Micelles Embedded in Multiphasic Protein Hydrogel Enable Efficient and Air-Tolerant Triplet Fusion Upconversion with Heavy-Atom and Spin-Orbit Charge-Transfer Sensitizers.

The applications of triplet-triplet annihilation-based photon upconversion (TTA-UC) in solar devices have been limited by the challenges in designing a TTA-UC system that is efficient under aerobic conditions. Efficient TTA-UC under aerobic conditions is typically accomplished by using soft matter or solid-state media, which succeed at protecting the triplet excited states of upconverters (sensitizer and annihilator) from quenching by molecular oxygen but fail at preserving their mobility, thus limiting the TTA-UC efficiency (ΦUC). We showcase a protein/lipid hydrogel that succeeded in doing both of the above due to its unique multiphasic design, with a high ΦUC of 19.0 ± 0.7% using a palladium octaethylporphyrin sensitizer. This hydrogel was made via an industrially compatible method using low-cost and eco-friendly materials: bovine serum albumin (BSA), sodium dodecyl sulfate (SDS), and water. A dense BSA network provided oxygen protection while the encapsulation of upconverters within a micellar SDS environment preserved upconverter mobility, ensuring near-unity triplet energy transfer efficiency. In addition to heavy atom-containing sensitizers, several completely organic, spin-orbit charge-transfer intersystem crossing (SOCT-ISC) Bodipy-based sensitizers were also studied; one of which achieved a ΦUC of 3.5 ± 0.2%, the only reported SOCT-ISC-sensitized ΦUC in soft matter to date. These high efficiencies showed that our multiphasic design was an excellent platform for air-tolerant TTA-UC and that it can be easily adapted to a variety of upconverters.