Direct observation of the intersystem crossing in poly(3-octylthiophene)

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Abstract
Using subpicosecond photoinduced absorption, we monitor the evolution of singlet excitons in solutions of regiorandom poly(3‐octylthiophene) (P3OT), regioregular poly(3‐hexylthiophene) (P3HT), and a well‐defined α‐oligothiophene with 12 repeat units (T12). We find a luminescence lifetime of 500 ps, and we observe the intersystem crossing from the singlet manifold to the triplet manifold, with a rate constant of k−1ISC=1.2 ns. By measuring the quantum efficiency of luminescence, we estimate an intrinsic lifetime of 2 ns for the singlet excitons, and a nonradiative decay rate of k−1nr=1.5 ns. We find no difference in the intersystem crossing time for the different thiophene derivatives, implying the intersystem crossing rate is determined primarily by the relatively large spin–orbit interaction due to the sulfur heteroatom and not by chain defects, chain ends, or effects due to the side groups. In addition, we find that for t<50 ps a fraction of the photoexcitations undergo one‐dimensional diffusion limited recombination.