Investigation of Hyperfine Interactions in Molecular Spin Qubits Constructed from A Nitronyl-Nitroxide Ligand and Transition Metal Ions

The qubit behavior of two S = 1/2 heterospin complexes with the general formula (Et₃NH)[M(hfac)₂L] has been investigated by pulse EPR methods (M = Zn (1) and Ni (2), hfac- is the coligand hexafluoroacetylacetonate and L- is the deprotonated nitrophenol-substituted NIT radical 2-(2-hydroxy-3-methoxy-5-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3-oxide-1-oxyl). Robust quantum coherence is observed in both compounds. At 100 K, 1 shows a longer phase memory time, Tm (0.9 μs) than 2 (0.12 μs), while at very low temperatures, the opposite is true (1: 1.78 μs at 5.2 K; 2: 3.7 μs at 5.5 K). With CPMG detection, longer Tm up to18 μs (1) and 7 μs (2) at 5 K is measured. The spin-lattice relaxation time (T₁) is also longer for 1 than for 2, due to strong spin-orbit coupling in the latter. HYSCORE and ENDOR investigations quantified the hyperfine couplings to ¹⁹F, ¹H, ⁶⁷Zn, and ¹⁴N, providing clear insights into the low temperature decoherence paths in the two qubits.