An obvious way to modify the electron density of a given molecular system is to introduce electron withdrawing (EW) and donating (ED) substituents. Such substitutions can be used to tailor the functioning of transition metal complexes for applications in photocatalysis, light-harvesting, luminescence, or spin crossover. Several recent studies investigated the mechanism by which electron withdraw-ing and donating substituents alter the electronic structure of pyridine-type ligands in spin-transition systems, either by shifting the spin-state equilibrium, or by modifying the Fe(II) singlet-quintet energy gap. Strong substituent effects for γ-substituted pyridine have been demonstrated by Halcrow and colleagues. Carefully selected substituents can be powerful aids for redesigning the potential energy surfaces (PESs) and thereby optimizing functionality. We capitalized on this in our previous study where we applied substitution to the tridentate terpyridine ligand in the 4' position to systematically modify the quintet lifetime of their homoleptic Fe(II) complexes by changing the relevant energy barriers. We theoretically examined how the potential energy surfaces, and in consequence the lifetime of the excited quintet state of [Fe(terpy)2] 2+ (terpy = 2,2':6',2"terpyridine) can be modified upon substitution with electron donating (ED) or withdrawing (EW) groups on the central pyridine (Py) ring in the 4' (γ) position (Scheme 1). The theoretical predictions were confirmed by transient optical absorption spectroscopy. The central theme of the present paper is to extend this study to a larger set of substituents to achieve the fine-tuning of the quintet lifetime. . The axial N atoms, which will be often referred to as Nax in the text, are in green.