Engineering Tunable Single and Dual Optical Emission from Ru(II)–Polypyridyl Complexes through Excited State Design

Excited state design is an efficient approach toward new applications in molecular electronics spanning solar cells, artificial photosynthesis and biomedical diagnostics. Ruthenium(II)–polypiridyl based complexes are an example of molecular building blocks with tunable single and dual wavelength emission that can be controlled by excited state engineering via selective ligand modification. Here we investigate three new heteroleptic [Ru(bpy)2X]+ complex ions, where X represents pyridinyl or pyrazinyl derivatives of diazolates, providing tunable emission in the visible and infrared region. The dual emission is shown to arise from the presence of two excited states consisting of a triplet metal-to-ligand charge transfer state localized on a bipyridine ligand, 3MLCT (bpy), and a state that either is entirely localized on the X ligand or is partially delocalized also spanning part of the bipyridine ligands, 3MLCT(X). By a suitable choice of the X ligand, emission from 3MLCT(bpy) and 3MLCT (X) states can be rat...