Functional chalcone-substituted tetrakis-metallophthalocyanines: Synthesis and spectroscopic characterization

Introduction

Metallophthalocyanines (MPcs) are useful and versatile types of organic dyes. Their substituted derivatives have been widely used as gas sensors, ¹ fuel cells, ² photovoltaic cell elements, ³ organic light-emitting diodes, ⁴ liquid crystals, ⁵ nonlinear optics, ⁶ electrodes, ⁷ electronic devices, ⁸ catalysis, ⁹ and photosensitizers for photodynamic therapy (PDT) of cancer.^10–12

Chalcones can easily be prepared via the Claisen–Schmidt condensation reaction. These compounds are α,β-unsaturated ketones having an enone system among two aromatic rings. They exhibit anticancer activity, ¹³ antimicrobial, ¹⁴ antibacterial, ¹⁵ enzyme inhibition, ¹⁶ thermal, fluorescence, and electrochromic properties.^17–19

The main problem with unsubstituted phthalocyanine (Pc) is its insolubility and aggregation. In addition, the aggregation and insolubility of Pcs restricts their use in many technological applications. Therefore, scientists have been focused on synthesizing non-aggregated and soluble Pcs. The solubility of phthalocyanines (Pcs) can be improved significantly by increasing the steric interactions in the Pc units having electron-withdrawing (–F, –Cl, –Br, –NO₂, etc.), electron-donating (–NH₂, Ar–S–, RO–, etc.), bulky, or long chain groups. ²⁰ From this point of view, we initially designed and synthesized new peripherally tetrasubstituted metallophthalocyanine derivatives 3a, 3b, 3c, and 3d carrying four bulky chalcone moieties. 3a, 3b, 3c, and 3d complexes are generally soluble in common organic solvents such as chloroform, dichloromethane, dimethyl sulfoxide, dimethyl formamide, and tetrahydrofuran.

Solubility in common organic solvents facilitates the characterization of the new compounds.

The objective of this research was to prepare and characterize new metallophthalocyanines (3a, 3b, 3c, and 3d) containing chalcone moieties by cyclotetramerization of a novel compound 3. The characterization of all new compounds was carried out by Fourier transform infrared spectroscopy (FTIR), 1H NMR, 13C NMR, mass spectroscopy (MS), and UV-Vis techniques.

Results and discussion

Novel compounds 3, 3a, 3b, 3c, and 3d were characterized using IR, NMR, UV-Vis, and mass spectroscopic techniques, and the proposed structures were confirmed from the spectroscopic data.

(E)-4-(3-(3-(3-Methoxyphenyl)acryloyl)phenoxy)phthalonitrile (3) was synthesized by heating (E)-1-(3-hydroxyphenyl)-3-(3-methoxyphenyl)prop-2-en-1-one (1) and 4-nitrophthalonitrile (2) in the presence of K₂CO₃ as a base in dry DMF at 60°C for 96 h. The crude product was filtered, washed with water, and dried in vacuo over P₂O₅. Finally, the product was crystallized from ethanol. The yield and melting point were 0.90 g (72%) and 87°C–88°C, respectively. The yield of the pure product is high and the melting point range is convenient for purity control. The synthetic route to 3 is summarized in Scheme 1.

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Scheme 1.

The synthetic route of (E)-4-(3-(3-(3-methoxyphenyl)acryloyl)phenoxy)phthalonitrile (3).

According to IR spectral data, a vibration appeared at 2361 cm⁻1, which belongs to the C≡N groups and supported the proposed structures of compound 3 (Figure S1). In the 13C NMR of 3, the new peaks at 116.6 and 117.8 ppm belonging to nitrile carbons indicated evidence of the successful substitution (Figure S2). In the 1H NMR spectra of phthalonitrile compound 3, the OH signal at 9.80 ppm for compound 1 disappeared (Figure S3). Mass spectral analysis of compound 3 confirmed that the target compound had been successfully prepared. The molecular ion [M + H]⁺ peak was observed m/z at 381 in the mass spectrum of the compound 3 (Figure S4).

Conversion of 3 into the peripherally tetrasubstituted metallophthalocyanine derivatives 3a, 3b, 3c, and 3d was accomplished through conventional cyclotetramerization reactions in the presence of metal salts (NiCl₂, Zn(CH₃COO)₂, CoCl₂, and CuCl₂), with n-pentanol as the solvent. Phthalocyanines 3a, 3b, 3c, and 3d were purified by column chromatography. They were obtained in satisfactory yields (40% for 3a, 32% for 3b, 30% for 3c, and 34% for 3d). The phthalocyanines 3a, 3b, 3c, and 3d show good solubility in common organic solvents, for example, dichloromethane, chloroform, and Dimethylformamide (DMF). The obtained pure compounds were characterized by IR, 1H NMR, 13C NMR, MS, and UV-Vis spectral data. The data are consistent with the assigned structures. The preparation of metallophthalocyanines 3a, 3b, 3c, and 3d is shown in Scheme 2.

