Electron Transfer Dynamics and Structural Effects in Benzonitrile Monolayers with Tuned Dipole Moments by Differently Positioned Fluorine Atoms.

To understand the influence of the molecular dipole moment on the electron transfer (ET) dynamics across the molecular framework, two series of differently fluorinated, benzonitrile-based self-assembled monolayers (SAMs) bound to Au(111) by either thiolate or selenolate anchoring groups were investigated. Within each series, the molecular structures were the same with the exception of the positions of two fluorine atoms affecting the dipole moment of the SAM-forming molecules. The SAMs exhibited a homogeneous anchoring to the substrate, nearly upright molecular orientations, and the outer interface comprised of the terminal nitrile groups. The ET dynamics was studied by resonant Auger electron spectroscopy in the framework of the core-hole clock method. Resonance excitation of the nitrile group unequivocally ensured an ET pathway from the tail group to the substrate. As only one of the π* orbitals of this group is hybridized with the π* system of the adjacent phenyl ring, two different ET times could be determined depending on the primary excited orbital being either localized at the nitrile group or delocalized over the entire benzonitrile moiety. The latter pathway turned out to be much more efficient, with the characteristic ET times being a factor 2.5-3 shorter than those for the localized orbital. The dynamic ET properties of the analogous thiolate- and selenolate-based adsorbates were found to be nearly identical. Finally and most importantly, these properties were found to be unaffected by the different patterns of the fluorine substitution used in the present study, thus showing no influence of the molecular dipole moment.

Concentration-Dependent Seeding as a Strategy for Fabrication of Densely Packed Surface-Mounted Metal-Organic Frameworks (SURMOF) Layers.

The layer-by-layer (LbL) method is a well-established method for the growth of surface-attached metal-organic frameworks (SURMOFs). Various experimental parameters, such as surface functionalization or temperature, have been identified as essential in the past. In this study, inspired by these recent insights regarding the LbL SURMOF growth mechanism, the impact of reactant solutions concentration on LbL growth of the Cu2 (F4 bdc)2 (dabco) SURMOF (F4 bdc2- =tetrafluorobenzene-1,4-dicarboxylate and dabco=1,4-diazabicyclo-[2.2.2]octane) in situ by using quartz-crystal microbalance and ex situ with a combination of spectroscopic, diffraction and microscopy techniques was investigated. It was found that number, size, and morphology of MOF crystallites are strongly influenced by the reagent concentration. By adjusting the interplay of nucleation and growth, we were able to produce densely packed, yet thin films, which are highly desired for a variety of SURMOF applications.

Liquid-Phase Epitaxial Growth of Highly Oriented and Multivariate Surface-Attached Metal-Organic Frameworks.

Multivariate metal-organic frameworks (MTV-MOFs) incorporating multiple chemical functionalities within single-phase crystalline materials show superior properties that arise from synergistic effects. Herein, we report an efficient and versatile method for the growth of highly oriented multivariate surface-attached MOFs (MTV-SURMOFs) by the combination of the liquid-epitaxial growth method (LPE) and the mixed-linker strategy. Twenty-six MTV-SURMOFs of the [M2L2P] type with a maximum of five different dicarboxylate linkers (L) were deposited onto suitably functionalized surfaces. Systematic studies by infrared reflection absorption (IRRA) spectroscopy and surface XRD provide evidence for the formation of highly oriented MTV-SURMOFs. Interestingly, the pKa's of the dicarboxylate linkers play a crucial role for the orientational quality of the MTV-SURMOFs. In addition, benzene uptake experiments showed that the MTV-SURMOFs exhibit up to 2.6 times higher adsorption capacity as compared to the single-linker SURMOFs, demonstrating the synergistic effects in these surface systems.

Triptycene-terminated thiolate and selenolate monolayers on Au(111).

To study the implications of highly space-demanding organic moieties on the properties of self-assembled monolayers (SAMs), triptycyl thiolates and selenolates with and without methylene spacers on Au(111) surfaces were comprehensively studied using ultra-high vacuum infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy and thermal desorption spectroscopy. Due to packing effects, the molecules in all monolayers are substantially tilted. In the presence of a methylene spacer the tilt is slightly less pronounced. The selenolate monolayers exhibit smaller defect densities and therefore are more densely packed than their thiolate analogues. The Se-Au binding energy in the investigated SAMs was found to be higher than the S-Au binding energy.

Self-Assembled Monolayers of Pseudo-C2v-Symmetric, Low-Band-Gap Areneoxazolethiolates on Gold Surfaces.

