**2. Kinetics and solvent catalysis**

When the proposed sensitizers, dicyanobis(2,2′-dipyridyl)iron(III) and dicyanobis(1,10-phenanthroline)iron(III), are introduced to the iodide solution in the binary solvent media, the oxidation of iodide is a spontaneous process that does not require any external triggering. When the reduction potential of the redox pair is taken into account, all of the reactions are electrochemically viable [30–32]. The visual color change of the solutions and the spectra of the products show the progress of the reactions and product(s) generation when compared to analogous iron(II) complexes (**Figure 1**) [33, 34]. **Figure 1** shows the wavelength maxima corresponding to dicyanobis(2,2′-dipyridyl)iron(II) and dicyanobis(1,10-phenanthroline)iron(II), which allows time course graphs to be drawn as the absorbance increases as a function of time after the products are formed. The reactions were investigated in the visible region at the wavelength maximum of dicyanobis(2,2′-dipyridyl)iron(II) and dicyanobis(1,10-phenanthroline)iron(II) (**Figure 2**). Each reaction was investigated using a pseudo-order kinetic model with an excess and changing concentration of the mediator, iodide, in comparison to a fixed and low concentration of either dicyanobis(2,2′-dipyridyl)iron(III) or dicyanobis(1,10-phenanthroline)iron(III). The reactions were studied at room temperature in all the reaction media. The oxidant concentration was held constant at 0.08 mM and the iodide (reductant) concentration was adjusted between 0.08 mM and 4 mM at 1:1, 1:2.5, 1:5, 1:7.5, 1:10, 1:20, 1:30, 1:40, and 1:50 times to preserve the pseudo-first order kinetic model in all reaction media at 0.06 M ionic strength (μ). The integration method was implemented on the absorbance data, and zero order kinetics was observed corresponding to the sensitizers i.e., oxidizing agents. The absorbance was plotted against time that yielded a slope "ɛ∙b∙*k*obs" that is the multiplication product of the molar absorptivity of either of dicyanobis(2,2′-dipyridyl)iron(II) and or dicyanobis(1,10-phenanthroline)iron(II),

**Figure 1.** *Visible absorption spectra of the products of the redox reactions in three different reaction media.*

**Figure 2.** *Representative time course graphs of reactions in different reaction media at 293 ± 1 K.*

the pathlength of the quartz cuvette (1 cm) and the observed zero order rate constant, respectively. The slope is a constant (ɛ∙b) times larger than the real value due to the inclusion of a constant mathematical number, which has no overall effect on the rate constant. Such a slope can be obtained by using absorbance without converting it to concentration (via implementing Beer-Lambert's law). When a graph is drawn between the concentrations as a function of time, it yields a straight line with a slope

equal to the zero order rate constant. **Figure 2** shows representative kinetic traces at a 1:5 sensitizer:mediator ratio for comparative examination in various solvent media. In 10% (v/v) TBA-water, the reaction between dicyanobis(2,2′-dipyridyl)iron(III) and iodide is the slowest, but dicyanobis(1,10-phenanthroline)iron(III)-iodide is the fastest in identical media.

Controlling the reaction within the DSSC with respect to potential sensitizers such as dicyanobis(2,2′-dipyridyl)iron(III) or dicyanobis(1,10-phenanthroline)iron(III) is aided by zero order kinetics corresponding to oxidizing agents in all reaction media. The mediator, such as iodide, plays the main role in controlling the reaction kinetics in such sensitizer-mediator interactions. When the reaction mechanism is known in all of the selected media, it becomes much easier to exploit this sensitizer-mediator interaction to get the most out of the reaction in a DSSC where the rate of the reaction is solely dependent on the mediator. The zero order rate constant obtained for each reaction in all reaction media was displayed as a function of the iodide ion concentration (**Figure 3**) for this experiment [4, 35, 36]. The redox reaction between dicyanobis(2,2′-dipyridyl)iron(III)-iodide in either 10% TBA-water (bpy-TBA in **Figure 3**) or 10% dioxane-water (bpy-dioxane in **Figure 3**) underwent a first order with the zero order rate constant increasing linearly with increasing iodide concentration, yielding a straight line passing through the origin. The overall first-order rate constant of the reaction is determined by the slope of the plot. In the meantime, a third order kinetics was found in the reaction of dicyanobis(1,10-phenanthroline) iron(III)-iodide in 10% TBA-water (phen-TBA in **Figure 3**). As a result, it has been discovered that in the selected reaction media, dicyanobis(1,10-phenanthroline) iron(III)-iodide reacts much faster than dicyanobis(2,2′-dipyridyl)iron(III)-iodide, implying that in a DSSC, the recombination process may be faster in the "phen" system rather than the "bpy" system. To avoid repeating long names, the former potential sensitizer is referred to as "bpy" and the latter as "phen". The main difference

