Photophysical, photochemical and aggregation behavior of novel peripherally tetra-substituted phthalocyanine derivatives
Highlights
► Synthesis and characterization of metal-free and zinc phthalocyanines.► Photophysical and photochemical properties in DMSO solution. ► Photodynamic therapy studies.
Introduction
Metallophthalocyanine (MPc) complexes are being extensively researched because of their diverse applications. These include their uses in electronic devices, non-linear optics, electrochromic devices, Langmuir–Blodgett films, as gas sensors and photosensitizers in photodynamic therapy (PDT) of cancer [1], [2]. The exceptional chemical and physical properties of phthalocyanines can be because of various substituents on the phthalocyanine framework. Over 70 elements can be included into the phthalocyanine core and its chemical versatility allows the introduction of many different substituents at peripheral positions [3], [4].
Metallophthalocyanines are known to have low solubility in most organic solvents. The solubility of these compounds can be improved via substitution of different groups on the phthalocyanine skeleton. It has been documented that tetra-substituted phthalocyanines are more soluble than their octa-substituted counterparts due to formation of constitutional isomers and their high dipole moments [5], [6].
Aggregation of the phthalocyanine compounds is an important phenomenon. The substituted metallophthalocyanines could be form two types of aggregations which affect on electronic and optical properties, namely face-to-face H-aggregation and side-to-side J-aggregation [1], [2]. Typically, phthalocyanine aggregation results in a decrease in intensity of the Q band corresponding to the monomeric species, meanwhile a new, broader and blue-shifted band is seen to increase in intensity. This shift to lower wavelengths indicates to H-type aggregation among the phthalocyanine molecules. Rare cases red-shifted bands have been observed corresponding to J-type aggregation of the phthalocyanine molecules. Generally, J-aggregates of Pc occurred by utilizing the coordination of the side substituent from one Pc molecule to the central metal ion in a neighbor [7], [8], [9], [10], [11]. The substituted zinc Pcs in non-coordinated organic solvents, e.g. chloroform and dichloromethane exhibit J-aggregation [12], [13]. The addition of coordinating solvents such as methanol or ethanol caused dissociation of the dimers, which implies that the absence of coordinating solvents is essential for J-aggregation of Pc [14]. UV–vis and MALDI-TOF-MS spectra could be used for determination presence of J-aggregation for Pc compounds [15].
Furthermore, owing to their extensively planar aromatic π system, phthalocyanines exhibit a high aggregation tendency which leads to insolubility in the case of unsubstituted parent derivatives or hinders purification and characterization of compounds in many respects together with a lower efficiency in their use in PDT [16], [17]. PDT is a binary therapy that involves the combination of visible light and a photosensitizer [18]. Diamagnetic ions such as Zn2+, Al3+, Ga3+ and Ti4+ give phthalocyanine complexes comprising both high triplet yields and long triplet lifetimes which are suitable for photodynamic therapy (PDT) applications [19]. Due to the intense absorption in the visible region, high efficiency to generate reactive oxygen species (such as singlet oxygen), and low dark toxicity, phthalocyanines have been used in this avenue for the treatment of various cancers and photoinactivation of viruses [18], [20].
Our previous studies have already reported synthesis, photophysical and photochemical properties of various substituted phthalocyanines [21], [22], [23], [24], [25]. These phthalocyanine complexes show interesting photophysical and photochemical properties especially high singlet oxygen quantum yields which are very important for PDT of cancer. In this work, we have been synthesized new metal-free (4) and zinc (5) phthalocyanines substituted with four 1-(2-oxyethyl)-4-piperidone ethylene ketal groups as potential PDT agents. Aggregation behavior, photophysical (fluorescence lifetime and quantum yields) and photochemical (singlet oxygen and photodegradation quantum yields) properties were investigated. This work has also been reported the effects of the substituents and the nature of the metal on the photophysical and photochemical parameters of 1-(2-oxyethyl)-4-piperidone ethylene ketal substituted phthalocyanine derivatives in DMSO. This work also explores the effects of substituents and nature of the central metal ions on the fluorescence properties of the phthalocyanines and on the quenching of the phthalocyanines by 1,4-benzoquinone (BQ) using the Stern–Volmer relationship.
Section snippets
Materials
All phthalocyanine reactions were carried out under nitrogen atmosphere using standard Schlenk techniques. 1,3-Diphenylisobenzofuran (DPBF) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) were purchased from Fluka. All solvents were dried and purified as described by reported procedure [26]. 1-(2-Hydroxyethyl)4-piperidone ethylene ketal (1) [27], 4-nitrophthalonitrile (2) [28] were prepared according to the literature procedure.
Equipment
FT-IR spectra were obtained on a Perkin Elmer 1600 FTIR
Synthesis and characterization
The general synthetic route was used for the new substituted phthalonitrile compound (3) starting from 1-(2-hydroxyethyl)4-piperidone ethylene ketal (1) and 4-nitrophthalonitrile (2) in the presence of dry K2CO3 as a base at 50 °C in dry DMF (Scheme 1). The tetramerization of the substituted phthalonitrile compound (3) in a high-boiling solvent in the presence of a few drops DBU as a strong base at reflux temperature under a nitrogen atmosphere afforded the metal-free phthalocyanine (4) as a
Conclusion
The novel metal-free (4) and zinc (II) (5) phthalocyanines bearing four 1-(2-oxyethyl)-4-piperidone ethylene ketal groups at peripheral position have been synthesized for the first time in this study. The studied compounds were characterized by UV–vis, IR, 1H-NMR, ESI mass spectroscopies and elemental analysis. It was concluded from the investigation of the spectroscopic behavior of these novel compounds in various solvents that the zinc (II) phthalocyanine compound (5) formed J-aggregates in
Acknowledgement
This study was supported by the Research Fund of Karadeniz Technical University, Project no: 2010.111.002.5 (Trabzon-Turkey).
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