Synthesis, electrochemical, and spectroelectrochemical properties of tetrakis(13,17-dioxa nonacosane-15-sulphanyl) phthalocyaninato zinc(II)
Graphical abstract
Tetrakis(13,17-dioxa nonacosane-15-sulphanyl) phthalocyaninato zinc(II), ZnPc(SCH(CH2OR)2)4, where R = C12H25, was synthesised and investigated electrochemically and spectroelectrochemically in dichloromethane. The well-defined UV–Vis spectra of the electro-reduced and electro-oxidised species of the complex obtained by applied potentials in a thin-layer cell provide a good evaluation of spectral changes. The former shows dimeric formation of the mono-cationic species, while the latter gives characteristic spectral changes corresponding to mono- and di-anionic species of zinc phthalocyanines.
Introduction
The importance of phthalocyanines in many fields, such as molecular electronics [1], [2], [3], non-linear optics [4], [5], [6], liquid crystals [7], [8], [9], [10], gas sensors [11], [12], [13], photosensitisers [14], [15], [16], catalysts [17], [18], [19], [20], semiconductive materials [21], [22], photovoltaic cells [23], [24], [25], and electrochromic displays [26], [27], [28], [29], is increasing rapidly as a result of novel compounds with functional groups on the phthalocyanine core or different types of metal in the centre. An important shortcoming of phthalocyanines is their low solubility in common organic solvents, which hinders their wider application. The introduction of alkoxy [30], [31] or alkylthio [32], [33], [34] substituents to the phthalocyanine cores provides the desired solubility and has turned out to be powerful means of modifying the chemical and physical properties of phthalocyanines. Electron-donating or electron-withdrawing groups on phthalocyanine macrocycle strongly affect not only the position of the longwave absorption bands in the electronic spectra but also the electrode potentials of their reduced or oxidised species in electrochemical reactions [30], [35], [36], [37]. Phthalocyanines with alkylthio substituents tend to shift the absorption bands to longer wavelengths in the near-IR compared with unsubstituted ones, thereby making an important contribution in semiconductor laser applications [38].
In most applications of phthalocyanine complexes, a good understanding of the redox properties of these complexes is important. Spectroelectrochemical studies of phthalocyanines facilitate understanding of the nature of the redox processes that may take place at either the central metal atom or the phthalocyanine ring, and usually cannot be distinguished by electrochemistry only. Spectroelectrochemical studies of phthalocyanines are also important with regard to their possible use as electrochromic materials, where several colours are displayed depending on the potential applied to the electrode surface [39], [40], [41], [42].
Our laboratories have recently reported the synthesis, and electrochemical and spectroelectrochemical properties of double-decker lutetium(III) phthalocyanines and tetrathia macrocycle-bridged metal phthalocyanines substituted with long-alkylthio chains in aqueous and non-aqueous media [39], [40], [41], [42]. In this work, the synthesis, and electrochemical and spectroelectrochemical characterisation of tetrakis(13,17-dioxa nonacosane-15-sulphanyl) phthalocyaninato zinc(II), ZnPc(SCH(CH2OR)2)4, where R = C12H25 (Fig. 1), are reported. Long-alkyl chains introduced into the peripheral position of the phthalocyanine unit provide high solubility in non-coordinating solvents (e.g., dichloromethane, chloroform, diethylether, n-hexane), which has been observed for long-alkyl chains or bulky-groups introduced into mono, unsymmetrical, double-decker phthalocyanines, and quinoxaline or some oxime complexes [43], [44], [45], [46], [47], [48].
The electrochemistry and spectroelectrochemistry of ZnPc(SCH(CH2OR)2)4 were determined in CH2Cl2 containing 0.1 M TBAP. Thus, detailed information is provided on several electrochemically generated redox species, such as [ZnPc(-3)(SCH(CH2OR)2)4]−, [ZnPc(-4)(SCH(CH2OR)2)4]2− and [ZnPc(-1)(SCH(CH2OR)2)4]+, which remain stable and keep their colour changes throughout the thin-layer spectroelectrochemical timescale. The first and second reduction products formed with the applied potentials (Eapp = −1.04 V and Eapp = −1.62 V) in a thin-layer cell showed characteristic spectral changes corresponding to mono- and dianionic species of zinc phthalocyanines. On the other hand, the appearance of the first oxidation product with the applied potential (Eapp = 1.20 V) in the thin-layer cell showed characteristic spectral changes corresponding to dimeric formation of mono-cationic species in CH2Cl2.
Section snippets
Chemicals and reagents
Dichloromethane (CH2Cl2) was treated with H2SO4 three times and distilled over P2O5 and CaH2 prior to use. Dimethylsulfoxide (DMSO) was freshly distilled after drying over alumina. Tetra-n-butylammonium perchlorate (TBAP, Fluka Chemical Co.) was recrystallised from ethyl alcohol and dried in a vacuum oven at 40 °C for at least 1 week prior to use. Ferrocene was recrystallised from ethyl alcohol before using. 4-(13,17-Dioxa nonacosane-15-sulphanyl) phthalonitrile was prepared according to the
Synthesis and spectral characterisation
Formation of tetrakis(13,17-dioxa nonacosane-15-sulphanyl) phthalocyaninato zinc(II), ZnPc(SCH(CH2OR)2)4 was achieved in a good yield by cyclotetramerisation reaction of 4-(13,17-dioxa nonacosane-15-sulphanyl) phthalonitrile [49] with anhydrous zinc acetate in a high-boiling solvent such as quinoline. Column chromatography with silica gel was employed to obtain pure product.
The desired compound, which had four long-alkylthio (SCH(CH2OR)2) groups on the periphery of phthalocyanine macrocycle,
Acknowledgements
The support of Research Fund of Technical University of Istanbul and State Planning Organization (DPT) is gratefully acknowledged. This work was also supported, in part, by a Grant-inAid from Turkish Academy of Science.
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