Structural and nonlinear optical properties of as-grown and annealed metallophthalocyanine thin films

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Structural and nonlinear optical properties of as-grown and annealed metallophthalocyanine thin ﬁ lms

A. Zawadzka

^a,

⁎ ^{, P. P} ł óciennik

^a

, J. Strzelecki

^a

, M. Pranaitis

^b

, S. Dabos-Seignon

^b

, B. Sahraoui

^b

aInstitute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland

bLUNAM Université, Université d'Angers, CNRS UMR 6200, Laboratoire MOLTECH-Anjou, 2 bd Lavoisier, 49045 Angers cedex, France

a b s t r a c t a r t i c l e i n f o

Article history:

Received 28 June 2012

Received in revised form 15 July 2013 Accepted 17 July 2013

Available online 25 July 2013 Keywords:

Metallophtalocyanine Physical vapor deposition Third harmonic generation

Third order nonlinear optical susceptibility Annealing process

The paper presents the Third Harmonic Generation investigation of four metallophtalocyanine (MPc, M = Cu, Co, Mg and Zn) thinfilms. The investigatedfilms were fabricated by Physical Vapor Deposition in high vacuum onto quartz substrates. MPc thinfilms were annealed after fabrication in ambient atmosphere for 12 h at the temperature equal to 150 °C or 250 °C. The Third Harmonic Generation spectra were measured to investigate the nonlinear optical properties and their dependence on the structure of the thinfilm after the annealing process. This approach allowed us to determine the electronic contribution of the third-order nonlinear optical susceptibility χ^b3Nelecof these MPcfilms and to investigate two theoretical models for explanation of the observed results. Wefind that the annealing process significantly changes the optical and structural properties of MPc thinfilms.

1. Introduction

In recent years, organic thinfilms have played an important role in finding materials for optical applications and devices. One kind of them are the thinfilms of phthalocyanines (Pcs) deposited on a solid state substrate. Phthalocyanine is an organic compound that forms stable complexes with several metals that have long been known as blue and green dyes and pigments. In addition to their traditional indus- trial applications, more often they are used as organic semiconductors [1], photodynamic medical therapies[2], optical recording materials [3], organic electroluminescence devices [4,5], gas sensors [6], and nonlinear optical materials [7–10]. Metallophthalocyanines (MPcs) also exhibit interesting physical and chemical properties that render them an important class of molecular functional materials. MPcs are p-type semiconductors characterized by a high thermal and chemical stability. The optical constants of MPc thinfilms provide information concerning microscopic characteristics of the material. The UV-VIS absorption spectra have been measured for thinfilms of phthalocyanine complex with Mg, Fe, Co, Zn, Cu and Ni[11,12]. For most MPcs,five transition bands, labeled as Q and B (Soret), N, L, and C, were identified. Its corresponding energies were estimated approximately 680, 380, 280, 250, and 205 nm[11]. Some MPcs may miss one or two of these bands. The optical properties of MPcs depend on their molecular com- position such as an atomic number, the position and nature of substitute atoms. The type of the central metal and its position relative to the ring

of metal-free Pc's particle play an important role in controlling their properties[13].

In the case of MPc thinfilms, the knowledge of the surface morphology and the preferred orientation of the crystallites are essential for their prospective applications. Especially, the analysis of preferred molecules orientation in MPc thinfilm plays an important role and is related to their optical properties (linear and nonlinear). It is well known that the metal-free Pc compounds have a large andflat molecular structure. Most of metallophthalocyanine complexes have a planar coordination around the central ion, which means that the metal and ligand atoms are generally in one plane. However, there are also cases where the large metal ions adopt a position outside the plane of the ring and significant doming occurs, giving these molecules' different symmetry. The diameter of phthalocyanine cavity can alsofluctuate and metallophthalocyanine compounds are able to form different phases[13,14]. The arrangement of molecular orientations during the film growth strongly depends on growth conditions, kind of substrates, and also thermal processes applied immediately after the thinfilm's deposition.

The mutual arrangement of molecules plays the most important role for thinfilm's structure and optical properties of the material. The knowledge of the linear (refractive index and proportional to the complex refractive index absorption coefficient) and nonlinear optical properties (susceptibility) is extremely important for applications of MPcs in modern optical technologies. Nonlinear optical materials with large susceptibility (second and third order) are essential for light emitting, modulating, and information technology devices because they determine the efficiency of these devices.

⁎ Corresponding author. Tel.: +48 56 6113244; fax: +48 56 622 5397.

