Kinetic and Thermodynamic Control in Porphyrin and Phthalocyanine Self-Assembled Monolayers

Effect of aromatic substituents on the H-bonded assembly of diketopyrrolopyrroles at solid-liquid interfaces

Hydrogen-bonded (H-bonded) self-assembly is a suitable approach for tailoring the solid-state packing and properties of organic semiconductors. Here we studied the H-bonded self-assembly of an important class of organic semiconductors, diketopyrrolopyrrole (DPP) derivatives, diselenophenylDPP (DSeDPP), dithiazolylDPP (DTzDPP), and dithienothiophenylDPP (DTTDPP), at solid-liquid interfaces using scanning tunneling microscopy (STM) and density functional theory (DFT). At the 1-octanoic acid/highly ordered pyrolytic graphite (HOPG) interface, DSeDPP and DTzDPP either co-assemble with the solvent via H-bonding between lactam and carboxyl groups or form homoassemblies through H-bonding between the lactam groups. However, DTTDPP forms two different homoassemblies involving H-bonding between lactam groups or weak H-bonding between the lactam group and the heteroaromatic ring. Enthalpic factors for the formation of homoassemblies and co-assemblies are investigated by evaluating the inter- and intramolecular interactions in the self-assembled lattices using DFT. A homoassembly with a twisted geometry of molecules with intermolecular π-interactions is only observed for DSeDPP. The absence of homoassembly with the twisted geometry of DTzDPP is attributed to the higher strain energy required to acquire out-of-plane twists in this molecule. Our study shows the profound effects aromatic substituents can impart in the supramolecular assembly of DPP molecules, which influences film morphology and hence its properties (e.g. charge transport).

Photocatalytic H2 Generation: Controlled and Optimized Dispersion of Single Atom Co-Catalysts Based on Pt-TCPP Planar Adsorption on TiO2

When using single atoms (SAs) as a co-catalyst in photocatalytic H2 generation, achieving a well-dispersed, evenly distributed and adjustable SA surface density on a semiconductor surface is a challenging task. In the present work we use the planar adsorption of tetrakis-(4-carboxyphenyl)-porphyrin (TCPP) and its platinum coordinated analogue, Pt-TCPP, onto anatase TiO2 surfaces to establish a spatially controlled decoration of SAs. We show that the surface Pt SA density can be very well controlled by co-adsorption of Pt-TCPP and TCPP in the planar monolayer regime, and by adjusting the Pt-TCPP to TCPP ratio a desired well dispersed surface density of SAs up to 2.6×105  atoms μm-2 can be established (which is the most effective Pt SA loading for photocatalysis). This distribution and the SA state are maintained after a thermal treatment in air, and an optimized SA density as well as a most active form of Pt for photocatalytic H2 evolution can be established and maintained.

STM studies on porphyrins and phthalocyanines at the liquid/solid interface for molecular-scale electronics

Porphyrins and phthalocyanines are promising candidates for single-molecule electronics. Among the many characterization tools, scanning tunneling microscopy (STM) represents a very powerful one to gain insight into the electronic properties at the molecular level, by correlating the charge transport behaviours of π-conjugated molecules with ultrahigh resolution imaging. In view of the sophistication of molecular self-assembly in the presence of a solution phase, in this frontier, we focus on STM studies on porphyrins and phthalocyanines at the liquid/solid interface, placing emphasis on the electronic and magnetic properties, as well as the switching behaviour of surface-confined or surface-anchored molecules. Furthermore, we have also addressed the topics of potential that can be exploited in this area.

