Bonnie Choi

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.

Enantioselective Copper-Catalyzed Reductive Coupling of Alkenylazaarenes with Ketones

Catalytic enantioselective methods for the preparation of chiral azaarene-containing compounds are of high value. By combining the utility of copper hydride catalysis with the ability of C═N-containing azaarenes to activate adjacent alkenes toward nucleophilic additions, the enantioselective reductive coupling of alkenylazaarenes with ketones has been developed. The process is tolerant of a wide variety of azaarenes and ketones, and provides aromatic heterocycles bearing tertiary-alcohol-containing side chains with high levels of diastereo- and enantioselection.

van der Waals Solids from Self-Assembled Nanoscale Building Blocks

Traditional atomic van der Waals materials such as graphene, hexagonal boron-nitride, and transition metal dichalcogenides have received widespread attention due to the wealth of unusual physical and chemical behaviors that arise when charges, spins, and vibrations are confined to a plane. Though not as widespread as their atomic counterparts, molecule-based two-dimensional (2D) layered solids offer significant benefits; their structural flexibility will enable the development of materials with tunable properties. Here we describe a layered van der Waals solid self-assembled from a structure-directing building block and C60 fullerene. The resulting crystalline solid contains a corrugated monolayer of neutral fullerenes and can be mechanically exfoliated. The absorption spectrum of the bulk solid shows an optical gap of 390 ± 40 meV that is consistent with thermal activation energy obtained from electrical transport measurement. We find that the dimensional confinement of fullerenes significantly modulates the optical and electronic properties compared to the bulk solid.

Tuning Singlet Fission in π-Bridge-π Chromophores

We have designed a series of pentacene dimers separated by homoconjugated or nonconjugated bridges that exhibit fast and efficient intramolecular singlet exciton fission (iSF). These materials are distinctive among reported iSF compounds because they exist in the unexplored regime of close spatial proximity but weak electronic coupling between the singlet exciton and triplet pair states. Using transient absorption spectroscopy to investigate photophysics in these molecules, we find that homoconjugated dimers display desirable excited-state dynamics, with significantly reduced recombination rates as compared to conjugated dimers with similar singlet fission rates. In addition, unlike conjugated dimers, the time constants for singlet fission are relatively insensitive to the interplanar angle between chromophores, since rotation about σ bonds negligibly affects the orbital overlap within the π-bonding network. In the nonconjugated dimer, where the iSF occurs with a time constant >10 ns, comparable to the fluorescence lifetime, we used electron spin resonance spectroscopy to unequivocally establish the formation of triplet-triplet multiexcitons and uncoupled triplet excitons through singlet fission. Together, these studies enable us to articulate the role of the conjugation motif in iSF.

Building Diatomic and Triatomic Superatom Molecules.

In this study, we have developed a method to create Co6Se8 superatoms in which we program the metal-ligand bonds. We exclusively form the Co6Se8 core under simple reaction conditions with a facile separation of products that contain differential substitution of the core. The combination of Co2(CO)8 and PR3 with excess Se gives the differentially and directionally substituted superatoms, Co6Se8(CO)x(PR3)(6-x). The CO groups on the superatom can be exchanged quantitatively with phosphines and isonitriles. Substitution of the CO allows us to manipulate the type and length of chemical bridge between two redox-active superatomic centers in order to modulate intersuperatomic coupling. Linking two superatoms together allows us to form the simplest superatom molecule: a diatomic molecule. We extend the superatom molecule concept to link three superatoms together in a linear arrangement to form acyclic triatomic molecules. These superatom molecules have a rich electrochemical profile and chart a clear path to a whole family of superatom molecules with new and unusual collective properties.

Ligand Control of Manganese Telluride Molecular Cluster Core Nuclearity

We report the synthesis, structural diversity, and chemical behavior of a family of manganese telluride molecular clusters whose charge-neutral cores are passivated by two-electron donor ligands. We describe three different core structures: a cubane-type Mn4Te4, a prismane Mn6Te6, and a dicubane Mn8Te8. We use various trialkylphosphines and N-heterocyclic carbenes (NHCs) as surface ligands and demonstrate that the formation of the different cluster core structures is controlled by the choice of ligand: bulky ligands such as P(i)Pr3, PCy3, or (i)Pr2NHC ((i)Pr2NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) form the cubane-type core, while the smaller PMe3 produces the prismane core. The intermediate-sized PEt3 produces both cubane and prismane species. These manganese telluride molecular clusters are labile, and the capping phosphines can be replaced by stronger ligands, while the internal core structure of the cluster remains intact. The interplay of structural diversity and ligand versatility and lability makes these clusters potentially useful building blocks for the assembly of larger aggregates and extended structures. We demonstrate the simplest prototype of these solid-forming reactions: the direct coupling of two Mn4Te4((i)Pr2NHC)4 units to form the dicubane Mn8Te8((i)Pr2NHC)6. We also postulate the prismatic Mn6Te6 as the common ancestor of both Chevrel-type M6E8 and octanuclear rhombododecahedral M8E6 molecular clusters (M = transition metal and E = chalcogen), and we discuss the core structure of our molecular clusters as recognizable building units for the zinc blende and the hypothetical wurtzite lattices of MnTe.

