A series of new ligands has been prepared in which pyridyl (L'-L5) or phenol (H2L7) end-groups are linked by oligothienyl chains: 2,5-bis(4-pyridyl)thiophene (L'); 5,5'-bis(4pyridyl)-2,2'-bithiophene (L'); 5,5"-bis(4-pyridyl)-2,2' : 5',2"-terthiophene (L3); 5,5"'-bis(4-pyridyl)-2,2' : 5',2" : 5",2"'-quaterthiophene (L4); 5,5'-bis[2-(4-pyridyl)ethenyl]-2,2'-bithiophene (L5) and 5,5'-bis(4-hydroxyphenyl)-2,2'-bithiophene (H2L7).The compounds L'-L5 undergo two one-electron reductions at potentials which converge as the compounds lengthen; H2L7 shows an irreversible oxidation ascribed to the formation of a quinone. They all have very strong n-n* transitions in their electronic spectra and are all strongly luminescent. Attachment of { Mo[HB(drnp~)~](N0)Cl} termini (dmpz = 3,5-dimethylpyrazolyl) to these compounds gives paramagnetic (for L'-L5) or diamagnetic (for L7) dinuclear complexes which show strong electrochemical interactions between the termini. the (formally) 17-electron/18-electron metal-based couples in the complex of L', for example, are separated by 450 mV, which may be ascribed to a substantial degree of delocalisation of the added electrons onto the bridging ligands. The electrochemical interactions across the ligands are significantly stronger than across those of comparable length with polyene bridges, which confirms the more effective ability of the oligothienyl groups to transmit electronic interactions. The EPR spectra of the paramagnetic dinuclear complexes of L'-L5 reveal the presence of magnetic exchange interactions between the unpaired spins. The complexes are all weakly luminescent, due to residual ligand-based luminescence which is not entirely quenched by the metal centres.
Tetrathiafulvalene and its derivatives can be chemically or electrochemically oxidized to yield stable radical cation salts which display unusual electrical properties. In order to increase the dimensionality of the system by extending the π‐conjugation and thus stabilizing the metallic state, while at the same time keeping the system small, the tetraiodo‐derivative (see Fig.) has been synthesized and characterized.
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