Thermal- vs Light-Induced On-Surface Polymerization.

Christophe Nacci,Silvia Nappini,Leonhard Grill,Donato Civita,Monika Schied,Elena Magnano,Igor Píš

doi:10.1021/acs.jpcc.1c06914

Abstract

On-surface polymerization is a powerful bottom-up approach that allows for the growth of covalent architectures with defined properties using the two-dimensional confinement of a highly defined single-crystal surface. Thermal heating is the preferred approach to initiate the reaction, often via cleavage of halogen substituents from the molecular building blocks. Light represents an alternative stimulus but has, thus far, only rarely been used. Here, we present a direct comparison of on-surface polymerization of dibromo-anthracene molecules, induced either thermally or by light, and study the differences between the two approaches. Insight is obtained by a combination of scanning tunneling microscopy, locally studying the polymer shape and size, and X-ray photoelectron spectroscopy, which identifies bond formation by averaging over large surface areas. While the polymer length increases slowly with the sample heating temperature, illumination promotes only the formation of short covalent structures, independent of the duration of light exposure. Moreover, irradiation with UV light at different sample temperatures highlights the important role of molecular diffusion across the surface.

Highlights

On-surface synthesis is a versatile method for the formation of covalently bound nanostructures on surfaces.[1−3] Ullmann coupling on surfaces is one of the preferred chemical reactions, leading to the formation of C−C bonds.[4−6] The activation of molecular precursors is operated by dissociating initial C−X
Molecules were deposited onto Au(111) kept at room temperature (RT), and the scanning tunneling microscopy measurements were conducted at 5 and 7.5 K and X-ray photoemission spectroscopy (XPS) measurements at RT
Scanning tunneling microscopy imaging of individual DBA molecules reveals a clear structural asymmetry related to the two C atoms linked to the Br atoms (C−Br) bonds pointing in different directions, as shown by the model structures superimposed on the image

Summary

Introduction

On-surface synthesis is a versatile method for the formation of covalently bound nanostructures on surfaces.[1−3] Ullmann coupling on surfaces is one of the preferred chemical reactions, leading to the formation of C−C bonds.[4−6] The activation of molecular precursors is operated by dissociating initial C−X (X= I, Br, ...) bonds at room temperature (RT) or at up to about 600 K.7 The halogen positions within the precursor define the positions at which new covalent bonds are formed and intrinsically encode the final polymer structure.[6,8,9] Heat, i.e., thermal energy, is the preferred stimulus for activation, but alternative activation modes have been explored, for instance, a scanning tunneling microscope (STM) tip (via voltage pulses),[10] an electron beam,[11] or light.[12]Light offers numerous advantages with respect to other approaches. On-surface synthesis is a versatile method for the formation of covalently bound nanostructures on surfaces.[1−3] Ullmann coupling on surfaces is one of the preferred chemical reactions, leading to the formation of C−C bonds.[4−6] The activation of molecular precursors is operated by dissociating initial C−X The halogen positions within the precursor define the positions at which new covalent bonds are formed and intrinsically encode the final polymer structure.[6,8,9] Heat, i.e., thermal energy, is the preferred stimulus for activation, but alternative activation modes have been explored, for instance, a scanning tunneling microscope (STM) tip (via voltage pulses),[10] an electron beam,[11] or light.[12]. Insight into these fundamental aspects is essential to gain control over light-induced processes on surfaces

Methods

Results

Conclusion