Unexpected Sign Inversion of the Circular Dichroism and Circularly Polarized Luminescence Response of Chiral Copolymer Thin Films by Tuning the Thickness and Annealing Conditions

Chiral copolymer thin films show a strong chiroptical response, in the present study with values for the circular dichroism (CD) reaching up to 16,000 mdeg and a dissymmetry parameter, g abs , of up to 0.7. This behavior renders these films highly attractive for optoelectronic applications, such as OLEDs with intrinsic circularly polarized emission. Such films of the achiral copolymer poly‐[(9,9‐di‐ n ‐octylfluorenyl‐2,7‐diyl)‐ alt ‐(benzo[2,1,3]thiadiazol‐4,8‐diyl)] (shortly, F8BT) blended with helicene‐like chiral inducers are investigated. These films exhibit a unique sign inversion effect of the CD and the circularly polarized luminescence (CPL) signals as a function of film thickness (with a threshold of about 100 nm) and annealing temperature (with a threshold of about 150 °C). It is shown that structural reorganization of the chiral supramolecular phase is the most likely reason for this effect. Detailed measurements of the CPL spectra as a function of the detection angle underline the importance of detecting the emission at normal incidence to the polymer surface to avoid distortion of the CPL signal, which is otherwise identified by a bisignate CPL response at larger detection angles due to waveguide emission from the edge of the polymer film.