Molecular Ruby under Pressure

The intensely luminescent chromium(III) complexes [Cr(ddpd)(2)](3+) and [Cr(H(2)tpda)(2)](3+) show surprising pressure-induced red shifts of up to -15cm(-1) kbar(-1) for their sharp spin-flip emission bands (ddpd=N,N-dimethyl-N,N-dipyridine-2-yl-pyridine-2,6-diamine; H(2)tpda=2,6-bis(2-pyridylamino)pyridine). These shifts surpass that of the established standard, ruby Al2O3:Cr3+, by a factor of 20. Beyond the common application in the crystalline state, the very high quantum yield of [Cr(ddpd)(2)](3+) enables optical pressure sensing in aqueous and methanolic solution. These unique features of the molecular rubies [Cr(ddpd)(2)](3+) and [Cr(H(2)tpda)(2)](3+) pave the way for highly sensitive optical pressure determination and unprecedented molecule-based pressure sensing with a single type of emitter.