**3. Catalytic effect of extended pi-conjugation**

The comparative analysis of both of the sensitizer-mediator redox reactions revealed the catalytic effect of pi-conjugation on the rate of reaction. The second order rate constant (*k*2) in case of ferricyphen-ferrocyanide has a greater value i.e.,

*Catalytic Behavior of Extended π-Conjugation in the Kinetics of Sensitizer-Mediator… DOI: http://dx.doi.org/10.5772/intechopen.106511*

1620 M�<sup>1</sup> s �<sup>1</sup> as compared to the ferricypyr-ferrocyanide that is 1050 M�<sup>1</sup> s �1 . The kinetic analysis of both of the reactions helped to identify the similar reaction mechanism of electron transfer between each sensitizer-mediator pair with a high rate in case of ferricyphen-ferrocyanide as compared to ferricypyr-ferrocyanide under similar experimental conditions. The sensitizer such as ferricyphen has extended π-conjugation in its structure that distributes the electron density over the entire chelate molecule while stabilizing it. As a result, in comparison to the 2,2<sup>0</sup> -dipyridyl chelate, the availability of the lone pair of electrons on the nitrogen atoms in the 1,10 phenanthroline chelate is reduced (**Figure 5**). The chelate such as 2,2<sup>0</sup> -dipyridyl does not carry an extended π-conjugation in its structure though its nitrogen atoms have high density of the lone pair of electrons that can be coordinated by the Fe(III) atom of ferricypyr relatively easily. However, in instance of 1,10-phennathroline the extended π-conjugation of the structure reduces the density of the lone electrons on the nitrogen atoms and therefore availability to Fe(III) in ferricyphen. As a result, in order to circumvent this electron deficiency, the electron affinity of Fe(III) in ferricyphen increases as compared to ferricypyr, which helps to enhance its electron accepting potential as compared to ferricypyr. Consequently, a catalytic effect of extended π-conjugation is observed in the reduction kinetics of the ferricyphen as compared to ferricypyr by ferrocyanide in aqueous medium.
