Magnetic field effect on excited-state spectroscopies of π-conjugated polymer films

The magnetic field effect in organic light-emitting diodes, such as magnetoconductance and magnetoelec-troluminescence, has been intensively explored in the last few years. Here, we demonstrate the magnetic field effect of two excited-state spectroscopies in films of a prototype pi-conjugated polymer, i.e., a soluble derivative of poly(phenylene vinylene), [2-methoxy-5-(2'-ethylhexyloxy)-poly(p-phenylene vinylene)] (MEH-PPV); these are magnetophoto-induced absorption (MPA) and magnetophotoluminescence (MPL). We study these novel magnetic field effects in pristine MEH-PPV films, MEH-PPV films subjected to prolonged illumination, and blend of MEH-PPV with a fullerene derivative. Being spectroscopic, MPA and MPL are determined by the photoexcitation spin density and thus may unravel the occurrence of myriad spin-mixing processes in organic semiconductors that include hyperfine interaction in polaron pairs, spin-sublevel mixing in triplet excitons, triplet-triplet annihilation, and triplet-singlet collision. The recently observed ultrasmall magnetic field effect at B < similar to 0.5 mT in organic diodes is also observed in the MPA response of MEH-PPV films that support polaron photoexcitations, thereby identifying the underlying mechanism as being due to spin mixing of polaron pairs by the hyperfine interaction.