Cyclic Voltammetry of Phthalocyanines

Phthalocyanines and their related compounds possess similar structures as porphyrins. They have been used as green to blue dyes and pigments since their discovery. In this decade, they are known to be utilized in important functional colorants for many fields such as catalyst, laser light absorbers in data storage systems, electrocharge carriers in photocopies, photo-antenna device in photosynthesis, photovoltaic cells and photosensitizers for dye-sensitized solar cells (DSSCs), and photodynamic therapy of cancer (PDT). The functions are attributed to high electron transfer abilities of phthalocyanines. Cyclic voltammograms were carried out for phthalocyanines in order to estimate their electron transfer abilities and electrochemical mechanism.


Introduction
A blue-colored insoluble compound was accidently observed as a by-product at the South Metropolitan Gas Company in London during the preparation of ocyanobenzamide from phthalimide and acetic acid at a high temperature by Braun and Tcherniac in 1907. The compound was later called phthalocyanine. In 1927, at the University of Fribourg, de Diesbach and von der Weid obtained stable blue material during the preparation of phthalonitrile from o-dibromobenzene with copper cyanide in refluxing pyridine. Later, the blue material was identified as copper phthalocyanine. In the following year, the blue impurity in the reaction products was formed during the industrial preparation of phthalimide from phthalic anhydride and ammonia in a glass-lined reaction vessel at the Grangemouth plant of Scottish Dyes Ltd. During the preparation, the glass-lined reaction vessel was cracked. By reason of the reaction carried out, outer steel casing of the reaction vessel, the accident results in the formation of blue impurity. This blue impurity is known to iron phthalocyanine at the present [1][2][3][4][5].
These blue materials determined the molecular structure, which was composed of four iminoisoindoline units with various central metal ions or di-hydrogen by Professor R. P. Linstead at University of London in 1929. Linstead named the byproduct phthalocyanine as a combination of Greek naphtha (rock oil) and cyanine (blue) in 1933. The molecular structure of phthalocyanine was confirmed later using X-ray diffraction analysis by Robertson in 1935 [1][2][3][4][5].
Phthalocyanines are an analogous molecular structure as natural colorant of porphyrins. In general, porphyrins consist of four pyrrole units, while phthalocyanines construct four isoindole and nitrogen atoms at meso positions. The central cavity of phthalocyanines can place 63 different elemental ions including di-hydrogen (metalfree phthalocyanine). Phthalocyanines containing one or two metal ions are called metal phthalocyanines. In phthalocyanine ring system and part of the atom numbering system, the 2, 3,9,10,16,17,23,24 positions are referred to as the peripheral sites and the 1,4,8,11,15,18,22,25 positions as the nonperipheral sites. M can be di-hydrogen or one of the 63 elements of the periodic table ( Figure 1) [1][2][3][4][5].
In order to utilize many applications, the absorption maxima of phthalocyanines are best if moved near infrared region. The strongest absorption of phthalocyanines in visible region called Q band can be attributed to allow from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO), which means π-π* transition. The Q-band of phthalocyanines can be moved by bathochromic effect through extension of the π conjugation system. Especially, phthalocyanines having bathochromic effect are useful for photosensitization purposes.
Particularly, phthalocyanines are known to have the potentials to utilize as second-generation photosensitizers for PDT because they have long life time triplet state and show strong absorption of the far-red light between 600 and 850 nm of which a greater penetration of tissue and satisfactory photosensitization of singlet oxygen take place [21][22][23].
No-substituted phthalocyanines are insoluble or lower solubility in common organic solvents. The weak points of phthalocyanines have been improved to introduce substituents onto the ring system. Alkyl-substituted phthalocyanines become soluble in organic solvents and they have a lipophilic property. The lipophilic phthalocyanines have a high tumor affinity [24]. Hydrophilic-substituted phthalocyanines show solubility in aqueous media. Phthalocyanines containing pyridine rings in place of one or more of the benzenoid rings expected amphiphilic properties [17].
In the second place, phthalocyanines have attractive attention for the conversion of solar to electricity, because dyes come into general used for DSSCs absorb only weakly in solar spectrum. Phthalocyanines for DSSCs are required to possess strong absorption of visible light in the far-red or near infrared region. Then, phthalocyanines have high conversion capability of solar energy to electricity in comparison to common sensitized dyes [25].