E-mail address:azawa@ﬁzyka.umk.pl(A. Zawadzka).

http://dx.doi.org/10.1016/j.tsf.2013.07.042

Contents lists available atScienceDirect

Thin Solid Films

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / t s f

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The third order susceptibility measured in thinﬁlms and crystals, as well as the isotropically averaged molecular third order polarizability measured in solution, have been obtained for a variety of metal's oxides- and metallo-substituted phthalocyanine compounds[15–18].

This paper is devoted to nonlinear optical properties of the Physical Vapor Deposition (PVD) deposited MPc thinﬁlms and mutual stacking of these compounds' molecules as a dependence on applied temperature during the annealing process. We want to show that the annealing process can modify third order nonlinear susceptibility of macrocycle organometallic compounds. The main aim of this study is a theoretical and experimental research of the Third Harmonic Generation (THG) process, which allows to understand the differences in nonlinear optical spectra of MPcs after the annealing process. On account of the aim, four different MPcs: copper phthalocyanine (CuPc), cobalt phthalocyanine (CoPc), magnesium phthalocyanine (MgPc), and zinc phthalocyanine (ZnPc) have been investigated.

2. Experimental details 2.1. Theﬁlm preparation

The MPc (M = Co, Zn, Cu, and Mg) thinﬁlms were prepared by a PVD technique using typical home-made equipment [11]. The thin ﬁlms were deposited on quartz substrate for both THG and Atomic Force Microscopy (AFM) measurements (for structural properties de- termination). The process of deposition was carried out under pressure about 2.66*10⁻³Pa. The powders of CoPc, ZnPc, CuPc, and MgPc (97%

Sigma-Aldrich Co.) were loaded onto a quartz effusion cell with a nozzle of 10 mm in diameter on the top. The source material was thermally evaporated from quartz crucible heated by tungsten resistance coil.

The temperature of the evaporation source was manually controlled (using K-type thermocouple and autotransformer). The temperatures of the source for metallophthalocyanine materials are inTable 1. The quartz plates were located on the substrate holder about 10 cm above the evaporation source. The film thickness in thermal evaporation depends on the time of evaporation and the distance between the source and the substrate. In our system the manual-control deposition assures approximately 75% accuracy of the desired value offilm thickness. The deposition rate was in the range 0.1–0.2 nm/s and depended on the source material and temperature. The thickness of fabricated MPcs thinfilms was in a range from 80 to 120 nm. During evaporation the substrates were held at room temperature. Selected thinfilms were annealed to assure a different phase of crystallization and related nonlinear optical properties.

2.2. Characterization of thinﬁlms

Third Harmonic Generation (THG) measurements were carried out using the rotational Maker fringe technique[19]in the transmission scheme shown inFig. 1. A fused silica glass plate was used as a reference material for THG measurements. A laser beam of a Q-switched mode- locked Nd:YAG laser working at 1064 nm with 16 ps pulse duration, 1.6 mJ power per pulse and the repetition frequency of 10 Hz was used as a fundamental beam. The intensity at the entrance to the sample was described by Gaussian distribution in space and time. The beam

diameter was 0.4 mm at thefilm and the applied power density was about 5 GW/cm². The fundamental beam was focused on the sample using a lens, whose focal distance was about 250 mm. A rotation stage with the mounted sample of MPc allowed the variation of the incidence angle, with a resolution of 0.5°. After passing the sample, the transmit- tingfilter was used to cut the pump laser beam before the photomultiplier. Detector saturation was prevented using linear neutral density filters, whose transmittance value was taken into account during datafitting. The third harmonic signals were detected by the photomultiplier tube model: HAMAMATSU R1828-01, integrated by a box-car average system and processed by a computer. A portion of the input beam was reflected and measured by a fast photodiode Ph2 to monitor the input energy. Finally, the so-called Maker fringes were gen- erated by rotating the sample through the range of ±70° to the normal and recorded.

The linear optical properties–the absorption spectra were measured at normal incidence in the spectral range 190–1100 nm using a double- beam spectrophotometer (Perkin Elmer Lambda 2 UV/VIS/NIR). The structural properties –AFM imaging was performed in the contact mode, with an Agilent 5500 instrument equipped with a MSNL–D Bruker cantilever.