Optimization of the adsorption and desorption processes of nickel octaethylporphyrin in carbon-based adsorbents

Despite being little explored for petroporphyrins recovery from oils and bituminous shales, adsorption and desorption processes can be feasible alternatives to obtain a similar synthetic material, and to characterize their original organic materials. Experimental designs were used to analyze the effects of qualitative (e.g., type of adsorbent, solvent, and diluent) and quantitative (e.g., temperature and solid/liquid ratio) variables on the adsorptive and desorptive performance regarding nickel octaethylporphyrin (Ni-OEP) removal using carbon-based adsorbents. The evaluation variables, adsorption capacity (qe ) and desorption percentage (%desorption ) were optimized by means of the Differential Evolution algorithm. The most efficient adsorbent for removing/recovery Ni-OEP was activated-carbon coconut shell, in which dispersive π-π type and acid-base interactions were likely formed. The highest values of qe and %desorption were obtained using toluene as solvent, chloroform as diluent, 293 K as temperature, and 0.5 mg.mL-1 as solid/liquid ratio for adsorption, and a higher temperature (323 K) and lower solid/liquid ratio (0.2 mg.mL-1) for desorption. The optimization process resulted in qe of 6.91 mg.g-1 and %desorption of 35.2%. In the adsorption-desorption cycles, approximately 77% of the adsorbed porphyrins were recovered. The results demonstrated the potential of carbon-based materials as adsorbent materials for obtaining porphyrin compounds from oils and bituminous shales.

Tandem Desulfurization/C-C Coupling Reaction of Tetrathienylbenzenes on Cu(111): Synthesis of Pentacene and an Exotic Ladder Polymer

Surface-confined reactions represent a powerful approach for the precise synthesis of low-dimensional organic materials. A complete understanding of the pathways of surface reactions would enable the rational synthesis of a wide range of molecules and polymers. Here, we report different reaction pathways of tetrathienylbenzene (T1TB) and its extended congener tetrakis(dithienyl)benzene (T2TB) on Cu(111), investigated using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. Both T1TB and T2TB undergo desulfurization when deposited on Cu(111) at room temperature. Deposition of T1TB at 453 K yields pentacene through desulfurization, hydrogen transfer, and a cascade of intramolecular cyclization. In contrast, for T2TB the intramolecular cyclization stops at anthracene and the following intermolecular C-C coupling produces a conjugated ladder polymer. We show that tandem desulfurization/C-C coupling provides a versatile approach for growing carbon-based nanostructures on metal surfaces.

Host-guest chemistry under confinement: peeking at early self-assembly events

Nanoscopic lateral confinement created on a graphite surface enabled the study of embryonic stages of molecular self-assembly on solid surfaces using scanning tunneling microscopy performed at the solution/solid interface.

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.

Quantifying reversible nitrogenous ligand binding to Co(ii) porphyrin receptors at the solution/solid interface and in solution

We present a quantitative study comparing the binding of 4-methoxypyridine, MeOPy, ligand to Co(ii)octaethylporphyrin, CoOEP, at the phenyloctane/HOPG interface and in toluene solution. Scanning tunneling microscopy (STM) was used to study the ligand binding to the porphyrin receptors adsorbed on graphite. Electronic spectroscopy was employed for examining this process in fluid solution. The on surface coordination reaction was completely reversible and followed a simple Langmuir adsorption isotherm. Ligand affinities (or ΔG) for the binding processes in the two different chemical environments were determined from the respective equilibrium constants. The free energy value of -13.0 ± 0.3 kJ mol^-1 for the ligation reaction of MeOPy to CoOEP at the solution/HOPG interface is less negative than the ΔG for cobalt porphyrin complexed to the ligand in solution, -16.8 ± 0.2 kJ mol^-1. This result indicates that the MeOPy-CoOEP complex is more stable in solution than on the surface. Additional thermodynamic values for the formation of the surface ligated species (ΔH_c = -50 kJ mol^-1 and ΔS_c = -120 J mol^-1) were extracted from temperature dependent STM measurements. Density functional computational methods were also employed to explore the energetics of both the solution and surface reactions. At high concentrations of MeOPy the monolayer was observed to be stripped from the surface. Computational results indicate that this is not because of a reduction in adsorption energy of the MeOPy-CoOEP complex. Nearest neighbor analysis of the MeOPy-CoOEP in the STM images revealed positive cooperative ligand binding behavior. Our studies bring new insights to the general principles of affinity and cooperativity in the ligand-receptor interactions at the solution/solid interface. Future applications of STM will pave the way for new strategies designing highly functional multisite receptor systems for sensing, catalysis, and pharmacological applications.

