The χT value determined at 300 K under a 0.1 T dc field (14.75 cm 3 K mol -1 , Figure ) is slightly higher than that determined at 298 K in fluorobenzene solution (12.97 cm 3 K mol -1 , Figure ) and the expected Dy(III) free ion value (14.17 cm 3 K mol -1 ). We observe a regular decrease in χT with decreasing temperature as excited CF states are thermally depopulated until ca. 10 K where there is a sharper decrease, reaching χT = 7.71 cm 3 K mol -1 at 2 K (Figure ); zero field-cooled (ZFC) and field-cooled (FC) data collected in a smaller 0.001 T or 0.005 T dc field show that this drop is mainly due to Zeeman depopulation effects, dropping only to χT = 11.3 cm 3 K mol -1 at 1.85 K (Figures and). Magnetization vs. field experiments show that the magnetization saturates at Msat = 5.42 μB under a 7 T applied dc field (Figure ), suggesting an mJ = ±15/2 ground state (Msat = 5.00 μB). Rapid quantum tunneling of magnetization (QTM) at zero field is also evident from the M vs. H hysteresis loops (Figure ), where 1-Dy shows a waistrestricted loop that is typical for Ln SMMs, and is closed at zero field at 2 K (Figure ). Ac susceptibility measurements of polycrystalline 1-Dy were performed up to 10 kHz to study the slow magnetic relaxation. Temperature-and frequency-dependent behavior were seen for the in-phase (χ') and out-of-phase (χ") components of ac susceptibility in zero dc field, with maxima in χ" due to slow relaxation of the magnetization present between 2 and 110 K (Figures and, Table ). The ac data were fit using the generalized Debye model to extract relaxation rates along with estimated standard deviations (ESDs) as a function of temperature. The temperature-dependence of the magnetic relaxation rates suggest Orbach relaxation at high temperatures, Raman-I relaxation at intermediate temperatures, and QTM at the lowest temperatures (Figure ). The temperature-dependent relaxation rates corresponding to these processes were modelled using Eqn 1 in CC-FIT2 5.6.4, giving: Ueff = 945 (24) K, τ0 = 10 -8.53 s, C = 10 -4.0(2) s -1 K -n , n = 3.9(1), 𝜏 𝑄𝑇𝑀 = 10 -1.00 s (Figure ); note that TB is not defined as τ < 100 s at all temperatures.