3. Theoretical models of THG

In the THG technique, an incident laser beam of high intensity at the frequencyωinteracts with a nonlinear medium and generates an addi- tional beam at a frequency 3ω. The third harmonic beam corresponds to a pure coherent electronic nonlinearity. This technique is one of the most informative methods for evaluating the electronic contribution of the real part of the third order nonlinear optical susceptibilitiesχ^b3Nelec. An advantage of the THG compared with Z-scan or Degenerate Four Wave Mixing (DFWM) is the fact that the response depends only on the instantaneous electronic contribution and not on the orientational and vibrational effects. The THG estimated value ofχ^b3Neleccan be at least one order of magnitude smaller than that measured via the DFWM experimental method. This difference does not usually exceed four orders of magnitude. Such difference values ofχ^b3Nare not surpris- ing, because it results from the different measurement techniques (DFWM and THG) and the operative nonlinear optical processes associ- ated with them. The degenerate four wave mixing (DFWM) method allows to measure different contributions, including: electronic, molecular orientation, and thermal effects. The THG method allows to determine only the electronic contribution to third order nonlinear optical susceptibility. The THG technique is sensitive to ultrafast electronic mechanisms of nonlinear response within femtosecond relaxation times and it is almost insensitive to slower effects such as thermal relaxation. Moreover, THG is a much more accurate technique because it allows to directly measure the nonlinear susceptibility and the result is undisturbed by the wavelength of fundamental laser beam. In the case of the DFWM technique, nonlinear signal from the sample is observed as an intensity decrease of the fundamental beams (all waves are focused on the sample and have the same wavelength).

A few theoretical models, using various approximations, have been described in order to determine the value ofχ^b3Nelecfrom the shape of the experimental curves of Maker fringes[19,20]obtained by the THG technique. We used two selected models (comparative model and Table 1

Parameters (temperature and time of deposition) of PVD process and obtained thickness of MPcs thinﬁlms.

Sample Temperature of deposition [°C] Time of deposition [min] Thickness[nm]

CuPc 350 20 100

MgPc 340 20 120

ZnPc 360 15 90

CoPc 360 15 80

Fig. 1.Experimental setup for THG measurements: BS1, BS2–beam splitters, Ph1, Ph2– photodiodes,λ/2–half wave plate, P–Glan polarizer, L–lens (f = 25 cm), RS–rotation stage, F–ﬁlters and PM–photomultiplier tube.

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model of Reintjes) for explanation of our results. The theoretical formu- las of these models are presented below.

3.1. Comparative model

This model compares[21]directly the maximum of light intensities amplitudes for third harmonic of nonlinear medium with those of the reference material used for its calibration of the experimental setup.

The value of the third order nonlinear susceptibilityχ^b3Nelecis derived by comparing third harmonic peak intensities of the sample and reference material (fused silica glass in this experiment).

The comparative model gives the order of magnitude of the third order nonlinear susceptibilityχ^b3Nelec. The refractive indices and third order susceptibility are considered real, so the weak absorption of the typical nonlinear sample is ignored. Theχ^b3Nelecvalue of the investigated material is calculated using the following Eq.(1):

χ^b³^Nelec¼χ^b³^NSð2=πÞ· LðCS=lÞ· Ið3ωS=I3ωÞ¹⁼² ð1Þ for the thinﬁlm whose thicknesslis much smaller than the coherence lengthLCSof fused silica. The indexScorresponds to the reference material,χ^b3NSis the third order nonlinear optical susceptibility andI3ωS

is the THG intensity of the reference measured under identical conditions to the sample.I_3ωis the absorption-corrected THG signal from the fringes of the sample. The value ofχ^b3NSfor fused silica glass equals 2.0 × 10⁻²²m²/V²and 3.11 × 10⁻¹⁴esu (atλω= 1064 nm) and is reported in the literature[20,22].

3.2. Theoretical model of Reintjes

The theoretical model of Reintjtes was developed in 1984[23]. In this model, wave equation is resolved in a homogenous and nonmagnetic nonlinear material, what explains the creation of Maker fringes. The Maker fringes become tighter when the angle of incidence θιincreases because the length of the optical path length in the sample increases nonlinearly with the angle whereas intensity of fringes decreases due to the increase of reﬂection coefﬁcient. Finally, the TH intensity is described by the following relation:

I₃_ω¼h576p⁶=n₃_ωn_ω³·λ_ω²·c²i

· χ^b3Nelec

2

· Ið Þ_ω²·L²

·

sinðπL=2L_CÞ=ðπL=2L_CÞg²

ð2Þ

where:L=l/cos[arcsin(sinθi/n_ω)] is the optical path length in sample,l stands for the thickness andnωfor the refractive index of the material, λω is the fundamental wavelength andLc the length of coherence, which corresponds to the distance along which bound and free waves gain a phase difference equal toπ. The length of coherence is described by the following relation: 6π(n_3ω−n_ω)/λω= π/LC.

