Photophysical behavior and intramolecular energy transfer in Os(ii) diimine complexes covalently linked to anthracene

Abstract

The photophysical behavior of two Os(II) complexes having a bipyridine ligand with anthracene attached directly to the bipyridine (4-(9-anthryl)-2,2′-bipyridine, bpy-AN) is reported. The two complexes [(bpy)₂Os(bpy-AN)]²⁺ and [(bpy-AN)₂Os(CO)Br]⁺ have ³MLCT excited states that differ in energy by less than 800 cm⁻¹. Despite this fact, the observed photophysical behavior of the two complexes is entirely different. The complex with the higher energy ³MCLT state, [(bpy-AN)₂Os(CO)Br]⁺, is nonemissive at room temperature, but has a long lived excited state that is localized on the ³(π–π*) state of the anthracene substituent. The other complex, [(bpy)₂Os(bpy-AN)]²⁺, exhibits emission at room temperature and has a transient absorption spectrum that is consistent with a localized ³MLCT state. The excited state decay behavior of the two complexes can be fit well assuming a model in which noninteracting ³MLCT and ³(π–π*) states are in equilibrium.