PDAFs defect emission

The blue emission from PDAFs is unstable, with the appearance of a strong emission band around 530 nm after annealing or upon running an EL device [14,15]. Initially this long wavelength emission band was believed to be due to emission from excimers [63,64], but subsequent work has shown that it is due to emission from fluorenone defects within the PDAF chain [15]. 

Other approaches that have been found to improve the stability of blue emission from PDAFs are to attach bulky substituents such as cyclohexyl groups [214] or polyhedral siloxanes [215-217] to the ends of the alkyl chains. These presumably work by reducing exciton migration to defect sites. 

Though the emission stability of 129 is much better than for PDAFs, long-wavelength emission is seen with time, possibly due to the susceptibility of the benzyl linkages to oxidation. Accordingly, a new class of polyfluorenes with aryl substituents at the 9-position has been developed (Scheme 61) [228]. Aryl substituents are much less susceptible to oxidation to lluorenone than alkyl groups and their bulk reduces interchain interactions, and hence exciton diffusion to any defect sites that may be formed. 

---