4. Results and discussion

Metallophthalocyanines can form many different polycrystalline structures connected with many polymorphic phases when evaporated under vacuum (10⁻³–10⁻⁴Pa) onto a substrate maintained at a room temperature. Several phases differing by the relative orientation of the macrocycles between adjacent columns may be distinguished. The most popular phases are: stableβform and metastableαform. Crystal- lization to theα-phase also occurs between 50 and 150 °C substrate's temperature. Further substrate's heating at 200 °C irreversibly trans- forms the material into theβform. The main differences between both these forms are the tilt angle of the molecule within the columns, the tilt angle relative to the substrate and arrangement of the common columns in the crystalline structure. Schematic representation of the main molecular stacking of MPc is shown inFig. 2. Majority of MPc molecules have a planar coordination around the central ion, which means

that the metal and ligand atoms are generally in one plane. These MPc molecules possess approximately D4hsymmetry. However, there are also cases where the large metal ions adopt a position outside the plane of the ring and significant doming occurs, giving these molecules C4vsymmetry. From these differences in geometries of MPc molecules, four different states of the phthalocyanine ring are identified: metal ion size is approximately equal to phthalocyanine cavity size, metal ion size is smaller than phthalocyanine cavity size (CoPc, CuPc), metal ion size is greater than phthalocyanine cavity size (ZnPc) and metal ion size is much larger than phthalocyanine cavity size. These four states exhibit four structural effects on the phthalocyanine ring: equilibrium ring geometry, ring contraction, ring expansion and metal non-planarity (ZnPc, MgPc). The two main differences between investigated MPc molecules are the metal ion size and the presence of d electron subshells. Zinc phthalocyanine is an example of a molecule, where the metal ion is larger than the equilibrium cavity size of the ring but not so large and giving the largest cavity diameters of the phthalocyanines (of around 3.96 Å)[11]. The molecule's structure shows non-planarity with the lowest energy state structure having the zinc atom of 0.075 Å out of the ring's plane. Magnesium phthalocyanine is an example, where the metal ion is smaller than the equilibrium cavity size of the ring and has a smaller cavity with a diameter of 3.86 Å. MgPc is the example of a structure exhibiting ring contraction, where to accommodate the smaller metal ion, the four isoindole groups are pulled in towards the magnesium atom. This gives the smaller cavity diameter and causes an effect of the C-N-C bridge bond's formation. Thus, to accommodate magnesium metal ion, a significant deformation of the phthalocyanine ring takes place. These bridge bonds are lengthened considerably relative to all of the other structures by around 0.05 Å and the C-N-C bond angle is decreased by around 5 degrees[24]. Thus, the molecular arrangements for the ZnPc and MgPc thinfilms are more complicated assuming distortion of planarity with different symmetry of molecule. All these parameters play a significant role in changes of physical properties along stacking molecules. Thus, the obtained thin films possessing different phases and different crystalline structure of MPc lead to different linear and nonlinear optical properties.

The different polymorphic phase of MPc thinﬁlm can be obtained by using the different substrates material, different substrates temperature, deposition rate, and deposition methods during the deposition process. In this study, it will be shown that a changing of the structure and the morphology can also be obtained by applying appropriate temperature after the deposition process. Annealing processes applied after the deposition are very important because in many cases they can lead to changes in polymorphic forms (a metastable form convert to the stable form) and arrangement of the structure.

The surface morphology of the deposited and annealed MPc thin films was analyzed by AFM, which confirms the differences of the polymorphic crystalline structure of the MPc thinfilm. Atomic Force Micros- copy imaging was performed in the contact mode. An AFM profile measurement was also used to check the surface roughness of the MPc Fig. 2.Schematic representation of the two main molecular phases of MPcs: stableβform and metastableαform.

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samples as well as tofind out the thickness of the thinfilms. In order to measure the thickness of the thinfilms, the samples were scratched by a needle and the scratched surfaces were examined using the AFM. Thick- ness measurements were also carried out using the coupled prism method, which allows to measure the thickness and refractive index of the material. Obtained thickness for MPc thinfilms are presented inTable 1.

