Rate Accelerations of Bromination Reactions with NaBr and H₂O₂ via the Addition of Catalytic Quantities of Diaryl Ditellurides

Diaryl ditellurides were oxidized in situ to give aryltellurinic acids, which catalyzed the oxidation of NaBr with H₂O₂ in buffered aqueous solutions. The aryltellurinic acids were slowly oxidized under the reaction conditions to the corresponding telluronic acids, which did not catalyze oxidation of NaBr with H₂O₂. Both 4-(methoxyphenyl)­tellurinic acid and 4-(methoxyphenyl)­telluronic acid were characterized in solution by ¹²⁵Te NMR and for their effectiveness as catalysts in kinetics studies. The effectiveness of the tellurinic acids as catalysts was very sensitive to electron demand in the intermediates present during the course of the reaction. Electron-withdrawing substituents favor the deprotonated tellurinic acid (tellurinate) in solution, while electron-donating substituents favor the protonated tellurinic acid. Of the nine ditellurides screened for their ability to accelerate the oxidation of NaBr with H₂O₂, diphenyl ditelluride emerged as the most active. The addition of only 0.20 mol % of this ditelluride (relative to substrate) promoted a 240-fold increase in the rate of oxidation of NaBr with H₂O₂, as measured by the bromination of 4-pentenoic acid. The “Br⁺” species prepared in situ were trapped by a series of alkenoic acids and activated aryl compounds

Influence of Surface-Attachment Functionality on the Aggregation, Persistence, and Electron-Transfer Reactivity of Chalcogenorhodamine Dyes on TiO₂

Chalcogenorhodamine dyes bearing phosphonic acids and carboxylic acids were compared as sensitizers of nanocrystalline TiO₂ in dye-sensitized solar cells (DSSCs). The dyes were constructed around a 3,6-bis­(dimethylamino)­chalcogenoxanthylium core and varied in the 9 substituent: 5-carboxythien-2-yl in dyes <b>1-E</b> (E = O, Se), 4-carboxyphenyl in dyes <b>2-E</b> (E = O, S), 5-phosphonothien-2-yl in dyes <b>3-E</b> (E = O, Se), and 4-phosphonophenyl in dyes <b>4-E</b> (E = O, Se). All dyes adsorbed to TiO₂ as mixtures of H aggregates and monomers, which exhibited broadened absorption spectra relative to those of purely amorphous monolayers. Surface coverages of dyes and the extent of H aggregation varied minimally with the surface-attachment functionality, the structure of the 9-aryl group, and the identity of the chalcogen heteroatom. Carboxylic acid-functionalized dyes <b>1-E</b> and <b>2-E</b> desorbed rapidly and completely from TiO₂ into acidified CH₃CN, but phosphonic acid-functionalized dyes <b>3-E</b> and <b>4-E</b> persisted on TiO₂ for days. Short-circuit photocurrent action spectra of DSSCs corresponded closely to the absorptance spectra of dye-functionalized films; thus, H aggregation did not decrease the electron-injection yield or charge-collection efficiency. Maximum monochromatic incident photon-to-current efficiencies (IPCEs) of DSSCs ranged from 53 to 95% and were slightly higher for carboxylic acid-functionalized dyes <b>1-E</b> and <b>2-E</b>. Power-conversion efficiencies of DSSCs under white-light illumination were low (<1%), suggesting that dye regeneration was inefficient at high light intensities. The photoelectrochemical performance (under monochromatic or white-light illumination) of <b>1-E</b> and <b>2-E</b> decayed significantly within 20–80 min of the assembly of DSSCs, primarily because of the desorption of the dyes. In contrast, the performance of phosphonic acid-functionalized dyes remained stable or improved slightly on similar timescales. Thus, replacing carboxylic acids with phosphonic acids increased the inertness of chalcogenorhodamine–TiO₂ interfaces without greatly impacting the aggregation of dyes or the interfacial electron-transfer reactivity

Intermolecular Charge Separation in Aggregated Rhodamine Dyes Used in Solar Hydrogen Production

