Altering the Conjugation Pathway for Improved Performance of Benzobisoxazole-Based Polymer Guest Emitters in Polymer Light-Emitting Diodes

Benzobisoxazoles (BBOs) are known to increase the electron
affinities and improve the electron transporting properties of materials
containing them. However, BBO copolymers generally do not perform
well as emissive guests in guest–host PLEDs due to inefficient
Förster resonance energy transfer (FRET) between host and guest.
The incomplete FRET results in a large amount of host emission and
limits the potential efficiencies of the devices. In all previously
reported BBO copolymers, the conjugation pathway was through the oxazole
rings. Herein we report six new BBO copolymers with backbone connectivity
directly on the central benzene ring, resulting in a conjugation pathway
for the polymers that is perpendicular to the previously reported
pathway. Guest–host PLEDs made using these polymers show that
the new conjugation pathway improves FRET between the poly­(N-vinylcarbazole) host and the BBO-containing polymer guest.
Because of highly efficient FRET, no host emission is observed even
at lower guest concentrations. The improved energy transfer results
in devices with luminous efficiencies up to 3.1 Cd/A, a 3-fold improvement
over previously reported BBO-based PLEDs. These results indicate that
the conjugation pathway plays a critical role in designing emissive
materials for guest–host PLEDs.