The rational design of molecular chains made of 4f ions and substituted 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical is presented. The reaction of Ln(hfac)·2HO (hfac = hexafluoroacetylacetonate) and the 4-cyano-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO-CN) radical affords air- and moisture-stable isostructural molecular chains of the formula [Ln(hfac)TEMPO-CN] for Ln = Gd and Tb, whereas zero-dimensional complexes of the formula [Dy(hfac)(TEMPO-CN)][Dy(hfac)(HO)] are obtained for Ln = Dy. To the best of our knowledge, the Gd derivative, Gd-TEMPO-CN, shows one of the strongest antiferromagnetic (AF) couplings for Gd-radical pairs ever reported with J/ k = -21.18 K, 14.72 cm ( H = - JS S spin Hamiltonian convention). The Tb derivative, Tb-TEMPO-CN, also shows strong Tb-radical AF coupling, which has been rationalized using the ab initio CASSCF approach ( J = -23.02 K, -16.7 cm) and confirmed by luminescence measurements. Tb-TEMPO-CN shows remarkable properties for a Tb-radical-based single-molecule magnet ( U = 69.3 ± 1 K; τ = 1.3 × 10 s) and two different relaxation processes triggered by interchain magnetic coupling.