New soluble 4-(4-formyl-2,6-dimethoxyphenoxy) substituted phthalocyanines: Synthesis,characterization,photophysical and photochemical properties

Abstract

This work describes the synthesis, spectral and fluorescence properties of bis 4-(4-formyl-2,6-dimethoxyphenoxy) substituted zinc (ZnPc) and magnesium (MgPc) phthalocyanines. The new compounds have been characterized by elemental analysis, UV-Vis, FT-IR, ¹H-NMR and mass spectra. Afterward, the effects of including metal ion on the photophysicochemical properties of the complexes were studied in biocompatible solvent DMSO to analyze their potential to use as a photosensitizer in photodynamic therapy (PDT). The fluorescence and singlet oxygen quantum yields were calculated as 0.04–0.15 and 0.70–0.52 for ZnPc and MgPc, respectively. According to the results, MgPc has higher fluorescence quantum yield than ZnPc, while ZnPc has higher singlet oxygen quantum yield than MgPc. The results show that the synthesized complexes can have therapeutic outcomes for cancer treatment.

Keywords

Phthalocyanines fluorescence photochemistry zinc magnesium

1 Introduction

Phthalocyanines (Pcs) are remarkable macrocyclic compounds having magnificent photophysical and photochemical properties to use in photodynamic therapy studies [1]. These dyes have also attracted a great deal of attention due to their intense color and diverse redox chemistry associated with both the 18π electron system of the phthalocyanine (Pc) ring and the central metal atom ion. Metallophthalocyanines (MPcs) has been extensively performed for many years due to their high chemical inertness and thermal stability have led to the exploration of their technological useful as industrial dyes, catalysts [2], as optical recording materials [3], in gas diffusion electrodes [4], in non-linear optics [5], as biosensors [6], as photosensitizers for photodynamic therapy of cancer [7] and solar cells [8].

A major disadvantage of Pcs is their low solubility in known organic solvents or in water. The solubility problems of these compounds severely limit their use in various fields of application. Solubility problems of these dyes can be solved by using various methods in order to obtain soluble phthalocyanine for the different technological applications. The solubility of phthalocyanines in nonpolar solvents can be improved by introducing different kinds of bulky substituents, such as crown ethers, alkyl, alkoxy and alkylthio; electron-withdrawing substituents (–F, –Cl, –Br, –NO₂, etc.) and electron donating atoms such as N and O at the periphery of the phthalocyanines [9, 10].

PDT has been used as an alternative and new method for cancer treatment [11, 12]. This technique uses visible-near inferred light to irradiate a photosensitizer and then transfers energy between the excited triple-state and ground-state molecular oxygen to generate singlet oxygen [13 –16]. PDT is a promising approach in terms of causing irreversible photodamage in tumor tissues [17 –19].

Phthalocyanine derivatives comprise the second generation of photosensitizer molecules employed in photodynamic therapy and have attracted much attention due to their outstanding photosensitizing performance [20]. These photosensitizers exhibit superior photochemical properties that make them a surely attractive class of photosensitizers for PDT [21].

In this study, it was aimed to investigate the synthesis of symmetric phthalocyanine compounds containing four aldehyde group (Scheme 1), and the effects of these groups on photodynamic therapy in comparison with our previous study [22, 23]. The selection of substituent and central metal atom are important to increase the efficiency of Pc as a photo sensitizer. For instance, the addition of electron-donating group, (sizing grup) bulky or long chain alkyl groups on phthalocyanine skeletons is aimed at enhancing the photosensitizer activity. Also, a heavy central atom such as zinc provides a high triplet yield because of its intense absorption in the red region of the visible light that can be photoactive. To improve emission properties and florescence quantum yields of the Pc molecule, magnesium atom is chosen as a central metal ion due to its small size for bioimaging applications [24, 25].

