Two-dimensional self-assembly and phase behavior of an alkoxylated sandwich-type bisphthalocyanine and its phthalocyanine analogues at the liquid-solid interface

A Paradigm Shift from 2D to 3D: Surface Supramolecular Assemblies and Their Electronic Properties Explored by Scanning Tunneling Microscopy and Spectroscopy

Exploring supramolecular architectures at surfaces plays an increasingly important role in contemporary science, especially for molecular electronics. A paradigm of research interest in this context is shifting from 2D to 3D that is expanding from monolayer, bilayers, to multilayers. Taking advantage of its high-resolution insight into monolayers and a few layers, scanning tunneling microscopy/spectroscopy (STM/STS) turns out a powerful tool for analyzing such thin films on a solid surface. This review summarizes the representative efforts of STM/STS studies of layered supramolecular assemblies and their unique electronic properties, especially at the liquid-solid interface. The superiority of the 3D molecular networks at surfaces is elucidated and an outlook on the challenges that still lie ahead is provided. This review not only highlights the profound progress in 3D supramolecular assemblies but also provides researchers with unusual concepts to design surface supramolecular structures with increasing complexity and desired functionality.

Anomalously Large Spectral Shifts near the Quantum Tunnelling Limit in Plasmonic Rulers with Subatomic Resolution

The resonance wavelength of a coupled plasmonic system is extremely sensitive to the distance between its metallic surfaces, resulting in "plasmon rulers". We explore this behavior in the subnanometer regime using self-assembled monolayers of bis-phthalocyanine molecules in a nanoparticle-on-mirror (NPoM) construct. These allow unprecedented subangstrom control over spacer thickness via choice of metal center, in a gap-size regime at the quantum-mechanical limit of plasmonic enhancement. A dramatic shift in the coupled plasmon resonance is observed as the gap size is varied from 0.39 to 0.41 nm. Existing theoretical models are unable to account for the observed spectral tuning, which requires inclusion of the quantum-classical interface, emphasizing the need for new treatments of light at the subnanoscale.

Bottom-Up Self-Assembled Supramolecular Structures Built by STM at the Solid/Liquid Interface

One of the lines of research on organic devices is focused on their miniaturization to obtain denser and faster electronic circuits. The challenge is to build devices adding atom by atom or molecule by molecule until the desired structures are achieved. To do this job, techniques able to see and manipulate matter at this scale are needed. Scanning tunneling microscopy (STM) has been the selected technique by scientists to develop smart and functional unimolecular devices. This review article compiles the latest developments in this field giving examples of supramolecular systems monitored and fabricated at the molecular scale by bottom-up approaches using STM at the solid/liquid interface.

Templated bilayer self-assembly of fully conjugated π-expanded macrocyclic oligothiophenes complexed with fullerenes

Fully conjugated macrocyclic oligothiophenes exhibit a combination of highly attractive structural, optical and electronic properties, and multifunctional molecular thin film architectures thereof are envisioned. However, control over the self-assembly of such systems becomes increasingly challenging, the more complex the target structures are. Here we show a robust self-assembly based on hierarchical non-covalent interactions. A self-assembled monolayer of hydrogen-bonded trimesic acid at the interface between an organic solution and graphite provides host-sites for the epitaxial ordering of Saturn-like complexes of fullerenes with oligothiophene macrocycles in mono- and bilayers. STM tomography verifies the formation of the templated layers. Molecular dynamics simulations corroborate the conformational stability and assign the adsorption sites of the adlayers. Scanning tunnelling spectroscopy determines their rectification characteristics. Current–voltage characteristics reveal the modification of the rectifying properties of the macrocycles by the formation of donor–acceptor complexes in a densely packed all-self-assembled supramolecular nanostructure.

Controlling the self-assembly of oligothiophene complexes that are used in multi-functional thin films can be challenging. Here the authors show a hierarchy of non-covalent interactions for robust self-assembly that orders Saturn-like complexes of fullerenes with oligothiophene macrocycles.

Controlling porphyrin nanoarchitectures at solid interfaces

Two complementary examples of porphyrin nanoarchitectonics are presented. The fabrication of binary molecular monolayers using two different porphyrin molecules, tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)porphyrin (1) and tetrakis(4-pyridyl)porphyrin (2), by deposition in ultrahigh vacuum was demonstrated. Two unusual heteromolecular monolayer structures were observed, with one exhibiting good separation of 1 molecules within the monolayer. Also, a synthetic nanoarchitectonic approach was used to prepare self-assembled molecular nanowires at a mica substrate. The nanowires could be observed to grow using atomic force microscopy (AFM), and the network structures of the nanowires could be influenced by manipulation using the AFM probe tip.

