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

Excited state design is an efficient approach towards 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 which 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 either a state that 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 rationally varied between 743 - 865 nm and from 555 - 679 nm, respectively. An increase in the nitrogen content of the six-membered ring of the X ligand results in a blueshift of the 3MLCT(bpy) emission but a redshift for the 3MLCT (X) emission. The wavelength difference between 3MLCT(bpy) and 3MLCT (X) emissions that can be tuned from 84–310 nm and is proportional to the difference in LUMOs energies (reduction potentials) of the isolated ligands. Our study provides key information towards new routes for the design of optically active dual wavelength molecular emitters