Best practice in determining key photophysical parameters in triplet–triplet annihilation photon upconversion

Triplet–triplet annihilation photon upconversion (TTA-UC) is a process in which low-energy light is transformed into light of higher energy. During the last two decades, it has gained increasing attention due to its potential in, e.g., biological applications and solar energy conversion. The highest efficiencies for TTA-UC systems have been achieved in liquid solution, owing to that several of the intermediate steps require close contact between the interacting species, something that is more easily achieved in diffusion-controlled environments. There is a good understanding of the kinetics dictating the performance in liquid TTA-UC systems, but so far, the community lacks cohesiveness in terms of how several important parameters are best determined experimentally. In this perspective, we discuss and present a “best practice” for the determination of several critical parameters in TTA-UC, namely triplet excited state energies, rate constants for triplet–triplet annihilation ( k_TTA ), triplet excited-state lifetimes ( τ_T ), and excitation threshold intensity ( I_th ). Finally, we introduce a newly developed method by which k_TTA , τ_T , and I_th may be determined simultaneously using the same set of time-resolved emission measurements. The experiment can be performed with a simple experimental setup, be ran under mild excitation conditions, and entirely circumvents the need for more challenging nanosecond transient absorption measurements, a technique that previously has been required to extract k_TTA . Our hope is that the discussions and methodologies presented herein will aid the photon upconversion community in performing more efficient and manageable experiments while maintaining—and sometimes increasing—the accuracy and validity of the extracted parameters.