Intermolecular Coupling Research Articles

Extending on-surface synthesis from 2D to 3D by cycloaddition with C60

As an efficient molecular engineering approach, on-surface synthesis (OSS) defines a special opportunity to investigate intermolecular coupling at the sub-molecular level and has delivered many appealing polymers. So far, all OSS is based on the lateral covalent bonding of molecular precursors within a single molecular layer; extending OSS from two to three dimensions is yet to be realized. Herein, we address this challenge by cycloaddition between C60 and an aromatic compound. The C60 layer is assembled on the well-defined molecular network, allowing appropriate molecular orbital hybridization. Upon thermal activation, covalent coupling perpendicular to the surface via [4 + 2] cycloaddition between C60 and the phenyl ring of the molecule is realized; the resultant adduct shows frozen orientation and distinct sub-molecular feature at room temperature and further enables lateral covalent bonding via [2 + 2] cycloaddition. This work unlocks an unconventional route for bottom-up precise synthesis of three-dimensional covalently-bonded organic architectures/devices on surfaces.

Energy relaxation pathways and their isotope effects of water bending mode in liquid phase: A nonequilibrium ab initio molecular dynamics simulation study

Vibrational energy relaxation pathways and the rates of excited bending mode of water molecules (H2O) in bulk liquid water (H2O), and the rates of the excited bending mode of HOD molecules in D2O (isotopically diluted water) were examined via non-equilibrium ab initio molecular dynamics (NE-AIMD) simulation. The NE-AIMD simulation showed a significantly fast relaxation of the bend vibration, at the timescale of 127 fs in pure water and 144 fs in isotopically diluted water. The main relaxation pathway for pure and isotopically diluted water was found to be driven by intramolecular bend–libration coupling (84% in pure water and 96% in isotopically diluted water). In the case of pure water, intermolecular coupling between bend–bend vibrations was small but non-negligible (11%), whereas such intermolecular coupling was negligible in isotopically diluted water owing to a large frequency mismatch between the excited bend vibration and the surrounding bend vibrations.

One-Pot Dual C-C Bond-Forming Cascade Process via Suzuki Coupling and Intramolecular Cyclocondensation: An Access to Functionalized Naphthalenes.

Naphthyl ketones encompass a class of essential compounds of biological and industrial significance. The development of synthetic strategies to achieve naphthyl ketones in single-step reactions from readily available materials is highly desirable. Herein, domino one-pot synthesis of functionalized naphthyl ketones via intermolecular Suzuki-Miyaura coupling, followed by intramolecular distal aldol-type condensation with the γ-methyl/methylene groups is reported. The present strategy displayed a comprehensive substrate scope and good functional group tolerance and enabled access to a variety of substituted naphthyl ketones, including aromatic-aliphatic hybrid frameworks of interesting structural features starting from the easily accessible starting materials such as α-iodoenone and 2-formylphenylboronic acids as well as 2-formylphenylboronic acid pinacol esters.

Multicopper Clusters Enable Oxidative Phenol Macrocyclization (OxPM) of Peptides.

The biosynthesis of glycopeptide antibiotics such as vancomycin and other biologically active biaryl-bridged and diaryl ether-linked macrocyclic peptides includes key enzymatic oxidative phenol macrocyclization(s) of linear precursors. However, a simple and step-economical biomimetic version of this transformation remains underdeveloped. Here, we report highly efficient conditions for preparing biaryl-bridged and diaryl ether-linked macrocyclic peptides based on multicopper(II) clusters. The selective syntheses of ring models of vancomycin and the arylomycin cyclic core illustrate the potential of this technology to facilitate the assembly of complex antibiotic macrocyclic peptides, whose syntheses are considered highly challenging. The unprecedented ability of multicopper(II) clusters to chelate tethered diphenols and promote intramolecular over intermolecular coupling reactions demonstrates that copper clusters can catalyze redox transformations that cannot be accessed by smaller metal catalysts.

