Spectroelectrochemical Methods Research Articles

Synthesis and Self-Assembly of β-Octa[(4-Diethoxyphosphoryl)phenyl]porphyrins.

The β-substituted porphyrinoids commonly used to form functional assembled systems in nature yet are still scarcely used in material chemistry probably due to the laborious synthesis of these compounds. In this work, β-octa[(4-diethoxyphosphoryl)phenyl]porphyrin (2HOPPP) and its metal (Zn(II), Cd(II), Cu(II), and Ni(II)) complexes were prepared in good yields. These highly soluble chromophores were characterized in solution using spectroscopic (NMR, UV-vis, fluorescence), electrochemical, and spectroelectrochemical methods. Attachment of the electron-deficient residue (ArP(O)(OEt)2) to the porphyrin macrocycle leads to easier reductions and harder oxidations of the macrocycle for all complexes studied as compared to corresponding meso-tetra[4-(diethoxyphosphoryl)phenyl]porphyrin derivatives reported previously. We demonstrated that the strong electron-deficient character of the MOPPP porphyrins results principally from the increase in the number of electron-withdrawing groups at the periphery of the tetrapyrrolic macrocycle. Electron-deficient porphyrins are highly required in supramolecular and material chemistry in part due to their ability to form supramolecular assemblies via the coordination of axial ligands to the central metal atom. According to single-crystal X-ray data, ZnOPPP forms in the crystalline phase dimers in which each of the two tetrapyrrolic macrocycles is connected through an unusual combination of hydrogen bonding of two phosphoryl groups and the water molecules axially coordinated to the zinc atom of the partner molecule. The involvement of water molecules in porphyrin binding allows for an increase of distance between two porphyrin mean N4 planes, up to 4.478 Å. The offset of phosphoryl groups attached to the macrocycle through a 1,4-phenylene spacer withdraws the whole porphyrin macrocycle of one molecule from spatial overlap with the macrocycle of a partner molecule and increases the Zn-Zn distance up to 10.372 Å. This still unknown type of porphyrin dimers allows one to get deeper insights into the organization of naturally occurring tetrapyrrolic macrocycles. ZnOPPP also forms a labile dimeric complex in 5.3 × 10-7-5.8 × 10-5 M chloroform solutions. In contrast, other complexes prepared in this work exist as monomeric species under these experimental conditions. The self-association constant of ZnOPPP has been determined by electronic absorption spectroscopy.

Spectroelectrochemical Studies of CTAB Adsorbed on Gold Surfaces in Perchloric Acid.

The behaviour of CTAB adsorbed on polycrystalline gold electrodes has been studied using a combination of spectroelectrochemical methods. The results indicate that the formation of the layer is the consequence of the precipitation of the CTAB micelles on the electrode surface as bromide ions, which stabilize the micelles, are replaced by perchlorate anions. This process leads to the formation of CTA+ layers in which perchlorate ions are intercalated, in which the adlayer suffers a continuous rearrangement that leads to the formation of micro-dominions of different types of hydrogen-bonded water populations throughout the adlayer. After prolonged cycling, a stable situation is reached. Under these conditions, water molecules permeate through the adlayer toward the electrode surface at potentials positive of the potential of zero charge, due to the repulsion between the CTA+ layer and the positive charge of the electrode.

Different electrochemical effect on photoluminescence in single-doped/dual-doped carbon quantum dots

Due to their application potential in novel optoelectronic devices, several methods have been used to understand or tune the photophysical properties of doped carbon quantum dots (CQDs). Spectroelectrochemical method is one of excellent means, which has been demonstrated in nitrogen-doped CQDs film to efficiently manipulate their photoluminescence. However, the role of different doping elements are still unclear. Therefore, in this work the optical properties of sulfur-doped CQDs (S-CQDs), sulfur-nitrogen co-doped CQDs (S,N-CQDs) and fluorine-nitrogen co-doped CQDs (F,N-CQDs) films were investigated by spectroelectrochemical method. Compared with S,N-CQDs and F,N-CQDs, S-CQDs have a more obvious photoluminescence (PL) enhancement phenomenon under the same bias voltage, and their PL intensity increases by 100% and 55.8% at +2.0 V and −2.0 V, respectively. We believe that the above PL enhancement is related to the weakening of the inter-molecular π-π interaction after electrochemical charges implantation and single-doped CQDs are more conducive to this process. In addition, the onset potential of PL enhancement is also affected by the doping element.

