From the viewpoint of applying water molecule as electron source for artificial photosynthesis, the information of the electrochemical oxidation properties in aqueous solution is of course requisite for the study. As described above attention has been focused on how to activate water molecule by only one-photon process. For the purpose the first oxidation wave of Si-porphyrins in the electrochemical behaviour should be the most curious one. In this context the oxidation waves of the Si-porphyrins were carefully examined.
Figure 89 Experimental set-up for CV measurement using a BDD electrode Si(IV)
OH
OH
H2N NH2 Si(IV)
OH
OH
H2N NH2
-
e
-+.
Attack on Oxygen by electron donor
- 84 - Experiment
Oxidation and reduction properties of Silicon Porphyrins were measured by cyclic voltammetry (CV) with a ALS electrochemical analyser (611DST) system, equipped with a boron doped diamond with wide electrochemical window43 or a glassy carbon as a working electrode, Ag/AgNO3 or Ag/AgCl (water) as a reference electrode, and Pt wire as a counter electrode in dimethylformamide (DMF), water, aqueous acetonitrile (CH3CN) containing 0.1 M supporting electrolyte, (C4H9)4N+PF6- or Na2SO4(water) The electrochemical studies were done in homogeneous condition with 0.2mM of Silicon porphyrin.
Result and Discussion
The cyclic voltamogram of silicon porphyrins measured in non-aqueous condition afforded a reversible CV for SiTMP and SiTFMP, while a quasi-reversible CV for SiTCPP, SiTPyP and SiTMPyP. However the addition of small percentage water i.e., aqueous condition resulted in irreversible process with higher anodic current. The case by case discussion the oxidation half reaction is given below;
SiTMP
In case of SiTMP the oxidation half reaction in acetonitrile was clearly reversible , also it is interesting to note that even in 10% water SiTMP shown quasi reversible type nature.
This is because the hydrophobic nature of the mesityl group may make the molecule not as much exposed to water and thus prevent attack at the axial –O after first oxidation, resulting in reversible peaks.
Figure 90 The CV of SiTMP in a) CH3CN and b) 10% Aqueous CH3CN SiTCPP
In case of SiTCPP the oxidation half reaction in DMF was quasi-reversible and in 100%
water SiTCPP shows irreversible type nature. The quasi-reversible peak is either due to the presence of trace amount of moisture in DMF or H2O in crystal structure of SiTCPP as observed in elemental analysis.
0.0 0.2 0.4 0.6 0.8 1.0 1.2
2 0 -2 -4 -6 -8 -10 -12 -14 -16
Current /
Potential v.s. Ag/AgNO3 / V
0.0 0.2 0.4 0.6 0.8 1.0 1.2
2 1 0 -1 -2 -3 -4 -5
Current /
Potential v.s. Ag/AgCl / V
a) b)
- 85 -
Figure 91 The CV of SiTCPP in a) DMF and b) H2O (pH=12) SiTFMP
In case of SiTFMP the oxidation half reaction in acetonitrile was reversible and in 20%
aqueous acetonitrile SiTFMP shows irreversible type nature. However the catalytic current was not high as expected considering the electron withdrawing nature of the –CF3
group and high oxidation potential, this may be due to the hydrophobic nature of the TFM porphyrin ring decreasing the ease of attack on axial oxygen atom.
Figure 92 The CV of SiTFMP in a) CH3CN and b) 20% Aqueous CH3CN SiTPyP
In case of SiTPyP the oxidation half reaction in acetonitrile was quasi-reversible and in 20% aqueous acetonitrile oxidation half reaction was irreversible. The reversible CV is largely effected by the presence of trace amount of moisture which was practically difficult to avoid, the high reactivity of SiTPyP towards water is obvious from this observation. The addition of water to resulted in catalytic increase of anodic current, especially at acidic pH. Due to the poorly defined peak in reverse cycle the non-catalytic potential is defined by oxidation peak potential rather than half wave potential.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 2
0 -2 -4 -6 -8 -10
Current /
Potential v.s. Ag/AgNO3 / V
0.0 0.2 0.4 0.6 0.8 1.0
0 -5 -10 -15 -20 -25 -30
Current /
Potential v.s. Ag/AgCl / V
a) b)
0.5 0.6 0.7 0.8 0.9 1.0 1.1
10 5 0 -5 -10 -15 -20 -25
Current /
Potential vs. Ag/AgNO3 / V
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0 -10 -20 -30 -40 -50
Current /
Potential v.s. Ag/AgNO3 / V
a) b)
- 86 -
Figure 93 The CV of SiTPyP in a) CH3CN and b) 20% Aqueous CH3CN SiTPyP
In case of SiTMPyP the oxidation half reaction in acetonitrile was quasi-reversible and in 20% aqueous acetonitrile oxidation half reaction was irreversible. The peak in non-catalytic CV is poorly defined owing to the cationic nature and also high hygroscopic nature of SiTMPYP, the experiments for the observation of good non catalytic CV is under progress. The addition of water to resulted in catalytic increase of anodic current.
Due to the poorly defined peak in reverse cycle the non-catalytic potential is defined by oxidation peak potential rather than half wave potential.
