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Sulfonamides have many important biological applications, yet their synthesis often involves long reaction times under dry and non-ambient conditions. Here we report the synthesis of a large range of sulfonamides at room temperature using 4-nitrophenyl benzylsulfonate as a starting material. Sulfonamides were prepared from a wide range of aliphatic, linear, and cyclic amines, anilines, and N-methylanilines. The yields and reaction times observed here were comparable to or better than those reported previously, establishing sulfur-phenolate exchange as a viable alternative.

SuFEx chemistry has recently evolved as next-generation click chemistry. However, in most SuFEx syntheses, additional reagents/catalysts and carefully controlled conditions are still needed. Here, we aim to further generalize S(VI) exchange chemistry, using 4-nitrophenyl phenylmethanesulfonate as example, in which the nitrophenolate group is exchanged for a wide range of (substituted) phenols and alkyl alcohols. Quantitative yields were reached within 10 min under ambient conditions and required only filtering through silica as workup.

Reaction rates of strained cycloalkynes and cycloalkenes with 1,2-quinones were quantified by stopped flow UV-Vis spectroscopy. Surprisingly, it was determined that 8-membered BCN-OH reacts substantially (16 times) faster than the more strained 7-membered THS. Thermodynamic activation parameters, obtained from the linearized Eyring equation and stopped-flow measurements, revealed that the reaction of both BCN-OH and THS with ortho-quinones is entropycontrolled, ruling out a relevant contribution of secondary orbital interactions (SOIs) as earlier proposed.

Click chemistry has been established rapidly as one of the most valuable methods for the chemical transformation of complex molecules. Due to the rapid rates, clean conversions to the products, and compatibility of the reagents and reaction conditions even in complex settings, it has found applications in many molecule-oriented disciplines. From the vast landscape of click reactions, approaches have emerged in the past decade centered around oxidative processes to generate in situ highly reactive synthons from dormant functionalities. These approaches have led to some of the fastest click reactions know to date. Here, we review the various methods that can be used for such oxidation-induced “one-pot” click chemistry for the transformation of small molecules, materials, and biomolecules. A comprehensive overview is provided of oxidation conditions that induce a click reaction, and oxidation conditions are orthogonal to other click reactions so that sequential “click-oxidation-click” derivatization of molecules can be performed in one pot. Our review of the relevant literature shows that this strategy is emerging as a powerful approach for the preparation of high-performance materials and the generation of complex biomolecules. As such, we expect that oxidation-induced “one-pot” click chemistry will widen in scope substantially in the forthcoming years.

Many biomedical fields rely on proteins that are selectively modified with novel chemical entities. These are often attached using reactive or catalytic moieties, but the position where these moieties are attached is often poorly controlled. In this work, we assess how catalyst position affects the efficiency and selectivity of protein modification. For this, we anchored a template DNA strand to the active site of three different proteins, which were subsequently hybridized to DNA strands that contained catalysts at different positions. We found that catalysts operating via a covalently bound reactant intermediate show a strong correlation between their distance to the protein surface and their efficiency and that the site-selectivity of the modification is affected by the position of the catalyst. Our results are rationalized using computational simulations, showing that one-point anchoring of the DNA construct leads to notable differences in the site of modification.

Herein, we describe the selective positioning of metal-organic framework (MOF) nanoparticles UiO-66 (Universitet i Oslo; Zr6O4(OH)4(bdc)6; bdc2- = benzene-1,4-dicarboxylate) and MIL-101 (Matérial Institut Lavoisier, Cr3O(OH) (H2O)2(bdc)3) at defined positions on a patterned substrate. For this purpose, patterned alkyne- and carboxylic acid-terminated self-assembled organic monolayer (SAM)-modified silicon surfaces were prepared by liquid immersion and microcontact printing (μCP). Preformed UiO-66 and MIL-101 nanometer-sized MOFs (NMOFs) were synthesized by solvothermal synthesis, and the nanocrystallite particles' exterior surface was functionalized in order to generate reactive sites (such as azides and amines) at the NMOFs. Copper-catalyzed alkyne azide cycloaddition and N-hydroxysuccinimide-mediated amide formation were used to selectively position the NMOFs at the surface of pre-patterned substrates. The resulting surfaces were thoroughly investigated by scanning electron microscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy, confirming the validity of the presented approach. We hope that our research paves the way for microsystem integration of NMOFs, for example, in microfluidic devices/reactors, and further investigation of their enhanced catalytic activity.

