Aggregation Control of Robust Water-Soluble Zinc(II) Phthalocyanine

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Aggregation Control of Robust Water-soluble Zinc(II) Phthalocyanine-based Photosensitizers Takuro Ikeuchi, John Mack, Tebello A Nyokong, Nagao Kobayashi, and Mutsumi Kimura Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03552 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 27, 2016

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Aggregation Control of Robust Water-soluble Zinc(II) Phthalocyanine-based Photosensitizers Takuro Ikeuchia, John Mackb, Tebello Nyokongb*, Nagao Kobayashia*, and Mutsumi Kimuraa* a

Department of Chemistry and Materials, Faculty of Textile Science and Technology,

Shinshu University, Ueda 386-8567, Japan b

Department of Chemistry, P. O. Box 94, Rhodes University, Grahamstown, South

Africa Graphical Abstract

Abstract: A water-soluble zinc phthalocyanine (ZnPc) complex with four negatively charged electron-withdrawing sulfonic acid substituents at the non-peripheral positions (α α-ZnTSPc) is found to have a high singlet oxygen (1O2) quantum yield and displays high photostability. The formation of aggregates is hindered and the HOMO is significantly stabilized, this making α-ZnTSPc potentially suitable for use as a photosensitizer for photodynamic therapy and photoimmunotherapy. Atomic force microscopy (AFM) reveals that mixtures of the negatively charged α-ZnTSPc complex with a similar positively charged ZnPc were found to result in the self-assembly of one-dimensional accordion-like fibers. Supramolecular fibers can be formed in aqueous solution through intermolecular electrostatic and donor-acceptor interactions between the two water-soluble ZnPcs.

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INTRODUCTION Metallophthalocyanines (MPcs) have been used as blue or green dyes and pigments, due to intense absorption bands that lie either at the red end of the visible or in the near-IR region,1 and have also found a wide range of other applications, such as their use as molecular semiconductors in various optoelectronic devices. Various substituents have been introduced at the ligand periphery to enhance the solubility of MPc complexes, since non-substituted MPcs are highly insoluble in aqueous solutions and most organic solvents.2 Water-soluble MPcs have been prepared by introducing ionic (-SO3−, -COO−, -N(CH3)3+, -NH3+) and non-ionic (oligo(ethylene glycol), -OH) groups.2-5 Water-soluble CuPcs have been used as the cyan coloring in inkjet printing systems. Although this water-based ink forms a clear blue color, it gradually fades upon irradiation with light due to exposure to ozone. To avoid this, the FUJIFILM Corporation have significantly improved the photostability of their water-soluble CuPc dyes by modulating the dye energy levels and promoting their aggregation.6

Scheme 1 Molecular structures of α-ZnTSPc, β-ZnTSPc and α-ZnTAPc. Only one of four possible positional isomers17 is shown for these structures. 2 ACS Paragon Plus Environment

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Water-soluble MPcs have also been investigated for use in near-infrared (NIR) light harvesting

photosensitizers

for

photodynamic

therapy

(PDT)7-11

and

photoimmunotherapy (PIT)12 in the treatment of cancers, macular degeneration, and infectious diseases. PDT photosensitizers generate single oxygen (1O2) upon irradiation with light of a suitable wavelength, and the subsequent oxidative cellular damage results in the destruction of tumor tissues. Since the aggregation of hydrophobic MPcs in aqueous solutions diminishes the efficiency of 1O2 generation through non-radiative relaxation pathways, the prevention of aggregation is one of the important strategies for enhancing the photophysical properties of water-soluble MPc sensitizers.8,11 Previously, photoactive MPcs have been encapsulated within micelles and dendritic architectures to attempt to achieve this.13-15 However, the addition of surfactants and the enlargement of the molecular size can have an influence on the bioavailability for PDT. Herein, we report

the

photophysical

properties

of

water-soluble

ZnPcs

with

four

electron-withdrawing groups substituted at the non-peripheral (or α) positions of the Pc ring (α α-ZnTSPc) and the peripheral (or β) positions (β β-ZnTSPc). The use of charged substituents provides scope for forming novel nanomaterials, since nanoscale architectures can be constructed through the use of suitable functional molecular building blocks,16 which can assemble spontaneously through non-covalently intermolecular interactions such as electrostatic, hydrogen-bonding, and donor-acceptor interactions. In this study, the formation of one-dimensional accordion-like assemblies by electrostatic and donor-acceptor interactions of α-ZnTSPc and a similar positively charged ZnPc complex (α α-ZnTAPc) (Scheme 1), is also reported. 3 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION

Scheme 2 Synthetic route to phthalonitrile 1. Reagents and conditions; (i) K2CO3, DMSO, rt, 1 h, (ii) H2O2, Na2WO4 2H2O, CH3COOH/H2O, 60°C, 1 h.

