A review focusing on phase change materials for thermal energy storage, particularly their nanoencapsulation, and insight into future research possibilities.

Quantum information processing (QIP) offers the potential to create new frontiers in fields ranging from quantum biology to cryptography. Two key figures of merit for electronic spin qubits, the smallest units of QIP, are the coherence time (T), the lifetime of the qubit, and the spin-lattice relaxation time (T), the thermally defined upper limit of T. To achieve QIP, processable qubits with long coherence times are required. Recent studies on (PhP-d)[V(CS)], a vanadium-based qubit, demonstrate that millisecond T times are achievable in transition metal complexes with nuclear spin-free environments. Applying these principles to vanadyl complexes offers a route to combine the previously established surface compatibility of the flatter vanadyl structures with a long T. Toward those ends, we investigated a series of four qubits, (PhP)[VO(CS)] (1), (PhP)[VO(β-CS)] (2), (PhP)[VO(α-CS)] (3), and (PhP)[VO(CSO)] (4), by pulsed electron paramagnetic resonance (EPR) spectroscopy and compared the performance of these species with our recently reported set of vanadium tris(dithiolene) complexes. Crucially we demonstrate that solutions of 1-4 in SO, a uniquely polar nuclear spin-free solvent, reveal T values of up to 152(6) μs, comparable to the best molecular qubit candidates. Upon transitioning to vanadyl species from the tris(dithiolene) analogues, we observe a remarkable order of magnitude increase in T, attributed to stronger solute-solvent interactions with the polar vanadium-oxo moiety. Simultaneously, we detect a small decrease in T for the vanadyl analogues relative to the tris(dithiolene) complexes. We attribute this decrease to the absence of one nuclear spin-free ligand, which served to shield the vanadium centers against solvent nuclear spins. Our results highlight new design principles for long T and T times by demonstrating the efficacy of ligand-based tuning of solute-solvent interactions.

The realization of quantum information processing would disrupt the status quo in the realm of computation; the extraordinary power of a hypothetical quantum computer motivates significant research efforts toward creating such a device. One promising route to enable quantum information processing involves employing electronic spins as the elementary unit of information, known as a qubit. Within this paradigm, paramagnetic defect sites in solid-state materials demonstrate appreciable promise, and recent developments in paramagnetic molecular coordination complexes illustrate an encouraging trajectory. While solid-state systems exhibit long spin coherence lifetimes, rational control of their properties remains challenging. Effecting synthetic control over qubit design prompted the study of tunable molecular species to develop design principles for spin coherence lifetimes. The challenge now lies in extending those molecular design principles to target new solid-state architectures that could enable device-scale systems. In this perspective, we detail recent progress in the rational design of molecular qubit complexes and highlight the advances that will be necessary in order to apply that progress to solid-state systems. We further examine the impact that the lessons learned from the study of qubits can have in the related fields of magnetic resonance imaging and biological sensing.

Enabling the rational synthesis of molecular candidates for quantum information processing requires design principles that minimize electron spin decoherence. Here we report a systematic investigation of decoherence via the synthesis of two series of paramagnetic coordination complexes. These complexes, [M(C2O4)3](3-) (M = Ru, Cr, Fe) and [M(CN)6](3-) (M = Fe, Ru, Os), were prepared and interrogated by pulsed electron paramagnetic resonance (EPR) spectroscopy to assess quantitatively the influence of the magnitude of spin (S = (1)/2, (3)/2, (5)/2) and spin-orbit coupling (ζ = 464, 880, 3100 cm(-1)) on quantum decoherence. Coherence times (T2) were collected via Hahn echo experiments and revealed a small dependence on the two variables studied, demonstrating that the magnitudes of spin and spin-orbit coupling are not the primary drivers of electron spin decoherence. On the basis of these conclusions, a proof-of-concept molecule, [Ru(C2O4)3](3-), was selected for further study. The two parameters establishing the viability of a qubit are a long coherence time, T2, and the presence of Rabi oscillations. The complex [Ru(C2O4)3](3-) exhibits both a coherence time of T2 = 3.4 μs and the rarely observed Rabi oscillations. These two features establish [Ru(C2O4)3](3-) as a molecular qubit candidate and mark the viability of coordination complexes as qubit platforms. Our results illustrate that the design of qubit candidates can be achieved with a wide range of paramagnetic ions and spin states while preserving a long-lived coherence.

Nuclear-electronic interactions are a fundamental phenomenon which impacts fields from magnetic resonance imaging to quantum information processing (QIP). The realization of QIP would transform diverse areas of research including accurate simulation of quantum dynamics and cryptography. One promising candidate for the smallest unit of QIP, a qubit, is electronic spin. Electronic spins in molecules offer significant advantages with regard to QIP, and for the emerging field of quantum sensing. Yet relative to other qubit candidates, they possess shorter superposition lifetimes, known as coherence times or T, due to interactions with nuclear spins in the local environment. Designing complexes with sufficiently long values of T requires an understanding of precisely how the position of nuclear spins relative to the electronic spin center affects decoherence. Herein, we report the first synthetic study of the relationship between nuclear spin-electron spin distance and decoherence. Through the synthesis of four vanadyl complexes, (PhP)[VO(CHS)] (1), (PhP)[VO(CHS)] (2), (PhP)[VO(CHS)] (3), and (PhP)[VO(CHS)] (4), we are able to synthetically place a spin-laden propyl moiety at well-defined distances from an electronic spin center by employing a spin-free carbon-sulfur scaffold. We interrogate this series of molecules with pulsed electron paramagnetic resonance (EPR) spectroscopy to determine their coherence times. Our studies demonstrate a sharp jump in T when the average V-H distance is decreased from 6.6(6) to 4.0(4) Å, indicating that spin-active nuclei sufficiently close to the electronic spin center do not contribute to decoherence. These results illustrate the power of synthetic chemistry in elucidating the fundamental mechanisms underlying electronic polarization transfer and provide vital principles for the rational design of long-coherence electronic qubits.