**Abstract**

This chapter discusses the catalytic effect of extended π-conjugation on the electron transfer process between ferricyphen-ferrocyanide and ferricypyr-ferrocyanide in an aqueous medium. Ferricyphen and ferricypyr may be feasible options for the sensitizer in dye-sensitized solar cells due to their high reduction potential, stability, capability as an outer-sphere oxidant, and photosensitivity. Meanwhile, ferrocyanide could be used as a mediator in DSSCs instead of iodide to avoid iodate production and achieve a similar reduction potential and stability. This chapter compared the ability of competent putative sensitizers to oxidize the likely mediator in water. In contrast to the 2,2<sup>0</sup> -dipyridyl chelate, the extended π-conjugation in 1,10-phenanthroline accelerated the redox process by increasing the electron affinity of ferricyphen as compared to ferricypyr. The reactions had the same kinetics but different rate constants, indicating that the ferricyphen-ferrocyanide reaction was several times faster than the ferricypyr-ferrocyanide reaction, revealing and confirming the catalytic influence of extended π-conjugation on the redox process.

**Keywords:** pi-conjugation, ferricyphen, ferricypyr, ferrocyanide, catalysis, kinetics, aqueous medium
