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Abstract Phosphiranes are weak Lewis bases reacting with only a limited number of electrophiles to produce the corresponding phosphiranium ions. These salts are recognized for their propensity to undergo reactions with oxygen pronucleophiles at the phosphorus site, leading to the formation of phosphine oxide adducts. Building on a thorough mechanistic understanding, we have developed an unprecedented approach that enables the selective reaction of carboxylic acids, and other nucleophiles, at the carbon site of phosphiranes. This method involves the photochemical generation of highly reactive carbenes, which react with 1‐mesitylphosphirane to yield ylides. The latter undergoes a stepwise reaction with carboxylic acids, resulting in the production of the desired phosphines. In addition to DFT calculations, we have successfully isolated and fully characterized the key intermediates involved in the reaction.

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Photocatalysis that uses the energy of light to promote chemical transformations by exploiting the reactivity of excited-state molecules is at the heart of a virtuous dynamic within the chemical community. Visible-light metal-based photosensitizers are most prominent in organic synthesis, thanks to their versatile ligand structure tunability allowing to adjust photocatalytic properties toward specific applications. Nevertheless, a large majority of these photocatalysts are cationic species whose counterion effects remain underestimated and overlooked. In this report, we show that modification of the X counterions constitutive of [Ru(bpy)<sub>3</sub>](X)<sub>2</sub> photocatalysts modulates their catalytic activities in intermolecular [2 + 2] cycloaddition reactions operating through triplet–triplet energy transfer (TTEnT). Particularly noteworthy is the dramatic impact observed in low-dielectric constant solvent over the excited-state quenching coefficient, which varies by two orders of magnitude depending on whether X is a large weakly bound (BAr$^F$<sub>4</sub>$^–$) or a tightly bound (TsO$^–$) anion. In addition, the counterion identity also greatly affects the photophysical properties of the cationic ruthenium complex, with [Ru(bpy)<sub>3</sub>](BAr$^F$<sub>4</sub>)<sub>2</sub> exhibiting the shortest 3MLCT excited-state lifetime, highest excited state energy, and highest photostability, enabling remarkably enhanced performance (up to &gt;1000 TON at a low 500 ppm catalyst loading) in TTEnT photocatalysis. These findings supported by density functional theory-based calculations demonstrate that counterions have a critical role in modulating cationic transition metal-based photocatalyst potency, a parameter that should be taken into consideration also when developing energy transfer-triggered processes.

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Abstract Chiral molecules, used in applications such as enantioselective photocatalysis 1 , circularly polarized light detection 2 and emission 3 and molecular switches 4,5 , exist in two geometrical configurations that are non-superimposable mirror images of each other. These so-called ( R ) and ( S ) enantiomers exhibit different physical and chemical properties when interacting with other chiral entities. Attosecond technology might enable influence over such interactions, given that it can probe and even direct electron motion within molecules on the intrinsic electronic timescale 6 and thereby control reactivity 7–9 . Electron currents in photoexcited chiral molecules have indeed been predicted to enable enantiosensitive molecular orientation 10 , but electron-driven chiral dynamics in neutral molecules have not yet been demonstrated owing to the lack of ultrashort, non-ionizing and perturbative light pulses. Here we use time-resolved photoelectron circular dichroism (TR-PECD) 11–15 with an unprecedented temporal resolution of 2.9 fs to map the coherent electronic motion initiated by ultraviolet (UV) excitation of neutral chiral molecules. We find that electronic beatings between Rydberg states lead to periodic modulations of the chiroptical response on the few-femtosecond timescale, showing a sign inversion in less than 10 fs. Calculations validate this and also confirm that the combination of the photoinduced chiral current with a circularly polarized probe pulse realizes an enantioselective filter of molecular orientations following photoionization. We anticipate that our approach will enable further investigations of ultrafast electron dynamics in chiral systems and reveal a route towards enantiosensitive charge-directed reactivity.

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Sujets

Multiple bonds Inorganic chemistry Modeling Chimie Théorique Photoisomerization Metalloporphyrin Iron Organic semiconductor MOLECULES NBO Phosphirane Mechanoresponsive luminescence 2+2 cycloaddition Ion-molecule reactions Photochemistry DFT computations Photodissociation INFRARED-SPECTRUM DENSITY-FUNCTIONAL THEORY Groundwaters PERTURBATION-THEORY APPROACH Computational Photochemistry Mechanism Hydrolysis Density Functional Theory DFT Redox reactions Electrochemical reduction Quinones Chimie Théorique et Computationnelle Photoisomérisation ICP-MS Mécanisme de Photoisomérisation Aggregation induced emission AIE solid state luminescence enhancement SLE ESIPT photoluminescence crystal structure SF-TD-DFT Photosubstitution Excited States Photoisomerization Mechanism Mécanisme de Photolibération Etats Excités Photoluminescence DFT Phosphirane phosphorus mechanism photochemistry selectivity Coordination compounds Oxidation Carbonate Density functional calculations DIMER Aggregation induced emission AIE Counterion effects Dithienylethene Photochromisme Complexes de Ruthénium à Ligand Nitrosyle Complexe de coordination Orbitales moléculaires Photochimie Ruthenium complex Ab initio calculations Chimie inorganique Excited states Photophysics Metal-centered excited states Molecular orbitals DER-WAALS COMPLEXES ESIPT RASPT2 Nitric oxide Electrochemical properties Actinides Phosphorus ACETYLENE Rhenium Density functional theory Photophysique Photocatalysis Ruthénium Ruthenium Photorelease Mechanism Chimie théorique Photochromes Nudged elastic band Quantum mechanics Diarylethenes Photosolvolysis mechanism Ab initio 3MLCT Lanthanides 3MC Mathematical methods IPEA Crystal Computational photochemistry Phosphorescence Electrochemistry CROSS-SECTIONS Photochimie Computationnelle Photorelease Photochromism Crystal structure Insertion reaction KOHN-SHAM ORBITALS Nitrosyl Ruthenium Complexes

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