Novel Alternating Fluorene-Based Conjugated Polymers Containing Oxadiazole Pendants with Various Terminal Groups

Abstract

A series of soluble alternating fluorene-based copolymers containing symmetrical and asymmetrical 1,3,4-oxadiazole (OXD) pendants with various terminal groups are synthesized by the palladium-catalyzed Suzuki coupling reaction. These polymers possess higher glass transition temperatures than that of the analogous dialkoxy-substituted polymer (PFPOC6) consisting of the same backbone without OXD pendants. The photophysical and electrochemical properties of these polymers are affected by the polar effect (electron-withdrawing group, −CN, and electron-donating group, −R or −OR) and the size effect (the size of the grafted side chain) of the OXD pendants. Owing to the large steric hindrance of OXD pendants, the aggregation of these polymers in solids is reduced, which results in almost identical PL emissions in both solution and solid states. The bulky OXD pendants on the polymer side chains can provide the polymer films with lower HOMO and LUMO energy levels and better electron injection property. Since only one emission peak is observed in both PL and EL spectra of these polymers, it is evidenced that effective energy transfer from the OXD pendants to the conjugated polymer backbones has occurred, thus eliminating the light emission from the OXD pendants. These asymmetrical OXD-substituted polymers have higher quantum yields and less aggregation in the solid state than the symmetrical OXD-substituted polymers. The symmetrical OXD-substituted polymer (P1) has a longer PL emission wavelength than the asymmetrical OXD-substituted polymers (P2−P8), which may be due to the improvement of the coplanarity between the polymer backbone and the symmetrical OXD pendants and/or the introduction of two electron-withdrawing OXD pendants.