A series of three homologous arene[2,3-d]-oxazole-2-thiols (benzoxazole-2-thiol (BOxSH), naphthaleneoxazole-2-thiol (NOxSH), and anthraceneoxazole-2-thiol (AOxSH)) were deposited onto Au(111) to obtain surfaces suitable as injection layers for organic electronics. The guiding idea was that the increasingly extended conjugated system would lower the band gap of the films while the introduction of the annulated heteroaromatic ring would provide the opportunity for pseudosymmetric attachment of the sulfur anchor, what should lower the conformational freedom of the system. In fact, the annulation of the oxazole ring lowers the optical band gaps of the parent compounds to 3.1-4.0 eV, depending on the number of benzene rings. To characterize the respective monolayers, a variety of spectroscopic techniques such as ellipsometry, infrared reflection-absorption spectroscopy, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy have been utilized. The monolayers of BOxS exhibit a lower film quality than those of NOxS and AOxS, with enhanced molecular density and more upright molecular orientation with increasing molecular length. Infrared spectroscopy suggests that the nitrogen atoms of the oxazole rings are located more closely to the Au(111) surface than the oxygen atoms, although no hints for an electronic interaction between the N atoms and the gold surface could be found. This preferred orientation could be tentatively traced to packing effects, solving a conundrum of the literature.

Self-Assembled Monolayers of Perfluoroanthracenylaminoalkane Thiolates on Gold as Potential Electron Injection Layers.

As a material with relatively small band gap and low lying valence orbitals, perfluoroanthracene (PFA) is of interest for the modification of electrode surfaces, for example, as charge injection layers for n-type organic semiconductors. To covalently attach PFA in the form of self-assembled monolayers (SAMs), we developed a synthesis of derivatives with a sulfur termination, linked to the 2-position of the PFA moieties by an -NH- group and a short alkane chain with two and three methylene groups, respectively. Spectroscopic characterization of the SAMs reveals that the molecules adopt an almost upright orientation on the gold surface, with the packing density mostly determined by the steric demands of the PFA units. The number of the methylene groups in the -NH-alkyl linker has only a minor impact on the SAM structure because of the nonsymmetric attachment of the PFA units, which permits the compensation of the orientational constraints imposed by the bending potential. The investigated SAMs alter the work function of gold by +(0.59-0.64) eV, suggesting comparably strong depolarization effects, affecting the extent of the work function modification.

Insight into the Oriented Growth of Surface-Attached Metal-Organic Frameworks: Surface Functionality, Deposition Temperature, and First Layer Order.

The layer-by-layer growth of a surface-attached metal-organic framework (SURMOF), [Cu2(F4bdc)2(dabco)] (F4bdc = tetrafluorobenzene-1,4-dicarboxylate and dabco = 1,4-diazabicyclo-[2.2.2]octane), on carboxylate- and pyridine-terminated surfaces has been investigated by various surface characterization techniques. Particular attention was paid to the dependency of the crystal orientation and morphology on surface functionality, deposition temperature, and first layer order. For the fully oriented deposition of SURMOFs, not only a suitable surface chemistry but also the appropriate temperature has to be chosen. In the case of carboxylate-terminated surfaces, the expected [100] oriented [Cu2(F4bdc)2(dabco)] SURMOF can be achieved at low temperatures (5 °C). In contrast, the predicted [001] oriented SURMOF on pyridine-terminated surface was obtained only at high deposition temperatures (60 °C). Interestingly, we found that rearrangement processes in the very first layer determine the final orientation (distribution) of the growing crystals. These effects could be explained by a surprisingly hampered substitution at the apical position of the Cu2-paddle wheel units, which requires significant thermal activation, as supported by quantum-chemical calculations.

Thiolate versus Selenolate: Structure, Stability, and Charge Transfer Properties.

Selenolate is considered as an alternative to thiolate to serve as a headgroup mediating the formation of self-assembled monolayers (SAMs) on coinage metal substrates. There are, however, ongoing vivid discussions regarding the advantages and disadvantages of these anchor groups, regarding, in particular, the energetics of the headgroup-substrate interface and their efficiency in terms of charge transport/transfer. Here we introduce a well-defined model system of 6-cyanonaphthalene-2-thiolate and -selenolate SAMs on Au(111) to resolve these controversies. The exact structural arrangements in both types of SAMs are somewhat different, suggesting a better SAM-building ability in the case of selenolates. At the same time, both types of SAMs have similar packing densities and molecular orientations. This permitted reliable competitive exchange and ion-beam-induced desorption experiments which provided unequivocal evidence for a stronger bonding of selenolates to the substrate as compared to the thiolates. Regardless of this difference, the dynamic charge transfer properties of the thiolate- and selenolate-based adsorbates were found to be nearly identical, as determined by the core-hole-clock approach, which is explained by a redistribution of electron density along the molecular framework, compensating the difference in the substrate-headgroup bond strength.

