Phosphonic acid anchored tripodal molecular films on indium tin oxide.

Whereas monopodal self-assembling monolayers (SAMs) are most frequently used for surface and interface engineering, tripodal SAMs are less popular due to the difficulty in achieving a reliable and homogeneous bonding configuration. In this context, in the present study, the potential of phosphonic acid (PA) decorated triptycene (TripPA) for formation of SAMs on oxide substrates was studied, using indium tin oxide (ITO) as a representative and application-relevant test support. A combination of several complementary experimental techniques was applied and a suitable monopodal reference system, benzylphosphonic acid (PPA), was used. The resulting data consistently show that TripPA forms well-defined, densely packed, and nearly contamination-free tripodal SAMs on ITO, with the similar parameters and properties as the monopodal reference system. Modification of wetting properties and work function of ITO by non-substituted and cyano-decorated TripPA SAMs was demonstrated, showing a potential of this tripodal system for surface engineering of oxide substrates.

Modular Approach to Creating Functionalized Surface Arrays of Molecular Qubits.

The quest for developing quantum technologies is driven by the promise of exponentially faster computations, ultrahigh performance sensing, and achieving thorough understanding of many-particle quantum systems. Molecular spins are excellent qubit candidates because they feature long coherence times, are widely tunable through chemical synthesis, and can be interfaced with other quantum platforms such as superconducting qubits. A present challenge for molecular spin qubits is their integration in quantum devices, which requires arranging them in thin films or monolayers on surfaces. However, clear proof of the survival of quantum properties of molecular qubits on surfaces has not been reported so far. Furthermore, little is known about the change in spin dynamics of molecular qubits going from the bulk to monolayers. Here, a versatile bottom-up method is reported to arrange molecular qubits as functional groups of self-assembled monolayers (SAMs) on surfaces, combining molecular self-organization and click chemistry. Coherence times of up to 13 µs demonstrate that qubit properties are maintained or even enhanced in the monolayer.

Effect of substitution on the charge transport properties of oligophenylenethiolate self-assembled monolayers.

Electrostatic effects in charge transport across the molecular framework, including those imposed by halogen atoms, have recently attracted noticeable attention of the molecular-electronics community. In this context, in the present work, we studied the effect of tail group (R) substitution on the charge transport properties of oligophenylenethiolate self-assembled monolayers (SAMs) on Au(111), with R = H, F, CH3, and CF3. The length of the molecular backbone was varied from one to three rings and the quality, basic parameters, and electrostatic properties of the SAMs were monitored. For a given length of the molecular backbone, the current density showed a strong dependence on R, being the highest for R = CH3, and then successively lower for R = H, R = CF3, and R = F. This tendency correlated neither with the molecular length of the precursors nor with the work function of the SAMs and was, therefore, exclusively ascribed to the identity of the tail group. In contrast to the current density, the tunneling decay coefficient, describing its dependence on the molecular length, was found to be independent of the identity of the tail group. The reasons behind the observed behavior are discussed and rationalized within the available experimental data and reasonable assumptions.

Rational Design of Porous Poly(ethylene glycol) Films as a Matrix for ssDNA Immobilization and Hybridization.

Poly(ethylene glycol) (PEG) films, fabricated by thermally induced crosslinking of amine- and epoxy-terminated four-arm STAR-PEG precursors, were used as porous and bioinert matrix for single-stranded DNA (ssDNA) immobilization and hybridization. The immobilization relied on the reaction between the amine groups in the films and N-hydroxy succinimide (NHS) ester groups of the NHS-ester-decorated ssDNA. Whereas the amount of reactive amine groups in the films with the standard 1:1 composition of the precursors turned out to be too low for efficient immobilization, it could be increased noticeably using an excess (2:1) concentration of the amine-terminated precursor. The respective films retained the bioinertness of the 1:1 prototype and could be successfully decorated with probe ssDNA, resulting in porous, 3D PEG-ssDNA sensing assemblies. These assemblies exhibited high selectivity with respect to the target ssDNA strands, with a hybridization efficiency of 78-89% for the matching sequences and full inertness for non-complementary strands. The respective strategy can be applied to the fabrication of DNA microarrays and DNA sensors. As a suitable transduction technique, requiring no ssDNA labeling and showing high sensitivity in the PEG-ssDNA case, electrochemical impedance spectroscopy is suggested.

