THG efficiency two magnitudes higher than reference material. Table 1 represents the thickness of studied samples, their absorption coefficients, and their third order susceptibilities, which were calculated using Equation 1.   Figure 4 illustrates the UV-Vis absorption spectra of Znq 2 , Cuq 2 , and Alq 3 thin films. All of the investigated compounds exhibit absorption in range of the blue light, with maxima positioned at λ abs = 380 nm for Znq 2 , λ abs = 390 nm for Alq 3 , and λ abs = 428 nm for Cuq 2 . These bands might be attributed to the combination of intraligand π→π * transition and the ligand to metal charge transfer (LMCT). The most important observation from this measurement is the fact that none of the compounds absorb wavelengths of 1064 nm, which was used to investigate the third nonlinear optical properties (THG response). These results suggest that in case of third order nonlinear susceptibility calculations, the use of the Kubodera-Kobayashi comparative model is justified, due to the strong absorption of generated third harmonic wavelength (355 nm).

THG Measurements
In Figures 5-7 we present curves with an oscillatory signal of measured sample and reference for two different polarizations of pump beam. There is a good symmetry of signals at incident angle 0 • visible for all of investigated samples. In every case, there is no significant difference between applied polarizations of initial laser beam. The angle range of −60 • to +60 • allowed us to measure a wide spectrum with observable growth and decay of Maker fringes intensity and phase matching. All of the presented compounds exhibit a THG efficiency two magnitudes higher than reference material. Table 1 represents the thickness of studied samples, their absorption coefficients, and their third order susceptibilities, which were calculated using Equation (1).   A few important conclusions can be drawn from comparing the results of calculated third order susceptibilities. Due to the strong absorption coefficient in the region of generated third harmonic   A few important conclusions can be drawn from comparing the results of calculated third order susceptibilities. Due to the strong absorption coefficient in the region of generated third harmonic A few important conclusions can be drawn from comparing the results of calculated third order susceptibilities. Due to the strong absorption coefficient in the region of generated third harmonic wavelength, the figure of merit was calculated (χ (3) /α) to allow easier comparison of NLO response for investigated compounds. This value provided a more accurate figure, which helped to conclude a clearer influence of the metal center on THG. Firstly, Alq 3 has the lowest THG response, whereas Cuq 2 have visibly the highest one. This order might be due to the metal's electronegativity, orbitals' shape, or extended delocalization path that are characteristic for coordination complexes. It should be noted that in the cases of both Cuq 2 and Znq 2 the clear intramolecular charge transfer is visible between ligand and metal (See Table 4), which increases THG response. Additionally, the unfilled d-shell in case of Cu enhances the NLO response, what can be associated with the additional electronic levels present in this system and increase of electron-accepting properties [51]. Moreover, in case of Alq 3 , the lowest THG response might originate from the lack of intramolecular charge transfer between ligand and metal center (See Table 4). Electron configuration of Al 3+ indicates that all orbitals are filled with electrons, hence its acceptor properties visibly decrease. For this sample quinolone ligands seem to act as a donor and an acceptor simultaneously. In the investigated compounds of Znq 2 and Cuq 2 , an organic ligand has more of an electron donating character, hence its orbitals have lower energy than metal orbitals that resolve in electron transfer from bonding π or antibonding π* orbital into σ* orbitals. Thus, the nature interactions between ligand and metal point to the ligand to metal charge transfer (LMCT) as an origin of high nonlinear optical response [52,53]. In the case of presented coordination complexes, the influence of the polarization of the incident beam is negligible. Results for p as well as s polarization vary in an insignificant way.

