The upconversion of near-infrared (NIR) to visible (vis) photons is of interest for display technologies and energy conversion. Although triplet–triplet annihilation (TTA) offers a mechanism for upconversion that works efficiently at low incident irradiance flux densities, current strategies for NIR–vis upconversion based on TTA have fundamental limitations. Herein, we report a strategy for NIR–vis TTA based on lanthanide-containing complexes to sensitize the upconversion. We demonstrate a β-diketonate complex of Yb3+ paired with rubrene that emits yellow (λem = 559 nm) under NIR excitation (λexc = 980 nm). This corresponds to an exceptional anti-Stokes shift of just less than 1 eV. Thus, lanthanide complexes could unlock high-performance NIR–vis upconversion, with lanthanide sensitizers overcoming the energy loss, reabsorption, and short triplet lifetime that fundamentally limit porphyrin, nanocrystals, and direct S0–T1 sensitizers.

Triplet-triplet annihilation up-conversion (TTA-UC) is a developing technology that can enable spectral conversion under sunlight. Previously, it was found that efficient TTA-UC can be realized in polymer hosts for temperatures above the polymer's glass transition (T > T). In contrast, TTA-UC with high quantum yield for temperatures below T is rarely reported. In this article, we report new polymer hosts in which efficient TTA-UC is observed well below T, when the polymer is in a fully solid state. The four poly(olefin sulfone) hosts were loaded with upconversion dyes, and absolute quantum yields of TTA-UC (η) were measured. The highest value of η = 2.1% was measured for poly(1-dodecene sulfone). Importantly, this value was the same in vacuum and at ambient conditions, indicating that the host material acts as a good oxygen barrier. We performed time-resolved luminescence experiments in order to elucidate the impact of elementary steps of TTA-UC. In addition to optical characterization, we used magic angle spinning solid-state NMR experiments to estimate the T2 transverse relaxation time. Relatively long T2 times measured for poly(olefin sulfone)s indicate an enhanced nanoscale fluidity in the studied (co)polymers, which unexpectedly coexists with a rigidity on the macroscale. This would explain the exceptional triplet energy transfer between the guest molecules, despite the macroscopic rigidity.

The triplet-triplet annihilation upconversion (TTA-UC) is an important type of optical process with applications in biophotonics, solar energy harvesting and photochemistry. In most of TTA-UC systems, the formation of triplet...

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.

An electron donor-acceptor dyad based on BODIPY (acceptor) and anthracene (donor) plays either the role of sensitizer or emitter in triplet-triplet annihilation photon up-conversion (TTA-UC). This Janus-like behavior was achieved via altering the relative ordering of charge-transfer and local excited state energies in the dyad through the polarity of TTA-UC media.