New π-cojugated materials for molecular electronic and light-emitting devices

The synthesis and characterisation of a range of oligo(aryleneethynylene)derivatives end-capped with cyanoethylthio and acetylthio groups are described. Sonogashira cross-coupling reactions have been key steps. These molecules are designed as molecular wires for conductance studies, including single-molecule conductance using Scanning Tunelling Microscopy (STM) and break junction techniques. Solution UV-V is absorption and fluorescence spectroscopy have been used to assess conjugation in the backbones, e.g. a sequential red shift is observed for molecules with 1, 2 and 3 fluorene units in the backbone (41 (RJ 11), 45 (RJ 13) and 47 (RJ 14); λ(_max) (abs.) 364, 371, 378nm, respectively) and a blue shift for molecules 35 (RJ 8) and 49 (RJ 17) is observed.The results of STM studies in Professor Ashwell's group (Bangor University) show symmetrical I-V characteristics for 35 (RJ 8), 41 (RJ 11), 43 (RJ 12) and 47 (RJ 14)assembled on gold. Preliminary STM results show length-independent current jumps for the molecules, which is inconsistent with theory: longer molecules should show lower conductance. The length independence suggests that the assembled molecules are being contacted by the STM tip along the backbone, not at the terminal sulfur. Most of the STM experiments show a single current jump, consistent with one molecule being contacted. However, some experiments with 43 (RJ 12) show double or even triple current jumps, suggesting simultaneous contact to two and three molecules. The conductance study of compound RJ 32 with I(t), I(s) and BJ techniques in Professor R. Nichols' group (Liverpool University) shows that the different techniques favour differing current peaks, with the BJ technique giving a higher propensity to higher current peaks, while the l(t)and I(s) method favour the lower current. A study using MCBJ experiments in Professor C. Schonenberger's group (University of Basel) show that oligo(phenyleneethynylene)(OPE) derivatives have lower conductance than oligo(phenylenevinylene) (OPV)analogues and alkoxy side chains on OPEs do not affect the single-molecule conductance

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

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