Luminescent Chromophores Research Articles

Liquid‐crystalline and light‐emitting polyacetylenes

AbstractRigid polymer backbones have often been considered to be detrimental to the packing of mesogenic pendants, and polyacetylenes have generally been regarded as unpromising materials for light‐emitting applications. Our group, however, has succeeded in creating a series of liquid‐crystalline polyacetylenes with rigid backbones and a variety of light‐emitting polyacetylenes with luminescent chromophores. Here we demonstrate that the rigid polyacetylene skeleton can play a constructive role in guiding the alignments of mesogenic pendants and prove that polyacetylenes can be highly emissive with photoluminescence quantum yields of up to 98% and electroluminescence performances comparable or superior to those of the best blue‐light‐emitting polymers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2607–2629, 2003

Luminescent, Cu 2+-binding chromophore–spacer–receptor conjugates derived from ruthenium-substituted lysine

Two luminescent chromophore–spacer–receptor conjugates, [H 2cyclenAcLys{Ru(bipy) 2m}OH](PF 6) 4 ( 3) and [mLys{Ru(bipy) 2m}OMe](PF 6) 2 ( 4) (m=4-carbonyl-4′-methyl-2,2′-bipyridyl; bipy=2,2′-bipyridyl) were synthesized. Both compounds bind copper(II) ions in aqueous solution with concomitant quenching of their ruthenium-based emission. A pentadentate acylated cyclen ligand (cyclen=1,4,7,10-tetraazacyclododecane) is the metal binding receptor site in 3. The conjugate 4 contains a bidentate 2,2′-bipyridyl subunit. Both compounds were prepared starting with the synthesis of the spacer–receptor subunits and subsequent coupling with the chromophore [Ru(bipy) 2(mOSu)](PF 6) 2 ( 8, HOSu= N-hydroxysuccinimide). The emission properties of 3 in aqueous solution are almost independent of the pH. Compound 4 shows a significant quenching below pH 5 which is most likely due to an electron transfer from Ru 2+ to the protonated bipyridyl ligand. Both ligands, 3 and 4, form 1:1 complexes with copper(II) ions. Higher aggregates are only formed at substoichiometric metal ion concentrations. The complex 3–Cu 2+ is stable over a range of pH 2–12. In contrast, the complex 4–Cu 2+ dissociates under basic conditions above approximately pH 10. Copper(II) is rebound upon acidification making 4 the first example for a receptor which exhibits reversibility at high pH.

Bridged triarylamines: a new class of heterohelicenes.

A series of bridged triarylamines, which constitute a new class of heterohelicenes, have been synthesized using a simple three-step procedure. These compounds are shown to be highly luminescent chromophores and are capable of being oxidized. The solid-state structures of these helicenes show a tendency for pi-stacking interactions into an overall zigzag motif.

Green Phosphorescent Dendrimer for Light-Emitting Diodes

Highly efficient organic LEDs made by solution processing are reported. It is shown that the dendritic architecture (see Figure) can be used to solubilize luminescent chromophores and form uniform films of blends. The simple device structures containing a light-emitting chromophore are amongst the most efficient solution-processed devices reported. Thanks to this technique, the inkjet printing of phosphorescent materials becomes feasible.

Green Phosphorescent Dendrimer for Light-Emitting Diodes

Highly efficient organic LEDs made by solution processing are reported. It is shown that the dendritic architecture (see Figure) can be used to solubilize luminescent chromophores and form uniform films of blends. The simple device structures containing a light-emitting chromophore are amongst the most efficient solution-processed devices reported. Thanks to this technique, the inkjet printing of phosphorescent materials becomes feasible.

Selective Recognition of Copper(II) by a Water-Soluble, Emitter-Receptor Conjugate Containing a Ruthenium Chromophore, a Lysine Bridge, and a Cyclen Unit

The amino acid derivative [H-Lys{Ru(bipy)2m}-cyclenH2](PF6)5 (7; Lys = lysine, bipy = 2,2′-bipyridyl, m = 4-carbonyl-4′-methyl-2,2′-bipyridyl, cyclen = 1,4,7,10-tetraazacyclododecane) has been synthesized. A modular approach was taken which involves only standard amide-coupling methods well-known from peptide synthesis. Compound 7 is readily soluble in water. It contains a luminescent ruthenium chromophore and a cyclen ligand which serves as a binding site for metal ions. The emission of 7 is pH-independent but efficiently quenched by Cu2+ ions in a pH range of 6−7. Copper(II) binding is reversible upon protonation of the ligand at pH values below 6. In contrast, no significant spectral changes are observed with Zn2+ and Ni2+, respectively. Thus, 7 selectively recognizes copper(II) in aqueous solution under slightly acidic to neutral conditions. Unfortunately, applications at higher pH values are limited by metal-promoted hydrolysis of 7 under mildly basic conditions.

