Exploring the triplet-to-singlet conversion mechanism in persistent luminescence: insights from a host-guest system

The afterglow phenomenon, characterized by persistent luminescence after the cessation of external excitation, is typically a result of late phosphorescence. However, recent research has explored the possibility of producing afterglow with delayed fluorescence resulting from triplet conversion mechanisms. The main mechanism is a reverse intersystem crossing (rISC), a monomolecular phenomenon in which triplet excitons are converted into singlets. However, triplet conversion can also happen via the intermolecular pathway of triplet-to-singlet (TTS) Förster transfers. For instance, this mechanism has been used to explain afterglow in a host-guest system composed of NPB and DCJTB molecules, but the mechanism behind the photophysics of this system has not been fully characterized. Here, we provide a full theoretical study of the photophysics of NPB and DCJTB molecules, employing a methodology that accounts for vibrational and medium effects to determine the rates of various intra- and intermolecular processes that determine the behavior of this system. We identify extremely low rISC and nonradiative decay rates in NPB as responsible for simultaneously making it an efficient dual emitter and an effective donor molecule for TTS exciton transfers. We also demonstrate how morphological conditions contribute to the pairing of energy levels between NPB and DCJTB, playing a key role in allowing for efficient TTS transfers. Finally, we use kinetic Monte Carlo simulations to prove that the TTS transfer mechanism is able to produce delayed fluorescence in a timescale of tenths of seconds, well-explaining the experimental observations.