Structure and emission studies of dimorphic crystals of [PtBr2(5,5′-bis(CF3CH2OCH2)-2,2′-bpy)] (1)

Abstract

The yellow (1y) and orange (1o) crystalline polymorphs of [PtBr₂(5,5′-bis(CF₃CH₂OCH₂)-2,2′-bipyridine)] exhibit surprisingly short nearest neighbour Pt⋯Pt separations of 3.526 Å and 3.590 Å, respectively, at 295 K. Both distances are much shorter than those found in structures of the unsubstituted [PtBr₂(2,2′-bipyridine)] analogue. Consistent with a linear chain structure in 1o and dimer formation in 1y, both solids exhibit emission spectra shifted to much longer wavelengths than that exhibited by the monomer in a low-temperature glass. Furthermore, the emission spectra of 1o and 1y shift to even longer wavelengths as the temperature decreases and the Pt⋯Pt separations contract. Till now delocalized emission of this type has been considered to be restricted to [PtCl₂(diimine)] systems and implausible in PtBr₂-containing analogues for steric reasons. Ironically, in the system at hand the bulky 5,5′-substituents apparently promote delocalization of the emission by forming a network of hydrogen-bonding-like C–H⋯F–C interactions that help shape the packing.