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.