Triptycene Tripods for the Formation of Highly Uniform and Densely Packed Self-Assembled Monolayers with Controlled Molecular Orientation

Characterize and Mediate Assembly of Triptycenes on Au(111) Surface

Herein, we report the assembly behavior of triptycenes with aldehyde (Trip-1) and amino (Trip-2) groups on pristine and iodine-passivated Au(111) surfaces by a combination of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and density functional theory (DFT) calculation. On Au(111) surface, Trip-1 forms long trimer chains and two-dimensional islands via aldehyde-aldehyde hydrogen bonding in one dimension and π-π stacking of adjacent benzene rings in the other dimension. In contrast, Trip-2 lies as individuals or in disorderly stacked islands. Trip-2 and Trip-1 can be mixed in an arbitrary ratio. And Trip-2 molecules disrupt the ordered self-assembly structure of Trip-1 due to the formation of stronger aldehyde-amino hydrogen bonding. DFT, XPS, and Raman spectra confirm the conformational difference of Trip-1 and -2, as well as the aldehyde-amino hydrogen bonding formation in Trip-1 and Trip-2 mixture. On the iodine-passivated Au(111) surface, Trip-1 forms single-molecule chains and a hexagonal closely packed structure due to iodine interlayer mediation. Trip-2 molecules disrupt the hexagonal closely packed structure of Trip-1.

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.

Alkoxylated Fluoranthene-Fused [3.3.3]Propellanes: Facile Film Formation against High π-Core Content

Solid-state assembling modes are as crucial as the chemical structures of single molecules for real applications. In this work, solid-state structures and phase-transition temperatures are investigated for a series of fluoranthene-fused [3.3.3]propellanes consisting of a rigid three-dimensional (3D) π-core and varying lengths of alkoxy groups. Compounds in this series with n-butoxy or longer alkoxy groups take an amorphous state at room temperature. In these molecules, rotatable biaryl-type bonds are not incorporated and high D3h molecular symmetry is retained. Therefore, π-fused [3.3.3]propellanes present a unique platform for amorphous molecular materials with low ratios of flexible alkoxy atoms to rigid π-core ones.

Rapid Access to Chiral and Tripodal Cavitands from β-Pinene

We report Pd-catalyzed cyclotrimerization of (+)-α-bromoenone, obtained from monoterpene β-pinene, into an enantiopure cyclotrimer. This C₃ symmetric compound has three bicyclo[3.1.1]heptane rings fused to its central benzene with each ring carrying a carbonyl group. The cyclotrimer undergoes diastereoselective threefold alkynylation with the lithium salts of five terminal alkynes (41-63 %, de=4-83 %). The addition enabled a rapid synthesis of a small library of novel chiral cavitands that, in shape, resemble a tripod stand. These molecular tripods include a tris-bicycloannelated benzene head attached to three alkyne legs twisted in one direction to form a nonpolar cavity with polar groups as feet. Tripods with methylpyridinium and methylisoquinolinium legs, respectively, form inclusion complexes with anti-inflammatory and chiral drugs (R)/(S)-ibuprofen and (R)/(S)-naproxen. The mode of binding shows drug molecules docked in the cavity of the host through ion-ion, cation-π, and C-H-π contacts that, in addition of desolvation, give rise to complexes having millimolar to micromolar stability in water. Our findings open the door to creating a myriad of enantiopure tripods with tunable functions that, in the future, might give novel chemosensors, catalysts or sequestering agents.

Substrate-Selective Intermolecular Interaction and the Molecular Self-Assemblies: 1,3,5-Tris(4-bromophenyl)benzene Molecules on the Ag(111) and Si(111) (√3 × √3)-Ag Surfaces

We report the formation processes of the self-assembled layer of 1,3,5-tris(4-bromophenyl)benzene (TBB) molecules on the Ag(111) and Si(111) (√3 × √3)-Ag surfaces by STM measurements and density functional theory (DFT) calculations. The self-assembled layers on the surfaces show characteristic structures controlled by the interplay between the intermolecular interaction and the molecule-substrate interaction. Through the cooperative interplay between the molecule-substrate interaction and the intermolecular halogen bond (XB), the periodic arrangement of TBB molecules appears on the Ag(111) surface. On the other hand, the two types of TBB arrangement appear on the Si(111) (√3 × √3)-Ag surface (phases 1 and 2). Phase 1 is the periodic arrangement of the TBB molecules and is derived from the cooperative interplay between the molecule-substrate interaction and the intermolecular van der Waals (vdW) interaction and the hydrogen bond (HB), and phase 2 is a random arrangement and is derived from the competitive interplay between the molecule-substrate interaction and the intermolecular XB and HB. Our present study specifies the role of the substrate in the molecular self-assembly of the substrate. Although the structure of the molecular self-assembly is controlled by the choice of the substrate, the cooperative interplay between the molecule-substrate interaction and the intermolecular interaction is necessary to realize the ideal periodic arrangement.

