Structural analysis of porphyrin multilayer films on ITO assembled using copper(I)-catalyzed azide-alkyne cycloaddition by ATR IR

Surface chemical reactions on self-assembled silane based monolayers

In this review, we aim to update our review "Chemical modification of self-assembled silane-based monolayers by surface reactions" which was published in 2010 and has developed into an important guiding tool for researchers working on the modification of solid substrate surface properties by chemical modification of silane-based self-assembled monolayers. Due to the rapid development of this field of research in the last decade, the utilization of chemical functionalities in self-assembled monolayers has been significantly improved and some new processes were introduced in chemical surface reactions for tailoring the properties of solid substrates. Thus, it is time to update the developments in the surface functionalization of silane-based molecules. Hence, after a short introduction on self-assembled monolayers, this review focuses on a series of chemical reactions, i.e., nucleophilic substitution, click chemistry, supramolecular modification, photochemical reaction, and other reactions, which have been applied for the modification of hydroxyl-terminated substrates, like silicon and glass, which have been reported during the last 10 years.

Dipyrrinato-Iridium(III) Complexes for Application in Photodynamic Therapy and Antimicrobial Photodynamic Inactivation

The generation of bio-targetable photosensitizers is of utmost importance to the emerging field of photodynamic therapy and antimicrobial (photo-)therapy. A synthetic strategy is presented in which chelating dipyrrin moieties are used to enhance the known photoactivity of iridium(III) metal complexes. Formed complexes can thus be functionalized in a facile manner with a range of targeting groups at their chemically active reaction sites. Dipyrrins with N- and O-substituents afforded (dipy)iridium(III) complexes via complexation with the respective Cp*-iridium(III) and ppy-iridium(III) precursors (dipy=dipyrrinato, Cp*=pentamethyl-η5 -cyclopentadienyl, ppy=2-phenylpyridyl). Similarly, electron-deficient [IrIII (dipy)(ppy)2 ] complexes could be used for post-functionalization, forming alkenyl, alkynyl and glyco-appended iridium(III) complexes. The phototoxic activity of these complexes has been assessed in cellular and bacterial assays with and without light; the [IrIII (Cl)(Cp*)(dipy)] complexes and the glyco-substituted iridium(III) complexes showing particular promise as photomedicine candidates. Representative crystal structures of the complexes are also presented.

Photo-controlled release of metal ions using triazoline-containing amphiphilic copolymers

Photo-controlled release of metal ions can be achieved by denitrogenation of triazoline from the micelles of amphiphilic copolymer, and has potential applications for biomedicines.

Functionalized Self-Assembled Monolayers Bearing Diiminate Complexes Immobilized through Covalently Anchored Ligands

The application of synthetic organic chemistry to the surface chemistry of monolayer arrays adds a novel dimension to the power of these systems for surface modification. This paper describes the elaboration of simple functionalized monolayers into dialdimine and dialdiminate ligands tethered to the monolayer surface. These ligands are then used to coordinate metal ions in an effort to form diiminate complexes with control over their environment and orientation. Ligand anchoring is best achieved through either thiol-ene photochemistry or azide-acetylene "click" chemistry. There is an influence of ligand bulk on some surface transformations, and in some cases reactions that have been reported to be effective on simple, homogeneous monolayer surfaces are not applicable to a more complex monolayer environment. The large excess of solution reagents relative to monolayer surface functionality adds another measure of difficulty to the control of interfacial reactions. In instances where the anchoring chain includes functional groups that can directly interact with metal ions, the metalation of ligand-bearing surfaces resulted in a higher metal ion content than would have been expected from binding only to the diimine ligands.

Multimolecular assemblies on high surface area metal oxides and their role in interfacial energy and electron transfer

High surface area metal oxides offer a unique substrate for the assembly of multiple molecular components at an interface.