Linearly polarized emission from colloidal semiconductor quantum rods.

Colloidal quantum rods of cadmium selenide (CdSe) exhibit linearly polarized emission. Empirical pseudopotential calculations predict that slightly elongated CdSe nanocrystals have polarized emission along the long axis, unlike spherical dots, which emit plane-polarized light. Single-molecule luminescence spectroscopy measurements on CdSe quantum rods with an aspect ratio between 1 and 30 confirm a sharp transition from nonpolarized to purely linearly polarized emission at an aspect ratio of 2. Linearly polarized luminescent chromophores are highly desirable in a variety of applications.

Control of mobility in molecular organic semiconductors by dendrimer generation

Conjugated dendrimers are of interest as novel materials for light-emitting diodes. They consist of a luminescent chromophore at the core with highly branched conjugated dendron sidegroups. In these materials, light emission occurs from the core and is independent of generation. The dendron branching controls the separation between the chromophores. We present here a family of conjugated dendrimers and investigate the effect of dendron branching on light emission and charge transport. We apply a number of transport measurement techniques to thin films of a conjugated dendrimer in a light-emitting diode configuration to determine the effect of chromophore spacing on charge transport. We find that the mobility is reduced by two orders of magnitude as the size of the molecule doubles with increased branching or dendrimer generation. The degree of branching allows a unique control of mobility by molecular structure. An increase in chromophore separation also results in a reduction of intermolecular interactions, which reduces the red emission tail in film photoluminescence. We find that the steady-state charge transport is well described by a simple device model incorporating the effect of generation, and use the materials to shed light on the interpretation of transient electroluminescence data. We demonstrate the significance of the ability to tune the mobility in bilayer devices, where a more balanced charge transport can be achieved.

Photophysics of segmented block PPV copolymer derivatives

The photophysics of the luminescent dimethoxy-substituted chromophore, [(2,6-dimethoxy-1,4 phenylene)-1,2 ethenylene-(2,5-dimethoxy-1,4 phenylene)-1,2 ethenylene-(3,5-dimethoxy-1,4 phenylene)] is studied in a series of segmented block copolymer, monomer, and polymer samples. Fluorescence lifetimes of the monomer and segmented block copolymer solution and thin film were determined over a temperature range from 77 to 298 K. Comparison of absorbance and emission spectra and time-resolved fluorescence studies indicate exciton confinement in the chromophore moities of the segmented block copolymer, leading to near unity quantum efficiency. Steady state absorption spectra show an increasing blue-shifted peak upon formation of the spin-coated thin film. Conformational geometry in the monomer thin film facilitates excimer formation, especially evident at low temperatures.

Supramolecular Materials from Triblock Rodcoil Molecules Containing Phenylene Vinylene

We report here on the synthesis and characterization of several triblock rodcoil molecules containing conformationally rigid and flexible sequences and luminescent chromophores based on phenylene vinylene. The molecules consist of blocks of oligostyrene, oligoisoprene, and a rigid rod block. The synthesis of the rod blocks required the preparation of new phenylene vinylene oligomers. On the basis of electron microscopy and X-ray diffraction experiments, these systems were found to self-organize into supramolecular nanostructures which in turn assemble into monolayers. The nanostructures have diameters between 3.5 and 5 nm, and the regularity of the nanostructures depends on rod-to-coil volume fraction. The layer spacing of one system is 6.0 nm when cast from solution but increases to 7.8 nm upon annealing at 150 °C, possibly as a result of an increase in aggregation number. Interestingly, the formation of supramolecular nanostructures and the observed hierarchical order in the materials formed by rodcoil ...

A Facile Iterative Procedure for the Preparation of Dendrimers Containing Luminescent Cores and Stilbene Dendrons

A simple two-step convergent iterative procedure has been developed for the preparation of stilbene dendrons containing an aldehyde at the center which can be coupled in a single step to give dendrimers that contain luminescent chromophores. The stilbene units in the dendrons are linked in a meta arrangement at the branching phenyl units allowing them to be treated, to a first approximation, as isolated chromophores. All the dendrimers are luminescent with emission observed from the core. The cores prepared were distyrylbenzene, distyrylanthracene, and meso-tetraphenylporphyrin which photoluminesce blue, yellow-green, and red, respectively. We found that all the dendrimers, independent of generation or core, could be spin-coated from solution to form good quality thin films, which shows that the processing conditions have been disengaged from the electronic properties in this series of materials.

