Dramatic Structural Enhancement of Chirality in Photopatternable Nanocomposites of Chiral Poly(fluorene-alt-benzothiadiazole) (PFBT) in Achiral SU-8 Photoresist

• Published in: Advanced functional materials Vol. 22; no. 24; pp. 5074 - 5080
• Main Authors: Oh, Heong Sub, Jee, Hongsub, Baev, Alexander, Swihart, Mark T., Prasad, Paras N.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 19.12.2012; WILEY‐VCH Verlag
• Subjects: conjugated polymers; metamaterials; optically active materials; photonics; supramolecular materials
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.201201273

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(fluorene‐alt‐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 |gabs| = 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. Nanocomposites of chiral poly(fluorene‐alt‐benzothiadiazole) (PFBT) dispersed in achiral SU‐8 photopolymer show dramatic enhancement of circular dichroism (CD). Thin films with a 1/20 to 1/40 ratio of PFBT/SU‐8 have maximum CD values up to 75 times that of a pure chiral PFBT film of equal optical absorbance. This is attributed to supramolecular organization of chiral polymer fibrils.