The photophysical properties of a series of homochelated complexes of the type Cu(NN)2+ are reported where (NN) denotes a heteroaromatic ligand. For the (NN) ligands 2,9-dimethyl-1,10-phenanthroline, 2,9-dimethyl-4,7-diphenyl-l,10phenanthroline, and 4,4,,6,6'-tetramethyl-2,2'-bipyridine, the emission quantum yields are 2.1 X 10"4, 2.5 X 104, and 0.5 X 104, respectively, in CH2C12 solution at 25 °C. Under these conditions the lifetimes are 90, 80, and 18 ns, respectively, for the same series of compounds. In each case the emission intensity is found to decrease as the solution temperature is lowered. This is interpreted in terms of two emitting states where the population of the upper level, which has a more favorable radiative rate constant, requires thermal energy. Detailed temperature studies of the 2,9-dimethyl-1,10-phenanthroline complex have been carried out, and the results have been analyzed by assuming that the two excited states are thermally equilibrated. We conclude that the lowest energy emitting state exhibits a radiative rate constant of ca. 103 s"\ typical of a triplet state. The other state lies some 1800 cm"1 to higher energy and exhibits a radiative rate constant of ca. 107 s"1. This state is ascribed singlet multiplicity. Tentatively, both states are assumed to derive from the lowest energy charge-transfer configuration of the complex.
