Tuning Optical Properties and Enhancing Solid-State Emission of Poly(thiophene)s by Molecular Control:  A Postfunctionalization Approach

Postfunctionalization of poly(3-hexylthiophene) (P3HT) enables the systematic study of electronic and steric effects of various functional groups on the optical and photophysical properties of 3,4-disubstituted poly(thiophene)s. In solution, these 3,4-disubstituted poly(thiophene)s exhibit a lower fluorescence yield (Φ fl ) than P3HT. In the solid state, bromo, chloro, and formyl groups increase Φ fl, whereas nitro groups completely quench fluorescence. Phenyl, p -tolyl, 2- and 4-methoxyphenyl, 2-thienyl, biphenyl, and 1-naphthyl substituents increase the solid-state fluorescence of P3HT by a factor of 2−3 as a result of steric interactions that force the aromatic substituents perpendicular to the main chain. An extensive enhancement in the solid-state Φ fl, 12 times greater than that of P3HT, is observed for polymers containing ortho -alkylphenyl or 2-(3-alkyl)thienyl groups, because of additional steric interactions between methyl or α-methylene hydrogens on the phenyl or thienyl substituents and main-chain thienyl groups. These interactions further increase the planarity of the polymer backbone and lead to an enlargement of the interplanar distance. Partial substitution of P3HT with o -tolyl groups affords polymer films with a solid-state Φ fl of up to 22%.