Photoexcited states in poly(p-phenylene vinylene): Comparison with trans,trans-distyrylbenzene, a model oligomer.

We present measurements of the photoexcited states of poly(p-phenylene vinylene), PPV, and a model oligomer, trans,trans-distyrylbenzene, probed by detection of photoluminescence and photoinduced absorption. Photoluminescence is strong in both materials and due to radiative recombination of the singlet exciton. Triplet excitons are also photogenerated, and in the case of the oligomer are seen directly in light-induced ESR measurements. We find a signal with a peak-to-peak width of 12 G at 1580 G for a probe frequency of 9.14 GHz. We assign this to the ``forbidden'' \ensuremath{\Delta}${\mathit{m}}_{\mathit{s}}$=2 transition between the triplet sublevels. From the position of the minimum field ${\mathit{H}}_{\mathrm{min}}$ for this microwave transition, we determine the zero-field splitting of the triplet sublevels to be 0.07 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. We find an associated photoinduced absorption at 1.95 eV in the oligomer which we attribute to a triplet-triplet transition of the excited state. For the polymer we find a photoinduced absorption response at 1.45 eV, very similar to that of the oligomer, and we consider that this is similarly due to a triplet exciton, although we have not been able yet to find direct ESR evidence. In the case of the polymer, charged photoexcitations are also seen in photoinduced absorption measurements, characteristic of bipolarons, with induced absorption at 0.6 and 1.6 eV. We do not find similar excitations in the oligomer, and we consider that intermolecular charge transport is necessary for the generation of the long-lived charged excitations probed in our experiments.