Triptycene-Based Molecular Rods for Langmuir-Blodgett Monolayers

Herein we introduce fully modular synthesis leading to three representative examples of rigid molecular rods that are intended to form sturdy monolayers on various surfaces. These molecules contain two triptycene units that are designed to interlock into a compact "double-decker" structure. Two of the three final products provided suitable crystals for X-ray diffraction (analyzed on synchrotron), allowing deeper insight into packing in the 3-D crystal lattice. The acidity of all three compounds were determined by capillary electrophoresis, and the pKa values ranged between 2.06-2.53. All three rigid rods easily formed Langmuir-Blodgett monolayers (LBMs) on the water-air interfaces, with the area per molecule equal to 55-59 Å2 /molecule, suggesting tight intermolecular packing. The thickness of all three films reached ∼19 Å after transfer to a gold (111) surface, meaning that individual molecules are tilted maximally 38° from the axis perpendicular to the surface. The structure of one of these films on a gold (111) surface was visualized by AFM. These geometrically unique molecules represent promising platforms with a wide scope of applicability in the supramolecular architecture.

Capturing the Trajectory of Metal-Ion-Cluster Formation: Stepwise Accumulation of Zn(II) Ions in a Robust Coordination Space Formed by a Rigid Tridentate Carboxylate Ligand

Organic ligand-directed synthesis of metal-ion clusters with a well-defined number and arrangement of metal ions is an important subject toward the development of functional inorganic-organic nanohybrids. Here we report the synthesis of multinuclear Zn-oxo clusters using a triptycene-based rigid ligand (H₃L) featuring three metal-coordination sites arranged in a triangular shape. Upon complexation of H₃L with zinc acetate dihydrate, a decanuclear Zn-oxo cluster and multinuclear Zn-oxo clusters with a smaller number of Zn(II) ions were formed as the final product and its intermediates, respectively. A comparison of the X-ray structure of the final product with those of the intermediates revealed the cluster-formation process, where four triptycene ligands preorganize to form a robust coordination space to which Zn(II) ions accumulate in a stepwise manner. This stepwise metal-ion accumulation, along with the formation of a large tetrahedral decanuclear Zn-oxo cluster, highlights the potential of ligand design using 1,8,13-substituted triptycenes for the development of various metal-ion clusters.

Triptycene Derivatives: From Their Synthesis to Their Unique Properties

Since the first preparation of triptycene, great progress has been made with respect to its synthesis and the understanding of its properties. Interest in triptycene-based systems is intense; in recent years, advances in the synthetic methodology and properties of new triptycenes have been reported by researchers from various fields of science. Here, an account of these new developments is given and placed in reference to earlier pivotal works that underpin the field. First, we discuss new approaches to the synthesis of new triptycenes. Progress in the regioselective synthesis of sterically demanding systems is discussed. The application of triptycenes in catalysis is also presented. Next, progress in the understanding of the relations between triptycene structures and their properties is discussed. The unique properties of triptycenes in the liquid and solid states are elaborated. Unique interactions, which involve triptycene molecular scaffolds, are presented. Molecular interactions within a triptycene unit, as well as between triptycenes or triptycenes and other molecules, are also evaluated. In particular, the summary of the synthesis and useful features will be helpful to researchers who are using triptycenes as building blocks in the chemical and materials sciences.

Introduction of Triptycene with a Particular Substitution Pattern into Polymer Chains Can Dramatically Improve the Structural and Rheological Properties

Although a large number of polymers that contain triptycene units in the main chains have been developed, no polymer design using 1,8-substituted triptycene has been reported to date. In this study, we investigated the properties of linear homo- and copolymers obtained by ring-opening polymerization of a triptycene monomer bearing a macrocyclic olefin linked at its 1,8-position and its copolymerization with cyclooctene, respectively. We found that the introduction of triptycene with this substitution pattern leads to nanoscale molecular ordering, thereby greatly improving the physical properties of the polymers. The key to this remarkable behavior of 1,8-substituted triptycene-containing polymers is the formation of a particular two-dimensional assembly of the triptycene units by nested hexagonal packing, which aligns one-dimensionally while folding the polymer chains into a well-defined layered structure. The polymer design using 1,8-substituted triptycene can be applied to other polymers, unless their main chain contains functional groups capable of a strong intermolecular interaction such as hydrogen bonding.

