Fundamentals of Crystalline Evolution and Properties of Carbon Nanotube-Reinforced Polyether Ether Ketone Nanocomposites in Fused Filament Fabrication

As fused filament fabrication (FFF) continues to gain popularity, many studies are turning to nanomaterials or optimization of printing parameters to improve the materials’ properties; however, many overlook how materials formulation and additive manufacturing (AM) processes cooperatively engineer the evolution of properties across length scales. Evaluating the in-process evolution of the nanocomposite using AM will provide a fundamental understanding of the material’s microstructure, which can be tailored to create unique characteristics in functionality and performance. In this study, the crystallinity behavior of polyetheretherketone (PEEK) was studied in the presence of carbon nanotubes (CNTs) as a nucleation aid for improved crystallization during FFF processing. Using various characterization techniques and molecular dynamics simulations, it was discovered that the crystallization behavior of extruded filaments is very different from that of 3D printed roads. Additionally, the printed material exhibited cold crystallization, and the CNT addition increased the crystallization of printed roads, which were amorphous without CNT addition. Tensile strength and modulus were increased by as much as 42 and 51%, respectively, due to higher crystallinity during printing. Detailed knowledge on the morphology of PEEK–CNT used in FFF allows gaining a fundamental understanding of the morphological evolution occurring during the AM process that in turn enables formulating materials for the AM process to achieve tailored mechanical and functional properties, such as crystallinity or conductivity

Mechanistic Investigations of the ZnCl₂-Mediated Tandem Mukaiyama Aldol Lactonization: Evidence for Asynchronous, Concerted Transition States and Discovery of 2-Oxopyridyl Ketene Acetal Variants

The ZnCl₂-mediated tandem Mukaiyama aldol lactonization (TMAL) reaction of aldehydes and thiopyridyl ketene acetals provides a versatile, highly diastereoselective approach to <i>trans</i>-1,2-disubstituted β-lactones. Mechanistic and theoretical studies described herein demonstrate that both the efficiency of this process and the high diastereoselectivity are highly dependent upon the type of ketene acetal employed but independent of ketene acetal geometry. Significantly, we propose a novel and distinct mechanistic pathway for the ZnCl₂-mediated TMAL process versus other Mukaiyama aldol reactions based on our experimental evidence to date and further supported by calculations (B3LYP/BSI). Contrary to the commonly invoked mechanistic extremes of [2+2] cycloaddition and aldol lactonization mechanisms, investigations of the TMAL process suggest a concerted but asynchronous transition state between aldehydes and thiopyridyl ketene acetals. These calculations support a boat-like transition state that differs from commonly invoked Mukaiyama “open” or Zimmerman–Traxler “chair-like” transition-state models. Furthermore, experimental studies support the beneficial effect of pre-coordination between ZnCl₂ and thiopyridyl ketene acetals prior to aldehyde addition for optimal reaction rates. Our previously proposed, silylated β-lactone intermediate that led to successful TMAL-based cascade sequences is also supported by the described calculations and ancillary experiments. These findings suggested that a similar TMAL process leading to β-lactones would be possible with an oxopyridyl ketene acetal, and this was confirmed experimentally, leading to a novel TMAL process that proceeds with efficiency comparable to that of the thiopyridyl system

Studies of Ligand Exchange in N-Heterocyclic Carbene Silver(I) Complexes

A series of N-heterocyclic carbene (NHC) Ag­(I) complexes have been prepared and used to study the dynamics of NHC ligand exchange in these Ag­(I) complexes. These studies used solution-state variable-temperature (VT) ¹³C NMR spectroscopy and the temperature-dependent changes in ¹³C–^107/109Ag coupling to determine activation energies for the ligand exchange process. The effects of concentration, bridging anions, and additives on the exchange process have been studied. The experimental activation energies for the NHC ligand exchange processes of these silver complexes are also compared with DFT calculations. The results are consistent with an associative mechanism for the Ag­(I)–NHC exchange process

Studies of Ligand Exchange in N-Heterocyclic Carbene Silver(I) Complexes

A series of N-heterocyclic carbene (NHC) Ag­(I) complexes have been prepared and used to study the dynamics of NHC ligand exchange in these Ag­(I) complexes. These studies used solution-state variable-temperature (VT) ¹³C NMR spectroscopy and the temperature-dependent changes in ¹³C–^107/109Ag coupling to determine activation energies for the ligand exchange process. The effects of concentration, bridging anions, and additives on the exchange process have been studied. The experimental activation energies for the NHC ligand exchange processes of these silver complexes are also compared with DFT calculations. The results are consistent with an associative mechanism for the Ag­(I)–NHC exchange process

