When molecular dimers, crystalline films or molecular aggregates absorb a photon to produce a singlet exciton, spin-allowed singlet fission may produce two triplet excitons that can be used to generate two electron–hole pairs, leading to a predicted ∼50% enhancement in maximum solar cell performance. The singlet fission mechanism is still not well understood. Here we report on the use of time-resolved optical and electron paramagnetic resonance spectroscopy to probe singlet fission in a pentacene dimer linked by a non-conjugated spacer. We observe the key intermediates in the singlet fission process, including the formation and decay of a quintet state that precedes formation of the pentacene triplet excitons. Using these combined data, we develop a single kinetic model that describes the data over seven temporal orders of magnitude both at room and cryogenic temperatures.

Silicon-based solar cells are approaching the thermodynamic limit of efficiency (Shockley-Queisser limit). Simultaneously, fossil fuels are strongly linked to climate changes. Consequently, new approaches are necessary to satisfy the world's steadily increasing energy demand. Singlet fission (SF) is a process overcoming the core assumptions that Shockley and Queisser postulated for their calculations: it is predicted to generate two charges per photon rather than only one! Basel et al. provide evidence for a charge-transfer-mediated mechanism of SF in a nonconjugated, rigid pentacene dimer.

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 understanding of the catalyst−support interactions has been an important challenge in heterogeneous catalysis since the supports can play a vital role in controlling the properties of the active species and hence their catalytic performance. Herein, a series of isostructural mesoporous metal−organic frameworks (MOFs) based on transition metals, lanthanides, and actinides (Zr, Hf, Ce, Th) were investigated as supports for a vanadium catalyst. The vanadium species was coordinated to the oxo groups of the MOF node in a single-ion fashion, as determined by single-crystal X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy, and diffuse reflectance UV−vis spectroscopy. The support effects of these isostructural MOFs were then probed using the aerobic oxidation of 4-methoxybenzyl alcohol as a model reaction. The turnover frequency was found to be correlated with the electronegativity and oxidation state of the metal cations on the supporting MOF nodes, highlighting an important consideration when designing catalyst supports.

Metal–organic frameworks enable the synthesis of arrays of atomically precise qubits.