Chemistry of electronically excited ruthenium polypyridine complexes: II. Photochemistry of the complex [Ru(2,2 '-bpy)(2)(4,4 '-bpy)(NH3)](BF4)(2)

As shown by H-1 NMR and electronic spectroscopy, the irradiation of acetoni trile solutions of the complex cis-[Ru(2,2 ' -bpy)(2)(4,4 ' -bpy)(NH3)](2+) at 450 +/- 10 nm in the region of the long-wave charge-transfer band Ru(II ) --> 2,2 ' -bipyridine results in the photosubstitution of the solvent mol ecule for 4,4 ' -bipyridine with the quantum efficiency of 0.21. A quantum- chemical simulation of the potential curves of detachment of the NH3, 2,2 ' -bpy, and 4,4 ' -bpy ligands for low-lying (MLCT)-M-3 charge-transfer and (3)dd ligand-field triplet electronically excited states showed that, as th e Ru-N distance increases, these states approach each other for any of the ligands owing to a reduction in the energy of the d --> d transitions. As a result, thermal occupation of the (3)dd triplet becomes possible, which no ticeably weakens the bonds between ruthenium and polypyridine ligands and s timulates the dissociation of the Ru-N bond involving 4,4 ' -bipyridine.