Triplet Orbital Coupling: Achieving Ultralong Lifetime and Color Control in Upconversion Luminescence

Lanthanide‐doped nanocrystals typically exhibit limited time‐gated nonlinear luminescence with microsecond lifetimes because of their parity‐forbidden 4f–4f transitions. Here the direct observation of ultralong‐lived upconversion luminescence by coupling molecular triplet orbitals to lanthanide‐doped nanocrystals is reported. Through careful manipulation of the molecular triplet states, nanohybrids capable of producing ultralong upconversion lifetimes up to 1.37 s and tunable color output are constructed, surpassing the intrinsic lifetime of lanthanide ions by a factor of 4500. In addition to the exciton migration pathway from the 4f levels of lanthanide ions to the molecular singlet and then to triplet states, direct activation of the triplet state by exciton from the 4f levels is observed. This phenomenon significantly reduces the required photon energy (by more than 0.83 eV) for stimulation of the triplet state compared to the process involving intersystem crossing. Taking advantage of the unique properties of these nanohybrids, a pulsed laser‐pumped encryption system with enhanced security is developed by embedding additional passwords that contain information about pulse frequency and width. This work provides a new perspective on time‐resolved nonlinear luminescence with an ultrawide time dimension by engineering molecular triplet states. Nonlinear luminescence with tunable color output and an ultralong lifetime of 1.37 s is achieved by coupling molecular triplet orbitals to lanthanide nanocrystals. In addition to the exciton transfer pathway from lanthanide ions to molecular singlet to triplet state, direct activation of the triplet state by excitons from lanthanide ions is also observed.