Nanostructured Aluminum Oxyhydroxide-A Prospective Support for Functional Porphyrin-Based Materials

A method for the grafting of unsymmetrical A2BC-type 5,15-bis(4-butoxyphenyl)-10-(4-carboxyphenyl)-20-(phenanthrenoimidazolyl)-porphyrin onto the surface of nanostructured aluminum oxyhydroxide modified with a single SiO2 layer (NAOM) was successfully developed. A straightforward procedure towards surface modification of NAOM allowed us to prepare a new porphyrin-containing hybrid material. The obtained 3D heterostructure was extensively characterized using XPS, TEM and diffuse reflectance spectroscopy. Structural and morphological peculiarities of the inorganic support before and after the immobilization procedure were studied and discussed in detail. The stability of the material against leaching and the porphyrin immobilization ratio ca. 14% by weight were also revealed.

Unlocking Long-Term Stability of Upconversion Nanoparticles with Biocompatible Phosphonate-Based Polymer Coatings

Achieving long-term (>3 months) colloidal stability of upconversion nanoparticles (UCNPs) in biologically relevant buffers has been a major challenge, which has severely limited practical implementation of UCNPs in bioimaging and nanomedicine applications. To address this challenge, nine unique copolymers formulations were prepared and evaluated as UCNP overcoatings. These polymers consisted of a poly(isobutylene-alt-maleic anhydride) (PIMA) backbone functionalized with different ratios and types of phosphonate anchoring groups and poly(ethylene glycol) (PEG) moieties. The syntheses were done as simple, one-pot nucleophilic addition reactions. These copolymers were subsequently coated onto NaYF₄:Yb³⁺,Er³⁺ UCNPs, and colloidal stability was evaluated in 1 × PBS, 10 × PBS, and other buffers. UCNP colloidal stability improved (up to 4 months) when coated with copolymers containing greater proportions of anchoring groups and higher phosphonate valences. Furthermore, small molecules could be conjugated to these overcoated UCNPs by use of copper-free click chemistry, as was done to demonstrate suitability for sensor and bioprobe development.

Mechanically Induced Switching between Two Discrete Conductance States: A Potential Single-Molecule Variable Resistor

The fabrication of solid-state single-molecule switches with high on-off conductance ratios has been proposed to advance conventional technology in areas such as molecular electronics. Herein, we employed the scanning tunneling microscope break junction (STM-BJ) technique to modulate conductance in single-molecule junctions using mechanically induced stretching. Compound 1a possesses two dihydrobenzothiophene (DHBT) anchoring groups at the opposite ends linked with rigid alkyne side arms to form a gold-molecule-gold junction, while 1b contains 4-pyridine-anchoring groups. The incorporation of ferrocene into the backbone of each compound allows rotational freedom to the cyclopentadienyl (Cp) rings to give two distinct conductance states (high and low) for each. Various control experiments and suspended junction compression/retraction measurements indicate that these high- and low-conductance plateaus are the results of conformational changes within the junctions (extended and folded states) brought about by mechanically induced stretching. A high-low switching factor of 42 was achieved for 1a, whereas an exceptional conductance ratio in excess of 2 orders of magnitude (205) was observed for 1b. To the best of our knowledge, this is the highest experimental on-off conductance switching ratio for a single-molecule junction exploiting the mechanically induced STM-BJ method. Computational studies indicated that the two disparate conductance states observed for 1a and 1b result from mechanically induced conformational changes due to an interplay between conductance and the dihedral angles associated with the electrode-molecule interfaces. Our study reveals the structure-function relationship that determines conductance in such flexible and dynamic systems and promotes the development of a single-molecule variable resistor with high on-off switching factors.

Selective Anchoring Groups for Molecular Electronic Junctions with ITO Electrodes

Indium tin oxide (ITO) is an attractive substrate for single-molecule electronics since it is transparent while maintaining electrical conductivity. Although it has been used before as a contacting electrode in single-molecule electrical studies, these studies have been limited to the use of carboxylic acid terminal groups for binding molecular wires to the ITO substrates. There is thus the need to investigate other anchoring groups with potential for binding effectively to ITO. With this aim, we have investigated the single-molecule conductance of a series of eight tolane or "tolane-like" molecular wires with a variety of surface binding groups. We first used gold-molecule-gold junctions to identify promising targets for ITO selectivity. We then assessed the propensity and selectivity of carboxylic acid, cyanoacrylic acid, and pyridinium-squarate to bind to ITO and promote the formation of molecular heterojunctions. We found that pyridinium squarate zwitterions display excellent selectivity for binding to ITO over gold surfaces, with contact resistivity comparable to that of carboxylic acids. These single-molecule experiments are complemented by surface chemical characterization with X-ray photoelectron spectroscopy, quartz crystal microbalance, contact angle determination, and nanolithography using an atomic force miscroscope. Finally, we report the first density-functional theory calculations involving ITO electrodes to model charge transport through ITO-molecule-gold heterojunctions.