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Scheme 2.

The synthetic route of the metallophthalocyanines 3a, 3b, 3c and 3d.

The IR spectra of metallophthalocyanines 3a, 3b (e.g. Figure S5), 3c and 3d are very similar. After conversion of the dinitrile derivative 3 into the metallophthalocyanines 3a, 3b, 3c, and 3d, the sharp peak for the C≡N vibration around 2361 cm⁻1 disappeared in the IR spectra.

The 1H NMR spectrum of 3a demonstrated aromatic protons between at 6.65 and 8.09 ppm as multiplet signals. The methoxy protons occurred at 3.37 ppm. The 1H NMR spectrum of compound 3b was almost the same as compound 3a, except for some minor shifts. NMR measurements of the cobalt(II) and copper(II) phthalocyanines 3c and 3d were precluded due to their paramagnetic nature. ²¹

UV-Vis spectroscopy is one of the best spectroscopic techniques for determination of the formation of phthalocyanines. Generally, two absorption bands are observed for phthalocyanine compounds in their electronic absorption spectra. One of them is observed at around 600–750 nm due to the π-π* transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the phthalocyanine ring and is known as Q. The other is observed in the ultraviolet region of the spectrum at around 300–450 nm, arising from deeper π levels/LUMO, and is known as the B or Soret band. ²² The ground state electronic absorption spectra of metallophthalocyanines 3a, 3b, 3c, and 3d in chloroform at room temperature show intense Q absorptions at λ_max = 681, 682, 703, and 697 nm, with weaker absorptions at λ_max = 612, 614, 630, and 626 nm, respectively. The single Q bands in metallophthalocyanine complexes 3a, 3b, 3c, and 3d are characteristic. The B band absorptions of compounds 3a, 3b, 3c, and 3d were observed at λ_max = 350, 347, 384, and 306 nm, respectively (Figure 1). This result is typical of metal complexes of substituted Pcs with D_4h symmetry. ²³

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Figure 1.

The UV-Vis spectra of nickel(II), zinc(II), cobalt(II), and copper(II) phthalocyanines 3a, 3b, 3c, and 3d in chloroform at room temperature (1 × 10⁻5 M).

The mass spectra of compounds 3a, 3b, 3c, and 3d were obtained using the matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) method; 2,5-Dihydroxybenzoic acid (DHB) was used as the matrix.^24,25 In the MALDI-TOF-MS of peripherally tetrasubstituted metallophthalocyanines 3a, 3b, 3c, and 3d, the molecular ion peaks were observed at m/z = 1735 as [M + H + DHB]⁺ (Figure S6), 1727 as [M + DHB–CH₃]⁺ (Figure S7), 1734 as [M + DHB]⁺ (Figure S8), and 1739 as [M + DHB]⁺ (Figure S9), respectively. The observation of these peaks clearly indicated the formation of the desired products.

Conclusion

The cyclotetramerization reaction of phthalonitrile derivative 3 resulted in the formation of novel tetrasubstituted Zn(II), Ni(II), Co(II), and Cu(II) phthalocyanines 3a, 3b, 3c, and 3d bearing four chalcone peripheral groups. The target symmetrical phthalocyanines were separated by column chromatography and characterized by a combination of FTIR, 1H NMR, 13C NMR, MS, and UV-Vis techniques.

Experimental

(E)-1-(3-Hydroxyphenyl)-3-(3-methoxyphenyl)prop-2-en-1-one (1) ²⁶ and 4-nitrophthalonitrile (2) ²⁷ were prepared according to the literature. All reagents were of reagent grade quality and were obtained from commercial suppliers. All solvents were purchased from Merck and freshly distilled. All reactions were carried out under a dry and oxygen-free nitrogen atmosphere using a Schlenk system. The IR spectra were determined using a Perkin Elmer 1600 Fourier Transform-Infrared (FTIR-ATR) spectrophotometer. 1H and 13C NMR spectra were recorded on a Bruker Avance III 400 MHz NMR spectrometer in CDCl₃, and chemical shifts (δ) are reported in ppm and coupling constants (J) are given in hertz (Hz). Mass spectra were measured on a Bruker Microflex LT MALDI-TOF MS spectrometer. Melting points were measured on a Barnstead electrothermal 9200 series digital apparatus. Electronic spectra in the UV-Vis region were recorded on a Perkin Elmer-Lambda 25 spectrophotometer, using 1 cm pathlength cuvettes at room temperature. Figures S1–S9 are supplied as Supplementary Information.

Supplemental Material