Hydrogen-bonding and vibrational coupling of water in a hydrophobic hydration shell: Significance of alkyl chain configuration and charge on the hydrophobe

The interaction between a molecular hydrophobe and water is crucial for their mutual structure and reactivity. Here, we have investigated the hydrogen-bonding (H-bonding) structure and intermolecular vibrational coupling i.e., the collective nature of water, in the hydration shell (HS) of charged and uncharged molecular hydrophobes, using polarization-resolved Raman difference spectroscopy with simultaneous curve fitting (Raman-DS-SCF) and isotopic dilution spectroscopy. Raman-DS-SCF provides the vibrational spectrum of water pertaining to the HS of the hydrophobes. The HS-spectra reveal that the positive charge on the hydrophobe (e.g., deuterated tetramethylammonium cation, d-TMA+) as well as the branching of the alkyl chain (n-butyl alcohol (BA) vs. tert-butyl alcohol (TBA)) reinforces the intermolecular vibrational coupling of water in the HS. The water-water H-bonding in the HS of d-TMA+ and d-TBA are similar to each other, but stronger than that of the bulk water. Apart from the strongly H-bonded water, the hydrophobic hydration shells exhibit weakly interacting dangling OH (ν¯max≈3670) which is intramolecularly coupled with the H-bonded OH of the same water molecule. The prevalence of the dangling-OH decreases with chain-branching and net positive charge on the hydrophobe: BA > d-TBA > d-TMA+. Altogether, the collective nature of water and its propensity for dangling-OH in a hydrophobic hydration shell strongly depend on the net charge and alkyl chain configuration of the hydrophobe.

Ketyl Radical Coupling Enabled by Polycyclic Aromatic Hydrocarbon Electrophotocatalysts.

Herein, we report a new class of electrophotocatalysts, polycyclic aromatic hydrocarbons, that promote the reduction of unactivated carbonyl compounds to generate versatile ketyl radical intermediates. This catalytic platform enables previously challenging intermolecular ketyl radical coupling reactions, including those that classic reductants (e.g., SmI2/HMPA) have failed to promote. More broadly, this study outlines an approach to fundamentally expand the array of reactive radical intermediates that can be generated via electrophotocatalysis by obviating the need for rapid mesolytic cleavage following substrate reduction.

TfOH‐Catalyzed Highly Regioselective Synthesis of Conjugated Enones by Coupling of Alkynes and Aldehydes

AbstractTriflic acid‐catalyzed regioselective enone synthesis from intermolecular coupling of aldehydes and alkynes has been developed. The present reaction is very efficient, effective, simple in nature and mild. The substrate scope is very diverse. This metathesis reaction exclusively provided trans product using aryl and aliphatic aldehydes and internal and terminal alkynes.

Surface modification of chitosan with Ni(II) Schiff base complex: A new heterogeneous catalyst for the synthesis of xanthones

A new biocomposite of chitosan, chitosan-supported di(pyridine-2-yl)methanone-Ni(II) complex, CS-DPM-Ni, is synthesized for the first time. The biocomposite is thoroughly characterized by FTIR, PXRD, XPS, FESEM, EDX, TGA, ICP-OES, and elemental analysis. The synthesized composite is successfully used as a heterogeneous catalyst in the synthesis of a library of xanthone derivatives by the intermolecular catalytical coupling of 2-substituted benzaldehydes and phenols. The catalyst could be retrieved from the reaction mixture by simple filtration and reused for up to four catalytic cycles. All products were isolated in good to high yields (65–90 %) with good turnover numbers (TONs), and fully characterized by 1H and 13C{1H} NMR spectroscopy. The green chemistry metrics values for the reaction were discerned and found to be close to the ideal values.

Intermolecular 3D-MoRSE Descriptors for Fast and Accurate Prediction of Electronic Couplings in Organic Semiconductors.

The theoretical rational design of organic semiconductors faces an obstacle in that the performance of organic semiconductors depends very much on their stacking and local morphology (for example, phase domains), which involves numerous molecules. Simulation becomes computationally expensive as intermolecular electronic couplings have to be calculated from density functional theory. Therefore, developing fast and accurate methods for intermolecular electronic coupling estimation is essential. In this work, by developing a series of new intermolecular 3D descriptors, we achieved fast and accurate prediction of electronic couplings in both crystalline and amorphous thin films. Three groups of developed descriptors could perform faster and higher accuracy prediction on electronic couplings than the most advanced state-of-the-art descriptors. This work paves the way for large-scale simulations, high-throughput calculations, and screening of organic semiconductors.

