Conformationally-restricted bicarbazoles with phenylene bridges displaying deep-blue emission and high triplet energies: systematic structure-property relationships.

The synthesis is reported of twelve new symmetrical carbazole dimers in which the carbazole units are linked via 1,4-phenylene spacers. There are two distinct series of compounds based on the position on the carbazole ring where the phenylene spacer is attached: this is either at carbazole C(3) (series 1a-1f) or at C(2) (series 2a-2f). The central phenylene ring is substituted with either two methyl, two methoxy or two cyano substituents which impart an intramolecular torsional angle between the phenylene and carbazole rings, thereby limiting the extent of π-conjugation between the carbazole units, and raising the triplet energies of the molecules to ET 2.6-3.0 eV, as determined from their phosphorescence spectra at 80 K. Structure-property relationships were studied by UV-vis and fluorescence spectroscopy, cyclic voltammetry and theoretical calculations. A notable observation is that substitution at the 2-position of carbazole (linear conjugation) exerts control over the position of the HOMO, while substitution at the 3-position of carbazole (meta conjugation) allows greater control over the LUMO. X-ray crystal structures are reported for two of the bicarbazoles. Compound 2d is shown to be a suitable host for the sky-blue emitter FIrpic in PhOLEDs, with improved device performance compared to CBP as host.

Pyridylpyrazole N

A series of blue iridium(iii) complexes (12-15) comprising sulfonyl-functionalised phenylpyridyl cyclometalating ligands and pyridylpyrazole N^N ligands are reported, with an X-ray crystal structure obtained for 12. The complexes are highly emissive with photoluminescence quantum yields of 0.52-0.70 in dichloromethane solutions: two of the complexes (12 and 14) show emissions at λ 457 nm which is considerably blue-shifted compared to the archetypal blue emitter FIrpic (λmax 468 nm). The short excited state lifetimes (1.8-3.3 µs) and spectral profiles are consistent with phosphorescence from a mixture of ligand-centred and MLCT excited states. Density functional (DFT) and time dependent DFT (TD-DFT) calculations are in agreement with the electrochemical properties and the blue phosphorescence of the complexes. The additional mesityl substituent on the pyridylpyrazole ligand of 12 and 13 enhances the solubility of the complexes facilitating thin film formation by solution processing. Phosphorescent organic light-emitting diodes (PhOLEDs) have been fabricated using 12 or 13 in a solution-processed single-emitting layer using either poly(vinylcarbazole) (PVK) or 1,3-bis(N-carbazolyl)benzene (mCP) as host. The most blue-shifted electroluminescence (λ 460 nm, CIEx,y 0.15, 0.21) is obtained for an OLED containing complex 12 and mCP, with a brightness of 5400 cd m-2 at 10 V which is high for PhOLEDs with similar blue CIE coordinates using a solution-processed emitter layer.

Electric Field Induce Blue Shift and Intensity Enhancement in 2D Exciplex Organic Light Emitting Diodes; Controlling Electron-Hole Separation.

A simple but novel method is designed to study the characteristics of the exciplex state pinned at a donor-acceptor abrupt interface and the effect an external electric field has on these excited states. The reverse Onsager process, where the field induces blue-shifted emission and increases the efficiency of the exciplex emission as the e-h separation reduces, is discussed.

Bimetallic cyclometalated iridium(III) diastereomers with non-innocent bridging ligands for high-efficiency phosphorescent OLEDs.

Two phosphorescent dinuclear iridium(III) diastereomers (ΛΔ/ΔΛ) and (ΛΛ/ΔΔ) are readily separated by making use of their different solubilities in hot hexane. The bridging diarylhydrazide ligand plays an important role in the electrochemistry and photophysics of the complexes. Organic light-emitting devices (OLEDs) that use these complexes as the green-emissive dopants in solution-processable single-active-layer architectures feature electroluminescence efficiencies that are remarkably high for dinuclear metal complexes, achieving maximum values of 37 cd A(-1), 14 lm W(-1), and 11% external quantum efficiency.

FRET and competing processes between conjugated polymer and dye substituted DNA strands: a comparative study of probe selection in DNA detection.

Fluorescence resonance energy transfer (FRET) between water-soluble conjugated polymer, poly-(9,9-bis(6'-N,N,N-trimethylammonium)-hexyl-fluorene phenylene) bromide, and ssDNA's labeled with four different types of dyes (Pacific-blue, Alexa-fluor 430, Fluorescein, and ROX) has been investigated. The effect of spectral overlap and Stokes-shift on the efficiency and properties of FRET were studied. In the DNA sequence detection technique that using cationic conductive polymer and the negatively charged DNA the electrostatic interaction leads to strong aggregation. The effective concentration of these aggregates is quite high leading to strong self-absorption. In this case, labeling with small Stokes shift dyes shows a strong output emission limitation even in extremely dilute system. The steady state fluorescence quenching of the CCP by FRET reveals that the competition between FRET and self-absorption plays a major role when accounting for the FRET ratio. Time-resolved fluorescence lifetime analysis was carried out to measure the energy transfer between the donor and the acceptor excluding self-absorption and quenching by ground-state complex formation. Time-resolved analysis indicates only around 30% of the total CCP excited-state population is quenched by FRET, whereas 55% is quenched by the DNA/buffer solution.

Improved single nucleotide polymorphisms detection using conjugated polymer/surfactant system and peptide nucleic acid.

An improved assay for the detection of single nucleotide polymorphisms (SNP) of mutant DNA using a combination of peptide nucleic acid (PNA) probes, a cationic conjugated polymer (CCP) and non-ionic surfactant is reported. A comparison between CCP/surfactant and CCP alone shows enhancement in the discrimination between mutant and wild type DNA by a factor of two. A discrimination factor of 70% and 92% was calculated for single and five bases mismatched mutants, respectively when using CCP/surfactant. Furthermore, CCP/surfactant provides a strong emissive donor which increases signal to noise ratio and prevents fluctuation in the output signal caused by the suspension nature of the CCP (due to polymer aggregation) in water. The fluorescence resonance energy transfer (FRET) ratio which defined as the ratio of PL emission of the acceptor to that of the donor, was found to be 20% better when the location of the mutation is five bases away from the duplex terminal compared to that in the centre of the duplex. The enhance discrimination referred to the difference in the FRET and reabsorption rates in different types of duplex. The FRET ratio can be very sensitive to the sample excitation strength, emission collection and spectrometer setting.