Electronic-Spectra Of Pyrazolyl-Bridged Binuclear Iridium(I) Complexes

The electronic absorption and emission spectra and emission lifetimes of lr2(mu-L)2(CO)4 (L = pyrazolyl, 3-methylpyrazolyl, 3,5-dimethylpyrazolyl) have been determined in the temperature range 77-300 K. Polarized emission excitation spectra of the complexes in rigid glassy solutions at 77 K are also reported. Like face-to-face d8-d8 systcms, these A-frame complexes display dipole-allowed transitions to emissive singlet and triplet excited states derived from monomer d(z)2 --> p, excitations; these dimer excited states are stabilized relative to monomer excited states by strong metal-metal bonding. There is a second singlet/triplet pair of excited states, also derived from monomer d(z)2 --> p, excitations; although transitions to these states are dipole forbidden in face-to-face structures, the corresponding bands in the absorption spectra of the A-frame complexes possess considerable intensity. These latter excited states have negligible metal-mctal bonding stabilization relative to the monomer states, but considerable metal-ligand charge-transfer character, as judged by well-resolvcd nu(Ir-CO) vibronic structure in the absorption bands. Overall, the spectroscopic results are consistent with a valence-bond model for the Ir-Ir interaction.