Stanislav Baluschev

Upconversion with ultrabroad excitation band: Simultaneous use of two sensitizers

The authors demonstrate the ability to combine sensitizers effectively working with single emitter in order to increase the excitation window for noncoherent upconversion. They show effective upconversion of the red part of the sun spectrum realized by ultralow excitation intensity (as low as 1Wcm−2) and ultrabroad excitation spectrum (Δλ∼80nm).

Towards the IR Limit of the Triplet-Triplet Annihilation-Supported Up-Conversion: Tetraanthraporphyrin

The processes by which locally (or in situ) up-converted photons are generated by NIR or IR excitation sources have been very intensively studied and have remarkable application potential in fields like up-conversion displays, biological imaging and sensing, and photodynamic therapy of cancer. The blue-shifted emission generated in the known and long-time studied up-conversion processes results from either two-photon absorption (TPA) in organic molecules, quantum dots or in proximity of metallic clusters, or sequential energy transfer (ETU) in rare-earth ion-doped glasses. All these processes have a common characteristic: they need an excitation source with very high brightness—in the case of TPA-based processes because of the virtual energy level used, in the case of the ETU-based processes because of the finite width of the ionic energy levels used. Additionally, both these processes need moderate or strong optical pumping, normally in order of many kWcm 2 up to MWcm . Recently, a different approach for up-conversion (UC), based on energetically conjoined triplet–triplet annihilation (TTA) was demonstrated. The fundamental advantage of the TTA-supported UC is its inherent independence on the coherence of the excitation light. The TTA-supported UC resolves also another demanding limitation of the above described “conventional” methods for UC (e.g., the ETU and all types of TPA)—the necessity to excite the samples with extremely bright optical sources (e.g., lasers). In contrast, for excitation of an efficient TTA–UC, optical sources with spectral power density of 125 mWnm 1 are sufficient and, in particular, the excitation source can be the Sun. The next advantage revealing the enormous application potential of the energetically conjoined TTA–UC is the very low intensity needed (on the order of 100 mWcm ) to achieve high quantum yields, on the order of 2–4% in organic solutions. In a further step, the efficiency of the TTA–UC in bulk solid-state films, composed of the sensitizer and emitter molecules blended in inactive polymer matrix, has to be optimized as it is significantly lower than in solutions. The TTA-supported up-conversion devices, based on organic solutions are very efficient, but cannot be easily sealed for the long term. The solid-state devices of this kind can be sealed easily, but they are not efficient enough. This obstacle can be avoided when highly viscous matrices are used. In fact, the energetically conjoined TTA–UC in highly viscous matrices possesses all the required characteristics: high quantum yield (comparable with those in liquid organic solution of the active species), very low excitation intensity ( 25 mWcm ), extremely low spectral power density optical sources ( 200 mWnm ), and versatility in excitation and emission wavelengths. These devices can be also sealed easily. The combination of all these unique characteristics and possibilities make energetically conjoined TTA–UC ready for diverse applications, such as all-organic, flexible, and transparent displays, up-converter devices for increasing the efficiency of, for example, dye-sensitized solar cells and local, in situ, generator of blue-shifted photons. To explore the above describe applications in their full, the IR limit, that is, the lowest energy photons able to serve as pumping source for the studied energetically conjoined TTA is of crucial importance. The highest excitation wave[a] Dr. V. Yakutkin, Dr. S. Baluschev Max-Planck-Institute for Polymer Research Ackermannweg 10, 55128 Mainz (Germany) Fax: (+49)6131-379-100 E-mail : balouche@mpip-mainz.mpg.de [b] S. Aleshchenkov, S. Chernov, Dr. A. Cheprakov Department of Chemistry, Moscow State University 119899 Moscow (Russia) Fax: (+7)495-939-1854 E-mail : avchep@elorg.chem.msu.ru [c] Dr. T. Miteva, Dr. G. Nelles Sony Deutschland GmbH Materials Science Laboratory, Hedelfingerstr. 61 70327 Stuttgart (Germany) Fax: (+49)711-5858-484 E-mail : miteva@sony.de Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200801305.

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.

