Carboxylic Anchor Groups Research Articles

Highly efficient novel unsymmetrical Zn and TiO naphthalenephthalocyanine and YD2 porphyrin dyes as co-sensitizers for DSSCs

Novel unsymmetrical zinc and oxotitanium phthalocyanine (NPcs) dyes containing 4-tert-butylsulfanyl group and carboxylic acid anchor group (ZnNSPPc and TiONSPPc) were synthesized and employed as sensitizing agents for dye-sensitized solar cells (DSSCs). Synthesized NPc dyes were characterized by FTIR, UV–Vis, 1H and 13C NMR techniques. Also, density functional theory (DFT) studies were utilized to investigate molecular structures and electronic parameters such as the highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) values. The DSSCs were characterized with linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). Furthermore, cocktails of synthesized NPc dyes with YD2 porphyrin at different ratios were prepared to acquire high-efficiency solar cells. DSSCs that were sensitized with only ZnNSPPc and TiONSPPc have photon conversion efficiency (PCE) values of 5.59 % and 6.23 %, respectively. For DSSCs sensitized with cocktail dyes, the highest efficiency belongs to the YD2-ZnNSPPc mixture (3:1), with a value of 11.64 % due to the panchromatic effect.

Efficient Exciton Dislocation and Ultrafast Charge Extraction in CsPbI3 Perovskite Quantum Dots by Using Fullerene Derivative as Semiconductor Ligand.

CsPbI3 quantum dots (QDs) are of great interest in new-generation photovoltaics (PVs) due to their excellent optoelectronic properties. The long and insulative ligands protect their phase stability and enable superior photoluminescence quantum yield, however, limiting charge transportation and extraction in PV devices. In this work, we use a fullerene derivative with the carboxylic anchor group ([SAM]C60) as the semiconductor ligand and build the type II heterojunction system of CsPbI3 QDs and [SAM]C60 molecules. We find their combination enables obvious exciton dislocation and highly efficient photogenerated charge extraction. After the introduction of [SAM]C60, the exciton-binding energy of CsPbI3 decreases from 30 meV to 7 meV and the fluorescence emission mechanism also exhibits obvious changes. Transient absorption spectroscopy visualizes a ~5 ps electron extraction rate in this system. The findings gained here may guide the development of perovskite QD devices.

The Effects of Perfluoroalkyl and Alkyl Backbone Chains, Spacers, and Anchor Groups on the Performance of Organic Compounds as Corrosion Inhibitors for Aluminum Investigated Using an Integrative Experimental-Modeling Approach

The ability of surfactant-like compounds to inhibit the corrosion of aluminum in NaCl solution was systematically investigated. The basic idea of this study was to scrutinize the effect of type of backbone chain (alkyl and perfluoroalkyl), length of backbone chain (number of carbon atoms 7, 10, and 17), various anchor groups (carboxylic, thiol, and imidazole) and presence of alkylene and benzene spacers between perfluoroalkyl chain and anchor group. To tackle these effects, three model studies were designed for alkaline etched, superhydrophilic aluminum surface and then approached experimentally and by density functional theory modeling. This enabled us to decouple the adsorption affinity of selected anchor groups on the hydroxylated aluminum surface from the lateral intermolecular cohesive interactions between hydrophobic backbone chains. Fourteen compounds were used to study the changes in the surface composition, wettability and the electrochemical barrier properties. For the carboxylic anchor group, the length and type of chain are important for barrier properties and also for tuning the wettability of the surface. The addition of alkylene spacer to perfluoroalkyl chain significantly affects the properties of the modified surface. Thiol and imidazole anchor groups, however, are not efficient inhibitors regardless the type and length of backbone chains.

