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

A series of protected and terminal dialkynes with extended pi-conjugation through the fused oligothienyl linker unit in the backbone, 2,5-bis(trimethylsilylethynyl)thieno3,2-b]thiophene 1a, 5,5'-bis(trimethylsilylethynyl)dithieno3,2-b:2',3'-d]thiophene 1b, 2,5-bis(ethynyl)thieno3,2-b]thiophene 2a, 5,5'-bis(ethynyl)dithieno3,2-b:2',3'-d]thiophene 2b, has been synthesized and characterised. The digold alkynyl complexes (Ph3P)Au(Ctriple bond]C)(C6H2S2)(Ctriple bond]C)Au(PPh3)] 3a and (Ph3P)Au(Ctriple bond]C)(C8H2S3)(Ctriple bond]C)Au(PPh3)] 3b have then been prepared by the reaction of two equivalents of Ph3PAuCl and a methanolic KOH solution of 1a 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 A, 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 pi-pi* interactions.