This paper reports the synthesis, characterization and spectroscopic and electrochemical study of a series of polypyridine Co(II) and Co(III) complexes. The effect on the redox potential of the presence of donor or acceptor substituents on the ligand, as well as the effect of an increased ligand aromaticity, was analyzed. Specifically, complexes of [Co(L-L)3]11+ type were prepared, with n = 2,3, and L-L = 1,10-phenanthroline (phen), 5-amino-1,10'-phenanthroline (phen-NH2), 3,4,7,8-tatramethyl phenantroline (phen-4Me), pyrazino[2,3- ][1,10]phenanthroline (ppl), 2-methylpyrazino[2,3- ][1,10]phenanthroline (ppl-Me), dipyrido[3,2-a:2',3'-c]phenazine (dppz), 7-methyldipyrido[3,2-a:2',3'-c]phenazine (dppz-Me) and 7-aminodipyrido[3,2-a:2',3'-c]phenazine (dppz-NH2). The results of this study open the possibility to have a battery of potential mediators for photoelectrochemical solar cells. Ultimately, the election of a determined couple as mediator will depend mainly on the electronic properties of the specific dye in a given cell.