Electrochemical Potential-Driven High-Throughput Molecular Electronic and Spintronic Devices: From Molecules to Applications

Molecules are fascinating candidates for constructing tunable and electrically conducting devices by the assembly of either a single molecule or an ensemble of molecules between two electrical contacts followed by current-voltage (I-V) analysis, which is often termed "molecular electronics". Recently, there has been also an upsurge of interest in spin-based electronics or spintronics across the molecules, which offer additional scope to create ultrafast responsive devices with less power consumption and lower heat generation using the intrinsic spin property rather than electronic charge. Researchers have been exploring this idea of utilizing organic molecules, organometallics, coordination complexes, polymers, and biomolecules (proteins, enzymes, oligopeptides, DNA) in integrating molecular electronics and spintronics devices. Although several methods exist to prepare molecular thin-films on suitable electrodes, the electrochemical potential-driven technique has emerged as highly efficient. In this Review we describe recent advances in the electrochemical potential driven growth of nanometric various molecular films on technologically relevant substrates, including non-magnetic and magnetic electrodes to investigate the stimuli-responsive charge and spin transport phenomena.

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.

Changing the structural and physical properties of 3-arm star poly(δ-valerolactone)s by a branch-point design

The chemical structure of a branch point of star-shaped polymers has been considered to have a small influence on the physical properties of the entire polymer. Contrary to this general notion, here we show that a 3-arm star polymer, composed of three poly(δ-valerolactone) arms extended from one side of a triptycene branch point, exhibits a remarkably high complex viscosity, compared to the analogous star-shaped polymers with a branch point of a triptycene isomer or triphenylethane.

Thermodynamic Phase-like Transition Effect of Molecular Self-assembly

The technique of self-assembled monolayers (SAMs) is frequently applied for grafting functional groups or area-selective deposition of thin films on a material surface. The formation and quality of SAMs are fundamentally determined by thermodynamic data, which are difficult to measure with available experimental methods. This work quantitatively extracted thermodynamic parameters including ΔH°, ΔG°, and ΔS° during the SAMs construction process with an ultrasensitive resonant microcantilever as molecule-surface interactions real-time recording tool. By correlating the thermodynamic parameters with self-assembling temperatures, a new thermodynamic phase-like transition effect of molecular self-assembly has been first revealed. The sharp transition of the thermodynamic parameters defines the critical condition for SAMs formation. The thermodynamic data further provide optimized reaction conditions for constructing high-quality SAMs. The explored quantitative thermodynamic analysis method not only plays as criterion for SAM growth but also helps to fundamentally elucidate physicochemical mechanism of spontaneous self-assembly.

Triptycene-Based Luminescent Materials in Homoconjugated Charge-Transfer Systems: Synthesis, Electronic Structures, AIE Activity, and Highly Tunable Emissions

We have developed a new family of luminescent materials featuring through-space charge transfer from electron donors to acceptors that are electronically separated by triptycene. Most of these molecules are highly fluorescent, and modulation of their emissions was achieved by tuning the electron-accepting strength in a range from the weak triptycene acceptor over triarylborane (BMes) to strongly accepting naphthalimide (Npa) moieties. Pz-Pz shows an aggregation-induced emission in aggregates and in the solid state coupled with a highly red-shifted broad emission (ca. 160 nm) of the excimer, indicating that phenothiazine (Pz) also plays a vital role in the emission responses as an electron donor. This work may help develop new approaches to photophysical mechanism based on the rigid, homoconjugated, and structurally unusual 3D triptycene scaffold.

