Luminescence of Transition Metal d6 Chelates

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 (“$\mathrm{d–d}$ ” 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 “$\mathrm{d–d}$ ” emission is less intense. Correlation of the relative emission intensities of the “$\mathrm{d–d}$ ” 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.