The AFM images of the same ZnPc, CuPc, MgPc and CoPc thinfilms before and after the annealing process and the profiles of the images for (ZnPc and CuPc) are shown inFig. 3. The image of ZnPc before annealing (Fig. 3a) shows a mixed morphology without the homogeneous region. After annealing, thefilm presented much more homogenous morphology with well-defined structures (Fig. 3b), which may be evidenced that most of the molecules are inflat-on or standing orientation. The particles tend to form neighboring domains with dimension of several hundred nanometers, regardless of the orientation. The profiles of the sample before and after annealing process are shown in Fig. 3c. Two other compounds (MgPc and CoPc) showed a similar effect of the phase change. The images of CoPc and MgPc before annealing (Fig. 3g, i) show mixed morphologies without homogeneous regions.

After annealing, thefilms presented almost a completely homogenous morphology with a well-defined standing form of nanostructures (Fig. 3h, j). This phenomenon is closely connected with an interaction between MPc molecule and substrate. In the case of CoPc and MgPc this molecule–molecule interaction is significantly stronger than the molecule-substrate interaction. Gaffo et al.[25]described the annealed and as-deposited ZnPc thinfilms. They reported the phase transition fromαtoβuponfilm annealing at 200 °C and confirmed this transition by Fourier transform infrared spectroscopy, UV–VIS spectroscopy, the AFM images and X-ray diffraction analysis. The results of this experiment show the same tendency.

Thinfilms of CuPc present a completely different morphology. The image of CuPc before annealing (Fig. 3d) shows a much more homogeneous morphology. Expressly visible is the tendency to form columns with a height of tens of nanometers. After annealing, the CuPc thin film presented almost completely homogenous morphology with well-definedflat structures (Fig. 3e). The profiles of the sample before and after the annealing process are shown inFig. 3f. The annealing process of CuPc and ZnPc led to the formation of well-defined flat form of molecules. This behavior is closely connected with an interaction between MPc molecule and substrate. In the case of CuPc this interaction is significantly higher in comparison with the molecule–molecule interaction[20].

The structure and the morphology orientation of MPc thinfilms are closely connected with linear and nonlinear optical properties. The MPc molecules are disoriented or oriented perpendicularly to the substrate surface after the deposition process and completely oriented parallel or perpendicular to the substrate surface after annealing, as was confirmed by AMF analyses. Linear optical absorption is strongly dependent on the crystal structure. The absorption spectra in the UV–VIS region as deposited and annealed (250 °C for 12 h) MPc thinfilms with different metal atoms are depicted in Fig. 4. The absorption spectra were measured at normal incidence in the spectral range 190–1100 nm.

Inspection ofFig. 4 shows that the absorption peaks intensities are comparable, while positions of the maxima change after the annealing process in comparison withﬁlm's absorbance for as-deposited samples.

In both cases, there are four absorption bands taking notations Q, B (Soret), N, and C[11]. The Q-band exists in the visible region of spectra while others (B, N, and C) exist in the UV region of spectra. Annealing at 250 °C shifts the peak position of all bands towards higher wavelengths side of spectra, except for MgPc. In this case, we observed the shift of the peak position towards shorter wavelengths side for the annealing

process temperature equal to 150 °C. A further increase of the temperature (250 °C) caused a partial decomposition of the thinﬁlm's structure, which allows concluding a lower thermal stability of this compound. It is also noted that the bands B, N, and C appear with higher intensities than of Q-band.

It is well known that molecules of metallophthalocyanines may exist mainly in two forms: monomer and dimer. In the dimer form, the close proximity of two rings can lead to coupling between the transition moments for two identical molecules. This interaction gives rise to new two levels of energy. The separation distance between them is re- ferred to the exciton splitting energy–so called Davydov splitting. In the previous studies, we reported the theoretical calculation and experimental results of this phenomenon[11].

Analysis of the spectra inFig. 4clearly shows the appearance of Q-band splitting into two peaks (around 600-700 nm) for all investigated MPcs, regardless of the fact whether the sample was subjected to the annealing process or not. With the increasing annealing temperature, the two Davydov splitting's peaks of Q-band were shifted to the same di- rection and changed gradually intensity compared to the no-annealing samples for ZnPc, MgPc and CuPc. A similar character of the shift but with a much smaller value occurred for CoPc. There was also no exchange in the intensity of the peaks for this material. Theα- andβ-phases of the different MPc thinﬁlms are shown both two Davydov splitting's peaks of

Fig. 4.UV-VIS absorption spectra of MPcs.

