Luminescence of Mn2+ ion in non-Oh and Td coordination environments: the missing case of square-pyramid
The first observation of luminescence for Mn(II) complexes with a square-pyramidal geometry (C4v) of the metal is reported. The complexes of such type, namely [Mn(L)2Hal]X, where L is a O,O'-chelating ligand Me2N-CH(Ph2P=O)2, and X = Cl, Br or [MnCl4], show at ambient temperature red photoluminescence (λ □(em¦max) ≈ 620 nm) with the millisecond lifetimes (3.8–7.6 ms). This emission, stemming from spin-forbidden 4T1 → 6A1 transitions in square-pyramidally coordinated Mn2+, is found to be responsive to the temperature. Upon cooling the complexes from 300 to 77 K, red-shift of the emission band by ≈ 20 nm takes place, which is accompanied by increasing of its integral intensity and lifetime. The peculiarities of the discovered Mn2+(C4v) luminescence are discussed against the classical one that occurs in octahedral (Oh) and tetrahedral (Td) surroundings.