Critical Sensitizer Quality Attributes for Efficient Triplet−Triplet Annihilation Upconversion with Low Power Density Thresholds

Triplet-triplet annihilation upconversion (TTAUC) is a power density-dependent process where photons of low energy are transformed into high energy ones. The most important attributes of efficient TTAUC are quantum yield φTTAUC, power density threshold Ith (photon flux at which 50% of φTTAUC is achieved), and the upconversion shift of emitted photons (anti-Stokes shift). To date, approaches to balance these parameters have remained unclear. Herein, the cumulative effect of sensitizer triplet lifetime (τ0 S), sensitizer-annihilator triplet energy gap (ΔET), and the total concentration of the sensitizer on the power density threshold at high TTAUC quantum yields is evaluated experimentally using Pt, Pd, and Zn tetraphenylporphyrin derivatives and a tetra-tert-butylperylene annihilator, and by kinetic rate modeling. The results suggest that a large energy gap (ΔET ≥ 4 kBT) and long sensitizer triplet lifetime make the triplet-triplet energy transfer (TTET) extremely efficient and allow the utilization of high sensitizer concentrations for low Ith. However, for large upconversion shifts, the triplet energy gap should be as small as possible. Smaller energy gap values result in slower forward TTET and faster reverse TTET, which together with high total sensitizer concentration can lead to a quenching of annihilator's triplet state and therefore elevate the Ith. In this regard, low concentration of a sensitizer is beneficial, making sensitizers with high molar extinction coefficients preferential. Sensitizers with a long living triplet state and a high molar extinction coefficient can work efficiently and have low Ith at 0 kBT or even negative ΔET. Kinetic rate modeling further helps to optimize the parameters for best possible TTAUC performance. Thus, the findings of the study pave the way for the design of TTAUC systems with superior performance, such as high φTTAUC at low excitation power densities with large anti-Stokes shift, for, for example, solar-driven photovoltaics, photocatalysis, bioimaging, and safe light-triggered drug-delivery systems.