Effective Suppression of Intra- and Interchain Triplet Energy Transfer to Polymer Backbone from the Attached Phosphor for Efficient Polymeric Electrophosphorescence

To systematically investigate the relationship between polymeric electrophosphorescence and polymer structures we designed and synthesized a series of copolymers with red-emitting iridium complex [(piq)(2)Irdbm] (where piq is 1-phenylisoquinolin and dbm is dibenzoylmethane) embedded onto the polymer backbone with different chain configurations. We found that triplet energy back transfer from an attached phosphor to polymer backbone call be suppressed not only by enhancing the triplet energy (E-T) of polymer backbone, but also by introducing a sterically hindered units on the backbone. Together with the improved charge transporting ability, high device efficiency can be achieved. A maximum external quantum efficiency of 2.93% with an emission peak of 629 nm is attained from the copolymer PFOxdIrpiq3 with fluorene-alt-oxadiazole backbone due to its relative high E-T (2.32 eV). The efficiency is further enhanced to 3.21 % under the identical device configuration for the copolymer PFSFIrpiq3 with fluorene-alt-spirobifluorene backbone regardless of its relative low E-T (2.18 eV). The promoted efficiency can be attributed to the effective inhibition of interchain triplet energy transfer because of sterically hindered spirobifluorene units on the backbone. The observation suggests a new design principle for electrophosphorescent polymers to suppress the triplet energy transfer to the polymer main chain from the attached phosphor.