Two pathways for photon upconversion in model organic compound systems

We have studied the phenomenon of photon upconversion in systems of two model compounds as highly efficient blue emitters sensitized with metallated macrocycle molecules in thin films. The bimolecular upconversion process in these systems is based on the presence of a metastable triplet excited state of the macrocycles giving rise to dramatically different photophysical characteristics relative to the other known methods for photon upconversion such as two-photon absorption, parametric processes, second harmonic generation, and sequential multiphoton absorption. The chosen blue emitter molecules have suitably positioned triplet levels: in the case of an oligofluorine—essentially higher and in the case of diphenylanthracene—lower than the sensitizer porphyrin platinum triplet level and thus two excitation pathways for photon upconversion were observed and investigated.

Annihilation Upconversion in Cells by Embedding the Dye System in Polymeric Nanocapsules

The first energetically conjoined TTA-assisted photon energy upconversion operating in cell tissue is described. The synthesized nanocapsules with the encapsulated UC dye system consisting of an emitter and a sensitizer show very efficient UC emission in aqueous dispersion under extremely low excitation intensity down to 0.05 W · cm(-2) so that tissue and cells are not affected by the excitation light. The demonstrated sub-linear intensity dependence of the UC emission is of crucial importance for life-science applications as the UC photon could serve as a local or in situ optical excitation source for subsequent light-triggered processes.

Annihilation assisted upconversion: all-organic, flexible and transparent multicolour display

In this paper, we demonstrate the first all-organic, transparent, flexible, versatile colour displays based upon triplet-triplet annihilation assisted photon energy upconversion in viscous polymeric matrix. The devices work with ultra-low excitation intensities down to 20mWcm 2 red or near-IR light. The displays are based on metallated-porphyrin sensitizers in combination with emitters dispersed in a transparent polymeric matrix and are driven by galvo- scanned laser diodes. The displays have external quantum yield as high as 3.2%. The response time can be adjusted to specific application requirements—up to 80µs allowing kHz-refreshment rate of the displayed information. It is possible to easily tune the optical density of the screens in order to obtain a desired transmittance for the excitation beam. We demonstrate the ability to achieve multicolour emission, using only one excitation source. There are practically no display size limitations.

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.

Micellar carrier for triplet–triplet annihilation-assisted photon energy upconversion in a water environment

In this paper, we demonstrate energetically conjoined triplet?triplet annihilation-assisted photon energy upconversion (UC) operating in an aqueous environment. Obtained micellar structures show very efficient UC emission in a water environment under extremely low excitation light intensity, down to 10?mW?cm?2. The demonstrated sub-linear intensity dependence of the UC emission is of crucial importance for life science applications, allowing upconverted photons to be generated even at low intensity that then serve as a local, in situ, optical excitation source for subsequent light-triggered processes.

Triplet-Triplet Annihilation Upconversion Based Nanocapsules for Bioimaging Under Excitation by Red and Deep-Red Light

Non-toxic and biocompatible triplet-triplet annihilation upconversion based nanocapsules (size less than 225 nm) were successfully fabricated by the combination of miniemulsion and solvent evaporation techniques. A first type of nanocapsules displays an upconversion spectrum characterized by the maximum of emission at λmax = 550 nm under illumination by red light, λexc = 633 nm. The second type of nanocapsules fluoresces at λmax = 555 nm when excited with deep-red light, λexc = 708 nm. Conventional confocal laser scanning microscopy (CLSM) and flow cytometry were applied to determine uptake and toxicity of the nanocapsules for various (mesenchymal stem and HeLa) cells. Red light (λexc = 633 nm) with extremely low optical power (less than 0.3 μW) or deep-red light (λexc = 708 nm) was used in CLSM experiments to generate green upconversion fluorescence. The cell images obtained with upconversion excitation demonstrate order of magnitude better signal to background ratio than the cell images obtained with direct excitation of the same fluorescence marker.

Upconversion photoluminescence in poly(ladder-type-pentaphenylene) doped with metal (II)-octaethyl porphyrins

We report on the optimization of the upconversion photoluminescence in films of conjugated polymers doped with platinum porphyrin. The upconversion emission was observed to take place at pump intensities as low as 0.5kW∕cm2. Comparison between the photoluminescence integral intensity of polyfluorene and ladder-type pentaphenylene polymer shows a five-fold increase of the efficiency of the upconversion process for the latter. The higher upconversion efficiency can be partially attributed to the reduced reabsorption of the photoluminescence in the case of the ladder-type pentaphenylene matrix.