Absorption and emission in tris(2,2′-bipyridyl)ruthenium(II); effects of excited-state asymmetry

Abstract

The absorption and emission spectra of [Ru(bipy)₃]²⁺(1)(bipy = 2,2′-bipyridyl), while assignable to a first approximation in D₃ symmetry, are more properly discussed using a model that allows for direct Franck–Condon overlap between the trigonal ground state and each of three equivalent asymmetric excited states. This singlet excited state distortion is inherited by (1)*, the emitting triplet, and can be semiquantitatively modelled on the asymmetry of [Ru^II(bipy⁰)₂(bipy^–)]⁺(1)^– and [Ir^III(bipy⁰)₂(bipy^–)]²⁺. The known polarisations of absorption and emission (including the thermally activated z-axis emission), the behaviour of the emission at low temperature in complex (1) and related mixed-ligand complexes, and the apparently anomalous photoselection behaviour of (1) are all natural consequences of such a model. The balance between electron trapping and delocalisation is briefly discussed. The classical theory of photoselection must be modified to cover the case of absorption-induced anisotropy, and changes in photoselection at very low temperatures are predicted.