Theoretical Simulations
The optimized structures of investigated compounds using the DFT method are shown in Figure 8. The linear optical properties are very important for the characterization of materials used in nonlinear optics. Figure 9 displays the calculated UV-Vis absorption spectra of Alq 3 , Znq 2 , and Cuq 2 complexes. The theoretical spectrum was made in the gas phase and the absorption peaks are shifted towards shorter wavelengths in respect to the experimental spectrum (see Table 2). The shift of the experimental absorption band towards the longer wavelengths is caused by the amorphous nature of the structure of films obtained by the PVD method. Comparing the experimental UV-visible absorption spectra and theoretical, it can be seen that intermolecular interactions cause a bathochromic effect on all molecules [54]. shell in case of Cu enhances the NLO response, what can be associated with the additional electronic levels present in this system and increase of electron-accepting properties [51]. Moreover, in case of Alq3, the lowest THG response might originate from the lack of intramolecular charge transfer between ligand and metal center (See Table 4). Electron configuration of Al 3+ indicates that all orbitals are filled with electrons, hence its acceptor properties visibly decrease. For this sample quinolone ligands seem to act as a donor and an acceptor simultaneously. In the investigated compounds of Znq2 and Cuq2, an organic ligand has more of an electron donating character, hence its orbitals have lower energy than metal orbitals that resolve in electron transfer from bonding π or antibonding π* orbital into σ* orbitals. Thus, the nature interactions between ligand and metal point to the ligand to metal charge transfer (LMCT) as an origin of high nonlinear optical response [52,53]. In the case of presented coordination complexes, the influence of the polarization of the incident beam is negligible. Results for p as well as s polarization vary in an insignificant way.

Theoretical Simulations
The optimized structures of investigated compounds using the DFT method are shown in Figure  8. The linear optical properties are very important for the characterization of materials used in nonlinear optics. Figure 9 displays the calculated UV-Vis absorption spectra of Alq3, Znq2, and Cuq2 complexes. The theoretical spectrum was made in the gas phase and the absorption peaks are shifted towards shorter wavelengths in respect to the experimental spectrum (see Table 2). The shift of the experimental absorption band towards the longer wavelengths is caused by the amorphous nature of the structure of films obtained by the PVD method. Comparing the experimental UV-visible absorption spectra and theoretical, it can be seen that intermolecular interactions cause a bathochromic effect on all molecules [54].    . Theoretically simulated UV-Vis spectra of Alq3, Znq2, and Cuq2. Figure 9. Theoretically simulated UV-Vis spectra of Alq 3 , Znq 2 , and Cuq 2 . Table 2. HOMO and LUMO energy levels and theoretical bandgap (E g ) HOMO-LUMO (B3LYP/6-31G(d)). Calculated values of HOMO and LUMO energy and the HOMO-LUMO energy gap (see Table 2) allows you to explore the relationship of non-linear properties with the structure of the studied materials.

Sample HOMO [eV] LUMO [eV] (E g ) HOMO-LUMO [eV]
The HOMO-LUMO gap of Znq 2 is relatively higher than the energy gap value for the remaining molecules. The high energy gap and the data here suggests that Znq 2 is relatively less reactive than the molecules Alq 3 and Cuq 2 . Decreasing the value of the energy gap increases the charge transfer within the particles. Persistent particles have large energy breaks, and the more reactive particles have small energy breaks HOMO-LUMO. Lower HOMO-LUMO gaps usually herald better nonlinear properties which are also visible in these complexes. It is known that the dependence of the energy gap on the third order nonlinear optical susceptibility is represented as the inverse of the optical break raised to 6th power [55].
The total third order hyperpolarizability γ tot values have been calculated from the formula presented in Reference [56]. As you can see from Table 3, the hyperpolarizability of complex Cuq 2 (γ tot = 8765 × 10 −36 esu) is about one order higher than for Alq 3 (γ tot = 359 × 10 −36 esu) and two orders higher than for Znq 2 (γ tot = 57 × 10 −36 esu). We compared our hyperpolarizability values with those reported by Leupacher et al. [57] of methylene blue (γ = 32 × 10 −36 esu) by means of THG measurements and the value of Cuq 2 is two orders higher. A significant value of the corresponding component points to the delocalization of the electrons in a given direction. The second order frequency-dependent hyperpolarizability for Cuq 2 is dominated by the oblong component of γ zzzz . The calculated values suggest that the hyperpolarizability is controlled by donor-acceptor strengths and the metal cation variability. As mentioned above, this complex exhibits an intramolecular charge transfer and produce polarization along the π-conjugated bonds. Table 4 shows the visualized structures of studied complexes and show the population of electrons on their orbitals. The left column shows molecular orbitals that are occupied in the ground state, with the lowest-energy orbital at the top. The orbital wave functions are positive in the red regions and negative in the green. The right column shows virtual molecular orbitals which may be occupied in excited states.

