Semin Lee

The activation of chalcogenoglycosides for O-glycosylation often requires strong electrophiles and a low temperature. However, visible-light irradiation of electron donor–acceptor (EDA) complexes between chalcogenoglycosides and less reactive electrophiles can circumvent this problem. Herein, we demonstrate high-yielding, room-temperature O-glycosylation with selenoglycosides in the presence of diphenyliodonium salts. Experimental and computational evidence indicates the formation of halogen-bonded EDA complexes and photoinduced electron transfer as critical steps.

A modular synthesis of a fluorene-based cycloparaphenyleneacetylene carbon nanohoop and its regioselective strain-promoted azide-alkyne cycloaddition is described herein.

In this work we report the synthesis, structure, and electronic properties of carbon‐rich compounds dehydrobiphenyleno[12]annulenes (DBP[12]As) comprising antiaromatic four‐membered rings (4MR) and 12‐membered ring (12MR). Ultraviolet–visible absorption spectra and electrochemical behaviors of DBP[12]As confirmed their relatively narrow highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap values and high HOMO energy levels, which were supported by density functional theory simulations. Parent DBP[12]A adopts a slipped herringbone structure in a crystalline state, with the molecules forming 1D stacks via π–π interactions. The experimentally derived bond lengths, bonding analyses using the Wiberg bond indices, and localized orbital locator calculation support a stronger double bond character for the 12MR bonds than the 4MR bonds in the inner six‐membered ring. The chemical shifts of hydrogens in ¹ H NMR spectra, as well as magnetically induced ring current analyses using quantum chemical calculations, indicate that the 4MRs have stronger antiaromatic character than the 12MR. The present information is useful for a fundamental understanding of carbon‐rich compounds with different antiaromatic units as well as designing novel molecules with unique electronic properties.

We report the use of nitrone C–H groups as hydrogen bond donors for binding anions. Acyclic anion receptors with two nitrone moieties were synthesized by condensation reactions between aryl-aldehydes and m-phenylenedihydroxylamines. The solid-state structure of dinitrones revealed C–H⋯O hydrogen bonds. Anion binding properties were investigated using NMR titration with halide and phosphate salts.

Large scale synthesis of cycloparaphenyleneacetylenes has been challenging due to low macrocyclization yields and harsh aromatization methods that often decompose strained alkynes. Herein, a cis-stilbene-based building block is subjected to alkyne metathesis macrocylization. The following sequence of alkene-selective bromination and dehydrobromination afforded a [8]cycloparaphenyleneacetylene derivative in high yield with good scalability. X-Ray crystal structure and computational analysis revealed a unique same-rim conformation for the eight methyl groups on the nanohoop.

The intermediacy of metallacyclobutadienes as part of a [2 + 2]/retro-[2 + 2] cycloaddition-based mechanism is a well-established paradigm in alkyne metathesis with alternative species viewed as off-cycle decomposition products that interfere with efficient product formation. Recent work has shown that the exclusive intermediate isolated from a siloxide podand-supported molybdenum-based catalyst was not the expected metallacyclobutadiene but instead a dynamic metallatetrahedrane. Despite their paucity in the chemical literature, theoretical work has shown these species to be thermodynamically more stable as well as having modest barriers for cycloaddition. Consequentially, we report the synthesis of a library of group VI alkylidynes as well as the roles metal identity, ligand flexibility, secondary coordination sphere, and substrate identity all have on isolable intermediates. Furthermore, we report the disparities in catalyst competency as a function of ligand sterics and metal choice. Dispersion-corrected DFT calculations are used to shed light on the mechanism and role of ligand and metal on the intermediacy of metallacyclobutadiene and metallatetrahedrane as well as their implications to alkyne metathesis.

Size selective synthesis of large cycloparaphenyleneacetylene carbon nanohoops was achieved using alkyne metathesis. The large nanohoops were stable in ambient conditions due to their reduced strain. The nanohoops exhibited blue fluorescence with high quantum yields.

The hierarchical assembly of colored cationic molecules with receptor-modified counteranions can be used to control optical properties in materials. However, our knowledge of when the optical properties emerge in the hierarchical organization and the variety of cation–anion salts that are available to create these materials is limited. In this work, we extend the salts from small halides to large inorganic anions and determine how the structure coevolves with the emission properties using solution assemblies. We study the chromogenic trioxatriangulenium (TOTA⁺) cation and its coassembly with cyanostar (CS) macrocycles selected to modify tetrafluoroborate (BF₄^–) counteranions through formation of 2:1 sandwich complexes. In the solid state, the TOTA⁺ cation stacks in an alternating manner with the sandwich complexes producing new red-shifted emission and absorption bands. Critical to assigning the structural origin of the new optical features across the four levels of organization (1° → 4°) is the selection of specific solvents to produce and characterize different assemblies present in the hierarchical structure. A key species is the electrostatically stabilized ion pair between the TOTA⁺ cation and sandwich complex. The red-shifted features only emerge when the ion pairs oligomerize together into larger (TOTA·[CS₂BF₄])_n assemblies. New electronic states emerge as a result of multiple copies of the TOTA⁺ making π-contact with cyanostar–anion complexes. Our findings and the ease with which the materials can be prepared as crystals and films by mixing the salt with a receptor provide a strong platform for the de novo design of new optical materials.

Porous materials provide a plethora of technologically important applications that encompass molecular separations, catalysis, and adsorption. The majority of research in this field involves network solids constructed from multitopic constituents that, when assembled either covalently or ionically, afford macromolecular arrangements with micro- or meso-porous apertures. Recently, porous solids fabricated from discrete organic cages have garnered much interest due to their ease of handling and solution processability. Although this class of materials is a promising alternative to network solids, fundamental studies are still required to elucidate critical structure–function relationships that govern microporosity. Here, we report a systematic investigation of the effects of building block shape-persistence on the porosity of molecular cages. Alkyne metathesis and edge-specific postsynthetic modifications afforded three organic cages with alkynyl, alkenyl, and alkyl edges, respectively. Nitrogen adsorption experiments conducted on rapidly crystallized and slowly crystallized solids illustrated a general trend in porosity: alkynyl > alkenyl > alkyl. To understand the molecular-scale origin of this trend, we investigated the short and long time scale molecular motions of the molecular cages using ab initio molecular dynamics (AIMD) and classical molecular dynamics (MD) simulations. Our combined experimental and computational results demonstrate that the microporosity of molecular cages directly correlates with shape persistence. These findings discern fundamental molecular requirements for rationally designing porous molecular solids.