Effect of quaternary ammonium salts on spectral properties of zinc octacarboxyphthalocyanine

Journal Pre-proof Effect of quaternary ammonium salts on spectral properties of zinc octacarboxyphthalocyanine

Joanna Nackiewicz PII:

S0022-2860(19)31641-2

DOI:

https://doi.org/10.1016/j.molstruc.2019.127532

Reference:

MOLSTR 127532

To appear in:

Journal of Molecular Structure

Received Date:

13 December 2018

Accepted Date:

03 December 2019

Please cite this article as: Joanna Nackiewicz, Effect of quaternary ammonium salts on spectral properties of zinc octacarboxyphthalocyanine, Journal of Molecular Structure (2019), https://doi.org /10.1016/j.molstruc.2019.127532

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Journal Pre-proof Effect of quaternary ammonium salts on spectral properties of zinc octacarboxyphthalocyanine Joanna Nackiewicz* Faculty of Chemistry, University of Opole, Oleska 48, Opole 45-052, Poland Corresponding

author. Tel.: +48 77 4527139; fax: +48 77 4527101.

E-mail address: [email protected].

Abstract The paper presents results of the research on the influence of quaternary ammonium salts on UV-vis absorption and emission spectra of zinc octacarboxyphthalocyanine (ZnPcOC). Quaternary ammonium salts which show biological activity play a crucial role. In ZnPcOC solutions with a proper concentration of tetrabutylammonium salts (TBAX), a strong bathochromic shift band (max= 760 nm – “R” band) showing red fluorescence appears in time. The presence of tetrabutylammonium salt causes pH increase and, consequently, a gradual dissociation of carboxyl groups in ZnPcOC molecule. It seems that –COO anions present in the ZnPcOC complex can interact with tetraalkylammonium cations, causing a decrease in symmetry in ZnPcOC complex. As a result, the “R”- band appears in the UV-vis spectrum of ZnPcOC. Keywords: zinc octacarboxyphthalocyanine, UV-vis spectra, fluorescence, quaternary ammonium salts, PDT

1. Introduction Phthalocyanines (Pcs) are a group of chemical compounds which are structural analogs of naturally occurring porphyrins. Phthalocyanine molecule is an aromatic, macrocyclic compound with 18 delocalized π electrons [1]. The UV-vis spectra of phthalocyanines are characterized by two absorption regions: the B band, in the 300-400 nm range and the intense Q band in the 550-750 nm range [1,2]. The bands occur as the result of electron transitions from occupied molecular orbitals (HOMO) to the doubly

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Journal Pre-proof degenerate, lowest unoccupied molecular orbital (LUMO) in the phthalocyanine ring (π – π* transitions) [1]. Phthalocyanines can be modified in a number of ways to obtain compounds with very desirable properties and applications. Due to their unique physicochemical properties, phthalocyanines are widely used in various fields of science, industry, technology and medicine [3,4]. Initially, due to their intense green-blue colour, the compounds were used as dyes and pigments [5]. Because of their similar structure to porphyrins, they can also serve as models of biologically active compounds [4]. For these reasons, phthalocyanines are intensively studied and considered to be second generation photosensitizers which could be used in photodynamic therapy (PDT) [6-9]. Photosensitizers are a key component influencing the process of photodynamic therapy. Photosensitizers used in PDT should have relatively intense absorption bands (ε > 2105 M-1cm-1) in order to minimize the dose needed to achieve a desired therapeutic effect and the Q band should has strong bathochromical shift. Moreover, the bands should be observed in the red part of the spectrum in the absorption range 650-850 nm, also known as “therapeutic window” [4]. Furthermore, phthalocyanine complexes with diamagnetic metal ions placed in the centre of a macrocycle (Zn2+, Al3+ or Ga3+) are particularly promising for PDT applications, because these complexes have a long triplet lifetime and high triplet quantum yield [4].

