SYNTHESIS AND UV-VISIBLE PROPERTIES OF SOLUBLE ALPHA-THIOPHENE OLIGOMERS - MONOMER TO OCTAMER

Described is the detailed synthesis of alpha-thiophene oligomers ranging from the monomer to the octamer that are silylated at the alpha' and omega-positions. The terminal trimethylsilyl groups allow the monomer, dimer, and trimer to be freely soluble in numerous organic solvents. The higher homologues, trimer through octamer, have, in addition to the terminal trimethylsilyl groups, methyl groups symmetrically substituting one or more of the thiophene units in order to enhance the solubility of the systems. Methyl substitution cannot be on the 3-position of terminal thiophene units or else rapid protodesilylation occurs in thiophene oligomeric intermediates greater than two units long. The UV-visible spectroscopic characteristics of the synthesized oligomers suggest that, in solution, electrochemically prepared poly(3-alkyl-alpha-thiophene) effectively has only 6-7 contiguous conjugated alpha-thiophene units. However, in the solid state, the effective conjugation path in the polymer is much longer. The effect of d-pi-p-pi-conjugation between the terminal silicon atom and the conjugated system was also investigated by executing the quantitative protodesilylation of the pentameric oligothiophene. The silicon groups do increase the UV absorption maxima.