Using Molecular Design to Engineer Solid-State Spin Qubits in Organic Semiconductors

Molecular spin qubits promise to enable breakthroughs in quantum information science due to the ability of chemistry to (1) create atomically precise structures using synthetic methodologies; (2) tailor the magnetic, optical, and electronic properties of the molecular qubits; and (3) construct large-scale ordered arrays of qubits via crystal growth and self-assembly. Here, we present our work on engineering atomically precise hierarchical systems with tunable quantum states to serve as spin qubit arrays.<br/> <br/>First, we engineer spin qubits based on high-spin multiexcitons photogenerated in organic semiconductors using singlet fission.¹ We engineer the packing of tetracene molecules within single crystals by using tailored linking groups to demonstrate multiexcitons that exhibit promising spin qubit properties, including a coherence time, <i>T</i>_m, of 3 μs at 10 K, a population lifetime, <i>T</i>_pop, of 130 μs at 5 K, and stability even at room temperature. The single-crystal platform also enables global alignment of the spins and, consequently, individual addressability of the spin-sublevel transitions. Decoherence mechanisms, including exciton diffusion, electronic dipolar coupling, and nuclear hyperfine interactions, are elucidated, providing design principles for increasing the spin coherence lifetime and operational temperature of the multiexciton spin qubits. By dynamically decoupling the qubits from the surrounding spin bath, <i>T</i>_m of 10 μs is achieved. This material system provides an exciting path to realize dense arrays of optically addressable qubits that are generated on demand at specific locations.<br/> <br/>Second, we harness a new class of organic framework materials, known as ion-paired frameworks, to develop atomically precise arrays of molecular spin qubits.² We learn how to control the density of the paramagnetic Cu(II) porphyrin spins by engineering the linker groups and crystal growth conditions. Pulse-electron paramagnetic resonance (EPR) spectroscopy is used to probe the spin coherence of these single crystals at temperatures up to 140 K. The crystals with the longest Cu−Cu distances exhibit a spin coherence time, <i>T</i>_m, of 207 ns and a spin−lattice relaxation time, <i>T</i>₁, of 1.8 ms at 5 K, which are records for qubits in an atomically precise molecular system. The mechanisms of spin decoherence and spin-lattice relaxation will also be discussed. Incorporating molecular electronic spin qubits in ion-paired frameworks enables control of composition, spacing, and interqubit interactions, providing a rational means to extend spin relaxation times for next-generation quantum technologies.<br/> <br/>[1] R. M. Jacobberger, et al. <i>J. Am. Chem. Soc.</i> <b>144</b>, 2276-2283 (2022).<br/>[2] C. M. Moisanu*, R. M. Jacobberger*, L. P. Skala*, et al. <i>J. Am. Chem. Soc.</i> <b>145</b>, 18447-18454 (2023).