Anaerobic Infrared Spectroelectrochemical Methods for Studying Oxygen-Sensitive [FeFe] Hydrogenases.

[FeFe] hydrogenases comprise an important class of H2 evolving enzymes; however, these proteins are often oxygen sensitive and require anaerobic environments for characterization. Understanding the electrochemical relationships between various active and inactive states of these enzymes is instrumental in uncovering the reaction mechanisms of the complex six-iron active center of [FeFe] hydrogenases called H-cluster. Since states of the H-cluster exhibit distinct fingerprint-like spectra in the mid-IR range, IR spectroelectrochemical experiments provide a powerful methodological framework for this goal. This chapter describes protocols for performing Fourier-transform infrared (FTIR) spectroelectrochemical experiments on [FeFe] hydrogenases under anaerobic conditions. Topics included experimental design, data acquisition, and data analysis.

Electrochemical polymerization, characterization and spectroelectrochemical studies of a N-substituted-2,5-dithienil-pyrrole bearing an aniline moiety for cross-linking (TPTBA)

N-substituted-2,5-dithienil-pyrroles have demonstrated to be an interesting alternative for the preparation of π-conjugated polymers with good electrochromic properties. Herein the 4-(4-(2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl)benzyl)aniline (TPTBA) was chemically synthetized and its electrochemical behavior and polymerization was studied. The obtained film was characterized by electrochemical and spectroelectrochemical methods. Poly-TPTBA presents a quasi-symmetrical reversible redox system characteristic to well-ordered systems. This explained by the presence of the aniline moiety which can favor a cross-linking of the polymer backbone. Spectroelectrochemical studies reveal a moderate rapid electrochromic transition between both redox states, requiring 2 seconds to alternate between each color: 430 nm (discharged state) and 800 nm (charged state).

Electrochromic properties of conductive polyporphyrin films based on 5,10,15,20-tetrakis(4-aminophenyl)porphyrin and 5,10,15,20-tetrakis(4-pyridyl)porphyrin

Polyporphyrin films based on 5,10,15,20-tetrakis(4-pyridyl)porphyrin and 5,10,15,20-tetrakis(4-aminophenyl)porphyrin were obtained by electrochemical polymerization during the electrooxidation of the respective monomers. The polyporphyrin formation involved the side substituents, with the [Formula: see text]-conjugated porphyrin platform being preserved. The obtained polyporphyrins form uniform electroconductive coatings, tightly adhered to the electrode surface. The electrochromic properties were studied by the spectroelectrochemical method. Electronic absorption spectra of the films were recorded under the action of electric current at the stepwise potential changes within several intervals. The obtained polyporphyrin films were shown to possess electrochromic properties. The films were oxidized when exposed to anodic potentials and reduced when exposed to cathodic potentials. The electronic absorption spectra were used to calculate the optical band gap of the initial, oxidized and reduced films and to evaluate their conductivity. The highest conductivity was found in the electrochemically oxidized poly-5,10,15,20-tetrakis(4-aminophenyl)porphyrin film.

Kinetics analysis of oxygen vacancies in TiO2 solar water reduction: Revealing effects and eliminating disadvantages

A consensus is yet to be reached on the effects of oxygen vacancy (VO) on the performance of TiO2 for photocatalytic water splitting as contrasting viewpoints have been presented in the latest researches. Herein, a comprehensive set of spectroelectrochemical methods are deployed to clearly reveal the advantages and disadvantages of VO on the performance of TiO2. The results indicate that surface VO improves the photocatalytic activity while bulk VO has a negative effect on the water reduction performance of TiO2. Intensity-modulated photocurrent spectroscopy (IMPS) and UV–vis spectroscopy provide compelling evidence that the improvement of H2 evolution can be attributed to the presence of defect level, while the low interface charge transfer efficiency caused by surface VO limits the further improvement of photocatalytical H2 evolution, which can be alleviated by an organic hole transport coating. The density functional theory (DFT) and surface photovoltaic (SPV) analyses confirm that the built-in field between TiO2 and hole molecules is the reason for the interface charge transfer efficiency improvement. Our findings provide a comprehensive understanding of VO in TiO2 by carrier behavior analysis and a scheme to further promote the photocatalytic performance.

