A complete set of the symmetrical bidiazine (bdz) and asymmetrical pyridyldiazine (pydz) ligands, along with the known 2,2′‐bipyridyl (bpy) ligand, and their respective rhenium(I)‐tricarbonyl‐bromo and ‐thiocyonato complexes are presented. A bathochromic shift is observed with an increasing number of nitrogen atoms, caused by a stabilization of the diimine based LUMO. As expected from the energy gap law, this results in an increase in the non‐radiative decay constant (knr) along the line bpy‐pydz‐bdz. Interestingly, the increase in knr for both pyridazine ligands (2‐(pyridin‐2‐yl)pyridazine (pypdz) and 3,3′‐bipyridazine (bpdz)) is much less pronounced as compared to the other pydz resp. bdz ligands. This results in overall longer lifetime and quantum yield, at the increased spectral overlap with sunlight – both properties sought for in the creation of light‐harvesting schemes in photochemistry.