To investigate the dynamic stereochemical inversion behavior of pillar[5]arenes (P[5]s) in more detail, we synthesized a series of novel rim-differentiated P[5]s with various substituents and examined their rapid rotations by variable-temperature NMR (203–298 K). These studies revealed for the first time the barrier of “methyl-through-the-annulus” rotation (ΔG‡ = 47.4 kJ·mol–1 in acetone) and indicated that for rim-differentiated P[5]s with two types of alkyl substituents, the smaller rim typically determines the rate of rotation. However, substituents with terminal C=C or C≡C bonds give rise to lower inversion barriers, presumably as a result of attractive π–π interactions in the transition state. Finally, data on a rim-differentiated penta-methyl-penta-propargyl P[5] exhibited the complexity of the overall inversion dynamics.

[Image: see text] The development of an efficient synthetic route toward rim-differentiated C(5)-symmetric pillar[5]arenes (P[5]s), whose two rims are decorated with different chemical functionalities, opens up successive transformations of this macrocyclic scaffold. This paper describes a gram-scale synthesis of a C(5)-symmetric penta-hydroxy P[5] precursor, and a range of highly efficient reactions that allow functionalizing either rim at will via, e.g., sulfur(VI) fluoride exchange (SuFEx) reactions, esterifications, or Suzuki–Miyaura coupling. Afterward, BBr(3) demethylation activates another rim for similar functionalizations.

Dendrimers are interesting candidates for various applications because of the high level of control over their architecture, the presence of internal cavities, and the possibility for multivalent interactions. More specifically, zwitterionic dendrimers modified with an equal number of oppositely charged groups have found use in in vivo biomedical applications. However, the design and control over the synthesis of these dendrimers remains challenging, in particular with respect to achieving full modification of the dendrimer. In this work, we show the design and subsequent synthesis of dendrimers that are highly charged while having zero net charge, that is zwitterionic dendrimers that are potential candidates for biomedical applications. First, we designed and fully optimized the synthesis of charge-neutral carboxybetaine and sulfobetaine zwitterionic dendrimers. Following their synthesis, the various zwitterionic dendrimers were extensively characterized. In this study, we also report for the first time the use of X-ray photoelectron spectroscopy as an easy-to-use and quantitative tool for the compositional analysis of this type of macromolecules that can complement techniques such as nuclear magnetic resonance and gel permeation chromatography. Finally, we designed and synthesized zwitterionic dendrimers that contain a variable number of alkyne and azide groups that allow straightforward (bio)functionalization via click chemistry.

The synthesis of "rim-differentiated" C5-symmetric pillar[5]arenes, whose two rims are decorated with different chemical functionalities, has remained a challenging task. This is due to the inherent statistical nature of the cyclization of 1,4-disubstituted alkoxybenzenes with different substituents, which leads to four constitutional isomers with only 1/16th being rim-differentiated. Herein, we report a "preoriented" synthetic protocol based on FeCl3-catalyzed cyclization of asymmetrically substituted 2,5-dialkoxybenzyl alcohols. This yields an unprecedented 55% selectivity of the C5-symmetric tiara-like pillar[5]arene isomer among four constitutional isomers. Based on this new method, a series of functionalizable tiara-pillar[5]arene derivatives with C5-symmetry was successfully synthesized, isolated, and fully characterized in the solid state.

Stimulated by its success in both bioconjugation and surface modification, we studied the strain-promoted oxidation-controlled cycloalkyne–1,2-quinone cycloaddition (SPOCQ) in three ways. First, the second-order rate constants and activation parameters (ΔH⧧) were determined of various cyclooctynes reacting with 4-tert-butyl-1,2-quinone in a SPOCQ reaction, yielding values for ΔH⧧ of 4.5, 7.3, and 12.1 kcal/mol, for bicyclo[6.1.0]non-4-yne (BCN), cyclooctyne (OCT), and dibenzoazacyclooctyne (DIBAC), respectively. Second, their reaction paths were investigated in detail by a range of quantum mechanical calculations. Single-configuration theoretical methods, like various DFT and a range of MP2-based methods, typically overestimate this barrier by 3–8 kcal/mol (after inclusion of zero-point energy, thermal, and solvation corrections), whereas MP2 itself underestimates the barrier significantly. Only dispersion-corrected DFT methods like B97D (yielding 4.9, 6.4, and 12.1 kcal/mol for these three reactions) and high-level CCSD(T) and multireference multiconfiguration AQCC ab initio approaches (both yielding 8.2 kcal/mol for BCN) give good approximations of experimental data. Finally, the multireference methods show that the radical character in the TS is rather small, thus rationalizing the use of single-reference methods like B97D and SCS-MP2 as intrinsically valid approaches.

