Determination of Upconversion Quantum Yields Using Charge-Transfer State Fluorescence of Heavy-Atom-Free Sensitizer...

The efficiency of photon upconversion via triplet-triplet annihilation is characterized by an upconversion quantum yield (Φ_UC); however, uncertainties remain for its determination. Here, we present a new approach for the relative measurement of Φ_UC 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 (Φ_ref = 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 Φ_UC of 6.9 ± 0.2% observed for heavy-atom-free sensitizers to date.