Various modern solar light-harvesting systems, including those used in photovoltaics and solar fuel production, depend on efficient electron transfer from a surface-bound molecular dye to nanoscopic semiconductor particles. However, the productive electron transfer competes with a variety of other relaxation pathways for the dye, and the dominant pathway can change dramatically depending on its environment. A new sulfur-substituted thiorhodamine dye was synthesized having exceptional light-harvesting qualities for solar energy applications and for solar hydrogen production in particular. The dye was created with a thiophene spacer bearing a phosphonate-ester (<b>1-Ester</b>) or phosphonic-acid (<b>1-Acid</b>) allowing for excellent solubility in MeCN or the ability to functionalize metal oxide semiconductor nanoparticles such as TiO₂. While <b>1-Ester</b> is found to be fully monomeric in MeCN, <b>1-Acid</b> readily forms H-aggregated dimers which, upon photoexcitation, undergo charge separation to an ion pair (IP) in 1.5 ps. For <b>1-Acid</b> dimers, the stabilization of the IP causes an increase in lifetime to 270 ps compared to the 75 ps lifetime of the monomer. When <b>1-Acid</b> is attached to TiO₂, the inhomogeneous surface creates a distribution of chromophore packing structures where a range of transition dipole coupling environments is present such that both excimers and IPs can form. In a variety of solvent environments, ultrafast electron injection was found to occur in <300 fs from the dye to the semiconductor while IP formation occurs in 2–4 ps. For all aggregates studied, the photophysics was the same whether pumped at 620 nm, exciting to the 0–0 absorption band, or at 565 nm to the 0–1 transition that is dramatically enhanced by transition-dipole coupling in the H-aggregate. Surprisingly, the long-time, >2 ns, persistent formation of the charge-separated state following charge injection to TiO₂ only accounts for ∼10% of the photoexcited population, with the dominant relaxation pathways being IP and excimer formation. IP and excimer formation lower the total energy of the aggregate below the conduction band edge of TiO₂, deactivating the electron transfer process. The implications of IP and excimer formation in systems for solar light harvesting are discussed

Synthesis and Photoelectrochemical Performance of Chalcogenopyrylium Monomethine Dyes Bearing Phosphonate/Phosphonic Acid Substituents

Chalcogenopyrylium monomethine dyes were prepared via condensation of a 4-methylchalcogenopyrylium compound with a chalcogenopyran-4-one bearing a 4-(diethoxyphosphoryl)­phenyl substituent (or the phosphonic acid derivative). The dyes have absorbance maxima of 603–697 nm in the window where the solar spectrum is most intense. The dyes formed H-aggregates on TiO₂, increasing the light-harvesting efficiency of the dyes. Shortcircuit photocurrent action spectra were acquired to evaluate the influence of dye structure on the photoelectrochemical performance

Selenorhodamine Dye-Sensitized Solar Cells: Influence of Structure and Surface-Anchoring Mode on Aggregation, Persistence, and Photoelectrochemical Performance

A library of six selenorhodamine dyes (<b>4-Se</b>–<b>9-Se</b>) were synthesized, characterized, and evaluated as photosensitizers of TiO₂ in dye-sensitized solar cells (DSSCs). The dyes were constructed around either a bis­(julolidyl)- or bis­(half-julolidyl)-modified selenoxanthylium core functionalized at the 9-position with a thienyl group bearing a carboxylic, hydroxamic, or phosphonic acid for attachment to TiO₂. Absorption bands of solvated dyes <b>4-Se</b>–<b>9-Se</b> were red-shifted relative to the dimethylamino analogues. The dyes adsorbed to TiO₂ as mixtures of monomeric and H-aggregated dyes, which exhibited broadened absorption spectra and increased light-harvesting efficiencies relative to the solvated monomeric dyes. Carboxylic acid-bearing dyes <b>4-Se</b> and <b>7-Se</b> initially exhibited the highest incident photon-to-current efficiencies (IPCEs) of 65–80% under monochromatic illumination, but the dyes desorbed rapidly from TiO₂ into solutions of HCl (0.1 M) in a CH₃CN:H₂O mixed solvent (120:1 v:v). The hydroxamic acid- and phosphonic acid-bearing dyes <b>5-Se</b>, <b>6-Se</b>, <b>8-Se</b>, and <b>9-Se</b> exhibited lower IPCEs (49–65%) immediately after preparation of DSSCs; however, the dyes were vastly more inert on TiO₂, and IPCEs decreased only minimally with successive measurements under constant illumination. Power-conversion efficiencies (PCEs) of the selenorhodamine-derived DSSCs were less than 1%, probably due to inefficient regeneration of the dyes following electron injection. For a given anchoring group, the bis­(half-julolidyl) dyes exhibited higher open-circuit photovoltages and PCEs than the corresponding bis­(julolidyl) dyes. The hydroxamic acid- and phosphonic acid-bearing dyes are intriguing photosensitizers of TiO₂ in light of their aggregation-induced spectral broadening, high monochromatic IPCEs, and relative inertness to desorption into acidic media

The performance of aminoalkyl/fluorocarbon/hydrocarbon-modified xerogel coatings against the marine alga <i>Ectocarpus crouaniorum</i>: relative roles of surface energy and charge

