Dört 14-üyeli tetraazalı makrosiklik halka içeren Sn(IV) ftolasiyanin`in siklik voltametri ile incelenmesi

Abstract

ÖZET Ftalosiyaninler, parlak mavi, yeşil tonlarında yüksek ısıya, ışığa ve asitlere karşı dayanıklı, çözünürlükleri az olan bileşiklerdir. Ftalosiyaninlerin stabiliteleri ve sentezlenen metal türevlerinin fiziksel ve kimyasal özellikleri kullanım alanlarını genişletmiştir. Ticari uygulamalarda kısmen ftalosiyaninin kendisi kullanılmakla birlikte daha çok metal türevleri kullanılmaktadır. Bu çalışmada dört 14-üyeli tetraazah makrosiklik halka içeren Sn(IV) ftalosiyaninin dimetilsülfoksid, dimetilformamid ve diklormetan içindeki siklik voltametrileri incelenmiştir. Destek elektrolit olarak 0.1 M tetrabutilamonyum perklorat ve tetraetilamonyum perklorat kullanılmıştır. Siklik voltametri incelemeleri üç-elektrodlu bir hücre içerisinde gerçekleştirilmiştir. Ölçümlerde bilgisayara bağlı PARC 273 potensiostat-galvanostat (EG&G)dan yararlanılmıştır. Voltamogramlar x-y kaydedicisi (RE 0091) üzerine kaydedilmiştir. V1U

SUMMARY CYCLIC VOLTAMMETRIC STUDIES OF Sn(IV) PHTHALOCYANİNES CONTAINING FOUR 14-MEMBERED TETRAAZA MACROCYCLES Phthalocyamnes are derivatives of the first new chromophore to be discovered in the twentieth century [32]. Phthalocyanine compounds are used in a variety of applications. Phthalocyanine complexes of 63 metals have been prepared and marketed. The color of most phthalocyanines varies from dark blue to a metallic bronze to green, depending on the chemical and crystalline form of the material. Most phthalocyanine compounds do not have melting point; they sublime and vaporize only under greaty reduced pressure and at temperatures above 500°C. The chemical properties of phthalocyanines depend to a large extent on the central metal or hydrogen atoms [31]. Phthalocyanines are stable to atmospheric oxidation up to 100°C or higher, depending on the metal derivative [32], Phthalocyanines are used primarily in inks, coloring for plastics and metal surfaces and dyestuffs. In addition to their extensive use as dyes and pigments, phthalocyanines have found wide applications in catalysis, in optical recording, in photoconductive materials, in photodynamic theraphy and as chemical centers. The physical and chemical properties of soluble phthalocyanines have recently attracted much attention from materials chemist for their potential use in semiconducting materials, non-linear optics, and other optical devices. At the same time, since they effectively absorb in the lower energy region of visible light, they have found extensive application as photoconductors in optical recording materials as well as photosensitizers in photodynamic theraphy. Introduction of functional groups into chemically stable phthalocyanine cores impart novel properties to the products suitable for various applications [34]. Among these we may cite supramolecular assemblies formed in liquid state or in solution as a result of the planar geometry encountered in these molecules, alkali or transition metal binding, and enhanced solubility in common organic solvents caused by bulky substituents. When these features are combined with the well-known delocalized electronic system and the useful electrochemical, photochemical, and optical behavior of phthalocyanines, a diversity of applications can be easily assumed. These compounds show mesogenic properties and form ion channels through the crown ether moieties capable of conducting alkali metal ions. Incorporation of monoaza crown ether units leads to products which additionally offer the advantage of being soluble in water over a wide pH range by quaternization of the aza function. IXTetraaza macrocycles show a tendency to form complexes with transition metal ions comparable with the interactions of crown ethers with alkali ions. Therefore, phthalocyanines peripherally containing these donor macrocycles are expected to form homo- or heteromultinuclear compounds. In this present work, the cyclic voltammetry of a Sn(IV) phthalocyanine containing four 14-membered tetraaza macrocycles has been investigated in dimethylsulphoxide and dimethylformamide solutions with two different supporting electrolyte (tetrabutylammonium perchlorate and tetraethylammonium perchlorate) and in dicholoromethane with 0.1 M tetrabutylammonium perchlororate as a supporing electrolyte. Triple-distalled and spectrosol