Phthalonitrile 1 was synthesized through a nucleophilic aromatic substitution reaction of 3-nitrophthalonitrile with sodium 3-mercapto-1-propanesulfonate, and the subsequent oxidation of the thioether group by H2O2 in the presence of sodium tungstate (Scheme 1).6

Water-soluble

α-ZnTSPc

bearing

four

3-sulfopropylsulfonyl

groups

at

non-peripheral positions was obtained by tetracyclization of 1 in the presence of Zn(CH3COO)2. A structural isomer with substituents at the peripheral positions (β β -ZnTSPc) was also synthesized from 4-nitrophthalonitrile by following the same procedures, so the effect of substituent position on the aggregation and photophysical properties could be assessed. The 1H NMR spectra of α-ZnTSPc and β -ZnTSPc exhibit complex peak patterns due to the four positional isomers that are formed for each of these compounds.17


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Fig. 1 a) Spectral changes of the absorption bands of β -ZnTSPc in water, DMSO, and DMSO-water of ratios 3:7, 4:6, and 5:5. The inset shows the absorbance changes at 682 (●) and 633 (▲) nm. b) Absorption (solid line), fluorescence (dotted line), and MCD spectra of α-ZnTSPc in water. Table 1 Photophysical Parameters of α-ZnTSPc and β-ZnTSPc in water and DMSO26

α-ZnTSPc β-ZnTSPc a

λmax /nm 660a (668b) 633a (682b)

φp x107

Φf

τF / ns

2.2a (2.0b) NDa (5.0)

0.09a (0.12b) NDa (0.22b)

2.7a (2.3b) NDa (2.9b)

φT

τT / ms

0.76b

0.50b

0.64b

0.70b

φ∆ 0.24a (0.62b) NDa (0.45b)

in water. b in DMSO.

The optical spectroscopy of phthalocyanines is typically understood in terms of the Q and B bands of Gouterman’s 4-orbital model.18 The absorption spectrum of β-ZnTSPc in water contains a broad Q band centered at 633 nm, in a manner that is typical of aggregated MPcs in a cofacial arrangement (Figure 1a and Table 1).19 When dimethyl sulfoxide (DMSO) was admixed, the Q band of the monomer at 682 nm


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increases as the absorption intensity of the peak at 633 nm decreases because of disaggregation. In contrast, the spectrum of α-ZnTSPc in water contains a narrow and intense Q band at 660 nm. This demonstrates that the introduction of bulky negatively charged side chains at the non-peripheral positions prevents aggregation in water.20 The magnetic circular dichroism (MCD) spectrum of α-ZnTSPc in water contains a single intense Faraday A-term in the Q band region, as is typically observed in the spectra of monomeric MPcs with D4h symmetry (Figure 1c).21 Upon excitation in the B (or Soret) band region in aqueous solution, a fluorescence emission band is observed at 665 nm that is a near mirror image of the analogous absorption band and there is a very small Stokes shift value. The fluorescence excitation spectrum of α-ZnTSPc is almost identical to the absorption spectrum, providing further evidence for the absence of aggregation. The fluorescence quantum efficiency (Φ f) of α-ZnTSPc in water is almost identical to what is observed in DMSO,22 and the fluorescence lifetime (τF) is in close agreement with the values that have been reported for non-aggregated ZnPcs (Table 1).8 This is highly noteworthy, since the low tendency of α-ZnTSPc to aggregate in aqueous solution can be expected to significantly enhance the conversion efficiency of 1O2 generation for biomedical applications such as PDT and PIT.


