**1. Introduction**

In the kinetics of reactions, particularly redox reactions, the solvent has a significant effect. Redox reactions occur when two responding entities exchange electrons. The electron giver, or reducing agent, is the one who contributes the electron; the electron acceptor, or oxidizing agent, is the one who accepts the electron. The donation and reception of electrons alter the oxidation states of the reactants since electrons are such small charged particles. As a result, the solvent plays an important role in electron transfer reactions. A few of the most influential characteristics that govern redox reactions include solvation, viscosity, and hydrogen bonding [1]. The solvent organizes and reorganizes itself around the reactants and products before and after the electron transfer event. Similarly, the solvent organizes and reorganizes around the reactants during the production of the transition state [2]. According to the transition state theory of reactions in solution and the double sphere model, the rate constant is related to the dielectric constant of a medium using the following expression [3].

$$
\ln k = \ln k\_0 - \frac{e^2 \mathcal{Z}\_A \mathcal{Z}\_B}{4\pi \varepsilon\_0 \varepsilon\_r r\_\ast k\_B T} \tag{1}
$$

*k* and *k*0 are the rate constants for any dielectric constant and infinite dielectric constant, respectively, in Eq. (1). The symbols *e, z*A*, z*B*,* ɛ0*,* ɛr*, r*#*, k*B and *T* represent the constant value of electric charge (a constant in coulombs), charge on reactants A and B, permittivity constant, dielectric constant of medium, inter-nuclear distance between the reacting entities that form the transition state complex, Boltzmann constant, and temperature in Kelvin scale, respectively. The Eq. (1) correlates the rate constant with charges on the reactants and the dielectric constant of the medium in three ways.


Other reaction parameters, such as the effect of ionic strength in a specific reaction, must be zero or close to zero in order to investigate the effect of the dielectric constant on the rate constant and, as a result, the rate of the reaction. Variation in ionic strength has a significant effect on the rate constant of any reaction, and we can find a theoretical value of the rate constant at zero ionic strength by extrapolating the graph to zero ionic strength, i.e., the intercept of the plot [4], using the transition state theory to formulate the primary salt effect. The theoretical value of the rate constant at zero ionic strength is known as the ideal value of the rate constant. The ideal rate constant for a reaction can be calculated in a variety of solvent systems with varied dielectric constants. The dielectric constant of reaction media can be changed by changing the proportion of one solvent to another. To determine the slope of the plot according to Eq. (1), it is advisable to plot the natural logarithm of the ideal rate constant (ln*k*) versus the reciprocal of the dielectric constant (1/ɛr) rather than the natural logarithm of the rate constant at any ionic strength. The value of the internuclear distance between the reactants that constitute the transition state complex and are involved in the rate determining step can be calculated using the slope of the plot. The inter-nuclear distance between the reactants of various reactions can be compared and used to control the kinetics of the reactions under certain experimental conditions.

As a result, it is clear that changing the nature of the reaction media affects the entire electron transfer mechanism. When dealing with the kinetics of redox reactions, it is also worth noting that changing the solvents' proportion in a reaction can change the viscosity and strength of hydrogen bonding, as well as the nature of hydrogen bonding, resulting in either the activation controlled mechanism or the diffusion controlled mechanism [5–12]. It is critical to have precise information on the kinetics of any reaction in order to manage it and make use of it as needed in a process. When it comes to dye-sensitized solar cells (DSSCs), the sensitizermediator reaction is crucial to the electron transfer cycle as well as the cell's

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

stability, durability, and efficiency. The influence of variations in the sensitizer on the stability and/or efficiency of DSSCs has been explored by a number of researchers [13–19]. A variety of natural and synthetic dyes were utilized. Others, however, have looked into the effect of mediator variation on DSSC efficiency [20–26]. To avoid flammable, poisonous, and volatile organic solvents, combinations of such solvents with water were tried to increase the DSSCs' stability and efficiency. Various organic solvents and their mixtures have been investigated for this purpose [15, 27, 28]. Aqueous-based dye sensitized solar cells are gaining popularity due to their environmentally beneficial and low-cost characteristics. In order to improve the DSSC's efficiency and stability, aqueous-based sensitizers and electrolytes have recently been explored [29]. The effect of dilute binary solvent media consisting of dilute organic percentage and excess water on two potential photosensitizers to oxidize iodide is described in this chapter. Dicyanobis(2,2′-dipyridyl)iron(III) and dicyanobis(1,10-phenanthroline)iron(III) could be a potential replacement for ruthenium-based dyes due to their stability, solubility, and cost-effectiveness. Furthermore, unlike hazardous ruthenium-based chemicals, the most likely ironbased sensitizers are not environmentally detrimental. The rate constants for the reactions were calculated using 10 volume percent of tertiary butyl alcohol in water and an equal volume percent of 1,4-dioxane in water. When the rate constants were compared, it was discovered that such sensitizer-mediator interactions could have an impact on DSSC's efficiency.
