Thiophene-Bipyridine Appended Diketopyrrolopyrrole Ligands and Platinum(II) Complexes

Straightforward palladium(II) catalyzed direct cross-coupling reaction between decyl, (S)-2-methyl-butyl, and dodecyl N-substituted diketopyrrolopyrrole thiophene (DPPT), including a 3-methoxy-thiophene derivative, and 6-bromo-2,2'-bipyridine afforded a series of mono- and bis-bipyridine substituted DPPT ligands 1-3. Complexation reactions with PtCl₂(DMSO)₂ provided ortho-metalated platinum(II) complexes 1-Pt and 2-Pt, together with the N^N^O complex 3d-Pt(N^N^O) resulted from the O-Me activation of the intermediary complex 3d-Pt(N^N). The ligand 1b and the mononuclear complexes 1a-Pt and 1b-Pt have been structurally characterized by single crystal X-ray structure, evidencing the establishment of numerous intermolecular π-π interactions in the solid state. Moreover, in the crystal structure of the model complex DMTB-Pt(N^N^O) (DMTB = 3,4-dimethoxy-(2,2'-bipyridine)) the chelating tridentate N^N^O mode is clearly evidenced. The chiral ligand 1b and its mononuclear complex 1b-Pt do not show any CD signal in solution, but they are CD active in the solid state with bisignate bands in the low energy region, opposite in sign between the ligand and the complex, suggesting helical supramolecular arrangement of the dpp chromophore in the solid state. Photophysical investigations demonstrate that all of the ligands are fluorescent with high quantum yields, while the emission is quenched for the complexes, except partially in 3d-Pt(N^N), very likely through an intersystem crossing mechanism promoted by the heavy metal. Density functional theory calculations support the differences observed between the absorption properties of the ligands, ortho- and non-ortho-metalated complexes. The highly fluorescent bipyridine ligands reported herein open the way toward multifunctional transition metal complexes and their use in organic electronics.