Synthesis of Carboxylated Viologen Cations and Their CT Salts with 1,3-Bis(dicyanomethylidene)Indan Anion

Charge transfer (CT) salt by combination of ionic organic donor and acceptor can be an interesting alternative to eliminate large voltage loss in bulk-heterojunction organic solar cells. We have succeeded in synthesis of novel CT co-crystals of 1,3 bis(dicyanomethylidene)indane anion (TCNIH-) donor and N-alkyl substituted viologen cations acceptor to exhibit broad and intense light absorption from visible to NIR. However, it is not straightforward to employ heavily crystallizing organic salts in thin film devices. As an alternative approach to prove the concept, here we propose attachment of the CT pair as a dye to porous oxide semiconductor to dissociate its CT exciton and to harvest it as current in a dye sensitized solar cell (DSSC) fashion. In this work, carboxylated viologen derivatives, N,N’-bis(carboxymethyl)-4,4’-bipyridinium chloride (CE2V) and N-(2-carboxymethyl)N’-methyl-4,4’-bipyridinium iodide (CEMV) (Fig. 1) were synthesized for their attachment to porous ZnO. Their formation of CT salts with TCNIH- has also been studied.CE2V was synthesized by carboxylation of 4,4’-bipyridinium with acrylic acid and subsequently converted to chloride with hydrochloric acid. CEMV iodide was prepared by monomethylating 4,4’-bipyridinium with methyl iodide and carboxylation with acrylic acid. The synthesized viologen was mixed with TCNIH- in 1 : 2 ratio in water, filtrated and recrystallized in ethanol to identify the structure of the co-crystals. Mesoporous ZnO film prepared by doctor blading was sequentially soaked in ethanolic solution of CE2V or CEMV, and acetone/toluene solution of TCNIH-, to obtain ZnO/viologen/TCNIH hybrid structure in the end. The attachment of respective molecules was checked by UV-vis absorption both for the film samples and the ammonia solution dissolving the entire film.Formation of CT salts was confirmed by significant broadening of the absorption towards NIR as compared to that of the Na+ salt of TCNIH- (Fig. 1), so that the presence of the carboxylic acid anchor group in viologens do not disturb their intimate ionic interaction with TCNIH-. Triclinic single crystalline structure of the co-crystals was identified by XRD. CE2V and CEMV were adsorbed at high concentrations (500 μM and 500 μM, respectively) onto ZnO, so that the carboxylic acid group indeed acted as an anchor for their attachment to ZnO. The anionic TCNIH- in fact could directly be adsorbed onto ZnO to yield bluish films, whose spectrum, however, differed from those with pre-attached viologens (Fig. 1). Those of ZnO/viologen/TCNIH indicate clear broadening towards red, which can be attributed to the formation of CT pairs. Photoelectrochemical analysis in the presence of suitable redox system is expected to result in photocurrent from the CT state. Figure 1

Visualization and Quantification of the Laser-Induced ART of TiO2 by Photoexcitation of Adsorbed Dyes

Dye-pretreated anatase TiO2 films, commonly used as photoanodes in dye sensitized solar cells (DSSC), were utilized as a model system to investigate the laser-induced anatase to rutile phase transformation (ART), using N719 dye, N749 dye, D149 dye, and MC540 dye as photo-sensitizers. The visible lasers (532 and 785 nm) used for Raman spectroscopy were able to transform pure anatase into rutile at the laser spot when excitation of the dye sensitizer caused an electron injection from the excited state of the dye molecule into the conduction band of the TiO2. The three dyes with carboxylic acid anchor groups (N719, N749 and D149 dyes) experienced ART upon dye excitation; diffuse reflectance infrared Fourier transform (DRIFTS) and Raman spectra validated that these dyes were chemisorbed to the semiconductor surface. The MC540 dye with a sulfonic acid anchor group did not experience ART and the DRIFTS and Raman spectra were inconclusive about the chemisorption of this dye to the TiO2. A TiO2 calibration curve and percent rutile contour plots developed for this project are able to quantify the amount of rutile created at the surface of the samples. These improved chemical images which map rutile concentration help to visualize how ART propagates from the center of the laser spot to the surroundings. Factors such as visible light absorption and anchor groups that covalently bind to the semiconductor play a key role in effective laser induced ART.

