Tuning the optical bandgap and piezoresistance in iridium-based molecular semiconductors through ligand modification

Abstract

Square-planar d⁸ metal complexes are known to stack with short metal–metal distances in the solid-state, forming linear molecular chains with conductive pathways that can be enhanced under pressure. Although the influence of the metallic centre on the behaviour of these materials has been previously studied, the role and significance of ligand choice has received less focus. Here, we study the relationship between the structural, optical and conductive properties of a series of d⁸ iridium dicarbonyl complexes with different β-diketonate ligands using a combination of experimental and computational methods. Our results show that ligand choice contributes significantly to the optical transitions of the molecules in solution by lowering the LUMO energy for complexes with π-conjugation or electronegative atoms. We also show that ligand choice is a pathway for band-structure tuning in the molecular crystal through ligand size selection and associated structural packing, with complexes packing in linear metal–metal stacks exhibiting a smaller optical bandgap in the solid state. With pressure-dependent measurements, we confirm that that favourable metal–metal stacking in the solid obtained by appropriate ligand choice leads to higher conductivity at lower pressures. Our results provide insight for the design and application of d⁸ metal complexes in optoelectronic devices and the development of future molecular materials.