Factorizing\nthe Nephelauxetic Effect in Heteroleptic\nMolecular Rubies

The interest in chromium(III) complexes has been renewed\nover the\npast decade for the design of efficient earth-abundant phosphorescent\nred-to-near-infrared spin-flip emitters and photocatalysts with long\nexcited state lifetimes. In this context, we report the energy tuning\nof spin-flip excited states based on heteroleptic bis(tridentate)\npolypyridine chromium(III) complexes <b>[3</b><sup><b>X,Y</b></sup><b>]</b><sup><b>3+</b></sup>, namely, <b>[3</b><sup><b>NMe,CH2</b></sup><b>]</b><sup><b>3+</b></sup>, <b>[3</b><sup><b>NMe,S</b></sup><b>]</b><sup><b>3+</b></sup> and <b>[3</b><sup><b>CH2,S</b></sup><b>]</b><sup><b>3+</b></sup> with the tridentate\nligands L<sup>X</sup> and L<sup>Y</sup> [X/Y = NMe, <i>N</i>,<i>N</i>′-dimethyl-<i>N</i>,<i>N</i>′-dipyridin-2-ylpyridine-2,6-diamine; X/Y = CH<sub>2</sub>, 2,6-bis(2-pyridylmethyl)pyridine and X/Y = S, 2,6-bis(pyridine-2-ylthio)pyridine].\nThe heteroleptic complexes <b>[3</b><sup><b>X,Y</b></sup><b>]</b><sup><b>3+</b></sup> are obtained via a novel\nsynthetic approach toward the required intermediate labile triflato\ncomplexes Cr(L<sup>X</sup>)(OTf)<sub>3</sub> (<b>2</b><sup><b>X</b></sup>) from the respective chlorido precursors CrCl<sub>3</sub>(L<sup>X</sup>) (<b>1</b><sup><b>X</b></sup>)\nusing trimethylsilyl trifluoromethanesulfonate. Spin-flip energies\nwere experimentally detected by vis/near-infrared absorption and emission\nspectroscopy as well as computationally derived by multireference\ncalculations. Together with the known homoleptic molecular ruby complexes,\nthe three resulting series of luminescent complexes <b>[3</b><sup><b>X,X</b></sup><b>]</b><sup><b>3+</b></sup>/<b>[3</b><sup><b>X,Y</b></sup><b>]</b><sup><b>3+</b></sup>/<b>[3</b><sup><b>Y,Y</b></sup><b>]</b><sup><b>3+</b></sup> allow delineation of an additive nephelauxetic\neffect of the ligands with chromium(III) ions and thus prediction\nof spin-flip emission energies of derived molecular rubies.