In this chapter, synthesis and cyclic voltammetry of soluble phthalocyanines and their homologs compounds, subphthalocyanines were described in order to utilize photosensitizers for PDT and DSSCs [10,21].
These phthalocyanines have been measured by cyclic voltammograms (CVs) and chronocoulometric analysis in order to estimate their electron transfer properties and corresponding mechanism.

Electrochemistry
3.1 Phthalocyanine-4,4 0 ,4″4‴-tetrasulfonic acids and phthalocyanine-2,3,9,10,16,17,23,24-octacarboxylic acids, octakis(hexoxymethyl) phthalocyanine and anthraquinocyanine CV is used in the estimation of electrochemistry. It is the electrochemical equivalent to spectroscopy. It is a useful tool for the characterization of reduction and oxidation systems. It consists of cyclic potential of a stationary electrode immersed in a quiescent solution and measuring the resulting current. The excitation signal is a linear potential scan with a triangular waveform. This triangular potential excitation signal sweeps the potential of the working electrode. The triangle returns at the same speed and permits the display of a compete voltammogram. Therefore, if a molecular is reduced in the forward scan, it will be re-oxidized on the reverse scan.  The CV value is the current response, which depends on the applied potential. The current response shows two kinds of peaks such as the upward cathodic and the downward anodic peaks.
The reported potentials are the midpoint potential of anodic and cathodic peaks for each couple, E 1/2 , and the peak potential for the irreversible step, which have a mark on superscript. The ΔE values are an anodic peak to cathodic peak separation located in the reduction (negative) potential region ( Figure 15).
The CV of cobalt phthalocyanine-4,4 0 ,4″4‴-tetrasulfonic acids showed two cathodic peaks and four anodic peaks. The peaks are attributed to four reduction stages. The first oxidation potential appeared at 0.67 V versus silver/silver chloride (Ag/AgCl) and the first reversible reduction potential at À0.62 V versus Ag/AgCl. The CV was sorted into five waves. The CVs consist of two reversible reduction couple, one irreversible reduction wave and two irreversible oxidation waves.
The CV of cobalt phthalocyanine-2,3,9,10,16,17,23,24-octacarboxylic acids, three cathodic, and six anodic peaks appeared. The peaks were sorted into three reversible reduction couples at À0.24, À0.66, and À1.39 V versus Ag/AgCl, and three irreversible oxidation waves at 0.67, 0.87, and 1.06 V versus Ag/AgCl. The reduction and oxidation of metal phthalocyanines are due to the interaction between the phthalocyanine macro-ring and the central metal. Sulfonic and carboxylic groups are electron-withdrawing groups, so they are expected to reduce the electro charge in the phthalocyanine macro-ring.
The relationship between the anodic and cathodic peak current ratio of a reversible couple, i a /i c and the scan rate, ν, provides a quick test for electrochemical mechanism associated with a preceding or succeeding reversible or irreversible chemical equilibrium. The scan rate varied from 0.05 to 0.3 Vs À1 (Figure 16).
Chronoamperometry is a current-time response to a potential step excitation signal. A large cathodic current flows immediately when the potential is stepped up from the initial value, after that it slowly attenuates. The reduction step exhibited that same behavior in comparison with both potential steps. The current-time curves are converted into the relation between the current and square root of time t 1/2 (Figures 17 and 18).
The current-time curve for chronoamperometry is expressed by the Cottrell equation (Eq. (1)).
where i is the current (A), n is the number of electrons transferred per ion or molecule (mol À1 ), F is Faraday's constant (96,485 C mol À1 ), A is the electrode    area (2.0 Á 10 À2 cm 2 ), C is the concentration (mol cm À3 ), D is the diffusion constant (cm s À1 ), and t is time (s). A plot of the current, i versus square root of time, t 1/2 gives a straight line. The slop means the diffusion constant in forward and reverse steps.
The current of the Cottrell plots is a measure of the rate for electrolysis at the electrode surface. Electrolysis is controlled with a mass transfer by diffusion on the electrode. The diffusion constant implies the rate of electrolysis. The slop means the diffusion constant in each step. The forward step indicates the reduction and the reverse step is oxidation ( Table 2).
