Molecular Cocrystal Packing Suppresses Hopping-Driven Decoherence of Excitonic Spin Qubits.

Molecular excitonic spins have garnered significant interest for quantum technologies because they can be initialized into addressable, multilevel quantum states through spin-selective intersystem crossing or singlet fission. However, excitonic spin coherence is difficult to maintain above liquid helium temperatures due to typical crystal packings, which promote decoherence through exciton hopping between magnetically inequivalent sites. Here, we engineer donor-acceptor cocrystals where molecular packing in isolated π-stacks of magnetically equivalent molecules suppresses hopping-induced decoherence. Pulse-electron paramagnetic resonance spectroscopy reveals that high-temperature spin coherence in this packing geometry is instead strongly influenced by mutual spin flip-flops between interacting excitons. Coherence anisotropy measurements indicate that spin-phonon coupling enhances the rate of spin flip-flops through dynamic reorientation of the zero field splitting tensor. As a result, coherence times decrease exponentially at elevated temperatures, with coherence times measurable up to 150 K. The combined results present generalized design strategies to preserve excitonic spin coherence at high temperatures.