By studying a single crystal of polydiacetylene-toluene-sulfonate, we have made the first identification of the triplet excited state of a conjugated polymer. The basis for this identification is the magnetic field dependence of the excited-state lifetime, which is measured by time-resolved optical absorption. Additionally, we have detected the $\Delta m=\pm 1$ transitions between triplet sublevels by measuring microwave-induced changes in the triplet-state lifetime.

• Received 16 December 1985

DOI:https://doi.org/10.1103/PhysRevLett.56.1850