Thickness, surface morphology, and optical properties of porphyrin multilayer thin films assembled on Si(100) using copper(I)-catalyzed azide-alkyne cycloaddition

Ligand-Controlled Electrochromic Diversification with Multilayer Coated Metallosupramolecular Polymer Assemblies

Designing surface-confined molecular systems capable of expressing changes in functional properties as a result of slight variations in chemical structure under the influence of an external stimulus is of contemporary interest. In this context, we have designed three tetraterpyridine ligands with variations in their core architecture (phenyl vs tetraphenylethynyl vs bithiophene) to create spray-coated electrochromic assemblies of iron(II)-based metallosupramolecular polymer network films on transparent conducting oxide substrates. These assemblies exhibited molecular permeability and spectroelectrochemical properties that are in turn dictated by the ligand structure. Electrochromic films with high coloration efficiencies (up to 1050 cm²/C) and superior optical contrast (up to 76%) with a concomitant color-to-color redox transition were readily achieved. These functional switching elements were integrated into sandwich-type electrochromic cells (CE up to 641 cm²/C) that exhibited high contrast ratios of up to 56%, with attractive ON-OFF ratios, fast switching kinetics, and high operational stability. Every measurable spectroelectrochemical property of the films and devices is an associated function of the ligand structure that coordinates the same metal ion to different extents. While exhibiting a ligand-structure induced differential metal coordination leading to porosity and spectroelectrochemical diversification, these assemblies allow the creation of electrochromic patterns and images by a simple spray-coating technique.

Probing the Roughness of Porphyrin Thin Films with X-ray Photoelectron Spectroscopy

Thin-film growth of molecular systems is of interest for many applications, such as for instance organic electronics. In this study, we demonstrate how X-ray photoelectron spectroscopy (XPS) can be used to study the growth behavior of such molecular systems. In XPS, coverages are often calculated assuming a uniform thickness across a surface. This results in an error for rough films, and the magnitude of this error depends on the kinetic energy of the photoelectrons analyzed. We have used this kinetic-energy dependency to estimate the roughnesses of thin porphyrin films grown on rutile TiO2 (110). We used two different molecules: cobalt (II) monocarboxyphenyl-10,15,20-triphenylporphyrin (CoMCTPP), with carboxylic-acid anchor groups, and cobalt (II) tetraphenylporphyrin (CoTPP), without anchor groups. We find CoMCTPP to grow as rough films at room temperature across the studied coverage range, whereas for CoTPP the first two layers remain smooth and even; depositing additional CoTPP results in rough films. Although, XPS is not a common technique for measuring roughness, it is fast and provides information of both roughness and thickness in one measurement.

Reactive films fabricated using click sulfur(vi)–fluoride exchange reactions via layer-by-layer assembly

We describe a novel and efficient method to generate tunable multifunctional polymer films with a wide range of potential biomedical applications using the “sulfur(vi)–fluoride exchange” (SuFEx) click reaction.

Surface Coupling of Octaethylporphyrin with Silicon Tetrachloride

The surface assembly of 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) using silicon tetrachloride as a coupling agent was investigated using atomic force microscopy (AFM). Nanopatterned films of Si-OEP were prepared by protocols of colloidal lithography to evaluate the morphology, thickness, and molecular orientation for samples prepared on Si(111). The natural self-stacking of porphyrins can pose a challenge for molecular patterning. When making films on surfaces, porphyrins will self-associate to form co-planar configurations of random stacks of molecules. There is a tendency for the flat molecules to orient spontaneously in a side-on arrangement that is mediated by physisorption to the substrate as well as by π-π interactions between macrocycles to form a layered arrangement of packed molecules, analogous to a stack of coins. When silicon tetrachloride is introduced to the reaction vessel, the coupling between the surface and porphyrins is mediated through covalent Si-O bonding. For these studies, surface structures of Si-OEP were formed that are connected with a Si-O-Si motif to a silicon atom coordinated to the center of the porphyrin macrocycles. Protocols of colloidal lithography were used as a tool to prepare surface structures and films of Si-OEP to facilitate surface characterizations. Conceptually, by arranging the macrocycles of porphyrins with defined orientation, local AFM surface measurements can be enabled to help address mechanistic questions about how molecules self-assemble and bind to substrates.

Evaluation of different dispersion models for correlation of spectroscopic ellipsometry and X-ray reflectometry

The importance of proper modeling of the materials' optical properties for interpretation of spectroscopic ellipsometry (SE) data is pretty much impossible to underestimate. In this study, the thickness-dependent titanium nitride (TiN) optical properties were represented by the multiple-oscillator Drude-Lorentz, Forouhi-Bloomer, and Lorentz optical dispersions with different numbers of parameters. The dielectric function of thin TiN films with intermediate behavior can be appropriately expressed in terms of 9-13 model parameters. Using X-ray reflectometry (XRR) as a reference technique and taking into account surface roughness of TiN films, it has been shown that three-term Lorentz dispersion model provides not only the best fit quality for the nominal thicknesses ranging from 125 Å to 350 Å but also an extremely good SE-XRR linear correlation with slope 1.05 ± 0.01, intercept -9.88 ± 2.06 Å, and R² = 0.9998. Use of other dispersion models results in a somewhat worse correlation with XRR measurements. Thus, an appropriate modeling of the film optical properties is one of the factors needed to be considered to establish well-grounded and credible SE and XRR correlation in case of using XRR as a reference technique, i.e., calibrating SE using x-ray reflectivity.

