Subpicosecond photoinduced absorption in poly[2,5-thienylene vinylene] and poly[3-methoxy-6-(2-ethyl-hexyloxy) phenylene vinylene

Abstract

The photoinduced absorption spectra of the conjugated polymers poly[3-methoxy-6-(2-ethyl-hexyloxy) phenylene vinylene] (MEH-PPV) and poly[2,5-thienylene vinylene] (PTV) have been measured using 100 fs laser pulses and white-light continuum generation. For MEH-PPV, a broad sub-gap absorption, peaking at ∼ 1.45 eV, appears within 1 ps of photoexcitation by a 2.05 eV pulse. With increasing time, the induced sub-gap absorption decays and sharpens slightly. These features are tentatively assigned to the photoproduction of triplet excitons. For PTV, excitation at 2.05 eV results in a sub-gap absorption whose maximum is somewhere below the low-energy cut-off of our experimental apparatus (1.3 eV). At early times, induced absorption and bleaching peaks are observed in the vicinity of the principal absorption edge. These features evolve with increasing time, and, by 100 ps, closely resemble the thermal modulation spectrum expected for complete transfer of the pump energy to the lattice due to the nonradiative recombination of the photoexcitations.