Promoting multiexciton interactions in singlet fission and triplet fusion upconversion dendrimers

Singlet fission and triplet-triplet annihilation upconversion are two multiexciton processes intimately related to the dynamic interaction between one high-lying energy singlet and two low-lying energy triplet excitons. Here, we introduce a series of dendritic macromolecules that serve as platform to study the effect of interchromophore interactions on the dynamics of multiexciton generation and decay as a function of dendrimer generation. The dendrimers (generations 1-4) consist of trimethylolpropane core and 2,2-bis(methylol)propionic acid (bis-MPA) dendrons that provide exponential growth of the branches, leading to a corona decorated with pentacenes for SF or anthracenes for TTA-UC. The findings reveal a trend where a few highly ordered sites emerge as the dendrimer generation grows, dominating the multiexciton dynamics, as deduced from optical spectra, and transient absorption spectroscopy. While the dendritic structures enhance TTA-UC at low annihilator concentrations in the largest dendrimers, the paired chromophore interactions induce a broadened and red-shifted excimer emission. In SF dendrimers of higher generations, the triplet dynamics become increasingly dominated by pairwise sites exhibiting strong coupling (Type II), which can be readily distinguished from sites with weaker coupling (Type I) by their spectral dynamics and decay kinetics.

Diphenylanthracene Dimers for Triplet-Triplet Annihilation Photon Upconversion: Mechanistic Insights for Intramolecular Pathways and the Importance of Molecular Geometry

Novel approaches to modify the spectral output of the sun have seen a surge in interest recently, with triplet–triplet annihilation driven photon upconversion (TTA-UC) gaining widespread recognition due to its ability to function under low-intensity, noncoherent light. Herein, four diphenylanthracene (DPA) dimers are investigated to explore how the structure of these dimers affects upconversion efficiency. Also, the mechanism responsible for intramolecular upconversion is elucidated. In particular, two models are compared using steady-state and time-resolved simulations of the TTA-UC emission intensities and kinetics. All dimers perform TTA-UC efficiently in the presence of the sensitizer platinum octaethylporphyrin. The meta-coupled dimer 1,3-DPA₂ performs best yielding a 21.2% upconversion quantum yield (out of a 50% maximum), which is close to that of the reference monomer DPA (24.0%). Its superior performance compared to the other dimers is primarily ascribed to the longer triplet lifetime of this dimer (4.7 ms), thus reinforcing the importance of this parameter. Comparisons between simulations and experiments reveal that the double-sensitization mechanism is part of the mechanism of intramolecular upconversion and that this additional pathway could be of great significance under specific conditions. The results from this study can thus act as a guide not only in terms of annihilator design but also for the design of future solid-state systems where intramolecular exciton migration is anticipated to play a major role.

Highly photoluminescent poly(norbornene)s carrying platinum–acetylide complex moieties in their side chains: evaluation of oxygen sensing and TTA–UC

Poly(norbornene)s carrying platinum–acetylide complex moieties change their photoluminescence colors in response to oxygen. The polymers serve as excellent sensitizers of TTA–UC with 9,10-diphenylanthracene.

Fluorescence upconversion by triplet-triplet annihilation in all-organic poly(methacrylate)-terpolymers

Fluorescence upconversion by triplet-triplet annihilation is demonstrated for a fully polymer-integrated material, i.e. in the limit of restricted diffusion. Organic sensitizer and acceptor are covalently attached to a poly(methacrylate) backbone, yielding a metal-free macromolecular all-in-one system for fluorescence upconversion. Due to the spatial confinement of the optically active molecular components, i.e. annihilator and sensitizer, UC by TTA in the constrained polymer system in solution is achieved at exceptionally low averaged annihilator concentrations. However, the UC quantum yield in the investigated systems is found to be low, highlighting that only chromophores in specific local surroundings yield upconversion in the limit of restricted diffusion. A photophysical model is proposed taking the heterogeneous local environment within the polymers into account.

