A versatile molecular layer-by-layer thin film fabrication technique utilizing copper(I)-catalyzed azide-alkyne cycloaddition

Fabrication of Fully Positively Charged Layer-by-Layer Polyelectrolyte Nanofilms with pH-Dependent Swelling Properties

Nanofilms fabricated by layer-by-layer (LbL) assembly from polyelectrolytes (PEs) are important materials for various applications. However, PE films cannot retain the charges along the polymer chains during fabrication, resulting in a low charge density. In this study, the preparation of LbL nanofilms with preserved positive charges via a controllable and efficient approach was achieved. To fabricate fully positively charged (FPC) LbL nanofilms, a polycation, poly-l-lysine, was partially grafted with azide and alkyne groups. Through copper-catalyzed azide-alkyne cycloaddition and the LbL procedure, nanofilms were fabricated with all of the individual layers covalently bonded, improving the pH stability of the nanofilms. Because the resulting nanofilms had a high charge density with positive charges both inside and on the surface, they showed unique pH-dependent swelling properties and adsorption of negatively charged molecules compared with those of traditional polyelectrolyte LbL nanofilms. This kind of FPC nanofilm has great potential for use in sensors, diagnostics, and filter nanomaterials in the biomedical and environmental fields.

Efficient hierarchical models for reactivity of organic layers on semiconductor surfaces

Computational modeling of organic interface formation on semiconductors poses a challenge to a density functional theory-based description due to structural and chemical complexity. A hierarchical approach is presented, where parts of the interface are successively removed in order to increase computational efficiency while maintaining the necessary accuracy. First, a benchmark is performed to probe the validity of this approach for three model reactions and five dispersion corrected density functionals. Reaction energies are generally well reproduced by generalized gradient approximation-type functionals but accurate reaction barriers require the use of hybrid functionals. Best performance is found for the model system that does not explicitly consider the substrate but includes its templating effects. Finally, this efficient model is used to provide coverage dependent reaction energies and suggest synthetic principles for the prevention of unwanted growth termination reactions for organic layers on semiconductor surfaces.

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.

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.

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.

Sequence-controlled molecular layers on surfaces by thiol–ene chemistry: synthesis and multitechnique characterization

Silicon surfaces were functionalized by thiol–ene chemistry using sequential reactions of different α,ω-dienes and α,ω-dithiols bearing marker moieties.

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.

π-Conjugated bis(terpyridine)metal complex molecular wires

Bottom-up approaches have gained significant attention recently for the creation of nano-sized, ordered functional structures and materials. Stepwise coordination techniques, in which ligand molecules and metal sources are reacted alternatively, offer several advantages. Coordination bonds are stable, reversible, and self-assembling, and the resultant metal complex motifs may contain functionalities unique to their own characteristics. This review focuses on metal complex wire systems, specifically the bottom-up fabrication of linear and branched bis(terpyridine)metal complex wires on electrode surfaces. This system possesses distinct and characteristic electronic functionalities, intra-wire redox conduction and excellent long-range electron transport ability. This series of comprehensive studies exploited the customizability of bis(terpyridine)metal complex wires, including examining the influence of building blocks. In addition, simple yet effective electron transfer models were established for redox conduction and long-range electron transport. A fabrication technique for an ultra-long bis(terpyridine)metal complex wire is also described, along with its properties and functionalities.

Advances in click chemistry for silica-based material construction

Click chemistry is undoubtedly the most powerful 1,3-dipolar cycloaddition reaction in organic synthesis.

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.

Photocurrent generation in organic photodetectors with tailor-made active layers fabricated by layer-by-layer deposition

Photodetectors supply an electric response when illuminated. The detectors in this study consist of an active layer and a polyvinylidene fluoride (PVDF) blocking layer, which are sandwiched by an aluminum and an indium tin oxide (ITO) electrode. The active layer was prepared of Zn porphyrins and assembled by covalent layer-by-layer (LbL) deposition. Layer growth was monitored by UV-vis absorbance, ellipsometry, and X-ray photoelectron spectroscopy. Upon exposure to chopped light, the detectors show an alternating transient photocurrent, which is limited by the accumulation of space charges at the blocking layer/active layer interface. We could show that the number of photoactive layers has a significant impact on device performance. The fastest response was achieved with fewer layers. The highest photocurrents were measured for detectors with an intermediate number of layers, beyond which, more layers did not lead to an increase in the photocurrent despite containing more active material.

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.

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.

