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The effects of electric field on absorption and photoluminescence (PL) of films of sulfide-substituted PPV derivative S3-PPV, poly[2-(phenyl)-3-(4'-(3,7-dimethyloctyloxy)phenyl)-1,4-phenylenevinylene-co-2-(11'-decyl sulfanylundecanyloxy)-5-methoxy-1,4-phenylene vinylene], were investigated. Electroabsorption (E-A) and electrophotoluminescence (E-PL) responses of S3-PPV show the Stark shifts, indicating a significant alternation in the molecular polarizability (Delta alpha) associated with the optical transitions. Field-induced enhancement or quenching is also observed for PL of S3-PPV, depending on the photoexcitation energy, whereas the shape of the PL spectra is independent of the excitation wavelength. The field effects on the decay profiles of PL indicate that the quenching results from a diminished population of the emitting states on excitation at 300 nm, whereas the PL is enhanced on excitation at 471 nm because the emitting state has an increased lifetime. The efficiency of field-assisted generation of electron-hole pairs produced through excitons monotonically increases with increasing excitation energy, and the nonradiative decay rate in the emitting state is diminished by electric fields in S3-PPV. The photoirradiation of S3-PPV in ambient air resulted in rapid degradation of the polymer film.