π‑Electron Conjugation in Two Dimensions

Organic oligomers and polymers with extended π-conjugation are the fundamental building blocks of organic electronic devices. Novel routes are being explored to create tailor-made organic materials, and recent progress in organic chemistry and surface chemistry has led to the synthesis of planar 2D polymers. Here we show how extending π-conjugation in the second dimension leads to novel materials with HOMO–LUMO gaps smaller than in 1D polymers built from the same parent molecular repeat unit. Density functional theory calculations on <i>experimentally realized</i> 2D polymers grant insight into HOMO–LUMO gap contraction with increasing oligomer size and show fundamental differences between 1D and 2D “band gap engineering”. We discuss how the effects of cross-conjugation and dihedral twists affect the electronic gaps

Molecular Orbital Gates for Plasmon Excitation

Future combinations of plasmonics with nanometer-sized electronic circuits require strategies to control the electrical excitation of plasmons at the length scale of individual molecules. A unique tool to study the electrical plasmon excitation with ultimate resolution is scanning tunneling microscopy (STM). Inelastic tunnel processes generate plasmons in the tunnel gap that partially radiate into the far field where they are detectable as photons. Here we employ STM to study individual tris-(phenylpyridine)-iridium complexes on a C₆₀ monolayer, and investigate the influence of their electronic structure on the plasmon excitation between the Ag(111) substrate and an Ag-covered Au tip. We demonstrate that the highest occupied molecular orbital serves as a spatially and energetically confined nanogate for plasmon excitation. This opens the way for using molecular tunnel junctions as electrically controlled plasmon sources