Influence of the CN Orientation on the Degree of Electron Delocalization of Ru-Ru-Ru Mixed-Valent Complexes.

Investigations on mixed-valent complexes in the Class II/Class III frontier have been a particularly interesting issue due to their special electron delocalization. In this work, a pair of cyanidometal-/isocyanidometal-bridged Ru-Ru-Ru compounds, cis-[Cp(dppe)Ru-B-Ru(dppe)Cp]2+ (B = NCRu(bpy)2CN, 12+; B = CNRu(bpy)2NC, 22+; Cp = 1,3-cyclopentadienyl, dppe = 1,2-bis(diphenlyphosphine)ethane, bpy = 2,2'-bipyridine), and one-electron oxidized 13+ and 23+ were synthesized and well characterized. For the two-electron oxidized 14+ and 24+, their Fourier transform infrared (FTIR) and UV-vis-NIR spectra were investigated by employing spectroelectrochemical methods. The time-dependent density-functional theory (TDDFT) calculations and the experimental results indicate that the one-/two-electron oxidized mixed-valent compounds belong to Class II-III systems.

The Role of Electrochemical and Spectroelectrochemical Techniques in the Preparation and Characterization of Conjugated Polymers: From Polyaniline to Modern Organic Semiconductors.

This review article presents different electrochemical and spectroelectrochemical techniques used to investigate conjugated polymers. The development of this research area is presented from an over 40-year perspective—the period of research carried out by Professor Mieczyslaw Lapkowski. Initial research involved polymers derived from simple aromatic compounds, such as polyaniline. Since then, scientific advances in the field of conductive polymers have led to the development of so-called organic electronics. Electrochemical and spectroelectrochemical methods have a great influence in the development of organic semiconductors. Their potential for explaining many phenomena is discussed and the most relevant examples are provided.

Development of a facile time-resolved spectroelectrochemical method: An application to determine the rate constant of protonation for anions of 9,10-diphenylanthracene, biphenyl, and p-quaterphenyl

• A simple time-resolved spectroelectrochmeical method was used. • Rate constant of protonation for organic radical anions and dianion was determined. • Two or three ethanol molecules are involved in the protonation reactions. • Comproportionation reaction between anion and dianion was directly measured. Using a conventional three-electrode system and electrochemical cell, we developed a simple spectroelectrochemical method that can be used to determine the rate constant of protonation reaction for several reduced species. A glassy carbon (GC) disk electrode as a working electrode and halogen lamp as a light source were employed. The reflected light on the GC electrode surface through the diffusion layer of electrochemically generated species was recorded by an electron multiplying charge coupled device camera equipped with a spectrometer. With this system, the absorbance change by the generation of the radical anion of 9,10-diphenylanthracene (DPA), biphenyl, and p -quaterphenyl was monitored. The molar extinction coefficient ( ε ) of the radical ions was calculated with the absorbance and charge during potential steps. The absorption spectrum of electrochemically generated dianion of p -quaterphenyl was also monitored, and the ε value was determined. The comproportionation reaction between p -quaterphenyl 2− and neutral p -quaterphenyl was also clearly observed on the spectroelectrochemical data. The absorbance change during the protonation at different concentration of ethanol ( c EtOH ) for DPA •− , biphenyl •− , p -quaterphenyl •− , and p -quaterphenyl 2− and the ε values were used to determine the first-order rate constant of the protonation reaction. The first-order rate constant increased linearly against c EtOH 2 for DPA •− , p -quaterphenyl •− , and p -quaterphenyl 2− , and against c EtOH 3 for biphenyl •− . From the slopes, the reaction rate was determined to be 0.45, 4.2×10 2 , and 4.3×10 4 M −2 s −1 , for DPA •− , p -quaterphenyl •− , and p -quaterphenyl 2− , respectively, and for biphenyl •− , 6.8×10 4 M −3 s −1 .

