Dramatic structural enhancement of chirality in photopatternable nanocomposites of chiral poly(fluorene-alt-benzothiadiazole) (PFBT) in achiral SU-8 photoresist

Materials with large optical activity at visible wavelengths are of great interest in photonics, particularly as one of the routes towards optical metamaterials. Here, dramatic structural enhancement of the optical activity of chiral poly(fluorenea alta benzothiadiazole) (PFBT) when dispersed in SU-8 to form a nanocomposite is reported. The supramolecular helical organization of PFBT chains in these optically clear nanocomposite films produces specific rotation at visible wavelengths that is 68 times that of a pure chiral PFBT film of the same optical absorbance. Photopatterning and development under standard conditions for SU-8 leave behind a residual film of dispersed PFBT/SU-8 aggregates in the nominally unexposed regions where the SU-8 matrix is removed. After annealing, the patterned film exhibits specific rotation 58 times that of a pure chiral PFBT film of the same optical absorbance. Photopatterned and annealed films have a dissymmetry ratio as high as |g_abs| = 0.32. This dramatic enhancement is attributed to supramolecular helical organization of the aggregates within the nanocomposites and of the aggregates liberated from the SU-8 matrix in the exposed regions.