Elisabeth Holder

Hybrid nanocomposite materials with organic and inorganic components for opto-electronic devices

Hybrid structures consisting of semiconducting organic polymers and strongly luminescent semiconductor nanocrystals offer favorable perspectives through highly saturated, tunable emission and tunable absorption, in combination with an easy processability from solution and low materials cost, which is of importance for several high-tech applications such as hybrid organic–inorganic light-emitting diodes and solar cells. This Feature Article describes selected examples of the design and synthesis of polymer compounds and inorganic semiconductor nanocrystals, followed by operation principles and material choices for devices that are based on combinations of polymers with semiconductor nanocrystals.

Red- and Green-Emitting Iridium(III) Complexes for a Dual Barometric and Temperature-Sensitive Paint

A new dual luminescent sensitive paint for barometric pressure and temperature (T) is presented. The green-emitting iridium(III) complex [Ir(ppy)(2)(carbac)] (ppy=2-phenylpyridine; carbac=1-(9H-carbazol-9-yl)-5,5-dimethylhexane-2,4-dione) was applied as a novel probe for T along with the red-emitting complex [Ir(btpy)(3)], (btpy=2-(benzo[b]thiophene-2-yl)pyridine) which functions as a barometric (in fact oxygen-sensitive) probe. Both iridium complexes were dissolved in different polymer materials to achieve optimal responses. The probe [Ir(ppy)(2)(carbac)] was dispersed in gas-blocking poly(acrylonitrile) microparticles in order to suppress any quenching of its luminescence by oxygen. The barometric probe [Ir(btpy)(3)], in turn, was incorporated in a cellulose acetate butyrate film which exhibits good permeability for oxygen. The effects of temperature on the response of the oxygen probe can be corrected by simultaneous optical determination of T, as the poly(acrylonitrile) microparticles containing the temperature indicator are incorporated into the film. The phosphorescent signals of the probes for T and barometric pressure, respectively, can be separated by optical filters due to the approximately 75 nm difference in their emission maxima. The dual sensor is applicable to luminescence lifetime imaging of T and barometric pressure. It is the first luminescent dual sensor material for barometric pressure/T based exclusively on the use of Ir(III) complexes in combination with luminescence lifetime imaging.

Efficient Synthesis of Carbazolyl- and Thienyl-Substituted β-Diketonates and Properties of Their Red- and Green-Light-Emitting Ir(III) Complexes

The efficient synthesis of novel beta-diketonates equipped with functional carbazolyl moieties and their subsequent transformations in 5-hexyl-thienyl substituted carbazole derivatives is presented by utilizing an effective Stille cross-coupling reaction. The introduced beta-diketonates served as ancillary ligands for novel heteroleptic red- and green-emitting Ir(III) complexes, when combined with 2-(naphthalen-1-yl)pyridine and 2-phenylpyridine as cyclometalating ligands. These novel Ir(III) complexes revealed color-tunability and a very good thermal stability until at least 207 degrees C. In polystyrene blends, the heteroleptic Ir(III) complexes revealed remarkable quantum yields up to 36% and suitably short phosphorescence lifetimes ranging from 1 to 4 micros. In the case of the orange-red Ir(III) emitter, equipped with 2-(naphthalen-1-yl)pyridine cyclometallating ligands, a luminous efficiency as high as 7.7 cd/A at 7.4 V was achieved. All fabricated diodes exhibited in addition favorable color stability.

Film thickness dependency of the emission colors of PPE–PPVs in inkjet printed libraries

The influence on the emission color of π-conjugated polymers varying side chains, film thicknesses and thermal treatment were investigated in a parallel fashion. The respective thickness libraries of six alkoxy-substituted poly(p-phenyleneethynylene)-alt-poly(p-phenylenevinylene)s (PPE–PPVs) were prepared by inkjet printing in a controlled way. The optical properties of the printed libraries were screened utilizing high-throughput methods. It was found that the emission colors of the investigated polymers strongly depend on the inter-chain interactions which are increased with increasing film thickness and influenced by the side chains. Moreover, upon annealing at 70 °C, white emission was observed from the printed films.