The chronocoulometry was taken by one treatment of chronoamperometry. The current response was integrated to give a response to the charge. The charge-time curve of the forward step for chronocoulometry is the integral of Eq. (1); this is called the Anson equation (Eq. (2)).
The reverse step is following equation (Eq. (3)): where τ is time of reverse potential step. The initial potential was À1.20 V versus Ag/AgCl and the step width was 250 ms. The step potential was 1.60 V versus Ag/AgCl ( Figure 19).
The Anson plot is a straight line with an intercept. Chronocoulometry is useful to study absorption on an electrode surface. When absorbed species exist on an electrode surface, it is electrolyzed immediately, whereas solution species must diffuse the electrode in order to react. The total charge Q total is measured in a potential step experiment.
where Q dl is the double layer charge (C), Q abs is the absorbed species charge (C), Γ is the amount absorbed (mol cm À3 ). Q total is obtained by summing Q, Q abs , and Q dl . As the expression of Q in Eq. (2), a plot of Q versus t 1/2 is a straight line. The  Anson plot should be linear with intercept that is equal to the second and third terms in Eq. (4). If Q dl is known, then, the value Q abs can be calculated for an electrode of the known electrode area. When double step chronocoulometry is used, the difference in the intercepts of forward and reverse steps is Q abs .
Only the value of Q in three terms depends upon the scanning time. The intercept of the Anson plot expresses the sum of Q dl and Q abs . The Q abs can take away Q dl , which is a value of the difference of intercepts between forward and reverse steps, since double step chronocoulometry is used. When no absorption of reactant or product, the intercept of Anson plot for both forward and reverse steps are equal (Q dl ). While reactant absorbs but product does not, the intercept of reverse is a measure of Q dl in the presence of absorbed reactant, and the intercept of forward step contains both Q dl and Q abs for absorbed reactant.
The relation between Q r /Q f and t 1/2 can be estimated by the mechanism and rate of the following chemical reaction ( Figure 21).
The oxidation of metal phthalocyanines having transition metal are electrochemically irreversible and electrons are added to the orbital of phthalocyanine ring or the central metal depending on the redox potential for reduction process.
After separation of regioisomers, fractions 1-3 have one pair of reversible oxidation peak and four irreversible waves. Fraction 4 has one pair of reversible and three irreversible waves.
The porphyrazine ring is influenced by the π-electrons about the closed system. Although the π-electron system of zinc bis(1,4-didecylbenzo)-bis(3,4-pyrido) porphyrazine and fractions 1-4 consists of one porphyrazine, two pyridinoid, and two didecyl-substituted benzenoid rings; the location of these rings except for porphyrazine are different from each regioisomer.
The ΔE values are around 100 mV and the reduction and oxidation processes are the same for regioisomers, except for fraction 4. The electron process of regioisomers between fractions 1 and 3 involves approximately one electron transfer. The ΔE values of fraction 4 show different behavior in comparison to the others. The different behavior for fraction 4 is attributable to the mixture of two types of C 2v regioisomers. The reduction and oxidation potentials of fraction 4 are based on the interaction between two types of C 2v regioisomers. No observation on the reversible couple in zinc bis(1,4-didecylbenzo)-bis(3,4-pyrido)porphyrazine resulted in interaction between regioisomers.
After quaternation of regioisomers, the shapes of CVs appeared clearly. The electron transfer ability of regioisomers has been increased remarkably by the acquisition of cation groups. The CV showed two anodic and two cathodic peaks, two anodic and three cathodic peaks, two anodic and two cathodic peaks, and one anodic and five cathodic peaks for fractions 1, 2, 3, and 4, respectively.
The CVs of nonperipheral arylsulfanyl-substituted lead phthalocyanines have similar shapes unconcerned in the presence of terminal groups such as methyl, methoxy, and tert-butyl, on the arylsulfanyl substituents. Similar phenomena were observed for other metal phthalocyanines.
Reduction and oxidation potentials of nonperipheral arylsulfanyl-substituted metal-free phthalocyanines and for those having a central metal of cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), and lead (Pb) were summarized. The CVs of  these compounds exhibited reduction and oxidation potentials, which were in accordance with their central metal ( Table 6).
