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

The qubit behavior of two <i>S</i> = 1/2 heterospin complexes with the general formula (Et₃NH)[M(hfac)₂L] has been investigated by pulse EPR methods (M = Zn (<b>1</b>) and Ni (<b>2</b>), hfac^<i>-</i> is the coligand hexafluoroacetylacetonate and L^<i>-</i> is the deprotonated nitrophenol-substituted NIT radical 2-(2-hydroxy-3-methoxy-5-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1<i>H</i>-imidazol-3-oxide-1-oxyl). Robust quantum coherence is observed in both compounds. At 100 K, <b>1</b> shows a longer phase memory time, <i>T</i>_m (0.9 μs) than <b>2 (</b>0.12 μs), while at very low temperatures, the opposite is true (<b>1:</b> 1.78 μs at 5.2 K; <b>2</b>: 3.7 μs at 5.5 K). With CPMG detection, longer <i>T</i>_m up to18 μs (<b>1</b>) and 7 μs (<b>2</b>) at 5 K is measured. The spin-lattice relaxation time (<i>T</i>₁) is also longer for <b>1</b> than for <b>2</b>, 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.