Photoluminescence Quenching and the Photochemical Oxidation of Porous Silicon by Molecular Oxygen

Exposure of luminescent n-type porous Si to gaseous molecular oxygen results in reversible quenching of the visible photoluminescence associated with this material. Steady-state and time-resolved photoluminescence quenching follow a dynamic Stern−Volmer model. From the Stern−Volmer analysis, the quenching rate constant, kq, was found to be 26 ± 9 Torr-1 s-1. The rate constant for quenching is not strongly dependent on the chemical composition of the surface. Hydride-, deuteride-, or oxide-terminated surfaces all display similar quenching rate constants. Quenching is attributed to electron transfer from the luminescent chromophore in porous Si to an O2 molecule weakly chemisorbed to a surface defect. In parallel with the reversible quenching process but on a much longer time scale (minutes to hours depending upon light intensity), porous Si samples also slowly photooxidize. Both the intensity (measured at steady state) and lifetime (measured by nanosecond-pulsed laser excitation) of photoluminescence decre...

Molecular design of asymmetric bichromophoric compounds

The goal of the present study is a better understanding of photoinduced electron or energy transfer in a single molecule. The growing interest of this field is caused by its implication in life phenomena such as photosynthesis and in the development of molecular devices. We present molecular design of new compounds consisting of a well-known luminescent organic moiety (porphyrin, carbazole, phthalocyanine) bound to an inorganic one by a bridging ligand. This latter is in the simplest case a triple bond that can be extended easily. In the present work, the inorganic part is a pentaammine—ruthenium which has been extensively studied previously. Our interest is focused on the possible ability of the ruthenium moiety to quench the light emission of the luminescent organic chromophore.

A Suzuki Coupling Approach to Pyrazines Related to Coelenterazine

2-Amino-3-benzoyl-5-bromopyrazine 6 is prepared from 2-chloropyrazine in 4 steps and its transition metal-catalysed Suzuki coupling to arylboronic acids is explored as a route to the pyrazine ring system found in the luminescent chromophore coelenterazine 2.

Phosphorescence Probes of the Glassy State in Amorphous Sucrose

AbstractWe have used phosphorescence from the luminescent chromophores N‐acetyltryp‐tophanamide (NATA) and erythrosin B (Ery B) to probe the molecular dynamics of the glassy state and the glass‐to‐rubber transition in amorphous sucrose. These measurements exploit the known sensitivity of phosphorescence intensity to the rate and extent of molecular collisions with the local environment. Both probes exhibit intense, well‐defined phosphorescence emission spectra in the glassy sugar matrix. Time‐resolved phosphorescence intensity decays for each probe were multiexponential both above and below the glass transition temperature, indicating that the sucrose matrix is heterogeneous on the molecular level; local microviscosities around the NATA probe were estimated in the range from 104 to 5 × 107 Pa·s in the glassy state at 25 °C. Plots of phosphorescence lifetime versus temperature for both probes are linear above and below the glass transition temperature of sucrose (62 °C), but exhibit a break in slope near approximately 50 °C characteristic of a dramatic change in the rate of molecular collision between the probe and the local environment. The similar sensitivity of two chemically different spectroscopic probes (the indole ring of NATA and the xanthene ring of Ery B) to molecular changes near the glass transition in this sugar indicates that phosphorescence may provide a generally applicable, site‐specific probe of the glassy state and changes in it in biological materials.

Fiber-optic Atrazine immunosensor

Fiber-optic sensors based on the excitation of luminescent chromophores by the evanescent field associated with light guiding in an optical fiber can be used for highly sensitive and selective biochemical affinity assays. Due to the small penetration depth of the evanescent field into the medrium, the generation and detection of luminescence are restricted to the close proximity of the fiber core, i.e., fluorophores in solution beyond the evanescent field will not contribute to the emission signal. Evanescent wave sensors allow the binding of fluorophores to the sensor surface to be monitored in real-time mode. These advantages make this approach especially useful for the determination of substances in complex media, such as blood, river water or soil extracts. An evanescent-wave fiber-optic immunosensor for the detection of the herbicide Atrazine has been developed. In the competitive assay format chosen, fluorescein-labeled and nonlabeled Atrazine in solution compete for the binding sites of anti-Atrazine antibodies immobilized on the surface of the optical fiber. A signal reproducibility of better than 5% within the working range of the sensor (0.5–200 nM Atrazine concentration) is achieved. The sensor performance in complex media has been investigated using samples of surface water and soil extracts.

