Luminescence properties of the structure built from 3-cyano-4-dicyanomethylene-5-oxo-4,5-dihydro-1H-pyrrol-2-olate and caesium(I).

The structure of caesium(I) 3-cyano-4-dicyanomethylene-5-oxo-4,5-dihydro-1H-pyrrol-2-olate (CsA), Cs(+).C(8)HN(4)O(2)(-), is related to its luminescence properties. The structure of CsA (triclinic, P-1) is not isomorphous with previously reported structures (monoclinic, P2(1)/c) of the KA and RbA salts. Nevertheless, the coordination numbers of the metals are equal for all salts (nine). Each anion in the CsA salt is connected by pairs of inversion-related N-H...O hydrogen bonds to another anion, forming a centrosymmetric dimer. The dimers are linked into infinite ribbons, stacked by means of pi-pi interactions, thus building up an anionic wall. Time-dependent density functional theory calculations show that the formation of the dimer shifts the wavelength of the luminescence maximum to the blue region. Shortening the distance between stacked anions in the row [from 3.431 (5) A for RbA to 3.388 (2) A for KA to 3.244 (10) A for CsA] correlates with a redshift of the luminescence maximum from 574 and 580 nm to 596 nm, respectively.