Johannes W. Hofstraat

Rare-earth doped polymers for planar optical amplifiers

Optical waveguide amplifiers based on polymer materials offer a low-cost alternative for inorganic waveguide amplifiers. Due to the fact that their refractive index is similar to that of standard optical fibers, they can be easily coupled to existing fibers with low coupling losses. Doping the polymer with rare-earth ions that yield optical gain is not straightforward, as the rare-earth salts are poorly soluble in the polymer matrix. This review article focuses on two different approaches to dope a polymer waveguide with rare-earth ions. The first approach is based on organic cage-like complexes that encapsulate the rare-earth ion and are designed to provide coordination sites to bind the rare-earth ion and to shield it from the surrounding matrix. These complexes also offer the possibility of attaching a highly absorbing antenna group, which increases the pump efficiency significantly. The second approach to fabricate rare-earth doped polymer waveguides is obtained by combining the excellent properties of SiO2 as a host for rare-earth ions with the easy processing of polymers. This is done by doping polymers with Er-doped silica colloidal spheres.

FLUORESCEIN AND EOSIN AS SENSITIZING CHROMOPHORES IN NEAR-INFRARED LUMINESCENT YTTERBIUM(III), NEODYMIUM(III) AND ERBIUM(III) CHELATES

Near-infrared luminescent ytterbium(III), neodymium(III) and erbium(III) chelates containing organic chromophores derived from fluorescein and eosin have been synthesized and studied spectroscopically. The complexes can be efficiently excited with visible light and show intense lanthanide luminescence at low concentrations (2≈ 10−6 mol l−1) in D2O as a result of energy transfer from the dye moiety to the rare earth ion. Quenching of the luminescence of the complexes by molecular oxygen reveals information on the rate of energy transfer from the “antenna” chromophore to the lanthanide ion.

Optical properties of erbium-doped organic polydentate cage complexes

The optical properties of different erbium (Er)-doped polydentate hemispherand organic cage complexes are studied, for use in polymer-based planar optical amplifiers. Room temperature photoluminescence at 1.54 um is observed, due to an intra-4 f transition in Er31. The Er is directly excited into one of the 4 f manifolds (at 488 nm), or indirectly (at 287 nm) via the aromatic part of the cage. The luminescence spectrum is 70 nm wide (full width at half maximum), the highest known for any Er-doped material, enabling high gain bandwidth for optical amplification. The absorption cross section at 1.54 um is 1.1x10 -20 cm2, higher than in most other Er-doped materials, which allows the attainment of high gain. Measurements were performed on complexes in KBr tablets, in which the complex is present in the form of small crystallites, or dissolved in the organic solvents dimethylformamide and butanol-OD. In KBr the luminescence lifetime at 1.54 um is <0.5 us, possibly due to concentration quenching effects. In butanol-OD solution, the lifetime is 0.8 us, still well below the radiative lifetime of 4 ms estimated from the measured absorption cross sections. Experiments on the selective deuteration of the near-neighbor C–H bonds around the Er3+-ion indicate that these are not the major quenching sites of the Er31 luminescence. Temperature dependent luminescence measurements indicate that temperature quenching is very small. It is therefore concluded that an alternative luminescence quenching mechanism takes place, presumably due to the presence of O–H groups on the Er-doped complex (originating either from the synthesis or from the solution). Finally a calculation is made of the gain performance of a planar polymer waveguide amplifier based on these Er complexes, resulting in a threshold pump power of 1.4 mW and a typical gain of 1.7 dB /cm.

Blue emitting iridium complexes : synthesis, photophysics and phosphorescent devices

Homoleptic Ir(Fnppy)3 and heteroleptic (Fnppy)2Ir(acac) complexes (n = 3: F3ppy = 2-(3′,4′,6′-trifluorophenyl)pyridine; n = 4: F4ppy = 2-(3′,4′,5′,6′-tetrafluorophenyl)pyridine; acac = acetylacetonate) have been synthesized and their spectroscopic properties investigated. The homoleptic complexes exist as two stereoisomers, facial (fac) and meridional (mer), that have been isolated and fully characterized. Their electrochemical and photophysical properties have been studied both in solution and in the solid state and electroluminescent devices have been fabricated. The emissive layers in devices have been obtained mixing the iridium complexes with a PVK [poly(9-vinylcarbazole)] host matrix, in the presence of the electron carrier Bu-PBD [2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole]. The application of a voltage (5.0–6.5 V) between the electrodes of devices leads to electro-generated blue luminescence which has similar energy to the solution emissions. Interestingly, the stability of the devices made with the homoleptic fluorinated iridium complexes strongly depends on the stereochemistry of these phosphors and high (up to 5.5%) external quantum efficiencies for the fac complexes are measured.