Synthesis and Functionalization of Porphyrins through Organometallic Methodologies

This review focuses on the postfunctionalization of porphyrins and related compounds through catalytic and stoichiometric organometallic methodologies. The employment of organometallic reactions has become common in porphyrin synthesis. Palladium-catalyzed cross-coupling reactions are now standard techniques for constructing carbon-carbon bonds in porphyrin synthesis. In addition, iridium- or palladium-catalyzed direct C-H functionalization of porphyrins is emerging as an efficient way to install various substituents onto porphyrins. Furthermore, the copper-mediated Huisgen cycloaddition reaction has become a frequent strategy to incorporate porphyrin units into functional molecules. The use of these organometallic techniques, along with the traditional porphyrin synthesis, now allows chemists to construct a wide range of highly elaborated and complex porphyrin architectures.

Electrochemical Rectification of Redox Mediators Using Porphyrin-Based Molecular Multilayered Films on ITO Electrodes

Electrochemical charge transfer through multilayer thin films of zinc and nickel 5,10,15,20-tetra(4-ethynylphenyl) porphyrin constructed via copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) "click" chemistry was examined. Current rectification toward various outer-sphere redox probes is revealed with increasing numbers of layers, as these films possess insulating properties over the neutral potential range of the porphyrin, then become conductive upon reaching its oxidation potential. Interfacial electron transfer rates of mediator-dye interactions toward [Co(bpy)3](2+), [Co(dmb)3](2+), [Co(NO2-phen)3](2+), [Fe(bpy)3](2+), and ferrocene (Fc), all outer-sphere redox species, were measured by hydrodynamic methods. The ability to modify electroactive films' interfacial electron transfer rates, as well as current rectification toward redox species, has broad applicability in a number of devices, particularly photovoltaics and photogalvanics.

Metal-Organic Coordination Network Thin Film by Surface-Induced Assembly

The growth of metal-organic coordination network thin films on surfaces has been pursued extensively and intensively to manipulate the molecular arrangement. For this study, the oriented multilayer thin films based on porphyrinic nanoarchitecture were synthesized toward metal-organic coordination networks using surface-induced assembly (SIA). Nanoscale molecular thin films were prepared at room temperature using cobalt(II) ion and porphyrin building blocks as precursors. Stepwise growth with a highly uniform layer was characterized using UV-vis, AFM, IR, and XPS studies. The grazing incidence small-angle X-ray scattering and X-ray reflectivity results remarkably suggested a periodic structure in in-plane direction with constant and high mass density (ca. 1.5 g/cm(3)) throughout the multilayer formation. We propose that orientation of the porphyrin macrocycle plane with a hexagonal packed model by single anchoring mode was tilted approximately 60° with respect to the surface substrate. It is noteworthy that the well-organized structure of porphyrin-based macrocyclic framework on the amine-terminated surface substrate can be achieved efficiently using a simple SIA approach under mild synthetic conditions. The synthesized thin film provides a different structure from that obtained using bulk synthesis. This result suggests that the SIA technique can control not only the film thickness but also the structural arrangement on the surface. This report of our research provides insight into the ordered porphyrin-based metal-organic coordination network thin films, which opens up opportunities for exploration of unique thin film materials for diverse applications.

A new route to fabricate biocompatible hydrogels with controlled drug delivery behavior

Hydrogels for drug delivery have attracted extensive interests since they can be used for biomaterials such as contact lenses. Here, we report that biocompatible hydrogels for contact lenses with controlled drug delivery behavior can be fabricated using copolymer hydrogels and Layer-by-Layer (LbL) surface modification technique. Methyl acrylic anhydride (MAA) modified β-cyclodextrin (β-CD) (MA-β-CD) was synthesized and copolymerized with hydroxyethyl methacrylate (HEMA) to form copolymer hydrogel. The introduction of second monomer of MA-β-CD would accelerate the polymerization of hydrogel, leading to increase of residual CC groups. The structure of copolymers was characterized by differential scanning calorimetry (DSC). Transparence, equilibrium swelling ratio and contact angle of copolymer hydrogel were also detailed discussed in the work. In vitro drug release results showed that copolymer hydrogel with higher MA-β-CD content exhibited a better drug loading capacity and drug release behaviors could be tuned by MA-β-CD/monomer ratio. Finally, alkynyl functional hyaluronic acid (HA-BP) and nitrine functional chitosan (CS-N3) were synthesized and covalently cross-linked to copolymer hydrogel surface using LbL technique through click chemistry. The successful LbL multilayers were confirmed by X-ray Photoelectron Spectroscopy (XPS). Resultsofcytotoxicityexperiment revealed that the hydrogels were biocompatible since they could support the growth of cells.