Almost all phthalocyanine complexes not having substituents in benzene rings have low solubility in the majority of common solvents. It results from strong π–π interactions between π electrons of adjacent phthalocyanine macrocycles [9]. Metal octacarboxyphthalocyanines, due to the presence of eight carboxyl groups, they are usually soluble in aqueous solutions, e.g. in biological systems [10]. Quaternary ammonium salts offer a wide range of applications including industry, agriculture and medicine. They are widely used as antiseptics, fungicides and catalysts in different chemical syntheses. Despite the fact that some quaternary ammonium salts show toxicity towards bacteria, the literature data point out that they are safe for people [11]. A special role is played by quaternary ammonium salts which show biological activity. Among them, lysosomotropic agents are of great significance [12]. 2

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The paper presents results of the research on UV-vis absorption and emission spectra of zinc octacarboxyphthalocyanine (fig. 1) in the presence of quaternary ammonium salts. The study has shown, that in the UV-vis absorption spectrum, in the presence of properly adjusted concentration of tetrabutylammonium salt, ZnPcOC obtains a strong bathochromical shift band, called the “R” band. In the presence of suitable concentration of quaternary ammonium salts, aqueous solution of ZnPcOC shows red luminescence.

Fig. 1 Structure of zinc octacarboxyphthalocyanine 2. Experimental 2.1 Reagents Zinc octacarboxyphthalocyanine was synthesized according to the method [13] and described earlier in [14]. Tetramethylammonium bromide (TMABr), 98%, Acros Organics, tetraethylammonium bromide (TEABr), for synthesis, Merck, tetrabutylammonium bromide (TBABr), pure, IE, England, tetrabutylammonium chloride (TBACl), pure, Fluka, tetrbutylammonium iodide (TBAI), pure, BDH, England, NaOH, a.p. grade, POCh Gliwice.

2.2 Measurements Absorption spectra in the UV–vis region were recorded with Jasco V-650 spectrophotometer. The spectra were measured in 10mm quartz cells at 25 °C. Julabo F25 thermostat was used for temperature

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Journal Pre-proof control. ZnPcOC concentration in the sample was 1·10−5 mol/L. The research was carried out for quaternary tetraammonium salts concentration 5·10−3 - 1·10−1 mol/L. The exposition of the solutions on visible light was carried out using the LED lamp by OPTELOPOLE, λmax=685nm, intensity 6.4 mW/cm2 at 25C. The emission spectra were measured in 10.00 mm quartz cells.

Fluorescence

excitation

spectra

were

recorded

on

Hitachi

F-7000

Fluorescence

spectrophotometer F-7000 equipped with 150 W xenon lamp, wavelength range: 600-800 nm, ex = 350 nm, scan speed 1200 nm/min; ex slit 5.0 nm, em slit: 5.0 nm.

3. Results 3.1 The influence of quaternary tetraammonium salts (TRAX) on UV-vis absorption spectra of ZnPcOC(aq) In the presence of quaternary tetraammonium salts, interesting changes in UV-vis absorption spectra of aqueous solutions of ZnPcOC can be observed. In the study, two factors were analysed: the influence of hydrocarbon chain length in tetraalkylammonium (tetramethyl-, tetraethyl-, tetrabutyl-) ion on UV-vis spectra of aqueous solutions of ZnPcOC and the influence of salt anion (bromide, iodide, chloride). The study shows that the changes in UV-vis spectra are caused by the presence of tetraalkylammonium cations. The UV-vis spectrum of aqueous solution of ZnPcOC is typical for its associated form with the Q band observed at max= 622 nm (fig. 2). The increase of chain length in quaternary ammonium salt causes a bathochromic shift in ZnPcOC spectrum to max= 642 nm and the increase of its intensity. However, the most interesting changes in the spectrum are caused by the presence of tetrabutylammonium cation. In the presence of tetrabutylammonium salts, the UV-vis absorption spectrum of ZnPcOC solution changes over time and a band, called the “R” band, occurs at max= 760 nm (fig. 2).

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Fig. 2 The influence of hydrocarbon chain length in quaternary ammonium salt on UV-vis absorption spectrum of ZnPcOC(aq), cZnPcOC=110-5 mol/L, cTRAX=110-2 mol/L.

