Chemical control of spin–lattice relaxation to discover a room temperature molecular qubit

The second quantum revolution harnesses exquisite quantum control for a slate of diverse applications including sensing, communication, and computation. Of the many candidates for building quantum systems, molecules offer both tunability and specificity, but the principles to enable high temperature operation are not well established. Spin–lattice relaxation, represented by the time constant T₁, is the primary factor dictating the high temperature performance of quantum bits (qubits), and serves as the upper limit on qubit coherence times (T₂). For molecular qubits at elevated temperatures (>100 K), molecular vibrations facilitate rapid spin–lattice relaxation which limits T₂ to well below operational minimums for certain quantum technologies. Here we identify the effects of controlling orbital angular momentum through metal coordination geometry and ligand rigidity via π-conjugation on T₁ relaxation in three four-coordinate Cu²⁺ S = ½ qubit candidates: bis(N,N′-dimethyl-4-amino-3-penten-2-imine) copper(II) (Me₂Nac)₂ (1), bis(acetylacetone)ethylenediamine copper(II) Cu(acacen) (2), and tetramethyltetraazaannulene copper(II) Cu(tmtaa) (3). We obtain significant T₁ improvement upon changing from tetrahedral to square planar geometries through changes in orbital angular momentum. T₁ is further improved with greater π-conjugation in the ligand framework. Our electronic structure calculations reveal that the reduced motion of low energy vibrations in the primary coordination sphere slows relaxation and increases T₁. These principles enable us to report a new molecular qubit candidate with room temperature T₂ = 0.43 μs, and establishes guidelines for designing novel qubit candidates operating above 100 K.