Fig. 3.AFM images (2 × 2μm area) of surface morphology of MPcs (M = Zn and Cu) compounds prepared using PVD technique: as deposited ZnPc sample (a), the same ZnPc sample after 12 h of annealing at 250 °C (b), proﬁles of the AFM images of ZnPc (c), as deposited CuPc sample (d), the same CuPc sample after 12 h of annealing at 250 °C (e), proﬁles of the AFM images CuPc (f), as deposited CoPc sample (g), the same CoPc sample after 12 h of annealing at 250 °C (h), as deposited MgPc sample (i) and the same MgPc sample after 12 h of annealing at 250 °C (j).

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Q-band absorption maxima, but the intensity and peak position vary with the phase exchange. These differences of the two peaks are closely related to the phase change due to the reorganization of thinﬁlms' molecular ar- chitecture, which have been mentioned several times in the literature [14,24–26].

The phase exchange affects not only linear, but also nonlinear optical properties. Until now, nonlinear optical spectra of MPcs have been studied mainly in a solution form[15,18]and in a solid state, but only as-deposited materials[16,17,19–21]. Manas et al.[15]and Derkowska et al.[18]have investigated the phthalocyanine as compounds in solution by the DFWM technique and observed a dependence of nonlinear susceptibility on the nature of the solvent, concentration of the solution, and the molecules' dimerization. They observed that the concentration and the kind of solvent can change the value of nonlinear susceptibility several times and the third order nonlinear optical susceptibility values of the ZnPc and MgPc are about three times larger than CuPc and CoPc.

They also noticed that the valueχ^b3Nelecdetected by the THG technique

had been approximately about two orders of magnitude smaller than that of detected by the DFWM experiment.

Typical DFWM technique allows determining different contributions of the nonlinear susceptibility, e.g.: electronic, orientation and thermal effects. The contributions of orientation and thermal parts strongly vary with duration of the laser pulse. The application of the short pulse laser (16 ps - in the case of presented experiment) causes that the orientation and thermal part are several rows smaller than the electronic part of the third order nonlinear optical susceptibility. Conse- quently, orientation and thermal contributions can be neglected and the obtained value is equal only to electronic contribution. THG method allows detecting the purely electronic contribution to third order nonlinear optical susceptibility. Thus, the results of both these experi- ments should give similar values ofχ^b3N. The results of presented experiment for as-deposited samples do not conﬁrm the signiﬁcant decreasing of the valueχ^b3Nelecdetected by the THG technique. Obtained values of the nonlinear optical susceptibility correspond to the Derkowska et al.

[18]results of the DFWM measurements.

An important factor inﬂuencing the nonlinear optical susceptibility is also a polymerization's process. For example, the dimerization's process can enhance linear and nonlinear properties of the material.

Manas and co-workers have also studied the effects of intermacrocycle interactions on the third order polarizabilities of cofacial Pc dimers and trimmers. From the observation, this mutual arrangement of the molecules in solution changed both linear and nonlinear optical properties.

They observed a shift of peak's position in the absorption spectra and the value of the third order nonlinear susceptibility. They observed a strong increase of susceptibility in all solutions, where the dimerization or trimmerization process appeared.

Cheng et al.[16]and Nalwa et al.[17]have investigated the metal oxides- and metallo-substituted phthalocyanines solid forms. They reported the preparation of a variety of free-base, metal's oxide-base and metallosubstituted Pc compounds fabricated by different methods, on different substrates, and in different forms (bulk crystal or amorphous Fig. 5.Typical example of Maker fringes for a sample of the fused silica glass (reference

sample).

Fig. 6.THG signal dependence as a function of the incident angle of the fundamental beam (S laser polarization) and temperature of annealing process: CoPc (a), CuPc (b), MgPc (c), ZnPc (d).

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ﬁlms). The value of the third order nonlinear optical susceptibility of presented solid forms were in the range from 10⁻²⁰to 10⁻¹⁷m²/V² and exhibited a dependence on kind of the substrate and thickness of the sample. Their theoretical and experimental results were based on the Maker fringe method. It can be noticed from the exploration of these papers, that the value of the third order nonlinear susceptibility is smaller by one or two rows for bulk crystal form in comparison with its thinﬁlm's forms.

In order to obtain information about the nonlinear optical properties four MPcs (M = Cu, Co, Zn and Mg) compounds were investigated.

Thinﬁlms of MPcs deposited on a 1 mm thick quartz substrate were measured. THG signals were analyzed using the Maker fringes technique with fused silica as a reference material.Fig. 5shows the Maker fringes patterns of THG observed by rotation of the reference sample.

Fig. 6shows the Maker fringes patterns of MPc samples for S - vertical polarization of laser radiation. It can be seen that the curve displays an oscillatory signal and intensities depend on the incidence angle. Good symmetry of THG signal for both as deposited and annealed MPc samples were found and proved the smooth surface and good crystallinity.

