Triplet Energy Back Transfer in Conjugated Polymers with Pendant Phosphorescent Iridium Complexes

The nature of Dexter triplet energy transfer between bonded systems of a red phosphorescent
iridium complex 13 and a conjugated polymer, polyfluorene, has been investigated in electrophosphorescent
organic light-emitting diodes. Red-emitting phosphorescent iridium complexes based on the [Ir(btp)2(acac)]
fragment (where btp is 2-(2‘-benzo[b]thienyl)pyridinato and acac is acetylacetonate) have been attached
either directly (spacerless) or through a −(CH2)8− chain (octamethylene-tethered) at the 9-position of a
9-octylfluorene host. The resulting dibromo-functionalized spacerless (8) or octamethylene-tethered (12)
fluorene monomers were chain extended by Suzuki polycondensations using the bis(boronate)-terminated
fluorene macromonomers 16 in the presence of end-capping chlorobenzene solvent to produce the statistical
spacerless (17) and octamethylene-tethered (18) copolymers containing an even dispersion of the pendant
phosphorescent fragments. The spacerless monomer 12 adopts a face-to-face conformation with a
separation of only 3.6 Å between the iridium complex and fluorenyl group, as shown by X-ray analysis of
a single crystal, and this facilitates intramolecular triplet energy transfer in the spacerless copolymers 17.
The photo- and electroluminescence efficiencies of the octamethylene-tethered copolymers 18 are double
those of the spacerless copolymers 17, and this is consistent with suppression of the back transfer of
triplets from the red phosphorescent iridium complex to the polyfluorene backbone in 18. The incorporation
of a −(CH2)8− chain between the polymer host and phosphorescent guest is thus an important design
principle for achieving higher efficiencies in those electrophosphorescent organic light-emitting diodes for
which the triplet energy levels of the host and guest are similar.