Synthesis, photophysics, and photochemistry of ...

Journal of Coordination Chemistry

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Synthesis, photophysics, and photochemistry of peripherally Schiff base-zinc complex substituted zinc phthalocyanine Pinar Sen, S. Zeki Yildiz, Göknur Yasa Atmaca & Ali Erdoğmuş To cite this article: Pinar Sen, S. Zeki Yildiz, Göknur Yasa Atmaca & Ali Erdoğmuş (2018) Synthesis, photophysics, and photochemistry of peripherally Schiff base-zinc complex substituted zinc phthalocyanine, Journal of Coordination Chemistry, 71:8, 1258-1267, DOI: 10.1080/00958972.2018.1455094 To link to this article: https://doi.org/10.1080/00958972.2018.1455094

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Journal of Coordination Chemistry, 2018 VOL. 71, NO. 8, 1258–1267 https://doi.org/10.1080/00958972.2018.1455094

Synthesis, photophysics, and photochemistry of peripherally Schiff base-zinc complex substituted zinc phthalocyanine Pinar Sena,b, S. Zeki Yildiza, Göknur Yasa Atmacac and Ali Erdoğmuşc a

Faculty of Arts and Sciences, Department of Chemistry, Sakarya University, Sakarya, Turkey; bFaculty of Engineering and Natural Sciences, Department of Forensic Science, Uskudar University, Istanbul, Turkey; c Department of Chemistry, Yildiz Technical University, Istanbul, Turkey

ABSTRACT

The content of this work is based on the introduction of the salicylhydrazone-zinc complexes into the phthalocyanine core. The reaction of the salicylhydrazone substituted ZnPc (1) with the related zinc(II) salt in basic conditions in DMF yielded bis[bis(salicyhydrazone) phenoxy)zinc(II)] phthalocyaninato zinc(II) (5) in which two salicylhydrazone-Zn complexes are linked through oxygen bridges to the macrocyclic core as three-nuclear complex. The novel compound synthesized in this study was fully characterized by general spectroscopic techniques such as FT-IR, UV-vis, 1H NMR, 13C NMR, elemental analysis and mass spectroscopy. In addition, spectral, photophysical (fluorescence quantum yields), and photochemical (generation of singlet oxygen and photo stability under light irradiation) properties of newly synthesized phthalocyanine (5) and the starting Pcs molecules used to obtain this molecule were investigated in DMSO solutions, comparatively.

ARTICLE HISTORY

Received 11 October 2017 Accepted 21 February 2018 KEYWORDS

Synthesis; metallo phthalocyanine; schiff base; salicylhydrazone metal complex; fluorescence property; singlet oxygen; photodegradation

CONTACT Pinar Sen [email protected] Supplemental data for this article can be accessed at https://doi.org/10.1080/00958972.2018.1455094. © 2018 Informa UK Limited, trading as Taylor & Francis Group

JOURNAL OF COORDINATION CHEMISTRY

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1. Introduction Phthalocyanines (Pcs), which are the synthetic analogs of porphyrins, have been the subject of extensive research in many different fields [1]. Phthalocyanines and many of their derivatives exhibit remarkable features that are outstanding within the fields of materials science [2]. The attractive properties of phthalocyanines are due to their thermal and chemical stability, as well as their versatility [3]. The applications of metallophthalocyanine (MPc) complexes are of interest in a variety of high-tech fields such as in non-linear optics [4], in Langmuir-Blodgett (LB) films [5], in optical data storage [6], in display devices as electrochromic substances [7] and in photodynamic therapy (PDT) of cancer [8]. More recently, functional group substituted phthalocyanines have emerged as molecular building blocks for the development of new molecular materials [9, 10]. As a result of the versatile chemistry of Pcs, the assemblies of Schiff base derivatives into phthalocyanine ring enhance the functionalities and applicability of this type of macrocycles by performing the formation of controlled supramolecular structures [11, 12]. Schiff base-ligand systems capable of binding metal centers are also of importance to form coordination compounds for various applications [13]. Hydrazones and their derivatives are a special group of compounds in Schiff base family as N/O donor ligands. They provide different modes of coordination with transition metal ions. Incorporation of such functional groups onto the phthalocyanine skeleton allows the development of further chemical reactions on Pc macrocycles in order to obtain targeted properties. So far, several novel coordination compounds of hydrazones derivatives have been synthesized and used in biological, pharmacological, clinical, and analytical applications [14, 15]. Although the field of application of phthalocyanines and hydrazone complexes is large, no such structures have been obtained involving the combination of these functional groups, except the studies performed by our group [16–18]. In this study, we aimed to examine how the properties of phthalocyanines, which are known for their photophysical and photochemical properties, will change if they are integrated with zinc-Schiff base complexes. We report the synthesis and characterization of phthalocyanine-based new material in which two salicylhydrazone-zinc complexes were attached to a phthalocyanine for the purpose of construction of multicomponent systems. The spectroscopic, photophysical, and photochemical properties of new functional compounds were also investigated to give an indication of the potential of the complexes as photosensitizers for PDT applications.

