Does Selectivity of Molecular Catalysts Change with Time? Polymerization Imaged by Single‐Molecule Spectroscopy

Abstract

The chemoselectivity of molecular catalysts underpins much of modern synthetic organic chemistry. However, little is known about the selectivity of individual catalysts because this single-catalyst-level behavior is hidden by the bulk catalytic behavior. Here, for the first time, the selectivity of individual molecular catalysts for two different reactions is imaged in real time at the single-catalyst level. This imaging is achieved through fluorescence microscopy paired with spectral probes that produce a snapshot of the instantaneous chemoselectivity of a single catalyst for either a single-chainelongation or a single-chain-termination event during ruthenium-catalyzed polymerization. Superresolution imaging of multiple selectivity events, each at a different single-molecular ruthenium catalyst, indicates that catalyst selectivity may be unexpectedly spatially and time-variable.

Exploring chemistry with single-molecule and -particle fluorescence microscopy

“…Single-polymer aggregate particles showed time-variable growth kinetics [8,41,42]. Furthermore, two-color single-molecule experiments investigated the selectivity of single molecular Grubbs catalysts for chain-elongation or chain-termination reactions; these experiments concluded that the selectivity of catalysts for these competing reactions varies with time [43]. In palladium catalysis, Goldsmith investigated the initiation-step kinetics of an immobilized pyridine-palladium-NHC ('PEPPSI' type) catalyst [44], while Jung studied the palladium-catalyzed Tsuji-Trost deallylation mechanism and discovered that the mechanism involves the formation of a π complex and that it proceeds through a long-lived Pd(II) intermediate [5,45].…”

Single-Molecule Colocalization of Redox Reactions on Semiconductor Photocatalysts Connects Surface Heterogeneity and Charge-Carrier Separation in Bismuth Oxybromide

“…For example, the photodeposition of metal salts has been used to identify facet-selective charge separation in microscale crystals where electrons and holes are preferentially extracted from different facets. ,− However, this ex-situ technique (where electron microscopy is used to image the deposits after the reaction) cannot quantify variations in photocatalytic activity among different particles. Furthermore, the initial deposition of metal or metal oxide particles on the semiconductor alters the pathways of subsequent photoexcited carriers, , and the observed crystal facet-selectivity can vary significantly with both the preparation method of the particles and their chemical environment during photodeposition (e.g., solution pH and ionic strength). − ,− In comparison, single-molecule localization microscopy uses fluorogenic probes that are chemically activated on the surface of the catalyst particle to provide noninvasive and real-time monitoring of individual reaction events. − This method quantifies how nanoscale variations in charge-carrier extraction across the surface of a single particle contribute to its photocatalytic activity.…”

Stochastic Noise in Single-Nanoparticle Catalysis

“…Over the past decade, tremendous progress has been made in this direction. In-situ microscopic, nanoscopic, and single-molecule-level spectroscopic tools − have elucidated the elementary catalytic events occurring on the surface of the catalyst in real time. ,− Owing to their improved spatial and temporal resolution, as compared to conventional bulk-level interrogations, these studies have revealed that catalysts are heterogeneous. Catalytic activity can vary spatiallyon nanometer and micrometer scalesand also sometimes in time. − Such spatial variation of catalytic behavior is often attributed to static differences in morphologies, structural features (facets, edges, or steps), or chemical environments of individual grains of the catalyst. − Temporal changes in activity are assigned to dynamics in the surface structure or composition of the catalyst resulting from processes such as adsorbate-induced surface restructuring, poisoning of the surface, or sint...…”