Spectroelectrochemistry of Electroactive Polymer Composite Materials.

In this review, we have summarized the main advantages of the method of spectroelectrochemistry as applied to recent studies on electrosynthesis and redox processes of electroactive polymer composite materials, which have found wide application in designing organic optoelectronic devices, batteries and sensors. These polymer composites include electroactive polymer complexes with large unmovable dopant anions such as polymer electrolytes, organic dyes, cyclodextrins, poly(β-hydroxyethers), as well as polymer-inorganic nanocomposites. The spectroelectrochemical methods reviewed include in situ electron absorption, Raman, infrared and electron spin resonance spectroscopies.

An insight into the polymerization process of the selected carbazole derivatives - why does it not always lead to a polymer formation?

Among numerous monomers of carbazole derivatives, some reveal intriguing differences in their electrochemical polymerization behavior. Two such monomers, namely, 9-(2-naphthalen-2-yl)-9H-carbazole, CNZ1, and 9-(naphthalen-2-yl)-3,6-di(thiophen-2-yl)-9H-carbazole, CNZ2, were herein studied to a greater detail. The main research focus was to elucidate the electropolymerization mechanism of both monomers. The monomers were electro-oxidized to form polymer films on electrodes. When CNZ1 and CNZ2 were electro-oxidized under the same conditions, the overall charge passed was 115 and 1500 µC, respectively, suggesting the formation of a very thin poly(CNZ1) film and, in contrast, a thick poly(CNZ2) film. The polymers electrochemically behaved reversibly at electrodes. They were characterized by a combined UV-vis-NIR and electron paramagnetic resonance (EPR) spectroelectrochemistry. The UV-vis-NIR absorption intensity for the poly(CNZ1) film was 10 times lower than for its monomer, but the poly(CNZ2) film behaved differently. The number of radicals formed in poly(CNZ1) during the electro-oxidation was ∼30 times lower than in poly(CNZ2). We examined the formation of different charge carriers and their concentration evolution using in-situ spectroelectrochemical methods. Scanning electron microscopy (SEM) imaging revealed a continuous and granular morphology of poly(CNZ1) and poly(CNZ2) films, respectively. UV-vis-NIR and EPR spectroelectrochemistry coupled with quantum-chemistry calculations enabled us to shine more light on differences in the behavior observed. For CNZ1, the radical is delocalized over the whole molecule. That can lead to a lower reactivity, thus preventing the formation of long polymer chains. In contrast, the radical delocalization is much more restricted to the carbazole-thiophene moiety for CNZ2. Consequently, this inccreased localization leads to the formation of long polymer chains.

Multiamperometric-SERS detection of melamine on gold screen-printed electrodes

• New multiamperometric-SERS detection method for analysis. • SERS substrates electrochemically prepared in few seconds. • Simple and reproducible methodology for SERS substrates preparation. • Good figures of merit for melamine determination. A new, simple and fast protocol to generate gold-based SERS substrates is presented in this work. Melamine is a compound widely used in the industry that can be toxic for humans if consumed even in low concentrations. EC-SERS is an excellent alternative to classical methods to detect and quantify this compound because Raman spectroscopy provides a fingerprint of the molecules, providing very good sensitivity. In this work, time-resolved Raman spectroelectrochemistry is employed to generate a SERS substrate and to detect melamine, all in a single experiment. The dynamic character of this technique provides valuable information about the interaction of the molecule with the generated substrate. An optimization of the spectroelectrochemical method based on a multi-pulse amperometric detection has been performed. The new protocol presented in this work shows significant figures of merit in a very short experiment time, just 25 s.

