Molecular Ruby under Pressure

The intensely luminescent chromium(III) complexes [Cr(ddpd)2]3+ and [Cr(H2tpda)2]3+ show surprising pressure‐induced red shifts of up to −15 cm−1 kbar−1 for their sharp spin‐flip emission bands (ddpd=N,N′‐dimethyl‐N,N′‐dipyridine‐2‐yl‐pyridine‐2,6‐diamine; H2tpda=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(H2tpda)2]3+ pave the way for highly sensitive optical pressure determination and unprecedented molecule‐based pressure sensing with a single type of emitter. Luminescence under pressure: The spin‐flip emission band of the highly luminescent molecular ruby [Cr(ddpd)2]3+ (ddpd=N,N′‐dimethyl‐N,N′‐dipyridine‐2‐yl‐pyridine‐2,6‐diamine) shifts by −15 cm−1 kbar−1 under hydrostatic pressure, surpassing that of the standard pressure calibrant, ruby Al2O3:Cr3+, by a factor of 20. Unique to the molecular ruby is its applicability also in solution (aqueous and methanolic), enabling optical pressure sensing in solution with a single dye.