Luminescence of the Mn²⁺ ion in non-<i>O</i>_h and <i>T</i>_d coordination environments: the missing case of square pyramid

Abstract

The first observation of luminescence for Mn(ii) complexes with a square-pyramidal geometry (C_4v) of the metal is reported. The complexes of such type, namely [Mn(L)₂Hal]X, where L is an O,O'-chelating ligand Me₂N-CH(Ph₂P[double bond, length as m-dash]O)₂, Hal = Cl or Br, and X = Br, I or [MnCl₄], show at ambient temperature red photoluminescence (λ≈ 620 nm) with millisecond lifetimes (3.8-7.6 ms). This emission, stemming from spin-forbidden ⁴E(G) →⁶A₁(S) transitions in square-pyramidally coordinated Mn²⁺, is found to be responsive to temperature. Upon cooling the complexes from 300 to 77 K, the red-shift of the emission band by ≈20 nm takes place, which is accompanied by the increase of its integral intensity and lifetime. The peculiarities of the discovered Mn²⁺(C_4v) luminescence are discussed against the classical luminescence that occurs in octahedral (O_h) and tetrahedral (T_d) surroundings.