Chiral switching by spontaneous conformational change in adsorbed organic molecules

Abstract

Self-assembly of adsorbed organic molecules is a promising route towards functional surface nano-architectures, and our understanding of associated dynamic processes has been significantly advanced by several scanning tunnelling microscopy (STM) investigations. Intramolecular degrees of freedom are widely accepted to influence ordering of complex adsorbates, but although molecular conformation has been identified and even manipulated by STM, the detailed dynamics of spontaneous conformational change in adsorbed molecules has hitherto not been addressed. Molecular surface structures often show important stereochemical effects as, aside from truly chiral molecules, a large class of so-called prochiral molecules become chiral once confined on a surface with an associated loss of symmetry. Here, we investigate a model system in which adsorbed molecules surprisingly switch between enantiomeric forms as they undergo thermally induced conformational changes. The associated kinetic parameters are quantified from time-resolved STM data whereas mechanistic insight is obtained from theoretical modelling. The chiral switching is demonstrated to enable an efficient channel towards formation of extended homochiral surface domains. Our results imply that appropriate prochiral molecules may be induced (for example, by seeding) to assume only one enantiomeric form in surface assemblies, which is of relevance for chiral amplification and asymmetric heterogenous catalysis.

Molecular chirality at surfaces

“…Such a case has been observed for prochiral bis-ethynyl-p-phenylene compounds on Au(111) (Fig. 30) [210]. Besides the two enantiomers, an achiral meso-form exists at the surface.…”

Structure and Energetics of Diphenylalanine Self-Assembling on Cu(110)

“…This rate is related to the energy barrier E associated with the conformational change by the Arrhenius equation k f ) k 0 exp(-E/k B T), where k B is the Boltzmann constant. Assuming a value of 10 11 -10 12 s -1 for the prefactor k 0 (consistent with the attempt frequency associated with a relatively soft phonon mode 40 ), the energy barrier E can be estimated to be about 0.8 eV. This value is possibly overestimated because of undersampling, as some more switching events may have been missed as a result of our limited image acquisition rate.…”

Contact Formation Dynamics: Mapping Chemical Bond Formation between a Molecule and a Metallic Probe

“…Similarly, the operation of catalysts and enzymes depends on understanding how particular molecules react with surface catalytic sites. [15][16][17] The manner in which molecules engage with and ultimately dock at these surface sites is critically important. In these and related systems, numerous studies have reported on the final state properties, that is, after the contact has been assembled or the reaction completed.…”