Poly thiophenes planar conjugated backbones

Qualitative comparison of poly(3-alkylthiophene) (PAT) solution UV spectra indicated a progressive red shift of kmax as the proportion of head-to-tail couplings increases. The rr-PATs show a red shift in the kmax compared with the ir-PATs. This is indicative of a lower energy k-k transition for the rr-PATs and, therefore, a longer conjugation length. The reason for the increase in orbital overlap is that rr-PATs establish a more planar conformation in solution that is more rod-like whereas ir-PATs are more coil-like, twisting the orbitals away from interaction. Different alkyl substituents on the thiophene backbone can drastically influence the absorption spectrum. 

In contrast, ionochromic effects were visible with head-to-tail poly[3-(2,5,8-trioxanonyl)thiophene] poly(6) both in solution and in the solid state [24 249]. With the addition of cations such as Pb " and Hg " ", the conjugation of this polymer, characterized by an absorption maximum at 439 nm in CHCI3 and 486 nm in the solid state, was completely eliminated [247]. Thus, the cationic complexation would induce the twisting of the polymer backbone away from planarity. However, although the response was chemoselective (Li+ and Zn + were found not to interfere), the ionochromic effects observed with Pb + and Hg " " were irreversible, as the conjugation of the polymer was not restored after the complexation. 

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