Conjugation Length\nDependence of Intramolecular Singlet\nFission in a Series of Regioregular Oligo 3‑Alkyl(thienylene-vinylene)s

Activated intramolecular singlet fission is known to\noccur in the\nconjugated polymer polythienylene-vinylene (P3TV). Instead, efficient\nintersystem crossing has been observed in a short 3-alkyl(thienylene-vinylene)\ndimer. Here, we investigate a series of oligomers covering the conjugation\nlength gap between the dimer and polymer. We confirm that the polymer\nand longer oligomers undergo activated intramolecular singlet fission,\nwhile the shorter oligomers with less than 3 units predominantly undergo\nefficient intersystem crossing. For the longer oligomers, the intermediate\nstate of singlet fission is assigned to the 3A<sub>g</sub><sup>–</sup> covalent state of triplet pair character, as predicted by several\ncomputational studies. Our results point to potential pitfalls when\nassigning triplet production pathways solely with transient absorption\nspectroscopy.