Photooxidation and photofading of dyed nylon-6,6 film: Influence of acid dye concentration and relationship with luminescent species

The effect of concentration on the photofading rates of two yellow azo acid dyes (C.I. Acid Yellow 135 and C.I. Acid Brown 226) has been examined in nylon-6,6 film and compared with the influence of the dyes on the photochemical oxidation of the polymer using u.v. absorption and viscometric analysis. The data have been related to the concentration of the fluorescent and phosphorescent unsaturated carbonyl chromophores in the polymer. For both dyes, the rate of photooxidation of the polymer was found to increase with decreasing dye concentration with the rate of dye fading exhibiting a similar but slower trend. In both cases, the dyes were more stable than the polymer with C. I. Acid Brown 226 exhibiting no significant fading above 0.2% w/w. The dye fading and polymer stability results show a close correlation with the ability of the dyes to quench the luminescent chromophores in the polymer. The ability of the dyes to quench the phosphorescent species was found to be most significant as indicated by the correlation coefficients. This result confirms previous findings on the importance of triplet energy transfer in the stabilization of acid dyed nylon-6,6 polymer, with the metal complex types being the most effective.

Influence of acid dyes on the photo-oxidation of nylon 6,6 film: relationship with rate of photofading and luminescent species

The photofading rates of three azo acid dyes (C.I. Acid Blue 62, C.I. Acid Yellow 135 and C.I. Acid Red 266) and four azo acid metal complex dyes (C.I. Acid Brown 266, C.I. Acid Blue 171, C.I. Acid Orange 162 and C.I. Acid Black 107) were examined in nylon 6,6 film and were compared with the influence of the dyes on the photochemical degradation of the polymer using UV—visible absorption and viscometric analysis. For the first three acid dyes, the relative rates of photofading correlate with the ability of the dyes to photostabilize the polymer, i.e. yellow > blue > red. However, all the dyes are more photostable than the polymer. The acid metal complex dyes also photostabilize the polymer in the order blue > black > orange > brown. The nature of the central metal atom is concluded to be an important factor. In the presence of the dyes the concentration of terminal amine groups has little influence on polymer stability, but dye bath treatment improves the photostability of the polymer due to extraction of photoactive impurities. There is a close correlation between the ability of the dyes to quench the luminescent chromophores and polymer stability; the ability to quench the phosphorescent species at 290 400 nm is most significant as indicated by the correlation coefficients. This suggests that triplet energy transfer may be important in stabilization of the polymer by the dye.

Liquid chromatographic and flow injection analysis of tetracycline using sensitized Europium(III) luminescence detection

Europium(III) can be used as a luminescent chromophore for detection in the liquid chromatographic and flow injection analysis of tetracycline. Detection is dependent upon an intramolecular energy transfer from the tetracycline to Eu(III). In liquid chromatography, the Eu(III) is added post-column as a complex with ethylenediaminetetraacetic acid. The post-column phase also serves to adjust the pH for optimum sensitivity. The method is highly selective for tetracycline since few compounds are capable of transferring energy to Eu(III). Fluorescent impurities that would otherwise interfere in flow injection analysis can be eliminated through the use of a delay time between the source pulse and the start of data acquisition. The detection limits for tetracycline using sensitized Eu(III) luminescence are better than those obtained using ultraviolet detection. The method is applied to the analysis of tetracycline in urine, blood serum, and gingival crevice fluid.

Lanthanide ions as luminescent chromophores for the liquid chromatographic detection of polynucleotides and nucleic acids

Europium(III) and terbium(III) can be used as luminescent chromophores for the liquid chromatographic detection of certain nucleotides and nucleic acids. The method is dependent upon an energy transfer from the nucleic acid to the lanthanide ion. Of the base moeities, only xanthine, quanine, and thiouridine have appropriate excited state energy levels for efficient energy transfer. The lanthanide ion can be added in pre- or post-column mode. The applicability of the method was demostrated for the detection of homologous polynucleotides such as poly X and poly G. The method was also used to detect transfer RNA from Escherichia coli.