Purification of ethanol for highly sensitive self-assembly experiments.

Ethanol is the preferred solvent for the formation of self-assembled monolayers (SAMs) of thiolates on gold. By applying a thin film sensor system, we could demonstrate that even the best commercial qualities of ethanol contain surface-active contaminants, which can compete with the desired thiolates for surface sites. Here we present that gold nanoparticles deposited onto zeolite X can be used to remove these contaminants by chemisorption. This nanoparticle-impregnated zeolite does not only show high capacities for surface-active contaminants, such as thiols, but can be fully regenerated via a simple pyrolysis protocol.

A 'dual click' strategy for the fabrication of bioselective, glycosylated self-assembled monolayers as glycocalyx models.

Solid surfaces decorated with specific saccharide patterns can serve as a model for the chemically and structurally highly complex glycocalyx of eukaryotic cells. Here we present an approach based on self-assembled monolayers on gold, which are built up in a three-step manner to provide a solid basis, a biorepulsive oligoethylene glycol part, and a specific carbohydrate terminus in a modular way. Of the different reaction sequences, the one with two consecutive 'click reactions' (the copper(i)-catalysed 1,3-dipolar cycloaddition of alkynes with azides and the thiourea-bridging of isothiocyanates with amines) directly 'on SAM' results in the densest layers, as demonstrated by infrared absorption reflection spectroscopy and ellipsometry. As a 'real life' test, the surfaces obtained this way were used for bacterial adhesion experiments. Here the biorepulsivity of the middle part of the SAMs as well as specific binding to the carbohydrate termini could be clearly demonstrated.

Dynamic double lattice of 1-adamantaneselenolate self-assembled monolayers on Au{111}.

We report a complex, dynamic double lattice for 1-adamantaneselenolate monolayers on Au{111}. Two lattices coexist, revealing two different binding modes for selenols on gold: molecules at bridge sites have lower conductance than molecules at three-fold hollow sites. The monolayer is dynamic, with molecules switching reversibly between the two site-dependent conductance states. Monolayer dynamics enable adsorbed molecules to reorganize according to the underlying gold electronic structure over long distances, which facilitates emergence of the self-organized rows of dimers. The low-conductance molecules assume a (7 × 7) all-bridge configuration, similar to the analogous 1-adamantanethiolate monolayers on Au{111}. The high-conductance molecules self-organize upon mild annealing into distinctive rows of dimers with long-range order, described by a (6√5 × 6√5)R15° unit cell.

A highly ordered, aromatic bidentate self-assembled monolayer on Au(111): a combined experimental and theoretical study.

Self assembled monolayers (SAMs) made from an aromatic organodithiol, 2-mercaptomethylbenzenethiol (C(6)H(4)SHCH(2)SH) on Au(111) have been investigated in the context of a combined experimental and theoretical approach. The SAMs prepared by immersion of an Au-substrate in corresponding ethanolic solutions were characterized using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS) and thermal desorption spectroscopy (TDS). Adapted from the experimentally obtained unit cell of the SAM, density functional theory (DFT) calculations were applied to get a deeper insight into the structure of these dithiolate based SAMs. On the basis of the experimental and theoretical findings we provide a detailed structural model for this aromatic SAM consisting of the phenyl-group rigidly anchored to the substrate by two thiolate-bonds.

Structural characterization of self-assembled monolayers of pyridine-terminated thiolates on gold.

Self-assembled monolayers (SAMs) fabricated on Au(111) substrates from a homologous series of pyridine-terminated organothiols have been investigated using ultra high vacuum infrared reflection adsorption spectroscopy (UHV-IRRAS), X-ray photoelectron spectroscopy (XPS), scanning tunnelling microscopy (STM) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. A total of 4 different pyridine-based organothiols have been investigated, consisting of a pyridine unit, one or two phenyl units, a spacer of between one and three methylene units and, finally, a thiol unit. For all pyridine-terminated thiols the immersion of Au-substrates in the corresponding ethanolic solutions was found to result in the formation of highly ordered and densely packed SAMs. For an even number of the methylene spacers between the SH group and the aromatic moieties, the SAM unit-cell is rather large, (5sq.rt(3) x 3)rect, whereas in case of an odd number of methylene units a smaller unit cell is adopted, (2sq.rt(3) x sq.rt(3))R30 degrees. The tilt angle of the molecules amounts to 15 degrees . In contrast to expectation, the pyridine-terminated organic surfaces exposed by the corresponding SAMs showed a surprisingly strong resistance with regard to protonation.