Photostudies of [Cu(bpy)(PPh₃)₂]⁺, [Cu(phen)(PPh₃)₂]⁺, and [Cu(dmp)(PPh₃)₂]⁺ in Solution and in Rigid, Low-Temperature Glasses. Simultaneous Multiple Emissions from Intraligand and Charge-Transfer States

Spectral studies of a series of complexes of the type [Cu(biL)(PPh₃)₂]⁺ have been carried out where biL denotes 2, 2'-bipyridine, 1, 10-phenanthroline, and 2, 9-dimethyl-1, 10-phenanthroline which will be abbreviated as bpy, phen, and dmp, respectively. The dmp complex, unlike the others, emits fairly efficiently in fluid solution and has been studied in detail. All three complexes exhibit metal-to-ligand charge transfer (d-π) transitions in solution around 360 nm, but solutions of the dmp complex give an extra visible absorption band which is attributable to a second copper complex formed according to the equilibrium: [Cu(dmp)(PPh₃)₂]⁺ = [Cu(dmp)]⁺ + 2PPh₃. This dissociation of PPh₃ is readily suppressed by the addition of excess PPh₃, and under these conditions, the E1/2 of the complex is estimated to be E0.7 V vs. SCE. Under the same conditions, the complex has an emission lifetime of 330 ns and an emission quantum yield of 1.4 X 10^-3. PPh₃ is not a quencher, but both electron-transfer and energy-transfer quenching have been observed. The reduction potential of the excited complex is estimated to be ca. -0.9 V vs. SCE. In rigid glasses at low temperature all three complexes exhibit broad, structureless emissions with lifetimes in the microsecond domain which can be assigned to³d-π states. Additional components with lifetimes in the millisecond domain have been observed for the phen and dmp complexes. The latter have characteristic vibronic structures which enable us to assign them as³π-π emissions involving the coordinated heterocyclic ligands. The occurrence of multiple emissions from these systems is rationalized in terms of current models from the literature. Finally, weak sub-nanosecond components have been time resolved from the total emission spectra of the phen and bpy complexes, and possible origins for these components are discussed.