Slow Electron Spin Relaxation at Ambient Temperatures with Copper Coordinated by a Rigid Macrocyclic Ligand

Area of Science:

• Quantum computing and sensing
• Materials science
• Coordination chemistry

Background:

• Paramagnetic transition metal complexes are promising quantum bits but suffer from decoherence and require low temperatures.
• Improving longitudinal relaxation time (T1) is crucial for enhancing phase coherence time (Tm).
• Existing strategies focus on reducing vibrational coupling and orbital angular momentum, with limited synthetic approaches for T1 improvement.

Purpose of the Study:

• To synthesize novel ligand frameworks for room-temperature quantum spin coherence.
• To investigate the impact of ligand design on the relaxation properties of Cu(II) spin centers.
• To develop a practical platform for designer macrocyclic ligands supporting coherent spin centers.

Main Methods:

• Synthesis of a modular macrocyclic ligand framework with four nitrogen donors (N4) based on phenanthroline.
• Characterization of the resulting Cu(II) complexes.
• Measurement of spin coherence properties, including longitudinal relaxation time (T1) and coherence time (Tm) at room temperature.

Main Results:

• A new N4 macrocyclic ligand framework supporting room-temperature coherent Cu(II) spin centers was synthesized.
• The optimized complex demonstrated more than double the T1 compared to the best previous Cu(II)-N4 compound.
• A room temperature coherence time (Tm) of 0.28 μs was achieved, approaching previously reported values.

Conclusions:

• The designer macrocyclic ligand platform enables significant enhancement of spin coherence at room temperature.
• Short Cu-N distances and a strong ligand field reduce vibrational coupling, improving T1 relaxation.
• This approach is critical for developing room-temperature quantum bits and biological sensors.