Covalent layer-by-layer thin films with charge-transfer chromophores: side chain engineering for efficient Ag+ ion recognition in aqueous solutions

Covalent layer-by-layer (LbL) thin films are fabricated by a highly efficient [2+2] cycloaddition-retroelectrocyclization (CA-RE) reaction of aniline-substituted alkyne and 7,7,8,8-tetracyanoquinodimethane (TCNQ) moieties. Polystyrene bearing aniline-substituted alkyne side chains and TCNQ polyester were prepared as precursor polymers, then sequentially deposited and fixed by covalent bonds on an indium-tin-oxide (ITO) substrate. The successful alternate growth of the films was demonstrated by many techniques including absorption spectroscopy, surface roughness, and redox activities. Interestingly, the water contact angles of the film surface were dependent on the side chains of the polystyrene derivatives. When hydrophilic triethylene glycol was employed, the surface hydrophilicity was similar to that of the TCNQ polyester. In contrast, the use of hydrophobic n-hexyl groups resulted in a significant difference in the water contact angles between the polystyrene and TCNQ polyester. A similar difference occurred when the Ag+ ion recognition was studied by soaking the LbL films in aqueous solutions. The LbL films constructed from the polystyrene with triethylene glycol chains displayed a more rapid and significant recognition of Ag+ ions than those constructed from the polystyrene with n-hexyl chains.

Functional Covalent Layer-by-Layer Thin Films by [2 + 2] Cycloaddition-Retroelectrocyclization

Covalent layer-by-layer (LbL) thin films with the well-defined charge-transfer (CT) chromophores were fabricated by the highly efficient [2 + 2] cycloaddition-retroelectrocyclization (CA-RE) reaction between the dialkylaniline-substituted alkynes and 7,7,8,8-tetracyanoquinodimethane (TCNQ) moieties. The resulting thin films showed potent redox activities and Ag⁺ ion sensing ability due to the formed CT chromophores.

A metal-lustrous porphyrin foil

A metal-lustrous self-standing film, named "porphyrin foil", was formed from a glass-forming polymeric porphyrin. The amorphous glass nature of the porphyrin foil played a key role in spontaneously producing a smooth surface. Its sharp contrast in intense absorption and specular reflection of light at each wavelength provided a brilliant metallic lustre.

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.

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.

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.

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.

Covalent layer-by-layer assembly of redox active molecular multilayers on silicon (100) by photochemical thiol-ene chemistry

The fabrication of thin organic films covalently grafted onto silicon substrates is of significant interest, as they are expected to give access to a broad range of new materials for integration into microelectronic applications. Covalent layer-by-layer (LbL) assembly offers a high degree of freedom when designing such thin films. In this work an approach for the preparation of covalent redox active molecular multilayers on silicon (100) surfaces is presented using a highly branched decaallylferrocene and thiol-ene click chemistry. The multilayers are analyzed by ellipsometry, X-ray photoelectron sprectroscopy, and cyclic voltammetry. The results indicate that the multilayer growth is linear for at least sixteen layers and the density of ferrocenes per layer is in the range of 6 × 10⁻¹¹ mol cm⁻². Moreover, this method for LbL assembly is extended to surfaces which have been locally passivated by microcontact printing. By atomic force microscopy measurements it is possible to show that the covalent LbL deposition proceeds exclusively in the nonpassivated areas.

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

We report the use of grazing-angle attenuated total reflectance (GATR) IR and polarized UV-vis to determine the molecular structure of porphyrin based molecular multilayer films grown in a layer-by-layer (LbL) fashion using copper-catalyzed azide-alkyne cycloaddition (CuAAC). The molecular orientation and bonding motif present in multilayer films of this type could impact their photophysical and electrochemical properties as well as potential applications. Multilayer films of M(II) 5,10,15,20-tetra(4-ethynylphenyl)porphyrin (1 M = Zn, 2 M = Cu) and azido based linkers 3-5 were used to fabricate the films on ITO substrates. Electrochemically determined coverage of films containing 2 match the trends observed in the absorbance. GATR-IR spectral analysis of the films indicate that CuAAC reactivity is leading to 1,4-triazole linked multilayers with increasing porphyrin and linker IR characteristic peaks. Films grown using all azido-linkers (3-5) display an oscillating trend in azide IR intensity suggesting that the surface bound azido group reacts with 1 and that further layering can occur through additional reaction with linkers, regenerating the azide surface. Films containing linker 5 in particular show an overall increase in azide content suggesting that only two of the three available groups react during multilayer fabrication, causing an overall buildup of azide content in the film. Films of 1 with linker 3 and 5 showed an average porphyrin plane angle of 46.4° with respect to the substrate as determined by GATR FT-IR. Polarized UV-vis absorbance measurements correlate well with the growth angle calculated by IR. The orientation of the porphyrin plane within the multilayer structures suggests that the CuAAC-LbL process results in a film with a trans bonding motif.