Polarons in ladder-type polymer films; Recombination channels and electron-phonon coupling

We applied the photomodulation spectroscopy for probe-photon energies From 0.05 to 2.5 eV in films of ladder-type poly(p-phenylene) and identified a photoinduced absorption (PA) band related to triplet excitons and two correlated PA bands due to charged polarons. The low energy polaron band peaks at a relatively low photon energy of similar to 0.1 eV, showing small polaronic relaxation energy. We employed photoinduced absorption-detected magnetic resonance spectroscopy to study the polaron recombination channels and found that spin 1/2 magnetic resonance enhances the bimolecular recombination of polarons to singlet byproducts (singlet excitons and bipolarons). However, it reduces polaron recombination to triplet byproducts, such as triplet excitons. The low-energy polaron PA shows antiresonances due to its overlap with Raman- and IR-active vibrations. The antiresonances spectrum is compared to the IR and Raman spectra and found to also contain modes that are silent in the latter spectra.