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.