Molecular Alignment of Homoleptic Iridium Phosphors in Organic Light-Emitting Diodes.

The orientation of facial (fac) tris-cyclometalated iridium complexes in doped films prepared by vacuum deposition is investigated by altering the physical shape and electronic asymmetry in the molecular structure. Angle-dependent photoluminescence spectroscopy and Fourier-plane imaging microscopy show that the orientation of roughly spherical fac-tris(2-phenylpyridyl)iridium (Ir(ppy)3 ) is isotropic, whereas complexes that are oblate spheroids, fac-tris(mesityl-2-phenyl-1H-imidazole)iridium (Ir(mi)3 ) and fac-tris((3,5-dimethyl-[1,1'-biphenyl]-4-yl)-2-phenyl-1H-imidazole)iridium (Ir(mip)3 ), have a net horizontal alignment of their transition dipole moments. Optical anisotropy factors of 0.26 and 0.15, respectively, are obtained from the latter complexes when doped into tris(4-(9H-carbazol-9-yl)phenyl)amine host thin films. The horizontal alignment is attributed to the favorable van der Waals interaction between the oblate Ir complexes and host material. Trifluoromethyl groups substituted on one polar face of the Ir(ppy)3 and Ir(mi)3 complexes introduce chemical asymmetries in the molecules at the expense of their oblate shapes. The anisotropy factors of films doped with these substituted derivatives are lower relative to the parent complexes, indicating that the fluorinated patches reinforce horizontal alignment during deposition. High efficiencies obtained from organic light emitting diodes prepared using the Ir dopants is attributed, in part, to improved outcoupling of electroluminescence brought about by molecular alignment.

Using Hybrid Organic-Inorganic Surface Technology to Mitigate Analyte Interactions with Metal Surfaces in UHPLC.

Interactions of analytes with metal surfaces in high-performance liquid chromatography (HPLC) instruments and columns have been reported to cause deleterious effects ranging from peak tailing to a complete loss of the analyte signal. These effects are due to the adsorption of certain analytes on the metal oxide layer on the surface of the metal components. We have developed a novel surface modification technology and applied it to the metal components in ultra-HPLC (UHPLC) instruments and columns to mitigate these interactions. A hybrid organic-inorganic surface, based on an ethylene-bridged siloxane chemistry, was developed for use with reversed-phase and hydrophilic interaction chromatography. We have characterized the performance of UHPLC instruments and columns that incorporate this surface technology and compared the results with those obtained using their conventional counterparts. We demonstrate improved performance when using the hybrid surface technology for separations of nucleotides, a phosphopeptide, and an oligonucleotide. The hybrid surface technology was found to result in higher and more consistent analyte peak areas and improved peak shape, particularly when using low analyte mass loads and acidic mobile phases. Reduced abundances of iron adducts in the mass spectrum of a peptide were also observed when using UHPLC systems and columns that incorporate hybrid surface technology. These results suggest that this technology will be particularly beneficial in UHPLC/mass spectrometry investigations of metal-sensitive analytes.

Highly Efficient Deep Blue Luminescence of 2-Coordinate Coinage Metal Complexes Bearing Bulky NHC Benzimidazolyl Carbene.

The structural, photophysical and electrochemical properties of three luminescent 2-coordinate coinage metal (i.e., M = Cu, Ag, Au) complexes bearing a sterically bulky benzimidazolyl carbene, 1,3-bis(2,6-diisopropylphenyl)-1-H-benzo[d]imidazol-2-ylidene (BZI), and carbazolide (Cz) as the anionic ligand were investigated. All the complexes emit in the deep blue region (~430 nm) with relatively narrow spectra (full width at half maximum = 44 nm, 2,300 cm-1) characterized by vibronic fine structure in nonpolar media (methylcyclohexane at room temperature), and with high photoluminescence quantum yields (ΦPL > 80%) and radiative rate constants (k r ~ 7.8 × 105 s-1). The luminescence is solvatochromic, undergoing a red-shift in a polar solvent (CH2Cl2) at room temperature that are accompanied by a decrease in quantum yields (ΦPL < 23%) and radiative rate constants (k r < 4.0 × 104 s-1), whereas the non-radiative rate constants remain nearly constant (k nr ~ 1.0 × 105 s-1). The radiative rate is controlled via thermally assisted delayed fluorescence (TADF) and temperature-dependent luminescence studies of the gold complex (Au BZI) in methylcyclohexane solution reveal an energy difference between the lowest singlet and triplet excited states of 920 cm-1. An organic light-emitting diode (OLED) fabricated using Au BZI as a luminescent dopant has an external quantum efficiency of 12% and narrow, deep-blue emission (CIE = 0.16, 0.06).

