EMISSION PHOTOSELECTION STUDIES OF TRANSITION-METAL COMPLEXES .1. BIS-(DIIMINE) COMPLEXES

The technique of emission photoselection spectroscopy utilized with organic systems to determine nonradiative selection rules and orbital relaxation pathways has seldom been used with transition-metal systems. The ability of the technique to acquire polarization data for randomly oriented samples can be useful in clarifying some of the unique excited-state relaxation pathways (multiple-state emission) observed for these inorganic complexes. Therefore, a polarization spectrometer designed and assembled in this laboratory has been used to acquire emission and excitation polarization spectra for the three [IrL2Cl2]+ complexes (where L is 2, 2‘-bipyridine (bpy), 1, 10-phenanthroline (phen), and 5, 6-dimethyl-l,10-phenanthroline (5, 6-Mephen)), [Ga(phen)2Cl2]+, and the three free ligands. The magnitude of the polarization, P, obtained for the three C2 Ir(III) complexes and the free ligands indicates that these emission chromophores all behave as linear oscillators. The slope of the emission polarization found for the three Ir(III) complexes indicates the presence of two emitting states, consistent with an earlier suggestion of a charge transfer and dd* emission from these complexes. The polarization spectra for the three Ir(III) are similar; however, this result cannot alone be used to distinguish between assignment of the emission origin as either dπ* (charge transfer) or ππ* (ligand localized). The photoselection spectra of the Ga complex is depolarized, implying an intramolecular hopping of the localized excited state from ligand to ligand. The dominant intensity mechanism for the phosphorescence of the free ligands apparently is a second-order vibronic scheme involving an out-of-plane vibration while, as expected, a first-order spin-orbit mechanism is utilized with the transition-metal complexes. © 1979, American Chemical Society. All rights reserved.