Density functional theory calculations of the local spin densities of 3-substituted thiophenes and the oligomerization mechanism of 3-methylsulfanyl thiophene

For obtaining a theoretical basis for the one-step formation of regioregular α-conjugated oligo- and polythiophenes, the electronic states of 3-substituted thiophenes and oligo(3-methylsulfanylthiophene)s were elucidated by molecular orbital calculations using density functional theory (DFT) with the Becke-type three parameters functionals (B3LYP) and the 6-31G(d) basis set. The reactivity for coupling reaction of mono- and oligo-thiophenes are inferred from the calculated unpaired electron spin densities of the respective radical cations, and the ionization potentials which correspond to the energies for generating radical cations during oxidative processes were estimated. The major regioselective products of the oligomerization of 3-(methylsulfanyl)thiophene [J. Org. Chem. 61 (1996) 8285] can be well understood in terms of the magnitude of spin densities. Since the steric hindrance between the methylsulfanyl side chains does not interfere with the coupling reaction occurring between 2-positions (C-2) of thiophene rings, the initiating reaction of 3-(methylsulfanyl)thiophene is the generation of a head-to-head (HH) dimer. On the other hand, the head-to-tail (HT) dimer would play an important role in the propagation reactions of 3-(4-butylphenyl)thiophene. This originates from the highest spin density at the 5′-position of the HT dimer and the low probability of the HH coupling due to the steric hindrance between thiophene ring and phenyl group.