Tailoring on-surface molecular reactions and assembly through hydrogen-modified synthesis: From triarylamine monomer to 2D covalent organic framework

TL;DR

This study demonstrates how controlling hydrogen environment during on-surface synthesis in ultrahigh vacuum enables precise manipulation of molecular reactions, leading to tailored 2D covalent organic frameworks with distinct electronic properties.

Contribution

It introduces a method to control on-surface reactions using hydrogen gas, allowing selective formation of molecular oligomers and insights into electronic structure evolution.

Findings

Hydrogen gas controls aryl-aryl bond formation during synthesis.

Large monomer islands, dimers, and hexamers can be selectively produced.

Electronic states evolve from monomer to 2D COF, revealing structural-electronic relationships.

Abstract

Relative to conventional wet-chemical synthesis techniques, on-surface synthesis of organic networks in ultrahigh vacuum has few control parameters. The molecular deposition rate and substrate temperature are typically the only synthesis variables to be adjusted dynamically. Here we demonstrate that reducing conditions in the vacuum environment can be created and controlled without dedicated sources -- relying only on backfilled hydrogen gas and ion gauge filaments -- and can dramatically influence the Ullmann-like on-surface reaction used for synthesizing two-dimensional covalent organic frameworks (2D COFs). Using tribromo dimethylmethylene-bridged triphenylamine ((Br$_3$)DTPA) as monomer precursors, we find that atomic hydrogen blocks aryl-aryl bond formation. Control of the relative monomer and hydrogen fluxes is used to produce large islands of self-assembled monomers, dimers, or macrocycle hexamers. On-surface synthesis of these oligomers, from a single precursor, circumvents potential challenges with protracted wet-chemical synthesis or low precursor volatility for large molecules. Using scanning tunneling microscopy and spectroscopy (STM/STS), we show that changes in the electronic states through this oligomer sequence provide an insightful view of the 2D-COF (synthesized in the absence of atomic hydrogen) as the endpoint in an evolution of electronic structures from the monomer.