Luminescent Vapochromism Due to a Change of the Ligand Field in a One-Dimensional Manganese(II) Coordination Polymer

The reactions of MnBr2 and ethane-1,2-diylbis(diphenylphosphine oxide) (dppeO2) in dichloromethane–methanol solutions gave colorless crystals with the general chemical formulas [MnBr2(dppeO2)]n (1), [MnBr2(dppeO2)(DMF)]n (1a), [Mn(dppeO2)3][MnBr4] (2), and Mn2Br4(dppeO2)2 (3) depending on the crystallization conditions. Compounds 1 and 1a display one-dimensional chain structures composed of Mn(II) ions linked by bridging dppeO2 to exhibit tetrahedral (1) or trigonal-bipyramidal (1a) coordination geometry, whereas 3 exhibits a cyclic dinuclear structure with two Mn(II) centers bridged by double dppeO2 to adopt tetrahedral geometry. Compound 2 consists of octahedrally coordinated cation [Mn(dppeO2)3]2+ and tetrahedrally arranged anion [MnBr4]2–. While 1 and 3 in crystalline and powder states are highly luminescent with green emission bands centered at ca. 510 nm, 2 shows intense orange luminescence peaking at 594 nm. Upon exposure of 1 to N,N-dimethylformamide vapor, the green emission centered at 510 nm is converted to red luminescence peaking at 630 nm, ascribed to the formation of DMF-coordinated compound 1a with a trigonal-bipyramidal ligand field, as demonstrated by X-ray crystallography. Red-emitting 1a could be reverted to the original green-emitting 1 with a tetrahedral ligand field upon heat at 160 °C, and such a reversible conversion could be perfectly repeated for several cycles. A new mechanism of luminescent vapochromism is thus proposed because of the reversible conversion of ligand fields in manganese(II) complexes.