Direct analysis of synthetic fibers under ambient conditions is highly desired to identify the polymer, the finishes applied and irregularities that may compromise its performance and value. In this paper, laser ablation electrospray ionization ion mobility time-of-flight mass spectrometry (LAESI-IMS-TOF-MS) was used for the analysis of synthetic polymers and fibers. The key to this analysis was the absorption of laser light by aliphatic and aromatic nitrogen functionalities in the polymers. Analysis of polyamide (PA) 6, 46, 66, and 12 pellets and PA 6, 66, polyaramid and M5 fibers yielded characteristic fragment ions without any sample pretreatment, enabling their unambiguous identification. Synthetic fibers are, in addition, commonly covered with a surface layer for improved adhesion and processing. The same setup, but operated in a transient infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI) mode, allowed the detailed characterization of the fiber finish layer and the underlying polymer. Differences in finish layer distribution may cause variations in local properties of synthetic fibers. Here we also show the feasibility of mass spectrometry imaging (MSI) of the distribution of a finish layer on the synthetic fiber and the successful detection of local surface defects.

Inspired by the homogeneous catalyst tris(pentafluorophenyl) borane [B(C6F5)3], which acts as a promotor of Si-H bond activation, we developed and studied a method of modifying silicon oxide surfaces using hydrosilanes with B(C6F5)3 as the catalyst. This dedihydrosiloxanation reaction yields complete surface coverage within 10 min at room temperature. Organic monolayers derived from hydrosilanes with varying carbon chain lengths (C8-C18) were prepared on oxidized Si(111) surfaces, and the thermal and hydrolytic stabilities of the obtained monolayers were investigated in acidic (pH 3) medium, basic (pH 11) medium, phosphate-buffered saline (PBS), and deionized water (neutral conditions) for up to 30 days. DFT calculations were carried out to gain insight into the mechanism, and the computational results support a mechanism involving silane activation with B(C6F5)3. This catalyzed reaction path proceeds through a low-barrier-height transition state compared to the noncatalyzed reaction path.

The first SuFEx click chemistry synthesis of SOF4-derived copolymers based upon the polymerization of bis(iminosulfur oxydifluorides) and bis(aryl silyl ethers) is described. This novel class of SuFEx polymer presents two key characteristics: First, the newly created [-N=S(=O)F-O-] polymer backbone linkages are themselves SuFExable and primed to undergo further high-yielding and precise SuFEx-based post-modification with phenols or amines to yield branched functional polymers. Second, studies of individual polymer chains of several of these new materials indicate the presence of helical polymer structures, which itself suggests a preferential approach of new monomers onto the growing polymer chain upon the formation of the stereogenic linking moiety.

The use of microreactor technology for the ring-opening polymerization of l-lactide catalyzed by 1,5,7-triazabicyclo[4.4.0]dec-5-ene allows for rapid optimization of reaction parameters (reaction temperature and residence time). At moderate catalyst loading, good control over the polymerization is demonstrated by high conversion of monomer (>95%) and low polydispersity (

Molecular simulations were used to compute the equilibrium concentrations of the different species in CO2/monoethanolamine solutions for different CO2 loadings. Simulations were performed in the Reaction Ensemble using the continuous fractional component Monte Carlo method at temperatures of 293, 333, and 353 K. The resulting computed equilibrium concentrations are in excellent agreement with experimental data. The effect of different reaction pathways was investigated. For a complete understanding of the equilibrium speciation, it is essential to take all elementary reactions into account because considering only the overall reaction of CO2 with MEA is insufficient. The effects of electrostatics and intermolecular van der Waals interactions were also studied, clearly showing that solvation of reactants and products is essential for the reaction. The Reaction Ensemble Monte Carlo using the continuous fractional component method opens the possibility of investigating the effects of the solvent on CO2 chemisorption by eliminating the need to study different reaction pathways and concentrate only on the thermodynamics of the system.