Grafting Copper and Gallium Corroles onto Zinc Oxide Nanoparticles

Two different copper and gallium arylcorroles have been functionalized using the Vilsmeier-Haack reaction. A further Knoevenagel reaction with cyanoacetic acid was performed on both complexes, affording the desired products with yields above 90 %. The newly synthesized compounds have been thoroughly characterized by a combination of spectroscopic methods, optical analyses, and X-ray crystallography. Moreover, they have been tested as anchoring groups for the hydrothermal synthesis of ZnO nanoparticles. The morphology of the heterogeneous composites has been studied by SEM, EDS and fluorescence microscopy analyses, thus confirming the presence of the corrole macrocycle in the hybrid material.

In Depth Analysis of Photovoltaic Performance of Chlorophyll Derivative-Based "All Solid-State" Dye-Sensitized Solar Cells

Chlorophyll a derivatives were integrated in "all solid-state" dye sensitized solar cells (DSSCs) with a mesoporous TiO2 electrode and 2',2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene as the hole-transport material. Despite modest power conversion efficiencies (PCEs) between 0.26% and 0.55% achieved for these chlorin dyes, a systematic investigation was carried out in order to elucidate their main limitations. To provide a comprehensive understanding of the parameters (structure, nature of the anchoring group, adsorption …) and their relationship with the PCEs, density functional theory (DFT) calculations, optical and photovoltaic studies and electron paramagnetic resonance analysis exploiting the 4-carboxy-TEMPO spin probe were combined. The recombination kinetics, the frontier molecular orbitals of these DSSCs and the adsorption efficiency onto the TiO2 surface were found to be the key parameters that govern their photovoltaic response.

Ab initio Study of Anchoring Groups for CuGaO2 Delafossite-Based p-Type Dye Sensitized Solar Cells

Here we report the first theoretical characterization of the interface between the CuGaO₂ delafossite oxide and the carboxylic (–COOH) and phosphonic acid (–PO₃H₂) anchoring groups. The promising use of delafossites as effective alternative to nickel oxide in p-type DSSC is still limited by practical difficulties in sensitizing the delafossite surface. Thus, this work provides atomistic insights on the structure and energetics of all the possible interactions between the anchoring functional groups and the CuGaO₂ surface species, including the effects of the Mg doping and of the solvent medium. Our results highlight the presence of a strong selectivity toward the monodentate binding mode on surface Ga atoms for both the carboxylic and phosphonic acid groups. Since the binding modes have a strong influence on the hole injection thermodynamics, these findings have direct implications for further development of delafossite based p-type DSSCs.

Engineering of Porphyrin Molecules for Use as Effective Cathode Interfacial Modifiers in Organic Solar Cells of Enhanced Efficiency and Stability

In the present work, we effectively modify the TiO₂ electron transport layer of organic solar cells with an inverted architecture using appropriately engineered porphyrin molecules. The results show that the optimized porphyrin modifier bearing two carboxylic acids as the anchoring groups and a triazine electron-withdrawing spacer significantly reduces the work function of TiO₂, thereby reducing the electron extraction barrier. Moreover, the lower surface energy of the porphyrin-modified substrate results in better physical compatibility between the latter and the photoactive blend. Upon employing porphyrin-modified TiO₂ electron transport layers in PTB7:PC₇₁BM-based organic solar cells we obtained an improved average power conversion efficiency up to 8.73%. Importantly, porphyrin modification significantly increased the lifetime of the devices, which retained 80% of their initial efficiency after 500 h of storage in the dark. Because of its simplicity and efficacy, this approach should give tantalizing glimpses and generate an impact into the potential of porphyrins to facilitate electron transfer in organic solar cells and related devices.

Synthesis and Single-Molecule Conductance Study of Redox-Active Ruthenium Complexes with Pyridyl and Dihydrobenzo[b]thiophene Anchoring Groups

The ancillary ligands 4'-(4-pyridyl)-2,2':6',2''-terpyridine and 4'-(2,3-dihydrobenzo[b]thiophene)-2,2'-6',2"-terpyridine were used to synthesize two series of mono- and dinuclear ruthenium complexes differing in their lengths and anchoring groups. The electrochemical and single-molecular conductance properties of these two series of ruthenium complexes were studied experimentally by means of cyclic voltammetry and the scanning tunneling microscopy-break junction technique (STM-BJ) and theoretically by means of density functional theory (DFT). Cyclic voltammetry data showed clear redox peaks corresponding to both the metal- and ligand-related redox reactions. Single-molecular conductance demonstrated an exponential decay of the molecular conductance with the increase in molecular length for both the series of ruthenium complexes, with decay constants of βPY =2.07±0.1 nm(-1) and βBT =2.16±0.1 nm(-1) , respectively. The contact resistance of complexes with 2,3-dihydrobenzo[b]thiophene (BT) anchoring groups is found to be smaller than the contact resistance of ruthenium complexes with pyridine (PY) anchors. DFT calculations support the experimental results and provided additional information on the electronic structure and charge transport properties in those metal|ruthenium complex|metal junctions.