Control of the distance between porphyrin sensitizers and the TiO2 surface in solar cells by designed anchoring groups

Unsymmetrical porphyrins were rationally-designed and synthesized to investigate the relation between their structure, properties and adsorption geometries, and their relative performance as dyes in dye-sensitized solar cells. Photophysics, electrochemical and TiO2 anchoring properties of the new unsymmetrical N-glycolic acid amino phenyl porphyrins were evaluated. Most dyes showed good energy matching between excited state energies and the TiO2 conduction band. Depending on the porphyrins, anchoring to TiO2 occurred with only one carboxyl anchor group or with two N-glycolic acid amino phenyl connected to opposite and adjacent phenyl groups. It was found that cell efficiencies normalized for surface coverage are strongly affected by the adsorption geometry and spacer linker flexibility. The effective distance between the porphyrin core and the TiO2 surface has key importance in cell efficiencies. The data is consistent with a through-space electron transfer and anchoring viaN-glycolic acid substituents located in adjacent phenyl groups results in higher surface coverage normalized cell efficiencies.

A Photochromic Benzothiadiazole‐Diarylethene System with Tunable On/Off Fluorescence Modulation

AbstractOrganic molecules with photoswitchable emission properties are of interest in the optical devices field. We present herein in‐solution studies for two dimethoxydithienylethene‐fluorophore (DTE‐fluorophore) conjugates with a carboxylic acid anchor group attachable to semiconductor surfaces (DTE‐BODIPY and BTD‐DTE, where BTD=benzothiadiazole). Both conjugates show almost quantitative reversible photoisomerization between open (OF) and closed forms (CF). With excitation of the fluorophores, both conjugates emit fluorescence with highly solvent‐dependent OFs (BTD‐DTE>DTE‐BODIPY) and minor solvent‐dependent CFs. In the polar solvent MeCN for BTD‐DTE a TICT state is clearly observed, which will be located in the twisted conjugated structure of the benzoic acid‐BTD‐fluorophore‐methoxy‐thiophene unit. BTD‐DTE shows positive solvatofluorochromism with a decrease of fluorescence quantum yields by increasing the Lippert orientation polarizability function (Δf) verified in Lippert‐Mataga plots. This leads to solvent dependent on‐off/off‐on (CF‐OF) emission modulation by Förster resonance energy transfer (FRET) with well‐separated emission bands for BTD‐DTE.

Charge transfer dynamics in CsPbBr3 perovskite quantum dots-anthraquinone/fullerene (C60) hybrids.

An advantage of colloidal quantum dots, particularly perovskite quantum dots (PQDs), as photoactive components is that they easily form complexes with functional organic molecules, which results in hybrids with enriched photophysical properties. Herein, we demonstrate the formation of stable ground state complexes of CsPbBr3 PQD with two widely used molecular electron acceptors, fullerene (C60) and anthraquinone, (AQ) which contain carboxylic anchor groups. Dynamics of the photo-induced electron transfer in the hybrids were compared. The use of carboxylic groups for binding results in stable complex formation and their photophysical properties depend on the ratio of components but not the absolute concentrations (up to micromolar concentrations). Time-resolved transient absorption (TA) spectroscopy shows that in both cases, a charge separated (CS) state is formed. Data analysis was aimed to evaluate the CS time constant in ideal one-to-one complexes and was found to be in the range of 30-190 ps. The CS state of PQD-AQ complexes recombines directly to the ground state in roughly one microsecond. Recombination of the CS state of PQD-C60 is more complex and points to strong inhomogeneity of these complexes. Majority of the CS states relax by first forming the C60 triplet state.