In the literature, the contribution of the methoxy group, whose both donor [22] and increase solubility in common organic solvents properties are known, to photophysicochemical efficiency has been examined [26, 27]. In addition, the presence of an aldehyde group in the structure may contribute to the increase of biological activity by allowing these compounds to be attached to biological systems containing amine groups such as amino acids. This work has described the synthesis, characterization and fluorescence properties of bis 4-(4-formyl-2,6-dimethoxyphenoxy) substituted phthalocyanines for the first time. The structure of synthesized symmetric phthalocyanine compounds was characterized by FTIR, UV-vis, fluorescence and MS analyzes. Besides, the photochemical and photophysical properties of the complexes were studied in DMSO, comparatively.

2 Experimental

4-hydroxy-3,5-dimethoxybenzaldehyde and 4-nitrophthalonitrile were purchased from Merck. The FTIR spectra were recorded with Perkin Elmer 1600 FTIR spectrophotometer. Absorption spectra were recorded with an Agilent 8453 UV–visible spectrophotometer. Mass spectra were determined with Bruker microflex LT MALDI-TOF MS at the Gebze Technical University MALDI-TOF Mass Laboratory and QTOF Agilent 6530 at Yildiz Technical University Central Laboratory. Fluorescence spectra were recorded on a Varian Eclipse spectrofluorometer using 1cm path length cuvettes at room temperature. Photo-irradiations for singlet oxygen determination were performed using a General Electric quartz line lamp (300 W). A 600 nm glass cut off filter (Schott) and a water filter were utilized to filter off ultraviolet and infrared radiations, respectively. An interference filter (Intor, 700 nm with a bandwidth of 40 nm) was additionally placed in the light path before the sample. Light intensities were measured with a POWER MAX 5100 (Mol electron detector incorporated) power meter.

3 Synthesis

3.1 4-(4-formyl-2,6-dimethoxyphenoxy)phthalonitrile

Syringaldehyde (0.10 g, 0.55 mmol), 4-nitrophthalonitrile (0.09 g, 0.55 mmol) and 20 ml of DMF were added into a 50 mL flask and stirring was started under Ar atmosphere. K₂CO₃ (0.08 g, 0.55 mmol) was slowly added to the reaction medium at 15 min intervals for a total of 2 hours. The reaction was continued under Ar atmosphere at room temperature for 48 hours. After the reaction was completed, it was poured into 50 mL of ice water to cause precipitation. After precipitation was complete, it was filtered through a glass filter to separate the particles, washed with distilled water and dried under vacuum. Yield: 0.14 g. (56 %). FTIR (cm^–1): 2232 (–C≡N), 1696 (C = O), 1251 (Ar–O–Ar), 1125 (Aliphatic ether), ¹H NMR (500 MHz, MeOD) δ 9.98 (s, H), 3.82 (s, 6H), 8.20–8.63 (m, 3H), 7.45 (m, 2H). Q-TOF m/z: 309.087 [M]⁺.

3.2 2(3),9(10),16(17),23(24)-Tetrakis[4-(4-formyl-2,6-dimethoxyphenoxy)] phthalocyaninato zinc(II) (ZnPc)

The phthalonitrile compound (0.15 g, 0.82 mmol) and anhydrous zinc acetate (0.09 g, 0.48 mmol) were dissolved in 2.0 mL n-pentanol in sealed tube. The reaction mixtures were stirred and heated at 150°C for 24 h under Argon atmosphere. The color started to turn green gradually after 15 minutes. The cooled reaction mixture was added drop by drop to 50 mL n-hexane. Then it was left in the refrigerator for 2 h to complete the precipitation. The precipitate was filtered through a sintered glass filter and then zinc(II) phthalocyanine on silica gel using THF as eluent and final product was dried in vacuum. Yield: 0.41 g. (46 %). FTIR (cm^–1): 2934 and 2861 (C–H), 1703 (C = O), 1590 (C = N), 1230 (Ar–O–Ar), 1125 (Aliphatic ether). MALDI-TOF m/z: 1298.428 [M]⁺.

