Luminescent digold ethynyl thienothiophene and dithienothiophene complexes; their synthesis and structural characterisation

A series of protected and terminal dialkynes with extended π-conjugation through the fused oligothienyl linker unit in the backbone, 2,5-bis(trimethylsilylethynyl)thieno[3,2-b]thiophene 1a, 5,5′- bis(trimethylsilylethynyl)dithieno[3,2-b: 2′,3′-d]thiophene 1b, 2,5-bis(ethynyl)thieno[3,2-b]thiophene 2a, 5,5′-bis(ethynyl)dithieno[3,2- b:2′,3′-d]thiophene 2b, has been synthesized and characterised. The digold alkynyl complexes [(Ph₃P)Au(C≡C)(C₆H ₂S₂)(C≡C)Au(PPh₃)] 3a and [(Ph ₃P)Au(C≡C)(C₈H₂S₃)(C≡C) Au(PPh3)] 3b have then been prepared by the reaction of two equivalents of Ph₃PAuCl and a methanolic KOH solution of la and 1b, respectively. The complexes have been characterised spectroscopically. The crystal structures show that the gold centres adopt a linear two-coordinate geometry appropriate for Au(I) complexes. Within the crystals adjacent molecules are linked by Au ⋯ S intermolecular interactions in the range 3.48-3.89 Å, but there are no short Au ⋯ Au contacts. The absence of Au ⋯ Au interactions in solution is confirmed by UV/visible absorption and emission spectroscopy, the spectra being dominated by ligand-centred π-π* interactions.