Tzenka Miteva

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.

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 0.05u2009Wu2009·u2009cm(-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.

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 225u2009nm) 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 λmaxu2009=u2009550u2009nm under illumination by red light, λexcu2009=u2009633u2009nm. The second type of nanocapsules fluoresces at λmaxu2009=u2009555u2009nm when excited with deep-red light, λexcu2009=u2009708u2009nm. 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 (λexcu2009=u2009633u2009nm) with extremely low optical power (less than 0.3u2009μW) or deep-red light (λexcu2009=u2009708u2009nm) 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.

Synergetic effect in triplet-triplet annihilation upconversion: highly efficient multi-chromophore emitter.

In the context of photonic applications using sunlight, specialattention is deservedly paid to organic multicomponent up-conversion (UC) systems comprised of an emitter (such as con-jugated semiconductor polymers or aromatic hydrocarbon de-rivatives) and a sensitizer part (such as metallated macrocy-cles).

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

A synthetic route to symmetrical tetraaryltetraanthra[2,3]porphyrins (Ar(4)TAPs) was developed. Ar(4)TAPs bearing various substituents in meso-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.

Novel rod-like fluorene-trimers exhibiting smectic LC mesophases

Two novel fluorene-trimers were synthesised following a Suzuki-type cross-coupling reaction. Methyl pendant chains in 9-position of the fluorene building blocks ensured a rod-like backbone of a small molecular diameter while the introduction of terminal, flexible n-octyl and n-hexadecyl alkyl chains in the 2- and 7″-positions causes the occurrence of two different liquid-crystalline (LC) mesophases for both trimers. The terminal chain length was found to strongly affect the phase transition temperatures but not the number and type of LC phases formed. By using several analytical techniques (differential scanning calorimetry DSC, polarizing microscopy, and X-ray diffractometry) the exact nature of the mesophases was determined. The n-hexadecyl- and n-octyl-substituted fluorene-trimers characteristically form smectic LC phases at transition temperatures of 108 and 171 °C, respectively, which when heated further are transformed into lower ordered nematic LC mesophases at 140 and 187 °C, respectively.

Biased-probe-induced water ion injection into amorphous polymers investigated by electric force microscopy

Although charging of insulators by atomic force microscopy (AFM) has found widespread interest, often with data storage or nanoxerography in mind, less attention has been paid to the charging mechanism and the nature of the charge. Here we present a systematic study on charging of amorphous polymer films by voltage pulses applied to conducting AFM probes. We find a quadratic space charge limited current law of Kelvin probe force microscopy and electrostatic force microscopy peak volumes in pulse height, offset by a threshold voltage, and a power law in pulse width of positive exponents smaller than one. We interpret the results by a charging mechanism of injection and surface near accumulation of aqueous ions stemming from field induced water adsorption, with threshold voltages linked to the water affinities of the polymers.

Sun-light upconversion in multi-component organic systems: development towards application for solar cells outcome enhancement

The specific application of photon upconversion (UC) in photovoltaics is only possible when the following requirements are fulfilled: First, the excitation intensity necessary for effective UC needs to be small (as low as 1Wcm-2) − comparable with light intensities obtainable from the moderate concentrated sunlight. Second, the excitation spectral power density required for effective UC needs to be comparable with those of the terrestrial sun irradiation (in order of 100μWnm-1). Third, efficient UC must be obtained by non-coherent light excitation (sunlight). And last but not least – compatibility between the UC device and the photovoltaic device, including good optical coupling has to be realized. Up to now the triplet-triplet annihilation-supported upconversion (TTA – UC) is the only upconversion process excited with moderate concentrated sunlight. Our group demonstrates UCd based on various UCmolecular systems efficiently transforming the NIR and IR-A part of the sun spectrum into the VIS-range, operating at moderate sunlight concentrations (10-50 suns, AM1.5). The next important accomplished requirement is the transfer of the hydrophobic UC-molecular system from an organic solvent to the aqueous environment. These new aqueous UC systems with high efficiencies ensure good sealing of the UC device and in this way its compatibility with different solar cells.