Phosphorus-Containing Dibenzonaphthanthrenes: Electronic Fine Tuning of Polycyclic Aromatic Hydrocarbons through Organophosphorus Chemistry

A concise synthetic route towards a new family of phosphorus-containing polycyclic aromatic hydrocarbons starting from the versatile acridophosphine has been established. The structural and optoelectronic properties of these compounds were efficiently modulated through derivatization of the phosphorus center. X-ray crystallographic analysis, UV/Vis spectroscopic, and electrochemical studies supported by DFT calculations identified the considerable potential of these scaffolds for the development of organophosphorus functional materials with tailored properties upon further functionalization.

On-Surface Assembly of Hydrogen- and Halogen-Bonded Supramolecular Graphyne-Like Networks

Demonstrated here is a supramolecular approach to fabricate highly ordered monolayered hydrogen‐ and halogen‐bonded graphyne‐like two‐dimensional (2D) materials from triethynyltriazine derivatives on Au(111) and Ag(111). The 2D networks are stabilized by N⋅⋅⋅H−C(sp) bonds and N⋅⋅⋅Br−C(sp) bonds to the triazine core. The structural properties and the binding energies of the supramolecular graphynes have been investigated by scanning tunneling microscopy in combination with density‐functional theory calculations. It is revealed that the N⋅⋅⋅Br−C(sp) bonds lead to significantly stronger bonded networks compared to the hydrogen‐bonded networks. A systematic analysis of the binding energies of triethynyltriazine and triethynylbenzene derivatives further demonstrates that the X₃‐synthon, which is commonly observed for bromobenzene derivatives, is weaker than the X₆‐synthon for our bromotriethynyl derivatives.

Isomeric Dithienophosphepines: The Impact of Ring Fusion on Electronic and Structural Properties

The synthesis and extensive experimental (X-ray crystallography, UV/Vis spectroscopy, cyclic voltammetry) and theoretical (DFT calculations) characterization of two isomeric dithieno[b,f]phosphepines (DTPs) are presented herein. The relative orientation of the phosphepine and the thiophene moieties has a decisive impact on the electronic and structural properties of these compounds. Moreover, the thiophene units allow for a facile subsequent functionalization through direct Pd-catalyzed C-H coupling, which renders DTPs highly promising building blocks for organophosphorus functional materials.

A Spherically Shielded Triphenylamine and Its Persistent Radical Cation

This work reports the design and synthesis of a sterically protected triphenylamine scaffold which undergoes one-electron oxidation to form an amine-centered radical cation of remarkable stability. Several structural adjustments were made to tame the inherent reactivity of the radical cation. First, the parent propeller-shaped triphenylamine was planarized with sterically demanding bridging units and, second, protecting groups were deployed to block the reactive positions. The efficiently shielded triphenylamine core can be reversibly oxidized at moderate potentials (+0.38 V, vs. Fc/Fc+ in CH2 Cl2 ). Spectroelectrochemistry and chemical oxidation studies were employed to monitor the evolution of characteristic photophysical features. To obtain a better understanding of the impact of one-electron oxidation on structural and electronic properties, joint experimental and computational studies were conducted, including X-ray structural analysis, electron paramagnetic resonance (EPR), and density functional theory (DFT) calculations. The sterically shielded radical cation combines various desirable attributes: A characteristic and unobstructed absorption in the visible region, high stability which enables storage for weeks without spectroscopically traceable degradation, and a reliable oxidation/re-reduction process due to effective screening of the planarized triphenylamine core from its environment.

5,7,12,14-Tetrafunctionalized 6,13-Diazapentacenes

The synthesis, property evaluation, and single crystal X-ray structures of four 5,7,12,14-tetrafunctionalized diazapentacenes are presented. The synthesis of these compounds either starts from tetrabromo-N,N-dihydrodiazapentacene or from a diazapentacene tetraketone. Pd-catalyzed coupling or addition of a lithium acetylide gave the precursors that furnish, after further redox reactions, the diazapentacenes as stable crystalline materials. The performance of the tetraphenyl-substituted compound as n-channel semiconductor was evaluated in organic field effect transistors.

