Functional π-Gelators and Their Applications

Cross-coupling reactions using samarium(II) iodide.

Tuning of multiple luminescence outputs and white-light emission from a single gelator molecule through an ESIPT coupled AIEE process

We report herein the synthesis and characterization of unmodified graphene oxide quantum dots (GOQDs) with white-light-emitting properties, upon photoexcitation at 340 nm. The Commission International de l’Eclairage (CIE) 1931 chromaticity coordinates for GOQDs (x = 0.29, y = 0.34) suggest that highly pure white-light emission was achieved. A detailed mechanistic study was carried out utilizing UV–visible absorption, steady-state and time-resolved fluorescence spectroscopy, and dynamic light scattering (DLS) techniques to understand the origin of the white-light emission. The results taken together suggest that GOQDs could self-assemble in solution and thus transform the luminescence behavior. Furthermore, the results indicate that the pH of the medium also plays a crucial role in assisting the aggregation to generate the white-light emission. The concentration-dependent DLS measurements support a cooperative mechanism for the aggregation kinetics in the system. More importantly, the study suggests that w...

White-Light Emission from Unmodified Graphene Oxide Quantum Dots

We report a general visible-light-mediated strategy that enables the construction of complex C(sp3)-rich N-heterospirocycles from feedstock aliphatic ketones and aldehydes with a broad selection of alkene-containing secondary amines. Key to the success of this approach was the utilization of a highly reducing Ir-photocatalyst and orchestration of the intrinsic reactivities of 1,4-cyclohexadiene and Hantzsch ester. This methodology provides streamlined access to complex C(sp3)-rich N-heterospirocycles displaying structural and functional features relevant to fragment-based lead identification programs.

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Streamlined synthesis of C(sp3)-Rich N-Heterospirocycles enabled by visible-light-mediated photocatalysis

https://www.rsc.org/suppdata/cc/c3/c3cc38419e/c3cc38419e.pdf

Supramolecular design for two-component hydrogels with intrinsic emission in the visible region.

Samarium diiodide (SmI2) is one of the most widely used single-electron reductants available to organic chemists because it is effective in reducing and coupling a wide range of functional groups. Despite the broad utility and application of SmI2 in synthesis, the reagent is used in stoichiometric amounts and has a high molecular weight, resulting in a large amount of material being used for reactions requiring one or more equivalents of electrons. Although few approaches to develop catalytic reactions have been designed, they are not widely used or require specialized conditions. As a consequence, general solutions to develop catalytic reactions of Sm(II) remain elusive. Herein, we report mechanistic studies on catalytic reactions of Sm(II) employing a terminal magnesium reductant and trimethylsilyl chloride in concert with a noncoordinating proton donor source. Reactions using this approach permitted reductions with as little as 1 mol % Sm. Mechanistic studies provide strong evidence that during the reaction, SmI2 transforms into SmCl2, therefore broadening the scope of accessible reactions. Furthermore, this mechanistic approach enabled catalysis employing HMPA as a ligand, facilitating the development of catalytic Sm(II) 5- exo- trig ketyl olefin cyclization reactions. The initial work described herein will enable further development of both useful and user-friendly catalytic reactions, a long-standing, but elusive goal in Sm(II) chemistry.

Mechanistic Study and Development of Catalytic Reactions of Sm(II)

Ligands that coordinate to SmI2 through oxygen are prevalent in the literature and make up a significant portion of additives employed with the reagent to perform reactions of great synthetic importance. In the present work a series of spectroscopic, calorimetric and kinetic studies demonstrate that nitrogen-based analogues of many common additives have a significantly higher affinity for Sm than the oxygen-based counterparts. In addition, electrochemical experiments show that nitrogen-based ligands significantly enhance the reducing power of SmI2 . Overall, this work demonstrates that the use of nitrogen-based ligands provides a useful alternative approach to enhance the reactivity of reductants based on SmII .

Aza versus Oxophilicity of SmI2 : A Break of a Paradigm.

Photoinduced electron transfer (PET) from europium(II)-complexes to variety of organic electron acceptors has been investigated in tetrahydrofuran (THF), 1,2-dimethoxyethane (DME) and acetonitrile (ACN). Laser flash photolysis (LFP) study indicates the formation of radical cation of the donor {Eu (II)} and radical/radical anion of corresponding acceptors, confirming the photoinduced electron transfer. Time resolved luminescence quenching experiments showed that rate constants of forward electron transfer are in the range of 10 8 –10 9  M −1  s −1 . Furthermore, experimentally obtained rate constants are in good agreement with calculated electron transfer rate constants using Marcus equation with a reorganization energy ( λ ) of 45 kcal/mol. Interestingly, the excited state decay from radical ion pair formed in Eu(II)-ketone systems revealed that back electron transfer (BET) rate constants are 4–5 order of magnitude less ( k bet  = 10 4 –10 5  s −1 ) compared to that of forward electron transfer, under the present experimental conditions. The significant decrease in rate is attributed to the large energy barrier for the back electron transfer process, which involves transformation of an otherwise stable trivalent lanthanide to its thermodynamically less stable divalent oxidation state. This investigation reveals, for the first time, that divalent europium complexes are promising candidates for generating long lived charge separated states in the excited state in the presence of suitable acceptors.

Photoinduced electron transfer from Eu(II)-complexes to organic molecules: Rate and mechanistic investigation

The reaction of SmI2 with the substrates 3-methyl-2-butanone, benzyl chloride, p-cyanobenzyl chloride, and anthracene were studied in the presence of water and an amine. In all cases, the water content versus rate profile shows a maximum at around 0.2 M H2 O. The rate versus amine content profile shows in all cases, except for benzyl chloride, saturation behavior, which is typical of a change in the identity of the rate-determining step. The mechanism that is in agreement with the observed data is that electron transfer occurs in the first step. With substrates that are not very electrophilic, the intermediate radical anions lose the added electron back to samarium(III) relatively quickly and the reaction cannot progress efficiently. However, in a mixture of water/amine, the amine deprotonates a molecule of water coordinated to samarium(III). The negatively charged hydroxide, which is coordinated to samarium(III), reduces its electrophilicity, and therefore, lowers the rate of back electron transfer, which allows the reaction to progress. In the case of benzyl chloride, in which electron transfer is rate determining, deprotonation by the amine is coupled to the electron-transfer step.

Sandeepan Maity

Papers

Supramolecular design for two-component hydrogels with intrinsic emission in the visible region.

Mechanistic Study and Development of Catalytic Reactions of Sm(II)

Aza versus Oxophilicity of SmI2 : A Break of a Paradigm.

Photoinduced electron transfer from Eu(II)-complexes to organic molecules: Rate and mechanistic investigation

Deciphering a 20-Year-Old Conundrum: The Mechanisms of Reduction by the Water/Amine/SmI2 Mixture.