White light generation tuned by dual hybridization of nanocrystals and conjugated polymers

Dual hybridization of highly fluorescent conjugated polymers and highly luminescent nanocrystals (NCs) is developed and demonstrated in multiple combinations for controlled white light generation with high color rendering index (CRI) (> 80) for the first time. The generated white light is tuned using layer-by-layer assembly of CdSe/ZnS core-shell NCs closely packed on polyfluorene, hybridized on near-UV emitting nitride-based light emitting diodes (LEDs). The design, synthesis, growth, fabrication and characterization of these hybrid inorganic?organic white LEDs are presented. The following experimental realizations are reported: (i) layer-by-layer hybridization of yellow NCs (?PL=580?nm) and blue polyfluorene (?PL=439?nm) with tristimulus coordinates of (x, y)=(0.31, 0.27), correlated color temperature of Tc=6962?K and CRI of Ra=53.4; (ii) layer-by-layer assembly of yellow and green NCs (?PL=580 and 540?nm) and blue polyfluorene (?PL=439?nm) with (x, y)=(0.23, 0.30), Tc=14395?K and Ra=65.7; and (iii) layer-by-layer deposition of yellow, green and red NCs (?PL=580, 540 and 620?nm) and blue polyfluorene (?PL=439?nm) with (x, y)=(0.38, 0.39), Tc=4052?K and Ra= 83.0. The CRI is demonstrated to be well controlled and significantly improved by increasing multi-chromaticity of the NC and polymer emitters.

Referenced dual pressure- and temperature-sensitive paint for digital color camera read out.

The first fluorescent material for the referenced simultaneous RGB (red green blue) imaging of barometric pressure (oxygen partial pressure) and temperature is presented. This sensitive coating consists of two platinum(II) complexes as indicators and a reference dye, each of which is incorporated in appropriate polymer nanoparticles. These particles are dispersed in a polyurethane hydrogel and spread onto a solid support. The emission of the (oxygen) pressure indicator, PtTFPP, matches the red channel of a RGB color camera, whilst the emission of the temperature indicator [Pt(II) (Br-thq)(acac)] matches the green channel. The reference dye, 9,10-diphenylanthracene, emits in the blue channel. In contrast to other dual-sensitive materials, this new coating allows for the simultaneous imaging of both indicator signals, as well as the reference signal, in one RGB color picture without having to separate the signals with additional optical filters. All of these dyes are excitable with a 405 nm light-emitting diode (LED). With this new composite material, barometric pressure can be determined with a resolution of 22 mbar; the temperature can be determined with a resolution of 4.3 °C.

Cyclometalated red iridium(III) complexes containing carbazolyl-acetylacetonate ligands : efficiency enhancement in polymer LED devices

The design, synthesis, photophysical and significantly improved electrooptical properties of a series of red emitting cyclometalated iridium(iii) complexes containing carbazolyl-acetylacetonate ligands are described.

Direct nitroxide mediated (co)polymerization of 4-vinylphenylboronic acid as route towards sugar sensors

The controlled polymerization of the unprotected, commercially available, 4-vinylphenylboronic acid (4-VBA) monomer is reported by nitroxide mediated polymerization. The glucose responsive pH window of poly(4-VBA) is reported based on turbidimetry. Finally, poly(4-VBA) polymeric pH sensors have been developed by the incorporation of solvatochromic dyes.

Screening structure–property correlations and device performance of Ir(III) complexes in multi-layer PhOLEDs

The structure-property correlations of a set of heteroleptic red- and green-emitting Ir(III) complexes with different temperature sensitivities and charge trapping capabilities are described, revealing superb performance in multi-layer phosphorescent organic light-emitting diodes (PhOLEDs) expressed by very high maximum luminous efficiencies up to 36.8 cd A(-1). Using 2-phenylpyridine and with 2-(naphthalen-1-yl)pyridine as the C^N ligand, the resulting red emitting complex featured a maximum luminous efficiency of 10.8 cd A(-1); one of the most excellent device performances within this class of red Ir(III) emitters.

Unveiling Photodeactivation Pathways for a New Iridium(III) Cyclometalated Complex

We report the synthesis and characterization of a neutral heteroleptic Ir(III) complex bearing 6-fluoro-2-phenylbenzo[d]thiazole as cyclometalating ligand and (Z)-6-(9H-carbazol-9-yl)-5-hydroxy-2,2-dimethylhex-4-en-3-one as ancillary ligand. The photodeactivation mechanisms have been elucidated through extensive density functional theory (DFT) calculations. The active role of metal-centered ((3) MC) triplet excited states in the nonradiative deactivation pathways is, for first time, confirmed in such complexes.