A better characterization of nanometer-thick organic layers (monolayers) as used for engineering surface properties, biosensing, nanomedicine, and smart materials will widen their application. The aim of this study was to develop direct analysis in real time high-resolution mass spectrometry (DART-HRMS) into a new and complementary analytical tool for characterizing organic monolayers. To assess the scope and formulate general interpretation rules, DART-HRMS was used to analyze a diverse set of monolayers having different chemistries (amides, esters, amines, acids, alcohols, alkanes, ethers, thioethers, polymers, sugars) on five different substrates (Si, Si3N4, glass, Al2O3, Au). The substrate did not play a major role except in the case of gold, for which breaking of the weak Au-S bond that tethers the monolayer to the surface, was observed. For monolayers with stronger covalent interfacial bonds, fragmentation around terminal groups was found. For ester and amide-terminated monolayers, in situ hydrolysis during DART resulted in the detection of ions characteristic of the terminal groups (alcohol, amine, carboxylic acid). For ether and thioether-terminated layers, scission of C-O or C-S bonds also led to the release of the terminal part of the monolayer in a predictable manner. Only the spectra of alkane monolayers could not be interpreted. DART-HRMS allowed for the analysis of and distinction between monolayers containing biologically relevant mono or disaccharides. Overall, DART-HRMS is a promising surface analysis technique that combines detailed structural information on nanomaterials and ultrathin films with fast analyses under ambient conditions.

Porous aluminum oxide (PAO), a nanostructured support for, among others, culturing microorganisms, was chemically modified in order to attach biomolecules that can selectively interact with target bacteria. We present the first comprehensive study of monolayer-modified PAO using conditions that are relevant to microbial growth with a range of functional groups (carboxylic acid, a-hydroxycarboxylic acid, alkyne, alkene, phosphonic acid, and silane). Their stability was initially assessed in phosphate-buffered saline (pH 7.0) at room temperature. The most stable combination (PAO with phosphonic acids) was further studied over a range of physiological pHs (4–8) and temperatures (up to 80 °C). Varying the pH had no significant effect on the stability, but it gradually decreased with increasing temperature. The stability of phosphonic acid-modified PAO surfaces was shown to depend strongly on the other terminal group of the monolayer structure: in general, hydrophilic monolayers were less stable than hydrophobic monolayers. Finally, an alkyne-terminated PAO surface was reacted with an azide-linked mannose derivative. The resulting mannose-presenting PAO surface showed the clearly increased adherence of a mannose-binding bacterium, Lactobacillus plantarum, and also allowed for bacterial outgrowth.

Langmuir has a long tradition of publishing excellent theory and modeling papers. As of the time of writing, for example, two modeling papers from 2013 are among the most cited 10 papers in Langmuir from that year [1,2] and the most cited paper from Langmuir in 2016 is a theory and modeling paper [3]. At the same time, many manuscripts using theory and modeling are submitted to Langmuir but are ultimately rejected. Although Langmuir has a mandate to publish the highest quality work of all kinds relevant to interface science, there are some issues that exist that are specific to considering work based on theory and modeling. In this Editorial, we give our perspectives as writers, readers and editors on how to write excellent theory and modeling papers for Langmuir. We hope these perspectives will be useful to authors and also to Langmuir’s reviewers.

The fabrication of chemically and mechanically stable monolayers on the surfaces of various inorganic hard materials is crucial to the development of biomedical/electronic devices. In this Article, monolayers based on the reactivity of silane, phosphonate, 1-alkene, and 1-alkyne moieties were obtained on the hydroxyl-terminated chromium nitride surface. Their chemical stability and tribology were systematically investigated. The chemical stability of the modified CrN surfaces was tested in aqueous media at 60 °C at pH 3, 7, and 11 and monitored by static water contact angle measurements, X-ray photoelectron spectroscopy (XPS), ellipsometry, and Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS). The tribological properties of the resulting organic monolayers with different end groups (fluorinated or nonfluorinated) were studied using atomic force microscopy (AFM). It was found that the fluorinated monolayers exhibit a dramatic reduction of adhesion and friction force as well as excellent wear resistance compared to those of nonfluorinated coatings and bare CrN substrates. The combination of remarkable chemical stability and superior tribological properties makes these fluorinated monolayers promising candidates for the development of robust high-performance devices.

Thiol-yne click (TYC) chemistry was utilized as a copper-free click reaction for the modification of alkyne-terminated monolayers on oxide-free Si(111) surfaces, and the results were compared with the analogous thiol–ene click (TEC) chemistry. A wide range of thiols such as 9-fluorenylmethoxy-carbonyl cysteine, thio-ß-d-glucose tetraacetate, thioacetic acid, thioglycerol, thioglycolic acid, and 1H,1H,2H,2H-perfluorodecanethiol was immobilized using TYC under photochemical conditions, and all modified surfaces were characterized by static water contact angle measurements, X-ray photoelectron spectroscopy (including a simulation thereof by density functional calculations), and infrared absorption reflection spectroscopy. Surface-bound TYC proceeds with an efficiency of up to 1.5 thiols per alkyne group. This high surface coverage proceeds without oxidizing the Si surface. TYC yielded consistently higher surface coverages than TEC, due to double addition of thiols to alkyne-terminated monolayers. This also allows for the sequential and highly efficient attachment of two different thiols onto an alkyne-terminated monolayer.

