Exploring On-Surface Synthesis under Mild Conditions.

ConspectusBottom-up approaches combining tailor-made molecular precursors and surface-mediated reactions under ultrahigh-vacuum (UHV) conditions attracted significant attention over the past decade as a promising strategy for synthesizing novel, functional, molecule-based materials. These methods have been remarkably successful in creating unconventional covalent products with atomic precision, though largely focusing on one-dimensional (1D) polymeric products. Extending the established protocols to synthesize two-dimensional (2D) covalent architectures presents a major challenge, primarily due to high annealing temperatures required that often entail competing reactions, high defect densities, and structural degradation.In this Account, we highlight the exciting potential of low-temperature (LT) on-surface reactions as an alternative pathway and discuss their largely unexploited capabilities. We summarize major recent advances, focusing on coinage metal surface-assisted chemical transformations at mild conditions in UHV, proceeding frequently near or below room temperature (RT). Special emphasis is placed on alkyne derivatives, either alone or combined with other functional groups, identified as versatile building blocks for next-generation carbon-rich nanomaterials such as graphyne or graphdiyne and their metalated derivatives, which offer immense potential for future technological applications.We discuss four major pathways for initiating LT on-surface reactions of alkyne species, following largely the chronological order of their discovery, and merging insights from high-resolution scanning probe microscopy, X-ray spectroscopies and density functional theory calculations: (i) Conversions catalyzed by in situ generated species and extrinsic elements; (ii) quantum tunneling-mediated reactions; (iii) reaction pathways involving surface-assisted radical or hydrogen transfer processes; and (iv) gas-mediated on-surface reactions. These and other selected examples of LT synthesis protocols offer significant advantages in terms of high selectivity and efficiency, notably enabling the controlled synthesis of extended, regular 2D organometallic and covalent compounds or architectures, and bearing promise for a multitude of all-carbon scaffolds, which currently remain challenging. We aim to inspire the development of functional robust nanoarchitectures with long-range order and atomic-scale precision, contributing to the advancement of molecule-based materials for diverse technological applications.