Metal–Phosphate Bilayers for Anatase Surface Modification

Compared to many other metal oxides, anatase TiO2 shows relatively lower reactivity toward carboxylic acid anchor groups. The latter is crucial for applications, for example, in dye-sensitized solar cells (DSSCs), where the most used dyes bind to the metal oxide surface through carboxylic acid terminations. To improve the surface reactivity, metal-phosphate bilayers of Ni or Co were synthesized on anatase TiO2 compact oxide and nanotubes. In both cases, time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) results showed that the bilayers were successfully formed and that the phosphate layer works as an intermediate between TiO2 and the other species. ToF-SIMS depth profiles of modified nanotubes showed that Ni and Co are present through the whole tube length and reduce in content after heat treatment, in agreement with XPS results. Phosphate groups, on the other hand, are more present in the tubes' depth, and their content on the surface is reduced upon exposure to temperature. The reactivity of the modified surfaces toward carboxylic acid-terminated molecules, as stearic acid and Ru-based N719 dye, was evaluated. Contact angle measurements together with dye desorption experiments demonstrated that the Co-phosphate bilayers heat-treated at 300 °C resulted in the largest enhancement compared to the reference. Bilayer-modified compact anatase TiO2 and anatase TiO2 nanotubes were utilized as photoanodes in DSSCs. An increase in efficiency was observed for all modified electrodes with phosphate-Co treatment, leading to the highest JSC values and an efficiency improvement of 48%.

Dissociative Adsorption of Benzoic Acid on Well-Ordered Cobalt Oxide Surfaces: Role of the Protons

We performed a surface science model study to reveal the role of the protons which are released upon linking of organic molecules to oxide surfaces via carboxylic acid anchor groups. Specifically, ...

Tuning Anatase Surface Reactivity toward Carboxylic Acid Anchor Groups.

The effect of different post-treatments on TiO2 anatase surface reactivity was investigated in order to obtain the best techniques for enhancing anatase performance in diverse applications, e.g., in photocatalysis and especially as photoelectrodes for dye-sensitized solar cells (DSSCs). Different post-treatments of compact anodic anatase TiO2 were compared, including O2 plasma, UV irradiation, immersion in H2O2, vapor thermal treatment, and post-anodization, evaluating the increase of the amount of OH reactive groups on the surface and removal of surface contamination. In XPS spectra, the increase of OH groups is evident by the O 1s peak at higher binding energy. ToF-SIMS principal component analysis demonstrated that treatments performed in aqueous media led to a cleaner surface, with substantial removal of electrolyte residues. Stearic acid and the organic dye N719 were adsorbed to the differently post-treated anatase, and adsorption was evaluated by contact angle and dye desorption measurements. A higher loading with molecules containing carboxylic acid functionalities was confirmed by both techniques on the treated samples. The post-treatments that presented the highest amounts of dye were used to prepare photoelectrodes, and these were tested in DSSCs where the efficiency values doubled in comparison with the non-post-treated electrode.

Photochemistry of Carboxylate on TiO2(110) Studied with Synchrotron Radiation Photoelectron Spectroscopy.

We present a dedicated synchrotron radiation photoelectron spectroscopy (SR-PES) study of a photochemical reaction on the surface of rutile TiO2(110). The photoreaction kinetics of carboxylate species (trimethyl acetate, TMA) upon irradiation by UV and soft X-rays were monitored, and we show that it is possible to control the reaction rates from UV light and soft X-rays independently. We directly observe Ti4+ → Ti3+ conversion upon irradiation, attributed to electron trapping at Ti sites close to surface OH groups formed by deprotonation of the parent molecule, trimethylacetic acid (TMAA). TMA photolysis on two surface preparations with different oxygen vacancy densities shows that the vacancy-related charge quenches the amount of charge that can be trapped at hydroxyls upon irradiation. During the initial stages of reaction the correlation between the amount of photodepleted TMA and the amount of charge trapped in the Ti 3d band gap state is nearly 1:1. A first-order kinetics analysis reveals that the reaction rate decreases with decreasing TMA coverage. There is also a coverage-dependent difference in the electronic structure of TMA moieties, primarily involving the carboxyl anchor group. These changes are consistent with a decreased hole affinity of the adsorbed TMA and hence a decreased reaction rate. This discovery adds to the previously presented picture of a reactivity that is inversely proportional to the number of surface hydroxyls, suggesting that the balance between the amounts of TMA, OH, and trapped charge needs to be considered.

The Synthesis of Diquinone and Dihydroquinone Derivatives of Calix[4]arene and Electrochemical Characterization on Au(111) surface.