Room-temperature current blockade in atomically defined single-cluster junctions

Fabricating nanoscopic devices capable of manipulating and processing single units of charge is an essential step towards creating functional devices where quantum effects dominate transport characteristics. The archetypal single-electron transistor comprises a small conducting or semiconducting island separated from two metallic reservoirs by insulating barriers. By enabling the transfer of a well-defined number of charge carriers between the island and the reservoirs, such a device may enable discrete single-electron operations. Here, we describe a single-molecule junction comprising a redox-active, atomically precise cobalt chalcogenide cluster wired between two nanoscopic electrodes. We observe current blockade at room temperature in thousands of single-cluster junctions. Below a threshold voltage, charge transfer across the junction is suppressed. The device is turned on when the temporary occupation of the core states by a transiting carrier is energetically enabled, resulting in a sequential tunnelling process and an increase in current by a factor of ∼600. We perform in situ and ex situ cyclic voltammetry as well as density functional theory calculations to unveil a two-step process mediated by an orbital localized on the core of the cluster in which charge carriers reside before tunnelling to the collector reservoir. As the bias window of the junction is opened wide enough to include one of the cluster frontier orbitals, the current blockade is lifted and charge carriers can tunnel sequentially across the junction.

Superatomic Two-Dimensional Semiconductor

Structural complexity is of fundamental interest in materials science because it often results in unique physical properties and functions. Founded on this idea, the field of solid state chemistry has a long history and continues to be highly active, with new compounds discovered daily. By contrast, the area of two-dimensional (2D) materials is young, but its expansion, although rapid, is limited by a severe lack of structural diversity and complexity. Here, we report a novel 2D semiconductor with a hierarchical structure composed of covalently linked Re6Se8 clusters. The material, a 2D structural analogue of the Chevrel phase, is prepared via mechanical exfoliation of the van der Waals solid Re6Se8Cl2. Using scanning tunneling spectroscopy, photoluminescence and ultraviolet photoelectron spectroscopy, and first-principles calculations, we determine the electronic bandgap (1.58 eV), optical bandgap (indirect, 1.48 eV), and exciton binding energy (100 meV) of the material. The latter is consistent with the partially 2D nature of the exciton. Re6Se8Cl2 is the first member of a new family of 2D semiconductors whose structure is built from superatomic building blocks instead of simply atoms; such structures will expand the conceptual design space for 2D materials research.

Two-Dimensional Hierarchical Semiconductor with Addressable Surfaces

Surfaces play a key role in determining material properties, and their importance is further magnified in the two-dimensional (2D) limit. Though monolayers are entirely composed of surfaces, there is no chemical approach to covalently address them without breaking intralayer bond. Here, we describe a 2D semiconductor that offers two unique features among 2D materials: structural hierarchy within the monolayer and surface reactive sites that enable functionalization. The 2D semiconductor is composed of a single layer of strongly interconnected Re6Se8 clusters arranged in an oblique lattice capped by substitutionally labile Cl atoms. We show that a simple ligand substitution strategy borrowed from traditional coordination chemistry can be used to modify the surface of the 2D material while preserving its internal structure. The potential generality of this approach establishes a promising route toward multifunctional 2D materials with tunable physical and chemical properties and may also facilitate better electrical top contact to 2D semiconductors.

CCDC 1579912: Experimental Crystal Structure Determination

Related Article: Elango Kumarasamy, Samuel N. Sanders, Murad J. Y. Tayebjee, Amir Asadpoordarvish, Timothy J. H. Hele, Eric G. Fuemmeler, Andrew B. Pun, Lauren M. Yablon, Jonathan Z. Low, Daniel W. Paley, Jacob C. Dean, Bonnie Choi, Gregory D. Scholes, Michael L. Steigerwald, NandiniAnanth, Dane R. McCamey, Matthew Y. Sfeir, and Luis M. Campos|2017|J.Am.Chem.Soc.|139|12488|doi:10.1021/jacs.7b05204