For tetrabutylammonium salt, the influence of anion type (X = Br-, Cl- and I-) on changes in the UV-vis absorption spectrum of ZnPcOC(aq) was investigated. With time, in the presence of Cl-, Br-, I- anions in the solution, the “R”- band occurs in all three cases, yet its intensity depends on anion type. For TBACl salt, the band intensity (at max=760 nm) is slight in comparison to the band intensity obtained after adding TBAI (fig. 3). The “R”-band is most intense in the presence of tetrabutylammonium iodide. Various “R”-band intensities may be caused by different pH of TBAX salts. For TBACl aqueous solutions, pH is 3.5, for TBABr pH=4.8 while for TBAI pH=6.1. Increase in pH affects the degree of dissociation of ZnPcOC carboxyl groups. Moreover, increase in pH causes the gradual breaking down of ZnPcOC aggregates. The ratio of band intensity at max= 760 nm to band intensity at max 700 nm is respectively 1.03 (for TBACl), 1.62 (for TBABr) and 2.19 (for TBAI).

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Fig. 3. The influence of an anion in tetrabutylammonium salt on UV-vis absorption spectrum of ZnPcOC(aq) cZnPcOC=110-5 mol/L, cTRAX=110-2 mol/L.

3.2 The influence of TBAX concentration and time on UV-vis absorption spectra of ZnPcOC(aq) The biggest and most interesting changes in the UV-vis absorption spectrum of ZnPcOC(aq) are caused by tetrabutylammonium salts (e.g. tetrabutylammonium bromide – TBABr). Moreover, it has been observed that in the presence of TBAX, the UV-vis absorption spectrum of ZnPcOC(aq) changes over time and various types of spectrum are obtained, depending on the concentration of the salt added. In the case of aqueous solution of ZnPcOC, when the salt is added to the solution and concentration of salt increases (0.510-2 – 9.010-2 mol/L) the shape of the UV-vis spectrum of ZnPcOC is changed (fig. 4). There is a decrease in absorbance of the absorption band at 646 nm, caused by dimers present in the solution, and an increase in the intensity of the monomer band (max = 684 nm). The increase of TBABr concentration causes also bathochromic shift of the Q band (from 684 nm to 694 nm).

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Fig. 4. The influence of TBABr concentration on UV-vis spectrum of ZnPcOC(aq), c=1.010-5mol/L; cTBABr [mol/L]: 1) 0.005; 2) 0.045; 3) 0.06; 4) 0.09.

The UV-vis spectrum of ZnPcOC changes over time, depending on the concentration of salt (e.g. TBABr), and three spectrum types can be observed. When the salt concentration is about 5.010-3 mol/L, both the monomer (max= 686 nm) and the dimer (max= 645 nm) are present in the solution (fig. 5a). The absorbance of both monomer and dimer bands increases in time. However, after about 400 min the system reaches equilibrium and the absorbance of respective bands does not change significantly (fig. 5b). Dimer forms of this complex dominant in the solution, and the designated intensity ratio of dimer band to monomer band is 1.2 after about 250 min and almost remains the same in time. Figure 5b shows that for concentration of TBAX at about 5.010-3 mol/L, the initial rates of dimer and monomer formation are comparable over time. However, the concentration of TBAX in the solution is insufficient for more dimers to break down.

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Fig.5a. The influence of time on ZnPcOC(aq)

Fig. 5b. The absorbance over time for Q region

spectrum containing of TBABr solution

(based on fig. 5a).

cZnPcOC=110-5mol/L, cTBABr=0.005mol/L.

In quaternary ammonium salt solutions of 0.7–2.010-2 mol/L concentration, ZnPcOC(aq) exhibits a different type of UV-vis spectrum over time. The UV-vis spectrum is more complex and shows strong absorption in its red part, at max = 760 nm. After about 100 min, there is a change in the monomer and dimer absorbance, and when the intensity ratio of dimer band to monomer band is equal to 1.1, a new, intense band starts to form (‘R”-band) at max = 760 nm with the absorbance increasing in time (fig. 6a). The “R”-band is bathochromic shift of the monomer band by about 14 nm (from 687 nm to 701 nm), with a slight increase in its intensity. After about 300 min, the system reaches equilibrium that exists for many hours (fig. 6b).

Fig. 6a. The influence of time on ZnPcOC(aq)

Fig. 6b. The absorbance over time for particular

spectrum containing TBABr solution cZnPcOC=110-

bands (based on fig. 6a).

