pi conjugation in 2,2'-bithiophene and its dimethyl derivatives: Model compounds of organic conducting polymers based on thiophene rings

Conformational properties of 2,2'-bithiophene and its 3,3'-, 4,4'-, and 5,5'-dimethyl derivatives have been investigated by means of quantum mechanical methods. Computations were performed at the ab initio HF/ 3-21G, HF/6-31G*, HF-6-311G**, and MP2/6-31G levels. Results indicate that 4,4'- and 5,5'-dimethyl-2,2'-bithienyl behave similarly to the unsubstituted compound. Thus, two minimum energy conformations were found for each compound which correspond to the anti-gauche and the syn-gauche, the latter being always less stable than the former. On the contrary, the preferences found for 3,3'-dimethyl-2,2'-bithienyl were drastically different, giving a unique energy minimum around the gauche-gauche conformation. This must be attributed to the strong repulsive interactions originated by the methyl substituents around the anti and syn conformations. Flexible geometry optimizations provided the torsional potentials for the four compounds. The barriers to internal rotation were analyzed in terms of the conjugative and nonbonded interactions between the rings.