Anisotropic Dielectric Tensor for Chiral Polyfluorene at Optical Frequencies

Molecular origin of high field-effect mobility in an indacenodithiophene-benzothiadiazole copolymer

One of the most inspiring and puzzling developments in the organic electronics community in the last few years has been the emergence of solution-processable semiconducting polymers that lack significant long-range order but outperform the best, high-mobility, ordered semiconducting polymers to date. Here we provide new insights into the charge-transport mechanism in semiconducting polymers and offer new molecular design guidelines by examining a state-of-the-art indacenodithiophene-benzothiadiazole copolymer having field-effect mobility of up to 3.6 cm(2) V(-1) s(-1) with a combination of diffraction and polarizing spectroscopic techniques. Our results reveal that its conjugated planes exhibit a common, comprehensive orientation in both the non-crystalline regions and the ordered crystallites, which is likely to originate from its superior backbone rigidity. We argue that charge transport in high-mobility semiconducting polymers is quasi one-dimensional, that is, predominantly occurring along the backbone, and requires only occasional intermolecular hopping through short π-stacking bridges.

Insights from chiral polyfluorene on the unification of molecular exciton and cholesteric liquid crystal theories for chiroptical phenomena

The Maugin-Oseen-DeVries theory accounts for chiroptical properties of cholesteric films with long-range order. For molecular systems with short-range structural correlation lengths, molecular exciton theory is used. A consistent description for systems with intermediate correlation lengths is lacking. Films of chiral polyfluorene behave according to Maugin-Oseen-DeVries theory when the film thickness exceeds 300 nm. Properties of thin films are consistent with molecular exciton theory. We describe the crossover in the optical properties of the film in a phenomenological way using a dielectric tensor that contains terms from Maugin-Oseen-DeVries and molecular exciton theory. Guided by the experimental findings, we explore the possibility of a unification of the Maugin-Oseen-DeVries and molecular exciton theory involving a coupled oscillator model.