Porphyrins as building blocks for single-molecule devices

Direct measurement of single-molecule electrical transparency by break junction experiments has become a major field of research over the two last decades. This review specifically and comprehensively highlights the use of porphyrins as molecular components and discusses their potential use for the construction of future devices. Throughout the review, the features provided by porphyrins, such as low level misalignments and very low attenuation factors, are shown with numerous examples, illustrating the potential and limitations of these molecular junctions, as well as differences emerging from applied integration/investigation techniques. QN/van der Zant La

Mechanical Fixation by Porphyrin Connection: Synthesis and Transport Studies of a Bicyclic Dimer

The bowl-shaped, 3-fold interlinked porphyrin dimer 2 was obtained in respectable yields during macrocyclization attempts. Its bicyclic structure, consisting of a macrocycle made of a pair of acetylene interlinked tetraphenylporphyrins which are additionally linked by a C-C bond interlinking two pyrrole subunits, has been confirmed spectroscopically (2D-NMR, UV/vis, HR-MALDI-ToF MS). Late-stage functionalization provided the structural analogue 1 with acetyl-protected terminal thiol anchor groups enabling single molecule transport investigations in a mechanically controlled break junction experiment. The formation of single-molecule junctions was observed, displaying large variations in the observed conductance values pointing at a rich diversity in the molecular junctions.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.QN/van der Zant La

Unravelling the conductance path through single-porphyrin junctions

Porphyrin derivatives are key components in natural machinery enabling us to store sunlight as chemical energy. In spite of their prominent role in cascades separating electrical charges and their potential as sensitizers in molecular devices, reports concerning their electronic transport characteristics are inconsistent. Here we report a systematic investigation of electronic transport paths through single porphyrin junctions. The transport through seven structurally related porphyrin derivatives was repeatedly measured in an automatized mechanically controlled break-junction set-up and the recorded data were analyzed by an unsupervised clustering algorithm. The correlation between the appearances of similar clusters in particular sub-sets of the porphyrins with a common structural motif allowed us to assign the corresponding current path. The small series of model porphyrins allowed us to identify and distinguish three different electronic paths covering more than four orders of magnitude in conductance.QN/van der Zant La

Charge Transport through a Single Molecule of trans-1-bis-Diazofluorene [60]fullerene

Fullerenes have attracted interest for their possible applications in various electronic, biological, and optoelectronic devices. However, for efficient use in such devices, a suitable anchoring group has to be employed that forms well-defined and stable contacts with the electrodes. In this work, we propose a novel fullerene tetramalonate derivate functionalized with trans-1 4,5-diazafluorene anchoring groups. The conductance of single-molecule junctions, investigated in two different setups with the mechanically controlled break junction technique, reveals the formation of molecular junctions at three conductance levels. We attribute the conductance peaks to three binding modes of the anchoring groups to the gold electrodes. Density functional theory calculations confirm the existence of multiple binding configurations and calculated transmission functions are consistent with experimentally determined conductance values.QN/van der Zant La

Electric-field induced bistability in single-molecule conductance measurements for boron coordinated curcuminoid compounds

We have studied the single-molecule conductance of a family of curcuminoid molecules (CCMs) using the mechanically controlled break junction (MCBJ) technique. The CCMs under study contain methylthio (MeS-) as anchoring groups: MeS-CCM (1), the free-ligand organic molecule, and two coordination compounds, MeS-CCM-BF2 (2) and MeS-CCM-Cu (3), where ligand 1 coordinates to a boron center (BF2 group) and to a CuII moiety, respectively. We found that the three molecules present stable molecular junctions allowing detailed statistical analysis of their electronic properties. Compound 3 shows a slight increase in the conductance with respect to free ligand 1, whereas incorporation of BF2 (compound 2) promotes the presence of two conductance states in the measurements. Additional experiments with control molecules point out that this bistability is related to the combination of MeS- anchoring groups and the BF2 moiety within the structure of the molecules. Theoretical calculations show that this can be explained by the presence of two conformers once compound 2 is anchored between the gold electrodes. An energy minimum is found for a flat structure but there is a dramatic change in the magnitude and orientation of dipole moment (favouring a non-flat conformer in the presence of an external electric field) due to a conformational change of one of the terminal MeS- groups. The results thus point to an intricate interplay between the applied bias voltage and the molecule dipole moment which could be the basis for designing new molecules aiming at controlling their conformation in devices.QN/van der Zant La

Mechanical Tuning of Through-Molecule Conductance in a Conjugated Calix[4]pyrrole

A conformationally flexible calix[4]pyrrole possessing a conjugated electronic structure (an N-substituted oxoporphyrinogen (OxP) related to porphyrin) was used to investigate the influence of mechanical stretching on the single-molecule conductance of these molecules using the mechanically-controlled break junction (MCBJ) technique. The results show that the molecule can be immobilized in a single-molecule break junction configuration, giving rise to different behaviours. These include step-like features in the conductance vs. displacement traces as well as conductance traces that exhibit a slower decay (‘downhill’ traces) than measured for direct tunneling. The latter class of traces can be associated with the mechanical manipulation (i. e., stretching) of the molecule with inter-electrode distances as long as 2 nm. Density functional theory (DFT) calculations reveal that OxP molecules are stable under stretching in the length regime studied.Accepted Author ManuscriptQN/van der Zant La