Factorizing the Nephelauxetic Effect in Heteroleptic Molecular Rubies

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 [3X,Y]3+, namely, [3NMe,CH2]3+, [3NMe,S]3+ and [3CH2,S]3+ with the tridentate
ligands LX and LY [X/Y = NMe, N,N′-dimethyl-N,N′-dipyridin-2-ylpyridine-2,6-diamine; X/Y = CH2, 2,6-bis(2-pyridylmethyl)pyridine and X/Y = S, 2,6-bis(pyridine-2-ylthio)pyridine].
The heteroleptic complexes [3X,Y]3+ are obtained via a novel
synthetic approach toward the required intermediate labile triflato
complexes Cr(LX)(OTf)3 (2X) from the respective chlorido precursors CrCl3(LX) (1X)
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 [3X,X]3+/[3X,Y]3+/[3Y,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.