In this paper, the influence of the lithium salt lithium-trifluoromethanesulfonate (LiOTf) on the photophysics of three different oligo(ethylene-oxide) side-chain grafted poly($p$-phenylene) (PPP) type polymers is studied by correlating polymer morphology to optical properties. It is shown that the introduction of the lithium salt leads to a phase transition from a lamellar-type ordered structure to a random amorphous phase, which is accompanied by an alteration of the optical properties of the polymers. Both the $\pi \text{−}{\pi }^{*}$ as well as the triplet exciton (TE) absorption, exhibit a hypsochromic shift upon the addition of LiOTf. Moreover, a detailed study of the TE behavior of the polymer/LiOTf blend systems for different LiOTf concentrations was performed, including the generation rate of TEs from singlet excitons, the TE monomolecular decay, and the TE bimolecular annihilation process. One finds that upon increasing LiOTf concentration, both the TE generation rate and the TE lifetime increase, whereas the bimolecular annihilation process is diminished. These behaviors are correlated with the above mentioned order-disorder transition of the polymer system, which was confirmed by x-ray measurements.

• Received 22 May 2006

DOI:https://doi.org/10.1103/PhysRevB.74.085208