Synthesis, Characterization, and Fluorescence Properties of Mixed Molecular Multilayer Films of BODIPY and Zn(II) Tetraphenylporphyrins

A new azido functionalized 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) has been synthesized to achieve spectral complementarity to a Zn(II) tetraphenylethynyl porphyrin (ZnTPEP). Mixed multilayer films were assembled on glass and quartz up to 10 bilayers thick in a layer-by-layer (LbL) fabrication process using copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) to couple these two dyes together with a tris-azido linker. By varying the amount of BODIPY in the CuAAC reaction solutions for the azido linker layers, we achieve tunable doping of BODIPY within the porphyrin films. We are able to demonstrate linear film growth and determine thickness by X-ray reflectivity (XRR). XRR data indicated that lower BODIPY loading leads to higher porphyrin content and slightly thicker films. Fluorescence emission and excitation spectra of the mixed multilayer films show efficient quenching of the BODIPY singlet and enhanced ZnTPEP emission, suggesting efficient energy transfer (EnT). The ease of fabrication and tunability of these films may serve as potential light harvesting arrays for molecular-based solar cells.

Host-guest interactions derived multilayer perylene diimide thin film constructed on a scaffolding porphyrin monolayer

The development of methods to grow well-ordered chromophore thin films on solid substrates is of importance because such surface-associated arrays have potential applications in the generation of functional electronic and optical materials and devices. In this article, we demonstrate a straightforward layer-by-layer (LBL) supramolecular deposition strategy to prepare numerous layers (up to 19) of functionalized perylene diimide (PDI) chromophores built upon a covalent scaffolding multivalent porphyrin monolayer. Our thin film formation strategy employs water as the immersion solvent and exploits the β-cyclodextrin-adamantane host-guest couple in addition to PDI based aromatic stacking. Within the resultant film the porphyrin scaffold is oriented close to parallel to the glass substrate while the PDI chromophores are aligned closer to the surface normal. Together, the porphyrin monolayer and the multi-PDI layers exhibit a large absorption bandwidth in the visible spectrum. Importantly, because a self-assembly strategy was utilized, when a single monolayer of PDI is deposited on the porphyrin scaffolding layer, this PDI monolayer can be readily disassembled by washing with DMF leading to the regeneration of the porphyrin monolayer. The PDI thin film can subsequently be regrown from the regenerated porphyrin surface. The reported LBL strategy will be of broad interest for researchers developing well-organized chromophoric films and materials due to its simplicity as well as the added advantage of being performed in sustainable and cost-effective aqueous media.