Computational Insights into Uranium Complexes Supported by Redox-Active α-Diimine Ligands

The electronic structures of two uranium compounds supported by redox-active α-diimine ligands, (^MesDAB^Me)₂U­(THF) (<b>1</b>) and Cp₂U­(^MesDAB^Me) (<b>2</b>) (^MesDAB^Me = [ArNC­(Me)­C­(Me)NAr]; Ar = 2,4,6-trimethylphenyl (Mes)), have been investigated using both density functional theory and multiconfigurational self-consistent field methods. Results from these studies have established that both uranium centers are tetravalent, that the ligands are reduced by two electrons, and that the ground states of these molecules are triplets. Energetically low-lying singlet states are accessible, and some transitions to these states are visible in the electronic absorption spectrum

Computational Insights into Uranium Complexes Supported by Redox-Active α-Diimine Ligands

The electronic structures of two uranium compounds supported by redox-active α-diimine ligands, (^MesDAB^Me)₂U­(THF) (<b>1</b>) and Cp₂U­(^MesDAB^Me) (<b>2</b>) (^MesDAB^Me = [ArNC­(Me)­C­(Me)NAr]; Ar = 2,4,6-trimethylphenyl (Mes)), have been investigated using both density functional theory and multiconfigurational self-consistent field methods. Results from these studies have established that both uranium centers are tetravalent, that the ligands are reduced by two electrons, and that the ground states of these molecules are triplets. Energetically low-lying singlet states are accessible, and some transitions to these states are visible in the electronic absorption spectrum

Utilizing Nearest-Neighbor Interactions To Alter Charge Transport Mechanisms in Molecular Assemblies of Porphyrins on Surfaces

When tunneling is the dominant mechanism of charge transport in a molecular junction, the conductivity of the junction is largely insensitive to chemical and structural perturbations which do not impact the overall length of the junction. This severely hampers the seemingly limitless potential of molecules to modulate charge transport at interfaces and their application in a host of device designs. This is a particular challenge for molecules baring insulating features like saturated hydrocarbons which decouple functional groups from the surface. Such decoupling groups increase the energy required to isolate charge on the molecule, pushing transport into the tunneling regime in many cases. Herein, we demonstrate that, through enhancement of nearest neighbor interactions, lateral delocalization of charge states in molecular islands can be used to shift transport out of the tunneling regime to the more efficient, and more chemically tunable, charge-hopping regime. In a previous study, it was found that through-bond tunneling was the dominant mechanism of charge transport through a hydrocarbon-tethered free-base porphyrin thiol. With coordination of zinc­(II), the formation of large molecular islands in an alkanethiol matrix on a Au(111) surface was facilitated. Bias-induced switching and unphysical tunneling efficiencies observed by scanning tunneling microscopy of these molecular islands, as well as Coulomb blockade observed in low-temperature crossed-wire tunnel junction measurements, indicate charge hopping becomes the dominant mechanism of transport in the molecular islands, whereas transport in single molecules was consistent with through-bond tunneling. These results elucidate the basis for functional conductivity–structure and supramolecular relationships that may be employed in the design of molecular junctions in organic thin films

Computational Insights into Uranium Complexes Supported by Redox-Active α-Diimine Ligands

The electronic structures of two uranium compounds supported by redox-active α-diimine ligands, (^MesDAB^Me)₂U­(THF) (<b>1</b>) and Cp₂U­(^MesDAB^Me) (<b>2</b>) (^MesDAB^Me = [ArNC­(Me)­C­(Me)NAr]; Ar = 2,4,6-trimethylphenyl (Mes)), have been investigated using both density functional theory and multiconfigurational self-consistent field methods. Results from these studies have established that both uranium centers are tetravalent, that the ligands are reduced by two electrons, and that the ground states of these molecules are triplets. Energetically low-lying singlet states are accessible, and some transitions to these states are visible in the electronic absorption spectrum

Diruthenium Naphthalene and Anthracene Complexes Containing a Doubly Linked Dicyclopentadienyl Ligand

