Determination of Upconversion Quantum Yields Using Charge-Transfer State Fluorescence of Heavy-Atom-Free Sensitizer as a Self-Reference

The efficiency of photon upconversion via triplet-Triplet annihilation is characterized by an upconversion quantum yield (φUC); however, uncertainties remain for its determination. Here, we present a new approach for the relative measurement of φUC for green-To-blue upconversion using BODIPY-pyrene donor-Acceptor dyad (BD1) as a heavy-Atom-free triplet sensitizer. This new approach exploits broad fluorescence from a charge-Transfer (CT) state of BD1, which possesses (i) a significant Stokes shift of 181 nm in dichloromethane and (ii) a comparably high CT-fluorescence quantum yield (φref = 7.0 ± 0.2%), which is independent from oxygen presence and emitter (perylene) concentration while also exhibiting a linear intensity dependence. On the basis of this, we developed an upconversion reference using the BD1 sensitizer mixed with perylene (1 × 10-5 M/1 × 10-4 M) in dichloromethane. With this reference system, we investigated the performance of three BODIPY donor-Acceptor dyads in the upconversion process and achieved one of the highest φUC of 6.9 ± 0.2% observed for heavy-Atom-free sensitizers to date.