Pyridine coordination chemistry for molecular assemblies on surfaces

CONSPECTUS: Since the first description of coordination complexes, many types of metal-ligand interactions have creatively been used in the chemical sciences. The rich coordination chemistry of pyridine-type ligands has contributed significantly to the incorporation of diverse metal ions into functional materials. Here we discuss molecular assemblies (MAs) formed with a variety of pyridine-type compounds and a metal containing cross-linker (e.g., PdCl2(PhCN2)). These MAs are formed using Layer-by-Layer (LbL) deposition from solution that allows for precise fitting of the assembly properties through molecular programming. The position of each component can be controlled by altering the assembly sequence, while the degree of intermolecular interactions can be varied by the level of π-conjugation and the availability of metal coordination sites. By setting the structural parameters (e.g., bond angles, number of coordination sites, geometry) of the ligand, control over MA structure was achieved, resulting in surface-confined metal-organic networks and oligomers. Unlike MAs that are constructed with organic ligands, MAs with polypyridyl complexes of ruthenium, osmium, and cobalt are active participants in their own formation and amplify the growth of the incoming molecular layer. Such a self-propagating behavior for molecular systems is rare, and the mechanism of their formation will be discussed. These exponentially growing MAs are capable of storing metal salts that can be used during the buildup of additional molecular layers. Various parameters influencing the film growth mechanism will be presented, including (i) the number of binding sites and geometry of the organic ligands, (ii) the metal and the structure of the polypyridyl complexes, (iii) the influence of the metal cross-linker (e.g., second or third row transition metals), and (iv) the deposition conditions. By systematic variation of these parameters, switching between linear and exponential growth could be demonstrated for MAs containing structurally well-defined polypyridyl complexes. The porosity of the MAs has been estimated by using electrochemically active probes. Incorporating multiple polypyridyl complexes of osmium and ruthenium into a single assembly give rise to composite materials that exhibit interesting electrochemical and electrochromic properties. These functional composites are especially attractive as they exhibit properties that neither of each metal complex possesses individually. Some of our MAs have very high coloration efficiencies, redox stability, fast responsive times and operate at voltages < 1.5 V. Moreover, their electrochemical properties are dependent on the deposition sequence of the polypyridyl complexes, resulting in MAs that possesses distinctive electron transfer pathways. Finally, some of these MAs are described in terms of their practical applications in electrochromic materials, storage-release chemistry, solar cells, and electron transport properties.

Attachment of a diruthenium compound to Au and SiO2/Si surfaces by "click" chemistry

Fabrication of electrodes with functionalized properties is of interest in many electronic applications with the surface impacting the electrical and electronic properties of devices. We report the formation of molecular monolayers containing a redox-active diruthenium(II,III) compound to gold and silicon surfaces via "click" chemistry. The use of Cu-catalyzed azide-alkyne cycloaddition enables modular design of molecular surfaces and interfaces and allows for a variety of substrates to be functionalized. Attachment of the diruthenium compound is monitored by using infrared and photoelectron spectroscopies. The highest occupied molecular (or system) orbital of the "clicked-on" diruthenium is clearly seen in the photoemission measurements and is mainly attributed to the presence of the Ru atoms. The "click" attachment is robust and provides a route to investigate the evolution of the electronic structure and properties of novel molecules attached to a variety of electrodes. The ability to attach this redox-active Ru molecule onto SiO2 and Au surfaces is important for the development of functional molecular devices such as charge-based memory devices.

Simultaneous electropolymerization and electro-click functionalization for highly versatile surface platforms

Simple preparation methods of chemically versatile and highly functionalizable surfaces remain rare and present a challenging research objective. Here, we demonstrate a simultaneous electropolymerization and electro-click functionalization process (SEEC) for one-pot self-construction of aniline- and naphthalene-based functional polymer films where both polymerization and click functionalization are triggered by applying electrochemical stimuli. Cyclic voltammetry (CV) can be applied for the simultaneous oxidation of 4-azidoaniline and the reduction of Cu(II) ions, resulting in polymerization of the former, and the Cu(I)-catalyzed alkyne/azide cycloaddition ("click" chemistry). Properties of the films obtained can be tuned by varying their morphology, their chemically "clicked" content, or by postconstruction functionalization. To demonstrate this, the CV scan rates, component monomers, and "clicked" molecules were varied. Covalent postconstruction immobilization of horseradish peroxidase was also performed. Consequently, pseudocapacitance and enzyme activity were affected. SEEC provides surface scientists an easy access to a wide range of functionalization possibilities in several fields including sensors, fuel cells, photovoltaics, and biomaterials.

Fast conversion of terminal thiocarbonylthio groups of RAFT polymers to “clickable” thiol groups via versatile sodium azide

A facile and fast way of converting thiocarbonylthio end groups of RAFT polymers to thiol groups was demonstrated.

