Dual Fluorescence of Octatetraene Hints at a Novel Type of Singlet-to-Singlet Thermally Activated Delayed Fluorescence Process

A novel type of singlet-to-singlet thermally activated delayed fluorescence (STS-TADF) from the optically bright 11Bu+ and dark 21Ag– states of octatetraene is proposed based on state-of-the-art ab initio simulations. The new phenomenon bears similarity with TADF, the key difference being that in the normal TADF the emissive state is repopulated from a nearby triplet, whereas in STS-TADF it occurs from a nearby dark singlet state (21Ag–). The nonadiabatic populations of the bright and dark singlet excited states are dynamically redistributed through a conical intersection mediated by an ultrafast skeleton C–C stretching vibration, establishing a steady state for a sufficiently long time to undergo radiative decay. The steady state manifests itself in the experimentally observed dual fluorescence. It is expected that the STS-TADF can be observed in other π-conjugated chromophores as well.