posted on 2004-10-19, 00:00authored byHsiao-Hsien Sung, Hong-Cheu Lin
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.