grade dimethylsulphoxide, dimethylformamide and dichloromethane, dried over 4A molecular sieves, were used in the voltammetric experiments. Tetrabutylammonium perchlorate (TBAP) (0.1 M) was used in dimethylsulphoxide and in dimethylformamide and in dicholoromethane as supporting electrolyte. Solutions were purged with nitrogen prior to each voltammetric measurement. Cyclovoltammetric measurements were performed on a PARC 273 potensiostat / galvanostat (EG&G) interfaced with an external computer. A standart three-electrode cell configuration was employed using a Pt plate (area 0.55 cm2) working electrode, a Pt wire counter electrode and a saturated calomel reference electrode (SCE). Woltammograms were recorded on the x-y recorder (RE 0091). After addition of a 0.1 M amount of tetrabutylammonium perchlorate and tetraethylammonium perchlorate (TEAP) as supporting electrolytes, the electrochemically available potential range was checked prior to use. The cyclic voltammograms of this Sn(IV) phthalocyanine in dimethylsulphoxide and dimethylformamide in the presence of tetrabutylammonium perchlorate and tetraethylammonium perchlorate as supporting electrolytes have been observed. On the other hand in dichloromethane the cyclic voltammetry experiments were carried out in 0.1 M tetrabutylammonium perchlorate. The cyclic voltammogram of this complex is characterized four one-electron reduction waves in dimethylsulphoxide. These waves show quasi-reversible behaviour at all sweep rate studied. The heterogenous electron transfer is relatively slow, so that the seperation between the cathodic and anodic peaks varies with the sweep rate. The ratio of anodic to cathodic peak currents differs from unity and depends on the switching potentials showing the presence of the coupled chemical reactions. Electron transfer is the rate-determining step in the anodic oxidation of the phthalocyanines. Therefore, the observed anodic current can be interpreted in terms of the expression for the potential dependence of the rate constant for electron transfer in eq 1 together with Fick's lows of diffusion [35]. k(Ep)=ksexp[-anF/RT(E-E0)] (1) The rate constant k(Ep) values were calculated by using the eq 2. xk(Ep) = 2.18 [DanFv/RT] `z (2) Since the bulky substituents in tetrabutylammonium perchlorate may cause the electron transfer to be slower, k(Ep) values of this species in dimethylsulphoxide and 0.1 M tetraethylammonium perchlorate system are higher than those in dimethylsulphoxide and 0. 1 M tetrabutylammonium perchlorate system. In dimethylsulphoxide with tetraethylammonium perchlorate as supporting electrolyte the cyclic voltammetry of this species showed four reduction peaks; the first reduction is two-electron while the others are all one-electron transfers. The green color of this species in dimethylsulphoxide and tetraethylammonium perchlorate converted to dark blue in the second day. The cyclic voltammogram of this dark blue solution showed three reduction and one oxidation while the first reduction is two-electron transfers and the others are all one-electron transfer. The fourth reduction peak does not appear in this cyclic voltammogram. The peaks shifted to more negative potentials with tetraethylammonium perchlorate. There is no great difference in the electronic spectra of green and dark blue solutions. The variation of Ep with sweep rate also changed from the quasi-reversible case to the charge transfer rate-determining case within the sweep rates studied. The values of diffusion coefficients of this species are low since it is consistent with the larger size of the molecule. If we would compare these values from the different supporting electrolytes point of wiev, we could conclude that the values are higher in dimethylsulphoxide and tetraethylammonium perchlorate system than those in dimethylsulphoxide and tetrabutylammonium perchlorate one. The cyclic voltammogram of this Sn(IV) species in dimethylformamide and 0.1 M tetrabutylammonium perchlorate as a supporting electrolyte four reduction peaks appeared. The first reduction is two-electron but the others are all one-electron transfers. The first and fourth reduction peaks are reversible and the third reduction wave has an irreversible character, i.e, reduction is intrinsically slow. In anodic region four oxidation waves appeared. Only the third peak is quasi-reversible while all the