Combining high-resolution scanning tunnelling microscopy and first-principles simulations to identify halogen bonding.

Scanning tunnelling microscopy (STM) is commonly used to identify on-surface molecular self-assembled structures. However, its limited ability to reveal only the overall shape of molecules and their relative positions is not always enough to fully solve a supramolecular structure. Here, we analyse the assembly of a brominated polycyclic aromatic molecule on Au(111) and demonstrate that standard STM measurements cannot conclusively establish the nature of the intermolecular interactions. By performing high-resolution STM with a CO-functionalised tip, we clearly identify the location of rings and halogen atoms, determining that halogen bonding governs the assemblies. This is supported by density functional theory calculations that predict a stronger interaction energy for halogen rather than hydrogen bonding and by an electron density topology analysis that identifies characteristic features of halogen bonding. A similar approach should be able to solve many complex 2D supramolecular structures, and we predict its increasing use in molecular nanoscience at surfaces.

Fluorinated carboxylic acids as powerful building blocks for the formation of bimolecular monolayers.

We compare the ability of a prototypical dicarboxylic acid and its fluorinated analogue to act as molecular building blocks for the formation of self-assembled monolayers. Whilst fluorination is found to prevent homomolecular self-assembly, it greatly increases the ability of the carboxylic acid to act as a hydrogen bond donor for the formation of bimolecular networks.

Sequencing conjugated polymers by eye.

The solid-state microstructure of a conjugated polymer is the most important parameter determining its properties and performance in (opto)-electronic devices. A huge amount of research has been dedicated to tuning and understanding how the sequence of monomers, the nature and frequency of defects, the exact backbone conformation, and the assembly and crystallinity of conjugated polymers affect their basic photophysics and charge transporting properties. However, because of the lack of reliable high-resolution analytical techniques, all the structure-property relations proposed in the literature are based either on molecular modeling or on indirect experimental data averaged on polydisperse samples. We show that a combination of electrospray vacuum deposition and high-resolution scanning tunneling microscopy allows the imaging of individual conjugated polymers with unprecedented detail, thereby unraveling structural and self-assembly characteristics that have so far been impossible to determine.

Gas Flow and Ion Transfer in Heated ESI Capillary Interfaces.

Transfer capillaries are the preferred means to transport ions, generated by electrospray ionization, from ambient conditions to vacuum. During the transfer of ions through the narrow, long tubes into vacuum, substantial losses are typical. However, recently it was demonstrated that these losses can be avoided altogether. To understand the experimental observation and provide a general model for the ion transport, here, we investigate the ion transport through capillaries by numerical simulation of interacting ions. The simulation encompasses all relevant factors, such as space charge, diffusion, gas flow, and heating. Special attention is paid to the influence of the gas flow on the transmission and especially the change imposed by heating. The gas flow is modeled by a one-dimensional gas dynamics description. A large number of ions are treated as point particles in this gas flow. This allows to investigate the influence of the capillary heating on the gas flow and by this on the ion transport. The results are compared with experimental findings. Graphical Abstract ᅟ.