Molecular Ruby under Pressure.

The intensely luminescent chromium(III) complexes [Cr(ddpd) ] and [Cr(H tpda) ] show surprising pressure-induced red shifts of up to -15 cm  kbar for their sharp spin-flip emission bands (ddpd=N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine; H tpda=2,6-bis(2-pyridylamino)pyridine). These shifts surpass that of the established standard, ruby Al O :Cr , by a factor of 20. Beyond the common application in the crystalline state, the very high quantum yield of [Cr(ddpd) ] enables optical pressure sensing in aqueous and methanolic solution. These unique features of the molecular rubies [Cr(ddpd) ] and [Cr(H tpda) ] pave the way for highly sensitive optical pressure determination and unprecedented molecule-based pressure sensing with a single type of emitter.