Several new electroactive diquinone and dihydroquinone derivatives of calix[4]arene bearing anchor functional groups were designed, synthesized and characterized. A method for selective protection of the hydroquinone -OH groups with trimethylsilyl groups (TMS) either on lower-rim or on upper-rim was developed. Four selected molecules - with sulfide anchor groups and carboxylic anchor groups - were adsorbed onto Au(111) single crystal surface using ex-situ and insitu self-assembly methods. Adsorbed molecules were then electrochemically probed with cyclic voltammetry. All adsorbed molecules showed redox response which changed during cycling. After conditioning CVs stabilized and showed two distinct current peaks for all molecules. Synthesized and electrochemically probed molecules are of interest to: Li-ion batteries (as cathode materials and overcharge protection), beyond Li-ion batteries and redox-flow batteries.

Charge transport through dicarboxylic-acid-terminated alkanes bound to graphene–gold nanogap electrodes

Graphene-based electrodes are attractive for single-molecule electronics due to their high stability and conductivity and reduced screening compared with metals. In this paper, we use the STM-based matrix isolation I(s) method to measure the performance of graphene in single-molecule junctions with one graphene electrode and one gold electrode. By measuring the length dependence of the electrical conductance of dicarboxylic-acid-terminated alkanes, we find that the transport is consistent with phase-coherent tunneling, but with an attenuation factor of βN = 0.69 per methyl unit, which is lower than the value measured for Au-molecule-Au junctions. Comparison with density-functional-theory calculations of electron transport through graphene-molecule-Au junctions and Au-molecule-Au junctions reveals that this difference is due to the difference in Fermi energies of the two types of junction, relative to the frontier orbitals of the molecules. For most molecules, their electrical conductance in graphene-molecule-Au junctions is higher than that in Au-molecule-Au junctions, which suggests that graphene offers superior electrode performance, when utilizing carboxylic acid anchor groups.

Symmetrically substituted phenothiazine as prospective candidate for UV responsive dye sensitized solar cells

A hybrid layer system consisting of anatase, an N-substituted dithienylated phenothiazine (DTPT) with a carboxylic anchor group as molecular donor–π–acceptor-system and poly(3-hexylthiophene) as hole transporting material is studied by means of X-ray and UV-photoelectron spectroscopy and UV/vis spectroscopy. The optoelectronic properties of the DTPT and the energy level alignment at the interface DTPT/TiO2 enable the design of an UV-responsive hybrid solar cell, which is conceptually presented.

Anchor Groups Effect on Spectroscopic and Electrochemical Properties of Quaternary Nanocrystals Cu–In–Zn–S Capped with Arylamine Derivatives

A two-step procedure is reported enabling preparation of quaternary Cu–In–Zn–S nanocrystals with electrochemically active ligands consisting of 4-dodecylphenylaminobenzene and amine, thiol, or carboxylic anchor groups. Detailed 1H NMR and IR studies of nanocrystals dispersion as well as free ligands recovered via nanocrystals dissolution indicate that in the organic shell of initial ligands weakly (1-octadecene (ODE)) and more strongly (1-dodecanethiol (DDT) and oleylamine (OLA)) bound ligands coexist. Treating the nanocrystals with pyridine removes weakly bound ligands; however, DDT and OLA molecules remain present as coligands with pyridine. Labile pyridine ligands can then be exchanged for the target 4-dodecylphenylaminobenzene derivatives with different anchor groups. 1H NMR lines of these ligands are broadened due to their restricted rotation; this broadening is especially pronounced for the lines corresponding to the anchor group protons. Electrochemical activity of the ligands is significantly altered after their binding to the nanocrystal surface. Strongly interacting anchor groups such as −Ph–SH or −Ph–CH2NH2 lose their electrochemical activity upon coordination to nanocrystals, and weakly interacting groups (−PhNH2) retain it. Secondary amine −Ph–NH–Ph– remains electrochemically active in all nanocrystals capped with the studied ligands; however, the potential of its oxidation depends on the conjugation with the anchor group.

