Molecular Design Principles for Achieving Strong Chiroptical Properties of Fluorene Copolymers in Thin Films

Understanding the molecular level origin of chiroptical properties in enantiopure pi-conjugated polymers is essential for tailoring these materials for application in organic light-emitting diodes and photonic devices. Here we have studied poly(9,9'-dialkylfluorene-alt-2,5-dialkoxyphenyl)s as prototypical copolymers to investigate their structure-chiroptical property relationship. The effect of a systematic variation of the location and configuration (S or R) of chiral and achiral side chains in the repeating units on the liquid crystalline ordering and its relationship to chiroptical properties were investigated. The results clearly indicate that enantiopure side chains on the fluorene units are critical for obtaining the cholesteric liquid crystalline ordering and thereby strong chiroptical properties in both absorption and photoluminescence in annealed thin films. Finally, solution-processed organic light-emitting diodes constructed from a liquid crystalline polymer show strong emission of circularly polarized light, demonstrating the potential application of these systems.