The intense absorption at λmax = 992 nm (10,080 cm -1 ), ε = 35 M -1 cm -1 , and the weaker absorption at λmax = 873 nm (11,460 cm -1 ), ε = 15 M -1 cm -1 , are 2 F7/2 → 2 F5/2 transitions belonging to 4-Yb, split by a 2 F5/2 CF of over 1,000 cm - 1 . The energies of the f-f transitions in 2-Ln and 4-Ln are similar, indicating that the ligand fields are of comparable strengths in the two series of complexes. Tm and 4-Tm. For 2-Yb, a broad S = ½ spectrum was observed, including a g1 feature with hyperfine structure at 90 mT and g2 and g3 features around 1150-1450 mT; the spectrum was overlaid with a sharp S = ½ feature, with hyperfine structure and centered at 334.5 mT, characteristic of a TEMPO • radical (Figure ). In Kramers ions such as Yb 3+ , all states are doubly degenerate, so the ground doublet can be considered as an effective spin ½ with three anisotropic g-values (g1 > g2 > g3). The spectrum was simulated in EasySpin,