Methoxy management of side chains on the carbazole-diphenylamine derivatives based hole transport materials for perovskite solar cells: A theoretical design and experimental research

Molecular design is the important approach for realizing the improvement of performance for hole transport materials (HTMs) in perovskite solar cells (PSCs). In this work, starting from the carbazole-diphenylamine derivatives based HTMs, three molecules (CX1-CX3) are designed through methoxy management on the side chains. The calculated results indicated that CX1-CX3 yield different molecular arrangements through methoxy management on the side chains, resulting in adjusting the intermolecular coupling strength and hole transport capabilities. In comparison with the designed CX2 and CX3, CX1 exhibits a higher hole mobility, stronger interfacial interactions and more efficient charge transfer at perovskite/HTMs interface. Hence, under the same conditions, PSC devices based on CX1 yields a PCE of 22.13% higher than those of other molecules CX2 (20.96%) and CX3 (19.72%). These findings highlight the significance of methoxy management on the side chains in HTMs for the photovoltaic performance of PSCs, and reveal the potential of molecular engineering for the design of efficient HTMs.

Vibrational Coupling and Hydrogen-Bond Structure of Water in the Extended Hydration Shell of Metal Ions.

Intra- and intermolecular vibrational coupling (VC) and hydrogen bonding (H-bonding) of water are sparsely understood in the hydration shell (HS) of a metal ion, though the corresponding knowledge for an anion is quite extensive. This is primarily due to the overwhelming effect of anions on water, which masks the subtle perturbing influence of most of the cations. Using Raman difference spectroscopy with simultaneous curve fitting (Raman-DS-SCF) in combination with isotopic dilution and polarized Raman spectroscopy, we have elucidated the VC and H-bonding of water in the HS of bi- and trivalent metal ions─Mg2+, Ca2+, La3+, Gd3+, Dy3+. Polarized Raman measurement of the HS water with VC "turned on" and "turned off" (using isotopically diluted water, HOD) reveals that water retains the intra- and intermolecular vibrational coupling in the HS of high-charge-density metal ions, which is in stark contrast to that of an anion. Hydration shell spectroscopy in HOD unambiguously shows that the average H-bonding of water becomes stronger in the HS than that of bulk water. The first HS water strongly donates two H-bonds to the second HS water (ν̅max ≈ 3200 cm-1) but weakly accepts a H-bond from the second HS water (ν̅max ≈ 3590 cm-1), which makes the HS water heterogeneous in terms of its H-bond structure. The weakly interacting OH (ν̅max 3585 cm-1 in HOD) red-shifts by ∼ 15 cm-1 while the VC is "turned on" (ν̅max ≈ 3600 cm-1 in H2O), revealing the intramolecular coupling of water in the HS of metal ions.

Controlling Intramolecular and Intermolecular Electronic Coupling of Radical Ligands in a Series of Cobaltoviologen Complexes.

Controlling electronic coupling between multiple redox sites is of interest for tuning the electronic properties of molecules and materials. While classic mixed-valence (MV) systems are highly tunable, e.g., via the organic bridges connecting the redox sites, metal-bridged MV systems are difficult to control because the electronics of the metal cannot usually be altered independently of redox-active moieties embedded in its ligands. Herein, this limitation was overcome by varying the donor strengths of ancillary ligands in a series of cobalt complexes without directly perturbing the electronics of viologen-like redox sites bridged by the cobalt ions. The cobaltoviologens [1X-Co]n+ feature four 4-X-pyridyl donor groups (X = CO2Me, Cl, H, Me, OMe, NMe2) that provide gradual electronic tuning of the bridging CoII centers, while a related complex [2-Co]n+ with NHC donors supports exclusively CoIII states even upon reduction of the viologen units. Electrochemistry and IVCT band analysis indicate that the MV states of these complexes have electronic structures ranging from fully localized ([2-Co]4+; Robin-Day Class I) to fully delocalized ([1CO2Me-Co]3+; Class III) descriptions, demonstrating unprecedented control over electronic coupling without changing the identity of the redox sites or bridging metal. Additionally, single-crystal XRD characterization of the homovalent complexes [1H-Co]2+ and [1H-Zn]2+ revealed radical-pairing interactions between the viologen ligands of adjacent complexes, representing a type of through-space electronic coupling commonly observed for organic viologen radicals but never before seen in metalloviologens. The extended solid-state packing of these complexes produces 3D networks of radical π-stacking interactions that impart unexpected mechanical flexibility to these crystals.

