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

Abstract

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.

Recent advances in materials for and applications of triplet–triplet annihilation-based upconversion

“…In addition to the above TTA-UC application, a variety of new attempts have been made to overcome the limitations of UC materials in aqueous environments such as gels (hydrogels, 10,43,87,88 organogels 89–91 ), metal–organic frameworks (MOFs) 27,92,93 and binary crystalline solid types. 44 Various gel types allow unique air-stable TTA-UC by proper stabilization with hydrogen bonding networks in TTA-UC chromophores.…”

Thermally activated delayed fluorescence in an optically accessed soft matter environment

“…This requirement is basic: the TTA–UC process is demonstrated up to now only in a hydrophobic/amphiphilic environment. In order to achieve TTA–UC in a water environment, it is necessary to embed the active moieties in oil-in-water microemulsions, 85 core/shell nanocapsules 86 or oil-laden microcapsules, 87 multiphasic protein hydrogels 88 or an enzymatic environment; 89 (iii) the SSOS – molecules must demonstrate relatively low viscosity and in ideal case be in the liquid state for the temperature region of interest; and (iv) SSOS-molecules must be inherently bio-compatible and non-toxic.…”

“…Also in multiphasic protein hydrogels, 88 the intensity of the delayed fluorescence decreases with the increase of the sample temperature. This experiment is performed in an air-saturated environment and no additional efforts for protection of the optically excited organic triplet ensembles against quenching by molecular oxygen present in the water environment and actively diffusing through the protein hydrogel were made.…”

“…Reprinted (adapted) with permission. 88 Copyright 2020 American Chemical Society. and therefore, simultaneously the signals of delayed fluorescence, as well as the signal of residual sensitizer phosphorescence will be observed.…”

Photon Upconversion in a Glowing Metal–Organic Framework

“…From a practical point of view, photon upconversion in solid-state materials is a highly desirable phenomenon, considering their potential optoelectronic applications . Typically, in order to obtain the TTA-UC in the solid state, the sensitizer and annihilator are doped in polymers or silica nanoparticles. − In these not-so-well-ordered systems, however, it is difficult to control the orientation of the chromophores and their separation distances so as to optimize energy-transfer or exciton-migration processes. This lack of control in the orientation of the chromophores requires the use of high excitation power, negating the advantage of these solid-state TTA-UC systems and reducing their potential to find applications.…”