ANALYSIS OF THE STRUCTURAL FEATURES OF A SYNTHESIZED COBALT–CALCIUM-CONTAINING PHTHALOCYANINE PIGMENT USING IR SPECTROSCOPY

ABSTRACT

In this study, phthalocyanine pigments synthesized based on cobalt and calcium were investigated using IR spectroscopy. The spectral analysis results enabled the identification of vibrational frequencies of the functional groups and the aromatic ring within the pigments. It was found that the coordination bonds of cobalt and calcium ions, acting as central metals, significantly alter the vibrational frequencies of the molecule. Peaks corresponding to aromatic C–H, C=C, C=N bonds, as well as metal–nitrogen bonds, were observed. The high thermal and chemical stability of the pigments enhances their practical significance for industrial applications.

АННОТАЦИЯ

В данной статье с использованием ИК-спектроскопии были исследованы фталоцианиновые пигменты, синтезированные на основе кобальта и кальция. Результаты спектрального анализа позволили определить колебательные частоты функциональных групп и ароматического кольца в составе пигментов. Было установлено, что координационные связи ионов кобальта и кальция, выступающих в роли центральных металлов, значительно изменяют колебательные частоты молекулы. Зафиксированы пики, соответствующие ароматическим связям C–H, C=C, C=N, а также металл–азотным связям. Высокая термическая и химическая устойчивость пигментов повышает их практическую значимость для промышленного применения.

Keywords: phthalocyanine pigment, IR spectrum (infrared spectrum), cobalt, calcium, coordination bond, thermal stability, chemical resistance.

Ключевые слова: фталоцианиновый пигмент, ИК-спектр (инфракрасный спектр), кобальт, кальций, координационная связь, термическая стабильность, химическая стойкость.

Introduction

In recent years, studies on dye pigments based on organic compounds have established that individual molecules belonging to the classes of triphenylmethane, xanthene, phthalocyanine, and thiazine compounds exhibit limited light absorption in the visible spectrum. It has been noted that such molecules do not display chromogenic properties in their isolated form and can be considered only as chromophoric fragments—necessary for coloration but insufficient to ensure the required color intensity. In particular, it has been scientifically substantiated that phthalocyanine groups in their monomolecular form are colorless and begin to absorb light effectively in the blue spectral range only when in the form of complexes, dimers, or supramolecular structures [1].

The phthalocyanine ring contains eighteen delocalized π-electrons, which characterizes it as a highly aromatic system. Due to this feature, phthalocyanines are classified among the compounds that form intensely colored pigments. Of particular practical importance are phthalocyanine complexes with various metal ions coordinated at the central position, as they exhibit high stability. For example, pigments based on copper phthalocyanine are widely used in modern industry—in the production of paints and coatings, printing inks, plastic products, as well as for dyeing leather and various synthetic fibers. These compounds also demonstrate excellent thermal stability, which enables their use across a broad range of technological processes [2].

The synthesis of the phthalocyanine ring primarily involves a cyclic tetramerization process. Starting materials for this reaction include compounds such as phthalonitrile, phthalimide, phthalic anhydride, 1,3-diiminoisoindole, among others (Figure 1).

Figure 1. Primary compounds used in the synthesis of phthalocyanines [3]

Phthalocyanines, which possess a structure similar to that of porphyrins, stand out for their widespread use as functional materials. Their practical significance is primarily associated with their high electronic conductivity [4]. Scientific studies have shown that the central cavity of the phthalocyanine molecule can accommodate various ions—including metal-free forms, as well as one or two metal atoms [5]. In recent years, due to their excellent charge transport properties, these compounds have found broad application in molecular photonics, nanoelectronics, optoelectronics, and other advanced technological fields [6].

Using a newly developed in-situ method and a sol-gel process, copper phthalocyanine was synthesized within a silica xerogel matrix. UV and IR spectroscopy confirmed that copper ions—present as complex ions [Cu(H₂O)₄]²⁺ and [CuCl₄]²⁻ during the drying and wet gel stages—are absent in the final product. Copper phthalocyanine molecules were gradually synthesized in-situ during the transition from the wet gel to the xerogel. In the resulting composite, the dimerization of CuPc is significantly reduced, as the in-situ synthesized CuPc molecules are well isolated within the micropores of the xerogel matrix. The addition of various auxiliary compounds to the pigments allows for the modification of their properties. For example, it has been demonstrated that applying green pigment G, based on copper phthalocyanine, to fabric surfaces increases their water and wear resistance [7–8].

