In an effort to develop tris-diimine Fe^II complexes displaying spin crossover properties in solution at room temperature, several asymmetric didentate units(benzimidazole-azines) were investigated. By changing the number of nitrogen atoms or their position within the 6-membered N-heterocyclic part of the ligands, or through steric programming of metal-ligand overlap, the spin transition can be finely tuned and rationalised through ligand field considerations. Fe^II complexes with unsymmetrical diimine ligands exist as a mixture of facial and meridional isomers in solution for which different magnetic properties were characterised thermodynamically to appreciate minute differences arising from the flipping of one ligand in the coordination sphere. Further, the introduction of a tridentate nucleating site by extension of the benzimidazole-azine ligands allows the coordination of a trivalent lanthanide, with the subsequent Eu-M triple helical assembly showing differential evolution in magnetic susceptibility with Fe^II and Zn^II in solution and solid-state. Finally, the emission of the luminescent Eu^III ion in the assembly could be quenched by the SCO-Fe^II, with its different spin-states acting as acceptors of different spectral overlaps, thus quenching efficiencies allowing Eu^III to be employed as a luminescent spin-state readout.