3.3 2(3),9(10),16(17),23(24)-Tetrakis[4-(4-formyl-2,6-dimethoxyphenoxy)] phthalocyaninato magnesium(II) (MgPc)

The phthalonitrile compound (0.20 g, 0.62 mmol), anhydrous metal salts MgCl₂ (0.13 g, 0.65 mmol) and catalytic amount of DBU (17μL) in 2.5 mL of dry n-pentanol was heated and stirred at 160°C in a standard Schlenk tube for 12 h under Argon atmosphere. After cooling to room temperature, the reaction mixture was precipitated by adding it drop-wise into n-hexane. After collecting by filtration, the green product was dissolved in THF and was precipitated by adding it drop-wise into n-hexane. Finally, pure of magnesium (II) phthalocyanine was chromatographed over a silica gel column using a mixture of CHCl₃: n-hexane (2/1 by volume) as eluent. Yield: 0.32 g. (39%). FTIR (KBr), v/cm^–1: 2948, 2862 (C–H), 1706 (C = O), 1590 (C = N), 1230 (Ar–O–Ar), 1125 (Aliphatic ether). MALDI-TOF m/z: 1257.62 [M]⁺.

4 Photophysical and Photochemical Studies

4.1 Fluorescence quantum yields (Φ_F)

Fluorescence quantum yields (Φ_F) were determined by applying the comparative method (Equation 1) [28], $Φ F = Φ F (Std) \frac{F . AStd . n^{2}}{FStd . A {. n}_{Std}^{2}}$ (1)

where F and F_Std are the area under the fluorescence emission curves of the sample and the standard, respectively. A and A_Std are the respective absorbances of the samples and standard (Unsubstituted R-ZnPc) at the excitation wavelengths, respectively. n² and n²_Std are the refractive indices of solvents used for the sample and standard, respectively. Unsubstituted R-ZnPc in DMSO (Φ_F= 0.20) [28, 29] was used as the standard. Both the samples and standard were excited at the same wavelength.

4.2 Singlet oxygen quantum yields (Phi_Δ)

Quantum yields of singlet oxygen photogeneration were determined in air (no oxygen bubbled) using the relative method (Equation 2) with R-ZnPc as reference and 1,3-Diphenylisobenzofuran (DPBF) as chemical quencher for singlet oxygen, $Φ_{Δ} = {Φ_{Δ}}^{Std} \frac{R {. I}_{abs}^{Std}}{R^{Std} . Iabs}$ (2)

where $Φ_{Δ}^{Std}$ is the singlet oxygen quantum yield for the standard R-ZnPc ( $Φ_{Δ}^{Std}$ = 0.67 in DMSO) [19]. R and R_Std are the DPBF photobleaching rates in the presence of the respective samples and standard, respectively. I_abs and $I_{abs}^{Std}$ are the rates of light absorption by the sample and standard, respectively. The samples containing DPBF were prepared in the dark and irradiated at the Q band region. The degradation of DPBF was monitored after each 5 s irradiation at 417 nm [24]. The light intensity of 7.05×7 10¹⁵ photons s^–1 cm^–2 was used for determination of Φ_Δ values. The absorption band of the DPBF reduced by light irradiation [19].

4.3 Photodegradation quantum yields (Φ_d)

Photodegradation quantum yields were determined using Equation 3, $Φ_{d} = \frac{(C_{0} - C_{t}) . V . N_{A}}{I_{abs} S . t}$ (3)

where “C₀” and “C_t” are the sample concentrations before and after irradiation respectively, “V” is the reaction volume, “N_A” is the Avogadro’s constant, “S” is the irradiated cell area, “t” is the irradiation time, “I_abs” is the overlap integral of the radiation source light intensity and the absorption of the sample. A light intensity of 7.05×10¹⁵ photons s^–1 cm^–2 was employed for determination of photodegradation [19, 24].

5 Result and discussions

In the FTIR spectra of phthalonitrile, the disappearance of the NO₂ band of 4-nitrophthalonitrile nearby 1350 cm^–1, and the appearance of new absorption at 1251 cm^–1 to Ar–O–Ar. In addition, the strong –C≡N bands also appeared at 2232 cm^–1. C = O bands appeared at 1696 cm^–1. Aliphatic ether bands also appeared at 1125 cm^–1. In the FTIR spectra of phthalocyanine C–H bonds appeared at 2934 and 2861 cm^–1. C = O bands appeared at 1703 cm^–1. Ar–O–Ar at 1230 cm^–1. Aliphatic ether bands also appeared at 1125 cm^–1. In addition, the C = N double bond in the Porphyrazine ring was observed at 1590 cm^–1. ¹H NMR spectra of phthalonitrile was given in Supplementary Information (S3) and recorded in methanol-d₄.