Synthesis, electrochemistry and photo-induced electron transfer of unsymmetrical dinuclear ruthenium osmium 2,2′-bipyridine complexes

Abstract Four unsymmetric dinuclear ruthenium and osmium complexes, Os(bpy) 3 -viologen-Ru(bpy) 3 ( 1 ; bpy = 2,2′-bipyridyl), Ru(bpy) 3 -viologen-Ru(bpy)(dcbpy) 2 ( 2 ; dcbpy = 4,4′-dicarboxyl-2,2-bipyridyl), Os(bpy) 3 -viologen-Ru(bpy)(dcbpy) 2 ( 3 ) and rotaxane 1⊂CB[7] (CB[7] = cucurbit[7]uril), were successfully synthesized. The NMR, electrochemistry and photochemistry studies of these complexes were performed in non-aqueous solutions. The results show that the CB[7] host mainly locates at the butyl linker part of the osmium side in an acetonitrile solution. This binding lowers the oxidation potential of osmium whereas the oxidation potential of ruthenium does not change. The oxidation potential of ruthenium in complexes 2 and 3 is higher due to the electron withdrawing property of carboxylic anchor groups. Analysis with UV–vis spectra shows the viologen radical formation and reversed process of these complexes in non-aqueous solutions.

Dynamics of a Covalently Conjoined FRET Dye Ensemble for Electron Injection into ZnO Nanorods

A low molecular weight purely organic Forster resonance energy transfer (FRET) sensitizer system combining multiple chromophores into a single molecule via covalent attachment is designed to adjust the ratio and relative position of the fluorol donor and coumarin acceptor units. A phenyl based scaffold accommodates both FRET partners and is connected to a carboxylic anchor group for adsorption on a semiconductor surface. The functionality of the complete ensemble is demonstrated by UV–vis and luminescence lifetime measurements. The energy and charge transfer dynamics of the FRET assemblies in solution and adsorbed on ZnO nanorods are measured by femtosecond transient absorption spectroscopy. Optical pump THz probe (OPTP) spectroscopy is applied to detect the injected electrons in the ZnO electrode, confirming the full electronic pathway from FRET light harvesting to subsequent charge carrier injection.

Efficient Electron Injection from Acyloin-Anchored Semisquarylium Dyes into Colloidal TiO2 Films for Organic Dye-Sensitized Solar Cells

Semisquarylium dyes use a novel acyloin anchor group to strongly bind to TiO2 semiconductors. Efficient acyloin anchor mediated electron injection into nanocrystalline TiO2 is demonstrated, allowing highly efficient dye-sensitized solar cells with IPCEs > 80%. The acyloin anchor can thus be viewed as a true alternative to the standard carboxylic acid anchor group. The opto-electronic and electron injection properties of the most basic semisquarylium dye SY404 are compared to the modified semisquarylium dye DD1 and the carboxylic acid anchored indoline dye D131 using a combination of ultrafast and photoemission spectroscopy. For SY404, ultrafast injection times of ∼50 fs are found despite a small energetic driving force between dye excited states and TiO2 conduction band minimum. This is possible due to the strong electronic coupling of the semisquarylium dyes to the TiO2 surface mediated by the acyloin anchor. For a better overlap with the solar spectrum, the semisquarylium dyes are modified by substitution with a larger donor moiety (DD1). While for DD1 the overall absorption increases, the injection process slightly slows down; however, it still proves fast enough for very efficient injection. Compared to the carboxylic acid anchored indoline dye D131, the SY404 dye injects more than seven times faster despite a ∼150 meV smaller driving force.

Surface excitons on a ZnO (000-1) thin film

Elementary excitations at the polar (000-1) surface of a 20 nm pseudomorphically grown ZnO thin film are examined by steady state and time-resolved photoluminescence spectroscopy at low temperature. We control the density of emission centers through the deposition of prototypical organic molecules with a carboxylic acid anchor group by the Langmuir-Blodgett technique. Knowledge of the precise film thickness, defect concentrations and number density of deposited molecules leads us to associate the surface exciton emission to defect-related localization centers that are generated through a photochemical process.