Intramolecular π Stacking in Cationic Iridium(III) Complexes with Phenyl-Functionalized Cyclometalated Ligands: Synthesis, Structure, Photophysical Properties, and Theoretical Studies

The syntheses of two new heteroleptic cationic iridium complexes containing 2,6-diphenylpyridine (Hdppy) and 2,4,6-triphenylpyridine (Htppy) as the cyclometalated ligands, namely, [Ir(dppy)2phen]PF6 (1, phen = 1,10-phenanthroline) and [Ir(tppy)2phen]PF6 (2), are described. The X-ray crystal structure of 2 reveals a distorted octahedral geometry around the Ir center and close intramolecular face-to-face π–π stacking interactions between the pendant phenyl rings at the 2-position of the cyclometalated ligands and the NN ancillary ligand. This represents a new π–π stacking mode in charged Ir complexes. Complexes 1 and 2 are green photoemitters: their photophysical and electrochemical properties are interpreted with the assistance of density functional theory (DFT) calculations. These calculations also establish that the observed intramolecular interactions cannot effectively prevent the lengthening of the Ir–N bonds of the complexes in their metal-centered (3MC) states. Complexes 1 and 2 do not emit light in light-emitting electrochemical cells (LECs) under conditions in which the model compound [Ir(ppy)2phen]PF6 (3) emits strongly. This is explained by degradation reactions of the 3MC state of 1 and 2 under the applied bias during LEC operation facilitated by the enhanced distortions in the geometry of the complexes. These observations have important implications for the future design of complexes for LEC applications

Highly Efficient Blue LECs Using Charged Iridium Complexes

Two heteroleptic charged iridium (III) species comprising two cyclometallating ligands and a neutral diimine ligand were synthesized and characterized, namely [(3,4,7,8-tetramethyl-1,10-phenanthroline-N-N')-bis-(2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine-$C^{6'}$,N)-iridium (III)]hexafluorophosphate (UM01) and [(3,4,7,8-tetramethyl-1,10-phenanthroline-N-N')-bis-(2-(2',4'-difluoro-phenyl)-1H-pyrazole-C^{6'},N)-iridium (III)]hexafluorophosphate (UM02). Both complexes were used as the blue emitter in OLED and LEC devices. We found that the optimized structure is ITO/PEDOT:PSS/complex:BMIMPF₆(1:1)/Al. The UM01 gave a current efficiency of 1.14 cd $A^{-1}$, whereas the UM02 shows a better CIE coordination at 0.19, 0.40

Identifying Diversity in Nanoscale Electrical Break Junctions

The realization of molecular-scale electronic devices will require the development of novel strategies for controlling electrical properties of metal|molecule|metal junctions, down to the single molecule level. Here, we show that it is possible to exert chemical control over the formation of metal|molecule...molecule|metal junctions in which the molecules interact by π-stacking. The tip of an STM is used to form one contact, and the substrate the other; the molecules are conjugated oligophenyleneethynylenes (OPEs). Supramolecular π−π interactions allow current to flow through the junction, but not if bulky tert-butyl substituents on the phenyl rings prevent such interactions. For the first time, we find evidence that π-stacked junctions can form even for OPEs with two thiol contacts. Furthermore, we find evidence for metal|molecule|metal junctions involving oligophenyleneethynylene monothiols, in which the second contact must be formed by the interaction of the π-electrons of the terminal phenyl ring with the metal surface

Ionic Iridium(III) Complexes with Bulky Side Groups for Use in Light Emitting Cells: Reduction of Concentration Quenching

Here, the photophysics and performance of single-layer light emitting cells (LECs) based on a series of ionic cyclometalated Ir(III) complexes of formulae [Ir(ppy)(2)(bpy)]+PF6- and [Ir(ppy)(2)(phen)]+PF6- where ppy, bpy, and phen are 2-phenylpyridine, substituted bipyridine and substituted phenanthroline ligands, respectively, are reported. Substitution at the NN Iigand has little effect on the emitting metal-ligand to ligand charge-transfer (MLLCT) states and functionalization at this site of the complex leads to only modest changes in emission color. For the more bulky complexes the increase in intermolecular separation leads to reduced exciton migration, which in turn, by suppressing concentration quenching, significantly increases the lifetime of the excited state. On the other hand, the larger intermolecular separation induced by bulky ligands reduces the charge carrier mobility of the materials, which means that higher bias fields are needed to drive the diodes. A brightness of ca. 1000cd m(-2) at 3 V is obtained for complex 5, which demonstrates a beneficial effect of bulky substituents

Variable contact gap single-molecule conductance determination for a series of conjugated molecular bridges

It is now becoming clear that the characteristics of the whole junction are important in determining the conductance of single molecules bound between two metal contacts. This paper shows through measurements on a series of seven conjugated molecular bridges that contact separation is an important factor in determining the electrical response of the molecular junction. These data are obtained using the I (t) method developed by Haiss et al since the scanning tunnelling microscope tip to substrate separation can be controlled through choice of the set-point (I-0) current and calibrated with current-distance curves and knowledge of the terminal to terminal length of the molecular wire. The contact gap separation dependence is interpreted as arising from tilting of these molecules in the junction and this model is underpinned by ab initio transport computations. In this respect we make the general observation that conductance increases rather dramatically at higher tilt angle away from the normal for conformationally rigid molecular wires and that this increase in conductance arises from increased electronic coupling between the molecular bridge and the gold contacts

Molecular Bridging of Silicon Nanogaps

The highly doped electrodes of a vertical silicon nanogap device have been bridged by a 5.85 nm long molecular wire, which was synthesized in situ by grafting 4-ethynylbenzaldehyde via C-Si links to the top and bottom electrodes and thereafter by coupling an amino-terminated fluorene unit to the aldehyde groups of the activated electrode surfaces. The number of bridging molecules is constrained by relying on surface roughness to match the 5.85 nm length with an electrode gap that is nominally 1 nm wider and may be controlled by varying the reaction time: the device current increases from <= 1 pA at 1 V following the initial grafting step to 10-100 nA at 1 V when reacted for 5-15 min with the amino-terminated linker and 10 mu A when reacted for 16-53 h. It is the first time that both ends of a molecular wire have been directly grafted to silicon electrodes, and these molecule-Induced changes are reversible. The bridges detach when the device Is rinsed with dilute add solution, which breaks the imine links of the in situ formed wire and causes the current to revert to the subpicoampere leakage value of the 4-ethynylbenzaldehyde-grafted nanogap structure