Advances in supramolecularly assembled nanostructures of fullerenes and porphyrins at surfaces

The ‘bottom-up’ strategy is an attractive and promising approach for the construction of nanoarchitectures. Supramolecular assemblies based on non-covalent interactions have been explored in an attempt to control surface properties. In this minireview, we focus on advances made in the past three years in the field of scanning tunneling microscopy (STM) on supramolecular assembly and the function of porphyrins, phthalocyanines, and fullerenes, non-covalently bound on metal single crystal surfaces. Well-defined adlayers, consisting of porphyrin and phthalocyanine for the design of supramolecular nanoarchitectures, supramolecular traps of C 60 on hydrogen bond networks, a unique approach for controlling molecular orientation by a 1:1 supramolecularly assembled film consisting of C 60 and the related derivatives and metallooctaethylporphyrins, and nanoapplications of fullerenes, either induced by tip manipulation or driven by thermal fluctuations at surfaces, were clearly visualized by STM.

Towards supramolecular engineering of functional nanomaterials: pre-programming multi-component 2D self-assembly at solid-liquid interfaces

Materials with a pre-programmed order at the supramolecular level can be engineered with a sub-nanometer precision making use of reversible non- covalent interactions. The intrinsic ability of supramolecular materials to recognize and exchange their constituents makes them constitutionally dynamic materials. The tailoring of the materials properties relies on the full control over the self-assembly behavior of molecular modules exposing recognition sites and incorporating functional units. In this review we focus on three classes of weak-interactions to form complex 2D architectures starting from properly designed molecular modules: van der Waals, metallo-ligand and hydrogen bonding. Scanning tunneling microscopy studies will provide evidence with a sub-nanometer resolution, on the formation of responsive multicomponent architectures with controlled geometries and properties. Such endeavor enriches the scientist capability of generating more and more complex smart materials featuring controlled functions and unprecedented properties.

Rotational libration of a double-decker porphyrin visualized

Scanning tunneling microscopy has revealed the reorientation of one of the macrocyclic rings of the double-decker porphyrin complex [Ce(TPP-Fc)(C(22)OPP)] [TPP-Fc = 5-(4-(4-ferrocenylphenylethynyl)phenyl)-10,15,20-triphenylporphyrin; C(22)OPP = 5,10,15,20-tetrakis(4-docosyloxyphenyl)porphyrin] by 90 degrees between scans when the other ring is fixed on a surface. This libration was evidenced by monitoring the location of the appended ferrocene unit, which functioned as a molecular beacon signaling its position.

Molecular Dynamics in Two-Dimensional Supramolecular Systems Observed by STM

Since the invention of scanning tunneling microscopy (STM), 2D supramolecular architectures have been observed under various experimental conditions. The construction of these architectures arises from the balance between interactions at the medium-solid interface. This review summarizes molecular motion observed in 2D-supramolecular structures on surfaces using nanospace resolution STM. The observation of molecular motion on surfaces provides a visual understanding of intermolecular interactions, which are the major driving force behind supramolecular arrangement.

Two-dimensional molecular patterning by surface-enhanced Zn-porphyrin coordination

In this contribution, we show how zinc-5,10,15,20-meso-tetradodecylporphyrins (Zn-TDPs) self-assemble into stable organized arrays on the surface of graphite, thus positioning their metal center at regular distances from each other, creating a molecular pattern, while retaining the possibility to coordinate additional ligands. We also demonstrate that Zn-TDPs coordinated to 3-nitropyridine display a higher tendency to be adsorbed at the surface of highly oriented pyrolytic graphite (HOPG) than noncoordinated ones. In order to investigate the two-dimensional (2D) self-assembly of coordinated Zn-TDPs, solutions with different relative concentrations of 3-nitropyridine and Zn-TDP were prepared and deposited on the surface of HOPG. STM measurements at the liquid-solid interface reveal that the ratio of coordinated Zn-TDPs over noncoordinated Zn-TDPs is higher at the n-tetradecane/HOPG interface than in n-tetradecane solution. This enhanced binding of the axial ligand at the liquid/solid interface is likely related to the fact that physisorbed Zn-TDPs are better binding sites for nitropyridines.

Scanning tunneling microscopy observation of self-assembled monolayers of strapped porphyrins

In this paper, we reveal that the free-base and zinc strapped porphyrins possessing long alkyl chains, C 24OPP-HQ and Zn(C 24OPP-HQ), respectively, can be arranged on surfaces. We used scanning tunneling microscopy (STM) to observe alkyl-chain-assisted self-assembled monolayers (SAMs) of these strapped porphyrins at the solid-liquid interface. STM images revealed that the strapped benzene moiety was detectable on the porphyrin core: that is, the strapped porphyrins could be differentiated from nonstrapped analogues. We compared the population of the nonstrapped porphyrin (C 24OPP) and either of the strapped porphyrins C 24OPP-HQ or Zn(C 24OPP-HQ) in the mixed SAMs. We then confirmed that Zn(C 24OPP-HQ) is more favorably incorporated in the mixed SAMs than C 24OPP-HQ. From (1)H NMR spectroscopic and X-ray crystallographic analyses, we concluded that the factors increasing the population of Zn(C 24OPP-HQ) in the mixed SAMs are the enhanced rigidity of the porphyrin core by the zinc coordination and the flat structure of the porphyrin moiety in the saddle conformation. This study demonstrates that strapped porphyrins possessing long alkyl chains are available to arrange the functional modules on the surface via chemical modification on the strapped moiety.