Photothermal laser processing of organic monolayers on oxide-free silicon substrates under ambient conditions is investigated. Organic monolayers on Si(100) and Si(111) substrates are prepared via hydrosilylation of H-terminated silicon samples in neat 1-hexadecene and 1-hexadecyne, respectively. Laser processing at lambda = 514 nm and a 1/e(2) spot diameter of 2.6 microm results in local decomposition of the monolayers and oxidation of the exposed substrate. In agreement with the high thermal and chemical stability of these monolayers, a thermokinetic analysis of the data from experiments at distinct laser powers and pulse lengths points to a highly activated process. As a result, processing is strongly nonlinear and allows for subwavelength patterning, with line widths between 0.4 and 1.4 microm. Most remarkably, upon fabrication of dense line patterns, narrow organic monolayer stripes with sharp edges and lateral dimensions of 80 nm are formed. This opens up new perspectives in photothermal engineering of organic/silicon interfaces, e.g., for hybrid microelectronic and sensor applications.

A new method is presented for the fast and reproducible functionalization of silicon and silicon nitride surfaces coated with covalently attached alkyl monolayers. After formation of a methyl-terminated 1-hexadecyl monolayer on H-terminated Si(100) and Si(111) surfaces, short plasma treatments (1-3 s) are sufficient to create oxidized functionalities without damaging the underlying oxide-free silicon. The new functional groups can, e.g., be derivatized using the reaction of surface aldehyde groups with primary amines to form imine bonds. In this way, plasma-treated monolayers on silicon or silicon nitride surfaces were successfully coated with nanoparticles, or proteins such as avidin. In addition, we demonstrate the possibility of micropatterning, using a soft contact mask during the plasma treatment. Using water contact angle measurements, ellipsometry, XPS, IRRAS, AFM, and reflectometry, proof of principle is demonstrated of a yet unexplored way to form patterned alkyl monolayers on oxide-free silicon surfaces.

The relation between the morphology, optical, and photoconductive properties of thin-film bulk heterojunctions of poly(3-hexylthiophene) (P3HT) with a series of electron-accepting siloxanes with a different number (x = 2, 4, 5) of pendant naphthalene diimide (NDIS) moieties is reported. All NDIS siloxanes show good electronaccepting properties, when blended with P3HT. Interestingly, the film (nano)morphology can be controlled by relatively small changes in the molecular structure of the NDIS siloxanes. Spin-coating from orthodichlorobenzene (ODCB) yields complex film (nano)morphologies, being correlated with the weight ratios of P3HT and NDIS siloxanes, and the molecular structure of the latter. On the other hand, all blends spincoated from chloroform (CHCl3) show good mixing of the components at the molecular level. It is inferred that the nanomorphology of the blends can greatly influence their photoconductive properties: samples spincoated from ODCB invariably display a higher photoconductance than corresponding samples spin-coated from CHCl3. This is explained in terms of a higher mobility of holes in samples spin-coated from ODCB, as measured by time-resolved microwave conductivity measurements. These data are useful to delineate the conditions for the research in strives for efficient organic photovoltaics. © 2009 American Chemical Society.

Blue-emitting amine-terminated Si nanoparticles (NPs; size, 1.57 ± 0.24 nm) are functionalized with a Ru(bpy)2(spb)2+ [bpy = 2,2′-bipyridine; spb = 4-(p-N-succinimidylcarboxyphenyl)-2,2′-bipyridine] complex. The distance between the dye and the Si core is controlled by different alkyl chain lengths (−C3H6, −C6H12, and −C11H22), and the thus formed Si NPs are two-chromophore systems that exhibit dual-emission in two separate regions: blue (∼450 nm, from Si core) and red (∼630 nm, from Ru dye). By measuring the Si/Ru ratio, the extinction coefficient of amino-terminated Si NPs was experimentally determined for the first time (2.6 × 105 M−1 cm−1). Energy transfer from Si NPs to acceptor molecules [Ru(bpy)2(spb)2+] is observed by steady-state and time-resolved fluorescence, and its distance-dependent efficiency is shown to be up to 55% in the case of a short alkyl spacer (−C3H6). Energy transfer rates are for all examined cases in the (0.2−2.2) × 109 s−1 range.

