A study on electronic excited states of iridium(III) complexes containing bipyridine and phenanthroline ligands. Solvent effect on triplet-triplet absorption spectra.

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In order to study the solvent effect on the lowest excited state of of cis-[IrCl2L2]Cl (L=2,2′-bipyridine, 1,10-phenanthroline, and 4,7-dimethyl-1,10-phenanthroline), the triplet-triplet absorption spectra (15000 cm−1—27000 cm−1) were measured by the use of N,N-dimethylformamide–water mixed solvents. For the triplet-triplet spectra of the three iridium complexes, remarkable spectral changes were observed with the change of the solvent polarity. An SCF calculation of the triplet-triplet transitions was carried out for free bipyridine and free phenanthroline. By comparing the calculated results and the observed spectra, the solvent effect can be interpreted in terms of a model invoking dπ*-ππ* interaction, as proposed by Crosby. The lowest triplet state of [IrCl2(phen)2]Cl is predominantly a dπ* state in 95% v/v DMF–water, but in water, by the reversal of the state ordering, the lowest state becomes a ππ* state. Furthermore, a quite similar solvent effect which was observed on the lowest triplet state of [IrCl2(bpy)2]Cl is discussed; this state is not expected to have a dπ*-ππ* interaction.
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