Structure Specific Adsorbate Interactions at Electrodepositing Cu Interfaces

Mechanistic understanding of interfaces during additive based electrochemical deposition of thin films is limited by lack of tools to discretely probe given features of deposits. Modern spectroelectrochemical methods that have enabled significant understanding, like surface-enhanced Raman and infrared spectroscopies (SERS and SEIRAS, respectively), are restricted by specimen geometry; SERS is closely linked to the use of rough plasmonically active surfaces while SEIRAS measurements is constrained to thin IR transparent metal films. More recently, the application of Au@SiO2 core-shell nanoparticles as “vibrational reporters” on plasmonically inactive surfaces has enabled collection of site-specific vibrational data on well-defined single crystal surfaces whereby the nanoparticle serves to channel the plasmonic energy to the nanoparticle-substrate interface. The present work utilizes this technique, Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy (SHINERS) to track relative coverages of sulfonate, chloride and methylene moieties following super-conformal Cu electrodeposition on a patterned interface (Figure 1 a,b,d). Corroborated by computational finite-element results (Figure 1 c,d) and x-ray photoelectron spectroscopy measurements, the SHINERS measurements demonstrate that during early stages of electrodeposition, on an accelerator functionalized surface, the regions with more deposition have higher coverage of sulfonate species. As deposition continues dilation of the sulfonate concentration occurs following the continual decrease in surface area of the interface. Ultimately, this work reveals the benefit from the combined use of SHINERs, photoelectron spectroscopy and computational models to further our understanding of composite electroplating and resolve complex site-specific interfacial interactions. Figure 1

Dynamic Chemistry Interactions: Controlled Single-Entity Electrochemistry.

Single-entity electrochemistry (SEE) provides powerful means to measure single cells, single particles, and even single molecules at the nanoscale by diverse well-defined interfaces. The nanoconfined electrode interface has significantly enhanced structural, electrical, and compositional characteristics that have great effects on the assay limitation and selectivity of single-entity measurement. In this Perspective, after introducing the dynamic chemistry interactions of the target and electrode interface, we present a fundamental understanding of how these dynamic interactions control the features of the electrode interface and thus the stochastic and discrete electrochemical responses of single entities under nanoconfinement. Both stochastic single-entity collision electrochemistry and nanopore electrochemistry as examples in this Perspective explore how these interactions alter the transient charge transfer and mass transport. Finally, we discuss the further challenges and opportunities in SEE, from the design of sensing interfaces to hybrid spectro-electrochemical methods, theoretical models, and advanced data processing.

Thin films of Zn-Tetrakis(4-Hydroxyphenyl) Porphyrin: Formation, morphology and electrochemical properties

This work presents the results of obtaining polyporphyrin Zn-tetrakis(4-hydroxyphenyl)porphyrin films by superoxide-assisted electrochemical deposition. The study determines the regions of potentials where the polyporphyrin film on the working electrode is formed. It also proposes mechanisms of formation of radical porphyrin species in these regions. The data obtained by electrochemical impedance spectroscopy show an increase in the charge transfer resistance and a decrease in the electrical double layer capacitance at the solid/liquid interface during the film formation. The resistance of the film formed was also determined in situ. The paper presents the results of determining the main type of porphyrin binding in the polymer by Raman spectroscopy and suggests variants of recombination of the resulting radical porphyrin forms. The atomic force microscopy is applied to obtain data on the microrelief and thickness of the films prepared. The calculation of specific conductivity gives the value about 1.2 × 10−4 S⋅m that is typical of semiconductors. The spectroelectrochemical method was used to show the RedOx processes of the porphyrin moieties in the obtained films at low potentials.

Electrochemistry and Electrochromic Performance of a Metallopolymer Formed by Electropolymerization of a Fe(II) Complex with a Triphenylamine-Hydrazone Ligand.