Figure 94 The CV of SiTMPyP in a) CH3CN and b) 20% Aqueous CH3CN
The Eoxp’s were well reflecting electronic properties of substituents on the meso-phenyl group in the order of –CH3 (SITMP) < -COO- (SiTCPP) < -CF3 (SiTFMP) < - Py (SiTPyP) < -Py+-CH3 (SiTMPyP). The oxidative water activation is substantially dependent on the oxidation ability of the catalyst. The criteria became obvious here that Si-porphyrins with the higher Eoxp than ~0.86 Volt vs SHE exhibit catalytic oxidation
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0 -5 -10 -15 -20 -25 -30
Current /
Potential v.s. Ag/AgNO3 / V
0.0 0.2 0.4 0.6 0.8 1.0
0 -10 -20 -30 -40 -50 -60 -70
Current /
Potential v.s. Ag/AgNO3 / V
a) b)
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0 -10 -20 -30 -40 -50 -60
Current /
Potential v.s. Ag/AgNO3 / V
0.0 0.2 0.4 0.6 0.8 1.0
0 -5 -10 -15 -20 -25 -30
Current /
Potential v.s. Ag/AgNO3 / V
a) b)
- 87 -
waves in aqueous solution. The critical Eoxp may be correlated with thermodynamic requirement for oxygen evolution from water (E0 =1.23 Volt and E = 0.82 Volt at pH = 7). Tuning the Eoxp of Si-porphyrins would thus be requisite for oxidative activation of water.
Table 10 Oxidation potential of Silicon Porphyrins non-aqueous and aqueous neutral condition
Figure 95 Scheme of oxidation half reaction taking place in electrochemical cell for ; a) non aqueous and b) aqueous conditions
Effect of new peak in reduction half reaction in oxidation cycle
The reduction half cycle coupled with oxidation half reaction of SiTPyP was observed in aqueous condition at pH 5 to check the effect on oxidation peak current. When oxidation half reaction was performed alone there was a rapid decrease of current after the first cycle. In contrast first followed by reduction, a new peak was appeared at -0.52 V (vs SHE) and the oxidation current after reduction process was more than the first cycle. This shows the new process at the reduction is having some effect on oxidation process, may
Silicon Porphyrins Non Aqueous E1/2vs SHE Aqueous Eoxp/ V vs SHE at pH 7
SiTMP 0.93 0.74a
SiTCPP 0.91b 0.88
SiTFMP 1.08 0.87
SiTPyP 1.16c 0.98
SiTMPyP 1.22c 1.11
Si-P
Si-P+
ELECTRODE X
H2O
ELECTRODE
Si-P
Si-P+
a) b)
- 88 -
be the intermediate formed during oxidation is reduced back at electrode surface by this reduction process to provide new molecule for oxidation. The absence of new reduction peak without oxidation process is indicative of the fact that new reduction peak is as result of oxidation process. When oxidation is coupled with reduction the rapid decrease of current for oxidation process is not observed, it is also interesting to note that if the reduction switching potential is less than -0.52 V vs. SHE there is decrease of current for oxidation process.
Figure 96 Plausible processes during oxidation-reduction coupled reaction
Figure 97 Decrease of current for SiTPyP at pH = 5 in water a) CV for 10 cycles and b) plot of peak current vs. number of cycle
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
80 60 40 20 0 -20 -40 -60 -80
Current /
A
Potential vs Ag/AgCl
/ VSi O- +.
Intermediate (may be peroxy)
Intermediate (may be peroxy) Reduced
No peak before oxidation
Increase of current
Blank Peak at aqueous pH = 5 Si- Por
reduction
Si O
O
-0.0 0.2 0.4 0.6 0.8 1.0 1.2
0 -5 -10 -15 -20 -25 -30 -35
Current /
Potential v.s. Ag/Cl / V
0 2 4 6 8 10
-5 -10 -15 -20 -25 -30 -35
Ic / A
Number of Cycles
a) b)
- 89 -
Figure 98 Decrease of current for SiTPyP at pH = 5 in water a) CV for 10 cycles for different reduction switching potential and b) plot of peak current vs. number of cycle
-1.0 -0.5 0.0 0.5 1.0
40 20 0 -20 -40
Current ,A
Potential , V vs Ag/AgCl
-0.5 0.0 0.5 1.0
40 20 0 -20 -40
Current ,A
Potential , V vs Ag/AgCl
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 20
10 0 -10 -20 -30
Current ,A
Potential , V vs Ag/AgCl
0 2 4 6 8 10
-30 -31 -32 -33 -34 -35 -36 -37
Ic , A
Number of Cycles
0 2 4 6 8 10
-10 -12 -14 -16 -18 -20 -22 -24 -26 -28 -30
Ic , A
Number of Cycles Reduction Switching Potential -1.2 V
Reduction Switching Potential -0.8 V
0 2 4 6 8 10
-8 -9 -10 -11 -12 -13 -14 -15 -16 -17 -18
Ic , A
Number of Cycles Reduction Switching Potential -0.5 V
(a) (b)
- 90 -