Enantiopure Supramolecular Motifs of Self-Assembled Diamine-Based Chiral Molecules on Cu(100)

The assembly of two diphenylethylenediamine enantiomers, separately deposited on Cu(100), is investigated from the first stages of two-dimensional crystallization to the nucleation and growth of the second layer. By means of scanning tunneling microscopy, we show that the chirality of enantiomers is expressed at different levels of molecular organization. Below the monolayer completion, a disordered phase coexists with domains of a square lattice aligned with the principal crystallographic directions of the substrate. The intrinsic chirality of the molecules is only manifested through specific features contained within the corresponding unit cell. For increasing coverage, this arrangement is accompanied by a second square structure, which appears to be clockwise or counterclockwise rotated with respect to the Cu(100) directions depending on the enantiomer. The nucleation of molecular chains on top of the aligned square structure gives rise to a second layer exhibiting a stripelike configuration with remarkable left- or right-handed helicity that mirrors that of the particular enantiomer. The nearly flat configuration of the first-layer molecules is confirmed by angular-dependent X-ray absorption experiments and supported by molecular dynamics simulations.
This work has been supported by the Spanish Government under projects MAT2013-47869-C4, MAT2014-59315-R, MAT2015-72848-EXP/AEI, FIS2016-74893-P, and MAT2016-77852-C2-1-R (AEI/FEDER, UE) and the Generalitat de Catalunya 2017 SGR 668. We also appreciate the specific agreement between ICMAB-CSIC and the Synchrotron Light Facility ALBA and the Spanish MINECO through the project MAT2015-68994-REDC and the “Severo Ochoa” Program for Centers of Excellence in R&D (SEV-2015-0496). The NEXAFS experiments were performed at BOREAS beamline at ALBA Synchrotron with the collaboration of ALBA staff. IMDEA Nanociencia acknowledges support from the “Severo Ochoa” Program for Centres of Excellence in R&D (MINECO, Grant SEV-2016-0686). R.P.-R. thanks the financial support through BES-2014-067942 (FPI fellowship). We acknowledge the CESGA Supercomputing Center for computational time at the Finisterrae Supercomputer and technical assistance.