Synthesis, fluorescence, and two-photon absorption of bidentate phosphane oxide derivatives: complexation with pb(2+) and cd(2+) cations

A series of fluorescent phosphane oxide derivatives based on diphenylphosphanoethane (DPPE) and diphenylphosphanomethane (DPPM) skeletons has been prepared by means of Grignard reactions and Sonogashira cross-couplings. The photophysical properties and the linear and nonlinear spectra of these compounds have been investigated. An edge-to-face conformation resulting in the formation of an excimer was confirmed by fluorescence lifetime measurements of these multichromophoric derivatives. Upon complexation with heavy metal ions such as Pb2+ and Cd2+, a red shift of the one- and two-photon excitation spectra was observed in the absorption and emission spectra. Furthermore, enhancement of the electron-withdrawing character of the phosphane oxide resulted in a significant enhancement of the two-photon absorption cross-section, leading to the first biphotonic Cd2+ sensors combining high affinity for Cd2+, large two-photon absorption cross-sections, and significant enhancement of the two-photon excited fluorescence in the presence of the cation. Such derivatives are highly promising for incorporation into devices for the detection of heavy metal ions in water and effluents.     

Why are water-hydrophobic interfaces charged?

We report ab initio molecular dynamics simulations of hydroxide and hydronium ions near a hydrophobic interface, indicating that both ions behave like amphiphilic surfactants that stick to a hydrophobic hydrocarbon surface with their hydrophobic side. We show that this behavior originates from the asymmetry of the molecular charge distribution which makes one end of the ions strongly hydrophobic while the other end is even more hydrophilic than the regular water (H2O) molecules. The effect is more pronounced for the hydroxide than for the hydronium. Our results are consistent with several experimental observations and explain why hydrophobic surfaces in contact with water acquire a net negative charge, a phenomenon that has important implications for biology and polymer science.     

Investigation of the possible origins of sulfur in 19th century salted paper photographs by x‐ray fluorescence spectroscopy

An X‐ray fluorescence method to determine whether sulfur is present in 19th century photographs due to intentional toning or to environmental deterioration is proposed. In the 19th century salted paper print photographic process, AgCl formed on the surface of a sheet of paper was exposed to sunlight in contact with a negative, leading to the printing out of a Ag image that was fixed by immersing it in a sodium thiosulfate solution or hypo. The improper execution of the fixing in these photographs may result in the presence of sulfur, mostly manifested in image fading, irregular staining, and discoloration. Also, 19th century artists produced salted paper prints with a variety of image tonalities, for example, by using an artificially aged thiosulfate bath. The presence of sulfur in photographs may also be due to sulfur‐containing environmental pollutants. Therefore, knowledge about the location and amounts of sulfur is important to understand the artistic technique and/or the deterioration processes. In this study, the amounts of sulfur and the silver to sulfur signal ratios were determined for a salted paper print made in the laboratory following a 19th century procedure and for two artistic salted paper photographs using X‐ray fluorescence and standards prepared with different amounts of retained hypo and quantified by inductively coupled plasma atomic emission spectroscopy. The amounts of sulfur and the normalized silver to sulfur ratios for the artistic photographs were evaluated based on the results obtained in the samples prepared in the laboratory and in the context of the 19th century practices. Copyright © 2016 John Wiley & Sons, Ltd.     

Synthetic,Optical and Theoretical Study of Alternating Ethylenedioxythiophene–Pyridine Oligomers: Evolution from Planar Conjugated to Helicoidal Structure towards a Chiral Configuration

A series of alternating 3,4‐ethylenedioxythiophene–alkynylpyridine oligomers (DA)_n with increased solubility are synthesized and their photophysical properties and nonlinear optical properties are investigated. Their quadratic polarizabilities are determined from hyper‐Rayleigh scattering experiments to obtain information on their conformations in solution. These chromophores, based on the alternation of electron‐rich (D) and electron‐deficient (A) moieties, exhibit optical properties that arise from the combination of dipolar and helicoidal features in the (DA)_n homologue series where n=1–4. The transition from dipolar conjugated planar structures (n=1, 2) to helicoidal structures (n=3, 4) is clearly evidenced by results from symmetry‐sensitive second‐order nonlinear optical experiments. This suggests an approach towards highly efficient chiral chromophores for second‐order nonlinear optics. Interestingly, this structural evolution also has significant impact on the photophysical properties: both absorption and fluorescence emission show bathochromic and hyperchromic shifts with increasing number of repeating units in the dipolar planar derivatives (n=1–2) but show saturation effects in the helicoidal structures (n=2–4). In addition, the helicoidal structures show sizeable two‐photon absorption at 700–750 nm (40–100 GM) for compounds lacking either electron‐donating or electron‐withdrawing substituents.     

Formal total syntheses of aspidosperma alkaloids via a novel and general synthetic pathway based on an intramolecular Heck cyclization

Cyclizations of bicyclic amides via an intramolecular Heck reaction followed by an oxidation reaction generate tricyclic spirocyclohexadienones. From these compounds, tetracyclic ketones can be synthesized to provide useful intermediates for the synthesis of indole alkaloids.     