Sensitized near-infrared luminescence from polydentate triphenylene-functionalized Nd3+, Yb3+, and Er3+ complexes

Hexa-deutero dimethylsulfoxide (DMSO-d6) solutions of terphenyl-based Nd3+ , Yb3+, and Er3+ complexes functionalized with a triphenylene antenna chromophore exhibit room temperature near-infrared luminescence at wavelengths of interest for the optical telecommunication network (~ 1330 and ~ 1550 nm). The sensitizing process takes place through the triplet state of triphenylene as can be concluded from the oxygen dependence of the sensitized luminescence. A significant fraction of the excited triphenylene triplet state is quenched by oxygen, instead of contributing to the population of the luminescent state of the lanthanide ion. The luminescence lifetimes of the triphenylene-functionalized lanthanide complexes ((2)Ln) are in the range of microseconds with a lifetime of 18.6 µs for (2)Yb, 3.4 µs for (2)Er, and 2.5 µs for (2)Nd in DMSO-d6. These luminescence lifetimes seem almost completely dominated by the vibrational quenching by the organic groups in the polydentate ligand and solvent molecules, which leads to low overall luminescence quantum yields.

Improving color purity and stability in a blue emitting polyfluorene by monomer purification

Monomer purification has been tailored to ensure reduced levels of fluorenone defects in the corresponding polyfluorene, which results in greater resistance to fluorescence degradation when exposed to high temperatures and demonstrates pure blue, more stable electroluminescence.

Optical properties of lissamine functionalized Nd3+ complexes in polymer waveguides and solution

Lissamine functionalized terphenyl-based Nd complexes are synthesized, and incorporated in deuterated dimethylsulfoxide solutions and partially fluorinated planar polymer waveguides. Optical excitation of the lissamine sensitizer around 500 nm, followed by intramolecular energy transfer to the Nd3+ ion, causes near-infrared photoluminescence (890, 1060, 1340 nm) due to intra-4f transitions in the Nd3+ ion. The intramolecular energy transfer rate is larger than 107 s−1. Due to the large absorption cross-section of the sensitizer (>10−17 cm2 around 500 nm), the Nd3+ is excited 104 times more efficiently than in a pure complex, without sensitizer. The Nd3+ luminescence lifetime is relatively short, both in solution (2.2 μs) and in a polymer host (0.8 μs), which is attributed to coupling to vibrational states of nearby C–H and O–H groups. Spincoated fluorinated polymer planar waveguides, doped with these sensitized organic Nd complexes show excellent waveguide properties. Upon continued illumination, photodegradation is observed in the doped polymer films.

Sensitized Near‐Infrared Emission from Nd3+ and Er3+ Complexes of Fluorescein‐Bearing Calix[4]arene Cages

Efficient intramolecular energy transfer (ET) from an excited chromophore to Nd3+ and Er3+ ions with subsequent emission in the near-infrared (h2) is possible in novel compounds (1). These are made by covalent attachment of fluorescein to a calix[4]arene possessing a cavity at the lower rim in which the trivalent lanthanoid ions are complexed.

Mono- and di-nuclear iridium(III) complexes. Synthesis and photophysics

This paper reports the synthesis and photophysical characterization of heteroleptic mono- and di-nuclear iridium(III) complexes. The complexes contain two ortho-metalating ligands, 2-phenylpyridine, with a bipyridine derivative as the third chelating unit. In the case of the dinuclear complexes the two iridium moieties are connected by a conjugated bridging ligand containing three or four phenyl units. All the complexes emit at room temperature and steady state and time resolved spectroscopy demonstrates that the lowest excited state is a metal-to-ligand charge transfer involving the bipyridine ligand.

Linear algorithms for stretched exponential decay analysis

Several non-iterative algorithms for the analysis of stretched exponential decay are proposed. Decay parameters are obtained by standard linear optimization methods, providing reasonable accuracy, high speed of processing and independence of the initial guesses. The latter feature ensures an excellent possibility to use them for generating initial guesses for iterative procedures, rendering the minimum search more reliable and faster. Proposed algorithms were investigated by fitting simulated data and then used for the analysis of fluorescence and anisotropy decays of dimethylaminonitrostilbene molecules dissolved in polymethyl methacrylate films.