A very mild method has been developed to obtain covalently attached alkyl monolayers from the attachment of 1-alkynes onto hydrogen-terminated silicon surfaces at room temperature in the dark. Apart from being the mildest method reported so far for the preparation of such monolayers, their quality, as indicated by water contact angles, XPS, and infrared spectroscopy, equals within experimental error that of the best reported alkyl monolayers on silicon.

A general, nanosecond equilibrium method is described for determining thermodynamically meaningful oxidation potentials in organic media for compounds that form highly reactive cation radicals upon one-electron oxidation. The method provides oxidation potentials with unusually high precision and accuracy. Redox ladders have been constructed of appropriate reference compounds in dichloromethane and in acetonitrile that can be used to set up electron-transfer equilibria with compounds with unknown oxidation potentials. The method has been successfully applied to determining equilibrium oxidation potentials for a series of aryl-alkylcyclopropanes, whose oxidation potentials were imprecisely known previously. Structure-property trends for oxidation potentials of the cyclopropanes are discussed.

[reaction: see text] The effects of pi-stacking on the stability of multiply hydrogen-bonded systems are investigated using hybrid DFT calculations on pi-stacked quadruply H-bonded dimers of ureidopyrimidinone in its different tautomeric forms. Both the strengths of the hydrogen bonds and the relative occurrence of tautomers are influenced by pi-stacking; electrostatics and natural bond orbital analysis are used to explain these observations. Finally, these conclusions are independent of the precise nature of the multiply hydrogen-bonded systems, including the DNA base pairs.

Molecular modeling was used to simulate various surfaces derived from the addition of 1-alkenes and 1-alkynes to Si=Si dimers on the Si(100)-2 × 1 surface. The primary aim was to better understand the interactions between adsorbates on the surface and distortions of the underlying silicon crystal due to functionalization. Random addition of ethylene and acetylene was used to determine how the addition of an adduct molecule affects subsequent additions for coverages up to one molecule per silicon dimer, that is, 100% coverage. Randomization subdues the effect that the relative positions of the adsorbates have on the enthalpy of the system. For ethylene and acetylene, the enthalpy of reaction changes less than 3 and 5 kcal/mol, respectively, from the first reacted species up to 100% coverage. As a result, a (near-)complete coverage is predicted, which is in line with experimental data. When 1-alkenes and 1-alkynes add by [2 + 2] addition, the hydrocarbon chains interact differently depending on the direction they project from the surface. These effects were investigated for four-carbon chains: 1-butene and 1-butyne. As expected, the chains that would otherwise intersect bend to avoid each other, raising the enthalpy of the system. For alkyl chains longer than four carbons, the chains are able to reorient themselves in a favorable manner, thus, resulting in a steady reduction in reaction enthalpy of about 2 kcal/mol for each additional methylene unit.

The effects of increasing concentration (10-7-2.5 x 10-3 mol/g) of Erythrosin B (Ery B) in poly(vinyl alcohol) films on its visible absorption spectrum have been investigated. In a concentration range of 2 x 10-7-10-5 mol/g, no effects on the absorption spectrum are found. By contrast, within this concentration range, time-resolved fluorescence experiments (fluorescence and anisotropy decay) reveal the presence of energy transfer between Ery B molecules, followed by fluorescence at low concentrations and fluorescence quenching at higher concentrations. At a concentration of 5 x 10-5 mol/g, the absorption spectrum broadens as compared to the monomeric spectrum and fluorescence is almost completely quenched. A further increase of the concentration results in a blue shift of the absorption spectrum. Using molecular mechanics calculations, it is shown that the initial broadening can be ascribed to excitonic interactions between randomly oriented molecules, whereas the blue shift at higher concentrations can be explained by the formation of oligomeric structures. At concentrations of 2.5 x 10-3 mol/g, the molecules are closely packed, resulting in a contribution at the red edge of the absorption spectrum.

Enantioselectivity experiments for the binding to chiral Cu(II) complexes have been performed for several α-amino acids using isothermal titration calorimetry. To a system containing nonionic micelles, Cu(II) ions, and cholesteryl glutamate as chiral selector, either the d- or l-amino acid was titrated. The highest enantioselectivities (KD/KL) were measured for the amino acids with relatively bulky R-groups [phenylalanine (1.24) and phenylglycine (1.26)], while an increase in enantioselectivity from 1.00 to 1.21 was measured upon increasing the size of the R-group in the amino acids alanine to leucine. For serine, no enantioselectivity was measured. Reciprocal enantioselectivities have been measured with phenylalanine upon inversion of the chirality of the glutamate headgroup of the chiral selector. Finally, it was found that the binding of the amino acid to the chiral−metal selector is an entropically driven process.

