Kinetic control over the chiral-selectivity in the formation of organometallic polymers on a Ag(110) surface

Abstract

Methods to control chiral-selectivity in molecular reactions through external inputs are of importance, both from a fundamental and technological point of view. Here, the self-assembly of prochiral 6,12-dibromochrysene monomers on Ag(110) is studied using scanning tunneling microscopy. Deposition of the monomers on a substrate held at room temperature leads to the formation of 1D achiral organometallic polymers. When the monomers are instead deposited on a substrate held at 373 K, homochiral organometallic polymers consisting of either the left- or right-handed enantiomer are formed. Post-deposition annealing of room temperature deposited samples at >373 K does not transform the achiral 1D organometallic polymers into homochiral ones and thus, does not yield the same final structure as if depositing onto a substrate held at the same elevated temperature. Furthermore, annealing promotes neither the formation of 1D covalently-coupled polymers nor the formation of graphene nanoribbons. Our results identify substrate temperature as an important factor in on-surface chiral synthesis, thereby demonstrating the importance of considering kinetic effects and the decisive role they can play in structure formation.

Fig. 1. Structure of the organometallic polymers formed at room temperature.

a Chemical structure of prochiral 6,12-dibromochrysene 1 showing both the left- and right-handed enantiomers which are available on the surface. b Schematic of the organometallic polymers 2 formed upon deposition of monomer 1 on Ag(110) held at RT. c, d Overview STM images of the organometallic polymers 2 formed on Ag(110) upon deposition of 1 onto Ag(110) held at RT. e Close-up STM image of two organometallic polymers at an edge of a molecular island with an overlaid molecular model. Deposition was done onto a Ag substrate kept at RT. Scanning parameters: (c) −1.9 V, 10 pA, (d) −1.9 V, 5 pA, (e) 0.5 V, 100 pA. Color code: Br, red; Ag, blue; C, gray; H, white.

Fig. 2. Structure of the organometallic polymers formed upon deposition on Ag(110) held at 373 K.

a Overview STM image showing organometallic polymers 2, 3L, and 3R. Both 3L and 3R are arranged in extended 2D islands. b, c Close-up STM images showing the organometallic polymers 3L and 3R, respectively with overlaid molecular models, Ag adatoms (blue) and split off Br atoms (red). d Schematic of both 1D homochiral organometallic polymers formed upon deposition of monomer 1 on Ag(110) held at 373 K. 3L consists of only left-handed enantiomers and 3R of only right-handed ones. Scanning parameters: (a) 1.0 V, 30 pA, (b) 1.0 V, 100 pA, (c) −1.7 V, 500 pA. Color code: Br, red; Ag, blue; C, gray; H, white.

Fig. 3. Structure of the samples upon heat treatment.

a Overview STM image showing the result of annealing a sample deposited at RT and annealed at 373 K. b Overview STM image of the same sample as in a, but further annealed at 423 K. Scanning parameters: (a) −1.9 V, 50 pA, (b) −1.9 V, 5 pA.