Microwave-assisted synthesis, electrochemistry and spectroelectrochemistry of amphiphilic phthalocyanines
Graphical abstract
Amphiphilic hexadeca-substituted metallophthalocyanines with three different substituents on each benzo moiety were synthesized by microwave irradiation.
Introduction
Phthalocyanines (Pcs) and their metalled congeners have attracted considerable interest in recent years because of their unique optical, electronic, catalytic and structural properties [1], [2], [3], [4]. Traditionally, phthalocyanines have been used as dyes and pigments [4] but recently they have found wide applications in the different scientific and technological areas [1] such as catalysis [5], liquid crystals [6], [7] chemical sensors [1], [8], photodynamic therapy of cancer [9] solar energy conversion [10] nonlinear optics [3], [11] optical data storage [12] and semiconductors [13].
Because of the π stacking (aggregation) between planar macrocycles, unsubstituted phthalocyanines are insoluble or slightly soluble in common organic solvents and water, thereby limiting their applications. Adding functional substituents on the periphery (β) or nonperiphery (α) of the macrocycles and axial substituents at the metal ion enhances their solubility in various solvents, since these substituents increase the distance between the 18-π electron conjugated systems of Pcs and make solvation applicable. While peripheral or nonperipheral substitution with alkyl, alkoxy, alkylthio, phenoxy, ester groups of different chain lengths or macrocyclic groups leads to phthalocyanine products soluble in nonpolar solvents [14], [15], [16], [17], [18], [19], [20], [21], [22], the incorporation of carboxyl, sulfonyl or amino groups results in water-soluble metallophthalocyanine derivatives [23], [24], [25], [26], [27]. However, the solubility in water can be accomplished only within certain pH ranges with these substituents. By quaternizing amino or aza groups of the substituents, products soluble in water over a wide pH range were obtained [28], [29], [30], [31], [32], [33].
The solubility, electronic and spectroscopic properties of the phthalocyanines do not only depend on the size and nature of the substituents but also depend on their positions (i.e., non-peripherally, peripherally and axially). Placing electron-donating substituents at the nonperipheral positions (1, 4, 8, 11, 15, 18, 22 and 25) of the phthalocyanine ring result in a shift of the Q-band toward the near-IR region. Substitution at the more sterically crowded non-peripheral positions reduces aggregation tendencies more than substitution at peripheral positions [1], [34], [35]. Axial ligands in MPcs are also useful in preventing or minimizing intermolecular interactions which result in aggregation in solution [36]. It has long been known that tetrasubstituted phthalocyanines are more soluble than the corresponding octa-substituted Pcs with identical substituents due to the formation of four constitutional isomers and the high dipole moment that results from the unsymmetrical arrangement of the substituents at the periphery [32], [33], [37].
Phthalocyanine derivatives exhibit maximum absorption in the red/far-red range between 600 and 800 nm, and have a great penetration into the tissues and a long triplet lifetime, and high singlet oxygen quantum yields. Closed shell diamagnetic ions such as Zn2+, Al3+, Ga3+, In3+ and Si4+, give phthalocyanine complexes with both high triplet yields and long lifetimes which are useful for PDT studies. Among these compounds, amphiphilic phthalocyanines are the most promising photosensitizers [38], [39], [40]. Sulfonated phthalocyanines with diamagnetic ions in the inner core have attracted considerable attention as second-generation photosensitizers for treatment of malignant tumors by photodynamic therapy (PDT) in the recent decades [40], [41].
Rapid synthesis of phthalocyanines with microwave irradiation has attracted much attention in recent years with respect to long reaction times and very high temperatures required in traditional synthetic routes to phthalocyanines [42], [43]. For this response, in a recent work, we have described microwave-assisted synthesis and in vitro PDT activity of a new hexadeca substituted zinc phthalocyanine bearing p-sulphonylphenoxy groups on peripheral positions [26]. In the present paper, to ensure the continuity of our previous studies, we have aimed to synthesize a new class of phthalocyanines with hexyloxy groups in non-peripheral and p-sulfonylphenoxy and chloro groups in peripheral positions and to investigate their electrochemical and spectroelectrochemical properties.
Section snippets
Chemicals and instrumentation
IR spectra were recorded on a PerkinElmer Spectrum One FT-IR (ATR sampling accessory) spectrophotometer, electronic spectra on a Scinco S-3100 spectrophotometer. 1H-NMR spectra were recorded on Agilent VNMRS 500 MHz and the spectrum was referenced internally by using the residual solvent resonances (δ = 2.49 ppm for DMSO-d6 and δ = 7.26 for CDCl3 in 1H NMR). Mass spectra were measured on a Micromass Quatro LC/Ultima LC–MS/MS spectrometer. Single mode reactor (CEM DISCOVER SP) were used for microwave
Synthesis and characterization
The synthesis of the phthalonitrile derivative bearing two hexyloxy groups on non-peripheral positions and p-sulfonylphenoxy and chloro groups on peripheral positions 1 was carried out by using the procedure previously presented [26]. Scheme 1 shows the synthetic procedure for the target amphiphilic phthalocyanine derivatives. Cyclotetramerization of dinitrile derivative 1 in the presence of 1,8-diazabicyclo[5,4,0]undec-7-ene and anhydrous metal salts [CuCl2, NiCl2, InCI3] at 140 °C in n
Conclusion
In this study, we report microwave-assisted synthesis characterization and electrochemical properties of novel hexadeca substituted nickel, copper and indium phthalocyanines having three different substituents on each benzo group. The synthesized phthalocyanine complexes show excellent solubility in polar organic solvents such as methanol, DMSO and DMF. The effect of the concentration on aggregation properties for copper phthalocyanine was investigated in methanol. No aggregation was observed
Acknowledgements
This work was supported by the Research Fund of the Istanbul Technical University and TUBITAK (Project no. 109T163 and 2010-01-02-GEP03). AG thanks Turkish Academy of Science (TUBA) for partial support.
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