Quantifying the Ultraslow Desorption Kinetics of 2,6-Naphthalenedicarboxylic Acid Monolayers at Liquid-Solid Interfaces

Kinetic effects in monolayer self-assembly at liquid-solid interfaces are not well explored but can provide unique insights. We use variable-temperature scanning tunneling microscopy (STM) to quantify the desorption kinetics of 2,6-naphthalenedicarboxylic acid (NDA) monolayers at nonanoic acid-graphite interfaces. Quantitative tracking of the decline of molecular coverages by STM between 57.5 and 65.0 °C unveiled single-exponential decays over the course of days. An Arrhenius plot of rate constants derived from fits results in a surprisingly high energy barrier of 208 kJ mol^-1 that strongly contrasts with the desorption energy of 16.4 kJ mol^-1 with respect to solution as determined from a Born-Haber cycle. This vast discrepancy indicates a high-energy transition state. Expanding these studies to further systems is the key to pinpointing the molecular origin of the remarkably large NDA desorption barrier.

Role of redox-active axial ligands of metal porphyrins adsorbed at solid-liquid interfaces in a liquid-STM setup

In a liquid-STM setup environment, the redox behavior of manganese porphyrins was studied at various solid-liquid interfaces. In the presence of a solution of Mn(III)Cl porphyrins in 1-phenyloctane, which was placed at a conductive surface, large and constant additional currents relative to a set tunneling current were observed, which varied with the magnitude of the applied bias voltage. These currents occurred regardless of the type of surface (HOPG or Au(111)) or tip material (PtIr, Au or W). The additional currents were ascribed to the occurrence of redox reactions in which chloride is oxidized to chlorine and the Mn(III) center of the porphyrin moiety is reduced to Mn(II). The resulting Mn(II) porphyrin products were identified by UV-vis analysis of the liquid phase. For solutions of Mn(III) porphyrins with non-redox active acetate instead of chloride axial ligands, the currents remained absent.

Single molecule level studies of reversible ligand binding to metal porphyrins at the solution/solid interface

Ligands bind reversibly to metal porphyrins in processes such as molecular recognition, electron transport and catalysis. These chemically relevant processes are ubiquitous in biology and are important in technological applications. In this article, we focus on the current advances in ligand binding to metal porphyrin receptors noncovalently bound at the solution/solid interface. In particular, we restrict ourselves to studies at the single molecule level. Dynamics of the binding/dissociation process can be monitored by scanning tunneling microscopy (STM) and can yield both qualitative and quantitative information about ligand binding affinity and the energetics that define a particular ligation reaction. Molecular and time dependent imaging can establish whether the process under study is at equilibrium. Ligand-concentration-dependent studies have been used to determine adsorption isotherms and thermodynamic data for processes occurring at the solution/solid interface. In several binding reactions, the solid support acted as an electron-donating fifth coordination site, thereby significantly changing the metal porphyrin receptor’s affinity for exogenous ligands. Supporting calculations provide insight into the metalloporphyrin/support and ligand–metalloporphyrin/support interactions and their energetics.

Binding and electronic level alignment of π-conjugated systems on metals

We review the binding and energy level alignment of π-conjugated systems on metals, a field which during the last two decades has seen tremendous progress both in terms of experimental characterization as well as in the depth of theoretical understanding. Precise measurements of vertical adsorption distances and the electronic structure together with ab initio calculations have shown that most of the molecular systems have to be considered as intermediate cases between weak physisorption and strong chemisorption. In this regime, the subtle interplay of different effects such as covalent bonding, charge transfer, electrostatic and van der Waals interactions yields a complex situation with different adsorption mechanisms. In order to establish a better understanding of the binding and the electronic level alignment of π-conjugated molecules on metals, we provide an up-to-date overview of the literature, explain the fundamental concepts as well as the experimental techniques and discuss typical case studies. Thereby, we relate the geometric with the electronic structure in a consistent picture and cover the entire range from weak to strong coupling.