**Figure 3.** *Kinetic study with respect to the reducing agent in different reaction media.*

between the two sensitizers is in the chelate, where the phen system has more piconjugation than the bpy system. The rest of the coordination sites and geometry, on the other hand, are similar. Both are octahedral complexes that are substitution inert. Furthermore, in the instance of dicyanobis(2,2′-dipyridyl)iron(III)-iodide, the first order rate constant is 13 times bigger in 10% dioxane-water than in 10% TBA-water. Consequently, the results reveal that dioxane has a catalytic influence on the redox kinetics of the sensitizer-mediator relationship when compared to TBA. Similarly, dicyanobis(1,10-phenanthroline)iron(III)-iodide displays faster electron transfer than dicyanobis(2,2′-dipyridyl)iron(III)-iodide in the identical reaction medium (10% TBA-water).

To evaluate the catalytic function of the solvent in the sensitizer-mediator interaction, it is obvious to calculate the ideal rate constant for each reaction in each solvent medium, such as the rate constant at zero ionic strength. **Figure 4** depicts the plots of the primary salt effect on the rate constant according to the formulation (2). The optimal value of the rate constant was determined by the intercept of the plots. When the effect of the solvent is significant and there is no effect of ions on the rate constant, the ionic strength is assumed to be zero, and the ideal value of the rate constant is obtained. When the experimental data is plotted and extrapolated to zero ionic strength, the theoretical value of the rate constant, or ideal rate constant, is produced.

$$\log\left(\varepsilon \cdot b \cdot k\_{\text{obs}}\right) = \log\left(\varepsilon \cdot b \cdot k\_{\text{obs}}\right)\_{\text{ideal}} + 2A z\_A z\_B \frac{\sqrt{\mu}}{1 + \sqrt{\mu}} \tag{2}$$

**Figure 4** shows the decelerating effect of increasing ionic strength on the observed zero order rate constant, indicating that opposite charges are involved in

**Figure 4.** *Plots of primary salt effect in different solvent media.*

*Solvent Catalysis in the Sensitizer-Mediator Redox Kinetics DOI: http://dx.doi.org/10.5772/intechopen.105393*

**Figure 5.** *Effect of solvent on the reaction kinetics of sensitizer-mediator interaction.*

the rate determining step that leads to the formation of the transition state complex. bpy-5%/10% /15% dioxane, bpy-5%/10%/15% TBA, and phen-5%/10%/20% TBA were used to depict the effect of increasing ionic strength in different solvent media for "bpy" and "phen" systems. The term bpy-5% dioxane, on the other hand, refers to a sensitizer-mediator interaction involving dicyanobis(2,2′-dipyridyl) iron(III)-iodide and a 5% (v/v) 1,4-dioxane-water solvent system. Meanwhile, dicyanobis(1,10-phenanthroline)iron(III)-iodide in a 5% (v/v) TBA-water solvent solution is phen-5% TBA. The remainder of the terms has comparable connotations as well. The ideal value of the rate constant was obtained from the intercept of each plot and was used to build a graph. According to Eq. (1), the natural logarithm of the ideal rate constant was drawn on the y-axis and the reciprocal of the dielectric constant was drawn on the x-axis for each system, including bpy-dioxane, bpy-TBA, and phen-TBA. **Figure 5** depicts the final results. The slope of the plots was used to calculate the inter-nuclear distance (*r*#) between the active species that constitute the transition state complex, and the results are presented in **Table 1**. **Table 1** demonstrates that in the reaction of dicyanobis(2,2′-dipyridyl)iron(III) with iodide in 10% (v/v) TBA-water versus 10% (v/v) dioxane-water, the inter-nuclear distance is very long enough. This exhibits the catalytic impact of dioxane over TBA by displaying the quick electron transfer kinetics between the sensitizer-mediator in dioxane-water as compared to TBA-water. The inter-nuclear distance between the active reactants that form the transition state complex and lead to the rate-determining step of the reaction in dicyanobis(1,10-phenanthroline)iron(III)-iodide is smaller (53 pm) than dicyanobis(2,2′-dipyridyl)iron(III)-iodide, which is 130 pm in 10% (v/v) TBA-water. This confirms that in the former situation, electron transfer between the sensitizer and mediator is faster than in the latter case, and that the solvent has a catalytic impact in the "phen" system rather than the "bpy" system. However, by utilizing 1,4-dioxane instead of tertiary butyl alcohol, the reaction of the "bpy" system accelerated almost to the level of the "phen" system, where *r*# is 59 pm in the former instance and 53 pm in the latter case. As a result, in order to accelerate a sensitizermediator interaction for rapid recombination in DSSC, solvent can be used in an environmentally friendly and cost-effective manner to increase the stability and efficiency of the solar cell.


**Table 1.**

*Catalytic effect of solvent in the sensitizer-mediator interaction.*
