Multiple pulse transient spectroscopy in luminescent π-conjugated polymers

We closely examined relaxation processes between even and odd parity excitons in poly(p-phenylene vinylene) derivative polymers using a multiple femtosecond pulse transient spectroscopy. By tuning the energy of re-excitation and probe pulses across a wide spectral range, we were able to measure a complete spectrum of the 1Bu exciton and also probe its counterparts: higher lying mAg and kAg excitons. Dramatically different relaxation paths were found for two later states: whereas the majority of mAg excitons experience an ultrafast internal conversion back to their 1Bu state, most of kAg excitons transform into a non-emissive state, which we attribute to weakly bound polaron pairs.