One-pot synthesis of symmetric octithiophenes from asymmetric β-alkylsulfanyl bithiophenes

Starting from 4-(octylsulfanyl)-2,2'-bithiophene, 4-bromo-4'-(octylsulfanyl)-2,2'-bithiophene, 4-iodo-4'-(octylsulfanyl)-2,2'-bithiophene, 4-bromo-4'-[(S)-2-methylbutylsulfanyl]-2,2'-bithiophene, and 4-iodo-4'-[(S)-2-methylbutylsulfanyl]-2,2'-bithiophene, a new series of symmetrically beta-substituted octithiophenes were synthesized by one-pot oxidative coupling with FeCl3. The octithiophenes obtained are soluble in common organic solvents and show different solvatochromic properties depending on the substitution type. In particular, the bromine atom exerts a positive influence on the supramolecular organization: the brominated octithiophenes display high filmability, solvatochromism, and CD induced by aggregation (when the chiral 2-methylbutylsulfanyl group is present), properties usually observed for polythiophenes. Density functional theory (DFT) calculations were carried out an a model bithiophene (4-substituted with a methylsulfanyl group) in order to understand the possible mechanism of the growth, the regiochemistry, and the reason for the polymerization leads to an octithiophene.