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) (dppeO(2)) in dichloromethane-methanol solutions gave colorless crystals with the general chemical formulas [MnBr2(dppeO(2))](n) (1), [MnBr2(dppeO(2))(DMF)]n (la), [Mn(dppeO(2))(3)]- [MnBr4] (2), and Mn2Br4 (dppeO(2))(2 )(3) depending on the crystallization conditions. Compounds 1 and la display one-dimensional chain structures composed of Mn(II) ions linked by bridging dppeO(2) to exhibit tetrahedral (1) or trigonal-bipyramidal (la) coordination geometry, whereas 3 exhibits a cyclic dinuclear structure with two Mn(II) centers bridged by double dppeO(2 )to adopt tetrahedral geometry. Compound 2 consists of octahedrally coordinated cation [Mn(dppeO(2))(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 la with a trigonal-bipyramidal ligand field, as demonstrated by X-ray crystallography. Red-emitting la could be reverted to the original green-emitting 1 with a tetrahedral ligand field upon heat at 160 degrees 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.