Alkynyl Linkers as a Design Tool to Gain Control over the Self-Assembly of Meso-Substituted Porphyrins on HOPG

Self-assembled monolayers (SAMs) fall generally into two broad categories: those that are covalently bound either to the surface or to each other and those that rely on weaker forces such as hydrogen bonding or van der Waals forces. The engineering of the structure of SAMs formed from weaker forces is an exciting and complex field that often utilizes long alkane substituents bound to core moieties. The core provides the unique optical, electronic, or catalytic property desired, while the interdigitation of the alkane chains provides the means for creating well-regulated patterns of cores on the substrate. This design technique sometimes fails because some of the alkane substituents remain extended into solution rather than become interdigitated on the substrate. One contributor to this is steric hindrance between elements of the core and of the alkane chain. It is shown that the use of an alkyne linker between the core and the alkane chain can, in the case of meso-substituted porphyrins, significantly reduce this steric barrier and allow more stable and predictable surface structures to form. In particular, 5,15-bis(1-octynyl)porphyrin and 5,15-bis(1-tetradecynyl)porphyrin are shown to form significantly more stable SAMs than their alkane-linked counterparts. Scanning tunneling microscopy is used to provide detailed surface structures. Temperature and solution concentration dependence of the surface coverage is also reported. Density functional theory (DFT) is used to determine the energetic effects associated with alkane substitution at both the meso and β positions and the beneficial energetic effects of the alkyne linker.

Restructuring of Porphyrin Networks Driven by Self-Assembled Octanoic Acid Monolayer on Au(111)

We report time-dependent surface restructuring of bicomponent domain structures of 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine (H₂OEP) and cobalt(II) 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine (CoOEP) (H₂/Co OEP) driven by self-assembled octanoic acid on the surface of Au(111). Scanning tunneling microscopy (STM) visualized molecular adsorption/desorption and rearrangement of supramolecular architectures in real-time in a solution of octanoic acid. We found that unique domain structures emerged at an initial state guided by adsorbed octanoic acid on the Au surface. Moreover, the desorption of octanoic acid occurred in solution, leading to the surface restructuring of porphyrin molecular networks. This molecular evidence is well-manifested in the time-dependent phase transitions, monitored by in situ STM.

Silk fibroin hydrogels for potential applications in photodynamic therapy

In this study, we prepared translucid hydrogels with different concentrations of silk fibroin, extracted from raw silk fibers, and used them as a matrix to incorporate the photosensitizer 5-(4-aminophenyl)-10,15,20-tris-(4-sulphonatophenyl) porphyrin trisodium for application in photodynamic therapy (PDT). The hydrogels obtained were characterized by rheology, spectrophotometry, and scattering techniques to elucidate the factors involved in the formation of the hydrogel, and to characterize the behavior of silk fibroin (SF) after incorporating of the porphyrin to the matrix. The rheology results demonstrated that the SF hydrogels had a shear thinning behavior. In addition, we were able to verify that the structure of the material was able to be recovered over time after shear deformation. The encapsulation of porphyrins in hydrogels leads to the formation of self-assembled peptide nanostructures that prevent porphyrin aggregation, thereby greatly increasing the generation of singlet oxygen. Also, our findings suggest that porphyrin can diffuse out of the hydrogel and permeate the outer skin layers. This evidence suggests that SF hydrogels could be used as porphyrin encapsulation and as a drug carrier for the sustained release of photosensitizers for PDT.

Intercalating Single-Atom Metal Centers into an Organic Monolayer with a Full-Sample Coverage

Thiolate self-assembled monolayers (SAMs) have been widely used as a straightforward method to functionalize the surface of a common substrate with selective organic functional groups. Here we describe a process that further introduces isolated metal centers into an organic SAM using solutions of metallic porphyrin so that different organic groups and metal single-atoms can be simultaneously exposed on top of the surface. The entire process employs only common laboratory equipment and mild-temperature (<100 °C) incubation to create a full-sample (>cm²) SAM coverage. Each step in this process is closely monitored and discussed using nm-scale scanning tunneling microscopy (STM) images. This work can be straightforwardly adopted by research groups interested in such a diversely customizable surface but without access to a vacuum-based deposition technology. The porphyrin molecules are shown to intercalate among closely packed thiolate SAM domains, and STM characterization shows that the entire mixed monolayer is stable in an ambient condition. This process also does not involve any tip-assisted desorption or lithography procedure and can thus be applied toward substrates of other shapes beyond a flat surface.

