Intensive chiroptical properties of chiral polyfluorenes associated with fibril formation

Thin films of chiral poly{9,9-bis[(3S)-3,7-dimethyloctyl]-2,7-fluorene} (1) were studied using circular dichroism (CD) spectroscopy. Films spin coated from chloroform solution, show CD with a degree of polarization gnbs (= +4 × 10-4 at 400 nm) that is independent of film thickness (50-290 nm). This implies that gnbs is an intensive property of the material and related to the chiral organization of the molecules on a length scale less than 50 nm. Atomic force microscopy (AFM) on the films reveals fibrils. Addition of nonsolvent methanol to a solution of 1 in chloroform leads to fibril formation in solution and results in CD similar in band shape to that of the pristine spin coated films from chloroform solution and a gnbs comparable in magnitude. Thus the chiral molecular arrangement leading to circular dichroism is part of the internal structure of these fibrils.