"Quick-Silver" from a Systematic Study of Highly Luminescent, Two-Coordinate, d10 Coinage Metal Complexes.

A systematic study is presented on the physical and photophysical properties of isoelectronic and isostructural Cu, Ag, and Au complexes with a common amide (N-carbazolyl) and two different carbene ligands (i.e., CAAC = (5 R,6 S)-2-(2,6-diisopropylphenyl)-6-isopropyl-3,3,9-trimethyl-2-azaspiro[4.5]decan-2-ylidene, MAC = 1,3-bis(2,6-diisopropylphenyl)-5,5-dimethyl-4-keto-tetrahydropyridylidene). The crystal structures of the (carbene)M(I)(N-carbazolyl) (MCAAC) and (MAC)M(I)(N-carbazolyl) (MMAC) complexes show coplanar carbene and carbzole ligands and C-M-N bond angles of ∼180°. The electrochemical properties and energies for charge transfer (CT) absorption and emission compounds are not significantly affected by the choice of metal ion. All six of the (carbene)M(Cz) complexes examined here display high photoluminescence quantum yields of 0.8-1.0. The compounds have short emission lifetimes (τ = 0.33-2.8 µs) that fall in the order Ag < Au < Cu, with the lifetimes of (carbene)Ag(Cz) roughly a factor of 10 shorter than for (carbene)Cu(Cz) complexes. Detailed temperature-dependent photophysical measurements (5-325 K) were carried out to determine the singlet and triplet emission lifetimes (τfl and τph, respectively) and the energy difference between the singlet and triplet excited state, Δ ES1-T1. The τfl values range between 20 and 85 ns, and the τph values are in the 50-200 µs regime. The emission at room temperature is due exclusively to E-type delayed fluorescence or TADF (i.e., T1→ΔS1→S0+hν ). The emission rate at room temperature is fully governed by Δ ES1-T1, with the silver complexes giving Δ ES1-T1 values of 150-180 cm-1 (18-23 meV), whereas the gold and copper complexes give values of 570-590 cm-1 (70-73 meV).

Molecular Orientation of Poly-3-hexylthiophene at the Buried Interface with Fullerene.

Molecular orientation at the donor-acceptor interface plays a crucial role in determining the efficiency of organic semiconductor materials. We have used vibrational sum frequency generation spectroscopy to determine the orientation of poly-3-hexylthiophene (P3HT) at the planar buried interface with fullerene (C60). The thiophene rings of P3HT have been found to tilt significantly toward C60, making an average angle θ ≈ 49° ± 10° between the plane of the ring and the interface. Such tilt may be attributed to π-π stacking interactions between P3HT and C60 and may facilitate efficient charge transfer between donor and acceptor. Upon annealing, the thiophene rings tilt away from the interface by Δθ = 12-19°. This may be attributed to higher crystallinity of annealed P3HT that propagates all the way to the interface, resulting in more "edge-on" orientation, which is consistent with the observed red-shift by ∼6 cm-1 and spectral narrowing of the C=C stretch bands.

Highly Efficient Photo- and Electroluminescence from Two-Coordinate Cu(I) Complexes Featuring Nonconventional N-Heterocyclic Carbenes.

A series of six luminescent two-coordinate Cu(I) complexes were investigated bearing nonconventional N-heterocyclic carbene ligands, monoamido-aminocarbene (MAC*) and diamidocarbene (DAC*), along with carbazolyl (Cz) as well as mono- and dicyano-substituted Cz derivatives. The emission color can be systematically varied over 270 nm, from violet to red, through proper choice of the acceptor (carbene) and donor (carbazolyl) groups. The compounds exhibit photoluminescent quantum efficiencies up to 100% in fluid solution and polystyrene films with short decay lifetimes (τ ≈ 1 µs). The radiative rate constants for the Cu(I) complexes ( kr = 105-106 s-1) are comparable to state of the art phosphorescent emitters with noble metals such as Ir and Pt. All complexes show strong solvatochromism due to the large dipole moment of the ground states and the transition dipole moment that is in the opposite direction. Temperature-dependent studies of (MAC*)Cu(Cz) reveal a small energy separation between the lowest singlet and triplet states (Δ ES1-T1 = 500 cm-1) and an exceptionally large zero-field splitting (ZFS = 85 cm-1). Organic light-emitting diodes (OLEDs) fabricated with (MAC*)Cu(Cz) as a green emissive dopant have high external quantum efficiencies (EQE = 19.4%) and brightness of 54 000 cd/m2 with modest roll-off at high currents. The complex can also serve as a neat emissive layer to make highly efficient OLEDs (EQE = 16.3%).