Annihilator dimers enhance triplet fusion upconversion

Optical upconversion is a net process by which two low energy photons are converted into one higher energy photon. There is vast potential to exploit upconversion in applications ranging from solar energy and biological imaging to data storage and photocatalysis. Here, we link two upconverting chromophores together to synthesize a series of novel tetracene dimers for use as annihilators. When compared with the monomer annihilator, TIPS-tetracene, the dimers yield a strong enhancement in the triplet fusion process, also known as triplet-triplet annihilation, as demonstrated via a large increase in upconversion efficiency and an order of magnitude reduction of the threshold power for maximum yield. Along with the ongoing rapid improvements to sensitizer materials, the dimerization improvements demonstrated here open the way to a wide variety of emerging upconversion applications.

Solid-State Photon Upconversion Materials: Structural Integrity and Triplet-Singlet Dual Energy Migration

Triplet-triplet annihilation-based photon upconversion (TTA-UC) is a process wherein longer-wavelength light (lower-energy photons) is converted into shorter-wavelength light (higher-energy photons) under low excitation intensity in multichromophore systems. There have been many reports on highly efficient TTA-UC in solution; however, significant challenges remain in the development of solid-state upconverters in order to explore real-world applications. In this Perspective, we discuss the advantages and challenges of different approaches for TTA-UC in solvent-free solid systems. We consider that the energy migration-based TTA-UC has the potential to achieve ideal materials with high UC efficiency at weak solar irradiance. While the UC performance of such systems is still limited at this moment, we introduce recently developed important concepts to improve it, including kinetic/thermodynamic donor dispersion in acceptor assemblies, defectless crystals, and triplet-singlet dual energy migration. Future integration of these concepts into a single material would realize the ideal TTA-UC system.

Plasmon-enhanced triplet-triplet annihilation upconversion of post-modified polymeric acceptors

We report the localized surface plasmon resonance (LSPR)-enhanced triplet-triplet annihilation upconversion (TTA-UC) of polymeric acceptors containing high percentages of acceptor units. A poly[(methyl methacrylate)-co-(glycidyl methacrylate)] copolymer series with increasing glycidyl methacrylate ratio was prepared using reversible addition-fragmentation chain transfer (RAFT) polymerization. After post-modification of the glycidyl group with anthracene, the acceptor unit, a series of poly[(methyl methacrylate)-co-(2-hydroxypropyl-9-anthroate methacrylate)] (polyACA) was produced with different numbers of acceptor units. These polymeric acceptors were grafted to silver nanoparticles in order to enhance the TTA-UC intensity in the polymers with higher percentages of acceptor units, where concentration quenching usually dominates. With the assistance of the silver nanoparticle LSPR, TTA-UC intensity was enhanced from the polymeric acceptor nanocomposites using platinum octaethylporphyrin as the sensitizer to form the TTA-UC systems. This method is anticipated to improve TTA-UC in the solid-state, where higher percentages of acceptor units are required, but usually cause chromophore concentration quenching, reducing TTA-UC efficiency.

Water-Dispersible Silica-Coated Upconverting Liposomes: Can a Thin Silica Layer Protect TTA-UC against Oxygen Quenching?

Light upconversion by triplet-triplet annihilation (TTA-UC) in nanoparticles has received considerable attention for bioimaging and light activation of prodrugs. However, the mechanism of TTA-UC is inherently sensitive for quenching by molecular oxygen. A potential oxygen protection strategy is the coating of TTA-UC nanoparticles with a layer of oxygen-impermeable material. In this work, we explore if (organo)silica can fulfill this protecting role. Three synthesis routes are described for preparing water-dispersible (organo)silica-coated red-to-blue upconverting liposomes. Their upconversion properties are investigated in solution and in A549 lung carcinoma cells. Although it was found that the silica offered no protection from oxygen in solution and after uptake in A549 cancer cells, upon drying of the silica-coated liposome dispersion in an excess of (organo)silica precursor, interesting liposome-silica nanocomposite materials were obtained that were capable of generating blue light upon red light excitation in air.

Room-Temperature High-Efficiency Solid-State Triplet-Triplet Annihilation Up-Conversion in Amorphous Poly(olefin sulfone)s

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_g). In contrast, TTA-UC with high quantum yield for temperatures below T_g is rarely reported. In this article, we report new polymer hosts in which efficient TTA-UC is observed well below T_g, 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_TTA-UC) were measured. The highest value of η_TTA-UC = 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.