Net-1,2-Hydrogen Atom Transfer of Amidyl Radicals: Toward the Synthesis of 1,2-Diamine Derivatives.

Hydrogen atom transfer (HAT) processes are among the most useful approaches for the selective construction of C(sp3)-C(sp3) bonds. 1,5-HAT with heteroatom-centered radicals (O•, N•) have been well established and are favored relative to other 1,n-HAT processes. In comparison, net 1,2-HAT processes have been observed infrequently. Herein, the first amidyl radicalls are reported that preferentially undergo a net 1,2-HAT over 1,5-HAT. Beginning with single electron transfer from 2-azaallyl anions to N-alkyl N-aryloxy amides, the latter generate amidyl radicals. The amidyl radical undergoes a net-1,2-HAT to generate a C-centered radical that participates in an intermolecular radical-radical coupling with the 2-azaallyl radical to generate 1,2-diamine derivatives. Mechanistic and EPR experiments point to radical intermediates. Density functional theory calculations provide support for a base-assisted, stepwise-1,2-HAT process. It is proposed that the generation of amidyl radicals under basic conditions can be greatly expanded to access α-amino C-centered radicals that will serve as valuable synthetic intermediates.

Spin-Based Chiral Separations and the Importance of Molecule-Solvent Interactions.

This work uses magneto-electrochemical quartz crystal microbalance methods to study the enantiospecific adsorption of chiral molecules onto a ferromagnetic substrate. The effects of solution conditions, pH, and solvent isotope composition indicate that the kinetics of the enantiomeric adsorption depend strongly on the charge state and geometry of the adsorbate, whereas no thermodynamic contributions to enantiospecificity are found. Density functional theory calculations reveal that an interplay between the adsorbate and solvent molecules is important for defining the observed enantiospecific preference with an applied magnetic field; however, it remains unclear if intermolecular vibrational couplings contribute to the phenomenon.

Hydrogen-Bonding Trends in a Bithiophene with 3- and/or 4-Pyridyl Substituents.

To improve the charge-carrier transport capabilities of thin-film organic materials, the intermolecular electronic couplings in the material should be maximized. Decreasing intermolecular distance while maintaining proper orbital overlap in highly conjugated aromatic molecules has so far been a successful way to increase electronic coupling. We attempted to decrease the intermolecular distance in this study by synthesizing cocrystals of simple benzoic acid coformers and dipyridyl-2,2'-bithiophene molecules to understand how the coformer identity and pyridine N atom placement affected solid-state properties. We found that with the 5-(3-pyridyl)-5'-(4-pyridyl)-isomer, the 4-pyridyl ring interacted with electrophiles and protons more strongly. Synthesized cocrystal powders were found to have reduced average crystallite size in reference to the parent compounds. The opposite was found for the intermolecular electronic couplings, as determined via density functional theory (DFT) calculations, which were relatively large in some of the cocrystals.

Electronic coarse-graining of long conjugated molecules: Case study of non-fullerene acceptors.

By considering only one electronic state per molecule, charge transport models of molecular solids neglect intramolecular charge transfer. This approximation excludes materials with quasi-degenerate spatially separated frontier orbitals, such as non-fullerene acceptors (NFAs) and symmetric thermally activated delayed fluorescence emitters. By analyzing the electronic structure of room-temperature molecular conformers of a prototypical NFA, ITIC-4F, we conclude that the electron is localized on one of the two acceptor blocks with the mean intramolecular transfer integral of 120meV, which is comparable with intermolecular couplings. Therefore, the minimal basis for acceptor-donor-acceptor (A-D-A) molecules consists of two molecular orbitals localized on the acceptor blocks. This basis is robust even with respect to geometry distortions in an amorphous solid, in contrast to the basis of two lowest unoccupied canonical molecular orbitals withstanding only thermal fluctuations in a crystal. The charge carrier mobility can be underestimated by a factor of two when using single site approximation for A-D-A molecules in their typical crystalline packings.