2. Experimental Section

The synthesis was carried out in a 1000 mL three-necked round-bottom flask equipped with a reflux condenser, mechanical stirrer, and thermometer. The reaction mixture consisted of phthalic anhydride (34 g), urea (80 g), cobalt(II) chloride (11 g), and boric acid (1 wt% relative to the mass of phthalic anhydride), with 200 mL of dimethylformamide (DMF) as the solvent. The mixture was heated to the boiling point of DMF and maintained under continuous stirring for 2 h. Upon completion, the reaction mixture was cooled to room temperature, and the resulting dark-blue phthalocyanine pigment was isolated by vacuum filtration using a Büchner funnel. The solid was repeatedly washed with distilled water and subsequently dried in an oven at 65 °C until constant weight was achieved.

The structural features of the synthesized pigment were characterized using Fourier-transform infrared (FTIR) spectroscopy. The spectra were recorded on a Shimadzu FTIR spectrometer within the 400–4000 cm⁻¹ range. This analysis enabled the identification of functional groups, vibrational modes, and metal–ligand coordination, thereby confirming the structural characteristics of the synthesized compound [9].

3. Results and Discussion

A high-frequency signal at 3047.53 cm⁻¹ corresponds to the stretching vibrations of C–H bonds with sp² hybridization in the aromatic rings of the phthalocyanine molecule. Peaks in this region indicate the integrity of the conjugated ring systems and a uniform distribution of electron density across the ring surface. This, in turn, reflects one of the key structural features of phthalocyanine compounds—a rigid molecular geometry. A strong, intense peak at 3345.75 cm⁻¹ is attributed to the stretching vibrations of N–H bonds. Peaks in the intermediate region around 2870.08 cm⁻¹ correspond to stretching vibrations of aliphatic C–H bonds. Since such bonds are not part of the phthalocyanine structure, this signal is likely due to residual organic substances or intermediate reaction products remaining after synthesis. The peak at 1697.36 cm⁻¹ corresponds to the stretching vibrations of C=N bonds within the phthalocyanine ring and plays a key role in identifying the coordination bond between the nitrogen atoms in the ring and the central metal ion. This signal indicates a high degree of order in the nitrogen-based conjugated system of the molecule. The signal at 1610.56 cm⁻¹, also attributed to C=N stretching vibrations, further confirms the delocalized distribution of electron density across the ring and the stability of the metal-coordinated complex structure. The signal at 1506.41 cm⁻¹ is associated with stretching and deformation vibrations of the pyrrolic segments that make up the phthalocyanine ring. This signal reflects the symmetrical and rigid geometry of the ring, as well as the stability of its electron density. The peak at 1463.97 cm⁻¹ corresponds to the stretching vibrations of C=C bonds in the aromatic ring, confirming the presence of a π-electron system and the preservation of the conjugated structure—an essential foundation for the optical and electronic properties of the molecule.

The region around 1332.81 cm⁻¹ is associated with deformation vibrations of aromatic C–H bonds, which describe the molecular geometry and deformation states related to the ring symmetry.The signal at 900.76 cm⁻¹ corresponds to out-of-plane deformation vibrations of C–H bonds in the benzene ring, indicating the integrity of the aromatic rings and the preservation of the π-electron system.The peak at 754.17 cm⁻¹ is attributed to stretching vibrations of the phthalocyanine ring and reflects the high conservation of ring symmetry and coordination bonds between the central metal and nitrogen ligands.The value at 719.45 cm⁻¹ corresponds to characteristic deformation vibrations of aromatic C–H bonds, confirming the integrity of the aromatic portion of the molecule.The signal at 543.93 cm⁻¹ corresponds to stretching vibrations of N–Co bonds. The presence of this peak clearly confirms coordination interactions between the nitrogen atoms of the phthalocyanine ring and the central cobalt ion, indicating the successful formation of the metal–ligand complex.The low-wavenumber peak at 480.28 cm⁻¹ corresponds to stretching vibrations of N–Ca bonds. This signal unambiguously indicates the coordinated arrangement of calcium ions with nitrogen atoms in the phthalocyanine structure and the formation of a polymetallic complex (see Figure 2).