Pepsin immobilized on high-strength hybrid particles for continuous flow online digestion at 10,000 psi.

Pepsin was immobilized on ethyl-bridged hybrid (BEH) particles, and digestion performance was evaluated in a completely online format, with the specific intent of using the particles for hydrogen-deuterium exchange mass spectrometry (HDX MS) experiments. Because the BEH particles are mechanically strong, they could withstand prolonged, continuous high-pressure at 10,000 psi. Online digestion was performed under isobaric conditions with continuous solvent flow, in contrast to other approaches where the pressure or flow is cycled. As expected, digestion efficiency at 10,000 psi was increased and reproducibly produced more peptic peptides versus digestion at 1000 psi. Prototype columns made with the BEH pepsin particles exhibited robust performance, and deuterium back-exchange was similar to that of other immobilized pepsin particles. These particles can be easily incorporated in existing HDX MS workflows to provide more peptide coverage in experiments where fast, efficient, and reproducible online pepsin digestion is desired.

Differential Expression of TGF-ß Isoforms in Human Kerationocytes by Narrow Band UVB.

BACKGROUND: Transforming growth factor-ß (TGF-ß), a multifunctional growth factor, has three isoforms: TGF-ß1, TGF-ß2, and TGF-ß3. Different isoforms of TGF-ß are associated with different proliferation and differentiation states of the epidermis. Narrow band ultraviolet B (NBUVB) emits a concentrated UVB source of 311 nm. NBUVB 1,000 mJ/cm(2) induces apoptosis in approximately 50% of keratinocytes. OBJECTIVE: The purpose of this study was to evaluate whether irradiation with NBUVB would alter the expression and production of TGF-ß1, 2, and 3. METHODS: We measured TGF-ß1, 2, and 3 mRNA and TGF-ß1 and 2 protein levels at 800, 1,000, and 1,200 mJ/cm(2) for 24 hours and 48 hours. RESULTS: TGF-ß1 mRNA levels were increased at both 24 hr and 48 hr, TGF-ß2 mRNA levels were decreased at both 24 hr and 48 hr, and TGF-ß3 mRNA levels were increased at 24 hr and similar to control at 48 hr. TGF-ß1 protein levels were increased at 48 hr but decreased at 24 hr. TGF-ß2 protein levels were decreased at both 24 hr and 48 hr. CONCLUSION: The results suggest a possible role for TGF-ß1 after NBUVB irradiation and opposing roles for TGF-ß1 and TGF-ß2 isoforms in NBUVB irradiation.

Simultaneous determination of biogenic monoamines in rat brain dialysates using capillary high-performance liquid chromatography with photoluminescence following electron transfer.

Simultaneous determination of biogenic monoamines such as dopamine, serotonin, and 3-methoxytyramine in brain is important in understanding neurotransmitter activity. This study presents a sensitive determination of biogenic monoamines in rat brain striatum microdialysates using capillary high-performance liquid chromatography with the photoluminescence following electron-transfer detection technique. Separation conditions were optimized by changing the concentration of an ion-interaction agent and the percentage of an organic modifier. The high concentration of ion-interaction agent enabled the amines as a class to be separated from interfering acids, but also made the separation very long. To shorten the separation time, 10% (v/v) acetonitrile was used as the organic modifier. Eight chromatographic runs during a 3-h period were analyzed in terms of retention times, peak heights, and peak widths. Chromatograms are very reproducible, with less than 1% changes in peak height over 3 h. Typical concentration detection limits at the optimum separation conditions were less than 100 pM for metabolic acids and approximately 200 pM for monoamines. The injection volume of the sample was 500 nL. Thus, the mass detection limits were less than 50 amol for metabolic acids and approximately 100 amol for monoamines. Typical separation time was less than 10 min. To validate the technique, the separation method was applied to the observation of drug-induced changes of monoamine concentrations in rat brain microdialysis samples. Local perfusion of tetrodotoxin, a sodium channel blocker, into the striatum of an anesthetized rat decreased dopamine, 3-methoxytyramine, and serotonin concentrations in dialysates. Successive monitoring of striatal dialysates at a temporal resolution of 7.7 min showed that the injection of nomifensine transiently increased dopamine and 3-methoxytyramine concentrations in rat brain dialysate.