Electric driven molecular switching of asymmetric tris(phthalocyaninato) lutetium triple-decker complex at the liquid/solid interface

Molecular structures are known to significantly impact the adsorption and assembling behavior of the adsorbates on surfaces. Precise control of the molecular orientation and ordering will enable us to tailor the physical and chemical properties of the molecular architectures. In this work, we present a strategy of attaching functional groups with dissimilar adsorption and assembling characteristics to the top and bottom phthalocyaninato moieties of a triple-decker complex, and orientational-dependent ordering of such molecules at the liquid/solid interface has been identified, which is attributed to the interaction of the intrinsic molecular dipole with the external electric field. In addition, isomerization of the noncentrosymmetric tris(phthalocyaninato) lutetium triple-decker complex has been revealed directly with STM and further confirmed by theoretical simulation. This approach provides a possible way for the preparation of organic films with switchable electronic and/or interface properties with external field.

Alkyl chain length dependence of the self-organized structure of alkyl-substituted phthalocyanines

The alkyl chain length on alkyl-substituted phthalocyanines (C(n)OPc) dependence of their self-organized structures was examined in this study. STM results indicated that the symmetry of ordered structures decreased as the alkyl chain became longer, with the exception of C(6)OPc, which preferentially formed a quasi-3-fold symmetrical structure. This could be explained by the fact that the C(n)OPc molecules are most likely to form densely packed structures. With C(n)OPc, when n = 4 to 10, the self-organized structures were dependent on the competition between how densely the molecules were arranged and how loose the intermolecular interaction energy was, caused by the formation of the densely packed structure. However, with C(n)OPc, when n = 10-18, the molecules tended to form densely packed structures by reducing the symmetry, even though the C(n)OPc molecules were distorted. When C(12)OPc and cobalt phthalocyanine were coadsorbed, the mixed system exhibited a four-fold symmetrical structure, which is rarely observed in C(12)OPc.

Homo- and heteroassemblies of lactim/lactam recognition patterns on highly ordered pyrolytic graphite: An STM investigation

The 2D assembly of phthalhydrazide 1 and aminopyrimidine 2 derivatives equipped with C16 and C8 alkyl chains, respectively, on highly ordered pyrolytic graphite (HOPG) was studied by scanning tunneling microscopy. Well-defined, rather complex surface layer patterns emerge resulting from a delicate balance of (self-) complementary (strong) hydrogen bonds and van der Waals force-driven ordering of the alkyl substituents on the HOPG surface. The four different compounds and their 1:1 mixtures yield seven different 2D structures. Phthalhydrazide offers in principle three tautomeric forms, with the lactim/lactam being the most stable. Depending on the solvent, different morphologies can be obtained. In one case, the special self-assembly of achiral 1a leads to a 2D chiral packing with the left- and right-hand motifs present in different domains. We assume that pure 1a is expressed in its lactim/lactam form, whereas in a 1:1 mixture with 2a it switches to the bislactam form. These features display a process of dynamic diversity generation through tautomerism resulting in different nanostructures in response to environmental parameters.

Arrays of double-decker porphyrins on highly oriented pyrolytic graphite

Three double-decker complexes of cerium(IV) were synthesized, which commonly have a 5,10,15,20-tetrakis(4-docosyloxyphenyl)porphyrin (C22OPP) moiety as one of the two tetrapyrrole rings. The three complexes-Ce(Pc)(C22OPP), Ce(C22OPP)2, and Ce(BPEPP)(C22OPP)-are distinguished by the other rings, which are Pc (=phthalocyanine), C22OPP, and BPEPP (=5,15-bis[4-(phenylethynyl)phenyl]porphyrin), respectively. The rate of inter-ring rotation of Ce(BPEPP)(C22OPP) was estimated to be approximately 3 s(-1) in solution at room temperature. These complexes assemble into ordered arrays at the interface of 1-phenyloctane and the highly oriented pyrolytic graphite surface, owing to the affinity of the long alkyl chains toward the surface, as revealed by means of scanning tunneling microscopy (STM) with molecular resolution. The shape of the upper ring is reflected in the STM image. Thus, Ce(Pc)(C22OPP), Ce(C22OPP)2, and Ce(BPEPP)(C22OPP) were observed as circular, square, and elliptic features, respectively. Possible molecular arrangements in the array of Ce(BPEPP)(C22OPP) are proposed by comparing STM images and molecular models. In the mixed arrays of Ce(BPEPP)(C22OPP) and H2(C22OPP), the double-decker complexes were distinguished by brighter features. Competitive adsorption experiments showed that the adsorption of Ce(BPEPP)(C22OPP) is less favorable than that of H2(C22OPP) by DeltaG(app) = 2.7 kJ mol(-1). Ce(BPEPP)(C22OPP) molecules appeared elliptic when placed within their own row, while they appeared isotropic when flanked by H2(C22OPP) molecules. Implications of the differences in the observed shapes to the inter-ring rotation are discussed.