It was found that the intensity of the signals is strongly dependent on the annealing process temperature and almost independent on the laser polarization for all THG measurements.

FromFigs. 5 and 6, one can see a non-monotonic and a symmetric angle dependence of the THG signal which reﬂects a substantial optical nonlinearity and these characteristic are typical for all investigated MPc compounds regardless of the annealing process. These measurements and theoretical calculation based on described Reintjes model allow es- timating the value of the real part of the third order nonlinear optical susceptibility.

The presented theoretical model of Reintjes is the most adapted for explaining the experimental values of THG for these organometallic compounds. This model takes into account the majority of parameters which inﬂuence the value ofχ^b3Neleclike the contribution of transmission factors on the interfaces of the nonlinear medium. In the case of presented experiment, the results obtained with two models (Compar- ative and Reintjes) give similar values of the third order nonlinear optical susceptibilityχ^b3Nelecfor both as deposited and annealed at temperature equal to 150 °C samples of metallophthalocyanine compounds except for the copper phthalocyanines (Fig. 7). In the case of these three metallophthalocyanine compounds (CoPc, MgPc and ZnPc), the value of the real part of the third order nonlinear optical susceptibility grows with the temperature's increase. The value ofχ^b3Nelecfor samples of CoPc, MgPc and ZnPc annealed at 250 °C are signiﬁcantly higher than the samples not undergoing the annealing process.

Magnesium phthalocyanine is an example, where the metal atom is devoid of the d valence orbital and the four isoindole groups are pulled in towards the magnesium atom causing an effect of the C-N-C bridge bond's formation. We supposed that the enhancement of χ^b3Nelec

value could originate from that the Mg atom is located out of the Pc ring plane. MgPc is very sensitive to the environment and may form a pyramidal structure because the Mg atom could easily ligate with other molecules. Therefore, too high temperature of the annealing process may destroy thinfilm's structures, what was observed for temperature equal to 250 °C. ZnPc is an example with a completelyfilled d-shell, where the metal ion is larger than the equilibrium cavity size of the ring and giving the largest cavity diameters of the phthalocyanines. The molecule's structure shows non-planarity and the zinc atom is of 0.075 Å out of the ring's plane. The 3d subshell of ZnPc isfilled and deep enough to form rather pure molecular orbitals. The valence electronic structures of ZnPc and MgPc are very similar. Theχ^b3Nelec

values and behavior after the annealing process in both these cases are very similar. Cobalt phthalocyanine is an example, where the central metal atom adopts a position inside the Pc ring's plane. The 3d subshell of CoPc is not completelyﬁlled, which caused a more complex electronic structure. In this case, metal 3d levels lie between the ligand's Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular

Orbital (LUMO). For such systems, there are extra absorption features that may arise from charge transfer transitions.

The case of CuPc is completely different. The highest values ofχ^b3Nelec

have the samples without the annealing process. The value of the real part of the third order nonlinear optical susceptibility decreases with the increase of the annealing process temperature and reaches a mini- mum value at the temperature equal to 250 °C (seeFig. 5). An explanation of this phenomenon is strongly connected with the phase change of the thinﬁlms structure.

Copper phthalocyanine is an example, where the central metal atom adopts a position inside the Pc ring's plane. The lowest energy molecular orbitals of CuPc are different from those of CoPc, ZnPc and MgPc, what may suggest the different electronic transport behavior. In this case, the 3d-like orbitals of CuPc is half occupied and positioned in the gap between the Pc HOMO and LUMO. The unﬁlled d valence orbital can be split into serials level due to the interaction between the π-conjugation electrons of Pc ring and d metal's electrons. This phenomenon will decrease the transition energy in low-lying d orbital-ligand or d–d transition. The existence of excited state with low transition energy will enhance the nonlinear optical susceptibilities of the material. The unﬁlled d valence shell of Cu atoms will couple with the conjugated electrons of Pc ring leading to the extension of the conjugated systems.

As a result, the CuPc with larger conjugated electron system will show a larger optical nonlinearity. This fact was conﬁrmed by the results of this experiment because the highest value of χ^b3Nelec was obtained for as-deposited CuPc samples.

The presented THG experiment was performed at 1064 nm, which gives a third harmonic at 355 nm. The 355 nm lies closely at the B Fig. 7.Comparison of the obtained third order nonlinear optical susceptibilities using two theoretical models on the studied MPcs compounds: S–polarization (a) and P–polarization (b).