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Fig. 2 a) Optimized structure of one of the C4 symmetry positional isomers of α-ZnTSPc (left: top view, right: side view). b) Calculated energy levels of unsubstituted ZnPc (left), α-ZnTSPc (middle), and β -ZnTSPc (right) at the B3LYP/6-31G(d) level of theory. The four frontier π-MOs that are associated with Gouterman’s 4-orbital model18 are highlighted with thicker black lines, while σ-MOs are offset to the right. The HOMO−LUMO gaps are highlighted with vertical dashed lines and their magnitudes are plotted with yellow diamonds against a secondary axis. c) The calculated TD-DFT spectra for unsubstituted ZnPc (bottom), β -ZnTSPc (middle), and α-ZnTSPc (top) at the CAM-B3LYP/6-31G(d) level of theory. Red diamonds are used to highlight the Q band of Gouterman’s 4-orbital model.18

Density functional theory (DFT) geometry optimization calculations of α-ZnTSPc and β -ZnTSPc have been carried out in vacuo using the B3LYP functional of the


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Gaussian 09 software package to identify trends in the electronic structures (Figures 2).23 The bulky sulfonyl groups of α-ZnTSPc help to prevent the molecular stacking of the hydrophobic Pc rings (Figure 2a). It is noteworthy that the Q band of α-ZnTSPc in DMSO was blue-shifted by ca. 15 nm relative to that of non-aggregated β -ZnTSPc (Table 1). A wider HOMO−LUMO gap is predicted for α-ZnTSPc in the DFT calculations (Figure 2b and Table S1) and a blue shift is predicted for its Q band in the time-dependent (TD) DFT calculations as would be anticipated on this basis. The predicted energies in vacuo of the HOMOs and LUMOs of α-ZnTSPc and β -ZnTSPc are significantly lower than those of unsubstituted ZnPc due to the strong inductive effect of the electron-withdrawing sulfonyl groups. The redox properties of α-ZnTSPc and β -ZnTSPc in solution were determined by using the differential pulse voltammetric (DPV) technique (Fig. S2). The first oxidation potentials of α-ZnTSPc (+0.28 V vs. Fc+/Fc) and β-ZnTSPc (+0.29 V vs. Fc+/Fc) are positively shifted as compared to the reported value of ZnPc (+0.10 V vs. Fc+/Fc),24 as would be anticipated based on the trends predicted in the theoretical calculations. When ZnPc is excited in the presence of O2 under light irradiation, 1O2 can be generated through energy transfer from the excited triplet state of ZnPc to O2.22,25 Singlet-oxygen quantum yield (φ∆) is the main parameter for evaluating the performance of photosensitizers for PDT.21,25 The value of φ∆ depends on the triplet quantum yield (φT), triplet lifetime (τT), triplet state energy, and the efficiency of energy transfer from the excited triplet state to the ground state of molecular oxygen with high

φT and long τT values usually resulting in high φ∆ values.26 The φT and τT values of 8 ACS Paragon Plus Environment

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α-ZnTSPc and β-ZnTSPc in DMSO were determined by laser flash photolysis analyses (Table 1),22 and were found to be higher than those that have been reported for non-substituted ZnPc.27 1O2 generation can be readily monitored by measuring the decrease

in

the

intense

absorption

bands

of

1

O2

quenchers

such

as

1,3-diphenylisobenzofuran (DPBF) in DMSO and anthracene-9,10-bis-methylmalonate (ADMA) in water.25 The φ∆ value of α-ZnTSPc in DMSO was determined to be 0.62 by studying the time-dependence of the absorbance change at the 416 nm band maximum for DPBF. The high φ∆ value of α-ZnTSPc in DMSO can be attributed to the higher φT and longer τT values relative to ZnPc. Although the φ∆ value of α-ZnTSPc in water is lower than that in DMSO, it is noteworthy that α-ZnTSPc can generate 1O2 in this manner without the use of surfactants to prevent aggregation. From these results, it can be concluded that α-ZnTSPc is a highly photostable 1O2 sensitizer that exhibits high conversion efficiency in water making it potentially suitable for use in biomedical applications such as PDT and PIT. Photobleaching quantum yields (φp) of α-ZnTSPc and β-ZnTSPc were determined in air-saturated DMSO solutions by monitoring the absorbance changes in the Q band region under laser light irradiation (Table 1).26 The φp values of α-ZnTSPc and β-ZnTSPc were much lower than those that have been reported for non-substituted ZnPc and water-soluble ZnPcs that are directly substituted with sulfonic acids,22 demonstrating their high stability. This was anticipated based on the theoretical calculations (Figure 2), since the electron-withdrawing sulfonyl groups stabilize the HOMO level and this makes the Pc ring less prone to oxidation (Fig. S2). 9 ACS Paragon Plus Environment

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Fig. 3 a) Absorption spectra of α-ZnTSPc (2.0 µM, blue line), α-ZnTAPc (2.0 µM, red line) and a mixture of α-ZnTSPc and α-ZnTAPc (black lines) in mixing molar ratios of 1:9, 1:4, 3:7, 2:3, 1:1 (green line), 3:2, 7:3, 4:1 ([α α-ZnTSPc]+[ α-ZnTAPc] = 2.0 µM) in water. b) AFM image of a casting α thin

film

of

a

α-ZnTSPc

and

α-ZnTAPc

mixture

([α α-ZnTSPc]+[ α-ZnTAPc] = 2.0 µM, 1:1 mol%) on mica and the α cross-cut profile at the white line in the AFM image.

