Molecular Heterojunctions of Oligo(phenylene ethynylene)s with Linear to Cruciform Framework

Abstract

Electrical transport properties of molecular junctions are fundamentally affected by the energy alignment between molecular frontier orbitals (highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO)) and Fermi level (or work function) of electrode metals. Dithiafulvene (DTF) is used as substituent group to the oligo(phenylene ethynylene) (OPE) molecular wires and different molecular structures based on OPE3 backbone (with linear to cruciform framework) are achieved, with viable molecular orbitals and HOMO–LUMO energy gaps. OPE3, OPE3–DTF, and OPE3–tetrathiafulvalene (TTF) can form good self‐assembled monolayers (SAMs) on Au substrates. Molecular heterojunctions based on these SAMs are investigated using conducting probe–atomic force microscopy with different tips (Ag, Au, and Pt) and Fermi levels. The calibrated conductance values follow the sequence OPE3–TTF > OPE3–DTF > OPE3 irrespective of the tip metal. Rectification properties (or diode behavior) are observed in case of the Ag tip for which the work function is furthest from the HOMO levels of the OPE3s. Quantum chemical calculations of the transmission qualitatively agree with the experimental data and reproduce the substituent effect of DTF. Zero‐bias conductance, and symmetric or asymmetric couplings to the electrodes are investigated. The results indicate that improved fidelity of molecular transport measurements may be achieved by systematic studies of homologues series of molecular wires applying several different metal electrodes.

Gold nanoparticles assembled with dithiocarbamate-anchored molecular wires

“…The 71 nm is based on estimating the diameter from the absorption spectrum using the method from Haiss et al 21 The surface area of a single OPE-2S (containing one dithiocarbamate anchoring group, see Fig. 1B ) in densely packed high quality self-assembled monolayers (SAMs) on (flat) Au-surfaces was found by Wei et al 18 to be 0.42 nm 2 /molecule. This gives an estimated maximum of about 38000 molecules for a full coverage of the AuNPs.…”

“…The OPE is terminated in both ends with a dithiocarbamate anchoring group ( Fig. 1A ) 18 , which has proven to be a good alternative to the commonly used thiol 19 20 . The charged nature of the sodium dithiocarbamate group(s) facilitates dissolution of the OPE wires in methanol and therefrom in the aqueous colloid solution, while – at the same time – ensuring a stable covalent binding to the AuNPs.…”

Direct observation of charge transfer between molecular heterojunctions based on inorganic semiconductor clusters

“…Over the past decade, crystalline molecular heterojunctions, [13][14][15][16][17] especially those with strongly bonded interfaces, have drawn much attention because of their ideal contact areas, bonding sites between adjacent molecules (or phases), and controllable compositions in ultra-small subunits. [18][19][20][21] This well-dened heterostructured conguration offers obvious advantages for photovoltaic and catalytic applications, as the sufficient hybridization of atomic orbitals permits efficient carrier delocalization and allows programmable band structures to meet the potential of catalytic reactions.…”

Exploring antiaromaticity in single-molecule junctions formed from biphenylene derivatives

“…Para-Linked OPE-3 systems (3 refers to the number of phenylene rings in the backbone) are benchmark molecules in this context. [15][16][17][18][19][20][21][22][23][24] OPEs are synthetically versatile and their conjugative and functional properties have been systematically tuned across many parameters. [25][26][27] The alkyne bonds in para-linked OPEs serve two main purposes: (i) they ensure a length-persistent rigid-rod structure with no possibility of geometrical isomerization (unlike oligophenylenevinylenes) (OPVs) and (ii) they space the phenyl rings apart, which enables the rings to rotate freely and achieve coplanarity, thereby maximizing the frontier orbital overlap along the molecule.…”