Vinyl cations substituted at the α-position (H2CC(+)R), with R = H, CHCH2, CH3, F, and Cl, and their neutral precursors (H2CCHR) have been studied using various quantum chemical methods to analyze the influence of these substituents on the thermodynamic stability and electronic properties. B3LYP data obtained with various basis sets are compared to those of post-HF computations, including MP2, MP4(SDQ), and QCISD(T) computations and the CBS-Q model chemistry. The results of those calculations are benchmarked against experimental results. The NBO and AIM population analysis methods are used for analysis of the electronic properties. The geometry, stability, and electronic structure of the vinyl cations under study are already accurately described at the B3LYP/6-311+G(d,p)//B3LYP/6-311G(d,p) and MP2/6-311+G(d,p)//MP2/6-311G(d,p) levels of theory. The results of the NBO calculations are shown to be preferable over AIM, since the latter, due to an artifact in that method, predicts counterintuitive charge dist...

The photopolymerization of spin-coated films of diacetylene derivatives containing phthalimido or naphthaldiimido moieties has been studied by monitoring the increase in the optical absorption in the visible region on irradiation at 308 nm. The phthalimido derivatives form blue polymers with absorption maxima at ca. 630 nm, corresponding to a highly conjugated, close to planar polydiacetylene (PDA) backbone configuration. The naphthaldiimido derivative yields a red polymer with an absorption maximum at 535 nm, indicating a backbone structure with a substantially reduced degree of -bond conjugation. The initial, "low-dose" quantum yields for monomer conversion, based on the total number of photons absorbed at 308 nm, range from 2 to 20. Monomer conversions up to ca. 50re found. The polymerized films are found to be photoconductive using the time-resolved microwave conductivity technique (TRMC). Mobile charge carriers are suggested to be formed via the triplet state of the arylimido moieties which undergoes long-distance charge transfer to polydiacetylene chains to form the arylimido radical anion and the mobile PDA radical cation or "hole". A maximum value of 1.4 x 10-2 cm2/(V s) for the product of the quantum yield for charge carrier formation and the hole mobility is found. The photoconductivity of the "blue" polymer is approximately an order of magnitude larger than for the "red" variety. This is attributed to a higher mobility of PDA holes in the former compound, resulting from the higher degree of backbone conjugation. The lifetime of the mobile carriers extends well into the microsecond region, which is considerably longer than previously found on direct ionization of PDA chains.

Alkyl cations of the form CH3C(+)HR have been investigated using high-level quantum chemical methods to study the influence of α-substituents R (R = H, CH3, CHCH2, C⋮CH, F, and Cl) on cation geometries and relative energies with respect to the neutral precursors CH3CH2R. The results of density-functional B3LYP computations with a variety of basis sets were compared with MP2, MP4, QCISD(T), and CBS-Q model chemistry results and benchmarked against experimental data. The results show that geometrical features are already accurately described using B3LYP/6-311G(d,p) or MP2/6-311G(d,p). For a systematic study of the energetics of α-substitution on alkyl cations, B3LYP/6-311+G(d,p) and MP2/6-311+G(d,p) computations form a useful compromise between accuracy (average deviation within 1 kcal/mol of the experimental error) and computational efficiency. The electronic structures of these species and their precursors CH3CH2R were studied using both natural bond orbital (NBO) and Atoms-in-molecules (AIM) analyses. Th...

The mechanism of photosensitized nucleophilic substitution reactions on arylcyclopropanes was investigated. Stereochemical experiments with methanol, water, and cyanide as nucleophiles showed that the reactions occurred stereospecifically with complete inversion of configuration at the carbon atom undergoing substitution. Independent generation of the arylcyclopropane cation radicals by nanosecond transient methods showed that they reacted rapidly with nucleophiles with kinetics that were first-order in both the cation radical and the nucleophiles. Through a combination of transient kinetics and steady-state Stern−Volmer quenching experiments, the reaction of the phenylcyclopropane cation radical with methanol was kinetically correlated with the formation of the substitution product. The reaction of phenylcyclopropane cation radical with a series of alcohols as nucleophiles showed small steric effects.

The nucleophilic substitution reactions on substituted arylcyclopropane cation radicals were studied by a combination of methods including product studies, time-resolved laser flash photolysis, kinetic isotope effects, and quantum chemical calculations. The reactions were found to proceed stereospecifically with inversion of configuration, with high regioselectivity for nucleophilic attack at the more substituted carbon atom, and with very small steric effects. Electronic effects on the nucleophilic substitution regiochemistry and the rate constants were found to be substantial for substituents on the cyclopropane moiety and on the aryl ring.

