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

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)(2)Os(bpy-AN)](2+) and [(bpy-AN)(2)Os(CO)Br](+) have (MLCT)-M-3 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 (MCLT)-M-3 state, [(bpy-AN)(2)Os(CO)Br](+), is nonemissive at room temperature, but has a long lived excited state that is localized on the (3)(pi-pi*) state of the anthracene substituent. The other complex, [(bpy)(2)Os(bpy-AN)](2+), exhibits emission at room temperature and has a transient absorption spectrum that is consistent with a localized (MLCT)-M-3 state. The excited state decay behavior of the two complexes can be fit well assuming a model in which noninteracting (MLCT)-M-3 and (3)(pi-pi*) states are in equilibrium.