ZnPc complexes with four positively-charged substituents at the peripheral positions (α α-ZnTAPc) were prepared from a phthalonitrile with a 3-dimethylaminopropoxy substituent. α-ZnTAPc is soluble in neutral-pH water due to partially protonation of the dimethylamino groups, and a Q band is observed at 704 nm. The Q band position of α-ZnTAPc is significantly red-shifted when compared with that of α-ZnTSPc due to the electron-donating nature of the alkoxy chains, which can be expected to destabilize


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the HOMO (Fig. S3). In contrast with α-ZnTSPc, α-ZnTAPc decomposes in a few days under ambient conditions (Fig. S4). The Φf values of α-ZnTAPc in water (Φ f = 0.02) and DMSO (Φ f = 0.06) were lower than those of α-ZnTSPc, due to intramolecular photo-induced electron transfer from the peripheral amines to the photoexcited ZnPc ring.28,29 When aqueous solutions of negatively charged α-ZnTSPc and positively charged α-ZnTAPc were mixed, a new broad absorption peak was observed at 623 nm, along with a weaker charge transfer band at ca. 720 nm (Figure 3a).30 These spectral changes can be ascribed to the formation of stacks of oppositely-charged molecules in a cofacial arrangement. The fluorescence emission of α-ZnTSPc was completely quenched after mixing of the two solutions, due to the formation of aggregates and electron transfer to the ternary amines of α-ZnTAPc. The absorption spectrum of the mixture was not altered by the addition of NaCl and changes in temperature, revealing that it is highly stable. The morphology of the aggregates was examined by atomic force microscopy (AFM). Samples were prepared by casting thin films of the α-ZnTSPc and α-ZnTAPc mixture from aqueous solution onto mica substrates. The mixture exhibited fibrous structures with a length of several micrometers and a height of about 1.5 nm (Figure 3b). In contrast, spreading solutions of only α-ZnTSPc or α-ZnTAPc onto mica substrates resulted in the formation of a rough surface with no obvious features. Since the observed height of fibrous assemblies is very close to the molecular dimensions of α-ZnTSPc and α-ZnTAPc, the fibrous structures can be considered to be composed of single one-dimensional stacks. Accordingly, it is reasonable to conclude that the mixing 11 ACS Paragon Plus Environment

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of two oppositely charged ZnPcs induced the formation of the one-dimensional accordion-like assemblies through intermolecular electrostatic and donor-acceptor interactions.

SUMMARY AND CONCLUSIONS Novel water-soluble ZnPcs with negatively charged sulfonic acids at the non-peripheral and peripheral positions of the Pc ring have been synthesized to investigate the effect of substitution position on their aggregation behavior and photophysical properties in water. Analysis of absorption and fluorescence spectral data demonstrates that β-ZnTSPc forms aggregates through π-π interactions between the hydrophobic ZnPc rings in aqueous solution, while in contrast there is no evidence for aggregation in the spectra of α-ZnTSPc. The electron-withdrawing sulfonyl groups of α-ZnTSPc stabilize the ZnPc ring against photodegradation because of a stabilization of the HOMO, while the high triplet quantum yield and long triplet lifetime values for α-ZnTSPc enhance the efficiency for 1O2 generation making the complex potentially suitable for use in PDT and PIT. The formation of highly photostable water-soluble phthalocyanine dyes provides a novel approach for designing highly effective sensitizers for PDT and PIT. A mixed solution of negatively charged α-ZnTSPc and positively charged α-ZnTAPc was found to be self-assemble into accordion-shaped fibers through intermolecular electrostatic and donor-acceptor interactions.

EXPERIMENTAL General. NMR spectra were recorded on a Bruker AVANCE 400 FT NMR 12 ACS Paragon Plus Environment

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spectrometer at 399.65 and 100.62 MHz for 1H and

13

C in DMSO-d6 solutions.

