Electrochemical studies of a series of substituted phenanthroline-Co(II) complexes all show generally similar behaviour, namely a chemically and electrochemically reversible Co-III/II redox couple, as well as a chemically and electrochemically reversible Co-II/I redox couple, followed by a ligand-based reduction. Electron donating- or -withdrawing substituents on the phenanthroline ligands which are coordinated to the Co metal, directly influence the electron density on the Co metal, due to good communication between these substituents and the Co metal via the aromatic rings of the heterocyclic substituted phenanthroline-Co(II) complexes, leading to either more negative (for electron donating groups) or more positive (for electron withdrawing groups) redox potentials respectively. Linear relationships relating E degrees'(Co-III/II) oxidation and E degrees'(Co-II/I) reduction to various experimental and empirical values, as well as to theoretically calculated energies, show that the electron density on Co is linearly influenced by the electronic properties of the ligands attached to the Co metal. All these established relationships can be used in the design of new substituted phenanthroline-Co(II) complexes with specific customized redox properties as required, for example, for the application of such Co(II) complexes as redox mediator for dye-sensitized solar cells. (C) 2018 Elsevier Ltd. All rights reserved.