Creation of chiral adsorbed structures using external inputs: results from lattice Monte Carlo simulations

Abstract

Patterning of solid surfaces with organic molecules has been recognized as a promising method to create functional 2D matrices with tunable structure and properties. In this work we use the lattice Monte Carlo simulations to study chiral pattern formation in adsorbed systems comprising simple molecular building blocks differing in shape. To that end we consider five-membered rigid isomers whose composite segments can occupy vertices of a triangular lattice and interact with short-range (nearest neighbors) forces. Our main focus is on those molecules which are prochiral, that is they can adopt mirror-image planar configurations when adsorbed. Moreover, the effect of orientational in-plane confinement of the molecules, which reflects their coupling with an external directional field, on the structure formation and chiral resolution in 2D is explored. The obtained results demonstrate that the confinement imposed on the surface enantiomers can induce their resolution and formation of extended homochiral domains. However, it is also shown that for certain molecular shapes the confinement can produce mixed racemic crystals instead of the homochiral assemblies. The insights from our simulation studies can be helpful in preliminary screening of molecular libraries to select optimal building blocks able to self-assembly into chiral 2D patterns with predefined architecture.

Heterogeneous Interactions Promote Crystallization and Spontaneous Resolution of Chiral Molecules

“…To limit the number of different molecular shapes, we only consider molecules with angles of 60, 120, or 180 deg between triplets of connected functional groups. Furthermore, we consider only chiral shapes, resulting in a total of 11 distinct molecular shapes, 16 which we order according to increasing radius of gyration. Functional groups interact via short-ranged pair potentials that represent effective interactions in solution, as illustrated in Figure 1b (see Methods); solvent effects are included in these interactions, and solvent species are not modeled explicitly.…”

Heterogeneous Interactions Promote Crystallization and Spontaneous Resolution of Chiral Molecules

“…Such molecules crystallize and separate better, as recently demonstrated by Woszczyk and coworkers. 16 We show in Table S3 that the number of low-energy polymorphs decreases dramatically when molecular orientation are constrained. A third example is the use of pressure, as recently demonstrated in the crystallization of B 2 O 3 , a material that forms a glass from the melt at ambient conditions but readily crystallizes into a single polymorph at elevated pressures.…”

“…To limit the number of different molecular shapes, we only consider molecules with angles of 60, 120, or 180 degrees between triplets of connected functional groups. Furthermore, we focus only on chiral shapes, resulting in a total of 11 distinct molecular shapes, 16 which we order according to increasing radius of gyration. Functional groups interact via short-ranged pair potentials that represent effective interactions in solution, as illustrated in Figure 1b (see Methods); solvent effects are included in these interactions and solvent species are not modeled explicitly.…”

“…[9][10][11] Despite much research activity, no definitive answers to these questions have been found. [12][13][14][15][16][17][18][19][20][21] In principle, thermodynamics dictates that the crystal with the lowest free energy prevails in equilibrium. Studies of chiral molecules a) Electronic mail: michael.gruenwald@utah.edu have shown, however, that thermodynamics alone cannot explain the bias towards racemic crystals observed in experiments.…”

“…In our efforts, we build on previous computa- tional work that has demonstrated the usefulness of simple molecular models in elucidating structure formation mechanisms of chiral molecules. 16,20,21,[42][43][44][45] Our study is based on a family of coarse-grained models of chiral molecules. The computational efficiency of these models allows us to simulate the crystallization dynamics of a wide range of molecular shapes and interactions, and to fully determine the thermodynamic landscape of crystal structures.…”