others have reversible character. In the second day voltammogram of this species the reduction peaks shifted to more negative potentials. These have all reversible character. The diffusion coefficients associated with these waves were calculated with the Randles-Sevcik equation. The values of the diffusion coefficients are higher in the second-day voltammogram than those obtained in the first day study. At the same time the higher k(Ep) values were also observed in the previous voltammogram. In dimethylformamide the cyclic voltammetry of this species showed four quasi-reversible one-electron reduction peaks with 0.1 M tetraethylammonium perchlorate as supporting electrolyte. In the anodic region the first oxidation peak XIafter switching potential is irreversible. The transfer coefficient (an) obtained from Ep vs log v and from Ep-Ep/2 were calculated as 0.270 and, 0.251 respectively [36]. So these values obtained by these two methods are within the experimental error. But the other two oxidation peaks are reversible character and two-electron transfers. In the second-day voltammogram of this species in dimethylformamide and 0.1M tetraethylammonium perchlorate three one-electron reduction waves appeared. If we would compare the reduction peaks in dimethylformamide and 0. 1 M tetraethylammonium perchlorate with those in dimethylformamide and 0.1 M tetrabutylammonium perchlorate system we could observe that the reduction waves in the previous case shifted to more negative potentials. Diffusion coefficients and k(Ep) values in both cases are not so different. The cathodic peak current Ip in the voltamogram is proportional to the square root of the sweep rate v. In the cyclic voltammograms of this species both in dimethylsulphoxide and dimethylformamide using 0.1 M tetrabutylammonium perchlorate as a supporting electrolyte the reductions around -0.7, -0.9 and -1.3 V appear approximately in the same regions and they may be attributed to the same type of reductions in the ring system. By changing the supporting electrolyte and using 0. 1 M tetraethylammonium perchlorate the same type of reductions were observed at somewhat shifted potential values. In dichloromethane electrochemical measurements were performed at around 10°C. The vapor pressure of this solvent is high, it is difficult to maintain constant composition. Aggregation of this species at that temperature makes the measurements comlicated. Thus the surface state of the solid electrode modified. So well defined redox responses are not obtained. In the cyclic voltammograms in dichloromethane two one-electron reduction waves were observed quasi-reversible character in tetrabutylammonium perchlorate. In this voltammogram the heterogenous electron transfer rate is relatively slow, so that the separation between the cathodic and anodic peaks varies with the sweep rate. The diffusion coefficients in dimethylformamide system are generally larger than those in dimethylsulphoxide system. Thus dimethylformamide has lower viscosity than dimethylsulphoxide. Solvents of low viscosity will generally give electrolyte solutions with greater conductivities. On the other hand, because diffusion is faster in such cases, diffusion-controlled chemical reactions will also be faster. In our case k(Ep) values in dimethylformamide are larger than those in dimethylsulphoxide. Diffusion coefficients of oxidation peaks of this species in dimethylsulphoxide and tetrabutylammonium perchlorate and tetraethylammonium perchlorate systems seem to be higher than those of the corresponding reduction waves. So the molecular weights in oxidation may become smaller than those in reductions. But in the case of dimethylformamide and tetrabutylammonium perchlorate and tetraethylammonium XUperchlorate systems the opposite result was observed, smaller values of diffusion coefficients The oxidation peaks have Diffusion coefficients decrease with charge. This is a reflection of two effects; as the charge increases, the size of the solvation shell increases; metal ions may coordinate the solvent molecules, but with increasing charge outer-sphere solvent molecules are also oriented about the ion. Thus a large aggregate with its solvation sheats moves through solution more slowly. R I ?N`.N' R-N N-R R-N N-R R-N N M N h' N R=Ts M=SnCl2 Figure 1 Sn(IV) phthalocyanine containing four 14-membred tetraaza macrocycles. Xlll