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)2Os(bpy-AN)]2+ and [(bpy-AN)2Os(CO)Br]+ have (3)MLCT 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(pi-pi*) 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(pi-pi*) states are in equilibrium.