Xavier Roy

Quantitative Intramolecular Singlet Fission in Bipentacenes

Singlet fission (SF) has the potential to significantly enhance the photocurrent in single-junction solar cells and thus raise the power conversion efficiency from the Shockley-Queisser limit of 33% to 44%. Until now, quantitative SF yield at room temperature has been observed only in crystalline solids or aggregates of oligoacenes. Here, we employ transient absorption spectroscopy, ultrafast photoluminescence spectroscopy, and triplet photosensitization to demonstrate intramolecular singlet fission (iSF) with triplet yields approaching 200% per absorbed photon in a series of bipentacenes. Crucially, in dilute solution of these systems, SF does not depend on intermolecular interactions. Instead, SF is an intrinsic property of the molecules, with both the fission rate and resulting triplet lifetime determined by the degree of electronic coupling between covalently linked pentacene molecules. We found that the triplet pair lifetime can be as short as 0.5 ns but can be extended up to 270 ns.

Nanoscale Atoms in Solid-State Chemistry

Ionic Materials via Charged Clusters The formation of salts from atomic and small molecular ions could in principle be replicated with larger inorganic clusters. However, many clusters are stabilized by organic ligands that create a barrier for charge transfer reactions to create ions. Roy et al. (p. 157, published online 6 June; see the Perspective by Batail) now report that chromium, cobalt, and nickel selenide and telluride clusters form materials by charge transfer with C60. The Co and Cr clusters formed a layered structure analogous to CdI2, while the Ni cluster formed a structure related to NaCl. Inorganic clusters combine with C60 to form layers and three-dimensional ionic materials through charge transfer. [Also see Perspective by Batail] We describe a solid-state material formed from binary assembly of atomically precise molecular clusters. [Co6Se8(PEt3)6][C60]2 and [Cr6Te8(PEt3)6][C60]2 assembled into a superatomic relative of the cadmium iodide (CdI2) structure type. These solid-state materials showed activated electronic transport with activation energies of 100 to 150 millielectron volts. The more reducing cluster Ni9Te6(PEt3)8 transferred more charge to the fullerene and formed a rock-salt–related structure. In this material, the constituent clusters are able to interact electronically to produce a magnetically ordered phase at low temperature, akin to atoms in a solid-state compound.

Prussian blue nanocontainers: selectively permeable hollow metal-organic capsules from block ionomer emulsion-induced assembly.

Hollow polymer-based particles are useful for the encapsulation, protection, and release of active compounds. Adding a metal-organic coordination framework shell to nanocontainers is an attractive goal because it should help control their stability and permeability while yielding new properties and functions. We have discovered that polymer capsules with a Prussian blue analogue inner shell can be synthesized by emulsion-induced assembly of a metal-containing amphiphilic block ionomer. The capsules are selectively permeable and were used as nanocontainers to encapsulate and release a model compound. Further, these nanomaterials are tunable in size and organize into 2-D close-packed arrays in the solid state. Potential applications for these materials include the encapsulation and nanopatterning of pharmaceutical, biological, and catalytic compounds.

Coordination Chemistry: New Routes to Mesostructured Materials

Although micro- and mesoporous materials have each been extensively studied separately, very little work has been done to combine both porosity ranges into a single material. Materials with bimodal porosity could have attractive properties for a wide range of applications and could open new areas of study. Here, we present a strategy to develop micro-mesoporous materials using coordination chemistry, and illustrate the concept with the formation of mesostructured Prussian blue analogues.

Orientational order controls crystalline and amorphous thermal transport in superatomic crystals

In the search for rationally assembled functional materials, superatomic crystals (SACs) have recently emerged as a unique class of compounds that combine programmable nanoscale building blocks and atomic precision. As such, they bridge traditional semiconductors, molecular solids, and nanocrystal arrays by combining their most attractive features. Here, we report the first study of thermal transport in SACs, a critical step towards their deployment as electronic, thermoelectric, and phononic materials. Using frequency domain thermoreflectance (FDTR), we measure thermal conductivity in two series of SACs: the unary compounds Co6E8(PEt3)6 (E = S, Se, Te) and the binary compounds [Co6E8(PEt3)6][C60]2. We find that phonons that emerge from the periodicity of the superstructures contribute to thermal transport. We also demonstrate a transformation from amorphous to crystalline thermal transport behaviour through manipulation of the vibrational landscape and orientational order of the superatoms. The structural control of orientational order enabled by the atomic precision of SACs expands the conceptual design space for thermal science.

