Synthesis, X-ray crystal structure and photophysics of butterfly shape orange and red emanating polynuclear complexes of tris(dibenzoylmethanato)Ln(III) (Ln = Sm/Eu) and exo-bidentate 4,4'-bipyridine.

Reaction of two equivalents of [Ln(dbm) 3 (H 2 O)] (Ln = Sm/Eu/Gd) with one equivalent of 4,4'-bipyridine (4,4'-bpy) led to the formation of rare polynuclear complexes of the type [Ln(dbm) 3 (4,4'-bpy)] n (dbm is the anion of 1,3-diphenyl-1,3-propanedione) instead of symmetrically bridged dinuclear complexes. The structure of the complexes has been established by the single crystal X-ray diffraction (SC-XRD) method and shows that the coordination sphere is composed of a LnO 6 N 2 core (octacoordinated). Shape analysis further revealed that the geometry around Ln(III) is distorted square anti-prismatic with SHAPE value 0.738 and 25.719 for [Sm(dbm) 3 (4,4'-bpy)] n and [Eu(dbm) 3 (4,4'-bpy)] n , respectively. Photoluminescence (PL) properties of [Sm(dbm) 3 (4,4'-bpy)] n and [Eu(dbm) 3 (4,4'-bpy)] n are discussed in the solid-state and PMMA hybrid film (w/w 6%). By employing theoretical modelling in conjunction with the experimental PL data and crystal structure and an energy transfer (ET) mechanism for the sensitized PL of [Eu(dbm) 3 (4,4'-bpy)] n is proposed and discussed in detail. Finally, the role of each ligand in sensitized PL of [Eu(dbm) 3 (4,4'-bpy)] n is calculated and discussed by the chemical partitions of the radiative decay.Graphical abstract.