Photocurrent enhancement by multilayered porphyrin sensitizers in a photoelectrochemical cell

Multilayer Zn(II) tetraphenylporphyrin chromophores, assembled using copper-catalyzed azide-alkyne cycloaddition (CuAAC), provide a new sensitization scheme that could be useful in dye-sensitized solar cells (DSSCs). We report on the photoelectrochemical responses of multilayer films of Zn(II) 5,10,15,20-tetra(4-ethynylphenyl)porphyrin (1) assembled on planar ITO substrates operating as a p-type DSSC using three different redox mediators. The traditional I(-)/I3(-) redox couple results in the greatest short circuit current densities (JSC) but very low open circuit potentials (VOC). The use of cobalt sepulchrate ([Co(sep)](2+/3+)) and cobalt tris-bipyridine ([Co(bpy)3](2+/3+)) as redox mediators generates higher VOC values, but at the expense of lower photocurrents. These results highlight the inherent differences in the interactions between the redox mediator and Zn(II) tetraphenylporphyrin multilayer films. Increasing the porphyrin content through multilayer growth proved to be effective in increasing the performance of photoelectrochemical cells with all three redox mediators. Cells using I(-)/I3(-) reached maximum performance (power output) at five porphyrin layers, [Co(bpy)3](2+/3+) at five layers, and [Co(sep)](2+/3+) at three layers. For all mediators, JSC increases with the addition of porphyrin layers beyond a monolayer. However, JSC reaches a maximum value at a point greater than one layer after which it decreases, presumably due to exciton diffusion limitations and the insulating effects of the multilayer film. Similarly, all cells also reach a maximum VOC beyond one porphyrin layer. We show that porphyrin arrays assembled using newly developed CuAAC layer-by-layer growth may be useful as a multilayer sensitization scheme for use in photoelectrochemical cells.

Efficient Förster resonance energy transfer in 1,2,3-triazole linked BODIPY-Zn(II) meso-tetraphenylporphyrin donor-acceptor arrays

Cu(I) catalyzed azide-alkyne cycloaddition (CuAAC) reactivity was successfully employed to synthesize three donor-acceptor energy transfer (EnT) arrays that contain one (Dyad), three (Tetrad) and four (Pentad) 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) donors connected to a Zn-tetraphenylporphyrin acceptor via 1,2,3-triazole linkages. The photophysical properties of the three arrays, along with individual donor and acceptor chromophores, were investigated by UV-vis absorption and emission spectroscopy, fluorescence lifetimes, and density functional theory (DFT) electronic structure modeling. Comparison of the UV-vis absorption spectra and frontier molecular orbitals from DFT calculations of the three arrays with ZnTPP, ZnTTrzlP, and Trzl-BODIPY shows that the electronic structure of the chromophores is essentially unperturbed by the 1,2,3-triazole linkage. Time-dependent DFT (TDDFT) calculations on the Dyad reproduce the absorption spectra in THF and show no evidence of excited state mixing of the donor and acceptor. The BODIPY singlet excited state emission is significantly quenched in all three arrays, consistent with EnT to the porphyrin core, with efficiencies of 95.8, 97.5, and 97.2% for the Dyad, Tetrad, and Pentad, respectively. Fluorescence excitation spectra of the three arrays, measured at the porphyrin emission, mirror the absorption profile of both the porphyrin and BODIPY chromophores and are consistent with the Förster resonance energy transfer (FRET) mechanism. Applying Förster theory to the spectroscopic data of the chromophores gives EnT efficiency estimates that are in close agreement with experimental values, suggesting that the through-space mechanism plays a dominant role in the three arrays.

Photochemical grafting and patterning of organic monolayers on indium tin oxide substrates

Covalently attached organic layers on indium tin oxide (ITO) surfaces were prepared by the photochemical grafting with 1-alkenes. The surface modification was monitored with static water contact angle, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) measurements. Hydrophobic methyl-terminated ITO surfaces can be obtained via the grafting of tetradec-1-ene, whereas the attachment of ω-functionalized 1-alkenes leads to functionalized ITO surfaces. The use of a C≡C-Ge(CH(3))(3) terminus allows for facile tagging of the surface with an azido group via a one-pot deprotection/click reaction, resulting in bio/electronically active interfaces. The combination of nonaggressive chemicals (alkenes), mild reaction conditions (room temperature), and a light-induced grafting that facilitates the direct patterning of organic layers makes this simple approach highly promising for the development of ITO-based (bio)electronic devices.