The equilibrium isotope effects (EIE) for the one-electron transfer between pyrene and seven regioselectively deuteriated pyrene isotopic isomers in dimethylformamide with 0.1 M tetrabutylammonium hexafluorophosphate were measured electrochemically. These data correlate linearly with the free energies (ΔG°) obtained in tetrahydrofuran using electron paramagnetic resonance (EPR) techniques. However, the slope of the resulting line is not unity, and it indicates that the EIE in the DMF system is only two-thirds of that in the THF system. PM3 calculated ΔG°'s, which would correspond to the gas phase electron transfers, also correlate linearly with both sets of experimental data, but the predicted magnitudes of the EIE's are smaller than those observed experimentally by either technique. The nonunity slopes probably reflect slight differences in ion solvation and/or ion association parameters between the anion radicals of the isotopic isomers. No general relationship between the EIE and the charge on the hydr...

A comparative computational study of the cation radicals of ethane and propane is made by using B3LYP density functional and high-level ab initio methods [up to QCISD(T)/6-311++G(2df,p) and CBS-APNO model chemistry]. The properties investigated include the structures and energetics of the cation radical isomers, their isotropic hyperfine coupling constants, and their carbon−carbon bond dissociation energies, as well as the vertical and adiabatic ionization potentials of the parent molecules. The computational results are compared with experimental data where possible. In general, the B3LYP method exhibits good agreement with experiment. All of the methods show that the potential energy surfaces of the two cation radicals are very flat. The electronic origin of this phenomenon and the relationship between the cation radical isomers of a given species (so-called electromers) are discussed in terms of a valence-bond model.

The solvolysis pattern of polyenol ethers in protic solvents was investigated using semiempirical molecular orbital methods with the recently developed PM3 parametrization. The complex mechanisms recently proposed for these solvolyses are substantiated and refined by our calculations. Most polyenol ethers react preferably via electron transfer with dioxygen. The radicals formed in this way combine regioselectively to form a hydroperoxide zwitterion. Two imitations to this electron-transfer pathway can be formulated: (1) if the polyene chain is short (mono- and dienol compounds), the oxidation potential is too high for electron transfer to occur and enol ether hydrolysis via protonation of the conjugated chain is observed; (2) if the alkoxy chain is protonated at the C β' -hydroxyl group, products resulting from a different ionic route are observed

A study of the charge distribution in the 1-hydropyrenyl anion (2 - ), and of its regioselectivity toward electrophilic attack was undertaken. In order to obtain reliable information on the reactive positions in 2 - , a model with the same conjugated system was prepared, which has its 1-position fixed with a spirocyclopentane ring. Highly resolved 1 H and 13 C NMR spectra of this spiro[cyclopentane-1,1'-[1H]pyrenyl] anion (1 - ) and of 2 - were obtained. These spectra could be completely assigned by means of COSY, NOESY, and 2D 1 H- 13 C COSY techniques. According to the 13 C NMR measurements, 5-Cof 1 - and 2 - bears the highest negative charge. The results of PM3 semiempirical calculations support this finding, and furthermore, a large HOMO coefficient was calculated at the quaternary 3a-C of 2 - , suggesting that this position is susceptible toward attack by soft electrophiles. In full agreement with the 13 C NMR and PM3 results, 5-C of 1 - and 2 - show the highest reactivity toward electrophilic attack, while the quaternary 3a-C of 1 - and 2 - is attacked by soft electrophiles

The effect of isotopic substitution on the electron affinity ({Delta}EA) of benzene and pyrene was studied using semiempirical MO calculations. Zero-point energy calculations were performed for various {sup 1}H/{sup 2}H- and {sup 12}C/{sup 13}C-substituted benzenes and their corresponding radical anions and for partially deuterated pyrenes and pyrene dianions. PM3 turned out to be appreciably better than either AM1 or MNDO in calculating the vibrational frequencies of neutral benzene. PM3 results were also in good agreement with the experimental IR data for the benzene radical anion The calculated values for {Delta}EA are 50-60% of the experimentally observed values. For example, {Delta}EA{sub calc} = 260 cal/mol for the benzene-h{sub 6}benzene-d{sub 6} couple, while {Delta}EA{sub exp} = 442 cal/mol. This shows that effects of this small size can reasonably be reproduced. A study of the relation between the charge on the carbon atom at which substitution takes place and the size of {Delta}EA showed no obvious general correlation to be present. 22 refs., 3 figs., 4 tabs.