Edge Phonon Excitations in a Chiral Self-Assembled Supramolecular Nanoribbon

By design, coupled mechanical oscillators offer a playground for the study of crystalline topology and related properties. Particularly, non-centrosymmetric, supramolecular nanocrystals feature a complex phonon spectrum where edge modes may evolve. Here we show, employing classical atomistic calculations, that the edges of a chiral supramolecular nanoribbon can host defined edge phonon states. We suggest that the topology of several edge modes in the phonon spectrum is nontrivial and thermally insulated from bulk states. By means of molecular dynamics, we excite a supramolecular bond to launch a directional excitation along the edge without considerable bulk or back-propagation. Our results suggest that supramolecular monolayers can be employed to engineer phonon states that are robust against backscattering, toward supramolecular thermal waveguides, diodes, and logics.

Triphenylene-Derived Electron Acceptors and Donors on Ag(111): Formation of Intermolecular Charge-Transfer Complexes with Common Unoccupied Molecular States

Over the past years, ultrathin films consisting of electron donating and accepting molecules have attracted increasing attention due to their potential usage in optoelectronic devices. Key parameters for understanding and tuning their performance are intermolecular and molecule-substrate interactions. Here, the formation of a monolayer thick blend of triphenylene-based organic donor and acceptor molecules from 2,3,6,7,10,11-hexamethoxytriphenylene (HAT) and 1,4,5,8,9,12-hexaazatriphenylenehexacarbonitrile (HATCN), respectively, on a silver (111) surface is reported. Scanning tunneling microscopy and spectroscopy, valence and core level photoelectron spectroscopy, as well as low-energy electron diffraction measurements are used, complemented by density functional theory calculations, to investigate both the electronic and structural properties of the homomolecular as well as the intermixed layers. The donor molecules are weakly interacting with the Ag(111) surface, while the acceptor molecules show a strong interaction with the substrate leading to charge transfer and substantial buckling of the top silver layer and of the adsorbates. Upon mixing acceptor and donor molecules, strong hybridization occurs between the two different molecules leading to the emergence of a common unoccupied molecular orbital located at both the donor and acceptor molecules. The donor acceptor blend studied here is, therefore, a compelling candidate for organic electronics based on self-assembled charge-transfer complexes.

Binary supramolecular networks of bridged triphenylamines with different substituents and identical scaffolds

Based on scanning tunneling microscopy experiments combined with density functional theory, we report the formation and the electronic structure of porous binary supramolecular networks on Au(111). The two triphenylamine derivatives with identical scaffolds intermix due to a maximization of the overall number of H-bonds instead of an optimization of the H-bond strength in the bonding motif. The HOMO-LUMO gap is defined by both molecules, which is typical for electron donor-acceptor networks.

Two-dimensional delocalized states in organometallic bis-acetylide networks on Ag(111)

The electronic structure of surface-supported organometallic networks with Ag-bis-acetylide bonds that are intermediate products in the bottom-up synthesis of graphdiyne and graphdiyne-like networks were studied. Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal a frontier, unoccupied electronic state that is delocalized along the entire organometallic network and proves the covalent nature of the Ag-bis-acetylide bonds. Density-functional theory (DFT) calculations corroborate the spatial distribution of the observed delocalized state and attribute it to band mixing of carbon and silver atoms combined with n-doping of the metal surface. The metal-bis-acetylide bonds are typical metal-organic bonds with mixed character containing covalent and strong ionic contributions. Moreover, the organometallic networks exhibit a characteristic graphene-like band structure with linear band dispersion at each K point.