5mol/L,

cTBABr=0.01mol/L.

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Journal Pre-proof Figure 6b shows that the rate of the “R”- band formation is low at the beginning and only after a certain period of time (about 30 min) it increases significantly. The curve shown in Fig. 6b takes a sigmoidal shape. Such a shape suggests that at some point the reaction speed reaches its maximum. Furthermore, the rate of the “R”- band formation depends on concentration of quaternary ammonium salts. The increase of TBAI concentration causes the increase of absorbance of the “R”- band. At the higher TBAI concentration, ion pairs are formed more quickly.

Fig. 7 The influence of quaternary ammonium salts concentration on the rate of the “R”- band formation, cTBAI [mol/L] c_1 = 0.014; c_2 = 0.012; c_3 = 0.01; c_4 = 0.007 In the case of solutions with higher salt concentrations ( 0.1 mol/L), the UV-vis spectrum resulting from ionized monomeric form is obtained. The monomer band is bathochromic shift to max= 708 nm (fig.8a). In these solutions, the equilibrium is reached very quickly (after about 1.5 h) and once it is reached, the band absorbance does not change in time (fig. 8b).

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Fig. 8a. The influence of time on ZnPcOC(aq) spectrum containing TBABr solution cZnPcOC=110-5mol/L, csalt=0.1mol/L.

Fig.8b. The absorbance over time for particular bands (based on fig. 8a).

3.3 Photostability of ZnPcOC in tetrabutylammonium salt solutions Phthalocyanines are compounds which are usually stable. However, with regard to zinc or magnesium complexes, when exposed to light, they can be destructed. [15,16]. The stability of Pc complexes is crucial for their potential use as photosensitizers in PDT. Kliber et al. presented the results regarding the influence of UV radiation on the monomeric form of ZnPcOC. It was demonstrated that when exposed to the radiation, it undergoes a significant photodegradation [10].

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Journal Pre-proof In the presence of tetrabutylammonium salt in ZnPcOC solution, a stable form of this complex with the “R”- band is created in time. When exposed to LED light, it does not show any substantial absorbance changes for the “R”-band (fig. 9), which means that a stable form of ZnPcOC complex has been obtained.

Fig. 9. The R band absorbance over time (after LED light exposure).

3. 4 Luminescence spectra Monomers of zinc phthalocyanine show fluorescence, whereas association causes fluorescence to quench [14]. This means that aqueous solution of ZnPcOC does not show significant fluorescence in comparison to NaOH solution in which the phthalocyanine exists as monomer (fig. 10). The emission band in NaOH solution is located at 706 nm. Furthermore, in the presence of a proper concentration of tetrabutylammonium salt (0.7–2.010-2 mol/L) and having obtained a complex form with the “R”- band, ZnPcOC also displays fluorescence. In the case of TBAI, the emission band was located at max = 704–706 nm for ZnPcOC complex. The intensity of emission rises with the increase of salt concentration. However, the excessive salt concentration causes a decrease of its intensity (fig. 10).

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Fig. 10. Emission spectra of ZnPcOC(aq in different solutions, cZnPcOC=110-5mol/L, ex= 350 nm.

4. Discussion Zinc phthalocyanine complexes have been studied for many years for their potential use as photosensitizers in PDT. The strong bathochromic shift (max= 760 nm) for ZnPcOC complex is one of the criteria which an “ideal photosensitizer” should meet [4]. The UV-vis spectrum of the aqueous solution of ZnPcOC in presence of quaternary ammonium salt changes location and intensity of Q and B bands. In water, quaternary ammonium salts occur in forms of ions, tetraalkyl cation and anion (X-). The increase of chain length in tetraalkylammonium cation causes gradual disintegration of associated molecules which is reflected by max shift of the Q band in the spectrum (from 622 nm to 642 nm) and the increase of its intensity. Large ammonium cations interact most likely with carboxyl groups of ZnPcOC complex. As a result, the strength of - interactions in associated molecules decreases. However, the most interesting changes in UV-vis spectra of aqueous solution of ZnPcOC are observed in time and are caused by the presence of the proper concentration of tetrabutylammonium salt. The “R”-band is strongly shifted to the red part and its intensity is about two times stronger than the intensity of dimer and monomer bands. Moreover, the less intense dimer band translates into the more intense “R”- band in time. This band shift results from the decrease of energy gap between the HOMO and LUMO level. Such 12