The complex of Fe(II) ions of general formula [FeL2 ](BF4 )2 with triphenylamine-hydrazone ligand L has been synthesized and characterized. Oxidative electropolymerization of the complex proceeded smoothly on the working electrode producing a homogenous thin film of metallopolymer. The film thickness and morphology of the layer was investigated by microscopy techniques such as scanning electron microscopy and atomic force microscopy, and the composition of the film was confirmed by X-ray photoelectron spectroscopy analysis. It was found that fifty successive oxidation/reduction cycles resulted in a 120 nm thick film on the electrode surface. The metallopolymer was also characterized using cyclic voltammetry and spectroelectrochemical methods. The film was found to change its color from yellow to green-blue, exhibit high change in transmittance of 60 % at 770 nm, and possess good electrochemical stability during 375 cycles of switching of the potential between -0.1 V and +1.5 V, owing to the presence of metal ions that link two ligand molecules resulting in formation of highly cross-linked film. The switching times (coloration and bleaching) were calculated to be 34.2 s and 7.3 s, respectively. Coloration efficiency of the formed film of polymeric complex was found to be 144 cm2 C-1 .

Localized Spectroelectrochemical Identification of Basal Plane and Defect-Related Charge-Transfer Processes in Graphene.

It is well-known that structural defects play a decisive role in electrochemical behavior of atomically thin materials, where all the defects are directly accessible by the electrolyte. However, the vast majority of experimental techniques do not allow disentanglement of the processes at the edges/defects from those at the intact basal plane. Therefore, to address this issue, we introduce a localized spectroelectrochemical method featuring a microdroplet electrochemical cell with simultaneous Raman spectroscopy monitoring. The electrochemical and spectral responses of the basal planes of monolayer graphene samples with varying levels of disorder were compared. Two contributions, stemming from the intact and defective areas on the surface, respectively, were discovered both in the Raman G band shifts and cyclic voltammetry using the hexaammineruthenium complex. Consequently, two independent electron transfer processes of slower and faster rates coexist in one sample, but they are restricted to the defect-free and defect-rich areas, respectively.

Electrochemical and spectroelectrochemical characterization of bacteria and bacterial systems.

Microbes, such as bacteria, can be described, at one level, as small, self-sustaining chemical factories. Based on the species, strain, and even the environment, bacteria can be useful, neutral or pathogenic to human life, so it is increasingly important that we be able to characterize them at the molecular level with chemical specificity and spatial and temporal resolution in order to understand their behavior. Bacterial metabolism involves a large number of internal and external electron transfer processes, so it is logical that electrochemical techniques have been employed to investigate these bacterial metabolites. In this mini-review, we focus on electrochemical and spectroelectrochemical methods that have been developed and used specifically to chemically characterize bacteria and their behavior. First, we discuss the latest mechanistic insights and current understanding of microbial electron transfer, including both direct and mediated electron transfer. Second, we summarize progress on approaches to spatiotemporal characterization of secreted factors, including both metabolites and signaling molecules, which can be used to discern how natural or external factors can alter metabolic states of bacterial cells and change either their individual or collective behavior. Finally, we address in situ methods of single-cell characterization, which can uncover how heterogeneity in cell behavior is reflected in the behavior and properties of collections of bacteria, e.g. bacterial communities. Recent advances in (spectro)electrochemical characterization of bacteria have yielded important new insights both at the ensemble and the single-entity levels, which are furthering our understanding of bacterial behavior. These insights, in turn, promise to benefit applications ranging from biosensors to the use of bacteria in bacteria-based bioenergy generation and storage.

Oligourea molecular lifter triggered by electric field

We have synthesized an oligourea foldamer containing seven urea residues, with a Cys-like residue introduced near the N-terminus of the helix. This enabled the immobilization of the molecules on gold electrode using gold-sulfur chemistry. Molecular layer was characterized by electrochemical and spectroelectrochemical methods. Electrochemical measurements confirmed the formation of a dielectric layer on the electrode surface and the determined potential of zero free charge proved that the molecules are adsorbed with the positive pole of the dipole pointing to the electrode surface. The measurements with the use of in situ surface-enhanced infrared absorption spectroscopy provided insight into the dynamics of molecules under the influence of the electric field. Namely, helical oligourea molecules undergo reversible potential-induced changes in the orientation of the molecules. Our results show that molecular layers of oligourea helical foldamers can be used as building blocks for assembly of stimuli-responsive molecular layers serving as molecular lifters triggered by electric field.