Reversing the Handedness of Self-Assembled Porous Molecular Networks through the Number of Identical Chiral Centres.

Scanning tunnelling microscope observations at the 1-phenyloctane/graphite interface reveal how chiral structural information at the molecular level is transferred and expressed structurally at the 2D supramolecular level for a porous system. The chirality of self-assembled molecular networks formed by chiral dehydrobenzo[12]annulene (cDBA) derivatives having three chiral chains and three achiral chains, alternatingly, is compared with those of cDBAs having six chiral chains reported previously. While for all cDBAs homochiral surfaces are formed, their handedness is not simply a reflection of the absolute configuration of the stereogenic centres. Both the number of stereogenic centres as well as the length of the achiral chains determine the supramolecular handedness, providing a deep insight into the supramolecular chirality induction mechanisms at play. Moreover, these cDBAs act to induce chirality in porous networks formed by achiral DBAs.

Dynamic control over supramolecular handedness by selecting chiral induction pathways at the solution-solid interface.

A dominant theme within the research on two-dimensional chirality is the sergeant-soldiers principle, wherein a small fraction of chiral molecules (sergeants) is used to skew the handedness of achiral molecules (soldiers) to generate a homochiral surface. Here, we have combined the sergeant-soldiers principle with temperature-dependent molecular self-assembly to unravel a peculiar chiral amplification mechanism at the solution-solid interface in which, depending on the concentration of a sergeant-soldiers solution, the majority handedness of the system can either be amplified or entirely reversed after an annealing step, furnishing a homochiral surface. Two discrete pathways that affect different stages of two-dimensional crystal growth are invoked for rationalizing this phenomenon and we present a set of experiments where the access to each pathway can be precisely controlled. These results demonstrate that a detailed understanding of subtle intermolecular and interfacial interactions can be used to induce drastic changes in the handedness of a supramolecular network.

On the stability of surface-confined nanoporous molecular networks.

Self-assembly of molecular building blocks into two-dimensional nanoporous networks has been a topic of broad interest for many years. However, various factors govern the specific outcome of the self-assembly process, and understanding and controlling these are key to successful creation. In this work, the self-assembly of two alkylated dehydrobenzo[12]annulene building blocks was compared at the liquid-solid interface. It turned out that only a small chemical modification within the building blocks resulted in enhanced domain sizes and stability of the porous packing relative to the dense linear packing. Applying a thermodynamic model for phase transition revealed some key aspects for network formation.

Towards enantioselective adsorption in surface-confined nanoporous systems.

The adsorption of chiral molecules in surface-confined chiral porous networks shows pronounced selectivity, as a result of complementary host-guest interactions.

A tale of tails: alkyl chain directed formation of 2D porous networks reveals odd-even effects and unexpected bicomponent phase behavior.

Supramolecular self-assembly of suitably functionalized building blocks on surfaces can serve as an excellent test-bed to gain understanding and control over multicomponent self-assembly in more complex matter. Here we employ a powerful combination of scanning tunnelling microscopy (STM) and molecular modeling to uncover two-dimensional (2D) crystallization and mixing behavior of a series of alkylated building blocks based on dehydrobenzo[12]annulene, forming arrays of nanowells. Thorough STM investigation employing high-resolution spatial imaging, use of specially designed marker molecules, statistical analysis and thermal stability measurements revealed rich and complex supramolecular chemistry, highlighting the impact of odd-even effects on the phase behavior. The methodology and analysis presented in this work can be easily adapted to the self-assembly of other alkylated building blocks.

Surface-induced diastereomeric complex formation of a nucleoside at the liquid/solid interface: stereoselective recognition and preferential adsorption.

With the aim of achieving surface-mediated enantioselective adsorption, the self-assembly of chiral oligo(p-phenylenevinylene) (OPV3T) with nucleosides is investigated at the liquid/solid interface by means of scanning tunneling microscopy and molecular modeling. OPV3T enantiomers form mirror related hexameric rosette patterns. The DNA nucleoside, thymidine, does not self-assemble into stable adlayers but coadsorbs with OPV3T on the surface, leading to a pattern transformation of OPV3T from rosettes to dimers, and a change in chiral expression as well. Diastereoselective recognition between OPV3T and thymidine enantiomers can be used to resolve thymidine enantiomers at an achiral surface with an OPV3T enantiomer as the resolving agent. The impact of molar ratio and concentration on the self-assembly and chiral resolution is systematically investigated. Because there is no interaction between OPV3T and thymidine in solution, the liquid/solid interface acts as the platform for the chiral resolution of thymidine enantiomers.

'Sergeants-and-Corporals' principle in chiral induction at an interface.

