Helical sexithiophenes:: An experimental and theoretical study implicating the alternating 2,2′:3,3′ regioisomer as a reliable helical motif

Perchlorinated sexithiophene regioisomer, 2,2′″″-diX-5,5′,5′′,5′″,5′″′,5′″″-hexachloro-[3,3′;2′,2′′;3′′,3′″;2′″,2′″′;3′″′,3′″″]sexithiophene (compound 1), demonstrates a reliable helical conformation in the solid state, regardless of a broad range of substituents, X. The synthesis and composition of compound 1a (X = H) synthetically accommodates substituent diversity at the 2- and 2′″″-sites. X-ray crystal structures (X = H, Cl, Br) and theoretical geometry optimizations (X = H, Cl, Br, I, Me, Et, t-Bu, and Ph) both confirm that the helical state, a conformation likely dictated by internal torsional strain, is predominant and unaffected by substituent X. It is predicted (ACID/B3LYP/6-31G(d) calculations and UV?visible spectra) that the helical structure exists as a fully conjugated system. Copyright © 2003 American Chemical Society.