Orientation transitions and chiral assemblies of para-terphenyl molecules on Cd(0001)

The interplay between orientation transition and chiral self-assemblies of para-terphenyl (P3P) molecules on the Cd(0001) surface has been investigated using low temperature scanning tunneling microscopy and density functional theory calculations. Three distinct molecular orientations have been discerned from the self-assembled thin films of P3P. At the low coverage, flat-lying molecules appear in the homochiral domains with the incommensurate registry to the substrate. With the coverage increasing, the incoming molecules are incorporated into the first layer with edge-on orientation and form the self-assembled zigzag chains. The alternative arrangement of zigzag chains with opposite chirality leads to the formation of a c(4 × 2) superstructure, in which the tilted molecules exhibit orientational frustration and fuzzy noises. The analysis of the tunneling spectra reveals that the electronic structure of P3P layers is contingent upon the hybridization between the electronic states of P3P molecules and the Cd(0001) surface. These results provide important insights into the interplay between orientational transition and chiral assembly of P3P molecules on metal substrates.

Planar in Brooker's mode and twisted in Reichardt's mode: defying the steric forces in biphenyl types of zwitterionic systems through metameric resonance stabilizations

To be planar or to be twisted at the bridge junctions in biphenyls or biaryl types of molecular systems depends on two conflicting forces: (1) steric repulsions (destabilizations) and (2) conjugation assisted electron delocalizations (resonance stabilizations). This work reports an unfamiliar kind of behaviour shown by metamers of a zwitterionic biphenyl type of system, where the Reichardt's metamer was found to be in an usual twisted conformation (delicate balance of conflicting forces), but the Brooker's metamer was found to be in a fully planar conformation. Interestingly, at the ωB97xD/aug-cc-pVDZ level, energetically (ΔE) the planar Brooker's metamer was found to be 16.7 kcal mol^-1 lower (22.9 kcal mol^-1 lower in the CASSCF method) in energy (more stable) than the isoelectronic twisted Reichardt's metamer, and also thermodynamic ΔG values were found to be close to ΔE values for various methods (for example, 15.6 kcal mol^-1 in the above case using the ωB97xD method). When the steric repulsions are in their full potentials at the ring junction site, attainment of a conformational planarity by any biaryl type of system has not been reported previously. Without reducing the steric constraints or even without inducing any attractive forces, determining what other factors were responsible for defying the steric forces is the main focus of this investigation. Using the results of quantum mechanical computations of NBO, rotational barriers, and other saddle points (metastable conformations in singlet and triplet surfaces) in the potential energy surfaces, the dominant contribution of the resonance stabilized quinonoid form to the ground state was delineated as the possible reason for this unusual behaviour.

Exploring the driving forces behind the structural assembly of biphenylthiolates on Au(111)

In this contribution, we use dispersion-corrected density functional theory to study inter- and intramolecular interactions in a prototypical self-assembled monolayer (SAM) consisting of biphenylthiolates bonded to Au(111) via thiolate groups. The goal is to identify the nature of the interactions that drive the monolayer into a specific conformation. Particular focus is laid on sampling realistic structures rather than high symmetry model configurations. This is achieved by studying conceptually different local minimum structures of the SAM that are obtained via exploring the potential energy surface from systematically varied starting geometries. The six obtained packing motifs differ in the relative arrangement of the two molecules in the unit cell (co-planar versus herringbone) and in the intramolecular configuration (twisted versus planar rings). We find that van der Waals interactions within the organic adsorbate and between the adsorbate and substrate are the main reason that these molecular assemblies can form stable structures at all. The van der Waals interactions are, however, very similar for all observed motifs; by analyzing various types of interactions in the course of three notional SAM-formation steps, we find that the main driving force stabilizing the actual global minimum structure originates from electrostatic interactions between the molecules.

Self-assembly of 1,3,5-benzenetribenzoic acid on Ag and Cu at the liquid/solid interface

Big difference: molecules of benzenetribenzoic acid are disordered on Cu but highly organised on Ag, forming a monopodal row structure.