On-Surface Synthesis of Square-Type Porphyrin Tetramers with Central Antiaromatic Cyclooctatetraene Moiety

Distance and Voltage Dependence of Orbital Density Imaging Using a CO-Functionalized Tip in Scanning Tunneling Microscopy

The appearance of frontier molecular ion resonances measured with scanning tunneling microscopy (STM)─often referred to as orbital density images─of single molecules was investigated using a CO-functionalized tip in dependence on bias voltage and tip-sample distance. As model systems, we studied pentacene and naphthalocyanine on bilayer NaCl on Cu(111). Absolute tip-sample distances were determined by means of atomic force microscopy (AFM). STM imaging revealed a transition from predominant p- to s-wave tip contrast upon increasing the tip-sample distance, but the contrast showed only small changes as a function of voltage. The distance-dependent contrast change is explained with the steeper decay of the tunneling matrix element for tunneling between two p-wave centers, compared to tunneling between two s-wave centers. In simulations with a fixed ratio of s- to p-wave tip states, we can reproduce the experimental data including the distance-dependent transition from predominant p- to s-wave tunneling contribution.

Solution Versus On-Surface Synthesis of Peripherally Oxygen-Annulated Porphyrins through C-O Bond Formation

This study investigates the synthesis of tetra- and octa-O-fused porphyrinoids employing an oxidative O-annulation approach through C-H activation. Despite encountering challenges such as overoxidation and instability in conventional solution protocols, successful synthesis was achieved on Au(111) surfaces under ultra-high vacuum (UHV) conditions. X-ray photoelectron spectroscopy, scanning tunneling microscopy, and non-contact atomic force microscopy elucidated the preferential formation of pyran moieties via C-O bond formation and subsequent self-assembly driven by C-H⋅⋅⋅O interactions. Furthermore, the O-annulation process was found to reduce the HOMO-LUMO gap by lifting the HOMO energy level, with the effect rising upon increasing the number of embedded O-atoms.

Planar and Curved π-Extended Porphyrins by On-Surface Cyclodehydrogenation

Recent advancements in on-surface synthesis have enabled the reliable and predictable preparation of atomically precise low-dimensional materials with remarkable properties, which are often unattainable through traditional wet chemistry. Among these materials, porphyrins stand out as a particularly intriguing class of molecules, extensively studied both in solution and on surfaces. Their appeal lies in the ability to fine-tune their unique chemical and physical properties through central metal exchange or peripheral functionalization. However, the synthesis of π-extended porphyrins featuring unsubstituted anthracenyl groups has remained elusive. Herein, we report an in vacuo temperature-controlled cyclodehydrogenation of bis- and tetraanthracenyl Zn(II) porphyrins on a gold(111) surface. By gradually increasing the temperature, sequential dehydrogenation leads to the formation of fused anthracenyl porphyrin products. Notably, at high molecular coverage, the formation of bowl-shaped porphyrins occurs, along with transmetalation of Zn with Au. These findings open the door to a variety of π-extended anthracenyl-containing porphyrin products via cyclodehydrogenation and transmetalation, offering significant potential in the fields of molecular (photo/electro)catalysis, (opto)electronics, and spintronics.

π-extended pyrenes: from an antiaromatic buckybowl to doubly curved nanocarbons with gulf architectures

The synthesis of π-extended pyrenes keeps attracting considerable attention. In particular, frameworks containing nonbenzenoid rings might display intriguing properties. Here, we report a practical synthetic pathway to access a new buckybowl (1), which is composed of four five-membered rings externally fused to a pyrene core. The buckybowl 1 exhibits antiaromaticity involving 22 π-electrons, a rapid bowl-to-bowl interconversion, and a small band gap. Furthermore, this buckybowl could be subjected to Scholl cyclodehydrogenation to prepare the doubly curved nanocarbons (2rac and 2meso), which exist as two diastereomers, as demonstrated by X-ray crystal structure determination. Variable temperature 1H NMR measurements reveal that 2meso can isomerize into 2rac under thermal conditions, with an activation free energy of 27.1 kcal mol-1. Both the enantiomers of 2rac can be separated by chiral HPLC and their chiroptical properties are thoroughly examined. In addition, the nanocarbon 2meso with two gulf architectures facilitates host-guest chemistry with a variety of guests, including PDI, TDI, C60 and C70.

On-Surface Synthesis of Polyene-Linked Porphyrin Cooligomer

π-Conjugated molecules are viewed as fundamental components in forthcoming molecular nanoelectronics in which semiconducting functional units are linked to each other via metallic molecular wires. However, it is still challenging to construct such block cooligomers on the surface. Here, we present a synthesis of [18]-polyene-linked Zn-porphyrin cooligomers via a two-step reaction of the alkyl groups on Cu(111) and Cu(110). Nonyl groups (-C9H19) substituted at the 5,15-meso positions of Zn-porphyrin were first transformed to alkenyl groups (-C9H10) by dehydrogenation. Subsequently, homocoupling of the terminal -CH2 groups resulted in the formation of extended [18]-polyene-linked porphyrin cooligomers. The structures of the products at each reaction step were investigated by bond-resolved scanning tunneling microscopy at low temperatures. A combination of angle-resolved photoemission spectroscopy and density functional theory calculations revealed the metallic property of the all trans [18]-polyene linker on Cu(110). This finding may provide an approach to fabricate complex nanocarbon structures on the surface.