Theoretical study of vertical van der Walls metal-porphyrin and metal free-porphyrin junctions

This paper provides a theoretical study of the thermoelectric properties of a vertical graphene/porphyrins/graphene architecture. It presents the details of calculating the conductance of metal-porphyrins, which is found to be enhanced by manipulating the metal central atom of the organic 125 porphyrin framework over the family and The results demonstrate that even when there is no direct inter-molecular coupling, indirect inter-molecular interactions mediated by the graphene electrodes produce quantum interference effects in the electronic structure of the molecular junction. These junctions are all observed to be HOMO-dominated, meaning that their Seebeck exhibit the same sign and similar behavior. The resulting single-molecule thermopowers range from almost +50 μV/K for both - and -porphyrin to +77 μV/K and +85 μV/K for - and -porphyrin, respectively. For these geometries, the effect of the metal complex with porphyrin on the conductance of the junctions can be seen. An extra resonance appeared in the HOMO-LOMO gap, and it can be disappeared or be shifted closer to Fermi energy to create a new path for the electron transmission. It only depends on the type of metal coordinating at the porphyrin center. Introducing such a new technique for designing high conductance and thermopower opens a door for high thermoelectric performance materials.

Dynamic Effects on Hole Transport in Amorphous Organic Semiconductors: a Combined QM/MM and kMC Study.

Small-molecule-based amorphous organic semiconductors (OSCs) are essential components of organic photovoltaics and organic light-emitting diodes. The charge carrier mobility of these materials is an integral and limiting factor in regard to their performance. Integrated computational models for the hole mobility, taking into account structural disorder in systems of several thousand molecules, have been the object of research in the past. Due to static and dynamic contributions to the total structural disorder, efficient strategies to sample the charge transfer parameters become necessary. In this paper, we investigate the impact of structural disorder in amorphous OSCs on the transfer parameters and charge mobilities in different materials. We present a sampling strategy for incorporating static and dynamic structural disorder which are based on QM/MM methods using semiempirical Hamiltonians and extensive MD sampling. We show how the disorder affects the distributions of HOMO energies and intermolecular couplings and validate the results using kinetic Monte Carlo simulations of the mobility. We find that dynamic disorder causes an order of magnitude difference in the calculated mobility between morphologies of the same material. Our method allows the sampling of disorder in HOMO energies and couplings, and the statistical analysis enables us to characterize the relevant time scales on which charge transfer occurs in these complex materials. The findings presented here shed light on the interplay of the fluctuating amorphous matrix with charge carrier transport and aid in the development of a better understanding of these complex processes.

Nickel-Catalyzed Intermolecular Reductive Coupling of Styrene and Its Derivatives with Imines Using Methanol as a Reductant

Catalytic reductive coupling of alkenes with imines has emerged as a powerful tool for amine synthesis via C–C bond formation with the aid of a reductant. However, many of these processes require the use of activated alkenes or hydrosilanes as reductants. Here, we report a nickel-catalyzed intermolecular reductive coupling of styrenes with imines utilizing methanol as a reductant. The key to the success of the reaction is the introduction of MeOLi as an additive, which promotes the ring opening of the aza-nickelacycle intermediate and facilitates hydride transfer from the methoxy group to nickel. This pathway, which involves the reductive transformation of saturated aza-nickelcycles, enables highly linear-selective reductive coupling of alkenes with imines.

Manipulating hydrogen bond dissociation rates and mechanisms in water dimer through vibrational strong coupling

The vibrational strong coupling (VSC) between molecular vibrations and cavity photon modes has recently emerged as a promising tool for influencing chemical reactivities. Despite numerous experimental and theoretical efforts, the underlying mechanism of VSC effects remains elusive. In this study, we combine state-of-art quantum cavity vibrational self-consistent field/configuration interaction theory (cav-VSCF/VCI), quasi-classical trajectory method, along with the quantum-chemical CCSD(T)-level machine learning potential, to simulate the hydrogen bond dissociation dynamics of water dimer under VSC. We observe that manipulating the light-matter coupling strength and cavity frequencies can either inhibit or accelerate the dissociation rate. Furthermore, we discover that the cavity surprisingly modifies the vibrational dissociation channels, with a pathway involving both water fragments in their ground vibrational states becoming the major channel, which is a minor one when the water dimer is outside the cavity. We elucidate the mechanisms behind these effects by investigating the critical role of the optical cavity in modifying the intramolecular and intermolecular coupling patterns. While our work focuses on single water dimer system, it provides direct and statistically significant evidence of VSC effects on molecular reaction dynamics.