Polarization memory decay spectroscopy of photoexcitations in π-conjugated polymers:: Evidence for excimers

We used the technique of ultrafast transient polarized photomodulation in the vi sible/near-infrared range and polarization memory decay (PMD) spectroscopy to study the primary photoexcitations in films and solutions of two poly(phenylene-vinylene) (PPV) derivatives, namely, 2-methoxy-5-(2'-ethylhexyloxy) PPV (MEH-PPV) and 2,5-dioctyloxy PPV (DOO-PPV). We found that the primary photoexcitations in DOO-PPV films and solutions and MEH-PPV solutions are intrachain singlet excitons that show the same PMD across the entire spectrum. In contrast, the ultrafast photoexcitations in MEH-PPV films are of two kinds; these are intrachain excitons with fast PMD kinetics and interchain species with very slow PMD kinetics, which is the property that identifies these photoexcitations as excimers (or interchain charge transfer exciton). We attribute the excimers in MEH-PPV films to the increased interchain coupling due to the tightly packed nanomorphology, which also explains the photoluminescence efficiency decrease and its odd kinetics.