A study of the electronic states of iridium(III) complexes containing 1,10-phenanthroline ligands. The contribution of a dd excited state to the lowest triplet state.

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To investigate the solvent dependence of the dd contribution to the lowest excited state of cis-[IrCl2L2]Cl (L=1,10-phenanthroline (phen) and 5,6-dimethyl-1,10-phenanthroline (5,6-Mephen)), the triplet-triplet absorption spectra (15000–25000 cm−1) were measured in N,N-dimethylformamide (DMF)–water mixed solvents. From a smaller solvent-dependence of the absorption intensity for [IrCl2(5,6-Mephen)2]Cl than for [IrCl2(phen)2]Cl, the lowest triplet state of the former is considered to have a ππ* character in 95% v/v DMF–water and 45% v/v DMF–water. However, in pure water a considerable decrease in the triplet-triplet absorbance was observed. Because the molar extinction coefficient of the triplet-triplet absorption of the dd-dd type is 10−2 times smaller than that of the ππ*-ππ* type, the observed decrease in the absorbance is elucidated by the increase in the contribution of the dd state to the lowest triplet state. Using ethanol–methanol (4 : 1, v/v) as a solvent, it was confirmed that the triplet-triplet absorption spectrum shows no change when the solutions are degassed and the temperature is lowered (77 K).
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