Turning on TTA: Tuning the Energy Landscape by Intermolecular Coupling

The development of efficient solid-state photon upconversion (UC) devices remains paramount for practical applications of the technology. In recent years, the incorporation of perovskite thin films as triplet sensitizers for triplet–triplet annihilation (TTA)-based UC has provided a promising solution. In the pursuit of finding an “ideal annihilator” to maximize the apparent anti-Stokes shift, we investigate naphtho[2,3- a ]pyrene (NaPy) as an annihilator in both solution-based and perovskite-sensitized TTA-UC systems. Surprisingly, we observe different emission behaviors of NaPy in the solid state based on the excitation wavelength. Under direct excitation, a high-energy transition S 1 ' dominates the emission spectrum, while UC results in increased emission from a lower lying state S 1 ''. We propose that this is the result of aggregation-related lowering of the singlet excited state thus changing the fundamental energetic landscape underlying TTA. Aggregation decreases the singlet energy below the energy level of the triplet pair state 1 (TT), yielding energetically favorable emission from the aggregated singlet state S 1 '' and weak emission from the higher lying singlet state S 1 ' through thermally or entropically driven TTA-UC.