In the mass spectrum, the molecular ion peak [M]⁺ of phthalonitrile was found at 309.087. In the MALDI-TOF mass spectra, the molecular ion peaks [M]⁺ of ZnPc and MgPc were found at 1298.42 and 1257.82 respectively. MALDI-TOF mass spectrum consists of a molecular ion peak at Supplementary Information S5, S7.

5.1 Ground-state electronic absorption and emission properties

UV-vis spectra are especially effective to determine the structure of the phthalocyanines. Generally, UV-Vis spectra of phthalocyanines show typical electronic spectra with two strong absorption bands known as Q and B bands. The Q-band in the visible region at ca. 600–750 nm is attributed to the π - π^* transition from HOMO (highest occupied molecular orbital) to the LUMO (lowest unoccupied molecular orbital) of the Pc (–2) ring and the B band in the UV region at ca. 300–400 nm arises from the deeper π - π^* transitions [27 , 30–33], but contains a small contribution from n - π^*transition.

UV-vis spectra of the solutions of Pc compounds prepared in chloroform (1×10^–3 M) were measured. In the UV-vis spectrum of Pcs, the Q band absorption of ZnPc and MgPc phthalocyanines are observed as a single band of high intensity in the visible region at 684 and 685 nm, respectively. B band absorption peak was also seen at 349 nm. The emission properties of the compounds were studied in DMSO. Both compounds showed similar emission characteristics (Emission maxima: 697 nm for ZnPc, 696 nm for MgPc) (Fig. 1, 2).

Scheme 1

Synthetic route of tetra 4-(4-formyl-2,6-dimethoxyphenoxy)substituted ZnPc and MgPc.

Fig. 1

Absorption (684), excitation (693) and emission (697) spectra of ZnPc in DMSO.

Fig. 2

Absorption (685), excitation (686) and emission (696) spectra of MgPc in DMSO.

5.2 Photophysical and Photochemical Studies

5.2.1 Fluorescence quantum yields (Φ_F)

The fluorescence quantum yield shows the efficiency of the fluorescence process, and so is an important property to determine a photosensitizer that is used in photodynamic therapy for bioimaging properties. In this way, the emission spectrums of the synthesized phthalocyanines were investigated in DMSO. The Φ_F values were calculated using the comparative method by unsubstituted zinc phthalocyanine (R-ZnPc) in DMSO as a standard, Table 1. According to the results, ZnPc has lower Φ_Fvalue (Φ_F= 0.04) than MgPc (Φ_F= 0.15) and both complexes have lower Φ_F value than the unsubstituted R-ZnPc (Φ_F= 0.20). When the literature was examined, similar properties were observed for magnesium phthalocyanines. Since the small magnesium atom does not affect the inter-system transition, it causes higher fluorescence quantum efficiency [26].

Table 1
Spectral parameters of photophysical and photochemical properties of the complexes

Sample Q band Excitation Emission Stokes shift Φ_F Φ_Δ (PDT) Φ_d(10^–4)

λ_max, nm λ_Ex, nm λ_Em, nm Δ_Stokes, nm

ZnPc 684 693 697 13 0.04 0.70 3.0

MgPc 685 686 696 11 0.15 0.52 6.0

Sample	Q band	Excitation	Emission	Stokes shift	Φ_F	Φ_Δ (PDT)	Φ_d(10^–4)
ZnPc	684	693	697	13	0.04	0.70	3.0
MgPc	685	686	696	11	0.15	0.52	6.0

