A central issue in molecular electronics in order to build functional devices is to assess whether changes in the electronic structure of isolated compounds by chemical derivatization are retained once the molecules are inserted into molecular junctions. Recent theoretical studies have suggested that this is not always the case due to the occurrence of pinning effects making the alignment of the transporting levels insensitive to the changes in the electronic structure of the isolated systems. We explore here this phenomenon by investigating at both the experimental and theoretical levels the I/ V characteristics of molecular junctions incorporating three different three-ring phenylene ethynylene derivatives designed to exhibit a significant variation of the HOMO level in the isolated state. At the theoretical level, our NEGF/DFT calculations performed on junctions including the three compounds show that, whereas the HOMO of the molecules varies by 0.61 eV in the isolated state, their alignment with respect to the Fermi level of the gold electrodes in the junction is very similar (within 0.1 eV). At the experimental level, the SAMs made of the three compounds have been contacted by a conducting AFM probe to measure their I/ V characteristics. The alignment of the HOMO with respect to the Fermi level of the gold electrodes has been deduced by fitting the I/ V curves, using a model based on a single-level description (Newns-Anderson model). The extracted values are found to be very similar for the three derivatives, in full consistency with the theoretical predictions, thus providing clear evidence for a HOMO level pinning effect.

The single‐molecule conductance of a series of BN‐acene‐like derivatives has been measured by using scanning tunneling break‐junction techniques. A strategic design of the target molecules has allowed us to include azaborine units in positions that unambiguously ensure electron transport through both heteroatoms, which is relevant for the development of customized BN‐doped nanographenes. We show that the conductance of the anthracene azaborine derivative is comparable to that of the pristine all‐carbon anthracene compound. Notably, this heteroatom substitution has also allowed us to perform similar measurements on the corresponding pentacene‐like compound, which is found to have a similar conductance, thus evidencing that B–N doping could also be used to stabilize and characterize larger acenes for molecular electronics applications. Our conclusions are supported by state‐of‐the‐art transport calculations.

A number of factors contribute to orbital energy alignment with respect to the Fermi level in molecular tunnel junctions. Here, we report a combined experimental and theoretical effort to quantify the effect of metal image potentials on the highest occupied molecular orbital to Fermi level offset, εh, for molecular junctions based on self-assembled monolayers (SAMs) of oligophenylene ethynylene dithiols (OPX) on Au. Our experimental approach involves the use of both transport and photoelectron spectroscopy to extract the offsets, εh trans and εh UPS, respectively. We take the difference in these quantities to be the image potential energy eV image. In the theoretical approach, we use density functional theory (DFT) to calculate directly eV image between positive charge on an OPX molecule and the negative image charge in the Au. Both approaches yield eV image ∼ −0.1 eV per metal contact, meaning that the total image potential energy is ∼−0.2 eV for an assembled junction with two Au contacts. Thus, we find that the total image potential energy is 25–30% of the total offset εh, which means that image charge effects are significant in OPX junctions. Our methods should be generally applicable to understanding image charge effects as a function of molecular size, for example, in a variety of SAM-based junctions.

A direct, efficient and versatile strategy for modulation of optoelectronic and magnetic properties of indeno[1,2-b]fluorene has been developed. 4-substituted-2,6-dimethylphenyl acetylene groups placed in the apical carbon of the five-membered rings...

We present a detailed theoretical characterization of the energetic alignment between the HOMO level of a series of thiolated oligophenylenes of increasing chain size, and the Fermi level of gold electrodes, using density functional theory (DFT) calculations for molecular self-assembled monolayers (SAMs) chemisorbed on an Au (111) surface, and the nonequilibrium Green's function (NEGF) formalism coupled to DFT for single molecule junctions. The additional role of the dynamic electronic polarization effects neglected in standard DFT calculations is also discussed. Interestingly, whereas the HOMO energy varies significantly among the unsubstituted oligomers in the gas phase, their alignment with respect to the Fermi level of the electrode is almost insensitive to chain size upon chemisorption, thus pointing to a strong pinning effect. The energy at which the HOMO is pinned strongly depends on the degree of interfacial hybridization, and hence on the contact geometry, as well as on the degree of surface coverage although a different mechanism enters into play.anchoring unit. Over the years, a large body of knowledge has been accumulated on the morphologies of such SAMs and on structure-property relationships for the work function shifts [3,4] ; in molecular electronics, many theoretical and experimental studies have focused on the changes in the transmission spectra and I/V characteristics when elongating the size of conjugated oligomers. [5][6][7] In contrast, less attention has been paid to structure-property relationships driving the alignment of the frontier electronic levels with respect to the Fermi level of the metallic electrodes; this energy offset can be deduced by ultraviolet photoelectron spectroscopy (UPS) measurements, [8,9] estimated from I/V characteristics of molecular junctions by using simple analytical models, [8,[10][11][12] or measured photocurrent spectra. [13] The alignment of the HOMO is clearly a key quantity in SAMs, providing electronic levels that favor charge injection in organic layers or in molecular junctions.Theory can prove useful in this context to shed light on this issue, even though it remains extremely challenging to predict the Fermi level alignment quantitatively via first-principles calculations, typically based on density functional theory (DFT). [14][15][16][17][18] This is due to the fact that the ionization potential/electron affinity (IP/EA) of SAM-forming molecules attached to metallic electrodes is governed by several effects not present for the isolated molecules: (i) the electronic polarization of the metal (i.e., image effects) and of the neighboring molecules in presence of a net charge, that both lower the IP and increase the EA of the molecules (i.e., stabilize the formation of a positive charge or negative charge on the molecules); these effects are neglected when performing standard DFT calculations on neutral systems; (ii) the hybridization of the orbitals of the anchoring group with the orbitals of the metallic atoms and the resulting charge reorganization ...

The single‐molecule conductance of a series of BN‐acene‐like derivatives has been measured by using scanning tunneling break‐junction techniques. A strategic design of the target molecules has allowed us to include azaborine units in positions that unambiguously ensure electron transport through both heteroatoms, which is relevant for the development of customized BN‐doped nanographenes. We show that the conductance of the anthracene azaborine derivative is comparable to that of the pristine all‐carbon anthracene compound. Notably, this heteroatom substitution has also allowed us to perform similar measurements on the corresponding pentacene‐like compound, which is found to have a similar conductance, thus evidencing that B–N doping could also be used to stabilize and characterize larger acenes for molecular electronics applications. Our conclusions are supported by state‐of‐the‐art transport calculations.

Understanding and controlling electrical conductivity at the single-molecule level is of fundamental importance for the development of new molecular electronic devices. This ideally requires considering the many different options offered...

In view of the development and the importance that the studies of conductance through molecular junctions is acquiring, robust, reliable and easy-to-use theoretical tools are the most required. Here, we present an efficient implementation of the self-energy correction to density functional theory (DFT) non-equilibrium Green functions (NEGF) method for TRANSIESTA package. We have assessed the validity of our implementation using as benchmark systems a family of acene complexes with increasing number of aromatic rings and several anchoring groups. Our theoretical results show an excellent agreement with experimentally available measurements assuring the robustness and accuracy of our implementation.