Monitoring the Onsets and Kinetics of β Phase Nucleation in Polydiarylfluorene Films Annealed at Different Temperatures

Polydiarylfluorene stands out as a prime example of a conjugated polymer, renowned for its remarkable light-emission properties and its capacity to form a distinct ordered crystalline phase-the β phase-with dramatic impact on the optoelectronic properties of this material. Despite extensive research on the morphology of this polymer, the initial stages of nucleation of polymer chains within β phase domains remain elusive. In this study, we investigate the β phase crystallization onset and evolution using a comprehensive set of structural and spectroscopic techniques. In particular, we probe the thermal onset for β phase nucleation by combining differential scanning calorimetry, grazing incidence wide-angle X-ray scattering, and absorption, photoluminescence, and Raman spectroscopies. Raman microscopy reveals that β domains are already emerging upon annealing at 210°C. Judicious control of β phase fraction is further shown to be essential for unlocking the optimal light-emitting properties by amplified spontaneous emission analysis. Remarkably, triplet exciton diffusion probed via continuous-wave photoinduced absorption spectroscopy uniquely provides definitive evidence for the presence of small, highly isolated β phase domains in films annealed at the lowest temperatures.