Use of tris(2,2'-bipyridine)osmium as a photoluminescence-following electron-transfer reagent for postcolumn detection in capillary high-performance liquid chromatography.

The photoluminescence-following electron-transfer (PFET) technique, developed in our laboratory, is a sensitive chromatographic detection method for oxidizable analytes. Because the oxidations are homogeneous, the technique avoids the problem of electrode fouling. A liquid-phase oxidant reacts with the electrochemically active analytes after separation, becoming capable of photoluminescence. Laser-induced photoluminescence is measured to quantitate the analytes. Thus, the electrochemical properties of the oxidant determine the detection selectivity, and the spectroscopic properties define the sensitivity. The properties of tris(2,2'-bipyridine)osmium (1) were investigated for use as the liquid-phase oxidant in the PFET system. The redox potential of the complex is less positive than that of tris(2,2'-bipyridine)ruthenium (2); thus, on-line generation of 1(3+) by reaction with PbO2, and selective oxidation of catechols by 1(3+), was possible. The mild oxidizing power of 1(3+) led to a lower background signal (compared to 2(3+)) when mixed with acidic mobile phases. Photoluminescence from 1(2+) was much weaker than that from 2(2+); nonetheless, the system achieved subnanomolar detection limits for dopamine, 3-methoxytyramine, and serotonin. Dopamine and 3-methoxytyramine in rat brain striatal dialysates were determined before and after the injection of nomifensine. The pH of the mobile phase can govern the detection selectivity, since oxidation of most organics is accompanied by proton transfer. Reaction of 1 with catechols showed pH-dependent sensitivity resulting from pH-dependent reaction rate changes. Since the reaction rate is also temperature dependent, increased temperature at the mixer resulted in higher sensitivity. However, the noise level also increased at elevated temperature; thus, the detection limit did not improve.

Influence of chemical kinetics on postcolumn reaction in a capillary Taylor reactor with catechol analytes and photoluminescence following electron transfer.

Postcolumn derivatization reactions can enhance detector sensitivity and selectivity, but their successful combination with capillary liquid chromatography has been limited because of the small peak volumes in capillary chromatography. A capillary Taylor reactor (CTR), developed in our laboratory, provides simple and effective mixing and reaction in a 25-microm-radius postcolumn capillary. Homogenization of reactant streams occurs by radial diffusion, and a chemical reaction follows. Three characteristic times for a given reaction process can be predicted using simple physical and chemical parameters. Two of these times are the homogenization time, which governs how long it takes the molecules in the analyte and reagent streams to mix, and the reaction time, which governs how long the molecules in a homogeneous solution take to react. The third characteristic time is an adjustment to the reaction time called the start time, which represents an estimate of the average time the analyte stream spends without exposure to reagent. In this study, laser-induced fluorescence monitored the extent of the postcolumn reaction (reduction of Os(bpy)3(3+) by analyte to the photoluminescent Os(bpy)3(2+)) in a CTR. The reaction time depends on the reaction rates. Analysis of product versus time data yielded second-order reaction rate constants between the PFET reagent, tris(2,2'-bipyridine)osmium, and standards ((ferrocenylmethyl)trimethylammonium cation and p-hydroquinone) or catechols (dopamine, epinephrine, norepinephrine, 3, 4-dihydroxyphenylacetic acid. The extent of the reactions in a CTR were then predicted from initial reaction conditions and compared to experimental results. Both the theory and experimental results suggested the reactions of catechols were generally kinetically controlled, while those of the standards were controlled by mixing time (1-2 s). Thus, the extent of homogenization can be monitored in a CTR using the relatively fast reaction of the reagent and p-hydroquinone. Kinetically controlled reactions of catechols, however, could be also completed in a reasonable time at increased reagent concentration. A satisfactory reactor, operating at 1.7 cm/s (2 microL/min) velocity with solutes having diffusion coefficients in the 5 x 10(-6) cm2/s range, can be constructed from 8.0 cm of 25-microm-radius capillary. Slower reactions require longer reaction times, but theoretical calculations expect that a CTR does not broaden a chromatographic peak (N = 14 000) from a 100-microm-capillary chromatography column by 10% if the pseudo-first-order rate constant is larger than 0.1 s(-1).