Surface-confined [2 + 2] cycloaddition towards one-dimensional polymers featuring cyclobutadiene units

Surface-confined synthesis has been offering a wide range of opportunities for the construction of novel molecular nanostructures. Exploring new types of on-surface coupling reactions is considered essential for being able to deliberately tune the materials properties. Here, we report on the formation of a covalent C-C bonding motif, namely 1,3-cyclobutadiene, via surface-confined [2 + 2] cycloaddition between pyrene moieties using low temperature scanning tunneling microscopy (LT-STM) and X-ray photoemission spectroscopy (XPS) measurements. By employing a hydrogen dosing treatment together with low-temperature activation, we were able to both eliminate residual byproducts and obtain covalent 1D polymers through the formation of 1,3-cyclobutadiene units. The resulting C-C bonding motif has so far hardly been explored in surface chemistry and substantial evidence is provided that the hydrogen treatment is crucial towards the removal of byproducts in surface-confined polymerization.

p-Doping of graphene in hybrid materials with 3,10-diazapicenium dications

N,N'-Didodecyl-substituted 3,10-diazapicenium salts featuring bromide and hexafluorophosphate counterions have been designed as novel dopants to realize individualized graphene sheets in a series of cutting edge experiments and to intrinsically stabilize them via p-doping. Importantly, electrochemical studies revealed two consecutive irreversible one-electron reductions of the N,N'-didodecyl-substituted 3,10-diazapicenium salts to yield the corresponding radical cation and neutral quinoidal species. Formation of both species was accompanied by characteristic changes in the absorption spectra. The 3,10-diazapicenium bromide was found to be a potent dopant to produce hybrid materials with exfoliated graphene. Microscopy based on AFM and TEM imaging and spectroscopy based on Raman probing corroborated that, upon drying, the hybrid material consists of few layer (5-8 layers) turbostratic graphene sheets that are p-doped. Our findings identify the newly synthesized N,N'-dialkylated 3,10-diazapicenium salts as highly promising candidates for the fabrication of functional graphene materials with tailored properties.

Organic Electron Acceptors Comprising a Dicyanomethylene-Bridged Acridophosphine Scaffold: The Impact of the Heteroatom

Stable two-electron acceptors comprising a dicyanomethylene-bridged acridophosphine scaffold were synthesized and their reversible reduction potentials were efficiently tuned through derivatization of the phosphorus center. X-ray crystallographic analysis combined with NMR, UV/Vis, IR spectroscopic, and electrochemical studies, supported by theoretical calculations, revealed the crucial role of the phosphorus atom for the unique redox, structural, and photophysical properties of these compounds. The results identify the potential of these electron deficient scaffolds for the development of functional n-type materials and redox active chromophores upon further functionalization.

Hierarchical on-surface synthesis and electronic structure of carbonyl-functionalized one- and two-dimensional covalent nanoarchitectures

The fabrication of nanostructures in a bottom-up approach from specific molecular precursors offers the opportunity to create tailored materials for applications in nanoelectronics. However, the formation of defect-free two-dimensional (2D) covalent networks remains a challenge, which makes it difficult to unveil their electronic structure. Here we report on the hierarchical on-surface synthesis of nearly defect-free 2D covalent architectures with carbonyl-functionalized pores on Au(111), which is investigated by low-temperature scanning tunnelling microscopy in combination with density functional theory calculations. The carbonyl-bridged triphenylamine precursors form six-membered macrocycles and one-dimensional (1D) chains as intermediates in an Ullmann-type coupling reaction that are subsequently interlinked to 2D networks. The electronic band gap is narrowed when going from the monomer to 1D and 2D surface-confined π-conjugated organic polymers comprising the same building block. The significant drop of the electronic gap from the monomer to the polymer confirms an efficient conjugation along the triphenylamine units within the nanostructures.

Stability of Odd- Versus Even-Electron Gas-Phase (Quasi)Molecular Ions Derived from Pyridine-Substituted N-Heterotriangulenes

Invited for this month's cover are the collaborating groups at the Friedrich-Alexander University Erlangen-Nürnberg (FAU), Germany and at the Max-Planck-Institut für Kohlenforschung, Germany. The cover picture shows the symbiosis of quantum chemical theory and gas-phase collision experiment investigating the influence of the electronic state on stability of the radical cation ([M]^{+ .} ) and protonated triangulene ([M+H]⁺ ). The dissociation of the radical cation requires less energy due to the formation of an energetically favored extended aromatic π-system. Read the full text of the article at 10.1002/cplu.201600416.