Exciton and biexciton signatures in the femtosecond transient absorption of phenylene-based polymers and oligomers

Summary form only given.The photophysics of conducting polymers from the poly(para-phenylene vinylene) (PPV) family are studied using femtosecond (fs) transient absorption (TA) to address several issues: what are the primary photoexcitations; how do they interact under high excitation; what are the signatures of biexcitons? We have measured solutions and films of the PPV oligomer MEH-DSB, and several soluble PPV derivatives, with similar results. MEH-DSB results are representative of the entire family. The fs differential transmission (DT) spectra of MEH-DSB show: (1) a positive band due to stimulated emission (SE) around 2.4 eV; (2) a photoinduced absorption (PA) band extending from 1.3-2.0 eV. In solutions at low intensities, both features show exponential decay, indicating that both originate from singlet excitons. In films, the decay is much faster and strongly nonexponential, with a second PA band at /spl sim/1.8 eV. The 1.4-eV band (PA/sub 1/) shows pronounced intensity-dependent dynamics, while PA/sub 2/ exhibits slower pump-independent decay, indicating the creation of two species. The matching PA/sub 1/ and SE features both arise from singlet excitons. We assign PA/sub 2/ to interchain excitons (geminate electron-hole pairs on adjacent chains).