Oriented Triplet Excitons as Long-Lived Electron Spin Qutrits in a Molecular Donor–Acceptor Single Cocrystal

Abstract

The photogeneration of multiple unpaired electron spins within molecules is a promising route to applications in quantum information science because they can be initialized into well-defined, multilevel quantum states (S > 1/2) and reproducibly fabricated by chemical synthesis. However, coherent manipulation of these spin states is difficult to realize in typical molecular systems due to the lack of selective addressability and short coherence times of the spin transitions.Here, these challenges are addressed by using donor−acceptor single cocrystals composed of pyrene and naphthalene dianhydride to host spatially oriented triplet excitons, which exhibit promising photogenerated qutrit properties. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy demonstrates that spatially orienting triplet excitons in a single crystal platform imparts narrow, well-resolved, tunable resonances in the triplet EPR spectrum, allowing selective addressability of the spin sublevel transitions. Pulse-EPR spectroscopy reveals that at temperatures above 30 K, spin decoherence of these triplet excitons is driven by exciton diffusion. However, coherence is limited by electronic spin dipolar coupling below 30 K, where T 2 varies nonlinearly with the optical excitation density due to exciton annihilation. Overall, an optimized coherence time of T 2 = 7.1 μs at 20 K is achieved. These results provide important insights into designing solid-state molecular excitonic materials with improved spin qutrit properties.

Alternant Hydrocarbon Diradicals as Optically Addressable Molecular Qubits

“…These problems can be tackled with molecular spin systems because they can be synthesized from bottom-up and extended into macromolecular systems. − Metal complexes have seen the most advancements, particularly with transition metals. − By contrast, little progress has been made with fully organic molecules − which, being metal-free, will be more cost-efficient and sustainable than their metal-based counterparts. Pioneering efforts were made by Gorgon et al using excited organic radicals; however, a drawback of microwave spin manipulation in an excited electronic state is the limited lifetime of the latter.…”

Structure-enabled long-lived charge separation in single crystals of an asymmetric donor–acceptor perylenediimide cyclophane

“…Numerous investigations on these cocrystals have shed light on the mechanisms underlying CT exciton generation, evolution, and charge migration. 1–3,12–15 Notably, two predominant structural motifs—alternating DADA π-stacks and segregated DDDD and AAAA stacks—emerge from non-covalent interactions like π–π stacking, CT interactions, as well as halogen and hydrogen bonding. 1–3,13,14,16 The intimate electronic coupling facilitated by close D–A packing often results in distinctive CT absorption bands in their electronic spectra, enabling direct photoinduced formation of the CT state.…”

Correlation between Radical and Quartet State Coherence Times in Photogenerated Triplet–Radical Conjugates

“…Compared to spin qubit systems, multilevel qubits (so-called qudits), as realized in molecular systems with S ≥ 1, have the advantage that, theoretically, the number of iterations in quantum algorithms as well as the error rate can be decreased, and more complex gate operations can be performed in a single physical qudit without the need of interqubit communication. − In particular, quartet states, generated by photoexcitation of organic chromophore–radical systems, have been shown to exhibit properties that make them promising molecular spin qudit candidates for applications in quantum information science. Pulse electron paramagnetic resonance (EPR) experiments demonstrate that they are long-lived and are characterized by spin coherence times that are longer than typical pulse microwave-driven gate operation times, even at moderate temperatures of 80 K. ,, …”