PICOSECOND PHOTOINDUCED ABSORPTION AND POLARIZATION MEMORY IN POLYTHIOPHENE DERIVATIVES

Although the fast dynamics of photoexcitations has been thoroughly explored in the conjugated polymer trans-polyacetylene, the experimental picture is much less complete for polymers in which the ground state is nondegenerate. We report time-resolved photoinduced-absorption data from films of one family of such polymers: the soluble polythiophene derivatives, poly(3-hexylthiophene-2,5- yl) and poly(3-octylthiophene-2,5-yl). Fast (<10-12 s) bleaching of the interband transition is observed at 2.06 eV, accompanied by photoinduced absorption in the band gap at 1.17 eV. The results indicate a shift of a fraction of the oscillator strength, consistent with rapid structural relaxation to form polarons with associated band-gap states. By varying the polarization of the optical probe relative to that of the pump, photoinduced dichroism is observed. The magnitude of the initial dichroism implies that in spin-cast films the polymers are (locally) chain extended and chain aligned. The time decay of the polarization memory is consistent with rapid trapping of the initially mobile charged excitations. © 1990 The American Physical Society.