Synthesis, fluorescence, and two-photon absorption of a series of elongated rodlike and banana-shaped quadrupolar fluorophores: a comprehensive study of structure-property relationships

An extensive series of push-push and pull-pull derivatives was prepared from the symmetrical functionalization of an ambivalent core with conjugated rods made from arylene-vinylene or arylene-ethynylene building blocks, bearing different acceptor or donor end-groups. Their absorption and photoluminescence, as well as their two-photon-absorption (TPA) properties in the near infrared (NIR) region, were systematically investigated to derive structure-property relationships and to lay the guidelines for both spectral tuning and amplification of molecular TPA in the target region. Whatever the nature of the core or of the connectors, push-push systems were found to be more efficient than pull-pull systems, and planarization of the core (fluorene versus biphenyl) always leads to an increase in the TPA cross sections. In contrast, increasing the conjugation length as well as replacement of a phenylene moiety by a thienylene moiety in the conjugated rods did not necessarily lead to increased TPA responses. The present study also demonstrated that the topology of the conjugated rods can dramatically influence the TPA properties. This is of particular interest in terms of molecular engineering for specific applications, as both TPA properties and photoluminescence characteristics can be considerably affected. Thus, it becomes possible to optimize the transparency/TPA and fluorescence/TPA efficiency trade-offs for optical limiting in the red-NIR region (700-900 nm) and for two-photon-excited fluorescence (TPEF) microscopy applications, respectively.     

Quantization of the dipole moment and of the end charges in push-pull polymers

A theorem for end-charge quantization in quasi-one-dimensional stereoregular chains is formulated and proved. It is a direct analog of the well-known theorem for surface charges in physics. The theorem states the following: (1) Regardless of the end groups, in stereoregular oligomers with a centrosymmetric bulk, the end charges can only be a multiple of 12 and the longitudinal dipole moment per monomer p can only be a multiple of 12 times the unit length a in the limit of long chains. (2) In oligomers with a noncentrosymmetric bulk, the end charges can assume any value set by the nature of the bulk. Nonetheless, by modifying the end groups, one can only change the end charge by an integer and the dipole moment p by an integer multiple of the unit length a. (3) When the entire bulk part of the system is modified, the end charges may change in an arbitrary way; however, if upon such a modification the system remains centrosymmetric, the end charges can only change by multiples of 12 as a direct consequence of (1). The above statements imply that-in all cases-the end charges are uniquely determined, modulo an integer, by a property of the bulk alone. The theorem's origin is a robust topological phenomenon related to the Berry phase. The effects of the quantization are first demonstrated in toy LiF chains and then in a series of trans-polyacetylene oligomers with neutral and charge-transfer end groups.     

Quenching of molecular fluorescence on the surface of monolayer-protected gold nanoparticles investigated using place exchange equilibria

The insertion of fluorescently labeled thiols into the protecting self-assembled monolayer on the surface of gold nanoparticles through place exchange reactions and the effects of this insertion on the photophysical properties of the fluorophores are investigated. Analysis of solution-phase fluorescence data using a dynamic equilibrium model yields the equilibrium constant for the place exchange equilibrium, as well as the relative fluorescence brightness of the fluorophores on the particle surface. In all cases we find a significant quenching of the fluorescence, and potential reasons for this quenching are discussed. In the case of these relatively small particles (4.5 nm diameter), the quenching appears to be mainly related to enhanced nonradiative deactivation pathways. The place exchange equilibrium constant reveals a reduced affinity of the fluorescently labeled thiols for insertion into the nonfluorescent alkylthiol monolayer (K(eq) approximately 0.2) compared to unlabeled alkylthiols.     

Exploiting Complex Fluorophore Interactions to Monitor Virus Capsid Disassembly

Supramolecular protein complexes are the corner stone of biological processes; they are essential for many biological functions. Unraveling the interactions responsible for the (dis)assembly of these complexes is required to understand nature and to exploit such systems in future applications. Virus capsids are well-defined assemblies of hundreds of proteins and form the outer shell of non-enveloped viruses. Due to their potential as a drug carriers or nano-reactors and the need for virus inactivation strategies, assessing the intactness of virus capsids is of great interest. Current methods to evaluate the (dis)assembly of these protein assemblies are experimentally demanding in terms of instrumentation, expertise and time. Here we investigate a new strategy to monitor the disassembly of fluorescently labeled virus capsids. To monitor surfactant-induced capsid disassembly, we exploit the complex photophysical interplay between multiple fluorophores conjugated to capsid proteins. The disassembly of the capsid changes the photophysical interactions between the fluorophores, and this can be spectrally monitored. The presented data show that this low complexity method can be used to study and monitor the disassembly of supramolecular protein complexes like virus capsids. However, the range of labeling densities that is suitable for this assay is surprisingly narrow.     

Computing a molecule in its environment: A mathematical viewpoint

This article is the sequel of a previous survey where we presented the mathematical results rigorously known on the models used in quantum chemistry to simulate an isolated molecular system. The emphasis here is on the models that take into account the environment of the molecule, namely, the models of the condensed phase and the models of interaction with electromagnetic fields. Some related issues are also dealt with. A bibliography of more than 150 references are provided. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 227–250, 1999