Low-lying electronic states of [Rh(bpy)₃]³⁺, [Pt(bpy)₂]²⁺, and [Ru(bpy)₃]²⁺. A Comparative study based on highly frequency-resolved and time-resolved spectra

Abstract

Optical emission and excitation spectra of [Rh(bpy-h₈)₃]³⁺, [Rh(bpy-h₈)₂(bpy-d₈)]³⁺, [Rh(bpy-d₈)₃]³⁺, [Pt(bpy-h₈)₂]²⁺, [Pt(bpy-h₈)(bpy-d₈)]²⁺, [Pt(bpy-d₈)₂]²⁺, [Ru(bpy-h₈)₃]²⁺, [Ru(bpy-h₈)₂(bpy-d₈)]²⁺, [Ru(bpy-d₈)₃]²⁺ are discussed. A series of trends - also including [Os(bpy)₃]²⁺ - is uncovered. These trends, which are connected with an increase in metal-d or MLCT character in the lowest triplet states, can for instance be seen in transition energies, emission lifetimes, zero-field splittings, rates of spin-lattice relaxation, vibronic satellite structures, and changes of nuclear equilibrium positions on excitation. Moreover, the compounds investigated are also appropriate for consideration of the concept of "dual emission", the relation between MLCT character and covalency, as well as the current models of localization/delocalization. In particular, a comparison of the properties of [Rh(bpy)₃]³⁺- and [Ru(bpy)₃]²⁺-doped [Zn(bpy)₃](ClO₄)₂ provides illustrative and very strong evidence for covalent delocalization in the lowest excited states of [Ru(bpy)₃]²⁺, while ligand-centered localization is found in the states of [Rh(bpy)₃]³⁺. Misinterpretations might occur if the aggregation effects of the chromophores are not taken into account.