LUMINESCENCE STUDIES OF PYRIDINE ALPHA-DIIMINE RHENIUM(I) TRICARBONYL COMPLEXES

The room- and low-temperature luminescences of ReL(CO)3X where L = 2,2′-bipyridine, 1,10-phenanthroline, or 5-phenyl-1,10-phenanthroline and X = substituted pyridine or quinoline were studied. Relatively small but useful variations in the state energies can be effected by altering the Hammett σ values of substituents on the pyridines. All complexes exhibit metal to ligand charge-transfer (MLCT) phosphorescences at room temperature. However, by choice of suitable ligands, the emissions can be switched to ligand-localized phosphorescence on cooling to 77 K. This behavior is explained on the basis of the proximity of the lowest MLCT and π-π* triplet states and the changes in energy of the MLCT state as a function of temperature. At room temperature the MLCT state can equilibrate to an energy that is lower than that of 3π-π* state and give MLCT luminescence. In rigid low-temperature media, however, the MLCT state cannot relax during the excited-state decay and emission is from the lower energy 3π-π* state. At room temperature, lifetimes are predominantly affected by alterations in the nonradiative rate constant, as described by the energy-gap law. From σ values of the substituents, both state energies and lifetimes can be predicted before synthesis. The design of new luminescent complexes is discussed. © 1990, American Chemical Society. All rights reserved.