The arylsulfanyl substituents in nonperipheral arylsulfanyl-substituted phthalocyanines influence the π electron density in the phthalocyanine ring. The effect of arylsulfanyl groups gives rise to the change of electron density of phthalocyanine ring in the molecule of nonperipheral arylsulfanyl-substituted phthalocyanines. Nonperipheral arylsulfanyl-substituted phthalocyanines exhibit excellent electron transfer properties.
The reduction and oxidation properties of nonperipheral arylsulfanylsubstituted phthalocyanines result from electron transfer from sulfur atoms in the arylsulfanyl substituents at the peripheral positions of phthalocyanine ring to the central metal atom, except in the case of Co as the central metal. In nonperipheral arylsulfanyl-substituted Co phthalocyanines, the irreversible peaks are attributed to the central metal and the reversible waves represent the reduction/oxidation of phthalocyanine ring, including arylsulfanyl groups. It appears that the electron transfer mechanism of nonperipheral arylsulfanylsubstituted phthalocyanines depend on the kind of central metal atom. In particular, the electron transfer mechanism of nonperipheral arylsulfanylsubstituted Co phthalocyanines and nonperipheral arylsulfanyl-substituted Zn phthalocyanines were attributed to reduction and oxidation of their central metal, while those of nonperipheral arylsulfanyl-substituted metal-free phthalocyanines, nonperipheral arylsulfanyl-substituted Cu phthalocyanines, nonperipheral arylsulfanyl-substituted Ni phthalocyanines, and nonperipheral arylsulfanyl-substituted Pb phthalocyanines resulted from HOMO to LUMO electron transitions [20].

Subphthalocyanines and nonperipheral arylsulfanyl-substituted subphthalocyanines
The CV of subphthalocyanine showed two cathodic peaks at À0.30 and À0.65 V versus Ag/AgCl, and two anodic peaks at 0.82 and À0.62 V versus Ag/AgCl. Subphthalocyanine has two irreversible oxidation and reduction at 0.82 and À0.30 V versus Ag/AgCl, and one pair of reversible reduction potential at À0.64 V versus Ag/AgCl ( Figure 24).
The reduction and oxidation potentials of subphthalocyanine and its derivatives are summarized ( Table 7).
In the case of metal phthalocyanines, their substituents influence the π electron environment in the molecule especially the four phenylene rings. The effect of substituents gives rise to the change of electron density of the four phenylene rings in the molecule. Electron transfer properties depend on the kind of the substituents.
While in the case of subphthalocyanine and its derivatives, reduction and oxidation potentials were of various values. However, one irreversible potential certainly appeared around À0.3 V versus Ag/AgCl, of which peaks are attributed to the reduction of the subphthalocyanine ring. The difference of CV between subphthalocyanine derivatives is attributed to the variation of the substituents depends on the subphthalocyanine ring [33].
As mentioned above, in general, metal phthalocyanines having transition metal behave as electrochemically irreversible, and exhibit reduction and oxidation properties resulting from interaction between the phthalocyanine ring and their central metal. The oxidation potential is about 1.0 V versus SHE. The reduction potential occurs between À0.3 and À0.8 V versus SHE. Electrons are added either to the molecular orbital of the phthalocyanine ring or the central metal, depending on the reduction and oxidation potential for reduction process.
As mentioned above, an irreversible reduction appeared around À0.3 V versus Ag/AgCl for subphthalocyanines [33]. The irreversible peaks of subphthalocyanines are attributed to the reduction of the subphthalocyanine ring. The reduction and oxidation potentials of subphthalocyanines result from their substituents.
The CVs of zinc bis(1,4-didecylbenzo)-bis(3,4-pyrido)porphyrazine and its regioisomers showed different oxidation and reduction potentials. After quaternation, the shapes of CVs appeared clearly. It is though that electron transfer ability has been increased remarkably by acquisition of cation groups.
The electron transfer properties of nonperipheral arylsulfanyl-substituted phthalocyanines were shown to be excellent estimated. The effect of arylsulfanyl groups gives rise to be significant.
For subphthalocyanine and its derivatives, reduction and oxidation potentials were various values. The difference of CV between subphthalocyanine derivatives is attributed to the variation of the substituents depends on the subphthalocyanine ring.
These results suggest that phthalocyanines will be appropriate materials for use in the next generation photosensitize for PDT and DSSCs.