Ligand modifications with electron-withdrawing and electron-donating groups were applied to afford three novel mononuclear cobalt-based catalysts [Co(TPA-R)](2+) (TPA = tris(2-pyridylmethyl) amine; R = tri-alpha F, 1; R = tri-alpha OMe, 2; R = mono-alpha F, 3) for water oxidation. Characterization of the catalysts shows that steric and electronic factors play important roles in inhibiting spontaneous intermolecular dimerization of two cobalt centers, and influence the catalytic behavior. Complex 1 exhibits the best catalytic ability and stability, showing a good efficiency with TOF of 6.03 +/- 0.02 mol (O-2)/(mol (cat)*s) in photo induced water oxidation experiments using Ru (bpy)(2/3)(+) as photosensitizer and Na2S2O8 as electron acceptor. The bulky electron donating groups in 2 led to degradation of the complex and formation of CoO. particles acting as the real catalyst. Electron-withdrawing substituents on the TPA ligand can stabilize the catalyst under both electrochemical and photo-induced conditions, with the enhancement increasing with the number of the electron-withdrawing groups. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.