2. Experimental 2.1. Chemicals and instruments The following chemicals were obtained from Sigma-Aldrich: methanol (MeOH), ethanol (EtOH), diethylether, ZnCl2, potassium hydroxide (KOH), tetrahydrofuran (THF), chloroform (CHCl3), dimethylformamide (DMF), dimethylsulfoxide (DMSO). All other reagents and solvents were reagent grade quality and obtained from commercial suppliers. All solvents were stored over molecular sieves (4 Å) after they dried and purified as described by Perrin and Armarego [19]. Oxygen-free inert atmosphere was supplied by argon through dual-bank

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vacuum-gas manifold system. Thin-Layer chromatography (TLC) was performed using silica gel 60-HF254 as an adsorbent. Melting points (m.p.) were determined using a BarnsteadElectrothermal 9200 apparatus and are uncorrected. Electronic spectra were recorded on a Shimadzu UV-2600 Pc-spectrophotometer with quartz cell of 1 cm. Infrared spectra were recorded on a Perkin Elmer Spectrum two FT-IR spectrophotometer equipped with Perkin Elmer UATR-TWO diamond ATR and corrected by applying the ATR-correction function of Perkin Elmer Spectrum software. 1H and 13C NMR spectra were recorded a Varian Mercury Plus 300 MHz spectrometer. For Maldi-TOF spectra, the experiments were carried out using a Bruker microTOF (Germany) in Gebze Institute of Technology. The compounds were ionized in the positive electro-spray ionization ion source (ESI+) of the mass-spectrometer. The elemental compositions of the samples were analyzed by an element analyzer (Flash 2000, Thermo Scientific). Compound 5 was digested using HNO3 at 170 °C for 1 h. The zinc content of 5 in the digested mixture was determined using an external standard with a Spectro Arcos inductively coupled plasma optical emission spectroscopy (ICP-OES) instrument (Spectro, Kleve, Germany). Fluorescence spectra were measured using a Varian Eclipse spectrofluorometer using 1 cm path length cuvettes at room temperature. Photo-irradiations for singlet oxygen determination were measured using a General Electric quartz line lamp (300 W). A 600 nm glass cut off filter (Schott) and a water filter were used 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.

2.2. Synthesis 2.2.1. Tetrakis [4-(salicylhydrazone)phenoxy)]phthalocyaninato zinc(II) (4) The preparation of 4 has been performed in three steps by applying our published literature procedure and reported recently [17]. 2.2.2. Bis[bis(salicylhydrazone)phenoxy)zinc(II)]phthalocyaninatozinc(II) (5) Compound 4 (100 mg, 0.07 mmol) and KOH (15 mg, 0.3 mmol) were added in dry DMF (10 mL). After stirring for 10 min under argon, ZnCl2 (19 mg, 0.14 mmol) was added and the final mixture was heated at 100 °C for 8 h and then allowed to cool to room temperature under argon atmosphere. The progress of the reaction was monitored by TLC using CHCl3/ THF (10/1) solvent system. When the reaction was completed, the reaction mixture was precipitated by adding diethylether (15 mL). The precipitate was collected by centrifugation, washed several times with CHCl3, ethanol and water to dissolve any unwanted organic impurity and any un-reacted metal salt and dried in vacuo. Yield: 48% (0.051 g). m.p. > 250 °C. FT-IR (UATR-TWOTM) ν(cm−1): 3302 (Ar-OH), 3054 (Ar, C-H), 1652 (C=N), 1595 (C=C), 1486-1373 (C-C), 1229 (Ar–O–Ar), 1159, 1087, 1042, 746 (in supporting information). UV-vis (DMSO, 1 × 10−5 M), λmax(nm) (log ε): 680 (5.17), 614 (4.49), 331 (5.02). MS (MALDI-TOF): m/z 1758.769 [M+1+2H2O]+, 1722.214 [M+1]+ (in Supporting Information). Anal. Calcd for C88H52N16O12Zn3 (%): C, 61.39; H, 3.04; N, 13.02; Zn, 11.39. Found (%): C, 60.82; H, 3.11; N, 12.89; Zn, 11.15 (ICP-OES).