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band in the absorption spectra of MPcs (seeFig. 4), which has a very strong absorption. Therefore, the large third order nonlinear susceptybility value of MPc thinﬁlms may be due to the three photon resonant contributions of the B band. As a consequence of the three- photon resonance, theχ^b3Nelecvalue of MPcs can be larger than normal- ly. We also notice that the third order nonlinear optical susceptibility values of the MPcs change at most three times after annealing and this increase is directly connected with the phase change of the thin ﬁlms' structure.

Investigated metallophthalocyanines demonstrate a good agreement between the experimental results and the theoretical calculation. The highest values ofχ^b3Nelecwere calculated for compounds form a well- defined, standing structure of the thinfilm. Thefilms with mixed (flat-on and standing domains) structure gave a smaller value ofχ^b3Nelec. The smallest value ofχ^b3Nelecwas calculated for thinfilms with a mixed morphology without a homogeneous region. These facts allow concluding that nonlinear optical properties are not only closely related to the phase (stableβand metastableα) of the molecular material but also to the morphology ofβstable phase.

The electronic contribution of the real part of the third order nonlinear optical susceptibility values for each studied compounds take into account the obtained values of the absorption coefﬁcients.

Comparing the measured and calculated susceptibilities of the MPcs with the known values for fused silica glass (reference material), one can see four orders higher values of the nonlinear optical susceptibilities for all metallophthalocyanine compounds (Table 2).

5. Conclusions

In this paper, the values of the electronic part of the third order nonlinear optical susceptibility for thinﬁlms of four MPcs (M = Cu,

Co, Zn and Mg) compounds were experimentally investigated and the- oretically calculated. Theχ^b3Nelecvalues were measured using the THG method at the fundamental wavelength of 1064 nm. Experimental results of these organometallic complexes were investigated and compared with two theoretical models. The obtained results showed that the value ofχ^b3Nelecdepends on the experiment's conditions such as the temperature of the annealing process as well as the type of central metallic atom in metallophthalocyanine molecular ring, planarity of the molecule andﬁlling d valence orbitals. It has been revealed that the polymorphic phase, the temperature stability, the symmetry of molecules, and the preferred arrangement of MPc thinﬁlm are closely related to their nonlinear optical properties. These effects were connected with the change of molecular symmetry during the annealing process.

It has been revealed that the annealing process affects the structural organization and morphology of the metalorganic thinﬁlms. The experimental results and theoretical calculation show that these compounds have aχ^b3Nelecvalue approximately larger of three orders of magnitude than the fused silica glass known as a reference material.

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Table 2

Fundamental beam polarization, Annealing Process Temperature and results of nonlinear optical susceptibilityχ^b3Nelecobtained for MPcs complexes from calculation based on Comparative and Reintjes models.

Sample Polari-zation Annealing Process

Temperature [°C]

χ^b3Nelec Model

[10⁻¹⁸m²/V²] [10⁻¹⁰esu]

CoPc P As deposited 4.90 ± 0.16 7.62 ± 0.24 Comparative

S As deposited 4.49 ± 0.15 6.99 ± 0.23

P 150 4.64 ± 0.21 7.21 ± 0.23

S 150 4.21 ± 0.19 6.54 ± 0.29

P 250 8.76 ± 0.34 13.62 ± 0.53

S 250 8.38 ± 0.34 13.03 ± 0.52

P – 4.54 ± 0.27 7.06 ± 0.29 Reintjes

S – 4.54 ± 0.27 7.06 ± 0.29

CuPc P As deposited 8.15 ± 0.34 12.67 ± 0.52 Comparative

S As deposited 7.79 ± 0.32 12.11 ± 0.50

P 150 4.63 ± 0.16 7.20 ± 0.25

S 150 4.25 ± 0.15 6.61 ± 0.23

P 250 3.63 ± 0.11 5.64 ± 0.17

S 250 3.23 ± 0.09 5.03 ± 0.12

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ZnPc P As deposited 3.71 ± 0.16 5.77 ± 0.25 Comparative

S As deposited 3.46 ± 0.12 5.38 ± 0.19

P 150 4.52 ± 0.21 7.03 ± 0.33

S 150 3.96 ± 0.17 6.16 ± 0.26

P 250 4.72 ± 0.18 7.34 ± 0.28

S 250 4.49 ± 0.16 6.98 ± 0.25

P – 3.61 ± 0.24 5.61 ± 0.23 Reintjes

S – 3.61 ± 0.24 5.61 ± 0.23

Silica P,S 0.0002 0.000311 Reference

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