Cuq2
The HOMO for Tris-(8-hydroxyquinoline) aluminum-Alq3 is mainly located on the three quinoxaline ligands. LUMO is mainly localized on the two nitrophenyl quinoxaline ligands. For the remaining two investigated molecules, the HOMO is also located on the ligands. LUMO is located all over the whole complex alike on the two linked ligands and as well as on Zn and Cu atoms. This electron delocalization can be attributed to the strong electron withdrawing nature of this group and is defined as ligand to metal charge transfer (LMCT) leading to high stability of complex. This effect is especially seen for the Znq2 and Cuq2 compounds. The presented calculation results reproduce the trend of the experimental results well and show clearly the increase of the nonlinear optical response upon Cu metal complexation [58].

Conclusions
The Alq3, Znq2, and Cuq2 complexes were studied both experimentally and theoretically, the HOMO and LUMO levels of compounds were analyzed by DFT/B3LYP/6-31G(d) method. In this paper, the main aim was to compare the third harmonic generation responses of three organometallic

Cuq2
The HOMO for Tris-(8-hydroxyquinoline) aluminum-Alq3 is mainly located on the three quinoxaline ligands. LUMO is mainly localized on the two nitrophenyl quinoxaline ligands. For the remaining two investigated molecules, the HOMO is also located on the ligands. LUMO is located all over the whole complex alike on the two linked ligands and as well as on Zn and Cu atoms. This electron delocalization can be attributed to the strong electron withdrawing nature of this group and is defined as ligand to metal charge transfer (LMCT) leading to high stability of complex. This effect is especially seen for the Znq2 and Cuq2 compounds. The presented calculation results reproduce the trend of the experimental results well and show clearly the increase of the nonlinear optical response upon Cu metal complexation [58].

Conclusions
The Alq3, Znq2, and Cuq2 complexes were studied both experimentally and theoretically, the HOMO and LUMO levels of compounds were analyzed by DFT/B3LYP/6-31G(d) method. In this paper, the main aim was to compare the third harmonic generation responses of three organometallic

Cuq2
The HOMO for Tris-(8-hydroxyquinoline) aluminum-Alq3 is mainly located on the three quinoxaline ligands. LUMO is mainly localized on the two nitrophenyl quinoxaline ligands. For the remaining two investigated molecules, the HOMO is also located on the ligands. LUMO is located all over the whole complex alike on the two linked ligands and as well as on Zn and Cu atoms. This electron delocalization can be attributed to the strong electron withdrawing nature of this group and is defined as ligand to metal charge transfer (LMCT) leading to high stability of complex. This effect is especially seen for the Znq2 and Cuq2 compounds. The presented calculation results reproduce the trend of the experimental results well and show clearly the increase of the nonlinear optical response upon Cu metal complexation [58].

Conclusions
The Alq3, Znq2, and Cuq2 complexes were studied both experimentally and theoretically, the HOMO and LUMO levels of compounds were analyzed by DFT/B3LYP/6-31G(d) method. In this

Cuq2
The HOMO for Tris-(8-hydroxyquinoline) aluminum-Alq3 is mainly located on the three quinoxaline ligands. LUMO is mainly localized on the two nitrophenyl quinoxaline ligands. For the remaining two investigated molecules, the HOMO is also located on the ligands. LUMO is located all over the whole complex alike on the two linked ligands and as well as on Zn and Cu atoms. This electron delocalization can be attributed to the strong electron withdrawing nature of this group and is defined as ligand to metal charge transfer (LMCT) leading to high stability of complex. This effect

Cuq2
The HOMO for Tris-(8-hydroxyquinoline) aluminum-Alq3 is mainly located on the three quinoxaline ligands. LUMO is mainly localized on the two nitrophenyl quinoxaline ligands. For the remaining two investigated molecules, the HOMO is also located on the ligands. LUMO is located all over the whole complex alike on the two linked ligands and as well as on Zn and Cu atoms. This electron delocalization can be attributed to the strong electron withdrawing nature of this group and is defined as ligand to metal charge transfer (LMCT) leading to high stability of complex. This effect is especially seen for the Znq2 and Cuq2 compounds. The presented calculation results reproduce the trend of the experimental results well and show clearly the increase of the nonlinear optical response upon Cu metal complexation [58].