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3. Results and discussion 3.1. Synthesis and spectroscopic characterization Scheme 1 shows the synthetic route for the target phthalocyanine 5. The preparation of 4 has been performed in three steps by applying our published literature procedure [17]. Peripherally tetra-substituted phthalocyanine (4) was prepared as a statistical mixture of four regio-isomers owing to the various possible positions of the substituents. Up to now, the successful separation of these four isomers with common column chromatography or by recrystallization has not been reported in the literature, since these mixtures are difficult to separate or characterize. No attempt has been made to separate the isomers of 4. Essentially, five different synthetic routes can be identified for the preparation of Schiff base metal complexes. Schiff base metal complexes can be obtained through the treatment of the Schiff base with the corresponding metal halides such with the direct exchange method. It consists of a two-step reaction involving the deprotonation of the Schiff bases and a successive reaction with metal halides. Deprotonation of the enolic hydrogen which is formed by the conversion of salicylhydrazone keto-form into enol-form in basic medium can be realized using with a base [20, 21]. The complexation of the salicylhydrazone groups

Scheme 1. Synthesis route: (i–iii) [13], (iv) DMF, KOH, ZnCl2.

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on the periphery positions of phthalocyanine 4 to give trinuclear-Pc was carried out with ZnCl2 as metal salt in DMF under basic conditions as shown in Scheme 1. The pure product was achieved with 48% yield after purification. Since this target complex is insoluble in common organic solvents and slightly soluble in DMSO, we could not get a satisfactory spectrum in the 1H NMR and 13C NMR. However, it was characterized by FT-IR spectroscopy, melting point, UV-vis, elemental analysis, mass spectroscopy and ICP-OES. All spectral data are in accord with the proposed structures. The melting point of the obtained compound of phthalocyanine is over 250 °C. Generally, the compounds of phthalocyanine, especially the metal complex substitute species, do not have a melting point [22]. When compared with similar studies, no specific melting point of the resulting phthalocyanine compounds could be determined [12, 13]. The FT-IR spectra of the metal complex-substituted phthalocyanine (5) showed that the (-C=O) and (N–H) vibration bands disappeared when compared to the free ligand (4). But, a new band appeared at 1652 cm−1 for 5 due to –C = N–N = C– stretching. This indicates that the keto oxygens in the hydrazone groups enolize and deprotonate during complexation by using a suitable base. This is also a further confirmation for the coordination of the ligands through the azomethine nitrogen and enolate oxygen atoms [16]. Phthalocyanines show typical electronic spectra with two absorption regions. One of them lies in the UV region at about 300–350 nm (B-band) and the other in the visible area at 600-700 nm (Q-band). The Q-band was attributed to π–π* transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the Pc ring. The other band called B-band arises from the deeper π levels to the LUMO [23]. The UV-vis spectra of the prepared phthalocyanine compound (5) showed intense Q-absorption band at 680 nm and B band at 331 nm. The UV-vis spectrum of 5 showed no splitting on the Q-bands which assigned that they were metallophthalocyanines with the D4h symmetry as expected (Figure 1) [24].

Figure 1. Absorption spectra of 2–4 [13] and 5 in DMSO (concentration 1 × 10−5 M).


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The elemental analysis result for the synthesized phthalocyanine (5) was consistent with the theoretically calculated values given in the Experimental section. The ICP-OES measurement was performed for the determination of the quantities of zinc(II) ions included in phthalocyanine structure (5). The theoretical percentage of Zn in 5 is 11.39% by weight. The experimental result showed that the percentage of coordinated zinc(II) ions was 11.15% by weight. Thus, the ICP-OES measurements provided an additional support for the molecular structure of 5. The mass spectrum of 5 was recorded by the MALDI-TOF Mass spectrometer to confirm the proposed structures. Dithranol (DIT) was used as matrix. In the mass spectrum of 5, the molecular ion peak and hydrated-molecular ion peak were observed at m/z: 1722.182 [M + 1]+ and 1758.021 [M + 1 + 2H2O]+ as a small peak, respectively. However, the base peak of the spectrum at m/z 1608.576 was attributed to fragment ion peak calculated as [M-C7H5O2 + 1/2H2O]+.

3.2. Photophysical and photochemical studies 3.2.1. Fluorescence quantum yields Fluorescence quantum yields (ΦF) were determined by comparative method [25], Eqn. (1): ΦF = ΦF (Std)

F ⋅ Astd ⋅ n2 FStd ⋅ A ⋅ n2Std

(1)

where F and FStd are the areas under the fluorescence curves of the MPc derivatives and the reference, respectively. A and AStd are the absorbances of the sample and reference at the excitation wavelength, and n2 and n2Std are the refractive indices of solvents used for the sample and standard, respectively. All of the samples and the standard were excited at the same relevant wavelength. We studied the quantum yield determinations in DMSO. The spectra were also compared under the same conditions (Figures 2 and 3). The phthalocyanine complexes 2-5 exhibited emission bands at 691, 690, 693, and 692 nm upon excitation at 620 nm, respectively. The observed Stokes shifts of 2–5 are 12 nm for 2 and 13 nm for 3–5. All complexes have typical fluorescence behavior. The quantum yield results in DMSO are not very different from each other, 0.08 for 2, 0.11 for 3, 0.09 for 4, and 0.12 for 5. We report that the new compounds have lower fluorescence quantum yields than standard ZnPc (ΦF = 0.20 in DMSO) [26] because of the bulky substituents which promote intersystem crossing.

