EPR and DEER Characterization of New Mixed Weakly Coupled Nitroxide Triradicals for Molecular Three-Spin Qubits

Three new mixed triradicals with small exchange coupling parameters (J ≪ A _N ) were obtained on the base of a coupling reaction between the derivative of spirofused 2,5-dihydroimidazol-type monoradical and the two mol. equivalents of carboxylic acid derivatives of PROXYL-, TEMPO- or 2,5-dihydro-1H-pyrrol-type nitroxides. Their intramolecular magnetic interactions were characterized in terms of comparison of the CW X, Q- and W-band EPR spectra with those of the monoradical precursors. The dipole–dipole coupling parameters of the triradicals were estimated on the base of the quantum chemical calculations at UB3LYP/6-31G(d) level of theory. Two types of the spin distances were found in the triradicals: short—with the distance of 13–17 Å (D ≈ 11–24 MHz) and long—with the distance of 21–23 Å (D ≈ 4–6 MHz). The longest spin–spin as well as spin–lattice relaxation times at 50 K were detected for the triradical carrying the two TEMPO fragments, indicating the potential usage of three-spin qubit models for quantum gate operations.