Factorizing the Nephelauxetic Effect in Heteroleptic Molecular Rubies
Florian Reichenauer, Dimitri Zorn, Robert Naumann, Christoph Förster, Katja Heinze
Abstract
The interest in chromium(III) complexes has been renewed over the past decade for the design of efficient earth-abundant phosphorescent red-to-near-infrared spin-flip emitters and photocatalysts with long excited state lifetimes. In this context, we report the energy tuning of spin-flip excited states based on heteroleptic bis(tridentate) polypyridine chromium(III) complexes [3 X,Y ] 3+, namely, [3 NMe,CH2 ] 3+, [3 NMe,S ] 3+ and [3 CH2,S ] 3+ with the tridentate ligands L X and L Y [X/Y = NMe, N, N ′-dimethyl- N, N ′-dipyridin-2-ylpyridine-2,6-diamine; X/Y = CH 2, 2,6-bis(2-pyridylmethyl)pyridine and X/Y = S, 2,6-bis(pyridine-2-ylthio)pyridine]. The heteroleptic complexes [3 X,Y ] 3+ are obtained via a novel synthetic approach toward the required intermediate labile triflato complexes Cr(L X )(OTf) 3 ( 2 X ) from the respective chlorido precursors CrCl 3 (L X ) ( 1 X ) using trimethylsilyl trifluoromethanesulfonate. Spin-flip energies were experimentally detected by vis/near-infrared absorption and emission spectroscopy as well as computationally derived by multireference calculations. Together with the known homoleptic molecular ruby complexes, the three resulting series of luminescent complexes [3 X,X ] 3+ / [3 X,Y ] 3+ / [3 Y,Y ] 3+ allow delineation of an additive nephelauxetic effect of the ligands with chromium(III) ions and thus prediction of spin-flip emission energies of derived molecular rubies.