Tailoring the structure of two-dimensional self-assembled nanoarchitectures based on ni(ii) -salen building blocks

Phenoxyaluminum(salophen) Scaffolds: Synthesis, Electrochemical Properties, and Self-Assembly at Surfaces of Multifunctional Systems

Salophens and Salens are Schiff bases generated through the condensation of two equivalents of salicylaldehyde with either 1,2-phenylenediamines or aliphatic diamines, respectively. Both ligands have been extensively exploited as key building blocks in coordination chemistry and catalysis. In particular, their metal complexes have been widely used for various catalytical transformations with high yield and selectivity. Through the modification of the phenol unit it is possible to tune the steric hindrance and electronic properties of Salophen and Salen. The introduction of long aliphatic chains in salicylaldehydes can be used to promote their self-assembly into ordered supramolecular structures on solid surfaces. Herein, we report a novel method towards the facile synthesis of robust and air-stable [Al(Salophen)] derivatives capable of undergoing spontaneous self-assembly at the graphite/solution interface forming highly-ordered nanopatterns. The new synthetic approach relies on the use of [MeAl^III (Salophen)] as a building unit to introduce, via a simple acid/base reaction with functionalized acidic phenol derivatives, selected frameworks integrating multiple functions for efficient surface decoration. STM imaging at the solid/liquid interface made it possible to monitor the formation of ordered supramolecular structures. In addition, the redox properties of the Salophen derivatives functionalized with ferrocene units in solution and on surface were unraveled by cyclic voltammetry. The use of a five-coordinate aluminum alkyl Salophen precursor enables the tailoring of new Salophen molecules capable of undergoing controlled self-assembly on HOPG, and thereby it can be exploited to introduce multiple functionalities with subnanometer precision at surfaces, ultimately forming ordered functional patterns.

Self-assembly of bis-salphen compounds: from semiflexible chains to webs of nanorings

The recently-observed self-assembly of certain salphen-based compounds into neuron-like networks of microrings interconnected with nano-thin strings may suggest a new highly-potent tool for nanoscale patterning. However, the mechanism behind such phenomena needs to be clarified before they can be applied in materials design. Here we show that, in contrast with what was initially presumed, the emergence of a "rings-and-rods" pattern is unlikely to be explained by merging, collapse and piercing of vesicles as in previously reported cases of nanorings self-assembly via non-bonding interactions. We propose an alternative explanation: the compounds under study form a 1D coordination polymer, the fibres of which are elastic enough to fold into toroidal globules upon solvent evaporation, while being able to link separate chains into extended networks. This becomes possible because the structure of the compound's scaffold is found to adopt a very different conformation from that inferred in the original work. Based on ab initio and molecular dynamics calculations we propose a step-by-step description of self-assembly process of a supramolecular structure which explains all the observed phenomena in a simple and clear way. The individual roles of the compound' s scaffold structure, coordination centres, functional groups and solvent effects are also explained, opening a route to control the morphology of self-assembled networks and to synthesize new compounds exhibiting similar behaviour.

Cooperation and competition between halogen bonding and van der Waals forces in supramolecular engineering at the aliphatic hydrocarbon/graphite interface: position and number of bromine group effects

Herein, the photophysical properties of two π-conjugated thienophenanthrene derivatives (6,9- and 5,10-DBTD) are reported. Their self-assembled monolayers in aliphatic hydrocarbon solvents under different concentrations were investigated by scanning tunneling microscopy on a graphite surface. The STM results revealed that the self-assembled structures of the two geometrical isomers exhibited absolutely different behaviors. At the aliphatic solvent/graphite interface, 6,9-DBTD produced almost a single stable coassembled linear structure, except for that with n-tridecane as the solvent. However, the self-assembly of 5,10-DBTD showed structural diversity, and it presented a gradient variety through increasing the chain length of the aliphatic solvents as well as the solution concentration. All ordered self-assembled adlayers critically depend on not only the interchain van der Waals (vdW) interactions, but also on multiple intermolecular interactions, including BrO[double bond, length as m-dash]C and BrS hetero-halogen bonds, homo-BrBr interactions, and HBr and HO hydrogen bonds. We proposed that the cooperation and competition of the intermolecular interactions involving a Br atom and interchain vdW forces induce this structural variety. Density functional theory calculations support to unravel the different elementary structural units based on halogen bonds and hydrogen bonds and were useful tools to dissect and explain the formation mechanism.