5.2.2 Singlet oxygen quantum yields (Φ_Δ)

Singlet oxygen leads to the death of cancer cells. Thus, the efficiency of singlet oxygen is one of the most important parameters in photodynamic therapy studies. In this study, the Φ_Δ values were obtained in the presence of two centre atoms (Mg and Zn) in biocompatible DMSO. 1,3-diphenylisobenzofuran (DPBF) was used as chemical quencher and the intensity of DPBF absorbance was decreased after each 5 s of light irradiation. The obtained spectra are shown in Figs. 3, 4 and the Φ_Δ values are listed in Table 1. The expected trend for the singlet oxygen quantum yield values is that heavy central metal atom increases the number of triple-state species [34]. When the Φ_Δ values are compared, ZnPc complex (Φ_Δ= 0.70) has higher Φ_Δ values than MgPc complex (Φ_Δ= 0.52) because of the zinc is a heavier metal which can increase the inter-system crossing (ISC).

Fig. 3

A typical spectrum for the determination of singlet oxygen quantum yield of the ZnPc in DMSO.

Fig. 4

A typical spectrum for the determination of singlet oxygen quantum yield of the MgPc in DMSO.

5.2.3 Photodegradation quantum yields

The photochemical stability of a photosensitizer is another very important parameter in photodynamic therapy studies to effective singlet oxygen generation. Photodegradation quantum yield is a measure of the photochemical stability of a complex under photoirradiation. During photodynamic therapy, the stability of the photosensitizer under the applied light is necessary for the effectiveness of the treatment. If the molecule undergoes photodegradation under the applied light power, both the dose of the drug and the yield of singlet oxygen decrease. In this part, the photochemical stability of the new complexes was calculated in DMSO. Following the degradation, both complexes have a decrease in the intensity of the maximum Q band and the spectral changes are shown in Figs. 5, 6 and Table 1. The Φ_d values were calculated as 3.0×10^–4 for ZnPc and 6.0×10^–4 for MgPc. According to the literature, the photodegradation quantum yields of stable molecules are between 10^–3 and 10^–6 [35]. It has been observed that both complexes have sufficient stability against photodegradation.

Fig. 5

A typical spectrum for the determination of photodegradation of the ZnPc in DMSO.

Fig.6

A typical spectrum for the determination of photodegradation of the MgPc in DMSO.

Footnotes

Acknowledgments

This study was supported by Yildiz Technical University (Project No: 2014-01-02-GEP02)

The supplementary material is available in the electronic version of this article: .

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New soluble 4-(4-formyl-2,6-dimethoxyphenoxy) substituted phthalocyanines: Synthesis,characterization,photophysical and photochemical properties

Abstract

Keywords

1 Introduction

2 Experimental

3 Synthesis

3.1 4-(4-formyl-2,6-dimethoxyphenoxy)phthalonitrile

3.2 2(3),9(10),16(17),23(24)-Tetrakis[4-(4-formyl-2,6-dimethoxyphenoxy)] phthalocyaninato zinc(II) (ZnPc)

3.3 2(3),9(10),16(17),23(24)-Tetrakis[4-(4-formyl-2,6-dimethoxyphenoxy)] phthalocyaninato magnesium(II) (MgPc)

4 Photophysical and Photochemical Studies

4.1 Fluorescence quantum yields (Φ F )

5.1 Ground-state electronic absorption and emission properties

5.2.1 Fluorescence quantum yields (Φ F )

Table 1 Spectral parameters of photophysical and photochemical properties of the complexes Sample Q band Excitation Emission Stokes shift Φ F ΦΔ (PDT) Φd(10–4) λmax, nm λEx, nm λEm, nm ΔStokes, nm ZnPc 684 693 697 13 0.04 0.70 3.0 MgPc 685 686 696 11 0.15 0.52 6.0

Footnotes

Acknowledgments

References

4.1 Fluorescence quantum yields (Φ_F)

5.2.1 Fluorescence quantum yields (Φ_F)

Table 1
Spectral parameters of photophysical and photochemical properties of the complexes

Sample Q band Excitation Emission Stokes shift Φ_F Φ_Δ (PDT) Φ_d(10^–4)

λ_max, nm λ_Ex, nm λ_Em, nm Δ_Stokes, nm

ZnPc 684 693 697 13 0.04 0.70 3.0

MgPc 685 686 696 11 0.15 0.52 6.0