PHYSICAL AND PHOTOPHYSICAL PROPERTIES OF RHENIUM(I) TETRACARBONYL COMPLEXES

The physical and photophysical properties of a series of rhenium(I) tetracarbonyl complexes [Re(CO)4(L-L)]-CF3SO3, where L-L = 2,2'-bipyrimidine (bpm), 4,4'-dimethyl-2,2'-bipyridine (dmb), 2,2'-bipyridine (bpy), and 1,10-phenanthroline (phen), are reported. The complexes displayed reductions but no oxidations within the +2 to -2 V vs SSCE electrochemical window. Electronic absorptions associated with dpi --> pi* transitions were observed between 300 and 400 nm; pi --> pi* transitions associated with the aromatic heterocyclic ligand were found at wavelengths <300 nm. Excitation at 355 nm resulted in the appearance of emission in solution at room temperature with emission quantum yields near 0.03 for the bpm, bpy, and dmb derivatives and 0.008 for the phen derivative. The bpy, dmb, and phen derivatives displayed structured emission located near 470 nm, while the bpm derivative displayed unstructured emission located near 521 nm. The complexes were photosensitive in methylene chloride, expelling one of the CO ligands. The photochemical quantum yields ranged from 0.03 to 0.9. Reductive quenching was observed with various methoxybenzene compounds and Cl- ion verifying the high [Re(CO)4(L-L)+*/0 redox potentials calculated to be on the order of 1.5 V. An excited-state model assigning structured emission to a 3LC state, unstructured emission to a 3MLCT state, and photosubstitution to a 3LF(dd) state or possibly the 3MLCT state is proposed.