Temperature Dependence of Triplet-Triplet Annihilation Upconversion in Phospholipid Membranes

Understanding the temperature dependency of triplet-triplet annihilation upconversion (TTA-UC) is important for optimizing biological applications of upconversion. Here the temperature dependency of red-to-blue TTA-UC is reported in a variety of neutral PEGylated phospholipid liposomes. In these systems a delicate balance between lateral diffusion rate of the dyes, annihilator aggregation, and sensitizer self-quenching leads to a volcano plot, with the maximum upconversion intensity occurring near the main order-disorder transition temperature of the lipid membrane.

Red Light Activation of Ru(II) Polypyridyl Prodrugs via Triplet-Triplet Annihilation Upconversion: Feasibility in Air and through Meat

Triplet-triplet annihilation upconversion (TTA-UC) is a promising photophysical tool to shift the activation wavelength of photopharmacological compounds to the red or near-infrared wavelength domain, in which light penetrates human tissue optimally. However, TTA-UC is sensitive to dioxygen, which quenches the triplet states needed for upconversion. Here, we demonstrate not only that the sensitivity of TTA-UC liposomes to dioxygen can be circumvented by adding antioxidants, but also that this strategy is compatible with the activation of ruthenium-based chemotherapeutic compounds. First, red-to-blue upconverting liposomes were functionalized with a blue-light sensitive, membrane-anchored ruthenium polypyridyl complex, and put in solution in presence of a cocktail of antioxidants composed of ascorbic acid and glutathione. Upon red light irradiation with a medical grade 630 nm PDT laser, enough blue light was produced by TTA-UC liposomes under air to efficiently trigger full activation of the Ru-based prodrug. Then, the blue light generated by TTA-UC liposomes under red light irradiation (630 nm, 0.57 W/cm²) through different thicknesses of pork or chicken meat was measured, showing that TTA-UC still occurred even beyond 10 mm of biological tissue. Overall, the rate of activation of the ruthenium compound in TTA-UC liposomes using either blue or red light (1.6 W/cm²) through 7 mm of pork fillet were found comparable, but the blue light caused significant tissue damage, whereas red light did not. Finally, full activation of the ruthenium prodrug in TTA-UC liposomes was obtained under red light irradiation through 7 mm of pork fillet, thereby underlining the in vivo applicability of the activation-by-upconversion strategy.

Imaging Upconverting Polymersomes in Cancer Cells: Biocompatible Antioxidants Brighten Triplet-Triplet Annihilation Upconversion

Light upconversion is a very powerful tool in bioimaging as it can eliminate autofluorescence, increase imaging contrast, reduce irradiation damage, and increase excitation penetration depth in vivo. In particular, triplet-triplet annihilation upconverting (TTA-UC) nanoparticles and liposomes offer high upconversion efficiency at low excitation power. However, TTA-UC is quenched in air by oxygen, which also leads to the formation of toxic singlet oxygen. In this work, polyisobutylene-monomethyl polyethylene glycol block copolymers are synthesized and used for preparing polymersomes that upconvert red light into blue light in absence of oxygen. In addition, it is demonstrated that biocompatible antioxidants such as l-ascorbate, glutathionate, l-histidine, sulfite, trolox, or even opti-MEM medium, can be used to protect the TTA-UC process in these polymersomes resulting in red-to-blue upconversion under aerobic conditions. Most importantly, this approach is also functional in living cells. When A549 lung carcinoma cells are treated with TTA-UC polymersomes in the presence of 5 × 10^-3 m ascorbate and glutathionate, upconversion in the living cells is one order of magnitude brighter than that observed without antioxidants. These results propose a simple chemical solution to the issue of oxygen sensitivity of TTA-UC, which is of paramount importance for the technological advancement of this technique in biology.

Recent emergence of photon upconversion based on triplet energy migration in molecular assemblies

This Feature Article reviews an emerging field of triplet energy migration-based photon upconversion (TEM-UC) that allows highly efficient photon upconversion at low excitation power.