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

Abstract

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.