Probing functional self-assembled molecular architectures with solution/solid scanning tunnelling microscopy

Over the past two decades, solution/solid STM has made clear contributions to our fundamental understanding of the thermodynamic and kinetic processes that occur in molecular self-assembly at surfaces. As the field matures, we provide an overview of how solution/solid STM is emerging as a tool to elucidate and guide the use of self-assembled molecular systems in practical applications, focusing on small molecule device engineering, molecular recognition and sensing and electronic modification of 2D materials.

Supramolecular Assemblies on Surfaces: Nanopatterning, Functionality, and Reactivity

Understanding how molecules interact to form large-scale hierarchical structures on surfaces holds promise for building designer nanoscale constructs with defined chemical and physical properties. Here, we describe early advances in this field and highlight upcoming opportunities and challenges. Both direct intermolecular interactions and those that are mediated by coordinated metal centers or substrates are discussed. These interactions can be additive, but they can also interfere with each other, leading to new assemblies in which electrical potentials vary at distances much larger than those of typical chemical interactions. Earlier spectroscopic and surface measurements have provided partial information on such interfacial effects. In the interim, scanning probe microscopies have assumed defining roles in the field of molecular organization on surfaces, delivering deeper understanding of interactions, structures, and local potentials. Self-assembly is a key strategy to form extended structures on surfaces, advancing nanolithography into the chemical dimension and providing simultaneous control at multiple scales. In parallel, the emergence of graphene and the resulting impetus to explore 2D materials have broadened the field, as surface-confined reactions of molecular building blocks provide access to such materials as 2D polymers and graphene nanoribbons. In this Review, we describe recent advances and point out promising directions that will lead to even greater and more robust capabilities to exploit designer surfaces.

In Situ Observations of UV-Induced Restructuring of Self-Assembled Porphyrin Monolayer on Liquid/Au(111) Interface at Molecular Level

Porphyrin-derived molecules have received much attention for use in solar energy conversion devices, such as artificial leaves and dye-sensitized solar cells. Because of their technological importance, a molecular-level understanding of the mechanism for supramolecular structure formation in a liquid, as well as their stability under ultraviolet (UV) irradiation, is important. Here, we observed the self-assembled structure of free-base, copper(II), and nickel(II) octaethylporphyrin formed on Au(111) in a dodecane solution using scanning tunneling microscopy (STM). As evident in the STM images, the self-assembled monolayers (SAMs) of these three porphyrins on the Au(111) surface showed hexagonal close-packed structures when in dodecane solution. Under UV irradiation (λ = 365 nm), the porphyrin molecules in the SAM or the dodecane solution move extensively and form new porphyrin clusters on the Au sites that have a high degree of freedom. Consequently, the Au(111) surface was covered with disordered porphyrin clusters. However, we found that the porphyrin molecules decomposed under UV irradiation at 254 nm. Molecular-scale observation of the morphological evolution of the porphyrin SAM under UV irradiation can provide a fundamental understanding of the degradation processes of porphyrin-based energy conversion devices.

How Equilibrium Gets Mimicked During Kinetic and Thermodynamic Control in Porphyrin and Phthalocyanine Self-Assembled Monolayers

The recent review of Hipps and Mazur is extended to consider the origins and significance of their conclusion that "surface structures vary with relative component concentration in a way that may mimic equilibria but is not". How this situation can arise during self-assembly is discussed, as well as a range of qualitative and quantitative observations and high-level free-energy calculations that document the effect for meso-tetraalkylporphyrins self-assembled monolayer (SAM) polymorphs. This leads to a discussion of modern challenges facing quantification of the effects caused by kinetic control, as well as to the usefulness of equilibrium mimicking in the design and synthesis of SAMs.

Periodic and nonperiodic chiral self-assembled networks from 1,3,5-benzenetricarboxylic acid on Ag(111)

An activated reaction can lead to a diversity of intermolecular bonding motifs through partially-reacted molecules.