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.