Excited state decay of tetrahalomanganese(ii) complexes

The excitation and emission spectra and decay times of several MnX^2-₄ (X = Cl^-, Br^-, 1^-) complexes of various tetraalkylammonium, -phosphonium, and -arsonium salts have been measured for the pure solids at 298°K and 77°K. High luminescence quantum yields (0.3-1.0) reveal that lifetimes fairly accurately reflect radiative decay rates. An impressive correlation exists between the lifetime, τ, of the ⁴T₁ (G) →⁶ A₁ emission and the ligand, X: for X = Cl^-, τ = 1.2 - 3,5 x 10^-3 sec; X = Br^-, τ = 0.35 - 0.43 X 10^-3 sec; X =l^-, τ = 0.036 - 0.055 X 10^-3 sec. We attribute this decreasing lifetime largely to the enhanced spin-orbital coupling associated with the heavier halide. We find that direct population of high energy charge-transfer (CT) states gives smaller emission yields than excitations in the ligand-field (LF) region.