The reaction of <i>cis</i>-{(η⁵-C₅H₃)₂(CMe₂)₂}­Ru₂(CO)₄Br₂ with naphthalene affords the <i>syn</i>-facial [<i>cis</i>-{(η⁵-C₅H₃)₂(CMe₂)₂}­Ru₂(μ-η⁶,η⁶-C₁₀H₈)]­[OTf]₂, (<b>2</b>^<b>2+</b>), a complex that appears to be two electrons short of the 18-electron rule. Density functional theory (DFT) calculations suggest that the Ru atoms satisfy their missing valence by a combination of a weak metal–metal bond and sharing electrons from the central π bond of the naphthalene. The one-electron reduction of <b>2</b>^<b>2+</b> yields <b>2</b>^<b>+</b>, a Class II mixed-valence complex, while the two-electron reduction of <b>2</b>^<b>2+</b> causes a hapticity change from η⁶ to η⁴ on one of the naphthalene rings and yields <i>cis</i>-{(η⁵-C₅H₃)₂(CMe₂)₂}­Ru₂(μ-η⁶,η⁴-C₁₀H₈<b>)</b> (<b>2</b>^<b>0</b>), a zwitterionic complex. The DFT calculations predict that the <i>C</i>_<i>s</i> isomer of <b>2⁰</b> is 4.69 kcal/mol lower in energy than the <i>C</i>_2<i>v</i> isomer, which is a transition state. Reaction of <i>cis</i>-{(η⁵-C₅H₃)₂(CMe₂)₂}­Ru₂(CO)₄Br₂ with anthracene affords the analogous <i>syn</i>-facial anthracene complex, [<i>cis</i>-{(η⁵-C₅H₃)₂(CMe₂)₂}­Ru₂(μ-η⁶,η⁶-C₁₄H₁₀)]­[OTf]₂, (<b>4</b>), and the tetranuclear dianthracene complex, [<i>cis</i>-{(η⁵-C₅H₃)₂(CMe₂)₂}­Ru₂(μ-η⁶,η⁶-C₁₄H₁₀)]₂[OTf]₄, (<b>5</b>). <b>2</b>^<b>2+</b>, <b>2</b>^<b>0</b>, and <b>5</b> were structurally characterized by X-ray diffraction

Diruthenium Naphthalene and Anthracene Complexes Containing a Doubly Linked Dicyclopentadienyl Ligand

The reaction of <i>cis</i>-{(η⁵-C₅H₃)₂(CMe₂)₂}­Ru₂(CO)₄Br₂ with naphthalene affords the <i>syn</i>-facial [<i>cis</i>-{(η⁵-C₅H₃)₂(CMe₂)₂}­Ru₂(μ-η⁶,η⁶-C₁₀H₈)]­[OTf]₂, (<b>2</b>^<b>2+</b>), a complex that appears to be two electrons short of the 18-electron rule. Density functional theory (DFT) calculations suggest that the Ru atoms satisfy their missing valence by a combination of a weak metal–metal bond and sharing electrons from the central π bond of the naphthalene. The one-electron reduction of <b>2</b>^<b>2+</b> yields <b>2</b>^<b>+</b>, a Class II mixed-valence complex, while the two-electron reduction of <b>2</b>^<b>2+</b> causes a hapticity change from η⁶ to η⁴ on one of the naphthalene rings and yields <i>cis</i>-{(η⁵-C₅H₃)₂(CMe₂)₂}­Ru₂(μ-η⁶,η⁴-C₁₀H₈<b>)</b> (<b>2</b>^<b>0</b>), a zwitterionic complex. The DFT calculations predict that the <i>C</i>_<i>s</i> isomer of <b>2⁰</b> is 4.69 kcal/mol lower in energy than the <i>C</i>_2<i>v</i> isomer, which is a transition state. Reaction of <i>cis</i>-{(η⁵-C₅H₃)₂(CMe₂)₂}­Ru₂(CO)₄Br₂ with anthracene affords the analogous <i>syn</i>-facial anthracene complex, [<i>cis</i>-{(η⁵-C₅H₃)₂(CMe₂)₂}­Ru₂(μ-η⁶,η⁶-C₁₄H₁₀)]­[OTf]₂, (<b>4</b>), and the tetranuclear dianthracene complex, [<i>cis</i>-{(η⁵-C₅H₃)₂(CMe₂)₂}­Ru₂(μ-η⁶,η⁶-C₁₄H₁₀)]₂[OTf]₄, (<b>5</b>). <b>2</b>^<b>2+</b>, <b>2</b>^<b>0</b>, and <b>5</b> were structurally characterized by X-ray diffraction