Photon Energy Upconverting Nanopaper: A Bioinspired Oxygen Protection Strategy

The development of solid materials which are able to upconvert optical radiation into photons of higher energy is attractive for many applications such as photocatalytic cells and photovoltaic devices. However, to fully exploit triplet–triplet annihilation photon energy upconversion (TTA-UC), oxygen protection is imperative because molecular oxygen is an ultimate quencher of the photon upconversion process. So far, reported solid TTA-UC materials have focused mainly on elastomeric matrices with low barrier properties because the TTA-UC efficiency generally drops significantly in glassy and semicrystalline matrices. To overcome this limit, for example, combine effective and sustainable annihilation upconversion with exhaustive oxygen protection of dyes, we prepare a sustainable solid-state-like material based on nanocellulose. Inspired by the structural buildup of leaves in Nature, we compartmentalize the dyes in the liquid core of nanocellulose-based capsules which are then further embedded in a cellulose nanofibers (NFC) matrix. Using pristine cellulose nanofibers, a sustainable and environmentally friendly functional nanomaterial with ultrahigh barrier properties is achieved. Also, an ensemble of sensitizers and emitter compounds are encapsulated, which allow harvesting of the energy of the whole deep-red sunlight region. The films demonstrate excellent lifetime in synthetic air (20.5/79.5, O₂/N₂)even after 1 h operation, the intensity of the TTA-UC signal decreased only 7.8% for the film with 8.8 μm thick NFC coating. The lifetime can be further modulated by the thickness of the protective NFC coating. For comparison, the lifetime of TTA-UC in liquids exposed to air is on the level of seconds to minutes due to fast oxygen quenching

All Organic Nanofibers As Ultralight Versatile Support for Triplet–Triplet Annihilation Upconversion

We present a method for the fabrication of ultralight upconverting mats consisting of rigid polymer nanofibers. The mats are prepared by simultaneously electrospinning an aqueous solution of a polymer with pronounced oxygen-barrier properties and functional nanocapsules containing a sensitizer/emitter couple optimized for triplet–triplet annihilation photon upconversion. The optical functionality of the nanocapsules is preserved during the electrospinning process. The nanofibers demonstrate efficient upconversion fluorescence centered at λ_max = 550 nm under low intensity excitation with a continuous wave laser (λ = 635 nm, power = 5 mW). The pronounced oxygen-barrier property of the polymer matrix may efficiently prevent the oxygen penetration so upconversion fluorescence is registered in ambient atmosphere. The demonstrated method can be used for the production of upconverting ultralight porous coatings for sensors or upconverting membranes with freely variable thickness for solar cells

Electron-Exchange-Assisted Photon Energy Up-Conversion in Thin Films of π-Conjugated Polymeric Composites

The mechanism of triplet–triplet annihilation (TTA)-induced up-converted (UC) delayed luminescence is studied in two different binary organic systems consisting of platinum(II) octaethyl porphyrin (PtOEP) mixed with either poly(fluorene) (PF26) or ladder-type pentaphenylene (L5Ph). Cyclic voltammetry and differential pulse voltammetry are employed for estimating the ionization potentials of PtOEP and L5Ph. Delayed luminescence spectroscopy sets the energy of the lowest excited triplet state of L5Ph 0.20 eV higher than the triplet state of PtOEP (1.90 eV). The different phosphorescence PtOEP lifetime indicates differences in PtOEP aggregation in the polymer matrices. The presented results propose that the difference in the relative intensities of the delayed UC luminescence is determined by the difference between the ionization potentials of PtOEP and the polymer matrix. In the solid state, the electric-field-induced quenching of the delayed L5Ph UC luminescence suggests the formation of an intermediate charge-transfer state after the TTA within the PtOEP domains

Tetraaryltetraanthra[2,3]porphyrins: Synthesis, Structure, and Optical Properties

A synthetic route to symmetrical tetra­aryl­tetra­anthra­[2,3]­porphyrins (Ar₄TAPs) was developed. Ar₄TAPs bearing various substituents in <i>meso</i>-phenyls and anthracene residues were prepared from the corresponding pyrrolic precursors. The synthesized porphyrins possess high solubility and exhibit remarkably strong absorption bands in the near-infrared region (790–950 nm). The scope of the method, selection of the peripheral substituents, choice of the metal, and their influence on the optical properties are discussed together with the first X-ray crystallographic data for anthraporphyrin

Tetraaryltetraanthra[2,3]porphyrins: Synthesis, Structure, and Optical Properties

A synthetic route to symmetrical tetra­aryl­tetra­anthra­[2,3]­porphyrins (Ar₄TAPs) was developed. Ar₄TAPs bearing various substituents in <i>meso</i>-phenyls and anthracene residues were prepared from the corresponding pyrrolic precursors. The synthesized porphyrins possess high solubility and exhibit remarkably strong absorption bands in the near-infrared region (790–950 nm). The scope of the method, selection of the peripheral substituents, choice of the metal, and their influence on the optical properties are discussed together with the first X-ray crystallographic data for anthraporphyrin

Hyperbranched Unsaturated Polyphosphates as a Protective Matrix for Long-Term Photon Upconversion in Air

The energy stored in the triplet states of organic molecules, capable of energy transfer via an emissive process (phosphorescence) or a nonemissive process (triplet–triplet transfer), is actively dissipated in the presence of molecular oxygen. The reason is that photoexcited singlet oxygen is highly reactive, so the photoactive molecules in the system are quickly oxidized. Oxidation leads to further loss of efficiency and various undesirable side effects. In this work we have developed a structurally diverse library of hyperbranched unsaturated poly­(phosphoester)­s that allow efficient scavenging of singlet oxygen, but do not react with molecular oxygen in the ground state, i.e., triplet state. The triplet–triplet annihilation photon upconversion was chosen as a highly oxygen-sensitive process as proof for a long-term protection against singlet oxygen quenching, with comparable efficiencies of the photon upconversion under ambient conditions as in an oxygen-free environment in several unsaturated polyphosphates. The experimental results are further correlated to NMR spectroscopy and theoretical calculations evidencing the importance of the phosphate center. These results open a technological window toward efficient solar cells but also for sustainable solar upconversion devices, harvesting a broad-band sunlight excitation spectrum