Final-State Simulations of Core-Level Binding Energies at Metal-Organic Hybrid Interfaces: Artifacts Caused by Spurious Collective Electrostatic Effects

Core-level energies are frequently calculated to explain the X-ray photoelectron spectra of metal-organic hybrid interfaces. The current paper describes how such simulations can be flawed when modeling interfaces between physisorbed organic molecules and metals. The problem occurs when applying periodic boundary conditions to correctly describe extended interfaces and simultaneously considering core hole excitations in the framework of a final-state approach to account for screening effects. Since the core hole is generated in every unit cell, an artificial dipole layer is formed. In this work, we study methane on an Al(100) surface as a deliberately chosen model system for hybrid interfaces to evaluate the impact of this computational artifact. We show that changing the supercell size leads to artificial shifts in the calculated core-level energies that can be well beyond 1 eV for small cells. The same applies to atoms at comparably large distances from the substrate, encountered, for example, in extended, upright-standing adsorbate molecules. We also argue that the calculated work function change due to a core-level excitation can serve as an indication for the occurrence of such an artifact and discuss possible remedies for the problem.

The Potential of X-ray Photoelectron Spectroscopy for Determining Interface Dipoles of Self-Assembled Monolayers

In the current manuscript we assess to what extent X-ray photoelectron spectroscopy (XPS) is a suitable tool for probing the dipoles formed at interfaces between self-assembled monolayers and metal substrates. To that aim, we perform dispersion-corrected, slab-type band-structure calculations on a number of biphenyl-based systems bonded to an Au(111) surface via different docking groups. In addition to changing the docking chemistry (and the associated interface dipoles), the impacts of polar tail group substituents and varying dipole densities are also investigated. We find that for densely packed monolayers the shifts of the peak positions of the simulated XP spectra are a direct measure for the interface dipoles. In the absence of polar tail group substituents they also directly correlate with adsorption-induced work function changes. At reduced dipole densities this correlation deteriorates, as work function measurements probe the difference between the Fermi level of the substrate and the electrostatic energy far above the interface, while core level shifts are determined by the local electrostatic energy in the region of the atom from which the photoelectron is excited.

Tetrapodal Diazatriptycene Enforces Orthogonal Orientation in Self-Assembled Monolayers

Conformationally rigid multipodal molecules should control the orientation and packing density of functional head groups upon self-assembly on solid supports. Common tripods frequently fail in this regard because of inhomogeneous bonding configuration and stochastic orientation. These issues are circumvented by a suitable tetrapodal diazatriptycene moiety, bearing four thiol-anchoring groups, as demonstrated in the present study. Such molecules form well-defined self-assembled monolayers (SAMs) on Au(111) substrates, whereby the tetrapodal scaffold enforces a nearly upright orientation of the terminal head group with respect to the substrate, with at least three of the four anchoring groups providing thiolate-like covalent attachment to the surface. Functionalization by condensation chemistry allows a large variety of functional head groups to be introduced to the tetrapod, paving the path toward advanced surface engineering and sensor fabrication.

Carbazole-Based Tetrapodal Anchor Groups for Gold Surfaces: Synthesis and Conductance Properties

As the field of molecular-scale electronics matures and the prospect of devices incorporating molecular wires becomes more feasible, it is necessary to progress from the simple anchor groups used in fundamental conductance studies to more elaborate anchors designed with device stability in mind. This study presents a series of oligo(phenylene-ethynylene) wires with one tetrapodal anchor and a phenyl or pyridyl head group. The new anchors are designed to bind strongly to gold surfaces without disrupting the conductance pathway of the wires. Conductive probe atomic force microscopy (cAFM) was used to determine the conductance of self-assembled monolayers (SAMs) of the wires in Au-SAM-Pt and Au-SAM-graphene junctions, from which the conductance per molecule was derived. For tolane-type wires, mean conductances per molecule of up to 10-4.37  G0 (Pt) and 10-3.78  G0 (graphene) were measured, despite limited electronic coupling to the Au electrode, demonstrating the potential of this approach. Computational studies of the surface binding geometry and transport properties rationalise and support the experimental results.

Multidentate Anchors for Surface Functionalization

The bottom-up functionalization of solid surfaces shows increasing importance for a wide range of interdisciplinary applications. Multidentate anchors with more than two contact points can bind to solid surfaces with strong chemisorption, well-defined upright configuration, and tailored functionality. The surface functionalization using multidentate anchors with three (tripodal), four (quadripodal), or more binding points is summarized herein, with a focus on those beyond classical tripodal anchors. In particular, the molecular design on how to achieve multisite interaction between anchor and substrate and the introduction of functional groups to thin films are discussed.

Optically active triptycenes containing hexa-peri-hexabenzocoronene units

We have successfully generated both left- and right-handed circularly polarized light using crystals prepared from a racemic triptycene containing hexa-peri-hexabenzocoronene units.