Self-assembly of an achiral porphyrin at the interface between a chiral solvent and an atomically flat substrate renders the monolayer chiral, and a non-racemic solvent can even overrule the intrinsic expression of chirality in the self-assembly of chiral molecules.

Chiral induction and amplification in supramolecular systems at the liquid-solid interface.

Chiral induction and amplification in surface-confined supramolecular monolayers are investigated at the liquid-solid interface. Scanning tunneling microscopy (STM) proves that achiral molecules can self-assemble into globally chiral patterns through a variety of approaches, including induction by chiral solvents or by a novel chiral amplification method. Our study demonstrates the aptness of both approaches, which have already been applied to (supramolecular) polymers in solution, to create chiral supramolecular monolayers at the liquid-solid interface.

Solvent-induced homochirality in surface-confined low-density nanoporous molecular networks.

Induction of chirality in achiral monolayers has garnered considerable attention in the recent past not only due to its importance in chiral resolutions and enantioselective heterogeneous catalysis but also because of its relevance to the origin of homochirality in life. In this contribution, we demonstrate the emergence of macroscopic chirality in multicomponent supramolecular networks formed by achiral molecules at the interface of a chiral solvent and an achiral substrate. The solvent-mediated chiral induction provides a simple, efficient, and versatile approach for the fabrication of homochiral surfaces using achiral building blocks.

Exploring the complexity of supramolecular interactions for patterning at the liquid-solid interface.

The use of self-assembly to fabricate surface-confined adsorbed layers (adlayers) from molecular components provides a simple means of producing complex functional surfaces. The molecular self-assembly process relies on supramolecular interactions sustained by noncovalent forces such as van der Waals, electrostatic, dipole-dipole, and hydrogen bonding interactions. Researchers have exploited these noncovalent bonding motifs to construct well-defined two-dimensional (2D) architectures at the liquid-solid interface. Despite myriad examples of 2D molecular assembly, most of these early findings were serendipitous because the intermolecular interactions involved in the process are often numerous, subtle, cooperative, and multifaceted. As a consequence, the ability to tailor supramolecular patterns has evolved slowly. Insight gained from various studies over the years has contributed significantly to the knowledge of supramolecular interactions, and the stage is now set to systematically engineer the 2D supramolecular networks in a "preprogrammed" fashion. The control over 2D self-assembly of molecules has many important implications. Through appropriate manipulation of supramolecular interactions, one can "encode" the information at the molecular level via structural features such as functional groups, substitution patterns, and chiral centers which could then be retrieved, transferred, or amplified at the supramolecular level through well-defined molecular recognition processes. This ability allows for precise control over the nanoscale structure and function of patterned surfaces. A clearer understanding and effective use of these interactions could lead to the development of functional surfaces with potential applications in molecular electronics, chiral separations, sensors based on host-guest systems, and thin film materials for lubrication. In this Account, we portray our various attempts to achieve rational design of self-assembled adlayers by exploiting the aforementioned complex interactions at the liquid-solid interface. The liquid-solid interface presents a unique medium to construct flawless networks of surface confined molecules. The presence of substrate and solvent provides an additional handle for steering the self-assembly of molecules. Scanning tunneling microscopy (STM) was used for probing these molecular layers, a technique that serves not only as a visualization tool but could also be employed for active manipulation of molecules. The supramolecular systems described here are only weakly adsorbed on a substrate, which is typically highly oriented pyrolytic graphite (HOPG). Starting with fundamental studies of substrate and solvent influence on molecular self-assembly, this Account describes progressively complex aspects such as multicomponent self-assembly via 2D crystal engineering, emergence, and induction of chirality and stimulus responsive supramolecular systems.

Formation of a non-crystalline bimolecular porous network at a liquid/solid interface.

An equimolar mixture of two structurally related molecular building blocks self-assembles into a 2D non-crystalline bimolecular porous pattern at a liquid-solid interface as revealed by scanning tunneling microscopy.

Control and induction of surface-confined homochiral porous molecular networks.

Homochirality is essential to many biological systems, and plays a pivotal role in various technological applications. The generation of homochirality and an understanding of its mechanism from the single-molecule to supramolecular level have received much attention. Two-dimensional chirality is a subject of intense interest due to the unique possibilities and consequences of confining molecular self-assembly to surfaces or interfaces. Here, we report the perfect generation of two-dimensional homochirality of porous molecular networks at the liquid-solid interface in two different ways: (i) by self-assembly of homochiral building blocks and (ii) by self-assembly of achiral building blocks in the presence of a chiral modifier via a hierarchical structural recognition process, as revealed by scanning tunnelling microscopy. The present results provide important impetus for the development of two-dimensional crystal engineering and may afford opportunities for the utilization of chiral nanowells in chiral recognition processes, as nanoreactors and as data storage systems.