The electrochemical and UV-visible spectroscopic properties of Rh(III) and Ir(III) complexes of the ortho-metalating (NC) ligands, 2-phenylpyridine (ppy) and benzo[A]quinone (bzq), have been studied. Cyclic voltammetric studies of several of the dimeric species, [M(NC)2C1]2, indicate metal-centered oxidation occurs at moderate potentials. Cationic monomers of the type M(NC)2(NN)+ where (NN) = 2,2'-bipyridine or 1,10-phenanthroline have been prepared by reaction of the chelating ligands with the parent dimers. Cyclic voltammetric studies of these monomers indicate that several reversible ligand-centered reductions are generally observed and that the chelating ligand is more easily reduced than is the ortho-metalating ligand. Spectroscopic studies of the mixed ligand monomers indicate that dual emissions from MLCT states associated with the ortho-metalating and chelating ligands occur in the Ir(III) complexes whereas a single emission from a ligand-localized excited state is observed in the Rh(III) complexes. These results are discussed in terms of electronic and nuclear coupling factors analogous to those encountered in descriptions of bimolecular energy and electron-transfer processes.

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Emission and absorption spectra have been determined for Rh(III) and Ir(1II) complexes of the nitrogen ligands, ethylenediamine (en) pyridine, 2,2′-dipyridyl (dip), and 1,10-phenanthroline (phen). Emission lifetimes at 77°K in glassy solution and at room temperature have been measured. From lifetime measurements and the energies of emission, luminescence from these heavy metal chelates is assigned as “phosphorescence,” indicating complete intersystem crossing, consistent with the heavy metal spin–orbit mixing of excited singlet and triplet states. Spin–orbit coupling is further manifested in the appearance of room temperature phosphorescence for the bis Ir(III) complexes of dip and phen. The emission can be classified as (1) metal localized (“d–d” emission) or (2) “delocalized molecular” (ligand localized or charge transfer). The metal localized emission appears at low energies, is broad and structureless, and exhibits a large Stokes shift. The delocalized molecular emission appears at higher energies and shows characteristic vibrational structure and a small Stokes shift. The intensity of the delocalized molecular emission is large, indicating a quantum yield approaching one. The “d–d” emission is less intense. Correlation of the relative emission intensities of the “d–d” emitters with the displacement of the emitting state from the ground state determined from the Stokes shifts suggest that recent Jortner and Rice theories of nonradiative transitions rationalizes these relative intensities. The “strong coupling” limit is illustrated by the tris and bis(en) complexes of Ir(III) and is consistent with the low intensity emission of these complexes. The larger emission intensity of the Rh(III) en complexes is consistent with the weaker vibrational coupling predicted for these complexes by the theory.

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further metal-localized oxidations and/or DPA'-localized oxidations. For 3 only irreversible reduction waves are seen at very negative potentials. Consistent with the other results is the appearance of an additional oxidation wave for 3 ( = 2). For 1 visible charge-transfer bands indicating transitions from Ru(II) to both the bpy :r* orbitals (~450 nm) and the HDPA pyridyl it* orbitals (350 nm) are observed. The emission remains dir* from bpy as in the = 0 complex. For 2 ( = 1 and 2) very low energy charge-transfer transitions (558 and 605 nm, respectively) assigned as dir* (bpy) are seen.The energies of these bands correlate well with the electrochemically predicted values. These results confirm that substantial changes occur in coordinated HDPA on deprotonation, and the results for 2 suggest that a metal-ligand interaction unique to Ru imine complexes exists.