Chemical shifts are reported relative to internal TMS. Absorption and fluorescence spectra were measured on a JASCO V-650 and a JASCO FP-8600 spectrometer. DPV data were recorded with an ALS 720C potentiostat, and electrochemical experiments were performed under purified nitrogen gas. Tetrabutylammonium perchlorate (TBAP) was used as an electrolyte for electrochemical measurements. Fluorescence lifetimes, singlet-oxygen quantum yields, triplet quantum yields, and triplet lifetimes were determined according to methods that have been reported previously by the Nyokong group.26 Atomic force microscopy images were acquired in tapping mode with a JEOL JSPM-5400 system. The samples for AFM were prepared by spin-coating aqueous solutions on a mica substrate. Theoretical calculations were performed with the Gaussian 09 software package.23 ASSOCIATED CONTENT Supporting Information Synthetic procedures and characterization data of α-ZnTSPc, β -ZnTSPc, and α-ZnTAPc. The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Author e-mail: [email protected], [email protected], and [email protected] Author Contribution These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was partially supported by a bilateral program between the Japan Society for the Promotion of Science (JSPS) and National Research Foundation (NRF) of South Africa (uid: 92425), JSPS KAKENHI grant numbers JP15H02172 & JP15H0091 and an NRF of South Africa CSUR grant (uid: 93627). Theoretical calculations were carried out in part at the Centre for High Performance Computing in Cape Town and 13 ACS Paragon Plus Environment

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photophysical measurements were made possible by the Laser Rental Pool Programme of the Council for Scientific and Industrial Research (CSIR) of South Africa. REFERENCES 1. Leznoff, C. C.; Lever, A. B. P. (Eds.), Phthalocyanines, Properties and Applications, VCH, 1989, vols. 1-4. 2. Dumoulin, F.; Durmuş, M.; Ahsen, V.; Nyokong, T. Synthetic pathways to water-soluble phthalocyanines and close analogs, Coord. Chem. Rev., 2010, 254, 2792-2847. 3. Sesalan, B. S.; Koca, A.; Gül, A. Water soluble novel phthalocyanines containing dodeca-amino groups, Dyes and Pigments, 2008, 79, 259-264. 4. de Oliverira, K. T.; de Assis, F. F.; Ribeiro, A. O.; Neri, C. R.; Fernandes, A. U.; Baptista, M. S.; Lopes, N. P.; Serra, O. A.; Iamamoto, Y. Synthesis of phthalocyanines-ALA conjugates: Water-soluble compounds with low aggregation, J. Org. Chem., 2009, 74, 7962-7965. 5. Tasso, T. T.; Furuyama, T.; Kobayashi, N. Absorption and electrochemical properties of cobalt and iron phthalocyanines and their quaternized derivatives: Aggregation equilibrium and oxygen reduction electrocatalysis, Inorg. Chem., 2013, 52, 9206-9215. 6. Yabuki, Y.; Fujiwara, T.; Tateishi, K.; Goto, M.; Sato, K. Development of high durability dyes for ink jet printing system and the establishment of an industrial production process, J. Syn. Org. Chem. Jpn. 2009, 67, 2-13 (The synthetic details were described in PCT Int. Appl. (2010), WO2010146381 A1). 7. Tuncel, S.; Trivella, A.; Atilla, D.; Bennis, K.; Savoie, H.; Albrieux, F.; Delort, L.; Billard, H.; Dubois, V.; Ahsen, V.; Caldefir-Chézet, F.; Richard, C.; Boyle, R. W.; Ducki, S.; Dumoulin, F. Assessing the dual activity of a chalcone-phthalocyanine conjugate: Design, synthesis, and antivascular and photodynamic properties, Mol. Pharmaceutics, 2013, 10, 3706-3716. 8. Makhseed, S.; Machacek, M.; Alfadly, W.; Tuhl, A.; Vinodh, M.; Simunek, T.; Novakova, V.; Kubat, P.; Rudolf, E.; Zimcik, P. Water-soluble non-aggregating zinc phthalocyanine and in vitro studies for photodynamic therapy, Chem. Commun., 2013, 49, 11149-11151. 9. Machacek, M.; Cidlina, A.; Novakova, V.; Svec, J.; Rudolf, E.; Miletin, M.; Kučera, 14 ACS Paragon Plus Environment

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Aggregation Control of Robust Water-Soluble Zinc(II) Phthalocyanine

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