Electrochemical and Electrocatalytical Properties of 3,7,13,17-Tetramethyl-2,8,12,18-Tetrabutylporphyrin in Alkaline Solution

Institute of Solution Chemistry of the Russian Academy of Sciences, 153045, Akademicheskaya, 1,Ivanovo, Russian FederationE-mail: ngm@ihnr.polytech.ivanovo.suReceived: 17 March 2000 / Accepted: 27 March 2000 / Published: 12 June 2000Abstract: The reduction and oxidation behaviour in alkaline solution of 3,7,13,17-tetramethyl-2,8,12,18-tetrabutylporphyrin (L) has been studied by cyclic voltammetry. Theshape of I-E curves was investigated at different terminal values of electrode potential and atdifferent scan rates. It was shown that it is possible to obtain two reversible one-electronelectroreduction processes accompanied by the formation of mono- and dianion forms of theporphyrin under specific experimental conditions. The reduction potentials of the LUL


Introduction
There are several publications concerning investigations of the electrochemical and electrocatalytical properties of porphyrin compounds [1][2][3][4][5].The reasons for this considerable interest in porphyrins and their metal complexes are their semiconductive, photosensitivity and catalytic properties as well as the possibility of modifying these properties by structural modification of the porphyrin ring.

Results and Discussion
As shown in Fig. 1, only one peak IIk (E=-1.3…-1.5V) can be found on the cathode curve.This curve was obtained after bubbling Ar through the electrolyte for 30 minutes.On the anode curve the inverse process peak IIa is easily detected at E=-1.0 … -1.3V.The ratios between the quantities of electricity as well as the difference between the potentials of peaks IIk and IIa do not correspond to a reversibility of process II.The peaks Ia (E=-0.6 … -0.8V) and IIIa (0 … 0.2V) also appear on the anode curve, whereas the corresponding peaks are absent on the cathode curve.An additional experiment was carried out to obtain the corresponding pairs of peaks Ia and IIIa.A preliminary electroreduction at E=0.5V and electrooxidation at E=-1.6V was imposed on the working electrode potential.As shown in Fig. 2 the two weak peaks for the process III appear if anode polarization at E=0.5V is performed.Electrode polarization at -1.6V affected the redistribution of the square peaks for the processes I and III (Fig. 3).This redistribution depends on the electrode polarization time, and the irreversibility of process II is seen to increase.At the same time the intensity of process I is growing (the area of peak Ia is increasing), but this process is still irreversible.It was thus shown that the shape of the I-E curves depends on the range of the electrode potential change (Fig. 4).Increasing the potential from -1.6V to  In Fig. 5 the shapes of the I-E curve at different terminal anode potential values are shown.The cathode peak corresponding to process I appears to narrow the range from 0.5V to negative value.The reversibility of the process I is also increasing as the range of the potential narrows.
The existence of the stable I-E curve with two peaks corresponding to one-electron processes allows us to investigate the dependence of peak position and the corresponding quantities of electricity for those processes upon scan rate (Fig. 6, Table1).The potentials for both oxidation and reduction were obtained by extrapolation of the potentials to zero scan rates.The effective numbers of electrons was calculated as well.
The comparison of obtained values E red/ox with the same values for other porphyrins [2][3][4][5] allows us to conclude that the pairs of peaks observed belong to one-electron processes of porphyrin electroreduction accompanied mono-and dianion form creation (L -and L 2-).The lower oxidation potential (E=0.25V) for process III is caused by its chemical structure and the electron-donor ability of methyl and butyl groups.
Additional peaks appeared on the I-E curve (Fig. 7) if molecular O 2 was present in the solution.These peaks correspond to the electroreduction of O 2 and peroxide ion creation.The analysis of the current ratio corresponding to these two stages showed that after 30 minutes of O 2 bubbling the electrolyte is saturated with oxygen and the equilibrium concentrations of the components in solution are established.The increasing ∆I O2 /∆I HO2 -ratio manifests itself about 10 cycles after start of scanning, and this fact testifies to the growth of the contribution of direct electroreduction process of dioxygen (Table 2).
Starting with the 10th cycle the ratio of the currents corresponding to the first and second stadium of molecular oxygen electroreduction is nearly a constant value.

Figure 7 .
Figure 7. Cyclic voltammogram when molecular O 2 has been added to the electrolyte.Scan rate 20mV/s.

Table 1 .
The parameters of electrochemical processes under investigation at the different scan rate.

Table 2 .
Current values after different numbers of scans for the molecular oxygen transformation process.