Solvatochromism on regiochemically substituted poly(Hexylthiophenes)

Poly (3, 3′ʹ-dihexylterthiophene) and poly (3′,4′-dihexylterthiophene) have been synthesized and studied with respect to their solvatochromic properties. While both polymers in chloroform solution have maximum absorption at approximately the same wavelength, they behave differently with respect to changes observed on their UV-visible spectra when the quality of the solvent is changed. A microscopic model for the solvation of both polymers has been developed based on semi-empirical quantum chemical calculations. The results are consistent with a high degree of conformational disorder in the 3′, 4′-substituted polymer even in the solid state, leading to chains of short conjugation length, and thus less sensitive to solvation effects. In 3,3′ʹ-substituted polythiophene on the other hand, the substituents impose smaller torsional barriers, similar to the ones found in regioregular poly (3-hexylthiophene). A rod-to-coil like transition is thus expected to occur as a function of solvent quality.