Conclusions
The Alq3, Znq2, and Cuq2 complexes were studied both experimentally and theoretically, the HOMO and LUMO levels of compounds were analyzed by DFT/B3LYP/6-31G(d) method. In this

Cuq2
The HOMO for Tris-(8-hydroxyquinoline) aluminum-Alq3 is mainly located on the three quinoxaline ligands. LUMO is mainly localized on the two nitrophenyl quinoxaline ligands. For the remaining two investigated molecules, the HOMO is also located on the ligands. LUMO is located all over the whole complex alike on the two linked ligands and as well as on Zn and Cu atoms. This electron delocalization can be attributed to the strong electron withdrawing nature of this group and is defined as ligand to metal charge transfer (LMCT) leading to high stability of complex. This effect is especially seen for the Znq2 and Cuq2 compounds. The presented calculation results reproduce the trend of the experimental results well and show clearly the increase of the nonlinear optical response upon Cu metal complexation [58].

Conclusions
The Alq3, Znq2, and Cuq2 complexes were studied both experimentally and theoretically, the HOMO and LUMO levels of compounds were analyzed by DFT/B3LYP/6-31G(d) method. In this The HOMO for Tris-(8-hydroxyquinoline) aluminum-Alq 3 is mainly located on the three quinoxaline ligands. LUMO is mainly localized on the two nitrophenyl quinoxaline ligands. For the remaining two investigated molecules, the HOMO is also located on the ligands. LUMO is located all over the whole complex alike on the two linked ligands and as well as on Zn and Cu atoms. This electron delocalization can be attributed to the strong electron withdrawing nature of this group and is defined as ligand to metal charge transfer (LMCT) leading to high stability of complex. This effect is especially seen for the Znq 2 and Cuq 2 compounds. The presented calculation results reproduce the trend of the experimental results well and show clearly the increase of the nonlinear optical response upon Cu metal complexation [58].

Conclusions
The Alq 3 , Znq 2 , and Cuq 2 complexes were studied both experimentally and theoretically, the HOMO and LUMO levels of compounds were analyzed by DFT/B3LYP/6-31G(d) method. In this paper, the main aim was to compare the third harmonic generation responses of three organometallic compounds investigated in the form of thin films. The films were obtained by using the PVD technique in a high vacuum on BK7 glass substrates. Structural properties showed that the number of quinoline ligands have a significant influence on the mechanism of thin film formation as well as the final microstructure of the film. The THG measurements were performed by means of the third harmonic generation (THG) technique at the fundamental wavelength of 1064 nm. The third order susceptibilities χ (3) THG were calculated using the Kubodera and Kobayashi comparative model. We obtained the highest THG response for Cuq 2 . These results indicate that the character of the metal as well as the shape of its orbital strongly influence the NLO response. The thin films fabricated by the Physical Vapor Deposition technique have a high quality, give correct experimental results, and have good compliance with the obtained theoretical data. It is clearly shown that the substitution by Cu atom has affected the lowering of the HOMO-LUMO bandgap and at the same time leads to an increasing of the hyperpolarizability γ tot . The HOMO-LUMO map indicates that electron delocalization in the studied complexes can be attributed to ligand to ligand charge transfer (LLCT) for Alq 3 and to ligand to metal charge transfer (LMCT) in the case of Cuq 2 as well as Znq 2 . The obtained values of second order hyperpolarizability γ, (which characterize the individual molecule) for Cuq 2 is much larger than the values obtained for the remaining molecules from investigated series (more than 22 times). The advantage of coordination complexes is the additional effects which are present in organometallics that result from efficient delocalization of π-electrons and influence the nonlinear optical behavior. The crucial advantage of the studied materials is the unique combination of good nonlinear optical and electronic properties and good optical quality of the studied films which are extremely difficult to be achieved. We believe that NLO properties of the presented organometallic thin films can be used in optoelectronic devices due to their high value of third harmonic generation.