Surface cis Effect: Influence of an Axial Ligand on Molecular Self-Assembly

Tuning Surface Organic Structures by Small Gas Molecules through Catassembly and Coassembly

The field of molecular assembly has seen remarkable advancements across various domains, such as materials science, nanotechnology, and biomedicine. Small gas molecules serve as pivotal modulators, capable of altering the architecture of assemblies via tuning a spectrum of intermolecular forces including hydrogen bonding, dipole-dipole interactions, and metal coordination. Surface techniques, notably scanning tunneling microscopy and atomic force microscopy, have proven instrumental in dissecting the structural metamorphosis and characteristic features of these assemblies at an unparalleled single-molecule resolution. Recent research has spotlighted two innovative approaches for modulating surface molecular assemblies with the aid of small gas molecules: "catassembly" and "coassembly". This Perspective delves into these methodologies through the lens of varying molecular interaction types. The strategies discussed here for regulating molecular assembly structures using small gas molecules can aid in understanding various complex assembly processes and structures and provide guidance for the further fabrication of complex surface structures.

Structure transformation from Sierpiński triangles to chains assisted by gas molecules

Reversible transformations between fractals and periodic structures are of fundamental importance for understanding the formation mechanism of fractals. Currently, it is still a challenge to controllably achieve such a transformation. We investigate the effect of CO and CO2 molecules on Sierpiński triangles (STs) assembled from Fe atoms and 4,4″-dicyano-1,1':3',1″-terphenyl (C3PC) molecules on Au surfaces. Using scanning tunneling microscopy, we discover that the gas molecules induce a transition from STs into 1D chains. Based on density functional theory modeling, we propose that the atomistic mechanism involves the transformation of a stable 3-fold coordination Fe(C3PC)3 motif to Fe(C3PC)4 with an axially bonded CO molecule. CO2 causes the structural transformation through a molecular catassembly process.

Assembly structures and electronic properties of truxene-porphyrin compounds studied by STM/STS

The self-assembly of functional molecules into uniform nanostructures with innovational properties has attracted extensive research interest. In the present work, the assembly structures and electronic properties of a novel type of truxene derivative, e.g. truxene-porphyrin derivatives, were studied, for the first time, on a highly oriented pyrolytic graphite (HOPG) surface. Scanning tunneling microscopy (STM) images revealed that the truxene-porphyrin compounds could be parallelly arranged into long-ranged lamellar patterns. Density functional theory (DFT) calculations helped explain the assembly mechanisms further. Besides, order distribution of the smaller compound 1T1P in the 1,3,5-tris(10-carboxydecyloxy)-benzene (TCDB) host network was achieved, which is a reflection of the dimensional effect in the host-guest assembly. Furthermore, together with theoretical analyses, scanning tunneling spectroscopy (STS) measurements were conducted to investigate the electronic properties of truxene-porphyrin compounds. Results showed that the metalation of the porphyrin units could have a significant effect on the band gap and the position of the gap center. The study enhances our understanding of the assembly mechanism of truxene derivatives at the molecular level and paves the way towards fabricating truxene-based functional nanodevices.

Stability of functionalized platform molecules on Au(111)

Trioxatriangulenium (TOTA) platform molecules were functionalized with methyl, ethyl, ethynyl, propynyl, and hydrogen and sublimated onto Au(111) surfaces. Low-temperature scanning tunneling microscopy data reveal that >99% of ethyl-TOTA and methyl-TOTA remain intact, whereas 60% of H-TOTA and >99% of propynyl-TOTA and ethynyl-TOTA decompose. The observed tendency toward fragmentation on Au(111) is opposite to the sequence of gas-phase stabilities of the molecules. Although Au(111) is the noblest of all metal surfaces, the binding energies of the decomposition products to Au(111) destabilize the functionalized platforms by 2 to 3.9 eV (190-370 kJ/mol) and even render some of them unstable as revealed by density functional theory calculations. Van der Waals forces are important, as they drive the adsorption of the platform molecules.

An STM/STS study of site-selective adsorption of C70 molecules onto arc-shaped BODIPY molecular-networks

In this article, a donor-acceptor H-T-BO/C70 system was studied by a STM/STS method on the molecular level. STM results revealed that H-T-BO, a BODIPY-based derivative, can form a semi-closed molecular network at the 1-phenyloctane/HOPG interface. After introducing C70 fullerene molecules into the network, two kinds of self-assembled nanoarrays were observed by STM. Density functional theory has been utilized to reveal the formation mechanism of the molecular nanoarrays. Scanning Tunneling Spectroscopy (STS) measurements were performed to investigate the electronic properties of H-T-BO/C70 systems. I-V spectra combined with theoretical analyses showed that the introduction of C70 into the H-T-BO system induced a great drop of the band gap, which should be a result of electrons transferring from the donor H-T-BO to the acceptor C70 molecules.