More Like this

1. An Ion‐Pairing Approach to Stereoselective Metal‐Free Ring‐Opening Metathesis Polymerization

   https://doi.org/10.1002/anie.202016393  

   Yang, Xuejin; Gitter, Sean R.; Roessler, Allison G.; Zimmerman, Paul M.; Boydston, Andrew J. (May 2021, Angewandte Chemie International Edition)

   Abstract Stereochemistry can have a profound impact on polymer and materials properties. Unfortunately, straightforward methods for realizing high levels of stereocontrolled polymerizations are often challenging to achieve. In a departure from traditional metal‐mediated ring‐opening metathesis polymerization (ROMP), we discovered a remarkably simple method for controlling alkene stereochemistry in photoredox mediated metal‐free ROMP. Ion‐pairing, initiator sterics, and solvation effects each had profound impact on the stereochemistry of polynorbornene (PNB). Simple modifications to the reaction conditions produced PNB withtransalkene content of 25 to >98 %. Highciscontent was obtained from relatively larger counterions, toluene as solvent, low temperatures (−78 °C), and initiators with low Charton values. Conversely, smaller counterions, dichloromethane as solvent, and enol ethers with higher Charton values enabled production of PNB with hightranscontent. Data from a combined experimental and computational investigation are consistent with the stereocontrolling step of the radical cationic mechanism proceeding under thermodynamic control. 

   more » « less
2. Stereomodulation of Poly(4‐methyl‐1‐pentene): Adoption of a Neglected and Misunderstood Commercial Polyolefin

   https://doi.org/10.1002/anie.202211992  

   Wentz, Charlotte M.; Fischbach, Danyon M.; Sita, Lawrence R. (October 2022, Angewandte Chemie International Edition)

   Abstract A toolbox of living coordination polymerization methods based on CPAM group 4 metal initiators has been applied for the stereomodulation of poly(4‐methyl‐1‐pentene) (PMP) tacticity to provide new fundamental forms in which thermal phase transitions,T_gandT_m, can be adjusted over a wide range. Also reported for the first time are the synthesis and properties of authenticated 1,2‐regioregular atactic PMP, which serves to correct past errors regarding this fundamental polyolefin. 

   more » « less
3. Deciphering Mg‐Surface Interactions with Unsaturated Hydrocarbons: An Integrated Experimental‐Theoretical Study

   https://doi.org/10.1002/cphc.202401061  

   Ghoshal, Sourav; Carnegie, Nathaniel; Ezeakunne, Chidozie; Dangi, Beni B; Kattel, Shyam (April 2025, ChemPhysChem)

   Abstract Here, we used a combination of laser‐induced experiments and density functional theory (DFT) calculations to study the mechanism of growth of carbonaceous species on the Mg surface. Experimental observations revealed that the carbon deposit forms upon laser illumination on the Mg surface, with the deposit being clearer and better structured in the presence of 1,3‐butadiene (C₄H₆) compared to ethylene (C₂H₄) gas. DFT thermodynamic and kinetic calculations of C₂−C₄hydrocarbons interaction on low‐index Mg(0001) were used to explain this experimental observation. Our results on Mg(0001) showed that the cis isomer of C₄H₆binds more strongly than its trans isomer via a [4+2] cycloaddition mechanism. We also investigated the adsorption of two units of C₂H₄and C₄H₆molecules, as well as the subsequent dehydrogenation stages that produce radical species responsible for chain growth mechanisms. The results showed that free energy change of dehydrogenation of two units of cis‐C₄H₆[i. e. cis‐C₈H₁₂] is lower than the dehydrogenation of trans conformer of C₄H₆and C₂H₄molecule, indicating that the dehydrogenation of two units of cis‐C₄H₆facilitates the initiation of growth of carbonaceous species on Mg surfaces. Therefore, the DFT calculations pinpoint the origin of the experimental observation of clearer carbon deposits on the Mg surface. 

   more » « less
4. Competition between Intra and Intermolecular Pnicogen Bonds: Complexes between Naphthalene Derivatives and Neutral or Anionic Bases

   https://doi.org/10.1002/cphc.202200173  

   Wysokiński, Rafał; Zierkiewicz, Wiktor; Michalczyk, Mariusz; Scheiner, Steve (April 2022, ChemPhysChem)

   Abstract The PnF₂(Pn=P,As,Sb,Bi) on a naphthalene scaffold can engage in an internal pnicogen Pn⋅⋅⋅N bond (PnB) with an NH₂group placed close to it on the adjoining ring. An approaching neutral NH₃molecule can engage in a second PnB with the central Pn, which tends to weaken the intramolecular bond. The presence of the latter in turn weakens the intermolecular PnB with respect to that formed in its absence. Replacement of the external NH₃by a CN⁻anion causes a fundamental change in the bonding pattern, producing a fourth covalent bond with Pn, which rearranges into a trigonal bipyramidal motif. This addition disrupts the internal Pn⋅⋅⋅N pnicogen bond, recasting the PnF₂⋅⋅⋅NH₂interaction into an NH⋅⋅⋅F H‐bond. 

   more » « less
5. Excited State Charge Separation in an Azobenzene‐Bridged Perylenediimide Dimer – Effect of Photochemical Trans‐Cis Isomerization

   https://doi.org/10.1002/chem.202102903  

   Zink‐Lorre, Nathalie; Seetharaman, Sairaman; Gutiérrez‐Moreno, David; Fernández‐Lázaro, Fernando; Karr, Paul_A; D'Souza, Francis (September 2021, Chemistry – A European Journal)

   Abstract Photoinduced charge transfer and separation events in a newly synthesized azobenzene‐bridged perylenediimide‐dimer (PDI‐dimer) are demonstrated. Trans‐to‐cis conversion (∼50 % efficiency) from the initial trans PDI‐dimer by 355 nm pulsed laser light, and its reversal, cis‐to‐trans, process by 435 nm laser light irradiation has been possible to accomplish. Efficient fluorescence quenching in the PDI‐dimer, more so for the cis isomer was witnessed, and such quenching increased with increasing solvent polarity. DFT‐calculated geometry and electronic structures helped in visualizing the charge transfer in the PDI‐dimer in both isomeric forms, and also revealed certain degree of participation of the azobenzene entity in the charge transfer events. Femtosecond transient absorption spectral studies confirmed occurrence of both charge transfer followed by charge separation in the studied PDI‐dimer in both trans and cis forms in polar solvents, and the evaluated time constants from Global target analysis revealed accelerated events in the cis PDI‐dimer due to proximity effects. The present study offers key insights on the role of the azobenzene bridge, and the dimer geometry in governing the excited state charge transfer and separation in symmetrically linked PDI dimer. 

   more » « less