Figure 2. IR spectrum of the phthalocyanine pigment containing cobalt and calcium

Table 1.

Comparative Table of IR Absorption Frequencies for Phthalocyanine Pigment Containing Cobalt and Calcium and Phthalimide

4. Conclusion

As a result of the conducted study, the phthalocyanine pigment synthesized based on cobalt and calcium was thoroughly analyzed using infrared (IR) spectroscopy. The spectral data confirmed the presence of stretching vibrations of C–H, C=C, and C=N bonds, as well as metal–nitrogen coordination bonds. The Co–N and Ca–N bonds observed in the low-frequency region indicate the formation of polymetallic complexes. The stable conjugated system of the phthalocyanine ring provided the pigment with high thermal and chemical stability. The obtained results open up opportunities for the application of these pigments as dyes and in the field of advanced functional coatings.

References:

1. Mikheev Y.A., Guseva L.N., Ershov Y.A. The nature of chromaticity of triphenylmethane, xanthene, phthalocyanine, and thiazine dyes // Russ. J. Phys. Chem. A 2010 8410. Springer, 2010. Vol. 84, № 10. - PP. 1778–1791.
2. Germinario G., Van Der Werf I.D., Sabbatini L. Pyrolysis gas chromatography mass spectrometry of two green phthalocyanine pigments and their identification in paint systems // J. Anal. Appl. Pyrolysis. Elsevier, 2015. Vol. 115. - PP. 175–183.
3. Nemykina V.N., Lukyanets E.A. Synthesis of substituted phthalocyanines // Arkivoc. Arkat, 2010. Vol. 2010, № 1. - PP. 136–208.
4. De la Torre, G.; Vazquez, P.; Torres, T. Role of structural factors in the nonlinear optical properties of phthalocyanines and related componds. Chem. Rev. 2004, 104. – PP. 3723-3750.
5. Cid, J.-J.; Yum, J.-H.; Jang, S.-R.; Nazeeruddin, M.K.; Martinez- Ferrero, E.; Palomares, E.; Ko, J.; Graetzel, M.; Torres, T. Molecular cosensitization for efficient panchromatic dye-sensitized solar cells. Angew. Chem. Int. Ed. 2007, 46. – PP. 8358–8362.
6. Pinzon, J.R.; Plonska-Brzezinska, M.E.; Cadona, C.M.; Athans, A.J.; Gayathri, S.S.; Guldi, D.M.; Herranz, M.A.; Martin, N.; Torres, T.; Echegoyen, L. Sc3N@C80-ferrocene electron-donor/acceptor conjugates as promising materials for photovoltaic applications. Angew. Chem. Int. Ed. 2008, 47. – PP. 4173-4176.
7. Tawiah B., Asinyo B.K., Badoe W., Zhang L., Fu S. Phthalocyanine green aluminum pigment prepared by inorganic acid radical/radical polymerization for waterborne textile applications // Int. J. Ind. Chem. Springer Berlin Heidelberg, 2017. Vol. 8, № 1. - PP. 17–28.
8. Chen W. et al. Study on catalytic oxidation of planar binuclear copper phthalocyanine on 2-mercaptoethanol // Sci. China Ser. B Chem. 2006 496. Springer, 2006. Vol. 49, № 6. - PP. 522–526.
9. Nabiev D.A., Turaev Kh.Kh., ‘Study of Synthesis and Pigment Characteristics of the Composition of Copper Phthalocyanine with Terephthalic Acid,’ International Journal of Engineering Trends and Technology, Vol. 70, No. 8. - PP. 1-9, 2022. Crossref. [Electronic resource] URL: https://doi.org/10.14445/22315381/IJETT-V70I8P201