Porous Honeycomb Self-Assembled Monolayers: Tripodal Adsorption and Hidden Chirality of Carboxylate Anchored Triptycenes on Ag.

Molecules with tripodal anchoring to substrates represent a versatile platform for the fabrication of robust self-assembled monolayers (SAMs), complementing the conventional monopodal approach. In this context, we studied the adsorption of 1,8,13-tricarboxytriptycene (Trip-CA) on Ag(111), mimicked by a bilayer of silver atoms underpotentially deposited on Au. While tripodal SAMs frequently suffer from poor structural quality and inhomogeneous bonding configurations, the triptycene scaffold featuring three carboxylic acid anchoring groups yields highly crystalline SAM structures. A pronounced polymorphism is observed, with the formation of distinctly different structures depending on preparation conditions. Besides hexagonal molecular arrangements, the occurrence of a honeycomb structure is particularly intriguing as such an open structure is unusual for SAMs consisting of upright-standing molecules. Advanced spectroscopic tools reveal an equivalent bonding of all carboxylic acid anchoring groups. Notably, density functional theory calculations predict a chiral arrangement of the molecules in the honeycomb network, which, surprisingly, is not apparent in experimental scanning tunneling microscopy (STM) images. This seeming discrepancy between theory and experiment can be resolved by considering the details of the actual electronic structure of the adsorbate layer. The presented results represent an exemplary showcase for the intricacy of interpreting STM images of complex molecular films. They are also further evidence for the potential of triptycenes as basic building blocks for generating well-defined layers with unusual structural motifs.

Macroscopic quantum electrodynamics and density functional theory approaches to dispersion interactions between fullerenes.

The processing and material properties of commercial organic semiconductors, for e.g. fullerenes is largely controlled by their precise arrangements, specially intermolecular symmetries, distances and orientations, more specifically, molecular polarisabilities. These supramolecular parameters heavily influence their electronic structure, thereby determining molecular photophysics and therefore dictating their usability as n-type semiconductors. In this article we evaluate van der Waals potentials of a fullerene dimer model system using two approaches: (a) Density Functional Theory and, (b) Macroscopic Quantum Electrodynamics, which is particularly suited for describing long-range van der Waals interactions. Essentially, we determine and explain the model symmetry, distance and rotational dependencies on binding energies and spectral changes. The resultant spectral tuning is compared using both methods showing correspondence within the constraints placed by the different model assumptions. We envision that the application of macroscopic methods and structure/property relationships laid forward in this article will find use in fundamental supramolecular electronics.

Controlling Electronic Transitions in Fullerene van der Waals Aggregates via Supramolecular Assembly.

Morphologies crucially determine the optoelectronic properties of organic semiconductors. Therefore, hierarchical and supramolecular approaches have been developed for targeted design of supramolecular ensembles of organic semiconducting molecules and performance improvement of, e.g., organic solar cells (OSCs), organic light emitting diodes (OLEDs), and organic field-effect transistors (OFETs). We demonstrate how the photonic properties of fullerenes change with the formation of van der Waals aggregates. We identified supramolecular structures with broadly tunable absorption in the visible spectral range and demonstrated how to form aggregates with targeted visible (vis) absorption. To control supramolecular structure formation, we functionalized the C60-backbone with polar (bis-polyethylene glycol malonate-MPEG) tails, thus yielding an amphiphilic fullerene derivative that self-assembles at interfaces. Aggregates of systematically tuned size were obtained from concentrating MPEGC60 in stearic acid matrices, while different supramolecular geometries were provoked via different thin film preparation methods, namely spin-casting and Langmuir-Blodgett (LB) deposition from an air-water interface. We demonstrated that differences in molecular orientation in LB films (C2v type point group aggregates) and spin-casting (stochastic aggregates) lead to huge changes in electronic absorption spectra due to symmetry and orientation reasons. These differences in the supramolecular structures, causing the different photonic properties of spin-cast and LB films, could be identified by means of quantum chemical calculations. Employing supramolecular assembly, we propounded that molecular symmetry in fullerene aggregates is extremely important in controlling vis absorption to harvest photons efficiently, when mixed with a donor molecule, thus improving active layer design and performance of OSCs.