Intramolecular mini-excitons in the series [Os(bpy)_3-x(bpy-d₈)_x]²⁺(x= 0-3 in [Zn(bpy)₃](ClO₄)₂and [Ru(bpy)₃](ClO₄)₂(bpy = 2,2′-bipyridine)

Abstract

A detailed examination of the spectroscopic properties of the lowest excited triplet metal-to-ligand charge transfer (3MLCT) states in the title systems is reported. The results are in accord with the C2/c single-crystal X-ray structure of [Zn(bpy)3](ClO4)2 and [Ru(bpy)3](ClO4)2 (bpy = 2,2′-bipyridine) which establishes one crystallographic site for the molecular cation. Two ligands are related by the twofold axis whereas the third ligand is unique. As a consequence, the partially deuterated complexes substitute in the lattice in two distinct ways. This leads to a superposition of spectra for the x = 1 and x = 2 systems. Spectroscopy of the series in both hosts can be described by using an exciton formalism with an excitation exchange interaction β between the two crystallographically equivalent metal-ligand subunits of ≈ 37 cm-1 and ≈ 78 cm-1 for levels I and II, respectively. These values of β are an order of magnitude larger than in the trigonal [Ru(bpy)3](PF6)2 hosts. Thus partial deuteration does not lead to full localization of the lowest-energy mini-excitons in the title systems.