<div><p>The effect of a series of xerogel coatings modified with aminoalkyl/fluorocarbon/hydrocarbon groups on the adhesion of a new test species, the filamentous brown alga <i>Ectocarpus crouaniorum</i>, has been explored, and compared with the green alga <i>Ulva linza</i>. The results showed that <i>E. crouaniorum</i> adhered weakly to the less polar, low wettability coatings in the series, but stronger adhesion was shown on polar, higher surface energy coatings containing aminoalkyl groups. The results from a separate series of coatings tuned to have similar surface energies and polarities after immersion in artificial seawater (ASW), but widely different surface charges, demonstrated that surface charge was more important than surface energy and polarity in determining the adhesion strength of both <i>E. crouaniorum</i> and <i>U. linza</i> on xerogel coatings. No correlation was found between adhesion and contact angle hysteresis. X-ray photoelectron spectroscopy analysis of samples after immersion in ASW confirmed the presence of charged ammonium groups on the surface of the aminoalkylated coatings.</p> </div

Probing Nanoscale Chemical Segregation and Surface Properties of Antifouling Hybrid Xerogel Films

Over the past decade there has been significant development in hybrid polymer coatings exhibiting tunable surface morphology, surface charge, and chemical segregationall believed to be key properties in antifouling (AF) coating performance. While a large body of research exists on these materials, there have yet to be studies on all the aforementioned properties in a colocalized manner with nanoscale spatial resolution. Here, we report colocalized atomic force microscopy, scanning Kelvin probe microscopy, and confocal Raman microscopy on a model AF xerogel film composed of 1:9:9 (mol:mol:mol) 3-aminopropyltriethoxysilane (APTES), <i>n</i>-octyltriethoxysilane (C8), and tetraethoxysilane (TEOS) formed on Al₂O₃. This AF film is found to consist of three regions that are chemically and physically unique in 2D and 3D across multiple length scales: (i) a 1.5 μm thick base layer derived from all three precursors; (ii) 2–4 μm diameter mesa-like features that are enriched in free amine (from APTES), depleted in the other species and that extend 150–400 nm above the base layer; and (iii) 1–2 μm diameter subsurface inclusions within the base layer that are enriched in hydrogen-bonded amine (from APTES) and depleted in the other species

Supporting Information from Sensitive SERS nanotags for use with a hand-held 1064 nm Raman spectrometer

General experimental details for the synthesis and characterisation of dyes 1-17 and HGNs; schematic detailing experimental setup, figures of extinction spectra for HGNs and selected chalcogenopyrylium dyes; chemical structures for dyes 1-17 and commercial reporters; SERS spectra and LOD plots for dye/HGN nanotags plus commercial reporter/HGN nanotags with 1064 nm excitation

GPx-Like Activity of Selenides and Selenoxides: Experimental Evidence for the Involvement of Hydroxy Perhydroxy Selenane as the Active Species

The reaction mechanism of the GPx-like oxidation of PhSH with H₂O₂ catalyzed by selenoxides proceeds via formation of the hydroxy perhydroxy selenane, which is a stronger oxidizing agent than selenoxide. A hydroxy perhydroxy selenane intermediate was observed by electrospray ionization mass spectrometry and ⁷⁷Se NMR spectroscopy in reactions of selenoxide <b>8</b> with H₂O₂.The initial velocity of oxidation of PhSH by H₂O₂ with selenoxide <b>8</b> is 4 orders of magnitude higher than that of <b>8</b> without peroxide. Selenoxide <b>8</b> is not reduced to selenide <b>6</b> by PhSH in the presence of H₂O₂. While electronic substituent effects have minimal impact on the catalytic performance of selenoxides, chelating groups increase the rate of catalysis

Hybrid Sol–Gel-Derived Films That Spontaneously Form Complex Surface Topographies

Surface patterns over multiple length scales are known to influence various biological processes. Here we report the synthesis and characterization of new, two-component xerogel thin films derived from carboxyethylsilanetriol (COE) and tetraethoxysilane (TEOS). Atomic force microscopy (AFM) reveals films surface with branched and hyper branched architectures that are ∼2 to 30 μm in diameter, that extend ∼3 to 1300 nm above the film base plane with surface densities that range from 2 to 77% surface area coverage. Colocalized AFM and Raman spectroscopy show that these branched structures are COE-rich domains, which are slightly stiffer (as shown from phase AFM imaging) and exhibit lower capacitive force in comparison with film base plane. Raman mapping reveals there are also discrete domains (≤300 nm in diameter) that are rich in COE dimers and densified TEOS, which do not appear to correspond with any surface structure seen by AFM