Outer Sphere Metal-to-Ligand Charge Transfer in Organometallic Ion Pairs

A series of organometallic salts which comprise a fac- (b)Re I (CO) 3 (py) + cation (b = 4,4‘,5,5‘-tetramethyl-2,2‘-bipyridine (tmb), 2,2‘-bipyridine (bpy), or 4,4‘-dicarbomethoxy-2,2‘-bipyridine (dmeb); py = pyridine) paired with the Co(CO) 4 - anion have been prepared and subjected to photophysical study. In nonpolar solvents the salts feature a broad, low-intensity ion-pair charge transfer (IPCT) absorption band. The energy of the IPCT band decreases with the LUMO energy of the diimine ligand, suggesting that the orbital basis of the transition is d (Co) → π* (b). An X-ray crystal structure of [(bpy)Re I (CO) 3 (py) + ][Co(CO) 4 - ] ( 2a ) reveals that the anion occupies a lattice position which is directly below (or above) the plane defined by the bpy ligand, which supports the d (Co) → π* (b) IPCT assignment. Luminescence studies of the salts indicate that the dπ (Re) → π* (b) metal-to-ligand charge transfer (MLCT) excited state is quenched by reductive electron transfer from Co(CO) 4 - . Nominally IPCT and MLCT excitation of the organometallic ion pairs afford the same geminate radical pair, [(b • - )Re I (CO) 3 (py),Co(CO) 4 • ]. However, laser flash photolysis studies reveal that the rate of charge recombination within the geminate radical pair is significantly slower when MLCT excitation is applied. The slower rate of charge recombination is attributed to the fact that triplet state geminate pairs are produced via the triplet MLCT excited state manifold.