Functionalization of SnO₂ crystals with a covalently-assembled porphyrin monolayer

The functionalization of micro- and nano-sized metal-oxide powders offers many advantages because of their large surface areas and, therefore, the large number of functional molecules that can be grafted onto the grain surfaces. Porphyrin molecules on large band-gap semiconducting metal oxides represent key materials for many different optical and electronic applications. Herein, we have proposed a general two-step procedure for the functionalization of metal-oxide crystals with dye-sensitizers. In particular, we functionalized SnO₂ nanoparticles with a monolayer of the bifunctional trichloro[4-(chloromethyl)phenyl]silane. Then, a monolayer of 5,10,15,20-tetrakis(4-hydroxyphenyl)-21H,23H-porphyne was covalently bound to the silanized SnO₂ grains. IR, UV/Vis, and luminescence measurements were used for optical characterization. The measured footprint of the grafted porphyrin molecules indicated total surface coverage of the grains. The surface electronic characterization was performed by using X-ray photoelectron spectroscopy. Emission measurements revealed two strong bands at 664.1 and 721.0 nm that were attributed to the porphyrin monolayer assembled on the surface of the SnO₂ crystals.

Surface-confined electroactive molecules for multistate charge storage information

Bi-stable molecular systems with potential for applications in binary memory devices are raising great interest for device miniaturization. Particular appealing are those systems that operate with electrical inputs since they are compatible with existing electronic technologies. The processing of higher memory densities in these devices could be accomplished by increasing the number of memory states in each cell, although this strategy has not been much explored yet. Here we highlight the recent advances devoted to the fabrication of charge-storage molecular surface-confined devices exhibiting multiple states. Mainly, this goal has been realized immobilizing a variety (or a combination) of electroactive molecules on a surface, although alternative approaches employing non-electroactive systems have also been described. Undoubtedly, the use of molecules with chemically tunable properties and nanoscale dimensions are raising great hopes for the devices of the future in which molecules can bring new perspectives such as multistability.

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.

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.

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

We report the structure, optical properties and surface morphology of Si(100) supported molecular multilayers resulting from a layer-by-layer (LbL) fabrication method utilizing copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC), also known as "click" chemistry. Molecular based multilayer films comprised of 5,10,15,20-tetra(4-ethynylphenyl)porphyrinzinc(II) (1) and either 1,3,5-tris(azidomethyl)benzene (2) or 4,4'-diazido-2,2'-stilbenedisulfonic acid disodium salt (3) as a linker layer, displayed linear growth properties up to 19 bilayers. With a high degree of linearity, specular X-ray reflectivity (XRR) measurements yield an average thickness of 1.87 nm/bilayer for multilayers of 1 and 2 and 2.41 nm/bilayer for multilayers of 1 and 3. Surface roughnesses as determined by XRR data fitting were found to increase with the number of layers and generally were around 12% of the film thickness. Tapping mode AFM measurements confirm the continuous nature of the thin films with roughness values slightly larger than those determined from XRR. Spectroscopic ellipsometry measurements utilizing a Cauchy model mirror the XRR data for multilayer growth but with a slightly higher thickness per bilayer. Modeling of the ellipsometric data over the full visible region using an oscillator model produces an absorption profile closely resembling that of a multilayer grown on silica glass. Comparing intramolecular distances from DFT modeling with experimental film thicknesses, the average molecular growth angles were estimated between 40° and 70° with respect to the substrate surface depending on the bonding configuration.

Synergism in multicomponent self-propagating molecular assemblies

Multicomponent self-propagating molecular assemblies (SPMAs) have been generated from an organic chromophore, a redox-active polypyridyl complex, and PdCl(2). The structure of the multicomponent SPMA is not a linear combination of two assemblies generated with a single molecular constituent. Surface-confined assemblies formed from only the organic chromophore and PdCl(2) are known to follow linear growth, whereas the combination of polypyridyl complexes and PdCl(2) results in exponential growth. The present study demonstrates that an iterative deposition of both molecular building blocks with PdCl(2) results in an exponentially growing assembly. The nature of the assembly mechanism is dictated by the polypyridyl complex and overrides the linear growth process of the organic component. Relatively smooth, multicomponent SPMAs have been obtained with a thickness of ∼20 nm on silicon, glass, and indium-tin oxide (ITO) coated glass. Detailed information of the structure and of the surface-assembly chemistry were obtained using transmission optical (UV/Vis) spectroscopy, ellipsometry, atomic force microscopy (AFM), synchrotron X-ray reflectivity (XRR), and electrochemistry.