Ferromagnetic ordering in superatomic solids.

In order to realize significant benefits from the assembly of solid-state materials from molecular cluster superatomic building blocks, several criteria must be met. Reproducible syntheses must reliably produce macroscopic amounts of pure material; the cluster-assembled solids must show properties that are more than simply averages of those of the constituent subunits; and rational changes to the chemical structures of the subunits must result in predictable changes in the collective properties of the solid. In this report we show that we can meet these requirements. Using a combination of magnetometry and muon spin relaxation measurements, we demonstrate that crystallographically defined superatomic solids assembled from molecular nickel telluride clusters and fullerenes undergo a ferromagnetic phase transition at low temperatures. Moreover, we show that when we modify the constituent superatoms, the cooperative magnetic properties change in predictable ways.

Limits of Carrier Diffusion in n-Type and p-Type CH3NH3PbI3 Perovskite Single Crystals

Using a combination of scanning photocurrent microscopy (SPCM) and time-resolved microwave conductivity (TRMC) measurements, we monitor the diffusion and recombination of photoexcited charges in CH3NH3PbI3 perovskite single crystals. The majority carrier type was controlled by growing crystals in the presence or absence of air, allowing the diffusion lengths of electrons (LDe–) and holes (LDh+) to be directly imaged with SPCM (LDe– = 10–28 μm, LDh+ = 27–65 μm). TRMC measurements reveal a photogenerated carrier mobility (μh + μe) of 115 ± 15 cm2 V–1 s–1 and recombination that depends on the excitation intensity. From the intensity dependence of the recombination kinetics and by accounting for carrier diffusion away from the point of photogeneration, we extract a second-order recombination rate constant (krad = 5 ± 3 × 10–10 cm3/s) that is consistent with the predicted radiative rate. First-order recombination at low photoexcited carrier density (knrp-type = 1.0 ± 0.3 × 105 s–1, knrn-type = 1.5 ± 0.3 × 105 ...

Assembling Hierarchical Cluster Solids with Atomic Precision

Hierarchical solids created from the binary assembly of cobalt chalcogenide and iron oxide molecular clusters are reported. Six different molecular clusters based on the octahedral Co6E8 (E = Se or Te) and the expanded cubane Fe8O4 units are used as superatomic building blocks to construct these crystals. The formation of the solid is driven by the transfer of charge between complementary electron-donating and electron-accepting clusters in solution that crystallize as binary ionic compounds. The hierarchical structures are investigated by single-crystal X-ray diffraction, providing atomic and superatomic resolution. We report two different superstructures: a superatomic relative of the CsCl lattice type and an unusual packing arrangement based on the double-hexagonal close-packed lattice. Within these superstructures, we demonstrate various compositions and orientations of the clusters.

Unbalanced Hole and Electron Diffusion in Lead Bromide Perovskites

We use scanning photocurrent microscopy and time-resolved microwave conductivity to measure the diffusion of holes and electrons in a series of lead bromide perovskite single crystals, APbBr3, with A = methylammonium (MA), formamidinium (FA), and Cs. We find that the diffusion length of holes (LDh+ ∼ 10-50 μm) is on average an order of magnitude longer than that of electrons (LDe- ∼ 1-5 μm), regardless of the A-type cation or applied bias. Furthermore, we observe a weak dependence of LD across the A-cation series MA > FA > Cs. When considering the role of the halide, we find that the diffusion of holes in MAPbBr3 is comparable to that in MAPbI3, but the electron diffusion length is up to five times shorter. This study shows that the disparity between hole and electron diffusion is a ubiquitous feature of lead halide perovskites. As with organic photovoltaics, this imbalance will likely become an important consideration in the optimization of lead halide perovskite solar cells.

Quantum Soldering of Individual Quantum Dots

Making contact to a quantum dot: Single quantum-dot electronic circuits are fabricated by wiring atomically precise metal chalcogenide clusters with conjugated molecular connectors. These wired clusters can couple electronically to nanoscale electrodes and be tuned to control the charge-transfer characteristics (see picture).