Journal Pre-proof spectral changes are not caused solely by the change of equilibrium between aggregate forms and monomer. It is likely that the molecular form (max= 760 nm) appears in the solution, causing a decrease in molecule symmetry from D4h to D2h, for example, therefore split of the Q-band can be observed in the spectrum. Cong et al. claim that the nature of the Q-band split is a complex issue [17]. It is known that the red-shift of the Q-bands sometimes results from the protonation of the of meso-nitrogen atoms of phthalocyanine macrocycle [18,19]. The shape and location of the “R”- band in UV-vis spectrum are similar to those obtained by Ogunsipe et al. in a series of protonated ZnPc derivatives [20]. The authors also obtained the Q-band split and bands shifted to the red part of the spectrum. It can be expected that in the presence of ammonium salts, meso- nitrogen atoms will also protonated in ZnPcOC complex. H+ cations present in the solution can contribute to nitrogen atoms protonation. Beeby et al. report that in solutions with pH =1, the first nitrogen atom of tetrasulfonated zinc phthalocyanine is protonated while at pH= 7, tetrasulfonated zinc phthalocyanine occurs in its nonprotonated form [21]. However, it seems that meso-nitrogen atom protonation in ZnPcOC complex is not responsible for the “R”- band formation and its red shift since ZnPcOC in aqueous solution appears in an associated form and, as Ogunsipe et al. presented, Pc associated complexes do not easily protonate [20]. The analysis of UV-vis spectra shows that ZnPcOC complex occurs mainly in an associated form in water and the pH of the complex is about 5.5. Suchan et al. indicated that at this pH value, carboxyl groups in ZnPcOC complex dissociate very slightly [14]. The pH of the solution significantly affects the equilibrium of the ZnPcOC solution. Kaliya et al. postulate that at pH=3, ZnPcOC occurs in its non-ionised form while at pH>6.5, it is completely deprotonated. They propose a two-step process of ZnPcOC ionization. They report that at pH 5, four out of eight carboxyl groups of ZnPcOC are dissociated [22]. However, in the presence of tetrabutylammonium salt, the pH increases (pH=6.2), and a carboxyl groups in ZnPcOC are gradually dissociated. It seems that –COO- anions present in ZnPcOC complex can interact with tetraalkylammonium cations (scheme 1), causing symmetry decrease in ZnPcOC complex. As a result, the “R”- band appears in time. The pH of this solution is stable over time (for about 800 min). Additionally, the emission spectra band observed in solutions ZnPcOC with TBAX

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Journal Pre-proof are caused by the formation of the monomer species, probably ion-pair species. The dimer and higher aggregates of ZnPcOC are nonfluorescent. Wang et al. noticed that unsymmetrical phthalocyanines show interesting chromophoric properties, which is pivotal to PDT [23]. It should be also noted that quaternary ammonium salts which show biological activity (especially lysosomotropic substances) have been studied for many years with regard to anticancer therapy [12, 24]. HO OO

O

=

N

OHO

N N

O

O

O

N

N

OH O-

Zn N N

N

O

O

O O- OH

=

N+

+

N+ HO OO

O N+

N

O

N

N

O

-

O HO

N O

N

OH O- N+

Zn N N

O

N

O

O O- OH N+

Scheme 1 Proposed complex formed with the tetrabutylammonium salt and ZnPcOC 14

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Joanna Nackiewicz “Effect of quaternary ammonium salts on spectral properties of zinc octacarboxyphthalocyanine” Individual author contribution: Conceptualization; Methodology; Investigation; Writing - Original Draft; Writing - Review and Editing; Supervision

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Declaration of interests x The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Journal Pre-proof Highlights: 

The influence of quaternary ammonium salts on UV-vis absorption and emission spectra of zinc octacarboxyphthalocyanine was studied



In UV-vis absorption spectra of ZnPcOC(aq) occur a strong bathochromic shift band, called the “R” band



The R band is stable over time and shows red luminescence