Automated Generation and Analysis of Molecular Images Using Generative Artificial Intelligence Models

The development of scanning probe microscopy (SPM) has enabled unprecedented scientific discoveries through high-resolution imaging. Simulations and theoretical analysis of SPM images are equally important as obtaining experimental images since their comparisons provide fruitful understandings of the structures and physical properties of the investigated systems. So far, SPM image simulations are conventionally based on quantum mechanical theories, which can take several days in tasks of large-scale systems. Here, we have developed a scanning tunneling microscopy (STM) molecular image simulation and analysis framework based on a generative adversarial model, CycleGAN. It allows efficient translations between STM data and molecular models. Our CycleGAN-based framework introduces an approach for high-fidelity STM image simulation, outperforming traditional quantum mechanical methods in efficiency and accuracy. We envision that the integration of generative networks and high-resolution molecular imaging opens avenues in materials discovery relying on SPM technologies.

Bespoke Tailoring of Graphenoid Sheets: A Rippled Molecular Carbon Comprising Cyclically Fused Nonbenzenoid Rings

The incorporation of nonbenzenoid rings into the hexagonal networks of graphenoid nanostructures is of immense importance for electronic, magnetic, and mechanical properties, but the underlying mechanisms of nonbenzenoid ring fusion are rather unexplored. Here, we report the synthesis and characterization of a rippled C84 molecular carbon, which contains 10 nonbenzenoid rings (five-, seven-, and eight-membered rings) that are contiguously fused to give a cyclic geometry. The fused nonbenzenoid rings impart high solubility, configurational stability, multiple reversible redox behaviors, unique aromaticity, and a narrow band gap to the system. Moreover, this carbon nanostructure allows for further functionalization via electrophilic substitution and metalation reactions, enabling access to finely tuned derivatives. Interestingly, both the bowl-shaped and planar conformations of the core in molecular carbon are observed in the solid state. Additionally, this molecular carbon displays ambipolar transport characteristics.

On-surface synthesis of Au-C4 and Au-O4 alternately arranged organometallic coordination networks via selective aromatic C-H bond activation

Selective activation of the C-H bond of aromatic hydrocarbons is significant in synthetic chemistry. However, achieving oriented C-H activation remains challenging due to the poor selectivity of aromatic C-H bonds. Herein, we successfully constructed alternately arranged Au-C4 and Au-O4 organometallic coordination networks through selective aromatic C-H bond activation on Au(111) substrate. The stepwise reaction process of the 5, 12-dibromopyrene 3,4,9, 10-tetracarboxylic dianhydride precursor is monitored by high-resolution scanning tunneling microscopy. Our results show that the gold atoms in C-Au-C organometallic chains play a crucial role in promoting the selective ortho C-H bonds activation and forming Au-C4 coordination structure, which is further demonstrated by a comparative experiment of PTCDA precursor on Au(111). Furthermore, our experiment of 2Br-PTCDA precursor on Cu(111) substrate confirms that copper atoms in C-Cu-C organometallic chains can also assist the formation of Cu-C4 coordination structure. Our results reveal the vital effect of organometallic coordination on selective C-H bond activation of reactants, which holds promising implications for controllable on-surface synthesis.

Porphene and porphite as porphyrin analogs of graphene and graphite

Two-dimensional materials have unusual properties and promise applications in nanoelectronics, spintronics, photonics, (electro)catalysis, separations, and elsewhere. Most are inorganic and their properties are difficult to tune. Here we report the preparation of Zn porphene, a member of the previously only hypothetical organic metalloporphene family. Similar to graphene, these also are fully conjugated two-dimensional polymers, but are composed of fused metalloporphyrin rings. Zn porphene is synthesized on water surface by two-dimensional oxidative polymerization of a Langmuir layer of Zn porphyrin with K2IrCl6, reminiscent of known one-dimensional polymerization of pyrroles. It is transferable to other substrates and bridges μm-sized pits. Contrary to previous theoretical predictions of metallic conductivity, it is a p-type semiconductor due to a predicted Peierls distortion of its unit cell from square to rectangular, analogous to the appearance of bond-length alternation in antiaromatic molecules. The observed reversible insertion of various metal ions, possibly carrying a fifth or sixth ligand, promises tunability and even patterning of circuits on an atomic canvas without removing any π centers from conjugation.

Orbital and Skeletal Structure of a Single Molecule on a Metal Surface Unveiled by Scanning Tunneling Microscopy

Atomic-scale spatial characteristics of a phthalocyanine orbital and skeleton are obtained on a metal surface with a scanning tunneling microscope and a CO-functionalized tip. Intriguingly, the high spatial resolution of the intramolecular electronic patterns is achieved without resonant tunneling into the orbital and despite the hybridization of the molecule with the reactive Cu substrate. The resolution can be fine-tuned by the tip-molecule distance, which controls the p-wave and s-wave contribution of the molecular probe to the imaging process. The detailed structure is deployed to minutely track the translation of the molecule in a reversible interconversion of rotational variants and to quantify relaxations of the adsorption geometry. Entering into the Pauli repulsion imaging mode, the intramolecular contrast loses its orbital character and reflects the molecular skeleton instead. The assignment of pyrrolic-hydrogen sites becomes possible, which in the orbital patterns remains elusive.