Since the first report of thiol-based self-assembled monolayers (SAMs) 30 years ago, these structures have been examined in a huge variety of applications. The oxidative and thermal instabilities of these systems are widely known, however, and are an impediment to their widespread commercial use. Here, we describe the generation of N-heterocyclic carbene (NHC)-based SAMs on gold that demonstrate considerably greater resistance to heat and chemical reagents than the thiol-based counterparts. This increased stability is related to the increased strength of the gold-carbon bond relative to that of a gold-sulfur bond, and to a different mode of bonding in the case of the carbene ligand. Once bound to gold, NHCs are not displaced by thiols or thioethers, and are stable to high temperatures, boiling water, organic solvents, pH extremes, electrochemical cycling above 0 V and 1% hydrogen peroxide. In particular, benzimidazole-derived carbenes provide films with the highest stabilities and evidence of short-range molecular ordering. Chemical derivatization can be employed to adjust the surface properties of NHC-based SAMs.

Numerous studies have focused on the mechanical control of solid structures and phase changes in molecular crystals. However, the molecular level understanding of how macroscopic forces affect the molecules in a solid remains incomplete. Here, we report that a small mechanical stimulus or solid seeding can trigger a single-crystal-to-single-crystal transformation from a kinetically isolated polymorph of phenyl(phenyl isocyanide)gold(I) exhibiting blue photoluminescence to a thermodynamically stable polymorph exhibiting yellow emission without the need for heating or solvent. The phase transformation initiates at the location of the mechanical stimulation or seed crystal, extends to adjacent crystals, and can be readily monitored visually by the accompanying photoluminescent color change from blue to yellow. The transformation was characterized using single crystal X-ray analysis. Our results suggest that the transformation proceeds through self-replication, causing the complex to behave as 'molecular dominoes'.3

Me, myself, and dye: Core‐unsubstituted perylene bisimide dyes bearing a melamine hydrogen‐bonding unit link with cyanurates giving H‐ (red, see picture) and J‐aggregates (green). In solution the aggregates can interconvert on thermal treatment or by changing the stoichiometry. In solid films the green to red color change occurs upon scratching the surface.

p-Conjugated compounds that exhibit tunable luminescence in the solid state under external mechanical stimuli have potential applications in sensors and imaging devices. However, no rational designs have been proposed that impart these mechano-responsive luminescent properties to p-conjugated compounds. Here we demonstrate a strategy for mechanoresponsive luminescent materials by imparting amphiphilic and dipolar characteristics to a luminescent p-conjugated system. The oligo(p-phenylenevinylene) luminophore with a didodecylamino group at one end and a tri(ethylene glycol) ester group at the other end yields segregated solid structures by separately aggregating its hydrophobic and hydrophilic moieties. The segregated structures force the molecules to align in the same direction, thereby generating a conflict between the side-chain aggregation and dipolar stabilization of the p-system. Consequently, these metastable solid structures can be transformed through mechanical stimulation to a more stable structure, from a p-p stacked aggregate to a liquid crystal and further to a crystalline phase with variable luminescence.

Green and blue polymorphs: A single-crystal-to-single-crystal (SCSC) phase transition of phenyl(3,5-dimethylphenyl isocyanide)gold(I) was triggered by mechanical picking or solid seeding and propagated spontaneously with a domino-like mechanism. As a result, one phase with intense green emission was transformed to another phase with weaker blue emission.