3.2.2. Singlet oxygen quantum yields Singlet oxygen molecule (1O2) is one of the important cytotoxic types for cancer cell death. A forceful photosensitizer must be produced efficiently to generate singlet oxygen for the treatment of cancer by PDT [27]. Generation of singlet oxygen ability of synthesized zinc(II) phthalocyanines (2–5) were measured in DMSO. Quantum yields of singlet oxygen were determined in air (no oxygen bubbled) using the relative method with ZnPc (in DMSO) as reference; DPBF as chemical quencher for singlet oxygen, using Eqn. (2): ΦΔ = ΦStd Δ

Std R ⋅ Iabs

RStd ⋅ Iabs

(2)

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Std where ΦStd Δ is the singlet oxygen quantum yield for the standard ZnPc, ΦΔ = 0.67 in DMSO [28]. R and RStd are the DPBF photo bleaching rates in the presence of the respective sample and standard, respectively. Iabs and IStd abs are the rates of light absorption by the sample and standard, respectively. Solutions, that contain DPBF, were prepared in the dark and were irradiated in the Q-band region. The degradation of DPBF at 417 nm was monitored after each 5 s irradiation. The light intensity of 7.05 × 1015 photons s−1 cm−2 was used for ΦΔ determinations. The absorption band of DPBF was reduced by light irradiation. Table 1 shows that, in general, the yields of the complexes have high values when compared to unsubstituted

Figure 2. Absorption (680 nm), excitation (683 nm), and emission (693 nm) spectra of 4 in DMSO.

Figure 3. Absorption (679 nm), excitation (680 nm), and emission (692 nm) spectra of 5 in DMSO.

Table 1. Photophysical and photochemical properties of 2–5 in DMSO. Complex 2 3 4 5

ΦF 0.08 0.11 0.09 0.12

Φd (10−5) 10 6.1 6.0 3.0

Φ∆ 0.61 0.78 0.62 0.71


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standard ZnPc, but the yield of 3 (ΦΔ = 0.78) is the highest. Substitution of phthalocyanine core with Schiff base and its zinc complex increased the generated singlet oxygen rate [20]. Singlet oxygen quantum yield of the multicomponent zinc phthalocyanine (5) is 0.71, which is very good result as a photosensitizer for PDT applications. Figures 4 and 5 show spectral changes observed during photolysis of 4 (Figure 4) and 5 (Figure 5) in DMSO in the presence of DPBF.

3.2.3. Photodegradation quantum yields Photodegradation quantum yield (Φd) determinations were carried out using the experimental set-up described in literature [29] Eqn. (3): Φd =

( ) C0 − Ct ⋅ V ⋅ NA Iabs ⋅ S ⋅ t

.

(3)

where “C0” and “Ct” are the sample concentrations before and after irradiation, respectively, “V” is the reaction volume, “NA” is Avogadro’s constant, “S” is the irradiated cell area, “t” is the

Figure 4. A typical spectrum for the determination of singlet oxygen quantum yield of 4 in DMSO.

Figure 5. A typical spectrum for the determination of singlet oxygen quantum yield of 5 in DMSO.

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Figure 6. A typical spectrum for the determination of photodegradation of 4 in DMSO.

irradiation time and “Iabs” is the overlap integral of the radiation source light intensity and the absorption of the sample. A light intensity of 2.42 × 1016 photons s−1 cm−2 was employed for Φd determinations [29–31]. The quantum yield values of the order 10−5 are given in Table 1 and the spectral changes observed for the complexes are shown in Figure 6. Photodegradation quantum yield (Φd) value of 2 in DMSO (Φd: 10 × 10−5) is less stable compared to the other complexes. We can say that all compounds (Schiff base and its zinc complex) are quite resistant to photochemical degradation, except for 2.

4. Conclusion In this paper, we have synthesized and characterized the phthalocyanines and investigated the effects of different functional groups on the photochemical and photophysical properties. A good photosensitizer must be very efficient in generating singlet oxygen. In this way, the new complexes are suitable to use as photosensitizers because of their high singlet oxygen yields, especially 3 and multicomponent pattern 5. The obtained high singlet oxygen production (Φ∆: 0.71) and enough photostability properties (Φd: 3 × 10−5) of peripherally Schiff base-zinc complex substituted novel multicomponent zinc phthalocyanine (5) suggested the hopeful photodynamic therapy studies. That is why the synthesized complexes can also be different alternatives with their bulky structures for PDT applications in comparison with the studies in the literature.

Disclosure statement No potential conflict of interest was reported by the authors.

Funding This work was supported by Research Fund of Sakarya University [project No. 2014-02-04 007].


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