Ligand Effects on the Spin Relaxation Dynamics and Coherent Manipulation of Organometallic La(II) Potential Qudits

We present pulsed electron paramagnetic resonance (EPR) studies on three La(II) complexes, [K(2.2.2-cryptand)][La(Cp′) ₃] (1), [K(2.2.2-cryptand)][La(Cp″) ₃] (2), and [K(2.2.2-cryptand)][La(Cp ^tt) ₃] (3), which feature cyclopentadienyl derivatives as ligands [Cp′ = C ₅H ₄SiMe ₃; Cp″ = C ₅H ₃(SiMe ₃) ₂; Cp ^tt = C ₅H ₃(CMe ₃) ₂] and display a C ₃ symmetry. Long spin-lattice relaxation (T ₁) and phase memory (T _m) times are observed for all three compounds, but with significant variation in T ₁ among 1-3, with 3 being the slowest relaxing due to higher s-character of the SOMO. The dephasing times can be extended by more than an order of magnitude via dynamical decoupling experiments using a Carr-Purcell-Meiboom-Gill (CPMG) sequence, reaching 161 μs (5 K) for 3. Coherent spin manipulation is performed by the observation of Rabi quantum oscillations up to 80 K in this nuclear spin-rich environment ( ¹H, ¹³C, and ²⁹Si). The high nuclear spin of ¹³⁹La (I = 7/2), and the ability to coherently manipulate all eight hyperfine transitions, makes these molecules promising candidates for application as qudits (multilevel quantum systems featuring d quantum states; d >2) for performing quantum operations within a single molecule. Application of HYSCORE techniques allows us to quantify the electron spin density at ligand nuclei and interrogate the role of functional groups to the electron spin relaxation properties.