Two New Armtype Polyoxometalates Grafted on Titanium Dioxide Films: Towards Enhanced Photoelectrochemical Performance

Two new carboxyethyltin-functionalized polyoxometalates (POMs) were successfully obtained and confirmed with physicochemical and spectroscopic methods including X-ray crystallography. The lowest unoccupied molecular orbitals of both compounds are higher in energy than that of TiO2 , and the optical band gaps of these compounds are smaller than that of TiO2 . Grafting them onto a TiO2 film created two kinds of novel photoanode materials that showed significantly enhanced photovoltaic and photocurrent responses, as well as improved photoelectrooxidation activities for methanol relative to that shown by a single TiO2 film. Further, P2 W15 -Co-SnR produced the largest photocurrent by exploring the photoelectric activities of a series of carboxyethyltin POM derivatives. This work provides new insight into the photoelectrochemical functionalization of POM-based organic-inorganic hybrids.

Systematic Investigations on the Roles of the Electron Acceptor and Neighboring Ethynylene Moiety in Porphyrins for Dye-Sensitized Solar Cells

Cyanoacrylic and carboxyl groups have been developed as the most extensively used electron acceptor and anchoring group for the design of sensitizers for dye-sensitized solar cells. In terms of the photoelectric conversion efficiency, each of them has been demonstrated to be superior to the other one in certain cases. Herein, to further understand the effect of these two groups on cell efficiencies, a series of porphyrin sensitizers were designed and synthesized, with the acceptors systematically varied, and the effect of the neighboring ethynylene unit was also investigated. Compared with the sensitizer XW5 which contains a carboxyphenyl anchoring moiety directly linked to the meso-position of the porphyrin framework, the separate introduction of a strongly electron-withdrawing cyanoacrylic acid as the anchoring group or the insertion of an ethynylene unit can achieve broadened light absorption and IPCE response, resulting in higher Jsc and higher efficiency. Thus, compared with the efficiency of 4.77% for XW5, dyes XW1 and XW6 exhibit higher efficiencies of 7.09% and 5.92%, respectively. Simultaneous introduction of the cyanoacrylic acid and the ethynylene units into XW7 can further broaden light absorption and thus further improve the Jsc. However, XW7 exhibits the lowest Voc value, which is not only related to the floppy structure of the cyanoacrylic group but also related to the aggravated dye aggregation effect due to the extended framework. As a result, XW7 exhibits a relatively low efficiency of 5.75%. These results indicate that the combination of the ethynylene and cyanoacrylic groups is an unsuccessful approach. To address this problem, a cyano substituent was introduced to XW8 at the ortho position of the carboxyl group in the carboxyphenyl acceptor. Thus, XW8 exhibits the highest efficiency of 7.59% among these dyes. Further cosensitization of XW8 with XS3 dramatically improved the efficiency to 9.31%.

Electrical properties and mechanical stability of anchoring groups for single-molecule electronics

We report on an experimental investigation of transport through single molecules, trapped between two gold nano-electrodes fabricated with the mechanically controlled break junction (MCBJ) technique. The four molecules studied share the same core structure, namely oligo(phenylene ethynylene) (OPE3), while having different aurophilic anchoring groups: thiol (SAc), methyl sulfide (SMe), pyridyl (Py) and amine (NH2). The focus of this paper is on the combined characterization of the electrical and mechanical properties determined by the anchoring groups. From conductance histograms we find that thiol anchored molecules provide the highest conductance; a single-level model fit to current-voltage characteristics suggests that SAc groups exhibit a higher electronic coupling to the electrodes, together with better level alignment than the other three groups. An analysis of the mechanical stability, recording the lifetime in a self-breaking method, shows that Py and SAc yield the most stable junctions while SMe form short-lived junctions. Density functional theory combined with non-equlibrium Green's function calculations help in elucidating the experimental findings.

What Makes Hydroxamate a Promising Anchoring Group in Dye-Sensitized Solar Cells? Insights from Theoretical Investigation

We report, from a theoretical point of view, the first comparative study between the highly water-stable hydroxamate and the widely used carboxylate, in addition to the robust phosphate anchors. Theoretical calculations reveal that hydroxamate would be better for photoabsorption. A quantum dynamics description of the interfacial electron transfer (IET), including the underlying nuclear motion effect, is presented. We find that both hydroxamate and carboxylate would have efficient IET character; for phosphate the injection time is significantly longer (several hundred femtoseconds). We also verified that the symmetry of the geometry of the anchoring group plays important roles in the electronic charge delocalization. We conclude that hydroxamate can be a promising anchoring group, as compared to carboxylate and phosphate, due to its better photoabsorption and comparable IET time scale as well as the experimental advantage of water stability. We expect the implications of these findings to be relevant for the design of more efficient anchoring groups for dye-sensitized solar cell (DSSC) application.