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 S1' dominates the emission spectrum, while UC results in increased emission from a lower lying state S1''. 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 S1'' and weak emission from the higher lying singlet state S1' through thermally or entropically driven TTA-UC.